The Measurement of the Intensity of the Gamma Rays of Radium in r-units

AFTER the unit of the roentgen-ray intensity, “one r,” was accepted by the International Congress of Radiology, it was desirable to determine, again, whether this unit might also be used to measure the intensities of gamma rays of radium. The first attempt to measure intensities of both roentgen and radium radiation in terms of the same unit was made by Friedrich and Opitz (1), who employed the “electrostatic” unit. This investigation was unsuccessful for two reasons: (1) The gamma-ray intensities were not measured in e-units directly, but were calculated indirectly by a comparison of the erythema skin reactions produced by both types of radiation with those produced by the roentgen-ray intensities which had been measured in e-units; (2) The Friedrich “e-unit” in use at that time was not the present international r-unit. In 1924, Glasser (2), using a modified e-unit, made a comparison of radium and roentgen-ray intensities. He found, by direct measurement, that 1 mg. e1 hr. = 1.69 e-units (or r-units) when using a radium pack of 4 cm. diameter and a filter of ¼ mm. lead plus 1 mm. brass at a distance of 2 cm. from the center of the radium capsule to the center of the ionization chamber. We now find that this value, obtained six years ago, agrees satisfactorily with the results obtained in our recent experiments, which were carried out with improved apparatus. This proves that the modified e-unit which was used at that time is practically identical with the r-unit of to-day. The results of our investigation are presented in this discussion. In 1925 Lahm (3) discussed this problem. His results, however, were not quite accurate, particularly because he also made an indirect calculation of the r intensities by comparison of the erythema reaction of radium with that produced by roentgen rays. It will be shown in this paper that this indirect method of estimating gamma-ray intensities in r-units may lead to considerable error. From his experiments, Lahm drew the conclusion that 450 mg. el hr. = 600 r = 1 skin-unit dose with a radium preparation 1.5 cm. in length, filtered by 1 mm. of brass, at a distance of 1 centimeter. In other words, 1 mg. el hr. = 1.34 r-units. This value was accepted by other investigators (Jona, 4, and Neeff, 5), and recently was again used by Lahm (6), but it does not agree with our results. Kessler and Sluys (7) have tried to make an absolute determination of the r-unit for gamma rays by employing a large air-ionization chamber constructed somewhat on the same general principles as that used for roentgen-ray measurements, though they have not published their results. Stahel (8) also made relative intensity measurements On gamma rays with a micro-ionization chamber; but his results are as yet unpublished. Recently Jona (9) presented some of the results of his investigations of the same problem. His values approximate those which we gave several years ago.