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Carbon and oxygen burning reactions, in particular, $^{12}\mathrm{C}+^{12}\mathrm{C}$ fusion, are important for the understanding and interpretation of the late phases of stellar evolution as well as the ignition and nucleosynthesis in cataclysmic binary systems such as type Ia supernovae and x-ray superbursts. A new measurement of this reaction has been performed at the University of Notre Dame using particle-$\ensuremath{\gamma}$ coincidence techniques with SAND (a silicon detector array) at the high-intensity 5U Pelletron accelerator. New results for $^{12}\mathrm{C}+^{12}\mathrm{C}$ fusion at low energies relevant to nuclear astrophysics are reported. They show strong disagreement with a recent measurement using the indirect Trojan Horse method. The impact on the carbon burning process under astrophysical scenarios will be discussed.

For central collisions of 17-115 A MeV {sup 40}Ar+Cu , Ag, Au, an overall balance is determined for the average mass, energy, and longitudinal momentum. Light charged particles and fragments are separated into forward-focused and isotropic components in the frame of the heaviest fragment. Energy removal by the isotropic component reaches 1-2 GeV. For such high deposition energies, statistical multifragmentation models predict much more extensive nuclear disassembly than is observed. (c) 1999 The American Physical Society.

The fusion of ${}^{6}\mathrm{He}$ with a ${}^{209}\mathrm{Bi}$ target has been studied at energies near to and below the Coulomb barrier. Despite the weak binding of the valence neutrons in ${}^{6}\mathrm{He}$, little evidence is found for suppression of fusion due to projectile breakup. Instead, a large enhancement of sub-barrier fusion is observed. It is suggested that this enhancement may arise from coupling to positive $Q$ value neutron transfer channels, resulting in ``neutron flow'' between the projectile and the target.

The determination of the equation of state of nuclear matter is one of the main objectives of relativistic heavy-ion physics. Insight into the equation of state has been gained through the study of collective phenomena, which can be attributed to the response of hot and dense nuclear matter from the hot and dense region formed by the overlap of projectile and target nuclei [1, 2]. In particular, directed sidewards flow is considered as an important signature of nuclear compression and thus is sensitive to the nuclear equation of state. The sidewards direction of the flow was described years ago on the basis of macroscopic thermaldynamic/hydrodynamical picture [3] and the more microscopic cascade model [4]. As two nuclei collide, the pressure and density increase in the interaction region. At a finite impact parameter there is an inherent asymmetry in the pressure, which results in a transverse flow of matter in the direction of lowest pressure. The amount of transverse flow is directly related to the stiffness of the nuclear equation of state and transport properties of the nuclear medium [5]. This flow reflects the interplay of collective and random motions. For a thermalized system, the random motions of emitted fragments are dictated by the thermal energy, which is independent of mass. Contributions to the fragment energy due to collective motion, on the other hand, increase linearly with mass, making the flow more easily observed for heavier fragments [6, 7].

Multifragment decays of central collisions in 84Kr + 197Au at E/A = 35, 55, and 70 MeV are studied. The dependence of the extracted emission time on the velocity of the fragment pair is investigated. More energetic pairs manifest a stronger Coulomb interaction indicating emission from a source of smaller spatial-temporal extent than less-energetic pairs. This trend can be understood in the context of a statistical model which allows the source characteristics to evolve as the fragments are emitted.

Experimental intermediate-mass-fragment multiplicity distributions for the [ital E]/[ital A]=80 and 110 MeV [sup 36]Ar+[sup 197]Au reactions are shown to be binomial at all excitation energies. From these distributions, a single binary event probability [ital p] can be extracted that has a thermal dependence. Thus, it is inferred that multifragmentation is reducible to a combination of nearly independent emission processes. If sequential decay is assumed, the increase of [ital p] with excitation energy implies a contraction of the time scale that is qualitatively consistent with recent fragment-fragment correlation data.

Fragment distributions have been measured for Au+Au collisions at [ital E]/[ital A]=100 and 1000 MeV. A high detection efficiency for fragments was obtained by combining the ALADIN spectrometer and the MSU-Miniball/WU-Miniwall array. At both energies the maximum multiplicity of intermediate mass fragments (IMF) normalized to the size of the decaying system is about one IMF per 30 nucleons but the element distributions show significant differences. Within a coalescence picture the suppression of heavy fragments in central collisions at [ital E]/[ital A]=100 MeV may be related to a reduction of the density in momentum space which is caused by the collective expansion.

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.

