H. Freiesleben

A Large area detector for high-energy neutrons

Abstract We present design studies, results of test measurements, and Monte Carlo simulations which served as a basis for the realization of a large area neutron detector (LAND). It has a front area of 2m×2m and a depth of 1 m, and features a multilayer structure of passive converter and active scintillator material. The detector is subdivided in independently operating paddles which allow time-of-flight and position measurement. An energy resolution of ΔT n / T n =5.3% for a flight path of 15 m and an overall detection efficiency of ϵ ≈ 1 is anticipated for neutrons with T n ≈ 1 GeV. The operation of LAND at the SIS facility of GSI is described.

Neutral-pion photoproduction off protons in the energy range 0.3 GeV<E(gamma)<3 GeV.

Single pi(0) photoproduction has been studied with the CB-ELSA experiment at Bonn using tagged photon energies between 0.3 and 3.0 GeV. The experimental setup covers a very large solid angle of approximately 98% of 4pi. Differential cross sections dsigma/dOmega have been measured. Complicated structures in the angular distributions indicate a variety of different resonances being produced in the s channel intermediate state gammap-->N(*)(Delta(*))-->ppi(0). A combined analysis including the data presented in this letter along with other data sets reveals contributions from known resonances and evidence for a new resonance N(2070)D15.

Photoproduction of eta mesons off protons for 0.75 GeV<E(gamma)<3 GeV.

Total and differential cross sections for the reaction p(gamma, eta)p have been measured for photon energies in the range from 750 MeV to 3 GeV. The low-energy data are dominated by the S11 wave which has two poles in the energy region below 2 GeV. Eleven nucleon resonances are observed in their decay into p eta. At medium energies we find evidence for a new resonance N(2070)D15 with (mass, width) = (2068+-22, 295+-40) MeV. At photon energies above 1.5 GeV, a strong peak in forward direction develops, signalling the exchange of vector mesons in the t channel.

Photoproduction of eta mesons off protons for 0.75 GeV

Total and differential cross sections for the reaction p(gamma, eta)p have been measured for photon energies in the range from 750 MeV to 3 GeV. The low-energy data are dominated by the S11 wave which has two poles in the energy region below 2 GeV. Eleven nucleon resonances are observed in their decay into p eta. At medium energies we find evidence for a new resonance N(2070)D15 with (mass, width) = (2068+-22, 295+-40) MeV. At photon energies above 1.5 GeV, a strong peak in forward direction develops, signalling the exchange of vector mesons in the t channel.

The reaction mechanism in the system132Xe+56Fe at 5.73 MeV/u: Evidence for a new type of strongly damped collisions

We present data for the132Xe+56Fe-system at 5.73 MeV/u laboratory energy. Due to inverted kinematics, where Xe is the projectile, we were able to measure energy spectra as well as angular distributions for reaction products with 20≦Z≦60 with unitZ-resolution; i.e. target- and projectile-like fragments have been investigated. The reaction shows a well focussed quasielastic component, where charge transfer from the light to the heavy collision partner dominates. This apparent tendency towards more asymmetric fragmentation is explained by a potential energy surface which favours such charge transfer in order to minimize the asymmetry energy of the liquid drop. The strongly damped component which constitutes the major part of the reaction cross section exhibits characteristics of a fusion-fission reaction with typical fission fragment kinetic energies and 1/sinΘc.m. angular distributions.The maximum cross section is found for the symmetric fragmentation, no clear indication is observed for a diffusion process leading to target- and projectile-like fragments. Our data are difficult to reconcile either with the standard diffusion models or with an equilibrated compound nucleus fission picture. We tentatively conclude that an essential part of the fully damped cross section originates from partial waves for which the compound nucleus has no fission barrier.

Longitudinal and transverse momentum distributions of 9Li fragments from break-up of 11Li

Transverse and longitudinal momentum distributions of Li-9 fragments from Li-11 break-up reactions in C, Al and Pb targets have been measured at 280 MeV/u. The two-neutron removal cross-section was measured to be sigma(-2n), = 0.26 +/- 0.02 b for the carbon target, sigma(-2n) = 0.47 +/- 0.08 b for the aluminum target and sigma(-2n), = 1.9 +/- 0.4 b for the lead target. No significant difference is observed between the narrow widths (FWHM approximate to 47 MeV/c) of the transverse and longitudinal momentum distributions of the Li-9 fragments. The physical implications of this are discussed.

Response of BaF2, CsI(Tl) and Pb-glass detectors to neutrons below 22 MeV

Abstract The properties of BaF 2 , CsI(Tl) and Pb-glass for neutron detection were studied using neutrons of kinetic energies up to 22 MeV produced in deuteron induced reactions on a thick LiF target. The neutron absorption length was extracted from a neutron transmission experiment. The absolute efficiency for neutron detection was determined by comparing neutron energy spectra (obtained from time-of-flight) with that of a calibrated liquid scintillator detector. For BaF 2 the detection efficiency as a function of the threshold on the light-output signal was studied as well as the possibility of pulse-shape discrimination between signals induced by γ-rays and neutrons. It is found that with increasing energies a detection mechanism comes into play which is different from the (n,γ) mechanism predominant at lower energies, and which might allow a (partial) discrimination of γ and neutron induced events at higher neutron energies.

The influence of the potential energy surface on neutron excess and mass equilibration in the 136Xe + 56Fe system

Abstract Using magnetic spectrometer first and second moments of the nuclide distributions of Xe-like reaction products from 5.9 MeVu 136Xe on 56Fe were measured as a function of excitation energy. The drift path in the N versus Z plane follows the gradient of the two-dimensional potential energy surface (PES). Qualitatively the reaction process can be understood as a stochastical PES-governed exchange of nucleons.

Energy dependence of collective flow of neutrons and protons in197Au+197Au collisions

We investigate the beam energy dependence of neutron and proton squeeze-out in collisions of197Au+197Au atE/A=400—800 MeV. The azimuthal anisotropy that describes the enhanced emission of mid-rapidity neutrons perpendicular to the reaction plane rises strongly with the transverse momentum of the neutrons. This dependence of the azimuthal anisotropy follows a universal curve — independent of beam energy — if the neutron momenta are measured in fractions of the projectile momentum per mass unit. Analogously, the kinetic energy spectra of mid-rapidity neutrons exhibit a universal behaviour as a function of the kinetic energy of the projectile.

Time-of-flight measurement of the evaporation residues from fusion of 5.9 mev/u 84kr and 27al

Abstract Mass and Z -distributions of the evaporation residues from compound-nucleus formation in the reaction 5.9 MeV/u 84 Kr on 27 Al were measured using a time-of-flight ΔE − E telescope, which is described in detail. The high recoil velocity attained by choosing the heavy reaction partner as projectile made it possible to resolve the reaction products by mass and atomic number. Data were taken in the angular range from 1.5 to 6°. The residue distributions are compared to evaporation calculations assuming the statistical decay by fission and particle evaporation of the compound nucleus 111 Mn formed at an excitation energy of 108 MeV with angular momenta up to L CN ≈ 69 h . The data are consistent with the assumption of statistical equilibrium. Details of the de-excitation process, in particular the fission competition and the influence of nuclear deformations at high angular momenta, are discussed.