Study and suppression of anomalous fast events in inorganic scintillators for dark matter searches

TL;DR

This paper reviews dark matter search efforts using inorganic scintillators, focusing on identifying and suppressing anomalous fast events caused by surface contamination, and reports promising initial results with unencapsulated NaI(Tl) crystals.

Contribution

It introduces a new method of using unencapsulated NaI(Tl) crystals in controlled atmospheres to effectively eliminate anomalous fast events in dark matter detection.

Findings

Anomalous fast events are linked to surface alpha contamination.

Unencapsulated NaI(Tl) crystals show no anomalous fast events in initial tests.

Surface control techniques can improve background suppression in dark matter searches.

Abstract

The status of dark matter searches with inorganic scintillator detectors at Boulby mine is reviewed and the results of tests with a CsI(Tl) crystal are presented. The objectives of the latter experiment were to study anomalous fast events previously observed and to identify ways to remove this background. Clear indications were found that these events were due to surface contamination of crystals by alphas, probably from radon decay. A new array of unencapsulated NaI(Tl) crystals immersed either in liquid paraffin or pure nitrogen atmosphere is under construction at Boulby. Such an approach allows complete control of the surface of the crystals and the ability to remove any surface contamination. First data from the unencapsulated NaI(Tl) do not show the presence of anomalous fast events.