Predicting narrow states in the spectrum of a nucleus beyond the proton drip line

Properties of particle-unstable nuclei lying beyond the proton drip line can be ascertained by considering the (usually known) properties of its mirror neutron-rich system. We have used a multichannel algebraic scattering theory to map the known properties of the neutron-14C system to those of the proton-14O one from which we deduce that the particle-unstable 15F will have a spectrum of two low-lying broad resonances of positive parity and, at higher excitation, three narrow negative-parity ones. A key feature is to use coupling to Pauli-hindered states in the target.     

Beta decay of the proton-rich nuclei 102Sn and 104Sn

The β decays of ¹⁰²Sn and ¹⁰⁴Sn were studied by using high-resolution germanium detectors as well as a Total Absorption Spectrometer (TAS). For ¹⁰⁴Sn, with three new β-delayed γ-rays identified, the total Gamow-Teller strength (B_GT) value of 2.7(3) was obtained. For ¹⁰²Sn, the γ-γ coincidence data were collected for the first time, allowing us to considerably extend the decay scheme. This scheme was used to unfold the TAS data and to deduce a B_GT value of 4.2(8) for this decay. This result is compared to shell model predictions, yielding a hindrance factor of 3.6(7) in agreement with those obtained previously for ⁹⁸Cd and ¹⁰⁰In. Together with the latter two, ¹⁰²Sn completes the triplet of Z ⩽ 50, N ≥ 50 nuclei with two proton holes, one proton hole and one neutron particle, and two neutron particles with respect to the doubly magic ¹⁰⁰Sn core.     

First Penning trap mass measurements beyond the proton drip line

The masses of six neutron-deficient rare holmium and thulium isotopes close to the proton drip line were determined with the SHIPTRAP Penning trap mass spectrometer. For the first time the masses of the proton-unbound isotopes 144,145Ho and 147,148Tm were directly measured. The proton separation energies were derived from the measured mass values and compared to predictions from mass formulas. The new values of the proton separation energies are used to determine the location of the proton drip line for holmium and thulium more accurately.     

Experimental studies of nuclei beyond the proton drip line by tracking technique

A novel experimental technique for measurements of in-flight decays of proton-unbound nuclei with pico-second lifetimes is described on the examples of the recent discovery of ¹⁹Mg and its two-proton (2p) radioactivity and the study of the reference 2p decay of the known isotope ¹⁶Ne . The method of measurements of 2p decays in flight by tracking all fragments with micro-strip detectors has also proven to be a potent tool for obtaining valuable spectroscopic information on exotic isotopes like ¹⁹Mg or ¹⁶Ne . Systematic studies of other 2p emitters predicted theoretically are foreseen with this powerful technique whose sensitivity is larger by factor of 20-30 in comparison with a conventional invariant-mass method. Information about the respective one-proton unbound subsystems can be obtained at the same time by evaluating proton-heavy-fragment correlations, which is illustrated on the example of the spectroscopy of ¹⁵F . This finding opens a way for systematic studies of exotic nuclei beyond the proton drip line, e.g., ⁶⁹Br . The properties of such nuclei may be important for the understanding of the element abundance in the Universe and may be used as input data for modeling the rp-process in various astrophysical sites.     

Nuclear spectroscopy near the proton drip line in the lanthanide region: The122La nucleus

Excited states in the very neutron-deficient¹²²La nucleus have been established for the first time using the⁹²Mo(⁴⁰Ca, 2α1p1n) reaction at beam energies of 190 and 200MeV. The observed transitions were grouped in three bands. Configurations have been assigned to the observed bands based on the properties extracted from the experimental data and on cranked shell model calculations.     

β-delayed proton decays near the proton drip line

We briefly reviewed the experimental study on β-delayed proton decays near the proton drip line published by our group during the period of 1996–2004, namely the first observation of the β-delayed proton decays of 9 new nuclides in the rare-earth region and the new measurements of β-delayed proton decays of 5 nuclides in the mass ∼90 region near theN=Z line with the aid of the “p-y” coincidence in combination with a He-jet tape transport system. In the meantime some important experimental technique details were supplemented. The experimental results, including the half-lives, spins, parities, deformations and production reaction cross sections for the 14 nuclei were summarized and compared with the current nuclear-model predictions, and then the following points were represented. (1) The experimental half-lives for⁸⁵Mo and⁹²Rh as well as the predicted “waiting point” nuclei⁸⁹Ru and⁹³Pd are 5–10 times longer than the theoretical predictions given by M?ller et al. using a macroscopic-microscopic model. It considerably influences the predictions of the abundances of the nuclides produced in the rp-process. (2) The current-model predictions are not consistent with the experimental assignments of the spins and parities for the proton drip-line nuclei¹⁴²Ho and¹²⁸Pm. However, the nuclear potential energy surface (PES) calculated by using a Woods-Saxon-Strutinsky method reproduced the experimental results. (3) The Alice code overestimated the production reaction cross sections of the studied 9 rare-earth nuclei by one order of magnitude or two, while HIVAP code overestimated them by one order of magnitude approximately.     

