D. Hilscher

Neutron emission in the reactionHo165+Fe56atElab=8.5MeV/u

Neutron emission and ..cap alpha..-particle emission in the /sup 165/Ho + /sup 56/Fe reaction have been measured for the damped-collision and fusion-fission components of this reaction at a bombarding energy of 8.5 MeV/u. The c.m. neutron energy spectra for damped collisions indicate equal temperatures for the light and heavy fragments, a result consistent with the equilibration of the excitation energy during the interaction time. Pre-equilibrium neutron emission is found to contribute less than 5%. An out-of-plane neutron anisotropy A/sub 2/ = 0.2 +- 0.1 is found. The measured ratios of the neutron multiplicities for the light and heavy fragments suggest a rapid approach of the fragment N/Z ratios during the collision to that corresponding to minimum potential energy of the composite system.

A combination of two 4π detectors for neutrons and charged particles.: Part I. The Berlin neutron ball—a neutron multiplicity meter and a reaction detector

Abstract Two 4π detectors for neutrons and charged particles have been combined for the investigation of GeV-proton and antiproton induced spallation reactions. It is a large neutron scintillator tank with a high-granularity charged-particle detector in its central vacuum chamber. Together they register the whole, neutral and charged, de-excitation cascade of the excited reaction complex as well as heavier reaction products, i.e. intermediate-mass and fission fragments and heavy residues. Part I of our communication describes the neutron detector Berlin neutron ball (BNB): Its spherical tank is filled with 1.5 m 3 scintillator liquid, loaded with 0.4 wt % Gd. BNB recognizes each reaction in the target with a low inelasticity threshold of 2.5 MeV ee and records event-wise the multiplicity of evaporated neutrons with ≈75% efficiency. Part II [12] describes the charged particle detector Berlin silicon ball (BSiB) assembled from 151 independent Si-detectors and six ancillary four-fold telescopes, considers its detection efficiency for various particle species and provides experience gained from the joint operation of both detectors, BNB and BSiB, as a filter on the excitation energy dissipated event-by-event in the spallation reaction.

Neutron yields of 6,7Li-induced reactions on thick 7Li-, 9Be-, 12C-, and natCu-targets

Abstract Neutron yield spectra were measured for 40 MeV 6,7 Li and deuterons on thick targets of 7 Li, 9 Be, 12 C, and nat Cu. The time-of-flight technique was employed and data were taken at angles between 0° and 120°. The neutron yield with energies larger than 1 MeV was obtained and integrated to get the total neutron yield. At 0° the 40 MeV 6,7 Li induced reactions on 9 Be and 7 Li result in neutron yields which are 50 to 70 times smaller and in mean neutron energies which are 2.5 times smaller than for 40 MeV deuteron induced reactions on 9 Be and 7 Li.

A combination of two 4π-detectors for neutrons and charged particles.: Part II. The Berlin silicon ball BSiB for light- and heavy-ion detection

Abstract A combination of two 4π-detectors for neutrons and charged particles is employed for the event-by-event determination of the thermal excitation energy of nuclei produced in spallation reactions. Here the highly efficient charged particle detector BSiB is described while the neutron detector is presented in the preceding paper (Jahnke et al., Nucl. Instr. and Meth. A, this issue). The Berlin Silicon Ball (BSiB) consists of 162 500 μm thick silicon detectors forming a self-supporting complete sphere with a diameter of 20 cm . By means of time of flight and energy H, He, IMFs, FFs, and HRs are identified. The lower detection thresholds are about 2 MeV for all detectors and about 0.5 MeV for 16 selected detectors in the forward direction. In addition, 6 ancillary four-fold detector-telescopes are employed with isotopic resolution up to C, and Z-identification up to Na. The geometric efficiency of BSiB has been determined experimentally to 80.4±1.0% for the standard configuration with 151 mounted detectors. Efficiency losses due to energy thresholds, target shadow, multiple hits and reaction kinematics are calculated for protons, helium, fission fragments, and heavy residues released in 1.2 and 2.5 GeV proton-induced spallation reactions on Fe and Au. The results are supported by experimental data for binary fission, which have been analysed as a function of excitation energy.

Light Particle Production in Spallation Reactions Induced by Protons of 0.8-2.5 GeV Incident Kinetic Energy

Absolute production cross sections have been measured simultaneously for neutrons and light charged particles in 0.8-2.5 GeV proton induced spallation reactions for a series of target nuclei from aluminum up to uranium. The high detection efficiency both for neutral and charged evaporative particles provides an event-wise access to the amount of projectile energy dissipated into nuclear excitation. Various intra nuclear cascade plus evaporation models have been confronted with the experimental data showing large discrepancies for hydrogen and helium production.

A new method to measure prescission neutrons in heavy-ion induced fission

Abstract Neutrons emitted in fusion-fission reactions were used to study the response of a 4π-neutron detector. This detector counts the total number of neutrons emitted in each reaction. Since the spherical scintillator tank of this detector is divided perpendicular to the beam axis into two identical half-spheres it is possible to count the number of captured neutrons in each half sphere separately. Employing the multiplicity in each half together with a Monte Carlo simulation of the detector it is shown that the forward to backward ratio of neutrons can be used to determine the number of neutrons emitted prior to scission. This yields (i) a new and efficient method for measuring prescission neutrons in fusion-fission-like reactions and (ii) the possibility to tag events with a large prescission neutron multiplicity.

Experimental Investigation of Neutron Generation in Thick Target Blocks of Pb, Hg and W with 0.4 to 2.5 GeV Proton Beams

Detailed experimental neutron data relevant to the design of the target station of neutron spallation sources have been gathered by the NESSI-collaboration at the COSY accelerator in FZ Jülich. Numerous neutron multiplicity distributions and reaction probabilities have been measured for 0.4 to 2.5 GeV protons bombarding highly segmented target blocks from Pb, Hg and W of up to 35 cm in length and 15 cm in diameter with the intention to provide a comprehensive data base for the improvement and validation of existing reaction simulation codes.

Neutron emission in the reaction /sup 165/Ho + /sup 56/Fe at E/sub lab/ = 8. 5 MeV/u

Neutron emission and ..cap alpha..-particle emission in the /sup 165/Ho + /sup 56/Fe reaction have been measured for the damped-collision and fusion-fission components of this reaction at a bombarding energy of 8.5 MeV/u. The c.m. neutron energy spectra for damped collisions indicate equal temperatures for the light and heavy fragments, a result consistent with the equilibration of the excitation energy during the interaction time. Pre-equilibrium neutron emission is found to contribute less than 5%. An out-of-plane neutron anisotropy A/sub 2/ = 0.2 +- 0.1 is found. The measured ratios of the neutron multiplicities for the light and heavy fragments suggest a rapid approach of the fragment N/Z ratios during the collision to that corresponding to minimum potential energy of the composite system.

A new method to measure neutron polarizations with energy discrimination

Abstract A new type of polarimeter for fast neutrons is described. The asymmetry in n-α scattering taking place between the plates of a henlium-filled parallel plate avalanche counter is determined by measuring the pulse height distribution caused by the recoil α-particles. Due to the mechanism of avalanche formation the pulse height depends strongly on the recoil direction, therefore the distributions differ for left and right scattering. The fast rising avalanche pulse allows the determination of neutron energies by time-of-flight measurements enabling one to measure polarizations for various neutron groups simultaneously.