The Neutron Time Of Flight (n_TOF) facility is a

neutron The neutron is a subatomic particle, symbol or , which has a neutral (not positive or negative) charge, and a mass slightly greater than that of a proton. Protons and neutrons constitute the nuclei of atoms. Since protons and neutrons beh ...

spectrometer A spectrometer () is a scientific instrument used to separate and measure spectral components of a physical phenomenon. Spectrometer is a broad term often used to describe instruments that measure a continuous variable of a phenomenon where the ...

CERN The European Organization for Nuclear Research, known as CERN (; ; ), is an intergovernmental organization that operates the largest particle physics laboratory in the world. Established in 1954, it is based in a northwestern suburb of Gene ...

, with the aim of studying neutron-nucleus interactions over a range of kinetic energies, using the

time of flight Time of flight (ToF) is the measurement of the time taken by an object, particle or wave (be it acoustic, electromagnetic, etc.) to travel a distance through a medium. This information can then be used to measure velocity or path length, or as a w ...

method. The research conducted at the facility has applications in

nuclear technology Nuclear technology is technology that involves the nuclear reactions of atomic nuclei. Among the notable nuclear technologies are nuclear reactors, nuclear medicine and nuclear weapons. It is also used, among other things, in smoke detectors an ...

and

nuclear astrophysics Nuclear astrophysics is an interdisciplinary part of both nuclear physics and astrophysics, involving close collaboration among researchers in various subfields of each of these fields. This includes, notably, nuclear reactions and their rates as ...

. The facility has been in operation at CERN since 2001, following a proposal from the former Director General, Carlo Rubbia, for a high-intensity neutron source.

Background

Time-of-flight

The time-of-flight technique is useful for studying

nuclear structure Understanding the structure of the atomic nucleus is one of the central challenges in nuclear physics. Models The liquid drop model The liquid drop model is one of the first models of nuclear structure, proposed by Carl Friedrich von Weizsäcke ...

at high energies. Neutron-induced reactions are measured by observing resonances in the yields of the reactions, specifically their cross sections. Below the energy required to remove a neutron from the nucleus (neutron separation energy), transition probabilities can be deduced from measuring the

gamma-ray A gamma ray, also known as gamma radiation (symbol γ or \gamma), is a penetrating form of electromagnetic radiation arising from the radioactive decay of atomic nucleus, atomic nuclei. It consists of the shortest wavelength electromagnetic wav ...

spectra produced by the decaying nuclei. This information can bemused to graph

resonances Resonance describes the phenomenon of increased amplitude that occurs when the frequency of an applied periodic force (or a Fourier component of it) is equal or close to a natural frequency of the system on which it acts. When an oscillatin ...

which directly correspond to nuclear levels in the decaying nucleus. For increasing mass of the nuclei, the spacing between nuclear levels decreases i.e. the level density increases. The level density is an important value for calculations involved in nuclear reaction rates, and have uses in astrophysical processes.

Nuclear astrophysics

Spectral analysis of light from stars is used to study stellar elemental composition, with most stars aligning along the main sequence of the Hertzsprung-Russell diagram. However, Red Giants have a large

luminosity Luminosity is an absolute measure of radiated electromagnetic power (light), the radiant power emitted by a light-emitting object over time. In astronomy, luminosity is the total amount of electromagnetic energy emitted per unit of time by a st ...

, and its spectra suggests heavy-element production takes place within the stars. Two major processes that take place in stars are rapid

neutron capture Neutron capture is a nuclear reaction in which an atomic nucleus and one or more neutrons collide and merge to form a heavier nucleus. Since neutrons have no electric charge, they can enter a nucleus more easily than positively charged protons, ...

(r-process) and slow neutron-capture process (

s-process The slow neutron-capture process, or ''s''-process, is a series of reactions in nuclear astrophysics that occur in stars, particularly asymptotic giant branch stars. The ''s''-process is responsible for the creation (nucleosynthesis) of approximat ...

). The s-process contributes significantly to element abundances in Red Giant, from

iron Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in f ...

to lead-bismuth. Neutron capture and beta decay occur during the s-process and leads to nucleosynthesis (creation of new atomic nuclei) along the neutron-rich side of the nuclear valley of stability. Neutron capture measurements are used to determine branching ratios of the s-process, which provide understanding of

stellar evolution Stellar evolution is the process by which a star changes over the course of time. Depending on the mass of the star, its lifetime can range from a few million years for the most massive to trillions of years for the least massive, which is cons ...

and the elemental history of stars.

Nuclear technology

Current

nuclear fission Nuclear fission is a reaction in which the nucleus of an atom splits into two or more smaller nuclei. The fission process often produces gamma photons, and releases a very large amount of energy even by the energetic standards of radio ...

technology has several limitations, including low

uranium Uranium is a chemical element with the symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. Uranium is weak ...

efficiency and a high-level of

nuclear waste Radioactive waste is a type of hazardous waste that contains radioactive material. Radioactive waste is a result of many activities, including nuclear medicine, nuclear research, nuclear power generation, rare-earth mining, and nuclear weapons r ...

production. Subcritical

Accelerator Driven Systems A subcritical reactor is a nuclear fission reactor concept that produces fission without achieving criticality. Instead of sustaining a chain reaction, a subcritical reactor uses additional neutrons from an outside source. There are two general c ...

(ADS) and Generation IV fast nuclear reactors are two solutions that may decrease these limitations. Obtaining accurate neutron data, particularly high-resolution and high-accuracy cross section data, is essential for nuclear system design.

Experimental setup

The n_TOF facility consists of a pulsed source, specifically a beam of 20 GeV/c protons from the

Proton Synchrotron The Proton Synchrotron (PS, sometimes also referred to as CPS) is a particle accelerator at CERN. It is CERN's first synchrotron, beginning its operation in 1959. For a brief period the PS was the world's highest energy particle accelerator. It ...

(PS) impinging on a lead target. The lead target used is cooled using

liquid nitrogen Liquid nitrogen—LN2—is nitrogen in a liquid state at low temperature. Liquid nitrogen has a boiling point of about . It is produced industrially by fractional distillation of liquid air. It is a colorless, low viscosity liquid that is wide ...

, previously water-cooled (before ''Long Shutdown 2''), and is made of several slabs. The facility also has a 185 m horizontal flight path, along with a 20 m vertical flight path added later in its operation. The flight path allows for a high energy resolution, optimised for measurements of samples with low mass and low cross section by the vertical flight path. NTOF_2

Neutrons are produced when the pulsed beam of protons is directed at the lead target, via neutron spallation reactions. For each proton, about 300 neutrons are expelled. The neutrons are slowed after being emitted, first by the lead target and afterwards by the slab containing water. From this, there is a wide range of neutron energies as neutrons will slow down by varying amounts, forming a spectrum of kinetic energies from meV to GeV. Finally, the neutrons are collimated and ejected through the flight path before they arrive at an experimental area. n_TOF uses a total absorption calorimeter (TAC), made of 42 pentagonal and hexagonal BaF₂ crystals, providing full solid angle coverage. The detector is used to measure the gamma ray cascades that are produced from neutron capture reactions, and has a near 100% efficiency for detecting these reactions.

References

External links

n-TOF Homepagen-TOF on INSPIRE
{{CERN CERN facilities Neutron facilities