Search

We have calculated the fission probabilities for 237-Np, 233,235,238-U, 232-Th, and nat-Pb following the absorption of photons with energies from 68 MeV to 3.77 GeV using the RELDIS Monte-Carlo code. This code implements the cascade-evaporation-fission model of intermediate-energy photonuclear reactions. It includes multiparticle production in photoreactions on intranuclear nucleons, pre-equilibrium emission, and the statistical decay of excited residual nuclei via competition of evaporation, fission, and multifragmentation processes. The calculations show that in the GeV energy region the fission process is not solely responsible for the entire total photoabsorption cross section, even for the actinides: ~55-70% for 232-Th, \~70-80% for 238-U, and ~80-95% for 233-U, 235-U, and 237-Np. This is because certain residual nuclei that are created by deep photospallation at GeV photon energies have relatively low fission probabilities. Using the recent experimental data on photofission cross sections for 237-Np and 233,235,238-U from the Saskatchewan and Jefferson Laboratories and our calculated fission probabilities, we infer the total photoabsorption cross sections for these four nuclei. The resulting cross sections per nucleon agree in shape and in magnitude with each other. However, disagreement in magnitude with total-photoabsorption cross-section data from previous measurements for nuclei from C to Pb calls into question the concept of a ``Universal Curve'' for the photoabsorption cross section per nucleon for all nuclei.

Neutron multiplicities after antiproton-induced fission have been determined as a function of the total mass of the fission fragments. Pre- and postscission neutron multiplicities of the reaction {bar p}+{sup 238}U have been deduced using the moving-source parametrization of the neutron energy spectra by Maxwellian distributions. A postscission multiplicity of 6.3(6) neutrons out of 20.0(10) in total emitted neutrons in fission indicates that fission is a rather slow process. The increase of the evaporative prescission neutron multiplicity is about 80{percent} of the total increase of evaporative neutrons as a measure of excitation energy. The postscission multiplicity gets only 20{percent} of the total increase, thus indicating that the nucleus has only a moderate excitation energy at the moment of scission. The results are compared with standard cascade-evaporation calculations predicting a much lower number of evaporated prescission neutrons as the experiment shows. This is a strong indication of the importance of dissipative effects in the fission process. Application of dissipative effects like a saddle-to-scission delay time and nuclear viscosity in the model calculation describes the experimental results much better. {copyright} {ital 1997} {ital The American Physical Society}

Information on the neutron distribution in the nuclear periphery was obtained by the annihilation of stopped antiprotons and the yield of residual nuclei. The last atomic transitions of the antiproton before annihilation gives complementary results. Properties of very hot nuclei (up to 1 GeV) after annihilation of stopped antiprotons were studied by neutron emission and fission. Absolute prob­ abilities of fission induced by stopped and fast antiprotons were determined. The experimental data are compared with elaborate calculations taking into account the annihilation process, the fast cascade and pre-equilibrium emission, thermalisation, particle evaporation and fission.

We present preliminary results of calculations by the phenomenological model of the estimated yield of some heavy actinide isotopes. It is assumed that these isotopes are produced as a result of multinucleon transfers followed by neutrons and charged particle emission A.S. Iljinov and E.A. Cherepanov (Proc. Int. Symp. on Synthesis and Properties of New Elements, Dubna, 1980, Joint Institute for Nuclear Research, Dubna, 1980, D7-80-556, p. 29). The yield P Z N (E∗) of primary excited actinides is found using the model of N.V. Antonenko and R.V. Jolos (Z. Phys. A, 338 (1991) 423). Absolute cross-sections for different binary reaction channels are obtained by summing the cross-sections for all subchannels with an appreciable yield according to J. Wilczynski et al. (Phys. Rev. Lett., 45 (1980) 606).

The double strangeness production has been observed in two final states of annihilation of antiprotons at momentum less than 0.9 GeV/c on Xe nuclei: K+K+X (8 events) and K+K0ΛX (6 events). The probabilities of the reaction pXe → K+K+X vary from 2 · 10−5 (at rest) up to 7 · 10−5 (in flight). The reaction pXe → K+K0ΛX is observed only in flight with probability 3 · 10−4. The properties of the observed reactions are similar to those resulting from the cascade process with production of Ξ hyperon: pN → K∗−K∗, K∗ → Kπ, −K∗N → ΞK, ΞN → ΛΛ. The new upper limit on the production probability of the stable H(S = −2) dibaryon in the reaction Xe → K+K+H(H → Σ− p)X was obtained to be < 2 · 10− (90%C.L.).