Broad states beyond the neutron drip line

Theoretical studies of broad states in the few-body systems beyond the neutron drip line have been performed. We introduce a theoretical model which allows to incorporate the initial structure of colliding nuclei, reaction mechanism, few-body effects and final-state interactions in studies of broad unbound states. The model is directly related to the sudden-removal approximation for high-energy knock-out or break-up reactions. We apply this model to qualitative studies of some general properties of broad few-body states including correlations for emitted fragments. The theoretical ideas are illustrated mainly using the example of ⁵H. The prospect for observation of broad continuum structures corresponding to the tetraneutron ⁴n is also discussed.Received: 9 April 2003, Revised: 21 August 2003, Published online: 26 January 2004PACS: 21.45. + v Few-body systems - 21.60.Gx Cluster models - 25.10. + s Nuclear reactions involving few-nucleon systems     

Quasi-elastic scattering of the proton drip line nucleus 17F on 12C at 60 MeV

The quasi-elastic scattering angular distribution of the proton drip line nucleus ¹⁷F on a ¹²C target was measured at 60 MeV. The experimental data have been compared with the theoretical analysis based onto optical model and continuum discretized coupled channels (CDCC). The couplings between breakup and elastic scattering channels, and between inelastic and elastic scattering channels resulted very weak. In order to explore the breakup effects the total reaction cross-section was deduced from the angular distribution of the quasi-elastic scattering data, and then compared with the existing data for the other weakly and tightly bound nuclei on ¹²C target using a universal function. From this comparison, we concluded that the breakup effect is not important for weakly bound projectiles on the light target as obtained also with the CDCC analysis.     

Structure of unstable nuclei near proton drip line

Gamov-Teller (GT) states in nuclei near the proton drip line are studied by using a microscopic Hartree-Fock (H-F) + Tamm-Dancoff approximation (TDA) (or random phase approximation (RPA)). The calculations predict that giant Gamow-Teller (GT) β-decays are possible for N=Z nuclei heavier than ₂₈⁵⁶Ni₂₈, carrying most of the sum rule strength. The amplitude of isospin T=1 admixed to the T=0 ground state in N=Z nuclei is also discussed in relation with Fermi β-decay sum rule. Finally, the shapes of unstable nuclei near the proton drip line are studied by using the finite-range droplet model (FRDM).     

Nuclear structure at the proton drip line: Advances with nuclear decay studies

The study of nuclear decay modes reveals a large panel of nuclear structure phenomena and allows us to investigate the behaviour of the atomic nucleus with an extreme imbalance of the number of neutrons and protons with respect to stable nuclei. In the present paper, we review certain aspects of β-delayed decay modes, of one- and two-proton radioactivity, and of the experimental techniques which allowed us to get deep insights into the organisation of the atomic nucleus. In most cases, the study of these decay modes is the only means to obtain the information searched for. The investigation of nuclear decay modes is shown to be a powerful tool to study the most proton-rich nuclei and their nuclear structure.     

Properties of Zr isotopes near the neutron drip line and beyond it

The properties of neutron-rich Zr isotopes up to the neutron drip line and beyond it have been investigated on the basis of the Hartree-Fock method with the Skyrme forces (Ska, Sly4), taking into account the deformation. By the example of chains of Zr isotopes, good agreement is shown for the two-neutron separation energies and mean-square radii with the known results of Hartree-Fock-Bogolyubov calculations with the Sly4 forces. For the extremely neutron-rich Zr isotopes, states with very large deformation parameters (β ≈ 0.4?0.45) of neutron and proton density distributions can be realized. Beyond the neutron drip line with respect to emission of two neutrons, the existence of ^150,152Zr isotopes, which are stable with respect to one-neutron emission, is predicted.     

