collision cascade is the slowing down of neutrons to increase the probability.
collision cascade is the slowing down of neutrons to increase the probability.
what is the slowing down of neutrons to increase the probability?
collision cascade
the "trick" to making a working reactor fuelled by natural or low enriched uranium is to slow enough of the neutrons to the point where their probability of causing nuclear fission in u increases to a level that permits a sustained chain reaction in the uranium as a whole. this requires the use of a neutron moderator, which absorbs some of the neutrons' kinetic energy, slowing them down to an energy comparable to the thermal energy of the moderator nuclei themselves (leading to the terminology of "thermal neutrons" and "thermal reactors"). during this slowing-down process it is beneficial to physically separate the neutrons from the uranium, since u nuclei have an enormous parasitic affinity for neutrons in this intermediate energy range (a reaction known as "resonance" absorption). this is a fundamental reason for designing reactors with discrete solid fuel separated by moderator, rather than employing a more homogeneous mixture of the two materials.
pressurized heavy-water reactor
exposure to free neutrons can be hazardous, since the interaction of neutrons with molecules in the body can cause disruption to molecules and atoms, and can also cause reactions that give rise to other forms of radiation (such as protons). the normal precautions of radiation protection apply: avoid exposure, stay as far from the source as possible, and keep exposure time to a minimum. some particular thought must be given to how to protect from neutron exposure, however. for other types of radiation, e.g., alpha particles, beta particles, or gamma rays, material of a high atomic number and with high density make for good shielding; frequently, lead is used. however, this approach will not work with neutrons, since the absorption of neutrons does not increase straightforwardly with atomic number, as it does with alpha, beta, and gamma radiation. instead one needs to look at the particular interactions neutrons have with matter (see the section on detection above). for example, hydrogen-rich materials are often used to shield against neutrons, since ordinary hydrogen both scatters and slows neutrons. this often means that simple concrete blocks or even paraffin-loaded plastic blocks afford better protection from neutrons than do far more dense materials. after slowing, neutrons may then be absorbed with an isotope that has high affinity for slow neutrons without causing secondary capture radiation, such as lithium-6.
neutron