Skt Petersburg Nuclear Physics Institute of RAS STUDIES

Skt. Petersburg Nuclear Physics Institute of RAS STUDIES OF FISSION DYNAMICS THROUGH THE SEARCH OF SCISSION NEUTRONS 12 -16. 04. 2010 Sacley "The scission process: The last stage of nuclear fission" 1

CONTENT 1. INTRODUCTION 2. SEARCH FOR “SCISSION RADIATIONS” AND INVESTIGATIONS OF THEIR CHARACTERISTICS 3. - Scission neutrons emitted near the rupture point - Scission gamma-radiation emitted near the rupture point 3. ESTIMATES OF SCISSION NEUTRONS YIELDS FROM 239 Pu(n, f) AND 252 Cf(s, f) REACTIONS 4. 233, 235 U(n, f) - Main results of the neutron energy and angular distributions measurements for different fragment energies and masses in 233, 235 U(n, f) reactions - Main results of the (n-n)-coincidence measurements in 233, 235 U(n, f), 239 Pu(n, f) and 252 Cf(s, f) reactions 5. 4. TRI- AND ROT-EFFECTS OF THE LIGHT CHARGED and NEUTRAL PARTICLE EMISSION ASYMMETRIES 6. - general mechanisms of the T-odd asymmetry effects appearance in ternary and binary fission of polarized heavy nucleus - first results of the effects investigations for the fast neutrons and gammarays in 233, 235 U(n, f) fission 12 -16. 04. 2010 Sacley 7. "The scission process: The last stage of nuclear fission" 2

Qualitative pictures of the low excitation energy fission TOTAL ENERGY Excitation energy Potential energy Kinetic energy Rupture point, 20 fm 12 -16. 04. 2010 Sacley "The scission process: The last stage of nuclear fission" R, fm 3

EXISTING SITUATION WITH SCISSION NEUTRON YIELDS IN FISSION 1. From general point of view one may expect relatively high probability of scission neutron emission near the time of fissioning nuclei rupture (no Coulomb barrier!) 2. In 1962 R. Fuller presented the first estimate of neutron emission as a result of the fast nonadiabatic change of nuclear potential in the rupture process (about 0. 4 neutron at t ~ 1, 5 10 -21 sec) 3. . Prompt neutron emission in non-adiabatic passage from the barrier top to the scission point (“pre-scission neutrons” ~10 -21 s. ) R. Fuller (1962). J. Boneh (1978). ( Well known effect in heavy ion induced fission, but smaller ~ 1% if > 10 -20 s. ) 4. Prompt neutron emission in rupture point (time scale – a few of ~10 -22 s) J. Negele (1982) 5. Instantaneous neutron emission as a result of the neck remnant “snatching” (Catapult ” mechanism of neutron emission. (Time scale – about ~10 -22 s. ). K. Dietrich (1981), Madler (1985). 6. G. Val’ski (2002) under statistical consideration of light particle emission in ternary fission and using interpolation method had obtained estimate about 0. 55(9)1/f for the case of 235 U(n, f) and 0. 18(4)1/f for 252 Cf spontaneous fission. 7. Prompt neutron emission from highly excited fission fragments (up to 90% of total neutron yield. 12 -16. 04. 2010 Sacley "The scission process: The last stage of nuclear fission" 4

EXPERIMENTAL RESULTS OF SCISSION NEUTRON SEARCH Nuclear reaction Type of experiment % of scission neutrons Typical average SN energy, Mev Reference (n-f) correlations 15% 1. 58 Skarsvag (1963) 10% 3. 2 Kapoor (1963) 235 U + nth (n-n) correlations 20% Franklyn (1978) 235 U + nth (n-f) correlations (10 2)% Samant (1995) 235 U + nth Compilations 14% 0. 98 and 2. 74 Kornilov (2001) 235 U + nth Total spectra 15% Kornilov (2002) 252 Cf sp. fission (n-n) correlations 10% Pringle (1975) 252 Cf sp. fission (n-f) correlations 20% > 1. 5 Piksaikin (1977) 252 Cf sp. fission 10% 2. 6 Bowman (1962) 252 Cf sp. fission (13. 2 3. 1)% 1 Riechs (1981) 252 Cf sp. fission “ 10% (1. 5 0. 3) Seregina (1985) 252 Cf sp. fission “ 1. 1% 0. 39 B. -Jorgenson (1988) 252 Cf sp. fission “ No scis. neutrons! Marten (1989) 252 Cf sp. fission “ < 1% Blinov (1989) 252 Cf sp. fission 10% 0. 9 and 3. 1 Kornilov (2001) “ “ “ Compilations 12 -16. 04. 2010 Sacley "The scission process: The last stage of nuclear fission" 5

