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Neutron and gamma ray measurements

for fusion experiments and spallation sources

Carlo Cazzaniga

Supervisor: prof.ssa Claudia RiccardiExternal supervisor: dr. Marco Tardocchi1

1) Istituto di Fisica del Plasma “Piero Caldirola”

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Overview

● Motivation

● Gamma ray spectroscopy for fusion plasmas with the LaBr3 detector

● Development of neutron detectors

– Gas detector for thermal neutrons

– Proton recoil spectrometers for fast neutrons

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Why is fast ion physics important in fusion research?

● The Tokamak is the most promising configuration for magnetic confinement fusion

Good confinement of alphas is essentially required in realizing a nuclear fusion reactor

Need for diagnostics → neutron and gamma spectrometers

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Neutron detectors for spallation sources

Why do we need new thermal neutron detectors?

● The shortage of 3He has triggered the search for effective alternative neutron detection technologies

Why wide do we need a wide energy range neutron spectormeter?

● Spectrum up to 800 MeV

● Study of single event effects due to atmospheric neutrons

ISIS spallation source

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Gamma ray spectroscopy for fusion plasmas

with the LaBr3 detector

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Why measuring γ-rays?

● Gamma-rays are emitted from reactions of fast ions (E>0.5 MeV) with impurities

4α + 9Be → 1n + 12C*

→ 12C + γ (4.44 MeV)

● Studies of fast ions physics → α particles confinement

Informations from gamma-ray spectroscopy:

● Good energy resolution → peak shape analysis (Doppler broadening)

● High rate → to follow transients associated with MHD instabilities

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The LaBr3 scintillator

Good energy resolution:

• High light yield (63000 photons per MeV)

High efficiency (25% for 4.44 MeV gammas)

• Big volume (3’’x6’’ cylinder)

• High Z and high density (5.08 g/cm3)

High rate capability (up to 2MHz):

• Fast scintillation time (16 nsec)

• PMT with active voltage divider

• ADC acquisition (400 MHz, 14 bit)

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Validation with radioactive sources

Peak Eγ (keV)

R (%)

137Cs 662 3.360Co 1173 2.560Co 1333 2.4

Good energy resolution thanks to high light yield.

Spectral shape as expected (full energy peaks, compton edges, backscatt. peak)

Good agreement with MCNP simulation.

Small difference is ascribed to minor differences between the MCNP model and the actual experimental setup

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Detector setup at JET

● The detector is now installed at JET above the roof lab.

● It shares the vertical line of sight with other gamma-ray detectors (HPGe, NaI)

~4m

LaBr3

LaBr3

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Fast ion studies using γ−ray emission spectroscopy at JET

● Low density plasmas with high ICRH (Ion Cyclotron Resonance Heating) RF power

● Several γ-ray peaks are observed from different reactions among fast ions and impurities, such as 9Be(α,nγ)12C, 12C(d,pγ)13C and 9Be(d,pγ )10Be, and have energies from Eγ = 3 to 4.5 MeV.

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Detector response to neutron background

● Inelastic scattering → gammas

● (n,2n) → gammas + more neutrons → inelastic scattering → gammas

● Delayed counts:

79Br (n,2n) 78Br → 78Se + β+ (τ = 9 min) (e.p. 2.6 MeV)

81Br (n,2n) 80Br (90%) → 80Kr + β- (τ = 25 min)(e.p. 2 MeV)

(10%) → 80Se + β+ (τ = 25 min)(e.p. 0.4 MeV)

elastic inelastic (n,2n) Radiative capture

others

2.5 MeV La139: 3 bBr81: 2 bBr79: 2 b

La139: 2 bBr81: 1.8 bBr79: 1.8 b

0 ≈ 10-2 b 0

14 MeV La139: 4 bBr81: 3 bBr79: 2 b

La139: 0.6 bBr81: 0.3 bBr79: 0.3 b

La139: 1.5 bBr81: 1 bBr79: 1 b

≈ 10-3 b < 0.2 b

Cross sections relevant for fusion neutron background

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Outline of measurements and simulations

Fusion plasmas

● AUG (DD reaction, 2.5 MeV neutrons)

● JET (DD reaction, 2.5 MeV neutrons)

Frascati Neutron Generator (FNG)

● DD reaction, 2.5 MeV neutrons

● DT reaction, 14 MeV neutrons

Simulations

● Simple model: mono-energetic neutrons on crystal only

● Simulation of produced gamma rays

● Simulation of detector response

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Neutron induced background spectrum

Simulations of 2.5 MeV neutrons agree with data

Different measurements confirm it's neutrons

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High rate neutron measurements at JET

• Temporal evolution of the counting rate of the LaBr3 spectrometer and JET total neutron yield measured with the fission chambers for discharge #82539.

