Search

The upgraded NIMROD–ISiS 4 π detector array is described. The array is composed of 228 detector modules annularly arranged into 14 rings. Telescopes of Si–CsI and Si–Si–CsI provide excellent isotopic resolution throughout the detector array. To enhance event characterization, the entire charged particle array is housed inside the TAMU Neutron Ball.

Transverse momentum spectra of protons and anti-protons measured in the rapidity range 0 y 3.1 from 0–10% central Au + Au collisions at s N N = 62.4 GeV are presented. The rapidity densities, d N / d y , of protons, anti-protons and net-protons ( N p – N p ¯ ) have been deduced from the spectra over a rapidity range wide enough to observe the expected maximum net-baryon density. From mid-rapidity to y = 1 the net-proton yield is roughly constant ( d N / d y ∼ 10 ), but rises to d N / d y ∼ 25 at 2.3 y 3.1 . The mean rapidity loss is 2.01 ± 0.14 ± 0.12 units from beam rapidity. The measured rapidity distributions are compared to model predictions. Systematics of net-baryon distributions and rapidity loss vs. collision energy are discussed.

12th International Conference on Nucleus-Nucleus Collisions, NN2015, Catania, Italy, 21 Jun 2015 - 26 Jun 2015; The European physical journal / Web of Conferences 117, 07018 (2016). doi:10.1051/epjconf/201611707018, Published by EDP Sciences, Les Ulis

We will show that the Thick Target Inverse Kinematics (TTIK) technique can be used to investigate the breakup of excited selfconjugate nuclei into many alpha particles. Two test runs were performed at Cyclotron Institute of Texas A&M University to study the reaction 20 Ne+α at maximum beam energies of 10 and 12 AMeV. Due to the limited statistics, only events with alpha multiplicity up to three were analyzed. The analysis of the three α -particle emission data allowed the identification of the Hoyle state and other 12 C excited states decaying into three alpha particles. The results will be shown and compared with other data available in the literature.

In this work, we present a new and general method for measuring the astrophysical $S$ factor of nuclear reactions in laser-induced plasmas and we apply it to $^{2}\mathrm{H}(d,n)^{3}\mathrm{He}$. The experiment was performed with the Texas Petawatt Laser, which delivered 150--270 fs pulses of energy ranging from 90 to 180 J to ${\mathrm{D}}_{2}$ or ${\mathrm{CD}}_{4}$ molecular clusters (where D denotes ${}^{2}\mathrm{H})$. After removing the background noise, we used the measured time-of-flight data of energetic deuterium ions to obtain their energy distribution. We derive the $S$ factor using the measured energy distribution of the ions, the measured volume of the fusion plasma, and the measured fusion yields. This method is model independent in the sense that no assumption on the state of the system is required, but it requires an accurate measurement of the ion energy distribution, especially at high energies, and of the relevant fusion yields. In the $^{2}\mathrm{H}(d,n)^{3}\mathrm{He}$ and $^{3}\mathrm{He}(d,p)^{4}\mathrm{He}$ cases discussed here, it is very important to apply the background subtraction for the energetic ions and to measure the fusion yields with high precision. While the available data on both ion distribution and fusion yields allow us to determine with good precision the $S$ factor in the $d+d$ case (lower Gamow energies), for the $d+^{3}\mathrm{He}$ case the data are not precise enough to obtain the $S$ factor using this method. Our results agree with other experiments within the experimental error, even though smaller values of the $S$ factor were obtained. This might be due to the plasma environment differing from the beam target conditions in a conventional accelerator experiment.

The detector system used at the first focus of the BigSol superconducting solenoid beam line at the Texas A & M superconducting cyclotron is presented. The system is composed of a position sensitive PPAC followed by an ionization chamber (IC) with a YAP(Ce) array in its back plane. The position sensitivity of the PPAC as well the energy resolution of the IC is investigated as a function of the counting rate. Pulse height, pulse height resolution and time resolution of the YAP(Ce) crystals are studied for a variety of heavy ion beams ranging from 20 Ne to 197 Au at energies from 15 AMeV to 40 AMeV. The pulse shape discrimination method using a Flash ADC is also explored, in order to identify decay products (alpha particles and electron/gamma) from the fragments implanted in the scintillator.

