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The design and performance of a new Compton-suppressed HPGe and charged-particle array, CLARION2-TRINITY, are described. The TRINITY charged-particle array is comprised of 64 Cerium-doped Gadolinium Aluminium Gallium Garnet (GAGG:Ce) crystals configured into five rings spanning 7-54 degrees, and two annular silicon detectors that can shadow or extend the angular coverage to backward angles with minimal $\gamma$-ray attenuation. GAGG:Ce is a non-hygroscopic, bright, and relatively fast scintillator with a light distribution well matched to SiPMs. Count rates up to 40 kHz per crystal are sustainable. Fundamental characteristics of GAGG:Ce are measured and presented, including light- and heavy-ion particle identification (PID) capability, pulse-height defects, radiation hardness, and emission spectra. The CLARION2 array consists of up to 16 Compton-suppressed HPGe Clover detectors ($\approx4\%$ efficiency at 1 MeV) configured into four rings (eight HPGe crystal rings) using a non-Archimedean geometry that suppresses back-to-back coincident 511-keV gamma rays. The entire array is instrumented with 100- and 500-MHz (14 bit) waveform digitizers which enable triggerless operation, pulse-shape discrimination, fast timing, and pileup correction. Finally, two examples of experimental data taken during the commissioning of the CLARION2-TRINITY system are given: a PID spectrum from $^{16}$O + $^{18}$O fusion-evaporation, and PID and Doppler-corrected $\gamma$-ray spectra from $^{48}$Ti + $^{12}$C Coulomb excitation., Comment: 14 pages, 21 figures

A measurement of the $^{50}\mathrm{Ti}(d,p)^{51}\mathrm{Ti}$ reaction at 16 MeV was performed using the Super Enge Split-Pole Spectrograph to measure the magnitude of the $N=32$ subshell gap in Ti. Seven states were observed that had not been observed in previous $(d,p)$ measurements, and the $L$ transfer values for six previously measured states were either changed or measured for the first time. The results were used to determine single neutron energies for the ${p}_{3/2}$, ${p}_{1/2}$, and ${f}_{5/2}$ orbitals. The resulting single neutron energies in $^{51}\mathrm{Ti}$ confirm the existence of the $N=32$ gap in Ti. These single neutron energies and those from previous measurements in $^{49}\mathrm{Ca}$, $^{53}\mathrm{Cr}$, and $^{55}\mathrm{Fe}$ are compared to values from a covariant density functional theory calculation.

The total fusion cross section for the $^{17}\mathrm{F}+^{12}\mathrm{C}$ system at incident energies near the top of the Coulomb barrier was studied using the newly developed Encore active-target detector at Florida State University. The $^{17}\mathrm{F}$ nucleus exhibits interesting nuclear structure properties in that although it has a low threshold against $^{17}\mathrm{F}\ensuremath{\rightarrow}^{16}\mathrm{O}+p$ breakup $({S}_{p}=600\phantom{\rule{0.28em}{0ex}}\mathrm{keV})$, the valence proton is in the $1{d}_{5/2}$ shell in the ground state so that the nuclear matter radius is predicted to be similar to that of the $^{16}\mathrm{O}$ core. By contrast, the low-lying $1/{2}^{+}$ first excited state (bounded by 105 keV) with the valence proton in the $2{\phantom{\rule{4pt}{0ex}}s}_{1/2}$ shell is considered to be a proton halo. In this paper possible influences of both the weak binding and the halo nature of the excited state on the total fusion cross section were investigated. The new data reported here complement existing measurements for the total fusion of $^{17}\mathrm{F}$ with heavy $(^{208}\mathrm{Pb})$ and medium mass ($^{58}\mathrm{Ni}$) targets by extending the range of systems studied to one where Coulomb effects should be minimal. Total fusion cross sections for the stable counterpart systems $^{16}\mathrm{O}+^{12}\mathrm{C}$ and $^{19}\mathrm{F}+^{12}\mathrm{C}$ were also measured to enable a systematic comparison. No significant influence of either the weak binding or the halo nature of the $^{17}\mathrm{F}\phantom{\rule{0.28em}{0ex}}1/{2}^{+}$ first excited state on the above barrier total fusion excitation function was observed when compared with the stable counterpart systems.

