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New Element Mendelevium, Atomic Number

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New Element Mendelevium, Atomic Number LETTE RS TO TH E EDITOR the 1.75-Mev level arises from the excitation of two various fractions were then examined with various protons to a (got&)' configiration, one may expect some types of counters. additional forbiddenness. In such a transition, the 39th The earliest experiments were con6ned to looking proton of Y" must change from a p~t2 to a gsts state, in for short-lived alpha-emitting isotopes of element 101 addition to the ordinary single particle transformation such as 101"' and 101'" that would be expected from of a de~ neutron into a g9~2 proton. For this reason, a (n, 2rt) and (cr,4rt) reactions. For this purpose it was reduced intensity for the transition to the 1.75-Mev sufficient to look quickly in the gross actinide fraction level may be expected. as separated by the anion resin column alone. No alpha Very precise measurements on the beta spectrum of activity was observed that could be attributed to ele- Y" gave no indication of any other group of beta rays ment 101 even when the time between the end of between 0.5 Mev and the end point at 2.26 Mev. This bombardment and the beginning of alpha pulse analysis fact, combined with the absence of any gamma radia- had been reduced to 5 minutes. To ascertain that a tion (other than bremsstrahlung) suggests that the proper recoil and chemical yield was being obtained, a 1.75-Mev level is the 6rst excited state of the even-even small amount of Cm'" was included in the target so nucleus Zr". that the Cf'" and Cf'" produced by the helium ion beam would serve I' This work was supported by the joint program of the Office as a monitor. of Naval Research and the U. S. Atomic Energy Commission, and Longer bombardments were then made, and a few by a grant from the Research Corporation. spontaneous fission events were observed over a period ' Kenneth W. Ford, preceding Letter LPhys. Rev. 98, 1516 (1955)j. We are indebted to Professor Ford for stimulating dis- of several hours in the actinide fractions. At this time cussions of the theoretical aspects of the problem. the cation resin column procedure was then added to the chemistry so that we could distinguish between elements 101 and 100. The initial experiments were New Element Mendelevium, confined to 6ssion counting of the trms-100 and the 100 fractions. Atomic Number 101* Several experiments were consistent in their demonstration of spontaneous fissions occurring in A. GHIoRsop 3 G HARvEYy G. R. CHoPPrN, both fractions. To determine more precisely the elution S. G. THQMPsoN, AND G. T. SKABoRG position of the element responsible for the trans-100 Radiation Laboratory and DePartment of Chemistry, spontaneous 6ssion activity a more elaborate experi- Umfoers~ty of Cal~forlr'a, Berkeley, Califorrlfa ment was undertaken. Three successive three-hour (Received April 18, 1955) bombardments were made and in turn their transmuta- tion products were separated quickly on a cation resin E have produced and chemically identi6ed for column. The isotope 99'" was added in each case so the 6rst time a few atoms of the element with that together with the Cf"' produced it would help to atomic number 101.Very intense helium ion bombard- calibrate the 99'" column run. Five spontaneous fission ments of tiny targets of have produced a few counters were then used to count simultaneously the 6ssionable atoms which in spontaneously elute the corresponding drops of eluent from the three runs. The gku-thulium position on a cation resin column. combined and normalized elution curve is shown in The method of production utilized the following Fig. 1. That the drops did not correspond exactly in techniques. In a special position in the Crocker Labora- each run is indicated on the spontaneous 6ssion histo- tory 60-inch cyclotron a very concentrated collimated gram the of the relative beam of 48-Mev helium ions (as much as 10 micro- by overlapping drop number limits of the activities that were placed in each counter. amperes in an area —s', &&4t inch) was allowed to pass combining the through a degrading absorber and then through a By data from all experiments, the half- 2-mil gold foil (yielding 41-Mev helium ions). On the life of the spontaneous 6ssion activity in the 101 and back. side of the gold foil, approximately 10' atoms' of the 20-day 99' 3 were electroplated in the beam area. From this target the nuclear transmutation recoils 246 ~~253 Cf C) were ejected in a narrow spray and caught on 0.1-mil LLI O gold foil adjacent to the target. The gold foil was IOO (t) B- 8 quickly dissolved in aqua regia, the gold extracted tL with ethyl acetate, and the aqueous phase eluted IOI 5 through a Dowex-1 anion resin column with 63f HC1 o 4- to complete the removal of gold and other impurities. 