Proceedings of the Third Eurasian Conference “Nuclear Science and its Application”, October 5 - 8 , 2004.
COMPARATIVE ANALYSIS OF THE CHARACTERISTICS OF FISSION
PROCESS ACTINIDES BY THERMAL NEUTRONS INDUCED
Kazantseva T.G., Koblik Yu.N., Pikul V.P., Khugaev A.V., Yuldashev B.S.
Institute o f Nuclear Physics, Tashkent, Uzbekistan
1. INTRODUCTION
Despite of large achievements in theory of fission process of heavy nucleus and huge experimental data, till now there is no unified picture of this process. The nuclear fission represents composite nuclear reaction, including practically all nuclear interactions known in low energy physics. This process is characterized by huge quantity (about 800) output channels. For analysis of dynamics of this reaction the experimental data is required, containing as much as possible information on an output channel. It is necessary with a high accuracy to know mass- energy and charge distributions, excitation energy, independent yields, odd-even effects, multiplicity of prompt neutrons.
On the basis of this data it is possible to observe dynamics of descent of a fissionable nucleus about a saddle point to a scission point within the framework of existing models to define shapes of a nuclei for different modes of fission.
The analysis of spontaneous and low energy fission is of concern from the point of view of analysis of a nuclear matter, as in this case the analysis of nucleon - nucleon interplays actually in vivo, i.e. without an external force.
The purpose of the present paper is the comparative analysis of the characteristics of fission process of uranium and plutonium isotopes by thermal neutrons induced with utilization of experimental data obtained for the last three decades. The values of total kinetic energies (TKE), kinetic energies of light and heavy fragments (E Kl, E Kh), half-width of mass and charge distributions (AA, AZ), odd-even effects on all mass distribution and separately for light and heavy groups were compared.
2. MASS DISTRIBUTIONS OF FISSION PRODUCTS
Mass distributions of fission products (FP) is one of the major characteristics of fission process. The main experimental data on mass fission yields FP was obtained by a radiochemical method with accuracy of 10-15 %. The mass-spectrometer technique has allowed considerably increasing measurement accuracy. The known precision installations LOHENGRIN [1] and COSI-FAN-TUTTE [2] Grenoble (France), a mass analyzer in Tashkent [3] allow alongside with mass yields to measure EK and Z of FP. In [4] the mass yields in a wide range of mass number compiled on all available experimental results and advised for use. Table 1 summarizes the average value of mass distributions FP and their dispersion of the U, U, Pu and 241Pu(nth,f) obtained from [4], Average values A and Z in heavy group for these divided systems are about identical. Probably, it is connected to the fact that a heavy fragment at first is reshaped.
Table 1
233U 235U 239Pu 241Pu
 L 93.27 95.12 99.87 101.29 Ah 138.29 138.49 137.37 137.77 CtAL 5.37 5.17 5.17 4.92 <^Ah 5.57 5.06 5.04 4.77 ZL 37.78 38.1 40.23 40.415 Zh 54.22 53.9 53.77 53.583 CtZL 1.99 2.04 2.08 1.95 Ctzh 2.70 2.20 2.21 1.99
Section I. Nuclear Energy: Present Status and Perspectives
Proceedings o f the Third Eurasian Conference “Nuclear Science and its Application”, October 5 - 8 , 2004.
In fig. 1 the values of half-widths AA for 233U, 235U (a), 239Pu, 241Pu (b) are given as a function of charge of FP. a). AA 18 1,6 1,4 1,2 1 0.8 b)
Fig. 1. The half-width A(A) as a function Z.
Increase of half-widths A(A) can be observed for Z=43, Z=49 in case of fission 233U, 235U and for Z=45 , Z=49 in case of fission 239Pu, 2,1Pu. It means that number of isotopes for these elements is very large as contrasted to by other ones. The formation of heavy fragments with Z=49 descends equally.
In fig. 2 FP mass yields of a heavy group with A=130-148 are given taken from [5](a) and [6](b). The form of these curves is practically identical to these divided systems.
FP with A=136 has anomalously low yield and maximum yield with A=134. a).
A
In fig. 3 the values of mass distributions dispersions o(A) are given at different values of EK 235U and 239Pu for light (a) [1] and heavy (b) [5] groups.
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Proceedings of the Third Eurasian Conference “Nuclear Science and its Application”, October 5 - 8 , 2004.
a). b).
At large EK values the considerable decrease in a (A) is observed. This can be possible because of a shorter time of descent from a saddle point to a scission point and small sizes of a neck. Besides our measurements [6] demonstrated increase in A with rise in EK for same Z.
3. CHARGE DISTRIBUTIONS
FP charge distributions is another major characteristics. Measurements of charge distributions are much more complex and labour-intensive problem as contrasted to by measurement of the mass-energy characteristics. Besides, it was supposed that influencing of charge distribution on dynamics of fission process is much easier than that for mass-energy distributions. There is less information on charge distributions than on mass-energy distributions. Such parameters as the most probable charge ZP, dispersion Gz and AZ=Zp-Zucd are usually
estimated. Zucd- Unchanged Charge Density.
With increase of EK the decreasing ZLP and AZ for the same mass number for all considered divided systems is observed. Probably it is connected to the fact that the proton subsystem in the beginning is formed and the neck consists of neutrons. At large EK the breaking of a neck becomes closer to a heavy fragment, in a consequent that quantity of neutrons for a light fragment is augmented.
Dispersions of charge distributions Gz in light group FP (fig. 4) have an oscillating type with the period ~5 a.m.u. The minimum values correspond to even-even nuclei, maximum values correspond to odd-odd nuclei.
Section I. Nuclear Energy: Present Status and Perspectives
Proceedings o f the Third Eurasian Conference “Nuclear Science and its Application”, October 5 - 8 , 2004.
Parameter AZ=ZP - Zucd also has an oscillating behaviour similar to az. With increase in EK
the value AZ decreases. Therefore, at small excitation energies the charge distribution in FP comes nearer to charge distribution in a compound nucleus. The calculations of deformations [7] for the most probable values EK and A have shown that the form of heavy fragments in a scission point is approximately identical, and form of light fragments notably differs.
REFERENCES
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3. U.A. Arifov, A.D. Belyaev et al. Proc. 8th Intern. Conf. on Low Energy Ion Accelerators. Goteborg, Sweden, 1973, p.226.
4. Internet address http://ie.gov/fission.html.
5. A.D. Belyaev, V.I. Kogan et al. Sov. Journal of Nucl. Phys., v.40, JV2 5, 1984, p. 1131.
6. A.D. Belyaev, V.I. Kogan et al. Sov. Journal of Nucl. Phys., v.48, JV2 2, 1988, p.324.
7. V.P. Pikul , U.Yu. Jovliev et al. Sov. Journal of Nucl. Phys., v.68, JV2 2, 2005, p.201.
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