Excited states of 154Tm

A high-spin level structure of the nucleus ¹⁵⁴Tm has been established for the first time up to 6.14 MeV. The ¹¹⁸Sn + ⁴⁰Ca reaction at 205 MeV associated with the recoil shadow method was used to identify the I ^π = (19)⁺ isomeric state at 2.74 MeV. In-beam γ-ray and conversion electron spectroscopy have been applied on the ¹⁴⁴Sm(¹⁴N, 4n)¹⁵⁴Tm reaction at 95 MeV to build the level scheme. The level structure of ¹⁵⁴Tm is compared to other structures observed in N = 85 neighbouring nuclei. Received: 4 January 2002 / Accepted: 15 March 2002     

Reaction mechanism of <Superscript>6</Superscript>Li scattering at 600 MeV

Elastic scattering and inclusive breakup of ⁶Li particles on ¹²C, ⁵⁸Ni, ⁹⁰Zr, and ²⁰⁸Pb targets are measured at 100A MeV. The elastic scattering data are compared with single channel and Coupled Discretized Continuum Channels calculations. The coupling-effect between the elastic and the breakup channels is important even at an incident energy of 100A MeV and cannot be neglected. The inclusive breakup data are investigated for orbital dispersion effects which are found to be less significant at 100A MeV. The longitudinal momentum distributions are broader than predicted by theoretical expectations. Received: 17 December 1999 / Revised version: 9 February 2000     

Inelastic scattering on 12Be and disappearance of the N = 8 magic number

Experimental studies on in-beam γ-ray spectroscopy using a ¹²Be radioactive beam are presented. Inelastic scattering of the neutron-rich ¹²Be nucleus on ²⁰⁸Pb, ¹²C, and (CH₂)_n targets has been studied by measuring de-excitation γ-rays in coincidence with scattered particles. The level schemes and transition probabilities are determined for low-lying excited states in ¹²Be. The present paper presents a brief review of the spectroscopic results, which may be associated with the N = 8 shell quenching near the drip line. Received: 1 May 2001 / Accepted: 4 December 2001     

2H( 3He, t)2p reaction at 2 GeV

The exclusive ²H( ³He, t)2p reaction has been studied at 2 GeV for energy transfers up to 500 MeV and triton angles up to 3.4^°. The protons were measured in the large acceptance magnetic detector DIOGENE, in coincidence with the forward tritons detected in a dedicated magnetic arm. The energy transfer spectra extend well above the pion threshold. However, in the region of Δ excitation, the yield is less than 10% of the inclusive ²H( ³He, t) cross-section, which indicates the small contribution of the ΔN ↦ NN process. The angular distributions of the two protons in their center of mass have been analysed as a function of energy transfer and triton angle and a Legendre polynomial decomposition has been achieved. These data have been compared to a model based on a coupled-channel approach for describing the NN and NΔ systems. Received: 21 October 2002 / Accepted: 15 November 2002 / Published online: 11 March 2003 RID="a" ID="a"e-mail: ramstein@ipno.in2p3.fr RID="†" ID="†"Deceased. RID="†" ID="†"Deceased. RID="d" ID="d"Present adress: School of Engineering, J?nk?ping University, P.O. Box 1026, S-551 11 J?nk?ping, Sweden. Communicated by M. Gar?on     

Properties unique in nuclei far from β stability line

Nucleons with very small binding energies present in nuclei far from the β stability line produce a unique shell structure, which leads to the disappearance of traditional magic numbers or to the creation of new magic numbers and new deformation regions. We study the shell structure in terms of the variation of two important ingredients, the kinetic energy and the spin-orbit splitting, as a function of the orbital angular momentum ℓ, when binding energies of neutrons decrease towards zero. It is also shown that for low-lying threshold strength, a negative sign is possible for the polarization charge coming from the coupling of one-particle to isoscalar shape oscillations. Received: 1 May 2001 / Accepted: 4 December 2001     

Spectroscopy of <Superscript>17</Superscript>C and (sd )<Superscript>3</Superscript> structures in heavy carbon isotopes

The structure of ¹⁷C has been investigated using the three-neutron transfer reaction (¹²C,⁹C) on a ¹⁴C target at 231MeV incident energy, the reaction Q-value is Q ₀ = - 46.930MeV. Eleven new states up to 16.3MeV excitation energy were identified. The same reaction has also been used on a ¹²C target ( Q ₀ = - 38.787MeV), and excited states in ¹⁵C up to 19MeV were observed. In ¹⁷C the three transferred neutrons populate (sd )³ configurations on the ¹⁴C core. The comparison of levels populated by the (¹²C,⁹C) reaction in ¹⁷C, ¹⁶C and ¹⁵C reveals a strong similarity of their properties. This concerns especially nine states in each of the three carbon isotopes, which show practically the same excitation energies except a constant mean shift of +5.82MeV for ¹⁶C and +6.65MeV for ¹⁵C with respect to ¹⁷C. The triples of states from the three isotopes, which correspond to each other, have also similar widths and cross-section ratios. It is concluded that the same (sd )³ structures are populated in the three carbon isotopes. The observed levels of ¹⁷C are also compared to the levels of ¹⁹O with known assignments and to shell model calculations, and their decay properties are discussed.     