2 The drops containing the actinide fraction were I I evaporated and the activity was then eluted through a 5k' . , O. IO I5 20 25 50 55 40 45 Dowex-50 resin cation column with ammonium n- ELUTION DROP NUMBER (COLUMN VOL. SUBTRACTED) hydroxy-isobutyrate' to separate the various actinide Fxe 1 Elution of elements 98-101 from Dowex-50 elements from each other. The radiations from the column with ammonium a-hydroxy-isobutyrate. LETTERS TO TH E E D I TOR 1519 70- 3. By this method of production, it would not be 60- possible to produce an element above 101. 50- We would like to suggest the name mendelevium, symbol Mv, for the new element in recognition of the ~ 30 pioneering role of the great Russian chemist, Dmitri Mendeleev, who was the first to use the periodic system 20 co of the elements to predict the chemical properties of O undiscovered elements, a principle which has been the O key to the discovery of the last seven transuranium 2 IO (actinide) elements. LJ 8 We should like to acknowledge the fact that this Q 7 LLi work was greatly facilitated by the reshaping of the 5 magnetic field of the Crocker Laboratory 60-inch cyclo- tron so as to secure a more concentrated ion beam. This modification was successfully carried out under ) 3 K the expert supervision of G. B. Rossi. We should like to express our appreciation to C. A. Corum for his (A 2 expert collaboration in the design of the special de- C) I-a Qector channel probe used in these bombardments. To G. B. Rossi, W. B. Jones, and the crew of the 60-inch I 0 400 800 I200 cyclotron, we extend our thank. for their patience and , MINUTES AFTER END OF BOMBARDMENT careful manipulation of the machine which made the FIG. 2. Spontaneous Gssion decay curve of element 100 bombardments successful. We should like to thank and element 101 fractions. T. O. Parsons, R. Silva, and A. Chetham-Strode for their considerable assistance. We should also like to 100 fractions was found to be approximately the same, express our appreciation to Professor J. G. Hamilton, perhaps 3 to 4 hours as shown in Fig. 2. director of the Crocker Laboratory, for his help and Obviously there is insufficient evidence to define the and cooperation and to Professor E. O. Lawrence for mass numbers concerned in this experiment. However, his continued interest in this program of research. recent experimental observations of the spontaneous *This work was performed under the auspices of the U. S. 6ssion decay rates' of odd-nucleon types indicate that Atomic Energy Commission. 'The 99'" they are hindered factors of thousands. we then target atoms were made by intense neutron bom- by If bardment of Cf2~2 and the subsequent beta decay of Cf'". We apply this principle to this experiment it is tempting would like to thank the personnel of the Materials Testing Re- to draw the following conclusions. By an (n, rt) reaction actor in Idaho for their very expeditious handling of this bom- 99'" 101"' bardment. on we have produced the isotope which s Choppin, Harvey, and Thompson (unpublished information). decays by electron capture with a half-life of the order '3 Unpublished data in this laboratory. of a half hour to 100"'; this isotope then decays by Choppin, Harvey, Thorn]~son, and Ghiorso, following Letter I Phys. Rev. 98, 1519 (1955)J. spontaneous fission with a half-life of the order of 3 to 4 hours. (Since no alpha activity is observed we can rule out the 3.2-hour isotope 100'" as being responsible s for the spontaneous fission activity on the basis of its Nuclear Properties of 100" known alpha-to-fission ratio of 1550.) In a total of G. R. CHorriN, B.G. HARvzv, S. G. THoMPsoN, and A. GHroRso eight separate experiments we have isolated only 17 Radiation Laboratory, UNioersity of Catiforrtia, atoms of 101 so it has not yet been possible to demon- Berkeley, CaHforniu strate this proposed genetic relationship between 101"' (Received April 18, 1955) and 100"'. However, another experiment' has shown shown that 100"' does have a spontaneous 6ssion half- HE nuclide 100"' ha, s been made by neutron life of about 3 hours. irradiation of 99"' in the Materials Testing The proof that these experiments resulted in the Reactor. One purpose of this experiment was to deter- identification of element 101 is as follows: mine the most probable mass assignment of the isotope of element 101 which has been produced recently. ' the elements 1. Only very heaviest decay by spon- Since the irradiated sample had been made earlier taneous fission with such short half-lives.
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