Measurement of supervoltage x-rays with the free-air ionization chamber

Abstract

A free-air ionization chamber for the measurement of supervoltage X-rays is described. It is built on the guarded-field principle and of such size as to permit measurements for X-ray excitation voltages from 150 to 350 kv at atmospheric pressure and 200 kv upward at elevated pressures. The electrode system is contained in a pressure cylinder of such size as to give an equivalent plate spacing of 4 m at a pressure of 10 atm. The chamber characteristics have been studied in the range of 200 to 400 kv. The most important finding is the required increase in plate separation at the higher voltages. While reasonable, this is at variance with the findings of other workers. From the m easurement of the plate separation for full electronic equilibrium, electron ranges may be obtained.To carry out these measurements it was necessary to design and construct a high-voltage plant capable of extreme steadiness of operation. This was accomplished with a four-stage rectifier having valves operated on a single pumping system. Potentials were carefully distributed through the system with the result that the voltage could be measured and controlled within ± ~ percent, which figure also represents the variation in tube output.The X-ray tube is of the simple four-stage type, made up of Pyrex-glass cylinders. By means of an insulated target, only the target current is measured.

Measurement, in roentgens, of the gamma radiation from radium by the free-air ionization chamber

“…Calculations were made for such leakage by dividing the exit face of the orifice into narrow concentric zones and then computing the transmission of each. The results given in table 1 are the maximum, and the actual transmission may possibly be less by as much as 10 percent. Since most of the other measurements were made at F = 62.5 cm, a 2-percent correction only was applied.…”

“…Kaye and Binks [13] found that at atmospheric pressure a distance of 7 m was apparently sufficient, whereas Friedrich's measurements [19] it was possible to test this over a range equivalent to 6 to 21 m at atmospheric pressure. Following our earlier procedures [1], ionization measurements were made for distances between the collector and the window over the above range. Inasmuch as the diaphragm was but 10 cm outside the aluminum window of the pressure tank, distances from the aluminum window to the collector were used.…”

“…247 arisen. In a recent publication [1] 2 the application of the free-air ionization chamber, with certain pertinent modifications in technique and necessary precautions, was described, in which was proved the adequacy of the free-air chamber for measuring X-rays up to 400-kv excitation potentials. This work still leaves its application to the higher frequency radiation untested.…”

“…The ionization chamber employed in this investigation has been described in detail [1] in connection with supervoltage X-ray investigations, no changes having been made for this study. Briefl;v, it consists of a guarded-field parallel-plate chamber (75 cm long WIth a maximum plate separation of about 40 cm) placed in a steel pressure tank 8 ft long and 2}6 ft in diameter, suitable for operating normally up to 10 atm and, if needed, at double that pressure.…”

“…Current measurements were made by essentially the same means ) used in our earlier X-ray measurements [1]. For comparative current measurements, the ion current was balanced against a known current through a very high (S. S. White) resistor.…”