Cluster formation is a fundamental aspect of the equation of state (EOS) of warm and dense nuclear matter such as can be found in supernovae (SNe). Similar matter can be studied in heavy-ion collisions (HIC). We use the experimental data of Qin et al. [Phys. Rev. Lett. 108, 172701 (2012)] to test calculations of cluster formation and the role of in-medium modifications of cluster properties in SN EOSs. For the comparison between theory and experiment we use chemical equilibrium constants as the main observables. This reduces some of the systematic uncertainties and allows deviations from ideal gas behavior to be identified clearly. In the analysis, we carefully account for the differences between matter in SNe and HICs. We find that, at the lowest densities, the experiment and all theoretical models are consistent with the ideal gas behavior. At higher densities ideal behavior is clearly ruled out and interaction effects have to be considered. The contributions of continuum correlations are of relevance in the virial expansion and remain a difficult problem to solve at higher densities. We conclude that at the densities and temperatures discussed mean-field interactions of nucleons, inclusion of all relevant light clusters, and a suppression mechanism of clusters at high densities have to be incorporated in the SN EOS., 20 pages, 15 figures, v2: matches published version, only minor editorial corrections

The response of YAP:Ce scintillators to energetic heavy ions in the mass range A = 20−200 has been studied. The pulse height and the pulse height resolution have been explored in details for a single YAP:Ce crystal, coupled to R4141 PMT, by using 20Ne, 40Ar, 84Kr, 129Xe and 197Au beams having energies ranging from 40 A MeV to 15 A MeV. Energy degraded beams have also been employed in order to cover a large energy range. Finally, timing properties were studied in the BIGSOL spectrometer at Texas A&M by measuring the heavy fragment time-of-flight against a PPAC detector.

We review the main results obtained by the BRAHMS collaboration on the properties of hot and dense hadronic and partonic matter produced in ultrarelativistic heavy ion collisions at RHIC. A particular focus of this paper is to discuss to what extent the results collected so far by BRAHMS, and by the other three experiments at RHIC, can be taken as evidence for the formation of a state of deconfined partonic matter, the so called quark-gluon-plasma (QGP). We also discuss evidence for a possible precursor state to the QGP, i.e. the proposed Color Glass Condensate., 32 pages, 18 figures

We present the results obtained by the BRAHMS experiment at the Relativistic Heavy Ion Collider (RHIC) for three colliding systems, namely: Au + Au , d + Au and p + p at [Formula: see text]. The main focus here is to give an overview of the main results on the reaction dynamics and on the properties of hot and high energy density matter produced in utra-relativistic heavy ion collisions. Measurement of particle production, particle spectra over a large rapidity interval as well as high pt measurements related to nuclear modification in Au + Au and d + Au collision are discussed. The observed number of charged particles produced per unit of rapidity at the central rapidity region indicates that a high energy density (> 5 GeV/fm3) system is created at the initial stage of the Au + Au reaction. From the particle spectra we deduced significant radial expansion (β ≈ 0.75) which is consistent with the large initial energy density. For Au + Au at η = 0 we observe the suppression of the high p T particles as compared to the elementary collisions, whereas for the d + Au reaction the Cronin type enhancement is observed. We also discuss to what extent these results can be taken as evidence for the quark gluon plasma (QGP). Finally, we present the nuclear modification effects in d + Au reaction as a function of rapidity and consider whether the observed suppression at forward rapidities constitutes sufficient evidence for a possible precursor state to the QGP, i.e. the Color Glass Condensate (CGC).

We present a measurement of π − / π + , K − / K + and p ¯ / p from p + p collisions at s = 200 GeV over the rapidity range 0 y 3.4 . For p T 2.0 GeV / c we see no significant transverse momentum dependence of the ratios. All three ratios are independent of rapidity for y ≲ 1.5 and then steadily decline from y ∼ 1.5 to y ∼ 3 . The π − / π + ratio is below unity for y > 2.0 . The p ¯ / p ratio is very similar for p + p and 20% central Au + Au collisions at all rapidities. In the fragmentation region the three ratios seem to be independent of beam energy when viewed from the rest frame of one of the protons. Theoretical models based on quark–diquark breaking mechanisms overestimate the p ¯ / p ratio up to y ≲ 3 . Including additional mechanisms for baryon number transport such as baryon junctions leads to a better description of the data.

Recent results on limiting temperatures and caloric curves are discussed with respect to the question of identifying critical behavior in the disassembly of nuclei. We suggest that the best candidates for exploring such behavior are light nuclei in which the Coulomb Energies are less disruptive.

