Search Results (60 total)

We report on a measurement of ϕ-meson production in Au+Au collisions at a beam momentum of 11.7A  GeV∕c by experiment 917 at the alternating gradient synchroton. The measurement covers the midrapidity region 1.2<y<1.6. Transverse-mass spectra and rapidity distributions are presented as functions of centrality characterized by the number of participant projectile nucleons. The yield of ϕ’s per participant projectile nucleon increases strongly in central collisions in a manner similar to that observed for kaons.

Transverse mass spectra of protons emitted in Au+Au collisions at beam energies of 6, 8, and 10.8 GeV/nucleon have been measured as a function of collision centrality over a rapidity range 0.5<y_lab<1.5. The spectra are well reproduced by Boltzmann distributions over the measured transverse mass region, which allows for extrapolation of the data to derive the rapidity density and apparent temperature of the emitting source. The shapes of the rapidity distributions suggest significant transparency or substantial longitudinal expansion in even the most central collisions at all three beam energies. The data are analyzed within a simple thermal source plus longitudinal expansion model.

We present results for antilambda and antiproton production in Au+Au collisions at 11.7 A GeV/c including spectra and extracted invariant yields for both species in central and peripheral collisions in the rapidity range 1.0<y<1.4. The antilambda yield increases from dN_Λ̅ /dy  =  1.2_-0.6-0.2^+0.7+0.2×10^-3 in peripheral collisions to 19_-5-2⁺⁴⁺³×10^-3 in central collisions. The direct antiproton yield is deduced from the measured total antiproton spectra to extract the ratio of antilambda-to-direct-antiproton production. The Λ̅ /p̅ ratio near midrapidity increases from 0.26_-0.15-0.4^+0.19+0.5 in peripheral collisions to 3.6_-1.8-1.1^+4.7+2.7 in central collisions, a value larger than current theoretical estimates.

An excitation function of proton rapidity distributions for different centralities is reported from AGS Experiment E917 for Au+Au collisions at 6, 8, and 10.8 GeV/nucleon. The rapidity distributions from peripheral collisions have a valley at midrapidity which smoothly change to distributions that display a broad peak at midrapidity for central collisions. The mean rapidity loss increases with increasing beam energy, whereas the fraction of protons consistent with isotropic emission from a stationary source at midrapidity decreases with increasing beam energy. The data suggest that the stopping is substantially less than complete at these energies.

The entry distribution in angular momentum and excitation energy for the formation of ²⁵⁴No has been measured after the ²⁰⁸Pb(⁴⁸Ca,2n) reaction at 215 and 219 MeV. This nucleus is populated up to spin 22ħ and excitation energy ≳6 MeV above the yrast line, with the half-maximum points of the energy distributions at ∼5 MeV for spins between 12ħ and 22ħ. This suggests that the fission barrier is ≳5 MeV and that the shell-correction energy persists to high spin.

The ground-state band of the Z  =  102 isotope ²⁵⁴No has been identified up to spin 14, indicating that the nucleus is deformed. The deduced quadrupole deformation, β  =  0.27, is in agreement with theoretical predictions. These observations confirm that the shell-correction energy responsible for the stability of transfermium nuclei is partly derived from deformation. The survival of ²⁵⁴No up to spin 14 means that its fission barrier persists at least up to that spin.

The single-particle character of states outside the doubly magic (radioactive) nucleus ⁵⁶Ni has been determined through a measurement of the (d,p) neutron transfer reaction using inverse kinematics. From the spectroscopic factors of the low-lying states in ⁵⁷Ni, the astrophysically interesting yield for the ⁵⁶Ni(p,γ) reaction to the mirror nucleus ⁵⁷Cu has been calculated, utilizing charge symmetry. The rate for this reaction in the temperature range typical of novae, supernovae, and x-ray bursts is found to be more than 10 times higher than previously assumed.

Inelastic proton scattering to the first excited 2⁺ state of the doubly magic ⁵⁶Ni nucleus was investigated in inverse kinematics, using a secondary beam of radioactive ⁵⁶Ni nuclei. At an incident energy of 101 MeV/nucleon, a value B(E2,0⁺→2⁺)=600±120 e² fm⁴ was measured. This result completes the set of experimental data for the first excited 2⁺ states in the 1f-2p shell with a closed shell of neutrons or protons. These data are compared to recent shell-model calculations.

In the very heavy collision system ¹⁹⁷Au+¹⁹⁷Au the K⁺ production process was studied as a function of impact parameter at 1 GeV/nucleon, a beam energy well below the free N-N threshold. The K⁺ multiplicity increases more than linearly with the number of participant nucleons and the K⁺/π⁺ ratio rises significantly when going from peripheral to central collisions. The measured K⁺ double differential cross section is enhanced by a factor of 6 compared to microscopic transport calculations if secondary processes (ΔN→KΛN and ΔΔ→KΛN) are ignored.

