1. Neutron spectrometry in fusion energy research
Göran Ericsson, Erik Andersson Sundén
M.Cecconello, S.Conroy, M.Gatu Johnson, L.Giacomelli, C.Hellesen, A.Hjalmarsson, J.Källne, E.Ronchi, H.Sjöstrand, M.Weiszflog
Uppsala University
G.Gorini, M.Tardocchi, J.Sousa, A.Murari, S.Popovichev
Milano-Bicocca, IST, JET
Outline:
Neutron emission in fusion experiments
Role of diagnostics, measurement conditions
Spectrometer design and techniques
The ToF technique; TOFOR
Thin-foil proton recoil technique; MPRu
Outlook and Conclusions
Frontiers …
Rome, 2009 1
See also poster by E.Andersson Sundén

2. Neutron emission Fusion experiments with D and T fuel: “Impurities”
Thermal RF RF
Simulation JET; D
Fusion experiments with D and T fuel:
d + d  3He + n (2.45 MeV)
d + t  4He + n (14.0 MeV)
“Impurities”
d + 3He, 4He, 9Be, 12C, ...  n + X
Plasma parameters: Pfus, Ti, f(vion),…
Fuel ion velocity populations:
Thermal  f(En) Gaussian
RF heating  f(En) anisotropic, double humped
Beam heating, alpha heating, …
Spectral components (ITER):
Thermal bulk Sn  1,
Beam heating Sn  0.1,
RF heating Sn  0.01,
a heating Sn  0.001,
Neutron emission variations:
Intensity; n/s (ITER)
Temporal (ms), spatial (cm)
Scatter
Background
T = total spectrum, B = thermal bulk
NBI = neutral beam AKN = alpha knock-on
Simulation ITER; DT
n rate JET; D
Rn [1015 s-1]
Frontiers …
Rome, 2009 2

3. Role and situation for diagnostics
Provide information on relevant plasma/fuel ion parameters
Feed-back for active control; ms time frame
Extended n source (100 m3), “continuous” n emission (min)
Collimated LOS, direct + scattered spectral contributions
Reliable, robust techniques
Harsh experimental conditions around the “reactor”
Neutron and gamma background
High-frequency EM interference
High levels of temperature, B-field
Competition over “real estate”; LOS, position, weight, space, …
Challenges for neutron spectroscopy
Results on ms  spectroscopy on MHz signal rates (Ccap)
High e OR close to reactor core
Access to weak emission components  high S/B ratio > 104
Peaked, well-known response function (0 – 20 MeV)
Real-time information in ms  data acq., processing, transfer
Frontiers …
Rome, 2009 3

4. Neutron spectroscopy techniques Most “standard” n spectr
Neutron spectroscopy techniques Most “standard” n spectr. techniques tested in fusion (JET)
NE213, Stilbene, nat. + CVD diamond
Reginatto, Zimbal, RSI 79 (2008)- PTB work
Krasilnikov, Rev Sci Instr 69 (1997)
Lattanzi, Angelone, Pillon , Fus Eng Des (2009)
TOFOR - UU
Gatu Johnson, NIM A591 (2008)
Frontiers …
Rome, 2009 4
TANDEM (TPR) - Harwell
Hawkes, RSI 70 (1999) 1134
MPRu - UU
Andersson Sundén, NIM A610 (2009)

5. Time-of-flight Optimized for Rate
Optimized for 2.45 MeV n in D plasmas
Continuous source of n:
Double scattering in S1 + S2
16m from plasma
2m concrete floor
Fast plastic scintillators:
5x S1 disks
32x S2 “umbrella”
S2 tilt to compensate for Dtlight
e ≈ 1%
Background = randoms
B  Rn2  S:B  Rn
Limitations:
“Paralysis” at high Rn
Rate in S1 (≈ MHz)
Ccap ≈ 500 kHz (S:B ≈ 1)
Cmax ≈ 44 kHz (Rn = 1.7∙1016 n/s)
Emphasis on rate capability:
Digital free-running time stamping
Separate, non-correlated p.h. spectra
En = 2mnR/ttof2 R n” n’
Frontiers …
Rome, 2009 5
n flux

6. TOFOR – count rate capability
Limiting sensitivity: random coincidences
No correlated time – p.h. information
 Randoms corrected for on statistical level
 Uniform level from ttof < 0
Digital time stamping electronics (IST, Portugal)
Dead time free: ALL signal events recorded (+ ALL randoms)
Event based correlations: reduce randoms, reduce timing walk S1 S2
Frontiers …
Rome, 2009 6

7. Thin-foil Magnetic Proton Recoil
Separation of functions:
n-to-p conversion in thin (mm) foil
Energy (momentum) separation in B-field
Counting in position resolved hodoscope (32 phoswich scint.)
Focal plane detector (FPD) can be shielded to any required level
Detectors need “only” count protons
Flexibility:
Multiple conversion foils – 2.5/14 MeV
Multiple p collimators
Background reduction:
Concrete + lead radiation shield
Phoswich scintillators, tdecay = 2, 180 ns
TR digital boards
Digital pulse shape discrimination
Performance:
Ccap >> MHz (Cmax = 0.61 MHz)
S:B  20000:1 (14-MeV in DT),
TOFOR
Frontiers …
Rome, 2009 7
5:1 (2.5-MeV in D)

8. MPR results Strong candidate for ITER Alpha heating in DT; MPR 1997
Observation of weak components
Alpha heating signature – knock-on n
Phoswich DPSD
Scattered n
Bgr/statistics
Background
14-MeV p
LED pulser
Phoswich DPSD
2.5 mm
0.3 mm
2.5-MeV p
14-MeV p
14-MeV n
Min. ionizing e-
Amplitude
Qshort Qlong
Preliminary phoswich DPSD analysis
Protons from T burn-up n (14-MeV)
Component at 1% of 2.5 MeV emission
LED for PMT gain monitoring system
Frontiers …
Rome, 2009 8
TR boards: 8 bit, 200 MSPS, 512 MB
Baseline restoration, pile-up rejection
Standard DPSD: 2D plot of Qlong/Qshort
Strong candidate for ITER

9. The future Conclusions
Combined pulse-height/time digitizing boards for ToF
Compact spectrometers for neutron camera; NE213, CVDD
Neutron spectroscopy system for ITER:
2.5-MeV n spectrometer for D operations; ToF
14-MeV n spectrometer for high power DT; MPR/TPR
Real-time applications
Innovative, new concepts …
Conclusions
Harsh experimental conditions; special requirements
Challenges for Fusion neutron spectrometry:
Count rate capability – provide plasma information
Background rejection – study weak emission comp.
Dynamic range/sensitivity – varying plasma cond.
Frontiers …
Rome, 2009 9