Search for forbiddenβ-decays of the drip line nucleus12Be

Beta-coincidentγ-rays have been measured from implanted pure samples of¹²Be separated at the LISE3 spectrometer at GANIL. An intensity of 0.040(26) % can be estimated for the branching ratio of the isospin forbidden pure-Fermi transition to the 0⁺ excited state of¹²B and of 0.008(6)% of the transition to the 1^?1 excited state. Both are taken to represent upper limits. The half-life has been re-measured to be 26.1(2.4) ms.     

Observation of a new rotational band in 104Nb nucleus

The high spin states in neutron-rich 104Nb have been investigated from study of prompt γ-rays in spontaneous fission of 252Cf with the Gammasphere detector array. A new rotational band has been identified for the first time. This band is proposed as a semi-decoupled band based on the configuration π5/2-[303](⊕)v1/2-[541].     

First decay study of the new isotope <Superscript>129</Superscript>Pm near the proton drip line

The very neutron-deficient nuclide ¹²⁹Pm was produced via the ⁹²Mo(⁴⁰Ca, p2n) reaction and identified for the first time by using the X- coincidence in combination with a He jet tape transport system. According to the decay curve of a 99 keV -ray which corresponds to the known transition in the daughter nucleus ¹²⁹Nd of the ¹²⁹Pm decay, the half-life of ¹²⁹Pm was determined to be 2.4(9) s. Based on the nuclear potential energy surface (PES) calculations, the ground-state spin-parity of ¹²⁹Pm was predicted as 5/2^- which is favorable to feed a 5/2^- low-lying state in the daughter nucleus ¹²⁹Nd via the (EC ) decay.Received: 20 November 2003, Revised: 7 January 2004, Published online: 29 June 2004PACS: 23.40.Hc Relation with nuclear matrix elements and nuclear structure - 21.10.Hw Spin, parity, and isobaric spin - 24.10.Pa Thermal and statistical models - 27.60. + j      

New isotopes,secondary reactions and spectroscopy for medium-mass nuclei at the proton drip line

This paper reviews two experiments performed at GSI and GANIL. Using a primary beam of ⁵⁸Ni at 650 MeV/nucleon impinging on a beryllium target, production cross sections of proton-rich fragments from projectile fragmentation have been measured at the projectile-fragment separator FRS at GSI. The production rates measured demonstrate that counting rates much higher than expected can be obtained at the proton drip line. The results from spectroscopy measurements show that no Thomas-Ehrmann effect is present in our data which means that the decay energies and masses can be well predicted by using e.g. the IMME. The secondary reactions evidence a slight increase of the interaction cross sections when approaching the proton drip line. Finally, in an experiment performed at the SISSI/LISE facility at GANIL using a ⁷⁸Kr primary beam, we have observed the 5 new isotopes ⁶⁰Ga, ⁶⁴As, ^69,70Kr, and ⁷⁴Sr. However, we have not found any evidence for ⁶⁹Br which was reported to be observed with a few counts at MSU. These new findings change our understanding of the path and of the ending point of the rp process.     

Laser spectroscopy and the properties of light nuclei near the neutron drip line

The interesting nuclear structure phenomena observed in some light nuclei at the neutron drip line suggest the measurement of basic ground state properties such as spins, magnetic moments and electric quadrupole moments. Here it will be discussed what experiments are presently feasable using laser spectroscopy. For the outstanding example of a halo nucleus,¹¹Li, the development of a technique combining -asymmetry detected optical pumping in a fast beam with NMR spectroscopy has yielded pertinent results, the most recent of which is an experimental value of the nuclear quadrupole moment.     

Precision Mass Measurements beyond ^{132}Sn: Anomalous Behavior of Odd-Even Staggering of Binding Energies

Atomic masses of the neutron-rich isotopes ^{121-128}Cd, ^{129,131}In, ^{130-135}Sn, ^{131-136}Sb, and ^{132-140}Te have been measured with high precision (10?ppb) using the Penning-trap mass spectrometer JYFLTRAP. Among these, the masses of four r-process nuclei ^{135}Sn, ^{136}Sb, and ^{139,140}Te were measured for the first time. An empirical neutron pairing gap expressed as the odd-even staggering of isotopic masses shows a strong quenching across N=82 for Sn, with a Z dependence that is unexplainable by the current theoretical models.     

Recoil Beta Tagging: Application to the study of odd-odd near proton drip line nuclei, 74Rb and 78Y

We present a study of low-spin states in ⁷⁴Rb and ⁷⁸Y using the new technique of Recoil Beta Tagging. This yielded new information on ⁷⁴Rb and has provided the first evidence for non-isomeric T= 1 states in ⁷⁸Y.