An unknown B emitter was counter in a simple Geiger set up using aluminium absorbers to determine the range of the beta particle and to attempt to identify the isotope. The thickness of aluminium needed to reduce the counts to background was 0.109 mm, the distance between the source and the end-window of the Geiger tube was 1.3cm and the end window density thickness was 1.7 mg/cm (the density of air is 1.293 mg/cm3 at STP and that of aluminium is 2.7 g/cm3). A repeat of the experiment after 3 weeks showed that the initial activity of the source has decreased by around 15%. With this information identify the unknown source.

An unknown B emitter was counter in a simple Geiger set up using aluminium absorbers to determine the range of the beta particle and to attempt to identify the isotope. The thickness of aluminium needed to reduce the counts to background was 0.109 mm, the distance between the source and the end-window of the Geiger tube was 1.3cm and the end window density thickness was 1.7 mg/cm (the density of air is 1.293 mg/cm3 at STP and that of aluminium is 2.7 g/cm3). A repeat of the experiment after 3 weeks showed that the initial activity of the source has decreased by around 15%. With this information identify the unknown source.