Excited states were investigated in 21 F and 25 Na using the 9 Be( 14 C,pnγ) reaction at 30, 35, and 45 MeV and the 9 Be( 18 O,pnγ) reaction at 35 MeV. Protons were detected and identified in an E-∆E telescope at 0 o in coincidence with one or more γ radiations in the FSU Compton-suppressed Ge detector array. Many new levels and electromagnetic decays were observed, especially among the higher spin states. Angular distributions and mean lifetimes were measured wherever possible in both nuclei. The energy levels of the positive-parity states in the two nuclei agree rather well with shell model calculations using both the USDA and WBP interactions up to the highest spins observed of 13/2h. Both a weak coupling approximation and shell model calculations using the WBP interaction generally reproduce the negative-parity states in 21 F. The shell model calculations reproduce relatively well the measured M1 and E2 transitions in both nuclei, but overpredict the parity-changing E1 transitions in 21 F, the only nucleus in which negative-parity states were observed in the present experiment.

The observation that Type Ia supernovae (SNe Ia) are fainter than expected given their red shifts has led to the conclusion that the expansion of the universe is accelerating. The widely accepted hypothesis is that this acceleration is caused by a cosmological constant or, more generally, some dark energy field that pervades the universe. In this paper, we explore what, on their own, the supernovae data tell us about this hypothesis. We do so by answering the following question: can these data be explained with a model in which the strength of gravity varies on a cosmic timescale?We conclude that they can. Consequently, the supernovae data alone are insufficient to distinguish between a model with a cosmological constant and one in which G varies. However, the varying-G models prove not to be viable when other data are taken into account. This topic is an ideal one for investigation by an undergraduate physics major because the entire chain of reasoning from models to data analysis is well within the mathematical and conceptual sophistication of a motivated undergraduate. arXiv:0909.5416v2 [astro-ph.CO]

Low-spin states in the neutron-rich, N = 90 nuclide 146 Ba were populated following β-decay of 146 Cs, with the goal of clarifying the development of deformation in Ba isotopes through delineation of their non-yrast structures. Fission fragments of 146 Cs were extracted from a 1.7-Ci 252 Cf source and mass-selected using the CARIBU facility. Low-energy ions were deposited at the center of a box of thin β detectors, surrounded by a high-efficiency HPGe array. The new 146 Ba decay scheme now contains 31 excited levels extending up to ∼2.5 MeV excitation energy, double what was previously known. These data are compared to predictions from the Interacting Boson Approximation (IBA) model. It appears that the abrupt shape change found at N = 90 in Sm and Gd is much more gradual in Ba and Ce, due to an enhanced role of the γ degree of freedom.

The structure of 29 Al and 27 Mg was investigated using the reactions 18 O( 14 C,p2n) and 18 O( 14 C,α n) at 40 MeV. The charged particles were detected and identified with a ∆E-E telescope in coincidence with γ radiation detected in the Florida State University (FSU) Compton suppressed γ detector array. The level and decay schemes of both nuclei have been expanded at higher spins and excitation energies. The positive-parity states up to 3.5 to 4.5 MeV agree well with shell model calculations using the USDA interaction. The negative-parity states in 27 Mg are reproduced relatively well by one-particle-hole calculations with the WBP-a interaction. Three 27 Mg states unbound by 0.4 to 1.4 MeV to neutron decay were observed to decay radiatively. One of these states had been previously observed to γ decay in a (d,pγ) experiment along with a surprising 16 other neutron unbound states. The competition between neutron and gamma decay in these states is discussed in terms of angular momentum barriers and spectroscopic factors.

High-spin states in 39 Ar were populated using the 27 Al(14 C, pn) reaction at 25.6 MeV. The deexciting γ rays were measured with the FSU γ detector array along with evaporation protons in a Si E-∆E telescope. The known high-spin level scheme was extended up to over 11 MeV with a dozen new levels above the neutron decay threshold. The decay pattern appears somewhat atypical for heavy-ion fusion-evaporation reactions. The structure of 39 Ar is discussed in terms of the new FSU cross-shell spsdpf interaction fitted to a wide range of nuclei. This interaction has proved quite successful in accounting for the level scheme of 39 Ar, including the previously suggested fully aligned πf 7/2 ⊗ νf 7/2 17/2 + state and previously discovered analogs of the lowest states in 39 Cl.

