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, J.-P. Meulders 4, R. Prieels 4 1.Uppsala University, Sweden 2.Khlopin Radium Institute, St.-Petersburg, Russia 3.Petersburg Nuclear Physics Institute, Gatchina, Russia 4.Université Catholique de Louvain, Louvain-la-Neuve, Belgium

Motivation ADS, FF hybride reactors Problem: build-up of FPs (close to symmetric region) which are out of concern in thermal nuclear reactors. Example: 126 Sn(T 1/2 ~ 10 5 years), 113 Cd (σ c ≈ b) Radioactive Beam Production (EURISOL, SPIRAL-II) Neutron-rich nuclei can be produced in fast neutron-induced fission of 238 U and 232 Th. Current status for E n >20 MeV: No data for 232 Th!

Measurement method The pre-neutron emission fragment masses are: whereis 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.

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)

1.Quasi-monoenergetic spectra with peak energies 32.8, 45.3 and 59.9 MeV. 2.The first fissile target at a distance of 375 (335) cm from the Li target. 3.The fluence rate of peak neutrons about 10 5 cm -2 s -1. Neutron Beam Facility

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

Wrap-around neutrons  MC folding: (n,f) XS (ENDF) + neutron spectrum (PTB) n U E peak = 33 MeV E wrap-around ≈ MeV Y wrap-around ≈ % Depending on the neutron energy and distance from the neutron source (for 238 U):

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

Correction for mass dispersion (32.8 MeV)

Measurement results: primary FFY

Comparison with EXFOR data n Zoller present C.V. Zöller, PhD thesis, TH Darmstadt, E peak =32.8 MeV, δE≈ MeV (22-33 MeV, Zöller) 2. E peak =45.3 MeV, δE≈ MeV (33-45 MeV, Zöller) 3. E peak =59.9 MeV, δE≈ MeV (55-71 MeV, Zöller)

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. TALYS simulations for fission fragment mass yields

To describe the experimental fission fragment mass yields, several modifications of the code have been done: 1.A systematics for the symmetric fission probability as a function of the incident neutron energy was introduced. 2.The masses of heavy fission fragments were adjusted to get better fit of the experimental data. TALYS simulations for fission fragment mass yields

Default TALYS calculation: TALYS simulations for fission fragment mass yields

Modified TALYS calculation: TALYS simulations for fission fragment mass yields

Non-peak neutron energies n U E peak = 33 MeV ∆t 1 ∆t 2 (∆t 1, ∆t 2 ) (E 1, E 2 )

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

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

Conclusions 1.We have measured neutron-induced fission fragment mass yields of 238 U and 232 Th at En=33, 45 and 60 MeV. The data for 232 Th were measured for the first time. 2. Data for neutron energies intervals 9-11, and MeV have been extracted. 2.Reasonable agreement with the EXFOR data. 3.Systematics of symmetric fission probability was introduced. 4.Modified TALYS describes fission fragments mass yields.