Anti-coincidence-shielded Ge

The NAA method can be divided into NAA (Instrumental NAA) and RNAA (Radiochemical NAA). In the latter, the various neutron-induced products are separated chemically to minimize interferences. There are several comprehensive review papers on INAA published in the literature 1,2,3,4). Briefly, the basic parameters controlling sensitivity for a multi-element determination are neutron flux, irradiation time, delay interval prior to counting, half-life and gamma-ray energy of the induced activity, and eflBciency and resolution of the detector. Table I outlines the irradiation parameters used for each of the two sequential irradiations. The final count occurring 40-50 days after the second irradiation is performed on an anti-coincidence-shielded Ge(Li) system developed recently in our laboratory. 

Figure 1. Anti-coincidence-shielded Ge(Li) y-ray spectra of irradiated U.S.G.S. Standard Basalt BCR-1...

The high sensitivity of anti-coincidence shielded spectrometers is of considerable value for the measurement of the natural radioactivity of, for example, environmental samples. A detector system similar to that described above was used to measure Cs in marine sediments down to the pCi level. The spectrometer was based on a 60 cm Ge(Li) detector with a 40 cm diameter x 40 cm long plastic scintillator as anti-coincidence shield, and this was further shielded by 10 cm of lead. At such low levels of activity the presence of radioactive isotopes in the materials of the detector system is clearly discernible in the y-spectra, and care must be taken in the choice of constructional materials. 

The y-ray spectroscopic information was obtained using an array of five Ge detectors with pentagonal Nal anti-Compton shields located at 63° to the beam and three additional Ge detectors at 24°. Two-fold or higher coincident events from these detectors were used to trigger the 72 Nal detectors of the Spin Spectrometer (SS) at ORNL. [JAA83] An average Compton suppression factor of 3.5 for the Co spectrum was obtained. The Ge detectors were placed at 20.8 cm from the target. 

Currie et al. have described a 30 cm Ge(Li) anti-coincidence spectrometer shielded by a split annular NaI(Tl) detector. This design of shield also allows pair-escape peak suppression, but has a lower background and Compton suppression efficiency than the plastic scintillator systems because of the lower solid angle around the detector for which shielding is provided. 

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