Experimental investigation of the effect of ionization on the 51V(p,n)51Cr reaction

The investigation of the effects of average atomic ionization on nuclear reactions is of prime importance for nuclear astrophysics. No direct experimental measurement using a plasma target has been done yet. In this regard, we measured for the first time the neutron production of a (p,n) reaction in different states of ionization. The studied nuclear reaction was 51V(p,n)51Cr. We measured a significantly lower neutron production than expected when the target was ionized, even when taking into account existing electron screening theory or the effect of the stopping power in the target on the injected proton beam. This experiment is a first step in the process to characterize the influence of ionization at astrophysically relevant energies.Comment: 20 pages, 10 figures, submitted to EP

Systematics of proton emission

A very simple formula is presented that relates the logarithm of the half-life, corrected by the centrifugal barrier, with the Coulomb parameter in proton decay processes. The corresponding experimental data lie on two straight lines which appear as a result of a sudden change in the nuclear shape marking two regions of deformation independently of the angular momentum of the outgoing proton. This feature provides a powerful tool to assign experimentally quantum numbers in proton emitters.Comment: 4 pages, 3 figure

High precision branching ratio measurement for the superallowed β decay of [Formula Presented] A prerequisite for exacting tests of the standard model

Nonanalog Fermi and Gamow-Teller branches in the superallowed β decay of [Formula Presented] have been investigated using γ-ray and conversion-electron spectroscopy. Nine observed transitions, in conjunction with a recent shell model calculation, determine the branching ratio of the analog transition to be 99.5(1)%. The experimental upper limits for the Fermi decay to the [Formula Presented] and [Formula Presented] levels are in agreement with recent theoretical predictions. The [Formula Presented] value for the [Formula Presented] β decay is predicted to be 10405(9) keV. © 2003 The American Physical Society

Fast Rotation of the N=Z Nucleus 36Ar

A highly-deformed rotational band has been identified in the N=Z nucleus 36Ar. At high spin the band is observed to its presumed termination at I=16+, while at low spin it has been firmly linked to previously known states in 36Ar. Spins, parities, and absolute excitation energies have thus been determined throughout the band. Lifetime measurements establish a large low-spin quadrupole deformation (beta_2=0.46+-0.03) and indicate a decreasing collectivity as the band termination is approached. With effectively complete spectroscopic information and a valence space large enough for significant collectivity to develop, yet small enough to be meaningfully approached from the shell model perspective, this rotational band in 36Ar provides many exciting opportunities to test and compare complementary models of collective motion in nuclei

First evidence of excited states in the near-drip-line nucleus 126Pr and signature inversion in A ≈ 130 nuclei

γ-ray transitions have been identified for the first time in the near-drip-line nucleus 126Pr, making it the lightest odd-odd praseodymium nucleus in which excited states have been reported. Evidence is presented for two rotational bands in 126Pr, one strongly coupled and the other doubly decoupled. In addition, the preliminary reports of a band in 128Pr are confirmed. The signature inversion phenomenon and trends in the energy staggering of the π11/2νh11/2 bands are discussed for the Cs, La, and Pr nuclei

Rotational structures in 129Nd and signature splitting systematics of the νh11/2 bands in A ∼ 130 nuclei

High-spin states in 129Nd were populated in the reaction 92Mo(40Ca, 2pn) with beam energies of 170 and 184 MeV in two separate experiments. Over 140 new transitions were placed in a level scheme that consists of four rotational structures, three of which have been observed for the first time. The bands were assigned configurations based on their B(M 1)/B(E2) ratios (for the strongly coupled bands), aligned angular momentum, observed band crossings, and signature splitting. Evidence for of the observation of the deformation driving νi13/2[660] 1/2 orbital is presented. Signature splitting trends in the νh11/2 bands of A ∼ 130 nuclei are investigated within the framework of the cranked shell model. Comparisons are made with observations in the πh11/2 bands near A ∼ 160

