Heavy-element chemistry --Status and perspectives

In the past ten years, nuclear chemists have made considerable progress in developing fast on-line separation techniques, which allowed to chemically characterize the first four transactinide elements Rf (rutherfordium, Z = 104), Db (dubnium, Z = 105), Sg (seaborgium, Z = 106), and recently also Bh (bohrium, Z = 107). In all cases the isolated nuclides were unambiguously identified by observing genetically linked decay chains. Nuclides with production cross-sections of less than 100 pb and half-lives as short as a few seconds have been chemically isolated. Thus, chemists have discovered or significantly contributed to the characterization of the nuclear-decay properties of a number of transactinide nuclei. New techniques with greatly improved overall efficiencies should allow chemists to extend their studies to even heavier elements such as Hs (hassium, Z = 108) and to the recently discovered superheavy elements with Z = 112 and 114, which can be produced only with picobarn cross-sections. Received: 21 March 2002 / Accepted: 16 May 2002 / Published online: 31 October 2002 RID="a" ID="a"e-mail: andreas.tuerler@psi.ch     

Fine structure in the α-decay of radium isotopes with mass numbers 209-212

Radium isotopes with mass numbers from 209 to 212 have been produced by heavy-ion fusion reactions ²⁰⁴Pb( ¹²C, xn) ^216-Ra. Radioactive decay properties were investigated by means of α- and α-γ-spectroscopy after in-flight separation of the evaporation residues from the projectile beam by the velocity filter SHIP and implantation into a 16-strip position-sensitive Si detector. For the even-even nuclei ^210,212Ra we identified α transitions into the first-excited 2⁺-state of the daughter nuclei ^206,208Rn. Weak α transitions into excited levels of the odd-A daughter nuclei ^205,207Rn were also observed. Received: 19 August 2002 / Accepted: 6 November 2002 / Published online: 18 February 2003 RID="a" ID="a"e-mail: f.p.hessberger@gsi.de Communicated by D. Guerreau     

Synthesis of superheavy nuclei in the reactions of 244Pu and 248Cm with 48Ca

This paper presents results of the experiments aimed at producing long-lived superheavy elements located near the spherical-shell closures with Z ⩾ 114 and N ⩾ 172 in the ²⁴⁴Pu + ⁴⁸Ca and ²⁴⁸Cm + ⁴⁸Ca reactions. The large measured α-particle energies of the newly observed nuclei, together with the long decay times and spontaneous fission terminating the chains, offer evidence of the decay of nuclei with high atomic numbers. The decay properties of the synthesized nuclei are consistent with the consecutive α-decays originating from the parent nuclides ^{288, 289}114 and ²⁹²116, produced in the 3n and 4n evaporation channels with cross-sections of about a picobarn. The present observations can be considered as experimental evidence of the existence of the “island of stability” of superheavy elements. Received: 21 March 2002 / Accepted: 16 May 2002 / Published online: 31 October 2002 RID="a" ID="a"e-mail: utyonkov@sungns.jinr.ru     

Decay properties of neutron-deficient nuclei in the region Z = 86–92

Neutron deficient isotopes of elements Z = 86-92 have been produced by heavy-ion fusion reactions ¹²C + ²⁰⁸Pb, ²⁰⁹Bi, ²²Ne + ²⁰⁸Pb, ⁵¹V + ¹⁷⁰Er, and ⁵⁰Ti + ¹⁷⁰Er. The evaporation residues were investigated by means of α- and α-γ-spectroscopy after in-flight separation from the projectile beam by the velocity filter SHIP and implantation into a 16-strip position-sensitive Si-detector. New or improved decay data for ^{225, 226}U, ^{216, 217m, 218}Pa, ^{215, 216, 217}Th, ^{214, 215, 216, 216m}Ac, ²¹⁴Ra and ²¹³Rn have been obtained. Received: 5 April 2000 / Accepted: 28 July 2000     

Chemical properties of the transactinide elements studied in liquid phase with SISAK

This article starts with a review of the current SISAK liquid-liquid extraction system, as used after the physical preseparator BGS at LBNL for chemical studies of transactinide elements. Emphasis will be on new additions and developments. Then the possibilities offered by the new TASCA separator at GSI and the use of actinide targets at both GSI and LBNL are discussed with respect to future SISAK transactinide experiments. Finally, current and future liquid-liquid extraction systems for studying elements Rf up to Hs are discussed.     