Particle production of charged kaons in central Au+Au collisions at has been studied as a function of rapidity by the BRAHMS collaboration at RHIC. The kaon spectral shapes are described well by an exponential in transverse mass for the rapidity range of 0 ≤ yK ≤ 3.3 with smoothly decreasing inverse slopes as the rapidity increases. For the charged kaon to pion ratios, while there is no significant rapidity dependence for the K+/π+ ratio, the K−/π− ratio shows a significant decrease from y ≈ 1 towards higher rapidities. The systematics of K−/K+ and ratios in the measured rapidity range demonstrates a strongly correlated behaviour which can be described by thermal–statistical models.

The BRAHMS experiment at RHIC was conceived to pursue the understanding of nuclear matter under extreme conditions by detailed measurements of charged hadrons over the widest possible range of rapidity and transverse momentum. The experiment consists of two spectrometers with complementary charged hadron detection capabilities as well as a series of global detectors for event characterization. A series of tracking detectors, time-of-flight arms and Cherenkov detectors enables momentum determination and particle identification over a wide range of rapidity and transverse momentum. Technical details and performance results are presented for the various detector subsystems. The performance of the entire system working together is shown to meet the goals of the experiment.

In this work we explore the possibility that the motion of the deuterium ions emitted from Coulomb cluster explosions is chaotic enough to resemble thermalization. We analyze the process of nuclear fusion reactions driven by laser-cluster interactions in experiments conducted at the Texas Petawatt laser facility using a mixture of D2+3He and CD4+3He cluster targets. When clusters explode by Coulomb repulsion, the emission of the energetic ions is nearly isotropic. In the framework of cluster Coulomb explosions, we analyze the energy distributions of the ions using a Maxwell- Boltzmann (MB) distribution, a shifted MB distribution (sMB) and the energy distribution derived from a log-normal (LN) size distribution of clusters. We show that the first two distributions reproduce well the experimentally measured ion energy distributions and the number of fusions from d-d and d-3He reactions. The LN distribution is a good representation of the ion kinetic energy distribution well up to high momenta where the noise becomes dominant, but overestimates both the neutron and the proton yields. If the parameters of the LN distributions are chosen to reproduce the fusion yields correctly, the experimentally measured high energy ion spectrum is not well represented. We conclude that the ion kinetic energy distribution is highly chaotic and practically not distinguishable from a thermalized one., Comment: 9 pages, 6 figures

A new method of accessing information on the symmetry free energy from yields of fragments produced in Fermi-energy heavy-ion collisions is proposed. Furthermore, by means of quantum fluctuation analysis techniques, correlations between extracted symmetry free-energy coefficients with temperature and density were studied. The obtained results are consistent with those of commonly used isoscaling techniques., Comment: 6 pages, 3 figures Heavy-ion nuclear reactions at Fermi energies, Nuclear equation of State, Fragmentation

Results from the BRAHMS Collaboration for the inaugural physics run with the relativistic heavy ion collider (RHIC), Au + Au collisions at √sNN = 130 GeV in the late summer of 2000, and some results of the very first data from the summer of 2001 at √sNN = 200 GeV are presented. Here the main focus is on the determination of the pseudorapidity distribution of charged particles as a function of collision centrality and on the measurement of the antiproton to proton and the K− to K+ ratios as a function of rapidity. The pseudorapidity density at midrapidity reaches dN(ch)/dη = 553 ± 36 for the most central collisions (0–5%) at √sNN = 130 GeV; the corresponding number for a higher energy is dN(ch)/dη = 632 ± 36, demonstrating an increase in multiplicity of about 14%. The measured antiproton/proton ratios peak at midrapidity around N()/N(p) = 0.64 ± 0.06 and drop to 0.41 ± 0.04 at y ≈ 2 for √sNN = 130 GeV reactions; the K− to K+ ratios are 0.90 ± 0.06 (y ≈ 0) and 0.83 ± 0.1 (y ≈ 2.5).

The reactions 11B + 198Pt →209Bi* and 10B + 198Pt →208Bi* have been investigated by measuring low- and high-energy γ-rays and proton-γ coincidences. The nuclear temperature of the 208Pb populated by first-chance proton emission from 209Bi has been determined by using an unfolding technique. The extracted value, T = 1.6±0.2 MeV, is compared with predictions from various level density parametrisations in which the shell effects play different roles. The comparison shows that, as expected in this excitation energy range, the shell effects in 208Pb are substantially washed out. Proton and high-energy γ-ray spectra as well as neutron average multiplicities have been also compared with predictions from Statistical Model calculations making use of a constant ( a = A/10MeV-1) or an excitation-energy-dependent level density parameter. The obtained results demonstrate that the high-energy γ-rays are more sensitive than protons to the level density variation with the excitation energy. However, Statistical Model calculations fail in reproducing satisfactorily the line-shape of the experimental γ-ray spectra, suggesting that the actual knowledge of the level densities in the 208Pb region is not sufficiently accurate.

