3158170
The Effects of Dissolved Methane upon Liquid Argon Scintillation Light
In this paper we report on measurements of the effects of dissolved methane upon argon scintillation light. We monitor the light yield from an alpha source held 20 cm from a cryogenic photomultiplier tube (PMT) assembly as methane is injected into a high-purity liquid argon volume. We observe significant suppression of the scintillation light yield by dissolved methane at the 10 part per billion (ppb) level. By examining the late scintillation light time constant, we determine that this loss is caused by an absorption process and also see some evidence of methane-induced scintillation quenching at higher concentrations (50-100 ppb). Using a second PMT assembly we look for visible re-emission features from the dissolved methane which have been reported in gas-phase argon methane mixtures, and we find no evidence of visible re-emission from liquid-phase argon methane mixtures at concentrations between 10 ppb and 0.1%.
121612933
Results from the Bo Liquid Argon Scintillation Test Stand at Fermilab
In these proceedings I discuss results from the Bo test stand at the Proton Assembly Building, Fermilab. This test stand has been used to characterize elements of the MicroBooNE optical system as well as to perform studies of processes affecting argon scintillation light such as scintillation quenching and optical absorption by impurities. I review in detail a recent measurement of the absorption of liquid argon scintillation light by dissolved nitrogen at the part-per-million level.
54840036
Demonstration of a Lightguide Detector for Liquid Argon TPCs
We report demonstration of light detection in liquid argon using an acrylic lightguide detector system. This opens the opportunity for development of an inexpensive, large-area light collection system for large liquid argon time projection chambers. The guides are constructed of acrylic, with TPB embedded in a surface coating with a matching index of refraction. We study the response to early scintillation light produced by a 5.3 MeV . We measure coating responses from 7 to 8 PE on average, compared to an ideal expectation of 10 PE on average. We estimate the attenuation length of light along the lightguide bar to be greater than 0.5 m. The coating response and the attenuation length can be improved; we show, however, that these results are already sucient for triggering in a large detector.
20950520
Photodegradation mechanisms of tetraphenyl butadiene coatings for liquid argon detectors
We report on studies of degradation mechanisms of tetraphenyl butadiene (TPB) coatings of the type used in neutrino and dark matter liquid argon experiments. Using gas chromatography coupled to mass spectrometry we have detected the ultraviolet-blocking impurity benzophenone. We monitored the drop in performance and increase of benzophenone concentration in TPB plates with exposure to ultraviolet (UV) light, and demonstrate the correlation between these two variables. Based on the presence and initially exponential increase in the concentration of benzophenone observed, we propose that TPB degradation is a free radical-mediated photooxidation reaction, which is subsequently confirmed by displaying delayed degradation using a free radical inhibitor. Finally we show that the performance of wavelength-shifting coatings of the type envisioned for the LBNE experiment can be improved by 10-20%, with significantly delayed UV degradation, by using a 20% admixture of 4-tert-Butylcatechol.
6463902
Dynamical pion collapse and the coherence of conventional neutrino beams
In this paper we consider the coherence properties of neutrinos produced by the decays of pions in conventional neutrino beams. Using a multiparticle density matrix formalism we derive the oscillation probability for neutrinos emitted by a decaying pion in an arbitrary quantum state. Then, using methods from decoherence theory, we calculate the pion state which evolves through interaction with decay-pipe gases in a typical accelerator neutrino experiment. These two ingredients are used to obtain the distance scales for neutrino beam coherence loss. We find that for the known neutrino mass splittings, no nonstandard oscillation effects are expected on terrestrial baselines. Heavy sterile neutrinos may experience terrestrial loss of coherence, and we calculate both the distance over which this occurs and the energy resolution required to observe the effect. By treating the pion-muon-neutrino-environment system quantum mechanically, neutrino beam coherence properties are obtained without assuming arbitrary spatial or temporal scales at the neutrino production vertex.
