gamma detection - FNDA 2011

Fast neutron inelastic scattering 

at the nELBE facility 

Roland Beyer, Helmholtz-Zentrum Dresden-Rossendorf 

www.hzdr.de/trakula

Fast neutron inelastic scattering at the nELBE facility 

Outline 

Seite 2 

� Motivation: Nuclear transmutation 

� The neutron source nELBE 

� The double time of flight setup and data analysis 

� Summary/Outlook


Outline 

Seite 3 




� Summary/Outlook


Data needs for transmutation facilities 

http://www.nea.fr/html/science/wpec/volume26/volume26.pdf 

Seite 4 

� for nuclei to be transmuted as well as 

for structural materials 

� fast neutron spectrum 

• neutron capture 

• neutron induced fission 

• neutron inelastic scattering 

� 56Fe (n,n'γ) 56Fe More details in the talk of 

Arnd Junghans!


Outline 

Seite 5 




� Summary/Outlook


ELBE = Electron Linac for beams with high Billiance and low Emittance 

E e = 6 … 40 MeV 

f = 13 MHz / 2 n 

Q = 77 pC / bunch 

Seite 6 

I max = 1 mA 

pulse duration = 5 ps


Seite 7 

An electron gets accelerated …


Seite 8 

… and hits the neutron source.


Inside … the and source hits the liquid neutron lead is source. circulating. 

Seite 9


Seite 10 

which There knocks the electron out a neutron loses energy from a 

in form lead of nucleus. 

γ-radiation, …


Seite 11 

The neutron leaves the source ...


Seite 12 

The neutron leaves the source …


… and flies into the experimental area. 

Seite 13


Seite 14 

The neutron hits the sample ...


Seite 15 

The … and remaining transfers energy part of is its released energy 

to an as atomic a photon. nucleus.


Seite 16 

Neutron und photon leave 

the sample …


Seite 17 

Neutron and are und registered photon leave by 

different the sample detectors. 

…


nELBE – neutron production 

Seite 18 

nELBE nELBE 

neutron neutron beam 

ELBE 

electron beam


nELBE – double ToF detector setup 

Seite 20 

BaF 2 array for gamma detection 

(16 crystals, 40 cm, Ø 5.3 cm) 

neutron beam 

•BaF 2 scintillator made of two 20 cm long hexagonal crystals (inner Ø = 53 mm) 

• active high voltage dividers � more stable due to reduced heat production 

• double sided readout � reduce trigger due to dark current 

sample: nat Fe (99.8%) � 91.754% 56 Fe 

mass: 19.82 g � 18.15 g 56 Fe



Seite 21 

BaF 2 array for gamma detection 


neutron beam 

5 plastic scintillators 

for neutron detection 

(1 m, 11 x 42 mm2 • EJ-200 plastic scintillator 1 m x 11 mm x 42 mm 

• double sided readout � reduce trigger due to dark current 

• active high voltage dividers � more stable due to reduced heat production 

• high gain photomultiplier + threshold just below single electron peak 

� neutron detection threshold approx. 20 keV 

• surrounded by 1 cm Pb shielding to reduce background 

) 

rate 


mass: 19.82 g � 18.15 g 56 Fe



PTB 235 U fission chamber (FC) 

for neutron flux determination 

Seite 22 

BaF2 array for gamma detection 

flight paths: 


source - FC: 

400 cm 

source - sample: 

600 cm 

• U-235 fission chamber (borrowed from PTB Braunschweig) 

sample - BaF 

• deposit = ten layers, 5 μg/mm 

2: 

30 cm 

neutron beam 

sample - plastics: 

100 cm 

2 U-235 (99.92%), Ø 76mm 

� 201.5 mg U-235 

• P10 gas flow 


mass: 19.82 g � 18.15 g 56 Fe 

5 plastic scintillators 

for neutron detection 

(1 m, 11 x 42 mm 2 )


Outline 

Seite 23 




� Summary/Outlook


Beam profile 

� measured with moveable plastic scintillator 

Seite 24 

4.7 m from source 6.2 m from source (target pos.)


