We studied the ${}^{\mathrm{12}}\mathrm{C}(p,2p+n)$ reaction at beam momenta of 5.9, 8.0, and $9.0\mathrm{G}\mathrm{e}\mathrm{V}/c$. For quasielastic ($p,2p$) events ${\mathbf{p}}_f$, the momentum of the knocked-out proton before the reaction, was compared (event by event) with ${\mathbf{p}}_n$, the coincident neutron momentum. For $|p_n|>k_F=0.220\mathrm{G}\mathrm{e}\mathrm{V}/c$ (the Fermi momentum) a strong back-to-back directional correlation between ${\mathbf{p}}_f$ and ${\mathbf{p}}_n$ was observed, indicative of short-range $n\mathrm{\text{−}}p$ correlations. From ${\mathbf{p}}_n$ and ${\mathbf{p}}_f$ we constructed the distributions of c.m. and relative motion in the longitudinal direction for correlated pairs. We also determined that $49\pm 13\%$ of events with $|p_f|>k_F$ had directionally correlated neutrons with $|p_n|>k_F$.

The development of nuclear shapes under the extreme conditions of high angular momentum and/or temperature is examined. Scaling properties are used to demonstrate universal properties of both thermal expectation values of nuclear shapes as well as the minima of the free energy, which can be used to understand the Jacobi transition. A universal correlation between the width of the giant-dipole resonance and quadrupole deformation is found, providing a novel probe to measure the nuclear deformation in hot nuclei.

Azimuthal anisotropy ($v_2$) and two-particle angular correlations of high $p_T$ charged hadrons have been measured in $\mathrm{A}\mathrm{u}+\mathrm{A}\mathrm{u}$ collisions at $\sqrt{s_{NN}}=130\mathrm{G}\mathrm{e}\mathrm{V}$ for transverse momenta up to $6\mathrm{G}\mathrm{e}\mathrm{V}/c$, where hard processes are expected to contribute significantly. The two-particle angular correlations exhibit elliptic flow and a structure suggestive of fragmentation of high $p_T$ partons. The monotonic rise of $v_2(p_T)$ for $p_T<2\mathrm{G}\mathrm{e}\mathrm{V}/c$ is consistent with collective hydrodynamical flow calculations. At $p_T>3\mathrm{G}\mathrm{e}\mathrm{V}/c$, a saturation of $v_2$ is observed which persists up to $p_T=6\mathrm{G}\mathrm{e}\mathrm{V}/c$.

Based on an evaluation of data on pion interferometry and on particle yields at midrapidity, we propose a universal condition for thermal freeze-out of pions in heavy-ion collisions. We show that freeze-out occurs when the mean free path of pions ${\lambda }_f$ reaches a value of about 1 fm, which is much smaller than the spatial extent of the system at freeze-out. This critical mean free path is independent of the centrality of the collision and beam energy from the Alternating Gradient Synchrotron to the Relativistic Heavy Ion Collider.

The ${}^7\mathrm{B}\mathrm{e}(p,\gamma ){}^8\mathrm{B}$ reaction plays a central role in the evaluation of solar neutrino fluxes. We report on a new precision measurement of the cross section of this reaction, following our previous experiment with an implanted ${}^7\mathrm{B}\mathrm{e}$ target, a raster-scanned beam, and the elimination of the backscattering loss. The new measurement incorporates a more abundant ${}^7\mathrm{B}\mathrm{e}$ target and a number of improvements in design and procedure. The point at $E_{\mathrm{l}\mathrm{a}\mathrm{b}}=991\mathrm{k}\mathrm{e}\mathrm{V}$ was measured several times under varying experimental conditions, yielding a value of $S_{17}(E_{\mathrm{c}\mathrm{.}\mathrm{m}\mathrm{.}}=850\mathrm{k}\mathrm{e}\mathrm{V})=24.0\pm 0.5\mathrm{e}\mathrm{V}\mathrm{b}$. Measurements were carried out at lower energies as well. Because of the precise knowledge of the implanted ${}^7\mathrm{B}\mathrm{e}$ density profile, it was possible to reconstitute both the off- and on-resonance parts of the cross section and to obtain from the entire set of measurements an extrapolated value of $S_{17}(0)=21.2\pm 0.7\mathrm{e}\mathrm{V}\mathrm{b}$.

Fine structure in proton emission from the $3.1(3)\mu \mathrm{s}$ activity of ${}^{\mathrm{145}}\mathrm{T}\mathrm{m}$ was discovered by using a novel technique of digital processing of overlapping recoil implantation and decay signals. Proton transitions to the ground state of ${}^{\mathrm{144}}\mathrm{E}\mathrm{r}$ and to its first excited $2^{+}$ state at 0.33(1) MeV with a branching ratio $I_p(2^{+})=9.6\pm 1.5\%$ were observed. The structure of the ${}^{\mathrm{145}}\mathrm{T}\mathrm{m}$ wave function and the emission process were analyzed by using particle-core vibration coupling models.

