A method for measuring the nuclear transformation rate of 109Cd

109Cd + sup(109m)Ag sources are measured in a large 4π proportional counter containing argon-methane at atmospheric pressure. The dimensions of the counter were chosen so that the internal conversion electrons from the 88-keV transition are totally absorbed in the gas. The internal conversion electron pulses are counted above a discrimination level selected to exclude K X-ray and Auger electron pulses from electron capture decay. Good resolution in the pulse height spectrum was observed for sources made at the centre of large diameter gold-coated VYNS foils. The calculated nuclear transformation rate is not critically dependent on the value used for the total internal conversion coefficient. (author)