Background: Resonance scattering has been extensively used to study the structure of exotic, neutron-deficient nuclei. Extension of the resonance scattering technique to neutron-rich nuclei was suggested more than 20 years ago. This development is based on the isospin conservation law. In spite of broad field of the application, it has never gained a wide-spread acceptance.Purpose: To benchmark the experimental approach to study the structure of exotic neutron-rich nuclei through resonance scattering on a proton target.Method: The excitation function for $p+^{8}\mathrm{Li}$ resonance scattering is measured using a thick target by recording coincidence between light and heavy recoils, populating $T=3/2$ isobaric analog states (IAS) in $^{9}\mathrm{Be}$.Results: A good fit of the $^{8}\mathrm{Li}(p,p)^{8}\mathrm{Li}$ resonance elastic scattering excitation function was obtained using previously tentatively known $5/{2}^{\ensuremath{-}}\phantom{\rule{4pt}{0ex}}T=3/2$ state at 18.65 MeV in $^{9}\mathrm{Be}$ and a new broad $T=3/2\phantom{\rule{4pt}{0ex}}s$-wave state---the $5/{2}^{+}$ at 18.5 MeV. These results fit the expected isomirror properties for the $T=3/2\phantom{\rule{4pt}{0ex}}A=9$ isoquartet.Conclusions: Our analysis confirmed isospin as a good quantum number for the investigated highly excited $T=3/2$ states and demonstrated that studying the structure of neutron-rich exotic nuclei through IAS is a promising approach.

The detection of the neutrinos produced in the p − p chain and in the CNO cycle can be used to test the Standard Solar Model. The 3He( α , γ )7Be reaction is the first reaction of the 2nd and 3rd branch of the p − p chain, therefore, the uncertainty of its cross section sensitively influences the prediction of the 7Be and 8B neutrino fluxes. Despite its importance and the large number of experimental and theoretical works devoted to this reaction, the knowledge on the reaction cross section at energies characterizing the core of the Sun (15 keV - 30 keV) is limited and further experimental efforts are needed to reach the desired (≈ 3%) accuracy. The precise knowledge on the external capture contribution to the 3He( α , γ )7Be reaction cross section is crucial for the theoretical description of the reaction mechanism. In the present work the indirect measurement of this external capture contribution using the Asymptotic Normalization Coefficient (ANC) technique is reported. To extract the ANC, the angular distributions of deuterons emitted in the 6Li(3He,d)7Be α-transfer reaction were measured with high precision at E H 3 e = 3.0 MeV and E H 3 e = 5.0 MeV. The ANCs were then extracted from comparison of DWBA calculations to the measured data and the zero energy astrophysical S-factor for 3He( α , γ )7Be reaction was found to be 0.534 ± 0.025 keVb.

The resoneut detector setup is described, which was developed for resonance spectroscopy using (d,n) reactions with radioactive beams in inverse kinematics and at energies around the Coulomb barrier. The goal of experiments with this setup is to determine the spectrum and proton-transfer strengths of the low-lying resonances, which have an impact on astrophysical reaction rates. The setup is optimized for l = 0 proton transfers in inverse kinematics, for which most neutrons are emitted at backward angles with energies in the 80–300 keV range. The detector system is comprised of 9 p-terphenyl scintillators as neutron detectors, two annular silicon-strip detectors for light charged particles, one position-resolving gas ionization chamber for heavy ion detection, and a barrel of NaI-detectors for the detection of γ -rays. The detector commissioning and performance characteristics are described with an emphasis on the neutron-detector components.

Measuring the fusion excitation function for an isotopic chain of projectile nuclei provides a stringent test of a microscopic description of fusion. We report the first measurement of the fusion excitation function at near-barrier energies for the 19 O + 12 C system. The measured excitation function is compared with the fusion excitation function of 18 O + 12 C. A significant enhancement in the fusion probability of 19 O ions with a 12 C target as compared to 18 O ions is observed. The experimental cross-sections observed at near-barrier energies are compared with a state-of-the-art microscopic model.

The cross sections of nuclear reactions between the radioisotope $^{7}\mathrm{Be}$ and deuterium, a possible mechanism of reducing the production of mass-7 nuclides in big-bang nucleosynthesis, were measured at center-of-mass energies between 0.2 and 1.5 MeV. The measured cross sections are dominated by the $(d,\ensuremath{\alpha})$ reaction channel, towards which prior experiments were mostly insensitive. A new resonance at 0.36(5) MeV with a strength of $\ensuremath{\omega}\ensuremath{\gamma}=1.7(5)\text{ }\text{ }\mathrm{keV}$ was observed inside the relevant Gamow window. Calculations of nucleosynthesis outcomes based on the experimental cross section show that the resonance reduces the predicted abundance of primordial $^{7}\mathrm{Li}$, but not sufficiently to solve the primordial lithium problem.

