GB1152985A - Improvements in methods of Liquid Scintillation and in Devices for Carrying Out the said method - Google Patents

Abstract

1,152,985. Liquid scintillation spectrometry. PICKER CORP. June 16, 1966 [June 26, 1965], No.27000/66. Heading G1A. In liquid scintillation spectrometry, compensation for colour and chemical quenching is affected by automatically restoring the count rate for a sample to what it would have been in the absence of quenching, e.g. by adjustment of the high voltage supply to the photo-multiplier and of the gain of the detector output amplification circuit. The apparatus used for determining the radioactivity of a sample containing a (3-emitting isotope, e.g. C 14 or H 3 , is a coincidence type liquid scintillation spectrometer. In one embodiment of the invention, Fig.1, two photo-multipliers 12 are located either side of the sample position 10. Conventional apparatus is used to present samples in seriatim order and also to control movement of an auxiliary source 39 as used in the "external standard" evaluation of the degree of quenching. Pulses from the photo-multipliers are summed in circuit 14 and fed through logarithmic and linear amplifiers 16, 19. Coincidence of pulses in both photo-multipliers opens gate 20 to let pulses through to the counting channels, each of which has a pulse height analyzer 24, dictating the width of the channel "window", and a scale 26. From the counts on these channels, the pulse height spectrum of a particular sample can be built up. An extra channel comprising an analyzer 30 and a logarithmic rate meter 32 is used to determine the degree of quenching and compensate for it by feedback to adjust the high voltage supplied to the photo-multipliers. In the case when the auxiliary source used is a γ-source, the first step involves obtaining a "normal" counting rate (i.e. with no quenching) using the y-source to produce scintillations from a standard scintillator solution. The "window" of channel 30, 32 is set above the spectrum of the most energetic isotope to be tested so that when the sample is irradiated by the auxiliary source, only scintillations due to the auxiliary source are counted. The "normal" count rate obtained from 32 may be recorded but is essentially used for manual adjustment of the reference voltage 34 so that the latter balances the output of rate meter 32 thereby causing initial setting of the high voltage fed to the photo-multiplier. A series of individual samples can then be examined as follows: a sample is irradiated by the γ-source and the induced count rate fed to comparator 33, where it is compared with the manual setting of reference voltage 34, the output of the comparator being a difference voltage which causes the high voltage value to be increased as required to compensate for quenching; the servo-loop is then disconnected and the ysource returned to a shielded position; the activity of the sample is then provided by the counts on the analyzer channels 24, 26. In a second embodiment, Fig.3 (not shown), the compensation channel 30, 32 of Fig. 1 is replaced by a circuit having two analyzer-logarithmic ratemeter channels supplying a differential amplifier, which feeds to the comparator. The channels are similar to those used in the "channels ratio" method of determining quenching (as indicated in Fig.2, not shown). In a third embodiment, Fig.4 (not shown), a second order compensation is introduced. Extra circuitry provides for adjustment of the gain of the detector output amplification circuit. A warning circuit indicates when automatic compensation cannot be achieved by the circuit. This occurs for example when there is severe quenching with a weak #-emitting sample e.g. tritium, Fig. 5 (not shown). Facility for rejection of such a sample may be provided. Other auxiliary sources besides y-sources, may be used e.g. α-emitters, #<SP>-</SP> and #<SP>+</SP> emitters, and neutron emitters (used with scintillator containing neutron absorption element). In the case of α-emitters, these may be immersed in the sample solution or located near the surface. Also, since there is no significant overlap in the pulse height spectra of the α-emitters and sample isotopes to be tested, the α-source need not be moved during counting of the sample. Examples of the use of the apparatus are described together with details of the scintillators and auxiliary sources, and quenching agents used.