Measurerment og Gamma Rays
The instrumentation used to measure gamma rays from radioactive samples generally consists of a semiconductor detector, associated electronics, and a computer-based, multi-channel analyzer (MCA/computer). Most NAA labs operate one or more hyperpure or intrinsic germanium (HpGe) detectors, which operate at liquid nitrogen temperatures (77 degrees K) by mounting the germanium crystal in a vacuum cryostat, thermally connected to a copper rod or "cold finger". Although HpGe detectors come in many different designs and sizes, the most common type of detector is the coaxial detector, which in NAA is useful for measurement of gamma-rays with energies over the range from about 50 keV to 3.0 MeV.
The two most important performance characteristics requiring consideration when purchasing a new HpGe detector are energy resolution and detection efficiency. Other characteristics to consider are peak shape, peak-to-Compton ratio, crystal dimensions or shape, and price.
The detector's energy resolution is a measure of its ability to separate closely spaced peaks in a gamma spectrum. In general, detector resolution is specified in terms of the full width at half maximum (FWHM) of the 122-keV photopeak of 57Co and the 1332-keV photopeak of 60Co. For most NAA applications, a detector with 1.0-keV resolution or below at 122 keV and 1.8 keV or below at 1332 keV is sufficient.
Detector efficiency depends on the energy of the measured radiation, the solid angle between sample and detector crystal, and the active volume of the crystal. A larger volume detector will have a higher efficiency. In general, in order to express the ability of the detector to register a gamma-ray hitting it, it is a convention to compare its efficiency with that of a 3-inch by 3-inch NaI(Tl)- detector using a 60Co source (1332-keV gamma ray) at a distance of 25 cm from the crystal face. A general rule of thumb for germanium detectors is 1 percent efficiency per each 5 cc of active volume. As detector volume increases, the detector resolution gradually decreases. For most NAA applications, an HpGe detector of 15-30 percent efficiency is adequate.
The true absolute “intrinsic” counting efficiency has to be measured by using certified standard gamma sources with known energies and intensities where the disintegration rate is known within narrow error limits.
Typical gamma-ray spectra from an irradiated pottery specimen are shown in Figures 3-4 using two different irradiation and measurement procedures.