Cosmogenic activation of xenon and copper

Laura Baudis,Francesco Piastra,Alexander Kish,Marc Schumann

doi:10.1140/epjc/s10052-015-3711-3

Abstract

Rare event search experiments using liquid xenon as target and detection medium require ultra-low background levels to fully exploit their physics potential. Cosmic ray induced activation of the detector components and, even more importantly, of the xenon itself during production, transportation and storage at the Earth's surface, might result in the production of radioactive isotopes with long half-lives, with a possible impact on the expected background. We present the first dedicated study on the cosmogenic activation of xenon after 345 days of exposure to cosmic rays at the Jungfraujoch research station at 3470m above sea level, complemented by a study of copper which has been activated simultaneously. We have directly observed the production of 7Be, 101Rh, 125Sb, 126I and 127Xe in xenon, out of which only 125Sb could potentially lead to background for a multi-ton scale dark matter search. The production rates for five out of eight studied radioactive isotopes in copper are in agreement with the only existing dedicated activation measurement, while we observe lower rates for the remaining ones. The specific saturation activities for both samples are also compared to predictions obtained with commonly used software packages, where we observe some underpredictions, especially for xenon activation.

Highlights

Liquid xenon (LXe) is used as detection medium in current and future rare event search experiments, such as direct dark matter [1,2,3,4,5] and neutrinoless double-beta decay searches [6,7,8,9]
The numbers are compared with our own predictions using the Activia and Cosmo codes, with another measurement performed at LNGS [42], and with additional predictions from the literature: these are based on the Activia package [12], semi-analytical calculations [44], and the TALYS 1.0 code [45,46]
We have carried out the first experiment to directly study the cosmogenic activation of xenon, which is employed as target and detection medium for rare event searches, and the second dedicated measurement on the activation of oxygen-free high thermal conductivity (OFHC) copper, often used as ultra-pure detector construction material

Summary

Introduction

Liquid xenon (LXe) is used as detection medium in current and future rare event search experiments, such as direct dark matter [1,2,3,4,5] and neutrinoless double-beta decay searches [6,7,8,9] It features a high scintillation and ionization yield [10,11], as well as a high radio-purity. For a given exposure time and altitude, the activation yield of materials can be predicted using software packages such as Activia [12] and Cosmo [13] Calculations with both codes were performed for the XENON100 experiment [1], the predicted production rates were too high to be compatible with the measured background rates [14,15,16].

Samples and preparation

Activation by cosmic rays

Measurement and data analysis

Copper

Literature values

Discussion and conclusions