The “Scission Neutron Emission” is last or
first stage of nuclear fission?
Nikolay Kornilov
Experiments
• Microscopic (differential)
experiments (as a rule TOF)
• Macroscopic experiments
• Integral experiments
Direct neutron beam
235U
Neutron detector
Thermal neutrons
IC with 235U
M=0.2mg-10g
Activated samples
    N ( E ) ( E ) dE
Pulsed neutron beam
with energy Eo
Neutron detector
Bulk sample,
M~100g
• Benchmark experiments
Keff= 1 ± β
IRMM-2007
Problem during more than 50 years of the
experimental efforts!
•
1972, Islam and Knitter motivated new
experiment "…though the results of the above
measurements, characterized by the average
fission neutron energy, <E>, agree reasonably
with each other, a number of integral
experiments shows higher values for <E>
indicating a harder fission neutron spectrum
than that given by differential measurements".
•
2003, Madland "…no calculated thermal
spectrum has been found that simultaneously
reproduces either of the two modern thermal
differential measurement and the set of
measured integral cross-sections to within an
acceptable level”
• PFNS from microscopic
experiments at thermal
energy do not agree with
integral data and can not
reproduce Keff for
benchmark experiments
ENDF/B-7
235
U: AVERAGE ENERGY OF FISSION NEUTRONS
2.2
evaluation 1999 (Kornilov et al)
evaluation 2004 (ENDF/B-7)
average for Eth (exp I)
IPPE+RI; 1990-95 (exp I)
Johansson, 1977 (exp II)
evaluation 2002 (Hambsch et al)
<E>, MeV
2.1
2.0
Keff=1
????????
1.9
0
1
2
3
4
5
INCIDENT NEUTRON ENERGY, MeV
Available differential data 235U
• Thermal point: Starostov et al (1983), 3 spectra,
Lajtai et al (1985), Yufeng et al (1989), Kornilov et al
(2008), 3 spectra;
• 0.5 MeV: Trufanov (1994), Staples (1995), IRMM
(2006-08) 8 spectra;
• 1-5MeV: Boykov (1991, 2.9 MeV), Trufanov (1994,
5MeV), Staples (1995, 1.5, 2.5, 3.0MeV)
Sources of information
1.6
1
1.4
R()
N(E,)
1.5
1.3
1.2
1.1
0
1.0
2
4
1,0
eV
M
E,
6
0,5
8
0,0
10
-0,5
12 -1,0

0.9
0.0
0.2
0.4
0.6

0.8
1.0
0.20
0.7
0.18
0.6
0.16
0.5
Yscn(),1/d
Yscn(), 1/d
Angular distribution of SCN relative to FF
0.14
0.12
0.10
0.4
0.3
0.2
0.08
0.1
0.06
0.0
-1.0
0.04
-1.0
-0.5
0.0
0.5
1.0

E1-E2=0.5 - 6 MeV
-0.5
0.0

0.5
1.0
Energy spectrum of SCN in LS
Yscn(E), 1/MeV
0.1
0.01
1E-3
0
2
4
6
E, MeV
8
10
Non “direct” method for SCN estimation
• Shape of the PFNS at thermal point requires the SCN incorporation
(problem #1).
R(E), <E>=1.988MeV
1.2
1.0
0.8
Starostov, 1982
IRMM, 2008
LANL, a=A/10.2
0.6
χ2=3.5
0
2
4
6
E (MeV)
8
10
12
3 sources model
In case of SCN emission, the resulting spectrum is composed of three sources (3
sources model):
N ( E )  N A1 ( E )  N A ( E )  N SCN ( E )
(1)
1. Neutrons from fragments after fission of the compound nucleus A+1
N A 1 ( E )  (1   )  WA 1 ( E )
(2)
where α is the share of scission neutrons and WA+1 is the spectrum which describes the
neutron emission from accelerated fragments;
2. Neutrons from accelerated fragments after fission of the nucleus A, which is formed
due to the emission of one SCN:
N A ( E )    (  1)  W A ( E ) /  .
(3)
3. Scission neutrons themselves:
N scn ( E ) 
 
