Scission Neutrons in Low Energy Fission
Experimental Search and Study of Scission Neutrons in Low Energy Fission
Tech Area / Field
- PHY-ANU/Atomic and Nuclear Physics/Physics
3 Approved without Funding
FEI (IPPE), Russia, Kaluga reg., Obninsk
- Nuclear Physics Institute, Russia, Leningrad reg., Gatchina
- European Commission / Joint Research Center / Institute for Reference Materials and Measurements, Belgium, Geel\nKinki University/Faculty of Science and Technology, Japan, Higashi-osaka
Project summaryThe purpose of the project is the investigation of non-adiabatic phenomena in nuclear fission at low excitation energies through precise measurements of the characteristics of fast neutrons accompanying the fission process. Prompt neutron emission is also an issue for neutron data evaluation, which is mainly based on model calculations and needs experimental input data for model improvement
As is well known the largest part of such neutrons is evaporating from the exited fragments after their acceleration in the strong Coulomb field. However, it seems to be evident that a relatively small amount of neutrons can be emitted by the fissioning system during its descent from the top of the external barrier to the scission point and near the moment of the strongly deformed nuclear drop rupture (so called Scission Neutrons, SCN).
The experimental investigations of the 252Cf spontaneous fission (Bowman , Riehs , and Seregina ) and the 235U neutron induced fission (Skarsvag  and Samant ) had demonstrated that about (10-15)% of fission neutrons are emitted near the scission point. But at the same time Knitter  and Blinov  had concluded that the share of the SCN is not higher than ~1%. As a consequence, it seemed to be impossible to come to the definite conclusion about SCN existence taking into account the results of all these experimental works.
However, Knitter's data being precisely re-evaluated recently (Kornilov ) have shown that Knitter's experimental data do not contradict to the results of Bowman  and Seregina  and one should incorporate about 10% of the SCN to explain common experimental peculiarities of fission neutron spectra. A similar procedure of data re-evaluation applied to the experimental results of Skarsvag  in the 235U neutron induced fission (Kornilov ) came to the same conclusion: there are ~0.4 n/fission, which were emitted by another mechanism, not from the fragments accelerated in the Coulomb field. The energy spectrum of these additional neutrons consists of two components as for the 252Cf and 235U fission. Their angular distribution is not isotropic in the Laboratory System of Coordinates.
Nevertheless the question of the SCN yields and in particular their detailed characteristic needs to be carefully investigated in dedicated precision experiments. The most important questions of the SCN properties are the following:
– what is the real figures of the SCN yield;
– how the properties of the SCN depend on fragment mass and kinetic energies;
– how they are changing from one fissile nuclei to another;
– what is the real mechanism of the emission of these neutrons.
The first theoretical papers (Fuller , Boneh ) came to the conclusion that the SCN neutrons can be emitted due to a non-adiabatic changing of the nuclear potential during fission. According to Boneh's calculation a descent time of 1.5Ч10-21s is required to provide for 0.44 SCN per fission. If one reduces this time to 3Ч10-21s the total amount of the SCN will be reduced by 7 times. So, in principal the investigations of the SCN may give unique information about properties of nuclear matter with strong deformation. The SCN may be a unique clock for the time scale determination of the low energy fission process.
On the other side some difficulties exist in the explanation of characteristics of the neutrons emitted from accelerated fragments. The detail statistical calculations cannot reproduce the dependence of the average center-of-mass neutron energy as a function of fragment masses. Since the first demonstration of this fact in 1985 (Rubchenya ) the problem was not solved up to now (Grudzevich ).
So, one may determine the purpose of this project as the study of the emission mechanism for all neutrons in fission, with especially careful investigation of the small contribution of the SCN neutron emission. This is the first investigation directly addressed to the study of the SCN. In any previous published experiments mainly post-scission neutrons have been investigated. The identification and careful study of the SCN emission may give new information about the non-adiabatic phenomena in low energy fission in addition to the light charged particle emission in ternary fission. It allows us to estimate the common features of these processes.
Besides the interest from a basic science point of view this study is also important to neutron data evaluation activities. Up to now the PFNS have been calculated on the basis of theoretical (or semi-empirical) models in which the absence of the SCN emission was compensated by an artificial change of model parameters. This fact complicates the data evaluation for the nuclei and energy range without experimental data. So, the new investigation of the neutron emission mechanism in fission would provide a good basis for practical application and increase the accuracy of the neutron data evaluation.
