The Shape of the Full-Energy Gamma Ray Peak in Germanium Detectors for Different Energies and Incident Geometries

Year
2002
Author(s)
Ronald M. Keyser - ORTEC
Abstract
The accurate analysis of gamma-ray spectra from germanium detectors requires that the peak shape of the full-energy gamma-ray peak be used in the determination of the peak area, especially in the difficult deconvolution regions needed in isotopic ratio programs such as MGA and PC/FRAM. Modeling programs such as MCNP also use the peak shape to predict the response of HPGe detectors. The resolution and peak shape are defined in the IEEE 325-1996 standard by the Full Width at Half Maximum (FWHM), Full Width at Tenth Maximum (FW.1M) and Full Width at Fiftieth Maximum (FW.02M) values. However, IEEE 325 only specifies the measurements at one geometry and two energies. Nearly all measurements are made in a different geometry and at other energies. Other investigators1 have shown that the peak FW.1M and FW.02M change with position of the incident gamma ray on the front of the detector. To quantify the resolution and peak shape as a function of energy and point of incidence, measurements have been made on several coaxial detectors of different sizes in various source-detector geometries. The full-energy peaks from 60 keV to 2.6 MeV were used. The sources used were an 241Am source, 60Co and a natural thorium oxide sample. The 241Am 59 keV gamma rays were collimated by a 2 cm thick, 1 mm diameter bore lead collimator. For one series, the 60Co 1332 keV gamma rays were uncollimated and taken at 25 cm for comparison with manufacturer’s specifications. The 60Co and 208Tl were collimated with a 10 cm thick, 2 mm diameter bore tungsten collimator. These collimated sources were used to collect spectra for the incident beam on the front and sides of the detectors. The detectors were placed in a low-background shield to reduce any contribution from external sources. None of the detectors tested was a low-background type. Data are presented to show that the peak shape changes with incident beam position and full peak energy.