For an experiment to study neutron radiative beta-decay, we operated large area avalanche photodiodes (APDs) at liquid nitrogen temperature to detect X-rays with energies between 0.2 keV and 15 keV. Whereas there are numerous reports of X-ray spectrometry using APDs at energies above 1 keV, operation at liquid nitrogen temperature allowed us to reach a nominal threshold of 0.1 keV. However, due to the short penetration depth of X-rays below 1 keV, the pulse height spectrum of the APD become complex. We studied the response using monochromatic X-ray beams and employed phenomenological fits of the pulse height spectrum to model the measurement of a continuum spectrum from a synchrotron. In addition the measured pulse height spectrum was modelled using a profile for the variation in efficiency of collection of photoelectrons with depth into the APD. The best results are obtained with the collection efficiency model.
Citation: Review of Scientific Instruments
Pub Type: Journals
avalanche photodiodes, beta-decay, X-ray