Search Results (29 total)

Inclusive double differential multiplicities of deuterons, ³H, ³He, and ⁴He measured by E877 for 11.5A GeV/c Au+Au collisions at the Brookhaven’s Alternating Gradient Synchrotron (AGS) are presented. Light fragments at beam rapidity are measured for the first time at AGS energies. Beam rapidity deuteron and ⁴He yields and transverse slope parameters are found to be strongly dependent on the impact parameter, and the shape of the deuteron spectra is not consistent with that expected for a simple thermal distribution. The deuteron yields relative to proton yields are analyzed in terms of a simple coalescence model. While results indicate an increase in source size compared to collisions of lighter systems at the same energy, they are inconsistent with a simple coalescence model reflected by a rapidity dependence of the coalescence parameter B_d. A new approach utilizing an expanding thermalized source combined with a coalescence code is developed for studying deuteron formation in heavy-ion collisions. The strong dependence of deuteron yields on collective motion implies that deuteron yields relative to those of protons can be used for constraining source parameters.

Two-proton correlation functions have been measured in Si+Pb collisions at 14.6A GeV/c and Au+Au collisions at 11.5A GeV/c by the E814/E877 Collaboration. Data are compared with predictions of the transport model RQMD and the source size is inferred from this comparison. Our analysis shows that, for both reactions, the characteristic size of the system at freeze-out exceeds the size of the projectile, suggesting that the fireball created in the collision has expanded. For Au+Au reactions, the observed centrality dependence of the two-proton correlation function implies that more central collisions lead to larger source sizes.

Inclusive single photon spectra have been measured in the transverse momentum (p_T) range from a few MeV/c up to approximately 1 GeV/c at various nucleon-nucleon center of momentum rapidities (y_NN) in the interval from -2.4<y_NN<+0.5 in 18 GeV/c p-Be and p-W collisions. These distributions were used to estimate the inclusive p_T and rapidity distributions for π⁰ production over approximately the same kinematic range. The measured photon spectra are compared with the expected distribution of photons from π⁰ decays and hadronic bremsstrahlung in order to search for new sources of ‘‘soft’’ photons at low transverse momentum. The results indicate no anomalous sources of soft photons in either p-Be or p-W interactions at these energies. The p-W data at forward angles set a limit on any excess of low p_T photons at <2.65 times hadronic bremsstrahlung at the 99% C.L. © 1996 The American Physical Society.

We have conducted a search for bound states of a negative pion and a number of neutrons (pineuts) using the E814 spectrometer. A beam of ²⁸Si at a momentum of 14.6A GeV/c was used to bombard targets of Al, Cu, Sn, and Pb. We describe our experimental technique, present measured upper limits for pineut production, and discuss the significance of our results.

We present the transverse energy distributions, dσ/dE_T and dE_T/dη, and the charged particle multiplicity distributions, dσ/dN_c and dN_c/dη, produced in p+Al and p+Pb collisions at 14.6 GeV/c. The data exhibit a weak correlation between these global variables. While a significant increase of the mean multiplicity with the mass of the target is observed, the transverse energy distributions show little target dependence. The dE_T/dη distribution shifts backward as the mass of the target increases, indicating the presence of rescattering. The data are compared to the predictions of the relativistic quantum molecular dynamic model (RQMD) and Fritiof event generators. RQMD reproduces well the main features of the data while Fritiof predicts too forward peaked transverse energy and particle multiplicity pseudorapidity distributions.

An investigation of the production of neutron-rich isotopes from the fragmentation of ²⁸Si projectiles at p_lab=14.6 GeV/c per nucleon was performed using the BNL-AGS-E814 spectrometer. We have measured the inclusive production cross sections of neutron-rich fragments (⁶He, ⁸He, ⁸Li, ⁹Li, ¹⁰Be, ¹¹Be, and ¹³B). We have also measured the transverse momentum distributions for ⁶He and ⁹Li, and the forward and transverse energy distributions associated with ⁶He production. The momentum distributions were analyzed in the context of the Goldhaber model. The question of whether the fragments are produced in the decay of the projectile following its electromagnetic excitation was also investigated.

