When a silver foil (Z=47) was used in an $\mathrm{\alpha }$ -ray scattering experiment, the number of $\mathrm{\alpha }$ -particles scattered at ${30}^0$ was found to be 200 per minute. If the silver foil is replaced by an Aluminium (z=13) foil of same thickness, the number of $\mathrm{\alpha }$ -particles scattered per minute at ${30}^0$ an is nearly equal to