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Double differential cross sections of neutrons from $d\phantom{\rule{0.16em}{0ex}}+{\phantom{\rule{0.16em}{0ex}}}^{63,65}$Cu reactions have been measured at deuteron energies of 6 and 7.5 MeV. The cross sections measured at backward angles have been compared to theoretical calculations in the framework of the statistical Hauser-Feshbach model. Three different level density models were tested: the Fermi-gas model, the Gilbert-Cameron model, and the microscopic approach through the Hartree-Fock-Bogoliubov method (HFBM). The calculations using the Gilbert-Cameron model are in best agreement with our experimental data. Level densities of the residual nuclei ${}^{64}$Zn and ${}^{66}$Zn have been obtained from statistical neutron evaporation spectra. The angle-integrated cross sections have been analyzed with the exciton model of nuclear reaction.

The level densities and $\ensuremath{\gamma}$-ray strength functions of ${}^{105,106,111,112}$Cd have been extracted from particle-$\ensuremath{\gamma}$ coincidence data using the Oslo method. The level densities are in very good agreement with known levels at low excitation energy. The $\ensuremath{\gamma}$-ray strength functions display no strong enhancement for low $\ensuremath{\gamma}$ energies. However, more low-energy strength is apparent for ${}^{105,106}$Cd than for ${}^{111,112}$Cd. For $\ensuremath{\gamma}$ energies above $\ensuremath{\approx}$4 MeV, there is evidence for some extra strength, similar to what has been previously observed for the Sn isotopes. The origin of this extra strength is unclear; it might be due to $E1$ and $M1$ transitions originating from neutron skin oscillations or the spin-flip resonance, respectively.

Evidence for the ground state of the neutron-unbound nucleus $^{26}\mathrm{O}$ was observed for the first time in the single proton-knockout reaction from a $82\text{ }\text{ }\mathrm{MeV}/\mathrm{u}$ $^{27}\mathrm{F}$ beam. Neutrons were measured in coincidence with $^{24}\mathrm{O}$ fragments. $^{26}\mathrm{O}$ was determined to be unbound by ${150}_{\ensuremath{-}150}^{+50}\text{ }\text{ }\mathrm{keV}$ from the observation of low-energy neutrons. This result agrees with recent shell-model calculations based on microscopic two- and three-nucleon forces.

Primary γ-ray spectra for a wide excitation-energy range have been extracted for 44Ti from particle-γ coincidence data of the 46Ti(p,tγ)44Ti reaction. These spectra reveal information on the γ-decay pattern of the nucleus and may be used to extract the level density and radiative strength function applying the Oslo method. Models of the level density and radiative strength function are used as input for cross-section calculations of the 40Ca(α,γ)44Ti reaction. Acceptable models should reproduce data on the 40Ca(α,γ)44Ti reaction cross section as well as the measured primary γ-ray spectra. This is only achieved when a coherent normalization of the slope of the level density and radiative strength function is performed. Thus, the overall shape of the experimental primary γ-ray spectra puts a constraint on the input models for the rate calculations. © 2012 American Physical Society.

A two-proton knockout reaction from a ${}^{17}$C beam at 55 MeV/u was used to produce the unbound nucleus ${}^{15}$Be. No significant number of events were observed for the decay of ${}^{15}$Be into a neutron and a ${}^{14}$Be fragment. An upper limit for the ${}^{14}$Be production cross section of 0.079$\phantom{\rule{0.16em}{0ex}}\ifmmode\pm\else\textpm\fi{}\phantom{\rule{0.16em}{0ex}}$0.038 mb was extracted, which is more than an order of magnitude smaller than the predicted two-proton knockout reaction cross section. Based on these results we conclude that any populated states in ${}^{15}$Be decay through three sequential neutron decays into ${}^{12}$Be, via the unbound first excited state of ${}^{14}$Be. Therefore, these states in ${}^{15}$Be must be neutron unbound by more than 1.54 MeV, which is the location of the first excited 2${}^{+}$ state in ${}^{14}$Be.

