▁imp ede ▁fast ▁neut rons , ▁this ▁might ▁be ▁reduced ▁to ▁. ▁Pe ier ls ▁also ▁attempted ▁to ▁simplify ▁the ▁problem ▁by ▁using ▁the ▁fast ▁neut rons ▁produced ▁by ▁f ission , ▁thus ▁om itting ▁consideration ▁of ▁moder ator . ▁He ▁then ▁calculated ▁the ▁critical ▁mass ▁of ▁a ▁sphere ▁of ▁u ran ium ▁metal ▁in ▁a ▁theoretical ▁paper ▁written ▁in ▁ 1 9 3 9 . ▁He ▁later ▁re called ▁that ▁the ▁size ▁of ▁the ▁critical ▁mass ▁" was ▁of ▁the ▁order ▁of ▁tons . ▁It ▁therefore ▁appeared ▁to ▁me ▁that ▁the ▁paper ▁had ▁no ▁relev ance ▁to ▁a ▁nuclear ▁weapon ." ▁ ▁However , ▁Bo hr ▁had ▁argued ▁that ▁the ▁u ran ium - 2 3 5 ▁is ot ope ▁was ▁far ▁more ▁likely ▁to ▁capture ▁neut rons , ▁so ▁f iss ile ▁even ▁using ▁neut rons ▁of ▁low ▁energy . ▁Fr isch ▁wonder ed ▁what ▁would ▁happen ▁if ▁he ▁were ▁able ▁to ▁produce ▁a ▁sphere ▁of ▁pure ▁u ran ium - 2 3 5 . ▁When ▁he ▁used ▁Pe ier ls ' ▁formula ▁to ▁calculate ▁this , ▁he ▁received ▁a ▁start ling ▁answer . ▁Pe ier ls ▁later ▁observed ▁that : ▁▁ ▁Re alis ing ▁the ▁sensitive ▁nature ▁of ▁the ▁document , ▁Pe ier ls ▁typed ▁it ▁himself . ▁One ▁carbon ▁copy ▁was ▁made . ▁Today ▁the ▁original ▁is ▁in ▁the ▁Bod le ian ▁Library ▁at ▁Oxford ▁University . ▁ ▁Non - techn ical ▁ ▁The ▁memor and um ▁was ▁written ▁in ▁two ▁parts . ▁The ▁first ▁was ▁an ▁elegant ▁and