Y. AKIBA, D. BEAVIS, P. BEERY, H.C. BRITT, B. BUDICK, C. CHASMAN, Z. CHEN, C.Y. CHI, Y.Y. CHU, V. CIANCIOLO, B.A. COLE, J.B. COSTALES, H.J. CRAWFORD, J.B. CUMMING R. DEBBE, J. ENGELAGE, S.Y. FUNG, M. GONIN, S. GUSHUE, H. HAMAGAKI, O. HANSEN, R.S. HAYANO, S. HAYASHI, S. HOMMA, H.HUANG,H. KANEKO, J. KANG, S. KAUFMAN, W. KEHOE, K. KURITA, R.J. LEDOUX, MJ. LEVINE, Y. MIAKE, D. MORRISON, R.J. MORSE, B. MOSKOWITZ, S. NAGAMIYA, M.N. NAMBOODIRI, T. NAYAK, J. OLNESS, C.G. PARSONS, L.P. REMSBERG, D. ROEHRICH, P. ROTHSCHILD, H. SAKURAI, T.C. SANGSTER, R. SETO, R. SOLTZ, S.G. STEADMAN, G.S.F. STEPHANS, T. SUNG, S. TANAKA, Y. TANAKA, M.J. TANNENBAUM, J. THOMAS, J.H. VAN DIJK, F. VIDEB^K, O. VOSSNACK, V. VUTSADAKIS, F.Q. WANG, Y. WANG, H.E. WEGNER, D.S. WOODRUFF, Y.D. WU, AND W. ZAJC

Neutron-induced fission cross sections have been deduced from measured fission probabilities for targets of Tb-231,Tb-233, U-234,U-235,U-235m,U-236,U-237,U-239, Np-236,Np-236m,Np-237,Np-238, Pu-237,Pu-237m,Pu-240,Pu-241,Pu-243, and (240,241,242,242m,243,244,244m)AM. The fission-probability data were measured for (t, pf) reactions, up to an equivalent incident neutron energy of E-n similar to 2.2 MeV, and for (He-3,xf) reactions, up to an equivalent E-n similar to 6 MeV. The procedure used to deduce cross sections for the corresponding (n,f) reactions, within the equivalent neutron-energy range of the fission-probability data, is briefly reviewed. The deduced (n,f) cross sections obtained to date are summarized. The limitations of the present formalism and proposed improvements are discussed, within the context of extending the (n,f) cross sections beyond the range of the fission-probability data, up to E-n = 20 MeV.

The purpose of this note is to combine existing information on the {sup 237}U(n,f) cross section to determine if some consistency can be obtained for the neutron induced fission excitation of {sup 237}U. The neutron induced fission cross section of the 6.8 day {sup 237}U was measured directly by McNally et al. in 1968 using the Pommard nuclear device test. At the same time critical assembly measurements were done at Los Alamos using the Flattop assembly. A previous measurement was also made at LASL in 1954 with two different neutron sources, each peaked near 200 keV. The results were 0.66 {+-} 0.10 b and 0.70 {+-} 0.07 b for the (n,f) cross section. More recently Younes and Britt have reanalyzed direct reaction charged particle data of Cramer and Britt that had determined the fission probability of the {sup 238}U compound nucleus as a function of nuclear excitation energy. They have combined fission probabilities with calculated neutron absorption cross sections, including corrections for the differences in angular momentum between the direct and neutron induced reactions. From this analysis they have extracted equivalent {sup 237}U(n,f) cross sections. The technique for extracting surrogate (n,f) cross sections from (t,pf) data has been demonstrated in a recent publication for the test case {sup 235}U(n,f). In addition to this experimental information, Lynn and Hayes have recently done a new theoretical study of the fission cross sections for a series of isotopes in this region. A summary plot of the data is shown in Fig. 1. Below 0.5 MeV the McNally, Cowan, and Younes-Britt results are in reasonable agreement. The average cross section in the Younes-Britt results, for En = 0.1 to 0.4 MeV, is 0.80 times the McNally values which is well within the errors of the McNally experiment. Above 0.5 MeV the McNally results diverge toward higher values. It should be noted that this divergence begins approximately at the {sup 237}Np threshold and that {sup 237}Np is the daughter of the 6.8 day {sup 237}U decay.