Toward complete spectroscopy of 128Pr and rotational structures in 126Pr

Over 240 transitions and three new rotational bands have been observed in the well-deformed, odd-odd 59128Pr69 nucleus. The high-spin states were populated in two experiments using the 92Mo(40Ca,3pn) reaction at beam energies of 170 and 184 MeV. Several structures were confirmed in the former experiment using the Clarion and HyBall arrays with the recoil mass spectrometer at Oak Ridge National Laboratory. Gammasphere, in conjunction with the Microball, were employed in the latter experiment at Argonne National Laboratory to extend the sequences to very high spins. Rotational structures in 126Pr were also identified in the α p n channel of the same reaction. The recent discrepancies of spin assignments for the yrast bands in 126,128Pr and the interpretation of the lowest crossing in the πh11/2 band in 127Pr are discussed. An adiabatic crossing of the intruder i13/2 neutron with one of the normal-deformed bands in 128Pr is observed at high rotational frequency. Experimental trends in the signature inversion phenomenon of the πh11/2νh11/2 bands in the A 130 region are defined and prove to be surprisingly irregular with respect to those found in the πh11/2νi13/2 bands of the A 160 region

Signature inversion in odd-odd nuclei

Trends in the energy staggering of the πh11/2νi13/2 and πh11/2νh11/2 bands in the mass 160 and 130 regions, respectively, have been investigated in order to better understand the origin of signature inversion. While the A ≈ 160 nuclei behave in a consistent manner, a more complicated'scenario is observed in the mass 130 region. As a result of our experiments on the lightest Pr nuclei, the systematics of these Z = 59 nuclei have been extended, which aids in the understanding of the latter region. Triaxial deformation, a pn interaction, and quadrupole pairing are considered as possible contributors to this effect. As all the chiral-twin bands that are known have signature inversion in the favored band, a possible link between the two phenomena should be considered

High precision branching ratio measurement for the superallowed beta decay of Rb-74: A prerequisite for exacting tests of the standard model

Journals published by the American Physical Society can be found at http://publish.aps.org/Nonanalog Fermi and Gamow-Teller branches in the superallowed beta decay of Rb-74 have been investigated using gamma-ray and conversion-electron spectroscopy. Nine observed transitions, in conjunction with a recent shell model calculation, determine the branching ratio of the analog transition to be 99.5(1)%. The experimental upper limits for the Fermi decay to the 0(2)(+) and (0(3)(+)) levels are in agreement with recent theoretical predictions. The Q(EC) value for the Rb-74 beta decay is predicted to be 10405(9) keV

Rigidity of the Doubly-Magic 100Sn Core

The doubly magic nucleus 100Sn represents a crucial test for the nuclear shell model, which is a widely used model for a quantitative description of nuclei. Excited states in 100Sn are experimentally not accessible with present day techniques, but it is possible to gain insight into its structure by studying the properties of excited states of neighboring nuclei. These nuclei are by themselves very interesting since they enable the determination of single-particle energies and two-body matrix elements, which are the basic parameters of the shell model. Excited states of nuclei in the vicinity of 100Sn have been studied in a series of experiments using three different experimental setups. The first experiment utilized the NORDBALL detector array where for the first time excited states in the Tz=-3/2 nuclei 99Cd and 101In were identified. In addition, the level schemes of several nuclei with previously known excited states were significantly extended. The nuclei 103In, 105In, 107In and 109In are described in this thesis. The following experiments used a recoil catcher setup in combination with two EUROBALL cluster detectors, where for the first time excited states in the Tz=-1 nuclei 98Cd and 102Sn were identified. These two nuclei are now the nearest neighbors of 100Sn with known excited states. The last three experiments used the Fragment Mass Analyzer at Argonne National Laboratory to study 102Sn in more detail. In all experiments a beam of 58Ni ions was used to bombard targets of 46Ti and 50Cr. The experimental level schemes of 98Cd, 99Cd, 102Sn and 103In are well reproduced by the shell model calculations. Isomeric states were found in 98Cd, 99Cd and 102Sn and their half-lives were measured. The isomeric states in all three nuclei decay via low energy E2 transitions. The deduced E2 transition rates lead to controversial results for the proton and neutron effective charges, which are also basic shell model parameters. In 102Sn the measured B(E2;6+-4+) value leads to a neutron effective charge of 2.0+0.5-0.3e, which is a fairly large value. The analysis of the isomeric 17/2+ state in 99Cd suggests a proton effective charge of 1.4(1) e, whereas the measured B(E2;8+-6+) value in 98Cd requires a proton effective charge of only 0.93+0.14-0.10e. The small proton effective charge in 98Cd is very surprising, since it is even smaller than the bare proton charge. In all other nuclei studied so far, the effective charges are larger than the bare nucleon charges, and usually the neutron effective charge is lower than the proton effective charge. The large neutron effective charge points to a softness of the 100Sn core with respect to quadrupole shape changes, whereas the two proton holes weakly polarize the core in 99Cd and not at all in 98Cd