In nuclear facilities utilizing plutonium, polymeric materials are subjected to long-term, close-contact, and continuous α radiation exposure, which can lead to compounding material degradation and eventual failure. Herein we model the attenuation of α particles by linear-low-density polyethylene (LLDPE), polyvinyl alcohol (PVA) thin films, and C60 using Monte Carlo N-Particle Extended (MCNPX) software. The degradation of these materials was investigated experimentally by irradiating them with a beam of α particles of 5.8 MeV energy at a tandem Van de Graaff accelerator delivering a dose rate of 2.95 × 106 rad s−1 over a 7.1 mm2 sample area. Our development of a method to test α particle-induced material degradation using a tandem accelerator is significant as degradation from naturally occurring α sources (i.e. Pu, Am) occurs too slowly for these sources to be used in practical experiments. Our results show that PVA nanocomposites containing 5 wt% C60 were found to withstand about 7 times the α dose of undoped PVA films before a puncture in the film was detected. When these films were adhered to a LLDPE sheet the dual layer polymer was capable of withstanding about 13 times the dose of LLDPE and nearly twice the dose of the doped PVA thin film alone. Doping polymers with C60 is an attractive way to generate more durable, radiation tolerant materials without increasing the thickness of the material which would lead to greater waste for disposal. Thus, the results herein help to resolve a prevalent technical challenge faced in nuclear facilities that utilize polymeric materials for nuclear processing and disposal.

A beam containing a substantial component of both the J^{π}=5^{+}, T_{1/2}=162 ns isomeric state of ^{18}F and its 1^{+}, 109.77-min ground state is utilized to study members of the ground-state rotational band in ^{19}F through the neutron transfer reaction (d,p) in inverse kinematics. The resulting spectroscopic strengths confirm the single-particle nature of the 13/2^{+} band-terminating state. The agreement between shell-model calculations using an interaction constructed within the sd shell, and our experimental results reinforces the idea of a single-particle-collective duality in the descriptions of the structure of atomic nuclei.

A beam containing a substantial component of both the $J^{\pi}=5^+$, $T_{1/2}=162$ ns isomeric state of $^{18}$F and its $1^+$, 109.77-min ground state has been utilized to study members of the ground-state rotational band in $^{19}$F through the neutron transfer reaction $(d$,$p)$ in inverse kinematics. The resulting spectroscopic strengths confirm the single-particle nature of the 13/2$^+$ band-terminating state. The agreement between shell-model calculations, using an interaction constructed within the $sd$ shell, and our experimental results reinforces the idea of a single-particle/collective duality in the descriptions of the structure of atomic nuclei.

Background: The $^{17}\mathrm{F}(p,\ensuremath{\gamma})^{18}\mathrm{Ne}$ reaction is part of the astrophysical ``hot CNO'' cycles that are important in astrophysical environments like novas. Its thermal reaction rate is low owing to the relatively high energy of the resonances and therefore is dominated by direct, nonresonant capture in stellar environments at temperatures below 0.4 GK.Purpose: An experimental method is established to extract the proton strength to bound and unbound states in experiments with radioactive ion beams and to determine the parameters of direct and resonant capture in the $^{17}\mathrm{F}(p,\ensuremath{\gamma})^{18}\mathrm{Ne}$ reaction.Method: The $^{17}\mathrm{F}(d,n)^{18}\mathrm{Ne}$ reaction is measured in inverse kinematics using a beam of the short-lived isotope $^{17}\mathrm{F}$ and a compact setup of neutron, proton, $\ensuremath{\gamma}$-ray, and heavy-ion detectors called resoneut.Results: The spectroscopic factors for the lowest $l=0$ proton resonances at ${\mathrm{E}}_{\mathrm{c}.\mathrm{m}.}=0.60$ and 1.17 MeV are determined, yielding results consistent within $1.4\ensuremath{\sigma}$ of previous proton elastic-scattering measurements. The asymptotic normalization coefficients of the bound ${2}_{1}^{+}$ and ${2}_{2}^{+}$ states in $^{18}\mathrm{Ne}$ are determined and the resulting direct-capture reaction rates are extracted.Conclusions: The direct-capture component of the $^{17}\mathrm{F}(p,\ensuremath{\gamma})^{18}\mathrm{Ne}$ reaction is determined for the first time from experimental data on $^{18}\mathrm{Ne}$.

Previous experiments observed a 4^{+} state in the N=28 nucleus ^{44}S and suggested that this state may exhibit a hindered E2-decay rate, inconsistent with being a member of the collective ground state band. We populate this state via two-proton knockout from a beam of exotic ^{46}Ar projectiles and measure its lifetime using the recoil distance method with the GRETINA γ-ray spectrometer. The result, 76(14)_{stat}(20)_{syst} ps, implies a hindered transition of B(E2;4^{+}→2_{1}^{+})=0.61(19) single-particle or Weisskopf units strength and supports the interpretation of the 4^{+} state as a K=4 isomer, the first example of a high-K isomer in a nucleus of such low mass.

