Search Results (28 total)

Light charged particles emitted at about 90^° in the frame of the projectile-like fragment in semiperipheral collisions of ⁹³Nb+⁹³Nb at 38A MeV give evidence for the occurrence, in the same class of events, of two different production mechanisms. This is demonstrated by differences in the kinetic energy spectra and in the isotopic composition of the particles. The emission with a softer kinetic energy spectrum and a low N/Z ratio for the hydrogen isotopes is attributed to an evaporation process. The harder emission, with a much higher N/Z ratio, can be attributed to a midvelocity process consisting of a nonisotropic emission, on a short time-scale, from the projectile-like fragment.

A systematic investigation of the average multiplicities of light charged particles and intermediate mass fragments emitted in peripheral and semiperipheral collisions is presented as a function of the beam energy, violence of the collision, and mass of the system. The data have been collected with the FIASCO setup in the reactions ⁹³Nb+⁹³Nb at (17,23,30,38)A MeV and ¹¹⁶Sn+¹¹⁶Sn at (30,38)A MeV. The midvelocity emission has been separated from the emission of the projectile-like fragment. This last component appears to be compatible with an evaporation from an equilibrated source at normal density, as described by the statistical code GEMINI at the appropriate excitation energy. On the contrary, the midvelocity emission presents remarkable differences in both the dependence of the multiplicities on the energy deposited in the midvelocity region and the isotopic composition of the emitted light charged particles.

Peripheral and semiperipheral collisions have been studied in the system ⁹³Nb+⁹³Nb at 38A  MeV. The evaporative and midvelocity components of the light charged particle and intermediate mass fragment emissions have been carefully disentangled. In this way it was possible to obtain the average amount not only of charge and mass, but also of energy, pertaining to the midvelocity emission, as a function of an impact parameter estimator. This emission has a very important role in the overall balance of the reaction, as it accounts for a large fraction of the emitted mass and for more than half of the dissipated energy. As such, it may give precious clues on the microscopic mechanism of energy transport from the interaction zone toward the target and projectile remnants.

The emission pattern in the v_perp-v_par plane of intermediate mass fragments with Z  =  3–7 (IMF) has been studied in the collision ¹¹⁶Sn+ ⁹³Nb at 29.5A  MeV as a function of the total kinetic energy loss of the reaction. This pattern shows that for peripheral reactions most IMF’s are emitted at velocities intermediate between those of the projectile- and target-like products. Coulomb trajectory calculations show that these IMF’s are produced in the interaction zone in a short time interval at the end of the target-projectile interaction.

Light charged particles emitted by the projectilelike fragment were measured in the direct and reverse collision of ⁹³Nb and ¹¹⁶Sn at 25A MeV. The experimental multiplicities of hydrogen and helium particles as a function of the primary mass of the emitting fragment show evidence for a correlation with net mass transfer. The ratio of hydrogen and helium multiplicities points to a dependence of the angular momentum sharing on the net mass transfer.

A Reply to the Comment by S. Chattopadhyay.

Primary and secondary masses of heavy reaction products have been deduced from kinematics and energy–time-of-flight measurements, respectively, for the direct and reverse collisions of ¹⁰⁰Mo with ¹²⁰Sn at 14.1A MeV. Direct experimental evidence of the correlation of energy sharing with net mass transfer and model-independent results on the evolution of the average excitation from equal-energy to equal-temperature partition are presented.

The ratio of the total charge bound in fragments with Z between 2 and 15 to the hydrogen yield, (tsum₂¹⁵M_iZi)/M_H, has been measured, and the neutron-to-proton ratio n/p has been estimated from the data of central Au + Au reactions between 100 and 400A MeV, measured with the phase I setup of the detector system FOPI and GSI, Darmstadt, in the polar-angle range between 7° and 30°. These two quantities were used to determine the entropy per nucleon S/A by comparing them with the predictions of the freesco code. The analysis allows the simulataneous extraction of the values of the baryonic entropy, temperature, and collective flow. The extracted values are in good agreement with the values obtained in earlier FOPI studies, and, for the baryonic entropy, with recent hydrodynamic calculations.

Relative populations of particle unstable states in ¹⁰B were measured for the normal kinematics reactions ¹⁴N+²⁷Al and ¹⁴N+¹⁹⁷Au at E/A=75 MeV, the nearly symmetric reaction ¹²⁹Xe+¹²²Sn at E/A=31 MeV, and the inverse kinematics reaction ¹²⁹Xe+²⁷Al at E/A=31 MeV. In all cases, populations of excited states are incompatible with statistical populations of the known excited states of this nucleus. For the ¹²⁹induced reactions, discrepancies are smaller than for the ¹⁴induced reactions. The overall agreement with statistical calculations is improved by assuming a proposed but not confirmed 3⁺ excited state contributing to the group of levels at E^*=6 MeV.

