Probing fission time scales with neutrons and GDR gamma rays

The time scales for nuclear fission have been explored using both pre-and postfission neutrons and GDR gamma rays. Four systems were investigated: 133-MeV ¹⁶O + ¹⁷⁶Yb and ²⁰⁸Pb and 104-MeV ⁴He + ¹⁸⁸Os and ²⁰⁹Bi. Fission fragments were measured in coincidence with PPACs. The neutrons were detected using eight detectors from the DEMON array, while gamma rays were measured using the US BaF2 array. The pre-and postfission gamma rays were determined using moving source fits parallel and perpendicular to the fission fragment emission directions. The time scales for fission for the neutrons were determined using the neutron clock technique. The gamma-ray data were fitted using a statistical model calculation based on the code CASCADE. The results of the fits from both data types were used to extract nuclear friction coefficients, γ, and fission time scales. The γ values ranged from 7 to 20, while the fission times were (31–105)×10^?21 s.     

Instabilities in Self-Focused Electron Beams

The appearance of large amplitude oscillations and attenuation of beam current have been observed in high perveance electrically self-focused beams. These phenomena are shown to be due to instabilities produced by the interaction of the beam with its self-generated plasma. The critical current which must be exceeded for the instabilities to appear and the frequencies of the oscillations have been related to beam and background parameters and the dimensions of the experimental system.     

Predissociation dynamics of Ã-state ammonia probed by two-photon excitation spectroscopy

Electronically excited ND₃(A¯) molecules have been prepared by laser two-photon excitation on theA¯¹A″₂—X¯¹′₁ transition and monitored via their resulting short-lived emission. The earlier observation of Douglas that ND₃(A¯) molecules carrying one quantum of out-of-plane bending vibration ν′₂ are least susceptible to predissociation, is confirmed. ND₃(A¯) predissociation rates are found to be both vibronic and rovibronic level dependent. Both observations may be understood by considering the likely form of the potential energy surface for ND₃(A¯) molecules in the region of the D₂N—D dissociation coordinate. At short D₂N—D separations this surface exhibits a barrier. The presence of a conical intersection (involving the ND₃ ground state surface) further out along the dissociation coordinate has a crucial influence on the magnitude of this barrier. The envisaged form of theA¯-state potential energy surface also provides a qualitative rationale for all previous experimental findings concerning electronic branching ratios and energy disposal amongst the primary photofragments arising in the photodissociation ofA¯-state ammonia.     

Organolithium addition to 5,6-disubstituted pyrimidines

The addition of organolithium reagents to the azomethine bond of 5,6-disubstituted pyrimidines provided, in every case, a single adduct. When reacted with either ethyl chloro-formate or hydrogen chloride, these adducts gave a single dihydro pyrimidine. The resulting N-H dihydro pyrimidines were oxidized with potassium permanganate to the corresponding pyrimidines.