The use of double-beam detection systems with pulsed sources of vacuum ultraviolet radiation

Pulsed sources of far ultraviolet radiation may show intensity variations of ±5% from flash to flash. The use of a double-beam optical system can reduce the effect of this variation on measured parameters of reactions produced by the ultraviolet radiation. This paper describes the principle of the double-beam method and considers a particular application to the measurement of reflectivity in the far ultraviolet where there are problems associated with collimation and the creation of the second beam. The alternative approaches of integrating and counting methods of detection are briefly described, and the adoption of the integrating method is justified for the case when there is a high photon flux in a short time interval: equations are derived for the signal produced by the photomultiplier dark current in this situation. Block diagrams and circuits are given for a detection system which can record on paper tape the relative magnitudes of the simultaneous charge pulses from the photomultipliers used to detect the two beams of radiation. The experimental results from single and double-beam systems are compared.