An improved measurement of electron-ion recombination in high-pressure xenon gas

Abstract

We report on results obtained with the NEXT-DEMO prototype of the NEXT-100 high-pressure xenon gas time projection chamber (TPC), filled with pure xenon gas at 10 bar pressure and exposed to an alpha decay calibration source. Compared to our previous measurements with alpha particles, an upgraded detector and improved analysis techniques have been used. We measure event-by-event correlated fluctuations between ionization and scintillation due to electronion recombination in the gas, with correlation coefficients between −0.80 and −0.56 depending on the drift field conditions. By combining the two signals, we obtain a 2.8% FWHM energy resolution for 5.49 MeV alpha particles and a measurement of the optical gain of the electroluminescent TPC. The improved energy resolution also allows us to measure the specific activity of the radon in the gas due to natural impurities. Finally, we measure the average ratio of excited to ionized atoms produced in the xenon gas by alpha particles to be 0.561 ± 0.045, translating into an average energy to produce a primary scintillation photon of W ex = (39.2 ± 3.2) eV.

A gas scintillator detector for 2D dose profile monitoring in pencil beam scanning and pulsed beam proton radiotherapy treatments

“…The normalization factor against an air ionization chamber is seen to decrease by about 7% for a 100 MeV reduction in beam energy, with roughly one-third of the reduction resulting from the known difference between xenon and air in the energy dependence of proton stopping power. The remainder of the effect might arise from energy dependence in the number of scintillation photons produced per keV of energy deposit in xenon (e.g., see Table 1 in Serra et al 2014), but there are no precise measurements in the literature of xenon scintillation yield vs. incident energy. Data for the number of ion pairs produced per MeV deposited in air are more precise (Jones et al 2007), but still cannot rule out a few percent change with incident proton energy.…”

“…By detecting the fast scintillation light directly, one bypasses problems associated with ion-ion recombination, which occurs over much longer time scales. For the GSD we have chosen xenon as the scintillating gas, due to its high scintillation light yield and negligible self-absorption of the resulting photons (Policarpo 1981, Serra et al 2014). The primary scintillation photons, in the vacuum ultraviolet (VUV) range peaked at a wavelength near 175 nm, result from the decay of the diatomic (dimer) states, Xe 2 *→2Xe + photon.…”

“…The resulting recombination luminescence contribution to the VUV scintillation light occurs after a typical electron thermalization delay time ≈ 2 μs in xenon gas at atmospheric pressure (Takahashi et al 1982), and then exhibits both fast (sub-microsecond) and slow (characteristic decay time of tens of μs) components (Mimura et al 2010). This recombination contribution has been partially suppressed in a number of previous experiments by application of a drift electric field to remove the electrons to a separate detector region reserved for an ionization measurement, with that separated ionization measurement sometimes carried out by accelerating the electrons to stimulate secondary xenon scintillation (e.g., see Serra et al 2014). In addition, recent searches for rare particle physics processes have used high-pressure xenon gas or liquid xenon to enhance the overall photon yield.…”

“…A single such proton produces ~10 3 primary scintillation photons (e.g., see Table 1 in Serra et al 2014), of which ~1 on average will be detected in one of the six PMTs. A single clinical beam pulse containing typically ~10 7 such protons will then produce a hefty signal in all the PMTs.…”

Design and performance of a high-pressure xenon gas TPC as a prototype for a large-scale neutrinoless double-beta decay search

“…Based on the results of previous studies on the relation between recombination and electric field [21,22], we chose a target uniformity of ±5% for the intensity of the drift field. Note that these previous studies are conducted with alpha particles from 222 Rn in [21] and 241 Am in [22]. The rate of recombination for ionization by alpha particles is higher than the one for ionization by electrons.…”