 E


 E 
exp
2
 T
T

1

1

 1 
 E

  T 2 exp
 T
2


2



 ,

(4)
where ω is the share of the low energy component and  is the neutron multiplicity.
“3 sources model” spectrum and thermal data
R(E), <E>=1.988MeV
1.4
this work
eq. 1
eq. 2
eq. 3
eq. 4
1.2
1.0
0.8
0.6
The residual chi-square is χ2=0.64
0.4
0.2
0.0
0
2
4
6
E (MeV)
8
10
12
Experimental results for 235U and 252Cf
Parameter
, 1/f
1
T1, МэV
T2, МэV
<E>, МэV
252
Cf,
Bowman,
1962
0.460.02
0.410.03
0.350.04
1.770.07
2.380.13
252
Cf, BudtzJorgensen,
1988
0.3820.003
0.6790.012
0.4780.008
1.540.04
1.640.04
235
U
Skarsvag,
1963
0.3780.006
0.6570.041
0.490.02
1.370.07
1.580.09
235
U
IRMM-2008
0.41
0.26
0.34
1.31
2.12
Vorob’ev (2007) et al estimated ~ 10% for 235U at thermal point
What is happened at 0.5MeV?
• The difference of the PFNS shape between thermal and 0.5 MeV input
energy can not be predicted with existing theoretical model
(problem #2).
•
•
<Eth> = 2.031MeV ; <E05> = 2.045MeV
IRMM(th) (2008) and Johansson (1977)
0.5MeV data
1.2
R(E), <E>=1.988MeV
1.4
R(E)
1.0
E0=thermal
0.8
E0=0.5MeV
0.6
Johansson
IRMM
ENDF/B-VII, a=A/11
1.2
1.0
0.8
0.6
0
2
4
6
E (MeV)
8
10
12
0
2
4
6
E (MeV)
8
10
12
What is happened at 0.5MeV (cont)?
1.2
b)
R(E)
1.0
0.8
IRMM, 2006, 0.5MeV
Staples, 1995, 0.5MeV
ENDF/B-7, a=A/11
IRMM, 2008, thermal
IRMM, 2007, 0.5MeV
0.6
0
2
4
6
E (MeV)
8
10
12
What does mean angular effect?
• Problem #3
R(E), <E>=2.002MeV
1.2
1.1
1.0
Jan08-R90
Jan08-L90
Jan08-R150
Apr07-R90
Apr07-L120
Apr07-R150
Jul06-R90
Jul06-L120
ENDF/B-VII
0.9
0.8
0.7
0
2
4
6
E (MeV)
8
10
What does mean left-right asymmetry?
Average spectral ratios <R> = N(E,R90)/N(E,L90)
Monitor
and their uncertainty for different energy intervals.
Pb bar
Sample
Pb bar
E1-E2,
MeV
0.8 -2
2-3
3-4
4-5
Detector 1
Detector 2
Proton
beam
<R>±δR
<R>±δR
E1-E2,
MeV
0.999±0.003 5 – 6
1.010±0.002 6 – 8
1.020±0.005 8 – 10
1.034±0.004
1.009±0.005
1.051±0.006
0.970±0.032
Detector 3
1.10
L03
000
R03
R90/L90
1.05
1.00
0.95
0.90
2
4
6
E (MeV)
8
10
Angular effect can be reproduced….
R(E), <E>=2.002MeV
1.2
=0.0
1.0
=0.45
0.8
0.6
0
2
4
6
8
10
12
E (MeV)
WHY share of SCN is changing……????
There is NO any idea to explain the difference between
differential and integral data! (problem#4)
1.2
IRMM-2008
Lajtai
Starostov
3 source model
ENDF/B-VII
1.2
ENDF/B-VII
experimental PFNS, eq. 1
1.1
R(E)=C/E
R(E), <E>=1.988MeV
1.4
1.0
0.8
1.0
0.9
0.6
0.8
0.01
0.1
1
E (MeV)
10
0
2
4
6
8
<E> (MeV)
10
12
14
Integral data
Ratio of the calculated average cross sections to experimental data for 252Cf
[Mannhart], and 235U with the ENDF/B-VII and “3 sources model” neutron
spectra. The ratio uncertainties δR include only experimental errors.
252
Reaction
235
Cf
ENDF/B-VII
19
F(n,2n)
Al(n,p)
46
Ti(n,p)
48
Ti(n,p)
51
V(n,α)
56
Fe(n,p)
59
Co(n,α)
58
Ni(n,p)
63
Cu(n,α)
90
Zr(n,2n)
93
Nb(n,n’)
93
Nb(n,2n)
115
In(n,n’)
204
Pb(n,n’)
235
U(n,f)
238
U(n,f)
237
Np(n,f)
<R>±σ/(N-1)1/2
27
<E>,
MeV
14.37
6.32
6.37
8.76
10.38
7.99
8.70
4.52
7.61
14.70
3.01
11.69
3.05
5.42
2.13
3.32
2.51
R±δR
1.009±0.033
1.005±0.022
0.982±0.018
1.003±0.019
0.987±0.022
0.998±0.018
0.996±0.019
0.998±0.013
1.005±0.020
0.991±0.029
0.989±0.017
1.027±0.051
0.970±0.017
0.974±0.057
1.006±0.012
0.979±0.016
0.999±0.016
0.995±0.004
<E>,
MeV
14.08
6.11
6.16
8.45
10.03
7.74
8.40
4.35
7.33
14.49
2.90
11.49
2.93
5.23
2.03
3.16
2.39
R±δR
1.019±0.139
0.989±0.018
0.994±0.017
0.997±0.066
1.005±0.023
0.973±0.062
1.012±0.022
0.992±0.013
1.006±0.048
1.108±0.076
0.972±0.047
0.964±0.049
1.001±0.012
0.940±0.106
1.013±0.019
0.994±0.023
0.998±0.021
0.998±0.009
    N ( E )   ( E )dE,
 E 
1
  