BASIC REASONS AND PECULIARITIES OF OUR INVESTIGATIONS Because of strong scattering of available experimental data about scission neutrons existence and their properties we concentrated our attention only on the scission neutron yields and on general type of their energy spectra. As this takes place: 1. We tried to use all known methods of scission neutron observation on the existed background of fast neutrons evaporated from the excited fission fragments, namely: - correlation measurements of neutron energy and angular distributions for different masses and energies of fission fragments - angular correlations of (n-n)-coincidences for different neutron energy thresholds under conditions of integration over all the other features of neutron emission - new TRI and ROT-effects of T-odd emission asymmetry recently observed for the light charged particles in ternary fission 2. We plan to perform, where possible, such types of investigations for the fission reactions 252 Cf(s. f. ), 233, 235 U(n, f), and 239 Pu(n, f) in the same experimental conditions. 3. Under the experimental data analysis and evaluation we used the following main suppositions: - taking into account very small probability of cascade emission we supposed Weiskopf energy spectra for scission neutrons isotropic emitted in CMS, - we have taken into account the big fragment angular momenta oriented relatively fission axis, - we have neglected possibility of neutron evaporation during fragments acceleration. 12 -16. 04. 2010 Sacley "The scission process: The last stage of nuclear fission" 6

SCHEMATIC VIEW OF EXPERIMENTAL SET-UP Reaction Chamber: 235 U target (Ø 15 mm) – 280 μg/сm 2 UF 4 onto 70 μg/сm 2 Ti backing; start MWPC: (68 x 92 mm 2) located within 7 mm range from the 235 U target; stop MWPC: (72 x 38 mm 2) located at a distance of 140 mm from the chamber axis. Neutron detectors: stilbene crystals (50 x 50 mm 2 and 40 x 60 mm 2 mounted on the Hamamatsu - R 6091) neutron registration threshold: – 150 200 ke. V; double-discrimination method: – pulse shape and time-offlight criteria time-of-flight distance: from 235 U target – ~ 50 cm 12 -16. 04. 2010 Sacley 8 1 7 2 6 3 5 4 4 5 3 6 2 7 1 "The scission process: The last " stage of nuclear fission" 8 77

ENERGY - ANGULAR DISTRIBUTIONS OF THE NEUTRONS IN FISSION 12 -16. 04. 2010 Sacley "The scission process: The last stage of nuclear fission" 235 U 8

The neutron yields from different fragment masses for 235 U(n, f) and total neutron yields as a function of pre-neutron fragment masses 12 -16. 04. 2010 Sacley "The scission process: The last stage of nuclear fission" 9

AVERAGE NEUTRON ENERGY AS A FUNCTION OF EMISSION ANGLE AND N(00)/N(900) AND N(1800)/N(900) RATIO FOR DIFFERENT NEUTRON ENERGIES IN 235 U FISSION 12 -16. 04. 2010 Sacley "The scission process: The last stage of nuclear fission" 10

Energy AVERAGE NEUTRON ENERGY AS A FUNCTION OF EMISSION ANGLE AND N(00)/N(900) AND N(1800)/N(900) RATIO FOR DIFFERENT NEUTRON ENERGIES IN 233 U FISSION 12 -16. 04. 2010 Sacley "The scission process: The last stage of nuclear fission" 11

ANISOTROPY OF NEUTRON EMISSION IN THE C. M. S. OF 235 U(n, f) FISSION FRAGMENTS (I. Guseva, PNPI) The red lines are the Monte-Carlo calculations of neutron emission anisotropy. The blue lines are neutron spectra in the center-of-mass system. At the average <JLF>=7ћ and <JHF> = 8ћ average values of anisotropy were found to be: 6. 3% - for light fragments and 9. 5% - for heavy fission fragments. 12 -16. 04. 2010 Sacley "The scission process: The last stage of nuclear fission" 12

The total prompt neutron spectra for 233, 235 U(n, f): experiment – full circles, calculations – the lines (see legends). Angular distribution of prompt neutrons in the center-of-mass system of fragments are given approximately by: φ(E c. m. s. , c. m. s. ) = 1 + A 2 Ec. m. s (3 cos 2( c. m. s ) - 1) / 2 12 -16. 04. 2010 Sacley "The scission process: The last stage of nuclear fission" 13

