Search Results (31 total)

The structure of ^19-22N nuclei was investigated by means of in-beam γ-ray spectroscopic technique using fragmentation reactions of both stable and radioactive beams. Based on particle-γ and particle-γγ coincidence data, level schemes are constructed for the neutron-rich nitrogen nuclei. The experimental results are compared with shell model calculations. The strength of the N=14 and Z=8 shell closures and the weakening of the shell model interaction WBT are discussed.

The decay of the ground-state two-proton emitter ⁴⁵Fe was studied with a time-projection chamber and the emission of two protons was unambiguously identified. The total decay energy and the half-life measured in this work agree with the results from previous experiments. The present result constitutes the first direct observation of the individual protons in the two-proton decay of a long-lived ground-state emitter. In parallel, we identified for the first time directly two-proton emission from ⁴³Cr, a known β-delayed two-proton emitter. The technique developed in the present work opens the way to a detailed study of the mechanism of ground state as well as β-delayed two-proton radioactivity.

A Reply to the Comment by Angelo Signoracci and B. Alex Brown.

The energies of the excited states in very neutron-rich ⁴²Si and ^41,43P have been measured using in-beam γ-ray spectroscopy from the fragmentation of secondary beams of ^42,44S at 39A  MeV. The low 2⁺ energy of ⁴²Si, 770(19) keV, together with the level schemes of ^41,43P, provides evidence for the disappearance of the Z=14 and N=28 spherical shell closures, which is ascribed mainly to the action of proton-neutron tensor forces. New shell model calculations indicate that ⁴²Si is best described as a well-deformed oblate rotor.

This paper demonstrates the feasibility of quadrupole moment measurements on isomeric levels populated in projectile fragmentation reactions. The neutron-rich ⁶¹Fe(9/2⁺) [E^*= 861 keV, T_1/2= 239(5) ns] isomer was produced and spin aligned by the intermediate energy fragmentation of a ⁶⁴Ni beam and implanted in a Cd single crystal. Its spectroscopic quadrupole moment |Q_s|=41(6)  e  fm² agrees with mean-field based calculations using the finite-range Gogny force, suggesting a moderately deformed shape characterized by an intrinsic charge quadrupole moment Q₀=-85  e  fm² or Q₀=+115  e  fm². The present measurement paves the way for future determinations of isomeric quadrupole moments in more exotic nuclei.

The N=28 shell closure has been investigated via the ⁴⁶Ar(d,p)⁴⁷Ar transfer reaction in inverse kinematics. Energies and spectroscopic factors of the neutron p_3/2, p_1/2, and f_5/2 states in ⁴⁷Ar were determined and compared to those of the ⁴⁹Ca isotone. We deduced a reduction of the N=28 gap by 330(90) keV and spin-orbit weakenings of ≃10(2) and 45(10)% for the f and p states, respectively. Such large variations for the f and p spin-orbit splittings could be accounted for by the proton-neutron tensor force and by the density dependence of the spin-orbit interaction, respectively. This contrasts with the picture of the spin-orbit interaction as a surface term only.

The β decay of the neutron-rich nuclei ^51-53K has been used to populate bound and unbound states in ^50-53Ca. Measurements of γ rays as well as β-delayed neutrons enabled detailed decay schemes to be established and levels identified in ^50-53Ca. A delayed one-neutron emission probability P_1n of 63±8% was determined for the decay of ⁵¹K. A total of seven new γ transitions were observed following the decay of ⁵¹K, and 25 neutron branches were found that enrich the level scheme of ⁵¹Ca. Delayed neutron emission probabilities of P_1n=74.4±9.3% and P_2n=2.3±0.3% were determined for the decay of ⁵²K, and 12 new γ transitions were observed in ^50,51,52Ca. Three new γ transitions were observed in ^52,53Ca following the β decay of ⁵³K. New limits on the P_1n and P_2n values were determined for the β decay of ⁵³K, and a decay scheme was established for ⁵³Ca for the first time. The data obtained here should help clarify the structure of neutron-rich fp-shell nuclei around the N=32-34 subshell closures.

The reduced transition probabilities B(E2;0⁺→2₁⁺) of the neutron-rich ⁷⁴Zn and ⁷⁰Ni nuclei have been measured by Coulomb excitation in a ²⁰⁸Pb target at intermediate energy. These nuclei have been produced at Grand Accélérateur National d’Ions Lourds via interactions of a 60A  MeV ⁷⁶Ge beam with a Be target. The B(E2) value for ⁷⁰Ni₄₂ is unexpectedly large, which indicates that neutrons added above N=40 strongly polarize the Z=28 proton core. In the Zn isotopic chain, the steep rise of B(E2) values beyond N=40 continues up to ⁷⁴Zn₄₄. The enhanced proton core polarization in ⁷⁰Ni is attributed to the monopole interaction between the neutron in the g_9/2 and protons in the f_7/2 and f_5/2 spin-orbit partner orbitals. This interaction could result in a weakening of magicity in ⁷⁸Ni₅₀.

