A neutron bomb or officially known as one type of Enhanced Radiation Weapon is a low yield fission-fusion thermonuclear weapon (hydrogen bomb) in which the burst of neutrons generated by a fusion reaction is intentionally allowed to escape the weapon, rather than being absorbed by its other components.
Compared to a pure fission bomb ... a neutron bomb would emit about 10x the amount of neutron radiation. In a fission bomb ... gamma rays and neutrons is approximately 5% of the entire energy released; in the neutron bomb it would be closer to 40%.
The US, USSR, France and China (and possibly Israel) have all developed them, and/or implemented them.
The lethal dose (LD50) of 600 Rads would extend to about 1350–1400 meters
But they also have substantial blast effects (so you don't get to keep the buildings, etc).
And, the neutrons get absorbed by the atmosphere (perhaps dictated by humidity).
But the basic function I was looking for was a way to destroy life (as we know it). Would carpet-nuking with Neutron bombs (or a better (reasonably feasible) solution*?) destroy surface/readily available insects, pollen, spores, bacteria and virii? How many would I need per square km?
- reasonably feasible; Serban points to an answer requiring 4,200x 1001kg antimatter bombs (admittedly to scour the whole surface of the planet)... which is several orders of magnitude more antimatter than humans have managed to produce so far...
Yes, some of these exist in deep areas, but I'm just asking about sterilizing the surface, not making the whole planet uninhabitable. And preferably with minimal side-effects, ie: trying not to push the whole planet into nuclear winter if it can be avoided.