AVERAGE NEUTRON YIELDS FOR DIFFERENT ANGLES IN LABORATORY COORDINATE SYSTEM Comparison of the 235 U and 233 U fission data obtained with two different neutron detectors 233 U 235 U Model calculations were performed with taking into account the angular anisotropy of fast neutron emission from excited fission fragments (A 2 = 0. 06) 12 -16. 04. 2010 Sacley "The scission process: The last stage of nuclear fission" 14

AVERAGE NEUTRON ENERGIES FOR DIFFERENT ANGLES IN LABORATORY COORDINATE SYSTEM Comparison of the 235 U and 233 U fission data obtained with two different neutron detectors Model calculations were performed with taking into account the angular anisotropy of fast neutron emission from excited fission fragments (A 2 = 0. 06) 12 -16. 04. 2010 Sacley "The scission process: The last stage of nuclear fission" 15

NEUTRON YIELDS AND AVERAGE ENERGIES FOR DIFFERENT ANGLES OF NEUTRON EMISSION IN 235 U(n, f) REACTION (SCISSION NEUTRONS YIELDS IS NOT MORE THEN 5%) 12 -16. 04. 2010 Sacley "The scission process: The last stage of nuclear fission" 16

NEUTRON YIELDS AND AVERAGE ENERGIES FOR DIFFERENT ANGLES OF NEUTRON EMISSION IN 233 U(n, f) REACTION (SCISSION NEUTRONS YIELDS IS NOT MORE THEN (4 -5%) 12 -16. 04. 2010 Sacley "The scission process: The last stage of nuclear fission" 17

SCHEMATIC VIEW OF EXPERIMENTAL SET-UP FOR (n-n)COINCIDENCE INVESTIGATIONS 12 -16. 04. 2010 Sacley "The scission process: The last stage of nuclear fission" 18

GENERAL VIEW OF EXPERIMENTAL SPECTRUM OF ( - ), ( -n), (n-n) COINCIDENCES AND ITS EXPANSION 12 -16. 04. 2010 Sacley "The scission process: The last stage of nuclear fission" 19

. SENSITIVITY OF (n-n)-COINCIDENCE METHOD TO THE SCISSION NEUTRON ADMIXTURE (a) AND TO THE ENERGY THRESHOLD OF NEUTRON REGISTRATIONS (b). Nsc/Ntot- 7% 12 -16. 04. 2010 Sacley "The scission process: The last stage of nuclear fission" 20

(n-n)-COINCIDENCES IN 252 Cf(s, f), 233, 235 U(n, f) AND 239 Pu(nf) REACTIONS Angular dependence of (n-n)-coincidences in 252 Cf fission Angular dependence of (n-n)- coincidences 233 U fission 12 -16. 04. 2010 Sacley Angular dependence of (n-n)-coincidences in 235 U fission Angular dependence of (n-n)-coincidences in 239 Pu fission "The scission process: The last stage of nuclear fission" 21

ESTIMATES FOR SCISSION NEUTRON YIELDS AND TEMPERATURES OF THE WEISKOPF ENERGY SPECTRA 252 -Cf 233 -U 235 -U *239 -Pu Scission neutron yields *(10 ± 2)% *(5 ± 2)% *(7 ± 2)% *(14 ± 2)% Parameters Temperature of spectrum *0. 8 Me. V NOT MORE THEN (4 -5)% *1. 1 Me. V *0. 9 Me. V • The data asterisked have been obtained from measurements (n-n)-coincidences • The yield estimates for 233, 235 U marked off by yellow color have been obtained from the energy and angular distributions of neutrons emitted from separated fragments 12 -16. 04. 2010 Sacley "The scission process: The last stage of nuclear fission" 22

TRI-effect of T-odd emission asymmetry for the third particle p. TP σ0 W( ) = 1 + DTRI∙σn∙[pfxp. TP] p. LF p. HF For the LCP: 235 U: DTRI= +(1. 7 0. 2)10 -3 233 U: DTRI = - (3. 9 0. 1)10 -3 From theoretical point of view such effect exists only for the particles appeared simultaneously! (A. Barabanov, 2001) 12 -16. 04. 2010 Sacley "The scission process: The last stage of nuclear fission" 23