The quantum-fluctuation method using Coulomb corrections is explored to extract temperatures and densities of hot nuclear systems produced in Fermi-energy heavy-ion collisions. The role of the neutron-proton asymmetry of the system was investigated. Temperature values were observed to not depend on the system neutron-proton asymmetry while a clear dependence is seen for density values. Coulomb corrections have been shown to lower temperature values by almost 2 MeV. On the other hand, density results exhibit a small variation.

The mass dependence of the transverse flow in the reactions of ${}^{40}\mathrm{Ca}+{}^{40}\mathrm{Ca}$ at 35 MeV/nucleon has been determined for emitted isotopes with $Z=1$ to 9. The observed flow is compared with that calculated using a constrained molecular dynamics (CoMD) simulation. With the application of the appropriate experimental filter, the general trend of the experimental mass-dependent flow is well reproduced by the simulation employing an effective interaction corresponding to a soft equation of state $(K=200$ MeV). The CoMD events are further utilized to study the mechanism of generation of the mass-dependent flow. It is found that the mass-dependent flow is generated by the interplay between the thermal and collective motions under a momentum conservation in the fragmenting system. With the help of the collective-thermal-interplay model, the mass-dependent flow scaled by the reduced mass of fragments $A/{A}_{\mathrm{sys}}$ is found to be almost independent of the size of the system.

The characteristic properties of the hot nuclear matter existing at the time of fragment formation in the multifragmentation events produced in the reaction $^{64}$Zn + $^{112}$Sn at 40 MeV/nucleon are studied. A kinematical focusing method is employed to determine the multiplicities of evaporated light particles, associated with isotopically identified detected fragments. From these data the primary isotopic yield distributions are reconstructed using a Monte Carlo method. The reconstructed yield distributions are in good agreement with the primary isotope distributions obtained from AMD transport model simulations. Utilizing the reconstructed yields, power distribution, Landau free energy, characteristic properties of the emitting source are examined. The primary mass distributions exhibit a power law distribution with the critical exponent, $A^{-2.3}$, for $A \geq 15$ isotopes, but significantly deviates from that for the lighter isotopes. Landau free energy plots show no strong signature of the first order phase transition. Based on the Modified Fisher Model, the ratios of the Coulomb and symmetry energy coefficients relative to the temperature, $a_{c}/T$ and $a_{sym}/T$, are extracted as a function of A. The extracted $a_{sym}/T$ values are compared with results of the AMD simulations using Gogny interactions with different density dependencies of the symmetry energy term. The calculated $a_{sym}/T$ values show a close relation to the symmetry energy at the density at the time of the fragment formation. From this relation the density of the fragmenting source is determined to be $\rho /\rho_{0} = (0.63 \pm 0.03 )$. Using this density, the symmetry energy coefficient and the temperature of fragmenting source are determined in a self-consistent manner as $a_{sym} = (24.7 \pm 3.4) MeV$ and $T=(4.9 \pm 0.2)$ MeV.

We explored alpha clustering in 24 Mg using the reaction 20 Ne+ and the Thick Target Inverse Kinematics (TTIK) technique. 20 Ne beams of energy 3.7 AMeV and 11 AMeV were delivered by the K150 cyclotron at Texas A&M University. The reaction chamber was filled with 4 He gas at a pressure sufficient to stop the beam before the detectors. The energy of the light reaction products was measured by three silicon detector telescopes. The time relative to the cyclotron radiofrequency was also measured. For the first time the TTIK method was used to study both single and multiple -particle decays. New results were obtained on elastic resonant  scattering, as well as on inelastic processes leading to high excitation energy systems decaying by multiple -particle emission. Preliminary results will be shown on events with -multiplicity one and two.

We present charged particle densities as a function of pseudorapidity and collision centrality for the 197Au+197Au reaction at sqrt{s_{NN}}=130 GeV. An integral charged particle multiplicity of 3860+/-300 is found for the 5% most central events within the pseudorapidity range -4.7

The transition of the level density parameter a off from the low excitation energy value a off=A/8 MeV−1 to the Fermi gas value a FG ∼ A/15 MeV−1 was discovered a few years ago studying particle spectra evaporated from hot compound systems of A∼ 160. A number of experiments have been recently performed to confirm the earlier findings and extend the investigation to other mass regions and to higher excitation energies. Furthermore, precision coincidence experiments have been done in the lead region in which evaporation residues are tagged by low energy gammarays. Those experiments open the possibility of a detailed study of the level densities in nuclei where the shell effects are important.

