Fission cross sections and the dynamics of the fission process F. -J Fission cross sections and the dynamics of the fission process F.-J. Hambsch

Overview Introduction Multi-Modal Random Neck Rupture (MM-RNR) model Statistical modeling of reaction cross sections Prediction of fission mode fluctuations Preliminary experimental results on 238U(n,f) Conclusions

Joint Research Center (JRC) Founded in 1957 under the Treaty of Rome. Opened on May 16, 1960. Five scientific research units and two support groups with a total staff of approximately 350 persons.

Introduction Mono-energetic neutron source 7 MV Van-de-Graaff accelerator 7LiF(p,n)7Be, TiT(p,n)3He, D2(d,n)3He, TiT(d,n)4He DC (Ip,d < 50 A), pulsed beam available 4 + 1 non-T beam line n < 109 /s/sr ionisation chambers, NE213 neutron/gamma-ray detectors, BF3 counters, HPGe detectors Bonner spheres fast rabbit systems (T1/2 > 1s) for activation studies

Introduction (cont.) GELINA neutron TOF spectrometer 70 - 140 MeV electron accelerator repetition frequency: 40 - 800 Hz neutron pulse: 2 s - 1 ns @ FWHM n = 3.4 1013/s @ 800 Hz 12 different flight paths with a length between 8 and 400 m ionisation chambers, C6D6 detectors high-resolution -ray detectors fission chambers for flux monitoring

Introduction (cont.) For a successful design of nuclear applications the precise knowledge of the involved reaction cross- sections is a pre-requisite cross-section data measurements  point wise data cross-section data evaluation  nuclear reaction models Successful modelling of reaction cross-section data requires precise knowledge about the nuclear physics involved. In case of nuclear fission (n- or x-particle induced) shape of the nuclear energy landscape probability of competing reaction mechanisms

The OECD nuclear data network IAEA - INDC WPEC JEFF ENDF JENDL CENDL BROND NEA Databank Nuclear Energy Agency Nucl. Sci. Committee WPEC: Working Party for Evaluation Co-operation JEFF: Joint European Fission + Fusion datafile

Modelling Scission point configuration(s): AFF, TKE Fission modes ??? Nuclear energy landscape fission valleys (fission modes) fission barrier height and penetrability: EA, EB, hA, hB ground state properties: EI, EII (hyper-deformation: EC, hC, EIII) Quantitative predictions of FF characteristics Y(A,TKE, p, d, ...)

Fission within the MM-RNR model (Multi-Modal Random Neck-Rupture) Lawrence shapes Dumbbell representation geometric parameters (l,r,s,z,c) several fission modes common inner barrier individual outer barriers mode-specific scission configurations fission-fragment deformation fission-fragment intrinsic excitation Potential energy surface U. Brosa, S. Grossmann and A. Müller, Physics Report 197 (1990) 167 S. Oberstedt, F.-J. Hambsch and F. Vives, Nucl. Phys. A644 (1998) 289

Calculated fission modes (239U)

Fission modes in nuclear fission ? parameterisation in terms of two asymmetric and one symmetric fission modes quite successful for the fission of the CN 236,239U and 240Pu including fission modes into fission cross-section models  quantitative description of FF emission yields for 252Cf (SF) more modes necessary !???

Mass distribution and modal fit

TKE distribution and modal fit

Statistical Model Below second chance fission the neutron interaction takes place through direct and compound nuclear mechanisms. Beside fission the following processes are possible : elastic and inelastic scattering and radiative capture. For the direct mechanism the coupled channel method (ECIS code) is used and for compound nuclear mechanism a statistical model (STATIS code which takes into account sub- barrier effects and the multi-modal fission concept) is used.

ECIS code provides: Raynal J. Statistical Model II ECIS code provides: Raynal J. STATIS code provides: Vladuca G., Tudora A., Hambsch F.-J., Oberstedt S., Nucl.Phys. A707, 32 (2002)

Schematic View of Barrier Structure

Total cross section calculation

Elastic cross section

Differential elastic cross section

Differential elastic cross section

Differential elastic cross section

Inelastic cross section

Differential inelastic cross section

Radiative capture cross section

235U(n,f) cross section of the S2 mode and total

Resonance region of 231Pa

Fission Cross-section for 238U(n,f)

Branching Ratio for 238U(n,f)

235U Mass Distribution at En=1.0 MeV

238U Mass Distribution at En=1.2 MeV

238U Mass Distribution at En=0.9 MeV

“Fission-mode fluctuations in a resonance” further experiments in summer 2006 more data being analysed (PhD thesis) En (MeV) fission cross-section (b)

Quality of measurements

Comparison to Vives et al.

Data analysis

Parasitic reactions cross sections 13.6 mb 276 mb 25% Photo fission events at En=0.9 MeV, estimation on Total cross section 39 mb * 3 7.9 mb 11.6 mb

Conclusions Statistical model calculations reproduce very well the different reaction cross sections Inclusion of fission modes adds additional predicting power to the model Experimental verification of fission mode changes under way for 238U(n,f) Preliminary data show variations of fission fragment properties at vibrational resonances in 238U(n,f)

Consistency of results ? From barrier parameters calculation of fission isomers fails Barrier parameters from fission cross-section analysis are in contradiction with those following from resonance parameters obtained for intermediate structure resonances 3rd minimum (HD) experimentally confirmed for 234U (Krasnahorkay et al.)

Motivation IRMM has longstanding tradition in measuring fission fragment properties and cross-sections. Experimental data are well described within the multi-modal fission model. Evaluation activity (and experimental activity) is diminishing. New evaluation approach based on model calculations with experimental input parameters. Prediction of fission fragment mass yield distributions.

Statistical Model III From the former relations it is obvious that correct calculations for the compound nucleus (fission, radiative capture, elastic and inelastic CN cross-sections) need good direct interaction calculations. The coupled channel parameters are determined in such a way to obtain the best possible agreement of the “direct” calculations with the total experimental cross- section and with experimental (differential) elastic and inelastic cross-sections at incident neutron energies E3 MeV, where the compound nucleus contributions is very small and can be neglected.