Spectrometer Compton suppression

A number of special techniques have evolved to increase the detection sensitivity in y-ray counting. One of the most important is the suppression of the Compton scattering events in the y-ray spectrum by the use of anticoincidence annulus around the central y-ray detector. The idea behind a Compton suppression spectrometer is that most events in which the incident photon undergoes one or more Compton scattering events in the central detector will result in partial energy deposition in the detector with a low-energy photon escaping the detector. 

Fig. 3. Portion of the 108Pd(a,2nY) 10Cd gamma-ray spectrum observed a) with a single diode and b) with the Compton suppression spectrometer. Some transitions are identified by their energy in keV.

C19. Cooper, J. A., Evaluation of lithium-drifted germanium Compton-suppression spectrometers for non-destructive radiochemical analysis. J. Radioanal. Chem. 6, 177-184 (1970). 

The Compton continuum, present in gamma energy spectra recorded either by a Nal(Tl) scintillator or by a Ge detector, is a nuisance that impedes the analysis of complex spectra. It is therefore desirable to eliminate or at least reduce that part of the spectrum relative to the gamma energy peak. One way to achieve this is to use two detectors and operate them in anticoincidence. Such an arrangement, known as the Compton-suppression spectrometer, is shown in Fig. 12.9. A large NaI(Tl) scintillator surrounds a Ge detector, and the two detectors are operated in anticoincidence. The energy spectrum of the central... 

Figure 12.9 Diagram of a Compton suppression spectrometer using a Nal(Tl) and a Ge detector. The two detectors are operated in anticoincidence, with the Ge recording the energy spectrum.

The detection limits can be further improved by lowering the background activity using a Compton suppression spectrometer (CSS). A fairly simple CSS would consist of a high resolution Ge detector surrounded by a Na guard detector and an array of electronic modules. The peak-to-Compton ratio can be improved to 550 to 650. The cost of a CSS can vary between US 75 000-100 000. 

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... 

Ge detectors each provided with a cylindrical BGO + Nal Compton suppression shield. The conversion electrons were measured with two spectrometers each consisting of a mini orange filter and a Si(Li) detector. The transmission of the filter was optimized for detection of electrons with energies in the range 0.9- 1.3 MeV. For 196Pb the intensity of the E0 0t transition+in +... 

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. 

When it reaches its full capability, TASCC will accelerate all ions between lithium and uranium to energies up to 50 MeV/u and 10 MeV/u, respectively, It will feed some major pieces of apparatus the Q3D magnetic spectrometer, the isotope separator, a growing array of gas and solid-state detectors housed in a 1.5 m diameter scattering chamber, and the 8ir" Y-ray spectrometer [AND 84], All are currently operational except the 8ir spectrometer, which is being built by a consortium of Canadian universities and AECL Chalk River, with completion scheduled for late 1986. It will comprise two subsystems i) a spin spectrometer of 72 bismuth germanate (BGO) detectors, and ii) an array of 20 Compton-suppressed hyperpure (HP) Ge detectors. 

A guided beam of cold neutrons will provide the highest intensity neutron beam with the lowest background at the sample position. Therefore, the use of a neutron guide is desirable. In addition, a pair spectrometer or a Compton suppression system is a desirable option, although this need is decreasing due to better detectors, MCAs and analysis software. 

A pair spectrometer or Compton suppression system, about US 150 000. 

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. 

Orphan et al. ° have reported the use of a Ge(Li) detector with split NaI(Tl) annular shield to provide y-spectrometry with Compton suppression and pair spectrometry facilities. The pair spectrometer mode of operation is of most value in the energy range 3—7 MeV where the double escape detection efficiency is comparable to the full energy peak efficiency. 

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