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The nuclear response of a medium-mass nucleus C^^Xe) to electromagnetic excitation in a nearrelativistic heavy-ion collision was investigated in the reaction ^^^XQ{OJA GeV)4-Pb. From an exclusive measurement of the neutron decay of the excited '^^Xe projectiles, strong excitations of giant resonances and, in particular, of the double isovector giant dipole resonance were identified. A resonance energy of 28.3 ±0.7 MeV, a width of 6.3 ± 1.6 MeV, and a total cross section of 215 ± 50 mb were found for the double giant dipole resonance.PACS numbers: 24.30.Cz, 25.75.+r Very large probabilities are expected for the electromagnetic excitation of high-lying collective nuclear modes, such as giant resonances, in peripheral heavy ion collisions at energies in the (near) relativistic regime. With such high probabilities for single-step excitations, also multistep excitations become probable. The excitation of multiphonon states, in particular of the double giant dipole resonance, are predicted to become observable with cross sections up to several hundred millibarns [1]. In a number of radiochemical measurements [2] it was shown that electromagnetic excitation followed by neutron emission contributes significantly to the process of fragmentation in peripheral heavy ion collisions. More recently, direct observations of the giant dipole resonance excitation in light nuclei were obtained with particle counter techniques [3,4]. Here, we report on a first exclusive measurement of the electromagnetic excitation and the decay of giant resonances in a medium-heavy nucleus. '^^Xe projectiles were excited in collisions with Pb target nuclei at a laboratory energy of 0.7^ GeV and from the exclusive measurement of the subsequent neutron decay the excitation energy was unambiguously reconstructed. We observe, in particular, the double isovector giant dipole resonance being excited with considerable cross section.The experiment was performed at the heavy ion synchrotron (SIS) facility at GSI, Darmstadt. We used '^^Xe projectiles of 0.7^ GeV with a typical beam intensity of 10"* particles per second, and lead and carbon targets with 1.38 and 0.26 g/cm^ thickness, respectively. The experimental setup is shown in Fig. 1. It meets the conjecture that the ^^^Xe projectile, excited in a peri-pheral collision, deexcites by emission of neutrons and subsequent gamma decay of the heavy residue. It is known that giant resonances in heavy nuclei almost exclusively decay via neutron emission. The primary projectile excitation energy is obtained by reconstructing the final state invariant mass from the four-momenta of the particles and of the emitted gamma rays in the laboratory frame.As shown in Fig. 1, the trajectories of the projectiles and of the projectile fragments were traced by means of four plastic scintillation counters, the thickness of which varied between 0.1 and 1 mm. These detectors delivered position information in both dimensions perpendicular to the beam axis and also provided an excellent time-offlight (TOF) resolution. Beam fragme...

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Abstract.We investigate the beam energy dependence of neutron and proton squeeze-out in collisions of 197Au a t-197Au at E/A =400-800 MeV. The azimuthal anisotropy that describes the enhanced emission of mid-rapidity neutrons perpendicular to the reaction plane rises strongly with the transverse momentum of the neutrons. This dependence of the azimuthal anisotropy follows a universal curve -independent of beam energy -if the neutron momenta are measured in fractions of the projectile momentum per mass unit. Analogously, the kinetic energy spectra of mid-rapidity neutrons exhibit a universal behaviour as a function of the kinetic energy of the projectile.

In patients with evidence of dual AV conduction, a premature atrial depolarization is normally conducted via a fast conducting pathway with a long refractory period. At a critically timed coupling interval, the effective refractory period is reached and now the impulse is conducted over a pathway with a slow conduction velocity and a short effective refractory period. At this moment in the His bundle electrocardiogram, a sudden increase in the AH interval occurs which is called the 'break' phenomenon. This phenomenon was studied in 14 patients, with a history of supraventricular paroxysmal tachycardia, at the patient's own heart rate, at different paced heart rates, and after the administration of atropine. In 11 patients the 'break' phenomenon could be elicited during sinus rhythm,; in 3 when the heart rate was accelerated. The effective refractory period of the fast conducting pathway was prolonged in 9 patients, remained unchanged in 3, and was shortened in 2 when the heart rate was increased. The effective refractory period of the slow conduting pathway was reached when block occurred proximal to the His bundle. This parameter was increased in 8 patients, remained unchanged in 3, and was decreased in 1 patient with augmentation of the heart rate. This effective refractory period was not measured in 2 patients. Atropine led to a shortening of the refractory period of both pathways in 8 patients studied. Atrial echo phenomena indicating the beginning of a re-entry circuit occurred at an AH prolongation that was not always identical to that measured at the point of 'break'. Widening, as well as narrowing, of the echo zone was found, indicating that the re-entry circuit was in a state of labile equilibrium. In cases where shortening of the pathways involved in the re-entry circuit was found, structures other than nodal may exist; therefore paranodal bypass must be considered.