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Diagnostic Capabilities of Line-Integrated Neutron Pulse Height Spectra Measurements

Daniele Marocco

Associazione Euratom-ENEA sulla Fusione, C.R. Frascati, C.P. 65, Frascati I-00044, Roma, Italy

D. Marocco Fusion Science And Engineering Doctorate – Garching October 2008

Preface

Two main quantities characterize neutron emission in fusion experiments:

• Neutron emissivity• Neutron spectrum

On present devices they are measured by separate diagnostics:• Neutron cameras (multi channel diagnostics providing neutron emissivity

along a plasma poloidal section)• Neutron spectrometers (single channel diagnostics providing line-integrated

neutron spectra)

Thanks to digital technologies new systems using compact spectrometers can be developed which will allow to combine neutron spectra and 2-D neutron profile measurementsThe present work aims at exploring the capability of a collimated compact spectrometer detector array equipped with a digital acquisition system


Outline

Introduction: • Plama neutron emissivity• Plasma neutron emission spectrum

Diagnostics for neutron spectrometry: organic liquid scintillators

Diagnostics neutron emissivity measurements: neutron cameras

Research activity


Introduction: neutron emissivity

Emissivity: plasma local neutron yield (n s-1 m-3) expressed as

D. Marocco Fusion science and engineering Doctorate

ABAB

BA σ vδ

(r)(r)nnY(r)+

=1

nA nB = particle densities; δAB= Kronecker symbol; <σ v>AB= neutron reactivity

Main nuclear reactions in plasma experiments:

D + D →t (1.01 MeV) + p (3.02MeV) D + D → 3He (0.82 MeV) + n (2.45MeV)

D + T → 4He (3.56 MeV) + n (14.03 MeV)

} Nearly equal probability: the emission of 2.5 MeV neutrons indicates the birth of 1.0 MeV tritons

When undergone by the fusion product tritons from the first D-D reaction branch = triton burn-up

Introduction: Neutron Emission Spectrum

The neutron emission spectrum in a tokamak reflects the velocitydistribution of the fusing ion pairs

Thermal plasma:Gaussian-shaped neutron spectrum (width W ∝ √Ti)


Non thermal plasma:Non-Gaussian tails and Doppler energy shifts

Diagnostics For Neutron Spectrometry

Large neutron spectrometers:

• Magnetic proton recoil (MPR): neutrons from the plasma are converted into recoil protons by means of a thin hydrogen-reach target; the recoil protons are momentum discriminated through a magnetic field

• Time of Flight (TOF): measurements of the times of correlated neutron scattering events in a start and stop detector

Compact neutron spectrometers:

• Diamond detectors (14 MeV only): based on the 12C(n, α) threshold reaction (∼8 MeV)

• Scintillator detectors

All measurements performed using a single collimated line of sight through the plasma: single line-integrated spectra


Liquid Scintillators (1/2)

Based on neutron scattering with hydrogen atoms: • Recoil protons excite scintillator molecular compounds with consequent

ligth emission• Ligth pulses are converted into electron signals and amplified coupling the

detector to photomultipliers (PMT)• γ pulses can be discriminated through pulse shape analysis

dN/dE

EEn

En scintillatorPMT

• Scintillator non linearity, finite energy resolution, multiple scattering from hydrogen atoms, alter the response function: specific codes or experimental measurements are needed

Scintillator energyresponse function


Liquid Scintillators (2/2)

For a non monocromatic neutron beam a Pulse Height Spectrum(PHS) given by the superposition of the different energy response function is provided by the scintillatorThe actual neutron spectrum is obtained by means of unfoldingcodes


PHS

Unfolding

Neutron Cameras: ITER RNC-VNC System

A neutron camera system equipped with organic liquid scintillatorsand digital electronics is foreseen for ITER:

Radial neutron camera (RNC) to be installed in equatorial port plug#1:

• Ex-port system: 12 LOS (x 3 on planes at different toroidal angles)

