Self Absorption and Backscattering of P- Particles

When a P emitter is counted as a solid on a planchet, some of the p particles hit the planchet and bounce back. The back-scattering increases as the atomic number of the planchet material increases. The solid P emitter will also reflect some of the P particles. This self-scattering depends on the presence of heavy atoms in the sample and on the shape and size of the sample crystals. When the layer of the P emitter is relatively thick, some of the P particles are absorbed by the sample. When the sample is very thin, the self-absorption is negligible and it is said to be infinitely thin. When the thickness of the sample is such that the P particles from the bottom cannot reach the top, the sample is described as infinitely thick. Self-absorption depends on the weight of the sample. The count rate can be kept fairly constant when the sample is infinitely thin or infinitely thick. 

In this experiment the weak P emitter is used to demonstrate the phenomena of back-scattering and self-absorption. An infinitely thin layer of Na2 S04 is deposited on two aluminium planchets and on a steel planchet. A constant activity is used for die three Na2 S04 samples. Comparing the count rates of the two aluminium planchets shows the reproducibility. Comparing the count rates of the aluminium and of the steel planchet demonstrates the effect of atomic number on the back scattering. Two Ba S04 precipitates, of different thickness are counted. Comparing their count rates demonstrates the phenomenon of self-absorption. 

I Wash two aluminium planchets and a steel planchet with alcohol and dry by vigorous wiping with tissue paper. The planchets should be veiy clean to ensure that the solution to be evaporated is evenly spread on them. This is essential to produce infinitely thin deposits of uniform size and thickness. 

The solid planchets used in the first part of the experiment are discarded in a special beaker. The solid Ba S04 precipitates and the filter paper are discarded in a second labelled container. Soak contaminated glassware in Decon 90 overnight after rinsing 

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