A new technique, full neutrino momentum reconstruction, is used to set limits on the admixture of heavy neutrinos into the electron neutrino. We measure coincidences between nuclear recoils and positrons from the beta decay of trapped radioactive atoms and deduce the neutrino momentum. A search for peaks in the reconstructed recoil time-of-flight spectrum as a function of positron energy is performed. The admixture upper limits range from $4\times {10}^{-3}$ to $2\times {10}^{-2}$ and are the best direct limits for neutrinos (as opposed to antineutrinos) for the mass region of 0.7 to 3.5 MeV.

Nine transitions of dipole character have been identified linking an excited superdeformed (SD) band in ${}^{\mathrm{152}}\mathrm{D}\mathrm{y}$ to the yrast SD band. As a result, the excitation energy of the lowest level in the excited SD band has been measured to be 14 238 keV. This corresponds to a 1.3 MeV excitation above the SD ground state. The levels in this band have tentatively been determined to be of negative parity and odd spin. The measured properties are consistent with an interpretation in terms of a rotational band built on a collective octupole vibration.

Evaporation residue cross sections have been measured following the fusion of ${}^{\mathrm{16}}\mathrm{O}$ with ${}^{\mathrm{204}}\mathrm{P}\mathrm{b}$, forming the compound nucleus ${}^{\mathrm{220}}\mathrm{T}\mathrm{h}$. These are compared with existing data for the same compound nucleus formed following fusion reactions with ${}^{\mathrm{40}}\mathrm{A}\mathrm{r}$, ${}^{\mathrm{48}}\mathrm{C}\mathrm{a}$, ${}^{\mathrm{82}}\mathrm{S}\mathrm{e}$, and ${}^{\mathrm{124}}\mathrm{S}\mathrm{n}$ projectiles. At energies where the reduced cross sections of $xn$ evaporation residues would be expected to be the same for all reactions, those for the heavier projectiles are typically a factor of 10 smaller than those for ${}^{\mathrm{16}}\mathrm{O}$. This inhibition is attributed to strong competition of quasifission with fusion at the low angular momenta associated with evaporation residue formation.

The STAR Collaboration reports the first observation of exclusive ${\rho }^0$ photoproduction, $\mathrm{A}\mathrm{u}\mathrm{A}\mathrm{u}\to \mathrm{A}\mathrm{u}\mathrm{A}\mathrm{u}{\rho }^0$, and ${\rho }^0$ production accompanied by mutual nuclear Coulomb excitation, $\mathrm{A}\mathrm{u}\mathrm{A}\mathrm{u}\to \mathrm{A}{\mathrm{u}}^{\mathrm{\star }}\mathrm{A}{\mathrm{u}}^{\mathrm{\star }}{\rho }^0$, in ultraperipheral heavy-ion collisions. The ${\rho }^0$ have low transverse momenta, consistent with coherent coupling to both nuclei. The cross sections at $\sqrt{s_{NN}}=130\mathrm{G}\mathrm{e}\mathrm{V}$ agree with theoretical predictions treating ${\rho }^0$ production and Coulomb excitation as independent processes.

We present the first measurements of the survival time of ultracold neutrons (UCNs) in solid deuterium (${\mathrm{S}\mathrm{D}}_2$). This critical parameter provides a fundamental limitation to the effectiveness of superthermal UCN sources that utilize solid ortho-deuterium as the source material. These measurements are performed utilizing a ${\mathrm{S}\mathrm{D}}_2$ source coupled to a spallation source of neutrons, providing a demonstration of UCN production in this geometry and permitting systematic studies of the influence of thermal up-scatter and contamination with para-deuterium on the UCN survival time.

A high-resolution $(\gamma ,{\gamma }^{\prime })$ study of the electric dipole response in ${}^{\mathrm{208}}\mathrm{P}\mathrm{b}$ at the S-DALINAC reveals a resonance structure centered around the neutron emission threshold. Microscopic quasiparticle phonon model calculations in realistic model spaces including the coupling to complex configurations are able to describe the data in great detail. The resonance is shown to result from surface density oscillations of the neutron skin relative to an approximately isospin-saturated core. It also forms an integral part of a toroidal $E1$ mode representing an example of vortex collective motion in nuclei.

The mechanism for the large suppression of complete fusion in the ${}^{\mathrm{9}}\mathrm{B}\mathrm{e}+{}^{\mathrm{208}}\mathrm{P}\mathrm{b}$ reaction has been investigated through measurement of sub-barrier breakup of ${}^{\mathrm{9}}\mathrm{B}\mathrm{e}$. Excluding breakup through the ${}^{\mathrm{8}}\mathrm{B}\mathrm{e}$ ground state, whose lifetime is too long, a prompt breakup component remains, having sufficient probability to explain the observed suppression of complete fusion. This appears to be associated with interactions at the nuclear surface. The fusion suppression is predicted to be almost proportional to the charge of the target nucleus, making it most significant in reactions with heavy nuclei.