3517583
Environmental effects on TPB wavelength-shifting coatings
The scintillation detection systems of liquid argon time projection chambers (LArTPCs) require wavelength shifters to detect the 128 nm scintillation light produced in liquid argon. Tetraphenyl butadiene (TPB) is a fluorescent material that can shift this light to a wavelength of 425 nm, lending itself well to use in these detectors. We can coat the glass of photomultiplier tubes (PMTs) with TPB or place TPB-coated plates in front of the PMTs. In this paper, we investigate the degradation of a solution-based, embedded TPB coating in a laboratory or factory environment to assess the viability of long-term TPB film storage prior to its initial installation in an LArTPC. We present evidence for severe degradation of this type of coating due to common fluorescent lights and ambient sunlight in laboratories, with potential losses at the 40% level in the first day and eventual losses at the 80% level after a month of exposure. We determine the degradation is due to wavelengths in the UV spectrum, and we demonstrate mitigating methods for retrofitting lab and factory environments.
118983427
Similarity flow of a neutral scalar coupled to a fixed source
A neutral scalar meson interacting with a fixed isoscalar nucleon is evolved according to the similarity renormalization group. The fixed source ends up being too singular and an appropriately dressed static source arises from the similarity flow. The fixed source is dressed by a virtual cloud of mesons extending out past the inverse meson mass according to the Yukawa potential. For low energies the source produces a Yukawa mean field which decouples from the remaining meson field fluctuations. The renormalization group procedure for effective particles is also illustrated for this fixed-source model.
17088589
Analytic treatment of positronium spin splittings in light-front QED
We study the QED bound-state problem in a light-front Hamiltonian approach. Starting with a bare cutoff QED Hamiltonian H{sub B} with matrix elements between free states of drastically different energies removed, we perform a similarity transformation that removes the matrix elements between free states with energy differences between the bare cutoff {Lambda} and effective cutoff {lambda} ({lambda}{lt}{Lambda}). This generates effective interactions in the renormalized Hamiltonian H{sub R}. These effective interactions are derived to order {alpha} in this work, with {alpha}{lt}1. H{sub R} is renormalized by requiring it to satisfy coupling coherence. A nonrelativistic limit of the theory is taken, and the resulting Hamiltonian is studied using bound-state perturbation theory. The effective cutoff {lambda}{sup 2} is fixed, and the limit 0{l_arrow}m{sup 2}{alpha}{sup 2}{lt}{lambda}{sup 2}{lt}m{sup 2}{alpha}{r_arrow}{infinity} is taken. This upper bound on {lambda}{sup 2} places the effects of low-energy (energy transfer below {lambda}) emission in the effective interactions in the {vert_bar}e{bar e}{r_angle} sector. This lower bound on {lambda}{sup 2} ensures that the nonperturbative scale of interest is not removed by the similarity transformation. As an explicit example of the general formalism introduced, we show that the Hamiltonian renormalized to O({alpha}) reproduces the exact spectrum of spin splittings, with degeneracies dictated by rotationalmore » symmetry, for the ground state through O({alpha}{sup 4}). The entire calculation is performed analytically, and gives the well-known singlet-triplet ground-state spin splitting of positronium, (7)/(6){alpha}{sup 2}Ryd. We discuss the remaining corrections other than the spin splittings and how they can be treated in calculating the spectrum with higher precision. {copyright} {ital 1997} {ital The American Physical Society}« less
118851759
Light-front Hamiltonian approach to the bound-state problem in quantum electrodynamics /
This dissertation presents the first theoretical investigation of the Lamb shift in a light-front hamiltonian approach: the dominant part of the splitting between the 2S(1/2) and 2P(1/2) energy levels in hydrogen is calculated. Also presented for the first time is an analytic calculation in a light-front hamiltonian approach of the singlet-triplet spin splitting in the ground state of positronium through fourth order in the fine-structure constant.
14842371
Singlet-Triplet Splitting of Positronium in Light-Front QED
We study the QED bound-state problem in a light-front hamiltonian approach. It is important to establish the equivalence (or not) of equal-time and light-front approaches in the well-understood arena of Quantum Electrodynamics. Along these lines, the singlet-triplet ground state spin splitting in positronium is calculated. The well-known result, \(\frac{7}{6}{\alpha ^2}\) Ryd, is obtained analytically, which establishes the equivalence between the equal-time and light-front approaches (at least to this order). The true equivalence of the two approaches can only be established after higher-order calculations. It was previously shown that this light-front result could be obtained analytically [1], but a simpler method is presented in this paper.