A new, very massive modular Liquid Argon Imaging Chamber to detect low
  energy off-axis neutrinos from the CNGS beam. (Project MODULAr)
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A new, very massive modular Liquid Argon Imaging Chamber to detect low 
energy off-axis neutrinos from the CNGS beam. 
 (Project  MODULAr) 
B. Baibussinov1, M. Baldo Ceolin1, G. Battistoni2, P. Benetti3, A. Borio3, E. Calligarich3, 
M. Cambiaghi3, F. Cavanna4, S. Centro1, A. G. Cocco5, R. Dolfini3, A. Gigli Berzolari3, 
C. Farnese1, A. Fava1, A. Ferrari2, G. Fiorillo5, D. Gibin1, A. Guglielmi1, G. Mannocchi6, 
F. Mauri3, A. Menegolli3, G. Meng1, C. Montanari3, O. Palamara4, L. Periale6, A. Piazzoli3, 
P. Picchi6, F. Pietropaolo1, A. Rappoldi3, G.L. Raselli3, C. Rubbia[A]4, P.Sala2, G. Satta6, 
F. Varanini1, S. Ventura1, C. Vignoli3 
1Dipartimento di Fisica e INFN, Università di Padova, via Marzolo 8, I-35131 
2Dipartimento di Fisica e INFN, Università di Milano, via Celoria 2, I-20123 
3Dipartimento di Fisica Nucleare, Teorica e INFN, Università di Pavia, via Bassi 6, I-27100 
4Laboratori Nazionali del Gran Sasso dell’INFN, Assergi (AQ), Italy 
5Dipartimento di Scienze Fisiche, INFN and University Federico II, Napoli, Italy 
6Laboratori Nazionali di Frascati (INFN), via Fermi 40, I-00044 
Abstract. 
The paper is considering an opportunity for the CERN/GranSasso (CNGS) neutrino complex, concurrent 
time-wise with T2K and NOvA. It is a preliminary description of a ≈ 20 kt fiducial volume LAr-TPC following 
very closely the technology developed for the ICARUS-T600, which will be operational as CNGS2 early in 
2008.  
The present preliminary proposal, called MODULAr, is focused on the following three main activities, 
for which we seek an extended international collaboration: 
 (1) the neutrino beam from the CERN 400 GeV proton beam and an optimised horn focussing, 
eventually with an increased intensity in the framework of the LHC accelerator improvement programme.  
 (2) A new experimental area LNGS-B, of at least 50’000 m3 at 10 km off-axis from the main 
Laboratory, eventually upgradable to larger sizes. As a comparison, the present LNGS laboratory has three halls 
for a total of 180’000 m3.  A location is under consideration at about 1.2 km equivalent water depth. The bubble 
chamber like imaging and the very fine calorimetry of the LAr-TPC detector will ensure the best background 
recognition not only from the off-axis neutrinos from the CNGS but also for proton decay and cosmic neutrinos.   
(3) A new LAr Imaging detector, at least initially with about 20 kt fiducial mass. Such an increase in the 
volume over the current ICARUS T600 needs to be carefully considered. It is concluded that a single, huge 
volume of such a magnitude is uneconomical and inoperable for many reasons.  A very large mass is best 
realised with a modular set of many identical, but independent units, each of about 5 kt, “cloning” the basic 
technology of the T600.  Several of such modular units will be such as to reach at least 20 kt as initial sensitive 
volume. Further phases may foresee extensions of MODULAr  to a mass required by the future physics goals.  
Compared with large water Cherenkov (T2K) and fine grained scintillators (NOvA), the LAr-TPC offers 
a higher detection efficiency for a given mass and lower backgrounds, since virtually all channels may be 
unambiguously recognized. In addition to the search for θ13 oscillations and CP violation, it would be possible 
to collect a large number of accurately identified cosmic ray neutrino events and perform search for proton 
decay in the exotic channels. 
The experiment might reasonably be operational in about 4/5 years, provided a new hall is excavated in 
the vicinity of the Gran Sasso Laboratory and adequate funding and participation are made available. 
(April 9,2007) 
                                                
[A] Corresponding author: Carlo.Rubbia@cern.ch 
Table of contents. 
1.— General considerations. ...............................................................................................3 
1.1. Physics introduction....................................................................................................3 
1.2. Comparing present and future detectors toward
. ....................................................3 
2.— The next LNGS neutrino detector................................................................................5 
2.1. General considerations. ..............................................................................................5 
2.2. A modular approach of the LAr-TPC detector. ............................................................6 
2.3. Double phase LAr-TPC signal collection? ..................................................................6 
2.4. A simplified structure for the modular detectors. .........................................................8 
2.5. The new experimental area. ......................................................................................11 
2.6. Initial filling procedures for the chamber. .................................................................13 
2.7. LAr purification. .......................................................................................................13 
2.8. Photo-multipliers for light collection.........................................................................14 
2.9. Electronic readout and trigger. .................................................................................14 
2.10. R&D developments. ................................................................................................15 
3.— The new low energy, off-axis neutrino beam (LNGS-B). ..........................................17 
3.1. The present high energy CNGS beam configuration. .................................................18 
3.2. A new, low energy focussing layout. ..........................................................................18 
3.3. Comparing the CNGS  and NOvA neutrino beams.....................................................19 
3.4. Detection efficiency for νe CC events and NC background rejection..........................21 
3.5. Comparisons with NOvA. ..........................................................................................23 
3.6. Evaluation of the beam associated 
 background. ...................................................24 
3.7. Comparing the ultimate  sensitivities to 
 and 
13( ). ......................................24 
4.— Tentative layout of LNGS-B. ....................................................................................26 
5.— Conclusions. .............................................................................................................28 
6.— References ................................................................................................................30 
7.— Appendix. General comments on the use of Perlite....................................................32 
1.— General considerations. 
1.1. Physics introduction. 
The understanding of neutrino has recently advanced remarkably with the observation 
that they have masses and that oscillate between each other. Oscillations arise in analogy to 
the CKM matrix for hadrons since the neutrino species 
 do not have specific masses, 
but are a combination of the mass eigenstates 
. Two of these oscillations, namely 
related to 
$( ) and 
 related to 
 have been experimentally observed by 
SK1. A third oscillation type characterized by 
, occurring around 
2  has not been 
observed. The observation of a non-zero value of 
 will open the way to the ordering of the 
neutrino masses and a determination of the CP violation phase 
"  in neutrino oscillations. CP 
violation in the lepton sector will be necessary in order to understand why matter is 
dominating over anti-matter in the Universe.  The very small but finite values of the neutrino 
masses require the existence of right-handed neutrino species and more generally neutrinos 
appear to be related to physics at an extremely high energy scale, beyond studies with 
accelerator beams. 
It is also possible that in addition to the indicated three types of neutrino species, other 
species could exist, oscillated by the 
.  There is unconfirmed evidence for the 
existence of this type of “sterile” neutrinos from the LNSD experiment [1] at Los Alamos 
National Laboratory. A search for evidence for sterile neutrinos is being pursued by 
MiniBooNE [2] at FNAL and ICARUS-600T [3] at LNGS.  
1.2. Comparing present and future detectors toward
First generation long baseline neutrino experiments are currently operational at K2K [4] 
over a baseline of 295 km, at FNAL and at CNGS with baselines of about 730 km. These 
developments should be further exploited in Japan and presumably also in the USA and 
Europe with some second generation experiments of much higher sensitivity, to become 
operational around 2010-2015. This requires major improvements both in the beam and in the 
detector mass and performance.  
The present detectors at FNAL (MINOS) [5] and CNGS2 (ICARUS) [3] are 
respectively a Iron-Scintillator sandwich of 2.5 cm iron and 4.1 cm wide scintillator strips 
with 5.4 kt total, 3.2 kt fiducial (MINOS, two modules) and a liquid Argon detector of a 
slightly lower mass of about 600 t of sensitive volume (ICARUS).  
 It is important to underline that in practice these two detectors have roughly 
comparable discovery potential in many channels because of the much higher resolution 
capabilities of LAr-TPC when compared with Fe-scintillation sandwich. The main beam 
requirement is the average target power of the incoming proton beam (POT) that are 
                                                
1 The experiment OPERA-CNGS1 is intended to observe explicitly the 
 appearance (M. Guler et 
al., [OPERA Coll.], CERN/SPSC 2000-028, SPSC/P318, LNGS P25/2000). 
presently comparable for the CERN SPS2 at 400 GeV and the FNAL main Injector at 120 
GeV, with about 170 kWatt on target. It is foreseen that a major improvement programme at 
FNAL will increase the beam intensity to up to 1MWatt of beam power and even beyond.  
An experimental proposal under consideration with a target date of circa 2012 is the 
NOvA [6] experiment, a totally active, fine grained scintillator of about 25 kt.  In comparison 
with MINOS, NOvA will have (1) a much greater mass; (2) a better identification of the 
electron type neutrinos, with a sampling of 0.15 r.l., compared to 1.5 r.l. for MINOS; (3) 
about 80% of the mass is active, when compared to 5% for MINOS; (4) the beam is located 
off-axis, in order to increase the number of events which are most sensitive to 
, namely 
= 2 ±1 GeV ; (5) a much higher intensity neutrino beam, corresponding to 5 ÷ 25 x 1020 
POT/y at 120 GeV, although this number is still subject to some uncertainties. The nominal 
quoted value is 6.5 x 1020 POT/y at 120 GeV.  
