Search

We have measured deconfined hadronic volumes, $4.4 < V < 13.0$ fm$^{3}$, produced by a one dimensional (1D) expansion. These volumes are directly proportional to the charged particle pseudorapidity densities $6.75 < dN_{c}/d\eta < 20.2$. The hadronization temperature is $T = 179.5 \pm 5$ (syst) MeV. Using Bjorken's 1D model,the hadronization energy density is $\epsilon_{F} = 1.10 \pm 0.26$ (stat) GeV/fm$^{3}$ corresponding to an excitation of $24.8 \pm 6.2$ (stat) quark-gluon degrees of freedom.

Preliminary results on photon production and forward-backward multiplicity correlations in p p collisions at √s = 1.8 TeV are presented. Some additional results obtained at lower energies are also shown.

Experiment E735{dagger} searched for evidence of the transition to quark-gluon plasma in p{bar p} collisions at {radical}{bar s} = 1.8 TeV. Using data from a high statistics run in 1988--1989, results are presented on multiplicity distributions, hyperon and phi production, and Bose-Einstein correlations. Some data was also taken at lower collision energies and results will be compared to previous experiments.

The transverse momentum of charged mesons and p's produced within the pseudorapidity range of η = −0.36 to η = +1.0 and azimuthal range of φ = +2° to φ = +18° has been measured in pp collisions at √s = 1.8 TeV. The charged multiplicity of each event was measured by either the 240 element cylindrical hodoscope covering the range −3.25 < η < +3.25 or the central drift chamber, which spans a pseudorapidity range of 3.2 units. The average transverse momentum as a function of the pseudorapidity density for mass-identified particles is presented. We have observed pseudorapidity densities as high as 30 particles per unit pseudorapidity.

E-735 is designed to study charged particle production in the central region of rapidity in proton-antiproton collisions at √ s = 1.8 TeV. Variations in transverse momentum and particle production ratios versus particle density are measured as a possible signature for a quack-gluon-plasma (QGP) phase transition. Preliminary results from the 1987 run are presented.

Thick-target recoil ranges and forward-to-backward ratios of some thirty target fragments from the interaction of Ag with 218 GeV {sup 16}O ions have been measured. The results were analyzed by means of the two-step model and compared with similar data for 18.5 GeV {sup 12}C ions and 400 GeV protons in order to test the applicability of factorization and limiting fragmentation.

We report on a search for metastable positively and negatively charged states of strange quark matter in Au+Pb reactions at 11.6 A GeV/c in experiment E864. We have sampled approximately six billion 10% most central Au+Pb interactions and have observed no strangelet states (baryon number A < 100 droplets of strange quark matter). We thus set upper limits on the production of these exotic states at the level of 1-6 x 10^{-8} per central collision. These limits are the best and most model independent for this colliding system. We discuss the implications of our results on strangelet production mechanisms, and also on the stability question of strange quark matter., 21 pages, 9 figures, to be published in Nuclear Physics A (Carl Dover memorial edition)

During the past year, our research program has involved continuing analysis of Fermilab E-735, search for quark-gluon plasma (QGP) in {bar p}-p collisions; continuing study of target fragments produced in the interaction of copper with intermediate-energy heavy ions; an exclusive study of multifragmentation using reverse kinematics at the Bevalac; and detector development for the STAR detector at RHIC.

The levels of 63 Cu and 65 Cu were populated in (p, p′), (p, α), (α, p) and (α, α′) reactions induced by 12.5–16.0 MeV protons and 16.8–21.3 MeV α-particles. Energy spectra were measured between 30° and 150° and the evaporation spectra were compared with the spin- and isospin- dependent statistical theory. The comparison permitted the determination of the level density parameter, a , and the pairing energy parameter, δ, in the Fermi gas model level density expression. It was found that while a given set of parameters could give a good fit to the spectra obtained at a given energy, the energy dependence of some of the differential cross sections was not very well reproduced. The implications of this disagreement for the validity of the Fermi gas model level density are discussed.

Excitations functions for the formation of /sup 126/I from /sup 127/I have been determined with 60--350 MeV ..pi..and ..pi../sup -/. The contributions of both direct nucleon knockout and pion inelastic scattering followed by neutron evaporation are visible in the excitation functions. The results are compared with an cascade-evaporation calculation, the Sternheim-Silbar nucleon charge exchange models, and the semiclassical model of Ohkubo and Porile.

The application of Ge(Li) γ−γ coincidence spectrometry to the assay of a uranium foil irradiated by 300 GeV protons is described. Digital gates were set on γ-rays from a number of possible reaction products and coincidence spectra were recorded. The technique was applied to the determination of both cross sections and thick-target recoil properties, and results are presented and compared with those obtained by conventional techniques.

