Patent GB-1226834-A (GB)
IPC: G01T1/204
Filed: 1968-07-03
Abstract: 1,226,834. Liquid scintillation spectrometry. BECKMAN INSTRUMENTS Inc. July 3, 1968 [July 3, 1967], No.31814/68. Heading G1A. In liquid scintillation spectrometry, the effects of quenching are compensated for automatically by variation of a detector circuit parameter in accordance with the degree of quenching encountered. The effects of quenching are especially troublesome in the counting of samples containing two different isotopes e.g. H<SP>3</SP> and C<SP>14</SP> having partially overlapping spectra, Fig. 1. In order to distinguish between the isotopes, counts are taken in three pulse height channels, determined by the levels set by discriminators in the detector circuit. However, if quenching occurs, a lowering of the energies of both the H<SP>3</SP> and C<SP>14</SP> spectra occurs with, in the worst case, the C<SP>14</SP> spectrum being shifted almost wholly into the H<SP>3</SP> counting channel in which the H<SP>3</SP> spectrum itself only occupies a small part Fig. 2 (not shown). By altering a parameter of the detector circuit e.g. the impedance of a device which follows the coincidence circuit, the high voltage supply to (affecting the gain of) the photo-multipliers, or the levels set in the discriminators, it can be arranged that the H<SP>3</SP> spectrum once more occupies the whole of its main counting channel, the C<SP>14</SP> spectrum being shifted to a certain extent back into the C<SP>14</SP> /H<SP>3</SP> counting channel. Counting can now be carried out on the sample, although with lower counting efficiencies than if there had been no quenching to be compensated for. The advantage of automatic quench compensation in evening out counting efficiencies over a range of externalstandard channels-ratio (which decreases as the degree of quenching increases is given by a graph, Fig. 4 (not shown). Similar graphs indicate the advantages of automatic quench compensation with respect to background and efficiency, Figs. 5-8 (not shown), and with respect to a dual-labels (H<SP>3</SP> and C<SP>14</SP>) count using two separated counting channels, Figs. 11-13 (not shown), The graphs also show that the H<SP>3</SP> spectrum does not shift as rapidly as the C<SP>14</SP> spectrum when quenching occurs, and reaches a minimum width before the C<SP>14</SP> spectrum does. Automatic compensations for quenching must take account of this, and so the parameter is varied in a non-linear fashion. The apparatus used is a conventional liquid scintillation spectrometer comprising a sample vial 21 and photo-multipliers 20 (having logarithmic responses) feeding coincidence circuitry 24 and discriminators 27, which set the pulse height levels for the counting channels 28. Further to this arrangement is provided a computer 29 receiving counts from the channels 28, in order to calculate the channels ratio when an external standard is moved into position adjacent the sample, thereby giving an indication of the amount of quenching in sample, Further, element 31 illustrates one example of a means for varying a detector circuit parameter - in this case, the gain, by the fact that 31 is a variable impedance device. Initial calibration for automatic quench compensation is carried out using two known samples, one the least quenched, and one the most highly quenched to be expected in the subsequent examination of unknown sample. With the former in position, the base gain of the system is set by adjusting potentiometer 26. An external standard is positioned near the sample and the channelsratio is measured and stored in memory 33. This value is then dialled into the reference side of converter 34 by use of device 35. The least quenched sample is replaced by the highly quenched sample and its external-standard channels ratio measured, and the value applied to converter 34 for comparison with the value in the reference side. Control 37 is adjusted until a slight overcompensation has been obtained. Measurement on unknown samples can now be carried out. For each sample, the computer obtains a measure of quenching by computing the external-standard channels-ratio for the sample and stores this in memory 33, so that on comparison of this with the reference value for the least quenched sample, an output from the converter is influenced by control 37 in such a way as to cause the required amount of gain to be introduced into the system by altering the impedance of device 31 to compensate for quenching when an actual count is subsequently taken. The variable impedance device may be a variable photo-resistor acted on by a lamp whose intensity is controlled by the summing circuit which comprises a differential transistor amplifier, Fig. 16 (not shown). The desired non- linearity in the characteristic is thus provided by the photo-electric link.