V. G. Lyapin

Breakthrough in pulse-shape based particle identification with silicon detectors

Identification of charged particles is an important method in nuclear spectroscopy. We have achieved a major breakthrough that makes the pulse-shape discrimination (PSD) method with a single solid-state detector comparable to and sometimes better than the traditional telescope technique. By using rear-side injection in over-biased surface barrier n-type Si detectors made from homogeneously doped n-TD silicon, and extracting the pulse-shape information already at the preamplifier level we have reached improved Z and even A discrimination over a wide dynamic range. Previously good separation with the PSD technique required a major degradation of time resolution and inferior energy resolution. Currently we have pushed down the dynamical time range to below 35 ns and reached time resolution of about 200 ps fwhm while maintaining good energy resolution characteristic of silicon detectors. The lowest energy threshold for Z separation of intermediate mass fragments (IMF) achieved with a 250 /spl mu/m thick detector is equivalent to a range of about 20 /spl mu/m in silicon. For IMFs with ranges higher than 80 /spl mu/m of silicon we got full isotope separation. Details of this study are presented, and the application of our method in recent nuclear physics experiments is briefly discussed.

Neutron multi-detector system: mutual influence of its modules

Abstract A position-sensitive neutron detector (PSND) has been designed and tested as a module of a multi-detector array. “Cross-talk” effects, contributing a major distortion factor for all closely packed neutron detector systems, have been measured. It was established, using neutrons from the spontaneous fission of 252 Cf (setting neutron energy threshold at 0.7 MeV), that the distortions of neutron energy and of neutron angular distribution are insignificant (within the level of statistical accuracy). The influence of cross-talk on the coincidence counting rate was measured to range from 48% to 16% for distances between detectors axes of 12 to 24 cm.

Neutron production in collisions between carbon nuclei of energy 2 GeV per nucleon and carbon, aluminum, copper, cadmium, and lead nuclei

Double-differential cross sections for neutron production were measured by the time-of-flight method for the interactions between carbon nuclei of energy 2 GeV per nucleon and carbon, aluminum, copper, cadmium, and lead nuclei. These measurements were performed for angles of 30°, 53°, and 90° in the neutron-energy range fromseveral hundred keVunits to 300MeV. The phenomenologicalmodel of four moving sources was used as a basis in analyzing experimental results and in estimating the contribution to neutron emission from various reaction stages. The temperature parameters determined from the slope of the neutron energy spectra proved to be 22 ± 2 MeV for a hot source (fireball) and 4.5 ± 0.3 MeV for the stage of thermal fragmentation of highly excited heavy nuclear residues. The relative contribution of these two sources to the total neutron yield is independent of the type of the target nucleus and is about 42%, on average.

Neutron production in the interaction of 2-GeV protons with nuclei

The double-differential cross sections for neutron production in the interactions of 2-GeV protons with Be, Al, Cu, Cd, and Pb nuclei were measured by the time-of-flight method in the region of angles larger than 30°. The respective experimental data are analyzed within the phenomenological model of four moving sources, including those associated with neutron emission in primary nucleon-nucleon collisions, the decay of a hot source (fireball), the multifragmentation process, and the deexcitation of nuclear fragments via neutron evaporation. Temperature-parameter values are universal for all sources and are virtually independent of the target nucleus and of the projectile energy in the region above 0.5 GeV. It is found that, for all of the above reactions, the relative contribution to the mean neutron multiplicity from the decay of a hot source and multifragmentation is about 41%.

Searching for Rare Decay Modes in the Reaction 238U+4He (40 MeV)

In the present paper we describe an experiment performed on the reaction 238U + 4He (40 MeV) using a two‐arm TOF‐E (time‐of‐flight vs. energy) spectrometer with micro‐ channel plate detectors and mosaics of PIN diodes. This experiment has aimed at searching for fine structures in the total kinetic energy vs. fragment mass (TKE ‐ M) distributions of binary events, and disclosing rare ternary decay modes others than already known from ternary fission. Details of the procedures of data handling are briefly described as well.

Energy distribution of ternary {alpha} particles in spontaneous fission of {sup 252}Cf

The energy distribution of the ternary {alpha} particles in spontaneous fission of {sup 252}Cf was measured. For the first time an energy threshold as low as 1 MeV was reached. The experiment used an array of unshielded silicon detectors measuring energy and time-of-flight (TOF) of ternary particles in coincidence with fission fragments. The TOF resolution of the system was sufficient for clear separation of {sup 6}He and tritons from {sup 4}He. The statistics were adequate to extract the {sup 6}He/{sup 4}He yield ratio. For both {sup 4}He and {sup 6}He, an excess in the yield (as compared to a Gaussian shape) was observed at energies below 9 MeV. The measured ternary {alpha} spectrum was corrected for the distortion induced by the detection geometry covering equatorial particle emission only. The emission angle was found to affect mainly the width of the energy distribution by up to 1 MeV.

HENDES-high efficiency neutron detection system for correlation measurements with HI beams

HENDES is a new device for correlation measurements of fission fragments, neutrons and light charged particles produced in HI induced reactions. Double differential neutron spectra are measured with the help of Position Sensitive Neutron Detectors, fission fragment spectra are recorded with large Position Sensitive Avalanche Counters, and light charge particle spectra are measured with arrays of dE-E telescopes. Collective dynamical parameters of nuclear matter are extracted from the data. New results were obtained from 40Ar+180Hf reaction at EAr=190–250 MeV. Work is in progress to study the influence of neutron excess on the fusion-fission dynamics in different combinations of Ni beams on Sn targets.

Pre- and post-scission neutron emission in the reaction180Hf(40Ar, fission) atElab=216 MeV

Neutron and fission fragment spectra following the180Hf(40Ar,f) reaction atElab=216MeV were measured. The following neutron total, pre- and post-scission multiplicities were extracted:Mntot=7.2±1.3,Mnpre=2.7±0.9,Mnpost=4.5±0.9. The average temperature parameters of the neutron spectra areTpre=(1.63±0.16) MeV andTpost=(1.14±0.26)MeV. The mean total kinetic energy of the fission fragments is TKE=(166±10) MeV and the measured width is σTKE=17.3MeV. The width of the fragment mass distribution is σA=18.3u. The same reaction was analyzed using a modified statistical code which includes nuclear dissipation effects and particle and γ-ray emission in the equilibrium compound nucleus state and saddle-to-scission transition. The extracted value of nuclear friction coefficient indicates over-damped large-scale fission motion. The obtained total fission time τfiss=9·10−20s is in agreement with other existing data.