Search Results (53 total)

The paper is devoted to the experimental determination of the space-time characteristics for the target multifragmentation in p(8.1  GeV)+Au collisions. The experimental data on the fragment charge distribution and kinetic energy spectra are analyzed within the framework of the statistical multifragmentation model. It is found that the partition of hot nuclei is specified after expansion of the target spectator to a volume equal to V_t=(2.9±0.2)V_o, with V_o as the volume at normal density. However, the freezeout volume is found to be V_f=(11±3)V_o. At freezeout, all the fragments are well separated and only the Coulomb force should be taken into account. The results are in accordance with a scenario of spinodal disintegration of hot nuclei.

The charge distribution of the intermediate mass fragments produced in p (8.1 GeV)+Au collisions is analyzed in the framework of the statistical multifragmentation model with the critical temperature for the nuclear liquid-gas phase transition T_c as a free parameter. It is found that T_c=20±3 MeV (90% C.L.).

The emission of light charged particles and intermediate-mass fragments (IMF’s) from central collisions of 45 MeV/nucleon ⁸⁴Kr with ⁹³Nb has been studied. Violent collisions have been selected using the total collected charge condition. The analysis of the primary IMF excitation energies has been performed for four bins of the detected IMF charge (2<Z_IMF<20). We find evidence that (a) the mean excitation energy per nucleon for these fragments is independent of fragment charge and approximately equal to 2.5 MeV, (b) the primary fragments at freeze-out preserve the entrance channel (combined system) N/Z ratio, and (c) the freeze-out volume itself is far from spherical.

The enhancement in the production of even-Z nuclei observed in nuclear fission has also been observed in fragments produced from heavy ion collsions. Beams of ⁴⁰Ar, ⁴⁰Cl, and ⁴⁰Ca at 25 MeV/nucleon were impinged on ⁵⁸Fe and ⁵⁸Ni targets. The resulting fragments were detected using the MSU 4π detector array, which had additional silicon detectors for better isotopic resolution. Comparison of the ratios of yields for each element showed enhancement of even-Z fragment production. The enhancement was more pronounced for reactions with a greater difference in the N/Z of the compound system. However, this effect was less for systems that were more neutron rich. The average N/Z for fragments also displayed an odd-even effect with a lower average N/Z for the even-Z fragments. This is related to the greater availability of neutron-poor isotopes for even-Z nuclei.

Nuclear disassembly into light and intermediate mass ejectiles is searched for in ⁴⁰Ar + Cu, Ag, Au reactions of 17-115 A MeV. Distributions in fragment mass versus velocity show that such nuclear disassembly does occur for 65-115 A MeV. However, it is very rare, with much less abundance than reactions leading to a massive residual nucleus.

The ensemble of charged isotropically emitted ejectiles is studied for central collisions of (17-115)A MeV ⁴⁰Ar + Cu, Ag, Au. Measurements of average multiplicities, spectral slopes, and masses of the heaviest fragments are compared to statistical models for multifragmentation or sequential evaporation. The multifragmentation models predict much more complete nuclear disassembly than is observed. The evaporation model reproduces the data much more closely except for the spectra of Z=1 ejectiles. The kinetic energies of Z=1 and 2 ejectiles are much less than found for 1A GeV ¹⁹⁷Au+¹²C for similar energy depositions. Entrance channel dynamics seem to affect the isotropic emission ensembles, often taken to define an equilibrated emission source.

The balance energy at which repulsive nucleon-nucleon scattering balances the attractive force due to the nuclear mean field is measured for the first time for the Au+Au reaction. The observed balance energy of 42±3_stat±1_sys MeV/nucleon agrees with the previously established power law dependence of E_bal on system mass, providing evidence that the Coulomb interaction does not suppress the attractive mean field near the balance energy, in contrast to recent theoretical predictions.

For central collisions of (17–115)A MeV ⁴⁰Ar+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.

