Speaker
Description
The n_TOF Collaboration operates the neutron time-of-flight facility at CERN [1], based on a 20 GeV/c pulsed proton beam impinging on a lead target employing water to moderate the spallation neutrons. The facility is characterized by a high-instantaneous neutron beam intensity, high energy resolution and a wide neutron energy spectrum, spanning from sub-thermal to GeV. The first experimental area, EAR1 [2], in operation since 2001, is located at 185 m from the spallation target nearly in the same direction as the incoming proton beam. In 2014, a new experimental area was commissioned, EAR2 [3], located at 20 m above the target in the perpendicular direction with respect to the proton beam. Thanks to the shorter flight path, this new beam line features a neutron flux around two orders of magnitudes bigger in the thermal region and about 30 times higher in the rest of the neutron energy spectrum compared to EAR1 [4]. Moreover, it presents a better signal to background ratio. This offers a unique opportunity of performing neutron induced cross section measurements for isotopes with very short half-life or small cross sections, paving the way for new challenging measurements in very diverse fields. Such is the case of the first measurement performed in EAR2 to determine the feasibility of neutron-induced fission, the 240Pu(n,f) cross section measurement [5], of special interest in nuclear waste management in the field of Nuclear Energy. In Astrophysics, the 7Be(n,α) and 7Be(n,p) reactions cross-section were measured for the first time in the energy regions of interest for the Big Bang nucleosynthesis [6-7], which are of importance to solve the Cosmological Lithium Problem or the 26Al(n,α) reaction which has a critical influence on the abundance of the 26Al cosmic γ-ray emitter [8]. Additionally, in the field of Medical Physics, the 33S(n,α) cross section was measured and data were provided for the first time from thermal to 10 keV [9].
In the course of CERN's second long shutdown (2019-2020), the facility has gone through a major upgrade comprised of the installation of a new spallation target design to fully optimise the features of the n_TOF experimental areas, unlike the previous one specifically designed for EAR1. 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 resolution is the main feature in the characterization of the resonance region of the measured cross-sections. Compared to the previous target, the energy resolution and the characteristics of the flux are significantly improved, enhancing the capabilities of the facility EAR2.
During a commissioning phase in 2021 the changes in the characteristics of EAR2 were investigated. This work presents the features of the neutron beam with the new spallation target: neutron energy flux and neutron energy resolution. The characterisation of the neutron flux required of the combination of a series of measurements, making use of several detection systems such as micro-megas, parallel plate avalanche counters and silicon monitors, with diverse neutron-converting reactions considered standard in different energy regions. The preliminary results of the neutron flux measurement in EAR2 compared to extensive Monte Carlo simulations, as well as an overview of the improvement of the neutron energy resolution will be presented.
[1] C. Rubbia et al., A High Resolution Spallation Driven Facility ATTHE CERN-PS to Measure Neutron Cross Sections in the Interval from 1 eV to 250 MeV: a Relative Performance Assessment, CERN/LHC/98-002-EET, 1998
[2] Guerrero C. et al, Performance of the neutron time-of-flight facility n_TOF at CERN, Eur. Phys. J. A 49, 2013
[3] Colonna N. et al, The Second Beam-Line and Experimental Area at n_TOF: A New Opportunity for Challenging Neutron Measurements at CERN, Nuclear Physics News, 25, 2015
[4] Sabaté-Gilarte M. et al, High-accuracy determination of the neutron flux in the new experimental area n_TOF-EAR2 at CERN, Eur. Phys. J. A 53, 2017
[5] Stamatopoulos A. et al. Investigation of the 240Pu(n,f) reaction at the n_TOF/EAR2 facility in the 9 meV–6 MeV range, Phys. Rev. C 102, 2020
[6] Barbagallo M. et al. Be(n,α)4He Reaction and the Cosmological Lithium Problem: Measurement of the Cross Section in a Wide Energy Range at n_TOF at CERN, Phys. Rev. Lett. 117, 2016
[7] Damone L. et al, 7Be(n,p)7Li Reaction and the Cosmological Lithium Problem: Measurement of the Cross Section in a Wide Energy Range at n_TOF at CERN. Phys. Rev. Lett. 121, 2018
[8] C. Lederer-Woods et al, Destruction of the cosmicγ-ray emitter 26Al in massive stars: Study of the key 26Al(n,α) reaction, Phys. Rev. Lett. C 104, 2021
[9] Sabaté-Gilarte M. et al, The 33S(n,α)30Si cross section measurement at n_TOF-EAR2 (CERN): From 0.01 eV to the resonance region, EPJ Web Conf. 146, 2017