Patent GB-1262759-A (GB)
Title: Scintillation detection
IPC: G01T1/164
Filed: 1969-06-19
Abstract: 1,262,759. Detecting position of radiation sources. PICKER CORP. 19 June, 1969 [25 June, 1968], No. 31135/69. Heading H4D.  Specification describes an improvement in tomographic scintillation detectors of the type described in U.S. Specification 3,011,057. In said detector, the subject (e.g. a person) to be investigated, is injected with a radio-active substance and the distribution of the ensuing radio-activity is detected by means of a scintillation crystal producing correspondingly distributed light flashes, and an array of phototubes 10, Fig. 1, viewing the crystal and giving outputs indicative of the X and Y co-ordinates of the flashes and, by means of a scanning device, outputs indicative of the intensity Z of the flashes. These co-ordinate signals are fed to a cathode-ray tube to give spots on the screen thereof indicative of the positions of the light flashes and thus of said radio-active distribution. In said detectors however, the X and Y coordinated signals are dependent on the intensity Z of the flash whereby two consecutive flashes at the same positions on the crystal but of different brightness, will give two differently positioned spots on the C.R.T. screen. To overcome this fault the present Specification advocates the feeding of the X and Y signals and the Z intensity (sum) signal to respective storage and decay circuits 30, 32 and 34, whereby the signals start simultaneously to decay and are then stopped when the Z signal has decayed to a reference level (as determined by comparison circuit 36) representing a standard intensity, the levels of the X and Y signals so reached being independent of the intensity of the light flash and giving correctly positioned spots on the C.R.T.12. The Z signal decay circuit 34 and comparison circuit 36 are shown in detail in Fig. 2. The start of the Z signal 162, Fig. 3, at time t 1  triggers, in monostable circuits 26 negative going and positive going pulses 94 and 140, Fig. 2, ending at time t 3 , and in monostable circuit 28, a positive going pulse 130 ending at time t 5 . A Darlington long tailed pair circuit 42 compares the amplitude of the Z signal 162 with the voltage across capacitor 40, such that from time t 1  to t 2  the collector potential of conducting transistor 54 is low and switches on transistor 74. Transistor 76 is switched on by pulse 94 such that a charging current flows to capacitor 40 via resistor 88, transistors 76, 74, diodes 82, 70 and resistor 68. Capacitor 40 thus charges up to the peak of signal 162 and then holds this charge until time t 3 . At time t 3 , transistor 134, in comparator 36, previously held conducting by pulse 140, becomes non-conducting, this raises the base potential of transistor 94 whereby said transistor starts to conduct and slowly discharges capacitor 40. The potential V z  on capacitor 40 is fed to one input of a voltage comparison logic block 136, via buffer transistors 112, 114, the other input of the block being fed with the reference voltage V R  from potentiometer 144. When V z  drops to V R , block 136 reduces its output voltage and shuts off transistor 94, whereby the capacitor's potential stores at the reference level. The output of the block 136 is fed to the X and Y delay circuits (similar to Fig. 2) to cause a similar storage of the potential reached by their capacitors and the subsequent application of said stored potential to the C.R.T. via outputs such as 124. The capacitor 40 continues to store, until time t 5 , when transistor 96 is rendered conductive by means of the end of pulse 130, and provides a rapid discharge path to ground.