Gateway for:

Member Countries

Fission Cross Sections of Tungsten Isotopes

#2213


Measurements of Proton- and Neutron-Induced Fission Cross Sections of Separated Tungsten Isotopes and Natural Tungsten in 50-200 MeV Energy Region

Tech Area / Field

  • FIR-NOT/Nuclear and Other Technical Data/Fission Reactors
  • FIR-REA/Reactor Concept/Fission Reactors
  • PHY-ANU/Atomic and Nuclear Physics/Physics

Status
8 Project completed

Registration date
25.05.2001

Completion date
07.09.2006

Senior Project Manager
Malakhov Yu I

Leading Institute
Khlopin Radium Institute, Russia, St Petersburg

Collaborators

  • University of Uppsala, Sweden, Uppsala\nEuropean Commission, Belgium, Brussels\nForschungszentrum Karlsruhe GmbH, Germany, Karlsruhe

Project summary

Nuclear data at the intermediate energies, their role for science and technology are being discussed widely for some years at the International conferences “Nuclear Data for Science and Technology” (Juelich, FRG, 1991; Gatlinburg, USA, 1994; Trieste, Italy, 1997). The demand for development of experimental and theoretical nuclear data bases for energies above 20 MeV was the object of special attention at specialized conferences on transmutation “Accelerator Driven Transmutation Technologies and Applications” (Las Vegas, USA, 1994; Kalmar, Sweden, 1996; Praha, Czech Republic, 1999).

For systems based on high-current accelerators (Accelerator Driven System – ADS) and intended for nuclear energy production and/or transmutation of long-life radioactive waste, these data are necessary first of all for calculations of the neutron-producing targets. At present two main options of neutron-producing targets are considered: fluid (Pb+Bi) eutectic and solid W [F. Venneri et al, LA-UR-99-3022]. For calculations of key parameters of these targets (number and spectrum of emitted neutrons, prompt and residual radioactivity, heat release, radiation stability (in the case of solid target)) the nuclear data are needed for a number of proton- and neutron-induced reactions (including fission reactions) at intermediate energies. In spite of relatively low cross section the fission reactions lead to products with high-energy release and, often, with long half-lives. Fission cross sections of W (isotope 184W) in the energy region up to the 200 MeV were included in the High Priority Request List due to their great importance to developing of ADS and nuclear reaction models [A.J. Koning et al.,Nucl. Instr. and Meth. in Phys. Res. A414 (1998) 49].

Recently evaluated cross section data libraries have been created at LANL for all stable tungsten isotopes up to 150 MeV (M. B. Chadwick et al., Report LA-UR-98-1825 (1998); Nucl. Sci. and Eng. 131 (1999) 293). However, there are no evaluated data sets for the fission cross sections, because the experimental database is extremely poor. Up to now only one experimental work has been performed in which the natW(p,f) cross section (together with others) was obtained at 190 MeV. (M.C. Duijvestijn et al., Phys. Rev. C59 (1999) 776). Recently the natW(n,f) cross sections have been measured at 66, 75, 94, 133, 144 and 174 MeV by the collaboration between the Khlopin Radium Institute and Uppsala University. The measurements were performed at the accelerator of the The Svedberg Laboratory, where the present work also will be carried out. The results have been presented in report [V. P. Eismont et al., Proc. of the 6th OECD/NEA Information Exchange Meeting on Partitioning and Transmutation of Actinides and Fission Products, Madrid, Spain, December 11-13, 2000, р.117 The measurements with neutrons are especially difficult because available fluxes of intermediate energy neutrons are lower than proton ones by many orders of magnitude.The (n,f) and (p,f) cross section measurements at intermediate energies are also complicated by light charged particles, which are produced by energetic nucleons. The relative contribution of these processes increases as the target mass decreases due to decrease of fissility. To get over these difficulties the experimental techniques have been elaborated for project #540 “Measurements of neutron induced fission cross sections in energy region 15<En<160MeV for basic and applied researches”, finished in 1999. Then the techniques have been improved in the current project #1309 “Measurements and Comparison Proton- and Neutron-induced Fission cross sections of Lead and Neighboring Nuclei in the 20-200 MeV Energy Region”. These methods will be used in the present work too. In the framework of the project # 540 the measurements of the (n,f) cross-sections for the range of nuclei from 181Ta to 243Am, including the 232Th(n,f) and 238U(n,f) cross sections have been carried out. In the framework of the current project # 1309 the (n,f) and (p,f) cross-sections for 208Pb and other lead isotopes will be measured at the intermediate energy region. Thus the fission cross section measurements of 184W will allow to close the list of nuclei (184W, 208Pb, 232Th and 238U) which fission cross sections are included in the High Priority Request List [A.J. Koning et al.] Thus a complete set of the fission cross sections needed for development of ADS conception, and creation of realistic model of nuclear reactions will be obtained in the energy region up to 200 MeV.

