An all sodium iodide anticoincidence shielded multidimensional gamma-ray spectrometer for low-activity samples

Abstract

A multidimensional gamma-ray spectrometer detector system has been developed which employs two 23 cm dia. by 20 cm thick NaI(Tl) cyrstals as its principal detectors. Each detector is viewed through 7.6 cm thick pure NaI light pipes by photomultiplier tubes and is surrounded by 38 cm dia. by 28 cm thick NaI(Tl) anticoincidence shields. Each principal detector, along with its light pipe and anticoincidence annulus, is hermetically sealed in a single container. The detector system is coupled to a 4096 word computer memory spectrometer and uses the gamma-ray decay characteristics of each radionuclide for its identification and measurement. This is accomplished by viewing the sample with the two NaI(Tl) detectors operated in coincidence and canceling those events not totally absorbed in the principal detectors by the anticoincidence annulus. Coincidence counts are stored in the energy-energy plane of the memory according to their photon energies, while single gamma rays which interact with either crystal are stored in a normal manner on the X or Y axis. The background response of this detector system has been measured at ground level and at 110 m below ground level where the cosmic-ray contribution is minimized. The two principal sodium iodide crystals have resolutions of 8.7% and 9.2% fwhm, respectively, at the 137Cs 0.662 MeV photopeak. The absolute efficiency for counting the 0.662 MeV 137Cs gamma ray is about 60%, while the coincidence counting efficiency for the 60Co gamma ray is about 14%. This detector system provides the most sensitive means available for the direct measurement of many radionuclides in environmental or extraterrestrial samples. The background contribution has been resolved to show the fraction caused by natural radioactivity in the detector system, that from the surrounding shielding, and that from cosmic radiation. The backscatter, Compton response, and efficiency parameters of the detector system are discussed and compared with other detector systems of somewhat similar design. The application of this instrument to several environmental studies is discussed.