# Critical mass of 235U and 239 Pu

## Homework Statement

Which of the two nuclides 235U and 239Pu has a smaller critical mass and why

## The Attempt at a Solution

i'm thinking 239Pu since it produces more neutrons on average per fission. Would that be correct? Are there any other reasons I can add?

## Answers and Replies

mgb_phys
Science Advisor
Homework Helper
What has to happen to the neutrons that are produced?

Hmmm cause more fissions? Ok so their lifetimes should matter too, as the longer neutrinos last the more fissions that could potentially cause. But should this be different for the two cases?

Or are you more leading me towards the respective cross-sections? As far as I know, in 235U fission can be induced even by a 0 energy neutron while high energy neutrons are needed for Pu. But then again, cross-section of fission falls for 235U for higher neutron energies. So the answer would depend on the neutron energies. :S.

mgb_phys
Science Advisor
Homework Helper
Or are you more leading me towards the respective cross-sections? ....so the answer would depend on the neutron energies. :S.
Yes .