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

1. Uppsala University, Sweden2. Khlopin Radium Institute, St.-Petersburg, Russia3. Petersburg Nuclear Physics Institute, Gatchina, Russia4. Université Catholique de Louvain, Louvain-la-Neuve, Belgium

Motivation• ADS, FF hybride reactorsProblem: 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!

Measurement method

The pre-neutron emission fragment masses are:

,)(,)( *1

*2*

2*1

*2*

1 mAmEE

EAm preCNpreCN

where CNA is the mass of the compound nucleus,

pre is the pre-fission neutron multiplicity ,*2,1E 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)

0 10 20 30 40 50 60 700.0

0.2

0.4

0.6

0.8

1.0

Schuhmacher et al., NIMA 421, 284 (1999)

E /

/ M

eV

-1Neutron energy / MeV

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 105 cm-2s-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

12

21

21 ,

MMM

EE

EMM

C

C

In C.M. system:

Method: double kinetic energy method

Wrap-around neutrons

MC folding: (n,f) XS (ENDF) + neutron spectrum (PTB)

n + 238UEpeak= 33 MeV

Ewrap-around ≈ 5.7-7.6 MeVYwrap-around ≈ 3.5-4.0%

Depending on the neutron energy and distance from the neutron source (for 238U):

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))

Correction for mass dispersion

...8,)( 22222 umaFWHMA INSTLMTCNFFTOT (32.8 MeV)

Measurement results: primary FFY

Comparison with EXFOR data

n

Zoller

present

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)

• 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.

U

UWaEaa x

)exp(11~)(

a~

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 + 238UEpeak= 33 MeV

∆t1

∆t2

(∆t1, ∆t2) (E1, E2)

Experimental results and TALYS simulations for fission fragment mass yields at 9-11, 16-18, and 24-26 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 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.2. Reasonable agreement with the EXFOR data.3. Systematics of symmetric fission probability was introduced.4. Modified TALYS describes fission fragments mass yields.