FR1452636A - Method for stabilizing the amplification of pulse spectrometers - Google Patents

Abstract

1,114,951. Pulse spectrometry. GESELLSCHAFT ZUR FORDERUNG DER FORSCHUNG AN DER EIDGENOSSISCHEN TECHNISCHEN HOCHSCHULE. June 4, 1965 [June 4, 1964], No. 23950/65. Heading G1A. The gain of a pulse spectrometer using a scintillation crystal and photo-multiplier is stabilized by controlling the gain retro-actively in dependence upon a pulse-count made of radiation from a constant intensity reference source superimposed upon the radiation being measured. To isolate the reference pulse-count from the total number of pulses counted for the reference radiation and radiation from the source being measured, the reference radiation is only superimposed periodically for small, so called first, time intervals and from the total count made during each first interval is subtracted a count made but a brief equal length (second) interval afterwards with the reference radiation absent. A succession of such subtractions yields the pulse-count of the reference source which has a time-wise mean value indicative of the variation in gain. In one arrangement the reference pulse-count is compared with a standard or gain pulse-count which is produced by a standard oscillator whose output is superimposed upon the measured radiation pulses from the multiplier during the second time intervals. The subtraction of the pulse-counts in corresponding first and second time intervals produces a series of pulse numbers which are the difference between the standard and reference pulse-counts and are indicative of the gain. The radiation of the source being measured is assumed not to vary in the short time between corresponding first and second intervals and the measured-source pulse-counts are therefore eliminated by subtraction. In another arrangement the standard pulses are obtained from the reference pulse source and are arranged to be independent of the gain of the multiplier by having a threshold limit after the multiplier, which, for the second intervals is of such a height that the dependency on gain of the reference pulses exceeding the limit is a minimum and negligible. By altering the limit the reference pulses are again made gain dependent. In a third arrangement which is suitable for multi-channel analysers the source of radiation being measured is assumed to vary over the small periods between first and second intervals. Four time intervals are used in which reference radiation is superimposed only in the first and fourth intervals. The pulses for each interval are received by two circuits each of which counts pulses in one of two energy bands placed one on either side of a line in the frequency-energy spectrum of the reference source. By suitably adding and subtracting the counts from the two circuits the radiation from the source being measured is eliminated and the difference in the number of reference pulses from the two energy bands is obtained. A change in gain will shift the spectrum and thereby change this difference value. The difference value is fed back via a digital-analogue converter to the cathode of the photo-multiplier. In all the embodiments a not-gate stops any pulses from reaching the main counter of the spectrometer during the intervals. The reference source chopper drive also operates an optical arrangement for producing electrical pulses corresponding to the timeintervals. In one arrangement Fig. 5 (not shown) a lamp housed inside a rotating diaphragm illuminates a photo-cell outside the diaphragm through appropriately spaced holes in the diaphragm wall.