A Multi-Contact, Low Capacitance HPGe Detector for High Rate Gamma Spectroscopy

The detection and identification of special nuclear materials and nuclear fission by-products is a critically important activity in support of nuclear non-proliferation programs. Gamma-ray spectroscopy is a key tool in this field, but no instrument exists that exploits the intrinsically high energy resolution and detection sensitivity of high purity germanium (HPGe) detectors while operating at high output count rates in the few 100 kcps to 1 Mcps range. The proposed work overcomes the high count-rate technical constraints of standard (high-capacitance) HPGe gamma-ray detectors by developing a multi-contact device with signal electrodes of a few pF capacitance. The technical approach combines the high electric field strength and intrinsically fast rise times of a planar detector with the collection efficiency of a coaxial device. Phase I will employ an electrostatic modeling program to investigate different multi-contact configurations on a 15 mm thick planar HPGe detector to calculate electrode capacitance and charge drift times, and to create a design that optimizes the geometry, contact width and spacing to meet or exceed the required specifications. The efficacy of the charge collection and capacitance model will be checked against measurements on an existing segmented HPGe planar detector. To gain sufficient collection efficiency for gamma-rays in the MeV range, the final detector design will use two back-to-back planar detectors. A second task in Phase I will be to design the detector housing, HPGe crystal mounts and internal connections, and explore options for the FETs and preamplifiers. An analysis of the electronic noise and the detection efficiency at 662 keV will also be made. Phase I will culminate with the design of the complete detector assembly and housing to be fabricated and tested in Phase II. We are in discussions with the leading segmented HPGe detector manufacturer for fabrication and eventual commercialization of the multi-contact detector. When combined with high rate multi-channel spectroscopy electronics, it is expected that the detector system will provide data throughput rates around 300 kcps per channel (at about 1 Mcps input) with electronic noise in the 500-600 eV FWHM range. This will present a significant advance for nuclear safeguards instrumentation with increased speed and accuracy of detection and identification in high count rate applications. As a unique tool for high rate and high resolution gamma spectroscopy, other applications are foreseen in areas such as medical imaging, and materials defect analysis by positron emission spectroscopy.