The previous analysis allows us to select the most convenient subjects for investigations. These are spontaneously fission of 252Cf and slow neutron induced fission of 235U. If to take into account the results of the literature the same amount of the SCN are emitted in the 235U and 252Cf fission. However, in the case of 235U fission the total average number of neutrons is smaller and they have a more anisotropic angular distribution in the LS to be compared with the case of spontaneous fission of 252Cf. It means that the additional SCN in the case of 235U may be investigated with higher accuracy. From the other side the fissioning nuclei in these cases are going to the scission point over different trajectories. The direct comparison of experimental results obtained with similar methods will be very fruitful as from the point of view of possible systematic error reduction and from the general physical point of view.
The thermal reactor (PNPI of RAS) with the neutron guide will be used as neutron source with an intensity on the 235U target of about 107 n/cm2/s. The Cf experiment will be carried out in IPPE.
The fragment masses, energy and emission angle will be measured with a double Frisch gridded ionization chamber. A 235U layer with a diameter of ~2 cm and a thickness of about ~50 g/cm2 and a 252Cf layer with a total fission of ~1000 1/s evaporated on a thin film will be mounted on the cathode of chamber. A new method based on digitalization of the anode pulses will be applied for the estimation of the fragments energy and cosine of emission angle between the electric field and the fragment direction. After the proper correction for energy losses inside the layer the collected events will be transformed to mass, energy and cosine distribution of fragments.
The neutron detector will be placed on the orthogonal axis to the chamber electrodes at the distance of 1m from the fissile layer. The neutron detector consists of an organic scintillator (NE218, stilbene, diameter of 10 cm and height of 5 cm) and a large phototube (like XP2041). The neutron spectra will be measured by the time-of-flight method with a total time resolution of ~2 ns. A fast waveform digitizer (1 Gs/s) will be applied for the neutron detector signals to get an estimation of time, energy and particle type.
The SCN energy-angular distribution will be estimated with the following iteration procedure. The neutron spectra at small angle, which have a small contribution of the SCN, will be transformed to the center-of-mass system. Than these spectra will be used for the calculation of the neutron energy-angular distribution corresponding to the main neutron emission mechanism. After the correction of the small angle spectra for the SCN contribution the calculation will be repeated. This procedure allows us to estimate the unknown anisotropic angular distribution and energy spectrum shape of the SCN in the laboratory system as well as the multiplicity and spectrum shape (average energy) of neutrons emitted from accelerated fragments.
The experimental method and evaluation procedure were investigated in previous papers (Kornilov [14,16]; Khriachkov ).
The project will be realized in the following steps:
1. Development of the Fission Fragment and Neutron Spectrometers (FFNS) on the basis of a new digital method applied for fission ionization chamber and neutron detector.
2. Investigation of the characteristics of the spectrometers. Testing the procedure for the estimation of the cosine of the emission angle between fragments and neutron direction and fragment masses.
3. Measurement of the angular distribution and energy spectra of neutrons relative to a fixed fragment direction for separate fragment mass and kinetic energies for 235U(th) and 252Cf(sf) fission.
2. Detailed analysis of the experimental procedure, estimation of all types of corrections such as neutron scattering on the detector materials, fast neutron recoil effect, background of fission gamma-rays and so on, measurement of the neutron detector efficiency.
3. Estimation of the energy-angular distribution of the SCN in the laboratory system, calculation of the total amount of SCN.
4. Estimation of the dependence for the SCN characteristics on fragment mass and fragment kinetic energy.
5. Estimation of the dependence for the post-fission neutron energy on fragment mass.
In this project the following results will be anticipated:
- Development of the new experimental technique based on digitalization of the experimental signals which will increase the accuracy of the experiments.
- Development of the new spectrometer for the fission fragment and neutron study (FFNS);
- New precise experimental data for the angular distribution and energy spectrum of the fast neutrons nad the SCN for 235U(th) and 252Cf(sf).
- Dependence of these distributions on fragment mass and kinetic energy.
- New experimental data for the dependence of the average center-of-mass neutron emission energy from accelerated fragments on their masses.
- Conclusion about the additional neutron existence and their emission mechanism.
Role of foreign Collaborators.
1. The independent analysis of the experimental data;
2. Development of the model for description of the experimental data.
3. Discussion and suggestion for further advancement of the study.
1. Evaluation of the prompt fission neutron multiplicity within the frame of the multi-modal fission model and in collaboration with candidate country scientists based on high-resolution fission fragment distributions measured at IRMM.
2. Estimation of the 252Cf standard prompt fission neutron spectrum on the basis of the new experimental data for differential properties of the scission neutrons.
3. Improved determination of the level densities as compared to older measurements performed at IRMM and important for the improvement of model codes for fission neutron spectrum evaluation.
4. Recommendations for the standard prompt fission neutron spectrum shape at low (<2 MeV) and high (>10 MeV) neutron energies.
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