$^{238}$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.

  • 1
    $\begingroup$ Emitting (spontaneous or not) neutrons is one thing. Absorbing them to obtain fission is another thing... $\endgroup$ – Jon Custer Apr 22 at 20:20

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