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The properties of cylindrical 1.5//×1.5//LaCl3:Cescintillation detectors were determined. For the energy resolution values between 3.8 and 4.2% at 662 keV were observed, following the expected E-1/ ...

Experiments on photo-induced fission of 238U and 234U using bremsstrahlung with endpoint energies between 6 MeV and 9 MeV have been carried out at the superconducting Darmstadt electron linear acce ...

The cross‐section for the neutron‐induced fission of 231Pa has recently been measured from the threshold to En = 3.5 MeV. The experimental results are described in terms of extended statistical model calculations.

The recoil mass spectrometer LOHENGRIN of the Laue‐Langevin Institute, Grenoble has been used to measure the light fission‐fragment mass yield and kinetic energy distributions from neutron‐induced 252Cf*, using 251Cf as target material.

Do The prompt neutron emission in spontaneous fission of 252Cf has been investigated applying cross correlation method and digital signal processing algorithms. A new mathematical approach for neutron/gamma pulse shape separation was developed and implemented for prompt fission neutron (PFN) time-of-flight measurement. The main goal was development of automated data analysis algorithms and procedures for data analysis with minimum human intervention. Experimental data was taken with a twin Frisch-grid ionization chamber and a NE213-equivalent neutron detector in an experimental setup similar to well work of C. Budtz-Jorgensen and H.-H. Knitter [1]. About 2*107 fission events were registered with 2*105 neutron/gamma detection in coincidence with fission fragments. Fission fragment kinetic energy, mass and angular distribution, neutron time-of-flight and pulse shape have been investigated using a 12 bit waveform digitizer.

In this work we report recent achievements in design of twin back-to-back ionization chamber (TIC) for fission fragment (FF) mass and kinetic energy spectroscopy. Correlated FF kinetic energies, their masses and the angle of the fission axes in 3D Cartesian coordinates can be determined from analysis of the heights and shapes of the pulses induced by the fission fragments on the anodes of TIC. Anodes of TIC were designed as consisting of isolated strips each having independent electronic circuitry and special multi-channel pulse processing apparatus. Mathematical algorithms were provided along with formulae derived for fission axis angles determination. It was shown how the point of fission fragments origin on the target plane may be determined using the same measured data. The last feature made the TIC a rather powerful tool for prompt fission neutron (PFN) emission investigation in event by event analysis of individual fission reactions from non point fissile source. Position sensitive neutron induced fission detector for neutron imaging applications with both thermal and low energy neutrons was found as another possible implementation of the designed TIC.

The purpose of the present paper was to report the recent results, obtained in development of digital pulse processing mathematics for prompt fission neutron (PFN) investigation using twin ionization chamber (TIC) along with fast neutron time‐of‐flight detector (ND). Due to well known ambiguities in literature (see refs. [4, 6, 9 and 11]), concerning a pulse induction on TIC electrodes by FF ionization, we first presented detailed mathematical analysis of fission fragment (FF) signal formation on TIC anode. The analysis was done using Ramo‐Shockley theorem, which gives relation between charged particle motion between TIC electrodes and so called weighting potential. Weighting potential was calculated by direct numerical solution of Laplace equation (neglecting space charge) for the TIC geometry and ionization, caused by FF. Formulae for grid inefficiency (GI) correction and digital pulse processing algorithms for PFN time‐of‐flight measurements and pulse shape analysis are presented and discussed.

The prompt neutron emission in spontaneous fission of 252Cf has been investigated applying digital signal electronics along with associated digital signal processing algorithms. The goal was to find out the reasons of a long time existing discrepancy between theoretical calculations and the measurements of prompt fission neutron (PFN) emission dependence on the total kinetic energy (TKE) of fission fragments (FF). On the one hand the 252Cf (sf) reaction is one of the main references for nuclear data, on the other hand the understanding of PFN emission mechanism is very important for nuclear fission theory. Using a twin Frisch‐grid ionization chamber for fission fragment (FF) detection and a NE213‐equivalent neutron detector in total about 107 fission fragment‐neutron coincidences have been registered. Fission fragment kinetic energy, mass and angular distribution, neutron time‐of‐flight and pulse shape have been investigated using a 12 bit waveform digitizer. The signal waveforms have been analyzed using ...

The prompt neutron emission in spontaneous fission of 252Cf has been investigated applying digital signal electronics. The goal was to compare the results of this digital data acquisition and digital signal processing analysis to the results of the pioneering work of Budtz‐Jo/rgensen and Knitter. Using a twin Frisch‐grid ionization chamber for fission fragment (FF) detection and a NE213 equivalent neutron detector in total about 10 neutron coincidences have been registered. Fission fragment kinetic energy, mass and angular distribution have been investigated using a 10 bit waveform digitizer. Neutron time‐ of‐ flight and pulse shape have been measured using analogue CAMAC modules, a 1 ns TDC and a pair of 12 bit charge‐to‐digital‐converters. The fission fragment signals have been analyzed using digital signal processing algorithms. The results are in very good agreement with literature. For the first time the dependence of the number of neutrons as a function of total kinetic energy (TKE) of the fragments...