Reactions on certain proton-rich, radioactive nuclei have been shown to have a significant influence on X-ray bursts. We provide an overview of two recent measurements of important X-ray burst reactions using in-flight radioactive ion beams from the RESOLUT facility at the J. D. Fox Superconducting Accelerator Laboratory at Florida State University. The 17 F(d,n) 18 Ne reaction was measured, and Asymptotic Normalization Coefficients were extracted for bound states in 18 Ne that determine the direct-capture cross section dominating the 17 F(p, γ ) 18 Ne reaction rate for T≲ 0.45 GK. Unbound resonant states were also studied, and the single-particle strength for the 4.523-MeV (3 + ) state was found to be consistent with previous results. The 19 Ne(d,n) 20 Na proton transfer reaction was used to study resonances in the 19 Ne(p, γ ) 20 Na reaction. The most important 2.65-MeV state in 20 Na was observed to decay by proton emission to both the ground and first-excited states in 19 Ne, providing strong evidence for a 3 + spin assignment and indicating that proton capture on the thermally-populated first-excited state in 19 Ne is an important contributor to the 19 Ne(p, γ ) 20 Na reaction rate.

Measurement of the fusion excitation function for 18 O + 12 C and 19 O + 12 C is described. The fusion cross-section is extracted through the direct measurement of evaporation residues resulting from the fusion process. At near barrier energies, the single additional neutron present in 19 O results in an enhancement in the fusion cross-section by a factor of over three as compared to 18 O.

The $^{19}\mathrm{Ne}(p,\ensuremath{\gamma})^{20}\mathrm{Na}$ reaction is the second step of a reaction chain which breaks out from the hot CNO cycle, following the $^{15}\mathrm{O}(\ensuremath{\alpha},\ensuremath{\gamma})^{19}\mathrm{Ne}$ reaction at the onset of x-ray burst events. We investigate the spectrum of the lowest proton-unbound states in $^{20}\mathrm{Na}$ in an effort to resolve contradictions in spin-parity assignments and extract reliable information about the thermal reaction rate. The proton-transfer reaction $^{19}\mathrm{Ne}(d,n)^{20}\mathrm{Na}$ is measured with a beam of the radioactive isotope $^{19}\mathrm{Ne}$ at an energy around the Coulomb barrier and in inverse kinematics. We observe three proton resonances with the $^{19}\mathrm{Ne}$ ground state, at 0.44, 0.66, and 0.82 MeV c.m. energies, which are assigned ${3}^{+}$, ${1}^{+}$, and (${0}^{+}$), respectively. In addition, we identify two resonances with the first excited state in $^{19}\mathrm{Ne}$, one at 0.20 MeV and one, tentatively, at 0.54 MeV. These observations allow us for the first time to experimentally quantify the astrophysical reaction rate on an excited nuclear state. Our experiment shows an efficient path for thermal proton capture in $^{19}\mathrm{Ne}(p,\ensuremath{\gamma})^{20}\mathrm{Na}$, which proceeds through ground state and excited-state capture in almost equal parts and eliminates the possibility for this reaction to create a bottleneck in the breakout from the hot CNO cycle.

A study of high spin states in the odd-neutron isotones 239 Pu and 237 U is reported. Striking differences were found in the high-spin properties of rotational bands built on the 1 /2 + [631] ground states in these two nuclei. These differences mirror those observed in the even–even Pu and U immediate neighbors and appear to be related to the strength of octupole correlations.

Deep-inelastic reactions between a beam of 1360 MeV 208Pb ions and a thick 238U target have been used to populate the neutron-rich nucleus 211Pb. The observation of its γ decay has allowed identification of excited states up to the highest spin which can be formed from the three valence neutrons, including identification of three high-spin isomers. Level energies and transition strengths are compared to shell-model calculations with empirical interactions and predictions are made for the expected behaviour of more neutron-rich lead isotopes. The evidence for a possible increase in the neutron effective charge moving away from the N = 126 shell gap is evaluated.

Particle gamma-ray angular correlation measurements have been used to study the spin alignment and magnetic-substate population parameters for the 2 + 1 (4.443 MeV) state in 12 C, populated in the 12 C ( 12 C , 12 C [0 + 2 ]) 12 C (2 + 1 ) inelastic scattering reaction in the vicinity of a prominent, narrow peak in the scattering excitation function. The data show a strong alignment of the spin with the orbital angular momentum, and suggest that the cross section peak corresponds to a spin 14 + resonance at E c.m. =28.0 MeV. This energy is close to that where a strong peak is also observed in the 0 + 1 +0 + 2 excitation function. A comparison between the data for these two channels lends some support to recent theoretical calculations of resonance behavior for angular-momentum-mismatched channels in 12 C + 12 C inelastic scattering.

