A new method of alpha ray measurement using a Quadrupole Mass
  Spectrometer
A new method of alpha ray measurement
using a Quadrupole Mass Spectrometer
Y. Iwata a,∗, Y. Inoue b, M. Minowa a
aDepartment of Physics, School of Science, University of Tokyo, 7-3-1, Hongo,
Bunkyo-ku, Tokyo 113-0033, Japan
bInternational Center for Elementary Particle Physics(ICEPP), University of
Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
Abstract
We propose a new method of alpha(α)-ray measurement that detects helium
atoms with a Quadrupole Mass Spectrometer(QMS). A demonstration is undertaken
with a plastic-covered 241Am α-emitting source to detect α-rays stopped in the
capsule. We successfully detect helium atoms that diffuse out of the capsule by
accumulating them for one to 20 hours in a closed chamber. The detected amount
is found to be proportional to the accumulation time. Our method is applicable to
probe α-emitting radioactivity in bulk material.
Key words: Quadrupole Mass Spectrometer, Helium atoms, Counting
1 Introduction
There are many ways to detect α rays such as gas-flow counters and solid-
state detectors[1], but all detect the corresponding ionization signal by the
incident α ray, instead of 4He itself. However, because α particles travel only
a few centimeters in the air and can be easily stopped by a piece of thin foil or
paper, it is harder to detect their rays than those of beta or gamma radiation.
In this work, we suggest a new method of α ray measurement that aims to
detect 4He neutral atoms. If one wants to measure α-emitting radioactivity
in bulk material with ordinary detectors, one has to rely on α rays emitted
∗ Corresponding author. Tel.: +81 3 5841 7622; fax: +81 3 5841 4186.
Email address: yiwata@icepp.s.u-tokyo.ac.jp (Y. Iwata).
Preprint submitted to Elsevier 13 November 2018
http://arxiv.org/abs/0704.1944v1
from the thin surface of the material because of their short range. However,
many materials diffuse stopped α particles out of their surface in the form of
neutral helium atoms. The amount of the released helium atoms can then be
measured by a Quadrupole Mass Spectrometer (QMS) in terms of the mass
number A = 4. Therefore, there is the advantage of being able to measure α-
emitting radioactivity in bulk materials. It can be seen that a higher sensitivity
for alphas can be expected.
2 Experimental setup and method
To examine our method of α-ray measurement, we used an Amersham X.825
disc type 241Am α source (Fig. 1) and a vacuum system including a QMS.
The 241Am source, an α-emitter with a half life of T1/2 = 432.2 y, is covered
with epoxy resin 25mm in diameter and 3mm in thickness. This source emits
3.25 × 105 alphas per second, which eventually stop in the resin and diffuse
out as 4He neutral atoms. The time needed for diffusion in resin is negligible 1
compared to the time elapsed of about 10 years or more after the production of
this α source. Therefore, the helium production rate of the source is thought to
be the same as that of alpha (3.25×105 s−1). A Quadrupole Mass Spectrometer
(QMS) is often used for trace element analysis because of its high isotopic
selectivity and efficiency[3]. The QMS we used is a Pfeiffer Vacuum QMS200
with a channeltron detector. The mass range is A = 1–100 and the detection
limit is 1× 10−12 Pa [4].
The experimental setup is shown in Figs. 2 and 3. A liquid nitrogen trap
was introduced to capture unwanted out-gas especially from the resin. We
controlled the valves, V1 and V2 (see Fig. 3), to measure the integrated QMS
channeltron current QHe of helium as follows:
(a) Draw a vacuum to ∼ 10−6Pa with both valves (V1, V2) open.
(b) Keep valve V1 closed for Tac =1h,4h,10h and 20h to accumulate helium
atoms from the source. The number of helium atoms expected was NHe =
1.17× 109, 4.68× 109, 1.17× 1010 and 2.34× 1010, respectively.
(c) After closing valve V2, open valve V1 to introduce helium atoms to the
QMS through an orifice for a couple of seconds.
