Structure of C 14 and B 14 from the C 14 , 15 ( d , He 3 ) B 13 , 14 reactions
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Bedoor, S.↗
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Results for “SHELL MODELS”
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Reexamining Gamow-Teller decays near Ni 78
Decays of neutron-rich nuclei 82,83 Zn and 82,83 Ga produced in proton-induced fission of 238 U were studied at the Holifield Radioactive Ion Beam Facility using on-line mass separation and β–γ spectroscopy techniques. New γ-ray transitions were identified and level schemes, which include states at high excitation energies in the range between 3–7 MeV were constructed. These high-energy levels were identified to be populated through allowed Gamow-Teller β transitions, and their structure was interpreted with new shell-model calculations. Furthermore, a β-delayed neutron branching ratio of 69±7% was deduced for 82 Zn and revised β-decay half-life values of 82 Zn [155(17)(20) ms] and 83 Zn [122(28) ms] were determined.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
Experimentally constrained ( p , γ ) Y 89 and ( n , γ ) Y 89 reaction rates relevant to p -process nucleosynthesis
The nuclear level density and the g-ray strength function have been extracted for 89 Y, using the Oslo Method on 89 Y(p,p'γ) 89 Y coincidence data. The g-ray strength function displays a low-energy enhancement consistent with previous observations in this mass region ( 93-98 Mo). Shell-model calculations give support that the observed enhancement is due to strong, low-energy M1 transitions at high excitation energies. The data were further used as input for calculations of the 88 Sr(p,γ) 89 Y and 88 Y(n,γ) 89 Y cross sections with the TALYS reaction code. Lastly, comparison with cross-section data, where available, as well as with values from the BRUSLIB library, shows a satisfying agreement.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
Low-energy enhancement in the γ -ray strength functions of Ge 73 , 74
Here, t≤he γ-ray strength functions and level densities of 73,74 Ge have been extracted up to the neutron-separation energy S n from particle-γ coincidence data using the Oslo method. Moreover, the γ-ray strength function of 74 Ge above S n has been determined from photoneutron measurements; hence these two experiments cover the range of Eγ ≈ 1–13 MeV for 74 Ge. The obtained data show that both 73,74 Ge display an increase in strength at low γ energies. The experimental γ-ray strength functions are compared with M1 strength functions deduced from average B(M1) values calculated within the shell model for a large number of transitions. The observed low-energy enhancements in 73,74 Ge are adopted in the calculations of the 72,73 Ge(n,γ) cross sections, where there are no direct experimental data. Calculated reaction rates for more neutron-rich germanium isotopes are shown to be strongly dependent on the presence of the low-energy enhancement.
46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
Change of Nuclear Configurations in the Neutrinoless Double-β Decay of 130 Te → 130 Xe and 136 Xe → 136 Ba
The change in the configuration of valence protons between the initial and final states in the neutrinoless double-beta decay of Te-130 -> Xe-130 and of Xe-136 -> Ba-136 has been determined by measuring the cross sections of the (d,He-3) reaction with 101-MeV deuterons. Together with our recent determination of the relevant neutron configurations involved in the process, a quantitative comparison with the latest shell-model and interacting-boson-model calculations reveals significant discrepancies. These are the same calculations used to determine the nuclear matrix elements governing the rate of neutrinoless double-beta decay in these systems.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
Total absorption spectroscopy of the β decay of 76 Ga
The β decay of 76 Ga was studied using the technique of total absorption spectroscopy for the first time. The experiment was performed at the National Superconducting Cyclotron Laboratory using the Summing NaI(Tl) detector. The extracted β-decay feeding intensity distribution and Gamow-Teller transition strength distribution are compared to shell-model calculations to help constrain nuclear matrix elements relevant to the neutrinoless double-β decay of 76 Ge.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
β and β – n decay of the neutron-rich Ge 84 nucleus
In this study, the β-decay properties of the very neutron-rich 84 Ge nucleus were studied at the Holifield Radioactive Ion Beam Facility at Oak Ridge National Laboratory. Several new γ-transitions and levels were added to its decay scheme and the order of the two lowest-lying levels in the daughter 84 As was corrected. For the first time γ radiation following β-delayed neutron emission was observed. The shell-model calculations and apparent β transition intensities were used to guide the spin assignment to the 84 As levels, in particular for the low-energy part of the level scheme. The new spin-parity (2 – ) proposed for the ground state of 84 As is supported also by the systematics of N = 51 isotones.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