Digital signal processing (DSP) algorithms are in high demand for modern nuclear fission investigation due to importance of increase the accuracy of fissile nuclear data for new generation of nuclear power stations. DSP algorithms for fission fragment (FF) and prompt fission neutron (PFN) spectroscopy are described in the present work. The twin Frisch-grid ionization chamber (GTIC) is used to measure the kinetic energy-, mass- and angular distributions of the FF in the {sup 252}Cf(SF) reaction. Along with the neutron time-of-flight (TOF) measurement the correlation between neutron emission and FF mass and energy is investigated. The TOF is measured between common cathode of the GTIC and the neutron detector (ND) pulses. Waveform digitizers (WFD) having 12 bit amplitude resolution and 100 MHz sampling frequency are used for the detector pulse sampling. DSP algorithms are developed as recursive procedures to perform the signal processing, similar to those available in various nuclear electronics modules, such as constant fraction discriminator (CFD), pulse shape discriminator (PSD), peak-sensitive analogue-to-digital converter (pADC) and pulse shaping amplifier (PSA). To measure the angle between FF and the cathode plane normal to the GTIC a new algorithm is developed having advantage over the traditional analogue pulse processing schemes. Algorithms aremore » tested by comparing the numerical simulation of the data analysis of the {sup 252}Cf(SF) reaction with data available from literature.« less

The prompt fission neutron multiplicity and spectra of actinides are nuclear data of crucial importance. Based on experimental fission yield and total kinetic energy data, new calculations for the prompt neutron multiplicity and spectra for 238U(n,f), 237Np(n,f) in the incident neutron energy range up to the second chance fission threshold, for 238U(n,f), up to 50 MeV, and for 252Cf(SF) have been performed.For the first time the multi‐modality of the fission process was taken into account up to the second fission chance (about 6 MeV). Additionally, for some isotopes a more realistic fission fragment residual temperature distribution as well as an anisotropy of the prompt neutron emission led to improved agreement between the calculation and experimental results. Also, the range of fission fragment pairs entering in the multiplicity and spectrum model was extended over the entire experimental fission‐fragment mass range. This led to an improved version of the Los Alamos (LA) model and especially to an impr...

Based on the recently measured cross‐section values for the neutron‐induced fission of 231Pa and our experience gained with other isotopes, new self consistent neutron cross section calculations for n+231Pa have been performed up to 30 MeV. The results are quite different to the existing evaluations, especially above the first chance fission threshold.

The prompt fission neutron multiplicity and spectrum for spontaneous fission and neutron induced reactions in the energy range of the first chance fission threshold are evaluated using the “point by point” approach that takes into account all possible fragmentations of the fissioning nucleus, the multi‐modal concept of fission and the most probable fragmentation approach. At higher incident energy when only the most probable fragmentation approach can be used, for the first time the model was extended to take into account the fission of the secondary compound nucleus chains formed by charged particle emission. The model parameters and their dependencies on the incident energy are determined by the study of the reactions where the respective nuclei are the main compounds. The linear dependence of the prompt gamma‐ray energy on prompt multiplicity is parameterized as function of the mass and charge numbers of the fissioning nucleus. The above models were successfully applied for many neutron induced reactions on actinides, giving also good results in integral benchmarks.

New, self‐consistent, neutron‐induced reaction cross‐section calculations for 235,238U, 237Np, and 231,232,233Pa have been performed. The statistical model code STATIS was extended to take into account the multi‐modality of the fission process. The three most dominant fission modes, the two asymmetric standard I (S1) and standard II (S2) modes, and the symmetric superlong (SL) mode have been taken into account. De‐convoluted fission cross sections for these modes in 235,238U(n,f) and 237Np(n,f) based on experimental branching ratios, were calculated for the first time up to the second chance fission threshold. For 235U(n,f) and 233Pa(n,f), the calculations being made up to 50 MeV and 20 MeV incident neutron energy, respectively, higher fission chances have been considered. This implied the need for additional calculations for the neighbouring isotopes.As a side product also mass yield distributions could be calculated at energies hitherto not accessible by experiment. Experimental validation of the predic...

