esi 2009, cern, genevag. bonheuremay. 15, 2009 1 neutron diagnostics for fusion experiments georges...

ESI 2009, CERN, GENEVA G. Bonheure May. 15, 2009 1

Neutron diagnostics for fusion experiments

Georges Bonheure


Outline• Introduction• Time-resolved neutron emission• Time-integrated neutron emission• Neutron profiles• Neutron spectra• Summary


Neutrons produced in fusion reactions:

D + T -> (4He + 3.56 MeV) + (n + 14.03 MeV) Q = 17.59 MeV D + D -> (3He + 0.82 MeV) + (n + 2.45 MeV) Q = 4.03 MeV T + T -> 4He + 2n Q = 11.33 MeV

What do neutrons do?

Introduction: fusion neutrons

D

T n

4He

fusion reactions cross sections

1.E-04

1.E-03

1.E-02

1.E-01

1.E+00

1.E+01

1 10 100 1000

kT (keV)

cro

ss s

ection (

barn

)

D-D D-TD-3HeT-T

10

1

10-1

10-2

10-3

10-4

Fusion energy: Neutron energy transferred to the

reactor coolant

Fuel generation: Breeding T from Li:nslow + 6Li -> 4He + Tnfast + 7Li -> 4He + T + nslow

To minimize: activation, radiation damage

D


The largest tokamak: JET (Joint European Torus: www.jet.efda.org)

JET: outside view

Record: Q = 0.8

Steady state: Q = 0.3

total output : max 16 MW


www.iter.org

The future

ITER site now!


Neutron source: progress in parameters

The jump to ITER

Biggest increase in neutron fluence!> Radiation hardness

Plasma volume

100 m3

850 m3

Neutron source strength

1010 – 5.7 1018 n s-1

1014 – 1020 n s-1

Neutron flux at first wallITER ~ 10x JET

Neutron fluenceITER ~ 104 x JET(1025 n m-2)


The plasma as a neutron source

BABABABBAAAB

BA ddtftftntn

tY vvvvvvrvrvrr

r

)(),,(),,(1

),(),(),(

D-D : )(5.82 keVTEin

D-T : )(180 keVTEin

))(1()()(

)()()(

,,

,, rrr

rrr k

vY

vY

n

n

btDTbtDD

btDDbtDT

D

T

Ion temperature:

Ion density ratio:


Access: ITER diagnostics are port-based where possible

Each diagnostic port-plug contains an integrated instrumentation package


Introduction: fast neutron diagnostic systems

• The variety of measurements that are possible are generally restricted due to:– Limited access to plasma

– Harsh radiation environment X,

– Strong magnetic fields, powerful high frequency wave generators and power supply

– Heat loads, mechanical stress– Timescale of measurements– Activation, tritium, beryllium


Neutron diagnostic systems: 4 types of systems

Time-resolved total emission(non-collimated flux)

Time-integrated emission(fluence)

2D-cameras (collimated flux along camera viewing lines)

Spectrometers (collimated flux along radial and tangential viewing lines)

Fusion power

Absolute emissionCalibration of time-resolved emission

Spatial distribution of emissiontomography

Plasma temperature and velocity

Combination of these measurements characterizes the plasma as a neutron source


• Short range of interactions: characteristic scale is the nucleus size: 1 fermi (fm) 10-13 cm!

• Elastic scattering: A(n,n)A• Inelastic scattering: A(n,n’)A*• Radiative capture: n + (Z,A) -> + (Z,A+1)• Fission: (n,f)• Other nucl.reactions: (n,p),(n,),…• High energy particle production (En > 100 MeV)

Interaction of neutrons


1. Time-resolved neutron emission

• Fission counters:

• 238U and 235U counters embedded in moderator and led shield

• Operate both in counting and current mode• Dynamic range: 10 orders of magnitude• 3 pairs installed at different positions around JET• Low sensitivity to X and radiation• No discrimination between 2.5 and 14 MeV neutron

emission• Calibrated originally in situ with californium 252Cf neutron

source, periodically cross-calibrated using activation techniqueU235 U238


Calibration with JET Remote Handling System

252Cf source strength: 109 n/sDuration : 3 days



• For mixed 14 MeV and 2.5 MeV neutron fields:– Silicon diode

• Fluence limit ~ 1012 cm-2

– Natural diamond detectors (NDD)

– Chemical vapor deposited (CVD) diamond detectors

• Radiation hardness >3.1015 cm-2

New radiation hard detectors are tested in JET



GEM based neutron detection


2. Time-integrated neutron emission

• Neutron activation method

Sample activity measurements:Sample activity measurements: 1) gamma spectroscopy measurements >>> most widely used reactions at JET: DD neutrons - 115In(n,n’)115mIn, DT neutrons - 28Si (n,p)28AL, 63Cu(n,2n)62Cu, 56Fe(n,p)56Mn >>> detectors : 3 NaI, HPGe (absolutely calibrated)2) delayed neutron counting (235U,238U,232Th)

>>>detectors: 2 stations with six 3He counters

Calibration: accuracy of the time-resolved measurements is typically ~ 8-10% for both DD and DT neutrons (7% at best using delayed neutron method) – after several years of work !!