Neutron-unbound states in ${}^{25}$F and ${}^{26}$F were populated via the reactions ${}^{9}$Be(${}^{26}$Ne,${}^{24}$F + n) and ${}^{9}$Be(${}^{26}$Ne,${}^{25}$F + n), respectively. A resonance close to the neutron separation energy in ${}^{25}$F was identified with a decay energy of $28\ifmmode\pm\else\textpm\fi{}4$ keV. This resonance corresponds to an excited state in ${}^{25}$F at $4249\ifmmode\pm\else\textpm\fi{}116$ keV assuming it decays to the ground state of ${}^{24}$F. Guided by shell-model calculations, a spin and parity of 1/2${}^{\ensuremath{-}}$ can be assigned to this state. In the spectrum of ${}^{26}$F, which was produced in a nucleon-exchange reaction, there are indications for an excited state with a decay energy of $\ensuremath{\sim}$270 keV.

Neutron-unbound resonant states of $^{11}\mathrm{Be}$ were populated in neutron knockout reactions from $^{12}\mathrm{Be}$ and identified by $^{10}\mathrm{Be}$--$n$ coincidence measurements. A resonance in the decay-energy spectrum at 80(2) keV was attributed to a highly excited unbound state in $^{11}\mathrm{Be}$ at 3.949(2) MeV decaying to the ${2}^{+}$ excited state in $^{10}\mathrm{Be}$. A knockout cross section of 15(3) mb was inferred for this 3.949(2) MeV state, suggesting a spectroscopic factor near unity for this $0p3/{2}^{\ensuremath{-}}$ level, consistent with the detailed shell model calculations.

A new resonance having a relative decay energy of $~0.6$ MeV has been observed above the two-neutron separation energy in $^{24}\mathrm{O}$ at an excitation energy of $~7.5$ MeV. The new level was found to directly feed the first excited state in $^{23}\mathrm{O}$, an $E=0.045(2)$-MeV resonance, through the measurement of neutron-neutron correlations. Energy level comparisons with $^{23}\mathrm{O}$ suggest that this new level in $^{24}\mathrm{O}$ is dominated by an unpaired neutron in the $\ensuremath{\nu}0{d}_{3/2}$ single-particle orbital coupled to a single hole in the $\ensuremath{\nu}0{d}_{5/2}$ single-particle orbital. Establishment of this two-neutron discrimination technique provides a means for investigating high-lying excited states in neutron-rich nuclei.

Particle-$\ensuremath{\gamma}$ coincidences from the $^{46}\mathrm{Ti}$($p,{p}^{\ensuremath{'}}\ensuremath{\gamma}$)$^{46}\mathrm{Ti}$ inelastic scattering reaction with 15-MeV protons are utilized to obtain $\ensuremath{\gamma}$-ray spectra as a function of excitation energy. The rich data set allows analyzsis of the coincidence data with various gates on excitation energy. For many independent data sets, this enables a simultaneous extraction of level density and radiative strength function (RSF). The results are consistent with one common level density. The data seem to exhibit a universal RSF as the deduced RSFs from different excitation energies show only small fluctuations provided that only excitation energies above $3$ MeV are taken into account. If transitions to well-separated low-energy levels are included, the deduced RSF may change by a factor of 2--3, which might be expected because of the involved Porter-Thomas fluctuations.

