Production and relevance of cosmogenic radionuclides in NaI(Tl) crystals

TL;DR

This study measures cosmogenic radionuclide production in NaI(Tl) detectors used in low-background experiments, identifying isotopes like 3H that significantly impact detector background levels.

Contribution

It provides new experimental data on cosmogenic isotope production rates in NaI(Tl) crystals and assesses their impact on detector backgrounds, enhancing understanding for low-background experiments.

Findings

Identified production rates of I, Te, Na isotopes with half-lives >10 days

Estimated sea level production rates using cosmic neutron flux data

Found 3H significantly contributes to background below 15 keV

Abstract

The cosmogenic production of long-lived radioactive isotopes in materials is an hazard for experiments demanding ultra-low background conditions. Although NaI(Tl) scintillators have been used in this context for a long time, very few activation data were available. We present results from two 12.5 kg NaI(Tl) detectors, developed within the ANAIS project and installed at the Canfranc Underground Laboratory. The prompt data taking starting made possible a reliable quantification of production of some I, Te and Na isotopes with half-lives larger than ten days. Initial activities underground were measured and then production rates at sea level were estimated following the history of detectors; a comparison of these rates with calculations using typical cosmic neutron flux at sea level and a selected description of excitation functions was also carried out. After including the contribution from the identified cosmogenic products in the detector background model, we found that the presence of 3H in the crystal bulk would help to fit much better our background model and experimental data. We have analyzed the cosmogenic production of 3H in NaI, and although precise quantification has not been attempted, we can conclude that it could imply a very relevant contribution to the total background below 15 keV in NaI detectors.