Accelerators – Halo Monitors


Air-filled ionization chambers, similar to those in use at the Paul Scherrer Institute in Switzerland, have been installed at 2 meter intervals in the high-energy beam lines leading to the radioisotope production vaults to detect stray-beam leaving the beam pipe. Each detector consists of two printed-circuit boards separated by a distance of 10 mm and with facing surfaces copper-plated. The ring-shaped copper surfaces have inner and outer diameters of 55.8 mm and 85.8 mm, respectively. On the one board the copper ring is divided in quadrants to indicate the position of the stray beam. The boards have been designed in such a way that they can be clamped onto the outside of the beam pipes without removing a pipe section. To detect stray particles the quadrant currents, using a 1 kV bias voltage, are measured, using integration techniques, and displayed with a computer program Halo. The system can measure 48 channels concurrently with an accuracy of a few pA. At the centre of the electronic system is a rabbit microprocessor that controls the components. The computer program connects to the microprocessor over the network using TCP/IP. The program displays the data graphically and controls the current range. The colour of the graphs changes when certain important conditions are met. Data from 8 stray-beam detectors are displayed. Using a client program the visual interface can be seen from any computer on the network. The stray-beam detectors now forms part of the safety interlocking system for protection of beam pipes and peripheral equipment. Stray-beam detectors, using PIN diodes, have been installed in the beam lines leading to the experimental areas.

Halo Monitormy1

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