13.06.2008A. Gagarski ISINN-16, Dubna 1 DETAILED STUDY OF THE EFFECTS FOLLOWING FROM ROTATION OF THE SCISSIONING NUCLEI IN TERNARY FISSION OF 235 U BY. - презентация

1 A. Gagarski ISINN-16, Dubna 1 DETAILED STUDY OF THE EFFECTS FOLLOWING FROM ROTATION OF THE SCISSIONING NUCLEI IN TERNARY FISSION OF 235 U BY COLD POLARISED NEUTRONS (ROT AND TRI EFFECTS) A.Gagarski, PNPI, Russia Petersburg Nuclear Physics Institute, Gatchina, RUSSIA Physikalisches Institut, Tübingen, GERMANY Institut für Kernphysik, TU Darmstadt,GERMANY Frank Laboratory, JINR, DubnaRUSSIA Khlopin Radium Institute, St.Petersburg, RUSSIA Department of Physics, University of Juvaskyla,FINLAND Institut Laue-Langevin, Grenoble, FRANCE

2 A. Gagarski ISINN-16, Dubna 2 Т-odd correlation in ternary fission by polarized neutrons (1998) W( LF, TP ) ~ (1 + D p f p TP ) P(θ) p f – momentum of light (heavy) fragment, p TP – momentum of light ternary particle (TP) σ – neutron spin θ– angle between p f and p TP P(θ)– conventional angular distribution of TP σ p lf p TP p hf θ

3 A. Gagarski ISINN-16, Dubna 3 Experimental method of the T-odd correlation study N i ( ) count rates of TP–FF coincidences for different selection criteria on angles, E TP and others recorded parameters of the fission event; ( ) ( ) – neutron spin direction, which is periodically flipped (~1 Hz). W( LF, TP ) ~ (1 + D p f p TP ) P(θ)

4 A. Gagarski ISINN-16, Dubna 4 Experimental method of the T-odd correlation study Measurement of all involved angles: Minimum diodes size 30 х 30 mm In some experiments position sensitive MWPCs (~ 2 mm) Spectroscopy of fission products: Energies of TPs, in some experiments в even their type (Alphas or Hydrogen isotopes) Masses and energies of FFs from time of flights Control and suppression of false setup asymmetries: Comparing of A i obtained for equivalent parameters i, but for events recorded by symmetrical detectors combinations

5 A. Gagarski ISINN-16, Dubna 5 The setup

6 A. Gagarski ISINN-16, Dubna 6 Asymmetry in 233 U ( ) θ TRI W( LF, TP ) ~ (1 + D p f p TP ) P(θ) Average values of the asymmetry = 3,9 ± 0,12 × 10 -3

7 A. Gagarski ISINN-16, Dubna snsn LR TRI Asymmetry in 235 U (2005) Average values of the asymmetry coefficient Detectors combination × ,4 ± 0,19+3,2± 0, snsn R L ROT θ, degree W( LF, TP ) ~ (1 + D p f p TP ) P(θ)

8 A. Gagarski ISINN-16, Dubna 8 Angular dependence of the asymmetry in 233 U and 235 U (comparison) 233 U 235 U 233 U TRI ~ – ROT ~ U TRI ~ ROT ~ 0.004

9 A. Gagarski ISINN-16, Dubna 9 Model of ROT-effect The compound nucleus having captured a polarized neutron will likewise be polarized with capture spins of 2+ or 3+ for 233 U (3+ or 4+ for 235 U) The quasi-stationary transition states at the saddle point of the fission barrier are lying in the gap between the barrier and single-particle excitations. The transition states are, hence, collective in nature. In an adiabatic process also at the scission point the nucleus retains a strong collective contribution to be described by (J, K) quantum numbers. The angular velocity ω of the rotation is found from the relation:

10 A. Gagarski ISINN-16, Dubna 10 Model of ROT-effect Δ θ ~0.2°

11 A. Gagarski ISINN-16, Dubna 11 Trajectory calculations Numerical integration of the motion equations. We performed such calculations for the first time for rotating nucleus. Starting scission configuration was optimized to describe known experimental angular and energy distributions of ternary particles. Then the ω, calculated on the base of known data about spins contribution into crossection and compound-nucleus polarization, was added to the conventional trajectory calculation. Obtained effective rotational momentum R ~1 h with the reasonable starting configuration gives the angular distribution shift ~ 0,2 o.