The excitation energy deposited in the reaction 209 Bi(α,α′) at 240 MeV has been determined using the TAMU Neutron Ball. The reconstructed average excitation energies as a function of the beam energy losses show that not all of the missing beam energy is usually deposited as thermal excitation in the target nucleus. Requiring an additional coincidence with a light charged particle or fission fragment leads to selection of a significant higher average excitation energy. This effect is extrapolated to explore results of previous GDR decay measurements in the case of a 208 Pb target.

We have undertaken a study of the thermal and the dynamical properties of fission and evaporation processes through the observation of neutron and light charged particle (LCP) emission (p, α) in collisions of 20 Ne + 159 Tb and 20 Ne + 169 Tm at 8, 10, 13 and 16 MeV/nucleon using the neutron multidetector DEMON. Both systems lead to the formation of compound nuclei (CN) with initial excitation energies ( E x * ) ranging between 100 and 250 MeV. These CN, formed through complete (CF) or incomplete (IF) fusion reactions, decay through either fission or evaporation residue (ER) formation. Our experimental goal in this study is to establish, as a function of E x * , i) σ ER /σ fission cross-section ratios, ii) pre- and post-scission neutron and LCP multiplicities with a clear cut assignment to CF and IF processes and iii) to try to consistently interpret these data in the framework of the recently modified statistical code GEMINI, including time-dependent fission width probabilities in order to establish the fission time scales.

Neutron fold distributions measured for the reaction 209 Bi(α,α′) at 240 MeV have been analyzed with the help of Statistical Model calculations to determine the distribution of excitation energy, E x , in the primary target fragments as a function of the projectile energy loss, EL. The reconstructed distributions in excitation energy feature a plateau which extends from the kinematical limit E x = EL to very small excitations, indicating a variety of interactions of the beam particles with the target nucleus. The requirement of an additional coincidence with a light charged particle leads to the selection of a significant higher average excitation energy. Those results are extrapolated to explore the effects of including the excitation energy distributions in the analysis of previous GDR measurements in 208 Pb. Corrections of the derived GDR parameters due to the partial transfer of excitation energy are suggested.

Neutron fold distributions measured for the reaction 209 Bi(α,α′) at 240 MeV have been analyzed with the help of Statistical Model calculations to determine the distribution of excitation energy in the primary target fragments as a function of the projectile energy loss, EL. Results show that the distributions in excitation energy feature a plateau which extends from the kinematical limit E x =EL to very small excitations, suggesting a variety of interactions of the beam particles with the target nucleus. Requiring an additional coincidence with a light charged particle leads to selection of a significant higher average excitation energy. This effect is extrapolated to explore results of previous GDR decay measurements in the case of a 208 Pb target. Corrections of derived GDR parameters due to the partial transfer of excitation energy are suggested.

The energy deposition associated with inelastic {alpha} particle scattering on {sup 209}Bi at 240 MeV has been determined using the TAMU neutron ball. A comparison of the reconstructed average excitation energies with the beam energy losses demonstrates that only part of the missing beam energy is usually deposited as thermal excitation in the target nucleus. Requiring an additional coincidence with a light charged particle or fission fragment leads to selection of a significant higher average excitation energy. {copyright} {ital 1998} {ital The American Physical Society}

International audience; The entrance channel dynamics of the 40Ca+40Ca reaction at 35A MeV is modeled with the recently developed QMD codes, EQMD and AMD-V. Drastic differences are observed in the entrance channel dynamics calculated by the two models. AMD-V with a Gogny force reproduces the experimental results over all. The importance of the introduction of the diffusion process to the centroid motion of wave packets is suggested.

Fusion-evaporation reactions induced by 110 MeV B-11 and radioactive C-11 on Rb-87 targets have been studied by measuring evaporation residue-light particle coincidences. The proton to alpha particle ratio in each reaction has been derived and compared with predictions from statistical model calculations. These calculations account rather well for the experimental data, when a small empirical adjustment of the emission barrier is performed, in agreement with earlier results. No evidence is found for predicted temperature and isospin modification of the binding energies. The possibility of a further study of isospin and temperature dependent effects in fusion-evaporation reactions with radioactive beams is discussed.