Nuclear structure of the neutron rich ^25-29Ne nuclei has been investigated through the in-beam γ-ray spectroscopy technique using fragmentation reactions of both stable and radioactive beams. Level schemes have been deduced for these Ne isotopes. In order to examine the importance of intruder fp configurations, they are compared to shell model calculations performed either in the restricted sd or in the larger sdpf valence space. The ^25,26Ne and ²⁷Ne nuclei were found to be in agreement with the sd shell model calculations, whereas ²⁸Ne exhibits signatures of the intruder fp shell contribution.

In an experiment at the SISSI/LISE3 facility of GANIL, we have studied the decay of the two proton-rich nuclei ⁴⁵Fe and ⁴⁸Ni. We identified 30 implantations of ⁴⁵Fe and observed for the second time four implantation events of ⁴⁸Ni. In 17 cases, ⁴⁵Fe decays by two-proton emission with a decay energy of 1.154(16) MeV and a half-life of T_1/2=1.6_-0.3^+0.5 ms. The observation of ⁴⁸Ni and of its decay allows us to deduce a half-life of T_1/2=2.1_-0.7^+2.1 ms. One out of four decay events is completely compatible with two-proton radioactivity and may therefore indicate that ⁴⁸Ni has a two-proton radioactivity branch. We discuss all information now available on two-proton radioactivity for ⁴⁵Fe and ⁴⁸Ni and compare it to theoretical models.

The β decay of ³¹Mg has been investigated at GANIL. The ions were implanted in a silicon detector array surrounded by germanium γ detectors. β particles and subsequent γ rays have been observed. The ³¹Mg half-life has been measured (T_1/2=237±25 ms) in good agreement with previously reported values. The decay scheme of ³¹Mg has been enriched with two new low-spin levels and seven new γ transitions in ³¹Al. Most importantly, very weak feeding to the ³¹Al ground and lowest excited states has been established. These results are compared to new shell-model calculations in the sd-fp valence space, using an interaction that was modified in order to reproduce the recently established low-energy level scheme of ³¹Mg. A good agreement for all observables is found, supporting the idea that ³¹Al is outside and ³¹Mg is inside the so-called “island of inversion.”

The nucleus ⁵⁴Zn has been observed for the first time in an experiment at the SISSI/LISE3 facility of GANIL in the quasifragmentation of a ⁵⁸Ni beam at 74.5  MeV/nucleon in a ^natNi target. The fragments were analyzed by means of the ALPHA-LISE3 separator and implanted in a silicon-strip detector where correlations in space and time between implantation and subsequent decay events allowed us to generate almost background free decay spectra for about 25 different nuclei at the same time. Eight ⁵⁴Zn implantation events were observed. From the correlated decay events, the half-life of ⁵⁴Zn is determined to be 3.2_-0.8^+1.8  ms. Seven of the eight implantations are followed by two-proton emission with a decay energy of 1.48(2) MeV. The decay energy and the partial half-life are compared to model predictions and allow for a test of these two-proton decay models.

A study of high-energy (43–68  MeV∕nucleon) one-neutron removal reactions on a range of neutron-rich psd-shell nuclei (Z=5–9,  A=12–25) has been undertaken. The inclusive longitudinal and transverse momentum distributions for the core fragments together with the cross sections have been measured for breakup on a carbon target. Momentum distributions for reactions on tantalum were also measured for a subset of nuclei. An extended version of the Glauber model incorporating second-order noneikonal corrections to the Jeukenne, Lejeune, and Mahaux parametrization of the optical potential has been used to describe the nuclear breakup, while the Coulomb dissociation is treated within first-order perturbation theory. The projectile structure has been taken into account via shell-model calculations employing the psd interaction of Warburton and Brown. Both the longitudinal and transverse momentum distributions together with the integrated cross sections were well reproduced by these calculations and spin-parity assignments are thus proposed for ¹⁵B,¹⁷C,^19–21N,^21,23O,^23–25F. In addition to the large spectroscopic amplitudes for the ν2s_1∕2 intruder configuration in the N=9 isotones, ¹⁴B and ¹⁵C, significant ν2s_1∕2² admixtures appear to occur in the ground state of the neighboring N=10 nuclei ¹⁵B and ¹⁶C. Similarly, crossing the N=14 subshell, the occupation of the ν2s_1∕2 orbital is observed for ²³O, ^24,25F. Recent claims of a modified shell structure for ²³O are investigated and the original suggestion of a ground state J^π=1∕2⁺ is confirmed. Analysis of the longitudinal and transverse momentum distributions reveals that both carry spectroscopic information, often of a complementary nature. The general utility of high-energy nucleon removal reactions as a spectroscopic tool is also examined.

