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We report on the generation of photon pairs at 1550 nm from free-standing epitaxially grown self-assisted micrometre long GaAs nanowires. The efficiency of the spontaneous parametric down-conversion process has a rate of 320 GHz/Wm normalized to the transmission of the setup, the pump intensity, and the volume of the nanostructure. GaAs is a high index dielectric that can support electromagnetic Mie modes, therefore we model how shorter nanowires could improve the second-harmonic signal and we found that sub-micro long nanowires (600 nm length and 250 nm diameter) can support quality factors up to 15 at the pump wavelength (780 nm). We anticipate that the near field enhancement compared to micrometre long nanowires will boost the second-harmonic generation and, correspondingly, the biphoton rate efficiency.

Facilities providing bright thermal neutron beams are of primary importance for various research topics such as condensed matter experiments, neutron-imaging or medical applications. Currently these are mainly spallation sources and nuclear reactors. However, these later facilities are ageing and the political context does not favor the building of new ones. This is the case in CEA-Saclay (France), where the Orphee reactor is planned to shutdown in 2019. Therefore, another local facility, affordable by one country, able to provide high brilliance neutron beams has to be built. At CEA-Saclay, a compact accelerator driven neutron source, SONATE, is investigated in taking advantage of the IPHI accelerator able to deliver a 3 MeV proton beam with an intensity up to 100 mA. In the future, SONATE is foreseen to operate with 20 MeV protons to increase the neutron brightness. In addition to the difficulties to operate such high intensity accelerators, the other challenges regard the target-moderator-reflector (TMR) design which is crucial to maximize the neutron flux at the detector location. At CEA-Saclay, several experiments were performed between 2016 and 2019 with the IPHI accelerator. Geant4 simulations were also developed. They demonstrate the feasibility of such concept and enable to find the best TMR configuration for the future SONATE facility. These developments are reported in this article.

We aim at building a compact accelerator-based neutron source (CANS) which would provide a thermal neutron flux on the order of 4x1012 n.s-1.cm-2.sr-1. Such a brilliance would put compact neutron sources on par with existing medium flux neutron research reactors for neutron scattering experiments. We performed the first neutron production tests on the IPHI proton accelerator at Saclay at a proton energy of 3 MeV. The thermal neutron flux were measured using gold foil activation and 3He detectors. The measured flux were compared with GEANT4 Monte Carlo simulations (10.4) in which the whole experimental setup was modelled. There is a good agreement between the experimental measurements and the Monte-Carlo simulations. The available modelling tools will allow us to optimize the whole Target Moderator Reflector assembly together with the neutron scattering spectrometer geometries for the design of the neutron scattering facility SONATE.

There is a worldwide growing interest for small-scale and reduced-cost neutron sources not based on nuclear fission. High-intensity proton or deuteron beams impinging on light materials could be used to produce such neutron sources with intensities or brightness comparable to nuclear reactor for dedicated experiments. To develop such technologies several key technological issues have to be addressed. Among them the neutron production and the maximization of the neutron extraction and transport to the instrument is a key parameter for the design of high-brightness sources adapted for the required application. This issue have to be addressed with validated and predictive Monte-Carlo simulations. In this paper we present preliminary results on the use of Geant4 in the context of Compact Accelerator based Neutron Source (CANS) developments.