The technique of invariant mass spectroscopy has been used to measure, for the first time, the ground state energy of neutron-unbound $^{28}\mathrm{F}$, determined to be a resonance in the $^{27}\mathrm{F}+n$ continuum at $2\underline{2}0(\underline{5}0)\text{ }\text{ }\mathrm{keV}$. States in $^{28}\mathrm{F}$ were populated by the reactions of a $62\text{ }\text{ }\mathrm{MeV}/\mathrm{u}$ $^{29}\mathrm{Ne}$ beam impinging on a $288\text{ }\text{ }\mathrm{mg}/{\mathrm{cm}}^{2}$ beryllium target. The measured $^{28}\mathrm{F}$ ground state energy is in good agreement with USDA/USDB shell model predictions, indicating that $pf$ shell intruder configurations play only a small role in the ground state structure of $^{28}\mathrm{F}$ and establishing a low-$Z$ boundary of the island of inversion for $N=19$ isotones.

Fusion cross sections were measured for the exotic proton-halo nucleus {sup 8}B incident on a {sup 58}Ni target at several energies near the Coulomb barrier. This is the first experiment to report on the fusion of a proton-halo nucleus. The resulting excitation function shows a striking enhancement with respect to expectations for normal projectiles. Evidence is presented that the sum of the fusion and breakup yields saturates the total reaction cross section.

Multifragmentation has been measured for [sup 197]Au+[sup 197]Au collisions at [ital E]/[ital A]=100, 250, and 400 MeV. The mean fragment multiplicity increases monotonically with the charged particle multiplicity at [ital E]/[ital A]=100 MeV, but decreases for central collisions with incident energy, consistent with the onset of nuclear vaporization. Molecular dynamics calculations follow some trends but underpredict the observed fragment multiplicities. Including the statistical decay of excited residues improves the agreement for peripheral collisions but worsens it for central collisions.

The momentum dependence of the two-proton correlation function is studied for central and peripheral [sup 36]Ar+[sup 45]Sc collisions at [ital E]/[ital A]=80 MeV. Calculations with the Boltzmann-Uehling-Uhlenbeck transport theory reproduce well the measured momentum dependence of the correlation functions for central collisions, but underpredict the momentum dependence for peripheral collisions.

Velocity distributions and fragment-fragment correlation functions have been measured as a function of charged particle multiplicity for $^{129}\mathrm{Xe}$${+}^{\mathrm{nat}}$Cu collisions at E/A=50 MeV. With increasing multiplicity, the velocity distributions evolve from a pattern exhibiting targetlike and projectilelike sources to an anisotropic distribution centered near the projectile-target center-of-mass velocity. Emission times extracted from the correlation functions are consistent with the decay of a projectilelike source in low multiplicity collisions, but the time scale for fragment emission in the high multiplicity collisions is too short for a separation of sources to occur.

Multifragment disintegrations following $^{129}\mathrm{Xe}$${+}^{197}$Au collisions at E/A=50 MeV have been studied with a multidetector system covering 88% of 4\ensuremath{\pi} in solid angle. The average number of intermediate-mass fragments (Z=3--20) increases strongly as a function of charged-particle multiplicity and reaches values larger than six for the most violent collisions. The results are compared to calculations with both dynamical and statistical models.

Multifragment events from 35, 40, 45, and 55 MeV/nucleon $^{139}\mathrm{La}$${+}^{12}$C, $^{27}\mathrm{Al}$, $^{40}\mathrm{Ca}$, $^{51}\mathrm{V}$, $^{\mathrm{nat}}\mathrm{Cu}$, and $^{139}\mathrm{La}$ reactions can be assigned to sources characterized by their velocity. At each bombarding energy, the probabilities of threefold, fourfold and fivefold events increase substantially with decreasing source velocity, but are independent of the target mass. To remove the bombarding-energy dependence, a simple transformation has been applied which gives the excitation energy of the fused system in the simple incomplete-fusion picture. These ``excitation functions'' appear to independent of both the system and bombarding energy.

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.

Isobaric analog states of $^{7}\mathrm{He}$ have been investigated by a novel technique involving the observation of the resonant yield of neutrons from the $^{6}\mathrm{He}(p,n)$ reaction in coincidence with $\ensuremath{\gamma}$ rays from the decay of the $({0}^{+},T=1)$ state in $^{6}\mathrm{Li}$. The $\ensuremath{\gamma}$ rays provide a clean signature for the isospin-conserving neutron decay of the low-lying isobaric analog resonances. It is conclusively shown that the analog of the recently observed low-lying spin-orbit partner of the $^{7}\mathrm{He}$ ground state does not exist. Evidence is presented that this state lies at much higher energies, in agreement with microscopic calculations.