One-neutron transfer study of Xe 137 and systematics of 13 / 2 1 + and 13 / 2 2 + levels in N = 83 nuclei
Excited states in 137 Xe have been studied by using the near-barrier single-neutron transfer reactions 13 C( 136 Xe, 12 C ) 137 Xe and 9 Be( 136 Xe, 8 Be ) 137 Xe in inverse kinematics.Particle- and particle- coincidence measurements have been performed with the Phoswich Wall and Digital Gammasphere detector arrays. Evidence is found for a 13/2 + 2 level (E = 3137 keV) and for additional high-lying 3/2 – and 5/2 – states. The results are discussed in the framework of realistic shell-model calculations. These calculations are also extended to the 13/2 + 1 and 13/2 + 2 levels in the N = 83 isotonic chain. Furthermore, they indicate that there is a need for a value of the neutron 0i 13/2 single-particle energy (E SPE = 2366 keV) lower than the one proposed in the literature. It is also demonstrated that the population patterns of the j = l ± 1/2 single-particle states in 137 Xe are different for the two targets used in these measurements and the implications of this effect are addressed.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
The Mg 30 ( t , p ) Mg 32 “puzzle” reexamined
Background: Competing interpretations of the results of a Mg 30 (t,p)Mg 32 measurement populating the ground state and 0$+\atop{2}$ state in Mg 32 , both limited to a two-state mixing description, have left an open question regarding the nature of the Mg 32 ground state. Purpose: Inspired by recent shell-model calculations, we explore the possibility of a consistent interpretation of the available data for the low-lying 0 + states in Mg 32 by expanding the description from two-level to three-level mixing. Methods: A phenomenological three-level mixing model of unperturbed 0p0h, 2p2h, and 4p4h states is applied to describe both the excitation energies in Mg 32 and the transfer reaction cross sections. Results: Within this approach, self-consistent solutions exist that provide good agreement with the available experimental information obtained from the Mg 30 (t,p)Mg 32 reaction. Conclusion: The inclusion of the third state, namely the 4p4h configuration, resolves the "puzzle" that results from a two-levelmodel interpretation of the same data. In our analysis, the Mg 32 ground state emerges naturally as dominated by intruder (2p2h and 4p4h) configurations, at the 95% level.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
First identification of excited states in Ba 117 using the recoil- β -delayed proton tagging technique
Excited states have been observed for the first time in the neutron-deficient nucleus 117 Ba using the recoil-decay tagging technique following the heavy-ion fusion-evaporation reaction 64 Zn( 58 Ni, 2p3n) 117 Ba. Prompt γ rays have been assigned to 117 Ba through correlations with β-delayed protons following the decay of A = 117 recoils. Through the analysis of the γ–γ coincidence relationships, a high-spin level scheme consisting of two bands has been established in 117 Ba. Based on the systematics of the level spacings in the neighboring barium isotopes, the two bands are proposed to have νh 11/2 [532]5/2 – and νd 5/2 [413]5/2 + configurations, respectively. Lastly, the observed band-crossing properties are interpreted in the framework of cranked shell model.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
β decay of Si 38 , 40 ( T z = + 5 , + 6 ) to low-lying core excited states in odd-odd P 38 , 40 isotopes
Low-lying excited states in P 38,40 have been identified in the β decay of T z =+5,+6, Si 38,40 . Based on the allowed nature of the Gamow-Teller (GT) decay observed, these states are assigned spin and parity of 1 + and are core-excited 1p1h intruder states with a parity opposite to the ground state. The occurrence of intruder states at low energies highlights the importance of pairing and quadrupole correlation energies in lowering the intruder states despite the N=20 shell gap. Configuration interaction shell model calculations with the state-of-art SDPF-MU effective interaction were performed to understand the structure of these 1p1h states in the even-A phosphorus isotopes. States in P 40 with N=25 were found to have very complex configurations involving all the fp orbitals leading to deformed states as seen in neutron-rich nuclei with N≈28. The calculated GT matrix elements for the β decay highlight the dominance of the decay of the core neutrons rather than the valence neutrons.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
