GB1041303A - Improvements in or relating to dosimetry - Google Patents

Abstract

1,041,303. Photo-electric dosage measurement. CONTROLS FOR RADIATION Inc. Feb. 14, 1964 [Feb. 18, 1963; Aug.29, 1963; Dec. 13, 1963], No.6345/64. Heading G1A. The radiation dosage that has been received by a thermoluminescent phosphor is measured by heating the phosphor and detecting and measuring the resultant light emitted. The phosphor may be lithium fluoride which has been exposed to an X- ray dosage of a few milli-roentgen. The phosphor crystals 88 are carried by a stainless steel tray 60 Fig. 5 (not shown) supported between two sets of contacts 26, 28 of beryllium copper, which grip the edges 62 of the tray 60. Oxygen-free gas, e.g. nitrogen, from a source 80 is passed over the phosphor specimen which is heated to 210‹ C. by passing a current through the resistive tray 60, the light emitted from the specimen being received by a photo-multiplier mounted above the specimen. The heating chamber has a recess in which slides a member 40 carrying the specimen tray. Felt blocks act as light seals against the sides of member 40, but do not restrict the flow of gas from the chamber, thus enabling all the oxygen to be flushed out. The measuring circuit Fig. 8, includes a timing motor 124 which controls the actuation Fig. 9 (not shown) of timer contacts in the circuit. When the specimen tray 60 has been slid in to position and the chamber filled with pure nitrogen, a button 94 is pushed to initiate heating, at which instant timer contacts 142 and 144 are opened and 114, 130, 132 and 136 are closed. The closing of contact 114 supplies power through switch 92 from a source 106 to the heating circuit. The closing of 130 completes a holding circuit for the motor 124, contact 132 connects the photo-multiplier 14 to the integrating circuit, while contact 136 removes any residual charge from capacitor 134. The light emitted by the specimen is sensed by the photo-multiplier, the signal from which is integrated by capacitor 134. The circuit is arranged to stop heating after a certain time and to disconnect the photo-multiplier (by the opening of switches 114 and 132 respectively). The charge stored by the capacitor 134 is then measured by an electrometer tube circuit, the output of which is applied to a digital voltmeter 112 and displayed on indicator 102. If the measured value is off the voltmeter scale, an extra capacitor may be brought into circuit by means of button 96 to reduce proportionally the charge on the capacitor 134 and to introduce a scaling factor of Î10 or x100, indication of these being provided by means of lamps 98, 100.