IMAGING WITH A MULTIPLANE MULTIWIRE PROPORTIONAL CHAMBER USING HEAVY-ION BEAMS

LBL-14143 IMAGING WITH A MULTIPLANE MULTIWIRE PROPORTIONAL CHAMBER USING HEAVY-ION BEAMS* U. T. Chu, J. R. Alonso, and C. A. Tobias Lawrence Berkeley Laboratory University of California, Berkeley, California 94720 I.EL--14143 DE82 014474 Abstract A 16-plane multiwire proportional chamber has been developed to accurately map intensity pro­ files of heavy ion beams at the Bevalac. The imaging capability of the system has been tested for reconstruction of 3-dimensional representa­ tion of a canir.e thorax region using heavy ion beams. Introduction Accelerated heavy ions from the Bevalac are used for biomedical experiments. Often the -a- diation field areas are large, of diameters of 20 cm or larger, and the radiation intensities across the fields are desired to be uniform. To map the dose uniformity quickly, a 1'j-plane multiwire proportional chamber has been construc­ ted and successfully used as a beam intensity monitoring device. It is named MEDUSA after MEdical Dose Uniformity SAmpler. The 16 chamber planes, each of which has 64 parallel wires placed 4 nm apart, are stacked with their wire directions staggered in such a way that they cover the ISO degree space as shown in Fig. 1. When the he^vy ions penetrate the chamber, the resulting ionization electrons are collected in each signal wire and stored in the integrating capacitor connected to the wire. The voltage on the capacitor is therefore proportio­ nal to the beam intensity over the area of a 4 mm-wide strip which is centered on the particu­ lar wire and extends midway toward the adjacent two wires. The voltage data collected on the 64 wires of a given chamber plane are the line in­ tegrals along the wires of the ionization resul­ ting from the penetrating heavy ions. In other words, the data represent the one dimensional projection, along the direction of the vires, of the two-dimensional beam intensity profile. Based on the 16 projections, each at a different angle, the beam intensity profile is readily re­ constructed. The reconstruction algorithm C33 783-3335 Fig. 1. MEI1USA. is based technique. on f i ltered ba-ik-oroj^ct'on *7nTs work was supported by the Assistant Secretary for Health and Environmental Research Division of the U.S. Department of Energy under Contract No, W-7405-ENG-48 and by the National Institute of Health Grant CA15184. The heavy ion beams at the Bevalac are pulsed at every 4 seconds, and MEDUSA is reset immedi­ ately prior to a beam pulse. At the end of each data collecting cycle, usually one or a few pulses, the capacitor voltages are sequentially sampled, digitized, and stored in a buffer menory under the control of an LSI—11 microprocessor. Upon command, the data in the buffer are serially transmitted to the host computer, a PDP 11/34 computer. The 1024 data points (16 pianes of 64 wires) are used to reconstruct the beam profile. •This work was supported by the Assistant Secretary for Health and Environmental Research Division of the U.S. Department of Energy under Contract No. DE-AC03-76SF00038, and by the National Institute of Health Grant CA15184.