Gamma-X (GMX) N-type High Purity Germanium (HPGe) Coaxial Radiation Detectors

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, energy resolution and/or efficiency 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 ⁶⁰Co.

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 ¹⁰⁹Cd and of the 22.16-keV Ag K x rays from the same source. The warranted window attenuation ratio is 18.

WE =	peak area at 22.16 keV
	peak area at 88 keV

             
22-keV Peak/88-keV Peak Area
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 ¹⁰⁹Cd source is ~21:1. A GAMMA-X detector typically displays a ration >18:1.
 
High-Voltage Shutdown and High-Rate Indicator
GAMMA-X detectors have high-voltage shutdown and high-rate indicator protection features. If the detector begins to warm while high-voltage bias is applied 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 under high neutron fluence.

Aluminum Window Option  (-A)
An all aluminum endcap can be chosen if the energies of interest exceed 20 keV.

Carbon Fiber Window Option  (-CW)
Carbon fiber has lower Z than Al and does not have any of the hazards associated with Be. GMX series carbon fiber windows are only 0.76 mm thick but extremely rugged.

SMART-1 Option  (-SMN)
The SMART-1 option monitors and reports on vital system functions, and can save authentication codes and report the code at a later time. It has the high voltage included, so none of the instruments require an external high-voltage power supply. The SMART-1 is housed in a rugged ABS molded plastic enclosure and is permanently attached to the detector endcap via a molded-strain-relieved sealed cable. This eliminates the possibility that the detector will suffer severe damage from moisture leaking into high-voltage connectors. The SMART-1 can be positioned in any convenient place and does not interfere with shielding or other mounting hardware.

Ultra-High Count-Rate Preamplifier Option  (-PL)
The Ultra-High Count-Rate Preamplifier (transistor-reset preamplifier), can handle input count rates up to 1,000,000 counts/s at 1 MeV, offers the added benefit of having no feedback resistor.

Remote Preamplifier Option  (-HJ)
This option allows all the preamplifier and high voltage connections to be outside a shield and removes the preamplifier and high voltage filter from the “line-of-sight” to the Ge crystal. For low background applications, this option eliminates any possible preamplifier or high voltage filter components that may add to the background inside a shield.

Low-Background Carbon Fiber Endcap Options  (-RB, -LB-C, and -XLB-C)
The Low-Background Carbon Fiber Endcap is as strong as Al, Mg, and Cu, creates less background, does not corrode, and can pass photons with energies less than 10 keV. This lower background material allows for lower Minimum Detectable Activity (MDA) for a specific counting time, which provides another step in increasing sample throughput in low-background counting applications. The lower Z of Carbon Fiber provides a low-energy window without the additional background found in most alloys.

See the GMX N-type HPGe Coaxial Radiation Detector Configuration Guide to configure your detector.