We have studied in detail the electromagnetic dissociation of ²⁸Si projectiles at 14.6 GeV/(c nucleon), interacting with Pb, Sn, Cu, and Al targets. Exclusive cross sections were measured for several decay channels, including final states involving the emission of protons, neutrons, and α particles. Excitation energy distributions for the 1n+²⁷Si and 2p+²⁶Mg decay channels were reconstructed with a resolution of 2 MeV, using a constrained kinematic fit. The energy distributions obtained for 1n+²⁷Si are in good agreement with the σ(γ,n) photoneutron cross sections multiplied by the virtual photon spectrum obtained in the Weizsäcker-Williams approximation. A search for the double photon excitation process, based on the dependence of the cross sections on the target atomic number, was performed.

Inclusive double differential multiplicities d²N/dy dp_t and related quantities have been measured for protons and deuterons produced in 14.6A GeV/c Si+Al and Si+Pb collisions using the E814 forward spectrometer at the AGS at BNL. Collision ‘‘centrality’’ is determined by measuring N_c, the total charged particle multiplicity in the pseudorapidity range 0.85<η<3.8. For both systems Si + Al and Si + Pb, an increase in the proton rapidity distribution dN/dy at midrapidity and a corresponding decrease at higher rapidities are observed with increasing N_c. For Si+Pb, Boltzmann slope parameters T_B increase significantly in the most central collisions. The measured distributions exhibit a centrality dependence even when σ/σ_geo≲10%, where full overlap between the Si and Pb nuclei occurs in a simple geometric picture. The proton rapidity distribution dN/dy is presented for the symmetric system Si+Al over the entire rapididty interval. The total number of protons, which is the integral of this quantity over rapidity, varies with N_c. Results are compared with various model calculations, mostly using the hadronic cascade codes ARC and RQMD. No significant nuclear transparency is observed, indicating that large baryon and energy densities are produced in these collisions.

Event shapes for Au + Au collisions at 11.4 GeV/c per nucleon were studied over nearly the full solid angle with the E877 apparatus. The analysis was performed by Fourier expansion of azimuthal distributions of the transverse energy (E_T) measured in different pseudorapidity intervals. For semicentral collisions a pronounced event anisotropy is identified with a clear forward-backward anticorrelation. In the forward pseudorapidity interval up to 9% of the E_T is in a directed component. The signal decreases for peripheral and very central collisions.

Coulomb-nuclear interference effects were investigated in the inelastic scattering of ¹⁶O and ¹⁸O by ⁶⁴Zn. Measurements of elastic and inelastic angular distributions of ¹⁸O were performed at a laboratory energy of 49 MeV, over the angular range from θ_lab∼30° to 85°. The excitation functions of ¹⁶O and ¹⁸O ions were measured at incident energies between 29 and 46 MeV at θ_lab=174°. The experimental angular distributions show structures which are more pronounced for projectile excitation than for target excitation. The interference minimum for the excitation of the ¹⁸O first 2⁺ state was found to be shifted towards forward angles by approximately 5° (c.m.) with respect to the distorted-wave Born approximation calculations and by approximately 3.5° (c.m.) with respect to the coupled-channels calculations. A pronounced Coulomb-nuclear interference minimum was seen in the excitation of ⁶⁴Zn(2⁺) state by inelastic scattering of ¹⁶O projectiles, whereas no pronounced minimum was observed in target excitation by ¹⁸O projectiles. The elastic scattering data were analyzed with the optical model. The inelastic differential cross sections for the excitation of the first 2⁺ states in the target and in the ¹⁸O projectile were analyzed using the distorted-wave Born approximation and also the coupled-channels approach with collective form factors.

