JINR Reports (1989), Dubna, Russia
A History of the Discovery of Element 103 
G. N. Flerov, I. Zvara, G. M.Ter-Akopian, V. A. Shchegolev and V. L. Mikheev
FLNR, JINR, Dubna, Russia

    1. The studies of "lawrencium" carried out at the Lawrence Radiation Laboratory  

    During 1958-61 at Berkeley several attempts were made to synthesize element 103 by bombarding curium and californium targets by the ions of nitrogen and boron, respectively. By that time the HILAC linear accelerator was put into operation and produced beams of the above ions with reasonable intensities. The targets were made from Cm and Cf accumulated in reactors with enhanced thermal neutron fluxes. 

    It was planned to perform the synthesis in reactions of the type (HI,xn). Their small cross sections predetermined the difficulties in the identification of the new elements detected according to their α radiation. The background from α emitters in the Po-Ra region produced in Pb and 8i impurities which could imitate the α particles due to the new element was especially troublesome. This fact was mentioned in papers by the group engaged in the synthesis of element 102 in Moscow [Ml]. Therefore the target decontamination and the choice of the method of the identification of the new element was of crucial importance. 

    Chemical identification was not feasible because of expected very short lifetimes of the isotopes which could hopefully be synthesized. In principle, a most credible physical method is the one which permits the establishment of genetic relationship between the unknown nuclide and its daughter product characterized by well known radioactive properties by means of α-decay [2]. Ghiorso's group used this approach in their preceding attempt to synthesize element 102 [83]. Another complementary evidence which may have independent demonstrative power under certain conditions is the established correspondence between the yield of the observed nuclide versus the projectile energy dependence and the expected excitation function of the reaction postulated. This method can certainly be used in some cases, for example, if the target is monoisotopic and the reaction is of the evaporation type, (HI,xn). 

    These were, in our view, the material and ideological prerequisites of the experiments designed for synthesis of element 103 at Berkeley in the early sixties. 

    The chronology of the attempts to synthesize element 103 is as follows. The first experiments date back to 1958 [B3]. A target composed of 244Cm (95%) and 246Cm (4%) was bombarded by 14N ions. The nuclear reaction products knocked out of the target and stopped in a gas were collected on a catcher by the eleotrostatic deposition of the ionized atoms. The α-radiation of the collected products was detected by photoemulsions. A weak α-activity with an energy of 9+-1 MeV and a half-life of about O.25 s was observed. It was supposed to be due to the isotope 2561O3 from the 244Cm (14N,4n)256l03 reaction. To prove this statement it was necessary to carry but additional experiments but the authors [B3] have never communicated a continuation of that study. It was possibly explained by the radiation accident which took place - the destruction of a curium target in one of the experiments [B4].  

    In 1960 Ghiorso [85] reported the experiments carried out to synthesize the isotope 259103. A 252Cf target about 1 mug in mass was bombarded with 11B ions. The identification of 259103 was attempted by establishing its relationship with the known nuclide 255Fm in the following chain 259103 α--->255Md EC--->255Fm by the "double recoil method" [83] (see Section 3 about element 102). 255Fm was detected on the catcher foil for daughter products and its distribution along the catcher corresponded to T1/2=0.2 sec for 259103. However control experiments showed that 255Fm could appear as a result of the "leakage" onto the collector directly from the zone of irradiation of the primary atoms of 255Md, produced in the reaction 252Cf( 11B,α4n)255Md. That followed from the fact that the 255Fm counts did not disappear when the velocity of transport of primary atoms to the zone of collection of daughter products was decreased substantially. The leakage was confirmed by the detection on the collector of 256Md produced by the reaction 252Cf(11B,α3n)256Md and observed by recording the spontaneous fission of the daughter nuclide 256Fm. All this indicated that the experimental method was not perfect.  

    In the third attempt the Berkeley team reverted to the direct observation of α-decay of element 103 amongst reaction products. In those experiments they used surface-barrier spectrometric detectors with a resolution of 50 keV to record α-particles. In 1961 Ghiorso and coworkers published a paper [86] in which they claimed for priority in the discovery of element 103 and proposed to name it "lawrencium". They aimed to synthesize the element in the reactions Cf(B,xn)103 by using 11B and 10B ion beams. The 3 mug target had the following isotopic composition: 3% 249Cf, 33% 250Cf, 12% 251Cf, and 51% 252Cf. Special measures were taken to remove Pb and Bi  


Pages 1 2 3 4 5 6 7 8

To the Input Page