Compton scattering shielding

To reduce interference from Compton scattering, an anticoincidence shield, 76 cm. X 76 cm., was constructed as shown in Figures 2 and 3. The shield consists of two independent type NE-102 plastic phosphor annuli. A 10-cm. bore through the top annulus accommodates the Ge(Li) detector chamber and the cryogenic assembly. The bottom annulus (i.d. diameter 25 cm.) houses a 20-cm. diameter by 15-cm. thick Nal(Tl) scintillator. Normally, the plastic phosphor is used in conjunction with the Nal(Tl) to form a well -shaped anticoincidence shield. Altema-... 

Equation (11.3) is valid for a narrow-beam geometry from a point source, and scattered radiations in the absorbing material are excluded. That is, each photon is either completely absorbed or transmitted. In reality, Compton scattering of 511 keV photons occurs within the shielding material, and some... 

The probability of fhese interactions depends not only on the energy of the gariuna ray but also on the atomic number, Z, of the material. Figure 25.6 plots the relative probability of Compton interactions, a, with photoelectric effect, t, and pair production, k. Higher Z materials, such as lead are more likely to have photoelectric effects than Compton scattering. From the standpoint of shielding gamma rays, photoelectric effect is preferable as discussed later. 

It is likely that you wiU need some sort of shelf or jig within the shield to hold the sample. This must be constructed of low Z materials to minimize fluorescence and should be as light as possible, consistent with secure holding of samples, to minimize bremsstrahlung and Compton scattering. Rigid plastics are fine. Aluminium, for the reasons stated above, should be avoided. 

Compton scattering within the detector shield - mainly from the sample itself. It also ignores the effect of poorer resolution of larger detectors. Under the conditions implied, bigger does appear to be better. However, the deductions made above would be completely different for detectors with much better, or worse, resolution than is typical. The deductions are also different if the continuum beneath the peaks is due primarily to sources of radiation external to the shielding. 

The broad continuum is due to Compton scattered y-rays of higher energy but poses no difficulty in the analysis except to decrease the signal to noise ratio. The Compton plateau can be reduced considerably with an anti-coincidence shield (Cooper and Rancitelli, 1972), but the use of this device does not appear to be widespread. The sensitivities in table 2 for a group separation can only serve as a guide, therefore, because the required count rate will depend on the extent of the Compton plateau due to other elements. 

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