The symmetry energy of nuclear matter is a fundamental ingredient in the investigation of exotic nuclei, heavy-ion collisions and astrophysical phenomena. A recently developed quantum statistical (QS) approach that takes the formation of clusters into account predicts low density symmetry energies far above the usually quoted mean field limits. A consistent description of the symmetry energy has been developed that joins the correct low-density limit with values calculated from quasi-particle approaches valid near the saturation density. The results are confronted with experimental values for free symmetry energies and internal symmetry energies, determined at sub-saturation densities and temperatures below 10 MeV using data from heavy-ion collisions. There is very good agreement between the experimental symmetry energy values and those calculated in the QS approach, Comment: 16 pages, 10 figures. arXiv admin note: text overlap with arXiv:0908.2344

Despite the long-standing interest in the symmetry energy by the nuclear physics community, much work remains to characterize the equation of state away from the valley of stability and normal density. Although the correlations between the thermodynamic properties (temperature, density, pressure) has been explored, the dependence of these correlations on the neutron-proton asymmetry has only recently been probed experimentally. In this work, we provide evidence for the asymmetry dependence of the nuclear caloric curve using multiple independent probes. Correlations between the temperature, density and pressure when normalized to their critical values exhibit scaling, allowing extraction of the critical point. The location of the critical point shows a dependence on the neutron-proton asymmetry.

In the last decade, the availability in high-intensity laser beams capable of producing plasmas with ion energies large enough to induce nuclear reactions has opened new research paths in nuclear physics. We studied the reactions 3He(d, p)4He and d(d,n)3He at temperatures of few keV in a plasma, generated by the interaction of intense ultrafast laser pulses with molecular deuterium or deuterated-methane clusters mixed with 3He atoms. The yield of 14.7 MeV protons from the 3He(d, p)4He reaction was used to extract the astrophysical S factor. Results of the experiment performed at the Center for High Energy Density Science at The University of Texas at Austin will be presented. © 2014 AIP Publishing LLC.

Ternary fission yields in the reaction 241Pu(nth,f) are calculated using a new model which assumes a nucleation-time moderated chemical equilibrium in the low density matter which constitutes the neck region of the scissioning system. The temperature, density, proton fraction and fission time required to fit the experimental data are derived and discussed. A reasonably good fit to the experimental data is obtained. This model provides a natural explanation for the observed yields of heavier isotopes relative to those of the lighter isotopes, the observation of low proton yields relative to 2H and 3H yields and the non-observation of 3He, all features which are shared by similar thermal neutron induced and spontaneous fissioning systems., Comment: 6 pages, 3 figures

The question "How heavy can an atomic nucleus be?" is a fundamental problem in nuclear physics. The possible existence of island(s) of stable super-heavy nuclei has been an inspiring problem in heavy ion physics for almost four decades. This paper is focused on the experimental search of Super/Hyper Heavy Elements (SHE/HHE) conducted at the Cyclotron Institute, Texas A&M University. A novel experimental idea and experimental set up introduced for this research will be presented.

The pre-scission and post-scission multiplicities of alpha particles were determined in coincidence with fission fragments for four windows of exit channel mass asymmetry in the reaction 20Ne+165Ho at 50A MeV. The technique used employed a kinematic analysis of the energy spectra of alpha particles as a function of the emission angle with respect to the fission axis. Using a procedure of fitting the experimental energy spectra with those resulting from a Monte Carlo simulation program, the total multiplicity of alpha particles was separated into pre-scission and post-scission components. Using the pre-scission and post-scission multiplicities, neutron multiplicity measurements and other empirical observations, both the initial excitation energy and the excitation energy at scission of the compound nucleus were determined. The pre-scission times in the de-excitation of a highly excited 178W compound nucleus with initial excition energy of 570 MeV were evaluated using statistical model calculations. Only a small decrease in scission time was derived for the most asymmetric events studied. The possible contribution of very damped deep inelastic processes is discussed.

The reaction has been studied , exploring the possibility of producing hot nuclei with low-angular momentum, in the mass region , suitable for studies of the level-density parameter. The possibility of using the energy of the inelastically scattered particles to determine the excitation energy deposited in the target nucleus has been checked by measuring evaporated charged particles, neutrons, fission fragments and -rays. Comparison of the data with results from the fusion reaction and statistical model calculations seems to indicate that, in the case of inelastic scattering, not all the energy dissipated in the collision is normally deposited as thermal excitation in the target nucleus. From evaporated proton spectra there is evidence for a strong pre-equilibrium component even at the most backward angles.

