| 摘要 |
1,252,971. Scintillation counting. NUCLEAR-CHICAGO CORP. Feb. 5, 1969 [Feb. 8, 1968], No.6146/69. Heading G1A. In an automatic liquid scintillation counting system including means for inserting into a chamber 16 Fig.1 containing a photo-multiplier, a sample 10 of a source in a scintillating liquid from a succession of samples of varying quench and for removing the same, and means for producing a measuring count and a conversion efficiency correction count which is subtracted from the measuring count to compensate for quench variation, means are provided for interrupting the testing of samples and introducing a standard source of light scintillation into the chamber at intervals shorter than the time required for substantial gain drift, for detecting shifts from a norm, of the amplitude spectrum of the photomultiplier resulting from the insertion of the standard source, and for altering the tranducer gain to return the amplitude spectrum to its norm and causing resumption of the measurement of samples. The conversion efficiency count is provided by a rack and pinion 32 moving a gamma source 26 to a position 30 beneath an inserted sample, a shutter 20 being provided to exclude ambient light. The gain adjustment is achieved by varying the high voltage supply 36 Fig. 4 to the photo-multipliers 18 in response to the departure of a pulse amplitude spectrum from its normal position when a standard vial or an alpha source and scintillating crystal are inserted in the chamber. The pulses from the photomultipliers 18 after summing may be directed by a switch 42 to a sample measuring and/or quench correcting circuit or to the automatic photomultiplier gain control circuit. In the last named, the spectrum amplitude shift is detected by discriminator 58, 60, 62, the difference count in the pulses passing between the level of discriminators 58-60 being compared with that of those passing between discriminators 60-62 and the difference itself being compared in 76 with a pre-set error count from a unit 78 the count of which is altered according to the elapsed standardisation time by a timer 80. The photomultiplier supply is adjusted by a digital to analogue converter 96 in accordance with the positions of bi-stable circuits FF1-FF4 which respond to the output of comparator 76. The correction effected may vary according to the time taken by the error count to reach the preset limit, or it may have one of two values according to the direction of the error relative to the error in the last correction. The limit count fed to comparator 76 may be continuously varied with time, or may be stepped and a correction signal is arranged to stop the count in the up-down counter 70 and to reset it for a further gain standardization check. Comparison in unit 76 may be analogue or digital. Testing of samples may be automatically resumed at the conclusion of a gain adjustment. The gain may be standardized automatically between successive samples or may be effected by manual switching as desired. The sample measurements and external standardization counts may be printed out or fed to a computer. |