Using the FOPI facility at GSI Darmstadt complete data of Au on Au collisions at 150A MeV were collected for charged products (Z=1–15) at laboratory angles 1°≤Θ_lab≤30°. Central collisions were selected by applying various criteria. The kinetic energy spectra of fragments from an isolated midrapidity source are investigated in detail for center-of-mass angles 25°≤Θ_c.m.≤45°. The heavy products (Z≥3) are used to determine the collective energy which is found to be at least 10A MeV.

Two-proton correlation functions for the inverse kinematics reaction ¹²⁹Xe+²⁷Al at E/A=31 MeV have been reanalyzed to search for differences between longitudinal and transverse correlations indicative of emission from a long-lived composite system. Evidence for such differences is found when tight angular cuts are applied in the compound nucleus rest frame and when the correlation functions are constructed by the event mixing technique.

Charged-particle correlations were measured for the reactions ¹⁴N+¹⁹⁷Au and ¹⁴N+²⁷Al at E/A=75 MeV and for ¹²⁹Xe+²⁷Al and ¹²⁹Xe+¹²²Sn at E/A=31 MeV. Relative populations of widely separated states in ⁴He, ⁵Li, and ⁸Be, only weakly affected by side feeding, indicate emission temperatures of T=4–5 MeV for the ¹⁴induced reactions and values about 1 MeV lower for the ¹²⁹induced reactions.

Both two-deuteron and two-proton correlation functions, at small relative momenta, have been measured for ¹⁴N+²⁷Al collisions at E/A=75 MeV. Data are compared to the predictions of a microscopic transport model which incorporates deuteron production and Coulomb interactions. The results indicate that deuterons are emitted over a shorter time span than protons.

Charged particles have been observed in collisions of Au on Au at an incident energy of 150A MeV using a high-granularity detector system covering approximately the forward hemisphere in the center-of-mass system. Highly central collisions have been studied using a double selection criterion which combines large charged-particle multiplicities with small transverse-momentum directivities. In this class of events about one quarter of the total nuclear charge emerges as intermediate-mass fragments with nuclear charges Z>2. These fragments are centered at midrapidity and are produced with large transverse velocities.

Exclusive measurements of two- and three-body events were performed for the system ¹²⁰Sn+¹⁰⁰Mo at 19.1 MeV/nucleon. Most ternary events are consistent with sequential processes in which one of the two deep-inelastic fragments fissions. For such events large differences are found between the fission probabilities of projectilelike and targetlike fragments of a given mass, this probability being larger for the nucleus which gained nucleons. This behavior demonstrates that there is a lack of equilibrium at the end of the deep-inelastic collision.

Two-proton correlation functions have been measured at θ_lab≊25° for the ‘‘forward kinematics’’ reactions ¹⁴N+²⁷Al, ¹⁴N+¹⁹⁷Au at E/A=75 MeV, for the ‘‘inverse kinematics’’ reaction ¹²⁹Xe+²⁷Al at E/A=31 MeV, and for the nearly symmetric reaction ¹²⁹Xe+¹²²Sn at E/A=31 MeV. For the reactions at 75 MeV per nucleon, the correlation functions exhibit pronounced maxima at relative proton momenta, q≊20 MeV/c, and minima at q≊0 MeV/c. These correlations indicate emission from fast, nonequilibrium processes. They are analyzed in terms of standard Gaussian source parametrizations and compared to microscopic simulations performed with the Boltzmann-Uehling-Uhlenbeck equation. For the reactions at 31 MeV per nucleon, the two-proton correlation functions do not exhibit maxima at q≊20 MeV/c, but only minima at q≊0 MeV/c. These correlations indicate emission on a slower time scale. They can be reproduced by calculations based on the Weisskopf formula for evaporative emission from fully equilibrated compound nuclei. For all reactions, the measured longitudinal and transverse correlation functions are very similar, in agreement with theoretical predictions.

Two-proton correlation functions measured for the ¹⁴N+²⁷Al reaction at E/A=75 MeV are compared to correlation functions predicted for collision geometries obtained from numerical solutions of the Boltzmann-Uehling-Uhlenbeck (BUU) equation. The calculations are in rather good agreement with the experimental correlation function, indicating that the BUU equation gives a reasonable description of the space-time evolution of the reaction.

A measurement of the circular polarization P_γ of the 1081 keV γ-ray in ¹⁸F is reported. The result is P_γ=(2.7±5.7)×10^-4. The reaction used was ¹⁶O(³He,p)¹⁸F with a 3.4 MeV ³He⁺ beam of an intensity of 7 μA impinging on a windowless water target. The circular polarization was measured with a four-prong transmission-type Compton polarimeter. The measurement of the analyzing power of the polarimeter is presented. Gamma rays were detected by means of 130 cm³ p-type germanium detectors placed behind the prongs. Four independent data acquisition channels were used: pile-up losses at a counting rate of 60 kcps were kept lower than 15%, owing to the use of trapezoidal shapers. Systematic effects of instrumental origin are discussed and found to give negligible contributions with respect to the statistical uncertainty. The calculation of the asymmetries is performed using different estimators; an extensive comparison of the various methods is presented. The result of the experiment, when averaged with those of the other experiments on ¹⁸F carried out until now, gives a grand average P_γ=(1.2±3.9)×10^-4. This result is compared with recent theoretical estimates.