Entry distributions in angular momentum and excitation energy have been measured for several very proton-rich isotopes of Pt and Au. This is the first systematic study of the energy-spin phase space for nuclei near and beyond the proton drip line. Comparisons are made between the distributions associated with proton-unbound Au nuclei and more stable Pt isobars. In 173 Au the first evidence is seen for the limits of excitation energy and angular momentum which a nucleus beyond the proton drip line can sustain.

The level structure of 5422Ti32 has been explored for the first time by combining β-decay measurements from fragmentation products with prompt γ-ray spectroscopy following deep inelastic reactions. The latter technique was also instrumental in tracing 52Ti30 to higher spin. The data provide new tests of effective interactions for full pf-shell calculations in neutron-rich nuclei above 48Ca. The data indicate the presence of a significant subshell gap at N=32 and comparisons between theory and experiment suggest an additional shell closure at N=34 in Ca and Ti isotopes.

Background: Measurement of the energy dependence of the fusion cross section at sub-barrier energies provides an important test for theoretical models of fusion.Purpose: The aim of the study is to extend the measurement of fusion cross sections in the sub-barrier domain for the $^{18}\mathrm{O}+^{12}\mathrm{C}$ system, and to use the new experimental data to confront microscopic calculations of fusion.Method: Evaporation residues produced in fusion of $^{18}\mathrm{O}$ ions with $^{12}\mathrm{C}$ target nuclei were detected with good geometric efficiency and identified by measuring their energy and time-of-flight. Theoretical calculations with a density-constrained time-dependent Hartree-Fock (DC-TDHF) theory include for the first time the effect of pairing on the fusion cross section.Results: Comparison of the measured fusion excitation function with the predictions of the DC-TDHF calculations reveal that the experimental data exhibit a smaller decrease in cross section with decreasing energy than is theoretically predicted.Conclusion: The larger cross sections observed at the lowest energies measured indicate a larger tunneling probability for the fusion process. This larger probability can be associated with a smaller, narrower fusion barrier than presently included in the theoretical calculations.

Excited states in the proton-unbound 173,175,177 Au nuclei were identified for the first time. Level structures associated with three different shapes were observed in 175 Au. While the yrast lines of 175 Au and 177 Au consist of a prolate band built upon the intruder 1/2 + [660] ( i 13/2 ) proton orbital, no sign of collectivity was observed in the lighter 173 Au isotope. Implications for the deformation associated with these structures are discussed with a focus on shape co-existence in the vicinity of the proton-drip line.

Three superdeformed (SD) bands have been found in 168Hf. Lifetime measurements revealed a large quadrupole moment, Q t ∼11.4 e b , for the strongest band (TSD1). Theoretical calculations using the Ultimate Cranker code predict high-spin SD minima (ϵ2∼0.43) with stable triaxial deformations of γ∼+20° and γ∼−15°. The measured Qt value suggests that band TSD1 corresponds, most likely, to a deformation with a positive γ value. This constitutes the first evidence for triaxial superdeformation in an even proton system.

The majority of experiments performed during the 2-year long stay of GAMMAS-PHERE at the Argonne National Laboratory aimed to study proton-rich nuclei far from the line of stability at and beyond the proton drip-line. A high reaction channel selectivity was required to assign in-beam {gamma}-ray transitions to weakly populated exotic nuclei in the presence of background from strong reaction channels. In many of the experiments this was achieved by using the Argonne fragment mass analyzer to separate heavy-ion fusion-evaporation reaction products from scattered beam and disperse them according to their mass-over-charge-state ratio. For medium mass and heavy a and proton emitters the Recoil-Decay Tagging method was implemented. In-beam {gamma}-ray transitions were observed in several proton emitters between Z=50 and Z=82. Among others, rotational bands were assigned to {sup 141}Ho and {sup 131}Eu. A quadruple deformation of {beta}=0.25(4) was deduced for the ground state in {sup 141}Ho from the extracted dynamic moment of inertia. Based on observed band crossings and signature splittings the 7/2{sup {minus}} [523] and 1/2{sup +}[411] configurations were proposed for the ground state and the isomeric state, respectively. Comparison with particle-rotor calculations indicates, however, that {sup 141}Ho may have significant hexadecapole deformation and could be triaxial.

