Neutron diagnostics for fusion experiments Georges Bonheure

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

Introduction: fusion neutrons 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? 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

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

The future ITER site now! www.iter.org

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 – 5.7 1018 n s-1 850 m3 1014 – 1020 n s-1 Biggest increase in neutron fluence! > Radiation hardness

The plasma as a neutron source 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, g 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) 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

Interaction of neutrons 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) -> g + (Z,A+1) Fission: (n,f) Other nucl.reactions: (n,p),(n,a),… High energy particle production (En > 100 MeV)

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.

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

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 >3.1015 cm-2 New radiation hard detectors are tested in JET

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

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 - 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 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%

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

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 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

Study of tritium diffusion nT/nD Pulse 61372: ne0 = 4.5 1019m-3 Pulse 61161: ne0 = 1.9 1019m-3 R (m) R (m) Time (s) Time (s) Theoretical predictions for D, v can be verified against measurements

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

Neutron spectroscopy: time of flight (TOFOR) Energy resolution for DD neutrons: ~5% Detection efficiency: 8 10-2 cm2 Count rate: < 500 kHz Simulated with GEANT code

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

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.

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

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…