The first excited states of 9Be and 9B

It is found here that the 1/2⁺ first excited state of ⁹Be is a virtual state with the energy of −23.5 KeV. The line shape for the excitation of the state is approximated with a simple analytic form based on the effective range expansion. The partner in ⁹B of this state is found to be a resonance with a maximum in the peak at about 1.1 MeV, FWHM of 1.5 MeV, and complex energy of 0.6 −i0.75 MeV. The line shape for its excitation is calculated in terms of the p−⁸Be phase shift. The phase shifts are obtained from N−⁸Be effective potentials deduced from the data on the photodisintegration of ⁹Be. A possibility for direct extraction of the energy of the resonant state from experimental data is also discussed, and an expression for a residue at a virtual state pole in terms of a quadrature taken over the virtual state eigenfunction is given. Received: 23 March 1998 / Revised version: 3 September 1998     

Mapping of the <Superscript>12</Superscript>C* states via the <Superscript>10</Superscript>B(<Superscript>3</Superscript>He,pααα) reaction

We have studied ¹²C in full kinematics via the ¹⁰B(³He,pααα) reaction at an energy of 2.45 MeV. In our data we have identified states in ¹²C from the ground state up to about 18 MeV, with spins ranging from 0 to 4. Due to the very good resolution, we are able to determine properties of these ¹²C resonances, such as their energy, width, and spin. In this contribution preliminary results from the ongoing analysis are presented. Main focus on the precise determination of the breakup spectra for all resonances.     

High-spin states in the <Superscript>97</Superscript>Tc nucleus

High-spin states in the ⁹⁷Tc nucleus have been studied by in-beam γ-ray spectroscopy with the reaction ⁸²Se(¹⁹F,4nγ) at 68 MeV incident energy. Excited states have been observed up to about 8 MeV excitation and spin 43/2. The observed level scheme is compared with results of shell model calculations. Received: 22 November 2002 / Accepted: 23 December 2002 / Published online: 18 March 2003 RID="a" ID="a"e-mail: bucurescu@tandem.nipne.ro RID="b" ID="b"Present address: INFN, Laboratori Nazionali di Legnaro, Legnaro, Italy. RID="b" ID="b"Present address: INFN, Laboratori Nazionali di Legnaro, Legnaro, Italy. RID="b" ID="b"Present address: INFN, Laboratori Nazionali di Legnaro, Legnaro, Italy. RID="c" ID="c"Present address: Università di Padova, Padova, Italy. Communicated by C. Signorini     

The angular distribution of the 12C(γ,pn) reaction for Eγ=120−150 MeV

The angular distribution of nucleons emitted in the ¹²C(γ,pn) reaction has been measured using tagged photons at the Mainz microtron MAMI. The variation of the reaction strength with the polar angles of the two emitted nucleons is reported for E_γ=120−150 MeV. The proton angular distribution peaks at more backward angles than the ²H(γ,p) differential cross section indicating a departure from the simple quasi-deuteron model of 2N photo-emission. The distribution shape is in reasonable agreement with microscopic theoretical models which include both π- and ρ-exchange. Received: 3 September 1997 / Revised version: 17 December 1997     

7Li quasi-free scattering off the α-cluster in 9Be nucleus

Spectra of coincident charged particles from the reactions induced by a 52 MeV ⁷Li beam incident on a beryllium target were measured. Strong contributions of the ⁷Li quasi-free scattering off the α-cluster in ⁹Be nucleus were observed. This observation supports the conclusions from the study of complete fusion of weakly bound light nuclei at low energies that the “fragility” of the nuclei makes their fusion less probable. Received: 1 June 1998     

Microscopic study of Gamow-Teller and spin-dipole states in 208Bi

Gamow-Teller (GT) and spin-dipole (SD) states in ²⁰⁸Bi are studied by using self-consistent Hartree-Fock + Tamm-Dancoff approximation taking into account the coupling to the continuum. Most of SD strength is found at the excitation energy E _x≈ 25MeV with a very broad width, which agrees with recent experimental observations. It is shown that Landau damping effect is responsible for the large width of SD peak, while the escape width is found to be at most 1MeV. We study also electric dipole (E1) transitions between GT and SD states in ²⁰⁸Bi. Main E1 transitions for 0^- and 1^- states are found near excitation energy expected from Brink's hypothesis in which SD states are considered as E1 resonances built on the GT state. Calculated E1 transition strengths between GT and SD states are compared with the analytic sum rules within one-particle one-hole (1p-1h) configuration space and within both 1p-1h and 2p-2h model space. Received: 11 April 2000 / Accepted: 30 August 2000     

