Numerical Investigation of a UCN Source Based on Solid Deuterium by Combining a Simulation Code with an Analytical Approach

At thermal ultra-cold neutron (UCN) sources (neutrons in thermal equilibrium with the moderator) only a very small fraction of neutrons have velocities ~6 m/s. Therefore, the UCN production rate cannot be substantially increased by simply lowering the temperature of the moderator. The new approach is to use the super-thermal principle, i.e., neutrons not in thermal equilibrium with the converter. We want to investigate scattering kernels for a super-thermal UCN source based on a two-layer arrangement of D2O and solid D2. The solid D2 (sD2) at temperature 8 K is kept in close contact with D2O moderator at room temperature. Using the MCNP code, the fast neutron flux on the spallation target, the thermal flux in the D2O near the sD2, and the cold flux in the sD2 are simulated. For a given cold flux, neutron transport equations are calculated. In order to obtain precise neutron scattering kernels, and consequently UCN flux and density, 330 neutron energy groups have been taken. The coupled energy dependent transport equations have been solved by combining MCNPX code with an analytical approach and using implicit method in MATLAB. We have obtained an optimal dimension for the UCN source. A suitable space step has been taken for the numerical stability.

Simple Woods-Saxon-type form for Ωα and Ξα interactions using folding model

We derive a simple Woods-Saxon-type form for potentials between Y=Ξ,Ωandαusing a single-folding potential method,based on a separable Y-nucleon Potential.The PotentialsΞ+αandΩ+αare accordingly obtained using the ESC08 c Nijmegens potential(in 3 S1 channel)and HAL QCD collaborationΩN interactions(in lattice QCD),respectively.In deriving the potential between Y andα,the same potential between Y and N is employed.The binding energy,scattering length,and effective range of the Y particle on the alpha particle are approximated by the resulting potentials.The depths of the potentials inΩαandΞαsystems are obtained at-61 MeV and-24.4 MeV,respectively.In the case of theΞαpotential,a fairly good agreement is observed between the single-folding potential method and the phenomenological potential of the Dover-Gal model.These potentials can be used in 3-,4-and 5-body cluster structures ofΩandΞhypernuclei.

Change in^(7)Be half-life in host media

First-principle calculations within the density functional theory framework are used to study the probability of electron capture for the^(7)Be nucleus.For this purpose,electron density at the^(7)Be nucleus is computed in Al,Au,Pd,Pt,and Pb environments.Our results show that the half-life of^(7)Be is changed by implanting^(7)Be in host environments.Electron affinity of the media and confinement effects are responsible for the change in the half-life of^(7)Be nucleus.Moreover,electric potential at the^(7)Be nucleus is calculated.Results show that variations in electric potential are usually consistent with those in electron density at the^(7)Be nucleus.

Effects of electric fields on^(7)Be half-life

First-principle calculations based on the density functional theory(DFT)method are adopted to investigate the influence of a strong electric field on the^(7)Be half-life.Accordingly,electronic structures of Be and BeO are examined in the presence of a homogeneous electric field.The electron density at the nucleus is estimated upon the geometry optimization.Our computations for the Be metal indicate a 0.02%increase in the decay rate of the^(7)Be nucleus,corresponding to a 0.02%decrease in the^(7)Be half-life,both at 5.14 V/A(0.1 a.u.).Furthermore,it is determined that the decay rate of^(7)Be is not considerably altered up to 3.6 V/Åin the BeO structure.Our results show that the screening energy of the electron can be dependent on the applied electric field strength.Furthermore,we predict variations in the Coulomb potential at the^(7)Be nucleus due to electric field application.