page_title: FERMIAC
section_title: Operation
hierarchical_section_title: FERMIAC / Operation
caption_attribution_description: Photograph of the the FERMIAC, or Monte Carlo trolley, an analog device invented by Enrico Fermi to implement studies of neutron transport.
context_page_description: The Monte Carlo trolley, or FERMIAC, was an analog computer invented by physicist Enrico Fermi to aid in his studies of neutron transport.
context_section_description: The FERMIAC employed the Monte Carlo method to model neutron transport in various types of nuclear systems. Given an initial distribution of neutrons, the goal of the process is to develop numerous "neutron genealogies", or models of the behavior of individual neutrons, including each collision, scattering, and fission.  When a fission occurs, the number of emerging neutrons is predicted, and the behavior of each of these neutrons is eventually modeled in the same manner as the first.  At each stage, pseudo-random numbers are used to make decisions that affect the behavior of each neutron.
  The FERMIAC used this method to create two-dimensional neutron genealogies on a scale diagram of a nuclear device.  A series of drums on the device were set according to the material being crossed and a random choice between fast and slow neutrons.  Random numbers also determined the direction of travel and the distance until the next collision.  Once the drums were set, the trolley was rolled across the diagram, drawing a path as it went.  Any time a change in material was indicated on the diagram, the drum settings were adjusted accordingly before continuing.
caption: The Fermiac in use
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