The Disintegration of Beryllium by Photons and Its Possible Bearing on the Mass of Be9

Abstract

Szilard and Chalmers [1] found that when beryllium was bombarded with the gamma-rays of radium in equilibrium with its decay products, neutrons were liberated, and could be detected by means of the radioactivity they excited in iodine. Meitner [2] showed that these neutrons excited radioactivity in I, Au and Ag, but not in Na, Si and Al. The former reactions involve the capture of a neutron, which, as Fermi [3] has shown, is most probable when the neutrons have little energy; while the latter three reactions involve the emission of an alpha-particle or proton, and probably have a higher probability when the bombarding neutrons have greater energies. Brasch and other [4], working with x-rays, have obtained this disintegration at voltages between 1.5 and 2 mev. In the present experiment, beryllium was bombarded with the x-rays from the tube in the Kellogg Radiation Laboratory. The tube is self-rectifying, and was supplied with 50 cycle a.c. at 0.9 mev peak. The electron current to the target was 2 m.a. The beryllium was located directly behind the tungsten target, in a bomb which was lowered inside the electrode. The ethyl iodide used as a detector for the neutrons was also in the bomb, just above the beryllium and in a Dewar vessel to prevent the heat generated at the target from reaching the ethyl iodide in the course of the irradiation, which lasted 40 minutes. The active iodine was separated .from the ethyl iodide by the method of Szilard and Chalmers [5]. 200 g of beryllium and 375 cc of ethyl iodide were employed; the inside diameter of the bomb was 6.2 cm, and its overall length 25 cm. The intensity of radiation at 1 cm behind the target is calculated to be 2.5X10^4 r/min., on the basis of the measured intensity at 50 cm from the target and the known filtration in both cases.

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