New excitations in Ba 142 and Ce 144 : Evolution of γ bands in the N = 86 isotones
New excited states in 142 Ba and 144 Ce are investigated by means of prompt γ-ray spectroscopy of the radiation following spontaneous fission of 252 Cf. Measurements of angular correlations and the observed branchings allowed the assignment of spins and parities with confidence. The new measurements are reinforced by shell-model calculations where energy levels, electric transitions, and magnetic moments are consistent with experimental data. Lastly, the presence of collectivity in the N = 86 isotones is confirmed by clear signatures of soft triaxial γ bands in both nuclei.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
Doubly magic Pb 208 : High-spin states, isomers, and E 3 collectivity in the yrast decay
Yrast and near-yrast levels up to spin values in excess of I = 30h have been delineated in the doubly-magic 208 Pb nucleus following deep-inelastic reactions involving 208 Pb targets and, mostly, 430-MeV 48 Ca and 1440-MeV 208 Pb beams. The level scheme was established up to an excitation energy of 16.4 MeV, based on multi-fold γ-ray coincidence relationships measured with the Gammasphere array. Below the well-known, 0.5-μs 10 + isomer, ten new transitions were added to earlier work. The delineation of the higher parts of the level sequence benefited from analyses involving a number of prompt- and delayed-coincidence conditions. Three new isomeric states were established along the yrast line with I π = 20 - (10342 keV), 23 + (11361 keV), and 28 - (13675 keV), and respective half-lives of 22(3), 12.7(2), and 60(6) ns. Gamma transitions were also identified preceding in time the 28 - isomer, however, only a few could be placed in the level scheme and no firm spin-parity quantum numbers could be proposed. In contrast, for most states below this 28 - isomer, firm spin-parity values were assigned, based on total electron-conversion coefficients, deduced for low-energy (<500 keV) transitions from γ-intensity balances, and on measured γ-ray angular distributions. The latter also enabled the quantitative determination of mixing ratios. The transition probabilities extracted for all isomeric transitions in 208 Pb have been reviewed and discussed in terms of the intrinsic structure of the initial and final levels involved. Particular emphasis was placed on the many observed E3 transitions as they often exhibit significant enhancements in strength (of the order of tens of W.u.) comparable to the one seen for the neutron j 15/2 →g 9/2 E3 transition in 209 Pb. In this context, the enhancement of the 725-keV E3 transition (56 W.u.) associated with the decay of the highest-lying 28 - isomer observed in this work remains particularly challenging to explain. Large-scale shell-model calculations were performed with two approaches, a first one where the 1, 2, and 3 particle-hole excitations do not mix with one another, and another more complex one, in which such mixing takes place. We compared the calculated levels with the data and a general agreement is observed for most of the 208 Pb level scheme. At the highest spins and energies, however, the 2 correspondence between theory and experiment is less satisfactory and the experimental yrast line appears to be more regular than the calculated one. This regularity is notable when the level energies are plotted versus the I(I+1) product and the observed, nearly linear, behavior was considered within a simple “rotational” interpretation. Furthermore, within this approximate picture, the extracted moment of inertia suggests that only the 76 valence nucleons participate in the “rotation” and that the 132 Sn spherical core remains inert.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
First-excited state g factor of Te 136 by the recoil in vacuum method
The g factor of the first 2 + state of radioactive 136 Te with two valence protons and two valence neutrons beyond double-magic 132 Sn has been measured by the recoil in vacuum (RIV) method. The lifetime of this state is an order of magnitude longer than the lifetimes of excited states recently measured by the RIV method in Sn and Te isotopes, requiring a new evaluation of the free-ion hyperfine interactions and methodology used to determine the g factor. In this paper, the calibration data are reported and the analysis procedures are described in detail. The resultant g factor has a similar magnitude to the g factors of other nuclei with an equal number of valence protons and neutrons in the major shell. However, an unexpected trend is found in the g factors of the N = 84 isotones, which decrease from 136 Te to 144 Nd. Finally, shell model calculations with interactions derived from the CD Bonn potential show good agreement with the g factors and E2 transition rates of 2 + states around 132 Sn, confirming earlier indications that 132 Sn is a good doubly magic core.