E  N ( E )   ( E ) dE
U
Experimental PFNS,
eq.1
<E>,
R±δR
MeV
14.03
0.868±0.139
6.07
0.941±0.018
6.12
0.948±0.017
8.41
0.927±0.066
9.97
0.915±0.023
7.70
0.913±0.062
8.35
0.942±0.022
4.33
0.959±0.013
7.28
0.948±0.048
14.45
0.937±0.076
2.88
0.951±0.047
11.45
0.860±0.049
2.91
0.977±0.012
5.20
0.903±0.106
1.99
1.013±0.019
3.14
0.968±0.023
2.36
0.985±0.021
0.938±0.010
Possible explanation ?
•
•
•
•
•
•
One may conclude that a factor exists which has a rather strong influence on the PFNS
shape and asymmetry effects but was not fixed in experimental investigations at 0.5MeV
input neutron energy
All experiments which results were used in the report were made with 7Li(p,n) reaction as
a neutron source and pulsed mode. One may assume that this factor is the neutron
polarization.
We should take into account the possible proton polarization also due to pulsed mode of the
accelerators (chopper, bunching high voltages, analyzing and switching magnets). In the
preparation stage of any PFNS experiment it was assumed that this factor is not important
or by definition should be equal to zero.
If this explanation is true, the transmission mechanism of the information from the incident
neutron to the secondary fission neutron should be found. The only possibility might be
scission neutron emission, a fast process without formation of the compound nucleus. This
may provide the link between the incident neutron and the secondary fission neutron.
So, for real clarification of this effect we need new experiments with polarized thermal
neutron beam. When we will confirm and verify this effect new theoretical model should
be developed.
The most difficult for understanding is the problem #4. There are not any realistic
ideas for its solving. May be they will come after new experimental efforts mentioned
above.
Neutron decay from excited states in the second
minimum?
Bni
Bn0
Conclusion
• New experimental and theoretical efforts are necessary to answer the
following very important questions:
• what is the mechanism of neutron emission in fission and fission process
itself. ~40% of fission should be happened without COMPOUND nucleus
formation;
• why the shape of the prompt fission neutron spectrum may change so
drastically. Thermal-0.5 MeV data, angular effect. SC Neutrons should be
emitted at beginning stage of fission, in any case, the low energy
component. We should construct the mechanism;
• what is the physical reason responsible for the formation of a more
energetic spectrum in the integral experiments in comparison with
microscopic data, and
• what is happening inside nuclear reactors.
Team
• F.-J. Hambsch, I. Fabry, S. Oberstedt
EC-JRC-Institute for Reference Materials and Measurements, Retieseweg
111, B-2440 Geel, Belgium
• T. Belgya, Z. Kis , L. Szentmiklosi
Institute of Isotopes HAS, Dept. of Nuclear Research, Budapest, Hungary
• S. Simakov
Forschungszentrum Karlsruhe, Institut für Neutronenphysik und
Reaktortechnik, D-76344 Eggenstein-Leopoldshafen, Germany