Compton continuum

The Ge(Li) detector is surrounded by a plastic phosphor shield, and the two are operated in anticoincidence, reducing the Compton continuum and increasing the sensitivity for detection of noncoincident photons. 

The use of anticoincidence shielding significantly reduces the Compton continuum and allows the detection of weak spectral lines usually masked by interfering Compton radiation. Further improvement resulted from separate recording of the coincidence and anticoincidence thus, radionuclides that normally decay with a coincidence scheme can be recorded without loss of efficiency. 

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. 

Figure 123 The measured pulse height spectrum for the source spectrum of Fig. 12.3. The statistical effects in the detector broaden both the peak and the Compton continuum part of the spectrum. The dashed line shows the spectrum that would have been recorded in the absence of the Compton continuum.

The magnitude of the Compton continuum is also affected by the size of the detector (Fig. 12.2). The larger the detector is, the greater the probability of a second Compton interaction. If the detector size could become infinite, the Compton continuum would disappear. 

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... 

Because gammas are detected by the NE 213 scintillator mainly through Compton interactions, the response function of the detector consists of the Compton continuum. The response function has been calculated and measured for several gamma energies. Figure 12.19 shows a comparison of calculated and measured response functions. 

Backscattering is minimal for alpha particles because they are mostly stopped in the backing material (NCRP 1985b). A typical factor is 1.03. Backscattered gamma rays add to the Compton continuum a broad peak at an energy of about 0.2 MeV. 

The spectral response of a detector is more complex than described in Section 2.4.4 because of the bulk of the detector. The observed Compton continuum consists of single plus multiple successive scattering interactions. When such multiple Compton scattering interactions are terminated by a photoelectric interaction, the pulse is added to the full-energy peak. Most of the counts in a full-energy peak for gamma rays above 100 keV are due to such multiple scattering plus a final photoelectric interaction. 

Germanium detectors are characterized by three parameters resolution, peak-to-Compton ratio, and efficiency. The resolution is typically given for the 1332-keV Co line and varies from 1.8 keV for the very best to 2.3 keV for the very large detectors. The peak-to-Compton ratio is measured as the ratio of the number of counts in the 1332-keV peak to the number of counts in a region of the Compton continuum. Values vary from 30 to 90 for the most expensive model. The efficiency is expressed as a relative efficiency compared with the 7.5x7.5-cm Nal(Tl) scintillation detector. Relative efficiencies of HP-Ge detectors vary from 10% up to 150%. The dead time of semiconductor detectors is low, so the count rate is limited largely by the electronic circuit. 

The smallest amount of an element that can be determined depends on the specific activity produced and the minimum activity measurable with sufficient precision. From Equation (10) the activity produced and measured per gram of an element can be calculated. However, the minimum activity that can be measured depends not only on the decay properties of the radionuclide and on the counting equipment, but also on the background of the detector or the Compton continuum on which the photopeak must be detected in gamma spectrometry. Thus the detection limit of an element for specific irradiation-counting conditions is not immutable since it deftends on the presence of other radionuclides. 

For measurement results giving a Poisson distribution. as in radioactive decay, the equation u.sed to calculate the standard deviation square root of the number of counts recorded when the number is large. The net signal 5 is obtained as the difference between the measured signal (5-i-J ) minus the background B (e.g.. Compton continuum in gamma spectrometry). Thus the statistically derived standard deviation is given as... 

Compton scattering - partial absorption giving rise to the Compton edge and Compton continuum. 

The larger the detector, the greater the probability of complete absorption of the gamma-ray and hence a larger full energy peak and lower Compton continuum (i.e. higher peak-to-Compton ratio). 

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