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RADIOACTIVITY of PROMETHEUM DISSERTATION Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosoph

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RADIOACTIVITY of PROMETHEUM DISSERTATION Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosoph RADIOACTIVITY OF PROMETHEUM DISSERTATION Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in the Graduate School of The Ohio State University By John K‘. ^ngj B. S. The Ohio State University 1963 Approved by* ACKNCWIEDGEMENTS I wiah to acknowledge the assistance of my adviser. Professor M. L. Pool, for his guidance and encouragement in the course of this work. I also wish to acknowledge the cooperation of ÎÆr. Paul Weiler and other numbers of the staff for their operation of the cyclotron. i i CONTENTS Page Introduction ...........................................................................................................................1 Apparatus .............................................................. .7 Half-lives ...................... ..................12 The 2,7-hour activity The 5.3-day activity The 40-day activity Activities of the order of one year Long activities Radiations ......................... 32 Radiations of the 2.7-hour activity Radiations of the 5.3-day activity Radiations of the 40-day activity Characteristics of the 1-year activities Characteristics of the 3.5-year activity Discussion .......................... 44 i i i INTRODUCTION Prometheum is element number 61. In the table of the elements, prometheum stands between the rare earths neodymium, number 60, and samarium, number 62. Persistent efforts to discover traces of pro­ metheum in association with other rare-earths in nature have failed. It is concluded that the element does not occur in nature (l), and all of its isotopes are unstable. When the cyclotron became available as a tool for the transmu­ tation of isotopes, attempts were made to produce and identify isotopes of prometheum. One of the most successful of the early attempts occurred at Ohio State University (2). This early investi- 148 gation reported some charaoteristies of the 5.3-day Pm activity which have not required revision since that time. Other prometheum activities which were established within the next few years included the 2.3-year Pm^^^ (S) and the 2-day Pm^^^ (4). Several reports on Pm^^^ were also made, but these showed some disagreement and rather inconclusive identification of the isotope involved. Wu and Segre (5) reported a 200-day activity resulting 141 from the bombardment of Pr withc(-partieles. Postulating that an (*t,2n) reaction would be most likely to proceed under the conditions 143 of their experiment, they attributed the new activity to Pm . Wilkinson and Hicks (6) performed a similar experiment and used ion- exchange chemistry to confirm that the observed activity belong to prometheum. They measured it as 285-days, however. It must be observed that the masses of the rare-earth isotopes in this region 2 are not known with enough precision to predict reaction thresholds. The mass-assignment of the (200-285)-day activity therefore remained in doubt. This was all of the reasonably reliable information available on the pranetheum isotopes at the start of the present investigation. It is summarized in the section of the nuclear chart shown in Figure 1. In addition, a great deal of misinformation about the prometheum isotopes had been published. The contradictory reports are noted in the article by Marinsky, Glendennin, and Coryell (4), These authors pointed out that some 15 activities had been reported in the prome­ theum region, most of them unconfirmed, and many of them oontradiototy. The early investigators were confronted with a situation compli­ cated by nature . Neodymium, the most useful element for the production of prometheum, has seven naturally occurring isotopes. The natural abundances of these isotopes are given in Figure 1, All of the abundances are greater than and less than ZOf»» If a prometheum activity is formed from bombardment of the natural mixture of neodym­ ium isotopes, the abundances give no clue concerning which isotope might be the parent. None of the isotopes can be said to predominate in the natural mixture, and on the other hand, none can be said to be present in a negligibly small amount. In some sections of the periodic table, the masses, or at least relative masses of the various isotopes are known with extreme pre­ cision. In these regions, the threshold energy requirements for given nuclear reactions can be accurately computed. This is often an aid in 142 145 W4 145 146 147 148 149 150 6 0 Nd 12.2 25.9 17.2 50d 5.5d 6 1 Pm .23 62 Sm 3.2 Fi[ure 1. I'uclear chart in prometheum region, 