The multiplicities of p and α particles detected in coincidence with fragments emitted in fully relaxed collisions in the reactions of 18.5A MeV ¹³⁶Xe+⁴⁸Ti have been measured for different exit channel mass asymmetries. A kinematic source analysis of the spectra and angular distributions of the light particles has been used to separate the total multiplicities into prescission and postscission contributions. From these results, the excitation energies at scission are determined using an empirical technique based upon previous measurements of light charged particle multiplicities observed in coincidence with evaporation residues. These excitation energies are found to decrease from ∼400 MeV to 110 MeV as the fragment mass asymmetry, A_H/A_L, varies from 4.8 to 1.0. A corresponding increase of the mean lifetime of the scissioning nucleus from ∼5×10^-22 s to ∼1×10^-20 s is derived using calculated statistical model decay widths. The extent to which this variation of lifetime with mass asymmetry may be attributed to completely damped deep inelastic collisions or to dynamic delays in the decay of a compound nucleus is discussed as is the need for inclusion of dynamics in the deexcitation calculations for hot nuclei. Observed three fragment events are also discussed.

We present data on collective flow of neutrons in collisons of ¹⁹⁷Au + ¹⁹⁷Au at 400 MeV/nucleon. The azimuthal distribution about the beam axis is investigated with respect to the reaction plane as determined from light charged particles. The ‘‘squeezeout’’ of neutrons, perpendicular to the reaction plane, is observed for the first time. Quantitative agreement to a high level of accuracy is found between the behavior of neutrons and hydrogen ions.

Triple differential cross sections d³σ/dp³ for charged pions produced in symmetric heavy-ion collisions were measured with the KaoS magnetic spectrometer at the heavy-ion synchrotron facility SIS at GSI. The correlations between the momentum vectors of charged pions and the reaction plane in ¹⁹⁷Au+¹⁹⁷Au collisions at an incident energy of 1 GeV/nucleon were determined. We observe, for the first time, an azimuthally anisotropic distribution of pions, with enhanced emission perpendicular to the reaction plane. The anisotropy is most pronounced for pions of high transverse momentum in semicentral collisions.

Measurements have been made of the total decay rates of the 14.4-keV level in the ⁵⁷Fe nucleus in highly charged Fe ions having one to six electrons outside the nucleus. Results are 10.0(9), 7.06(18), 7.36(14), 7.16(11), 7.10(23), 7.28(25), and 6.92(46), in units of 10⁶ s^-1, for ions of charge states q from 25 to 19, respectively. For the q=25 single-electron ion the result quoted is for the ion in which the spin of the 1s_1/2 electron and the excited nuclear state are coupled to give quantum number F=1. Measurements have also been made of the K-shell fluorescent yields for ions in charge states q (before K-vacancy production) from 22 to 19. Results are 0.62(1), 0.50(1), 0.48(2), and 0.50(6), respectively. These data are discussed and compared with theoretical predictions.

The nuclear response of a medium-mass nucleus (¹³⁶Xe) to electromagnetic excitation in a near-relativistic heavy-ion collision was investigated in the reaction¹³⁶Xe(0.7A GeV)+Pb. From an exclusive measurement of the neutron decay of the excited ¹³⁶Xe projectiles, strong excitations of giant resonances and, in particular, of the double isovector giant dipole resonance were identified. A resonance energy of 28.3±0.7 MeV, a width of 6.3±1.6 MeV, and a total cross section of 215±50 mb were found for the double giant dipole resonance.

We present results from an experimental study of reabsorption effects in subthreshold π⁰ production in the reaction ¹²⁹Xe+¹⁹⁷Au at 44 MeV/nucleon. Within the picture of pion generation in nucleon-nucleon scattering we deduce, from our data and from a comparison with the systematics of production cross sections available for lighter reaction systems, information on the π⁰ absorption length in nuclear matter. For the π⁰ kinetic-energy range ≃5–100 MeV the energy-averaged λ_abs and its momentum dependence are obtained, and compared with optical-model calculations.

Based on the systematics for hard photon production a scheme is suggested which relates the hard-photon multiplicity observed in intermediate energy heavy ion reactions to the impact parameter. The method is applied to data obtained in an exclusive experiment of the reaction ⁴⁰Ar+¹⁵⁸Gd at 44 MeV/nucleon, demonstrating the usefulness of this new approach.

High-energy γ rays have been measured in coincidence with heavy fragents in deeply inelastic reactions of ¹³⁶Xe+⁴⁸Ti at 18.5 MeV/nucleon. The giant dipole resonance (GDR) strength function is deduced from an analysis of the photon spectra within the statistical model. The GDR width Γ is studied as a function of the fragment excitation energy E^*. A saturation at about Γ=10 MeV is observed for E^*/A≥1.0 MeV/nucleon.