A new measurement of the circular polarization P_γ of the 1081-keV γ line in ¹⁸F has been performed. The result is P_γ(1081 keV)=(2.7±5.7)×10^-4. This result, when averaged with those of recent measurements in ¹⁸F, sets an upper constraint for the absolute value of the coupling constant f_π of the weak πNN interaction of |f_π|<~1.5×10^-7.

Levels of ⁸³Rb and ⁸⁶Rb have been populated by bombarding natural krypton gas with protons of E_p=2830 keV. Doppler shift attenuation measurements in gas were performed using the same krypton gas as stopper medium. Lifetimes or lifetime limits are determined for four levels in ⁸³Rb and for the first excited level in ⁸⁶Rb. The collective properties of positive parity states in ⁸³Rb are confirmed.

NUCLEAR REACTIONS ⁸²Kr(p, γ), ⁸³Kr(p, n), ⁸⁶Kr(p, n), E_p=2.8 MeV. Natural krypton gas target; p=10⁴-10⁷ Pa. Doppler shift attenuation in gas. Measured: γ transition energies, lifetimes.

Double differential cross sections have been measured for energetic p, d, t, and α particles emitted in ¹⁶O-induced reactions on targets of A1, Zr, and Au at incident energies of 140, 215, and 310 MeV. The energy and angular distributions are well described by isotropic emission from a moving thermal source. The extracted temperature and velocity parameters are found to vary systematically with the incident energy per nucleon above the Coulomb barrier. The observed trends cannot be explained by compound nucleus emission but instead suggest emission from a source which consists of comparable contributions from target and projectile. Alternatively, the proton energy spectra are compared with a precompound model and with a simple knockout model. The d, t, and α-particle cross sections are also described in terms of a generalized coalescence relation which takes into account Coulomb repulsion from the target nucleus.

NUCLEAR REACTIONS ²⁷Al(¹⁶O,x), E=140, 215, and 310 MeV; ⁹⁰Zr(¹⁶O,x), E=215 and 310 MeV; ¹⁹⁷Au(¹⁶O,x), E=140, 215, and 310 MeV; x=p, d, t, and α. Measured σ(E_x,θ_x). Analysis in terms of moving source, precompound, knockout, and coalescence models.

The decay scheme of ⁸⁵Rb up to 2.4 MeV excitation energy was investigated by the ⁸⁴Kr(p,γ) reaction and Coulomb excitation with alpha particles. Singles and γ-γ coincidence spectra were recorded. Doppler shift attenuation measurements in gas were performed using the same gas as stopper medium. Lifetimes or lifetime limits were determined for 20 levels by Coulomb excitation and the Doppler shift attenuation method. An existing ambiguity referring to the lifetime of the second excited level has been removed.

NUCLEAR REACTIONS ⁸⁴Kr(p,γ,) Coulomb excitation by alpha bombardment. E_p=1.85-2.87 MeV. E_α=3 MeV. Natural Kr gas target; p=0.1-106×10⁵ Pa. Doppler shift attenuation in gas. Measured: level energies, branching ratios, decay scheme, lifetimes.

Double differential cross sections have been measured for energetic p, d, t, and α particles emitted in reactions of 315 MeV ¹⁶O ions on ²³⁸U. In coincidence with light-particle emission, the momentum transfer to the target is determined by measuring the folding angle between the two fission fragments resulting from the sequential decay of the target nucleus. It is concluded that the emission of these particles occurs predominantly in fusionlike "central" collisions and at an early stage of the reaction. The energy and angular distributions are described by thermal emission from a source moving with approximately half of the beam velocity. Alternatively, the energy spectra can be explained by emission from a rotating hot spot. The cross sections for d, t, and α emission can be described in terms of a generalized form of the coalescence model which takes into account the Coulomb repulsion from the target nucleus.

[NUCLEAR REACTIONS ²³⁸U(¹⁶O,Xf), x=p,d,t,α, E=315 MeV; measured σ(E_x,θ_x) and fission fragment folding angle distribution. Analysis in terms of hot spot, moving source, and coalescence models.]

The decay scheme of ⁴⁹Ti up to 5.2 MeV excitation energy was investigated by the (d, pγ) and (p, p^′γ) reactions and by Coulomb excitation. Singles spectra and p-γ coincidences were recorded. Lifetimes or lifetime limits were determined for 15 levels by Coulomb excitation or by the Doppler shift attenuation method. A consistent decay scheme for levels up to E_x=5.2 MeV was deduced including many so far unobserved transitions. On the basis of these results J^π assignments are proposed and a comparison is made with the predictions of the deformed configuration mixing model.

NUCLEAR REACTIONS ⁴⁸Ti(d,pγ), ⁴⁹Ti(p,p^′γ), Coulomb excitation by alpha bombardment. E_d=6 MeV, E_p=6 MeV, E_α=5.25-5.8 MeV. Enriched ⁴⁹Ti target. p-γ coincidences. Doppler shift attenuation. Measured level energies, decay scheme, lifetimes.