As an example of the improbablity of this
Americium
the isotope 242m1Am (half-life 141 years) has the largest cross sections for absorption of thermal neutrons (5,700
barns),that results in a small
critical mass for a sustained
nuclear chain reaction.
The critical mass for a bare 242m1Am sphere is about 9–14 kg.
It can be lowered to 3–5 kg with a metal reflector and should become even smaller with a water reflector.
Such small critical mass is favourable for portable
nuclear weapons, but those based on 242m1Am are not known yet, probably because of its scarcity and high price.
The critical masses of two other readily available isotopes, 241Am and 243Am, are relatively high – 57.6 to
75.6 kg for 241Am and 209 kg for 243Am.
[86] Scarcity and high price yet hinder application of americium as a
nuclear fuel in
nuclear reactors.
Americium is the only synthetic element to have found its way into the household, where one common type of
smoke detector uses 241Am in the form of americium dioxide as its source of
ionizing radiation.
This isotope is preferred against 226
Ra because it emits 5 times more alpha particles and relatively little of harmful γ-radiation.
The amount of Americium in a typical new smoke detector is 1
microcurie (37
kBq) or
0.28 microgram.
So to obtain sufficient 241 Am for a critical mass you would require at least 30,000,000,000 smoke detectors at a rough guess.