Neutron flux measured by fission chamber 

Seite 26 

Integral at target: 2x10 4 n/s/cm 2


Experimental method 

Seite 27 

56 

Fe(n, n'γ 

) 

56 

Fe 

{ 

56 

Fe+ 

n→ 

56 

56 

* 

Fe + n' 

* 

Fe → 

56 

Fe+ 

γ



Seite 28 

56 

Fe(n, n'γ 

) 

56 

Fe 

{ 

56 

Fe+ 

n→ 

56 

56 

* 

Fe + n' 

* 

Fe → 

56 

Fe+ 

γ 

without sample (82 h live time)



Seite 29 

56 

Fe(n, n'γ 

) 

56 

Fe 

{ 

56 

Fe+ 

n→ 

56 

56 

* 

Fe + n' 

* 

Fe → 

56 

Fe+ 

γ 

with sample (78 h live time)


Kinematic calculations 

Seite 30 

-Fe-56 (1.,2.,3. Level) 

(847, 2085, 2658 keV) 

-Fe-54 (1. Level) 

(1408 keV) 

-Fe-56 (2 x 1. Level) 

(1694 keV)


Plastics efficiency 

Seite 34 

Measurement at PTB: 

- Mono-energetic neutrons 

- Beyer et al., NIMA 575 (2007) 449 

Measurement at HZDR: 

- nELBE spectrum 

- Relative to 235U fission chamber 

NEFF7: 

- well established code for neutron 

detection efficiency simulation 

- developed at PTB 

Modified NEFF7: 

- Cuboid detector geometry 

- Double sided readout 

- Scintillation light propagation and 

attenuation 

- PMT Quantum efficiency 

- Threshold = one photo electron per 

PMT 

Problems: 

In simulation: 

- Unknown light output function at low energy transfer 

In measurement: 

- Collimated beam at nELBE 

- Influence of lead shielding


Plastics efficiency 

Seite 35 

Measurement at PTB: 

- Mono-energetic neutrons 

- Beyer et al., NIMA 575 (2007) 449 

Measurement at HZDR: 

- nELBE spectrum 

- Relative to 235U fission chamber 

NEFF7: 

- well established code for neutron 

detection efficiency simulation 

- developed at PTB 

Modified NEFF7: 

- Cuboid detector geometry 

- Double sided readout 

- Scintillation light propagation and 

attenuation 

- PMT Quantum efficiency 

- Threshold = one photo electron per 

PMT 

Problems: 

In simulation: 

- Unknown light output function at low energy transfer 

In measurement: 

- Collimated beam at nELBE 

- Influence of lead shielding


The 56 Fe(n,n’γ) cross section for the 1 st excited state 

Seite 36 

� absolute normalization still missing


Measurements of photon production cross section 

Seite 37


Measurements of photon production cross section 

Seite 38 

with target without target


Summary and outlook 

• nELBE is intended to deliver data on fast neutron induced reactions 

• the ELBE electron beam delivers a high neutron flux with very good time structure 

• different kinds of experiments can be done: 

• inelastic scattering using a double time of flight setup: Fe-56 and Na-23 

• neutron transmission: Al, Ta, Pb 

• elastic scattering: D(n,n)D 

• fission: U, Am, Np, Pu � Future 

• planned improvements: 

• LaBr 2 detectors instead of BaF 2 � better photon energy resolution 

• new bigger experimental area within extension of ELBE facility 

Seite 40


National Center for High-Power Radiation sources 

• New Neutron Time-of-Flight Facility 

• X-ray source using Laser-Compton-Backscattering 

• High-Power Laser (PW) for Ion Acceleration 

Seite 41


Thanks to all collaborators 

HZDR, Institute of Radiation Physics: 

A.R. Junghans, D. Bemmerer, E. Grosse, R. Hannaske, A. Hartmann, K. Heidel, M. Kempe, T. Kögler, 

R. Massarczyk, R. Schwengner, M. Sobiella, A. Wagner, The ELBE Crew 

HZDR, Institute of Safety Research: 

