Robert HaightLANSCE-NS

Workshop on Statistical Nuclear Physics

and Applications in Astrophysics and Technology

Ohio University

July 8-11, 2008

LA-UR-08-4399

Statistical Neutron-Induced Reactions

Studied by Neutron, Proton, and Alpha-Particle Emission

• Context

• Neutron-induced reactions

• Charged particle emission

• Neutron emission

• (Gamma-ray emission)

• Concentrate around A ~ 56

Outline

Context – history, data bases, previous works

• Lots of data on charged-particle induced reactions– Protons, 3He, alphas, heavy ions, etc.– Emission spectra, angular distributions, etc. for charged particles and

neutrons – Major experimental efforts in the 1960’s, 1970’s; continuing at lower

intensity through the present time– Major analyses of data

– Gilbert & Cameron– Backshifted Fermi Gas

– E.g. Vonach, Dilg, etc.– Superfluid models

• Neutron-induced reactions – Lots at 14 MeV incident energy– Some at other energies– Evaluated data files – ENDF, JEFF, JENDL, BROND, etc.

Why then study more neutron-induced reactions?

• Applications– Neutron transport for many applications– Radiation damage in fast fission reactors (AFCI, GNEP) and

fusion reactors of the future from (n,H) and (n,He) reactions– a.k.a. “Gas Production”

– Requirements on accuracy of data

• Basic physics– Learn more about reaction models, level densities– Other data (e.g. total cross sections, known very well) constrain

reaction models– Reactions can be studied over a wide range of incident energies

in the same experiment – use “white” neutron source

(n,xp) and (n,xalpha) reactions are in competition with neutron emission (n,n’), (n,2n), etc.

Physics:

• Optical model for transmission coefficients

• Nuclear levels

• spectroscopy

• level densities

• We measure as a function of incident neutron energy

•Traces out competition with excitation energy

•Insights into non-statistical reactions, e.g. direct and pre-equilibrium

61Ni

60Ni + n(target)

59Ni + 2n

En

11.388

-7.820

0.000

2+

4+

0+

1.332

2.5062.042

60Co + p

57Fe +

-1.354

(J ,Ex)

(J ,Ex)

(J ,Ex)

n

p

Visitor center

Isotope production facility

800-MeV proton linac

Proton radiography and UCN

Target-2

Target-4

Proton storage ring

MLNSC -neutron scattering

Visitor center

Isotope production facility

800-MeV proton linac

Proton radiography and UCN

Target-2

Target-4

Proton storage ring

MLNSC -neutron scattering

Los Alamos Neutron Science Center - LANSCE

LANSCE Neutron Sources cover 16 orders of magnitude in neutron energy

Lujan centerEn < 500 keV

PSR

Target-2“Blue Room”

Target-4(Fast neutrons)

LINAC

• Lujan - ultra-cold to epithermal neutrons up to ~500 keV

• Target 2 - UCN to fast neutrons; also protons

• Target 4 – Fast neutrons from 0.1 to 800 MeV

Proton storage ring

To Areas A, B and C

We use the 30-degree flight path at Target 4 (WNR)

Proton beam

Charged-particle emission@ 15. 1 meters“NZ”

Neutron emission@ 22.7 meters“FIGARO”

• Neutron energy range can be studied in one experiment

• Covers energies of statistical reactions – up to ~ 10 MeV

• Covers much higher energies where direct and other pre-equilibrium reactions become important

Our incident neutron energy range is from 1 to 100 MeV

High-energy tailFission spectrum

1.0E-05

1.0E-04

1.0E-03

1.0E-02

1.0E-01

1.0E+00

1.0E-01 1.0E+00 1.0E+01 1.0E+02 1.0E+03

En (MeV)

Y (n

/p/M

eV/S

r)

15 Deg

30 Deg

60 Deg

90 Deg

0.1 1 10 100 1000

100

10 -1

10 -2

10 -3

10 -4

10 -5

Y (

n/p

/sr/

MeV

)

E n (M eV)

1.0E-05

1.0E-04

1.0E-03

1.0E-02

1.0E-01

1.0E+00

1.0E-01 1.0E+00 1.0E+01 1.0E+02 1.0E+03

En (MeV)

Y (n

/p/M

eV/S

r)

15 Deg

30 Deg

60 Deg

90 Deg

0.1 1 10 100 1000

100

10 -1

10 -2

10 -3

10 -4

10 -5

Y (

n/p

/sr/

MeV

)

E n (M eV)

Fast neutron source spectra at LANSCE

Light charged-particle emission

p, d, t, 3He, alpha

Many approaches have been used to measure charged-particle emission in reactions induced by fast neutrons

• Gas accumulation: irradiate and then measure by mass spectrometry

• only for Helium (hydrogen contamination is everywhere)• need a monoenergetic neutron source or the result is an average over the spectrum

• Activation: e.g. 56Fe(n,p)56Mn (2.579 hours)• Need monoenergetic source (as above)• Need a radioactive product – e.g. not 56Fe(n,alpha)53Cr(stable)• Not complete when other channels are open, e.g. 56Fe(n,n’p)55Mn (stable); 56Fe(n,n +alpha)52Cr(stable)

• Detect protons, deuterons, tritons, 3He and alpha particles • Monoenergetic source • White source and time-of-flight techniques LANSCE

Cu & Wshielding

Cu

Mylarwindow

Mo window

Reaction chamber55.9 cm I.D.

