GAMMA-X Series detectors are manufactured from ORTEC-grown germanium crystals processed in our advanced manufacturing facility in Oak Ridge, TN. The detectors are fabricated from N-type germanium with an inner contact of diffused Li and an outer, ultra thin, contact of ion-implanted boron.
The GAMMA-X detector, uniquely, demonstrates excellent efficiency at both high and low energies. ORTEC offers GMX Series HPGe detectors with relative efficiencies from 10% to 100% and beyond.
ORTEC maintains a large stocklist of HPGe detectors
. Some of these have "super specifications," that is, a warranted energy resolution better than the usual warranted specifications.
High- and Low-Energy Performance of the GAMMA-X Detector
The high-energy performance of a GAMMA-X detector is defined by its relative efficiency, resolution, and peak-to-Compton ratio at 60
The low-energy performance of this detector is defined by its resolution at 5.9 keV, its active surface area, and the detector window thickness.
The thickness of the entrance contact of the GAMMA-X detector is described by the ratio of the areas of two peaks of a readily available source. The peaks chosen are those of the 88-keV gamma rays from the 109
Cd and of the 22.16-keV Ag K x rays from the same source. The warranted window attenuation ratio is 20.
22-keV Peak/88-keV Peak Area
||peak area at 22.16 keV
| peak area at 88 keV
This specification quantifies the thinness of the entrance window in GAMMA-X detectors. The natural ratio of gamma rays from the 22-keV and 88-keV lines of a 109
Cd source is ~21:1. A GAMMA-X detector typically displays a ration >20:1.
GMX detectors in 70-mm (2.75-in.) or 76-mm (3-in.) diameter endcaps (10 to ~35%) are supplied with 51-mm (2-in.) diameter Be windows. GMX detectors in 83-mm (3.25-in.) diameter endcaps (~30 to 65%) are supplied with 64-mm (2.5-in.) diameter Be windows. These windows are 0.020-in. thick and have a transmission coefficient of ~95% at 5.9-keV. Detectors in 95-mm (3.75-in.) diameter endcaps (~60 to 100%) receive a 84-mm (3.3-in.) diameter Be window that is 0.030-in. thick.
High-Voltage Shutdown and High-Rate Indicator
GAMMA-X detectors have high-voltage shutdown and high-rate indicator protection features. If the LN2
supply is exhausted and the detector begins to warm while high-voltage bias is applied (when using the Model 659 Bias Supply
), the high voltage automatically shuts off, thus protecting the FET from damage. This is accomplished with a temperature sensor (located on the mount behind the detector) that shuts down the high voltage before the molecular sieve can outgas and cause a dangerous high-voltage arc. Using the high-leakage current of a warming detector to shut down the high voltage can result in FET and detector damage.
Neutron Damage Resistance
In the GAMMA-X detector, electron collection is the dominant process. Fast neutrons generate hole-trapping centers; that is, negatively charged defects that trap holes but not electrons.
Therefore, the GAMMA-X detector, in which the hole collection process is of secondary importance, is basically less sensitive to radiation damage than coaxial Ge devices in which the hole collection process is of primary importance. These theoretical considerations have been experimentally confirmed.
It should be noted that once severe radiation damage has occurred, the “longest mileage” is obtained by avoiding cycling the detector to room temperature. This is true for either p- or n-type Ge detectors. However, for slightly damaged GAMMA-X detectors (~0.1 keV degradation), cycling, or even leaving the detector warm for an extended period, will have no unfavorable effect.
GAMMA-X detectors should be maintained at a temperature as close to 77 K as possible to minimize the extent of radiation damage. Therefore a streamline cryostat, with one less thermal connection, is a better choice than a PopTop for this purpose.
Repair of neutron-damaged GAMMA-X detectors can be performed at any of our worldwide repair facilities, or by you in your own laboratory. Contact us for information about our Customer-Neutron-Damage-Repairable GAMMA-X detectors.