Benchmarking shielding simulations for an accelerator-driven spallation neutron source

Nataliia Cherkashyna,Tobias Panzner,Georg Ehlers,Uwe Filges,Phillip M Bentley,Emmanouela Rantsiou,Douglas D Dijulio

doi:10.1103/physrevstab.18.083501

Abstract

The shielding at an accelerator-driven spallation neutron facility plays a critical role in the performance of the neutron scattering instruments, the overall safety, and the total cost of the facility. Accurate simulation of shielding components is thus key for the design of upcoming facilities, such as the European Spallation Source (ESS), currently in construction in Lund, Sweden. In this paper, we present a comparative study between the measured and the simulated neutron background at the Swiss Spallation Neutron Source (SINQ), at the Paul Scherrer Institute (PSI), Villigen, Switzerland. The measurements were carried out at several positions along the SINQ monolith wall with the neutron dosimeter WENDI-2, which has a well-characterized response up to 5 GeV. The simulations were performed using the Monte-Carlo radiation transport code Geant4, and include a complete transport from the proton beam to the measurement locations in a single calculation. An agreement between measurements and simulations is about a factor of 2 for the points where the measured radiation dose is above the background level, which is a satisfactory result for such simulations spanning many energy regimes, different physics processes and transport through several meters of shielding materials. The neutrons contributing to the radiation field emanating from the monolith were confirmed to originate from neutrons with energies above 1 MeV in the target region. The current work validates Geant4 as being well suited for deep-shielding calculations at accelerator-based spallation sources. We also extrapolate what the simulated flux levels might imply for short (several tens of meters) instruments at ESS.

Highlights

Accelerator-driven pulsed-spallation neutron source facilities have substantial shielding requirements for both safety and scientific purposes
We present the results of measurements carried out at the Swiss Spallation Neutron Source (SINQ) [11] at the Paul Scherrer Institute, Villigen, Switzerland, in order to understand the neutron background leaking from the SINQ monolith
For the points where the measured radiation dose is above the background level, the agreement between measurements and simulations is about a factor of 2

Summary

Introduction

Accelerator-driven pulsed-spallation neutron source facilities have substantial shielding requirements for both safety and scientific purposes. At these types of facilities, neutrons are generated via interactions between a highenergy (GeV) proton beam with a heavy metal target. A significant fraction of the neutrons escape the target-moderator assembly and continue into the bulk shielding and the beamport entrances. At a pulsed-spallation source, experiments measuring across the frame boundary (i.e., when the proton pulse arrives) may reveal a spike of “prompt” neutrons [2,3].

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