Cosmogenic production of tritium in dark matter detectors

Abstract

The direct detection of dark matter particles requires ultra-low background conditions at energies below a few tens of keV. Radioactive isotopes are produced via cosmogenic activation in detectors and other materials and those isotopes constitute a background source which has to be under control. In particular, tritium is specially relevant due to its decay properties (very low endpoint energy and long half-life) when induced in the detector medium, and because it can be generated in any material as a spallation product. Quantification of cosmogenic production of tritium is not straightforward, neither experimentally nor by calculations. In this work, a method for the calculation of production rates at sea level has been developed and applied to some of the materials typically used as targets in dark matter detectors (germanium, sodium iodide, argon and neon); it is based on a selected description of tritium production cross sections over the entire energy range of cosmic nucleons. Results have been compared to available data in the literature, either based on other calculations or from measurements. The obtained tritium production rates, ranging from a few tens to a few hundreds of nuclei per kg and per day at sea level, point to a significant contribution to the background in dark matter experiments, requiring the application of specific protocols for target material purification, material storing underground and limiting the time the detector is on surface during the building process in order to minimize the exposure to the most dangerous cosmic ray components.Comment: Final version as publishe

Cosmogenic activation of materials

“…The range derived in Ref. 71 is well compatible with the measured rates by EDELWEISS and CDMSlite. For enriched germanium, as used in double beta decay experiments, the measured production rate is higher than in natural germanium; this can be due to the fact that cross sections increase with the mass number of the germanium isotope in all the energy range above ∼50 MeV, according to TENDL-2013 and HEAD-2009 data.…”

“…As it can be seen in Table 3, this rate is higher than previous estimates, but the required exposure times to get the deduced tritium activities for ANAIS detectors based on that rate roughly agree with the time lapse between sodium iodide raw material purification starting and detector shipment, according to the company. 71 …”

“…In the calculations of Ref. 71, based on a selection of excitation functions mainly from TENDL for neutrons and HEAD-2009 libraries, there is a significant difference in the medium energy range between the two estimates of cross sections for I, being those from TENDL much higher; this fact is responsible of the large uncertainty in the production rate estimated for NaI. As it can be seen in Table 3, this rate is higher than previous estimates, but the required exposure times to get the deduced tritium activities for ANAIS detectors based on that rate roughly agree with the time lapse between sodium iodide raw material purification starting and detector shipment, according to the company.…”

“…71, the study of tritium production performed for other targets like germanium and sodium iodide was applied also to argon and neon, selecting the excitation functions and considering the Gordon et al cosmic neutron spectrum. If saturation activity was reached for tritium, according to the production rates deduced in this study and presented in Table 3 too, tritium could be very problematic.…”

“…In Ref. 71, dedicated calculations of production rates of tritium at sea level have been performed for some of the materials typically used as targets in dark matter detectors (germanium, sodium iodide, argon and neon), based on a selection of excitation functions over the entire energy range of cosmic neutrons, mainly from TENDL for neutrons and HEAD-2009 libraries. All these results on the production rate of tritium in both natural and enriched germanium are summarized in Table 3.…”