Pulse shape analysis of scintillation signals from pure and xenon-doped liquid argon for radioactive background identification

Abstract

The response of a 19 kg active mass liquid argon scintillation detector to gamma, alpha and neutron radiation and its characteristic scintillation photon time distribution has been studied in the framework of the GERDA double beta decay research and development program. The achieved photo-electron yield of 1.24 pe/keV was stable during the two years of operation. The detector exhibits excellent energy resolution of 8.7/17.2 keV (σ ) for gamma energies of 60/239 keV. Radon was loaded into the liquid argon to study alpha energy quenching and decay time correlation of its progenies. A robust pulse shape analysis method was used to identify and discriminate amongst the different radiation types. 60 keV gamma signals could be discriminated against neutron recoils of the same visible energy with a miss-identification probability of < 5 • 10 −4 limited by statistics and ambient backgrounds. Xenon doping of liquid argon increased the photo-electron yield and improved the spectroscopic performance of the detector leading to an energy resolution of 7.2/15.4 keV (σ ) for 60/239 keV. The discrimination power improved slightly with the addition of xenon up to concentrations of 300 ppm. Applications for background identification and discrimination in double beta decay search with 76 Ge crystals, as well as for Dark Matter search with liquid argon are discussed.

Ion-beam excitation of liquid argon

“…As the electron density, and consequently the probability for collisions, is different for different particles [10] the intensity ratio between singlet and triplet decays provides an applicable parameter for particle discrimination. This has been shown in studies on the pulse shape in wavelength-integrated measurements [23,24]. However, the fast light emission from the third excimer continuum at longer wavelengths might mimic the fast singlet component in such measurements, influencing the discrimination potential.…”

“…This, however, has to be confirmed in wavelength-and timeresolved measurements at low energies deposited. In fact, for α-particles a dependence of I s I t on the particle's energy has been reported [23].…”

“…The time constants for the singlet and triplet decay of the neutral excimer molecules in the liquid phase are very different: light emission in allowed singlet decays typically happens within a few nanoseconds, while for triplet decays life times of more than 1 µs have been reported [7,8,23,52] in the absence of impurities. The direct transition from triplet states into the ground state is forbidden, and only mixing between 3 Σ + u and 1 Π u states through spin-orbit coupling [53] renders the decay possible.…”

“…γ 10 ± 5 1200 ± 20 0.30 ± 0.01 [23] e − <6.2 1300 ± 60 0.51 ± 0.05 [7,8] α 10 ± 5 1200 ± 20 2.6 ± 0.1 [ 23] n 10 ± 5 1200 ± 20 3.5 ± 0.2 [ 23] fission fragments 6.8 ± 1.0 1550 ± 100 3 [54] the excitation [55],…”

Pulse-shape discrimination and energy resolution of a liquid-argon scintillator with xenon doping

“…Interactions in liquid argon produce large amounts of scintillation, comparable to the light output of NaI(Tl). Therefore, energy resolution can be as low as a few percent FWHM [1]. Unlike NaI(Tl), which has a slow 230-ns decay time, argon has a fast component with a 7-ns decay time, followed by a slower component with 1.6-µs decay time [2].…”