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Commissioning of the new n_TOF spallation neutron source at CERN


Department 6.4 “Neutron Radiation” is member of n_TOF, the collaboration that operates CERN's pulsed “white” neutron source. At the n_TOF facility, neutrons with energies covering almost 11 orders of magnitude are produced by spallation with 20 GeV/c proton pulses impinging on a massive lead target. The neutrons are then directed to two experimental areas (EAR1 and EAR2 ) where scientists can study neutron‑nucleus interactions that are of interest in nuclear astrophysics, medical applications, nuclear technologies, and neutron metrology.

From 2019 to mid‑2021, CERN implemented what was known as the Long Shutdown 2 (LS2), a two‑and‑a‑half‑year break in experimental activities that was dedicated to the upgrade and maintenance of the accelerator complex. During LS2, the n_TOF facility also underwent a major upgrade, including the installation of a new neutron producing target [1]. This is the third generation of targets used at n_TOF, improving upon previous generations by further optimizing both radiation protection and energy resolution. The commissioning of the new target started in July 2021 and PTB joined with several other institutes from across Europe to provide scientific and technical support while also supplying two fission chambers to measure the neutron flux in both EAR1 and EAR2.

two fission ionization chambers

Figure 1: PTB's H19 fission ionization chamber (left) was used in EAR1 and the low‑mass fission ionization chamber (right) in EAR2 of the n_TOF neutron source at CERN to characterize the neutron energy distribution.

Fission chambers are neutron detectors that contain a small amount (usually not more than a few milligrams) of fissile material. These devices take advantage of the relatively large amount of energy released in neutron‑induced fission reactions to produce easily detectable signals that indicate the presence of neutrons. The two PTB chambers contain very thin layers of the fissile isotope U‑235, which has no fission energy threshold and can therefore be used to detect neutrons over the full n_TOF neutron energy range, from the thermal part at 25.3 meV up to several hundred MeV. The H19 chamber used in EAR1 is a secondary standard for neutron fluence measurements. It has been maintained by Department 6.4 since the 1990s and has already been used for the characterization of the neutron fields at numerous other external facilities. The second chamber is a recent development from 2017/2018 but is as yet not as well characterized as the H19. In this ionization chamber, the amount of structural material that interferes with neutron flux measurements has been minimized, and the chamber has been segmented into several sections that can be read out separately. It is therefore well suited for use in EAR2, where the neutron intensity is about two orders of magnitude higher than in EAR1 due to the smaller distance to the spallation target.

The measurement campaign for commissioning was led by the local n_TOF team and lasted throughout the summer and autumn of 2021 before being put on hold in mid‑November 2021 for the CERN winter shutdown. It will be completed in 2022. Hundreds of terabytes of data have already been collected and the analysis has begun, with the various tasks distributed among the collaboration partners. Initial results are expected to be presented starting next summer at international conferences and in peer‑reviewed journal publications.


[1] R. Esposito, M. Calviani, et al. (for the n_TOF collaboration), Physical Review Accelerators And Beams 24(2021) 093001.


Opens local program for sending emailDr. E. Pirovano, Department 6.4, Working Group 6.42