Backscattering of p Particles

Stick with sellotape a piece of Sr (pure p emitter) ( Sr) foil to a thick aluminium plate (AI) ( 5 mm thick) and hold it in position (with thick tape) to a thin aluminium plate which fits just under the end-window of a Geiger-MOller (G-M) counter inside its lead castle. The Sr foil will be separated from the counter by thick aluminium which absorbs its p particles. Both aluminium plates have a circular hole opposite the end window of the counter. (See Fig. 18.1) 

Place an aluminium absorber (say 21.0 mg cm ) (M) on the second shelf of the counter s lead castle and count for 200 seconds. Repeat this after replacing the aluminium absorber by thicker absorbers in turn (e.g. 55, 82, 240, 712 and 1335 mg 

Replace the aluminium absorber (1335 mg cm ) by a copper absorber of the same thickness on the second shelf and count for 200 seconds. Repeat using instead of copper, lead absorbers of the same thickness. Repeat using an iron absorber instead. 

Change the position of the Sr source by moving it to alter the angle of scatter and place the lead absorber used previously and count for 200 seconds. 

To determine the background stick the Sr source at the bottom of aluminium absorber (1335 mg cm ) and count for 600 seconds. Calculate the count rate and subtract it from all count rates determined. 

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For counting low-energy j3 radiation, the crystal is substituted by a scintillating liquid, and the sample is dissolved in the liquid (internal-source liquid scintillation counting). The method is also used to measure weak X-ray and y-ray emitters. Under these conditions, self-absorption of the radiation in the sample, absorption of the radiation in the air and the window of the detector, and backscattering of p particles are avoided. 

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