Patent GB-1095162-A (GB)
Title: Apparatus for measuring the strength of radioactive sources
Inventor: BRADLEY JOHN ERNEST STOBART
IPC: G01T1/178
Filed: 1964-03-19
Abstract: 1,095,162. Measuring radio-activity. NATIONAL RESEARCH DEVELOPMENT CORPORATION. March 19, 1965 [March 19, 1964], No.11650/64. Heading G1A. In order to avoid the effects of background radiation, apparatus for measuring the strength (activity) of a radio-active source (particularly a relatively weak source in the presence of a large background) includes means for modulating the response of the detector system to the radiation flux from the source, without substantially modulating its response to the background, the measuring apparatus also including limiting means to reduce relatively large amplitude variations in the signal measured e.g. due to statistical fluctuations in source emission or in the background, during the last part of the measuring period. The specimen to be investigated is placed in a tubular holder (10), Fig. 1 (not shown) mounted on a circular platform (9) with a similar holder (11) containing a non-radio-active or standard specimen, between two scintillation counters (1, 2), the crystals of which face one another. The platform (9) is rotated by a motor (14) to move every 1¢ seconds between two positions at 180‹ to one another, micro-switches (16, 17) controlling the stopping of the platform at each position. The flux reaching a detector (1 or 2) is then modulated at a frequency of <SP>1</SP>/ 3  c/s. The presence of a shielding plate (12) between the specimen holders and collars (8) around the scintillator crystals ensures that there is no substantial variation in the intensity of background radiation reaching the detectors. These produce signals, representative of the flux intensity received, which are transmitted through respective channels comprising amplifiers 22, 25, pulse height analysers 23, 26 and monostable trigger circuits 24, 27, pulses from the latter being applied in opposite senses to an integrating circuit 28 which develops an output of sinusoidal waveform and frequency <SP>1</SP>/ 3 rd c/s, and having an amplitude representing the difference in the intensities of the two specimens. The measurement of this amplitude is carried out in two periods, controlled by a process timer 33. During the first period, lasting 10 seconds, to obtain an initial measurement, much of the circuit is inoperative e.g. amplitude limiters 35, 37 &c., and the gain of the main amplifier 35 is set to the higher of its two values. A voltage built up in the phase sensitive detector 36 at the end of the 10 seconds is representative of this measurement, if there is any detectable difference in the two intensities. The timer then operates to prepare the apparatus for the next measuring period, of 110 seconds, by reducing the gain of the amplifier 29 to a value ten times less than the higher value and actuating the amplitude limiters &c. Any voltage build-up in detector 36 is applied to a relay 33 which, now made operative by control from the timer, produces a unidirectional output which is fed to a control circuit 39 to control the setting of a variable attentuator 31, and to a switching circuit 30, whose indicator 42 is energized representing the existence of a difference signal, and which causes a signal to be applied from the master generator 19 to the attenuator 31. The control circuit signal causes rapid adjustment of the attenuator setting from its previously-set centre position such that a signal is fed to amplifier 29, which is of the same amplitude, but opposite in phase, to the integrating circuit output, resulting in a null output from amplifier 29. The attenuator setting is indicated at 32 and represents the initial measurement. During the following 110 seconds, any differences in the outputs of the integrating circuit 28 and the attenuator 31 are channelled back to adjust the attenuator setting. The measurement gradually increases in accuracy due to the averaging effect of the time constants of the amplifier 29 (60 seconds) and the detector 36 (200 seconds). Fluctuations, of large amplitude, but statistically insignificant, are reduced by the amplitude limitations of the circuit. The final measurement is given by the attenuator indicator 32, but if the setting moves during a further checking period of 15 seconds of measurement (movement being detected by sensing circuit 40) the whole measurement is repeated.