1. Neutron diagnostics for fusion experiments
Georges Bonheure

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

3. Introduction: fusion neutrons
Neutrons produced in fusion reactions: D + T -> (4He MeV) + (n MeV) Q = MeV D + D -> (3He MeV) + (n MeV) Q = 4.03 MeV T + T -> 4He + 2n Q = MeV What do neutrons do? D
Fusion energy: Neutron energy transferred to the
reactor coolant
Fuel generation: Breeding T from Li: nslow + 6Li -> 4He + T nfast + 7Li -> 4He + T + nslow
To minimize: activation, radiation damage

4. JET: outside view
Record: Q = 0.8
Steady state: Q = 0.3
total output : max 16 MW
The largest tokamak: JET (Joint European Torus:

5. The future
ITER site now!

6. Neutron source: progress in parameters
The jump to ITER
Plasma volume
Neutron source strength
Neutron flux at first wall
ITER ~ 10x JET
100 m3
Neutron fluence
ITER ~ 104 x JET
(1025 n m-2)
1010 – n s-1
850 m3
1014 – n s-1
Biggest increase in neutron fluence!
> Radiation hardness

7. The plasma as a neutron source
Ion temperature:
Ion density ratio:

8. Access: ITER diagnostics are port-based where possible
Each diagnostic port-plug contains an integrated instrumentation package

9. 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, g
Strong magnetic fields, powerful high frequency wave generators and power supply
Heat loads, mechanical stress
Timescale of measurements
Activation, tritium, beryllium

10. Neutron diagnostic systems: 4 types of systems
Time-resolved total emission
(non-collimated flux)
Fusion power
Absolute emission
Calibration of time-resolved
emission
Time-integrated emission
(fluence)
2D-cameras (collimated flux along camera viewing lines)
Spatial distribution of emission
tomography
Spectrometers (collimated
flux along radial and tangential
viewing lines)
Plasma temperature and velocity
Combination of these measurements characterizes the plasma as a neutron source

11. Interaction of neutrons
Short range of interactions: characteristic scale is the nucleus size: 1 fermi (fm) cm!
Elastic scattering: A(n,n)A
Inelastic scattering: A(n,n’)A*
Radiative capture: n + (Z,A) -> g + (Z,A+1)
Fission: (n,f)
Other nucl.reactions: (n,p),(n,a),…
High energy particle production (En > 100 MeV)

12. 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 g 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 technique
U235
U238
(Note : to improve this slide see slide (4) from Sergei ITPA presentation/ 2 pictures on fission chamber)
The solution adopted (what were the alternatives) in JET for the measurement of the time-resolved total neutron emission is based on
fission counters containing $^{238}U$ and $^{235}U$.
Counters are embedded in polyethylene moderators and lead shield\cite{swin:nim1}.
This system covers a large dynamical range of 10 orders of magnitude in the neutron flux.
There is no discrimination between 2.5 and 14 MeV neutrons and it is fairly insensitive to X and $\gamma$ radiation.
To ensure good reliability, redundancy is provided with three pairs of
fission counters installed at three different positions around the machine and with non-collimated view to the plasma .
The system was originally absolutely calibrated to 10\% in situ using a Californium $^{252}Cf$ neutron source.
It is periodically recalibrated using the activation technique.
ITER neutron diagnostics will include a number of fission counters
to monitor the total neutron flux and the fusion power.

13. Calibration with JET Remote Handling System
252Cf source strength: 109 n/s
Duration : 3 days

14. 1. Time-resolved neutron emission
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 > cm-2
New radiation hard detectors are tested in JET

15. 1. Time-resolved neutron emission
GEM based neutron detection m

16. 2. Time-integrated neutron emission
Neutron activation method
Samples used as flux monitors are automatically transferred to 8 Irradiation ends
Sample activity measurements:
1) gamma spectroscopy measurements
>>> most widely used reactions at JET:
DD neutrons - 115In(n,n’)115mIn,
DT neutrons Si (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
Neutron transport calculations with MCNP
to obtain the response coefficient for the samples
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 !!
MIX composition:
Se-16%, Fe-20%, Al-16%, Y-48%

17. Activation technique PRINCIPLE
Escaping light charged particles p, t, d, 3He or a 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
HpGe detector
Example of JET results: Gamma spectrometry of a natural Titanium target
Activation probe (targets holder)
48Ti(p,n)48V Ep > 4.9 MeV
A measurement challenge: Escaping alpha particles

18. 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

19. Digital pulse shape discrimination technique
Benefits
Detailed post processing possible (events identification, pile-up,…)
Deconvolution of spectrum information
Increase dynamic range in both energy and count-rate g 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

20. Study of tritium diffusion
nT/nD
Pulse 61372: ne0 = m-3
Pulse 61161: ne0 = m-3
R (m)
R (m)
Time (s)
Time (s)
Theoretical predictions for D, v can be verified against measurements

21. Time of flight Proton recoil 4. Neutron spectroscopy
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
I am now speaking about the last category of systems
I must say that many different techniques and detectors were tried at JET for neutron spectrometry.
Presently, the activity in neutron spectrometry at JET is mainly concentrated in the development of three different spectrometers:
One spectrometer is based on the time of
flight technique.
And two spectrometer are based on the proton recoil technique. The proton recoil is based on the n-p scattering reaction.
Two different approach are used at JET.
The first approach uses a thick hydrogenous target such as the whole of the
energy of any recoiling proton is deposited. The target is the detector itself.
The energy spectrum of the recoil
protons is recorded with no information on the recoil angle.
Therefore, the neutron energy spectrum is recovered by an
unfolding process.
The second approach uses a thin hydrogenous target or proton radiator. The neutron beam is converted into a proton beam and
The recoil protons are magnetically analyzed.
Trade off: energy resolution vs detection efficiency

22. Neutron spectroscopy: time of flight (TOFOR)
Energy resolution for DD neutrons: ~5%
Detection efficiency: cm2
Count rate: < 500 kHz
Simulated with GEANT code

23. 4. Neutron spectroscopy NE213 TOFOR TANDEM MPR TG Diagnostics Garching
April, 2009 23 23

24. Neutron spectroscopy: spectral unfolding techniques
Comparison between different unfolding techniques:
Maximum entropy (MAXED)
Minimum fisher regularisation (MFR)
In order to make use of NE213 spectrometer, a lot of work is devoted to unfolding techniques.
Neutron energy spectra are obtained using several
unfolding techniques including the Maximum Entropy unfolding procedure\cite{reginatto:nim1} and the newly developed
Minimum fisher regularisation technique. Comparison has been done between the two unfolding procedures
See on this topic contribution from Jan and Andreas.

25. Summary: neutron diagnostic systems
JET
ITER
Time-resolved
total emission
Total: fission counters
14 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:
NE213 and stilbene
Magnetic proton recoil
To be defined

26. 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: ‘Neutron diagnostics for reactor scale fusion experiments’

27. Thanks…