NUCLEAR INSTRUMENTS AND METHODS II 5 (1974) 47-55; © NORTH-HOLLAND PUBLISHING CO. A CALIBRATION PROCEDURE FOR THE RESPONSE OF SILICON SURFACE-BARRIER DETECTORS TO HEAVY IONS* S. B. KAUFMAN, E. P. STEINBERG, B. D WILK1NS, J. UNIK, A.J. GORSKI Chermstry Diviston, Argonne National Laboratory, Argonne, llhnozs 60439, U.S.A. and M.J. FLUSS Chemical Engineering Division, Argonne National Laboratory, Argonne, Ilhnow 60439, U.S.A. Received 16 July 1973 The response of slhcon surface-barrier and other semiconductor detectors to heavy ions is complicated by the presence of a pulse-height defect, such that heavy ions produce a smaller pulse height than lighter ions of the same kinetic energy Based on the results of measurements of this phenomenon w~th a variety of ions, a new calibration technique for such detectors is proposed. Unhke the widely used calibration technique proposed by Schmitt and co-workers, which assumes the pulse- height response to be linear in both mass and kinetic energy, the present procedure reproduces the observed non-linearities. It is based on separating the energy of the ion into two terms, one of which is strictly proportional to the pulse height and is, m fact, the energy which a hght ion, such as an alpha particle, must have to give the same pulse height. The second term is the energy defect of the ion, and is a function of its kinetic energy, mass and atomic number. Applications of the technique to experimental data are presented, including energy and time-of- flight mass measurements of energy-degraded fission fragments, and double-energy measurements of the fissioning systems 235U(n,f) and 2a~Cf(sf). 1. Introduction The use of semiconductor detectors for the measure- ment of the energy of fission fragments and other heavy ions is comphcated by the effects of the pulse-height defect (PHD). The energy-response characteristics for heavy ions are both non-linear and charge- and mass- dependent, thus requiring special calibration techmques for their energy determinations. The most widely used techmque has been that proposed by Schmltt et al. 1"2). It was based on measurements of the response of semiconductor detectors to ions of bromine and iodine, and was expressed by the following equation: E(x,M) = (a+a'M)x+b+b'M, (1) where x is the pulse height and M is the mass. The coefficients a, a', b and b' in this equation were experimentally determined for a group of surface- barrier detectors by measuring the pulse-height spectrum of a standard fissioning source, either 252Cf spontaneous fission or thermal neutron fission of /3SU. The pulse heights of the average light and heavy fission fragment groups, PL and PH respectwely, were then related to the four coefficients in eq. (1), so that a measurement of one of the standard fission spectra sufficed to determine these coefficients. Although eq. (1) has been used successfully in the * Work performed under the auspices of the U.S. Atomic Energy Commission. fission fragment range of masses and energies, its originators warned 2) that it may not be valid outside of this range. In two previous pubhcations 3'4) data were presented on the response of a number of silicon surface-barrier detectors to monoenergetic heavy ions, including ions as heavy as uranium. The emphasis of that work was on ions and energies outside &the range of validity of eq. (1), in order to learn how far it could be extended. In this paper we propose a new calibration technique which is applicable to a much wider range of masses and energies, in particular to lower energies and lighter masses. In addition, it is well suited for extrapolation to higher energies and heavier ions. The proposed technique includes a dependency on both mass and charge (atomic number) of the ion, which leads to different results than eq. (1) in the fission fragment region due to the neutron excess nature of the prompt fission fragments. Several experimental tests of the technique will be presented to illustrate how it differs from eq. (1). 2. The calibration technique As was suggested previously3) the energy of an ion may be separated into two terms, its apparent energy (Ea), which is the energy of a light ion, such as an alpha particle, yieldmg the same pulse height, and its energy defect (AE), which is the difference between its true energy (Et) and its apparent energy. The energy of an ion of mass M, atomic number Z, and pulse height x 47