New results on elements 111 and 112

Experiments on the synthesis and identification of the nuclei ²⁷²111 and ²⁷⁷112 were performed in order to confirm previous results. Three additional decay chains were measured in the reaction ⁶⁴Ni + ²⁰⁹Bi →²⁷³111^*. The study revealed considerably improved data on the decay chain originating from ²⁷²111. One additional chain was measured in the reaction ⁷⁰Zn + ²⁰⁸Pb →²⁷⁸112^*. The decay properties of the chain starting at ²⁷⁷112 are in excellent agreement with the second chain of the first experiment down to ²⁶⁵Sg, where the new chain ends by a previously unknown spontaneous-fission branch. A re-analysis of all the data on elements 110, 111, and 112 measured at GSI since 1994 (a total of 34 decay chains was investigated) revealed that for 2 chains (the second chain of ²⁶⁹110 measured in 1994 and the first chain of ²⁷⁷112 measured in 1996) the results of the new analysis differed from the previous one. In all other cases the earlier data are exactly reproduced. Received: 21 December 2001 / Accepted: 15 January 2002     

New calculations of α-decay half-lives by the Viola-Seaborg formula

The Viola-Seaborg formula is a well-known formula for α-decay half-lives of heavy nuclei. In this work we obtain new parameters of this formula through a least-square fit to even-even nuclei between Z = 84 and Z = 110 with N greater than 126. On average, the formula can reproduce the half-lives of heavy even-even nuclei within a factor of 1.3. The formula with new parameters works well for the superheavy region which is a hot topic of nuclear physics. The numerical results from the formula are compared with those from the cluster model.     

The new isotope 270110 and its decay products 266Hs and 262Sg

The even-even nucleus ²⁷⁰110 was synthesized using the reaction ⁶⁴Ni + ²⁰⁷Pb. A total of eight α-decay chains was measured during an irradiation time of seven days. Decay data were obtained for the ground-state and a high-spin K isomer. The new nuclei ²⁶⁶Hs and ²⁶²Sg were identified as daughter products after α-decay. Spontaneous fission of ²⁶²Sg terminates the decay chain. The measured data are in agreement with calculations using the macroscopic-microscopic model and with self-consistent HFB calculations with Skyrme-Sly4 interaction. Received: 27 November 2000 / Accepted: 16 January 2001     

Nuclear levels in 228Th populated in the decay of 228Pa (II)

The electron-capture decay of ²²⁸Pa to levels in ²²⁸Th was studied using mass-separated sources and a γ-ray detection system consisting of five Compton-suppressed Ge detectors. A total of 87 levels were observed up to an excitation energy of 2 MeV which are connected by approximately 500 γ-ray transitions. The complete octupole quadruplet, three excited K ^π=0⁺ bands and two K ^π=2⁺ bands were identified below 1.4 MeV. The observed level structure is compared to calculations within the quasiparticle-phonon nuclear model. The surprisingly good agreement indicates that ²²⁸Th has less transitional character than hitherto assumed. Received: 30 March 1998     

The new isotope 217U

The new neutron deficient isotope ²¹⁷U was produced in the bombardment of the ¹⁸²W target with ⁴⁰Ar ions and identified using a recoil-- correlation method. The -decay energy and the half-life of ²¹⁷U were determined to be 8005 ± 20 keV and 15.6 ^+21.3 _-5.7 ms, respectively. Received: 18 May 2000 / Accepted: 21 June 2000     

Behavior of intruder based states in light Bi and Tl isotopes: the study of 187Bi α decay