Long steelCollimator

15.1 m to neutron source

neutronbeam

0.94 m

Fissionchamber

Sample

CsI(Tl) + photodiode

Silicon surface barrier detector

Low pressure proportionalcounter or thin silicon surfacebarrier detector

2.2 cm diam.

Brass "cleanup"collimator

.

2.7 cm diam.

Charged particles emitted in the reactions are identified by E detectors and their energies are determined by stopping detectors of silicon or CsI(Tl)

We choose detectors to give information on the complete charged-particle spectra

• Low pressure proportional counters allow identification of helium ions to below 3 MeV

• Silicon detectors stop alpha particles up to 33 MeV

• CsI(Tl) scintillators – 3 cm thick -- stop 100 MeV protons

A large dynamic range of particles is detected. The range is defined by low energy helium ions and high energy protons

59Co(n,xalpha) angle-integrated emission spectra are described well by calculations

Ref: S. M. Grimes et al., Nucl. Sci. Eng. 124, 271 (1996)

Excitation function for 59Co(n,xalpha) is described well by calculations up to > 20 MeV

However, level density parameters needed to be modified to fit the 59Co(n,xalpha) data

Nucleus a ( / MeV ) (MeV)

57Co 4.70 1.2758Co 5.30 0.00

54Mn 5.03 0.0055Mn 5.46 1.2756Mn 6.05 0.00

57Fe 5.72 1.5458Fe 6.19 2.83

Gilbert & Cameron systematics

Measurements on nickel isotopes show problems with evaluated data libraries

58Ni(n,xalpha)

0

50

100

150

200

0 5 10 15 20

En (MeV)

Cro

ss s

ecti

on

(m

b)

.

LANSCE data

Grimes

Kneff

Graham

Dolya

Haight

Tsabaris

Goverdovski

Qaim84

Fessler

ENDF/B-VI

Peter Fu analyzed the differences in the evaluated cross sections for 58Ni(n,alpha) -- 1995

Fu’s analysis shows variations in the used level densities

Ra

tio

of

lev

el d

en

sit

ies

Uhl / Fu BSFG / GC

Low energy 58Ni(n,alpha) data (Tohoku) could be fit well

• T. Kawano, et al., J. Nucl. Sci. Tech. 36, 256 (1999)

• Baysian analysis (KALMAN)

Parameter Prior Posterior

a(58Ni) (/MeV)

7.994 6.163

a(58Co) (/MeV)

8.942 7.957

a(55Fe) (/MeV)

8.143 8.163

60Ni(n,xalpha)

0

20

40

60

80

100

120

0 5 10 15 20En (MeV)

Cro

ss S

ecti

on

(m

b)

LANSCE Data

Grimes

Kneff

Graham

Dolya

Fischer

Haight

ENDF/B-VI

The situation with 60Ni is similar with regard to evaluated data libraries

Recent results for iron also show a problem with the ENDF evaluation

Iron (n,Helium)

0

0.01

0.02

0.03

0.04

0.05

0 2 4 6 8 10 12 14 16 18 20

Neutron Energy (MeV)

Cro

ss S

ecti

on

(b

)

FZK/INPE56Fe

NRG-2003

JENDL-HE56Fe

IEAF-2001

LANSCE data

ENDF/B-VII56Fe

Haight/Kneff + He-accumulation

Results for hydrogen production are in agreement with ENDF and also confirm LA150 evaluation up to 50 MeV

Fe(n,Hydrogen)

0

200

400

600

800

1000

1200

1400

0 20 40 60 80 100 120 140

En(MeV)

Sig

(m

b)

.

