Anisotropic neutron evaporation from spinning fission fragments Kazimierz Kaz 01 E. Chernysheva, Frank Lab Dubna O. Dorvaux, In2P3 Strasbourg F.-J. Hambsch, IRMM Geel F.Hanappe, ULB Bruxelles J. Itkis, Flerov Lab Dubna Y. Kopatch, Frank Lab Dubna M. Mutterer, TU Darmstadt L. Stuttgé, In2P3 Strasbourg H.-J. Wollersheim, GSI Darmstadt F. Gönnenwein, Univ. Tübingen for the collaboration

Kinematical anisotropy of neutrons in lab system Assume neutrons are evaporated from fully accelerated fission fragments but with the emission in their own cms being isotropic θ cm θ lab Centre of mass system of LF Laboratory system velocityn velocities of LF fragment In the transformation from the cms to the lab system the neutrons are thrown into the forward hemisphere. In the lab the angular distribution of neutrons is no longer isotropic as shown here for LF moving to the left Likewise the energy distribution of neutrons is transformed fission axis LF HF Experimental Result for 252 Cf(sf) Density of neutrons in velocity space: ρ(V)d³V = ρ(V,θ lab ) V² dV dω for FF masses ~ 120 and ~ 132 H.R.Bowman et al Kaz 2

adiabatic Mass Fission Fragments carry large angular momenta J.O.Rasmussen et al 1969 M.Zielinska-Pfabé, K. Dietrich 1974 Bending Model: Coulomb + nuclear forces bring about a potential pocket which aligns deformed fragments on the fission axis. Angular vibrations are excited as zero point oscillations or non-adiabatically at neck rupture or thermally. 235U(n,f) 238U(γ,f) J.L.Durell 1997D.DeFrenne 1984 / ħ Mass Large angular momenta Note: Angular momenta are perpendicular to fission axis ● ● Non-deformed (spherical) fragments acquire very large angular momenta via single particle excitations 2008 Kaz 3

Neutron evaporation from rotating nuclei Neutrons evaporated from a rotating nucleus will preferentially be emitted in the equatorial plane of spin z x I fission axis n n cms n anisotropy ~ (1 + A sin² θ) θ Averaging over all possible orientations of spin ┴ fission axis, a forward – backward preference along fission axis results in cms Averaged over spin cms n anisotropy ~ (1 + b cos² θ cm ) For fixed spin spin where θ is polar angle relative to spin where θ cm is polar angle relative to fi-axis Calculated anisotropy in cms of fragment Isotropic evaporation for l = 0 neutrons. Anisotropic evaporation for l > 0. Note that the yield of neutrons is rapidly decreasing for l increasing From V. Bunakov, I. Guseva et al Kaz 4

non-zero n-anisotropy in cms observed from fit to n-spectrum Ratio of n spectra with/without anisotropy where cms anisotropy ~ ( 1 + b cos² θ cm ) b = 0 n-spectrum from evaporation theory J. Terrell 1959: “it is probably not possible to prove anything about anisotropy from the fission neutron spectrum alone” 235 U(n,f) E n = thermal without cms anisotropy From F.-J. Hambsch et al Kaz 5

ISSUE Scission Neutrons and/orcms Anisotropy The cms anisotropy will reinforce the kinematical anisotropy in the lab However, the effect is very small, prohibitively small for experiments From V. Bunakov, I.Guseva et al U(n,f) Isotropy in cms No scission n % scission n 50% from LF 35% from HF 235 U(n,f) Angular distribution of neutrons in lab as a function of angle θ lab relative to LF. The intensities with both LF and HF contributing at 0° : 90° : 180° are 9 : 1 : 4 From K. Skarsvag et al Kaz 6

z x I fission axis n n θ spin Concept of “CORA” experiment How to disentangle in the lab the contributions of kinematical and cms anisotropy? Analyse triple coincidences between 1 fission fragment and 2 fission neutrons z x y I O ● ● ● ● Assume - for the sake of argument - extreme cms anisotropy: all n in a plane spin Project fission axis and all n events on a plane perpendicular to fi axis. All neutron events will be aligned on a single line, e.g. the x-axis. as observed in the projection plane Extreme cms ANISOTROPYPerfect ISOTROPY triple coincidences Assume - for the sake of argument - orientation of fragment spin is fixed x y ● O ●● ● ●● ● x y O ● ● ● ● ● ● ● Φ1Φ1 Φ2Φ2 all n-events on a line n-events distributed isotropically in plane Due to kinematical focussing density of events is enhanced near origin O Plane of projection Plane of projection Fission axis 2008 Kaz 7

Setup of “CORA” experiment 252 Cf(sf) fission source CODIS back-to-back Ionisation Chamber with 5-sector cathode DEMON neutron detectors Up to 100 units ● Fission axis x y O Projection plane for evaluation Only fission events in cone ± 15° are considered VIEW FROM TOP 2008 Kaz 8

252 Cf spont. fission source 2008 Kaz 9 Layout of experiment CORA II (June 2008) Fragment detector CODIS In green: modular neutron detectors DEMON

Predictions from Theory n-anisotropy in FF cm system A sin² θ cm heavy fragment light fragment Distribution of ΔΦ with ΔΦ the difference in Φ-angle for 2 neutrons from same fi event relative to fragment spin I. Guseva Kaz 10

MC simulation for isotropic neutron distribution Due to the modular pattern of the neutron detectors of DEMON the counting efficiency can not be expected to be perfectly flat Neutron counting efficiency versus detection angle Φ Difference ΔΦ = Φ 2 – Φ 1 in Φ- angle for two neutrons detected per fission Contrary to the Φ-distribution the ΔΦ- distribution is rather smooth 2008 Kaz 11

Summary ● WELL KNOWN: fission neutrons being emitted from moving fragments exhibit a kinematical anisotropy in lab ● ISSUE: are neutrons evaporated isotropically or anisotropically in their own cms system ? ● SUGGESTION: cms anisotropy may be attributed to large angular momenta of fission fragments ● PROBLEM: how to disentangle in experiment kinematical and cms anisotropy ? ● CORA: is a project to directly observe the cms anisotropy based on triple correlation data (fragment, n, n) ● EVALUATION: particular scheme of evaluation allows to have model-free access to cm anisotropy with kinematical anisotropy being “switched off” ● EXPERIMENT: underway since July 2008 ● FINAL AIM: find from experimental ΔΦ distribution the cms anisotropy (1 + A sin² θ) relative to FF spin and by averaging over spin the cms anisotropy (1 + b cos² θ cm ) relative to fission axis. Calculate n-spectra taking into account anisotropy b and compare to experiment 2008 Kaz 12