Absorption of scintillation light in a 100 l liquid xenon γ-ray detector and expected detector performance

Abstract

An 800 ℓ liquid xenon scintillation γ ray detector is being developed for the MEG experiment which will search for µ + → e + γ decay at the Paul Scherrer Institut. Absorption of scintillation light of xenon by impurities might possibly limit the performance of such a detector. We used a 100 ℓ prototype with an active volume of 372×372×496 mm 3 to study the scintillation light absorption. We have developed a method to evaluate the light absorption, separately from elastic scattering of light, by measuring cosmic rays and α sources. By using a suitable purification technique, an absorption length longer than 100 cm has been achieved. The effects of the light absorption on the energy resolution are estimated by Monte Carlo simulation.

The MEG detector for μ +→e+ γ decay search

“…A comparison of the observed light distribution on each PMT with the one obtained by a detailed Monte Carlo simulation results in a measurement of the LXe attenuation length [41]. This has turned out to be much larger than the dimension of our detector during all physics runs (λ Abs > 300 cm).…”

“…The first used 2.3 of LXe with 32 PMTs, and the second used 68.6 of LXe with 228 PMTs [41]. The purpose of the first prototype was a proof-of-principle detection of γ -rays at ∼2 MeV from radioactive sources in LXe with adequate resolutions.…”

“…6.1, scintillation light may be absorbed by impurities in LXe, such as H 2 O and O 2 at ppm level. We have developed two purification methods, gasphase [41] and liquid-phase [45], to remove those impurities. Gas-phase purification removes impurities in Gaseous Xenon (GXe) by means of a metal-heated getter.…”

“…To minimise the position dependence of the detected photons, 11 LEDs are flashed simultaneously. In dedicated gain measurement runs, performed every second day on average, LEDs are flashed at several different intensities, and the PMT gains are evaluated from the photoelectron statistics following the method described in [41]. There are two kinds of known PMT gain instabilities, a long-term gain decrease, and a rate-dependent gain shift.…”

Leakage tests for 241Am solid sources used for liquid xenon detector monitoring

“…The MEG experiment [3,4] is the first that extensively used alpha sources distributed within the sensitive volume of a 100 L prototype liquid Xenon calorimeter. Larger calorimeters (800 L) are under construction.…”