LA150

ENDF/B-VI

New WNR data

Neutron emission

With gamma-ray detectors near the sample, we trigger off the prompt gamma-rays to study neutron emission

FIGARO (n,xn+)

n

x

• 20 Neutron detectors

• “Double time-of-flight“ experiment

• Incident neutron energy from TOF from souce

• En’ emitted from TOF ~ 1m

• Neutron emission in coincidence with gamma rays

sample 22 m from WNR source

BaF2

HPGe

BaF2

HPGe

A+1Z

AZ + n(target)

E n

2+

4+

0+

(J,E x) n'

Trigger

A+1Z

AZ + n(target)

E n

2+

4+

0+

(J,E x) n'

Trigger

Neutron emission contingent on one specific -transition

Nickel data were described well with EMPIRE calculation, with modified level density

58,60Ni(n,n’) (natural elemental isotopes)

Ref: D. Rochman, Nucl. Instr. Meth. in Phys. Res. A523, 102 (2004)

Iron data are obtained by triggering on the lowest 2+ ground state gamma ray

• Dietrich noted that nearly all of the excited states in 56Fe decay through the 847 keV 2+ state

Fe57

(target)

4 +

(J,E

x)

56Fe + n

2 +

+

0

+

(J,E

x) n'

0.847Trigger

En

2.085

2.6572+

Iron data are being analyzed

One neutron detector, binned in incident neutron energies

1–1.5 MeV 1.5-2 MeV 2–2.5 MeV

2.5–3 MeV 3–3.5 MeV 3.5-4 MeV

4–4.5 MeV 5–5.5 MeV

6–6.5 MeV

7–7.5 MeV

6.5-7 MeV5.5-6 MeV

7.5-8 MeV 13-15 MeV

4.5–5 MeV

3-3.5 MeV Iron

0

200

400

600

800

0 1 2 3 4 5

En (MeV)

Ne

utr

on

s/M

eV

(S

cale

d)

5-5.5 MeV Iron

0

50

100

150

200

250

300

0 1 2 3 4 5 6

En (MeV)

Neu

tro

ns/

MeV

(sc

aled

)

4-4.5 MeV Iron

0

100

200

300

0 1 2 3 4 5 6

En (MeV)

Ne

utr

on

s/M

eV (

scal

ed)

13-15 MeV Iron

0

50

100

150

200

0 2 4 6 8 10 12 14 16En (MeV)

Ne

utr

on

s/ M

eV

(sc

ale

d)

Takahashi data (1992)

Examples of preliminary data for 56Fe

0.847

0.8470.847

• Emission spectra are measured as a function of incident neutron energy

• Only part of data are analyzed so far (one of 3 gamma-ray detectors) and better statistics are on the way

• Energy resolution is good for neutrons of a few MeV

• Gating on other gamma-rays is possible to test angular momentum distribution of states populated by (n,n’)

Preliminary data for 56Fe are encouraging

Show distribution of “a”

Ref. Dilg et al., Nucl. Phys. A217, 269 (1973)

Systematics give an estimate of accuracy of level density inputs to calculations

Some observations

• Emission data (p, alpha, n,…) neutron-induced reactions can be described by statistical reactions with suitable parameter selection.

• Can these data be predicted ab initio with confidence from global or other parameters?

• Competition among reaction channels can reduce the errors in the calculated results, but, given “bad” parameters, it is hard to predict the outcome.

• Users need data to some accuracy

Path forward

• New approach: Measure and model neutron emission spectrum contingent on the following gamma cascade going through a given level (or set of levels)

-- angular momentum selection -- need new reaction model code

(Monte Carlo HF)

A+1Z

AZ + n(target)

E n

2+

4+

0+

(J,E x) n'

Trigger

A+1Z

AZ + n(target)

E n

2+

4+

0+

(J,E x) n'

Trigger

• Increasingly large set of data to test reaction models–Neutron reaction data complement charged-particle data–Will the fits be physics or parameterizations?

• To provide data for GNEP – “Gas Production” by neutrons on structural and other materials – e.g. Fe, Cr, Ni, Zr, Ta, W etc.

- The cross sections are “source terms” for assessing radiation damage of materials

- Gas production is an important component of radiation damage in materials irradiated to high fluences in advanced fuel concepts.

• Other applications

• Neutron interrogation – transport though containers, etc.

• Shielding

• Fusion

• Criticality safety

• Detector development. For example, neutron output detection same as for fission neutrons

Applications motivate (fund) this work

Effects of Helium are observed at temperatures above 0.5 Tmelt

Thanks to Stuart Maloy

0.5 TmeltCopper

Summary

• Increasingly large data set for nucleon-induced reactions on nuclides with A ~ 56 can be used to test reaction model calculations• Charged- particle emission• Neutron emission• Gamma - ray emission

• Model calculations can describe data if suitable parameters are used

• Nuclear level densities are the largest uncertainty in the reaction model calculations

• Some widely used evaluations for helium production are in disagreement with our results … and with others.

• Evaluations for hydrogen production are in somewhat better shape

An aside -- higher energy data for iron test new evaluations – different physics at higher energies

Iron (n,Helium)

0

0.05

0.1

0.15

0.2

0 20 40 60 80 100

Neutron Energy (MeV)

Cro

ss S

ecti

on

(b

)

FZK/INPE56Fe

NRG-2003

JENDL-HE56Fe

IEAF-2001

LA-15056Fe

LANSCE

TSL-Uppsala