It is demonstrated that the fragmentation functions at large momentum fraction play a key role in hard hadron production from relativistic proton-proton collisions. We find that this region of the fragmentation functions is not strongly constrained by the electron-positron data. This freedom can be used (together with the transverse-momentum distribution of partons) to reproduce hard pion-to-proton ratio data in relativistic proton-proton collisions.

The number of levels of a many-fermion system confined by a harmonic-oscillator potential is computed as a function of excitation energy. Because of its exact nature, the formalism accounts for effects of shell structure on the level density. The method is easily extended to a variety of situations as is illustrated with the inclusion of isospin and deformation effects as well as a calculation of the number of spurious states.

Production of the radioisotope ${}^{\mathrm{18}}\mathrm{F}$ in novae is severely constrained by the rate of the ${}^{\mathrm{18}}\mathrm{F}(p,\alpha ){}^{\mathrm{15}}\mathrm{O}$ reaction. A resonance at $E_{\mathrm{c}\mathrm{.}\mathrm{m}\mathrm{.}}=330\mathrm{k}\mathrm{e}\mathrm{V}$ may strongly enhance the ${}^{\mathrm{18}}\mathrm{F}(p,\alpha ){}^{\mathrm{15}}\mathrm{O}$ reaction rate, but its strength has been very uncertain. We have determined the strength of this important resonance by measuring the ${}^{\mathrm{18}}\mathrm{F}(p,\alpha ){}^{\mathrm{15}}\mathrm{O}$ cross section on and off resonance using a radioactive ${}^{\mathrm{18}}\mathrm{F}$ beam at the ORNL Holifield Radioactive Ion Beam Facility. We find that its resonance strength is $1.48\pm 0.46\mathrm{e}\mathrm{V}$, and that it dominates the ${}^{\mathrm{18}}\mathrm{F}(p,\alpha ){}^{\mathrm{15}}\mathrm{O}$ reaction rate over a significant range of temperatures characteristic of ONeMg novae.

The interplay of nuclear effects on the $p_T>2\mathrm{G}\mathrm{e}\mathrm{V}$ inclusive hadron spectra in $d+\mathrm{A}\mathrm{u}$ and $\mathrm{A}\mathrm{u}+\mathrm{A}\mathrm{u}$ reactions at $\sqrt{s_{NN}}=17$, 200, and 5500 GeV is compared to leading order perturbative QCD calculations for elementary $p+p$ ($\overline{p}+p$) collisions. The competition between nuclear shadowing, Cronin effect, and jet energy loss due to medium-induced gluon radiation is predicted to lead to a striking energy dependence of the nuclear suppression/enhancement pattern in $A+A$ reactions. We show that future $d+\mathrm{A}\mathrm{u}$ data can be used to disentangle the initial and final state effects.

We report the first observation of double radiative capture on pionic hydrogen. The experiment was conducted at the TRIUMF cyclotron using the RMC spectrometer and detected $\gamma$-ray coincidences following ${\pi }^{-}$ stops in liquid hydrogen. We found the branching ratio for double radiative capture to be $[3.05\pm 0.27(\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t})\pm 0.31(\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t})]\times {10}^{-5}$. The measured branching ratio and angle-energy distributions support the theoretical prediction of a dominant contribution from the $\pi \pi \to \gamma \gamma$ annihilation mechanism.

We present data on the inclusive scattering of polarized electrons from a polarized ${}^{\mathrm{3}}\mathrm{H}\mathrm{e}$ target at energies from 0.862 to 5.06 GeV, obtained at a scattering angle of $15.5°$. Our data include measurements from the quasielastic peak, through the nucleon resonance region, and beyond, and were used to determine the virtual photon cross-section difference ${\sigma }_{1/2}-{\sigma }_{3/2}$. We extract the extended Gerasimov-Drell-Hearn integral for the neutron in the range of four-momentum transfer squared $Q^2$ of $0.1–0.9{\mathrm{G}\mathrm{e}\mathrm{V}}^2$.

Three distinctly different mechanisms are shown to populate the $K^{\pi }=6^{+}$ ($t_{1/2}=77\mathrm{n}\mathrm{s}$), ${16}^{+}$ (31 yr), and $8^{-}$ (4 s) isomer bands of ${}^{\mathrm{178}}\mathrm{H}\mathrm{f}$ by Coulomb excitation. High spin states of the three isomer bands were populated by Coulomb excitation of a hafnium target with a 650 MeV ${}^{\mathrm{136}}\mathrm{X}\mathrm{e}$ beam. Although direct population of high-$K$ bands is highly $K$-forbidden, isomer bands in ${}^{\mathrm{178}}\mathrm{H}\mathrm{f}$ were populated up to spins ${13}_{K=6}^{+}$, ${20}_{K=16}^{+}$, and ${14}_{K=8}^{-}$ with in-band $\gamma$ yields of $\sim {10}^{-4}$ of the ground state band. The data are consistent with a rapid increase in $K$ mixing with increasing spin in the isomer bands.