12 A. Gagarski ISINN-16, Dubna 12 New results – detailed study of the asymmetries in 235 U ternary fission (2007 г.) 235 U

13 A. Gagarski ISINN-16, Dubna 13 Angular dependence of the asymmetry in 235 U ROT TRI S ROT = 0.215(5) o D TRI = (2)

14 A. Gagarski ISINN-16, Dubna 14 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 )

15 A. Gagarski ISINN-16, Dubna 15 TRI-effect in 235 U (D TRI SCALING coefficient )

16 A. Gagarski ISINN-16, Dubna 16 ROT-effect in 235 U (S ROT angular SHIFT)

17 A. Gagarski ISINN-16, Dubna 17 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

18 A. Gagarski ISINN-16, Dubna 18 ROT and TRI in 235 U and 233 U 233U ROT: 2Δ~0.05 o TRI: (12) 235U ROT: 2Δ=0.215(5) o TRI: (2) Main puzzle -- why the two neighboring isotopes behave so different in polarized neutron induced ternary fission? Big ROT and small TRI in 235 U Small ROT and big TRI in 233 U Same sign of ROT in both of them, but opposite TRI effect

19 A. Gagarski ISINN-16, Dubna 19 Naive explanation of TRI-asymmetry 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 another degrees of freedom just before rupture or directly at rupture moment F catap = m [r dω/dt] F Cori = 2m [v ω] Difference between 235 U and 233 U could be in different collective motion at rupture moment (f.ex. in 235 U there can be no transverse vibrations (К=0)) May be the difference could be linked to difference in total ternary-to-binary yields for the two nuclei? (20 % difference in ternary-to-binary yields)

20 A. Gagarski ISINN-16, Dubna 20 Perspectives Search for ROT- effect in 239 Pu ternary fission { 240 Pu* (0+,1+)} Study of the influence of transition states spins and parities onto TRI- и ROT-asymmetries signs and values : { 236 U* (3–, 4–) – 246 Cm*(3+,4+)}; { 234 U* (2+, 3+) – 242 Pu* (2+, 3+)} Comparison of the TRI- и ROT-effects neutron energy, eV 236 U* total σ, b Search for TRI- и ROT-effects for fast neutrons and gammas from 235 U(n,f) in resonances of 236 U*

21 A. Gagarski ISINN-16, Dubna 21 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 light fragment direction). Both effects were studied and separated in 235 U(n th,f). The results shows that the effects can give a new possibility to study fission dynamic at low excitation energies. Theoretical study of the T-odd asymmetries is very important (!)

22 A. Gagarski ISINN-16, Dubna 22

23 A. Gagarski ISINN-16, Dubna 23 Ternary Fission In Ternary Fission (TF) besides the two Fission Fragments a third light charged particle (LCP) is emitted (discovered in 1946) LCP is "witness" of nuclear matter breaking possibility to get information on a configuration of nuclear system at rupture moment TF was intensively studied (Yields, Energies, Angles). The probability of TF relative to Binary Fission is ~97% of LCPs are He and Н isotopes, and ~90% – alpha particles. Average angle of emission is ~82 o relative to LF Mean energy ~16 MeV (for alpha particles) 252 Cf(sf) from M. Mutterer et al.

24 A. Gagarski ISINN-16, Dubna 24 Основные экспериментальные результаты (TRI эффект в 233 U) ( г.) = – и p-d-t = –

25 A. Gagarski ISINN-16, Dubna 25 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

26 A. Gagarski ISINN-16, Dubna 26 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 – )]

27 A. Gagarski ISINN-16, Dubna 27 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

28 A. Gagarski ISINN-16, Dubna 28 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)

29 A. Gagarski ISINN-16, Dubna 29 Модель ROT-эффекта