For example: http://nuclearweaponarchive.or...
With regard to "thermal equilibrium" we have from the above, Quote: "[Note: Many descriptions in the open literature exist dating back to the late seventies claiming that energetic X-rays from the primary are absorbed by the radiation casing (or plastic foam), and are re-emitted at a lower energy - implying that some sort of energy down-shifting mechanism (like X- ray fluorescence) is at work. This is a misconception. The lining of the casing is in local thermal equilibrium with the energy flux impinging on it, and re-radiates X-rays with the same spectrum. The X-ray spectrum softens simply because the photon gas cools as it expands to fill the entire radiation channel.] In physics a closed container of radiation, like the radiation case, is called a "hohlraum". This German word for "cavity" (which has the obvious English cognates "hole" and "room") has been attached to the study of the thermodynamics of radiation since the last century in connection with blackbody radiation. German physicists early in this century used it as a theoretical model for deriving the blackbody radiation laws from quantum mechanics. Energy in a hohlraum necessarily comes into thermal equilibrium and assumes a blackbody spectrum. This is important for obtaining the necessary symmetry for an efficient implosion. Regardless of how uneven the initial energy distribution within the casing is, the radiation field will quickly establish thermal equilibrium throughout the casing - heating all parts to the same temperature."
Hans Bethe said they made complicated bombs in those days. Hmmmm...
If all else fails, lower your standards.