We consider here the possibility of a substantial and equivalent upgrade of a LAr-TPC 
detector for CNGS, having in mind competition and timetable comparable to the ones of 
NOvA [6] and of T2K [7]. We keep in mind that the key process is the observation of the 
oscillation driven 
events. As already pointed out, the use of the imaging capability of 
the LAr-TPC ensures a much higher discovery potential than in the case of scintillator (or 
water) detectors, i.e. a comparable sensitivity may be achieved with a much smaller sensitive 
mass.  
The scientific community at large is presently considering also conceptual designs for 
huge, “ultimate” detectors in the order of one or more hundreds of kiloton [8] [10], with huge 
costs, comparable to the ones of the LHC experiments: some R&D efforts are presently going 
toward this long distance goal [9]. Amongst them one has described a bi-phase LAr-TPC 
detector of a mass of 100 kt with amplification in the gas phase (GLACIER) [10], a huge 
liquid scintillator of about 50 kt (LENA) [8] and a water Cherenkov counter of 440÷730 kt 
(MEMPHYS) [8]. In USA and Japan two analogous projects (UNO and HyperKamiokande) 
have been proposed [8].  
But, no doubt the next practical steps for the period 2011÷2013 are still of more modest 
magnitude, of the order 20 ÷ 30 kt fiducial mass. Two programmes are already under 
development in Japan and in the US, namely the T2K combination of a new high intensity 50 
GeV proton accelerator aiming at the well known SK water Cherenkov detector [7] and the 
NOvA scintillation detector of similar fiducial mass with neutrinos from the 120 GeV Full 
Energy Injector at FNAL [6]. They both are intended to operate with off-axis neutrino beams 
and for an optimum neutrino energy window of 2 ± 1 GeV.  
The present paper is considering opportunities for the CNGS neutrino complex after the 
completion of the present OPERA/ICARUS phase, which should be completed by about 
2011, concurrent time-wise with T2K and NOvA.   
                                                
2 However the fraction of time dedicated to neutrino beam is smaller at CERN than at FNAL The 
assumed efficiency for the parasitic operation at CERN is ≈ 50%, corresponding to a nominal 4.5 
x1019 POT/y, equivalent to 400/120*4.5 1019 = 1. 5 x1020 POT/y at FNAL and 120 GeV.   The neutrino 
event rate is of 3000 ev/kt/y for FNAL and 2800 ev/kt/y for CERN.  
2.— The next LNGS neutrino detector. 
2.1. General considerations. 
The T600 detector is now readied in Hall B and it is expected to become operational 
before the end of 2007. We will collect in the subsequent years a large number of beam 
associated and of cosmic ray events that will perfect the technology and provide a rich 
amount of experimental physics.  The T600 is therefore considered a necessary step toward 
the realisation of any much larger LAr-TPC detector. Running of the T600 will provide 
absolutely essential experience, which is required in order to develop sensibly such a “next 
step”. Evidently, a number of modifications are required in order to ensure the scalability of a 
detector to much larger sizes.  These are presently under active consideration. 
The present CNGS proposal is focused on the following three main activities, for which 
we seek a larger international collaboration: 
1. a new neutrino beam configuration derived from the existing horn focussing and the 
existing proton beam line from the 400 GeV SPS, eventually with an increased 
intensity in the framework of the LHC related accelerator improvement programme. 
Relatively modest changes in the neutrino beam focussing of CERN will produce a 
nearly optimal beam configuration.  
2. A new experimental area, eventually enlarged in future phases, which we indicate as 
LNGS-B to be realised about 10 km off-axis from the main laboratory, away from the 
protected area of the Gran Sasso National Park, without significant underground 
waters and with a minimal environmental impact. A provisional location is under 
consideration, corresponding to about 1.2 km of equivalent water depth. The high 
event rejection power of the LAr-TPC detector will ensure the absence of 
backgrounds not only for the neutrinos from the CNGS but also for proton decay and 
cosmic neutrinos.  
3. A new LAr-TPC Imaging underground detector made of several modular units, each 
of about 5 kt fiducial mass. As a first step, a total of about 20 kt will be realised with 
appropriate safety requirements and along the lines of the vast R&D work carried out 
over the last decades by INFN and other International Institutes and culminating in 
the actual operation of the T600, foreseen in 2008 with the ICARUS experiment.  
This programme may eventually be improved further on with additional modules, 
depending on the developments of the programmes with and without accelerators. 
The forthcoming operation of the T600 detector in the real experiment LNGS-B will 
represent the completion of a development of the LAr-TPC chamber over more than two 
decades. As it is described in this paper, the operation of the T600 evidences the large 
number of important milestones which have been already achieved in the last several years, 
opening the way to the development of this new line of modular elements and which may be 
extrapolated progressively to the largest conceivable LAr-TPC sensitive masses.  
As described later on, the new detector will maintain the majority of components that 
have been developed with industry for the T600. The detector should be easily upgraded in 
the far future to a larger scale, depending on the potential physics goals.  
The off-axis physics programme is not making obsolete the on-axis searches, presently 
concentrated on the T600, which both contribute to a wider physics programme and may also 
profit of the advances offered by the MODULAr concept.   
2.2. A modular approach of the LAr-TPC detector. 
Conceptual designs have been described in the literature [11] with a single LAr 
container of a huge size, up to 100 kt. But already in the case of containers of few thousand 
ton the geometrical dimensions of most types of events under study (beam-
" , cosmic ray-
proton decays) are relatively confined, i.e. much smaller than the fiducial volume. Hence 
increasing the container’s size does not appreciably affect the acceptance in fiducial volume 
of each event and introduces no significant physics arguments in its favour. We believe that 
there are instead serious arguments for which such an huge size approach cannot be easily 
realised in practice and that suggest instead the use of a modular structure of several separate 
(identical) vessels, each one however of the size of a few thousands ton.  
In case of an accidental leak of the ultra-pure LAr, the amount of liquid that is spoiled 
is proportional to the actual volume of the container. Segmentation is therefore useful in 
overcoming events due to poisoning of the liquid. In the case of a major damage of the 
detector, the liquid can be provisionally transferred to another container.  An additional, 
reserve vessel of the order of 100 kt is, on the other hand, not realistic.  In addition, the safety 
requirements of an underground vessel are strongly dependent on its size.   
One of the most relevant features of LAr-TPC is its ability to detect accurately 
ionisation losses at the percent level. Over the very large volume, the inevitable in-
homogeneities in electron lifetime due to even modest variations in purity of the LAr produce 
very large fluctuations in the actual value of the collected charge and hamper the possibility 
of charge determination along the tracks. Therefore we have chosen to use a modular 
approach of sufficient size in order to reduce the effects due to the non-uniformity of the 
electron collection due to the emergence of negative ions, which impose a reasonably short 
maximum drift distance of each gap.  
As it will be discussed further on, it has been assumed that a reasonable sensitive 
volume should be of 8 x 8 m2 cross section and a length of about 60 m, corresponding to 
3840 m3 of liquid or 5370 t of LAr. The drift length is 4 m. A field shaping grid should be 
added in the middle of the HV gap in order to reduce the effects due to the space charges to a 
negligible level. A reasonable three-plane wire pitch for such a large container should be of 
the order of 6 mm, twice the value of the T300.  
2.3. Double phase LAr-TPC signal collection? 
Several years ago [12] the ICARUS collaboration had studied a double phase Noble gas 
arrangement, in which ionization electrons from the tracks are drifted from the liquid to a 
superimposed gaseous phase. Electrons were further accelerated and ionised with the help of 
a grid, like in an ordinary gaseous TPC, before being collected by the readout wire planes. 
Dark matter searches in Argon (WARP) [13] and in Xenon (XENON) [14] have profited of 
this technique.  
Essentially the same idea has been also envisaged some time ago for very large (≥ 100 
kt) monolithic LAr-TPC detectors.  In the specific case of GLACIER [10], it has been 
proposed a very large electron drift length of 20 m at 1 kV/cm in LAr, corresponding to a 
drift voltage of as much as 2 MV, about a factor ten larger than the one discussed in the 
present proposal.  If one assumes that the free electron lifetime is at least 2 ms [10], one 
expects an attenuation of the free electrons due to ion recombination in the impurities, 
presumably Oxygen, of as much as a factor ∼150 after 20 m. This residual free electron 
component cannot be directly recorded electronically (as in ICARUS) over the whole drift 
distance and must be therefore first amplified by the proportional gain profiting of the 
gaseous phase.  In practice, taking into account the large capacitance of the extremely 
extended read-out electrodes (up to 70 m) and consequently of the larger noise signal from 
the input FET, gains of the order of 103 are typical required values. The instrumentally 
increased dynamic range of the signals collected must take in full account this huge dynamic 
factor along the drift time extent.  