Energy spectra of protons emitted in the reactions of 64 Zn, 66 Zn, 68 Zn, and 70 Zn with 12.5 MeV protons and 18 MeV 4 He ions were measured between 30° and 150° to the beam. The contribution of the precompound process was determined on the basis of the asymmetry in the angular distribution and by comparison of the spectra with calculations based on the compound and precompound models. Both methods indicate that the contribution of this process increases from ≈5% for 64 Zn to ≈20% for 70 Zn. The ratio of proton to neutron evaporation widths is found to depend strongly on target neutron number and a correlation is established with the corresponding seperation energies.

Excitation functions have been determined for the formation of ten iodine nuclides (/sup 117/I-- /sup 126/I) in the interaction of 60--350 MeV ..pi../sup +/ and ..pi../sup -/ with /sup 127/I. The contribution of various reaction mechanisms has been estimated from the systematic features of the excitation functions and isotopic yield distributions with the aid of cross sections obtained for the production of various isobaric xenon nuclides, cross-section results for 500 MeV proton bombardment of /sup 127/I, and comparison with Monte Carlo cascade-evaporation calculations. It is found that, with varying importance, mechanisms involving direct neutron knockout, pion inelastic scattering plus evaporation, pion absorption, and pion charge exchange contribute to the observed yields.

The 64 Zn compound nucleus was formed at energies of 17.6–23.5 MeV in the reactions 63 Cu + p and 60 Ni + α and energy spectra of the emitted protons and α-particles were recorded. The extent of isospin mixing between the T > and T states of the compound nucleus was determined by comparison of the quantity σ(p, p')σ(α, α')/σ(p, σ)σ(α, p) with theory. The mixing fraction decreases from 0.8 to 0.4 over the above energy interval. The mixing matrix element shows a corresponding decrease from 0.34 to 0.07 keV.

The semiclassical trajectory model of Benioff, which treats the nuclear structure of the target by means of the harmonic oscillator model, has been adapted to the (..pi..,..pi..N) reaction in the (3,3) resonance region. Excitation functions have been calculated for comparison with the experimental results on the following reactions: /sup 12/C(..pi../sup + -/,..pi../sup + -/n), /sup 25/Mg(..pi../sup + -/,..pi../sup + -/p), and /sup 197/Au(..pi../sup + -/,..pi../sup + -/n). The nucleon charge exchange probability P was treated as an adjustable parameter and found to decrease from approx.0.2 for light elements to < or approx. =0.05 for heavy elements. The value of P for /sup 12/C is supported by a parameter-free estimate on the basis of a modified free nucleon-nucleon scattering model. The decrease in P with increasing target A is attributed to nuclear size and structure effects. The calculated excitation functions are in good agreement with the experimental results for both light and heavy target nuclei. Experiments that would constitute a more stringent test of various aspects of the model are proposed.

Energy spectra and angular distributions of protons and α-particles emitted in the decay of 56 Fe, 60 Ni, 63 Cu, 64 Zn, and 66 Zn were measured. These compound nuclei were produced with a given excitation energy by irradiation of appropriate targets with 14.0 MeV protons and 15–19 MeV α-particles. The proton spectra featured a low-energy peak ascribable to the (p, np) or (α, np) reaction as well as a higher energy peak due to the (p, p') or (α, p) reaction. These peaks were sufficiently well resolved that differential cross sections could be obtained for the p, np, as well as α exit channels. The α and np exit channels were found to be free of precompound effects and are not affected by isospin conservation. Comparison of these particular cross sections for both entrance channels permits a test of the independence hypothesis provided a correction is made for the different angular momentum distributions in the entrance channels. When this correction is applied on the basis of the spin-dependent statistical theory it is found that the independence hypothesis is confirmed in all cases. Comparison of the p and α exit channels leads to values of the isospin mixing fraction, μ, between the T > and T states of the compound nucleus. The values of μ are 0.4±0.2 for all five compound nuclei and correspond to Coulomb mixing widths whose value is approximately 20 keV.

Partial cumulativc yields and thick-target recoil properties of 14 xenon nuclides formed in the interaction of /sup 238/U with 11.5-GeV protons and ranging from /sup 122/Xe to /sup 136/Xe have been measured. Independent yields and recoil ranges were deduced with the aid of reported data. The isotopic-yield distribution consists of two peaks with a broad region of near-constant yields between them. The recoil ranges exhibit the charactcristic dropoff on the near neutron-deficient side of stability. Comparisons with other isotopic- and isobaric-yield distributions and ranges of products in the same mass region are presented. (auth)

The average ranges projected in the forward, backward and perpendicular directions have been measured for 11 C from the 12 C(p, pn) reactionm from threshold to 150 MeV. The results as well as similar data available for slightly higher energies have been compared with a cascade-evaporation calculation based on the Vegas code. The calculation predicts ranges that are in fairly good agreement with experiment above 100 MeV, although the relative importance of the direct knock-out process appears to be underestimated. It is shown that compound nucleus formation is not the principal reaction mechanism below 50 MeV.

