Search

We have for the first time identified e + e − -pairs produced by strongly varying electromagnetic fields in distant SPt collisions at 200 GeV/u projectile energy at the CERN-SPS. The differential cross section for the mass range 10 MeV/ c 2 ≤ m ee ≤ 100 MeV/ c 2 and the polar angle range 141 mrad ≤ θ e ≤ 260 mrad agrees well with a lowest-order perturbation QED-calculation.

UV photons from a Cherenkov radiator are multiplied in a multistep avalanche chamber operating in a gated mode at low gas pressure (40 Torr). The gas mixture is C 2 H 6 -argon ( 80 20 ) and TMAE at 34°C. Visible light emitted from single photoelectron avalanches is detected by a CCD camera coupled to an image intensifier system. The detector was tested with 5 GeV c electrons, using a CH 4 radiator gas at 1 atm. Cherenkov rings essentially free of particle background and of secondary photon feedback were obtained in this mode of operation with a mean number n ≅ 11.5 ( N 0 ≅ 76 cm −1 ). We present this new method and discuss its performance.

We have detected in nine different positions of space the neutrons associated with the collision of 63 Cu on 197 Au at 400 MeV bombarding energy. The deep inelastic products were detected at two different angles: close to the grazing angle and 30° forwards of it. Their masses were measured using a time-of-flight technique. The neutrons were detected in coincidence with the fragments - the efficiency of the neutron detectors was measured relatively to a 252 Cf source during beam time. The neutron threshold was set at 300 keV. Within an accuracy of 10% all the emitted neutrons are evaporated by the fully accellerated deep inelastic fragments. It is shown that the excitation energy is shared between the fragments in proportion to their masses and that the relaxation time for internal equilibration of the composite system is very short (∼10 −22 s).

The angular distribution of gamma rays excited by both (p, n) and (α, n) reactions leading to the final nuclei 59 Ni, 63 Zn and 68 Ga have been investigated. The observed angular distributions are consistent with those theoretically predicted by the compound nucleus statistical model whenever the spin sequence is previously known. The (p, n) and (α, n) reactions yield identical values of the multipole mixing ratios for all the transitions. The variation of the gamma-ray angular distributions with incident energy following the (p, n) reaction is correctly predicted by the model. The energies of the low-lying levels of 68 Ga have been accurately determined, and the first excited state at 175 keV excitation is found to have a spin 2 (+) . The results indicate that the level at 375 keV is probably a doublet.

The angular distribution of gamma rays excited by both the (p,n) and (α,n) reactions leading to the final nuclei 63 Zn and 68 Ga have been measured. The results have been compared with the compound nucleus statistical model and it is found that: a) the correct spin for the 190 keV level of 63 Zn is uniquely determined; b) the (p,n) and (α,n) reactions yield identical values for the multipole mixing ratios; c) the variation of the gamma-ray angular distributions with incident energy, following the (p,n) reaction, is correctly predicted by the model.