Therefore in the double phase arrangement, all three different types of charged particles 
have to be simultaneously considered, namely (1) the initial free electrons from Argon which 
are attenuated over the distance by as much as two orders of magnitude, (2) the accumulated 
negative ions from recombination by impurities and (3) the positive Argon ions especially 
from the multiplication near the wire in the gas. The ion speeds in liquid are extremely slow, 
typically far less than 2 mm/s3 at 1 kV/cm (with 2 MV over 20 m the drift time is ≈ 10’000 s, 
i.e. about 3 hours!). 
It has been demonstrated in WARP and XENON experiments that free electrons can 
overcome the liquid to gas barrier in the presence of a sufficiently strong electric field of a 
few kV/cm. It is expected that both the positive ions travelling from gas to liquid and the 
negative ions travelling from liquid to gas, because of their larger masses and hence smaller 
speed, will be ultimately trapped and accumulate at each side of the liquid-gas boundaries for 
a so far unknown period of time.  
The free electrons crossing the double ion layer cloud are presently under study with 
the 2.3 litre WARP detector underground at the LNGS. There is some preliminary evidence 
already in this small detector that space charges due to ion crossing at the boundary may 
introduce additional fluctuation in the electron ionisation signal. This will introduce a 
substantial worsening of one of the most relevant features of LAr-TPC, namely its ability to 
detect accurately ionisation losses. 
 The consequence for a detector of the size of 70 million litres and a diameter of 70 m, 
is an enormous extrapolation, which requires a very extensive R&D. The phenomenon of 
transfer of ions through the interface is expected to be rather complicated, not well 
understood and it has not been conclusively measured experimentally [15]. In the case 
relevant to GLACIER, it is likely that some trapping times may be ultimately occurring, but 
                                                
3 We remark that on such a timescale of hours convective motions inside the vessel become very 
relevant and they may seriously modify the time of drift of positive ions in either direction. 
experimental studies are needed to assess how much.  This is an absolutely crucial point prior 
to a successful, practical realization of a huge dual phase experiment in Argon.  
A first step is the 5 m long prototype ARGONTUBE [16] under study on surface in 
Bern, which will allow to experimentally verify these hypotheses and prove the feasibility of 
detectors with long drift paths, representing a very important milestone in the conceptual 
proof of the feasibility of the dual phase detector in Argon. 
It is therefore concluded that at least at the present stage of the LAr-TPC, the single 
phase geometry which has been already very well developed experimentally [3] is vastly 
preferable and associated to a drift path length which could minimize the extent of the 
negative and positive ions. Negative ions in this configuration are smoothly drifting and 
directly captured by the collecting wires with a negligibly small signal (the electric signal is 
proportional to the drift speed, a factor ≈10-5 smaller for ions). Positive ions are 
straightforwardly collected at the cathode. For a sufficiently small drift volume, like the one 
described in the present proposal, the electric field distortions due to the slow ion motion can 
be made to be negligible. 
2.4. A simplified structure for the modular detectors. 
The structure of the detector has been considerably streamlined in order to reduce the 
number of components, its cost and increase the reliability of the system.  The modular 
structure permits to repeat the initial engineering design of the prototype to a series of several 
subsequent units, reducing progressively their costs and their construction time.  
Clearly the main aim of the detector is the one of filling and maintaining over many 
years a very large amount of ultra-pure LAr in stable conditions inside a dedicated 
underground cave, within very rigid safety conditions.  The initial filling procedure is 
determined by the supply rate currently provided by the supplying industry. We believe that 
the maximum rate available in the European market is of the order of 200 m3/d. At the rate of 
100 m3/d, the initial filling of the required about 25 kt of ultra-pure LAr (corresponding to a 
volume of 17’000 m3) is about 170 days, which is acceptable. At the commercial cost of 0.7 
Euro/l, the value of the LAr for the initial filling is about 12 MEuro, which is also quite 
acceptable.  The mechanical structure of each of the modular units should be as simple as 
possible, keeping the costs of the various components commensurate to the relatively modest 
initial investment for the LAr.  
The wire arrangement is scaled out from the industrial realisation of the existing 
ICARUS-T600, which is taken as a reference design. As well known ICARUS is made of 
two identical modules (T300). In each of the T300 made of two readout planes and a high 
voltage plane in a double gap configuration, the three readout planes have coordinates at 0° 
and ± 60° with respect to the horizontal direction. This identical geometry is “cloned” into a 
larger modular detector, with the linear dimensions scaled by a factor 8/3 =2.66, namely the 
cross sectional area of the planes is now 8 x 8 m2 rather than 3 x 3 m2. The wires in the 
longitudinal direction were originally 9.4 m long with the wire planes subdivided in two 
equal segments.  In the next step the length will be quantised also into two individual wire 
sets, but 25 m long, corresponding again to the ratio 25/9.4 = 2.66.  The longer wires have a 
higher capacitance and the signal/noise ratio is significantly decreased (wires, of the order of 
10pF/m; cables, of the order 50pF/m). This factor is compensated widening the pitch to 6 
mm, to be compared to the previous 3 mm, doubling the
dE dx  signals.  Therefore we expect 
signal/noise ratios which are rather similar to the ones of the T600, namely of the order of 
10/1. As it will be discussed later on, in collaboration with industry (CAEN), over the last 
several years the electronic chain from “wire to computer” has been considerably improved 
in performance and reduced in cost4.  
Each wire is now observing a time projected volume which is a factor 2.66 x (4/1.5) x 2 
= 14.2 larger than in the case of the T600 (wire length x drift length x wire pitch). Therefore 
the average LAr mass observed by each TPC readout wire is about 200 kg/channel. A 20 kt 
sensitive volume will then require of the order of 105 wires.  
At this stage the configuration of the modules may not be considered as absolutely 
frozen and a number of possible configurations are possible, as shown in Figure 1, 
maintaining as a reference a readout plane dimension of 8 x 8 m2. 
 Figure 1a represent the previously indicated basic configuration of a scaled T300 
double gap arrangement. The nominal voltage of the T300 is 75 kV for the 1.5 m long drift, 
corresponding to a drift field of 500 V/cm, although the field-shaping electrodes have been 
currently operated without problems up to 150 kV. The engineering design for a T1200, 
never constructed, required a 3 m drift length. At the same nominal electron drift velocity 
(500 V/cm), for the present choice of 4 m drift, the HV would be 200 kV.  However a 
significantly higher field, like for instance 350 kV, will shorten the drift time, 
                                                
4 The estimated commercial cost of each channel “wire to computer” is now about 60 Euro. Assuming 
for each wire a sensitive volume of 200 kg, the levelized cost of the electronics is 60/200 = 0.3 
Euro/kg, which is about ½ of the cost of the Argon procurement. The low cost achieved for such a 
sophisticated electronics is an additional argument in favour of a single phase LAr configuration with a 
relatively short drift gap distance, along the lines of the T600, rather than the double phase GLACIER 
arrangement.   
Figure 1. Various possible alternative arrangements for a modular unit. In (a) we show a scaled up 
T300 configuration with 4 m drift time and about 25’000 readout wires. The sensitive mass is about 
5000 ton. In (b) the configuration is the same as in (a), except the drift distance has been reduced to 
2 m and doubling the number of readout wires. In (c) we show a scaled up T600 configuration with a 
twin module, 4 m drift time and about 50’000 wires. The sensitive mass is then 10’000 tons.   
drift
# sqrt(E
drift
)) and reduce correspondingly the requirements of purity for the LAr to the 
case already optimised of a T1200 with a 3 m drift.  
In Figure 1b we have doubled the number of wire planes in order to reduce the drift 
distance to 2 m. In this configuration the halving of the drift time to the already successful 
configuration of the T300 is performed doubling the number of signal wires to 50’000, with a 
significant increase in the cost of the channels. In order to maintain an electron drift time 
exactly the same as the one of T300 since 
drift
# sqrt(E
drift
)  we need an increase of the drift 
field to
75kV " 2 /1.5( )
= 89kV .  Note that the HV of the T300 has been tested up to 150 kV 
without any problem. Although we believe that the drift distance can be safely extended to 4 
m, this alternative shows that the choice of the electron drift length is not determinant. 
Solution 1b, eventually with an even higher drift field to reduce the maximum drift time to 
values below the ones of the present T600, is perfectly possible in case that some unforeseen 
problem may develop, obviously at the cost of doubling the number of electronic channels.  
Finally in Figure 1c we show a scaled up T600 twin volume configuration, with 4 m 
drift time and about 50’000 wires. The two volumes are physically separated, but they are 
both kept in the same cryogenic volume. The total sensitive mass of one 1c module is then 
10’000 tons. The new proposed halls of the LNGS and 20 kt could host two modules of type 
1c in one container.  
Considerable experience of the ICARUS collaboration has shown that free electron 
drift times 
drift
 of several milliseconds are currently realised with commercial purification 
systems based on Oxysorb™. The effects on the electron attenuation are shown in Figure 2 
where the drifting charge attenuation versus drift path at different electric field intensities are 
given for 
drift
 = 10 ms and for different electron lifetimes at 0.5 kV/cm.  
Figure 2. The maximum free electron attenuation into negative ions is shown for different values 
of the maximum electron drift path, respectively (1) for different values of the free electron 
lifetime at E = 0.5 kV/cm and (2) for different electric drift fields and a 10 ms electron lifetime.  