Intermediate mass fragment (IMF) production is different for the reaction pair ⁵⁸Fe+⁵⁸Fe and ⁵⁸Ni+⁵⁸Ni for which the only difference is the ratio N/Z, 1.23 for ⁵⁸Fe and 1.07 for ⁵⁸Ni. For beam-velocity fragments at 5.4° with Z from 3 to 15 the more proton-rich Ni reaction produces more even-Z IMFs than the Fe reaction. The ratio (number of IMFs from ⁵⁸Ni+⁵⁸Ni)/(number of IMFs from ⁵⁸Fe+⁵⁸Fe) as a function of Z is about 10% larger for even-Z values than for odd-Z values. The magnitude of this odd-even effect is about the same for beam energies with E/A=45, 75, and 105 MeV.

In the reactions ⁴⁰Ar+²³²Th at 44A and 77A MeV heavy fragments (HF) are produced in events where a large fraction of the incident energy is thermalized. These events are accompanied by, on the average, two intermediate mass fragments (IMF). The IMF’s have various origins: evaporation from a rather massive complex, projectile remnants, and from partially damped collisions. It is concluded that the heavy fragments are formed in semicentral collisions with light particle and IMF emissions which reduce the mass and excitation energy of the residual system such that fission becomes improbable. The HF production is found to be low at 27A MeV.

Measurements are reported for fragment masses and velocities from the reactions (17–115)A MeV ⁴⁰Ar+Cu, Ag, and Au. Charged particle multiplicities were used to select the most violent reactions, and systematics are reported for the momentum and energy deposition. These reactions are fusionlike for ≲44A MeV ⁴⁰Ar with a large fraction of momentum and energy deposition in the heavy nuclear system. However, for ≳44A MeV ⁴⁰Ar, a majority of the projectile energy and momentum is carried away by a multibody spray of light ejectiles and only a minority is deposited in the heavy nucleus. Nevertheless heavy composite systems are formed with up to 9–12 MeV per source nucleon for 115A MeV ⁴⁰Ar.

The energy at which collective transverse flow in the reaction plane disappears, the balance energy E_bal, is found to depend on the isospin of the system using the reactions ⁵⁸Fe+⁵⁸Fe and ⁵⁸Ni+⁵⁸Ni. The more neutron-rich system exhibits higher balance energies for all measured impact parameters, in agreement with the predictions of a transport model which incorporates an isospin dependent mean field and isospin dependent in-medium nucleon-nucleon cross sections.

Collective transverse flow of nuclear matter was measured as a function of the ratio of neutrons to protons ( N/Z) of the interacting system for the first time. The collisions of three isotopically pure beams of A  =  58 nuclei with two A  =  58 targets were studied at 55 MeV/nucleon. The results for the flow variables demonstrate the sensitivity of transport models to elementary aspects of the nucleon-nucleon collisions.

We have experimentally studied small impact parameter heavy-ion collisions in the (nearly) symmetry entrance channels ¹²C+¹²C, ²⁰Ne+²⁷Al, ⁴⁰Ar+⁴⁵Sc, ⁸⁴Kr+⁹³Nb, and ¹²⁹Xe+¹³⁹La, each at many intermediate beam energies. The results from a number of analyses based on a projection of the ‘‘shapes’’ of the experimental events called the sphericity are presented. Comparisons of the relative efficiencies of various experimental methods for the selection of central events are made. The importance of autocorrelations between the sphericity and the various impact-parameter–dependent variables is evaluated. Searches for beam energy-dependent transitions from sequential binary disassembly to multifragmentation in the central events are described. Comparisons to dynamic and hybrid model code calculations will be discussed. The average sphericities of the intermediate mass fragments (IMF’s, for which 3≤Z≲20), are presented. The possibility that the IMF emission occurs following the formation of transient toroidal or disk-like geometries in the central events is explored. Increases in the average sphericities of the central events for increasing beam energies are observed which is attributed to transitions from sequential binary disassembly to multifragmentation. The transitional beam energies for the central ⁴⁰Ar+⁴⁵Sc, ⁸⁴Kr+⁹³Nb, and ¹²⁹Xe+¹³⁹La reactions are near ∼50, ∼40, and ∼40 MeV/nucleon, respectively.

