The back-scattering of beta radiation

Principle

Because of their charge, beta particles are deflected from their direction when they interact with the electrical fields of the shell or the nucleus of the atoms of the irradiated substance. Back-scattering occurs, when the angle of deflection is greater than 90°. The back-scattering rate is predominately dependent on the nuclear-charge number C of the back-scattering material. With an atom of high nuclear-charge number, the scattering occurs at a large angle and with little loss of energy. The back-scattering factor R is approximately proportional to the square root of the nuclear-charge number C. The mass per unit area f (see the experiment "Shielding from beta-radiation") or the thickness x of the irradiated material only influence the back-scattering factor up to a saturation value. The maximum back-scattering is practically attained at a mass per unit area which is smaller than half the radius of the beta particles in the material., because larger layer thicknesses lead to absorption of the scattered electrons. The saturation value is below 200 mg/cm2 for all materials used. This corresponds to a saturation layer thickness of x = f /? ? 0.74 for aluminium and ? 0.17 mm for lead. The layer thicknesses of all the metals provided are above the saturation value, so that their influence on the back-scattering cannot be  demonstrated.

Task

To examine from which materials beta radiation is best back-scattered.

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