B. Kohlmeyer

The kaon spectrometer at SIS

A double-focussing QD magnetic spectrometer has been developed and installed at the SIS heavy-ion facility at GSI in Darmstadt. The spectrometers primary purpose is to study meson production in energetic nucleus-nucleus collisions in detail. Its compact design is matched to the requirements of kaon detection with short flight path (5–6.5 m), large solid angle (up to 35 msr), wide momentum acceptance (pmaxpmin ⋍ 2), maximum momentum 1.6 GeV/c (1.9 GeV/c at reduced solid angle) and reasonable momentum resolution (⋍ 1% without and ⋍ 10−3 with ray tracing). A focal-plane length of about 1.5 m allows the efficient use of the detectors necessary for particle identification and ray-tracing, involving wire chambers, time-of-flight scintillators and Cherenkov detectors. Collisions can be characterized by two multiple-module plastic-scintillator hodoscopes detecting reaction fragments in the forward hemisphere. While the primary purpose for the construction of the spectrometer is the measurement of kaons, it can serve as a general purpose magnetic spectrometer. Its large solid angle also allows the study of two-particle correlations.

Enhanced out-of-plane emission of K+ mesons observed in Au + Au collisions at 1-A/GeV

The azimuthal angular distribution of K+ mesons has been measured in Au + Au collisions at 1 AGeV. In peripheral and semi-central collisions, K+ mesons preferentially are emitted perpendicular to the reaction plane. The strength of the azimuthal anisotropy of K+ emission is comparable to the one of pions. No in-plane flow was found for K+ mesons near projectile and target rapidity.

First evidence for different freeze-out conditions for kaons and antikaons observed in heavy-ion collisions.

Differential production cross sections of K- and K+ mesons have been measured in Ni+Ni and Au+Au collisions at a beam energy of 1.5 A GeV. The K(-)/K(+) ratio is found to be nearly constant as a function of the collision centrality and system size. The spectral slopes and the polar emission pattern differ for K- and K+ mesons. These observations indicate that K+ mesons decouple earlier from the fireball than K- mesons.

Observation of different azimuthal emission patterns of K+and of K- mesons in heavy-ion collisions at 1-2A GeV

Azimuthal distributions of pi+, K+, and K- mesons have been measured in Au+Au reactions at 1.5A GeV and Ni+Ni reactions at 1.93 A GeV. In semicentral collisions at midrapidity, pi+ and K+ mesons are emitted preferentially perpendicular to the reaction plane in both collision systems. In contrast for K- mesons in Ni+Ni reactions, an in-plane elliptic flow was observed for the first time at these incident energies.

In-Medium Effects on Phase Space Distributions of Antikaons Measured in Proton-Nucleus Collisions

Differential production cross sections of K+/- mesons have been measured in p + C and p + Au collisions at 1.6, 2.5, and 3.5 GeV proton beam energy. At beam energies close to the production threshold, the K- multiplicity is strongly enhanced with respect to proton-proton collisions. According to microscopic transport calculations, this enhancement is caused by two effects: the strangeness exchange reaction NY --> K- NN and an attractive in-medium K- N potential at saturation density.

Time-of-flight measurement of the evaporation residues from fusion of 5.9 mev/u 84kr and 27al

Abstract Mass and Z -distributions of the evaporation residues from compound-nucleus formation in the reaction 5.9 MeV/u 84 Kr on 27 Al were measured using a time-of-flight ΔE − E telescope, which is described in detail. The high recoil velocity attained by choosing the heavy reaction partner as projectile made it possible to resolve the reaction products by mass and atomic number. Data were taken in the angular range from 1.5 to 6°. The residue distributions are compared to evaporation calculations assuming the statistical decay by fission and particle evaporation of the compound nucleus 111 Mn formed at an excitation energy of 108 MeV with angular momenta up to L CN ≈ 69 h . The data are consistent with the assumption of statistical equilibrium. Details of the de-excitation process, in particular the fission competition and the influence of nuclear deformations at high angular momenta, are discussed.

Properties of high-energy pions emitted from heavy-ion collisions at 1 GeV/nucleon

Positively charged pions and protons from collisions of Ne+NaF and Au+Au at 1 GeV/nucleon incident energy were measured near midrapidity. The center-of-mass pion spectra deviate from a Maxwell-Boltzmann distribution. The slope of the high-energy part of the pion spectra varies significantly with the system mass and little with the size of the reaction zone. While the total pion yield rises linearly with the number of participant nucleons, the highenergy component increases more than linearly.

Kaon production in heavy ion collisions and the nuclear equation of state

Abstract Double differential cross sections for protons, π + and K + are measured in Au + Au collisions at 1.0 GeV/u and Θ lab =44°±4° with a new magnetic spectrometer at SIS. The pion spectra show an enhancement at low momenta with respect to a thermal distribution and little dependence on the collision centrality. The high-energetic pions measured in semi-central collisions at approximately midrapidity exhibit an anisotropic azimuthal angular distribution, peaking at 90° and 270° with respect to the reaction plane. The same feature is observed for protons. The kaon to proton ratio increases with the number of participating nucleons, i.e. with the centrality of the collision, whereas the pion to proton ratio stays constant. The first data on subthreshold K + production in a heavy mass collision system are discussed with respect to nuclear matter properties by a comparison to microscopic transport calculations.

A fast-fission component with small mass drift in the reaction84Kr+27Al atElab=5.9 MeV/u

All reaction products in the range from target- and projectile-like to fission-like fragments were measured for the system84Kr+27Al at 5.9 MeV/u beam energy. They are assigned to the various reaction mechanisms on the basis of experimental signatures (energy dissipation, mass and angular distribution). The sum of the measured partial cross sections, including the evaporation residue yield obtained previously, agrees with the total reaction cross section derived from elastic scattering. A small fast-fission component was found, discernible from deep-inelastic reactions by its 1/sin θ angular distribution, and distinguished from compound-nucleus fission by an incomplete mass asymmetry relaxation.

A study of the influence of shell effects in nuclear level densities on the decay of the compound nucleus58Ni

Mass andZ-distributions of fusion products from the reaction 5.8 MeV/u46Ti+12C were measured in the angular range betweenθlab=1.5° and 8° using a time-of-flightΔE−E telescope. The results are compared with an earlier measurement of the reaction32S+26Mg, in which the same compound nucleus was produced at higher excitation energy. The residual nuclide distributions are interpreted with the aid of evaporation calculations and the influence of shell effects in the level densities is discussed. The data are consistent with the assumption that the shell effects vanish above 15 to 20 MeV excitation energy, that is, that they are essentially only important for the last evaporation step.