The nuclear level densities of $^{118,119}\mathrm{Sn}$ and the $\ensuremath{\gamma}$-ray strength functions of $^{116,118,119}\mathrm{Sn}$ below the neutron separation energy are extracted with the Oslo method using the ($^{3}\mathrm{He}$,$\ensuremath{\alpha}\ensuremath{\gamma}$) and ($^{3}\mathrm{He}$,$^{3}\mathrm{He}$${}^{\ensuremath{'}}\ensuremath{\gamma}$) reactions. The level-density function of $^{119}\mathrm{Sn}$ displays steplike structures. The microcanonical entropies are deduced from the level densities, and the single neutron entropy of $^{119}\mathrm{Sn}$ is determined to be $1.7$ $\ifmmode\pm\else\textpm\fi{}$ $0.2$ ${k}_{B}$. Results from a combinatorial model support the interpretation that some of the low-energy steps in the level density function are caused by neutron pair breaking. An enhancement in all the $\ensuremath{\gamma}$-ray strength functions of $^{116\ensuremath{-}119}\mathrm{Sn}$, compared to standard models for radiative strength, is observed for the $\ensuremath{\gamma}$-ray energy region of $\ensuremath{\simeq}$$4\ensuremath{-}11$ MeV. These small resonances all have a centroid energy of $8.0(1)$ MeV and an integrated strength corresponding to $1.7(9)%$ of the classical Thomas-Reiche-Kuhn sum rule. The Sn resonances may be due to electric dipole neutron skin oscillations or to an enhancement of the giant magnetic dipole resonance.

An excited state of $^{22}\mathrm{N}$ unbound with respect to neutron emission was observed in a stripping reaction from a 85 MeV/nucleon $^{26}\mathrm{F}$ beam. The observed decay energy of 650(50) keV places the level, which is interpreted to be the first ${3}^{\ensuremath{-}}$ state, at an excitation energy of 1.93(22) MeV. Together with the previously measured bound states of $^{22}\mathrm{N}$, reduction of the $N=14$ shell gap compared to less neutron-rich nitrogen isotopes at the neutron dripline is observed. Based on the magnitude of the reduction of the shell gap for $^{22}\mathrm{N}$, a disappearance of the gap and even a level inversion of the $\ensuremath{\nu}1{s}_{1/2}$ and the $\ensuremath{\nu}0{d}_{5/2}$ levels in the neutron-unbound nucleus $^{21}\mathrm{C}$ seems likely.

Neutron and proton evaporation spectra from the {sup 6}Li+{sup 55}Mn and d+{sup 59}Co reactions have been analyzed with the Hauser-Feshbach approach using different input models for nuclear level densities of {sup 60}Ni and {sup 60}Co nuclei. It has been found that models with a Fermi-gas like temperature dependence fail to reproduce particle spectra from both reactions simultaneously. We obtained the surprising result that the only way to describe our data is to assume the independence of the nuclear temperature on the excitation energy up to about the 20 MeV energy range.

The nuclear level densities of $^{116,117}\mathrm{Sn}$ below the neutron separation energy have been determined experimentally from the $(^{3}\mathrm{He},\ensuremath{\alpha}\ensuremath{\gamma})$ and $(^{3}\mathrm{He},^{3}\mathrm{He}{}^{'}\ensuremath{\gamma})$ reactions, respectively. The level densities show a characteristic exponential increase and a difference in magnitude due to the odd-even effect of the nuclear systems. In addition, the level densities display pronounced step-like structures that are interpreted as signatures of subsequent breaking of nucleon pairs.

The ground state energy and width of $^{7}\mathrm{He}$ has been measured with the Modular Neutron Array (MoNA) and superconducting dipole Sweeper magnet experimental setup at the National Superconducting Cyclotron Laboratory. $^{7}\mathrm{He}$ was produced by proton knockout from a secondary $^{8}\mathrm{Li}$ beam. The measured decay energy spectrum is compared to simulations based on Breit-Wigner line shape with an energy-dependent width for the resonant state. The energy of the ground state is found to be 400(10) keV with a full-width at half-maximum of $125({}_{\ensuremath{-}15}^{+40})$ keV.