The nucleus ${}^{94}\mathrm{Mo}$ was investigated using a powerful combination of $\ensuremath{\gamma}$-singles photon scattering experiments and $\ensuremath{\gamma}\ensuremath{\gamma}$-coincidence studies following the $\ensuremath{\beta}$ decay of ${}^{94}{\mathrm{Tc}}^{m}$. The data survey short-lived ${J}^{\ensuremath{\pi}}{\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1}^{+}{,2}^{+}$ states and include branching ratios, $E2/M1$ mixing ratios, lifetimes, and transition strengths. The proton-neutron mixed-symmetry (MS) ${1}^{+}$ scissors mode and the ${2}^{+}$ MS state are identified from $M1$ strengths. A $\ensuremath{\gamma}$ transition between MS states was observed and its rate was measured. Nine $M1$ and $E2$ strengths involving MS states agree with the O(6) limit of the interacting boson model-2 using the proton boson $E2$ charge as the only free parameter.

Excited states of the nuclei 122,126,130 Te were populated via the (γ, γ′) reaction at endpoint energies of the bremsstrahlung between 4.5 and 5.5 MeV. Gamma rays were detected with a EUROBALL CLUSTER detector and a single HPGe detector. In all investigated nuclei two or three prominent dipole transitions were identified at E γ ≈ 3 MeV. The corresponding low-lying J = 1 states are interpreted as two-phonon excitations. Quasiparticle-phonon-model calculations predict at about 3 MeV one 1 − state arising from the coupling of the first quadrupole and the first octupole phonon, and one 1 + state arising from the coupling of the first and the second quadrupole phonon, where the latter has isovector character. Such an excitation mode can be considered as an analogue of the scissors mode in vibrational nuclei. The calculated transition strengths are compatible with experimental ones within a factor of about 1.5.

The E1 response of the semi-magic nucleus 140Ce below the particle threshold was measured in a (γ,γ′) experiment utilizing the new Euroball Cluster detector at the S-DALINAC. While the energy averaged data are in good agreement with tagged photon results, here they are resolved for the first time into 54 individual transitions. A quasiparticle-phonon model calculation including up to three-phonon configurations compares well to the extracted strength distribution. The interference between one- and two-phonon contributions is essential for a quantitative reproduction.

Dramatic advances are being made in nuclear structure physics because of the availability of facilities producing beams of exotic radioisotopes. Of these, silicon-42 is of particular interest for study of the effects of packing extra neutrons into a nucleus. Neutrons and protons in atomic nuclei are arranged in a series of energy levels or ‘shells’. When a shell has a full quota of particles, the nucleus is particularly stable: the ‘magic numbers’ for stability are 2, 8, 20, 28, 50, 82 and 126. Silicon-42 owes its existence to its 28 neutrons, but is still a short-lived atom. This short lifetime has been explained as a consequence of strong deformation but this is contradicted in a new study that suggests that silicon-42 is almost spherical. Nuclear shell structures—the distribution of the quantum states of individual protons and neutrons—provide one of our most important guides for understanding the stability of atomic nuclei. Nuclei with ‘magic numbers’ of protons and/or neutrons (corresponding to closed shells of strongly bound nucleons) are particularly stable1,2. Whether the major shell closures and magic numbers change in very neutron-rich nuclei (potentially causing shape deformations) is a fundamental, and at present open, question3,4. A unique opportunity to study these shell effects is offered by the 42Si nucleus, which has 28 neutrons—a magic number in stable nuclei—and 14 protons. This nucleus has a 12-neutron excess over the heaviest stable silicon nuclide, and has only one neutron fewer than the heaviest silicon nuclide observed so far5. Here we report measurements of 42Si and two neighbouring nuclei using a technique involving one- and two-nucleon knockout from beams of exotic nuclei6,7. We present strong evidence for a well-developed proton subshell closure at Z = 14 (14 protons), the near degeneracy of two different (s1/2 and d3/2) proton orbits in the vicinity of 42Si, and a nearly spherical shape for 42Si.

Motivation: Detailed experimental knowledge of the level structure of light weakly bound nuclei is necessary to guide the development of new theoretical approaches that combine nuclear structure with reaction dynamics. Purpose: The resonant structure of 8 B is studied in this work. Method: Excitation functions for elastic and inelastic 7 Be+p scattering were measured using a 7 Be rare isotope beam. Excitation energies ranging between 1.6 and 3.4 MeV were investigated. An R-matrix analysis of the excitation functions was performed. Results: New low-lying resonances at 1.9, 2.5, and 3.3 MeV in 8 B are reported with spin-parity assignment 0 + , 2 + , and 1 + , respectively. Comparison to the Time Dependent Continuum Shell (TDCSM) model and ab initio no-core shell model/resonating-group method (NCSM/RGM) calculations is performed. This work is a more detailed analysis of the data first published as a Rapid Communication. [J.P. Mitchell, et al, Phys. Rev. C 82, 011601(R) (2010)] Conclusions: Identification of the 0 + , 2 + , 1 + states that were predicted by some models at relatively low energy but never observed experimentally is an important step toward understanding the structure of 8 B. Their identification was aided by having both elastic and inelastic scattering data. Direct comparison of the cross sections and phase shifts predicted by the TDCSM and ab initio No Core Shell Model coupled with the resonating group method is of particular interest and provides a good test for these theoretical approaches.

