Search Results (30 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.

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.

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 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.

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.

The yrast J^π=8⁺ states in neutron-rich ^70,72,74,76Ni nuclei are predicted to be isomeric. The present paper describes two GANIL experiments. In the first of them a search was made for the 8⁺ isomeric states in ^72,74Ni nuclei via fragmentation of ⁷⁶Ge using the ion γ-decay correlation technique. Although these states were not observed, limits for their lifetimes were determined. In the second experiment the decay spectroscopy of ^70,72Co nuclei was performed using fragmentation of a ⁸⁶Kr³⁶⁺ beam and the new LISE2000 spectrometer. The β delayed γ rays from the decay of ^70,72Co to ^70,72Ni were observed using the EXOGAM germanium detectors. The half life of ⁷²Co was measured to be 62(3) ms and the level sequence of the lowest excited states in ⁷²Ni was suggested, with the 2⁺ state at 1096  keV. An attempt to reproduce the level scheme in terms of shell-model calculations was undertaken. The reasons for the disappearance of the 8⁺ isomer in ⁷²Ni are discussed.

A new isomeric 0⁺ state was identified as the first excited state in the self-conjugate (N=Z) nucleus ⁷²Kr. By combining for the first time conversion-electron and gamma-ray spectroscopy with the production of metastable states in high-energy fragmentation, the electric-monopole decay of the new isomer to the ground state was established. The new 0⁺ state is understood as the band head of the known prolate rotational structure, which strongly supports the interpretation that ⁷²Kr is one of the rare nuclei having an oblate-deformed ground state. This observation gives in fact the first evidence for a shape isomer in a N=Z nucleus.

The structure of neutron-rich ^40,42,44S nuclei has been investigated through in-beam γ-ray spectroscopy using the fragmentation reaction of a 60 MeV A ⁴⁸Ca beam on a thin Be target. E_γ, I_γ, γγ-coincidence, and γ-ray angular distributions were measured for each produced fragment. The level schemes previously containing only a single γ transition were extended, and spin values were proposed for the new states. The experimental results were interpreted by use of microscopic collective-model and large-scale shell-model calculations. The results of the model calculations are consistent with each other, and give a reasonable description of the experimental results. Both models predict an erosion of the N=28 shell closure at Z=16 and suggest a deformed ground state for ^40,42S and a spherical-deformed mixed configuration for ⁴⁴S.

In an experiment at the SISSI-LISE3 facility of GANIL, the decay of the proton drip line nucleus ⁴⁵Fe has been studied. Fragment-implantation events have been correlated with radioactive decay events in a 16×16 pixel silicon-strip detector. The decay-energy spectrum of ⁴⁵Fe implants shows a distinct peak at (1.14±0.04)  MeV with a half-life of T_1/2=(4.7_-1.4^+3.4)  ms. None of the events in this peak is in coincidence with β particles. For a longer correlation interval, daughter decays of the two-proton daughter ⁴³Cr can be observed after ⁴⁵Fe implantation. The decay energy for ⁴⁵Fe agrees nicely with several theoretical predictions for two-proton radioactivity.

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.

Mass measurements of high precision have been performed on sodium isotopes out to ³⁰Na using a new technique of radio-frequency excitation of ion trajectories in a homogeneous magnetic field. This method, especially suited to very short-lived nuclides, has allowed us to significantly reduce the uncertainty in mass of the most exotic Na isotopes: a relative error of 5×10^-7 was achieved for ²⁸Na having a half-life of only 30.5 ms and 9×10^-7 for the weakly produced ³⁰Na. Verifying and minimizing binding energy uncertainties in this region of the nuclear chart is important for clarification of a long-standing problem concerning the strength of the N=20 magic shell closure. These results are the fruit of the commissioning of the new experimental program MISTRAL.

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.

Isomeric ratios and momentum distributions of nuclei produced in the fragmentation of a 60A MeV ⁹²Mo beam on a thin ²⁷Al target have been studied in detail. A strong dependence of the isomeric ratio on the structure of the isomer and on the reaction mechanism has been observed for the first time at intermediate energies. The results are quantitatively reproduced in a framework of kinematical and statistical models of nuclear reactions.

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 masses of 31 neutron-rich nuclei in the range A  =  29–47 have been measured. The precision of 19 masses has been significantly improved and 12 masses were measured for the first time. The neutron-rich Cl, S, and P isotopes are seen to exhibit a change in shell structure around N  =  28. Comparison with shell model and relativistic mean field calculations demonstrate that the observed effects arise from deformed prolate ground state configurations associated with shape coexistence. Evidence for shape coexistence is provided by the observation of an isomer in ⁴³S.

