Search Results (75 total)

Direct density measurements were made from shock-loaded aluminum and copper samples by combining plate-impact experiments with proton radiography at the Los Alamos Neutron Science Center. Flyer plates were accelerated using a 40 mm bore powder gun to create a shock wave in a sample. The sample material was then interrogated in real time using the proton radiography facility. The increase in density behind the shock front causes a measurable change in the transmission of protons through the sample, which can then be quantified as a density value in the material. Hugoniot values were calculated using more traditional techniques to evaluate the accuracy of the radiographically obtained density measurements.

We present a new phenomenology for burn propagation inside a thermal explosion based on dynamic radiography. Radiographic images were obtained of an aluminum cased solid cylindrical sample of a plastic bonded formulation of octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine. The phenomenology observed is ignition followed by cracking in the solid accompanied by the propagation of a radially symmetric front of increasing proton transmission. This is followed by a further increase in transmission through the sample, ending after approximately 100  μs. We show that these processes are consistent with the propagation of a convective burn front followed by consumption of the remaining solid by conductive particle burning.

Breakup densities of hot ¹⁹⁷Au-like residues have been deduced from the systematic trends of Coulomb parameters required to fit intermediate-mass-fragment kinetic-energy spectra. The results indicate emission from nuclei near normal nuclear density below an excitation energy E^*/A≲2  MeV, followed by a gradual decrease to a near-constant value of ρ/ρ₀∼0.3 for E^*/A≳5  MeV. Temperatures derived from these data with a density-dependent Fermi-gas model yield a nuclear caloric curve that is generally consistent with those derived from isotope ratios.

Effects of in-medium cross sections and of optical potential on preequilibrium emission and on formation of a thermal source are investigated by comparing the results of transport simulations with experimental results from the p+¹⁹⁷Au reaction at 6.2–14.6  GeV∕c. The employed transport model includes light-composite-particle production and allows for inclusion of in-medium particle-particle cross-section reduction and of momentum dependence in the particle optical potentials. Compared to the past, the model incorporates improved parametrizations of elementary high-energy processes. The simulations indicate that the majority of energy deposition occurs during the first 25  fm∕c of a reaction. This is followed by a preequilibrium emission and readjustment of system density and momentum distribution toward an equilibrated system. Within different variants of calculations, the best agreement with data, on the d∕p and t∕p yield ratios and on the residue mass and charge numbers, is obtained at the time of about 65  fm∕c from the start of a reaction, for simulations employing reduced in-medium cross sections and momentum-dependent optical potentials. By that time, the preequilibrium nucleon and cluster emission, as well as mean field readjustments, drive the system to a state of depleted average density, ρ∕ρ₀∼1∕4–1∕3 for central collisions, and low-to-moderate excitation, i.e., the region of nuclear liquid-gas phase transition.

Multifragment breakup of ^natAg and ¹⁹⁷Au nuclei bombarded by 1.8–4.8  GeV ³He ions has been studied with the Indiana Silicon Sphere 4π detector array. To investigate the properties of the emitting source as a function of excitation energy, a two-component moving-source analysis has been performed on the intermediate-mass-fragment spectra, gating on excitation energy. The results provide evidence for nuclear expansion∕dilution to a value of ρ∕ρ₀≲1∕3 prior to breakup. For the most violent events, relatively low source velocities of v∕c~0.01 and slope temperatures of T∼15  MeV are obtained for the dominant thermal-like source. The dependence of isotope ratios on deposition energy and ejectile kinetic energy is examined for H and He isotopes, and the caloric curves for the 4.8  GeV data are presented.

The relationship between nuclear temperature and excitation energy of hot nuclei formed by 8 GeV/c negative pion and antiproton beams incident on ¹⁹⁷Au has been investigated with the ISiS 4π detector array at the BNL AGS accelerator. The double-isotope-ratio technique was used to calculate the temperature of the hot system. The two thermometers used, (p/d-³He/⁴He) and (d/t-³He/⁴He), are in agreement below E^*/A∼8 MeV when corrected for secondary decay. Caloric curves derived from successive segments of the H and He kinetic energy spectra show a systematic decrease in temperature as the kinetic energy bin decreases, consistent with “cooling curve” behavior. When extrapolated to the evaporative-peak region, these results provide good agreement with caloric curves measured for similar systems. The caloric curves from this experiment are also compared with the predictions from the SMM multifragmentation model.

