Absolute calibration of photodiodes with a monochromatic beam measured with an electrical-substitution radiometer

Abstract

International audienceTraditional efficiency calibration of spectrometers is based on the measurement of sources of radionuclides calibrated in activity and whose emission probabilities are known beforehand. However, there are regions of the spectrum where there is a lack of reliable tabulated emission probabilities. Furthermore, if one aims to improve the measurement of emission probabilities, this approach implies the use of the same parameters that one intends to measure. The solution to these limitations is the use of photon fluxes whose intensity has been measured in an absolute way by another technique. Photodiodes constitute a very suitable choice for measuring the intensity of these fluxes, but they need to be previously calibrated in terms of amount of current generated per unit power. In this work, we present the calibration of several photodiodes with a monochromatic photon beam whose intensity is previously measured in an absolute way by a cryogenic electrical-substitution radiometer.Cryogenic detectors are based on the measurement of the temperature rise experienced by an absorber when the radiation interacts with it. In electrical-substitution radiometers, the amount of incident energy is determined by finding the electrical power that must be dissipated in the material to get the same temperature rise obtained during the photonic heating. In order to monitor temperature, a thermistor is polarized by a constant current, so the diminution of resistance induced by the temperature rise implies a decrease of its potential.In this work, the intensity of monochromatic beams is measured by means of the radiometer BOLUX (BOLometer for Use in the field of X-rays). This measurement is corrected by transmission, escape and scattering in BOLUX's absorber. Afterwards the current generated in the photodiode is measured when the same beam impinges on it, in order to obtain its efficiency at that photon energy. Measurements were performed at two different beamlines (Métrologie and PUMA) at synchrotron SOLEIL, covering an energy range from 3 keV to 60 keV. Absolute calibration of photodiodes also plays an important role in all those applications that rely on well-characterized photon fluxes, such as reference-free X-ray spectrometry