A new experimental approach was developed that can reduce the uncertainties in astrophysical rapid proton capture ($rp$) process calculations due to nuclear data. This approach utilizes neutron removal from a radioactive ion beam to populate the nuclear states of interest. Excited states were deduced by the $\ensuremath{\gamma}$-decay spectra measured in a semiconductor Ge-detector array. In the first case studied, $^{33}\mathrm{Ar}$, excited states were measured with uncertainties of several keV. The 2 orders of magnitude improvement in the uncertainty of the level energies resulted in a 3 orders of magnitude improvement in the uncertainty of the calculated $^{32}\mathrm{Cl}(p,\ensuremath{\gamma})^{33}\mathrm{Ar}$ rate that is critical to the modeling of the $rp$ process. This approach has the potential to measure key properties of almost all interesting nuclei on the $rp$-process path.

Multifragment decays of central collisions in ${}^{84}\mathrm{Kr}{+}^{197}\mathrm{Au}$ at $E/A\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}70\mathrm{MeV}$ are studied. By utilizing a technique sensitive to the emission order of fragments, it is deduced that carbon fragments are emitted prior to beryllium fragments when these fragments have the same velocity. This observation is consistent with the cooling of a thermally decaying source.

A new, sensitive method allows one to search for the enhancement of events with nearly equal-sized fragments as predicted by theoretical calculations based on volume or surface instabilities. Simulations have been performed to investigate the sensitivity of the procedure. Experimentally, charge correlations of intermediate mass fragments emitted from heavy ion reactions at intermediate energies have been studied. No evidence for a preferred breakup into equal-sized fragments has been found., Comment: 12 pages, TeX type, psfig, submitted to Phys. Rev. Lett, also available at http://csa5.lbl.gov/moretto/ps/zcor_pp.ps

Intermediate-mass-fragment (IMF, 3 {le} Z {le} 20) emission probabilities and IMF charge distributions were recently shown to be reducible to the corresponding one fragment quantities. Furthermore, a strong thermal scaling was shown to control the energy dependence of the same quantities. The multifragmentation scenario painted by these experimental observations is that of a process controlled by a largely independent emission of individual fragments, which, in turn is dominated by phase space. Fragment-fragment angular correlations have been used to study the space-time extension of the emitting source. In particular, small angle repulsion (the Coulomb hole) has been interpreted in terms of the Coulomb repulsion of fragments emitted near each other, both in space and time. We will show in this paper that, except at small angles, the particle-particle angular correlations and their dependence on excitation energy are interpretable in terms of nearly independently emitted fragments whose angular distributions are controlled by phase space. Thus we show that the angular correlations are reducible and thermally scalable. A mass scaling of the angular correlations will also be demonstrated and its possible implications discussed.

Two-proton correlation functions for central collisions of {sup 36}Ar+{sup 45}Sc at {ital E}/{ital A}=80, 120, and 160 MeV are compared to calculations with the Boltzmann-Uehling-Uhlenbeck (BUU) transport model. Agreement is found at {ital E}/{ital A}=80MeV, but the model predicts too large correlations at {ital E}/{ital A}=120 and 160 MeV. The discrepancy may be due to delayed emission of protons from particle-unstable states not modeled in BUU. {copyright} {ital 1995} {ital The} {ital American} {ital Physical} {ital Society}.

A strong thermal signature is found in the charge distributions associated with multifragmentation from the reaction ${}^{36}\mathrm{Ar}{+}^{197}\mathrm{Au}$ at $E/A\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}110$ MeV. The $n$-fold charge distributions are reducible to the onefold charge distributions through a simple scaling that is dictated by fold number and charge conservation.

Energy spectra for intermediate mass fragments produced in central Au+Au collisions at [ital E]/[ital A]=100MeV indicate a collective expansion at breakup. For the first time, values for this collective expansion energy per nucleon are extracted independently for each charge. Typically, these values are one-third to one-half of the incident kinetic energy per nucleon in the c.m. system, but they decrease with [ital Z][sub [ital f]], suggesting that all fragments do not participate equally in the collective expansion.

Intermediate mass fragment (IMF: 3[le][ital Z][le]20) emission from necklike structures joining projectilelike and targetlike residues has been observed for peripheral [sup 129]Xe+[sup nat]Cu collisions at [ital E]/[ital A]=50 MeV. These fragments are emitted primarily at velocities between those of the projectilelike and targetlike residues. Relative to the charge distribution of fragments evaporated from projectilelike residues, the distribution for neck'' emission shows an enhanced emission for fragments with 4[le][ital Z][sub IMF][le]12. This feature is consistent with expectations for the fragmentation of a noncompact cylindrical configuration.

Differences between longitudinal and transverse two-proton correlation functions, consistent with finite lifetime effects, are observed for central [sup 36]Ar+[sup 45]Sc collisions at [ital E]/[ital A]=80 MeV. The correlations between two protons emitted with low total momenta are consistent with emission from a source of Gaussian radius [ital r][sub 0][approx]4.7[plus minus]0.3 fm and characteristic lifetime [tau][approx]25[plus minus]15 fm/[ital c].