Investigation of the heavy nuclei fission with anomalously high values of the fission fragments total kinetic energy

Binary fission of 232Th and 238U induced by fast neutrons were under intent investigation in the IPPE during recent years. These measurements were performed with a twin ionization chamber with Frisch grids. Signals from the detector were digitized for further processing with a specially developed software. It results in information of kinetic energies, masses, directions and Bragg curves of registered fission fragments. Total statistics of a few million fission events were collected during each experiment. It was discovered that for several combinations of fission fragment masses their total kinetic energy was very close to total free energy of the fissioning system. The probability of such fission events for the fast neutron induced fission was found to be much higher than for spontaneous fission of 252Cf and thermal neutron induced fission of 235U. For experiments with 238U target the energy of incident neutrons were 5 MeV and 6.5 MeV. Close analysis of dependence of fission fragment distribution on compound nucleus excitation energy gave us some explanation of the phenomenon. It could be a process in highly excited compound nucleus which leads the fissioning system from the scission point into the fusion valley with high probability

Measurement of (n, α

A novel spectrometer was developed and used to measure the cross section for the (n,α) reaction at IPPE. Direct measurements of the α-particles yield from solid isotopic pure targets of 50, 52 and 53 chromium, 54 and 57 iron, 60 nickel, and 64 zinc were carried out in the neutron energy range from 4.7 to 7.2 MeV. For some isotopes the (n,α) reaction cross-section for neutron energies less than 14 MeV were measured for the first time. The result of the comparison of new experimental data with the evaluated data from libraries ENDF/B VII, JENDL 4.0, JEFF 3.1, ROSFOND 2010 and BROND 3 and with the experimental data of other authors is presented

SPECTROMETER OF HEAVY AND LIGHT CHARGED PARTICLES ON THE BASE OF A DOUBLE IONIZATION CHAMBER WITH NETS OF FRISH

The spectrometer creation for the energy measurement of mass and angular distributions of charged particles is the aim of the paper. Theoretical bases of the ionization method of the charged particle indication have been developed. Partial sections of (n, alpha ) on *995*998Ni and *993*996Ar reactions have been measured for the first time. Properties of cool fragmentations at the *992*993*996*99,*992*993*995U division by neutrons with 1 MeV energy have been investigated.The spectrometer of alpha -particles has been introduced into operation. The spectrometer is being used for the determination of *992*993*999Pu in samples. The paper results may find their field of application in organizations, engaged by the design of reactors of quick neutrons and thermonuclear reactorsAvailable from VNTIC / VNTIC - Scientific & Technical Information Centre of RussiaSIGLERURussian Federatio

The Cross-section of the 10B(n,alpha)7Li Reaction Measured in the MeV Energy Range

The excitation function of the 10B(n,alpha)7Li reaction was measured between 1.5 MeV and 5.6 MeV within the frame of the IAEA Coordinated Research project (CRP) on "Improvement of the Standard Cross Sections for Light Elements";. An ionisation chamber and signal digitisation were used for the spectrometry of the reaction products, which were forwards emitted by bombarding a thin boron target with neutrons produced at the IRMM Van de Graff accelerator. The neutron flux was determined by measuring the backwards emitted fission fragments from a 238U sample mounted in a back-to-back geometry relative to the boron target. The highlight of this study was the discovery of the kinematic effect of particle leaking, which is very important for the accurate determination of the number of the boron reaction events. The experiment is described and the results are discussed and compared with evaluations and experimental data of other groups.JRC.D.5-Neutron physic

The Cross Section of the 16O(n,alpha) 13C Reaction in the MeV Energy Range

A novel spectrometer was developed and used to measure the cross section for the 16O(n,alpha) reaction at IRMM. The basic parts of the new instrument are an ionisation chamber, a gas oxygen target, and signal digitisation. It is shown that simultaneous digitisation of the anode and cathode signals allows an effective background suppression and the accurate determination of the number of reaction events and the number of atoms in the gas target. Cross section values for the 16O(n,alpha) reaction measured in the energy range 3.95-9.0MeV are presented. None of the existing nuclear data libraries describes well the IRMM data in the entire energy range.JRC.D.5-Neutron physic

