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CYCLOTRON BOMBARDMENTS WITH HE3 a n d r 3 DISSERTATION Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philisophy in the Graduate School of The Ohio State University By Thomas William Donaven, 3.S., M.A The Ohio State University 1952 Approved by Adviser ( fe/\J TABLE OF CONTENTS Cyclotron Bombardments with He^ or H3 Acknowledgements Page Introduction 1 Description of Apparatus 3 Method of Operation 13 Purification of He^ or 17 Results 20 Conclusions 30 References 32 Miscellaneous Photographs 36 Autobiography 1*0 i 918254 Acknowledgements It is with deep appreciation that I extend thanks to Professor M. L. Pool for his guidance in this work. Special thanks is also due to Dr. D. N. Kundu for his advice, help, and cooperation in this project. Acknowledgement is also made to Mr. Paul Weiler and Mr. Donald Moore of the cyclotron staff, and to the machine shop under Mr. Carl McWhirt, for their aid in completing this work. The support given me through fellowships by The Ohio State University Physics Department and by the Oak Ridge Institute of Nuclear Studies is gratefully acknowledged. ii INTRODUCTION The naturally occurring elements whose atoms are heavier than Bismuth, i.e. Po, At, Rn, Fr, Th, Pa, U, etc. are all radioactive. The remainder of the elements may be made radioactive by hitting them with high speed nuclei of other elements. The elements whose nuclei are normally used to create radioactivity are hydrogen and helium. In order to give a nucleus a high speed with the minimum of equipment, it is necessary to strip all of the electrons from the atom, leaving its bare nucleus to be accelerated. It becomes increasingly difficult to strip a nucleus as the number of electrons surrounding it increases. Hence hydrogen, with one electron, and helium, with two electrons, are normally used to bombard other elements. The neutron, with no charge, is also used to produce radioactivity; it is effective at any speed and is derived from nuclear reactions. Protons give reactions such as (p, n), (p, 2n), (p, ci ), and (p, He^). These are produced by the proton entering the nucleus that it hits, becoming part of it, and then this "compound nucleus'" emits one or more particles. The hydrogen isotope of mass two, the deuteron, gives similar reactions. However, in some cases instead of the deuteron and the bombarded nucleus forming a "compound nucleus", the deuteron is polarized. Thereby, the neutron or proton part of the deuteron touches the nuclear surface and then sticks to the target nucleus; the remaining part of the deuteron is left behind. This left out part is afterwards observed as a repelled proton or outcoming neutron from the nuclear event. The ci particle has also been used to cause nuclear reactions of the types (cx, n), ( c(, p), (o{ , 2n), etc. It would, therefore, be /• -2- interesting to see what type of nuclear reactions occur for the bom barding particles of intermediate mass, i.e., the heaviest isotope, H-3, of hydrogen, consisting of a proton and two neutrons, and for the lightest isotope of helium, He3, consisting of two protons and a neutron. One would also expect to be able to produce new radioactive isotopes of various elements since the reaction (h3} p) would seem to be a feasible method of introducing two neutrons into a nucleus and the reaction (He3, n) is equivalent to (He^ 2n). The (He^, 2n) reaction requires comparatively high energy particles, while the (He3, n) reaction should occur at low energies* DESCRIPTION OF APPARATUS The design of a workable system of circulating the few available cubic centimeter s of He3 and h3 was one of the main problems of this investigation. As is the case so many times, once suitable equipment is at hand the remaining problems are comparatively easy to solve. The equipment described here works as well as could be reasonably expected and at present there are only very minor changes in it being considered. These changes will be described later. There are at present only a few cubic centimeters of He3 or H3 gas available for cyclotron bombardments here at The Ohio State University. This is the maximum amount of He3 or h3 that the Atomic Energy Commission will allow anyone institution to have at one time. Hence if cyclotron bombardments are to be made, they must be made with not more than this amount. Ordinarily about 200 cubic centimeters per hour of gas are used in making a bombardment with protons, deuterons or alphas. In an ordinary run the gas is pumped through the cyclotron tank by two eight inch oil diffusion pumps and then out into