Detecting High Energy Gamma Rays from Neutron Interactions: Neutron Damage and HPGe Detectors

High Purity Germanium (HPGe) Detectors for Neutron Activation Measurements
In cases where prompt- or delayed-neutron activation measurements are made which produce high energy elemental capture gamma-rays, n-type (GMX) HPGe is generally selected over scintillators because HPGe provides 30 to 40X better energy resolution. Although overall HPGe system pulse processing throughput can be a factor of 10 less than what is possible with fast scintillator technology, HPGe’s high Z contributes to increased primary photopeak efficiency for similar sized scintillator detector volumes and gives better peak to Compton ratios than can be obtained using scintillators.

The disparity between HPGe and Sodium Iodide (NaI) resolution is greatly enhanced as the size/length (volume) of the detector increases. The ultimate measured result is that HPGe has a much higher signal to noise ratio than does any existing scintillator technology, thus making it easier to detect high energy gamma events with low intrinsic detection efficiency that have few counts in the key photopeak.

Neutron Damage to HPGe Detectors

For measuring high energy (5-20 MeV) photons produced by neutron activation, selecting the largest volume n-type HPGe detector possible to improve detection efficiency is not the best choice when the detector is exposed to a moderate to high neutron flux for extended periods. The practical concern when using HPGe with isotopic or generator-produced neutrons is accumulated charge collection damage from neutron interactions. Neutron damage annealing provided by ORTEC’s Service Center can restore the charge collection properties of an n-type detector to an almost factory-fresh state. Note that Neutron damage annealing can be performed in the cryostat when the user has proper training and equipment.

In applications where an n-type (GMX) HPGe detector used for detecting high energy gammas from prompt neutron interactions, where the detector is exposed to neutrons, ORTEC recommends a coaxial detector no greater than about 50% efficient. This detector volume optimizes efficiency while minimizing neutron damage susceptibility. p-type (GEM) detectors are not recommended for use in a neutron flux since they are not easily restored by the thermal neutron damage annealing process.

As indicated by the data below, the greater the detector volume, the more susceptible the detector is to rapid neutron damage. Note that when HPGe detectors are operated in a neutron flux at elevated cryogenic temperatures, due to cooling problems, the rate of neutron damage may increase. Neutron resistance in a GMX detector is best when the detector is as cold as possible in the configuration required.

2 X 108 cm2 for GEM detectors up to 20% in efficiency
1 X 107 cm2 for GEM detectors up to 70% in efficiency
4 X 109cm2 for GMX detectors up to 30% in efficiency
1 X 109 cm2 for GMX detectors up to 70% in efficiency
1 X 109cm2 for GLP (planar) detectors

Measurable Changes to Detector Performance from Neutron Damage

Neutron damage in an HPGe detector causes a change in the charge collection properties within the crystal. Typically, this can be identified when resolution and/or peak shape ratios begin to degrade for the 1.33-MeV peak when no concurrent change in the system noise is seen.

Using a pulser to verify that system electronic noise remains constant, then collecting spectral data and comparing measured peak shape ratios at the1.33-MeV peak before and after neutron exposure will allow the user to determine if neutron damage to the crystal has occurred.

Because measurable neutron damage is incremental it is sometimes difficult to determine if significant neutron damage has occurred. The more neutrons that a detector is exposed to, the worse the damage may become. Generally, until a detector is “thermal cycled” or warmed to room temperature, followed by cooling, neutron damage resolution degradation may not be apparent. If kept continuously cold, a detector with neutron damage may continue to produce “good enough” data for an application. If desired, a thermal cycle will confirm whether neutron damage has occurred if the resolution and peak shape ratios worsen with no significant change in system noise after the thermal cycle. A reasonable rule of thumb ORTEC uses to assess whether a detector has neutron damage after a thermal cycle is to see if measured peak shape ratios at 1.33 MeV are 10 percent poorer than before neutron exposure.

Detecting High Energy Gamma Rays from Neutron Interactions: Neutron Damage and HPGe Detectors