The excitation energies of the single-particle normal and intruder levels in both ¹⁸³Tl and ¹⁸⁷Bi were measured for the first time via the α decay of ¹⁸⁷Bi produced in the ⁹⁷Mo(⁹²Mo,pn)¹⁸⁷Bi reaction. The previously unobserved ¹⁸⁷Bi ground state (h_9/2) to ¹⁸³Tl ground state (s_1/2) α transition was identified, establishing the ¹⁸⁷Bi intruder state excitation energy to be 112(21) keV, 70 keV less than that of the same level in ¹⁸⁹Bi. Received: 10 September 1998 / Revised version: 18 December 1998     

Alpha-decay half-life of <Superscript>221</Superscript>Fr in different environments

A possible dependence of the alpha decay process on the solid-state environment of the decaying nucleus has been investigated by measuring the alpha-decay half-life of ²²¹Fr at room temperature in different materials ( Au, W, Si and polyethylene). The change in half-life between the insulator Si and the metals Au/ W is found to be 0.30(17)% / 0.42(21)%, respectivly. Furthermore an improved value of 4.79(2)min was found for the absolute half-life of ²²¹Fr.     

Experimental evidence on the ground-state energy of 229Pa

The E1 transition from the 3/2⁻ state to the ground state of ²²⁹Pa was identified in the (p,tγ) reaction. Its energy is found to be 11.6(3) keV. This leads to a value of Q_g.s.=−4133(2) keV and to a mass defect of ²²⁹Pa of 29894(3) keV. It is concluded that the ground-state configuration of ²²⁹Pa is 3/2[651]. Received: 29 January 1998     

A new alpha-particle-emitting isotope 259Db

An isotope of the element 105 with mass number 259 has been produced via the reaction ²⁴¹Am(²²Ne, 4n)²⁵⁹Db at E _lab = 118 MeV. The reaction products were transported and collected using the helium-jet technique and the rotating wheel apparatus. The α-decays of the products and their daughter nuclides were detected by a set of Si(Au) detectors arranged ingeniously. The Z and A of the nuclide have been unambiguously identified by the genetic relationship between the new activity and the known nuclide ²⁵⁵Lr established by α-recoiled milking measurement. The new nuclide ²⁵⁹Db has a half-life of 0.51±0.16 s and decays by alpha-particle emission of E _α = 9.47 MeV. Furthermore, the nuclide ²⁵⁸Db and its daughter ²⁵⁴Lr have also been clearly observed using the same projectile-target combination. Their half-lives and α-particle energies determined in this work are in agreement with previous known data, thus also proving the reliability of our assignment of ²⁵⁹Db. Received: 7 December 2000 / Accepted: 6 February 2001     

On the quest of production of superheavy nuclei in reactions of 48Ca with the heaviest cctinide targets

The sequence of radioactive decays of an unknown isotope produced in a rare fusion reaction to known lighter isotopes is used to identify mass and atomic number of the mother isotope, which has been separated before from the bulk of other reaction products by an in-flight recoil separator. By this technique the elements 107 to 112 were produced by single atom decay-chain analysis. Such a correlation technique reaches its limit by the occurrence of accidental sequences and it collapses beyond a maximum possible correlation time, at which a true event cannot be distinguished anymore from a random event. ⁴⁸Ca-induced fusion reactions with actinides are discussed. In 1983 at GSI, Darmstadt and LBL, Berkeley, ⁴⁸Ca/²⁴⁸Cm-experiments (II) were performed, which are compared to recent ⁴⁸Ca-experiments at FLNR-Dubna (I) irradiating ²⁴⁴Pu, ²⁴²Pu, and ²³⁸U. In these experiments production of isotopes of superheavy elements 112 and 114 is claimed. Our analysis of accidental sequences in ⁴⁸Ca-induced reactions is presented, which is at variance with the published analysis from FLNR-Dubna. We find that the maximum correlation time using continuous beams at today existing separation systems is not in the one-hour regime, but in the few-minute regime. The five spontaneous fission events observed in the FLNR experiments are preceded by signals in the (1–16)-minute range. These times are shown to be longer than the maximum possible correlation times. The preceding signals are decoupled from the spontaneous fission signal and carry no information on the spontaneous fission events observed. Moreover, random probabilities of 0.2 to 0.6 for the signals preceding the fission events indicate that the correlations are of random origin. The evidence to have discovered element 114 in the reported experiments is classified “very weak”. Received: 13 October 1999     

