Characterisation of the n_TOF 20 m beam line at CERN with the new spallation target

. The n_TOF facility hosts CERN’s pulsed neutron source, comprising two beam lines of di ff erent flight paths and one activation station. It is based on a proton beam delivered by the PS accelerator impinging on a lead spallation target. During Long Shutdown 2 (LS2) at CERN (2019-2021), a major upgrade of the spallation target was carried out in order to optimize the performances of the neutron beam. Therefore, the characteristics of n_TOF two experimental areas were investigated in detail. In this work, the focus is on the second experimental area (EAR2), located 20 m above the spallation target. Preliminary results of the neutron energy distribution and beam line energy resolution are presented, compared to previous experimental campaigns and Monte Carlo simulations with the FLUKA code. Moreover, preliminary results of the spatial beam profile measurements are shown.


Introduction
The n_TOF Collaboration operates the neutron time-offlight facility at CERN [1], based on a 20 GeV/c pulsed proton beam impinging on a lead target using water to moderate the energy of the neutrons generated in the spallation process. The facility is characterised by a highinstantaneous neutron beam intensity, high energy resolution and a wide neutron energy spectrum, spanning from sub-thermal to GeV.
The facility comprises two neutron beam lines, the first experimental area (EAR1) [2], in operation since 2001, located at 185 m from the spallation target, nearly in the same direction as the incoming proton beam, and the second experimental area (EAR2) [3,4], located at 20 m above the target, in a perpendicular direction with respect to the proton beam, commissioned in 2014. Thanks to its shorter flight path, EAR2 features a neutron flux around two orders of magnitude bigger in the thermal region and about 30 times higher in the rest of the neutron energy spectrum compared to EAR1 [5], as well as a better signalto-background ratio, in the case the background is dominated by the natural activity of the sample. This offers a unique opportunity of performing neutron-induced cross section measurements for isotopes with very short halflife [6] or small cross sections, enabling new challenging measurements in very diverse fields. EAR2 beam line is shown in detail in Figure 1.
In the course of the Long Shutdown 2 (LS2) at CERN (2019-2021), the facility went through a major upgrade consisting in the installation of a new spallation target [7], shown in Figure 2. It has been designed to fully optimise the features of the EAR2 without detriment of EAR1 performance, unlike the previous one specifically designed for EAR1. In particular, this new target presents a flat lead wedge at the top and a dedicated water container to act as moderator for EAR2. These changes impact the characteristics of the neutron beam, i.e. the neutron flux, the energy resolution and the beam profile. The flux plays a key role in the determination of the energy dependence of the neutron-induced cross section. The energy resolu- Figure 1: Schematic of EAR2 neutron beam line [4]. tion is an important constraint in the characterisation of the resonance region of the measured cross sections. And the beam profile determines the size of samples and setups to be used.
During a commissioning phase in 2021, the changes in the characteristics of EAR2 were studied. This work presents the features of the neutron beam with the new spallation target.

Neutron flux
The neutron flux measurement was carried out with several detector systems: SiMon (Silicon Monitor) [9], Micromegas (micro-mesh gaseous structure) [10][11][12], and PPAC (parallel plate avalanche counters) [13,14]. In Figure 3, a picture of the setup in the experimental hall  is shown. The neutron beam comes from below, passing through SiMon, then Micromegas, and lastly, PPAC. The detectors are loaded with samples of isotopes whose neutron-induced cross sections for various reactions are considered as standard in different energy regions. The choice of sample and reaction for every detector is summarised in Table 1.
The neutron flux is extracted from the combination of the different data sets in the regions of interest, indicated in Table 1. Moreover, an activation measurement of a 197 Au sample has been carried out to determine the absolute value of the neutron flux.
In Figure 4 preliminary results of the neutron flux, measured with SiMon and Micromegas, are presented together with Monte Carlo simulations performed with the FLUKA code [15][16][17]. The results of the previous operational phase, Phase 3 (2014-2018), are also shown for reference. A general increase in the absolute value of the flux can be observed with the new spallation target. More precisely, an increase of 45% is observed at the epithermal region and evaporation peak, while an increase of 25% is observed at the thermal peak.

Beam profile
The beam profile is the spatial distribution of the neutrons in the beam. It was measured at 20 m from the target using the position-sensitive PPAC detectors loaded with 235 U. In Figure 5, a preliminary result of this measurement is shown. As can be seen, the beam has a diameter of 5 cm, marked by red dashed lines.

Energy resolution
In order to study the energy resolution (ER) in EAR2, C 6 D 6 scintillators were used to measure the neutron capture cross section of several isotopes. In Figure 6, a picture of the setup is shown. The cross sections of the various isotopes employed present resonances in a wide energy range and these were used to study the energy resolution at different energy regions. Those regions are indicated in Table 2. In Figure 7a, a first comparison of a resonance in the 197 Au(n,γ) yield measured in the previous operational phase, in 2015, and the current one, in 2021, is shown.  From this figure, a clear improvement of the resolution is noticeable, as the resonance is narrower. The improved energy resolution in EAR2, together with the recent detector R&D projects [18,19], opens the way to new and more challenging measurements [20].
The counting rate, as resulting from the measurements, is also compared against the expected yield obtained by means of convoluting the ENDF/B-VIII.0 evaluation [21] with the resolution function. The latter is extracted from FLUKA simulations of the whole neutron production and moderation process in the spallation target. In Figure 7b, a measured resonance of 197 Au(n,γ) is compared against its expected yield.

Summary
During LS2 at CERN, a new spallation target was installed at the n_TOF Facility, requiring a complete study of the