A gauge for the measurement of the density variation in a shock wave is described. It is based on the absorption of low energy beta radiation from an169Er source. The gauge was used to study the density variation through the contact zone between the expansion chamber gas and the driver gas of a valve driven shock tube. Comparisons are made between the measured density and that calculated from pressure measurements in terms of the initial condition of the test and the driver gases. There are significant differences between these densities due to mixing of the two gases in the contact zone.