Samples used as flux monitors are automatically

transferred to 88 Irradiation ends

Neutron transport calculations with MCNPto obtain the response coefficient for the samples

•MIX composition:

•Se-16%, Fe-20%, Al-16%, Y-48%


PRINCIPLE• Escaping light charged particles p, t, d, 3He

or hit selected targets and produce nuclear reactions of type A(z, n)B*, A(z,γ)B*,…

• B* radioactive decay (gamma photons) are measured using high purity germanium detectors

Activation techniqueActivation technique

Activation probe (targets holder)

Example of JET results: Gamma spectrometry of a natural Titanium target

HpGe detector

48Ti(p,n)48V Ep > 4.9 MeV

A measurement challenge: Escaping alpha particles


3. Neutron profiles: 2D cameras

• Two multi-collimator arrays (60tons each) with 19 channels available in total , 10 horizontal and 9 vertical

• Adjustable collimators: Ø10 and 21 mm

• Detectors: – Liquid organic scintillators

NE213 with pulse shape discrimination

– BC 418 plastic scintillators– CsI scintillators for γ rays

• Calibration: embedded sodium (22Na) sources

• γ / n separation control: movable americium beryllium 241Am/Be source


Digital pulse shape discrimination technique

n

n/γ separation obtained with a 14 bits- 200 MegaS/s DPSD prototype

One NE213 detector of neutron camera is exposed to a plasma pulse

• Benefits – Detailed post

processing possible (events identification, pile-up,…)

– Deconvolution of spectrum information

– Increase dynamic range in both energy and count-rate


Study of tritium diffusion

vnr

nD

trrrt

n

)(1

Pulse 61161: ne0 = 1.9 1019m-3Pulse 61372: ne0 = 4.5 1019m-3

Theoretical predictions for D, v can be verified against measurements

Time (s) Time (s)

R (

m)

R (

m)

nT/nD


4. Neutron spectroscopy

• Time of flight• Proton recoil

– 1) ‘thick hydrogenous target’ (high efficiency)

• No information on recoil angle : energy spectrum recovered by unfolding

– 2) ‘thin hydrogenous target’ (low efficiency)

• Analysis of recoil proton momentum

Trade off: energy resolution vs detection efficiency


Neutron spectroscopy: time of flight (TOFOR)

Energy resolution for DD neutrons: ~5%Detection efficiency: 8 10-2 cm2

Count rate: < 500 kHzSimulated with GEANT code


23

TANDEM

TOFOR

NE213

MPR

TG DiagnosticsGarching

April, 2009

23

4. Neutron spectroscopy


Neutron spectroscopy: spectral unfolding techniques

Comparison between different unfolding techniques:

• Maximum entropy (MAXED)

• Minimum fisher regularisation (MFR)


Summary: neutron diagnostic systems

Systems JET ITER

Time-resolved total emission

Total: fission counters14 MeV: Silicon diodes

fission counters Diamond detectors

Time-integrated emission

Foil activation Foil activation

2D-cameras Liquid scintillators NE213 Plastic scintillators BC418

Diamond detectors Stilbene, NE213,U238 fission counter, fast plastic

Spectrometers Time of flight Proton recoil systems:1) NE213 and stilbene2) Magnetic proton

recoil

To be defined


Final remarks

• With the move towards ITER– role of fast neutron diagnostics will increase– Capabilities of those systems need to accommodate an increase in

fluence by 4 orders of magnitude and in flux by 1 order of magnitude

• JET has an extensive set of fast neutron diagnostics, more than 2 decades of accumulated experience, and it will continue to play a leading role in development of fast neutron measurements for fusion applications

• Active research areas include new radiation hard detectors, new electronics and acquisition systems, spectrometers, tomography and unfolding techniques

• Neutron measurements contribute to advanced physics studies e.g in the field of plasma particle transport

• For references see in: http://pos.sissa.it/ ‘Neutron diagnostics for reactor scale fusion experiments’


Thanks…

esi 2009, cern, genevag. bonheuremay. 15, 2009 1 neutron diagnostics for fusion experiments georges...

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