The β-delayed one- and two-proton emission of 27S

In an experiment performed at the GANIL LISE3 facility, radioactive ²⁷S isotopes have been produced by projectile fragmentation of a 95 AMeV ³⁶Ar primary beam. After selection by means of the LISE3 separator, the isotope of interest was implanted in a silicon-detector telescope where its half-life ( T _1/2 = 15.5(15) ms) and its main decay branches were measured. Received: 19 October 2001 / Accepted: 5 December 2001     

The β-decay of 22Al

In an experiment performed at the LISE3 facility of GANIL, we studied the decay of ²²Al produced by the fragmentation of a ³⁶Ar primary beam. A β-decay half-life of T_1/2 = 91.1±0.5ms was measured. The β-delayed one- and two-proton emission as well as β-α and β-delayed γ-decays were measured and allowed us to establish a partial decay scheme for this nucleus. New levels were determined in the daughter nucleus ²²Mg. The comparison with model calculations strongly favours a spin-parity of I^π = 4⁺ for the ground state of ²²Al.     

High-resolution in-beam particle spectroscopy --New results on prompt proton emission from 58Cu

Prompt proton decay lines in ⁵⁸Cu have been studied by means of high-resolution in-beam particle-γ coincidence spectroscopy using the GAMMASPHERE Ge-detector array in conjunction with a dedicated set of ancillary detectors including four ΔE-E silicon-strip telescopes. High-spin states in ⁵⁸Cu have been populated via the heavy-ion fusion-evaporation reaction ²⁸Si(³⁶Ar, 1α1p1n) at 148 MeV beam energy. The full-width at half maximum for the proton peak could be reduced significantly compared to earlier experiments. The results indicate that only one prompt proton decay branch exists in the decay-out of the well-deformed band of ⁵⁸Cu. Received: 11 March 2002 / Accepted: 10 April 2002     

New results on proton emission from odd-odd nuclei

⁷⁸Kr beams were used to bombard ⁵⁸Ni, ⁹²Mo and ⁹⁶Ru, producing three new, odd-odd, proton-emitting isotopes, ¹³⁰Eu, ^164mIr and ¹⁷⁰Au. Preliminary experimental and theoretical results will be presented in this paper. Received: 21 March 2002 / Accepted: 16 May 2002 / Published online: 31 October 2002 RID="a" ID="a"e-mail: mahmud@ph.ed.ac.uk     

Systematics of half-lives for proton radioactivity

Half-life measurements for both ground-state and isomeric transitions in proton radioactivity are systematized by using a semiempirical, one-parameter model based on tunneling through a potential barrier, where the centrifugal and overlapping effects are taken into account within the spherical nucleus approximation. This approach, which has been successfully applied to alpha decay cases covering ∼ 30 orders of magnitude in half-life, has shown, in addition, very adequate at fitting all existing data on partial half-life, T _1/2p , of proton emission from nuclei. Nearly 70 measured half-life values have been analysed, and the data could be described by two straight lines relating the pure Coulomb contribution to half-life with the quantity (Z _d is the atomic number of the daughter nucleus, μ₀ is the reduced mass, and Q _p is the total nuclear energy available for decay). These straight lines are shown to correspond to different degrees of deformation, namely, very prolate ( δ ≳ 0.1 , and other shaped ( δ ≲ 0.1 parent nuclei. The goodness in reproducing the data attained in the present systematics allows for half-life predictions for a few possible cases of proton radioactivity not yet experimentally accessed.     

First evidence for the two-proton decay of 45Fe

Decays of five ⁴⁵Fe atoms have been observed at the fragment separator of GSI. Whereas one event is consistent with the β-decay of ⁴⁵Fe accompanied by the emission of a 10 MeV proton, four decays are consistent with the emission of particle(s) of total energy 1.1±0.1 MeV. This observation represents the first evidence for two-proton ground-state radioactivity, a decay mode predicted for ⁴⁵Fe. The time distribution of the observed decay events corresponds to a half-life of 3.2^+2.6 _-1.0 ms. Received: 17 May 2002 / Accepted: 28 May 2002     

<Superscript>18</Superscript>Ne diproton decay

Two proton radioactivity studies have been performed on excited states of ¹⁸Ne produced, among other fragments, by ²⁰Ne projectile fragmentation and excited via Coulomb excitation on a Pb target. Every incoming ion was tagged before interacting with the lead target on an event by event basis in order to discriminate the secondary reactions according to the projectile. Decay of ¹⁸Ne levels has been studied by complete kinematical reconstruction. In spite of the low statistics a couple of events looks very promising for two proton correlated emission.     