Cases (a) and (b) represent respectively the configurations of Figures 1a and 1b with 4 m and 2 
m drift paths.   
The longitudinal r.m.s. diffusion spread 
 after an electron drift path 
x  and moving at 
a speed 
 is given by 
= sqrt 2D x v
D( ) , where 
D = 4.06  cm
"1 . In more practical units, 
# 0.9sqrt $
drift
ms( )[ ]  mm . For a drift field of 0.5 kV/cm and a 4 m path the average value 
=1.1 mm  and the maximum value is 
D max
=1.6  mm .  
The new mechanical structure, which has been highly streamlined, is essentially made 
of only three main mechanical components: 
(1) An external insulating vessel made of two metallic concentric volumes, filled 
in between with perlite (see Appendix for details). Perlite is a mineral which 
is vastly used industrially, the world consumption being of the order of 2 
million tons annually5. The environmental aspects of perlite are not severe: 
mining generally takes place in remote areas, and airborne dust is captured 
by bag-houses, and there is practically no runoff that contributes to water 
pollution. In order to ensure an adequate thermal insulation, about 1.5 m 
thickness is required, corresponding to over 3000 m3 for a container. The 
bottom-supporting layer is made out of low conductivity light bricks. The 
specific heat loss is 3.86 W/m2 for a nominal thermal conductivity of 0.029 
(0.025-0.029) W/m/K. This is significantly smaller than the specific heat loss 
of the T600. Taking into account the dimensions of the vessel, the total heat 
loss is 8.28 kW. At present in the LNGS the cryogenic plant of ICARUS 
T600 is made of 10 units, each with 4 kW of (cold) power.  Three of such 
units (≤ 12 kW) should be adequate to ensure cooling of the walls of the 
vessel during normal operation. Evacuation of the perlite is therefore 
unnecessary.  
(2) A linear supporting holding structure frame with wire planes at each lateral 
side and the high voltage plane at the centre. The photomultipliers for the 
light trigger are also mounted on this frame behind the wire planes. The 
structure of the planes is identical to the one already developed for the T600, 
except that only one wire out of two is installed in order to go from 3 mm to 
6 mm pitch. The inner structure of the huge container is therefore extremely 
simple, being primarily a linear wire structure along the edges of the 
container, the rest remaining essentially free of structures.  
(3) The liquid Ar and N2 supply and refrigeration, provided with cooling and 
purification both in the liquid and gas phases, with an appropriate re-
circulating system to ensure that the whole liquid is moving orderly inside 
the vessel volume to unsure uniformity of the free electron lifetime.    
2.5. The new experimental area. 
The ICARUS-T600 detector is located inside the Hall B of the LNGS laboratory in an 
appropriate containment tank constructed above the floor of the Hall.  The new experimental 
                                                
5 The present production in Greece, where vast resources are available, is about 500 thousand t/y. 
The estimated cost is about 40 $/ton.  
area, which we indicate as LNGS-B, to be realised about 10 km off-axis from the main 
laboratory, away from the protected area of the Gran Sasso National Park, without significant 
underground waters and with a minimal environmental impact. A provisional location is 
under consideration, corresponding to about 1.2 km of equivalent water depth. The high 
event rejection power of the LAr-TPC detector will ensure the absence of backgrounds not 
only from the neutrinos from the CNGS but also for proton decay and cosmic neutrinos. 
The total volume excavated for the original LNGS was of about 180’000 m3. It is 
foreseen that the new LNGS-B could be about one half of this volume, namely initially about 
50’000 m3.  However, as a difference from the main LNGS, the shapes of the cavities, rather 
than being vast, general purpose halls, are tailored to the specific experiment.  
Each modular detector unit is located in an appropriate “swimming pool” cave in the 
rock concentric to the perlite walls, where the liquid tank is contained: therefore there is no 
realistic possibility of leak outside the walls of the rock for any foreseeable circumstance. 
The worst case is the total loss of external cryogenic cooling both of N2 and of Ar.  Therefore 
Figure 3. Indicative cross section of the T600 “clone” in the dedicated “swimming pool like” 
underground hall. The lower part is made of two twin separate LAr containers made of 
Aluminum extruded structures, thermally stabilized with forced N2 circulation. Outside the 
structure an about 1.5 m thick perlite wall provides spontaneous, passive heat insulation. The 
region on top of the “swimming pool” is accessible to auxiliary equipments. Personnel access 
is strictly controlled. 
the tank will spontaneously warm up in contact with the heat leaks of the surrounding 
components through the 1.5 m thick perlite wall. Assuming a heat leak rate of 10 kWatt, the 
LAr evaporation rate is of 220 kg/h, negligibly small with respect to the 5 kt of the stored 
LAr tank. Therefore the tank will remain stable in its liquid form for any specified length of 
time.  More generally there is not even the most remote possibility to provide from the 
environment around the cavern a sufficiently large amount of heat in order to cause a 
catastrophic evaporation of a massive amount of LAr.  
For a configuration of the type 1c, the cross section is 11 x 20 m2 (shown in Figure 1) 
and the length is about 60 m. Different containers may have entirely separate halls since the 
event containment is anyway very good. An exhaust pipe is necessary in order to evacuate 
the evaporated liquid into the atmosphere in case of an accidental leak, although a risk 
analysis will certainly show that the probability of such events is very small.  
2.6. Initial filling procedures for the chamber.  
In the present T600 the vessel is evacuated in order to inject ultra pure Argon. The new 
detector, in view of its large size is very hard to evacuate and a new method has to be applied. 
The idea is to perform successive flushing in the gaseous phase in order to attenuate the 
presence of gases other than Argon with an approximately exponential chain.  This method of 
flushing with pure Argon gas is widely used already in gaseous wire and drift chambers 
which are generally not evacuated. In the present case, additional problems may arise in view 
of the magnitude of the volume and the possibility of creating for instance “dead” spots, in 
which the gas may not circulate. A suitable small scale test dewar container is under 
construction in order to perfect the method. 
In the idealised case of complete turbulent and continuous uniform mixing through the 
container, the transition air-argon is an exponential with a factor ≈ 1/2 at each passage. 
Therefore, in order to achieve an attenuation of the order of 10-6, 14 cycles are necessary. If 
instead the Argon is injected with little or no turbulence, for instance uniformly from the 
bottom with the extraction of initial air on the top, the transition argon-air moves orderly 
from the bottom to the top and only pure air exits from the top, producing a faster and more 
orderly transition. These are limiting cases and the efficiency of the actual filling will need 
some model studies and some hydrodynamic calculations to be perfected. Some preliminary 
considerations indicate that about 6 cycles may be necessary. The density of the gaseous 
Argon at room temperature is about 600 times smaller than the one of the liquid. Hence a 
ultimate gas purification of the order of 10-6 would correspond to an increment due to filling 
of the order of 2 x 10-9 (2 ppb) with pure liquid, which is adequate for the initial filling before 
local purification.  
2.7. LAr purification.  
In order to ensure a free electron lifetime adequate for the longest ≈ 3ms fly-path, a 
vigorous purification of the LAr must be kept at all times with filtering methods based on 
Oxysorb™ and molecular sieves. In analogy with what is currently performed with T600 and 
all previously constructed detectors, the purification is performed both in the liquid and in the 
gaseous phase. An improvement in the purification system is needed to enlarge in a 
significant way the TPC volume. New purification devices have to be implemented, possibly 
operating either near or directly inside the cryostat. They should be simple, robust and 
without moving parts, to guarantee a total reliability. 
An order of magnitude of the liquid re-circulation rate needed to reach safely the 
running condition in few months (for example 60 days) could be of the order of several 
percent of volume per day. For a volume of 4000 m3, this rate means a re-circulation rate of 
the order of 240 m3/day (6%/day or purification cycle in 16.6 days), which is in the range 
possible with Oxysorb™6. 
In conclusion, some thousands cubic meters is a reasonable limit for a single TPC 
volume.  It could be cleaned, cooled and filled in few months and then kept completely 
operative (with an adequate LAr drift length) after few months. Altogether, such a detector 
could be put in operation in a reasonable period of time. 
2.8. Photo-multipliers for light collection. 
Like in the case of T600 a number of photomultipliers located behind the readout wires 
are used to provide a t=0 trigger. This is particularly important for the cosmic rays and proton 
decay events in which no starting signal is provided, but could be as well very useful in order 
to tag events coming from the CNGS beam. The technique already used in the case of the 
T600 consists in glass phototubes with a thin deposit of wave-shifter in order to record the 
scintillation light from the LAr. A significant contribution is also due to the Cherenkov light 
emitted directly in the visible by relativistic particles.  
2.9. Electronic readout and trigger. 
The ICARUS T600 detector has a DAQ system (5·104 channels) designed at University 
of Padova/INFN, engineered and built by CAEN. It has proven to perform quite satisfactory 
in the test run performed in Pavia during summer 2001. 
The electronics has been described in various papers and technical notes. We remind 
here that it is based on an analogue front-end followed by a multiplexed AD converter (10bit) 
and eventually by a digital VME module that performs local storage and data compression. 
In the following, starting from the experience gained in the T600 operation, we discuss 
performance and limits of the actual system with the aim of improving its characteristics in 
view a multi-kton TPC with a number of channels in the order of several times x 105. 
