16.05.2009FISSION-2009 1 The Influence of Rotation of the Scissioning Nucleus onto Angular Distribution of Light Particles in Ternary Fission Induced by. - презентация

1 FISSION The Influence of Rotation of the Scissioning Nucleus onto Angular Distribution of Light Particles in Ternary Fission Induced by Cold Polarized Neutrons A.Gagarski, PNPI, Russia Petersburg Nuclear Physics Institute, Gatchina, RUSSIA Physikalisches Institut, Tübingen, GERMANY Institut für Kernphysik, TU Darmstadt,GERMANY Frank Laboratory, JINR, Dubna,RUSSIA Khlopin Radium Institute, St.Petersburg, RUSSIA Department of Physics, University of Jyväskylä,FINLAND Institut Laue-Langevin, Grenoble, FRANCE

2 FISSION Introduction p LF p HF p TP ~82 o For 252 Cf from M. Mutterer et al.

3 FISSION Ternary fission by polarized neutrons p LF p HF W1W1 σ0σ0 p TP W0W0 Questions: Is W1 = W0 ? Does the rotational (or mirror) symmetry hold? What happens if we reverse the spin? Answers: W1 W0 (up to ~10 –2 ) Symmetry is violated The angular distribution will change.

4 FISSION ROT effect p LF p HF σ0σ0 p TP

5 FISSION TRI effect p LF σ0σ0 p TP p HF

6 FISSION Experimental method Measurement of all involved angles: Diodes size ~30x30 mm 2 Position sensitive MWPCs (~ 2 mm) Spectroscopy of fission products: Energies of TPs, Masses and energies of FFs from times of flight Neutron spin flip frequency 1 Hz Relative measurements Control and suppression of false setup asymmetries: Comparing of A obtained for events recorded by symmetrical detector combinations Depolarized neutrons beam SD- array LF HF MWPC TP SD- array σ+σ+ σ–σ– MWPC

7 FISSION The setup

8 FISSION The ROT asymmetry in the counts rates LF HF TP σ+σ+ σ–σ–

9 FISSION The TRI asymmetry in the counts rates LF HF TP σ+σ+ σ–σ–

10 FISSION Dependences of the Asymmetries on LF-LCP angle Y(θ) – the experimental angular distribution Y'(θ)– its derivative S ROT is characteristic parameter of ROT effect– the angular shift between TP distributions for the two spin polarizations D TRI characterized TRI effect – it is one half of the relative difference in total probabilities for TPs to be emitted towards the upper (lower) hemisphere for the two spin directions Corrections: – degree of polarization of the neutron beam – admixture of accidental coincidences – portion of wrongly identified LF-HF – the geometrical efficiency

11 FISSION Experimental dependencies and their fits 235 U 233 U 239 Pu TRI and ROT effects parameters ReactionJS ROT ( o )D TRI (×10 3 ) 235 U(n,f)3 –, 4 – U(n,f)2 +, – Pu(n,f)0 +, –

12 FISSION Experimental dependencies and their fits

13 FISSION Model of ROT-effect

14 FISSION Model of ROT-effect S ROT The collective rotation lasts also during the acceleration phase of TPs in Coulomb field of the FFs. The rotation deflects the LCP trajectory in the coordinate frame linked with FFs towards or away from the LF depending on the rotation direction.

15 FISSION Model of ROT-effect The rotation is being polarized after capturing of the polarized cold neutron Effective R – averaged over (J,K)

16 FISSION Trajectory calculations Calculations for rotating nucleus (!) Starting scission configuration was optimized to describe known experimental angular and energy distributions of ternary particles σ(J,K) R ω σ(4–) / σ(3–) = 1.8 (Kopach et al.); for J = 4 K = 0 and for J = 3 K = 2 were taken R ~ 0.7 h for 235 U Details can be found in contribution made by I. Guseva

17 FISSION ω P LF Naive explanation for TRI-effect ROT: motion in Coulomb field after scission of rotating nucleus, direct indicator of the rotation, allow to determine the velocity and direction of the rotation TRI: influence of the rotation onto axial symmetry in the neck just before rupture or directly at rupture moment v F Corio v F Catap r ω P LF

18 FISSION Values of ROT- and TRI-effects TRI and ROT effects parameters ReactionJS ROT ( o )D TRI (×10 3 ) 235 U(n,f)3 –, 4 – U(n,f)2 +, – Pu(n,f)0 +, – Variation of ROT values: different mixtures of (J,K) channels in the cross-section Variation of TRI values : interplay between Coriolis and Catapult forces. Catapult is always there if there is a rotation, while Coriolis needs vibrations related with K. Higher K lower R! 239Pu is especially interesting --- less contributed channels