Schematic diagram of ROT- effect appearance in ternary fission (Shift of LCP angular distributions) Observed shift of LCP angular distribution θ 2 In 235 U: 2 - 1 ~ 0. 20 in 239 Pu: 2 - 1 ~ 0. 020 θ 1 Anticlockwise rotation Clockwise rotation 12 -16. 04. 2010 Sacley "The scission process: The last stage of nuclear fission" 24

Shift of the third particle angular distribution (ROT- effect) p. TP σ0 ROT: 0. 215(5)o TRI: + 0. 0017 JL p. LF 235 U JH p. HF W( ) = 1+DROT∙σn∙[pfxp. TP]∙(pf∙p. TP) In the contrast to TRI-effect ROT-effect can exist for neutrons and -rays emitted from fragments as well (big oriented angular momenta!) 12 -16. 04. 2010 Sacley "The scission process: The last stage of nuclear fission" 25

TRI-effect of Т-odd asymmetry of scission neutron emission • The search for TRI and ROT- effects in scission neutron emission is a subject of much current interest. • Non zero value of these effects would indicate the scission neutrons existence in fission process. PM MWPC start n MWPC stop LF Existing information: <Dn> = - (9 ± 5)· 10 -4 233 U: <D > = - (3 ± 7)· 10 -4 PNPI-2005 n 235 U: │<D >│ < 4 10 -5 FRM-2 -2010 n HF 235 U: Expected value of TRI-effect at 5% scission neutron admixture has to be about 10 -4 The absence of TRI-effects for neutrons may be indicative of different mechanism for scission neutrons emission compared to the LCP in ternary fission 12 -16. 04. 2010 Sacley "The scission process: The last stage of nuclear fission" σ+ σ– MWPC n PM stop Fissile target Experimental set-up 26

Where “scission” -rays may be emitted? ● During fissioning system descent from the barrier top to the rupture Just in the rupture of strongly deformed fissioning system In the moment of the excited light charged particle decay (for example 5 He* ~ 10 -21 s. ; 7 He* ~ 4∙ 10 -21 s. ; 8 Li ~ 2∙ 10 -20 s. ) In the process of fission fragments acceleration (“bremstraglung”) But in all these cases the yield of such “scission” -rays compared to the -rays from fission fragments is extremely low !!! (less then 10 -2) 12 -16. 04. 2010 Sacley "The scission process: The last stage of nuclear fission" 27

MECHANIZM OF FALSE “ROT-EFFECT” APPEARENCE FOR NEUTRONS AND -RAYS System rotation J J l 90+ 90 - -φ + - Neutrons or γ-rays from fragments +φ J h -90 -45 0 45 90 System rotation In the contrast to the ROT-effect of asymmetry for LCP emission in ternary fission, the “ROT-effect” for neutrons and -rays from fragments may arise as a result of existence of large oriented moments in fission fragments appeared in the rupture process! 12 -16. 04. 2010 Sacley "The scission process: The last stage of nuclear fission" 28

Results of the search investigations of ROT-effect for -rays Result of G. Danilyan et al. FRM-2 reactor Result of G. Petrov et al. WWR-M reactor Observed angular distribution shift for -rays: 0. 10(3)0 compared to 0. 215(3)0 for LCP 12 -16. 04. 2010 Sacley "The scission process: The last stage of nuclear fission" 29

Expected ROT-effects for the neutrons emitted in polarized fission in comparison with -rays one 236 U* Expected form of “ROTeffect” for neutrons from fission fragments Expected form of ROTeffect for scission neutrons Observed form and value of “ROT-effect” for -rays ROT-effect form for “scission neutrons” has to be similar to the -rays one 12 -16. 04. 2010 Sacley "The scission process: The last stage of nuclear fission" 30

Perspectives of the further search investigations of ROTeffects for neutrons and -rays Observed shift of -rays angular distribution is direct confirmation of fissioning system rotation around the direction of its polarization. From this point of view this effect together with ROT- and TRI-effects for the LCPs in ternary fission is useful for fission dynamics investigations. Although shift of -ray angular distribution observed now can not testify ”scission” -rays existence in fission, they can be emitted in principal from physical point of view. However, it is doubtful if they may be selected in the further investigations at the background of the shift effect for -rays from fission fragments. Unlike -rays scission neutrons yield my be estimated through the ROTeffect 12 -16. 04. 2010 Sacley "The scission process: The last stage of nuclear fission" 31

THANK YOU FOR ATTENTION 12 -16. 04. 2010 Sacley "The scission process: The last stage of nuclear fission" 32

12 -16. 04. 2010 Sacley "The scission process: The last stage of nuclear fission" 33