The prompt neutron emission probability from neutron‐induced fission in the resonance region is being investigated at the time‐of‐flight facility GELINA of the IRMM. A double Frisch‐gridded ionization chamber is used as a fission‐fragment detector. For the data acquisition of both the fission‐fragment signals as well as the neutron detector signals the fast digitization technique has been applied. For the neutron detection, large‐volume liquid scintillation detectors from the DEMON collaboration are used. A specialized data analysis program taking advantage of the digital filtering technique has been developed to treat the acquired data.Neutron multiplicity investigations for actinides, especially in resonance neutron‐induced fission, are rather scarce. They are, however, important for reactor control and safety issues as well as for understanding the basic physics of the fission process. Fission yield measurements on both 235U and 239Pu without prompt neutron emission coincidence have shown that fluctuat...

In the frame of the exploratory research initiative of the JRC a new pulsing device has been installed at the Van‐de‐Graaff of the IRMM to produce pulsed quasi mono‐energetic neutron beams in the MeV range. The pulse width may be tuned from 10 μs up to several hundreds of μs with a repetition rate between 1 Hz and 5 kHz. The aim of the device is to study the decay of short‐lived activation products between pulses in an essentially neutron‐free environment. In a first application the shape isomer in 239U was searched.

A systematic calculation of the fission mode characteristics of the light actinides in the frame of the multi-modal random neck-rupture model has been started. In particular the isotopes 220,226,232Th and 220,226Ac are under investigation. A clear competition between the outer barrier heights of the asymmetric standard and the symmetric superlong fission mode has been found. Their systematic variation with the compound nuclear mass might explain the drastic changes in the nuclear charge distributions recently observed at GSI. The current status will be reported and discussed in the light of the experimental results.

The nuclear charge distribution in the thermal neutron-induced fission of the odd Z nuclides 239Np(Z=93) and 242mAm(Z=95) has been studied in the range of very light fission products using the mass separator Lohengrin of the Institut Laue-Langevin (Grenoble). Odd-even effects of a new kind and shell effects have been observed and are analysed quantitatively. The physical relevance of the observed effects for the understanding of the fission process is discussed.

Ternary events in the thermal neutron induced fission of 241Pu were studied with the recoil separator LOHENGRIN at the Institut Laue Langevin in Grenoble. Yields and energy distributions could be determined for most isotopes of the elements hydrogen to oxygen. Also several heavier nuclei up to 30Mg could be observed. Yields were measured for 42 isotopes, for further 17 isotopes upper limits could be deduced. For the first time the halo nuclei 11Li and 14Be were found in neutron induced fission with yields of some 10−10 per fission. This is orders of magnitude lower than predicted by most of the ternary fission models.

Several μs isomers have been observed in the mass range A=88–109. The detection is based on time correlation between reaction products selected by the LOHENGRIN spectrometer and the γ-rays depopulating the isomers. Three new isomers have been observed in 94Y, 96Rb, and 106Nb.

Energy distributions and yields of ternary particles (TP’s) emitted in the thermal neutron induced fission of 229Th, 233U and 239Pu have been determined. Special emphasis was put on the study of heavy ternary particles. The measurements were performed at the high flux reactor of the Institut Laue-Langevin (ILL). For identification of particles the mass separator Lohengrin and a ΔE-E ionisation chamber were used. In the case of 233U(nth,f) it was possible to detect 25 different TP’s, with 19 of them having never been detected before in this reaction. For 229Th(nth,f) all 17 TP’s that were observed were new. From the reaction 239Pu(nth,f) 21 out of 34 detected TP’s were new. The energy distributions and yields of the TP’s are given. The yields of TP’s as a function of mass number depend on the mass of the decaying compound nucleus. Heavy TP’s are increasingly favored in the decay of heavy mother nuclei.

The reaction 238U(n,f) has been studied at IRMM at different incident neutron energies ranging from En=1.2 to 5.8 MeV. The existence of vibrational resonances in the region of the threshold of the fission cross-section and the proton pairing effect should induce variations in the fission fragment properties. The fission fragment mass, mean total kinetic energy (TKE¯) and angular distributions have been investigated with a double Frisch-gridded ionization chamber. For each incident neutron energy, more than 105 events have been accumulated. The TKE¯ shows an increasing trend up to En=3.5 MeV with a sudden drop at roughly En=3.8 MeV which might be related to the onset of pair breaking. Above En=3.8 MeV TKE¯ is again continually increasing. The two-dimensional mass-TKE distributions have been compared by means of a fit with theoretical calculations performed recently in the frame of the multi-modal random neck-rupture model. Actually, two solutions are possible with assuming either two or three Gaussians for...

At the mass separator LOHENGRIN of the Institut Laue-Langevin, fragment mass, charge and kinitic energies have been measured for the target nuclei {sup 245}Cm and {sup 241}Pu. The mass range for the Curium measurement was 76{