Fusion Engineering and Design 83 (2008) 1818–1821 Contents lists available at ScienceDirect Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes Multi-scattering time-of-flight neutron spectrometer for deuterium to tritium fuel ratio measurement in fusion experimental reactors K. Asai a,∗ , K. Yukawa a , T. Iguchi a , N. Naoi a , K. Watanabe a , J. Kawarabayashi a , M. Yamauchi b , C. Konno b a Quantum Engineering, Nagoya University, Furo-chou, Chikusa-ku, Nagoya, Aichi 464-8603, Japan b Japan Atomic Energy Agency, Tokai-mura, Naka, Ibaraki 319-1195, Japan article info Article history: Available online 13 September 2008 Keywords: Fuel ion ratio Burning plasma Plasma diagnostics Neutron spectrometer Time-of-flight ITER abstract A time-of-flight (TOF) neutron spectrometer has potential as a fuel ratio (n D /n T ) measurement system in the International Thermonuclear Experimental Reactor (ITER). A new neutron spectrometer is proposed to monitor the fuel ratio in the core of the ITER plasma. This system is based on a conventional TOF method and is composed of a water cell and a few tens of scintillator pairs. The water cell is inserted before the first scintillator of the TOF system and serves as a neutron scattering material. A trial experiment demonstrates the feasibility of this system for detecting trace-DD neutrons within a DT neutron beam. The system responses to DT and DD neutrons are also presented. © 2008 Elsevier B.V. All rights reserved. 1. Introduction The DT burn control in the International Thermonuclear Experi- mental Reactor (ITER) requires real-time information on the fusion power and fuel ratio (the ratio of deuterium density n D and tri- tium density n T ) in the plasma. For instance, fuel ratio monitoring in the plasma core is used to tailor the isotope ratio D/T of the fuel to be injected [1]. Several types of tools have been proposed to monitor the fuel ratio [2–7]. Monitoring of fuel ratio with neu- tron spectroscopy [8–12] is superior in the plasma core where fusion reactions most frequently occur. The fuel ratio in a burning plasma can be derived from the intensity ratio of DD/DT neutron where detecting trace amounts of DD neutrons that are due to the DT burning plasma is a key issue. Refs. [13–15] have reported feasibility considerations for this kind of measurement. A major concern is the down scattering of the overwhelming amount of DT neutrons inside the ITER machine structure and Radial Neu- tron Camera (RNC) [16], which is called “Wall Emission”. This Wall Emission creates a background signal that prevents DD neutron detection. Fuel ratio monitoring based on neutron spectroscopy is not yet practically applied to the actual Tokamaks. Although the applica- bility of the existing neutron spectrometers to monitor the fuel ratio has been discussed, specific techniques or designs for this pur- pose are not yet presented. A double crystal time-of-flight (DC-TOF) ∗ Corresponding author. Tel.: +81 52 789 4688; fax: +81 52 789 5127. E-mail address: asai@avocet.nucl.nagoya-u.ac.jp (K. Asai). system is a potential neutron spectrometer for this purpose. The DC-TOF system is composed of a pair of scintillators used to mea- sure the flight time of a neutron between them. This simple system can easily distinguish neutron energy without complicated pro- cesses such as the spectrum unfolding method, and typically has a higher detection efficiency than other methods based on recoil proton detection. On the other hand, the DC-TOF method where the first detector is placed in the beam line of the incident neutron is disadvantageous under a high radiation flux. In the ITER experi- ments, the radiation intensity such as neutrons and -rays changes as the reactor power increases. In a high reactor power region, the thickness of the first scintillator and/or the aperture of the neutron collimator must be adjusted. Otherwise an accidental count due to high radiation fluxes can be a major background source of the TOF system [17]. The fuel ratio in the plasma core has been predicted to change from 0.1 to 3 [18], which correspond to 0.05% to 1.5% of the DD/DT neutron intensity ratio. Because the relative intensity of the DD neutrons is very small, a typical value is 0.5% compared to the rest of the spectrum, accidental counts due to the high event rate of the first scintillator will be another background source when detecting DD neutrons, which will lead to poor measurement accuracy. Hence, a new approach for a time-of-flight (TOF) neutron spec- trometer to monitor the fuel ratio in a burning plasma core is proposed and discussed. The proposed system has a radiator or neu- tron scattering material in front of the TOF system. The radiator has a larger cross-section for DD neutrons than for DT neutrons, which enhances the relative intensity of the DD neutron after scattering. The feasibility of this new concept is successfully demonstrated 0920-3796/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2008.08.002