Search

The production of neutral pions has been studied in the reactions 40 Ar + nat Ca , 86 Kr + nat Zr and 197 Au + 197 Au at 1 A GeV. For high energy pions emitted from the heavier systems a steeper than linear rise of the pion multiplicity with the centrality of the reaction is observed, indicating a pion production process other than binary nucleon-nucleon collisions. At low transverse momenta an enhancement of the π 0 -multiplicity increasing with the mass of the collision system is found. Systematic discrepancies between the experimental results and recent BUU, QMD and Cascade calculations are discussed.

The production of pi-0 and eta-mesons has been studied in the reactions Ne-20 + Al at 350 MeV/u and Ar-40 + Ca at 1.0 GeV/u. Rapidity distributions and transverse momentum spectra have been measured and are compared to thermal distributions.

The azimuthal angle distributions of neutral pions at midrapidity from Au+Au reactions at 1 GeV/nucleon incident energy have been measured. An enhanced emission of [pi][sup 0]'s perpendicular to the reaction plane is observed. The azimuthal asymmetry is dependent on the [pi][sup 0] momentum: the [pi][sup 0] spectrum perpendicular to the reaction plane is harder than in the reaction plane. The strength of the observed asymmetry appears to be more pronounced for [pi][sup 0] than for charged particles and neutrons.

π0 -production probability of (9± 2)· 10− 3 per20Ne+Al reaction has been measured at 350 MeV/u using the Two Arm Photon Spectrometer TAPS. This yield is consistent with an interpolation of published π-production rates in heavy ion collisions.

Isotopically resolved yields of complex particles and fragments from the reaction /sup 12/C on /sup 197/Au at three bombarding energies E/A = 30, 48, and 84 MeV were measured. Specific entropies S/A were derived from the isotope yield ratios using a quantum statistical model. The entropy increases with the bombarding energy in the case of preequilibrium light particles but, for intermediate mass fragments, has an energy-independent value of S/Aapprox. =2.3.

Energy spectra of light particles emitted in the interaction of 84 MeV· A 18 O and 48 MeV· A 20 Ne ions with different targets have been measured with telescopes consisting of NaI, BGO and surface-barrier detector stacks. The inclusive data have been analysed within a moving-source parametrization. By using a large-area plastic-scintillator detector arrangement covering the target with a solid angle of about 2π, light-charged-particle multiplicity distributions have been measured and multiplicity-selected light-particle spectra have been obtained. It is shown that multiplicity selection allows a rough separation of peripheral and central processes and will be a suitable means for probing different theoretical models of nucleon emission.

An inclusive measurement of light and medium heavy fragments for eight projectile target combinations and incoming energies is presented. The energy spectra of all fragments are fitted consistently with a moving source parametrization. The results are compared to several model predictions. No evidence for the liquid gas phase transition of nuclear matter is found and considerable doubts in thermal interpretations arise. An alternative model of a cold shattering of the target nucleus is able to explain the energy spectra well but fails to reproduce the charge spectra.

Correlations between intermediate-mass fragments (IMF's) and heavy fragments were measured for $^{18}\mathrm{O}$-induced reactions on $^{\mathrm{nat}}\mathrm{Ag}$ and $^{197}\mathrm{Au}$ at $\frac{E}{A}=84$ MeV. Correlations of two coincident IMF's show a depletion at small relative velocities ${v}_{\mathrm{rel}}\ensuremath{\le}1$ cm/ns, reflecting the proximity of the two IMF's at the point of creation. However, the time scales for IMF emission derived from this correlation are similar to those of binary-IMF-heavy-recoil coincidences which indicates a sequential nature of multifragment emission in these reactions.

Isotopically resolved yields of intermediate-mass fragments from $^{12}\mathrm{\ensuremath{-}}$ and $^{18}\mathrm{induced}$ reactions on $^{58}$,64Ni, $^{\mathrm{nat}}\mathrm{Ag}$, and $^{197}\mathrm{Au}$ at E/A=84 MeV were measured. The systematic variation of the yield ratios with projectile and target shows that these fragments are produced by a mechanism that involves the whole target nucleus. It is demonstrated that, within a statistical interpretation, the isotope-yield ratios are a measure of the entropy and determine a relation between temperature and density at breakup.

For the WA80 fixed-target heavy-ion experiment at the CERN-SPS, a 1278 element lead-glass calorimeter has been constructed to measure photons in the energy range from 0.2 to 20 GeV. We describe the design of the detector, the achieved energy and position resolution, the gain monitoring system and the performance of the high-energy photon trigger. The quality of photon identification and π0 invariant mass reconstruction is discussed.

Isotopically resolved yields of particles and complex fragments from 12 C and 18 O induced reactions on 58 Ni, 64 Ni, Ag and 197 Au in the intermediate range of bombarding energies 30 MeV ≤ E / A ≤ 84 MeV were measured. The systematic variation of the deduced isotope yield ratios with projectile and target is used to determine the degree of N/Z equilibration achieved and to establish time scales for the reaction process. A quantum statistical model is employed in order to derive entropies of the emitting systems from the measured isotope yield ratios.