The azimuthal distributions of light particles (Z=1,2) with respect to the entrance channel reaction plane are investigated for Ar+V collisions in order to characterize the modes of collective motion in intermediate energy heavy-ion reactions. At a beam energy of 35 MeV/nucleon, light charged particles are found to exhibit an enhanced emission in the reaction plane which increases with the mass of the detected particle. As the beam energy is increased to 100 MeV/nucleon, the anisotropy nearly vanishes, providing insight into the dynamics of these reactions in a transitional energy regime. The observed anisotropy exhibits signatures of two distinct modes of collective motion: attractive mean-field deflection and rotation of the fused system. A microscopic calculation based on mean-field mediated interactions plus nucleon-nucleon collisions reproduces both forms of collective motion and their associated azimuthal anisotropies. The calculation also suggests that the anisotropy due to mean-field deflection is established during the initial stages of the collision.

The average multiplicities of intermediate mass fragments (IMFs) for central heavy-ion collisions in the (nearly) symmetric entrance channels ²⁰Ne+²⁷Al, ⁴⁰Ar+⁴⁵Sc, ⁸⁴Kr+⁹³Nb, and ¹²⁹Xe+¹³⁹La, are systematically studied over a wide range of intermediate beam energies. Cuts on experimental variables commonly assumed to be correlated with the impact parameter are used to select the most central collisions. The results for six different centrality variables are compared, and the extent to which measurements of the multiplicities of IMFs in small impact parameter collisions are affected by the variable used to select the central events is discussed. General methods for locating such ‘‘autocorrelations’’ are described. The two centrality observables that are the least autocorrelated with the number of intermediate mass fragments are identified, and these variables are used to select the most central collisions. The entrance channel mass and beam energy dependence of the experimental IMF multiplicities are presented and compared to a variety of model predictions. The models picturing the disassembly as a sequential binary process always underpredict the experimental IMF multiplicities. A generally more accurate reproduction of these multiplicities is provided by several similar chemical equilibrium models commonly assumed to be the theoretical description of multifragmentation.

Multifragmentation of the system α+¹⁹⁷Au has been studied at incident energies of 1 and 3.6 GeV/nucleon with the new 4π setup, FASA. Mass, energy, and velocity of a few fragments are measured with high precision using time-of-flight telescopes, while for the other fragments, global multiplicity information is obtained from 55 CsI(Tl) detectors covering the main part of 4π. Very high mean IMF (intermediate mass fragment) multiplicities of 〈M〉_IMF=5.1±0.8 are observed at the higher incident energy. Compared to the lower incident energy, they increase by 40%, indicating an increasing energy deposition with increasing incident energy.

The Z distributions of fragments emitted from central collisions of ⁴⁰Ar+⁴⁵Sc at beam energies from 15 to 115 MeV/nucleon have been fitted to power laws σ(Z)∝Z^-λ. The λ parameter reaches a minimum at a beam energy of 23.9±0.7 MeV/nucleon. A percolation model calculation reproduces the observed Z distributions for all beam energies, using the mean excitation energy as extracted from proton kinetic energy spectra. We extract the critical value of the deposited excitation energy for our system and make predictions for the dependence of this quantity on the size of the fragmenting system.

The disappearance of collective flow in nucleus-nucleus collisions occurs at an incident energy (E_bal) where the attractive scattering dominant at low energies balances the repulsive scattering dominant at high energies. We have performed the first systematic study of the entrance-channel mass dependence of the disappearance of flow and hence E_bal. The new data presented for the C+C, Ne+Al, Ar+Sc, and Kr+Nb systems show that E_bal scales as A^-1/3 where A is the mass of the combined system. Boltzmann-Uehling-Uehlenbeck model calculations show trends which are in qualitative agreement with these new results.