E. Altstadt, C. Beckert, A. Ferrari, V. Galindo, K. Noack, F.-P. Weiss 

HZDR, Department Radiation Protection and Safety: 

B. Naumann 

HZDR, Department Research Technology: 

R. Schlenk, S. Schneider 

TU Dresden: 

H. Freiesleben, D. Gehre, M. Greschner, A. Klix, K. Seidel 

Physikalisch Technische Bundesanstalt Braunschweig: 

M. Mosconi, S. Löb, M. Erhard, R. Nolte, S. Röttger 

Others: 

Th. Beyer, E. Birgersson, J. Klug, K. Kossev, M. Marta, A. Matic, 

C. Nair, C. Rouki, G. Rusev, K.-D. Schilling, G. Schramm, 

Seite 43 

www.erinda.org 

www.hzdr.de/trakula


The End 

� Was one slide before! 

Seite 44


235 U Fission chamber efficiency 

� calculation from ENDF (n,fis) cross section 

� simulation with MCNP 

(includes multiple processes and beam attenuation) 

� incredients 

• number of fissionable nuclei 

• fission detection efficiency 

• homogeneity 

Seite 45 

Nolte, Nucl.Sci.Eng.156(2007)197 

Gayther, Metrologia 27 (1990) 221



Seite 46 

� absolute normalization still missing 

Uncertainties: @ 2 MeV 

Fission chamber efficiency 2.1 % 

Fission chamber counts 1.5 % 

Fission chamber background 1.8 % 

Loss due to ADC range 0.1 % 

Scaling factor FCTarget 0.3 % 

Attenuation factor 1.1 % 

� Neutron flux 2.9 % 

Sample in counts 2.3 % 

Sample out counts 15.9 % 

Normalization factor 1.5 % 

BaF 2 efficiency 1.3 % 

Plastic efficiency 2.2 % 

� Reaction rate 3.8 % 

� Cross section 4.8 %


Detector geometry - details 

� borated polyethylene block between BaF 2 and plastics 

� number of random coincidences reduced by one order of 

magnitude 

Seite 47 

Plastics BaF 2-Setup 

angular coverage: 

- θ n = 60°…120° 

- ϕ n = - 12°…+12° 

- θ γ = 50°…130° 

- ϕ γ = +/- (30°…130°)


Outline 

Seite 48 


� Basics: Neutron production and inelastic scattering 

� nELBE – The source and the double time of flight setup 

� Experiment analysis and results 

� Summary/Outlook


How to produce neutrons 

� Sources using radioactive isotopes: 

Seite 49 

• mixed sources, e.g. 244 Cm � alpha decay � 13 C(α,n) 

• spontaneous fission, e.g. 252 Cf 

� Reactor Neutrons 

� Sources using accelerators: 

• quasi mono-energetic sources: 

- 7 Li(p,n) 7 Be � e.g. PTB Braunschweig 

- 3 H(d,n) 4 He � e.g. D-T-Generator of TU Dresden at HZDR 

• continuous sources: 

- bremsstrahlung: 238 U(γ,n) � e.g. GELINA at IRMM Geel, Belgium 

- spallation: Pb(p,xn) � e.g. CERN nTOF


How to measure inelastic neutron scattering 

56 

� mono-energetic neutron-source and detection of the 

scattered neutron 

• e.g. Van-de-Graaff in Studsvik, Nyköping, Sweden 

• T(p,n) 3He + each excited state can be identified 

- new setting for each point 

Seite 51 

Fe(n, n'γ 

) 

56 

Fe 

{ 

56 

Fe+ 

n→ 

56 

56 

* 

Fe + n' 

* 

Fe → 

56 

Fe+ 

γ 

56 Fe



56 




• T(p,n) 3He + each excited state can be identified 


� continuous spectrum and detection de-excitation 

gamma 

• e.g. Gelina in Geel, Belgium 

• electron Linac + Uranium target 

+ all energies in one experiment 

- knowledge on decay scheme necessary 

Seite 53 

Fe(n, n'γ 

) 

