GB893907A - Improvements in or relating to ionisation chamber circuits - Google Patents

Abstract

893,907. Radiation detecting systems. UNITED KINGDOM ATOMIC ENERGY AUTHORITY. April 5, 1960 [April 10, 1959], No. 12297/59. Class 40(3). The present invention seeks to avoid the problems inherent in the measurement of small direct currents derived from ionization chambers to which a steady D.C. polarizing potential is applied. According to the invention the polarizing potential for an ionization chamber is provided by an A.C. generator and the A.C. current passing is amplified and fed to measuring apparatus. In this, phase-sensitive means are used separating the in-phase or anti-phase and quadrature components. The former is used to give an indication of radiation flux and the latter to check the functioning of the system. The A.C. polarized chamber may be used in a bridge circuit. In one arrangement it is proposed to apply to a chamber an A.C. polarizing potential exceeding in amplitude the saturated condition for the chamber. The output in such a case is rich in harmonics and substantially independent of the peak amplitude of the applied potential. Its fundamental component may be taken as proportional to the radiation (such as neutron) flux. Another possible arrangement is to apply a very small alternating polarizing potential and this gives an almost pure sinusoidal output proportional in amplitude to the potential over a limited range. The fundamental component of the output is in this case approximately proportional to the square root of the radiation flux and there is a low capacitive current component. The resistance of electrode coatings of boron may considerably affect the output current when low potentials are applied. As the resistance of the coating is not stable it is preferred to reduce this to a low value by depositing a metal coating in a boron coating which is absent from a plurality of small areas of the electrode so that the deposited coating is in good electrical contact with the electrode. In a reactor trip circuit embodiment using a polarizing potential of one volt or less at a frequency of 60 c/sec. the chamber 1 is supplied with this potential from generator 2 of push-pull type through variable resistor R. This resistor is set to provide a null output at 3 when the inphase current component reaches a predetermined level corresponding to a given neutron flux level. The output of amplifier 4 is fed to phase-sensitive meter 5 which indicates the in-phase or anti-phase component amplitude and has a scale reading in both directions from zero. Phase sensitive relay 6 is also connected to the amplifier output, is responsive to the quadrature current component and controls an alarm device. A further amplifier 7 and relay 8 is provided to exercise a control function if the neutron flux exceeds the given value.