TABLE IV EXAMPLE OF ADVANTAGE OF ENRICHMENT OF TARGET OF SPECIFIC GRAVITY •Note: It was not feasible to consider making these isotopes using normal target material. They have become available only as a result of the enrichment of stable isotope Based on the use of the Oak Ridge Research Reactor (maximum thermal flux, LIX 106 quired shielding and output changes, if a specified teletherapy head can be loaded with pure cesium 137 to give a 1,000 rhm output, it can be loaded with a cesium source containing 2 per cent cesium 134 to give only 260 rhm. It has thus far been difficult to supply cesium 137 with a very low 134 content or even to assure that a desired cesium source will have a certain percentage of the 134 contaminant. Recently, we have undertaken to produce large cesium 137 heat sources for use in thermoelectric generators to supply small but reliable amounts of electric power (isotopic power) in remote locations on land or under the sea. Fortunately for medical users, this has resulted in development of a method to extract cesium 137 from underground tanks at the Commission's Hanford Atomic Power Operations. These aged wastes have been stored long enough for the cesium 134 to decay to minor proportions (1.4 per cent or less). Unlike other long lived fission products which eventually form a cake in the waste tanks, the cesium remains in the supernate. The technique developed is to elute off the supernate, run it through a centrifuge to remove the particulate matter and absorb it on "Decalso," an inorganic alumino-silicate ion exchange bed. The cesium 137 can, therefore, be shipped essentially as a solid material. Recently, the largest single shipment of fission products, over 200,000 curies of cesium 137, was made using this technique. The relative simplicity of cesium 137 separation and preparation for shipment indicates that massive quantities of the material can be made available at reasonable cost. ACCELERATOR-PRODUCED RADIOISOTOPES Throughout the period 1934-1945, most radioisotopes used were produced in cyclotrons. Today, the cyclotron complements rather than competes with the nuclear reactor for production of radioisotopes. The cyclotron is an important source of certain medically useful isotopes that cannot be produced in a reactor. Cyclotronproduced isotopes frequently are those that lie on the neutron-deficient side of the nuclear stability line. These decay principally by positron emission or orbital electron capture. Cyclotron-produced radioisotopes, in general, have a distinct advantage of high specific activity since they ordinarily are isotopes of a different chemical element than the target and hence can be chemically separated in carrier-free form. While numerous cyclotrons in the United States produce radioisotopes to a certain extent, the principal source of those made available commercially is the unique highbeam-intensity 86 inch cyclotron at Oak Ridge National Laboratory.12 The construction of the 86 inch cyclotron is novel in that it is a vertical cyclotron in which the dees hang suspended between the poles of a 400 ton magnet. It is otherwise a conventional fixed-frequency machine which produces protons with energies up to 23 mev. The important feature is that high beam currents of 1 to 2.6 milliamperes can be routinely employed for the production of neutron-deficient radioisotopes by proton transmutations. Special target cooling is necessary, however, because of the difficult problems of heat dissipation in the target, involving 50 kilowatts over an area of less than one square inch. Not only can curie quantities of certain radioisotopes (for example, cobalt 57 and iron 55) be produced in a single run, but specific activities may readily be obtained which far surpass those of reactor-produced isotopes. For example, cobalt 57 may be produced with a purity such that for every Io radioactive cobalt atoms, there is only one stable atom present. Iron 55 is produced with a specific activity between 3 and 10 times that available with present reactor techniques. The 86 inch cyclotron can also be used to achieve great versatility in radioisotope production. As already mentioned, over 250 stable isotopes are available as targets for proton bombardment. Many reaction types, e.g., p,n; p,2n; p,2p; p,d; p,y; and P,a; are possible. Facilities are provided for rapid removal of very short life material. The wide variety of neutron-deficient isotopes of considerable importance in research and in medical diagnosis which can be produced with high isotopic purity and specific activity underscores the usefulness of a high-current cyclotron. For example, the positron emitter arsenic 74 is regularly produced in 200 to 300 millicurie quantities and routinely supplied on a 3 week schedule adapted to its 17.5 