M. Hoefman

Dynamical effects in proton–proton bremsstrahlung for non-coplanar geometries

Abstract The proton–proton bremsstrahlung process, including both coplanar and non-coplanar kinematics, has been measured with polarized protons of 190 MeV. High-precision cross sections and analyzing powers have been obtained. Cross sections as a function of non-coplanarity angle are presented and show large variations of dynamical origin. Non-coplanar analyzing powers have been measured for the first time and are compared to theoretical predictions.

Cross sections and electromagnetic response functions for radiative proton capture in

Abstract Exclusive differential cross sections of virtual-photon radiative proton capture in pd→ 3 He +e + e − at a proton energy of 190 MeV have been measured for the first time. The leptonic-angle dependence of the measured cross section is exploited in order to determine the electromagnetic response functions in the time-like region. The data are compared to a relativistic gauge-invariant impulse approximation and a Faddeev calculation. Differences between theoretical predictions and data are discussed in terms of enhanced magnetic radiation possibly originating from virtual Δ excitations.

Evidence for thermal equilibration in multifragmentation reactions probed with bremsstrahlung photons

The production of nuclear bremsstrahlung photons (E{sub {gamma}}>30 MeV) has been studied in inclusive and exclusive measurements in four heavy-ion reactions at 60A MeV . The measured photon spectra, angular distributions, and multiplicities indicate that a significant fraction of the hard photons is emitted in secondary nucleon-nucleon collisions from a thermally equilibrated system. The observation of the thermal component in multifragment {sup 36}Ar+{sup 197}Au reactions suggests that the breakup of the thermalized source produced in this system occurs on a rather long time scale.

Photon Production in Heavy-ion Collisions Close to the Pion Threshold

Abstract We report on a measurement of hard photons ( E γ >30 MeV) in the reaction Ar+Ca at 180 A MeV at an energy in which photons from the decay of π ° mesons are dominating. Simultaneous measurement with the TAPS spectrometer of the photon spectrum and photon-photon coincidences used for the identification of π ° enabled the subtraction of π ° contribution. The resulting photon spectrum exhibits an exponential shape with an inverse slope of E 0 =(53±2 (stat) −5 +8 (syst) ) MeV. The photon multiplicity equal to (1.21±0.03 (stat) +0.3 −0.2 (syst) )·10 −2 is roughly one order of magnitude larger than the value extrapolated from existing systematics. This enhancement of the hard photon production is attributed to a strong increase in the contribution of secondary np collisions to the total photon yield. We conclude that, on average, the number of np collisions which contribute to the hard photon production is 7 times larger than the number of first chance np collisions in the reaction Ar+Ca at 180A MeV.

Suppression of soft nuclear bremsstrahlung in proton-nucleus collisions

Photon energy spectra up to the kinematic limit have been measured in 190 MeV proton reactions with light and heavy nuclei to investigate the influence of the multiple-scattering process on the photon production. Relative to the predictions of models based on a quasifree production mechanism, a strong suppression of bremsstrahlung is observed in the low-energy region of the photon spectrum. We attribute this effect to the interference of photon amplitudes due to multiple scattering of nucleons in the nuclear medium.

Measurements of the virtual bremsstrahlung yields in the p+p system

Abstract In this paper recent results obtained from the p + p → p + p + e + + e − experiment are presented. This experiment has been performed with a 190 MeV polarized proton beam obtained from the new cyclotron AGOR at KVI in Groningen. Differential cross sections have been obtained in exclusive measurements in which all four exit particles have been measured in a coincidence setup between SALAD and TAPS. The data are compared with LET calculations. A reasonable agreement is found for virtual-photon invariant masses up to 80 MeV/c 2 .

Thermal bremsstrahlung probing the thermodynamical state of multifragmenting systems

Inclusive and exclusive hard-photon (E-gamma > 30 MeV) production in five different heavy-ion reactions (Ar-36+Au-197, Ag-107, Ni-58, C-12 at 60A MeV and Xe-129+Sn-120 at 50A MeV) has been studied coupling the TAPS photon spectrometer with several charged-particle multidetectors covering more than 80% of 4 pi. The measured spectra, slope parameters and source velocities as well as their target-dependence, confirm the existence of thermal bremsstrahlung emission from secondary nucleon-nucleon collisions that accounts for roughly 20% of the total hard-photon yield. The thermal slopes are a direct measure of the temperature of the excited nuclear systems produced during the reaction.

Coherent Bremsstrahlung in \alpha + p Reactions at 50 MeV\nucleon

Bremsstrahlung emitted in the α + p reaction at 50 MeV/nucleon was studied using a 200 MeV α -particle beam and a liquid hydrogen target. Double-differential photon cross sections were measured over a wide range of angles and energies. Coherent bremsstrahlung was found to be strong for all photon energies. The cross section is enhanced at the highest photon energies and is characteristic of direct capture to the unbound 5 Li ground and first excited state. With direct capture as a limiting case of coherent bremsstrahlung a consistent reaction picture can be obtained.

Probing Few-body Systems with Bremsstrahlung

A series of bremsstrahlung measurements have been performed at KVI with the 190 MeV polarized proton beam of the new superconducting cyclotron, AGOR. The aim of these measurements is to investigate the dynamics of interactions involving intermediate off-shell nucleons. Nucleon-nucleon bremsstrahlung is the most fundamental process used for such studies as it involves the strong interaction between two nucleons and the well-known electromagnetic interaction. The present experiment provides the most accurate data on the proton-proton system. In addition the first exclusive proton-proton virtual-bremsstrahlung data ever is presented. The accuracy of the real bremsstrahlung data is high enough to investigate higher-order effects such as the virtual Δ-isobar and meson-exchange currents. None of the microscopic calculations is able to describe the data both in magnitude and in shape. The virtual-bremsstrahlung data are integrated over the detector acceptance. This same integration is done for two different soft-photon calculations for comparison.

Transverse momentum distributions of neutral pions produced at deep subthreshold energies

We present in this paper transverse momentum distributions of π0-mesons measured in 36Ar+197Au reactions at 25A and 35A MeV beam energies with the spectrometer TAPS at the KVI AGOR accelerator. Our data are consistent with thermal-like emission of π0-mesons with temperature parameters much lower than in earlier measurements.