Whistler wave excitation in a magnetized laboratory plasma by a density modulated electron beam is studied for frequency modulation below, but in the range of, the electron gyrofrequency. When the beam velocity differs from the phase velocity of the whistler at the frequency modulation of the beam, a whistler wave, and, by Čerenkov emission, a beam wave at the same frequency are excited as single poles. When both velocities are equal, a resonance occurs leading to a maximum in the emission of whistler waves. The experiment performed in a magnetized plasma (np≊1011 cm−3, Te≊0.2 eV, B≊100 G) with a modulated electron beam (nb≊109 cm−3, Eb≊300 eV) is in agreement with a simple model which predicts the characteristics of the excited waves.
Nuclei with Z up to 82 emitted in the 208 Pb+ 197 Au reaction at isiab s== 29 MeV per nucleon have been measured as a function of the associated neutron multiplicity. The data reveal the presence of strong correlations between character of a collision and neutron multiplicity. The trends suggest a disassembly of the nuclear system into a large number of nucleons and small fragments in the events with the highest neutron multiplicity. In such events, approximately one-third of the neutrons are released from the system and fragment yields decrease in an exponential fashion with increasing mass.PACS numbers: 25.70.Np, 25.70.Lm Recent experiments 1 studying interactions of intermediate-energy Ar projectiles with massive targets (Au,Th) have shown that, above a bombarding energy of 30 MeV per nucleon, the measured most probable multiplicity of neutrons emitted from the reaction in dissipative collisions remains almost independent of the bombarding energy. Since for heavy systems the neutron multiplicity is a good measure of the total excitation energy introduced into the system, such a "saturation" effect could have represented a manifestation of limits to the temperature attainable by a heavy nucleus. 2 However, in subsequent experiments it has been observed 3 that a further increase in excitation energy per nucleon is possible when heavier projectiles such as Kr are used at similar velocities (E\ 3L \y= s 32 MeV per nucleon). Such a dependence of the limiting temperature on the projectile-target combination suggests that the limits reached in the above experiments are not yet those characteristic of nuclear systems in general but, rather, are a reflection of the influence of the collision dynamics on the efficiency of heat-generation mechanisms.The purpose of the present work is to extend the above systematics to very-heavy-projectile-target combinations and to search for evidence for changes in reaction mechanisms, at the higher temperatures that can presumably be generated in such heavier systems. Therefore, an experiment was carried out at GANIL, using the 29-MeV-per-nucleon beam of Pb, the heaviest projectile available, to bombard a Au target. Charged reaction products, ranging from protons to projectilelike fragments, were measured with an array of different types of detector telescopes, ensuring a wide dynamic range and a broad angular coverage. In the following, charged-fragment data are discussed as obtained with a forward telescope consisting of 200-^m (SE) and 500-jum (E) Sistrip detectors. The 24x24-mm 2 strip detectors (with six strips each) were aligned so as to ensure X-Y position sensitivity of the telescope. The detector covered the angular range from 6.1° to 20°, including the grazing angle of 6.2°. The 200-jum detector imposed a detection threshold of 16-18 MeV per nucleon for fragments with atomic numbers from Z =30 to 82.The sensitive An neutron-multiplicity detector ORION was employed to provide a measure of the number of neutrons produced in a collision. A more detailed description of this d...
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