day half life. A list of some of the cyclotron-produced TABLE V SOME RADIOISOTOPES ROUTINELY PRODUCED BY OAK RIDGE NATIONAL LABORATORY Isotope Beryllium 7 Sodium 22 Iron 55 Vanadium 48 Palladium 103 Promethium 150 Europium 147 Europium 146 Europium 149 Gold 195 Thallium 202 Bismuth 207 86 INCH CYCLOTRON" EC, electron capture. * Energy values may be found in various nuclear data tables which are conveniently listed in reference 19. radioisotopes routinely produced by Oak Ridge National Laboratory is given in Table v. PHYSICAL SEPARATION OF ISOTOPIC PRODUCTS The specific activity of a radioisotope product is limited by the residue of stable isotopes of the same element. Radioisotopes produced by neutron capture reactions include unreacted stable isotopes, e.g., cobalt 59 in cobalt 60. Fission products also contain stable isotopes produced during the fission process, either as direct fragments or as the end products of decay chains. Thus, iodine 131 with high specific activity cannot be made from long irradiated uranium because of the accumulation TABLE VI SOME REACTOR-PRODUCED SHORT LIVED Isotope Silicon 31 of long lived (16 million year) iodine 129 SPECIAL CATEGORIES BY PROPERTIES Yttrium 90 Potassium 42 Manganese 56 Copper 64 Nickel 65 Gallium. 72 Arsenic 76 Arsenic 77 В β, γ Radioisotopes may be classified in certain categories based on their physical Tunesten 187 properties. Thus, radioisotopes with short halflives have certain advantages or unique applications that make it worthwhile to consider their availability and potentialities as a group. Similarly, low energy radiation emitters or positron emitters may be considered as groups. Awareness of production progress in such special categories of radioisotopes may lead to employing them in broader applications. EC, electron canture. IC, internal conversion electrons. • Energy values may be found in various nuclear data tables, which are conveniently listed in reference 19. portation. Almost any point in the United States is less than one day's distance by air freight, thus a great variety of short lived radioisotopes can be shipped and used successfully. Table vi lists some moderately short lived reactor-produced, regularly available radioisotopes. The use of short lived isotopes in the medical field can be greatly extended by making them available in the user's laboratory. A considerable number of small research and isotope production reactors are now available in universities and research laboratories throughout the United States and other countries. This should permit more attention to local use of short lived isotopes. A special way of achieving local avail 1 NEWACHECK, R. L., BEAUFAIT, L. J., and ANDERSON, E. E. Nucleonics, 1957, 15, No. 5, 122–125. * GLEASON, G. I. Intern. J. Appl. Radiation and Isotopes, 1960, 8, 90. MAYER, W. J., and ANDERSON, R. L. Ind. Eng. Chem., 1960, 52, 993-994 * Reference 14, this article. 'MYERS, W. G. J. Nuclear Med., 1960, 1, 125. ability is to milk a short lived daughter activity from a longer lived parent in a fashion similar to the classical milking of radon from radium. There are some two dozen pairs of parent-daughter related isotopes, where the daughter's half life is shorter than that of the parent and the relative half lives make a milking system feasible. A few of these for which practical radioactive "cows" have been made are listed in Table VII. Three of these radioactive cows are routinely available from Brookhaven National Laboratory: iodine 132, technetium 99m, and yttrium 90.14 To obtain iodine 132 and technetium 99m, the parent tellurium and molybdenum activities are produced as fission products and are separated carrier-free from reactor-irradiated uranium. The parent materials are loaded on beds of alumina in a glass ion exchange column. After the iodine 132 reaches equilibrium in about 12 hours, it may be milked by pouring ammonium hydroxide solution through the column. The technetium 99m column is similar but because of the chemical form of the isotopic material, milking is done with dilute nitric acid. These two short lived products are uniquely suited to applications in the medical field. Iodine 132 has several advantages over the more familiar 8 day iodine 131. Because of its short half life (it 28 yr. 80 hr. decays by a factor of 1,000 in 24 hours), a series of tests may be made on the same subject on successive days. Its shorter half life may result in a total radiation dosage that is 1 to 2 per cent of that received from iodine 131 for the same diagnostic test. Technetium 99m should be a useful research tool; it combines a short half life and unique radiation characteristics. It decays by isomeric transition with the emission of a single gamma ray of about 140 kev. The absence of beta particles reduces