From event-by-event records of observed photoneutron multiplicities for photons from 30 to 140 MeV on several heavy targets (Sn, Ce, Ta and Pb), it was possible to determine the mean number of photoneutrons, gn, for each photon energy and the widths W of the multiplicity distributions. The mean neutron numbers increase smoothly from about three to six over the photon energy span for all four targets. The widths go from about one to two neutrons in the same interval. When these measurements are combined with other photonuclear information, it is possible to extract the average numbers of fast neutrons and fast protons and the average number of evaporation neutrons emitted per photoabsorption.
The study of photon-induced reactions in collisions of heavy nuclei at RHIC and the LHC has become an important direction of the research program of these facilities in recent years. In particular, the production of vector mesons in ultra-peripheral collisions (UPC) has been intensively studied. Owing to the intense photon fluxes, the two nuclei participating in such processes undergo electromagnetic dissociation producing neutrons at beam rapidities. Here, we introduce the (pronounced noon) Monte Carlo program, which generates events containing such neutrons. is a ROOT based program that can be interfaced with existing generators of vector meson production in UPC or with theoretical calculations of such photonuclear processes. can also be easily integrated with the simulation programs of the experiments at RHIC and the LHC.
External routines: The generator is based on ROOT.
Nature of problem: The electromagnetic fields of nuclei at RHIC and the LHC can be described as a flux of quasi-real photons. These photons may interact with one of the nucleus in the opposite beam. There are events of interest where two independent interactions occur, one involving a hard scattering and one from the exchange of soft photons. As a result of the latter, the nucleus get excited and upon de-excitation it may emit neutrons, which are boosted to beam rapidities. The program computes the probability of neutron emission based on existing measurements and some mild modelling; it then generates neutrons in a per-event basis.
Solution method: The break-up probabilities are computed using existing data and stored in ROOT objects (graphs and histograms). Photon energies from the accompanying hard process, e.g. vector meson production, are loaded into the program and a catalogue of specific break-up probabilities is constructed. The number of neutrons emitted in the event is generated and the neutrons are produced and boosted into the laboratory frame. The output is a TTree with a TClonesArray of TParticles per event, which can be easily interfaced to the simulation programs of the RHIC and LHC collaborations.
Restrictions: At the moment only emission from Pb is available.
The average photoneutron muliplicities of Au, Ta and 182W were deduced from their previously measured excitation energies , from 160 to 250 MeV. A combined analysis of these data and those measured at Saclay up to 140 MeV allowed the extraction of information on at the “pure evaporation” and quasideuteron energy regions. A theoretical approach for the study of above 140 MeV, which incorporates photopion reabsorption processes by two-body, was proposed, allowing a tentative delineation of the pion mean free path in the nucleus.
The cross sections for nine separate 232Th(γ, xnyp) reactions over the energy range 40–150 MeV have been determined from the residual α-activity of their reaction products after electron and bremsstrahlung irradiation. The results are compared with the intranuclear cascade and hybrid exciton models for pre-equilibrium nuclear decay.
The absolute photofission cross sections for 232Th and 235, 238 U has been studied from 20 to 110 MeV using the photon tagging technique at the Saclay linear electron accelerator (ALS). The fission fragments were directly detected using a parallel plate avalanche detector (PPAD). The basic experimental results are a set of data giving σ(Eγ, fission). These results are compared with the few other measurements performed with monochromatic photons. From a comparison of the present fission data with the total absorption cross section, the total fission probability was deduced. The behaviour of this probability was discussed in terms of a cascade model for fission-evaporation competition. Quite good agreement is found with a calculation based on the hybrid model of pre-equilibrium reactions.
Absolute photofission cross sections of 235,238U have been measured in the energy range 40–105 MeV with “tagged” photons, using parallel plate avalanche counters for a direct detection of the fission fragments. The measured cross sections are nearly constant in the investigated energy range and amount to about 17 mb for both isotopes. The results are compared with recent photoneutron data.