 The task of the metallurgists was to determine how to cast plutonium into a sphere . The difficulties became apparent when attempts to measure the density of plutonium gave inconsistent results . At first contamination was believed to be the cause , but it was soon determined that there were multiple allotropes of plutonium . The brittle α phase that exists at room temperature changes to the plastic β phase at higher temperatures . Attention then shifted to the even more malleable δ phase that normally exists in the 300 – 450 ° C ( 570 – 840 ° F ) range . It was found that this was stable at room temperature when alloyed with aluminum , but aluminum emits neutrons when bombarded with alpha particles , which would exacerbate the pre @-@ ignition problem . The metallurgists then hit upon a plutonium – gallium alloy , which stabilized the δ phase and could be hot pressed into the desired spherical shape . As plutonium was found to corrode readily , the sphere was coated with nickel . 
