Effect of Reentrant Hole Perturbation on Angular Thermal Neutron Spectra in Time of Flight Measurement

The effect of the presence of a reentrant hole for extracting the neutron beam from within experimental systems of two different geometries is analyzed theoretically with use made of multi-group 2- dimensional discrete S n method without resorting to bold assumptions for neutron transport nor drastic simplification of geometry. One of the two experimental systems is a rectangular light water prism 12 cm high of 40 × 40 cm 2 cross section, poisoned with Cd and/or In, and provided with a 1, 2 or 3 cm diameter reentrant hole. The other system is a 1″ thick natural uranium plate sandwiched between two layers of pure light water, each 4.6 cm thick, which also is provided with a 1cm diameter reentrant hole. The following is concluded by comparing the angular neutron flux with and without the reentrant holes. With the first experimental system, perturbations of the order 10∼25% is caused, which is particularly strong below about 0.3 eV, except when the hole diameter is 1cm. The perturbation effect increases as the reentrant hole becomes larger in diameter and shallower in depth. In the case of the second experimental system, the effect results in about 2% increase of the neutron flux at the bottom of the reentrant hole when the bottom is located in the natural uranium plate. On the other hand, if the bottom is in the light water region, the neutron flux is reduced by about 2∼4% at the peak of the thermal neutron spectra.