Evidence for a supershort fission mode in the reactions "2"4"2"mAm(n_t_h,f) and "2"4"4Cm(sf)

Evidence for a new fission mode has been found in the mass-energy distribution of fragments as from the thermal neutron induced fission of "2"4"2"mAm as from the spontaneous fission (sf) of "2"4"4Cm. In both data sets, which were measured using different methods, a couple of events is grouped around the mass numbers A_L_F=81/82 with total kinetic energies (TKE) near the Q-value of the reaction. The yield of these events amounts to several units times 10"-"5 of all fissions. The mass of this light fission fragment nearly corresponds to one third of the mass of the fissioning nucleus. An attempt is made to understand this mode in the framework of a cluster description of the fission process. Within this model, a strong influence of the magic neutron number N=50 may cause a clusterization into three prefragments of nearly equal mass. One of them escapes and leads to the effect observed. (orig.)Available from TIB Hannover: RR 1847(98) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Spontaneous fission of "2"4"4Cm studied by the two-velocities method at the FOBOS spectrometer

The two - velocities method was used to measure the mass - energy distribution of fission fragments (FF) originating from the spontaneous fission (sf) of "2"4"4Cm. Especially, the energy regions of cold compact (CCF) and cold deformed fission (CDF) have been investigated. The FF mass distributions of CCF-and CDF products are basically different for spontaneous and thermal neutron induced fission of Cm isotopes. Evidence for the existence of a supershort fission mode has been observed. (orig.)SIGLEAvailable from TIB Hannover: RR 1847(95-99) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

The 4#pi#-fragment-spectrometer FOBOS

The 4#pi# - fragment - spectrometer FOBOS developed for heavy-ion research at beam energies of 10 + 100 AMeV has been commissioned for physical experiments at the Flerov Laboratory of Nuclear Reactions of the Joint Institute for Nuclear Research in Dubna. Based on the logarithmic detector principle, it is able to register charged fragments from protons up to heavy residual nuclei in a large dynamical range. Position-sensitive avalanche counters, axial ionization chambers and CsI(Tl) scintillation detectors are arranged in three concentric detector shells. An array of phoswich detectors is used as a more granular forward detector at narrow polar angles. The modular concept of FOBOS allows for different experimental application in the field of exclusive fragment spectroscopy at medium multiplicities. For illustration the fragment spectroscopy studies concerning the spontaneous fission process and the fragmentation of hot nuclei by means of the FOBOS set-up are considered. (orig.)Available from FIZ Karlsruhe / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEBundesministerium fuer Bildung, Wissenschaft, Forschung und Technologie, Bonn (Germany)DEGerman

The COMBAS fragment separator of radioactive nuclei and the FOBOS 4#pi#-detector for charged particles

The projectile-like fragment separator COMBAS is being designed at the Flerov Laboratory of Nuclear Reactions, JINR Dubna, for providing radioactive nuclear beams. COMBAS is a compact achromatic beam line with a high resolving power of 4360. It accepts fragments within 6.4 msr solid angle and with a momentum spread of 10%. The method of isotopic separation is based on a combination of magnetic rigidity and energy loss analysis. The separated radioactive beam is planned to be transported either into a time-projection chamber in the regime gas target - gas detector or to a secondary target positioned in the centre of the FOBOS 4#pi#-array or to another set-up. The FOBOS detector is intended for heavy ion reaction studies in the bombarding energy range of 10-100 AMeV at the cyclotron U-400M of the FLNR. Presently, only primary cyclotron beams are used because COMBAS is not yet complete. FOBOS consists of a gas-detector ball of 30 position-sensitive avalanche counters and 30 axial ionization chambers behind them and an outer scintillator shell of 210 CsI(Tl) counters surrounding the gas detectors. An array of 96 phoswich counters cover the very forward angles. All charged reaction products can be measured in a wide dynamic range and in a geometry convering a substantial part of 4#pi#. First data have been taken concerning fission and emission of intermediate-mass fragments in the reactions "7Li (43 AMeV) on "2"3"2Th and "1"4N (34 AMeV) on "1"9"7Au. (orig.)15 refs.SIGLEAvailable from TIB Hannover: RR 1847(103) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekBundesministerium fuer Bildung, Wissenschaft, Forschung und Technologie, Bonn (Germany)DEGerman