I muri i te pakanga, i hoki a Leona Marshall ki te Whare Wananga o Chicago, i te wa i arotahi ai ia ki te rangahau i te Fermi's Institute for Nuclear Studies. I te tau 1960, i uru ia ki te Whare Wananga o Chicago Pile 3, i kitea e ia he huarahi ki te 100 ōrau o te porotiti porotiti porotiti porotiti, me te whakatau i te taupū whakaheke o nga neutron mo nga momo rauemi. I whanau tana tamaiti tuarua, ko John Marshall III, i te 1949. I noho ia hei kaiwhakaako awhina i te 1953. I muri i te matenga o Fermi i te 1954, ka wehe nga Marshalls. I hoki mai a John Marshall ki te Laboratory Los Alamos, i te wa i riro ai a Leona, he whaea kotahi, i te Institute for Advanced Study i Princeton, New Jersey i te 1957. I te tau e whai ake nei ka noho ia ki te Brookhaven National Laboratory, i te wa e aro atu ana te rangahau i roto i te ahupūngao i te karihi mai i te karihi me nga matūriki.
Ko John Marshall Tuatoru