Calibration of neutron detectors at ASDEX Upgrade, measurement and model

https://doi.org/10.1016/j.fusengdes.2021.112702Get rights and content

Highlights

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    A different absolute calibration procedure for the 3He detector at ASDEX Upgrade.

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    Reproducible geometry and better count statistics.

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    A detailed Monte Carlo simulation of the calibration using the Serpent code.

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    Varying discrepancy factor with components and source position.

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    The factor is sensitive to moderator thickness and neutron scattering.

Abstract

The neutron production in ASDEX Upgrade (AUG) neutral beam injection (NBI) heated discharges is dominated by beam-target fusion reactions. Hence, the neutron rate (NR) and energy distributions are footprints of the fast ion distribution. This motivates to establish a reliable neutron rate calibration. Comparisons at AUG between the experimental NR and the one predicted by the TRANSP code show systematic variations from campaign to campaign. Potential reason for this is the delicate absolute calibration of the neutron detectors. Therefore, a different calibration technique was performed, enabling longer acquisition time, uniform geometry, better statistics and thus less uncertainty. A toy train carrying a radioactive source (238Pu/B) over two radial positions on the equatorial plane shows a periodical NR on the epithermal 3He neutron detector. The calibration results are compared to a neutron transport simulation using the Monte Carlo (MC) code Serpent. Preliminary comparisons for one source position on the outer railway track show a discrepancy factor of about 130 in the position of least material inside the simulation, in the direct line of sight to the detector. For a better understanding of these results, two additional measurements were performed. The results were again compared to a detailed Serpent simulation. This paper describes the calibration set-up for the neutron measurements in AUG, provides a brief simulation background on reaction rate estimations and a survey on the comparison between the measured and calculated neutron rates.

Introduction

Research fusion facilities mostly operate with deuterium (D-D) fuel which produces neutrons of 2.45 MeV energy. In neutral beam injection (NBI) heated discharges, the fast ions reacting with the bulk plasma (beam-target reactions) dominate the neutron rate (NR). Thus, the NR is an imprint of the fast particles, which can drive instabilities and damage the plasma facing components (PFC) [1], [2]. Accurate neutron measurements and a reasonable agreement between experiment and theoretical calculations are therefore essential to understanding the fast ion dynamics.

Over the course of NR investigations at ASDEX Upgrade (AUG), comparisons between the experimental and the NR predicted by the TRANSP code [3] show systematic deviations between calibration campaigns, which may point to potential calibration errors in the neutron detectors. The calibration has been found accurate to about 40% [2]. In comparison, the statistical uncertainty in 1 ms binning, which is roughly 4%, and could be minimized with the choice of binning, is negligible. The usual way to calibrate involved the probing of a few discrete source positions inside the machine which did not have high statistics and were hard to reproduce precisely over the years. This has prompted us to carry out an ab initio absolute calibration with more reproducible geometry, better count statistics and the utilization of a comprehensive Monte Carlo simulation [4], [5], [6] for neutron transport and detection using the Serpent code [7].

Section snippets

Calibration set-up for neutron measurements at AUG

Complex geometries like tokamaks and stellarators are organized in sectors and consist of various materials and components. Moreover, plasma heating systems and diagnostics add up to the packed surrounding in the reactor hall. This plays a significant role in neutron measurements mainly due to the strong scattering of neutrons. Further, neutron interactions depend on material composition and thickness. To capture these effects, a toy train carrying a radioactive source (238Pu/B) was

Simulation of the n-rate in AUG with the Serpent code

Neutron transport models rely on statistical methods that track the particle paths and calculate probabilities for numerous events. Monte Carlo transport codes are one example of such approach that simulates histories of particles traversing modelled geometries.

The most convenient way to incorporate a detailed ASDEX Upgrade model inside Serpent is using stereolithography (STL) files. For that purpose the whole machine was decomposed into smaller parts using the CAD software CATIA [9] and

Comparison between measurement and Serpent simulation

In this section we present the results from three calibration measurements - two inside the vacuum vessel and one outside. Measurements with the toy train were carried out for a total of one weekend (one day one radial position). The recorded data acquisition exceeds 130k seconds. The smoothed neutron rate from the outer railway track for 5000 s and the corresponding average NR are shown in Fig. 4.

The average period of a full toroidal turn is ≈ 280 s. The displayed background noise is roughly

Summary and conclusions

Three sets of calibration measurements of the 3He thermal neutron detector were performed at ASDEX Upgrade and simulated using the Monte Carlo transport code Serpent. A detailed geometry of AUG was decomposed, converted to STL files and implemented in the code. Comparisons between the experimental NR and the simulation outside the vacuum vessel resulted in a varying factor (between 5 and 15) with respect to the radial source position. The paper proves that the difference likely depends on the

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training program 2014–2018 and 2019–2020 under grant agreement No. 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission.

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