abstract

The real- and virtual-photon emission during interactions between few-nucleon systems have been investigated at KVI with a 190 MeV proton beam. Here I will concentrate the discussion on the results of the virtual-photon emission for the proton–proton system and proton–deuteron capture. Predictions of a fully-relativistic microscopic-model of the proton–proton interaction are discussed. For the proton–deuteron capture process the data are compared with predictions of a relativistic gauge-invariant impulse approximation and a Faddeev calculation. For the virtual photon processes, the nucleonic electromagnetic response functions were obtained for the first time and are compared to model predictions.

abstract

The photon asymmetry in the reaction \(p(\vec {\gamma },\pi ^{0})p\) has been measured with the photon spectrometer TAPS using linearly polarized photons from the tagged-photon facility at the Mainz Microtron MAMI close to the pion threshold and in the \({\mit \Delta }(1232)\)-resonance region. The total and differential cross sections were also measured simultaneously with the photon asymmetry. This allowed determination of the \(S\)-wave and all three \(P\)-wave amplitudes. The results in the threshold region are compared to the predictions of ChPT. With the photon spectrometer TAPS the full polar angular range of the pion could be covered and from the new results in the \({\mit \Delta }(1232)\) region a E2/M1-ratio of \(-(2.4 \pm 0.16_{\rm stat.} \pm 0.24_{\rm sys.})\,\%\) is extracted.

abstract

The production of nuclear bremsstrahlung (\(E_{\gamma }\gt 30\) MeV) has been studied in heavy-ion collisions, as well as proton and \(\alpha \)-particle collisions with nuclei. In heavy-ion reactions the measured photon spectra show an exponential shape dominated by the incoherent sum of photons produced in first-chance collisions. Photon spectra, angular distributions and multiplicities at 60A MeV indicate that a significant fraction of photons is emitted in secondary nucleon–nucleon collisions from a thermally equilibrated system. In 200 MeV \(\alpha +p\) collisions the incoherent contribution to the photon spectrum is observed as well at low photon energies, while coherent bremsstrahlung is observed at the highest photon energies from radiative capture into unbound states of \(^5\)Li. In 190 MeV proton reactions with light and heavy targets photon spectra have been measured up to the kinematic limit. At high photon energies the spectra show the expected behaviour from first-chance collisions. Below ca. 80 MeV a significant suppression of the photon yield is observed. We attribute this effect to the interference of photon amplitudes due to multiple scattering of nucleons in the nuclear medium.

abstract

Microscopic calculations of Compton scattering on the free proton and light nuclei are presented. For the description of Compton scattering on the proton the conventional \(K\)-matrix approach and the “Dressed \(K\)-Matrix” model are introduced. The latter approach can be used to calculate polarizabilities as well as Compton scattering for photon energies upto 1 GeV since it obeys the symmetry properties which are appropriate in the different energy regions. In particular, crossing symmetry, gauge invariance and unitarity are satisfied. The extent of violation of analyticity (causality) is used as an expansion parameter. Coherent Compton scattering on light nuclei at 200–300 MeV is studied in the impulse approximation and is shown to be a sensitive probe of the in-medium properties of the \({\mit \Delta }\)-resonance. Modifications of the properties of the \({\mit \Delta }\)-resonance due to the nuclear medium are accounted for through the self-energy operator of the \({\mit \Delta }\). The dominant medium effects such as the Pauli blocking effects in the decay width, effective nucleon mass and particle–hole excitations in the pion propagator are consistently included in nuclear matter.

abstract

The experiments with the electromagnetic calorimeter TAPS using tagged photon beams focus on meson production from the free proton and from nucleons bound in nuclei. Following an introduction, results from the recent TAPS campaign at the tagged photon facility MAMI (Mainz) on double pion photo-production are presented. The observations using a proton target provide direct evidence for a \(\rho \) strength in the decay of the \(D_{13}\) resonance. Furthermore, double pion production from nuclei is used as a tool to study in-medium modifications of the \(\pi \pi \) interaction which might indicate a partial restoration of chiral symmetry.

abstract

Recent measurements of the \(pd\rightarrow \,^{3}\textrm {He}\,\eta \) reaction at four beam energies: 930, 965, 1037 and 1100 MeV have shown that the total cross section is nearly constant throughout the whole energy region, despite the CM angular distributions becoming more anisotropic with increasing energy. The data join smoothly onto the results of near-threshold measurements, which are dominated by the \(s\)-wave \(\eta \,^3\)He final state interaction. At all the energies the differential cross section is maximal for \(\cos \theta _{\eta }\approx +0.5\). The data are used to develop a Monte Carlo description of the \(pd\to \,^{3}\textrm {He}\,\eta \) reaction based on a two-step model proposed by Kilian and Nann. With one free parameter the model qualitatively reproduces the excitation function in the energy range of this experiment but fails to reproduce the angular distributions.

abstract

For the combined setup of the Crystal Barrel and TAPS at ELSA in Bonn we have proposed to study the reaction \(\gamma p \rightarrow K^0 {\mit \Sigma }^+\). The reaction is characterised by the final state of 6 photons and a forward emitted proton. Here we report on results of simulations to demonstrate the feasibility of the experiment. From the threshold behaviour of the cross section and angular distributions we aim to search for a \(3^{\rm rd}\) \(S_{11}\) resonance just above the \(K {\mit \Sigma }\) threshold, which may mix with the two lower lying \(S_{11}\) resonances and thus provide an explanation for the unusually strong \(\eta \) branching of the \(S_{11}\)(1535) resonance. The hyperon polarisation can be studied as a sensitive tool to determine the various resonance admixtures.

