# Bare Critical Masses of $^{241}$Am, $^{242m}$Am, $^{238}$Pu and $^{242}$Pu

$$^{238}$$Pu has 144 neutrons, and it has 2.8$$\times$$10$$^{3}$$ g$$^{-1}$$.s$$^{-1}$$ spontaneous fission neutrons and bare critical mass in 10 kg. $$^{242}$$Pu has 148 neutrons, and it has 1.7$$\times$$10$$^{3}$$ g$$^{-1}$$.s$$^{-1}$$ spontaneous fission neutrons (slightly less than $$^{238}$$Pu).

On the other hand, $$^{241}$$Am has 146 neutrons but its bare critical mass is 75.7 and 73.3 kg using MONK and MCNP (using JEF 2.2). $$^{242m}$$Am has 147 neutrons (1 extra) but its bare critical mass is 13.9 and 14.2 kg using MONK and MCNP. I am not sure about their spontaneous fission neutrons values.

My questions are

1). Why do $$^{241}$$Am and $$^{242m}$$Am have so different bare critical masses. Does the explanation lie in the values of spontaneous fission neutrons?

2). Similarly, why do $$^{238}$$Pu and $$^{242}$$Pu have so different bare critical masses (10 kg and 100 kg, respectively) although the spontaneous fission neutrons are similar.

• Emitting (spontaneous or not) neutrons is one thing. Absorbing them to obtain fission is another thing... – Jon Custer Apr 22 at 20:20