In a previous publication [T. Abbott , E802 Collaboration, Phys. Rev. C 63, 064602 (2001); 64, 029901(E) (2001)], measurements of the A dependence and pseudorapidity interval (deltaeta) dependence of midrapidity E-T distributions in a half-azimuth (Deltaphi=pi) electromagnetic calorimeter were presented for p+Be, p+Au, O+Cu, Si+Au, and Au+Au collisions at the BNL-AGS. The validity of the "nuclear geometry" characterization versus deltaeta was illustrated by plots of the E-T(deltaeta) distribution in each deltaeta interval in units of the measured (p+Au) in the same deltaeta interval for p+Au collisions. These plots, with aperture corrected scale in the physically meaningful units of number of average observed p+Au collisions, were nearly universal as a function of deltaeta, confirming that the reaction dynamics for E-T production at midrapidity at AGS energies is governed by the number of projectile participants and can be well characterized by measurements in apertures as small as Deltaphi=pi,deltaeta=0.3. A key ingredient in these analyses is the probability p(0) for no signal to be detected in a given aperture deltaeta for the fundamental p+Au collision. In fact the measured (p+Au) is biased and the true (true)(p+Au) for the detector aperture is the measured value times 1-p(0). The issues and merits of measuring the E-T(deltaeta) distribution in units of (p+Au) or (true)(p+Au) in the same deltaeta interval are presented and discussed. This method has application at RHIC, where p-p data could be used as the reference distribution for two participants. The E-T distributions for B+A collisions, with E-T(deltaeta) scale normalized by (true)(p-p) in the same aperture for p-p collisions, would then be given directly in the popular unit "per participant-pair" [K. Adcox , PHENIX Collaboration, Phys. Rev. Lett. 86, 3500 (2001); I. G. Bearden , BRAHMS Collaboration, Phys. Lett. B523, 227 (2001); B. B. Back , PHOBOS Collaboration, Phys. Rev. C 65, 031901(R) (2002); C. Adler , STAR Collaboration, Phys. Rev. Lett. 89, 202301 (2002)].

Neutron-induced fission cross sections have been extracted for targets of Pu-240,Pu-241,Pu-243, U-234,U-236,U-237,U-239, and Th-231,Th-233 from E-n=100 keV to approximate to2.5 MeV using surrogate (t,pf) fission-probability data and a detailed statistical model to compensate for the difference between neutron-induced and (t,p) reactions. This paper extends the results of previous work on the U-235(n,f) cross section, which serves as a proof-of-principle study. The (n,f) cross sections are compared to earlier estimates based on the same surrogate data, but obtained using a more simplistic approach. The cross sections are also compared to accepted values where direct measurements exist and are consistently accurate to within 20% below E(n)approximate to0.5 MeV and 10% at higher energies. The case of the U-237(n,f) cross section, simulated from surrogate (t,pf) data, is investigated in greater detail to reconcile contradictory measurements in the literature.

Neutron-induced fission cross sections on U-235 and U-235(m) targets in the incident neutron energy range E-n=0.1-2.5 MeV have been deduced from surrogate U-234(t,pf) measurements. The surrogate (t,pf) reaction is used to populate the same compound system as the (n,f) reaction before fission, and modeling is used to compensate for the difference in population mechanisms. The calculations presented in this paper improve on previous results by incorporating realistic angular momentum and parity distributions for the (t,p) channel, and by updating transmission-coefficient values used in the neutron-capture and emission contributions that compete with the fission process. The results are generally reliable within the 10% systematic uncertainties of the (t,pf) data.