Gamma-ray spectroscopy of the superheavy nucleus 256Rutherfordium reveals details of its nonspherical shape and internal structure—the heaviest element for which such information has been obtained.

We describe the investigation of high-spin states in ${}^{24}$Mg populated with the ${}^{12}\mathrm{C}{{(}^{16}\mathrm{O},\ensuremath{\alpha})}^{24}\mathrm{Mg}$ reaction at 62- and 68-MeV beam energy. The excited states were established through the coincident detection of up to four $\ensuremath{\alpha}$ and $\ensuremath{\gamma}$ particles in complete decay cascades toward a final state of angular momentum zero, the ground state of either ${}^{20}$Ne or ${}^{16}$O. We describe a new analysis method for the angular correlations in such events and apply it with the goal to assign spin and parity to $\ensuremath{\alpha}$-unbound states in ${}^{24}$Mg. We establish a number of high-spin, natural-parity states between spins 6 and 12, including the lowest known ${10}^{+}$ and ${12}^{+}$ levels. The energy systematics of positive- and negative-parity high-spin states are compared to the predictions of modern shell-model calculations.

The low spin structure of 130Ba was investigated via the emitted γ-radiation following the β-decay of 130La. New “K = 0”- and “K = 4”- bands were identified. A comparison with the O(6) symmetry of the IBM is shown. A discussion of the low lying levels of the O(6) symmetry in terms of multiple quadrupole “phonon” excitations built on the ground state is given.

The neutron-rich $N=28$ nucleus $^{44}\mathrm{S}$ was studied using the two-proton knockout reaction from $^{46}\mathrm{Ar}$ at intermediate beam energy. We report the observation of four new excited states, one of which is a strongly prolate deformed 4${}^{+}$ state, as indicated by a shell-model calculation. Its deformation originates in a neutron configuration which is fundamentally different from the ``intruder'' configuration responsible for the ground-state deformation. Consequently, we do not have three coexisting shapes in $^{44}\mathrm{S}$, but three coexisting configurations, corresponding to zero-, one-, and two-neutron particle-hole excitations.

Excitation functions of elastic and inelastic $^{7}\mathrm{Be}+p$ scattering were measured in the energy range between 1.6 and 2.8 MeV in the c.m. An $R$-matrix analysis of the excitation functions provides strong evidence for new positive parity states in $^{8}\mathrm{B}$. A new ${2}^{+}$ state at an excitation energy of 2.55 MeV was observed, and a new ${0}^{+}$ state at 1.9 MeV is tentatively suggested. The $R$-matrix and time-dependent continuum shell model were used in the analysis of the excitation functions. The new results are compared to the calculations of contemporary theoretical models.

Using a beam of the radioactive isotope $^{25}\mathrm{Al}$, produced with the new RESOLUT facility, we measured the direct $(d,n)$ proton-transfer reaction leading to low-lying proton resonances in $^{26}\mathrm{Si}$. We observed the lowest $l=0$ proton resonance, identified with the ${3}^{+}$ state at 5.914-MeV excitation energy. This result eliminates the largest uncertainty in astrophysical reaction rates involved in the nucleosynthesis of $^{26}\mathrm{Al}$.

The expanded level structure of $^{240}$Pu available from the present study highlights the role of strong octupole correlations in this nucleus. Besides a delayed alignment in the yrast band, the observations include the presence of both $I^{+}{\to}(I-1)^{-}$ and $I^{-}{\to}(I-1)^{+}$ E1 transitions linking states of the yrast and negative-parity bands at high spin and the presence of an additional even-spin, positive-parity band deexciting exclusively to the negative parity sequence. The observations appear to be consistent with expectations based on the recently proposed concept of octupole phonon condensation., Comment: Accepted by Physical Review Letters on February 20, 2009

Measurements of the $N=28$ isotones $^{42}\mathrm{Si}$, $^{43}\mathrm{P}$, and $^{44}\mathrm{S}$ using one- and two-proton knockout reactions from the radioactive beam nuclei $^{44}\mathrm{S}$ and $^{46}\mathrm{Ar}$ are reported. The knockout reaction cross sections for populating $^{42}\mathrm{Si}$ and $^{43}\mathrm{P}$ and a 184 keV \ensuremath{\gamma}-ray observed in $^{43}\mathrm{P}$ establish that the ${d}_{3/2}$ and ${s}_{1/2}$ proton orbits are nearly degenerate in these nuclei and that there is a substantial $Z=14$ subshell closure separating these two orbits from the ${d}_{5/2}$ orbit. The increase in the inclusive two-proton knockout cross section from $^{42}\mathrm{Si}$ to $^{44}\mathrm{S}$ demonstrates the importance of the availability of valence protons for determining the cross section. New calculations of the two-proton knockout reactions that include diffractive effects are presented. In addition, it is proposed that a search for the ${d}_{5/2}$ proton strength in $^{43}\mathrm{P}$ via a higher statistics one-proton knockout experiment could help determine the size of the $Z=14$ closure.