abstract

New data on deep subthreshold pion and hard photon production in \(^{36}\)Ar on \(^{197}\)Au collisions at 25\( A\) MeV beam energy are presented. The experiment was performed with the spectrometer TAPS (Two-Arm-Photon-Spectrometer) at the AGOR cyclotron at KVI (The Netherlands). Preliminary results of hard photon and pion production at this very deep subthreshold energy are reported.

abstract

The current electronics for TAPS is more than 10 years old with the expected deficiencies of such an old hardware. To overcome these problems, a new 4 channel per module electronics has been designed which will replace and improve the currently used electronics. The main goals for the new design have been the need of a better handling, easier maintainance, improved performance and the capability to handle higher readout rates. A basic requirement was the compatibility with the HADES experiment to perform combined experiments. This is achieved by using similar VME-motherboards. The readout of a typical TAPS-BaF\({_2}\)-signal comprises 4 QACs for the separate integration of the long and short scintillation components in two different ranges, a TAC and 2 LEDs for more sophisticated trigger conditions. A CFD is included to optimize the timing information. Several tests were performed with prototypes. Initial problems were identified and have been removed.

abstract

A compact photon detector with nearly 4\(\pi \)-coverage in solid angle has been proposed to be implemented into the magnetic spectrometer ANKE at COSY, Jülich. Based on the physics program and experimental constraints a first concept will be presented. As part of the R&D program, the response to monoenergetic photons between 45 and 770 MeV has been measured at the tagged photon facility at MAMI, Mainz for arrays consisting of large CeF\(_3\) and PbWO\(_4\) scintillation detectors. The applicability of a fine-mesh photomultiplier (Hamamatsu R5505) has been tested in static and variable magnetic fields.

abstract

Double pion-photoproduction has been studied on \(p, ^{12}\)C and \(^{\rm nat}\)Pb at incident photon energies up to 820 MeV at the MAMI facility in Mainz. The pions were detected in coincidence using the TAPS calorimeter. First preliminary results are available. The invariant mass distribution in the 2\(\pi ^{0}\)-photoproduction channel changes shape with increasing nuclear mass. This \(A\) dependence is not seen in the \(\pi ^{0} \pi ^{\pm }\) channel. The shift is consistent with theoretical predictions for a partial restoration of chiral symmetry at normal nuclear matter density.

abstract

Photoproduction data on various final states involving neutral mesons have been taken by the CB-ELSA-experiment at the Electron Stretcher Accelerator ELSA (Bonn). The data show clear structures due to resonance production. Evidence for successive decays of high-mass nucleon resonances via \({\mit \Delta }(1232)\pi ^{0}\), \({\mit \Delta }(1232)\eta \) and \(N(1535)S_{11}\pi ^{0}\) have been observed. There is evidence for production of \(a_{0}(980)\)-mesons in their \(\pi ^{0}\eta \) decay.

abstract

Statistical emission is the main source of high-energy photons measured in heavy-ion reactions at projectile energies up to 5 MeV/\(u\), as it is shown for \(^{12}\)C + \(^{58,64}\)Ni reactions. At higher projectile energies another important process occurs — the bremsstrahlung emission. Besides that, the Giant Dipole Resonance can be excited in hot nuclei produced not only via complete fusion reactions but also by incomplete fusion and/or after - preequilibrium emission of light particles. The importance of those processes is demonstrated for \(^{12}\)C + \(^{58,64}\)Ni and other reactions.

abstract

The reaction \(\gamma \; p\rightarrow \pi ^0\gamma \,^\prime \;p\) has been measured with the TAPS calorimeter at the Mainz microtron facility. This reaction channel provides access to the static magnetic moment of the \({\mit \Delta }^{+}(1232)\) resonance. Preliminary energy differential cross sections are presented and compared to recent calculations of the \(\gamma \; p\rightarrow \pi ^0\gamma \,^\prime \;p\) reaction.

abstract

We present the results of the analysis of the hard photon production in the \(^{129}\)Xe+\(^{\rm nat}\)Sn at 50A MeV system studied in the GANIL E300 experiment. The energy and angular hard photon distributions confirm the existence of a thermal component which follows the recently measured thermal bremsstrahlung systematics. Exploiting the performances of our complete detection system, consisting of TAPS and 3 charged particle multidetectors, we have also measured the hard photon multiplicity as a function of the charged particle multiplicity.

abstract

The neutral pion production was studied in subthreshold 60\(A\) MeV Ar+C, Ni, Ag, Au reactions. A kinematical fit procedure was applied for a reliable reconstruction of neutral pion kinematics. The transverse momentum spectra are very similar for all studied systems and they are reasonably described by Boltzmann distributions. The temperature parameter does not depend on the total energy available in the system. Perpendicular to backward emission of neutral pions is well described by a simple geometrical model of pion reabsorption.

abstract

The paper raises the problem of reabsorption of neutral pions in nuclear matter. Sophisticated transport models are unable to reconstruct the energy spectra and angular distributions of subthreshold particles. In order to reproduce angular distributions of subthreshold pions we have developed a simple, geometrical model and have tried to compare its predictions to the wealth of experimental data. Although the assumptions of the model are very simple, the agreement between data and the model results is generally very good. The angular distribution of primordial pions was described by the formula \(1+A_2P_2(\cos \vartheta )\). For each data set the \(A_2\) parameter was determined from the comparison to experimental distribution. No apparent trend is observed and we concluded that the angular distribution of primordial pions has a universal value of \(A_2\) parameter.