Beta particle penetrating ability. At left is a disc of strontium 90 (green), a radioactive isotope of the element, and at right is a Geiger counter. Between the two is a piece of aluminium foil. The strontium 90 disc is emitting beta particles, which are electrons emitted by unstable atomic nuclei. Not all of the beta particles are able to penetrate the aluminium and reach the detector in the Geiger counter. To see this apparatus without the aluminium shield see clip K004/4567.