a Physics explanation " The plutonium in the first American atomic bombs was stabilized in the low density delta phase (density 16.9) by alloying it with 3% gallium (by molar content, 0.8% by weight), but was otherwise of high purity. The advantages of using delta phase plutonium over using the high density alpha phase (density 19.2), which is stable in pure plutonium below 115 degrees C, are that the delta phase is malleable while the alpha phase is brittle, and that delta phase stabilization prevents the dramatic shrinkage during cooling that distorts cast or hot-worked pure plutonium. In addition stablization eliminates any possibility of phase transition expansion due to inadvertent overheating of the pit after manufacture, which would distort and ruin it for weapon's use"