studies of fission dynamics through the search o f scission neutrons

12-16.04.2010 Sacley "The scission process: The last stage of nuclear fission"

1

STUDIES OF FISSION DYNAMICS THROUGH THE SEARCH OF SCISSION NEUTRONS

Skt. Petersburg Nuclear Physics Institute of RAS


2

CONTENT1. INTRODUCTION

2. SEARCH FOR “SCISSION RADIATIONS” AND INVESTIGATIONS OF THEIR CHARACTERISTICS

- Scission neutrons emitted near the rupture point - Scission gamma-radiation emitted near the rupture point

3. ESTIMATES OF SCISSION NEUTRONS YIELDS FROM 233,235U(n,f) 239Pu(n,f) AND 252Cf(s,f) REACTIONS

- Main results of the neutron energy and angular distributions measurements for different fragment energies and masses in 233,235U(n,f) reactions - Main results of the (n-n)-coincidence measurements in 233,235U(n,f), 239Pu(n,f) and 252Cf(s,f) reactions

4. TRI- AND ROT-EFFECTS OF THE LIGHT CHARGED and NEUTRAL PARTICLE EMISSION ASYMMETRIES

- 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 gamma-rays in 233,235U(n,f) fission


3

Excitation energy

Kinetic energy

Potential energy

Rupture point, 20 fmR, fm

TO

TA

L E

NE

RG

Y

Qualitative pictures of the low excitation energy fission


4

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. In1962 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” ~10-21s. ) 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 ~10-22s.). 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 235U(n,f) and 0.18(4)1/f for 252Cf spontaneous fission.

7. Prompt neutron emission from highly excited fission fragments (up to 90% of total neutron yield.


5

Nuclear reaction

Type of experiment % of scission neutrons

Typical average SN energy, Mev

Reference

235U + nth (n-f) correlations 15% 1.58 Skarsvag (1963)

235U + nth “ 10% 3.2 Kapoor (1963)

235U + nth (n-n) correlations 20% Franklyn (1978)

235U + nth (n-f) correlations (10 2)% Samant (1995)

235U + nth Compilations 14% 0.98 and 2.74 Kornilov (2001)

235U + nth Total spectra 15% Kornilov (2002)

252Cf sp. fission (n-n) correlations 10% Pringle (1975)

252Cf sp. fission (n-f) correlations 20% > 1.5 Piksaikin (1977)

252Cf sp. fission “ 10% 2.6 Bowman (1962)

252Cf sp. fission “ (13.2 3.1)% 1 Riechs (1981)

252Cf sp. fission “ 10% (1.5 0.3) Seregina (1985)

252Cf sp. fission “ 1.1% 0.39 B.-Jorgenson (1988)

252Cf sp. fission “ No scis. neutrons! Marten (1989)

252Cf sp. fission “ < 1% Blinov (1989)

252Cf sp. fission Compilations 10% 0.9 and 3.1 Kornilov (2001)

EXPERIMENTAL RESULTS OF SCISSION NEUTRON SEARCH


6

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 252Cf(s.f.), 233,235U(n,f), and 239Pu(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.


7" 7

1

2

2

3

4

5

6

7

8

8

3

4

5

6

7

1

Reaction Chamber:235U target (Ø15mm) – 280

μg/сm2 UF4 onto 70 μg/сm2 Ti backing;

start MWPC: (68 x 92 mm2) located within 7 mm range from the 235U target;

stop MWPC: (72 x 38 mm2) located at a distance of 140 mm from the chamber axis.

Neutron detectors: stilbene crystals (50 x 50 mm2

and 40 x 60 mm2 mounted onthe Hamamatsu - R6091)neutron registration threshold:

–150 200 keV;double-discrimination method:

– pulse shape and time-of-flight criteria

time-of-flight distance: from 235U target – ~ 50 cm

SCHEMATIC VIEW OF EXPERIMENTAL SET-UP


8

ENERGY - ANGULAR DISTRIBUTIONS OF THE NEUTRONS IN 235U FISSION


9

80 100 120 140 160

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

< to

t(m

)>

Muller (2E-2V - method) [12] Maslin 2-geometry [13] Nishio et.al. [14] Present data

<(

m)>

Pre-neutron fragment mass, m [a.m.u.]120 130 140 150 160

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Present data Maslin 4-geometry Maslin 2-geometry

Pre-neutron fragment mass, m [a.m.u.]