(d) Open valve V2 to introduce most of the remaining atoms at one time for
helium detection.
In our QMS system, the bypass valve V2 and the orifice(φ0.3mm) are placed
for a high pressure sample gas. Here, we utilized them in step (c) to avoid a
1 Diffusion time of helium through various samples of epoxy resin adhesives is found
in ref [2].
drastic change in the channeltron current measured by the QMS, which was
observed with just V1 open after step (b). Considering the conductance of the
orifice, the amount of helium atoms lost in step (c) is estimated to be less than
2%, so we measured the integration of the channeltron currents for 5 seconds
after opening valve V2 in step (d). The integration time of 5s was determined
after considering the time needed for opening valve V2 by hand (∼ 1s) and
the evacuation time T0 ∼ 0.3s (≪5s) expected by the pumping speed of our
vacuum system.
To estimate the amount of spurious signal caused by the out-gas from the
resin, we also measured channeltron currents against a 57Co source as a control
covered with the same epoxy capsule as the 241Am source. Assuming that
both sources are emitting out-gas of similar composition and amount, the net
integrated channeltron current QHe can be defined as the difference between
the integrated current QAm and QCo with the
241Am and the 57Co source,
respectively, under the same accumulation time Tac. In terms of NHe described
above, QHe can be written as
QHe = QAm −QCo = eGRiNHe, (1)
where e is the elementary charge, G ∼ 7 × 103 is the amplification factor of
the channeltron, and Ri is the detection efficiency of the QMS, i.e., the ratio
of the number of helium atoms detected by the QMS compared to the initial
quantity NHe. Note that NHe and QHe are proportional to the accumulation
time Tac.
3 Result and analysis
We measured a set of QAm and QCo twice for each Tac =1h,4h,10h and 20h.
Fig. 4 shows a few examples of the experimental result for Tac = 20h, 10h and
4h. A clear difference between two sources can be seen for the data of Tac =
20h and 10h. The difference can also be seen in Fig. 4-(C) (Tac = 4h), though
a drastic change in the channeltron current makes it unclear.
Fig. 5 shows the relationship between the accumulation time Tac and the net
amount of helium QHe = QAm − QCo with the best fit under the assumption
of linearity. Here, because of the difficulty in evaluating the exact error of
QHe, we estimated it at QCo for each data point, which may be a conservative
overestimation. The unclarity for the data of Tac = 4h described above is
reflected in the error bars in this figure.
The best-fit proportionality coefficient (1.9±0.2)× 10−12[C/h] corresponds to
Ri ∼ 2 × 10
−6 according to Eq.(1). We can see certain degree of linearity
between Tac and QHe, but the obtained detection efficiency Ri was rather
small, probably because most of the helium atoms were evacuated by the
vacuum pump before they got ionized.
To estimate the ultimate detection limit under this system apart from the
influence of the out-gas coming from the epoxy capsule, we measured the
distribution of QBG, the background channeltron current without any sources
integrated for 5 seconds, as shown in Fig. 6. The best fit to the data by
the normal distribution is also shown in the figure. In this way, the standard
deviation σ = 9.3 × 10−14 [C] was obtained. We defined the detection limit
Qlimit as
Qlimit = (1.645σ)×
2 = 2.2× 10−13 [C] (2)
at 95% confidence level. Here, the factor
2 is introduced to take into account
the two sets of the data, the 241Am and the 57Co source, that were used for
detecting the helium signal. The obtained Qlimit corresponds to ∼ 10
8 helium
atoms under our system.
Although we did not try to obtain higher detection efficiency Ri with this mea-
surement, since the valves were controlled manually, there are some possible
solutions to improve Ri:
• Atom buncher [5]
An atom buncher is a cold trap to capture target gas atoms. It consists of a
metal surface that often is cooled by liquid helium. Some kinds of laser can
be used to temporarily heat a spot on the surface to evaporate gas atoms.