• In-port system: 9 LOS

Vertical neutron camera (VNC) to be installed in a lower divertorport:

• 10 LOS

PHS covering a whole poloidal plasma section: possibility to observe spatial and time evolution of neutron spectra


Neutron Cameras: JET Neutron Profile Monitor (1/2)2 concrete shields each including a fan-shaped array of collimators: 10collimated channels with a horizontal view; 9 channels with a vertical view

Each LOS equipped with:• NE213 liquid organic scintillator

• Analogue pulse shape discrimination (PSD) electronics for simultaneous measurements of the 2.5MeV-14MeV neutrons and γ -rays


Neutron Cameras: JET Neutron Profile Monitor (2/2)

The JET profile monitor due to limitations imposed by analog electronics:

• Works just as a counter calibrated to provide neutron counts in specific energy windows (1.8 MeV - 3.7 MeV for DD; > 7 MeV for DT). No PHS during discharges

• Is limited to count rates ~200 kHz

An ENEA project for a full digital upgrade of all neutron profile monitor channels has been approved


As a part of this project the development of a single channel digitizer (SCD) to be installed at JET has been carried out

Doctorate Research: Scope & Program

Scope: Investigate the diagnostic capability of multiple line integrated neutron PHS (scintillators+digital electronics) byscanning with the SCD all the JET neutron profile monitor channels

Program:• Set up of SCD and elaboration software

• SCD benchmarking

• SCD installation at JET

• Data acquisition and Analysis (on-going)

• Modeling activity: feasibility study of deriving a 2D neutron energy profile using a combined inversion-unfolding technique (to be started)


Program: SCD and elaboration software set up (1/2)

200 MSamples/s &14-bit resolution

FPGA- based (Altera 1S25) (mainly used for data compression)

Data transfer to PC through PCI

DPSD rack unit

Estimated Sustainable CountRates: ~ 500 kHz DT~ 900 kHz DD


Program: SCD and elaboration software set up (2/2)

LabVIEWTM code managing off-line data analysis :• n and γ separation through digital charge comparison

• Separated n & γ count rates and PHS provided via pulse integration


n

γ

n/γ separation plot

Program: SCD Benchmarking

12 kHz acquisition

The system has been benchmarked at the PTB accelerator (Braunshweig–Germany) with 2.5 and 14 MeV neutrons against an analog acquisition chain using a fully caraterized liquid scintillator:

• comparable energy resolution up to ~ 420 kHz


Program: SCD installation at JET

Scintillator PMT

Splitter//

Analog PSD Module SCD

The digitizer can be connected to a single profile monitor channel in place of the analog PSD module normally used to detect 14 MeV neutrons


Acquisition of data (Na-22 γ sources and plasma discharges) obtained placing the digitizer on each channel of neutron profile monitor presently on–going

Identify a data sub-sets with quasi-similar plasma conditions and:

• Characterize single PHS

• Compare PHS from different channels (channel to channel variations & temporal evolution)

• Perform PHS unfolding

Program: Data Acquisition and Analysis


Counts converted to brightness by meansof intrinsic efficiency of the detectors and inverted:

A 2D profile of the fuel ratio can be obtained(Ratio Method)

2D information on 1 MeV tritons slowingdown can be obtained from the comparison of the time evolution of DD and DT signals

Program: Data Acquisition and Analysis - 14 MeV to 2.5 MeV Ratio


DD

DT

DD

DT

D

T

vv

YY

nn

σσ2

=1

10

100

1000

10 4

10 5

0 2 4 6 8 10 12 14 16

SHOT # 68569

Proton energy (MeV)

channel #15

14 MeV neutron signal due to triton burn-up reactions

2.5 MeV neutrons +14 MeV contribution

Rough estimate of the actual DD and DT counts for each channel using PHS

channel #15

Program: Data Acquisition and Analysis - Fast neutron tails


PHS unfolding: •A Line-integrated ion temperature profile can be obtained during the ohmic phase

