Search Results (65 total)

The fusion-evaporation reaction ⁹Be(¹¹B,2p) was used to populate excited states in ¹⁸N. New gamma-ray transitions were added to the ¹⁸N level scheme. The mean lifetime of the first excited state was measured to be 582(165) ps and its transition rate to the ground state was determined to be B(M1)=0.036(10) W.u. Shell model calculations in the full p-sd model space were used to investigate the low-lying configurations in ¹⁸N and in the N=11 isotones ¹⁷C and ¹⁹O. It was found that the role of the proton-neutron interaction is important in determining the ground state and low-lying excited state properties. The ground state spin inversion in these isotones is attributed to the increased importance of the quadrupole relative to the pairing interaction and is discussed within the framework of a schematic pairing + quadrupole model.

Using both the absolute and relative surrogate techniques, the ²³⁶U(n,f) cross section was deduced over an equivalent neutron energy range of 0 to 20 MeV. A 42 MeV ³He beam from the 88 Inch Cyclotron at Lawrence Berkeley National Laboratory was used to perform a (³He,α) pickup reaction on targets of ²³⁵U (J^π=7/2^-) and ²³⁸U (J^π=0⁺) and the fission decay probabilities were determined. The ²³⁵U(³He,αf) and ²³⁸U(³He,αf) were surrogates for ²³³U(n,f) and ²³⁶U(n,f), respectively. The cross sections extracted using the surrogate method were compared to directly measured cross sections. The sensitivity of these cross sections to the J^π-population distributions was explored.

We have deduced the cross section for ²³⁷U(n,f) over an equivalent neutron energy range from 0 to 20 MeV using the surrogate ratio method. A 55 MeV ⁴He beam from the 88 inch cyclotron at Lawrence Berkeley National Laboratory was used to induce fission in the following reactions: ²³⁸U(α,α^′f) and ²³⁶U(α,α^′f). The ²³⁸U reaction was a surrogate for ²³⁷U(n,f), and the ²³⁶U reaction was used as a surrogate for ²³⁵U(n,f). Scattered α particles were detected in a fully depleted segmented silicon telescope array over an angle range of 35^° to 60^° with respect to the beam axis. The fission fragments were detected in a third independent silicon detector located at backward angles between 106^° and 131^°.

We have investigated the population of nuclei formed in binary reactions involving ⁷Li beams on targets of ¹⁶⁰Gd and ¹⁸⁴W. The ⁷Li+¹⁸⁴W data were taken in the first experiment to use the LIBERACE Ge array in combination with the STARS Si ΔE-E telescope system at the 88-Inch Cyclotron of the Lawrence Berkeley National Laboratory. By using the Wilczyński binary transfer model, in combination with a standard evaporation model, we are able to reproduce the experimental results. This is a useful method for predicting the population of neutron-rich heavy nuclei formed in binary reactions involving beams of weakly bound nuclei and will be of use in future spectroscopic studies.

The energy levels of ¹⁵F have been measured by the p(¹⁴O,p)¹⁴O reaction. The 120 MeV ¹⁴O radioactive ion beam was produced by the BEARS coupled cyclotron system at an intensity averaging 1×10⁴ particles/second on target. Energy calibration was obtained using resonances from the p(¹⁴N,p)¹⁴N reaction. The two lowest resonances in ¹⁵F were fitted with an R-matrix calculation. The fit to the ground state had J^π=1/2⁺ at 1.23 ± 0.05 MeV (width 0.5 − 0.84 MeV), and the first excited state was J^π=5/2⁺ at 2.81 ± 0.02 MeV (width 0.30 ± 0.06 MeV), both relative to the mass-energy of the proton and ¹⁴O. The ¹⁵F ground state energy supports the disappearance of the Z = 8 proton magic number for odd Z,T_z=-3/2 nuclei.

The g factor of the 2₁⁺ state in ⁴⁰Ar has been measured by use of Coulomb excitation in inverse kinematics and the transient magnetic-field technique. The resulting g factor, g(2₁⁺)=-0.015(42), is in reasonable agreement with shell-model calculations within the (full sd)_π(full fp)_ν space, without including core excitations. Although highly deformed admixtures in the wave function cannot be completely ruled out, they are small, in contrast to the case of ⁴²Ca.

