Anisotropic neutron evaporation from spinning fission fragments

1. Anisotropic neutron evaporation from spinning fission fragments Kazimierz 2008 F. Gönnenwein, Univ. Tübingen for the collaboration 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 2008 Kaz 01

2. Kinematicalanisotropy of neutrons in lab system Assume neutrons are evaporated from fully accelerated fission fragments but with the emission in their own cms being isotropic Experimental Result for 252Cf(sf) n velocities velocity n velocities of LF fragment θlab θcm HF LF fission axis Centre of mass Laboratory system of LF system 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 Density of neutrons in velocity space: ρ(V)d³V = ρ(V,θlab) V² dV dω for FF masses <ML> ~ 120 and <MH> ~ 132 Likewise the energy distribution of neutrons is transformed H.R.Bowman et al. 1963 2008 Kaz 2

3. 12 adiabatic 8 4 84 108 132 156 Mass Fission Fragments carry large angular momenta J.L.Durell 1997 D.DeFrenne 1984 Note: 238U(γ,f) 235U(n,f) 12 Large angular momenta 8 Angular momenta are perpendicular to fission axis <Iprim> / ħ <Iprim> / ħ 4 80 160 120 Mass 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. ● ● J.O.Rasmussen et al 1969 M.Zielinska-Pfabé, K. Dietrich 1974 Non-deformed (spherical) fragments acquire very large angular momenta via single particle excitations 2008 Kaz 3

4. Neutron evaporation from rotating nuclei Neutrons evaporated from a rotating nucleus will preferentially be emitted in the equatorial plane of spin Calculated anisotropy in cms of fragment I spin x n fission axis θ z n For fixed spin cms n anisotropy ~ (1 + A sin² θ) where θ is polar angle relative to spin Averaging over all possible orientations of spin ┴ fission axis, a forward – backward preference along fission axis results in cms Isotropic evaporation for l = 0 neutrons. Anisotropic evaporation for l > 0. Note that the yield of neutrons is rapidly decreasing for l increasing Averaged over spin cms n anisotropy ~ (1 + b cos² θcm) From V. Bunakov, I. Guseva et al. 2004 where θcm is polar angle relative to fi-axis 2008 Kaz 4

5. non-zero n-anisotropy in cms observed from fit to n-spectrum 235U(n,f) En = thermal without cms anisotropy Ratio of n spectra with/without anisotropy where cms anisotropy ~ ( 1 + b cos² θcm) n-spectrum from evaporation theory b = 0 J. Terrell 1959: “it is probably not possible to prove anything about anisotropy from the fission neutron spectrum alone” From F.-J. Hambsch et al. 2003 2008 Kaz 5

6. ISSUE Scission Neutrons and/or cms Anisotropy Isotropy in cms No scission n - - - - 15% scission n 50% from LF 35% from HF 235U(n,f) 235U(n,f) The cms anisotropy will reinforce the kinematical anisotropy in the lab Angular distribution of neutrons in lab as a function of angle θlab relative to LF. The intensities with both LF and HF contributing at 0° : 90° : 180° are 9 : 1 : 4 However, the effect is very small, prohibitively small for experiments From V. Bunakov , I.Guseva et al 2005 From K. Skarsvag et al 1963 2008 Kaz 6

7. 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 Assume - for the sake of argument - orientation of fragment spin is fixed 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. I spin y I x n x ● All neutron events will be aligned on a single line, e.g. the x-axis. fission axis θ ● ● z O z ● n Extreme cms ANISOTROPY Perfect ISOTROPY y y triple coincidences ● Plane of as observed in the projection plane projection Plane of ● Φ1 Φ2 ● Fission axis ● ● ● ● ● ● ● ● Due to kinematical focussing density of events is enhanced near origin O O ● x O x ● ● projection all n-events on a line n-events distributed isotropically in plane 2008 Kaz 7

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

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

10. Predictions from Theory n-anisotropy in FF cm system Distribution of ΔΦ with ΔΦ the difference in Φ-angle for 2 neutrons from same fi event relative to fragment spin A sin² θcm heavy fragment I. Guseva 2007 light fragment 2008 Kaz 10

11. MC simulation for isotropic neutron distribution Neutron counting efficiency versus detection angle Φ Difference ΔΦ = Φ2 – Φ1 in Φ- angle for two neutrons detected per fission Due to the modular pattern of the neutron detectors of DEMON the counting efficiency can not be expected to be perfectly flat Contrary to the Φ-distribution the ΔΦ- distribution is rather smooth 2008 Kaz 11

12. 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 θcm) relative to fission axis. Calculate n-spectra taking into account anisotropy b and compare to experiment 2008 Kaz 12