CAGE: An Internal Source Scanning Cryostat for HPGe Characterization

TL;DR

This work addresses the challenge of surface backgrounds in LEGEND by introducing CAGE, an internal-source scanning vacuum cryostat that positions a collimated source on HPGe detector surfaces with tunable incidence angles. The system combines a rotatable IR shield, three motion stages, a cryogenic detector mount, and comprehensive slow controls to map surface interactions and extract pulse-shape features for background discrimination. Commissioning with a 241Am source demonstrates a beam-spot precision of about 3.1 mm and reveals radially dependent risetimes for 60 keV gammas, along with alpha discrimination via the DCR parameter. These results support developing pulse-shape-based cuts to mitigate surface backgrounds, enhancing LEGEND’s sensitivity for both LEGEND-200 and LEGEND-1000 experiments.

Abstract

The success of current and future-generation neutrinoless double beta decay experiments relies on the ability to eliminate or reduce extraneous backgrounds. In addition to constructing experiments using radiopure materials and handling in underground laboratories, it is necessary to understand and reduce known backgrounds in data analysis. The Large Enriched Germanium Experiment for Neutrinoless double beta Decay is searching for this decay using 76Ge-enriched high-purity germanium detectors submerged in an active liquid argon veto. A significant background in LEGEND is surface events from shallowly-impinging radiation on detector surfaces. In this paper we introduce the Collimated Alphas, Gammas, and Electrons (CAGE) scanning system, an internal-source scanning vacuum cryostat, designed to perform studies of surface events on sensitive surfaces of HPGe in a surface-lab. CAGE features a collimated radionuclide source inside a movable infrared shield that is able to perform precision scans of detector surfaces by utilizing three independent motor stages for source positioning. This allows detailed studies of pulse shapes as a function of source position and incident angle, where defining features can be extracted and exploited for removing surface backgrounds in data analysis in LEGEND. In this paper, we describe CAGE and demonstrate its performance with a commissioning run with 241Am. The commissioning run was completed with the source at normal incidence, and we estimate a beam spot precision of 3.1 mm, which includes positioning uncertainties and the beam-spot size. Using the 59.5 keV gamma population from 241Am, we show that low-energy photon events near the passivated surface feature risetimes that increase with radial distance from the detector center. We suggest a specific metric that can be used to discriminate low-energy gamma backgrounds in LEGEND with similar characteristics.

Figures (15)

• Figure 1: Two-dimensional cross-section of PC detector technologies used in the LEGEND-200 experiment, with weighting field magnitude, $|E_w|$, indicated by the colorbar. The heavy grey lines denote the N+ surfaces, the heavy black lines indicate the P+ contacts, and all surfaces with no heavy lines are bare or passivated. (left) Majorana style PPC detector with large passivated surface; (center) GERDA Broad Energy Germanium (BEGe) detector, with a passivated ditch and large P+ contact; (right) Inverted Coaxial Point Contact (ICPC) detector, implemented for LEGEND. Figure from Comellato:2020ljj.
• Figure 2: External view of the assembled CAGE vessel.
• Figure 3: Detector mount on the CAGE cold-plate, shown here with the OPPI-1 PPC detector, described in \ref{['sec:cage_collim']}.
• Figure 4: CAD rendering of the IR shield and motor assembly. (a) Shows the components, (b) shows the movement directions of the vacuum motors, and (c) shows the movement of the rack and pinion system to lift the IR shield assembly.
• Figure 5: Left: IR shield with the motor assembly. The positions of the limit switches for the rotary stage and source motors are indicated here. The limit switches for the linear stage are internal and are not visible without disassembling the stage. Right: Underside of IR shield with G-10 shaft, collimator (pointed away from the camera), and copper braid.