Neutron intensity gains for converging guide systems

Neutron focusing, or increased current density at the expense of increased beam divergence, can be obtained by the addition of a converging guide at the end of a straight guide. The greatest gains are achieved when the critical angle of the coating of the converging guide is much greater than that of the straight guide. The gain is limited, however, by the increase in angle that the neutron trajectory makes with the axis upon successive reflections. Acceptance diagrams are used for deriving analytical expressions for describing the transmission properties of such systems. The results can be reduced to nine basic formulae depending on the geometric configuration and the coatings of the guides. A graphical method is presented for determining the appropriate expression for the current density gain for a particular guide configuration and relative surface coatings, as a function of wavelength. The results are in agreement with earlier measurements obtained with a straight-converging guide system. (orig.)