A variety of moderator structures composed of natural uranium and polyethylene were calculated

showing that the best arrangement is to wrap the 14 MeV neutron source with a layer of 2-2.5cm natural uranium and to use thermal neutrons at a distance of 2-3cm from the surface of uranium. When the enriched uranium is used as the intensifying material

only at a certain distance from the source can it have better effect than natural uranium. The subcritical multiplying systems composed of polyethylene and fuel （polyethylene mixed with enriched uranium） with the same dimension as mentioned above were also calculated. Natural uranium and lead were found to still have significant enhancement for 14 MeV neutron source in the systems. In a subcritical system with proper structure

the intensifying factor M can be close to or even greater than （1-k）-1 where k is effective multiplication constant of the system.