Author Institution: Associate Director, Westinghouse Research Laboratories, East Pittsburgh, Pa

Condon, E. U.

The Ohio Journal of Science

KnowledgeBank at OSU

THE DEVELOPMENT OF HIGH VOLTAGE FOR THEPRODUCTION OF NEUTRONS ANDARTIFICIAL RADIOACTIVITYE. U. CONDON, Ph.D.,Associate Director, Westinghouse Research Laboratories,East Pittsburgh, Pa.Eight years ago neutrons and the phenomenon of artificial radio-activity were discovered, following closely on the first work on artificialtransmutation of the elements. In the years that have followed therehas been a tremendous advance in our knowledge of fundamentalphysics of the atom as a result of these discoveries, a big developmentof equipment for producing and measuring the new radiations, and awide recognition of their great potentialities for medicine. I under-stand it to be the function of our symposium this morning to make arapid survey of this field from the particular standpoint of the cancerproblem. In opening the discussion, I shall undertake only to presentthe physical side of the subject, properly leaving to my associates adiscussion of the biological and medical phases.Let us start by recalling that we now know that the atoms of matterare made of a central nucleus, composed of protons and neutrons andhence charged electrically positive, surrounded by enough electrons tomake the atom as a whole electrically neutral. All the development ofatomic physics from 1912 to 1930 was concerned with the study of theouter electronic structure—the decade from 1930 to 1940 has seen theinterest shift almost entirely to the internal structure of the nucleus.In solid bodies the atoms are packed in about one hundred million tothe inch and the nucleus itself is about ten thousand times smaller indiameter. The atomic nucleus is the smallest thing whose structure isstudied by man. The principal means of study has been by observationof what happens when atomic nuclei are struck by ions of hydrogen orhelium which have been artificially accelerated to high speeds by meansof high voltages. The bombarding ions need the high speed becausethey are charged positively and so are repelled by the nuclei, henceunless they have very high speeds they do not get close enough toproduce any effect.In the latter 1920's when physicists began to turn their attention tothe nucleus, it was thought that several million volts at least would beneeded to give the particles enough energy to effect nuclear transmu-tation. And at that time there was no known way that appearedpracticable for getting more than one million volts. Cascaded trans-formers had been used up to one million volts or so. Electronic rectifiersin combination with transformers had been developed up to severalhundred kilo volts. But then two developments occurred which openedup the subject. In the first place, theoretical applications of wavemechanics showed that the voltages needed were not nearly so high as atfirst thought. This theoretical prediction led to discovery that light131132 E. U. CONDON Vol. XLIelements could be transmuted with relatively low voltages—severalhundred thousand—although at such voltages the yield is very low andrises exponentially with the voltage.The other important development was the introduction of two meansof getting particles of extremely high energy. These are the cyclotronof Professor E. O. Lawrence of Berkeley and the belt-type electrostaticgenerator of Professor R. J. Van de Graaff of the Massachusetts Insti-tute of Technology. These made possible the production of high energybeams of positive ions for production of neutrons, artificial radio-activematerials, and nuclear research in general. The principle of operation ofeach of these is well-known to this audience. In the cyclotron there areno extremely high voltages on different parts of the apparatus. Insteadthe ions are caused by a magnetic field to move in a spiral path whilegaining energy from an alternating electric field. In the belt generatorthe high voltage is obtained by carrying charge to an electrode with anendless cloth belt. The maximum attained voltage depends on thebreakdown strength of the dielectric around this high voltage design.This requirement leads at once to very large sizes and special means ofinsulation. Both types of equipment, when built on a scale sufficient toprovide several millions of volts, are quite expensive—somewhere in theneighborhood of $100,000 being the amount needed.The cyclotron started in 1930 with a very small model built byLawrence, then a working instrument built by Lawrence and Livingstonwhich gave 1 million electron volts (Mev.) protons. This was followed byanother with a magnet having pole-pieces 37 inches in diameter whichattained about 6 Mev. and last year by completion of another having 60inch pole pieces which gives 16 Mev. deuterons and 32 Mev. alphaparticles. Now Lawrence is busy with the details of a huge projectinvolving a cyclotron with pole pieces 184 inches in diameter, themagnet of which will weigh close to 5000 tons, which will producedeuterons of some 150 Mev. energy. The project is estimated to cost inthe neighborhood of $1,500,000 of which some two-thirds has beenprovided by the Rockefeller Foundation. In the same period there hasbeen a very widespread recognition of the importance of this researchtool as evidenced by the fact that there are some thirty cyclotrons inthe world, either built or building, mostly in the United States, andmostly