At present the theoretical models of subactinide fission cannot be used for practice. For instance, the natW(p,f) cross section predicted by code LAHET is 20 times lower than the experimental result obtained by Duijvestijn et al. At the same time, the experimental results are already obtained, mainly in framework of the mentioned projects, which may have importance for creating models of nuclear reactions including models of nuclear fission. A comparison of the proton and neutron cross-sections is of a special interest for getting knowledge about mechanisms of nuclear reactions and fission process at intermediate energies. The problem connected to the observed differences in proton- and neutron-induced fission cross sections has been discussed first in the analysis of (p,f),(n,f) (and also (g,f)) cross section measurements for nuclei from Th to Am in the energy region above 20 MeV [V.P. Eismont et al. Proc. of Int. Conf. on Nucl. Data in Sci. and Techn., Gatlinburg, Tennessee, USA, May 9 - 13, 1994, ed. J.K.Dickens, USA, Illinois, 1994, v. 1, p. 397]. Later it became the object of a number of our works. It was found in these works that for all nuclei from Ta to Am at nucleon energies E well above the proton Coulomb barrier of these nuclei (V=10ё15 MeV), i.e. at the energies of several tens of MeV and more, where the Coulomb factor, 1-V/E, is close to unity, the (p,f) cross sections are higher than the (n,f) cross sections at the same energies. Moreover the difference increases as the mass of fissioning nuclei decreases. This phenomenon has been explained as a “memory effect” when the composition of fissioning nuclei (sum over all nuclei formed in the processes preceding the fission) “remembers” by which type of interaction it has been created: (p,f) or (n,f). The average parameter of fissionability of these compositions, Z2/A, for protons is more than for neutrons. Moreover it was noted that the properties of the (g,f) reaction, for which the parameter of fissionability has an intermediate value ((Z2/A)p > (Z2/A)g> (Z2/A)n), agree with this general picture too. The observed increase of the difference between the (p,f) and (n,f) cross sections has been explained as a result of increasing rate of fission probability change, Pf » Гfn, with the decrease of mass (charge) of the nucleus. The analogous explanation has been given later in the work [V.E. Bunakov et al., The European Journal, A8, (2000) No 4, 447].

However at present the experimental data in the region of Pb (and lighter) have large errors. So far as Pf » Гfn, » exp {[Bn-Ef(Z2/A)]/T}, where Bn is the neutron binding energy and Ef is the fission barrier, and Ef(Z2/A) has a ‘plateau’ in the interval of change of parameter Z2/A from 208Pb to W (as a result of the effect of large shell correction for double magic nucleus 208Pb), the dependence spf/snf on Z2/A in the region of W may have some features, and it would be interesting to determine them experimentally by means of more precise measurements.

Data on tungsten isotopes may be of a particular interest for the development of nuclear reaction models, because nuclei with atomic weight A » 180 (as 182,183,184W) belong to the region of strongly deformed nuclei (with deformation parameter b2 ~ 0.25-0.30). The comparison of fission cross sections of these nuclei, and therefore their fissilities, Pf, with ones of transitional nuclei with A » 190-200, for example 197Au (b2 ~ 0.10-0.15), as well as with ones of the spherical double magic nucleus (208Pb) will be of interest for development of an accurate model of fission process.

The Project will give possibility to use the specialists, facilities and equipment for peace goals. The Project will promote the development of fundamental and applied researches, including solution of the international technical problem of transition to ecologically clean and internally safe nuclear technologies. The total efforts of weapon scientists are 69% of full employment.

Planning scope of activities includes manufacturing of thin targets (with thickness of 1mg/cm2) of separated isotopes of 182,183,184,186W and natW, their weighing and analysis on of high fissility impurities, preparation to measurements and fission cross-section measurements at neutron and protons beams with energies 50-180 MeV. Thus the experimental data base will be created and physical foundation will be determined for creating files of evaluated data in the energy region of 20-200 MeV, and also new tests for nuclear models and their parameters will be found, which will allow in aggregate to create accurate methods for fission cross-sections calculations for practically all materials which are interesting as neutron-producing targets for acceleration transmutation.

The Uppsala University (Sweden), Karlsruhe Institute of Nuclear Research (Germany) and European Comission (Belgium) will participate as foreign collaborators. Researchers of the Uppsala University (Dr. Olson and others) are the participants of all experimental works which are jointly carried out at the TSL accelerator. They are co-authors of all the publications of the common research results. Representatives of Karlsruhe Nuclear Research Institute and the European Comission participate in exchange of information during realization of the Project, produce comments to quarter, annual reports and final reports, take part in joint symposiums and workshops.


Back

The International Science and Technology Center (ISTC) is an intergovernmental organization connecting scientists from Kazakhstan, Armenia, Tajikistan, Kyrgyzstan, and Georgia with their peers and research organizations in the EU, Japan, Republic of Korea, Norway and the United States.

 

ISTC facilitates international science projects and assists the global scientific and business community to source and engage with CIS and Georgian institutes that develop or possess an excellence of scientific know-how.

Promotional Material

Значимы проект

See ISTC's new Promotional video view