Patent GB-754572-A (GB)
Title: Apparatus for the examination of objects with the aid of x-rays or gamma rays
IPC: G01N23/18
Filed: 1954-04-26
Abstract: 754,572. Examination of objects of radiation beams. AKTIEBOLAGET BOFORS. April 26, 1954 [April 28, 1953], No. 12036/54. Class 40 (3). [Also in Group XX] In the examination of objects by X-rays or γ-rays the object under test is mounted in a holder which gives it axial and/or rotational movement and a beam of radiation is transmitted through the object, this beam being split before and after its traverse of the object into two beams, each of which is received by a radiation sensitive device the outputs of the two devices being compared. As shown, a shell 21 under examination is supported by four rollers 20 which are so supported that they may either lift the shell or rotate it or both. Lifting with the shell is a compensating plate 23 which equalizes the absorption of the shell over its length. A beam of X-rays or γ-rays from an X-ray tube or radio-active isotope 24 is shaped by a screen 25 and passes through the shell to a beam-splitting plate 26 where four separate beams of radiation are formed by slits 27, 28, 29, 30. The beams from the outer slits are received on fluorescent materials 37, 38 associated with photomultipliers 35, 36 while those from the inner pass through a further apertured plate 46 to photo-multipliers 49 and 50 respectively. Each pair of cells is provided with a light source 53, 39 from which light is conducted to the cells by light guides to assist in avoiding amplification changes in the cells. The output of each cell is compared with its associated cell so as to detect irregularities in the filling or casing of the shell. Circuits, (Fig. 2). The cathode of photomultiplier 35 is fed through variable resistor 66 and resistor 67 while that of multiplier 18 is fed through resistor 68. All the dynodes of tube 35 except the last are connected to a voltage divider while the anode is connected to a relatively positive source. The last dynode is connected to the anode of tube 36 while the potential of the last dynode of 36 has a potential approximately equal to that of the penultimate dynode of 35. Thus the currents at the last dynode of cell 35 and the anode of cell 36 are in opposite directions so that if there is no defect in the shell no potential is developed across resistor 75 in the earth return connection. Unbalance in the two multiplier outputs causes a voltage pulse to be developed across resistor 75. The pulse is fed through a condenser 76 to an amplifier 77. Resistor 78 condenser 76 and 79 act as a filter to eliminate both high-frequency static fluctuations of the tube output currents and low-frequency fluctuations resulting from change in photo-cell sensitivity. The amplifier is connected to one of the anodes of a double diode 78 and through a phase inverter triode 81 to the other. Positive pulses pass directly to the diode. Negative ones are inverted. The pulses then pass through contact 85 operated by the workpiece driving mechanism to operate an indicator 44. A blocking voltage (not shown) is applied to the diode to eliminate pulses of insufficient amplitude. The apparatus is compensated for sensitivity variations of the cells. This is done by chopping the voltage developed across condenser 69 by an A.C. vibrated contact 88, one contact of which is connected to one of the cathodes of a double diode 90, the other cathode being earthed. The double diode is necessary in case the voltage on the side of the condenser not earthed becomes positive. The anodes of the diodes are interconnected and earthed through a paralleled resistor and condenser 91, 92. The voltage developed at the anode is fed to a two-stage triode amplifier 93. The primary of a transformer is in the anode circuit of the second triode. The secondary of the transformer feeds a detector circuit comprising two series connected diodes 103 which provided a high blocking resistance and a high line constant circuit comprising condenser 108 and resistor 107. Thus the signal takes account only of slow changes in photo-tube amplifications and is used to control the conductivity of a triode 106 which by taking a higher current through resistor 67 changes the sensitivity of multiplier 35. Relay 86 may be operated by contacts, in the work driving mechanism to render the indicator operative only at selected periods.