Neutrons in full flight at CERN’s n_TOF facility

Accurate knowledge of the interaction probability of neutrons with nuclei is a key parameter in many fields of research. At CERN, pulsed bunches from the Proton Synchrotron (PS) hit the spallation target and produce beams of neutrons with unique characteristics. This allows scientists to perform high-resolution measurements, particularly on radioactive samples.

The story of the n_TOF facility goes back to 1998, when Carlo Rubbia and colleagues proposed the idea of building a neutron facility to measure neutron-reaction data needed for the development of an energy amplifier. The facility eventually became fully operational in 2001, with a scientific programme covering neutron-induced reactions relevant for nuclear astrophysics, nuclear technology and basic nuclear science. During the first major upgrade of the facility in 2009, the old spallation target was removed and replaced by a new target with an optimised design, which included a decoupled cooling and moderation circuit that allowed the use of borated water to reduce the background due to in-beam hydrogen-capture γ rays. A second improvement was the construction of a long-awaited “class-A” workplace, which made it possible to use unsealed radioactive isotopes in the first experimental area (EAR1) at 200 m from the spallation target. In 2014, n_TOF was completed with the construction of a second, vertical beamline and a new experimental area – EAR2.

One of the most striking features of neutron–nucleus interactions is the resonance structures observed in the reaction cross-sections at low-incident neutron energies. Because the electrically neutral neutron has no Coulomb barrier to overcome, and has a negligible interaction with the electrons in matter, it can directly penetrate and interact with the atomic nucleus, even at very low kinetic energies in the order of electron-volts. The cross-sections can show variations of several orders of magnitude on an energy scale of only a few eV. The origin of these resonances is related to the excitation of nuclear states in the compound nuclear system formed by the neutron and the target nucleus, at excitation energies lying above the neutron binding energy of typically several MeV. In figure 1, the main cross-sections for a typical heavy nucleus are shown as a function of energy. The position and extent of the resonance structures depend on the nucleus. Also shown on the same energy scale are Maxwellian neutron energy distributions for fully moderated neutrons by water at room temperature, for fission neutrons, and for typical neutron spectra in the region from 5 to 100 keV, corresponding to the temperatures in stellar environments of importance for nucleosynthesis.

The wide neutron energy range is one of the key features of the n_TOF facility.

In nuclear astrophysics, an intriguing topic is understanding the formation of nuclei present in the universe and the origin of chemical elements. Hydrogen and smaller amounts of He and Li were created in the early universe by primordial nucleosynthesis. Nuclear reactions in stars are at the origin of nearly all other nuclei, and most nuclei heavier than iron are produced by neutron capture in stellar nucleosynthesis. Neutron-induced reaction cross-sections also reveal the nuclear-level structure in the vicinity of the neutron binding energy of nuclei. Insight into the properties of these levels brings crucial input to nuclear-level density models. Finally, neutron-induced reaction cross-sections are a key ingredient in applications of nuclear technology, including future developments in medical applications and the transmutation of nuclear waste, accelerator-driven systems and nuclear-fuel-cycle investigations.

The wide neutron energy range is one of the key features of the n_TOF facility. The kinetic energy of the particles is directly related to their time-of-flight: the start time is given by the impact of the proton beam on the spallation target and the arrival time is measured in the EAR1 and EAR2 experimental areas. The high neutron energies are directly related to the 20 GeV/c proton-induced spallation reactions in the lead target. Neutrons are subsequently partially moderated to cover the full energy range. Energies as low as about 10 MeV corresponding to long times of flight can be exploited and measured at n_TOF because of its pulsed bunches spaced by multiples of 1.2 s, sent by the PS. This allows long times of flight to be measured without any overlap into the next neutron cycle.

Higher flux

Another unique characteristic of n_TOF is the very high number of neutrons per proton burst, also called instantaneous neutron flux. In the case of research with radioactive samples irradiated with the neutron beam, the high flux results in a very favourable ratio between the number of signals due to neutron-induced reactions and those due to radioactive decay events, which contribute to the background. While the long flight path of EAR1 (200 m from the spallation target) results in a very high kinetic-energy resolution, the short flight path of EAR2 (20 m from the target) has a neutron flux that is higher than that of EAR1 by a factor of about 25. The neutron fluxes in EAR1 and EAR2 are shown in figure 2. The higher flux opens the possibility for measurements on nuclei with very low mass or low reaction cross-sections within a reasonable time. The shorter flight distance of about a factor 10 also ensures that the entire neutron energy region is measured in a 10 times shorter interval. For measurements of neutron-induced cross-sections on radioactive nuclei, this means 10 times less acquired detector signals due to radioactivity. Therefore the combination of the higher flux and the shorter time interval results in an increase of the signal-to-noise ratio of a factor 250 for radioactive samples. This characteristic of EAR2 was, for example, used in the first cross-section measurement in 2014, when the fission cross-section of the highly radioactive isotope ²⁴⁰Pu was successfully measured. An earlier attempt of this measurement in EAR1 was not conclusive. An example from 2015 is the measurement of the (n,α) cross-section of the also highly radioactive isotope ⁷Be, relevant for the cosmological Li problem in Big Bang nucleosynthesis.

The most important neutron-induced reactions that are measured at n_TOF are neutron-capture and neutron-fission reactions. Several detectors have been developed for this purpose. A 4π calorimeter consisting of 40 BaF₂ crystals has been in use for capture measurements since 2004. Several types of C₆D₆-based liquid-scintillator detectors are also used for measurements of capture γ rays. Different detectors have been developed for charged particles. For fission measurements, ionisation chambers, parallel-plate avalanche counters and the fission-fragment spectrometer STEFF have been operational. MicroMegas-based detectors have been used for fission and (n,α) measurements. Silicon detectors for measuring (n,α) and (n,p) reactions have been developed and used more recently, even for in-beam measurements.

The measurements at CERN’s neutron time-of-flight facility n_TOF, with its unique features, contribute substantially to our knowledge of neutron-induced reactions. This goes together with cutting-edge developments in detector technology and analysis techniques, the design of challenging experiments, and training a new generation of physicists working in neutron physics. This work has been actively supported since the beginning of n_TOF by the European Framework Programmes. A future development currently being studied is a possible upgrade of the spallation target, to optimise the characteristics of the neutron beam in EAR2. The n_TOF collaboration, consisting of about 150 researchers from 40 institutes, looks forward to another year of experiments from its scientific programme in both EAR1 and EAR2, continuing its 15 year history of measuring high-quality neutron-induced reaction data.

Further Reading
CERN-Proceedings-2015-001, p32