In efforts to determine phase transitions in the disintegration of highly excited heavy nuclei, a popular practice is to parametrize the yields of isotopes as a function of temperature in the form Y(z)=z^-τf(z^σ(T-T₀)), where Y(z)’s are the measured yields and τ, σ, and T₀ are fitted to the yields. Here T₀ would be interpreted as the phase transition temperature. For finite systems such as those obtained in nuclear collisions, this parametrization is only approximate and hence allows for extraction of T₀ in more than one way. In this work we look in detail at how values of T₀ differ, depending on methods of extraction. It should be mentioned that for finite systems, this approximate parametrization works not only at the critical point, but also for first-order phase transitions (at least in some models). Thus the approximate fit is no guarantee that one is seeing a critical phenomenon. A different but more conventional search for the nuclear phase transition would look for a maximum in the specific heat as a function of temperature T₂. In this case T₂ is interpreted as the phase transition temperature. Ideally T₀ and T₂ would coincide. We invesigate this possibility, both in theory and from the ISiS data, performing both canonical (T) and microcanonical (e=E^*/A) calculations. Although more than one value of T₀ can be extracted from the approximate parametrization, the work here points to the best value from among the choices. Several interesting results, seen in theoretical calculations, are borne out in experiment.

Intermediate mass fragments (IMFS) (IMF: 3<~Z_IMF<~20) observed in coincidence with two correlated fission fragments following incomplete fusion in ¹²C+²³²Th at E/A=16 and 22 MeV are investigated. IMFs emitted prior to significant deformation of the fissioning system, as well as IMFs emitted near scission, are distinguished based upon their characteristic kinetic energy and angular distributions. The yield distributions of IMFs emitted near scission in these¹²C induced reactions are compared with near-scission IMF yields in spontaneous and low-energy ternary fission. Comparisons are made to both experimental fusion-evaporation data and theoretical predictions of a statistical model. The excitation energy dependence of relative IMF yields for both isotropic and near-scission emission is also presented. Our results for near-scission emission suggest that the production of IMFs near scission is inconsistent with a statistical emission mechanism in which emission barriers follow a standard Z dependence. Dynamical model calculations are used to investigate the role of dissipation, angular momentum, N/Z, and kinetic energy on the fragment formation near scission.

The analyzing power for proton-carbon elastic scattering in the Coulomb-nuclear interference region of momentum transfer, 9.0×10^-3<-t<4.1×10^-2 (GeV/c)², was measured with a 21.7  GeV/c polarized proton beam at the Alternating Gradient Synchrotron of Brookhaven National Laboratory. The ratio of hadronic spin-flip to nonflip amplitude, r₅, was obtained from the analyzing power to be Rer₅=0.088±0.058 and Imr₅=-0.161±0.226.

Experimental data from the reaction of an 8.0 GeV/c π^- beam incident on a ¹⁹⁷Au target have been analyzed in order to investigate the breakup time scale for hot residues. Helium nuclei angular distributions and energy spectra supported by a momentum tensor analysis suggest that at large excitation energy, above 3-5 MeV/nucleon, highly excited heavy fragments are separated promptly after the thermalization. A binary fission-like mechanism fits the experimental data at low excitation energies, but seems unable to reproduce the data at excitation energies above 3-5 MeV/nucleon.

The thermal component of the 8 GeV/c π+ Au data of the ISiS Collaboration is shown to follow the scaling predicted by Fisher’s model when Coulomb energy is taken into account. Critical exponents τ and σ, the critical point (p_c,ρ_c,T_c), surface energy coefficient c₀, enthalpy of evaporation ΔH, and critical compressibility factor C_c^F are determined. For the first time, the experimental phase diagrams, (p,T) and (T,ρ), describing the liquid vapor coexistence of finite neutral nuclear matter have been constructed.

A percolation model of nuclear fragmentation is used to interpret 10.2 GeV/c p+¹⁹⁷Au multifragmentation data. Emphasis is put on finding signatures of a continuous nuclear matter phase transition in finite nuclear systems. Based on model calculations, corrections accounting for physical constraints of the fragment detection and sequential decay processes are derived. Strong circumstantial evidence for a continuous phase transition is found, and the values of two critical exponents, σ  =  0.5±0.1 and τ  =  2.35±0.05, are extracted from the data. A critical temperature of T_c  =  8.3±0.2 MeV is found.