radiation damage to biological systems. The yttrium 90 generator is somewhat similar but is specially designed to prevent contamination of the laboratory or of the yttrium 90 product with strontium 90. Milking produces yttrium 90 containing less than 10-6 per cent of strontium 90, thus the product is suitable for general use in clinical laboratories. Yttrium 90 has been of interest in the control of effusions in the pleural and peritoneal cavities and has been suggested for routine use as a follow-up after abdominal cancer surgery. It is a pure beta emitter, having no gamma radiation. Its beta radiation, with a maximum energy of 2.26 mev., is higher than that of either gold 198 or phosphorus 32 and therefore penetrates more deeply in the tissue. The half life of 2.7 days is considerably shorter than that of 14.3 day phosphorus 32. About 5,000 millicuries of yttrium 90 can be milked from a 100 millicurie strontium 90 generator in one year; hence it is potentially one of the cheapest radioisotopes available. Brookhaven National Laboratory also makes available two short lived isotopes, magnesium 28 and fluorine 18, of particular interest because they are produced by triton reactions in a nuclear reactor. Both can be cyclotron produced, but reactor production is advantageous when quantity production and low price are considerations. Magnesium 28 (21.4 hour half life) is produced by the Mg(t,p)Mg" reaction using tritons generated by the reaction Li‘(n,t)He*. The target material is an alloy of 75 per cent lithium 6 and 25 per cent magnesium by weight. At a thermal neutron flux of approximately 1X1013, specific activities ranging from 1 to 2 millicuries of magnesium 28 per gram of stable magnesium are produced by this method. Fluorine 18 (112 minute half life) is produced in a reactor by the reaction O1(t,n) F18 using tritons from the Li (n,t)He* reaction. It decays by positron emission and has the characteristic annihilation gamma rays associated with it. Although the fluorine 18 half life is short, it is the only isotope of fluorine sufficiently long lived to serve as a tracer. It may also find application in localization work using the annihilation gamma scanning technique. Iodine 133, with a 21 hour half life, is another short lived isotope now being supplied by Brookhaven National Laboratory. It is a fission product that decays by beta emission and associated gamma rays. Of all the available radioisotopes of iodine, it has the highest ratio of average beta energy to average gamma energy, which makes it useful where maximum local radiation dosage is desired with a minimum of whole body irradiation. Iodine 133 has also found medical application for double-tagging experiments in conjunction with iodine 131. Isotopes of even shorter life can be produced in the medical laboratory if a source of neutrons is available. Sources of thermal neutrons moderated by paraffin are sold commercially. More recently, acceleratortype neutron generators of convenient size have become available. Some are as small as a few feet in length by 4 inches in diameter. Completely portable, such a device gives up to 103 neutrons per second with an energy of 14 mev. Short lived isotopes that can be produced by these neutron generators include sodium 24 (15 hr.), magnesium 27 (9.5 min.), and aluminum 28 (2.3 min.). Cyclotrons, too, have been brought within the budget of the small college or research laboratory. One such commercially available cyclotron is only 3X3X5 feet in outside dimensions, yet can provide a 25 microampere beam of 2 mev. protons. Only 2 kilowatts input power is needed. With higher power and minor modifications, proton energies of 8 mev. and beam currents of I ma. can be attained. Such low cost machines provide a broad capability for production of short lived isotopes as well as permitting activation analysis and other nuclear techniques. LOW ENERGY GAMMA AND X RAY Radioisotopes that emit only one type of energy of radiation permit specific applications without complicating side effects. Thus, isotopes that emit only low energy gamma or x rays have special advantages for diagnosis or therapy, while giving little radiation dose to surrounding healthy tissue and reducing the radiation hazard to operating personnel. An example of a low energy emitter is palladium 103 which has been used for internal tumor therapy at the Argonne National Laboratory in a number of patients. Some substantial palliation resulted with no serious complications. Palladium 103 emits 20 kev. x rays and has a 17 day half life. In contrast to higher energy gamma emitters, it is easy to work with since a cumbersome lead syringe to protect the surgeon's hands is not necessary. The palladium is injected as palladium black in particles of 100 to 500 microns. Being |