We investigate the importance of the quantum statistics and deexcitation of primary fragments on the isotope yield ratio temperature determination. A phenomenological formula is presented which allows derivation of the temperature of the decaying nuclear system at the freeze-out time from the measured double yield ratios of two isotope pairs. This prescription is applied to the recent ALADIN and EOS Collaboration data. {copyright} {ital 1997} {ital The American Physical Society}

A study of the decay of hot and heavy composite nuclei produced in the violent collisions between {sup 63}Cu and {sup 232}Th at 35A MeV is presented. The measurement of fission fragment correlations indicates that {approximately} 70{percent} of the projectile linear momentum can be transferred to the fissioning system. Heavy reaction products were observed at a laboratory angle of {theta}=6{degree} in coincidence with neutrons, light charged particles, and intermediate mass fragments. The dynamical aspects of the collisions between the projectile and target nuclei were investigated using the computer code CHIMERA which is based upon the molecular dynamics concept. Asymptotic characteristics of the reaction products were confronted with results of calculations of the tandem CHIMERA plus GEMINI codes. The data and model comparisons show that a composite system of mass as high as 275 amu and with an excitation energy {approximately} 1 GeV is formed in the most violent collisions. Some of the heavy reaction remnants are located on the fragment mass versus velocity plane inside the area where the evaporation residues resulting from the decay of the hot composite system are expected. A high neutron multiplicity associated with these events indicates their origin in the most dissipative events. However, amore » low cross section for the production of these remnants and the close similarity of their characteristics to the fission fragments do not allow more conclusive statements. {copyright} {ital 1997} {ital The American Physical Society}« less

Observations of heavy remnants emitted at forward angles with high velocities and high associated particle multiplicities have been used to select central collisions of 35A MeV {sup 63}Cu with {sup 197}Au. The data indicate that these remnants, both fission fragments and evaporation residuelike products, result from the deexcitation of nuclei with A{approximately}225{endash}240 having excitation energies of {approximately} 800{endash}1300 MeV. Similar particle multiplicities are observed for both evaporative and fission decay channels. Modeling the results with hybrid codes which treat entrance channel dynamics followed by sequential statistical decay requires the inclusion of some delay in the fission channel to produce heavy remnants with mass A{ge}130, but the trend of the predicted velocities of these heavy remnants is different from that of the experiments. Calculations with a dynamic model based on the molecular dynamics approach have also been performed and lead to similar results. He and Li isotope yield ratios and the apparent temperatures derived from those ratios are similar to those previously reported for excited nuclei in this mass region. Temperatures derived from other yield ratios are also similar once a self-consistent treatment, taking into account population and decay of known excited states, is applied. The derived temperatures show little variation more » with excitation energy, suggesting that a limiting temperature may be reached at relatively low excitation energy, although the interpretation of this result and the determination of the actual initial value of this temperature is model dependent. Comments on the application of the double isotope yield ratio technique to extraction of the nuclear caloric curve are made. {copyright} {ital 1997} {ital The American Physical Society} « less

We report on experiments in which the Texas Petawatt laser irradiated a mixture of deuterium or deuterated methane clusters and helium-3 gas, generating three types of nuclear fusion reactions: D($d$,${}^{3}$He)$n$, D($d$,$t$)$p$, and ${}^{3}$He($d$,$p$)${}^{4}$He. We measured the yields of fusion neutrons and protons from these reactions and found them to agree with yields based on a simple cylindrical plasma model using known cross sections and measured plasma parameters. Within our measurement errors, the fusion products were isotropically distributed. Plasma temperatures, important for the cross sections, were determined by two independent methods: (1) deuterium ion time of flight and (2) utilizing the ratio of neutron yield to proton yield from D($d$,${}^{3}$He)$n$ and ${}^{3}$He($d$,$p$)${}^{4}$He reactions, respectively. This experiment produced the highest ion temperature ever achieved with laser-irradiated deuterium clusters.

