VOLUME 6) NUMBER 9 PHYSICAL REVIEW LETTERS Mav l, 1961
NEW ELEMENT, LAWRENCIUM, ATOMIC NUMBER 103 Albert Ghiorso, Torbjgrn Sikkeland, Almon E. Larsh, and Robert M. Latimer Lawrence Radiation Laboratory, University of California, Berkeley, California (Received April 13, 1961)
Bombardments of californium with boron ions because heavy-ion bombardment of these elements have produced alpha-particle activity which can produces in high yield an alpha activity with an only be ascribed to decay of a new element with 8.8-Mev alpha-particle energy and a 25-second atomic number 103. This activity at best amounts half-life which can obscure the lower energy to only a few alpha counts per hour (@=1 micro- alpha activity of element 103. The heavy-ion barn), so it has not yet been possible to detect beam of either B' or B"was collimated so as the mendelevium decay product to prove further to pass through the tiny target and typically was the atomic number of the new activity. For the limited to 0.5 microampere dc to avoid melting present, attribution of this activity to element the target foil. The transmuted atoms recoiled number 103 must rest entirely on nuclear rather from the target into an atmosphere of helium. than chemical evidence. This gas flowed slowly through a nearby 0.050- The method used to produce and identify radi- inch orifice and carried the electrically charged ations from element 103 decay is shown schemat- transmutation products to a thin copper conveyor ically in Fig. 1 and is based on the one used for tape. This tape was periodically pulled a short the element 102 experiment. ' The 3-microgram distance to place the groups of collected atoms californium target has an isotopic composition to positions successively in front of each of five at present of 3.'3% 249, 32.8% 250, 12.3 'fo 251, solid-state Au-Si surface-barrier detectors. The and 50.8% 252. It was electroplated in an area pulses caused by passage of alpha particles into 0.10 inch in diameter onto nickel foil 50 micro- each detector were amplified, except for a few inches thick. The purification of the target, cru- milliseconds during the beam bursts, by sepa- cially important for the success of the experi- rate preamplifiers in the shielded bombardment ment, consisted in careful use of ion exchange area and then sent to a main counting area to be columns with specially purified reagents. The further amplified and analyzed. In the counting last step in the purging of undesirable lead and area, the pulses were passed through separate bismuth impurities was accomplished by heating window amplifiers and then analyzed by two the final target in vacuo by electron bombardment. separate electronic systems. One system con- Lead and bismuth impurities must be reduced sisted of five separate 100-channel pulse ana-
DEGRADER FOILS MULTIPLE D ETECTO R COL LIMATOR SYSTEM FARADAY CUP FOR SHORT-LIVED ALPHA EMITTERS BEAM ENERGY MEASURING SILICON DETECTOR RADE R FOILS
LI CON- GOLD ALPHA VACUUM DETECTOR S
PULLER HELIUM ATMOSPHE R IC BRAKE NO. 2 P RESSURE FIG. 1. Schematic diagram of apparatus. SUPPLY REEL COPPER TAPE SUCTION I" SIDE xO.5 MIL" THICK
TAK E-UP RE E L SUCTION
BRAKE NO. I VOLUME 6, NUMBER 9 PHYSICAL REVIEW LETTERS Max 1, 1961 lyzers, and the other consisted of a multiplex the detector assembly. Studies of the method unit using five punched paper tape storage units. with heavy-ion reactions were made by bom- With the multiplex unit, it was possible to de- barding Sm' ' to produce short-lived holmium termine the time when each event occurred, alpha emitters or Pb and Bi to produce various since the conveyor tape was last advanced. alpha emitters with energy between 7 and 9 Mev. The silicon crystal detectors vitally necessary In the bombardment of Cf with B ions, the for the experiment were brought to the authors' activity attributed to element 103 consists of attention by C. J. Borkowski and J. L. Blanken- alpha particles with an energy of 8.6 Mev de- ship of the Oak Ridge National Laboratory. With caying with a half-life of 8+ 2 seconds. Also their very kind help, it has been possible in our observed are alpha particles of 8.4 and 8.2 Mev laboratory to make silicon detectors suitable for with similar half-lives of about 15 seconds, these experiments. The present detectors are which are probably due to element 102. Figure 2 made of 800-1800 ohm-cm silicon, 6x10 mm, shows an alpha-particle spectrum from the first suitably etched and mounted, and are covered detector obtained during the most recent set of with a layer of gold about 20 micrograms per runs. These activities have been observed re- cm' thick. Initially, it was found that operation peatedly during many weeks of bombardment of of these detectors was very erratic because of the californium target with both B"and B" ions. the helium atmosphere and the very intense beta Similar bombardments of Pb, Bi, Pu", and Am'~' radiation fields. The use of charge-sensitive do not produce the new activities. preamplifiers reduced this effect, but it is still The mass number of the element 103 isotope is found that because of the circumstances of the thought to be 257 for the following reasons. B" it the crys- experiment, is necessary to replace bombardments of Cf y Cf y and Cf cause tal detectors occasionally. compound nucleus reactions which lead to 103'" The energy of the boron ions was changed by by the emission of 4, 5, or 6 neutrons, while degradation of the 10.3-Mev per nucleon beam with B"this same result is accomplished with in aluminum absorbers. Because the beam is 3, 4, or 5 neutrons. These are known from other so intensely concentrated, it was found that the experiments to be the most prominent neutron-out