We have measured cross sections for the synthesis of nuclei of mass A≤4 in collisions of 14.6A GeV/c ²⁸Si nuclei with targets of Pb, Cu, and Al. The data are measured at close to center-of-mass rapidities, and are unique in their exploration of the centrality dependence of nucleosynthesis. Simple coalescence models that were used to study nucleosynthesis at lower energies are inadequate for the description of our measurements. Our data and improved models are used to extract parameters related to the size of the interaction volume at freeze-out.

We present the results of an analysis of two charged particle and transverse energy correlations in Si+Pb collisions at BNL AGS at 14.6 GeV/c per nucleon. The measured semi-inclusive normalized two particle pseudorapidity corrleation function exhibits short range correlations similar to the correlations observed in hadron-hadron and hadron-nucleus collisions at higher energies, although the observed correlations are smaller than the values scaled from hp and hA data. Estimates, provided by the observed correlations, of the intermittency indices as well as of the parameters of the cluster model are presented. Predictions using the fritiof event generator, which at this level of statistical accuracy show no pseudorapidity correlations, are not in agreement with our data. Azimuthal angle two particle correlations show nonzero back-to-back correlations in the central region (consistent with fritiof predictions) and are almost flat in the projectile fragmentation region. We also present results on the transverse energy azimuthal correlation function, which are similar to those from the two particle correlation function.

We present a systematic study of transverse energy (E_T) production in collisions of 11.4A GeV/c Au and 14.6A GeV/c Si ions with targets of Al, Au, and Pb. Comparison of data for Au+Au and Si+Al indicates that, for the heavier system, there is an increase in the amount of stopping which is accompanied by a decrease in the width of the dE_T/dη distribution. The ratio of the maximum E_T observed for the two systems is significantly greater than the ratio of the total energy available in the center of mass frame.

We have measured antiproton production cross sections as functions of centrality in collisions of 14.6 GeV/c per nucleon ²⁸Si ions with targets of Al, Cu, and Pb. For all targets, the antiproton yields increase linearly with the number of projectile nucleons that have interacted, and show little target dependence. We discuss the implications of this result on the production and absorption of antiprotons within the nuclear medium.

Collisions of ²⁸Si+Al, Cu, and Pb at E_lab=14.6 GeV/nucleon were studied at the Brookhaven National Laboratory Alternating Gradient Synchrotron. Charged particle multiplicity was measured over the pseudorapidity interval 0.875≤η≤3.86 with a silicon pad detector. A strong correlation is seen between the multiplicity and the transverse energy measured in the interval -0.5≤η≤0.8. Correlation with the energy going forward after the collision and comparison with calculations indicate that rescattering is required to explain the data. The data are compared under the assumption of Koba-Nielson-Olesen scaling. The measured multiplicity scales approximately with the total number of participant nucleons and less well with the available center-of-mass kinetic energy.

We report a direct measurement of the final-state energy spectrum in the electromagnetic dissociation of ²⁸Si into p+ ²⁷Al at an energy of 14.6 GeV/nucleon. The final-state energy is obtained through a calculation of the p-²⁷Al invariant mass in kinematically reconstructed events. The final-state energy spectrum for all targets is peaked near the isovector giant-dipole resonance in ²⁸Si and the dependence of the magnitude of the cross section on target charge confirms that the excitation is largely electromagnetic. By exploiting the expected scaling behavior on target Z and A, the background from nuclear interactions is evaluated and subtracted, leaving a pure electromagnetic dissociation final-state energy distribution. This distribution is well reproduced by simulated events, in which the photon spectrum calculated in the Weiszäcker-Williams approximation is combined with experimental data on the photonuclear reaction ²⁸Si(γ,p) ²⁷Al, and slight differences are observed only at low final-state energy.

We present the rapidity and transverse momentum distributions of protons and neutrons from collisions between 14.6 GeV/nucleon beams of ²⁸Si and targets of Al, Cu, and Pb. The data were measured in the forward spectrometer/target calorimeter detectors of the E814 apparatus. The results indicate the existence of two distinct domains, one of beam rapidity projectilelike nucleons, and the second of participant nucleons. From the former, the in-medium inelastic nucleon-nucleon cross section is deduced. It is found to agree, within 10%, with the ‘‘free’’ value of 30 mb although under present conditions one of the two colliding nucleons has been struck before with a high probability. We compare with the present data the predictions of a fragmentation model as well as of models dealing explicitly with the heavy-ion collision and particle creation and emission.

