Text: The adjective "cold" has been applied to neutrons with an average energy less than that of thermal neutrons. The quotation marks indicate that these neutrons are not produced by refrigeration but by a device which depends on the coherent scattering of slow neutrons. From the Bragg law n = 2d sin 0 It is apparent that when L, the neutron wavelength, exceeds 2d, where d is the grating spacing of the crystal, the value of sin 0 becomes greater than unity, which is impossible mathematically. Therefore there is no reflection. If a beam of thermal neutrons traverses a column of polycrystalline graphite, the neutrons which have wavelengths less than 2d will be reflected and ultimately be removed from the column. Thus the neutrons with the higher energies are not transmitted. Since 2d for graphite is 6.7 A the computed maximum energy of the transmitted neutrons is 0.002 ev. This is well below the average energy of thermal agitation for neutrons at room temperature.

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