The relation between excitation energy and reaction observables has been examined for (6.0–14.6)-GeV/c protons, (5.0–9.2)-GeV π^-, and 8.0-GeV/c antiprotons incident on a ¹⁹⁷Au target. Relative to proton and π^- beams, 8.0-GeV/c antiprotons are found to be the most effective projectile for depositing high excitation energies in the targetlike residue. For protons and π^- the excitation-energy distributions are nearly identical and appear to be independent of beam momentum above 6–8 GeV/c. It is found that total measured charge, total thermal energy, and total charged-particle multiplicity scale most directly with excitation energy, whereas IMF multiplicity and total transverse energy exhibit large fluctuations. Correlations of the observed fragment multiplicity, charge, and kinetic-energy distributions with excitation energy indicate a transition in the reaction observables near E^*/A≈4–6 MeV. These experimental signals are consistent with a multifragmentation mechanism that becomes the dominant deexcitation mode above in the range E^*/A∼4–6 MeV.

The event-by-event reconstruction procedure and related uncertainties involved in the derivation of excitation energy and source-size distributions are investigated for GeV hadron-induced reactions. The analysis is performed for the 5.0–14.6 GeV/c proton-, π^- and antiproton-induced reactions on ¹⁹⁷Au, measured with the Indiana silicon sphere charged-particle detector array at the Brookhaven AGS accelerator. The relative contributions of the three major components of the excitation-energy calorimetry: charged-particle kinetic-energy sums, neutrons, and Q values from reconstructed events, are found to be relatively constant for excitation energies above about 500 MeV. Effects on the results imposed by various assumptions necessary to account for experimental factors are examined and a corresponding deconvolution of the excitation-energy distribution is performed. The major uncertainties in the calorimetry are found to be (1) separation of nonequilibrium and thermal-like charged particles, and (2) the unmeasured neutron component. The self-consistency of the procedure is tested via comparisons with the SMM and SIMON codes for the disintegration of hot nuclei.

A binomial reducibility and thermal scaling analysis is performed on well-chacracterized thermal-like sources formed in 8 GeV/c π^-+¹⁹⁷Au reactions. The fragment probability distributions are shown to be binomial when plotted as a function of the measured excitation energy E^* and the binomial elementary probability p is shown to follow the expected Boltzmann factor: ln(p)∝exp(-B/sqrt[E^*/A]). Binomial reducibility and thermal scaling are explored also using global variables other than E^*, and the effect of source size on the binomial parameter p and m is shown. Finally, the extracted probability p is found to be correlated with the experimentally deduced fragment emission time up to about 6A MeV of excitation energy, hinting at a possible transition in decay mechanism above that excitation energy.

Fragment kinetic energy spectra for reactions induced by 8.0 GeV/c π^- beams on a ¹⁹⁷Au target have been analyzed. The average fragment kinetic energies are observed to increase systematically with fragment charge but are nearly independent of excitation energy. Near E*/A=5 MeV, the data are well accounted for by two statistical multifragmentation models, SMM and SIMON-explosion. However, at higher excitation energies, a small amount of extra energy, proportional to the fragment mass, is required in the models in order to match the experimental fragment’s kinetic energies. This extra expansion energy is small relative to the radial expansion observed in heavy-ion-induced reactions, consistent with the interpretation that the latter expansion may be driven primarily by collective dynamical effects that are not present in light-ion-induced collisions.

Excitation-energy-gated two-fragment correlation functions have been studied between E^*/A  =  (2–9)A MeV for equilibriumlike sources formed in 8–10 GeV/c π^- and p+¹⁹⁷Au reactions. Comparison with an N-body Coulomb-trajectory code shows an order of magnitude decrease in the fragment emission time in the interval E^*/A  =  (2–5)A MeV, followed by a nearly constant breakup time at higher excitation energy. The decrease in emission time is strongly correlated with the onset of multifragmentation and thermally induced radial expansion, consistent with a transition from surface-dominated to bulk emission expected for spinodal decomposition.

Comparison of the heating effect produced by 8 GeV/c π^- and antiproton beams incident on ¹⁹⁷Au nuclei has been conducted with the Indiana silicon sphere 4π detector array. Event reconstruction indicates formation of thermal-like heavy residues with excitation energies up to 1.7 GeV. Enhanced energy deposition is observed for antiprotons relative to negative pions. For events with excitation energies that exceed 1000 MeV, there is a 50% increase in cross section for the antiproton beam relative to the π^- beam. The predominant decay mode at these high excitation energies is multifragmentation in which three or more Z≥3 fragments are emitted.

