ron ▁density ▁to ▁multiply ▁by ▁a ▁factor ▁e ." ▁The ▁available ▁data ▁was ▁very ▁approximate , ▁but ▁their ▁central ▁point   – ▁that ▁a ▁bomb ▁was ▁possible ▁using ▁fast ▁( ~ 2 Me V ) ▁neut rons   – ▁remains . ▁Jer emy ▁Bern stein ▁remarked ▁of ▁this ▁effort : ▁" Let ▁me ▁make ▁the ▁same ▁point ▁by ▁asking ▁a ▁somewhat ▁different ▁question ▁but ▁using ▁the ▁correct ▁numbers . ▁How ▁much ▁time ▁does ▁it ▁take ▁to ▁f ission ▁a ▁kil ogram ▁of ▁ 2 3 5 U ▁using ▁fast ▁neut rons ?" ▁Using ▁modern ▁values ▁he ▁found ▁that ▁to ▁be ▁" equal ▁to ▁about ▁a ▁micro second , ▁which ▁makes ▁the ▁point ▁about ▁the ▁rapid ity ▁of ▁f ission ▁with ▁fact ▁[ s ic ] ▁neut rons ". ▁ ▁In ▁the ▁original ▁memor and um , ▁if ▁the ▁neut rons ▁had ▁veloc ities ▁of ▁ 1 0 9 ▁cm / s , ▁then ▁they ▁would ▁have ▁an ▁average ▁time ▁between ▁f ission ▁coll isions ▁of ▁. ▁Therefore , ▁Bern stein ' s ▁time ▁for ▁a ▁kil ogram ▁of ▁u ran ium - 2 3 5 ▁to ▁f ission ▁is ▁found ▁by ▁solving : ▁ ▁where ▁ ▁was ▁the ▁mean ▁time ▁for ▁f ission ▁neut ron ▁density ▁to ▁increase ▁by ▁e . ▁Given ▁the ▁dou bling - time ▁ ▁this ▁imp lied ▁a ▁mean ▁f ission ▁exponential ▁fol ding ▁time ▁of ▁ ▁This ▁led ▁to ▁a ▁calculation ▁of ▁the ▁energy ▁released , ▁which ▁Pe ier ls ▁reck oned ▁as ▁approximately : ▁ ▁where ▁M