In-beam γ-ray spectroscopy using fragmentation reactions of both stable and radioactive beams has been performed in order to study the structure of excited states in neutron-rich oxygen isotopes with masses ranging from A=20 to 24. For the produced fragments, γ-ray energies, intensities, and γ-γ coincidences have been measured. Based on this information new level schemes are proposed for ^21,22O up to the neutron separation energy. The nonobservation of any γ-decay branch from ²³O and ²⁴O suggests that their excited states lie above the neutron decay thresholds. From this, as well as from the level schemes proposed for ²¹O and ²²O, the size of the N=14 and 16 shell gaps in oxygen isotopes is discussed in the light of shell-model calculations.

A new approach to the production and detection of bound neutron clusters is presented. The technique is based on the breakup of beams of very neutron-rich nuclei and the subsequent detection of the recoiling proton in a liquid scintillator. The method has been tested in the breakup of intermediate energy (30–50 MeV/nucleon) ¹¹Li, ¹⁴Be, and ¹⁵B beams. Some six events were observed that exhibit the characteristics of a multineutron cluster liberated in the breakup of ¹⁴Be, most probably in the channel ¹⁰Be+⁴n. The various backgrounds that may mimic such a signal are discussed in detail.

The neutron-rich ^66,68Ni have been produced at GANIL via interactions of a 65.9A MeV ⁷⁰Zn beam with a ⁵⁸Ni target. Their reduced transition probability B(E2;0₁⁺→2⁺) has been measured for the first time by Coulomb excitation in a ²⁰⁸Pb target at intermediate energy. The B(E2) value for ⁶⁸Ni₄₀ is unexpectedly small. An analysis in terms of large scale shell model calculations stresses the importance of proton core excitations to reproduce the B(E2) values and indicates the erosion of the N  =  40 harmonic-oscillator subshell by neutron-pair scattering.

Three-body correlations in the dissociation of two-neutron halo nuclei are explored using a technique based on intensity interferometry and Dalitz plots. This approach provides for the combined treatment of both the n-n and core-n interactions in the exit channel. As an example, the breakup of ¹⁴Be into ¹²Be+n+n by Pb and C targets has been analyzed and the halo n-n separation extracted. Evidence for a finite delay between the emission of the neutrons in the reaction on the C target was observed and is attributed to ¹³Be resonances populated in sequential breakup.

Radiative capture of protons is investigated as a probe of clustering in nuclei far from stability. The first such measurement on a halo nucleus is reported here for the reaction ⁶He( p,γ) at 40 MeV. Capture into ⁷Li is observed as the strongest channel. In addition, events have been recorded that may be described by quasifree capture on a halo neutron, the α core, and ⁵He. The possibility of describing such events by capture into the continuum of ⁷Li is also discussed.

A new transition is reported in the β-delayed neutron decay of ¹⁶C. The energy of the associated neutrons is 3.29±0.03 MeV, leading to the feeding, with a branching ratio of ∼1%, of a probable 1⁺ level in ¹⁶N at 6.00±0.03 MeV. Such an observation is in good accordance with shell model calculations carried out within the 0p1s0d model space.

The breakup of ^10,12Be into He clusters has been studied using the p,¹²C(¹²Be,⁶He,⁶He) and ¹²C(¹²Be,⁴He,⁶He) inelastic scattering and two neutron transfer reactions with a 378 MeV ¹²Be beam incident on ¹²C and (CH₂)_n targets. Evidence has been found for three new states in ¹⁰Be at excitation energies of 13.2, 14.8, and 16.1 MeV, which may be associated with a ⁴He+⁶He cluster structure. The evidence for He cluster states in ¹²Be in the excitation energy range 12 to 25 MeV is also discussed.

The two-neutron halo nucleus ¹⁴Be has been investigated in a kinematically complete measurement of the fragments ( ¹²Be and neutrons) produced in dissociation at 35 MeV/nucleon on C and Pb targets. Two-neutron removal cross sections, neutron angular distributions, and invariant mass spectra were measured, and the contributions from electromagnetic dissociation (EMD) were deduced. Comparison with three-body model calculations suggests that the halo wave function contains a large ν(2s_1/2)² admixture. The EMD invariant mass spectrum exhibited enhanced strength near threshold consistent with a nonresonant soft-dipole excitation.

The level structure of the unbound nucleus ¹¹N has been studied by ¹⁰C+p elastic resonance scattering in inverse geometry with the LISE3 spectrometer at GANIL, using a ¹⁰C beam with an energy of 9.0 MeV/nucleon. An additional measurement was done at the A1200 spectrometer at MSU. The excitation function above the ¹⁰C+p threshold has been determined up to 5 MeV. A potential-model analysis revealed three resonance states at energies 1.27_-0.05^+0.18 MeV (Γ=1.44±0.2 MeV), 2.01_-0.05^+0.15 MeV (Γ=0.84±0.2 MeV), and 3.75±0.05 MeV (Γ=0.60±0.05 MeV) with the spin-parity assignments I^π=1 / 2⁺,1 / 2^-,5 / 2⁺, respectively. Hence, ¹¹N is shown to have a ground state parity inversion completely analogous to its mirror partner ¹¹Be. A narrow resonance in the excitation function at 4.33±0.05 MeV was also observed and assigned spin parity 3 / 2^-.