Patent GB-1185071-A (GB)
Title: Improvements in or relating to Liquid Scintillation Spectrometry
IPC: G01T7/08
Filed: 1967-04-06
Abstract: 1,185,071. Source array, sample support. PACKARD INSTRUMENT CO. Inc. 6 April, 1967 [11 April, 1966], No. 15902/67. Heading H5R. [Also in Division G1] External standardization of asample in liquid scintillation spectrometry (i.e. compensation for the reduction in counting efficiency due to quenching by the sample, using a standard radio-active source external of the sample vial) is carried out with two external standard sources adjacent the sample vial, one emitting radiation of both relatively high and relatively low energies, and the other, located at a different height relative to the sample as compared with the first source, emitting mainly relatively low energy radiation. Such an arrangement of sources permits the drawing and use of calibration curves, for determining counting efficiency, which are virtually independent of the volume of sample in the vial, over an appreciable practical range, e.g. of the order of 15 ml. Prior art arrangement.-The system described is a modified form of that described in Specification 1,100,931 in which a single external standard source is used and which is illustrated in Figs. 1, 2 and 3 (not shown). Using this prior art system, before measurements in an unknown sample of a particular #-emitting isotope can be made, calibration curves for that isotope must be drawn defining the variation in counting efficiency as the amount of quenching material in the sample changes (the counting efficiency being given by the ratio of the recorded count in the sample pulse channel I to the known activity of the sample and the amount of quenching by the ratio of counts obtained in the two channels II, III used to measure the scintillations due to the external standard γ-source). Curves are obtained for different sample volumes, since the counting efficiency varies with the volume at any parricular value of external standard channels ratio due to the fact that high energy emissions from the standard source are more effective in producing scintillations than low energy emissions, as the sample volume increases. Volume independence by present arrangement.- By locating two external standard sources of the above-mentioned different energy characteristics, adjacent the sample vial, the volume dependence of the counting efficiency is reduced. Thus, a standard source 48 of, e.g. Ra 226 or Cobalt emitting low and high energy γ-rays, is located adjacent the bottom of the sample vial 24 during standardization and a low energy γ- source, e.g. of Am 241 or Ba 133, near the top, as in Fig. 11 and in Fig. 7 (not shown). These sources may alternatively be on either side of the vial, Fig. 7a (not shown). The calibration curves obtained show sample volume independence for a range of higher volumes as illustrated in Fig. 8 (not shown). In a further arrangement, Fig. 9 (not shown) the low energy source 98 is at the midpoint of the vial and the volume independent range is for lower volumes, as indicated in Fig. 10 (not shown). To enable spacing of the standard sources at the required positions for use in apparatus similar to the prior art arrangement of Fig. 2, in which the standard sources are brought into and out of position adjacent the sample by a fluid pressure transport system, the sources may be of the pellet type and of dumbell shape, Fig. 13 (not shown). Spacing of the two sources can be by similarly shaped dummy pellets, Fig. 14 (not shown) or by a long spacer rod, Fig. 15 (not shown). Alternatively, the sources may be mounted in either end of an elongate rod, Figs. 16, 16A (not shown). The low energy emitting source 98 may be of any required length, e.g. longer than source 48, and, if necessary, in the case of the pellet type, may comprise more than one pellet. A fluid pressure transport system is described, with reference to Fig. 12 (not shown), and as associated with the prior art elevator system of Figs. 1, 2 and 3, for changing of sample vials. The control for this transport system utilizes multivibrators and time delay circuits for obtaining the right sequence of operations. Specification 1,041,413, which relates to an automatic sample vial presentation system suitable for use with the present system, is referred to.