The excitation energies of the primary hot isotopes in multifragmentation events are experimentally reconstructed in the reaction system 64Zn + 112Sn at 40 MeV/nucleon. A kinematical focusing method is employed to evaluate the multiplicities of the evaporated light particles associated with isotopically identified fragments with 3≤Z≤14. Angular distributions of the velocity spectra of light charged particles and neutrons associated with trigger isotopes are examined. A moving source fit is used to separate the kinematically correlated particles, evaporated from the parents of the detected isotopes, from the uncorrelated particles originating from other sources. The latter are evaluated experimentally relative to those in coincidence with the Li isotopes. A parameter, k, is used to adjust the yield of the uncorrelated particles for different trigger isotopes. For each experimentally detected isotope, the multiplicities, apparent temperatures, and k values for n, p, d, t, and α particles are extracted. Using the extracted values, the excitation energies of the primary hot isotopes are reconstructed employing a Monte Carlo method. The extracted excitation energies are in the range of 1 to 4 MeV/nucleon but show a significant decreasing trend as a function of A for a given Z of the isotopes. The results are compared with those of antisymmetrized molecular dynamics (AMD) and statistical multifragmentation model (SMM) simulations. While some of the experimental characteristics are predicted partially by each model, neither simulation reproduces the overall characteristics of the experimental results. © 2013 American Physical Society.

The plasma astrophysical $S$ factor for the $^{3}\mathrm{He}(d,p)^{4}\mathrm{He}$ fusion reaction was measured for the first time at temperatures of few keV, using the interaction of intense ultrafast laser pulses with molecular deuterium clusters mixed with $^{3}\mathrm{He}$ atoms. Different proportions of ${\mathrm{D}}_{2}$ and $^{3}\mathrm{He}$ or ${\mathrm{CD}}_{4}$ and $^{3}\mathrm{He}$ were mixed in the gas target in order to allow the measurement of the cross section for the $^{3}\mathrm{He}(d,p)^{4}\mathrm{He}$ reaction. The yield of 14.7 MeV protons from the $^{3}\mathrm{He}(d,p)^{4}\mathrm{He}$ reaction was measured in order to extract the astrophysical $S$ factor at low energies. Our result is in agreement with other $S$ factor parametrizations found in the literature.

We report on experiments in which the Texas Petawatt laser irradiated a mixture of deuterium or deuterated methane clusters and helium-3 gas, generating three types of nuclear fusion reactions: D(d,^{3}He)n, D(d,t)p, and ^{3}He(d,p)^{4}He. We measured the yields of fusion neutrons and protons from these reactions and found them to agree with yields based on a simple cylindrical plasma model using known cross sections and measured plasma parameters. Within our measurement errors, the fusion products were isotropically distributed. Plasma temperatures, important for the cross sections, were determined by two independent methods: (1) deuterium ion time of flight and (2) utilizing the ratio of neutron yield to proton yield from D(d,^{3}He)n and ^{3}He(d,p)^{4}He reactions, respectively. This experiment produced the highest ion temperature ever achieved with laser-irradiated deuterium clusters.

Two different methods have been employed to determine the plasma temperature in a laser-cluster fusion experiment on the Texas Petawatt laser. In the first, the temperature was derived from time-of-flight data of deuterium ions ejected from exploding ${\mathrm{D}}_{2}$ or ${\mathrm{CD}}_{4}$ clusters. In the second, the temperature was measured from the ratio of the rates of two different nuclear fusion reactions occurring in the plasma at the same time: $\mathrm{D}(d,^{3}\mathrm{He})n$ and $^{3}\mathrm{He}(d,p)^{4}\mathrm{He}$. The temperatures determined by these two methods agree well, which indicates that (i) the ion energy distribution is not significantly distorted when ions travel in the disassembling plasma; (ii) the kinetic energy of deuterium ions, especially the ``hottest part'' responsible for nuclear fusion, is well described by a near-Maxwellian distribution.

The experimental determination of freeze-out temperatures and densities from the yields of light elements emitted in heavy ion collisions is discussed. Results from different experimental approaches are compared with those of model calculations carried out with and without the inclusion of medium effects. Medium effects become of relevance for baryon densities above $\approx 5 \times 10^{-4}$ fm$^{-3}$. A quantum statistical (QS) model incorporating medium effects is in good agreement with the experimentally derived results at higher densities. A densitometer based on calculated chemical equilibrium constants is proposed., 5 pages, 3 figures

The reactions 10 MeV/u 86 Kr + 76 Ge and 27 MeV/u 16 O + 150 Sm were used to form the same composite system with A ∼ 160 at about 300 MeV of excitation energy, as derived from measurements of both the linear momentum transfer and the neutron multiplicity. The comparison of the evaporative spectra of the α particles in coincidence with evaporation residues does not show evidence for effects related to the entrance channel of the reaction, such as those predicted by dynamical models.