Energy calibration of surface barrier detectors for fission fragments

Abstract

The pulseheight, X, delivered in surface barrier detectors by incoming fission fragments of given energy, E, and mass, M, has been studied. Fragments separated as to energy and mass from the 235U(n, f) reaction induced by thermal neutrons were available from the Lohengrin spectrometer of the ILL. The energies on Lohengrin were calibrated by a time-of-flight technique. It could be shown that for the mass and energy ranges of unslowed fragments encountered in nuclear fission there is a linear relationship between E and X for any fixed M, and similarly a linear relationship holds between E and M for any fixed X. The calibration equation, therefore, reads E = ( a + a′ M) X + b + b′ M with a through b′ being constants characterizing the detector. In parallel to the above measurements, the pulseheight spectra from a 252Cf(sf) source were taken. The pulseheights X = P L and X = P H for the light and heavy fragment peaks of this spectrum were determined by a fitting procedure with Gaussians. Several detectors having been tested, it appears that at least for detectors of the same brand, a universal calibration scheme can be set up which correlates the detector constants a through b′ to the Cf spectrum parameters P L and P H. The accuracy of this calibration is sufficient for most practical purposes. The above calibration method has been put forward and in general use since many years [1]. However, the revised constants of the present investigation yield kinetic energies of fission fragments which are lower by about 1–2%. These energies are in excellent agreement with recent independent measurements of the kinetic energy release in fission. Along with the calibration the energy resolution of surface barrier detectors has been studied as a function of bias voltage on the detector and mass or energy of incoming ions.