backThe nuclear reactions most frequently used for the generation of mono-energetic neutrons are 7Li(p,n)7Be, 3H(p,n)3He, 2H(d,n)3He and 3H(d,n)4He. The energy and the angular distribution of the neutrons are defined by the differential cross section of the nuclear.
The energy distribution of the neutrons in monoenergetic neutron fields is very narrow and depends only on the emission angle and target thickness. This distribution can, however, easily be disturbed due to the strong interaction of neutrons with matter. In a so-called reference field, these disturbances are, as far as possible, minimized by the use of low-mass experimental facilities.
Therefore, the large experimental area of the accelerator facility is equipped with a grid floor with a large hole below the neutron-production target. Due to the large distances of the position of the experiment to walls, the floor and the ceiling, and due to the low-mass target construction, a very small fraction of low-energy neutrons (< 5 %) is present at the site of the experiment.
Low-scatter experimental area of PTBxs PIAF accelerator facility. The neutron-production target is mounted at the end of the beamline above the opening in the grid floor.
The measurement of the neutron fluence, i.e., of the "intensity" of the neutron field, is carried out with recoil proton detectors so that all measurements are traceable to the neutron-proton scattering cross section which is the primary reference in neutron metrology. The energy distribution of the neutrons is determined with liquid scintillators and by the time-of-flight method, i.e., by measurement of the neutron velocity. For that purpose, the accelerators used must be operated in such a way that the beam pulses have a duration of only a few nanoseconds so that the moment of formation of the neutrons is well defined.
The figure shows the time-of-flight distribution of a 19 MeV neutron field with the dominating intensity maximum of the mono-energetic neutrons and the background produced by disturbing reactions in the neutron production target.
Time-of-flight distribution of the 19 MeV neutron field which is produced by the T(d,n)4He reaction with a tritium-loaded titanium target. In addition to the intensity maximum of the mono-energetic 19 MeV neutrons, there is also a continuum of low-energy neutrons which is formed by reactions of the deuteron beam with the target materials and by neutron scattering in the target.
The reference fields are used to determine the energy dependence of the response of neutron detectors. The wide rang of systems under investigation ranges from radiation protection dosemeters to detector systems for fundamental research in fusion technology or in nuclear and high-energy physics. The monoenergetic neutron fields with energies between 24 keV and 19 MeV meet the requirements described in ISO 8529.
In addition to these reference fields, also other (p,n), (d,n) and (a,n) nuclear reactions can be used for neutron production. Therefore, both monoenergetic and also broad (white) neutron energy distributions are generated and used, mainly, to investigate neutron detectors for fundamental research. The figure shows an experiment aiming at the calibration of a detector for β-delayed neutrons (BELEN) with the nuclear reactions 51V(p,n), 13C(p,n) and 13C(a,n). The BELEN detector is being constructed for the 
accelerator facility FAIR at the GSI in Darmstadt within the scope of a Spanish-German collaboration.
Calibration of the BELEN neutron detector at the accelerator facility of PTB. The neutron production target is located in the centre of the detector.
Literature
- R. Nolte and D.J. Thomas:
Monoenergetic fast neutron reference fields: I. Neutron production
Metrologia 48 (2011) S263-S273
- R. Nolte and D.J. Thomas:
Monoenergetic fast neutron reference fields: II. Field characterization
Metrologia 48 (2011) S274-S291
- R. Nolte, M.S. Allie, R. Böttger, F.D. Brooks, A. Buffler, V. Dangendorf, H. Friedrich, S. Guldbakke, H. Klein, J.P. Meulders, D. Schlegel, H. Schuhmacher and F.D. Smit:
Quasi-Monoenergetic Neutron Calibration Fields in the Energy Range from Thermal to 200 MeV
Radiat. Prot. Dosim. 110 (2004) 97 - 102