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
D. Marocco Fusion Science And Engineering Doctorate – Garching October 2008
Outline
Introduction: • Plama neutron emissivity• Plasma neutron emission spectrum
Diagnostics for neutron spectrometry: organic liquid scintillators
Diagnostics neutron emissivity measurements: neutron cameras
Research activity
D. Marocco Fusion Science And Engineering Doctorate – Garching October 2008
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)
D. Marocco Fusion Science And Engineering Doctorate – Garching October 2008
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
D. Marocco Fusion Science And Engineering Doctorate – Garching October 2008
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
D. Marocco Fusion Science And Engineering Doctorate – Garching October 2008
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
D. Marocco Fusion Science And Engineering Doctorate – Garching October 2008
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
D. Marocco Fusion Science And Engineering Doctorate – Garching October 2008
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
D. Marocco Fusion Science And Engineering Doctorate – Garching October 2008
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
D. Marocco Fusion Science And Engineering Doctorate – Garching October 2008
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)
D. Marocco Fusion science and engineering Doctorate
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
D. Marocco Fusion Science And Engineering Doctorate – Garching October 2008
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
D. Marocco Fusion Science And Engineering Doctorate – Garching October 2008
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
D. Marocco Fusion Science And Engineering Doctorate – Garching October 2008
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
D. Marocco Fusion Science And Engineering Doctorate – Garching October 2008
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
D. Marocco Fusion Science And Engineering Doctorate – Garching October 2008
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
D. Marocco Fusion Science And Engineering Doctorate – Garching October 2008
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
D. Marocco Fusion Science And Engineering Doctorate – Garching October 2008
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)
D. Marocco Fusion Science And Engineering Doctorate – Garching October 2008
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
Program: Modeling Activity (2/3)
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
D. Marocco Fusion Science And Engineering Doctorate – Garching October 2008
)()( 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
Program: Modeling Activity (3/3)
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
D. Marocco Fusion Science And Engineering Doctorate – Garching October 2008
Francesco Activity
F. Marocco My cradle – Rome – when mammy and daddy are lucky
Bring My Daddy Back Home !
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
D. Marocco Fusion science and engineering Doctorate
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
D. Marocco Fusion science and engineering Doctorate
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
D. Marocco Fusion Science And Engineering Doctorate – Garching October 2008
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
D. Marocco Fusion Science And Engineering Doctorate – Garching October 2008
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