The neutron unbound ground state of $^{25}\mathrm{O}$ ($Z=8$, $N=17$) was observed for the first time in a proton knockout reaction from a $^{26}\mathrm{F}$ beam. A single resonance was found in the invariant mass spectrum corresponding to a neutron decay energy of ${770}_{\ensuremath{-}10}^{+20}\text{ }\text{ }\mathrm{keV}$ with a total width of 172(30) keV. The $N=16$ shell gap was established to be 4.86(13) MeV by the energy difference between the $\ensuremath{\nu}1{s}_{1/2}$ and $\ensuremath{\nu}0{d}_{3/2}$ orbitals. The neutron separation energies for $^{25}\mathrm{O}$ agree with the calculations of the universal $sd$ shell model interaction. This interaction incorrectly predicts an $^{26}\mathrm{O}$ ground state that is bound to two-neutron decay by 1 MeV, leading to a discrepancy between the theoretical calculations and experiment as to the particle stability of $^{26}\mathrm{O}$. The observed decay width was found to be on the order of a factor of 2 larger than the calculated single-particle width using a Woods-Saxon potential.

The level densities of {sup 44}Sc and {sup 47}Ti have been determined from measurements of particle evaporation spectra from the compound nuclear reaction {sup 3}He+{sup 45}Sc with an 11 MeV {sup 3}He beam. The level density of {sup 44}Sc has been compared to the level density obtained from an independent experimental method based on an analysis of {alpha}-{gamma} coincidences from the transfer reaction {sup 45}Sc({sup 3}He,{alpha}{gamma}){sup 44}Sc. The good agreement between the two experiments indicates the reliability of the level density obtained. Some level density systematics have been tested against the experimental data. New Fermi-gas level density parameters have been derived.

Rare isotope beams of neutron-deficient $^{106,108,110}\mathrm{Sn}$ from the fragmentation of $^{124}\mathrm{Xe}$ were employed in an intermediate-energy Coulomb excitation experiment. The measured $B(E2,{0}_{1}^{+}\ensuremath{\rightarrow}{2}_{1}^{+})$ values for $^{108}\mathrm{Sn}$ and $^{110}\mathrm{Sn}$ and the results obtained for the $^{106}\mathrm{Sn}$ show that the transition strengths for these nuclei are larger than predicted by current state-of-the-art shell-model calculations. This discrepancy might be explained by contributions of the protons from within the $Z=50$ shell to the structure of low-energy excited states in this region.

The scandium isotopes $^{44,45}\mathrm{Sc}$ were studied with the $^{45}\mathrm{Sc}$($^{3}\mathrm{He}$, $\ensuremath{\alpha}\ensuremath{\gamma}$)$^{44}\mathrm{Sc}$ and $^{45}\mathrm{Sc}$($^{3}\mathrm{He}$, $^{3}\mathrm{He}$${}^{'}\ensuremath{\gamma}$)$^{45}\mathrm{Sc}$ reactions, respectively. The nuclear level densities and \ensuremath{\gamma}-ray strength functions have been extracted using the Oslo method. The experimental level densities are compared to calculated level densities obtained from a microscopic model based on BCS quasiparticles within the Nilsson level scheme. This model also gives information about the parity distribution and the number of broken Cooper pairs as a function of excitation energy. The experimental \ensuremath{\gamma}-ray strength functions are compared to theoretical models of the $E1$, $M1$, and $E2$ strength and to data from (\ensuremath{\gamma}, $n$) and (\ensuremath{\gamma},$p$) experiments. The strength functions show an enhancement at low \ensuremath{\gamma} energies that cannot be explained by the present standard models.

We have observed a resonance in neutron-fragment coincidence measurements that is presumably the first excited state of $^{23}\mathrm{O}$ at 2.8(1) MeV excitation energy which decays into the ground state of $^{22}\mathrm{O}$. This interpretation is consistent with theory. The reaction mechanism supports the assignment of the observed state as the $5/{2}^{+}$ hole state. This assignment and the recently observed $3/{2}^{+}$ particle state advance the understanding of $^{23}\mathrm{O}$.

