An indicator for ion beam cross-section

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AN INDICATOR FOR ION BEAM CROSS-SECTION P. W. NICHOLSON

Physics Department, Middlesex Hospital Medical School, London, W. 1., U.K. Received 29 May 1968

A simple arrangement is described to enable the cross-sectional distribution of an ion beam to be continuously monitored. In the course of generating 14 MeV neutrons by accelerating deuterons onto a tritiated target it was found that target burn-up was occurring due to oversharp focusing of the ion beam. In addition a knowledge of the ion beam cross-section was needed to obtain reproducible geometry when samples were placed nearthetargetfor irradiation in the neutron flux. The arrangement described below gives a continuous indication of the beam cross-section and is simpler than other methods 1-3) that have been described.

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Fig. 1. Diaphragm and array of pick-up wires in the ion beam.

The method is to arrange a regular array of fixed horizontal and vertical wires normal to the path of the ion beam (fig. 1). These pick-up wires are connected, by way of a screened multicore cable, to their remote indicators. The circuit for the indicator for a single channe[ is shown in fig. 2. The current collected from the ion beam by each pick-up wire develops a voltage across the load R and causes the DM 160 indicator glow tube (Mullard Ltd) to glow with proportional intensity. The OA 200 silicon diode and 100 kf2 resistor protect the glow tube from excessive positive

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Fig. 2. Circuit of remote indicator for one pick-up wire.

grid voltages. The switch allows different loads (and hence sensitivities) to be selected and the switches for all the channels are physically coupled by using a multiple wafer rotary switch. The arrangement was used in a 0.4 mA ion beam of 120 keV deuterons which were incident on a 2.5 cm dia. target. The pick-up wires of 0.5 m m dia. tinned copper were mounted adjacent to the grounded target but behind a metal diaphragm with a 2.5 cm dia. aperture (fig. 1). The potentiometer (fig. 2) was included in each channel to allow the output of each pick-up wire to be weighted according to the length of wire exposed in the diaphragm aperture. Thus a uniform ion beam distribution was indicated by all the tubes glowing equally. To prevent secondary electrons from the target being collected by the pick-up wires and to allow the secondary electron current from the pick-up wires to attain saturation it was found necessary to bias all wires at - 5 0 V to ground (fig. 2). Application of more negative voltage had no further effect on the current picked up. Five horizontal and five vertical pick-up wires were sufficient for the present purposes. The glow tubes were arranged in an L-shaped array as if forming the axes of an X Y plot. The arrangement described was tested by allowing the ion beam to fall on a quartz target and observing the resultant fluorescence by closed circuit television. For the present application the arrangement gave a perfectly adequate indication of the cross-sectional distribution of the ion beam. For other applications certain modifications might be made. Obviously the number, spacing and diameter of the pick-up wires could be varied to suit other ion beam characteristics. Tubes other than the DM 160 could be used. For ion beam currents with a high ripple content it might be necessary to include smoothing capacitors in the circuits. References 1) j. W. Jagger, J. P. Page and P. J. Riley, Nucl. Instr. and Meth.

49 (1967) 121. z) P. H. Rose, A. B. Wittkower, R. P. Bastida and A. Galejs, Nucl. Instr. and Meth. 14 (1961) 79. z) K. O. Nielson and O. Skilbreid, Nucl. Instr. 1 (1957) 159. 357