financed in the expectation that they will prove valuable researchtools in medicine.Going along with this story of steadily increasing voltage, the" cyclotroneers" have made an enviable record in improving the totalbeam current, the latest being a total of 300 microamperes of deuteronsat 16 Mev. on the 60-inch cyclotron. This means practically 5 kilowattsof power in a form available for intense nuclear disintegrations. Just afew years ago we heard of currents of 0.1 microampere. Now currentswell above 10 are commonplace and many cyclotrons are getting closeto 100 microamperes of beam current.The first large belt generator is the one built by Van de Graaff atRound Hill, Massachusetts. This attained very great voltages, aroundten million, but the great difficulties associated with building a largevacuum tube in which to accelerate the ions with this generator wereNo. 3 ARTIFICIAL RADIOACTIVITY 133never successfully overcome. The tube was designed for horizontaloperation, which introduced great difficulties of support. The first suc-cessful application of the belt generator principle to the study of nuclearphysics is due to Tuve and Hafstad of the Carnegie Institution ofWashington. Their generator with a two-meter sphere worked success-fully to about 1200 kilovolts, and they usually have had availableabout 1 microampere of analyzed beam current. Later the Round Hilloutfit was moved to the campus of the Massachusetts Institute ofTechnology and sat up in a different way to permit the installation of avertical discharge tube. In this form it gets about 2500 kilovolts. Thislaboratory has principally applied the outfit to studies on the ultra hardX-rays produced when electrons of this voltage strike a metal target.Most X-ray physics and all radiological experience hitherto has stoppedat 1000 kv., so we may hope for many interesting results from Cambridge.About five years ago, Herb at Madison, Wisconsin, made the firstreally successful application of the use of compressed air as an insulatingmedium to improve the attainable voltage with a belt generator. Work-ing with a pressure tank about 6 feet in diameter and 14 feet long, hebuilt a machine which gives 2.5 million volts. The great gain in com-pactness which compressed air insulation gives is apparent when werealize that Herb's outfit in a small basement laboratory room givesessentially the same voltage as the large M.I.T. outfit which requires agood-sized building to house it.With this experience to go on, the decision was made to build a largepressure insulated belt generator at the Westinghouse laboratories.This has been in operation about a year, as has also another similar oneat the Carnegie Institution in Washington. Our machine is operatedsuccessfully at 4000 kilovolts with proton beam currents of about 1microampere. The Washington machine has shown a similar perform-ance. Two more pressure insulated belt generator outfits of intermediatesize are now nearing completion, at the University of Minnesota and atthe University of Pennsylvania.I am sure that people who are closely interested in this subject wouldlike me to say something about the relative merits of the two means ofobtaining high energy particles. This is a quite complicated subject—and there seems to be no one who has had close working experience withboth types. The Washington group soon will be in this position, for theyare now building a large cyclotron like Lawrence's present large one tosupplement the work of their large belt generator.Since yields of neutrons or of radio-active materials go up roughlyexponentially with voltage and directly with current, there is no questionbut that a cyclotron is at present superior to the belt generator as astrong producer of radio-active materials or of neutrons. And it doesnot seem likely that the belt type generator will catch up in total voltageand current for some time to come, so as a production instrument thecyclotron seems quite superior. In the belt generator the generatoritself is capable of providing much greater currents—the crux of theproblem is in the ion source at the high voltage end of the vacuum tube.The main advantages of the belt generator seem to be as a scientificinstrument and as a means of studying ultra-hard X-rays. The cyclotron134 E. U. CONDON Vol. X L Icannot accelerate electrons owing to their light mass and high relativisticvariation of mass. Moreover, the different particles in a cyclotron beamdo not appear to be nearly so homogeneous in energy as the particlesin the beam in a steady belt generator. In our outfit at Westinghouse,we feel the variation is scarcely more than =*= one-tenth per cent,whereas I think all cyclotron beams show a range of about =»= 10 per cent,although it is hard to get definite information on this point. This makesthe belt outfit an ideal instrument for precision measurements of res-onance levels in nuclei and for onset thresholds of certain endoergicreactions.It appears, therefore, that the two machines are more comple-mentary than competitive, and that each is destined to play an importantpart in the continuing development of nuclear physics.

The Development of High Voltage for the Production of Neutrons and Artificial Radioactivity

https://kb.osu.edu/bitstream/1811/3147/1/V41N03_131.pdf

The Development of High Voltage for the Production of Neutrons and Artificial Radioactivity

Abstract

Similar works

Full text

Available Versions

KnowledgeBank at OSU