 This gave a value for a uranium-235 gadget with a perfect tamper of 25 kg; but this was only an approximation.It was based on simplifying assumptions, notably that all neutrons had the same speed, that all collisions were elastic, that they were scattered isotropically, and that the mean free path of neutrons in the core and tamper were the same.Bethe's T Division, particularly Serber's T-2 (Diffusion Theory) Group and Feynman's T-4 (Diffusion Problems) Groups, would spend the next few months working on improved models.