Synthesis and decay properties of superheavy atoms in nuclear reactions induced by stable and radioactive ion beams

This talk consists of two parts. The first one presents the results of investigations performed in 1998-2000 in Dubna on the synthesis of superheavy nuclei in reactions induced by ⁴⁸Ca ions. The radioactive decay properties of the nuclei, indicating a considerable increase in the α-decay and spontaneous fission half-lives of the isotopes of elements 110-116 when approaching the closed neutron shell at N = 184, are given. In the second part the possible ways of advancing into the region of more neutron-rich nuclei, using stable and radioactive ion beams, are discussed. Since so far no intense radioactive ion beams are available, some experiments with stable beams are considered as a test for the suggested ideas. Received: 1 May 2001 / Accepted: 4 December 2001     

Identification of the new isotope 241Bk

A new neutron-deficient berkelium isotope ²⁴¹Bk produced in the ²³⁹Pu(⁶Li, 4n) reaction has been identified using a gas-jet coupled on-line isotope separator. Cm K and L X-rays associated with the EC decay of ²⁴¹Bk were observed in the mass-241 fraction, and three γ transitions were attributed to the EC decay of ²⁴¹Bk through X-γ coincidences. The half-life of ²⁴¹Bk was determined to be 4.6±0.4 min which is 1/2-1/4 of that of theoretical predictions. The half-life value and the observed γ transitions can be consistently explained as a consequence of the allowed EC transition of π7/2⁺[633] → ν7/2⁺[624]. Received: 14 October 2002 / Accepted: 6 November 2002 / Published online: 17 January 2003 RID="a" ID="a"e-mail: asai@tandem.tokai.jaeri.go.jp Communicated by J. ?yst?     

Energy systematics of low-lying Nilsson levels in odd-mass einsteinium isotopes

The decay of odd-mass mendelevium isotopes ( A = 247-255) has been studied by means of α-γ spectroscopy. Strong evidence for a small α branch in the decay of ²⁵³Md was found. γ lines in coincidence with α-decays of ^{247, 249, 251, 253}Md have been observed for the first time. Levels in the einsteinium daughter nuclei were assigned on the basis of α-γ coincidence measurements. An energy systematics of the 7/2^-[514] Nilsson level could be established, showing a correlation with the deformation parameter β₂ of the corresponding nuclei. The results are compared with theoretical predictions     

Alpha-gamma decay studies of 255Rf, 251No and 247Fm

The decay of the isotopes ²⁵⁵Rf, ²⁵¹No and ²⁴⁷Fm produced in the reactions , and was investigated by means of α-γ spectroscopy. Previously observed γ transitions in coincidence with α decays of ²⁵⁵Rf were confirmed, their energies and line intensities were measured more precisely, and their multipolarities were determined as E1. In ²⁵¹No a new isomeric state at E ^* > 1700keV with a half-life of ≈ 2μs was identified. The decay of ²⁴⁷Fm was measured more precisely. A partial level scheme of the daughter nucleus ²⁴³Cf could be established.     

Isomeric decay of 67Fe --Evidence for deformation

Decay-spectroscopy study of the ^67mFe isomer has been performed at GANIL. This isomer is found to have an energy of 387 keV and a half-life of 75(21) μs. An intermediate excited state is introduced at 367 keV. The results are interpreted in terms of various nuclear models, and a deformed shape is inferred for ⁶⁷Fe. Received: 12 August 2002 / Accepted: 12 September 2002 / Published online: 17 January 2003 RID="a" ID="a"e-mail: sawicka@ganil.fr and sawicka@zsjlin.igf.fuw. edu.pl Communicated by W. Henning