Decay studies of N ≈ Z nuclei from 75Sr to 102Sn

Neutron deficient nuclei near ¹⁰⁰Sn have been produced by fragmentation of a 1 ^. AGeV ¹¹²Sn beam. The fragments were separated, identified and stopped in a highly segmented silicon strip detector stack. This detector measured the total energy of emitted β⁺-particles. γ-radiation was measured with surrounding detectors. The half-lives for many nuclides have been determined for the first time and give important information for the following topics: For the heaviest particle-stable odd-odd nuclei ⁹⁰Rh, ⁹⁴Ag and ⁹⁸In we observed for the first time fast β-decays, compatible with superallowed Fermi transitions and confirmed such decays for ⁷⁸Y, ⁸²Nb and ⁸⁶Tc. We have also observed long-lived T = 0 states in some of these nuclei. We measured the half-lives of all rp-process waiting-point nuclei from ⁸⁰Zr up to ^{92, 93}Pd. In addition we find the proton drip line nucleus ⁷⁷Y to decay dominantly via β-decay. To study the Gamov-Teller strength in the β-decay near the doubly magic ¹⁰⁰Sn we measured the half-life, β- and γ-spectrum of ¹⁰²Sn. We propose a level scheme for the daughter nuclide ¹⁰²In and deduce the Gamov-Teller strength (B _GT = 4.0±0.6). This is one of the largest values known. Received: 21 March 2002 / Accepted: 16 May 2002 / Published online: 31 October 2002 RID="a" ID="a"e-mail: thomas.faestermann@ph.tum.de     

Systematics of high-spin isomers in N = 83 isotones and a high-spin isomer beam

Isomers in N = 83 isotones of Z = 60-66, were studied systematically. Their spins and parities are 49/2⁺ and 27⁺ for odd and odd-odd nuclei, respectively. Nearly constant excitation energies of these isomers indicated a decrease of a Z = 64 shell gap energy as Z decreases from 64 to 60 within the framework of a deformed independent-particle model (DIPM). Their configurations are [ν(f _7/2 h _9/2 i _13/2),π(h _11/2)²]_49/2+ and [ν(f _7/2 h _9/2 i _13/2),π(h _11/2)²(d _5/2)^-1]₂₇₊ for odd and odd-odd nuclei, respectively. The shape of the yrast states changes suddenly at spin 49/2(odd) and 27(odd-odd) from a near spherical to an oblate shape. Transitions from isomers are highly hindered because of the shape changes. They may be categorized to be shape isomers. The development of a secondary beam produced by using these high-spin isomers is also described. Received: 1 May 2001 / Accepted: 4 December 2001     

Search for molecular bands in 13C

In a systematic search through experimental results for transfer reactions, inelastic excitations and other data for ¹³C we identify states which have no simple structure related to the low-lying states of ¹²C. We propose that these states have strong α-clustering and that they are related to chain states expected in ^13-14C. The properties of these states are discussed on the basis of a molecular orbital model. Received: 12 December 2001 / Accepted: 3 April 2002     

Molecular-orbital structure in neutron-rich Be and C isotopes

The structure of Be and C isotopes are investigated based on the molecular-orbit (MO) model. The low-lying states are characterized by several configurations of valence neutrons, which are constructed as combinations of basic orbits. In ¹⁰Be, all of the observed positive-parity bands and the negative-parity bands are described within the model. The second 0⁺ state of ¹⁰Be has a large α-α cluster structure, and this is characterized by a (1/2⁺ _σ)² configuration. An enlargement of the α-α distance due to two-valence neutrons along the α-α axis makes their wave function smooth and reduces the kinetic energy drastically. Furthermore, the contribution of the spin-orbit interaction due to coupling between the S _z = 0 and the S _z = 1 configurations, is important. In the ground state of ¹²Be, the calculated energy exhibits similar characteristics, that the remarkable α clustering and the contribution of the spin-orbit interaction make the binding of the state with (3/2^- _π)²(1/2⁺ _σ)² configuration properly stronger in comparison with the closed p-shell (3/2^- _π)²(1/2^- _π)² configuration. This is related to the breaking of the N = 8 (closed p-shell) neutron magic number. Also, the molecule-like structure of the C isotopes is investigated using a microscopic α+α+α+n+n+ ^{. . .} model. The combination of the valence neutrons in the π- and the σ-orbit is promising to stabilize the linear-chain state against the breathing and bending modes, and it is found that the excited states of ¹⁶C are the most promising candidates for such structure. Received: 1 May 2001 / Accepted: 4 December 2001     

Spectroscopy of nuclei far from stability at GANIL: Recent experiments

The structure of neutron-rich light nuclei around N = 20, 28 has been investigated at GANIL by means of in-beam gamma spectroscopy using fragmentation reactions of ³⁶S and ⁴⁸Ca beams on a Be target. Gamma decay of relatively high-lying excited states have been measured for the first time in nuclei around ³²Mg and ⁴⁴S. Level schemes are proposed and discussed for a large number of these neutron-rich nuclei around N = 20 and N = 28. Received: 1 May 2001 / Accepted: 4 December 2001     

