nuclear physics in poland 1996 – 2006

HARD PHOTONS FROM NUCLEUS-NUCLEUSAND PROTON-NUCLEUS COLLISIONST. Matulewicz, K. PiaseckiInstitute of Experimental Physics, Warsaw University, WarszawaExperimental facilities: GANIL Caen, AGOR KVI Groningen, SIS18 GSI DarmstadtThe properties of the hot and dense zone formedin (central) nucleus-nucleus collisions can be beststudied with the probes which do not suffer fromstrong final state interactions. Bremsstrahlungphotons can provide relatively undistorted insightinto the physical conditions of the hot zone, buttheir usage is restricted at low energies (below~20A MeV) by dramatically low production crosssection, and at higher energies (above 100A MeV),by photons stemming from electromagneticdecays of produced hadrons like π 0 and η mesons.The total spectrum of photons (Fig. 1) consists ofthe low energy part (below ~10 MeV) originatingfrom statistical decays of excited fragments,photons from the deexcitation of the Giant DipoleResonance (around 15 to 20 MeV) and hardphotons (above 30 MeV). Hard photons comepredominantly from the bremsstrahlung processin proton-neutron interactions and they canwitness the early phase of the collision. Studies ofthe photon spectra were done for nucleus-nucleuscollisions in the energy range from 40A to 100AMeV using the TAPS spectrometer. TAPS consistsof approx. 400 BaF 2 scintillator modules, whichcan be arranged in various experiment-specificconfigurations. Excellent time resolution andpulse-shape analysis allows for unambiguousphoton identification and spectroscopy.undergoes photoabsorption on another nucleon,what releases also the pion rest mass and createsphotons of extreme energies.The measurements realized for 180A MeV Ar+Casystem showed a significant enhancement of thehard photon cross section with respect to theextrapolations based on lower energy data.The second-order quantum interference effect,known as Hanbury-Brown and Twiss (HBT) effector intensity interferometry, allows to extract thesource size on the basis of the analysis of twobodycorrelation function. Pairs of bremsstrahlungphotons (E>25 MeV) have beenmeasured. While the initial experiments of limitedstatistics indicated the oscillatory character of thecorrelation function (suggesting secondaryrecompression of the nuclear matter during thecollision), higher statistics data show a flatcorrelation function. This shape can beunderstood only as a peculiar interferencebetween photons from first-chance collisions andphotons from target or projectile-like fragments.Only in the case of central collisions (selected viacharged particles multiplicity) the indications forthe standard HBT effect can be found.Detailed analysis of the shape of the photonenergy spectrum above the region influenced bythe Giant Dipole resonance revealed the presenceof a second, softer, component. According to thetransport model calculations, these photons areemitted at a later stage of the collision, when theexcited zone approaches the thermalequilibration. The extracted source temperatureagrees quite well with the caloric curve. Also, thethermalization time can be evaluated.At the high-end of the spectrum, even the fullyconstructive superposition of the Fermi motionwith the beam momentum does not allow toexplain the origin of most energetic photons.According to the transport model, photons abovethe Fermi-motion related kinematical limit (190MeV for 60A MeV beam) are predominantlyproduced by a two-step process: a pion producedin nucleon-nucleon interaction subsequently.Fig. 1: Photon energy spectrum measured with TAPS for the Kr+Nicollisions at 60A MeV.139