We study nuclear reactions induced by virtual photons associated with Lorentz-boosted Coulomb fields of ultrarelativistic heavy ions. Evaporation, fission and multifragmentation mechanisms are included in a new RELDIS code, which describes the deexcitation of residual nuclei formed after single and double photon absorption in peripheral heavy-ion collisions. Partial cross sections for different dissociation channels, including the multiple neutron emission ones, are calculated and compared with data when available. Rapidity and transverse momentum distributions of nucleons, nuclear fragments and pions, produced electromagnetically, are also calculated. These results provide important information for designing large-rapidity detectors and zero-degree calorimeters at RHIC and LHC. The electromagnetic dissociation of nuclei imposes some constrains on the investigation of exotic particle production in gamma-gamma fusion reactions., Comment: 26 LaTeX pages including 8 figures, uses epsf.sty

We study the disintegration of nuclei by strong electromagnetic fields induced by ultrarelativistic heavy ions. The proposed multi-step model includes 1) the absorption of a virtual photon by a nucleus, 2) intranuclear cascades of produced hadrons and 3) statistical decay of the excited residual nucleus. The combined model describes well existing data on projectile fragmentation at energy 200 GeV per nucleon. Electromagnetic multifragmentation of nuclei is predicted to be an important reaction mechanism at RHIC and LHC energies., Comment: 18 LaTeX pages including 4 figures, uses epsf.sty. Submitted to Phys.Rev. C

The $\omega$- and $\rho$-resonance production and their dileptonic decay in $\pi^- A$ reactions at GSI energies are calculated within the intranuclear cascade (INC) approach. The invariant mass distribution of the dilepton pair for each resonance is found to have two components which correspond to the decay of the resonances outside and inside the target nucleus. The latter components are strongly distorted by the nuclear medium due to resonance-nucleon scattering and a possible mass shift at finite baryon density. These medium modifications are compared to background sources in the dilepton spectrum from $\pi N$ bremsstrahlung and the Dalitz decays of $\Delta$'s and $\eta$ mesons produced in the reaction., Comment: 10 pages, LaTeX, plus 4 postscript figures, submitted to Phys. Lett. B

The intranuclear cascade (INC) model for photonuclear reactions has been extended at energies up to 10 GeV. To this end a phenomenological method for exclusive description of the elementary γN-interaction has been developed taking into account both the resonance contribution from the two-body channels γN → πB ∗ and γN → M ∗ N ( B ∗ and M ∗ being baryon and meson resonances) and the non-resonant statistical contribution from multibody channels γN → iπN (2 ⩽ i ⩽ 8). This method well describes a wide range of γN data up to Eγ ∼ 10 GeV. The INC model takes into account rescattering and absorption in a nucleus of pions, nucleons, and η-, ϱ-, ω-mesons generated in the γN-interaction. The evaporation, fission and multifragmentation of highly excited residual nucleus have also been taken into account. As a result of the Monte Carlo simulation an event of γA-reaction is generated which contains mesons, nucleons, nuclear fragments, fission fragments and final nuclide. The model well describes the available data on meson production and proton emission obtained with quasi-monochromatic photons.

Simultaneous measurements of inclusive energy spectra and multiplicities of ${\mathrm{\ensuremath{\pi}}}^{\ifmmode\pm\else\textpm\fi{}}$, ${\mathit{K}}^{\ifmmode\pm\else\textpm\fi{}}$, n, p, d, and t following antiproton annihilation on nuclei over a wide energy range and in the case of neutrons down to the evaporative part of the spectra are reported. Thirteen targets in the mass range of A=12--238 were used in a target mass dependent investigation of the fast stage of the antiproton-nucleus interaction. The deduced transferred, preequilibrium and equilibrium excitation energies agree very well with the dynamical picture drawn by the intranuclear cascade model (INC). Ratios of directly emitted neutrons to protons have been determined to be about twice the N/Z ratio in the target nucleus nearly independently of its mass. These unexpected values for this new sensitive observable are not completely understood in the standard framework of INC. Possible effects of isospin and nucleon densities as well as further schemes beyond the INC are discussed.

The photofission cross section of Au was determined in the energy range 100--300 MeV by means of a quasimonochromatic photon beam. The nuclear fissility ${P}_{f}$ was calculated using the recently measured total photoabsorption cross sections. The nuclear excitation energy ${E}^{\mathrm{*}}$, charge and mass of compound nucleus were obtained by means of an intranuclear cascade Monte Carlo calculation. The fissility values determined for Au, Bi, and U were compared with the predictions of the cascade-evaporation model and remarkably fitted by the calculation.

The annihilation of energetic (1.2 GeV) antiprotons is exploited to deposit maximum thermal excitation (up to 1000 MeV) in massive nuclei (Cu, Ho, Au, and U) while minimizing the contribution from collective excitation such as rotation, shape distortion, and compression. Excitation energy distributions {ital d}{sigma}/{ital dE}{asterisk} are deduced from eventwise observation of the whole nuclear evaporation chain with two 4{pi} detectors for neutrons and charged particles. The nuclei produced in this way are found to decay predominantly statistically, i.e., by evaporation. {copyright} {ital 1996 The American Physical Society.}