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In 1953, the International Commission on Radiological Units recommended [1] the continued use of the roentgen up to 3 Mev for the measurement of Xand gamma-ray dose. The physical requirements of standard ionization chambers for measuring roentgens in the X-ray region up to 250 lev have already been reported [2,3,4]. The calibration of secondary instruments in at least two laboratories also seems to be in agreement [5] for cobalt -60 and radium, on the assumption of a fixed value for t he emission constant (roentgens pCI' milligram hour at 1 cm) of radium. However, there is some question about the value of t he emission constant for radium [6 ,7] when its determination is based upon measurements with small cavity ionization chambers. In addition , many of the commercial secondary instruments show a difference in calibrat ion of the order of 5 to 20 percent between 250-kv X-rays and cobalt, so an interpolat ion of the calibration for the 300to 4001\:v X-ray therapy range seems risky. Extension of the range of standard instruments at least up to cobalt energies, therefore seems desirable. The present report contains t he design criteria for t he construction of a standard chamber for 250to 500-1\:v X-rays. A plan view of one type of a standard free-air chamber is shown schemat ically in figure 1. A parallel-plate system is co ntained in a radiationshielded box, B . A known area of the horizontal X-ray beam is defined by the diaphragm, D , so that a beam of photons passes centrally between the plates. A high potential (field strength of the order of 100 v /cm ) on plate H , with respect to the other plates, sweeps out the ionization produced in the air between the plates. The ionization is measured for a length, L, of beam determin ed by the limit ing lines of force to t he edges of the collector, C. These lines arc made straight and perpendicular to the coll ector by t he guard plates, G, and auxiliary wires or strips, VV. The latter are co nnected to a resistancedividing network to grade the potential uniformly across the end, top, and bottom gaps between C or G, and H. Generally guard plates are not con-

Standard ionization-chamber requirements for 250- to 500-kilovolt x-rays