DISCUSSION: A SIMPLE CALORIMETER FOR

6-MEV ELECTRONS

Paul Goodwin

Columbia- Presbyterian Medical Center New York. N . Y .

The extremely high intensity of the electron beams from linear accelerators makes it possible t o use calorimetry as a method of intercomparison as well a s a method of calibration of electron beams. I n Part I I of this monograph, Dr. D. K . Bewley' briefly described a simple calorimeter he has been using for 8-mev elec- trons. Unaware of his design, we had constructed a calorimeter quite similar to his, but perhaps even simpler. As shown in F I G U R E I , the calorimeter consists of an aluminum block, 6 cm x 6 cm x 1.5 cm, in the center of a Styrofoam block approximately 20 x 20 x 10 cm. The electron beam enters from above, through a mylar window, and is totally absorbed by the aluminum. A large thermistor is mounted on the lower side of the block; eight resistors mounted in the block serve as heaters for calibration purposes. The thermistor is connected as one arm of a Wheatstone bridge. Irradiation with 6-mev electrons from an accelerator produces bridge currents of the order of 0.1 ma. Calibration is achieved by comparing the straight-line portion of the heating curve with that resulting from the passing of a known current through the resistor chain. In this way the total energy in the beam

FIGURE I . Calorimeter for 6-mev electrons. 99

100 Annals New York Academy of Sciences

may be accurately measured. The approximate dose in rads may be obtained by dividing this value by the irradiated area times the 50% range, which is about 2,800 mg/cm2 for 6-mev electrons. The dose is 90-100% of this value, from the surface to a depth of 2,000 mg/cm2.

The Varian 6-mev accelerator normally used for x-ray therapy contains an ar- rangement whereby the target, ionization chamber, and beam flattener may be easily removed, allowing the electron beam to exit, unmonitored. Using this cal- orimeter, the output of our Clinac was found to be about 0.25 x lo6 rads per min at 1 m. Although not practical for therapy, such an output may be useful for radiobiology and for radiation sterilization of such things as bone chips and plastic heart valves.

This calorimeter is quite portable; in fact, with its weight of less than a pound, it is even mailable, and thus might serve as a means of calibration and intercom- parison among owners of Clinacs who might wish to use their electron beams for radiobiology or radiation sterilization.

Reference

1. BEWLEY, D. K. Discussion: A simple calorimeter for the calibration of solid state do- simeters. Ann. N.Y. Acad. Sci. This monograph.