Background model for a NaI(Tl) detector devoted to dark matter searches
Background model for a NaI(Tl) detector devoted to dark matter searches
Astrop. Phys. 37 (2012) 60-69
NaI (Tl) is a well known high light yield scintillator. Very large crystals can be grown to be used in a wide
range of applications. In particular, such large crystals are very good-performing detectors in the search
for dark matter, where they have been used for a long time and reported first evidence of the presence of
an annual modulation in the detection rate, compatible with that expected for a dark matter signal. In the
frame of the ANAIS (Annual modulation with NaI Scintillators) dark matter search project, a large and
long effort has been carried out in order to characterize the background of sodium iodide crystals. In this
paper we present in detail our background model for a 9.6 kg NaI (Tl) detector taking data at the Canfranc
Underground Laboratory (LSC): most of the contaminations contributing to the background have been
precisely identified and quantified by different complementary techniques such as HPGe spectrometry,
discrimination of alpha particles vs. beta/gamma background by Pulse Shape Analysis (PSA) and coincidence
techniques; then, Monte Carlo (MC) simulations using Geant4 package have been carried out for
the different contributions. Only a few assumptions are required in order to explain most of the measured
background at high energy, supporting the goodness of the proposed model for the present ANAIS prototype
whose background is dominated by 40K bulk contamination. At low energy, some non-explained
background components are still present and additional work is required to improve background understanding,
but some plausible background sources contributing in this range have been studied in this
work. Prospects of achievable backgrounds, at low and high energy, for the ANAIS-upgraded detectors,
relying on the proposed background model conveniently scaled, are also presented.
NaI (Tl) is a well known high light yield scintillator. Very large crystals can be grown to be used in a wide
range of applications. In particular, such large crystals are very good-performing detectors in the search
for dark matter, where they have been used for a long time and reported first evidence of the presence of
an annual modulation in the detection rate, compatible with that expected for a dark matter signal. In the
frame of the ANAIS (Annual modulation with NaI Scintillators) dark matter search project, a large and
long effort has been carried out in order to characterize the background of sodium iodide crystals. In this
paper we present in detail our background model for a 9.6 kg NaI (Tl) detector taking data at the Canfranc
Underground Laboratory (LSC): most of the contaminations contributing to the background have been
precisely identified and quantified by different complementary techniques such as HPGe spectrometry,
discrimination of alpha particles vs. beta/gamma background by Pulse Shape Analysis (PSA) and coincidence
techniques; then, Monte Carlo (MC) simulations using Geant4 package have been carried out for
the different contributions. Only a few assumptions are required in order to explain most of the measured
background at high energy, supporting the goodness of the proposed model for the present ANAIS prototype
whose background is dominated by 40K bulk contamination. At low energy, some non-explained
background components are still present and additional work is required to improve background understanding,
but some plausible background sources contributing in this range have been studied in this
work. Prospects of achievable backgrounds, at low and high energy, for the ANAIS-upgraded detectors,
relying on the proposed background model conveniently scaled, are also presented.