Gamma ray imaging for environmental remediation. 1998 annual progress report

Abstract

'The objective of this research is the development of high resolution germanium detector systems for direct imaging of spent nuclear fuels and fissile materials and Compton scatter imaging of large objects of arbitrary size. A small configuration of detectors shall be assembled to demonstrate the performance of such a system. The efforts in this first year of a three year program have been focused on the design and procurement of a four detector array of germanium strip detectors. The basic detector is a 5 cm x 5 cm x 1 cm thick planar germanium detector with segmented anode and cathode. These segmentations are in the form of strips with a pitch of 2 millimeters. The strips on the anode side run orthogonal to the strips on the cathode side so that, in a gamma-ray interaction, the collection of electrons and holes on opposite electrodes produces signals which identify the interaction position in two dimensions. Each detector in the array has 25 anode contacts and 25 cathode contacts. The detector array has been designed as a planar array providing 100 square centimeters of active area mounted in a single cryostat. To reduce electronics channel count, parallel strips from adjacent detectors are electrically joined together effectively making strips that are 10 cm long. In this configuration, 100 channels of electronics are required to readout the array. In a related project, the authors are investigating processes to produce improved contacts for germanium strip detectors which would offer both more reliable extended use and the potential for finer strip pitch or position resolution. Investigating amorphous contacts, they have, to date, prepared two Ge test detectors. One was produced using n-type Ge and the other p-type Ge. The p-type detector has a Li diffused contact on one side while the n-type detector has a B ion-implanted contact. Amorphous contacts have been formed on the opposite side of each detector by RF sputtering. They are currently conducting a series of leakage current versus operating temperature measurements in order to determine the barrier heights of the amorphous contacts. The use of n and p type detectors will allow us to measure the electron and hole blocking behavior of the contacts respectively. This will provide important information on the nature of these contacts and will determine if double-sided strip detectors can be produced using only amorphous contacts.'