#StudentSeminar: New technologies for High Purity Germanium detectors

por Stefano Bertoldo

jueves 27 jul. 2023 15:00 → 16:30 Europe/Madrid

zoom-0-0 - zoom (Virtual)

Gamma ray spectroscopy is a fundamental tool for radiation detection in the study of nuclear structure, in the identification of hazardous materials in the security sector, for different medical diagnoses or industrial metal casts defect identification. High Purity Germanium (HPGe) gamma ray detectors are those with the higher energy resolution and can be fabricated with segmented contacts to exploit tracking capabilities. Contact processing is nowadays limited to a few technologies since the high purity of the bulk (1010 atoms/cm3) is hardly preserved by high temperature treatments. Most common procedures are Lithium diffusion at 300-400C° for n+ contact and room temperature Boron implantation for p+. Lithium diffusion however creates a thick doped layer which is thermally unstable under neutron damage recovery annealing and prevent fine segmentation. In this work we show the effectiveness of the Pulse Laser Melting (PLM) technique creating Sb highly doped n+ junctions without inducing bulk contamination contrary to what happens with many other standard doping processes. First prototypes of segmented detectors produced starting from bare crystals and exploiting PLM, are described. Several processing steps have been optimized. First, crystal polishing using mechanical and chemical abrasion has to be performed to obtain a smooth surface where different elements (Al as p+ dopant and Sb as n+ dopants) are deposited via sputtering. Dopants are then incorporated in the first hundred nanometers of Ge crystal structure with UV Pulse Laser Melting technique through an ultrafast melt and regrowth. Gold plating and junction segmentation are needed to achieve good electrical contacts for gamma imaging devices: photolithography is tested to produce a detector capable of precisely locating a gamma event within the active volume. A final surface passivation is then required to avoid surface leakage currents between electrodes: commercial devices passivation layers rely on amorphous germanium or amorphous silica depositions, but we obtained a chemical passivation which, although thermally unstable, forms a good insulation layer. Detectors are then cooled down to liquid nitrogen temperature within prototyped cryostats in order to freeze Ge thermal carriers. Several tests are performed through I- V characteristic curves: each segment, often surrounded by a guard ring, is inversely polarized as a single diode with respect to the opposite side to find its breakdown voltage, and electrical insulation between segments is obtained polarizing a segment with respect to its neighbor. Spectroscopic measurements using weak gamma sources are finally used to test the resolution of the HPGe detector. Very high- resolution on segmented detectors is demonstrated. Moreover, transient charge collection events demonstrate the feasibility of gamma interaction localization. Several upgrades are also performed to reproduce these processes on coaxial detectors similar to the devices in arrays like AGATA.