RESEARCHES IN NUCLEAR FISSION AND THEIR INTERACTIONS WITH NEUTRONS, CONSIDERED AS FUNDAMENTAL GROUNDS FOR ATOMIC ENERGY ENGINEERING

1. Measuring of fission cross-sections, initiated by neutrons and protons under intermediate energies
2. Measuring of number of fission neutrons and cross-sections averaged in neutron fission spectrum
3. The study of differential momentary neutron spectra at nuclear fission by charged particles
4. Measuring of heavy nuclei fission cross-sections, caused by neutrons with energies up to 200 MeV

1. Measuring of fission cross-sections, initiated by neutrons and protons under intermediate energies

• c.p.-m.s. Smirnov Andrey Nickolaevich
• c.p.-m.s. Tootin Gennadiy Alexsandrovich

The Institute takes part in solving of serious international problem related to creation of internally safe and pollution-free nuclear-power engineering on the basis of accelerator-controlled reactor. The works were started in 1994, as a part of important ISTC native Project No 17. Since 1997 they have been continued within the frameworks of personnel Projects No 540, 1309, 2213.

Scientists of Radium Institute were the first succeeded with the help of synchrocyclotron installed at Uppsala University (Sweden), designed to reach both protons and quasi-monochromatic neutrons with energies up to 180 MeV, in measuring of total and differential fission cross-sections for 14 nuclides from ¹⁸¹фБ to ²⁴³бm, initiated by quasi-monochromatic neutrons with energies within this range.

Their own measurements, as well as literary data systematization and new patterns of relationships, obtained, have formed the basement for the present-day data base on nuclear fission cross-sections under intermediate energies.

Multi-sectional ionization chamber allowing to simultaneous measuring of fission characteristics for up to seven different nuclei, or 14 single element targets with total mass not exceeding 1 gram was designed and made at the Laboratory with the purpose to study the processes of weak-fissionable nuclei fission by intermediate energy neutrons within the range of lead and more lightweight nuclei.

In order to carry out measurements of cross-sections caused by protons and neutrons they use a chamber based on thin film punch counter devices representing detectors, which insensitivity to weak-ionizing particles is similar to that of solid-state tracking detectors, such as silicate glasses, but working in on-line environment (they give immediate response to a fission fragment). The advantage of these detectors consists in their fine time resolution at a counting rate sufficient for measuring of relatively small fission cross-sections (within sub-actinoid range).

The chamber operated by intermediate energy neutron beam, installed by Radium Institute's employees at Teo Swedberg's Laboratory (the city of Uppsala in Sweden)

The fission initiated by lightweight particles (neutrons and protons) is embarrassed with multiple mechanisms of interaction between particles and nuclei. The results obtained give us an opportunity to find out a role of impinging particle isospin, i.e. to present theoretical interpretation of the difference observed between proton and neutron cross-sections. Measuring of cross-sections for lead isotopes separated, including twice magical nucleus of ²⁰⁸Pb, suggests a key to understanding of a function of nuclei internal structure. This is the reason why the most important physical problems related to the works ongoing are included into "Nuclear Physics" Fundamental Research Program under the Ministry for Industry, Science, and Technologies.

2. Measuring of number of fission neutrons and cross-sections averaged in neutron fission spectrum

From 1995 to the present time within the frameworks of ISTC Projects No 183, No 183B, No 1828 they carry out at Laboratory precision measurements of momentary neutron spectra at spontaneous fission of ^244,246Cm and ^240,242Puisotopes, at fission of ^243,245Cm and ^242mAm isotopes, caused by thermal neutrons, and also momentary neutron spectra measurements for ^241,243Am isotopes at fission caused by fast neutrons with energies of 2.6, 4.5, and 14.6 MeV.

They carry out spectra measurements by the method of transit time, relative to international standard - momentary neutron spectra from spontaneous fission of ²⁵²Cf, at that, they measure simultaneously both spectra to be studied and reference. At present time this method is the most rigorous and foolproof.

3. The study of differential momentary neutron spectra at nuclear fission by charged particles

With the purpose to measure the spectra of neutrons and fission fragments they use at Radium Institute various types of appropriate detectors. At the Laboratory there is created multi-parameter spectrometer, which gives opportunity to carry out high-precision measurements of neutron and fission fragment distribution characteristics.

At the present time they evolve collaboration with Oregon University (professor V. Loveland) and Uppsala University (professor K. Aleclet) in the field of studying the processes of nuclear fusion, using radioactive beams.

Multi-parameter spectrometer	Vacuum chamber with detectors

4. Measuring of heavy nuclei fission cross-sections, caused by neutrons with energies up to 200 MeV

With the purpose to improve nuclear databases for transmutation technologies they carry out experimental and theoretical investigations of heavy nuclei fission reaction.

They carry out experiments in synchrocyclotron (PINP, the city of Gatchina) on a beam of fast neutrons with 1-GeV energy. The installation gives opportunity to measure heavy nuclear fission cross-sections, caused by fast neutrons within energy range of 0.5 - 200 MeV with 1-MeV interval.

Two ISTC Projects, which have allowed to measuring of neutron fission crass-sections for ²³⁸U and ²³³U, ²³²Th, ²³⁹Pu, ²⁴⁰Pu, ²⁴³Am, Pb, Bi, and W with respect to ²³⁵, are completed.
The Laboratory plans to modernize this installation:

• enlarge energy range available for measurements, towards both low and high energies;
• measure fission cross-section of the standard, ²³⁵U, with respect to cross-section initiated by hydrogen.