56 

Fe 

{ 

56 

Fe+ 

n→ 

56 

56 

* 

Fe + n' 

* 

Fe → 

56 

Fe+ 

γ 

56 Fe



56 




• T(p,n) 3 He 

+ each excited state can be identified 


� continuous spectrum and detection de-excitation 

gamma 

• e.g. Gelina in Geel, Belgium 

+ all energies in one experiment 

- knowledge on decay scheme necessary 

� continuous spectrum and detection of both emitted 

particles 

• e.g. FIGARO in Los Alamos, USA 

• spallation source 

+ all energies, level identification 

- low count rate 

Seite 54 

Fe(n, n'γ 

) 

56 

Fe 

{ 

56 

Fe+ 

n→ 

56 

56 

* 

Fe + n' 

* 

Fe → 

56 

Fe+ 

γ 

56 Fe


Neutron facility comparison 

n-ToF-device 

Seite 55 

CERN 

n-ToF 

LANL 

NSC 

ORNL 

SNS 

FZK 

VdG 

ORNL 

ORELA 

IRMM 

GELINA 

ELBE ELBE with 

SRF-gun 

beam power / kW 10 60 1000 0.4 8 7 5 40 

rep. rate / s -1 0.4 20 60 

2.5⋅10 

5 500 800 

1.6⋅10 

6 

pulse charge / nC ≈ 10 3 4⋅10 3 3⋅10 4 0.01 ≈ 100 ≈ 100 0.08 1.8 

flight path / m 183 ≈ 20 60 84 0.8 40 20 6 6 

n-pulse length / ns > 7 125 100-700 ≈ 1 > 4 > 1 < 0.4 < 0.4 

E min / eV 0.1 0.1 1 0.1 10 3 10 10 2⋅10 5 2⋅10 4 

E max /eV 3⋅10 8 ≈10 8 ≈10 8 2⋅10 5 5⋅10 6 4⋅10 6 7⋅10 6 7⋅10 6 

∆E/E @ 1MeV 

n-flux 

(s⋅cm 2 ⋅E-decade) - 

1 

0.5 

% 

5⋅10 5 

5% ≈ 10 % >10 % ≈ 10 % < 1 % < 2 % ≈ 1 % ≈ 1 % 

10 5 ≈ 10 7 ≈ 10 6 10 6 -10 7 ≈ 10 4 10 4 4⋅10 4 10 5 10 6


nELBE – neutron facility at ELBE 

e - 

Seite 57 

γ 

A Pb 

n 

A-1 Pb


Determination of the inelastic scattering cross section 

� Incoming Neutron Flux 

Seite 62 

• 235 U fission chamber neutron detection efficiency � calculation/simulation 

- number of fissionable nuclei 

- fission detection efficiency 

- homogeneity 

• count rate in time of flight spectrum � statistics + background subtraction 

• corrections for loss due to ADC gate 

• scaling factor for ratio of beam area at fission chamber and sample size 

• corrections for flux attenuation inside fission chamber, air and sample 

fission chamber neutron beam sample



� Number of detected reactions 

Seite 63 

• measurements with and without sample in beam 

- background normalization


Background subtraction 

Seite 64 

sample in beam 

sample off beam 

kinematic window 

for event counting 

“Banana” for 

background 

determination




Seite 65 


- background normalization 

• efficiency of neutron detectors




Seite 66 


- background normalization 

• efficiency of neutron detectors 

• solid angle of neutron detectors � calculation 

• efficiency of photon detectors 

- measured with mono energetic radioactive sources 

- 54 Mn: 835 keV is similar to 56 Fe 1 st excited level at 847 keV 

- solid angle included in source measurement 

� Number of target nuclei 

• target mass 

• purity 

• isotopic enrichment



Seite 67


Inelastic neutron scattering on 23 Na 

Seite 68 

without target Na-23 

E thr = 2391 keV 

E thr = 2076 keV 

E thr = 440 keV



Seite 69



Seite 70

gamma detection - FNDA 2011

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