Abstract
Materials and structures of a collimator for a new neutron emission profile monitor in JT-60SA are examined through Monte Carlo simulations using the Monte Carlo N-Particle transport code. First, the shielding properties of various material combinations are compared in order to determine a combination with high shielding performances against both neutrons and gamma-rays. It is found that a collimator consisting of borated polyethylene and lead has a high shielding performance against neutrons. Moreover, a high shielding performance against gamma-rays is obtained when a lead pipe with a radial thickness of 0.01 m is inserted into a collimation tube. Second, we demonstrate that it is possible to improve the spatial resolution to a desired level by installing a thin tubular extension structure that fits into the limited space available between the main collimator block and the tokamak device. Finally, the collimator structures that meet both the targeted spatial resolutions (<10% of the plasma minor radius) and the targeted counting rate (105 cps order) are discussed.
Highlights
Deuterium (D) plasma experiments will be performed in the superconducting tokamak JT-60SA [5]
Materials and structures of a collimator for a new neutron emission profile monitor in JT-60SA are examined through Monte-Carlo simulations using the MCNP code
For the sake of completeness and to systematically delineate trends, we included the combination of heavy concrete and lead, its weight exceeds the 23-ton weight limit of the stage
Summary
Monitor is composed of a fast time-response neutron detector, a photomultiplier and a magnetic shield. In large-sized fusion devices, the main shielding materials that are often adopted for the collimators of neutron emission profile monitors are heavy concrete or high-density polyethylene. A collimator having a high shielding performance, a high spatial resolution and a high time resolution is desirable Given these targets and constraints, the main purpose of this study is to identify suitable materials and a suitable structural form for such a collimator. The shielding performance of various combinations of these materials will be evaluated using Monte-Carlo neutron and gamma-ray transport simulations. Simulation results are presented and discussed, beginning with a comparison between several collimator material combinations with respect to their shielding performances against both neutrons and gamma-rays under the limited installation conditions. Few works exist, where the shielding performance of the lead pipe insertion against the gamma-ray flux generated in the collimator is investigated using Monte-Carlo simulations.
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