Covalently bound molecular structures in the α + 16O system

Using the concept of covalent molecular orbitals for neutrons and the known properties of the local α + ¹⁶O potential the formation of asymmetric molecular structures in neon isotopes is discussed. Experimental evidence for parity doublets in ²¹Ne is reviewed and a corresponding band structure for the states in ²¹Ne at moderate excitation energy of E _x = 0-8 MeV is proposed. The structure of some bands can be interpreted as consisting of an instrinsic asymmetric ( ⁴He + ¹⁶O) structure bound by a covalent neutron in σ and π orbitals. An extension of the observed structures to symmetric molecular structures in isotopes of Mg and heavier nuclei is suggested. In particular shape isomers in isotopes of magnesium, namely (He)₂O molecules, can be predicted and an extended Ikeda diagram is proposed. Received: 11 July 2001 / Accepted: 22 August 2001     

Three-body decays of light nuclei: 6Be, 8Li, 9Be, 12O, 16Ne, and 17Ne

The theoretical approach to the two-proton radioactivity and three-body decays developed in (L.V. Grigorenko, R.C. Johnson, I.G. Mukha, I.J. Thompson, M.V. Zhukov, Phys. Rev. Lett. 85, 22 (2000) and to be published in Phys. Rev. C) is applied to the range of light nuclear systems. We study nuclear structures, widths, and momentum correlations for the decay fragments. Strong contradictions with experiment, as well as effects of special interest, are found in ¹²O and ¹⁶Ne nuclei. Received: 21 March 2002 / Accepted: 16 May 2002 / Published online: 31 October 2002 RID="a" ID="a"e-mail: l.grigorenko@surrey.ac.uk     

Magic gaps and intruder levels in triaxially superdeformed nuclei

Nuclear shell model calculations based on a modified harmonic-oscillator potential result in amazingly stable triaxial nuclear shapes. Major gaps in the single-particle energy spectra at proton number 71 and neutron number 94 combine constructively at low and intermediate rotational frequencies. At high frequencies, gaps at proton number 72 and neutron number 97 combine in an equally favourable way. The sizes of the gaps may be as large as 35% of the values for the gaps at the classical magic numbers 50 and 82 at spherical shape. The dependence on the positions of the intruder levels in forming the gaps is discussed. Experimentally observed rotational bands in lutetium (Z = 71) and hafnium (Z = 72) appear in isotopes and frequency ranges, which are consistent with the gaps in the theoretical single-particle energy spectra.     

Count fluctuation analysis in the triaxial strongly deformed potential well of 163Lu

High-spin properties of the triaxial strongly deformed potential well in ¹⁶³Lu at excitation energies above resolvable bands have been investigated. Gated E _γ-E _γ spectra display clear ridges with moments of inertia corresponding to those observed for the discrete strongly deformed bands. A fluctuation analysis of the ridges yields a number of two-step paths of ≈ 40 and ≈ 20, when gating on triaxial strongly or normally deformed bands, respectively. These results show that a potential well at large deformation coexists with the normally deformed well, and indicate a mixing between states in the two wells at higher excitation energy. Received: 4 January 2002 / Accepted: 6 May 2002     

Beta decay of 93Pd

The very neutron-deficient isotope ⁹³Pd was produced in fusion-evaporation reactions of a ⁴⁰Ca beam on a ⁵⁸Ni target. The reaction products were separated at the GSI Online Mass Separator, using ion sources of the FEBIAD type. The -decay properties of ⁹³Pd were studied by detecting -delayed protons and -delayed -rays. The feeding of excited levels in the daughter nucleus ⁹³Rh and the -decay half-life of ⁹³Pd were determined. The experimental results are discussed in comparison to shell model predictions. Received: 19 April 2000 / Accepted: 17 June 2000     

Decay of proton-rich nuclei between 39Ti and 49Ni

Decay studies of very neutron-deficient nuclei ranging from ³⁹Ti to ⁴⁹Ni have been performed during a projectile fragmentation experiment at the GANIL/LISE3 separator. For all nuclei studied in this work, ^{39, 40}Ti, ^{42, 43}Cr, ⁴⁶Mn, ^{45, 46, 47}Fe and ⁴⁹Ni, half-lives and decay spectra have been measured. In a few cases, γ coincidence measurements helped to successfully identify the initial and final states of transitions. In these cases, partial decay scheme are proposed. For the most exotic isotopes, ³⁹Ti, ⁴²Cr, ⁴⁵Fe and ⁴⁹Ni, which are candidates for two-proton radioactivity from the ground state, no clear evidence of this process is seen in our spectra and we conclude rather on a delayed particle decay. Received: 26 July 2000 / Accepted: 20 December 2000     

Neutron-rich nuclei and fission; recent developments and future aspects

Production and studies of neutron-rich nuclei produced in fission are reviewed. Some recent experiments performed with the ISOL technique at the IGISOL and the ISOLDE facilities are reviewed. The manipulation of neutron-rich nuclei is discussed with a special focus on radioactive ion cooling and trapping techniques under construction worldwide. Perspectives of obtaining intense post accelarated beams of fission products are discussed. Received: 1 May 2001 / Accepted: 4 December 2001