Exclusive studies of angular distributions for intermediate-mass fragments (IMFs) produced in GeV hadron-induced reactions have been performed with the Indiana Silicon Sphere (ISiS) 4π detector array. Special emphasis has been given to understanding the origin of sideways peaking, which becomes prominent in the angular distributions for beam momenta above about 10 GeV/c. Both the magnitude of the effect and the peak angle increase as a function of fragment multiplicity and charge. When gated on IMF kinetic energy, the angular distributions evolve from forward-peaked to near isotropy as the fragment kinetic energy decreases. Fragment-fragment angular-correlation analyses show no obvious evidence for a dynamic mechanism that might signal shock wave effects or the breakup of exotic geometric shapes such as bubbles or toroids. Moving-source and intranuclear cascade simulations suggest that the observed sideways peaking is of kinematic origin, arising from significant transverse momentum imparted to the heavy recoil nucleus during the fast cascade stage of the collision. A two-step cascade and statistical multifragmentation calculation is consistent with this assumption.

To investigate the source size and time dependence of multifragmentation reactions, small- and large-angle relative velocity correlations between coincident complex fragments have been measured for the 1.8–4.8 GeV ³He+^natAg, ¹⁹⁷Au systems. The results support an evolutionary scenario for the fragment emission process in which lighter IMFs (Z≲6) are emitted from a hot, more dense source prior to breakup of an expanded residue. For the most highly excited residues, for which there is a significant yield of fragments with very soft energy spectra (E/A<~3 MeV), comparisons with an N-body simulation suggest a breakup time of τ∼50 fm/c for the expanded residue. Comparison of these data with both the evolutionary expanding emitting source model and the Copenhagen statistical multifragmentation model shows good agreement for heavier IMF’s formed in the final breakup stage, but only the evolutionary model is successful in accounting for the lighter IMFs.

Ternary fission in the reaction ¹²C+²³²Th at E_lab  =  22A MeV reveals evidence for dynamical decay. The relative emission probability of intermediate-mass fragments (IMF: 3≤Z≤20) as a function of the initial excitation of the composite system is examined. While IMFs emitted pre-scission exhibit behavior consistent with statistical emission, near-scission IMFs, characterized by unique angular and energy distributions, clearly exhibit a behavior consistent with dynamical decay.

Isotope ratios are examined for several inclusive studies of light-ion and ¹⁴N-induced reactions that involve significantly different reaction dynamics and bombarding energies. For adjacent isotope pairs that have one nuclide with N<Z, the ratios show a strong dependence on emission angle. Pairs with only N>~Z isotopes depend weakly on N/Z of the colliding system, but are otherwise not sensitive to angle of observation or beam energy. The double isotope-ratio method of Albergo has been used to determine apparent nuclear temperatures from these data. When empirically corrected for secondary decay effects, values in the range T_iso≈4.0±0.4 MeV are found for forward-angle measurements and T_iso≈2.4±0.4 MeV for backward angles. The double isotope-ratio temperatures are found to be systematically lower than temperatures derived from spectral shape analyses and Fermi gas estimates. This difference suggests the importance of time evolution in the application of temperature gauges. Relative emission-time differences between neutron-deficient and heavier isotopes arise from both nonequilibrium emission processes and cooling of the system during statistical decay. The importance of secondary feeding is also pointed out. These effects are illustrated by expanding, emitting source calculations.

The charge-moment technique has been used to study the fragment charge distribution for the ³He(4.8 GeV)+¹⁹⁷Au reaction. A large variety of fragment charges characterized by a relative variance ∼2.3, is observed for excitation energies around 5.5 MeV/nucleon. Similar signals related to a phase transition are predicted by the percolation model and the statistical multifragmentation model. Effects of detector acceptance and contribution from fission are discussed.

Exclusive studies of sideways-peaked angular distributions for intermediate-mass fragments (IMFs) produced in hadron-induced reactions have been performed with the Indiana silicon sphere (ISiS) detector array. The effect becomes prominent for beam momenta above about 10 GeV/c. Both the magnitude of the effect and the peak angle increase as a function of fragment multiplicity and charge. When gated on IMF kinetic energy, the angular distributions evolve from forward peaked to nearly isotropic as the fragment energy decreases. Fragment-fragment correlation studies show no evidence for a preferred angle that might signal a fast dynamic breakup mechanism. Moving-source and intranuclear cascade simulations suggest a possible kinematic origin arising from significant transverse momentum imparted to the recoil nucleus during the fast cascade. A two-step cascade and statistical multifragmentation calculation is consistent with the data.

Small-angle relative velocity correlations have been measured for the emission of soft intermediate mass fragments (0.7<~E/A<~3.0 MeV) in high-deposition-energy events produced in the 4.8 GeV ³He+¹⁹⁷Au reaction. The experimental correlations are compared with an N-body Coulomb-trajectory simulation that accounts for the fragment spectra, multiplicity, and charge distributions. In order to reproduce the data, a small residue (Z∼10–20) that is located randomly in the breakup volume is required. This analysis indicates breakup time scales of order τ=20–50 fm/c.