19 FISSION Perspectives Study of the influence of transition states spins and parities onto TRI- и ROT-asymmetries signs and values : 233 U(n,f) (J =2+, 3+) have to be re-measured with better accuracy; interesting to compare it with 241 Pu(n,f) (J =2+, 3+); another possible candidate is 245 Cm(n,f) (J=3+,4+) Comparison of the TRI- и ROT-effects in resonances of 236 U* neutron energy, eV 236 U* total σ, b Search for TRI- и ROT-effects for fast neutrons and gammas from 235 U(n,f)

20 FISSION Conclusion The angular distribution of light particles in ternary fission by polarized neutrons exhibits two type of effects while flipping the neutron spin: TRI-effect (different probabilities to be emitted into opposite hemispheres), and ROT-effect (shift of the angular distributions relative to the light fragment direction). Both effects were studied and separated in 233 U(n th,f), 235 U(n th,f), 239 Pu(n th,f). The results show that the effects are very sensitive to the scission configuration and parameters of transition states, and hence they can give a new possibility to study fission process. Theoretical study of the asymmetries is very important

21 FISSION

22 FISSION Fitting for individual slices of the parameters The fission events data were sorted into slices over parameters: E TP, M LF, E tot Fitting by model function D ROT (E TP ), D ROT (M LF ), D ROT (E tot ), S ROT (E TP ), S ROT (M LF ), S ROT (E tot )

23 FISSION TRI-effect in 235 U (D TRI SCALING coefficient )

24 FISSION Trajectory calculations of ROT effect in 235 U Effective momentum R was calculated from: σ(J 4– ) / σ(J 3– ) = 1.8 (Kopach et al.) for J 4– K = 0 and for J 3– K = 2 were taken

25 FISSION Основные экспериментальные результаты (TRI эффект в 233 U) ( г.) = – и p-d-t = –

26 FISSION Статистическая модель Бунакова для TRI- асммметрии Есть некоторый вклад начального спина нейтрона в соответствующую проекцию углового момента осколков в момент деления TP уносит угловой момент из делящейся системы В зависимости от направления эмиссии TP, соответствующая проекция углового момента осколков увеличивается или уменьшается плотность уровней системы зависит этой проекции, а в статистической модели плотность уровней определяет вероятность JLJL JHJH l

27 FISSION Статистическая модель Бунакова для TRI- асммметрии Спин компаунд ядра Угловой момент TP Параметр плотности уровней Поляризация компаунд ядра Момент инерции Внутреннее возбуждение осколков Фактор передачи поляризации P(J + ) = (2I + 3) / [3 (2I + 1)] P n дляJ + = I + 1/2 P(J – ) = –1/3 P n дляJ – = I – 1/2 Сечение при низких энергиях – суперпозиция резонансов D = [D(J + ) σ(J + )+D(J – ) σ(J – )] / [σ(J + )+σ(J – )]

28 FISSION Энергия возбуждения Кинетическая энергия Потенциальная энергия R, ферми E tot = E sc + Δ V E k tot = E k sc + E k coul J J J КАЧЕСТВЕННАЯ КАРТИНА ПРОЦЕССА ДЕЛЕНИЯ Точка разрыва ~20 ферми) Деформация, ферми Наиболее актуальная проблема современной физики деления – динамика спуска сильно деформированного ядра с барьера и его разрыв! ЕрЕр ЕрЕр ЕсЕс ЕсЕс Е*Е* Е*Е* Е k Е * tot E k tot

29 FISSION Исследование Т-нечётной асимметрии в зависимости от параметров продуктов деления Нейтронный пучок (PF1 в ИЛЛ): ~ 4.5Å; Φ capture ~ n/cm 2 s; продольно поляризован ~ 94 1 %; радиочастотный флиппер 1 Гц Мишень ~3.4 мг 233 U (UF 4 ) ~100 мкг/см 2 на тонкой титановую пленке (~100 мкг/см 2 ) PIN диодов для TP, каждый мм, толщина 380 мкм Определение типа частицы по времени нарастания сигналов с PIN диодов Координатная чувствительность MWPC (~2 мм по обеим координатам) положение на мишени и углы можно определить: –массу осколков : M1/M2 T1/T2, –кинетическую энергию: E=E1+E2 L2 A/2T1 T2 (Разрешение невелико, поскольку T/T~1/10)

30 FISSION Model of ROT-effect S ROT LF HF The collective rotation lasts also during the acceleration phase of TPs in Coulomb field of the FFs. The rotation deflects the LCP trajectory in the coordinate frame linked with FFs towards or away from the LF depending on the rotation direction.

31 FISSION Модель ROT-эффекта