Luminescence quenching of the triplet excimer state by air traces in gaseous argon

Luminescence quenching of the triplet excimer state by air traces in gaseous argon C. Amsler a,∗, V. Boccone a, A. B¨uchler a, R. Chandrasekharan b, C. Regenfus a, J. Rochet a aPhysics Institute, University of Z¨urich, CH–8057 Z¨urich, Switzerland bInstitute for Particle Physics, ETH-Z¨urich, CH–8093 Z¨urich, Switzerland Abstract While developing a liquid argon detector for dark matter searches we investigate the influence of air contamination on the VUV scintillation yield in gaseous argon at atmospheric pressure. We determine with a radioactive α- source the photon yield for various partial air pressures and different reflectors and wavelength shifters. We find for the fast scintillation component a time constant τ1 = 11.3 ± 2.8 ns, independent of gas purity. However, the decay time of the slow component depends on gas purity and is a good indicator for the total VUV light yield. This dependence is attributed to impurities destroying the long-lived argon excimer states. The population ratio between the slowly and the fast decaying excimer states is determined for α-particles to be 5.5 ± 0.6 in argon gas at 1100 mbar and room temperature. The measured mean life of the slow component is τ2 = 3.140 ± 0.067 µs at a partial air pressure of 2 × 10−6 mbar. Key words: Argon scintillation, VUV detection, Excimer, Dark matter search PACS: 32.50.+d, 52.25.Os, 29.40.Cs 1. Introduction Noble liquids such as argon (or xenon) can act as targets for WIMPs (Weak Interacting Massive Particles), the most popular candidates for dark matter in the universe. These elements have high scintillation yields and are also suitable for charge detection because of their relatively low ionization potentials. Both ionization and scintillation light can be detected [1,2]. Argon (40Ar) is cheap com- pared to xenon and is therefore competitive for ∗Corresponding author Email address: claude.amsler@cern.ch (C. Amsler). large volumes, in spite of its contamination by the 39Ar β-emitter. Here we present measurements on gaseous argon done while developing the scintilla- tion light read out of a 1 ton liquid argon TPC to search for dark matter (ArDM, [1]). The light yield and the mechanism for the lu- minescence of noble gases and liquids are compa- rable to that of alkali halide crystals [3,4] and are described in the literature for dense gases [5, 6] and liquids [7–10]. Fundamental to the scintilla- tion process is the formation of excited dimers (ex- cimers) which decay radiatively to the unbound ground state of two argon atoms. Figure 1 shows Preprint submitted to Elsevier Science 23 October 2018 arXiv:0708.2621v1 [physics.ins-det] 20 Aug 2007 schematically the two mechanisms leading to light emission in argon, excitation and ionization. Exci- tation leads through collisions with neighbouring atoms to neutral excimers Ar∗ 2 which decay radia- tively into two argon atoms. Ionization leads to the formation of charged excimers which are neutral- ized by thermalized electrons (recombination lu- minescence). Both processes are strongly pressure and density dependent. For gaseous argon at room temperature and normal pressure, at which we op- erate here, excitation dominates [11,12], while re- combination luminescence becomes important at high pressures or in liquid. Fig. 1. The two mechanisms leading to the emission of 128 nm photons (adapted from ref. [6]). The argon excimers are created in three nearly degenerate spin states, two singlets (1Σ− u and 1Σ+ u ) and a triplet (3Σ+ u ). The 1Σ− u state cannot decay radiatively by parity conservation, and the 3Σ+ u is expected to have a much longer lifetime than the 1Σ+ u since it has to decay into two spin-0 ar- gon atoms. The 3Σ+ u and 3Σ+ u states decay radia- tively by emitting VUV photons in a ≈10 nm band around 128 nm. These photons are not absorbed by atomic argon and can therefore be detected. Light at higher wavelengths (mostly in the near IR) is also produced from transitions between highly ex- cited argon atomic states. The production times of the triplet and singlet states and their production ratio vary with argon density and also depend on the type of projec- tile, e.g. electron, α-particle or fission fragment [9]. However, the decay times are not affected. The time constants of the singlet and triplet states have been measured in dense gases with 160 keV electrons [5]. The production time of the sin- glet excimer is around 40 ns at 3 atm, decreasing with increasing gas pressure, and the mean life is about 4 ns. The mean life of the triplet excimer state is substantially larger, 3.2 ± 0.3 µs. In liquid argon the mean lives scatter in the range between 4 and 7 ns for the singlet state, and between 1.0 and 1.7 µs for the triplet state. The triplet to singlet production ratios are 0.3, 1.3 and 3 for electrons, α-particles and fission fragments, respectively (for a compilation see ref. [9]). The large difference in time constants between singlet and triplet states is unique for argon among noble g

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