Since 1988, in the ICARUS proposal, the main characteristics of the signals were 
described and subsequently they were confirmed by tests on small chambers and eventually 
by the operation of the T600. In a multi-kton TPC we can foresee wires (or in general 
electrodes) with a pitch larger than the 3mm used in the T600. A reasonable assumption 
would be a 6mm pitch that will allow using most of the tooling already built and designed for 
the T600. 
                                                
6 The standard rate of a single Oxysorb™ pack is about 120 m3/d. Therefore two of such units are 
sufficient for the chosen size of the vessel. 
The capacitance associated to each channel will be determined by the capacitance of the 
wires, in the order of 10pF/m, in parallel with the capacitance of the cable, in the order 
50pF/m. Let's assume in the following discussion ~600 pF as a reasonable number for 10m 
electrode wires and average 8m of cable.  
The dominant noise in a high capacitance detector is the series noise esn (voltage noise) 
linearly increasing with the input total capacitance (CD) while the parallel noise (current 
noise) contribution is proportional to the shaping time of the signal. 
We propose to use the present IC taking into account that due to the need of spares for 
the T600 a silicon run of the specific BiCMOS process of 6 wafers has been recently made. 
Each wafer, taking into account the known yield, contains some 12000 good circuits which 
means 24000 channels. The total number of channels that could be equipped is about 
140*103. These wafers, kept in inert atmosphere, can be easily packaged in very small cases 
(4x4 mm2). A R&D program is also proposed for the development of a hybrid sub module 
hosting eight or more channels of amplification and eventually, as it will be described later, 
also the analogue to digital converter. At present a revision of the ICARUS analogue 
electronics is underway with the aim of further improving the front-end performance.  
In the T600 collaboration a novel technique for the realization of feed-throughs has 
been developed. INFN holds a patent (RM2006A000406) for this technology that allows easy 
realization of feed-through with high density vias and different shapes.  
The ADCs work at 20Mhz sampling rate, interleaved so the 10bit digital output has a 
40Mhz frequency that means that each channel is sampled every 400ns. The power dissipated 
is significant: 500mW. The required bandwidth taking into account that two sets of 16 
channels are merged in a 20 bit word, is 800Mbit/s for 32 channels. 
The main feature of the new design is to move into digital domain all the conversion 
process at a very early stage and to exploit the use of numerical digital filtering techniques. 
The final quality of the converted data is highly dependent on the sampling frequency and 
numerical filtering. 
The trigger system will divide the detector volume in sub-volumes to cope with the data 
acquisition rate required by shallow depth location. The basic structure will reproduce the 
one already implemented in the T600 and it will be based both on analogue signals from 
wires (sums of set of wires) and scintillation light detected by PMs inside the liquid Argon. 
All together will merge with the DAQ architecture taking into account that time resolution 
required is low (must be compared with drift time) and anyway the absolute time will be 
associated to each triggered event. 
2.10. R&D developments. 
The increase of the active LAr volume of about one order of magnitude with respect to 
T600, the streamlining and simplification of the mechanical structures and the new 
developments previously described in the structure of the detector require some specific 
R&D developments, which are not of very substantial nature and could be implemented in 
parallel with the detailed engineering design of MODULAr. These developments (see Figure 
4) are intended to simulate on a small scale the basic innovations with respect to the present 
T600, namely: 
(1) The filling process starting from air to pure LAr, taking into account the 
motion of the gas, optimising the inlet and outlet geometries and minimising 
the number of cycles. 
(2) The thermal convections of the LAr, in order to optimise the temperature 
gradients and to ensure a convincing circulation in all regions of the dewar, 
both in the cooldown phase and in the stationary state. 
(3) The outgassing rate and the recirculation processes required in order to 
achieve the required electron lifetime  
(4) The geometry of the compact re-circulators both in the liquid and in the 
gaseous phases. 
The cryostat is based on a foam glass platform (the simplest solution, industrially well 
tested and implemented) and is surrounded by a perlite-insulated walls, about 1-meter thick.  
This approach for the thermal insulation should be cheaper even if bulkier. The 
instrumentation must be well developed in order to be able to determine both thermal and 
convective measurements in a variety of conditions. 
Finally, also electronics, starting from the positive experience of the T600 may require 
some specific development mainly on the layout improvement of the analogue part. 
Figure 4. Layout of the test unit. This small unit is necessary in order to certify all the major 
changes with respect to the present T600 unit. The cross section is 5 x 5 m2 and the height is 
about 8 m. 
3.— The new low energy, off-axis neutrino beam (LNGS-B).  
The primary goals of future experimental programmes in Japan (T2K), US (NOvA) and 
our present LOI at CNGS are related to the so far unknown angle 
13( ), as a pre-
requisite for a non zero CP-violation phase 
 in the lepton sector. They are all based on a 
neutrino beam with horn focussing and a fine grained detector of about 20-25 kt of fiducial 
volume. 
As it is well known, the highest sensitivity occurs at distances corresponding to the 
maxima and minima of the cosmic neutrino oscillations, namely at the energy interval (2 ± 1) 
GeV for neutrino distances of 830 km (120 GeV) and 730 km (400 GeV) respectively at the 
NOvA and CNGS detectors since the oscillation maximum for the CNGS baseline, assuming 
Δm223 =2.5 10
-3 eV2, is at 1.5 GeV.  The T2K detector will be driven by a 50 GeV proton 
beam and a smaller distance of 295 km and therefore should require correspondingly smaller 
energies.  
In order to optimize a horn driven, conventional neutrino energy spectrum it is now 
universally agreed that the next configuration may be obtained using a high energy beam and 
looking at an off-axis direction in order to shift the Lorentz boost of the relativistic beam 
toward the required energy range, typically in the case of the LNGS and FNAL with 
displaced path distances of several kilometers from the main beam axis.  
As well known this detector will also produce a large amount of additional non-
accelerator physics results, and in particular of (1) proton decay especially in the SUSY 
channels and (2) of cosmic rays 
neutrino events, with considerable 
improvements with respect to SuperK.  
In order to be able to record 
simultaneously (pulsed) accelerator 
and (continuous) non-accelerator 
events, the detector must be triggered 
by the inner photo-multiplier array, as 
already indicated by the T600.   
On a longer timescale it may be 
possible to realize the entirely new 
technology of beta-beams [17], where 
the purity of 
 is extremely high, 
namely a 
 contamination well 
below 10-4. Evidently the residual 
contamination due to neutral currents 
must also be reduced correspondingly. 
As shown in Ref. [18] the presently 
developed LAr technology will be 
capable of realising such required 
Figure 5. The present 400 GeV CNGS neutrino CC 
interaction signal in all its oscillated channels at 731 
km, at three different distances off-axis and on-axis. 
For instance, the value for 12 km off axis 
corresponds to 16 mrad or 0.94°.  
powerful identification capabilities. Therefore the massive LAr-TPC detector presently under 
consideration may continue to be used also in this ‘ultimate” phase, eventually increasing the 
number of modular units.  
3.1. The present high energy CNGS beam configuration. 
The present 400 GeV CNGS beam, optimized for τ appearance experiment, is a high-
energy wide band beam with an average on-axis energy of 18 GeV. The horn/reflector optics 
is designed to focus in the forward direction secondary particle in the 20-50 GeV energy 
range with an angular acceptance of 20-30 mrad. The thin target is optimized to minimize 
pion/kaon re-interactions. The integrated 
contamination is 0.6%. From comparisons with 
the previous WANF beam and the early observations with CNGS, the systematic uncertainty 
on the calculated νe /νµ ratio is expected to be very good, of the order of 5% [19]. Therefore 
the intrinsic 
 contamination, although significant in the search for 
 oscillations, 
may be very precisely calibrated.  
In order to visualize the situation we consider the present CNGS beam geometry 
(Figure 5). Displacing the detector off-axis with respect to the neutrino beam direction by 
several kilometres at the location of the detector strongly reduces the shape of the neutrino 
energy spectrum, which, as expected, becomes progressively softer as the distance from axis 
is increased. The strong 
"  CC signal at 0 km is progressively shifted to smaller energies with 
a much smaller over-all rate, although significantly enhanced at the optimum energy. 
3.2. A new, low energy focussing layout. 
The standard 400 GeV CNGS optics as such is not optimized for 
e  oscillation 
searches, neither on-axis nor off-axis.  In particular, the small angular acceptance of the 
magnetic lenses, around 20-30 mrad, limits the neutrino fluxes at low energies. Optimization 
of the target and beam optics with low energy focussing has been calculated and it is in 
progress. It indicates that a design close to that proposed by the NOvA collaboration is also 
applicable in the case of CNGS. The main changes with respect to the present CNGS design 
are: 
(1) A more compact (without air gaps) and thicker target, to increase tertiary production 
at low energy;  
(2) Larger acceptance (up to 100 mrad) of the horns system in the 7 – 20 GeV pion 
energy (this momentum range offers a kinematically efficient production of 2 – 3 Gev 
neutrinos at 10 –15 mrad off-axis). 
A shorter tunnel length (between one half and two third) could also be considered 
because it would marginally reduce the 
 flux due to muon decays that primarily happen in 
the downstream part of the decay tunnel; this major upgrade of the civil engineering has not 
been considered in the present study.  
