Radioactive contamination of BaF2 crystal scintillator

Barium fluoride (BaF$_2$) crystal scintillators are promising detectors to search for double beta decay processes in $^{130}$Ba ($Q_{2{\beta}}$ = 2619(3) keV) and $^{132}$Ba ($Q_{2{\beta}}$ = 844(1) keV). The $^{130}$Ba isotope is of particular interest because of the indications on 2${\beta}$ decay found in two geochemical experiments. The radioactive contamination of BaF$_2$ scintillation crystal with mass of 1.714 kg was measured over 113.4 hours in a low-background DAMA/R&D set-up deep underground (3600 m w.e.) at the Gran Sasso National Laboratories of INFN (LNGS, Italy). The half-life of $^{212}$Po (present in the crystal scintillator due to contamination by radium) was estimated as $T_{1/2}$ = 298.8 $\pm$ 0.8(stat.) $\pm$ 1.4(syst.) ns by analysis of the events pulse profiles.

The high level of radioactive contamination of BaF 2 scintillation crystal by uranium and thorium is the main source of background of the detectors [4], however, this feature allows to use the detector for measurements of some short-lived isotopes in U/Th chains (e.g. of 212 Po). Results of measurements of radioactive contamination of a large volume BaF 2 crystal scintillator are presented here. We have also derived a half-life value of 212 Po from the data by using pulse-shape analysis of 212 Bi − 212 Po events.

Experiments
Radioactive contamination of BaF 2 (3" × 3", 1.714 kg) was measured over 113.4 hours in a low-background DAMA/R&D set-up deep underground (3600 m w.e.) at the Gran Sasso National Laboratories of INFN (LNGS, Italy). The BaF 2 crystal scintillator was viewed through two light-guides (∅3" × 100 mm) by two low radioactive 3" photomultipliers (PMT, ETL 9302FLA). The detector was surrounded by Cu bricks and sealed in a low radioactive air-tight Cu box continuously flushed by high purity nitrogen gas (stored deep underground for a long time) to avoid the presence of residual environmental radon. The Cu box was surrounded by a passive shield made of high purity Cu, 10 cm of thickness, 15 cm of low radioactive lead, 1.5 mm of cadmium and 4 to 10 cm of polyethylene/paraffin to reduce the external background. The shield was contained inside a Plexiglas box, also continuously flushed by high purity nitrogen gas. An event-by-event data acquisition system recorded amplitude and pulse profile of events at the sampling rate of 1 GSPS over a time window of 4000 ns.
The energy scale of the BaF 2 detector and its energy resolution in the range of interest have been determined by means of 22 Na (511, 1275 keV), 137 Cs (662 keV), 241 Am (60 keV), 60 Co (1173, 1333 keV), 133 Ba (356 keV) and 228 Th (239, 2615 keV) gamma sources. The energy resolution (full width at half maximum, FWHM) of 15.5% was obtained for 662 keV γ quanta of 137 Cs, while for 511 and 1275 keV γ lines of 22 Na source the energy resolution was 16.4% and 10.8%, respectively (see Fig. 1; in all figures, energy is given in gamma scale). The energy dependence of the energy resolution can be approximated as: FWHM (keV) = [397(54) + 15.6(3) × E γ ] 1/2 , where E γ is energy of gamma quanta in keV.

Pulse-shape discrimination of α, β and Bi-Po events
The mean time method was used to separate alpha, gamma(beta) and Bi-Po events in the BaF 2 scintillator. The scatter plot of the mean time versus energy (see Fig.  2) demonstrates pulse-shape discrimination ability of the detector. The background energy spectrum accumulated over 113.4 hours with the BaF 2 crystal scintillator is presented in Fig. 3. A substantial increase of the counting rate in the energy interval 1.2 − 3.4 MeV is due to α activity of 238 U and 232 Th daughters from uranium and thorium contamination of the BaF 2 crystal. Energy spectrum of α events selected from the background data with the help of the pulse-shape discrimination is presented in Fig. 4. The spectrum was fitted by a model built of α peaks of 238 U and 232 Th and their daughters, assuming broken equilibrium of the chains. Therefore the activities of the following nuclides and sub-chains ( 232 Th, 228 Th-212 Pb; 238 U, 234 U, 230 Th, 226 Ra-214 Po, 210 Po; 235 U, 231 Pa, 227 Ac-211 Bi) were taken as free parameters of the fit. Besides, values of the energy resolution of the detector to alpha particles and alpha/beta ratio (relative light output for α particles as compared with that for β particles (γ rays)) were also introduced into the fit as free parameters. According to the fit, the α/β ratio for the BaF 2 scintillation detector depends on energy of α particles as α/β = 0.200(1) + 0.0245(1) × E α , where E α is energy of α particles in MeV. The radioactive contamination of the BaF 2 crystal obtained from the fit is presented in Table 1. One can conclude that the BaF 2 crystal is contaminated by radium.

Half-life of 212 Po
A distribution of the time intervals selected by the pulseshape analysis of the Bi-Po events is presented in Fig. 7. In order to suppress a contribution of the 214 Bi− 214 Po chain (with the half-life of 214 Po T 1/2 = 164 µs), the energy of the second event was selected from the energy interval 3000-3800 keV (see lower part in Fig. 6). An energy threshold of 300 keV was chosen for the first events to decrease jitter of the event time determination. The time spectrum was fitted by sum of two exponential functions that represent the decays of 212 Po and 214 Po. The decay constant of the second exponent was bounded taking into account the table uncertainty of the 214 Po halflife ± 2.0 μs [9].  [9], and in a reasonable agreement with the recent result of the Borexino collaboration 294.7 ± 0.6(stat.) ± 0.8(syst.) ns [10].
We have also estimated activity of 228 Th from the Bi-Po analysis as 1.04(10) Bq/kg, which is in a reasonable agreement with the result obtained from the fit of the alpha spectrum presented in Fig. 6.

Conclusions
The radioactive contamination of the BaF 2 crystal scintillator was estimated to be on the level of few Bq/kg for 226 Ra and 228 Th. Taking into account 3 orders of magnitude lower activity of 238 U and 232 Th (only limits <0.0002 Bq/kg for 238 U and <0.004 Bq/kg for 232 Th were obtained) and broken equilibrium in the chains, one can conclude that the BaF 2 crystal is contaminated by radium. The response of the BaF 2 crystal scintillator to α particles has been investigated in the wide energy interval and the capability of pulse-shape discrimination between α particles and γ quanta (electrons) has been demonstrated.
Analysis of the time intervals distribution between β and α decays in the fast Bi-Po chains allowed us to estimate half-life of 212 Po as T 1/2 = 298.8 ± 0.8(stat.) ± 1.4(syst.) ns, which is in an agreement with the table value [9].
The contamination of the BaF 2 crystal by 226 Ra and 228 Th is the main problem in applications of this scintillator to search for double beta decay of barium. An R&D of methods to purify barium from radium traces is in progress at the Gran Sasso National Laboratories with an aim to develop radiopure BaF 2 crystal scintillators to search for double beta decay of 130,132 Ba. Such a counting experiment is of particular interest, taking into account indications of two geochemical experiments on double beta decay of 130 Ba.