The heavy-ion radiative capture reaction, ¹²C(¹²C,γ), has been investigated at beam energies around 16 MeV. Two different experiments were performed. Capture cross sections were obtained by measuring fused ²⁴Mg residues and were found to significantly exceed values reported earlier. Subsequently, the decay pathways associated with radiative capture were delineated using the Gammasphere array. A substantial fraction of the decay was found to proceed through a few high-lying doorway states near 10 MeV in excitation.

The fragment yields from the multifragmentation of gold, lanthanum, and krypton nuclei obtained by the EOS Collaboration are examined in terms of Fisher’s droplet formalism modified to account for Coulomb energy. The critical exponents σ and τ and the surface energy coefficient c₀ are obtained. Estimates are made of the pressure-temperature and temperature-density coexistence curve of finite neutral nuclear matter as well as the location of the critical point.

A systematic analysis of multifragmentation (MF) in fully reconstructed events from 1A GeV Au, La, and Kr collisions with C has been performed. These data are used to provide a definitive test of the variable volume version of the statistical multifragmentation model (SMM). A single set of SMM parameters directly determined by the data and the semi-empirical mass formula are used after the adjustable inverse level density parameter ε₀ is determined by the fragment distributions. The results from SMM for second stage multiplicity, size of the biggest fragment, and the intermediate mass fragments are in excellent agreement with the data. Multifragmentation thresholds have been obtained for all three systems using SMM prior to secondary decay. The data indicate that both thermal excitation energy E_th^* and the isotope ratio temperature T_He-DT decrease with increase in system size at the critical point. The breakup temperature obtained from SMM also shows the same trend as seen in the data. The SMM model is used to study the nature of the MF phase transition. The caloric curve for Kr exhibits back-bending (finite latent heat) while the caloric curves for Au and La are consistent with a continuous phase transition (nearly zero latent heat) and the values of the critical exponents τ, β, and γ, both from data and SMM, are close to those for a “liquid-gas” system for Au and La. We conclude that the larger Coulomb expansion energy in Au and La reduces the latent heat required for MF and changes the nature of the phase transition. Thus the Coulomb energy plays a major role in nuclear MF.

The g factors of 2₁⁺, 4₁⁺, and 2₂⁺ states in the stable ^{130,132,134,136}Xe isotopes have been measured via projectile Coulomb excitation in inverse kinematics in combination with the transient field technique. The results show a steady decrease in g(2₁⁺) as the number of neutron holes increases in the lighter nuclei below the closed N=82 neutron shell. The g factors of the 4₁⁺ states in ^132,134Xe are consistently larger than the g factors of the 2₁⁺ states, a characteristic of proton excitation. The g factors of the 2₁⁺, 4₁⁺, and 2₂⁺ states in ¹³⁰Xe are approximately equal as would be expected for vibrational excitations.

Multifragmentation MF results from 1A GeV Au on C have been compared with the Copenhagen statistical multifragmentation model (SMM). The complete charge, mass, and momentum reconstruction of the Au projectile was used to identify high momentum ejectiles leaving an excited remnant of mass A, charge Z, and excitation energy E^* which subsequently multifragments. Measurement of the magnitude and multiplicity (energy) dependence of the initial free volume and the breakup volume determines the variable volume parametrization of SMM. Very good agreement is obtained using SMM with the standard values of the SMM parameters. A large number of observables, including the fragment charge yield distributions, fragment multiplicity distributions, caloric curve, critical exponents, and the critical scaling function are explored in this comparison. The two stage structure of SMM is used to determine the effect of cooling of the primary hot fragments. Average fragment yields with Z>~3 are essentially unaffected when the excitation energy is ≤7 MeV/nucleon. SMM studies suggest that the experimental critical exponents are largely unaffected by cooling and event mixing. The nature of the phase transition in SMM is studied as a function of the remnant mass and charge using the microcanonical equation of state. For light remnants A<~100, backbending is observed indicating negative specific heat, while for A>~170 the effective latent heat approaches zero. Thus for heavier systems this transition can be identified as a continuous thermal phase transition where a large nucleus breaks up into a number of smaller nuclei with only a minimal release of constituent nucleons. Z<~2 particles are primarily emitted in the initial collision and after MF in the fragment deexcitation process.

