Measurement of Muon Annual Modulation and Muon-Induced Phosphorescence in NaI(Tl) Crystals with DM-Ice17

TL;DR

This study measures muon interactions and induced phosphorescence in NaI(Tl) crystals used for dark matter detection, finding that phosphorescence does not mimic the annual modulation signal observed by DAMA.

Contribution

First measurement of muon-induced phosphorescence in NaI(Tl) crystals at the South Pole, analyzing its properties and impact on dark matter detection signals.

Findings

Muon rate of 2.93 +/- 0.04 muons/crystal/day with 12.3% modulation

Long-lived phosphorescence with 5.5 +/- 0.5 s decay time

Phosphorescence characteristics do not mimic DAMA's annual modulation

Abstract

We report the measurement of muons and muon-induced phosphorescence in DM-Ice17, a NaI(Tl) direct detection dark matter experiment at the South Pole. Muon interactions in the crystal are identified by their observed pulse shape and large energy depositions. The measured muon rate in DM-Ice17 is 2.93 +/- 0.04 muons/crystal/day with a modulation amplitude of 12.3 +/- 1.7%, consistent with expectation. Following muon interactions, we observe long-lived phosphorescence in the NaI(Tl) crystals with a decay time of 5.5 +/- 0.5 s. The prompt energy deposited by a muon is correlated to the amount of delayed phosphorescence, the brightest of which consist of tens of millions of photons. These photons are distributed over tens of seconds with a rate and arrival timing that do not mimic a scintillation signal above 2 keVee. While the properties of phosphorescence vary among individual crystals, the annually-modulating signal observed by DAMA cannot be accounted for by phosphorescence with the characteristics observed in DM-Ice17.