The Beta stopper is an ingenious idea from our friends at CLEAPSS. The Beta stopper is an aluminium cylinder made to fit over a standard 14mm diameter Radium source. The source is inserted in a hollow and secured in place with a nylon screw. The Aluminium is thick enough to block all the alpha and beta emissions, allowing only the gamma radiation through. The Beta stopper is a very inexpensive alternative to buying a new Co-60 source and gives great results when used for the gamma radiation inverse square law experiment.