A systematic analysis of the moments of the fragment size distribution has been carried out for the multifragmentation of 1A GeV Au, La, and Kr on carbon. The breakup of Au and La is consistent with a continuous thermal phase transition. The data indicate that the excitation energy per nucleon and isotopic temperature at the critical point decrease with increasing system size. This trend is attributed primarily to the increasing Coulomb energy with finite size effects playing a smaller role.

The g factors of the 2₁⁺ and 4₁⁺ states in ^{78,80,82,84,86}Kr have been measured for the first time, using Coulomb excitation of isotopic Kr beams and the transient field technique. The measured g factors of 2₁⁺ states in ^78,80,82Kr are well described, in both magnitude and progression with neutron number, by the IBA-II model. Whereas the lighter isotopes show a dominant collective structure with g factor values close to Z/A, the large g(2₁⁺)=1.12(14) value of ⁸⁶Kr, with its closed N=50 shell, is unequivocally dominated by specific proton configurations. The g factor of the 2₁⁺ state in ⁸⁴Kr, with two holes in the 1g_9/2 neutron orbit, reflects both proton and neutron components in the wave function. In addition, the lifetimes of several 2₁⁺ and 4₁⁺ states were remeasured by the Doppler shift attenuation method, yielding values which, in some cases, differ from those in the literature.

The transverse momenta (p_x,p_y) of projectile fragments produced by 1.0A GeV ¹⁹⁷Au nuclei incident on Au and C targets have been measured. The medium and heavy fragments have p_x and p_y distributions, which are wider than predicted by models. For the Au target the widths of the distributions are significantly larger than those for C, particularly for the heavy fragments. The C distributions show a different gross structure, which may be due to the target-projectile size difference.

The g factors of the 4₁⁺ and 6₁⁺ states in ^144,148Nd, the 4₁⁺ state in ¹⁴⁶Nd and the 6₁⁺, 8₁⁺, and 10₁⁺ states in ¹⁵⁰Nd have been measured for the first time by projectile Coulomb excitation coupled to the transient field technique. The g factors of the 2₁⁺ states of ^{144,146,148,150}Nd have been remeasured with high precision. The data clearly indicate that, while ^148,150Nd are well described by collective excitations, the structure of the low lying levels in the lighter isotopes is dominated by the 2f_7/2 neutron configuration.

The cluster distributions of three different systems are examined to search for signatures of a continuous phase transition. In a system known to possess such a phase transition, both sensitive and insensitive signatures are present; while in systems known not to possess such a phase transition, only insensitive signatures are present. It is shown that nuclear multifragmentation results in cluster distributions belonging to the former category, suggesting that the fragments are the result of a continuous phase transition.

It is shown that the Fisher droplet model, percolation, and nuclear multifragmentation share the common features of reducibility (stochasticity in multiplicity distributions) and thermal scaling (one-fragment production probabilities are Boltzmann factors). Barriers obtained, for cluster production on percolation lattices, from the Boltzmann factors show a power-law dependence on cluster size with an exponent of 0.42±0.02. The EOS Collaboration Au multifragmentation data yield barriers with a power-law exponent of 0.68±0.03. Values of the surface energy coefficient of a low density nuclear system are also extracted.

Multifragmentation in fully reconstructed events from 1A GeV Kr and La collisions with C has been studied. Results are compared with similar data for 1A GeV Au+C. The emitted charged particles and fragments are identified with emission from either a prompt first stage or a second stage in which the remnant resulting from the first stage breaks up. The nuclear charge, mass, and excitation energy distributions of the remnant are determined. The total charged multiplicity, as well as those of the first and second stages are obtained. Freeze-out temperatures and thermal excitation energy permit the determination of the caloric curve. The fragment charge distribution as well as the IMF multiplicity distribution and those of individual fragments are obtained. The various results are examined as to the extent of universal behavior when scaled for varying system size. Comparisons are made with intranuclear cascade and statistical multifragmentation model calculations.