In Figure 6 we show the expected event rate, for a hypothetical 
 oscillation 
equal to the present CHOOZ upper limit 
=11°,  #
= 0  and the intrinsic 
 contamination 
from the beam. Of course this is the highest expected signal and most likely the oscillatory 
signal is of much smaller size. Calculations are the result of a full beam simulation based on 
the FLUKA MonteCarlo code [20]. 
 Such new low energy CNGS 
optics (see Figure 7) is very similar to 
the one proposed for NOvA at 120 GeV, 
taking into account that the energies of 
the protons on target (POT) are in the 
ratio 400/120 = 3.3. Preliminary 
calculations at 14.8 mrad off-axis and at 
a baseline of 732 km demonstrate that 
the neutrino spectra obtained with a 
proton beam of 400 GeV are very 
similar in shape to those obtained with 
120 GeV protons, except for the yield, 
substantially higher at 400 GeV.  
The relative behaviour can be 
easily factorized in a good 
approximation. While the resulting 
neutrino beam intensity grows roughly 
proportionally with proton energy and 
therefore the neutrino flux in the 
interesting beam energy region is 
roughly proportional to the beam power 
on target, the detailed shapes of both the initial 
 and of the 
 intrinsic beam 
contamination are relatively unaffected by the proton energy and depend primarily on the 
choices of the target/horn system, the focused momentum range and the secondary particle 
acceptances.  
The beam intensities at 400 GeV and 120 GeV are shown in Figure 7. The plot shows 
that in the focused energy range the ratio of the muon fluxes due to the higher proton energy 
is about 2.6, namely about 80% of the linear increase expected on the energy alone, slowly 
growing to the full proton energy factor for the higher energy tail.  
An entirely similar effect is observed for the ratio of the 
 intrinsic beam 
contaminations and therefore no appreciable difference is also observed in the 
background. 
3.3. Comparing the CNGS  and NOvA neutrino beams.  
The CNGS neutrino beam is presently given for a rate of 4.5 x 1019 POT/y at 400 GeV. 
This rate is believed as generally insufficient for the future off-axis neutrino programmes. A 
vigorous programme is on its way at FNAL in order to accumulate in line with the NOvA’s 
nominal assumption7 a yearly rate of 6.5 x 1020  POT/y at 120 GeV. On the basis of the 
                                                
7 Ref [6]. chap. 11,page 75  
Figure 6. Calculated beam spectra for low energy 
focusing. Rates are estimated for 1019 protons and 400 
GeV on target. Both the event rate for an hypothetical 
 oscillations equal to the present CHOOZ 
upper limit 
=11°,  #
= 0  and the intrinsic 
contamination from the beam are shown.  
previous considerations (factor 2.6) this corresponds to approximately 2.5 x 1020  POT/y at 
400 GeV, about a factor 5.5 larger than the 
present CNGS yearly performance. 
Evidently such a factor must be recovered at 
CERN. 
We should compare more in detail 
dedicated repetition rates and energies at 
FNAL and CNGS. The nominal NOvA’s is 
given for a beam intensity of 6.0 x 1013 ppp 
and a repetition rate of 1.5 s, corresponding 
to a cycle integrated beam power of 768 
kW.  The nominal values for CNGS today 
are two proton batches 50 ms apart, in total 
4.8 x 1013 ppp, every 6 s, corresponding to a 
beam power of 512 kW, which is 2/3 of the 
FNAL value. Therefore the instantaneous 
cycles for fully dedicated accelerators are 
quite comparable.  
The differences (a factor 5.5 rather 
than 1.5) therefore come primarily because of the present CNGS parasitic operation with 
fixed SPS target, an efficiency factor of 55% in the operation of the SPS complex and a less 
intensive operation of the accelerator complex because of the LHC/fixed target sharing. 
Such “human factors” should be supported in order to reduce the gap between the two 
accelerators. We can assume that in several years from now, a dedicated 6 s cycle rate for the 
neutrino beam and an efficiency factor of 80% rather than 50% may become possible.  These 
factors should bring the integrated intensity to 1.2 x 1020 POT/y at 400 GeV, corresponding to 
about 4.0 x 1020 POT/y at 120 GeV. This value is not far from the figure assumed by NOvA’s 
of 6.5 x 1020  POT/y at 120 GeV. With such improvements, presumably possible in several 
Figure 7. Muon neutrino CC event rates for 1019 
protons at 120 GeV (FNAL) and 400 GeV 
(CNGS), calculated with the low energy focusing 
optics, 14.8 mrad off-axis and a baseline of 732 
km and without oscillation mixing. 
Table 1. Rates for 5 years, 20 kt and 1.2 1020 POT /year. Oscillation with sin2(2θ13)=0.1. The upper 
integration limit, Εlim, has been chosen to get the best sensitivity, 
S sqrt bkg[ ]   
 0 <  Ε  < Εlim 0 <  Ε  < 10 GeV 
 Εlim (GeV) µ e 
bkg[ ]  Signal 
S  S/√(bg) µ e 
bkg[ ]  Signal, 
Present high energy configuration 
7 km 3.5 6200 34 190 33 1000 200 240 
10 km 2.5 2300 15 101 26 4600 160 130 
Low energy focussing 
7 km 3.5 13000 70 390 47 17500 340 430 
10 km 2.5 5700 28 250 47 7800 230 280 
years from now, the present CNGS beam may become roughly competitive with FNAL, the 
neutrino fluxes at the detector being then in the ratio 1.6 to 1.  
Integrated CC event rates for 5 years 
at 1.2 1020 POT/year in a 20 kt fiducial 
volume of a LAr-TPC detector are 
summarized in Table 1. Signal events 
have been calculated for sin2(2θ13)=0.1 to 
allow easy scaling. The actual size of the 
 oscillation driven 
13( ) is 
obviously unknown. In order to estimate 
the sensitivity for a small signal 
S  in the 
presence of a significant beam associated 
 background 
bkg[ ]  we consider the 
quantity 
S sqrt bkg[ ]  as a figure of merit 
as a function of the off-axis distance (see 
Figure 8). 
The upper integration limit, Εlim, has 
been chosen to get the best sensitivity, 
S sqrt bkg[ ] .  It varies with the off-axis 
distance because of the different spectrum 
shape of signal and background. As a 
reference the rates, integrated up to 10 GeV, are also shown for an estimation of the signal 
selection efficiency in the case of the optimum cut (80% in the τ optics vs. 90% in the low 
focus case). 
Both at CERN and at FNAL increases in the proton beam intensity are conceivable 
with relatively modest efforts, but require additional money. A further increase to as much as 
a factor 4 has been considered at FNAL with a new 8 GeV Proton Driver, provided it will be 
built in the near future. Several increases of the accelerated intensity may be considered also 
in the case of CERN, especially based on the improvements of the CPS and in connection 
with the several LHC improvement programmes.  In both cases the main limit may not be 
however the proton accelerator but rather the capability of the target/horns complex to 
withstand the required power.  
3.4. Detection efficiency for νe CC events and NC background rejection. 
In the previous section, the NC background has been assumed to be negligible. This 
assumption is supported by extensive studies performed by ICARUS collaboration [3,21] 
both for beam and atmospheric neutrinos.  In particular, the analysis of NC background 
rejection in a low energy on-axis neutrino beam [22] which has been extensively performed 
for the ICARUS configuration can be applied as well to the off-axis beam described in the 
present work.  We summarize here this analysis and its results. A full simulation of the events 
in LAr-TPC was performed to study the background of neutral pions in both neutral current 
and charged current interactions, which may simulate a background of 
 induced CC.  
Figure 8. Sensitivity to oscillation expressed in 
terms of the 
S sqrt bkg[ ]  (intrinsic plus tau), as 
a function of the off-axis distance. The plot 
indicates the off-axis distances 7 ÷ 12 km (0.550 
÷ 0.94°, 9.6 ÷ 16.4 mrad) are the optimum. 
For each neutrino event was recorded the visible energy, defined as the energy 
deposited by ionization in the sensitive LAr-TPC volume, with the exception of the 
ionization due to heavily quenched recoils. The optimization of the fiducial volume was 
based on three criteria: energy reconstruction, electron identification and 
o identification. 
(1) The energy reconstruction due to non-
containment of the neutrino events 
affects the signal/background ratio, 
especially when the signal is restricted 
in a narrow energy range. However, 
this non-containment is severe only for 
interactions occurring near the end of 
the detector and in a few centimetres 
lateral skin. A minimal cut of 50 cm in 
the longitudinal direction and 5 cm on 
the sides of the sensitive volume has 
been found sufficient to preserve the 
signal/background ratio.  In the large 
modules as described in the present 
proposal, these cuts would reduce the 
fiducial volume by a (negligible) 2%. 
(2) Electron identification is also assured 
under these geometrical cuts. Indeed, 
due to the directionality of the neutrino 
beam the probability that an electron 
escapes from the instrumented volume 
before initiating a shower is extremely 
small: only 2 % of the electrons 
“travel “ through a LAr-TPC thickness smaller than 3 X0, and 0.3 % travel less than 1 X0 
in the instrumented volume. Therefore we can safely assume to identify electrons with 
almost 100 % efficiency. 