High-spin states in ⁷³Se were populated via the fusion-evaporation reaction ⁶³Cu(¹⁶O,αpn)⁷³Se at a beam energy of 69 MeV. Several new levels were found in bands of both negative and positive parity. The yrast band of ⁷³Se shows evidence of shape coexistence at low spin. The results of cranked, deformed Woods-Saxon shell-model calculations with self-consistent pairing are compared to the data.

It is demonstrated that the naive expectation of Z_T² dependence of the cross section for reactions produced via Coulomb dissociation in heavy ions is modified substantially due to the ‘‘apparent’’ charge distribution. The deviations are shown to be important in predicting both the energy and the Z_T dependence of the cross sections. The effect increases with Q value of a given dissociation channel; for a given Q value the deviation is larger for lower incident energy.

Free-induction decays of ⁸⁹Y NMR are observed in magnetically dilute alloys Gd_xY_1-xBa₂Cu₃O_7-δ for x=0, 0.005, and 0.01. It is shown that the excess decay by Gd is almost completely due to its direct dipolar interactions. From an analysis of the indirect Ruderman-Kittel-Kasuya-Yosida interaction, the exchange coupling constant is estimated to be less than 0.01 eV.

Electromagnetic dissociation cross sections for one, two, and three nucleon removal were measured for an E_lab/A=14.6 GeV ²⁸Si beam impinging on ²⁷Al, ¹²⁰Sn, and ²⁰⁸Pb targets. The measured cross sections exhibit an approximate Z_T² dependence and are well reproduced by calculations convoluting the virtual photon spectrum with the experimental σ(γ,N) photonuclear cross sections. Comparison with other experimental results and energy dependence of the cross sections are discussed.

Collisions of ²⁸Si+Al, Cu, Pb at E_lab/A=14.6 GeV were studied in a calorimetry-based experiment at the BNL Alternating Gradient Synchrotron. Transverse-energy production was measured for pseudorapidities -0.5<η<0.8. Correlations with the spectra and multiplicity of neutrons and protons emitted into a forward 0.8° cone demonstrate quantitatively the large amount of nuclear stopping observed in these reactions. Calculations in hadronic-fireball or nucleon-nucleon based models underpredict the measured transverse-energy production for Si+Pb and indicate the need to include rescattering of secondaries and /or contributions from target fragmentation.

High-spin states were excited in ⁷⁶Br via the reaction ⁶³Cu(¹⁶t/rO,n2p) at a beam energy of 69 MeV. Several new high-spin states were found, including four levels in a previously unreported rotational band. A previously predicted signature inversion is confirmed.

Excited states in ⁷⁵Kr were populated via the reaction ⁴⁶S,2pn) at a beam energy of 97 MeV. Several new high-spin states were found. Evidence for band crossings was found in both the positive- and negative-parity bands. Calculations using a cranked Strutinsky-Bogolyubov model with a Woods-Saxon potential indicate noticeably different deformations for the opposite parity bands: β₂=0.40, γ=-8° for the positive-parity band and β₂=0.36, γ=3° for the negative-parity band. These deformations are comparable to superdeformations reported by other authors in the cerium region. Signature splitting in both positive- and negative-parity bands was well reproduced by the calculations. Indirect evidence suggests shape coexistence within the positive-parity band at low spin.

Angular distributions for the elastic scattering of ¹⁶O from ¹⁰B have been measured at E_c.m.=14.17, 16.15, and 18.65 MeV over an angular range from 20° to 176°. The angular distributions, which exhibit an oscillatory enhancement of the cross section at backward angles, are described by a scattering matrix parametrization, by adding a very narrow l window to the normal, strong absorption S matrix.