A light-element radioactive ion-beam capability has been developed at the LBNL 88-Inch Cyclotron. The system is based on the coupled-cyclotrons method and utilizes short-lived species, e.g., ¹¹C, ¹⁴O, ¹³N produced by (p,n) and (p,α) reactions at the LBNL Biomedical Isotope Facility Cyclotron. In a first experiment, ¹⁹⁷Au(¹¹C,xn)^208-xAt excitation functions have been measured for energies ranging from the Coulomb barrier up to 110 MeV using a beam of ¹¹C with intensities up to (1–2)×10⁸ ions/sec on target. The results of this experiment are compared to measurements of ¹⁹⁷Au(¹²C,xn)^209-xAt excitation functions.

Exclusive measurements of light-charged-particle (¹H, ²H, and ⁴He) energy spectra, angular distributions, and emission multiplicities are reported for the two reactions ⁴⁰Ar+²⁷Al and ⁵⁵Mn+¹²C at a matched excitation energy of 127 MeV. Comparisons are made with statistical model predictions for the evaporative processes in these reactions, which can be characterized as emissions from rotational-energy-dominated systems. The model simulations do well in reproducing a broad range of angular distribution data and the ⁴He/¹H cross-section ratio, using spin distributions derived from fusion cross-section systematics. The same model parameters, however, predict particle energy spectra and coincidence cross sections which are inconsistent with the measurements for both reactions. These results support previous conclusions from model comparisons with inclusive data, and suggest fundamental flaws in the statistical model as applied to light-mass, high-spin, nuclear systems.

The inclusive light fragment (Z<~7) yield data in Au+Au reactions, measured by the EOS Collaboration at the LBNL Bevalac, are presented as a function of multiplicity. Moving from central to peripheral collisions the measured charge distributions develop progressively according to a power law which can be fitted, within errors, by a single τ exponent independently of the bombarding energy except for the data at 250A MeV. In addition, the location of the maximum in the individual yields of different charged fragments, for a given beam energy, shifts towards lower multiplicity as the fragment charge increases from Z=3 to Z=7. This trend is common to all six measured beam energies. Moments of charge distribution are also reported. The universal features observed in the present Au + Au data are consistent with previous experimental findings in the Au + C multifragmentation reaction at 1A GeV.

In order to test the statistical model’s ability to predict the behavior of relatively light mass systems (A≈67) with large angular momenta, two matched heavy ion nuclear reactions were used to produce ⁶⁷Ga^* composite nuclei at an excitation energy of 127 MeV. Light charged particles (protons, deuterons, tritons, and α particles) were used as probes to characterize the composite systems and track the deexcitation processes. From these measurements, energy spectra, cross sections, angular distributions, anisotropy ratios, and particle multiplicities were deduced. Measuring many degrees of freedom provides a stringent test for the statistical models. What is found is that models which did well in predicting the behavior of heavy composite systems (A≈150), are unable to simultaneously reproduce energy spectra, angular distributions, and particle multiplicities for the lighter systems (A≈67), where angular momentum plays a dominant role. This implies that more rigorous models and/or additional physics are needed to understand the behavior of the hot, high-spin nuclear matter in this mass region.

The properties of the remnant resulting from the emission of prompt particles in the interaction of 1A GeV ¹⁹⁷Au+C interactions have been compared with intranuclear cascade and Boltzmann-Uehling-Uhlenback transport calculations. The number of first-stage particles and the energy spectra of first-stage protons are also compared. Both models can fit the general but not the detailed features of the data.

Triple-differential cross sections for neutrons from high-multiplicity La-La collisions at 250 and 400 MeV per nucleon and Nb-Nb collisions at 400 MeV per nucleon were measured at several polar angles as a function of the azimuthal angle with respect to the reaction plane of the collision. The reaction plane was determined by a transverse-velocity method with the capability of identifying charged-particles with Z=1, Z=2, and Z>2. The flow of neutrons was extracted from the slope at mid-rapidity of the curve of the average in-plane momentum vs the center-of-mass rapidity. The squeeze-out of the participant neutrons was observed in a direction normal to the reaction plane in the normalized momentum coordinates in the center-of-mass system. Experimental results of the neutron squeeze-out were compared with BUU calculations. The polar-angle dependence of the maximum azimuthal anisotropy ratio r(θ) was found to be insensitive to the mass of the colliding nuclei and the beam energy. Comparison of the observed polar-angle dependence of the maximum azimuthal anisotropy ratio r(θ) with BUU calculations for free neutrons revealed that r(θ) is insensitive also to the incompressibility modulus in the nuclear equation of state.