Range and shielding of alpha radiation

Principle

Because of their high specific ionization capacity in air, alpha particles only have a range of a few centimetres. This is dependant upon their energy E, and can be calculated from the following empirical relationship:

R = 0.476 E1.5 (cm/MeV) (valid for air at standard pressure and 15°C.) In nuclear transformations, all alpha particles are given the same amount of energy, so their range is uniform and sharply limited, provided their energy is not affected by covering or material layers above them. The source of radiation used in this experiment emits alpha radiation of differing energies and also beta radiation, to which the Geiger-Müller counter reacts in the same way. A differentiation between them is therefore not possible with this counter. The method proposed here, in which the relative influence of a sheet of paper on the count rates is utilized to identify the type of radiation, has the following problems: 

1. The decrease in the count rates at increased distances cannot be clearly attributed to exceeding the range. Although the sheet of paper completely absorbs all alpha particles, it also weakens the intensity of the gamma radiation. The count rates are therefore also decreased by the sheet of paper  past the range of the alpha particles. 
2. The differences between the count rates with and without absorption is very small, particularly at longer distances, and so at lower count rates. Considerable variations can therefore result in the calculation of the percentage count rate reductions. This makes it necessary to lengthen the time of measurement when count rates are low.

Task

Alpha particles are completely absorbed by a sheet of paper, whereas beta radiation is only slightly weakened by it. The decrease in the radiation intensity which is caused by a sheet of paper therefore enables the presence of alpha radiation to be determined. The objectives are: 

1. To determine the percentage reduction in the count rates caused by a sheet of paper at distances of from 1 cm to 7 cm. 
2. To determine the range of alpha particles from a comparison of the radiation intensities with and without a weakening sheet of paper.

Literature for this article als follows

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