The neutron yields from different fragment masses for 235U(n,f) and total neutron yields as a function of pre-neutron fragment

masses


10

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 235U FISSION

1 2 3 4 5 6 71

10

100

235U

N(00) / N(90

0)

N(1800) / N(90

0)

Our data (2009) calculated with A

2 = 0.06

calculated with A2 = 0

Rat

io

Neutron energy, En [Ì ýÂ]

0 18 36 54 72 90 108 126 144 162 180

1.4

1.6

1.8

2.0

2.2

2.4

2.6

2.8

Skarsvag data (1963) Our data (2009) Calculation

235U

Ave

rage

neu

tron

ene

rgy,

<E

n(

)> [

MeV

]

[degree]


11

0 18 36 54 72 90 108 126 144 162 180

1.4

1.6

1.8

2.0

2.2

2.4

2.6

2.8

233U

Model calculation

Ave

rage

neu

tron

ene

rgy,

<E

n()

> [

MeV

]

neutron detector N1 neutron detector N2 average

[degree]1 2 3 4 5 6 7

1

10

100

233U

N(00) / N(90

0)

N(1800) / N(90

0)


2 = 0.06


Rat

io

Neutron energy, En [Ì ýÂ]

En

erg

y

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 233U FISSION


12

ANISOTROPY OF NEUTRON EMISSION IN THE C.M.S. OF 235U(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.


13

0 2 4 6 8 10

0.2

0.4

0.6

0.8

235U

experimental data calculated with A2 = 0.06 calculated with A2 = 0

n (

En)

[ne

utro

n / f

issi

on /

MeV

]

Neutron energy, En [MeV]0 2 4 6 8 10

233U

Neutron energy, En [MeV]

experimental data calculated with A2 = 0.06 calculated with A2 = 0

The total prompt neutron spectra for 233,235U(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 + A2 Ec.m.s (3 cos2( c.m.s ) - 1) / 2


1414

Model calculations were performed with taking into account the angular anisotropy of fast neutron emission from excited fission fragments (A2 = 0.06)

0 18 36 54 72 90 108 126 144 162 180

0.2

0.4

0.6

0.8

Model calculation

Skarsvag (1963)


n(

) [ne

utro

n / f

issi

on /

sr]

[degree]0 18 36 54 72 90 108 126 144 162 180

0.0

0.2

0.4

0.6

0.8

Model calculation


n(

) [ne

utro

n / f

issi

on /

sr]

[degree]

AVERAGE NEUTRON YIELDS FOR DIFFERENT ANGLES IN LABORATORY COORDINATE SYSTEM Comparison of the 235U and 233U fission data obtained with two different

neutron detectors

233U235U


15

AVERAGE NEUTRON ENERGIES FOR DIFFERENT ANGLES IN LABORATORY COORDINATE SYSTEM

Comparison of the 235U and 233U fission data obtained with two different neutron detectors

0 18 36 54 72 90 108 126 144 162 180

1.4

1.6

1.8

2.0

2.2

2.4

2.6

2.8

235U

Model calculation

Skarsvag (1963)


Ave

rag

e n

eutr

on

en

erg

y, <

En(

)> [

MeV

]

[degree]0 18 36 54 72 90 108 126 144 162 180

1.4

1.6

1.8

2.0

2.2

2.4

2.6

2.8

233U

Model calculation

Ave

rage

neu

tron

ene

rgy,

<E

n()

> [

MeV

]


[degree]

Model calculations were performed with taking into account the angular anisotropy of fast neutron emission from excited fission fragments (A2 = 0.06)


16

0 18 36 54 72 90 108 126 144 162 180

0.2

0.4

0.6

0.8

235U

Skarsvag data (1963) Our data (2009) calculated with A

2= 0.06

n(

) [ne

utro

n / f

issi

on /

sr]

[degree]

0 18 36 54 72 90 108 126 144 162 180

0.9

1.0

1.1

235U

calculated with A2 = 0.06


error "corridor" due to uncertainty of neutron c.m.s spectra

n(

) exp /

n(

) calc

[degree]

0 18 36 54 72 90 108 126 144 162 180

1.4

1.6

1.8

2.0

2.2

2.4

2.6

2.8

Skarsvag data (1963) Our data (2009) Calculation

235U

Ave

rage

neu

tron

ene

rgy,

<E

n(

)> [

MeV

]

[degree]

0 18 36 54 72 90 108 126 144 162 180

0.95

1.00

1.05



235U

<E

n(

)>ex

p /

<E

n(

)>ca

lc

[degree]

NEUTRON YIELDS AND AVERAGE ENERGIES FOR DIFFERENT ANGLES OF NEUTRON EMISSION IN 235U(n,f) REACTION

(SCISSION NEUTRONS YIELDS IS NOT MORE THEN 5%)


