Design and Testing of a Pulse Tube Based Cooling System for High Purity Germanium Detectors

Abstract

A cooling package based on a new type of production cryocooler was designed and tested at the Technology Development Laboratory (TDL) of the Houston Advanced Research Center (HARC) in collaboration with the company Kelvin International. The prototype is based on an Iwatani CryoMini® Pulse Tube Cryocooler provided by Kelvin International. The CryoMini cryocooler was integrated with an EG&G Pop-Top High Purity Germanium (HPGe) detector within a TDL designed cooling package. One of the key features of this new type of cryocooler is the small size of its cold head allowing for the combination of the cooling package prototype and an EG&G Pop-Top detector to fit within a cylindrical volume with a diameter slightly larger than 3 inches and a length of less than 2 1/2 feet long. The compact design of the cryocooler is attractive for environmental surveys, shallow hole investigations, and other applications that require long uninterrupted operating time, minimal space and ease of operation. The Cooling package temperature histories were recorded during operation, and gamma ray spectra were acquired with the prototype. A steady state temperature of 78 K was achieved in the Pop-Top capsule. This temperature is well within the operating range of a standard HPGe Pop-Top detector and provides a comfortable margin to convert the prototype into a more rugged field device. The energy resolution of an HPGe detector is compared for spectra acquired with the prototype and with a standard liquid nitrogen dewar for calibration sources and Naturally Occurring Radioactive Material (NORM) samples. The energy resolution of the prototype was measured at 2.08 keV for the 1.332 MeV line of a Cobalt-60 gamma ray source. Although somewhat higher than the resolution measured for the same detector cooled with a liquid nitrogen cryostat (1.73 keV) the measured energy resolution is within the warranted range for the HPGe detector used in this study.