• Line-integrated profile information on intensities and temperature of the different ion components can be derived during the additionally heated phase

Rough estimate of the fast neutron component in eachchannel boxing the PHS

Program: Modeling Activity (1/3)

Aim: develop a combined unfolding - inversion technique to derive a 2D neutron energy profile starting from a set of line integrated PHS (feasibility study)


ill-posed problem: small variations in input data produce high variations in the solutions

regularization techniques are needed

bk = brigthness measurement from chord kej = neutron emissivity ψ = normalized poloidal flux coordinate


Each brightness measurement bk can be thought as the energy integral of a line integrated neutron spectrum Sk

∫= dEEsb kk )( unfolding of the line integrated PHS representing the measurement of a camera LOS

Each emissivity eJ can be thought as the energy integral of a local energy spectrum hJ

∫= dEEhe jj )( Local energy spectrum to be derived


)()( EhLEs =With respect to typical inversionproblems the response matrix L connects energy functions rather than real numbers

Inserting back these definitions in the emissivity linear equation system


If a feasible method is identified its robustness will be tested through simulation using a reference plasma and neutron camera layout (ITER, JET):

• Set up a reference 2D neutron enegy profile (phantom)

• Derive a set of synthetic meaurements (line-integrated PHS):• Integration along LOS • Folding with the detector response functions (including statistical,

background and random errors)

• Apply the combined unfolding-inversion algorithm to obtain an “inverted”2D neutron energy profile

• Compare the phantom and the inverted profile


Francesco Activity

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Liquid scintillators: detection principleLiquid organic Scintillators: molecular compounds characterized by a molecular structure in which unbound π-electrons are prone to excitation by incident radiation

Fluorescence

Delayed fluorescence

S2

γ


S0

S3

S1

1 2

S0

S3

S1T3

T1

T21

2

3

4

Liquid scintillators: n/γ separation

The proportion of delayed fluorescence of the pulse is related to the triplet density in the wake of the incident particle (determined by the rate of energy loss,dE/dx, of the incident particle)

Heavier particle

Greater energy loss rate

More delayed fluorescence in the output

Pulses that decay more slowly then those

from lighter particles

4000

10

100

1000

5000 50 100 150 200 250 300 350 400 450

time (ns)ch

anne

l

ΔtFΔtS

n

γ


4000

10

100

1000

5000 50 100 150 200 250 300 350 400 450

time (ns)

chan

nel

Δt FΔt S

Charge comparison

Operates on an endless ring of digitized data performing:Offset RemovalDynamic data window creationWindow cutting

Input ± 2.8 VCoupled in Interleaved mode (5 ns delay) giving an actual sampling rate of 200 MSamples/s on one input channel

Packs window data and timing information (time of the first over threshold sample)

Data first stored in a portion of the RAM (∼1.2 GB).When the programmed acquisition time is reached the acquisition stops and data are saved to disk

Matches the different speed in input and output

The acquisition process

The Elaboration Process

bk = lkje jj=1

N

∑ k =1, M

b = L e

If the bk coefficients are affected by noise the system could be inconsistent meaning that there is no emissivity that exactly solves the system.

Le − b 2 LT b = LT Le Least squares soultion

Thikonov regularization Le − b 2 + α D e 2

LT b = (LT L + αDT D)e

Program: Modeling Activity (3/3)Magnetic Surfaces

LayoutIon Temperature

Profile

Reference 2D EnergyProfile

Gaussian ShapedSpectra

Line IntegratedSpectra

Line Integrated PHS: Synthetic Measurements

Detector ResponseFunctions

Noise:Statistics

BackgroundRandom

2D Ion Temperatureprofile

Los Layout

Combinedunfolding - inversion

Inverted 2D EnergyProfile


Neutron Cameras

Diagnostic providing line-integrated neutron counts (brightness, ns-1m-2)along a large number lines of sight (LOS)

The emissivity over a poloidal section of the plasma is obtained by Inversion/tomographic techniques


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