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Characterisation of the neutron beam in the n_TOF-EAR2 experimental area at CERN following the spallation target upgrade

J. A. Pavon-Rodriguez, J. Lerendegui-Marco, A. Manna, S. Amaducci, M. Sabate-Gilarte, E. Musacchio-Gonzalez, M. Bacak, V. Alcayne, M. A., Cortes-Giraldo, V. Vlachoudis, R. Zarrella, F. Garcia-Infantes, E. Stamati, A. Casanovas, N. Patronis, L. Tassan-Got, J. M. Quesada, the n_TOF Collaboration

TL;DR

This study characterises the neutron beam delivered by the upgraded n_TOF-EAR2 setup after the new spallation target was installed, focusing on neutron flux, spatial profile, and energy resolution. A comprehensive commissioning campaign combines FLUKA Monte Carlo simulations with measurements from multiple detectors (SiMon2, MGAS, PPACMon) and activation checks to validate the flux and the energy–TOF relationship via the energy resolution function. The results show a broad energy spectrum (from sub-meV to hundreds of MeV) with a ~3 cm FWHM beam at the reference position, and a significant improvement in energy resolution compared with the previous target, enabling more precise resonance analyses (e.g., Au and Fe n,γ resonances) and enabling ambitious measurements with radioactive isotopes. The FLUKA simulations reproduce the flux shape within a few percent over much of the spectrum and generally agree with the measured beam profile, reinforcing the reliability of MC-based beam optimization and supporting high-precision cross-section studies for astrophysics, nuclear technology, and medical physics.

Abstract

The n_TOF facility at CERN has undergone a major upgrade after the installation of a new spallation target, designed to improve the features of both neutron beamlines at the experimental areas 1 and 2 (EAR1 and EAR2) and the commissioning of a new experimental area (NEAR). Due to improved coupling of the spallation target with the EAR2 beamline, the upgrade resulted in a significantly increased neutron flux and improved neutron energy resolution. This paper presents the results of the commissioning phase that followed to characterise the EAR2 neutron beamline and validate the FLUKA Monte Carlo simulations of the facility. The main features of the neutron beam, namely the neutron flux, spatial profile and energy resolution, are evaluated and compared to the previous target. The neutron flux presents a general increase of 20% below 1 eV, 40% between 1 eV and 100 keV and 50% between 100 keV and 10 MeV. The measured width of the beam profile was 3 cm (FWHM) at the reference position for neutron capture measurements. The energy resolution with the new spallation target shows a significant improvement compared to the previous one. Moreover, FLUKA Monte Carlo simulations present a good agreement with the measured neutron flux and profile within uncertainties, and a remarkable reproduction of the energy resolution.

Characterisation of the neutron beam in the n_TOF-EAR2 experimental area at CERN following the spallation target upgrade

TL;DR

This study characterises the neutron beam delivered by the upgraded n_TOF-EAR2 setup after the new spallation target was installed, focusing on neutron flux, spatial profile, and energy resolution. A comprehensive commissioning campaign combines FLUKA Monte Carlo simulations with measurements from multiple detectors (SiMon2, MGAS, PPACMon) and activation checks to validate the flux and the energy–TOF relationship via the energy resolution function. The results show a broad energy spectrum (from sub-meV to hundreds of MeV) with a ~3 cm FWHM beam at the reference position, and a significant improvement in energy resolution compared with the previous target, enabling more precise resonance analyses (e.g., Au and Fe n,γ resonances) and enabling ambitious measurements with radioactive isotopes. The FLUKA simulations reproduce the flux shape within a few percent over much of the spectrum and generally agree with the measured beam profile, reinforcing the reliability of MC-based beam optimization and supporting high-precision cross-section studies for astrophysics, nuclear technology, and medical physics.

Abstract

The n_TOF facility at CERN has undergone a major upgrade after the installation of a new spallation target, designed to improve the features of both neutron beamlines at the experimental areas 1 and 2 (EAR1 and EAR2) and the commissioning of a new experimental area (NEAR). Due to improved coupling of the spallation target with the EAR2 beamline, the upgrade resulted in a significantly increased neutron flux and improved neutron energy resolution. This paper presents the results of the commissioning phase that followed to characterise the EAR2 neutron beamline and validate the FLUKA Monte Carlo simulations of the facility. The main features of the neutron beam, namely the neutron flux, spatial profile and energy resolution, are evaluated and compared to the previous target. The neutron flux presents a general increase of 20% below 1 eV, 40% between 1 eV and 100 keV and 50% between 100 keV and 10 MeV. The measured width of the beam profile was 3 cm (FWHM) at the reference position for neutron capture measurements. The energy resolution with the new spallation target shows a significant improvement compared to the previous one. Moreover, FLUKA Monte Carlo simulations present a good agreement with the measured neutron flux and profile within uncertainties, and a remarkable reproduction of the energy resolution.
Paper Structure (10 sections, 7 equations, 18 figures, 4 tables)

This paper contains 10 sections, 7 equations, 18 figures, 4 tables.

Table of Contents

  1. Introduction
  2. EAR2: the n_TOF 20 m neutron beamline
  3. Monte Carlo simulations with FLUKA
  4. Neutron flux
  5. Methodology
  6. Flux evaluation
  7. Comparison with the previous spallation target
  8. Spatial beam profile
  9. Energy resolution
  10. Conclusions

Figures (18)

  • Figure 1: Vertical layout of the n_TOF-EAR2 beamline (not to scale). Distances on the left indicate the upstream position of the elements with respect to the centre of the target.
  • Figure 2: Exploded view (3D model) of the n_TOF new spallation target Esposito2020. The red arrow indicates the direction of the impinging proton beam.
  • Figure 3: 3D model of the FLUKA geometry of the target-moderator assembly generated with Flair, the advanced FLUKA graphical user interface.
  • Figure 4: FLUKA simulated $\lambda$ distribution as a function of $E_\textrm{n}$.
  • Figure 5: Experimental setup for the determination of the neutron flux at EAR2, consisting of SiMon2 (green), MGAS (red) and PPACMon (blue), placed in upstream order.
  • ...and 13 more figures