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

• Benchmark experiments

Keff= 1 ± β

Neutron detector

Neutron detector

IC with 235U M=0.2mg-10g

Bulk sample, M~100g

Direct neutron beam

Pulsed neutron beamwith energy Eo

Thermal neutrons

235U

Activated samples

dEEEN )()(

IRMM-2007

Problem during more than 50 years of the experimental efforts!

• PFNS from microscopic experiments at thermal energy do not agree with integral data and can not reproduce Keff for benchmark experiments

• 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”

ENDF/B-7235U: AVERAGE ENERGY OF FISSION NEUTRONS

INCIDENT NEUTRON ENERGY, MeV

0 1 2 3 4 5

<E

>, M

eV

1.9

2.0

2.1

2.2evaluation 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)

Keff=1

????????

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

Angular distribution of SCN relative to FF

-1.0 -0.5 0.0 0.5 1.0

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Ysc

n()

,1/d

-1.0 -0.5 0.0 0.5 1.00.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

0.20

Ysc

n(),

1/d

E1-E2=0.5 - 6 MeV

Energy spectrum of SCN in LS

0 2 4 6 8 10

1E-3

0.01

0.1

Ysc

n(E),

1/M

eV

E, MeV

Non “direct” method for SCN estimation • Shape of the PFNS at thermal point requires the SCN incorporation

(problem #1).

0 2 4 6 8 10 12

0.6

0.8

1.0

1.2

Starostov, 1982 IRMM, 2008 LANL, a=A/10.2

R(E

), <

E>

=1.

988M

eV

E (MeV)

χ2=3.5

3 sources modelIn case of SCN emission, the resulting spectrum is composed of three sources (3

sources model):

)()()()( 1 ENENENEN SCNAA (1)

1. Neutrons from fragments after fission of the compound nucleus A+1

)()1()( 11 EWEN AA (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:

/)()1()( EWEN AA . (3)

3. Scission neutrons themselves:

22

212

1

exp1

exp)(T

E

TT

E

TEEN scn

, (4)

where ω is the share of the low energy component and is the neutron multiplicity.

“3 sources model” spectrum and thermal data

0 2 4 6 8 10 120.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4 this work eq. 1 eq. 2 eq. 3 eq. 4

R

(E),

<E

>=

1.98

8MeV

E (MeV)

The residual chi-square is χ2=0.64

Experimental results for 235U and 252Cf

Vorob’ev (2007) et al estimated ~ 10% for 235U at thermal point

Parameter

252Cf, Bowman,

1962

252Cf, Budtz-Jorgensen,

1988

235U Skarsvag,

1963

235U IRMM-2008

, 1/f 0.460.02 0.3820.003 0.3780.006 0.41 1 0.410.03 0.6790.012 0.6570.041 0.26 T1, МэV 0.350.04 0.4780.008 0.490.02 0.34 T2, МэV 1.770.07 1.540.04 1.370.07 1.31 <E>, МэV 2.380.13 1.640.04 1.580.09 2.12

What is happened at 0.5MeV?

• <Eth> = 2.031MeV ; <E05> = 2.045MeV • IRMM(th) (2008) and Johansson (1977) 0.5MeV data

0 2 4 6 8 10 12

0.6

0.8

1.0

1.2

E0=thermal

E0=0.5MeV

R(E

)

E (MeV)

• The difference of the PFNS shape between thermal and 0.5 MeV input energy can not be predicted with existing theoretical model (problem #2).

0 2 4 6 8 10 12

0.6

0.8

1.0

1.2

1.4 Johansson IRMM ENDF/B-VII, a=A/11

R(E

), <

E>

=1.

988M

eV

E (MeV)

What is happened at 0.5MeV (cont)?

0 2 4 6 8 10 12

0.6

0.8

1.0

1.2

IRMM, 2006, 0.5MeV Staples, 1995, 0.5MeV ENDF/B-7, a=A/11 IRMM, 2008, thermal IRMM, 2007, 0.5MeV

R

(E)

E (MeV)

b)

What does mean angular effect? • Problem #3

0 2 4 6 8 100.7

0.8

0.9

1.0

1.1

1.2

Jan08-R90 Jan08-L90 Jan08-R150 Apr07-R90 Apr07-L120 Apr07-R150 Jul06-R90 Jul06-L120 ENDF/B-VII

R(E

), <

E>

=2.