Investigation of the heavy nuclei fission with anomalously high values of the fission fragments total kinetic energy

Binary fission of 232Th and 238U induced by fast neutrons were under intent investigation in the IPPE during recent years. These measurements were performed with a twin ionization chamber with Frisch grids. Signals from the detector were digitized for further processing with a specially developed software. It results in information of kinetic energies, masses, directions and Bragg curves of registered fission fragments. Total statistics of a few million fission events were collected during each experiment. It was discovered that for several combinations of fission fragment masses their total kinetic energy was very close to total free energy of the fissioning system. The probability of such fission events for the fast neutron induced fission was found to be much higher than for spontaneous fission of 252Cf and thermal neutron induced fission of 235U. For experiments with 238U target the energy of incident neutrons were 5 MeV and 6.5 MeV. Close analysis of dependence of fission fragment distribution on compound nucleus excitation energy gave us some explanation of the phenomenon. It could be a process in highly excited compound nucleus which leads the fissioning system from the scission point into the fusion valley with high probability

Digital spectrometer for prompt fission neutron spectrum measurements

This paper presents a digital neutron spectrometer based on simultaneous digitizing of the signals from the fission chamber and a scintillation detector. The scintillation detector is based on stilbene crystal. The intrinsic detection efficiency of the used stilbene crystal and the energy dependence of the light output for the recoil protons were measured. It is shown, that the method allowed us to achieve time resolution of 1.5 ns and an good n/γ separation down to neutron energies of 400 keV

Digital spectrometer for prompt fission neutron spectrum measurements

This paper presents a digital neutron spectrometer based on simultaneous digitizing of the signals from the fission chamber and a scintillation detector. The scintillation detector is based on stilbene crystal. The intrinsic detection efficiency of the used stilbene crystal and the energy dependence of the light output for the recoil protons were measured. It is shown, that the method allowed us to achieve time resolution of 1.5 ns and an good n/γ separation down to neutron energies of 400 keV

Investigation of the heavy nuclei fission with anomalously high values of the fission fragments total kinetic energy

Binary fission of 232Th and 238U induced by fast neutrons were under intent investigation in the IPPE during recent years. These measurements were performed with a twin ionization chamber with Frisch grids. Signals from the detector were digitized for further processing with a specially developed software. It results in information of kinetic energies, masses, directions and Bragg curves of registered fission fragments. Total statistics of a few million fission events were collected during each experiment. It was discovered that for several combinations of fission fragment masses their total kinetic energy was very close to total free energy of the fissioning system. The probability of such fission events for the fast neutron induced fission was found to be much higher than for spontaneous fission of 252Cf and thermal neutron induced fission of 235U. For experiments with 238U target the energy of incident neutrons were 5 MeV and 6.5 MeV. Close analysis of dependence of fission fragment distribution on compound nucleus excitation energy gave us some explanation of the phenomenon. It could be a process in highly excited compound nucleus which leads the fissioning system from the scission point into the fusion valley with high probability

Experimental Study of the (n,alpha) Reaction on a Set of Light Nuclei

An experimental setup based on an ionization chamber with Frisch grid and waveform digitizer was used for (n,a) cross section measurements. Use of digital signal processing allowed us to select a gaseous cell inside the sensitive area of the ionisation chamber with high accuracy. This kind of approach provides a powerful method to suppress background from detector components and parasitic reactions on the working gas. The new method is especially interesting for the study of reactions on elements for which solid target preparation is difficult (e.g. noble gases). Additionally it has the advantage of an accurate determination of the number of nonradioactive nuclei in the selected gas cell. In the present experiments a set of working gases was used, which contained admixtures of nitrogen, oxygen, neon, argon and boron. Fission of 238U was used as neutron flux monitor. The cross section of the (n,a) reaction for 16O, 14N, 20Ne, 36Ar, 40Ar and the branching ratio alpha0/alpha1 of the 10B(n,alpha0) to 10B(n,alpha1) reactions were measured for neutron energies between 1.5 and 7 MeV.JRC.D.5-Nuclear physic