(3)  
o identification. Neutral pions from νµ NC events could be misidentified as electrons. 
But because of the superior imaging capability of LAr-TPC technology, all events where 
both photon conversion points can be distinguished from the ν interaction vertex can be 
rejected.  To be conservative, in the ICARUS analysis only photons converting at more 
than 2 cm from the ν vertex were rejected. The remaining neutral pion background was 
further reduced by assuming that events where the parent 
o mass can be reconstructed 
within 10 % accuracy are discarded. The effect of the last requirement obviously depends 
on the assumed fiducial volume. Only 4 % of 
o’s survive the cuts when all interaction 
vertexes are accepted, and a further decrease to 3% is obtained when the fiducial volume 
is restricted as described above. On the remaining photon sample we apply the results 
obtained in Ref. [23] on the possibility to discriminate electrons from photons on the 
basis of dE/dx. This method provides a 90% electron identification efficiency with photon 
misidentification probability of 3% at relatively low energies. The misidentification 
Figure 9. Background sources at LNGS on-axis 
with a low energy CNGS beam as studied for 
the ICARUS detector. This calculation is very 
similar to the expectations from the off-axis low 
energy beam, with the exception of the 
background, which is much smaller. The 
residual NC background in the LAR-TPC, after 
rejection of neutral pions, is about two orders of 
magnitude smaller than the intrinsic beam 
associated 
 background. 
probability is expected to decrease with energy due to the decreasing contribution of 
Compton scattering. A track length of 2.5 cm is sufficient to achieve the discrimination. 
After all cuts, the final 
o mis-interpretation probability is 0.1 %, while the 
corresponding electron identification efficiency is 90 %. 
The residual background, after all the above cuts are applied, is shown in Figure 9 in 
the case of the study performed for the ICARUS detector exposed at a CNGS beam on-axis 
[22].  Given the very similar energy spectrum and background contaminations, the same 
signal efficiency and NC rejection power can be applied in the case of the off-axis beams. 
The residual NC background, after rejection of neutral pions in the LAR-TPC is about two 
orders of magnitude smaller than the intrinsic beam associated 
 background. 
3.5. Comparisons with NOvA. 
In the present letter of intent we consider the possibility of a substantial and equivalent 
upgrade of a LAr-TPC detector for CNGS, having in mind competition and timetable 
comparable to the ones of NOvA. We keep in mind that the key process is the observation of 
the oscillation driven 
events. As already pointed out, the use of the imaging 
capability of the LAr-TPC ensures a much higher discovery potential than it is the case of 
scintillator (or water) detectors, i.e. a comparable sensitivity may be achieved with a much 
smaller sensitive mass. The higher performance of LAr-TPC introduces important advantages 
with respect to NOvA, namely: 
• the NOvA detector is mostly limited to elastic events while LAr-TPC may 
collect also all kinds of inelastic events. An elaborate MC simulation of the 
NOvA proposal8 of the signals and backgrounds for oscillations using relevant 
parts of the MINOS experiment software, the NEUGEN3 neutrino interaction 
generator and the GEANT3 detector simulation show that the efficiency for 
accepting an event from 
oscillations is approximately 24%.  This 
introduces about a factor 4 in rate with respect to LAr-TPC in which virtually 
all event configurations are identified, for the same fiducial mass;  
• NOvA in contrast with LAr-TPC is may be contaminated by neutral current 
events that fake electron events, while in reality they are due to 
o. The 
background is typically about two-thirds from beam's produced from muon and 
kaon decay and one-third from neutral-current events. This increases the 
background of 
 events and increases further the level of the over-all 
discovery potential by a factor 
sqrt 1.5 1( ) =1.22 . We conclude that a ≈ 5 kt 
LAr-TPC detector should have performances comparable to the ones of NOvA. 
We underline that the NC cross sections are today only poorly known and 
therefore the magnitude of the effect, absent for LAr-TPC has to be carefully 
measured in separate experiments, at least as long the  
13( ) signal is close 
to the sensitivity limit.  
                                                
8  Ref [6]. chap. 12, page 78 
Similar considerations apply also about the water Cherenkov counter at T2K, where 
apparently more stringent cuts are necessary with a corresponding reduction of the rates in 
order to improve the sensitivity for the small signal 
 oscillation driven 
13( ), so 
far obviously unknown, with respect with the 
o related backgrounds. 
3.6. Evaluation of the beam associated 
 background. 
 The future CNGS low energy neutrino beam most likely will lack of an appropriate 
near detector. As a consequence the estimations of νe/νµ background ratio will be primarily 
based on the Monte Carlo simulation of the beam. The on-axis beam flux could however be 
determined experimentally (Figure 9) with the help of ICARUS-T600 neutrino detector 
presently in Hall B, measuring the neutrino spectra components with relatively high statistics 
(~100 νµ CC events/kt/10
19POT, peaked at 7 ± 2 GeV). Once the agreement is confirmed on-
axis, it may be possible to normalize and tune the simulation for the beam off-axis. The 
possibility of operating simultaneously the on-axis and the off-axis events, both with very 
similar LAr-TPC detectors, represent a very important correlation amongst the two 
measurements. 
Experience with the CERN-WANF beam has demonstrated that FLUKA based 
calculations, compared with measurements of the νµ spectrum, are able to provide an 
accuracy on the νe background normalization at the level of 3 % as well as a error on the 
spectrum shape better than 4% [24]. Similar considerations should bring precious new 
information for the future low energy off-axis neutrino beam. 
When applied to the off-axis beam the overall errors are reduced since the kinematics 
of the meson decay confine the neutrino spectra in a much narrower energy range, hence the 
error associated with the spectrum shape knowledge is strongly suppressed. 
3.7. Comparing the ultimate  sensitivities to 
 and 
13( ).  
The value of 
13( ) plane to be searched upon is currently unknown, although 
the experimental upper limit is 0.14. In order to be detectable, the number of 
 must 
substantially exceed the intrinsic 
 beam contamination. For instance, even in absence of 
NC contamination, which is the case of LAr-TPC, equal rates of oscillations and of 
contamination (the results for FNAL and CNGS are quite similar) correspond to 
",sin
13( ) 
≈ 1.6 x 10-2. 
As already pointed out, sensitivity to smaller values of 
13( ) implies an 
accurate knowledge of the actual 
 contamination and of the neutrino cross sections. The 
sensitivity to a non-zero value of 
13( ) is then proportional only to the square root of 
the number of events. 
The sensitivities for 
13( )( ) plane at 3σ for T2K, NOvA and the proposed 
future scenarios at CNGS, all based on 5 years of operation, 20 kt LAr-TPC detector 10 km 
off-axis are shown in Figure 10 with 1.2 1020 POT/year at 400 GeV (1) and with a delivered 
proton intensity of 4.3 1020 POT/year (2). 
Calculations have been performed with the GLoBES code [25], assuming for the 
energy resolution a conservative value of 15% and a global 5% uncertainty on the beam 
composition. 
As expected, the sensitivity with the new CNGS beam is definitely better than the one 
of T2K and NOvA. 
Figure 10. Comparison of the sensitivities for 
,  sin
13( ) plane at 3σ, all based on 5 years of 
neutrino operation for T2K, NOvA-1 and two proposed future scenarios at CNGS, 20 kt fiducial LAr 
detector. The CNGS-1 beam configuration is for (1) 1.2 1020 POT/year at 400 GeV, (2) a cycle 
integrated beam power of 512 kWatt, (3) a new target/optics configuration optimized for a low energy 
neutrino beam, (4) 10 km off-axis detector and (5) without substantial increases of the SPS 
performance. The CNGS-2 configuration assumes an hypothetical improvement of the SPS and CPS 
to 4.33 1020 POT/year corresponding to 1.6 MWatt beam power.  The intensity of the NOvA-1 
experiment is 6.5 x 1020 POT/year at 120 GeV and a a cycle integrated beam power of 768 kWatt.  
The corresponding intensity improvements for NOvA-2 are not completely identified and therefore the 
sensitivity is not shown. . All cases are computed for Δm > 0. Note the much higher sensitivity offered 
by the LAr approach which ensures higher discovery potentials, since every type of event is then 
clearly recognized and identified, contributing to the determination of the oscillation phenomenon. As 
a comparison, for instance in the case of NOvA, only 24% of the events may be used and a sizeable 
contaminant of mis-interpreted neutral current events add to the intrinsic 
emission from the beam. 
Similar inefficiency considerations apply to the water Cherenkov counter at T2K. 
4.— Tentative layout of LNGS-B. 
As described, the off-axis arrangement implies the realisation of a separate 
underground cave at about a significant distance with respect to the main CNGS beam line. A 
preliminary study has been conducted9 in order to identify the most appropriate new location, 
which we indicate with LNGS-B, keeping in mind a number of conditions:  
• The underground laboratory may be at a depth, which is much shallower than 
the one of the main Laboratory. The high event rejection power of the LAr-TPC 
detector will ensure the absence of backgrounds not only from the neutrinos 
from the CNGS but also for proton decay and cosmic neutrinos.  A depth of 
about 400 m of rock, corresponding to about 1.2 km of equivalent water depth 
has been chosen. 