Measurements of conversion rates are reported for the first time in one- and two-electron systems, for the 14.4-keV nuclear state in ⁵⁷Fe. The total half-life of this state deduced for the two-electron ion is 100±5 ns, and that deduced for the total spin F=1 state of the one-electron ion is 79±6 ns.

Evaporation residues following the interaction of ¹²C with targets of ¹⁹⁷Au, ¹⁶⁰Gd, and ¹²⁰Sn were measured in the energy range of E_lab=5.5-10 MeV/nucleon. The experimental system allowed the determination of the cross sections of the complete fusion (CF) and the incomplete fusion (ICF) processes with a sensitivity to ICF processes below the percent level. The ICF processes were observed at all bombarding energies studied, down to energies slightly above the Coulomb barrier. The two processes have very different angular distributions, with the ICF peaked at a significantly larger angle than the CF.

Quasi-elastic reactions were studied for ⁵⁸Ni beams at 330 MeV and ⁶⁴Ni beams at 341 and 380 MeV incident energy, respectively, on even-A Sn isotopes. Angular distributions were measured in the range 20°≤θ_lab≤55° using a magnetic spectrograph with a gas-filled focal-plane detector yielding single mass and charge resolution; generally individual states were not resolved. Cross sections for quasi-elastic one-neutron pickup and stripping reactions vary smoothly with the ground-state Q value. Total quasi-elastic transfer cross sections range from 340 to 640 mb, corresponding to 20–40 % of the total reaction cross section determined from the elastic scattering angular distributions. The distribution of the total cross section into the various reaction channels is discussed.

Nucleon-transfer cross sections have been measured at subbarrier energies in collisions between Sn target nuclei and light (¹⁶,18O) and heavy (⁵⁸Ni) projectile nuclei. Large differences in cross sections correlate directly with the subbarrier fusion enhancement, suggesting nucleon transfer as an important doorway to fusion with a direct dependence on the overall transfer strengths. Simple two-level mixing considerations support this picture.

Excitation functions for fission were measured for ^58,64Ni beams incident on the even ^112-124Sn targets at energies extending from well below to about 1.5 times the Coulomb barrier. Fission was identified by kinematic coincidence between fission fragments. Angle integrated fission cross sections were obtained from angular distributions taken at several energies for all systems. From these and the previously measured cross sections for evaporation residues, we obtain the total fusion cross sections and fission probabilities over the energy range 150≲E_c.m.≲240 MeV. The competition between particle evaporation and fission in the compound nuclei is compared to statistical model calculations. A good description of the data for all 14 systems is achieved with the use of a single set of parameters. The model includes fission barriers with finite range and nuclear diffuseness effects, and partial-wave distributions for fusion that are qualitatively consistent with those from microscopic reaction model calculations. The fusion excitation functions are analyzed in terms of the dynamical fusion model of Swiatecki et al. Within this model we extract new values for the "extra-push" parameters.

Quasielastic nucleon-transfer cross sections have been mea- sured at energies ≊35% above the Coulomb barrier for ⁵⁸,64Ni + ¹¹²,116,120,124Sn. The systematic trends were studied over the range of systems, which span a factor of 2 in neutron excess N-Z. The observed magnitude of the cross sections and their surface localization suggest that quasielastic processes play an important role in the average nucleus-nucleus potential, a quantity of considerable recent interest in near-barrier collisions of heavy systems.

The spontaneous emission of ¹⁴C from ²²³Ra, recently discovered by Rose and Jones, has been confirmed, and the mass of the emitted particles unambiguously identified. The present measurement was performed with a ²²⁷Th source containing 9.2 mCi of ²²³Ra. An Enge split-pole magnetic spectrograph was used to suppress the intense alpha radiation and to identify the ¹⁴C particles. The spectrograph was calibrated with tandem-accelerated beams of ¹⁴C, ¹³C, and ¹²C. In six days of decay counting, twenty-four ¹⁴C events were observed yielding a branching ratio of (4.7±1.3)×10^-10 for the emission of ¹⁴C from ²²³Ra relative to that of alpha particles. The value is in fair agreement with the result of Rose and Jones, (8.5±2.5)×10^-10, and with more recent measurements from other laboratories.

We have measured fission and particle-evaporation cross sections for the nuclei ^170-188Pt formed with the reactions ^58,64Ni + ^112-124Sn. This represents the first systematic study of the decay of compound nuclei formed with heavy projectiles (A_proj≳1 / 2A_target), and thus endowed with large angular momenta at relatively low excitation energies. Using a statistical model and a single set of input parameters, we have reproduced all the data.