The fusion-fission cross sections of the ⁴He+²³⁸U and ⁶He+²³⁸U systems have been measured, at Louvain-la-Neuve, for energies around and below the Coulomb barrier, using an array of Si detectors surrounding a UF₄ target. The data taken with ⁴He are in good agreement with previous data and with the coupled channel fusion calculation performed with ECIS. The ⁶He data show a regular trend with a large enhancement below the barrier which is attributed to the halo structure of the ⁶He nucleus.

In an experiment at the SISSI/LISE3 facility of GANIL, we used the projectile fragmentation of a primary ⁵⁸Ni²⁶⁺ beam at 74.5 MeV/nucleon with an average current of 3 μA on a natural nickel target to produce very neutron-deficient isotopes. In a 10-day experiment, 287 ⁴²Cr isotopes, 53 ⁴⁵Fe isotopes, 106 ⁴⁹Ni isotopes, and 4 ⁴⁸Ni isotopes were unambiguously identified. The doubly magic nucleus ⁴⁸Ni, observed for the first time, is the most proton-rich isotope ever identified with an isospin projection T_z  =  -4. It is probably the last doubly magic nucleus with “classical” shell closures accessible for present-day facilities. Its observation allows us to deduce a lower limit for the half-life of ⁴⁸Ni of 0.5 μs.

The γ radiation and neutrons emitted following the β decays of ²⁴O, ^25–27F, and ^28-30Ne have been measured. The nuclides were produced in the quasifragmentation of a 2.8 GeV ³⁶S beam, separated in-flight and identified through time-of-flight and energy-loss measurements. The ions were stopped in a silicon detector telescope, which was used to detect the β particles emitted in their subsequent radioactive decay. The coincident γ rays were measured using four large volume germanium detectors mounted close to the implantation point and the neutrons were detected using 42 ³He proportional counters. The measured γ-ray energy spectra are compared with shell model calculations and, where available, the level energies deduced from multinucleon transfer reactions.

The halo neutron breakup cross section for ¹¹Be on Si has been obtained in a wide energy range by applying an integral method and separately determining the contributions of stripping and dissociation mechanisms. A new breakup mechanism, for which the core energy is strongly dumped, has also been observed. Parallel momentum distributions of ¹⁰Be resulting from breakup have been deduced for both stripping and dissociation and angular and energy distributions of the neutrons coincident with different reaction products have been measured. Charge changing cross sections for ^10,11Be complemented the measurements. An extended Glauber model has been elaborated in order to provide a unitary interpretation for all the data. It takes into account both the specific structure of ¹¹Be and the reaction mechanism, practically without free parameters. The effects of reaction mechanisms on the widths of observed momentum distributions are particularly important.

New isomeric states in the neutron-rich nuclei near the Z  =  28 and N  =  40 shell closures have been identified among the reaction products of a 60.3A MeV ⁸⁶Kr beam on a ^natNi target. From the measured isomeric decay properties information about the excited states and their nuclear structure has been obtained. The isomerism is related mostly to the occupation of the neutron g_9/2 orbital, an intruder level in the N  =  3 fp shell. It is illustrated with the decay properties of ⁶⁹Ni^m, ⁷⁰Ni^m, and ⁷¹Cu^m interpreted within the nuclear shell model.

A spectroscopic study of the proton-rich, particle unstable nucleus ¹¹N has been performed using the multinucleon transfer reaction ¹²C(¹⁴N,¹⁵C)¹¹N at 30A MeV incident energy at GANIL. Levels of ¹¹N are observed as well defined resonances in the spectrum of the ¹⁵C ejectiles. They are localized at 2.18(5), 3.63(5), 4.39(5), 5.12(8), and 5.87(15) MeV above the ¹⁰C+p threshold. The comparison of the measured widths with R-matrix calculations allows the estimation of spins and parities for these resonances.

The short-lived isomeric states in the T_Z=1 nuclei, ^94mPd [T_1/2=0.6(1) μs] and ^96mAg [T_1/2=0.7(2) μs], were identified among the fragmentation products of the ¹¹²Sn (63A MeV) + ^natNi (93.5 mg/cm²) reaction at GANIL. The separation and identification of the reaction products was done by means of the Alpha and LISE3 magnetic spectrometers combined with time-of-flight, energy-loss, and total kinetic energy measurements. Evidence for isomeric states in ⁸⁰Y, ⁹⁸Cd, and ¹⁰²Sn was also obtained.