• DD plasmas with high NBI (Neutral Beam Injection) have a high neutron yield, mostly from beam-plasma reactions

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Measured (linear fit) Predicted (MCNP + transport)

slope 0.88 · 10-10 (0.9 ± 0.3) · 10-10

intercept 2 · 103 1.5 · 103

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How to handle neutron background on ITER?

TO DO NEXT

● Detailed MCNP simulations

● Characterization of neutron shielding (6LiH)

● Line of sight optimization

● Neutron flux at this position: up to ~ 108 – 109 neutron cm-2 s-1

on the detector: 5 109 – 5 1010 neutrons / sec

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Gas Electron Multiplier for

thermal neutron detection

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bGEM for thermal neutrons

The 10 B(n,α)7 Li reaction → thermal neutrons

Neutron conversion Charge particle detector

GEM (Gas Electron Multiplier) ElectronsElectrons

IonsIons

60 %

40 %

● Single Boron Layer: Low efficiency (1%)

● Three-dimensional structures: What efficiency can we achieve?

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Testing the prototype

IN THE LAB

● Calibration with X-ray sources

● Gain as function of measurement parameters

● Response to background gamma rays

AT THE NEUTRON SOURCE

● First measurement of thermal neutron with single converter layer

Assembly of the bGEM detector using the cathode with single converter layer.

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Characterization of a GEM detector

Peak position changes with the gain

Photo-peak of 55Fe (6 keV)

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Experimental test at ISIS● The bGEM detector successfully measured the thermal neutron beam

profile

● a TOF measurement was performed

● Beam Profile Measurement and detector counting rate using Cd filters (imaging)

● Thermal neutron efficiency as a function of the detector gain

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Telescope Proton Recoilspectrometers

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TPR detection principle

● Energy range

● Efficiency

● Energy resolution

choose

● Target thickness and area

● Angle and distance

● Proton spectrometer (type, material, dimensions)

● Acquisition system

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TPR for fusion● Two prototype of proton spectrometers

(YAP and LaBr3 scintillators) tested in the

lab with X and gamma rays

● Gamma and neutron background evaluated through MCNP simulations and measurements at FNG

Peak Energy

LaBr3

Resolution

YAP Resolution

Cs137 662 keV 4.2% 5.5%Co60 1172 keV 3.5% 3.8%Co60 1333 keV 3.7% 3.8%

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TPR for spallation sources

● neutrons

n

YAP 1''x1''

Phototube

Mounting in Milano

Test at ISIS spallation source

Pulse acquired with digital system

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Results of first test at ISIS

● Some recoil protons observed, but bkg is too high

● T must stand for Telescope → need for delta E detector

Gamma flashesFast neutron events

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Conclusion

The LaBr3 gamma ray

spectrometer

Gas detector with boron layer

Proton recoil spectrometers

Diamond detectors

What is the right measurement technique for your application?

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Courses

● Schools

● Sokendai Asian Winter School 2011: " New direction of plasma physics and fusion science for future energy "

● 11th Kudowa Summer School: "Towards Fusion Energy"● MCNPX Intermediate Workshop

● Classes

● Radioattività● Introduzione alla fisica dello stato solido

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Publications

Papers on peer-reviewed journals

● C. Cazzaniga et al. “Response of the LaBr3 gamma-ray spectrometer to fusion neutrons”, to be submitted

● M. Nocente, M. Garcia-Munoz, G. Gorini, M. Tardocchi, A. Weller, S. Akaslompolo, R. Bilato, V. Bobkov, C. Cazzaniga, B. Geiger, G. Grosso, A. Herrmann, V.G. Kiptily, M. Maraschek, R. McDermott, J.M. Noterdaeme, Y. Podoba, G.Tardini and the ASDEX Upgrade Team, "Gamma-ray spectroscopy measurements of fast ions on ASDEX Upgrade", Nucl. Fusion 52 (2012) 094021

Conference proceedings

● C. Cazzaniga, M. Nocente, M. Tardocchi, L. Giacomelli, G. Croci, C. Riccardi and G. Gorini, “The LaBr3 Gamma Ray Spectrometer for

ITER diagnostics”, proceedings of the 11th Kudowa Summer School, “Towards Fusion Energy”, June 11-15, 2012

● Nocente, M. Angelone, C. Cazzaniga, I. Chugunov, R.C. Pereira, G. Croci, D. Gin, G. Grosso, A. Neto, A. Olariu, S. Olariu, M. Pillon, A. Shevelev, J. Sousa, M. Tardocchi and G. Gorini,“Gamma-ray measurements and neutron sensitivity in a fusion environment”, Proceedings of the 1st International Conference on “Fusion for Neutrons and Subcritical Nuclear Fission”, Varenna, 2011

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