Two-pion correlation functions are analyzed at mid-rapidity for three systems (14.6 A-GeV Si+Al, Si+Au, and 11.6 A-GeV Au+Au), seven distinct centrality conditions, and different kT bins in the range 0.1--0.5 GeV/c. Source reference frames are determined from fits to the Yano-Koonin source parameterization. Bertsch-Pratt radius parameters are shown to scale linearly with both number of projectile and total participants as obtained from a Glauber model calculation. A finite emission duration that increases linearly with system/centrality is also reported. The mT dependence of the Bertsch-Pratt radii is measured for the central Si+Au and Au+Au systems. The system/centrality dependence is investigated separately for both high and low mT regions.

The population cross section of the T{sub 1/2} = 25-minute, E{sub x} = 76.8-keV isomer in {sup 235}U via the {sup 235}U(n,n{prime}{gamma}) reaction has been estimated in the E{sub n} = 2.1-20-MeV range. Gamma rays populating both isomer and ground states were detected using the GEANIE spectrometer at the LANSCE/WNR ''white-source'' neutron facility. Partial {gamma}-ray cross sections were obtained as a function of incident neutron energy, using {gamma}-ray spectroscopy and the time-of-flight technique. A correction for unobserved transitions was applied to the measured partial cross sections using the Hauser-Feshbach code GNASH to produce population cross sections for the isomer- and ground-state levels. The deduced isomer population cross section at E{sub n} = 2.1 MeV is 1.1(1) b, and the isomer-to-ground state population ratio decreases from 0.9 to 0.06 over the E{sub n} = 2.1-20-MeV range. The details of the measurement and recommendations to improve the current results are discussed.

Measurements of the A dependence and pseudorapidity interval (delta eta) dependence of midrapidity E-T distributions in a half-azimuth (Delta phi = pi) electromagnetic calorimeter are presented for p + Be, p + Au, O + Cu, Si + Au, and Au + Au collisions at the BNL-AGS (Alternating-Gradient Synchrotron). The shapes of the upper edges of midrapidity E-T distributions as a function of the pseudorapidity interval delta eta in the range 0.3 to 1.3, roughly centered at midrapidity, are observed to vary with delta eta. like multiplicity-the upper edges of the distributions flatten as delta eta is reduced. At the typical fixed upper percentiles of E-T distributions used for nuclear geometry characterization by centrality definition-7 percentile, 4 percentile, 2 percentile, I percentile, 0.5 percentile-the effect of this variation in shape on the measured projectile A(p) dependence for O-16, Si-28, Au-197 projectiles on an Au target is small for the ranges of delta eta and percentile examined. The E-T distributions for p + Au and p + Be change in shape with delta eta; but in each delta eta interval the shapes of the p + Au and p + Be distributions remain indentical with each other-a striking confirmation of the absence of multiple-collision effects at midrapidity at AGS energies. The validity of the nuclear geometry characterization versus delta eta is illustrated by plots of the E-T(delta eta) distribution in each delta eta interval in units of the measured (p + Au) same delta eta interval for p + Au collisions. These plots, in the physically meaningful units of "number of average p + Au collisions," are nearly universal as a function of delta eta confirming that the reaction dynamics for E-T production at midrapidity at AGS energies is governed by the number of projectile participants and can be well characterized by measurements in apertures as small as Delta phi = pi, delta eta = 0.3.

Proton and deuteron distributions from 11.6A GeV/c Au + Au collisions measured by the E802 Collaboration in experiment E-866 are presented. The invariant yield of protons and deuterons is studied as a function of the transverse mass for different cuts of rapidity and centrality. At low m(t)-m(0) the proton and deuteron invariant spectra deviate from a single exponential shape. The average m(t) as function of centrality and rapidity is used to explore the effect of collective transverse flow in the reaction. The ratio of the deuteron to squared proton yield as a function of transverse momentum, rapidity, and centrality is used to probe the coalescence model of deuteron production. This ratio is constant as a function of rapidity only for the most central cuts and decreases with the centrality for every rapidity cut. The ratio of the differential cross section of the deuteron to the squared differential cross section of the proton, for the most central cut, is not constant as a function of m(t)-m(0). [S0556-2813(99)04911-0].