• The location of the experimental halls should be between 7 km and 12 km from 
the axis of the beam.  
• The new laboratory should be out of the protected area of the Gran Sasso Park.  
• The neighbouring rock should not imply any presence of significant 
underground water and a minimal environmental impact. 
The general layout of the landscape across Gran Sasso massif side is shown in the top 
of Figure 11. Two potential locations at 10 km from the CNGS beam axis have been 
identified; location A is on the Teramo side of the mountain (close to L’Aquilano village), 
location B is on the L’Aquila side (close to Camarda village). Both locations fullfil the 
requirements of being outside the Gran Sasso National Park Area in a water free rock at a 
sufficient rock depth (400 m), providing an adequate shielding to cosmic radiation (as shown 
at the bottom-left of Figure 11); moreover they are easily reachable through the ordinary 
roads. Site A (shown at the bottom-right of Figure 11) is preferred because it requires a 
shorter access tunnel for a given depth.  
The new cavern must have a gas exhaust of an adequate cross section to the surface and 
be organised with the instrumental entry from the top, well above the perlite insulated LAr-
TPC container.  The entrance to the cave may be made with a strong door which can be 
closed and undergo pressurisation in case of a major accident. In this case the only exit of the 
gas will be through the exhaust pipeline. 
                                                
9 The study of the choice of the new hall locations as well as the preliminary layout design have been 
commissioned by the Prof. E. Coccia (director of LNGS) and performed by Ing. Roberto Guercio 
(Univ. Roma I, Italy). 
Figure 11. General layout for a potential new site to detect low energy off-axis neutrinos from the 
CNGS beam. In the top picture two potential locations, one at each side of the existing laboratory have 
been indicated with (A) and (B) located at 10 km off axis from the main CNGS beam, respectively in 
location Aquilano and Camarda. The natural muon background is shown next (equiv. water depth ≈ 
1.2 km) and compared both with the flux at the surface and with several existing underground 
laboratories. The Aquilano location is also shown, since it offers the shortest distance of the tunnel. 
They are both outside the natural park and they can be easily reached from main roads. 
5.— Conclusions. 
The forthcoming operation of the T600 detector in the real experiment CNGS2 will 
represent the completion of a development of the LAr-TPC chamber over more than two 
decades and it opens realistically the way to truly massive detectors for accelerator and non 
accelerator driven phenomena. As it has been described in this paper, the operation of the 
T600 evidences that a number of important milestones have been already achieved in the last 
several years, opening the way to the development of a new line of modular elements and 
which can be extrapolated progressively to the largest conceivable LAr-TPC sensitive 
masses.  
The new detector will maintain the majority of components we have already developed, 
in particular: 
• The readout electronics and the data acquisition of the T600, which has been 
developed in collaboration with the CAEN company. The 50’000 channels of 
electronics already at hand are adequate for about 10’000 tons of sensitive mass 
of new modular elements.  
• The signal feed-throughs for the very large number of signal wires, which have 
been developed for the T600 can be applicable directly to the new modular 
elements.  The technology, which has been patented by INFN/Padova, has 
shown itself extremely reliable and capable of withstanding the extreme leakage 
rate of the many tens of thousands of feed-throughs at LAr temperatures.  
• The original technology for the realisation of the wire planes and associated 
structures to withstand the very large changes of temperature during filling of 
the detector and which has been industrially realized by the company CINEL. A 
detailed engineering design, which has been developed for the module T1200 
but never realized in practice, is also completely compatible and ready for the 
present design. 
• An original and extremely robust technology for the readout wires has been 
developed, capable of withstanding the wide variations in temperature (-200 K) 
in the cooling and warm up phases. This technology has been developed in 
collaboration with industry. So far in the T600 not a single wire has broken, in 
spite of the many operations and of the transport on road for about 600 km.  
Although the wires are about 2.66 times longer, which does not constitute a 
problem, the same method will be cloned to the new modular elements. The 
realisation is simple, fast and cheap and it is realised with the help of an 
automatic machine.  
• The high voltage feed-through and the appropriate > 100 dB noise filtering to 
remove the electric noise from the very small signals of the wire chambers has 
been developed and fully operated with no problem at 150 kV, which is twice 
the design voltage of the T600. The design voltage of the new modular elements 
is 200 kV.  
• The purity of the LAr is generally well below the required level of 3 x 10-10 of 
equivalent Oxygen purity, which has required a dedicated technological 
development over the many years. The LAr is continuously purified both in the 
gas and in the liquid phase and circulated with adequate low temperature 
pumps. The purification system can be expanded in a straightforward way to 
become adequate to the new modular elements. A great deal of experience has 
taught us how to remove materials that are producing significant leakages.  
• Photomultipliers, which are wave shifting the 128 nm Argon light into the 
visible, have been designed with the help of the EMI Company in order to 
ensure an appropriate photocathode efficiency at the LAr temperature.  
• An appropriate method of high precision purity monitors, in order to monitor in 
real time the purity of the LAr.  
As pointed out already the main domain of remaining developments, to ensure the 
correct realization of the new modular elements, is related to the streamlining and 
simplification of the mechanical structures, to the reduction of the overall costs and to the 
new developments, previously described, of the structure of the detector, which are: 
• The use of perlite for the cryogenic structure. Perlite is vastly in use in the 
cryogenic industry and should represent no problem. 
• The filling process starting from air to pure LAr, taking into account the motion 
of the gas, optimising the inlet and outlet geometries and minimising the 
number of cycles. 
• The thermal convections of the LAr, in order to optimise the temperature 
gradients and to insure a convincing circulation in all regions of the dewar, both 
in the cooldown phase and in the stationary state. 
• The outgassing rate and the recirculation processes required in order to achieve 
the required electron lifetime. 
• The geometry of the compact re-circulators both in the liquid and in the gaseous 
phases. 
The realization of these developments should not rise any insurmountable problem and 
the detailed engineering design of the first new modular elements should proceed smoothly 
and rapidly.  
The experiment might reasonably be operational in about 5 years, provided a new hall 
is excavated in the vicinity of the Gran Sasso Laboratory and appropriate funding is made 
available. 
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[2] E. Church et.al., [MiniBooNE Coll.] [arXiv:nucl-ex/9706011] ;M. Sorel, 
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7.— Appendix. General comments on the use of Perlite. 
Perlite is a generic term for naturally occurring siliceous volcanic rock. The 
distinguishing feature which sets perlite apart from other volcanic glasses is that when heated 
to a suitable point in its softening range, it expands from four to twenty times its original 
volume. 
This expansion process is due to the presence of two to six percent combined water in 
the crude perlite rock. When quickly heated to above 870 C the crude rock pops in a manner 
similar to popcorn as the combined water vaporizes and creates countless tiny bubbles in the 
softened glassy particles. It is these tiny glass-sealed bubbles which account for the amazing 
lightweight and other exceptional physical properties of expanded perlite. 
The expansion process also creates one of perlite's most distinguishing characteristics: 
its white color. While the crude perlite rock may range from transparent to light gray to 
glossy black, the color of expanded perlite ranges from snowy white to grayish white. 
Expanded perlite can be manufactured to weigh from 32 kg/m3 to 240 kg/m3. Because 
of its unique properties, perlite insulation has found wide acceptance in the insulating of 
cryogenic and low temperature storage tanks, in shipping containers, cold boxes, test 
chambers, and in food processing. 
Perlite insulation suitable for non evacuated cryogenic or low temperature use exhibits 
low thermal conductivity throughout a range of densities, however, the normal recommended 
density range is 48 to 72 kg/m3, that is about 1/20 of the density of water. In addition to its 
excellent thermal properties, perlite insulation is relatively low in cost, easy to handle and 
install, and does not shrink, swell, warp, or slump. Perlite is non-combustible, meets fire 
regulations, and can lower insurance rates. Because it is an inorganic material, it is rot and 
vermin proof. As a result of its closed cell structure, the material does not retain moisture. 
Thermal conductivity varies with temperature, density, pressure, and conductivity of 
the gas which fills the insulation spaces at mean temperature -126 C°, but it is typically in the 
interval 0.025-0.029 W/m/K.  
In the present design the volume of perlite is 3928 m3, corresponding to a mass of 235 t 
at an expanded density of 60 (48 to 72) kg/m3. At the thickness of 1.5 m and a temperature 
difference of 200 K, the specific heat loss is 3.86 W/m2 for a nominal thermal conductivity of 
0.029 (0.025-0.029) W/m/K. This is significantly smaller than the specific heat loss of the 
T600.  Taking into account the dimensions of the vessel, the total heat loss is 8.28 kW. At 
present in the CNGS the cryogenic plant is made of 10 units, each with 4 kW of (cold) 
power.  Three of such units (≤ 12 kW) should be adequate to ensure cooling of the walls of 
the vessel during normal operation.  
A higher level of insulation is possible evacuating the perlite, with a resulting thermal 
conductivity up to 40 times less than 0.029 W/m/K depending on vacuum and temperature. It 
also may be used for storage of oxygen, nitrogen, and LNG when especially low thermal 
conductivities are desired. At the present stage we believe that such an improvement is not 
necessary, since additional losses are anyway due to the circulation of the liquid and the 
presence of cables and other ducts at cryogenic temperature.
