A detailed concept of a project of the compact neutron source Dedicated for Academical Research and Industrial Application (DARIA) is presented. The main units of the DARIA neutron source are: a proton source, a proton accelerator, a beryllium target, cold neutron sources, combined and water neutron moderators, neutron guides, and neutron scattering stations. To obtain neutrons, the reaction of collision of protons with beryllium nuclei is used, which provides a neutron flux sufficient for conducting experiments in condensed matter physics with moderate heat release on the target. Instruments for studying the crystal and magnetic structure of matter (diffractometers), spectrometers for studying the dynamics of the crystal lattice, the dynamics of magnetic structures (phonon and magnon spectra), as well as small-angle scattering devices for studying nanoobjects and nanostructures are being developed as neutron scattering stations. Each of the stations has its own neutron moderator, which forms the optimal spectrum of the neutron flux specific for it.
Options for optimizing the neutron source to increase the neutron flux and luminosity on the sample are considered. A new approach to the design of the compact neutron source DARIA has been formed, which starts with a sample and ends with a proton source. Thus, taking into account the real physical and technical constraints, all elements of the CNS (proton accelerator, target assembly, moderators and neutron stations) are optimized as a whole and each channel leading to the neutron scattering facility is optimized separately. Pivot points were set for further optimization. The advantages and disadvantages of a pulsed and continuous linear accelerator are described, and the parameters of the proton accelerator for the CNS DARIA are selected.
The issue of using a gas-dynamic source of protons based on electron cyclotron resonance (ECR) in a compact neutron source with a proton accelerator of the DARIA project was considered. It has been shown experimentally that a gas-dynamic proton source provides a beam with a current of ~ 100 mA, a duration of more than 100 μs at a repetition rate of up to 1000 pulses/s. Using the results of measuring the emittance of the proton beam, the development of an accelerator channel operating in a pulsed mode and providing acceleration of this beam to an energy of 13 MeV was carried out.
The target assembly includes a target itself, a moderator, a reflector and a cold moderator. To study its geometric parameters, which directly affect the neutron flux, numerical modeling was carried out in the PHITS program using the JENDL-4.0 library. The calculation of heat release was carried out using similarity equations applicable to the specified cooling conditions. With the conditional parameters of a proton accelerator with an energy of 13 MeV, a maximum pulse current of 100 mA, and a filling factor of 5% for a beryllium target, presented in the form of a cylinder with a radius of 0.5 cm and a thickness of 0.2 cm, placed in a vacuum, simulation was performed to determine the Bragg peak and the neutron exit. The neutron yield is determined at different target thicknesses. The optimum thickness was found as 1.1 mm. With such a thickness, the blistering effect (accumulation of hydrogen in the target due to the incidence of a proton beam on it) is excluded and the heat release from the target is reduced to 10%, but the number of neutrons remains practically at the maximum level.
The calculation of the spherical reflector is performed. It is shown that the beryllium reflector copes with its task more efficiently than the graphite one. In this case, the dimensions of the beryllium reflector will be smaller, since for the effective operation of the reflector, its dimensions must fit into 2 ... 3 diffusion lengths; for beryllium it is 22 cm, and for graphite 54 cm. The very presence of a reflector increases the thermal neutron flux density several times. In the calculation of the reflector, a sphere with a water moderator with a radius of 4 cm was assumed. The thermal calculation of the target assembly showed that it is possible to remove all the heat generated in it (6.5 kW) either using a rotating structure or when water moves at a sufficient speed, but in any case the surface the heat transfer must be larger than the target into which the proton beam hits.
Optimization of different versions of cold neutron sources was carried out based on the requirements of specific neutron scattering instruments. According to the formed requirements for the phase space element and the optimized functionality, for three instruments (SANS, LASSO, BASSET), the most suitable cold moderator for further optimization should be parahydrogen. Parahydrogen possesses the property of "directivity" of cold neutrons, which makes it possible to obtain low divergences in comparison with other cold moderators. The optimal dimensions for a parahydrogen moderator for a design with two cold moderators is a cylinder L = 16 cm long and d = 3.4 cm in diameter. For the MONOPOLY diffractometer and the INDIGO reverse geometry spectrometer, mesitylene should be chosen as cold moderators. To achieve the maximum brightness of a cold source, the dimensions of the mesitylene moderator should exceed the dimensions of the channel. The concept and terms of reference for the design of a neutron moderator for a neutron diffractometer are presented. Based on these calculations, a prototype cold neutron moderator for DARIA was developed.
The prerequisites have been created for the development of a preliminary design of a compact university-type neutron source based on 4 neutron scattering installations in 2021.