Charged kaon production has been measured in Si+Al and Si+Au collisions at 14.6 A GeV/c, and Au+Au collisions at 11.1 A GeV/c by Experiments 859 and 866 (the E--802 Collaboration) at the BNL AGS. Invariant transverse mass spectra and rapidity distributions for both K+ and K- are presented. The centrality dependence of rapidity-integrated kaon yields is studied. Strangeness enhancement is observed as an increase in the slope of the kaon yield with the total number of participants as well as the yield per participant. The enhancement starts with peripheral Si+Al and Si+Au collisions (relative to N+N) and appears to saturate for a moderate number of participating nucleons in Si+Au collisions. It is also observed to increase slowly with centrality in Au+Au collisions, to a level in the most central Au+Au collisions that is greater than that found in central Si+A collisions. The enhancement factor for $K^+$ production are 3.0 +-0.2(stat.) +-0.4(syst.) and 4.0 +-0.3(stat.) +-0.5(syst.), respectively, for the most central 7% Si+Au collisions and the most central 4% Au+Au collisions relative to N+N at the correponding beam energy.

Analysis of directed flow observable for protons and pions from Au+Au collisions at 10.8 GeV/nucleon from experiment E917 at the AGS is presented. Using a Fourier series expansion, the first Fourier component, {nu}{sub 1},was extracted as a function of rapidity for mid-central collisions (17-24%). Clear evidence for positive directed flow is found in the proton data, and a weak, possibly negative directed flow signal is observed for {pi}{sup +} and {pi}{sup {minus}}.

In this paper Au+Au collisions at 11.6A GeV/c are characterized by two global observables: the energy measured near zero degrees (E-ZCAL) and the total event multiplicity. Particle spectra are measured for different event classes that are defined in a two-dimensional grid of both global observables. For moderately central events (sigma/sigma(int) < 12%) the proton dN/dy distributions do not depend on E-ZCAL but only on the event multiplicity. In contrast the shape of the proton transverse spectra shows little dependence on the event multiplicity. The change in the proton dN/dy distributions suggests that different conditions are formed in the collision for different event classes. These event classes are studied for signals of new physics by measuring pion and kaon spectra and yields. In the event classes doubly selected on E-ZCAL and multiplicity there is no indication of any unusual pion or kaon yields, spectra, or K/pi ratio even in the events with extreme multiplicity. [S0556-2813(99)03704-8].

Strangeness production in Au+Au collisions has been measured via the yields of K+ and K- at 6, 8 AGeV and of anti-Lambda at 10.8 AGeV beam kinetic energy in experiment E917. By varying the collision centrality and beam energy, a systematic search for indications of new phenomena and in-medium effects under high baryon density is undertaken.

Strangeness production in heavy ion collisions, when compared with proton-proton collisions, is potentially a sensitive probe for collective energy deposition and therefore for reaction mechanisms in general. It may therefore provide insight into possible quark-gluon plasma formation in dense nuclear matter. To establish an understanding of the observed yields, a systematic study of high density baryon matter at different beam energies is essential. This might also reveal possible discontinuities in the energy dependence of the reaction mechanism. We present preliminary results for kaon production in Au + Au collisions at beam kinetic energies of 6, 8, and 10.7 GeV/u obtained by the E917 experiment at the AGS (BNL). These measurements complement those carried out by the E866 Collaboration at 2, 4, and 10.7 GeV/u with a significantly enlarged data sample. In both experiments a large range of rapidities was covered by taking data at different angular settings of the magnetic spectrometer.

Two-particle correlations between pions in Au+Au collisions have been measured at beam kinetic energies of 6, 8, and 10.8 GeV/u at the Alternating Gradient Synchrotron (AGS) over a wide range of rapidities using a magnetic spectrometer. The data have been analyzed in the Hanbury-Brown and Twiss (HBT) framework to extract source parameters. The event-by-event orientation of the reaction plane has also been measured using a scintillator hodoscope at far forward rapidities, and beam vertexing detectors upstream of the target. A preliminary analysis of the dependence of the source parameters on the reaction plane is presented.