The 9 Be( 14 C, αγ) reaction at E Lab =30 and 35 MeV was used to study excited states of 19 O. The Florida State University (FSU) γ detector array was used to detect γ radiation in coincidence with charged particles detected and identified with a silicon ∆E-E particle telescope. Gamma decays have been observed for the first time from six states ranging from 368 to 2147 keV above the neutron separation energy (S n =3962 keV) in 19 O. The γ decaying states are interspersed among states previously observed to decay by neutron emission. The ability of electromagnetic decay to compete successfully with neutron decay is explained in terms of neutron angular momentum barriers and small spectroscopic factors implying higher spin and complex structure for these intruder states. These results illustrate the need for complementary experimental approaches to best illuminate the complete nuclear structure. 1The decay mode(s) of a nuclear quantum state is (are) one of its most important properties after energy. Usually nuclear decay follows the hierarchy of the fundamental forces of nature.Decay by emission of one or more particles mediated by the strong nuclear interaction is normally the dominant mode for those states unbound to particle emission. Bound excited states usually decay by emission of electromagnetic radiation to the ground state. The ground state, in turn, decays much more slowly by β decay mediated by the weak nuclear interaction until the lowest energy neutron-to-proton ratio has been reached. Of course, there are exceptions: lower energy charged particle decay from unbound states is inhibited by the Coulomb barrier, and large spin change or low emission energy can inhibit γ decay so much that β decay occurs first. Even decay by neutrons which face no Coulomb barrier can be inhibited by large spin change, as is well known in high-spin spectroscopy of medium and heavy nuclei. However, based on the simple picture of barrier penetrability, neutron decay is usually assumed to dominate over radiative decay when angular momentum barriers are not very high. But Fermi's Golden Rule [1] states that the decay rate is the product of the coupling strength, the density of final states, and the matrix element between the wavefunction of the parent state and the daughter state (usually called spectroscopic factor (S)). What is often overlooked is that this latter factor due to the nuclear structure may be more instrumental than the angular momentum barrier in limiting neutron decay rates below those of electromagnetic decay. Examples are states with complex structure often involving intruder configurations.The role of factors beyond barrier penetrability is highlighted by observations reported in this paper of the decays of unbound states in 19 O populated in the 9 Be( 14 C,αγ) reaction which favors higher-spin states, which are typically more complex and may involve intruder configurations. The importance of 19 O with a closed major proton shell and an exactly halffilled neutron d 5/2 orbital has been recognized wi...

We report the results of a study of rotational bands in 219 Ra via the 208 Pb( 14 C,3n) reaction to look for evidence that this nucleus is statically octupole deformed. We add 19 γ rays not previously observed to the level scheme and extend the two most strongly populated alternating parity bands to J=51/2 and 45/2. The magnitude of the energy splitting between the spin-parity doublets in the two bands appears to exclude the possibility that 219 Ra has a static octupole deformation.PACS numbers: May be entered using the \pacs{#1} command.

β − decay of T z = + 11 2 nuclei, 37 Al and 39 Si, was studied to obtain information about excited states in the daughter and granddaughter nuclei. New information on excited states has been obtained for 37,39 P while level-schemes for 37 Si and 37,39 S were confirmed and expanded on as compared to the most recent investigations. For the nuclei studied, the valence proton and neutron occupy different major shells with opposite parity, hence the β − decay preferentially and primarily populates core-excited 1p1h intruder states with a parity opposite to the ground state of the daughter nucleus. These low-lying intruder states shed light on the N = 20 shell gap and its evolution with neutron number. The β − decay of very neutron rich nuclei also illustrates how the Gamow Teller (GT) strength distribution changes with neutron excess. Comparing β − decay of 37,39 Si presented here with 38,40 Si from a previous study, we clearly see an even-odd effect in the GT strength distribution with the decay from even-even nuclei showing strong low lying strength while for the odd A being very fragmented. Configuration interaction shell model calculations with the state-of-art SDPFSDG-MU effective interaction were performed to understand the structure of the 1p1h states and explore the GT strength distribution.