The Technique of High Intensity Bombardment with Fast Particles

Abstract

Recent improvements in cyclotron technique have led, in the case of the 37″ cyclotron at the University of California, to the utilization of beams of such intensity that nearly a kilowatt of power must be dissipated in the target. To maintain high operating efficiency it is desirable that targets should be bombarded in elementary form rather than as compounds, and that the cyclotron vacuum chamber should be protected from the debris of bombardment at all times. The former requirement puts a severe limitation on the designer because many elements are neither refractory nor are they good heat conductors. The second requirement necessitates that a thin metal window be used to isolate the target from the vacuum chamber, because most elements sublime or spatter in one way or another no matter how well cooled they may be. To meet this situation a bombarding chamber has been developed (a ``bell-jar window''). It consists of a rectangular copper box with one end closed by a water-cooled copper plate carrying the target and the other by a thin metal window. The window end is bolted to the cyclotron's target orifice and the interior of the box is filled with hydrogen or helium at a pressure of a few centimeters. The presence of the gas serves to keep the window cool, to keep the target from being damaged by the beam and to protect the cyclotron in the event of a sudden failure of the window. The metal windows are supported on a grid of elongated holes which has a geometrical aperture of about 93 percent. The target plate is heavily knurled and the element being bombarded may either be fused on it or pressed into the knurlings with a spatula. The surface of the target can further be protected by covering it with thin gold foil. With this arrangement it has been found possible to bombard red phosphorus, sodium metal, lithium metal and many other difficult targets with the full intensity of the cyclotron.