Intermediate-mass fragments formed in reactions of ³He ions with ^natAg and ¹⁹⁷Au targets have been studied at five energies between 0.48 and 3.6 GeV. Inclusive measurements show that as the bombarding energy increases, there is a strong enhancement in fragment cross sections and a trend toward isotropic angular distributions. Between 0.90 and 1.8 GeV, a change in the emission mechanism is suggested by (1) kinetic energy spectra with high-energy tails that become distinctly flatter, (2) a broadening of the spectral Coulomb peaks toward lower energies, and (3) charge distributions that become constant, exhibiting a power-law exponent τ≊2.0. Exclusive studies of the ³He+^natAg system at 0.90 and 3.6 GeV detected multifragment events with multiplicities up to four. The probability for high-multiplicity events increases about 40-fold between 0.90 and 3.6 GeV. At both energies, the kinetic energy spectra depend on multiplicity, especially when triggering on backward-emitted fragments. For multiplicity three events, a rapidity analysis of the data at 3.6 GeV is consistent with a single, relatively low source velocity, v_S≊0.4 cm/ns. The data are compared with predictions of a coplanarity-sphericity calculation, the sequential statistical decay code GEMINI, and a hybrid intranuclear cascade/percolation model.

We have measured two-fragment reduced-velocity correlation functions of the intermediate mass fragments (IMF: 3≤Z≤7) produced in multifragment final states for the Kr+Nb system (E/A=35, 45, 55, 65, and 75 MeV). From the measured correlation functions we extract mean IMF emission lifetimes (τ) which are observed to decrease from τ≊400 fm/c at E/A=35 MeV to τ≊125 fm/c at E/A=55 MeV. For beam energies in excess of E/A=55 MeV, no further decrease in τ is observed, indicating a possible saturation of the mean emission lifetime for IMF produced in multifragment exit channels.

We present Z distributions for fragments with 1≤Z≤12 from central collisions of ⁴⁰ Ar+⁴⁵Sc at incident energies ranging from 35 to 115 MeV/nucleon. We find that the Z distributions can be described by a power law or an exponential and steepen with increasing incident energy. Over the range of incident energies studied, the average number of intermediate mass fragments decreases while the average number of particles increases. When combined with previous results for the charge distributions, a minimum is observed in the extracted power-law parameter.

Multifragment azimuthal correlation functions have been measured as a function of beam energy and impact parameter for the Ar+Sc system (E/A=35 to 115 MeV). The observed azimuthal correlation functions—which do not require corrections for dispersion of the reaction plane—exhibit strong asymmetries which are dependent on impact parameter and beam energy. Rotational collective motion and flow seem to dominate the correlation functions at low beam energies. It is proposed that multifragment azimuthal correlation functions can provide a useful probe for intermediate energy heavy ion reaction dynamics.

Multifragment events for intermediate-mass fragments (3≤Z≤12) with multiplicity up to four have been observed in the reaction of 0.90 and 3.6 GeV ³He ions with ^natAg nuclei. Fragment energy spectra and angular distributions are found to be dependent on event multiplicity. The data suggest significant modification of the Coulomb field of the emitting source.

We have added a new measurement at 100 MeV/nucleon to our previous excitation function for collective flow of light fragments from ⁴⁰Ar+⁵¹V collisions. In the earlier work, flow decreased as the beam energy was raised from 45 to 85 MeV/nucleon. This provided hints to the disappearance of flow, but the lack of measurements at higher beam energies precluded the observation of the reappearance of flow. At a beam energy of 100 MeV/nucleon the flow has reappeared and this allows an experimental determination of the region where attractive scattering balances with repulsive scattering in these collisions. A simultaneous least-squares fit of the flow data for light fragments yields (85±10) MeV/nucleon for the balance energy.