Patent GB-1335401-A (GB)
Title: Method and device for scintillating spectrometry
IPC: G01T1/204
Filed: 1970-11-18
Abstract: 1335401 Liquid scintillation spectrometry INTERTECHNIQUE S A 18 Nov 1970 [21 Nov 1969 29 Oct 1970] 54860/70 Heading G1A In liquid scintillation spectrometry, the measurement of the radio-activity of a sample is corrected to compensate for the degree of quenching in the sample, the latter being determined by measuring the amplitude value r, Fig.1, of the centroid G of that portion, of the curve of net count rate against amplitude of the scintillations, within a predetermined amplitude range L 1  to L 2 . The measurement of r may be carried out with the aid of an external standard source. From the position of r within the window L 1  to L 2 , the counting efficiency E (which is dependent on the degree of quenching) may be calculated. Typical correction procedure may be manual or automatic adjustment of a system operating parameter in dependence on the calculated value of E, e.g. photo-multiplier high voltage, amplifier gain, pulse height analyzer windows. When automatic measurement and compensation for quenching are to be carried out two successive counts are required. Apparatus. A suitable system is as shown in Fig.2 which comprises a conventional liquid scintillation spectrometer having a multi-channel pulse weight analyzer 28-28", computer arithmetic unit 30, store 32, display 34 and input data means 36, 38. Added to this are the units 40-48 which provide for calculation of r and calibration of the system. They comprise a pulse height analyzer having a window equivalent to L 1  to L 2 , a gate 44 allowing only pulses in this window to pass via A/D converter 42 to the computer 30. Units 46, 48 are for controlling a display of the background and mean pulse height. The value of r is obtained by dividing the accumulated amplitude of pulses within window L 1  to L 2  by their total number within this window. In an alternative additional (calibration) circuit, Fig.8 (not shown) operating on an analog basis instead of a digital one as in Fig.2, the circuit includes a capacitor integrating pulses shaped previously into constant width pulses. Calibration and measurement procedure. In the case of a series of samples to be tested which all only contain one isotope e.g. carbon-14, the apparatus is first supplied with a series of reference samples of known different activities and degrees of quenching. The computer is also supplied with the necessary data to form from this and the counts taken on the reference samples a calibration curve, comprising a number of points, of counting efficiency E against r. When the test samples are measured, the value of r for each sample is calculated and correlated with the calibration curve according to the program of Fig.7 (not shown) to determine E for the sample, which value is used to correct the count. With samples to be counted which all contain a mixture of isotopes, e.g. Carbon 14 and tritium, three calibration curves (i.e. for <SP>14</SP>C in its own and in the tritium counting window and for tritium in its own window) of E against r are read into the computer store using reference samples. If the series of samples is a mixture of double-label and single label (low energy) samples, two calibration systems 40-48 are provided, the pulse height analyzer windows being set to correspond with the optimum pulse height ranges for the two isotopes.