Spectroscopy of 13B, 14B, 15B and 16B using multi-nucleon transfer reactions

New information has been obtained on excited states of the neutron-rich boron isotopes ¹⁴B, ¹⁵B and ¹⁶B, using the reactions ¹²C(¹⁴C,¹²N)¹⁴B, ¹³C(¹⁴C,¹²N)¹⁵B and ¹⁴C(¹⁴C,¹²N)¹⁶B at about 24 MeV/A. The mass excess of ¹⁶B has been measured for the first time, it is 37.08(6) MeV. This means that ¹⁶B is unbound by only 0.04(6) MeV. Furthermore, the nucleus ¹³B has been investigated with the four reactions ¹⁶O(¹⁴C,¹⁷F), ¹²C(¹⁴C,¹³N), ¹²C(¹³C,¹²N) and ¹²C(¹⁵N,¹⁴O). Choosing different target-projectile combinations, it was possible to populate states with different selectivity. New states are observed in ¹³B at excitation energies above the threshold for two-neutron decay. Received: 27 December 1999 / Revised version: 11 February 2000     

Quantum Monte Carlo calculations of light nuclei

Quantum Monte Carlo calculations using realistic two- and three-nucleon interactions are presented for nuclei with up to ten nucleons. Our Green's function Monte Carlo calculations are accurate to ∼1-2% for the binding energy. We have constructed Hamiltonians using the Argonne v₁₈ NN interaction and reasonable three-nucleon interactions that reproduce the energies of these nuclear states with only ∼500 keV rms error. Other predictions, such as form factors, decay rates, and spectroscopic factors also agree well with data. Some of these results are presented to show that ab initio calculations of light nuclei are now well in hand. Received: 1 May 2001 / Accepted: 4 December 2001     

Thick skin in neutron/proton-rich sodium isotopes

Nucleon (both neutron and proton) density distributions of the chain of sodium isotopes are calculated using a semi-phenomenological model of nuclear density which incorporates correctly the asymptotic behaviour and the behaviour near the centre. The experimental charge root-mean-square radii and the single neutron and proton separation energies, required as input, are used. The calculated interaction cross-sections using these densities in the Glauber model agree well with the experiment. The calculated neutron rms radii r _n and the nuclear skin thickness ( r _n - r _p) closely agree with the corresponding experimental values and also are consistent with the Relativistic Hartree-Bogoliubov (RHB) calculations. Received: 24 April 2001 / Accepted: 28 June 2001     

Gamow-Teller transitions from 58Ni to discrete states of 58Cu

Under the assumption that isospin is a good quantum number, symmetry is expected for the transitions from the ground states of T = 1, T _z = ±1 nuclei to the common excited states of the T _z = 0 nucleus situated between the two nuclei. The symmetry can be studied by comparing the strengths of Gamow-Teller (GT) transitions obtained from a (p, n)-type charge-exchange reaction on a target nucleus with T _z = 1 with those from the β-decay of the T _z = - 1 nucleus. The A = 58 system is the heaviest for which such a comparison is possible. As a part of the symmetry study, we measured the GT transitions from ⁵⁸Ni (T _z = 1) to ⁵⁸Cu (T _z = 0) by using the zero-degree (³ He, t) reaction at 150 MeV/nucleon. With the achieved resolution of 50 keV, many hitherto unresolved GT states have been identified. The GT transition strengths were obtained for states up to 8 MeV excitation, i.e., near to the Q window limitation ( Q _EC = 9.37 MeV) of the β-decay from ⁵⁸Zn (T _z = - 1) to ⁵⁸Cu. The strength distribution is compared with that from shell-model calculations. Received: 24 November 2001 / Accepted: 30 January 2002     

Measurement of the 24Mg(p, t)22Mg reaction for the states near the 21Na + p threshold

Differential cross-sections of the ²⁴Mg(p, t)²²Mg reaction were measured at 34.68 MeV for the states near the proton threshold at 5.502 MeV in ²²Mg. Among them, the new states at 5.962, 6.046, 6.246 and 6.323 MeV, which were reported previously, have been confirmed. Angular distributions for these states were analyzed by distorted-wave Born-approximation calculations to deduce the spins and parities. The angular distribution for the 5.714 MeV state, which is considered to be most crucial for the stellar reaction ²¹Na(p, γ)²²Mg, has been found to be consistent with J ^π = 2⁺ assignment. The 6.046 MeV state is newly assigned to have J ^π = 0⁺, and the 5.962 MeV state is tentatively assigned to have J ^π = (1^-). These two states will also play an important role for ²²Mg production in novae. Received: 7 March 2002 / Accepted: 7 May 2002