Excitation functions have been measured for a number of (p, xn), (p, pxn), (p, 2pxn) and (p, 3pxn) reactions of Ga69 and Ga71 with 13–56 MeV protons. The following peak cross-sections were determined: Ga69-(p, 2n)-465 mb, (p, 3n)-65 mb, (p, 4n)-7.2 mb, (p, pn)-420 mb, (p, p2n)-370 mb, (p, p3n)-193 mb, (p, αn)-134 mb, (p, 3p)-39 mb, (p, αpn)-42 mb; Ga71-(p, 3n)-600 mb, (p, 4n)-137 mb, (p, pn)-300 mb, (p, p3n)-240 mb, (p, α3n)-80 mb, (p, αp)-2.4 mb. The results have been compared with (1) statistical theory calculations, (2) cascade-evaporation calculations, and (3) a similar study of the reactions of Zn66+α-particles. It is concluded that the reactions exhibit both compound nuclear and direct interaction features with the latter being consistent with a cascade-evaporation mechanism. Values have been obtained for the total reaction cross-section of Ga69 and they are in agreement with continuum theory values for r0=1.4−1.5 fm.

Differential cross sections have been measured between 30° and 150° for the (p, p′) and (p, α) reactions induced by 14 MeV protons in the following targets: 35 Mn, 36 Fe, 58 Ni, 59 Co, 62 Ni, 63 Cu and 64 Zn. The results have been compared with the spin-dependent statistical theory and good agreement was obtained for the (p, α) reactions with a level density parameter a = (0.138±0.008) A MeV −1 . This value is substantially lower than that obtained in a spin-independent analysis of the same data. The agreement of the (p, p′) spectra with theory is poor and it is found that a 15–40% contribution from precompound emission is required. The emission of low-energy protons from the (p, np) reaction was observed in all cases and comparison of the cross sections with these of the corresponding (p, n) reactions yields information on the competition between γ-ray and proton emission.

The angular distributions, cross sections and recoil ranges of 207 Po from the 206 pb ( 4 He, 3n) and 207 Pb( 3 He, 3n) reactions have been measured over the energy intervals 32–44 MeV and 21–33 MeV, respectively. The compound nuclear nature of these reactions was established by comparison of the average ranges with values expected for this mechanism. The angular distributions were used to derive values of the average total kinetic energy of the neutrons and photons emitted in these reactions. The partition of the excitation energy between these two decay modes was found to be equal for the two reactions to within at most 1.5 MeV. The expected difference in energy partition arising from angular momentum differences is approximately 0.5 MeV.

Oxygen and carbon impurities in metal foils have been assayed with a sensitivity of 2–25 ppm by measurement of proton energy spectra in reactions induced by 14 MeV protons. Energy spectra of α-particles from (p, α) reactions have been used to distinguish between surface and body impurities.

The following studies, currently in progress, are discussed: angular distributions and differential ranges of Sc and Ba fragments emitted in the interaction of 0.8-400 GeV protons with /sup 238/U; comparative study of the emission of light (/sup 24/Na-/sup 52/Mn) fragments in the interaction of Cu - U with relativistic /sup 12/C ions and protons; comparative study of the recoil properties of products of the spallation of silver by relativistic /sup 12/C ions and protons; measurement of the excitation functions of the /sup 127/I(..pi../sup + -/,..pi../sup + -/xn) reactions; energy spectra of isotopically resolved fragments emitted in the interaction of high-energy protons with complex nuclei. 9 references.

A number of nuclear reaction studies are in progress. Measurements of the energy dependence of recoil properties of /sup 131/Ce, /sup 131/La, and /sup 131/Ba formed in the interaction of /sup 238/U with 0.8-400 GeV protons are described. Preliminary results on the angular distribution and differential ranges of Sc and Ba nuclides formed in the interaction of /sup 238/U with 0.8, 3.0, and 11.5 GeV protons are given. Preliminary angular distribution measurements of products formed in the interaction of /sup 238/U with 400 GeV protons are described. A comparative study of the interaction of silver with 25.2 GeV /sup 12/C ions and high energy protons is described and results of the analysis of the proton data are presented. The formation of /sup 24/Na in the interaction of gold with protons and ..pi../sup +/ was studied and preliminary results are reported. A list of publications is included. (JFP)

The following studies, currently in progress, are discussed: angular distributions and differential ranges of Ba fragments from the interaction of /sup 238/U with 0.8-to 400- GeV protons; target A dependence of the angular distribution of Sc fragments emitted in 400-GeV proton interactions; energy spectra of isotopically resolved fragments emitted in high-energy proton interactions with noble gases; comparative study of fragments emission in the interaction of Cu-U with relativistic /sup 12/C ions and high-energy protons; excitation functions of the /sup 127/I(..pi../sup + -/,..pi../sup + -/xn) reactions. 7 figures.