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The design and operation of a current integrator and a radiation monitor for use with U.B.C.'s 4 M.E.V. Van de Graaff generator. Edwards, Martin Hassall

Abstract

The project has been to design, build and calibrate a current integrator for use in the measurement of the target ion current of a Van de Graaff generator and a 50 K.V. ion accelerator. Various methods of measuring small fluctuating currents used by previous workers are discussed with reference to the particular needs of this problem. Errors due to the target cooling system, target contamination, neutralization of the beam at low energies and secondary electron emission are also discussed. The integrator was designed using a Miller integrating circuit consisting of a pair of condensers connected from the grid of the input tube to the plate of the output tube of an amplifier having an unfed-back gain of approximately 4000, The conditions for accurate operation of the Miller circuit have been derived. To comply with them the input tube of the amplifier is connected as an electrometer tube and the condensers used are low leakage polystyrene condensers. Any incoming charge is stored on the condensers and the integral of this charge is indicated by the decreasing anode voltage of the last tube of the amplifier. At the end of a rundown a Schmitt trigger circuit operates a relay which recharges one of the pair of condensers to a fixed potential and thereby adds a known quantity of charge to it, and then reconnects it, also actuating a mechanical counter. The virtue of this scheme is that there is always one condenser in the Miller circuit and thus no dead time errors arise. Currents from 4 x 10ˉ⁸ amperes to 2 x 10ˉ⁴ amperes are measurable on four overlapping ranges. The integrator sensitivity (in coulombs required to produce a count) is found to vary with the counting rate. Mean sensitivities accurate to ± 3% on each of the ranges are given. For higher accuracy (± 1%) it is necessary to read the sensitivity appropriate to the counting rate from a calibration graph. The variation in sensitivity with counting rate appears to arise from the time required for the relay to switch the recharging condenser. The sensitivity varies less than ± ½% during the day, and about ± 2% in a week. If all voltages are adjusted as recommended, the sensitivity is found to be constant to 1% or better two months after the original calibration. The lower limit of measurable currents is set by drift currents of the order of 3 x l0ˉ¹⁰ amps. The upper limit of measurable currents is set by the bottoming of the amplifier during the recharging time. Two simple air ionization chamber gamma radiation health monitors have also been built (using a shielded electrometer tube in a d.c, amplifier) and calibrated.

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