002M

eV

E (MeV)

What does mean left-right asymmetry?Average spectral ratios <R> = N(E,R90)/N(E,L90)

and their uncertainty for different energy intervals.

E1-E2, MeV

<R>±δR E1-E2, MeV

<R>±δR

0.8 -2 0.999±0.003 5 – 6 1.009±0.005 2 - 3 1.010±0.002 6 – 8 1.051±0.006 3 - 4 1.020±0.005 8 – 10 0.970±0.032 4 - 5 1.034±0.004

Sample

Detector 1

Detector 2

Detector 3

Proton beam

Pb bar

Pb bar

Monitor

2 4 6 8 100.90

0.95

1.00

1.05

1.10

R90

/L90

L03 000 R03

E (MeV)

Angular effect can be reproduced….

0 2 4 6 8 10 12

0.6

0.8

1.0

1.2=0.0

R(E

), <

E>

=2.

002M

eV

E (MeV)

=0.45

WHY share of SCN is changing……????

There is NO any idea to explain the difference between differential and integral data! (problem#4)

0 2 4 6 8 10 12 140.8

0.9

1.0

1.1

1.2

ENDF/B-VII experimental PFNS, eq. 1

R(E

)=C

/E

<E> (MeV)0.01 0.1 1 10

0.6

0.8

1.0

1.2

1.4 IRMM-2008 Lajtai Starostov 3 source model ENDF/B-VII

R(E

), <

E>

=1.

988M

eV

E (MeV)

Integral dataRatio 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.

235U 252Cf ENDF/B-VII Experimental PFNS,

eq.1

Reaction

<E>, MeV

R±δR <E>, MeV

R±δR <E>, MeV

R±δR

19F(n,2n) 14.37 1.009±0.033 14.08 1.019±0.139 14.03 0.868±0.139 27Al(n,p) 6.32 1.005±0.022 6.11 0.989±0.018 6.07 0.941±0.018 46Ti(n,p) 6.37 0.982±0.018 6.16 0.994±0.017 6.12 0.948±0.017 48Ti(n,p) 8.76 1.003±0.019 8.45 0.997±0.066 8.41 0.927±0.066 51V(n,α) 10.38 0.987±0.022 10.03 1.005±0.023 9.97 0.915±0.023 56Fe(n,p) 7.99 0.998±0.018 7.74 0.973±0.062 7.70 0.913±0.062 59Co(n,α) 8.70 0.996±0.019 8.40 1.012±0.022 8.35 0.942±0.022 58Ni(n,p) 4.52 0.998±0.013 4.35 0.992±0.013 4.33 0.959±0.013 63Cu(n,α) 7.61 1.005±0.020 7.33 1.006±0.048 7.28 0.948±0.048 90Zr(n,2n) 14.70 0.991±0.029 14.49 1.108±0.076 14.45 0.937±0.076 93Nb(n,n’) 3.01 0.989±0.017 2.90 0.972±0.047 2.88 0.951±0.047 93Nb(n,2n) 11.69 1.027±0.051 11.49 0.964±0.049 11.45 0.860±0.049 115In(n,n’) 3.05 0.970±0.017 2.93 1.001±0.012 2.91 0.977±0.012 204Pb(n,n’) 5.42 0.974±0.057 5.23 0.940±0.106 5.20 0.903±0.106

235U(n,f) 2.13 1.006±0.012 2.03 1.013±0.019 1.99 1.013±0.019 238U(n,f) 3.32 0.979±0.016 3.16 0.994±0.023 3.14 0.968±0.023

237Np(n,f) 2.51 0.999±0.016 2.39 0.998±0.021 2.36 0.985±0.021 <R>±σ/(N-1)1/2 0.995±0.004 0.998±0.009 0.938±0.010

dEEENEE

dEEEN

)()(1

,)()(

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?

Bn0

Bni

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