amount of energy loss in a given absorber could reactions of boron with the transuranium ele- change with time. To monitor the energy of the ments. Excitation functions with B"and B"ions ions striking the target, another Si detector was for production of the 8.6-Mev alpha activity are calibrated against nuclear emulsions, and used consistent with the above deduction. at a small solid angle to measure the energy of These excitation functions were, of necessity, those ions scattered forward at 20 by the Fara- very broad because the same activity could be day cup window. produced by several reactions; consequently, Calibration and study of the total system with- these data could not rule out conclusively (B,Pxn) out beam was accomplished with either U". ' re- reactions which would produce light isotopes of coil products collected onto the tape or with Po'" element 102. The final proof was then accom- alpha particles from samples held in front of plished by accentuating the element 102 produc-
5000— —50 ~ C, O 4000— 6.8 Mev (from Pb) -40 " 8.I Mev (from Pb) 0) CL 5000— —50 C FIG. Cf+B C 0 2. . Alpha spectrum from first de- o 2000 —RO tector. Summation of O 8.2 Mev Mev CL, 8.6 Runs Nos. 139-148. Total IOOO IO bombardment = 5. 0 pa hr. 8.8 Mev (fro m Pb) Spontaneous fissi ns Cycle time = 15 seconds. J IZL 00 IO 20 50 40 50 60 70 80 90 IOO
Cha nne I number
474 VOLUME 6, NUMBER 9 PHYSICAL REVIEW LETTERS MAv 1, 1961 tion by bombarding the californium target with gineering and maintenance personnel for their C" ions. It was found that the 8.6-Mev activity patient production of the many hundreds of hours was decreased by more than a factor of 2, and of boron beam operation; to C. Corum for his the 8.2-Mev activity (thought to be mostly 102"') always ingenious mechanical designs which were was increased by a factor of about 20. This was the backbone of the experiment; to S. G. Thomp- to be expected for the element assignments given. son, L. Phillips, R. Gatti, F. McCarthy, and Experiments with Pu'40 had shown that the (C", T. Parsons for their separation of the californium axn) cross sections would be larger, while the from the "napkin-ring" material; to R. Garrett (C",pxn) cross sections would be smaller in for her continuous capable assistance; to A. Wyd- comparison with the boron bombardments of ler for his very able multiplex design and con- californium. Possible light isotopes of mende- struction; to %. Goldsworthy for his competent levium that could be produced and conceivably amplifier designs; to W. Stockton for his indi. s- might emit alpha particles in the 8.2-8.6 Mev pensable silicon detectors; and to B. Isaacs region were ruled out by bombardments of Am'4' and S. Hargis for their very good-natured help with C'2 ions. in the data processing. Special acknowledgment In honor of the late Ernest O. Lawrence, we is due to the Health Chemistry Department under respectfully suggest that the new element be P. Howe for its skillful handling of a very diffi- named lawrencium with the symbol Lw. cult radioactivity protection problem. We are The element 103 experiment has been in the also indebted to F. Asaro, I. Perlman, and G. T. process of development for almost three years, Seaborg for helpful discussions regarding inter- and its successful culmination has been due in no pretation of these experiments. small part to the help of all of our colleagues. We would like to express our appreciation par- for his ticularly to the following: to F. Grobelch ~A. Ghiorso, T. Sikkeland, J. R. Walton, and G. T. tireless assistance in innumerable ways, both Seaborg, Phys. Rev. Letters 1, 18 (1958). with the accelerator and the experimental equip- We are indebted to M. C. Michel for the mass ment; to J. Gavin, the HILAC operators, its en- analysis.
TWO- PHOTON DE-EXCITATION OF THE 0+ LEVEL IN Zr" Hans Ryde, Pedro Thieberger, and Torsten Alvager~ Nobel Institute of Physics, Stockholm, Sweden (Received February 21, 1961; revised manuscript received March 27, 1961) Recently great interest has been focused on transition has to be chosen in which the single- the second-order radiation process in which quantum emission is forbidden or strongly the de-excitation of a nuclear level takes place hindered. In the decay of the 1.73-Mev 0+ level by the simultaneous emission of two photons, to the 0+ ground state in Zr all electromagnetic two conversion electrons, or one photon and one radiation is, to the first order, absolutely for- conversion electron, in competition with a single- bidden. The only modes of decay are then in- quantum transition. The effect has been theoret- ternal conversion (EO transition), internal pair ically treated by various authors' ' and experi- formation, or two-photon de-excitation. The mentally studied by several workers. ' " These radioactive decay of the Y" nucleus by beta processes are described as taking place via emission to levels in Zr" with a half-life of virtually reached, intermediate levels of higher 64 hours has been thoroughly studied by, among excitation energy and a continuous energy distri- others, Yuasa et aL" They report that the bution of the emitted radiation is expected. The main beta transition to the ground state of Zre total energy of each pair of quanta is the same has a maximum energy of 2.26 Mev and that a as the energy available for the single-quantum weak beta component feeds the 1.73-Mev level transition. with a relative intensity of 2.2 x10 4. The in- To study the two-photon de-excitation experi- ternal pair formation in the decay of this level mentally under favorable circumstances, a takes place with a relative intensity of about
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