The detection efficiency Ri would improve if the heated area is placed close
to the ionization chamber of the QMS. This device can be applied to the
detection of helium by cooling the surface to a temperature low enough to
trap the helium atoms.
• Pulsed Supersonic Valve [6]
The Pulsed Supersonic Valve(PSV) is an electromagnetic device to generate
supersonic free gas jet. This valve consists of two parallel metallic plates
as a gate for the sample gas. The gas is only allowed to be temporarily
introduced when the gate is opened by electromagnetic repulsion between
these two plates. The jet of sample gas is then injected into the ionization
chamber. Similar to the atom buncher described above, higher Ri can be
achieved by locating the PSV near the ionization region of the QMS.
Attaching devices such as an atom buncher and a PSV to improve detection
efficiency and so obtain a higher Ri, much smaller quantities of helium atoms
could be detected than under our present system. Also, some QMS are said to
be able to count target atoms one by one[4], so our system could theoretically
be improved to detect a single helium atom; that is, each α ray regardless of
its energy if the detection efficiency Ri gets close to one.
Our method can be applied to the accurate estimations of α-induced soft errors
in very-large-scale integrated circuit (VLSI). Soft errors are caused by α rays
from a minute amount of radioactive substance in LSI packages, and have
become a serious problem in VLSI circuits[7]. Using mass analysis in vacuum
may enable us to reduce the detection limit of α-emitting radioactivity in the
packages compared to the conventional method with a gas flow proportional
counter.
4 Conclusion
We proposed a new method for alpha(α) ray measurement by detecting helium
atoms with a QMS. An 241Am α source and a 57Co source as a control were
used to examine our method. The result showed that we could successfully
detect helium atoms, but the detection efficiency was only 2× 10−6 under our
system. Additional devices such as an atom buncher and a PSV to improve
detection efficiency may allow us to reduce the detection limit. This detector
cannot measure the energy of the α particles, but that feature is reasonably
common to many other conventional α detectors and is not a shortcoming
in most applications. Our method may become practicable to choose low α-
active material for LSI packages, which is essential to reduce α-induced soft
error rates in VLSI circuits.
References
[1] Kai Siegbahn, Alpha-, beta- and gamma-ray spectroscopy, Amsterdam, 1965.
[2] A. Gerlach, W. Keller, J. Schulz and K. Schumacher, Microsystem Technologies
7 (2001) 17.
[3] N. Trautmann, G. Passler and K. Wendt, Anal. Bioanal. Chem. 378 (2004)
[4] Pfeiffer Vacuum catalog: Mass spectrometer, 2005.
[5] G. S. Hurst, et al., J. Appl. Phys. 55 (1984) 1278.
[6] H. Pauly, Atom, Molecule, and Cluster Beams I, Springer, 2000.
[7] Y. Tosaka, et al., Jpn. J. Appl. Phys. 45(4B) (2006) 3185.
Fig. 1. Plastic-covered 241Am α source. The central “point” is 241Am.
Fig. 2. Schematic view of our detection method.
Fig. 3. Photograph of the experimental setup.
 0  3  6  9  12  15
Time [s]
V1 open
V2 open
241Am α source 20h
57Co source (control) 20h
(A) Tac = 20h.
 0  3  6  9  12  15
Time [s]
V1 open
V2 open
241Am α source 10h
57Co source (control) 10h
(B) Tac = 10h.
 0  3  6  9  12  15
Time [s]
V1 open
V2 open
241Am α source 4h
57Co source (control) 4h
(C) Tac = 4h.
Fig. 4. A few examples of the experimental result.
0 4 8 12 16 20
Tac [h]
f(Tac)=1.9*10
-12Tac
fit f(Tac)
Fig. 5. Linearity between Tac and QHe (Tac = 1h, 4h, 10h, 20h).
2.4 2.6 2.8 3.0 3.2
BG integral (5s)   [10-12C]
σ = 9.3*10-14  [C]
data for fitting
Fig. 6. Distribution of QBG.
	Introduction
	Experimental setup and method
	Result and analysis
	Conclusion
	References
