Measurement of the intrinsic radiopurity of 137Cs/235U/238U/232Th in CsI(Tl) crystal scintillators

Abstract

The inorganic crystal scintillator CsI(Tl) has been used for low energy neutrino and Dark Matter experiments, where the intrinsic radiopurity is an issue of major importance. Low-background data were taken with a CsI(Tl) crystal array at the Kuo-Sheng Reactor Neutrino Laboratory. The pulse shape discrimination capabilities of the crystal, as well as the temporal and spatial correlations of the events, provide powerful means of measuring the intrinsic radiopurity of 137 Cs as well as the 235 U, 238 U and 232 Th series. The event selection algorithms are described, with which the decay half-lives of 218 Po, 214 Po, 220 Rn, 216 Po and 212 Po were derived. The measurements of the contamination levels, their concentration gradients with the crystal growth axis, and the uniformity among different crystal samples, are reported. The radiopurity in the 238 U and 232 Th series are comparable to those of the best reported in other crystal scintillators. Significant improvements in measurement sensitivities were achieved, similar to those from dedicated massive liquid scintillator detector. This analysis also provides in situ measurements of the detector performance parameters, such as spatial resolution, quenching factors, and data acquisition dead time.

Radioactive contamination of scintillators

“…Such an approach provides high detection efficiency, especially for α and β particles. A typical low background scintillation set-up (see, for instance, [199,200,201,202,203,44,204,205,206,207,208,209,41]) consists of scintillator, light-guide to shield the scintillator from radioactivity of photomultiplier tubes (typically, the most contaminated details of a low background scintillation set-up), passive shield. Background of the detector can be further suppressed by using of active shield counters surrounding the main detector, and anti-muon veto counters [45,195].…”

“…Data on the energy and arrival time of events can be used to select fast decay chains from the 232 Th, 235 U and 238 U families. The method of time-amplitude analysis is described in detail in [223,224,44,45,208]. For instance, the following sequence of α decays from the 232 Th family can be selected:…”

“…The mean time method is also widely used to discriminate β(γ) and α particles in scintillation detectors (see e.g. [187,202,208,228]). The following formula can be applied to calculate the mean time parameter t for each pulse:…”

“…Very low background NaI(Tl) scintillators have been produced by Saint-Gobain for the DAMA/LIBRA dark matter experiment [209,279,280]; several R&D are in progress to develop radiopure NaI(Tl) scintillators [248,252,253,256,257]. Low background CsI(Tl) were developed in the framework of the KIMS dark matter experiment [187,208]. Dependence of radiopurity of CsI(Tl) on crystal growth conditions was studied in [208].…”

Measurement of neutrino-electron scattering cross section and the EW parameters at the Kuo-Sheng reactor neutrino laboratory

“…In situ calibration was achieved using the measured γ-lines from 137 Cs, 40 K and 208 Tl. The internal contaminations of the 238 U and 238 Th series were measured [8] and found to be negligible compared to the observed background rates. Residual background at the relevant 3−8 MeV range are cosmic-ray induced and full energy deposition of ambient gamma such as due to neutron capture by 63 Cu or due to coincidence of γ-emissions following 208 Tl decays.…”