An improved X-ray phototimer.

Abstract

In the first x-ray phototimers, which were developed in this laboratory (University of Chicago) approximately six years ago, Morgan (1) employed a hot cathode thyratron (RCA 2051) as the trigger and connected its grid to the 9th dynode of the multiplier phototube (931). He was forced to abandon this arrangement, however, because of relatively poor internal insulation in the dynodes of the phototubes available at that time and to employ instead as the control electrode the better insulated anode. The output of this anode fed the control grid of a pentode (6J7) and thus an amplified voltage of the desired sign was supplied to the grid of a cold cathode trigger tube (1C21). Now that better insulated phototubes have become available (1P21 with mica-filled base), it is possible to revert to the 9th dynode as the control electrode. Recently, aided by the expert advice and assistance of Professor Robert Moon and Mr. Karl Norris of our Institute of Radiobiology and Biophysics, we have designed new phototimers to replace those long used at the University of Chicago (2). The new sets differ from the old ones in the following respects: 1. The 9th dynode of the multiplier phototube rather than the anode is used as the output electrode, thus eliminating the amplifier and polarity inverter (6J7). 2. A hot cathode thyratron (2D21) is substituted for the cold cathode tube (1C21). 3. A precisely stabilized two-stage inverse feed-back high-voltage supply replaces the old supply in which stabilization depended on an OA4G in the primary circuit of the high-voltage transformer. 4. A resistor has been introduced in series with the metering condenser to compensate for the inherent lag in the opening time of the x-ray contactor. 5. The kv. compensating circuit has been eliminated because clinical experience has shown that when we employ multiple stops and multiple potentiometers for each stop the necessary voltage range for each combination of stop and potentiometer is so small that kv. compensation is not required. 6. The impedance and the shielding of critical circuits within the chassis and particularly in the detector cable have been improved, with resulting increased immunity to humidity and to high frequency transients. Theory of Circuit Figure 1 is a simplified schematic diagram of the improved circuits. Section I is a filtered 105-volt d.c. source in which the negative pole is grounded. Section II is a filtered stabilized high-voltage d.c. source capable of precise regulation over a range of 300 volts (from 700 to 1,000 volts). Section III is the detector and trigger tube assembly. Section I: When contacts 1A close, point W of the circuit receives filtered d.c. that is positive to ground and has a voltage maintained at 105 volts by the regulator tube VR105. Section II: The positive side of the filtered high-voltage supply is at P and the negative side at N.