Comparison of CDMS [100] and [111] oriented germanium detectors

TL;DR

This paper compares the effects of [100] and [111] orientations on germanium detectors used in dark matter searches, focusing on electron transport and fiducial volume determination.

Contribution

It provides a detailed analysis of how crystal orientation influences charge transport and detector performance in CDMS germanium detectors.

Findings

Electron transport varies significantly between [100] and [111] orientations.

Detector orientation impacts the fiducial volume and event classification accuracy.

Electric field fringing effects influence charge collection at detector edges.

Abstract

The Cryogenic Dark Matter Search (CDMS) utilizes large mass, 3" diameter $\times$ 1" thick target masses as particle detectors. The target is instrumented with both phonon and ionization sensors and comparison of energy in each channel provides event-by-event classification of electron and nuclear recoils. Fiducial volume is determined by the ability to obtain good phonon and ionization signal at a particular location. Due to electronic band structure in germanium, electron mass is described by an anisotropic tensor with heavy mass aligned along the symmetry axis defined by the [111] Miller index (L valley), resulting in large lateral component to the transport. The spatial distribution of electrons varies significantly for detectors which have their longitudinal axis orientations described by either the [100] or [111] Miller indices. Electric fields with large fringing component at high detector radius also affect the spatial distribution of electrons and holes. Both effects are studied in a 3 dimensional Monte Carlo and the impact on fiducial volume is discussed.