The results of measurements of the production of neutron-rich nuclei by the fragmentation of a {sup 48}Ca beam at 142 MeV/nucleon are presented. Evidence was found for the production of a new isotope that is the most neutron-rich silicon nuclide, {sup 44}Si, in a net neutron pickup process. A simple systematic framework was found to describe the production cross sections based on thermal evaporation from excited prefragments that allows extrapolation to other weak reaction products.

The $\mathcal{S}$ shape of the canonical heat-capacity curve is known as a signature of the pairing transition, and along an isotopic chain it is significantly more pronounced for nuclei with an even number of neutrons than for those with an odd number. Although the heat capacities extracted from experimental level densities in $^{93\ensuremath{-}98}\mathrm{Mo}$ exhibit a clear $\mathcal{S}$ shape, they do not show such an odd-even staggering. To understand the underlying physics, we analyze thermal quantities evaluated from the partition function calculated using the static-path plus random-phase approximation (SPA+RPA) in a monopole pairing model with number-parity projection. The calculated level densities reproduce very well the experimental data, and they also agree with estimates made using the back-shifted Fermi-gas model. We clarify the reason why the heat capacities for Mo isotopes do not show odd-even staggering of the $\mathcal{S}$ shape. We also discuss thermal odd-even mass differences in $^{94\ensuremath{-}97}\mathrm{Mo}$ that were calculated using the three-, four-, and five-point formulas. These thermal mass differences are regarded as indicators of pairing correlations at finite temperature.

The $^{55}\mathrm{Mn}$$(d,n)^{56}\mathrm{Fe}$ differential cross section is measured at ${E}_{d}=7$ MeV. The $^{56}\mathrm{Fe}$ level density obtained from neutron evaporation spectra is compared to the level density extracted from the $^{57}\mathrm{Fe}$$(^{3}\mathrm{He}$, $\ensuremath{\alpha}\ensuremath{\gamma})^{56}\mathrm{Fe}$ reaction by the Oslo-type technique. Good agreement is found between the level densities determined by the two methods. With the level density function obtained from the neutron evaporation spectra, the $^{56}\mathrm{Fe}$ \ensuremath{\gamma}-strength function is also determined from the first-generation \ensuremath{\gamma} matrix of the Oslo experiment. The good agreement between the past and present results for the \ensuremath{\gamma}-strength function supports the validity of both methods and is consistent with the low-energy enhancement of the \ensuremath{\gamma} strength below $~4$ MeV that was first discovered by the Oslo method in iron and molybdenum isotopes.

The level densities and radiative strength functions (RSFs) of $^{50,51}\mathrm{V}$ have been extracted using the ($^{3}\mathrm{He}$,$\ensuremath{\alpha}\ensuremath{\gamma}$) and ($^{3}\mathrm{He}$,$^{3}\mathrm{He}$${}^{\ensuremath{'}}\ensuremath{\gamma}$) reactions, respectively. From the level densities, microcanonical entropies are deduced. The high \ensuremath{\gamma}-energy part of the measured RSF fits well with the tail of the giant electric dipole resonance. A significant enhancement over the predicted strength in the region of ${E}_{\ensuremath{\gamma}}\ensuremath{\lesssim}3$ MeV is seen, which at present has no theoretical explanation.

We have searched for particle-bound {sup 26}O and {sup 28}F isotopes in the reaction products of secondary {sup 27}F and {sup 29}Ne beams, respectively. No events have been observed. Upper limits of 3.3 mb and 1.2 mb for the respective production cross sections of {sup 26}O and {sup 28}F by one p-stripping reactions are established. Since in the case of {sup 28}F, this upper limit is sufficiently small compared to common estimates, we conclude that {sup 28}F is most likely unbound. For the case of {sup 26}O, our result is not statistically significant, thus, the question cannot be answered whether {sup 26}O is a particle-unbound nucleus or not.