the air by a Hyper Vac 100 mechanical pump that backs up the oil diffusion pumps. Thus it is obvious that in a He3 or H3 bombardment that the He3 or h 3 gas must be reinserted into the cyclotron after it has been pumped out. It would also be desirable to free it from the air which has leaked into the cyclotron and diluted the He3 or H3. There is also a certain amount of hydrogen which outgasses from the metal in the cyclotron tank. The circulation system now to be described is of course the last of a series of systems tried; the others did not work satisfactorily. -k- The main requirements of a successful circulation system are the followings it must be able to produce a vacuum of at least 10“3 millimeters of mercury in the cyclotron tank; it must return the gas to the cyclotron tank as fast as it is pumped out; and it must have a control to regulate the rate at which the He3 or h3 is returned to the cyclotron tank so that the pressure may be regulated to produce as large a beam of He3 or h3 particles as possible* The circulation system consists of one of the regular eight inch oil diffusion pumps backed by two DPI mercury diffusion pumps of 20 liters per second pumping speed each. The output of the mercury diffusion pumps goes to a ballast tank of about 30 liters capacity, thence through a one and a quarter inch copper tube to the control valves and from there directly back into the cyclotron. Figure 1 shows a diagram of the system. There is a liquid air trap between the mercury diffusion pumps and the oil diffusion pumps and also one between the ballast tank and the control valves. There is also a liquid hydrogen trap which sits on the top of an unused oil diffusion pump. Mercury diffusion pumps are used to back up the oil diffusion pump since they can operate with a backing pressure as high as 10 millimeters of mercury. This pressure 'would require the presence of almost 500 cubic centimeters of gas in the circulation system. It would require only about 50 - ?5 cubic centimeters of gas to dilute the 2 cubic centimeters of He3 or H3 used in a bombardment to the point where the beam of He3 or H3 particles would be so weak that it would be useless to continue the bombardment. Thus, there is a large reserve capacity available to insure that the mercury pumps will not stop operating because of too high a backing pressure. If they were -5- CIRCULATION SYSTEM FOR He & H’ D iffu sio n Pumfr IX' Cyclotron Tank Figure 1 -6- to stop operating because of too high a backing pressure, the mercury in them would probably be distributed through-out the system. If they are operating and connected to the cyclotron without liquid air in the traps, it takes only a few seconds for enough mercury to diffuse into the cyclotron to make its operation impossible because of the high vapor pressure of mercury. Even with liquid air in the traps, great care must also be taken to insure that the pressure in the mercury pumps is lower than that in the cyclotron before the valve between them is opened3 otherwise the rush of gas from the mercury system to the cyclotron carries mercury vapor past the trap and into the cyclotron. The line between the oil diffusion pump and the mercury pumps consists of two parts. The first is a three inch line which leads through a valve to the mechanical pump used on regular runs and the second part consists of a two inch valve and a large three inch diameter liquid air trap. It is necessary that the line between the oil diffusion pump and the mercury pumps be of fairly large diameter and unobstructed; otherwise the mercury pumps will not Ixave enough pumping speed to back the oil diffusion pumps. During a He3 or H3 bombardment the valve in the three inch line to the mechanical pump is closed and the two inch valve to the mercury diffusion pumps is opened. The two inch valve between the cyclotron and the mercury diffusion pumps deserves special mention because of its excellent operating characteristics. Ordinary valves, such as those used between the oil diffusion pumps of the cyclotron and the Hyper Vac 100 mechanical pump which produced their backing pressure, frequently leak air into -7- the system around, the stem when they are being opened or closed, or even when they are stationary, either open or closed. The ordinary valves are made vacuum tight by packing high vacuum wax around the stem. However, this makeshift procedure does not prevent the valves from leaking while they are being opened or closed, which is the time that they most frequently leak. The two inch valve mentioned above, however, has not been found to leak at any time.