19^0 M IÉ 4 the identification of and mass assignment of new activities:. Such is not the case in the neodymium and samarium region. Mass spectrographs are not capable of measuring masses with the degree of precision necessary for predicting reaction thresholds in this region. Chemical purification of neodymium is a difficult task. The end result is never entirely free of the other rare earths. When chemi­ cally purified neodymium is used as a starting material, stray activities due to impurities invariably are formed. In view of the above difficulties it is not surprising that so few prometheum activities were accurately known in 1950, In the spring of 1951, however, enrichments of the neodymium isotopes became avail­ able from the AEG in quantities sufficient for cyclotron research. These enrichments are made on the large mass-spectrographs known as Calutrons, The AEG furnishes the isotopic analysis and a spoctro- graphic analysis with each enrichment. The materials are of a chemical purity previously unavailable in chemically separated neodymium. Each enrichment features one of the isotopes of neodymium in high abundance, but the enrichments also contain small amounts of the other isotopes. Samples of each enrichment were loaned to this laboratory for research. The analyses of these samples as given by the ABC are shown in Table 1, These analyses made it appear that the enriched samples would furnish the key to the prometheum activities, A research progreun was designed with the purpose of verifying the reported prometheum activities and adding as much as possible to the nuclear data on pro­ metheum, The proposed program was to consist of 7-Mev proton bombardments on each of the enriched samples as well as the natural Table 1 Analyses of Neodymium Samples Per oent Principal Natural Isotope 142 143 144 145 146 148 150 Mixture Ingredient 93.0 4.04 0.7 1.2 0.6 1.2 1.1 27.1 3.2 83.9 0.94 0.8 0.3 0.6 0.5 12.2 ::: 2.9 8.6 93.4 4.8 0.9 1.3 1.0 23.9 Nd^^® 0.4 1.8 2.3 78.6 1.0 0.7 0.4 8.3 Ndl^ 0.4 1.2 2.4 13.7 95.6 2.5 1.0 17.2 NdjJ® 0.08 0.15 0.15 0,6 1.4 89.8 1.2 5.7 Nd^®° 0.07 0.11 0.07 0.2 0.2 3.9 94.8 5.6 La 0.15 0.15 0.15 0.15 0.15 -- Mg 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 Ni 0.08 0.08 0.08 0.08 0.08 - Pt 0.04 - -- - -— Sm -- 0.31 0.31 0.31 - - Ba ---- - 0.04 - Fe - -- - - 0.08 - Ho - -- - - 0.31 - Mn - -- - - 0.04 - Si —--——— 0.08 mixture. By comparison with other elements in the same region, it was expected that 7-Mev protons would probably be sufficient to bring about (p,n), (p,Y)» possibly, (p,oC) reactions. The (p,zn) reaction would not be expected at all with energies less than 8.5-Mev and usually is not prominent in rare earth bombardments unless 10 or 11- Mev protons are used. The 7-Me? protons should therefore produce a small number of new isotopes which should be identifiable from the isotopio analyses of the starting material. Short bombardments (10 minutes) would be tried first in order to find short (20 second - 2 hour) activities. Longer bombardments (3 - 10 hour) were then to be tried for the study of activities longer 6 than 2-hours. It was planned that these proton bombardments on neodymium could then be followed by whatever other bombardments ( oi-particles, neutrons, etc. on Ed, Pr, Sm) were indicated as cross checks of the results. In general this scheme has been carried out. Some of the activities found have been followed for three years and are s till being followed. APPARATUS The hombardmen'ts were performed on the Ohio State University cyclotron. This instrument has a 42-inoh magnet pole face diameter. It will accelerate protons to slightly over 7-Mev at about 13.8 megacycles. Counting equipment included shielded G-M tubes, both the fh -sensitive (Tracerlab TGC-2) and x-ray sensitive (Tracerlab TGO-3) types. The long half-lives were followed on a Wulf electrometer as long as their activity was high enough. This is an extremely stable instrument over long periods of time. It therefore served as a valu­ able check on the more variable G-M tubes y^en following long half- lives. The magnetic ^-ray spectrometer used in the study of has been described by Sooville (12). It is a single focusing, 180° deflec­ tion instrument with a battery powered magnet. Its use is limited to samples with a rather high activity. A scintillation spectrometer employing a sodium iodide crystal was 146. used for the measurement of the -ray energy of Rn * In this in­ strument, a scintillation caused by the passage of a y -ray through a crystal is amplified by a photo-multiplier tube. The pulse from the photo-multiplier is transmitted to an electronic gating circuit which rejects all pulses except those within a narrow band.
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