74 ATOMIC AND MOLECULAR PHYSICS↗
High-spin structures in Xe 132 and Xe 133 and evidence for isomers along the N = 79 isotones
In this study, the transitional nuclei 132 Xe and 133 Xe are investigated after multinucleon-transfer (MNT) and fusion-evaporation reactions. Both nuclei are populated (i) in 136 Xe + 208 Pb MNT reactions employing the high-resolution Advanced GAmma Tracking Array (AGATA) coupled to the magnetic spectrometer PRISMA, (ii) in the 136 Xe + 198 Pt MNT reaction employing the GAMMASPHERE spectrometer in combination with the gas-detector array CHICO, and (iii) as an evaporation residue after a 130 Te (α,xn) 134-xn Xe fusion-evaporation reaction employing the HORUS γ -ray array at the University of Cologne. The high-spin level schemes are considerably extended above the J π = (7 - ) and (10 + ) isomers in 132 Xe and above the 11/2 - isomer in 133 Xe. The results are compared to the high-spin systematics of the Z = 54 as well as the N = 78 and N = 79 chains. Furthermore, evidence is found for a long-lived (T 1/ 2 » 1 μs) isomer in 133 Xe which closes a gap along the N = 79 isotones. Finally, shell-model calculations employing the SN100PN and PQM130 effective interactions reproduce the experimental findings and provide guidance to the interpretation of the observed high-spin features.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
Quasiparticle and γ -band structures in Dy 156
Excited band structures recently observed in 156 Dy are investigated using the microscopic triaxial projected shell model (TPSM) approach and the quasiparticle random phase approximation (QRPA) based on the rotating mean field. It is demonstrated that new observed excited bands, tracking the ground-state band, are the γ bands based on the excited two-quasineutron configurations as conjectured in the experimental work.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
Quantum Monte Carlo calculations of weak transitions in A = 6 – 10 nuclei
{\it Ab initio} calculations of the Gamow-Teller (GT) matrix elements in the $\beta$ decays of $^6$He and $^{10}$C and electron captures in $^7$Be are carried out using both variational and Green's function Monte Carlo wave functions obtained from the Argonne $v_{18}$ two-nucleon and Illinois-7 three-nucleon interactions, and axial many-body currents derived from either meson-exchange phenomenology or chiral effective field theory. The agreement with experimental data is excellent for the electron captures in $^7$Be, while theory overestimates the $^6$He and $^{10}$C data by $\sim 2\%$ and $\sim 10\%$, respectively. We show that for these systems correlations in the nuclear wave functions are crucial to explain the data, while many-body currents increase by $\sim 2$--$3\%$ the one-body GT contributions. These findings suggest that the longstanding $g_A$-problem, {\it i.e.}, the systematic overprediction ($\sim 20 \%$ in $A\le 18$ nuclei) of GT matrix elements in shell-model calculations, may be resolved, at least partially, by correlation effects.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