17

0 18 36 54 72 90 108 126 144 162 180

0.2

0.4

0.6

0.8

233U


2= 0.06

n

() [

neut

ron

/ fis

sion

/ sr

]

[degree]

0 18 36 54 72 90 108 126 144 162 180

0.9

1.0

1.1

233U



error "corridor" due to uncertainty of neutron c.m.s spectra

n(

) exp /

n(

) calc

[degree]

0 18 36 54 72 90 108 126 144 162 180

1.4

1.6

1.8

2.0

2.2

2.4

2.6

2.8


2 = 0.06

233U

Ave

rage

neu

tron

ene

rgy,

<E

n(

)> [

MeV

]

[degree]

0 18 36 54 72 90 108 126 144 162 180

0.95

1.00

1.05

<E

n(

)>ex

p /

<En(

)>ca

lc



233U

[degree]

NEUTRON YIELDS AND AVERAGE ENERGIES FOR DIFFERENT ANGLES OF NEUTRON EMISSION IN 233U(n,f) REACTION

(SCISSION NEUTRONS YIELDS IS NOT MORE THEN (4 -5%)


18

( 2 0

- 1 8

0 )

T a r g e t U - 2 3 5 6 x 2 0

D e t e c t o r 2

S t e e l

P b

C o l l i m a t o r 1 0 x 4 0

H e - 3 n e u t r o n m o n i t o r

f

D e t e c t o r 1

f

B + p o l y e t h y l e n e

C d N e u t r o n b e a m

SCHEMATIC VIEW OF EXPERIMENTAL SET-UP FOR (n-n)-COINCIDENCE INVESTIGATIONS


19

GENERAL VIEW OF EXPERIMENTAL SPECTRUM OF (-), (-n), (n-), (n-n) COINCIDENCES AND ITS EXPANSION

100 200 300 400 500

50

100

150

200

250

300Neutrons

- quanta

Total Integral [arb. units]

Part

ial In

teg

ral [a

rb. u

nit

s]


20

. SENSITIVITY OF (n-n)-COINCIDENCE METHOD TO THE SCISSION NEUTRON ADMIXTURE (a) AND TO THE ENERGY THRESHOLD OF

NEUTRON REGISTRATIONS (b).

Nsc/Ntot- 7%


21

0 20 40 60 80 100 120 140 160 180

0 20 40 60 80 100 120 140 160 180

1,0

1,5

2,0

2,5

3,0

3,5

4,0

4,5

1,0

1,5

2,0

2,5

3,0

3,5

4,0

4,5 252-Cf

Enf

= 1600 keV

Enf

= 1200keV

Enf

= 800 keV

Enf

= 550 keV

Enf

= 425 keV

NO

RM

AL

IZE

D

n-n

C

OIN

CID

EN

CE

S

(arb

. u

nits

)

ANGLE nn

(degrees)

0 20 40 60 80 100 120 140 160 180

0 20 40 60 80 100 120 140 160 180

1

2

3

4

5

6

7

8

9

1

2

3

4

5

6

7

8

9 235-U E

nf = 2030 keV

Enf = 1700 keV

Enf = 1350keV

Enf = 900 keV

Enf = 600 keV

Enf = 425 keV

N

OR

MA

LIZ

ED

n-n

CO

INC

IDE

NC

ES

(a

rb. u

nits

)

ANGLE (degrees)

0 30 60 90 120 150 180

1

2

3

4

5

6 233-U

Eth = 2000 keV

Eth = 1700 keV

Eth = 1300keV

Eth = 850 keV

Eth = 580 keV

Eth = 425 keV

N

OR

MA

LIZ

ED

n-

n C

OIN

CID

EN

CE

S (

arb.

uni

ts)

ANGLE (degrees)

(n-n)-COINCIDENCES IN 252Cf(s,f), 233,235U(n,f) AND 239Pu(nf) REACTIONS

Angular dependence of (n-n)-coincidences in 252Cf fission

Angular dependence of (n-n)-coincidences in 235U fission

Angular dependence of (n-n)- coincidences 233U fission

Angular dependence of (n-n)-coincidences in 239Pu fission

0 30 60 90 120 150 180

1

2

3

4

5

6

7

8

1

2

3

4

5

6

7

8 239-Pu

Eth= 2030 keV

Eth= 1690 keV

Eth= 1330keV

Eth = 800 keV

Eth = 560 keV

Eth = 425 keV

N

OR

MA

LIZ

ED

n-n

CO

INC

IDE

NC

ES

ANGLE (DEGREES)


