Ethan L. Hull

Temperature sensitivity of surface channel effects on high-purity germanium detectors

The temperature sensitivity of surface channel effects on planar high-purity germanium detectors was measured using 60-keV gamma-ray scanning techniques, as part of a radiation damage study. When measured in this manner, the surface effects on most detectors showed extreme temperature sensitivity in the 72–95 K region. The effect of the surface channel increased with increasing temperature to such an extent that the efficiency, as measured by the count rate in the 1332-keV peak from a 60Co source, decreased by a factor of over two in some cases. Since the peak efficiency for the 1332-keV gamma ray decreased as the temperature increased throughout the operating range (72–120 K) the effect of the surface channel must continue to increase beyond the temperature (95 K) at which the 60-keV scan loses its sensitivity because of the strong attenuation of these much lower energy gamma rays. Radiation damage had no measurable effect on the surface characteristics. No correlation between the surface effects and the resolution changes of the 1332-keV peak was observed.

Radiation Effects in CdZnTe Gamma-Ray Detectors Produced by 199 MeV Protons

Many future space missions will use cadmium zinc telluride (CdZnTe) gamma-ray detectors because their operation at room temperature makes compact, lightweight detector systems possible. Even though instruments for space using CdZnTe detectors have already been built, the effect of the high- energy particle space environment on these detectors has not been measured. To determine the effect of energetic charged particles on these detectors, we have bombarded several CdZnTe detectors with 199 MeV protons at the Indianan University Cyclotron Facility. Planar detectors of area 1 cm2 and thickness 2-3 mm from both eV products and Digirad were irradiated, along with a 2 multiplied by 2 array of proprietary design from Digirad. Using standard gamma-ray sources, the response of the detectors was measured before and after bombardment in steps up to fluences of 5 multiplied by 109 p cm-2. Significant effects from the proton irradiation were observed in the gamma-ray spectra. In particular, the peak positions of the lines in the spectrum were shifted downward proportional to the fluence. The explanation is almost certainly the production of electron traps by the high energy proton interactions, resulting in a decrease of the mobility-lifetime ((mu) (tau) ) product of the electrons. Calculations were made to model the effect of a decrease in electron trapping length on the spectrum.

First-generation hybrid compact Compton imager

At Lawrence Livermore National Laboratory, we are pursuing the development of a gamma-ray imaging system using the Compton effect. We have built our first generation hybrid Compton imaging system, and we have conducted initial calibration and image measurements using this system. In this paper, we present the details of the hybrid Compton imaging system and initial calibration and image measurements

Gamma-ray escape peak characteristics of radiation-damaged reverse-electrode germanium coaxial detectors

Abstract A comparison of the characteristics of full-energy gamma-ray peaks and their corresponding escape peaks when high energy photons interact in radiation damaged reverse-electrode (n-type) germanium coaxial detectors is presented. Coaxial detector geometry is the dominant factor, causing charge collection to be dramatically better for interactions occurring near the outer periphery of the detector as well as increasing of the probability of escape events occurring in this region. It follows that the resolution of escape peaks is better than that of ordinary gamma-ray peaks. This is experimentally verified. A nearly identical but undamaged detector exhibited significant Doppler broadening of single escape peaks. Because double escape events preferentially occur at outer radii, energy shifts of double escape reflect extremely small amounts of charge trapping in undamaged detectors.

Pulse-shape discrimination techniques for correcting the effects of radiation damage in germanium coaxial detectors

Abstract Pusle-shape discrimination (PSD) techniques on current pulses from coaxial germanium detectors can significantly correct for charge losses due to hole trapping caused by radiation damage. Numerical simulations of PSD indicate that by measuring the two largest photon interaction locations and correcting the energy depositions separately, a significant recovery of the energy resolution and the Gaussian line shape of narrow lines in radiation damaged detectors can be obtained.

Laboratory tests of pulse shape discrimination techniques for correcting the effects of radiation damage in germanium coaxial detectors

Abstract A reverse-electrode closed-end germanium coaxial detector was irradiated with 183-MeV neutrons to evaluate the value of Pulse Shape Discrimination (PSD) techniques in restoring the energy resolution and line shape of the radiation damaged detector. Two consecutive irradiations were performed for total fluences of 5.0×10 8 and 10.4×10 8 n / cm 2 , with PSD tests performed after each irradiation. These irradiations degraded the energy resolution and line shapes; however, PSD corrections significantly restored the performance, even after severe damage. These PSD techniques delay and potentially eliminate, in some experimental situations, the need to anneal germanium detectors in damaging radiation environments.

A 16N gamma-ray facility

Abstract A practical 16N gamma-ray source is created in a medium-energy cyclotron environment. A 16N source emits 6129 and 7115 keV gamma rays. The viability of this several μCi source for detector calibration and studying detector physics is established.

MECHANICALLY COOLED LARGE-VOLUME GERMANIUM DETECTOR SYSTEMS FOR NUCLEAR EXPLOSION MONITORING

Compact maintenance free mechanical cooling systems are being developed to operate large volume (~570 cm3, ~3 kg, 140% or larger) germanium detectors for field applications. We are using a new generation of Stirling-cycle mechanical coolers for operating the very largest volume germanium detectors with absolutely no maintenance or liquid nitrogen requirements. The user will be able to leave these systems unplugged on the shelf until needed. The flip of a switch will bring a system to life in ~1 hour for measurements. The maintenance-free operating lifetime of these detector systems will exceed five years. These features are necessary for remote long-duration liquid-nitrogen free deployment of large-volume germanium gamma-ray detector systems for Nuclear Explosion Monitoring (NEM). The Radionuclide Aerosol Sampler/Analyzer (RASA) will greatly benefit from the availability of such detectors by eliminating the need for liquid nitrogen at RASA sites while still allowing the very largest available germanium detectors to be utilized. These mechanically cooled germanium detector systems being developed here will provide the largest, most sensitive detectors possible for use with the RASA. To provide such systems, the appropriate technical fundamentals are being researched. Mechanical cooling of germanium detectors has historically been a difficult endeavor. The success or failure of morexa0» mechanically cooled germanium detectors stems from three main technical issues: temperature, vacuum, and vibration. These factors affect one another. There is a particularly crucial relationship between vacuum and temperature. These factors will be experimentally studied both separately and together to insure a solid understanding of the physical limitations each factor places on a practical mechanically cooled germanium detector system for field use. Using this knowledge, a series of mechanically cooled germanium detector prototype systems are being designed and fabricated. Our collaborators at Pacific Northwest National Laboratory (PNNL) will evaluate these detector systems on the bench top and eventually in RASA systems to insure reliable and practical operation. «xa0less