A search has been made for the $^{6}\mathrm{He}+^{6}\mathrm{He}$ and \ensuremath{\alpha} + $^{8}\mathrm{He}$ decay of the molecular rotational band in $^{12}\mathrm{Be}$ using the $^{10}\mathrm{Be}$($^{14}\mathrm{C}$,$^{12}\mathrm{Be}$${}^{*}$)$^{12}\mathrm{C}$ reaction at 88.5 MeV. Although the \ensuremath{\alpha} + $^{6}\mathrm{He}$ decay of $^{10}\mathrm{Be}$ was observed in the data set there is no evidence for the breakup of $^{12}\mathrm{Be}$. The cross-section upper limits for the $^{10}\mathrm{Be}$($^{14}\mathrm{C}$,$^{6}\mathrm{He}$ $^{6}\mathrm{He}$)$^{12}\mathrm{C}$ and $^{10}\mathrm{Be}$($^{14}\mathrm{C}$,\ensuremath{\alpha} $^{8}\mathrm{He}$)$^{12}\mathrm{C}$ reactions are 50 and 300 nb respectively.

$B(E3;{0}_{1}^{+}\ensuremath{\rightarrow}{3}_{1}^{\ensuremath{-}})$ transition matrix elements have been measured for even-mass $^{124\ensuremath{-}134}\mathrm{Xe}$ nuclei using subbarrier Coulomb excitation in inverse kinematics. The trends in energy $E({3}^{\ensuremath{-}})$ and $B(E3;{0}_{1}^{+}\ensuremath{\rightarrow}{3}_{1}^{\ensuremath{-}})$ excitation strengths are well reproduced using phenomenological models based on a strong coupling picture with a soft quadrupole mode and an increasing occupation of the intruder ${h}_{11/2}$ orbital.

Clustering phenomena in 10Be and 18O were studied by means of resonance elastic scattering of α-particles on 6He and 14C. Excitation functions for α+6He and α+14C were measured and detailed R-matrix analyses of the excitation functions was performed. We compare the experimental results with the predictions of modern theoretical approaches and discuss properties of cluster rotational bands.

{gamma} rays from neutron-rich Ti nuclei in the vicinity of N=32 have been studied at Gammasphere using deep-inelastic reactions induced by {sup 48}Ca beams on thick {sup 208}Pb and {sup 238}U targets. The {sup 53}Ti yrast cascade was identified for the first time, and the location in energy of states with spin up to I = 21/2 was determined. The excitations of {sup 53}Ti provide new tests of effective interactions for full-pf-shell calculations. Comparisons between theory and experiment for the highest spin states in {sup 53}Ti favor the recently proposed GXPF1A interaction, which accounts for the appearance of a neutron subshell closure at N=32 in Ti isotopes as well as for the lack of a similar subshell closure at N=34, as predicted by earlier calculations with another interaction.

Excited states in $^{20}\mathrm{O}$ were populated in the reaction $^{10}\mathrm{Be}(^{14}\mathrm{C},\ensuremath{\alpha})$ at Florida State University (FSU). Charged particles were detected with a particle telescope consisting of 4 annularly segmented Si surface barrier detectors and $\ensuremath{\gamma}$ radiation was detected with the FSU $\ensuremath{\gamma}$ detector array. Five new states were observed below 6 MeV from the $\ensuremath{\alpha}\mathrm{\text{\ensuremath{-}}}\ensuremath{\gamma}$ and $\ensuremath{\alpha}\mathrm{\text{\ensuremath{-}}}\ensuremath{\gamma}\mathrm{\text{\ensuremath{-}}}\ensuremath{\gamma}$ coincidence data. Shell model calculations suggest that most of the newly observed states are core-excited $1p\mathrm{\text{\ensuremath{-}}}1h$ excitations across the $N=Z=8$ shell gap. Comparisons between experimental data and calculations for the neutron-rich O and F isotopes imply a steady reduction of the $p\mathrm{\text{\ensuremath{-}}}sd$ shell gap as neutrons are added.