22

ESTIMATES FOR SCISSION NEUTRON YIELDS AND TEMPERATURES OF THE WEISKOPF ENERGY SPECTRA

Parameters 252-Cf 233-U 235-U *239-Pu

Scission neutron yields

Temperature of spectrum

*(5 ± 2)%*(10 ± 2)% *(14 ± 2)%*(7 ± 2)%

*1 MeV*1.1 MeV*0.8 MeV

•The data asterisked have been obtained from measurements (n-n)-coincidences

• The yield estimates for 233,235U marked off by yellow color have been obtained from the energy and angular distributions of neutrons emitted from separated fragments

NOT MORE THEN (4-5)%

*0.9 MeV


23

TRI-effect of T-odd emission asymmetry for the third particle

pLF

σ0

pTP

pHF

W() = 1 + DTRI∙σn∙[pfxpTP]

From theoretical point of view such effect exists only for the particles appeared simultaneously!

(A. Barabanov, 2001)

For the LCP:

235U: DTRI= +(1.7 0.2)10-3

233U: DTRI = - (3.9 0.1)10-3

)()(

)()()(exp

NN

NND


24

θ 1

θ 2

Observed shift of LCP angular distribution

In 235U: 2 - 1 ~ 0.20

in 239Pu: 2 - 1 ~ 0.020

Clockwise rotation

Anticlockwise rotation

Schematic diagram of ROT- effect appearance in ternary fission

(Shift of LCP angular distributions)

))1(( 22222 KJJR


25

Shift of the third particle angular distribution (ROT- effect)

pLF pHF

σ0

pTP

W() = 1+DROT∙σn∙[pfxpTP]∙(pf∙pTP)

ROT: 0.215(5)o

TRI: + 0.0017

In the contrast to TRI-effect ROT-effect can exist for

neutrons and -rays emitted from fragments as well (big oriented angular momenta!)

JH

JL 235U


26

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.

Experimental set-up

σ–

MWPCstop

Fissile target

PM

PM

n

n

LFHF

σ+MWPCstop

MWPCstart

Existing information: 235U: <Dn> = - (9 ± 5)·10-4

233U: <Dn> = - (3 ± 7)·10-4 PNPI-2005235U: │<Dn>│ < 4 10-5 FRM-2-2010

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


27

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 5He* ~ 10-21 s.; 7He* ~ 4∙10-21 s.; 8Li ~ 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)


28

System rotation

Neutrons or γ-rays from fragments

+ φJ

h

J

J

l

MECHANIZM OF FALSE “ROT-EFFECT” APPEARENCE FOR NEUTRONS AND -RAYS

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!

- φ

System rotation

50 60 70 80 90 100 1100

200

400

600

800

1000

N+Z

N-Z

N, c

ount

s

angle, degree

-90 -45 0 45 90

+ -

90-

90+


29

Results of the search investigations of ROT-effect for -rays

-180 -150 -120 -90 -60 -30 0 30 60 90 120 150 180

-0,0005

-0,0004

-0,0003

-0,0002

-0,0001

0,0000

0,0001

0,0002

0,0003

0,0004

0,0005

0,0006

0,0007

0,0008

(N1

-N2

)/(N

1+

N2

)

Angle, deg.

Result of G. Danilyan et al. FRM-2 reactor

Result of G. Petrov et al. WWR-M reactor

)]'(cos1/[)'2sin()( 2exp AAD

Observed angular distribution shift for -rays: 0.10(3)0 compared to 0.215(3)0 for LCP

)()(

)()()(exp

NN

NND


30

Expected ROT-effects for the neutrons emitted in polarized 236U* fission in comparison with -rays one

ROT-effect form for “scission neutrons” has to be similar to the -rays one

Expected form of “ROT-effect” for neutrons from

fission fragments

Expected form of ROT-effect for scission

neutrons

Observed form and value of “ROT-effect” for -rays


31

Perspectives of the further search investigations of ROT-effects 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 ROT-effect


32

THANK YOU FOR ATTENTION


33

Possible mechanism of TRI-effect appearance

FCori = − 2m [v ω] Fcatap = m [r dω/dt]Fcentr = mω [r ω]

TRI

CoriF

catapF

~R

r

v

TRI

CoriF

catapF

~R

r

v

studies of fission dynamics through the search o f scission neutrons

Documents

scission neutron yields

neutron energy

scission radiations

prompt neutron emission

estimate of neutron

ternary fission

f reactions4

instantaneous neutron