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Measurement of fragment mass yields in neutron-induced fission of 232 Th and 238 U

Measurement of fragment mass yields in neutron-induced fission of 232 Th and 238 U at 33, 45 and 60 MeV. V.D. Simutkin 1 , I.V. Ryzhov 2 , G.A. Tutin 2 , M.S. Onegin 3 , L.A. Vaishnene 3 , J. Blomgren 1 , S. Pomp 1 , M.Österlund 1 , P. Andersson 1 , R. Bevilacqua 1 ,

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Measurement of fragment mass yields in neutron-induced fission of 232 Th and 238 U

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  1. Measurement of fragment mass yields in neutron-induced fission of 232Th and 238U at 33, 45 and 60 MeV V.D. Simutkin1, I.V. Ryzhov2, G.A. Tutin2, M.S. Onegin3, L.A. Vaishnene3, J. Blomgren1, S. Pomp1, M.Österlund1, P. Andersson1, R. Bevilacqua1, J.-P. Meulders4, R. Prieels4 • Uppsala University, Sweden • Khlopin Radium Institute, St.-Petersburg, Russia • Petersburg Nuclear Physics Institute, Gatchina, Russia • Université Catholique de Louvain, Louvain-la-Neuve, Belgium

  2. Motivation • ADS, FF hybride reactors • Problem: build-up of FPs (close to symmetric region) which are out of concern in thermal nuclear reactors. • Example: 126Sn(T1/2 ~ 105 years), 113Cd (σc≈ 20650 b) • Radioactive Beam Production (EURISOL, SPIRAL-II) • Neutron-rich nuclei can be produced in fast neutron-induced fission of 238U and 232Th. Current status for En>20 MeV: No data for 232Th!

  3. Measurement method The pre-neutron emission fragment masses are: where is the mass of the compound nucleus, is the pre-fission neutron multiplicity , are the pre-neutron emission fragment kinetic energies in the center mass system.

  4. Multisection Frisch-Gridded Ionization Chamber • Seven units. Each unit constitutes a twin Frisch-gridded ionization chamber with a common cathode. • Anodes of two adjacent chambers are common. Ryzhov et al., NIMA 562, 439(2006)

  5. Neutron Beam Facility • Quasi-monoenergetic spectra with peak energies 32.8, 45.3 and 59.9 MeV. • The first fissile target at a distance of 375 (335) cm from the Li target. • The fluence rate of peak neutrons about 105 cm-2s-1.

  6. Data analysis Method: double kinetic energy method In C.M. system: Data analysis: • Pre-fission neutrons • Pulse height effect • Frisch grid inefficiency • Prompt neutrons • Energy losses in the target • Momentum transferred to the nucleus

  7. Wrap-around neutrons n + 238U Epeak= 33 MeV MC folding: (n,f) XS (ENDF) + neutron spectrum (PTB) Depending on the neutron energy and distance from the neutron source (for 238U): Ewrap-around≈ 5.7-7.6 MeV Ywrap-around ≈ 3.5-4.0%

  8. Wrap-around neutrons (cont.) Wrap-around neutron mass yields (En>5 MeV): evaluation, code PYF (D. M. Gorodisskiy et al., Ann. Nucl. Energ. 35, 238 (2008))

  9. Correction for mass dispersion (32.8 MeV)

  10. Measurement results: primary FFY

  11. Zoller n present Comparison with EXFOR data C.V. Zöller, PhD thesis, TH Darmstadt, 1995 1. Epeak=32.8 MeV, δE≈31.5-34.5 MeV (22-33 MeV, Zöller) 2. Epeak=45.3 MeV, δE≈43.5-46.5 MeV (33-45 MeV, Zöller) 3. Epeak=59.9 MeV, δE≈58.0-62.0 MeV (55-71 MeV, Zöller)

  12. TALYS simulations for fission fragment mass yields • The calculation of level densities of the deformed states by multiplication of the ground state level densities on the collective enhancement factors has been disabled. • Nuclear densities of deformed states were considered independently on the ground state level density for each Th and U isotope which undergoes fission. • Parameters and in • were varied to describe fission cross-section.

  13. TALYS simulations for fission fragment mass yields • To describe the experimental fission fragment massyields, severalmodifications of the codehavebeendone: • A systematics for the symmetric fission probability as a function of the incident neutron energywasintroduced. • The masses of heavy fission fragments wereadjusted to get betterfit of the experimental data.

  14. TALYS simulations for fission fragment mass yields Default TALYS calculation:

  15. TALYS simulations for fission fragment mass yields Modified TALYS calculation:

  16. Non-peak neutron energies n + 238U Epeak= 33 MeV ∆t1 ∆t2 (∆t1, ∆t2) (E1, E2)

  17. Experimental results and TALYS simulations for fission fragment mass yields at 9-11, 16-18, and 24-26 MeV

  18. Comparison with EXFOR data C.M. Zöller, PhD thesis, TH Darmstadt, 1995

  19. Conclusions • We have measured neutron-induced fission fragment mass yields • of 238U and 232Th at En=33, 45 and 60 MeV. The data for 232Th • were measured for the first time. • 2. Data for neutron energies intervals 9-11, 16-18 and 24-26 MeV have been extracted. • Reasonable agreement with the EXFOR data. • Systematics of symmetric fission probability was introduced. • Modified TALYS describes fission fragments mass yields.

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