Anti-Compton spectrometer

Detail tests on nuclear models require not only a knowledge of energy, spin and parity of many levels, but also the determination of transition multipolarities and branching ratios. Precise intensities are thus needed. The well shielded anti-Compton spectrometer offers a rather simple solution especially for accurate angular distribution measurements. When the spectra are very complex, like in the case of final doubly odd nuclei, intensities cannot be determined without use of high resolution instruments. The curved crystal spectrometer provides a powerful solution at, unfortunately, non negligible cost. 

An application of anticoincidence circuits is the anti-Compton spectrometer. The Compton continuum in y spectra can be reduced relative to the photopeaks by placing the Ge detector inside a second detector, usually a scintillation detector, connected in anticoincidence, so that only pulses in the central detector that are not coincident with those in the outer detector are recorded. Anti-Compton spectrometers are very useful for measurement of y rays of very high energy. 

By coincidence or anticoincidence methods the background of a detector may by decreased by a factor of about 100 or more. The application of an anticoincidence circuit is the same as in an anti-Compton spectrometer. 

In some cases, it may be necessary to position shielding or other detectors behind the detector itself (for example, in an anti-Compton spectrometer system - see Chapter 13). For this purpose, detector systems are available with the preamplifier mounted close to the dewar leaving a clear length of cold finger between it and the detector. 

Mortier et al. (38) obtained precisions ranging from 3.5 to 10.7 % for concentrations between 1.02 and 0.16 g/g. The detection limit is 0.033 Mg/g, but can be lowered by carrying out the measurements with an anti-compton spectrometer. In this way an upper limit of 0.015 /ig/g was obtained for undoped zirconium. 

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. 

Cooper has evaluated the use of various types of Compton-suppression spectrometer for instrumental radiochemical analysis. It appears that the simple anti-coincidence shielded spectrometer with the sample located inside... 

---