# Why can “slow” neutrons trigger fission?

My understanding of nuclear fission is that some massive isotopes such as uranium-235 are unstable and when split via fission there will be a "slow" neutron. This slow neutron will hit another unstable isotope and this vicious cycle is repeated until you have a chain reaction.

The unstable isotopes are very dense and require high energy to split them - in this case, I would think, ideally a fast neutron. Why does a slow neutron work?

• Q2b: Would both slow and fast neutrons 'work'? Why not? – Hennes Apr 11 '15 at 11:50
• @ACuriousMind thank you very much for correcting my grammar – user6760 Apr 11 '15 at 12:48

You have a lump of playdoh$^{235}$. Now a slow ball of playdoh - let's call it "neutron" - comes along and is captured by the big lump. Since the lump is now bigger by one playdoh unit you have highly unstable playdoh$^{236}$, which then fissions into two smaller lumps. In this process two to three fast neutrons are emitted. Now, since you want the fission to go on you need moderators which slow the neutrons down, or else the neutrons will scatter of the other lumps without combining with them. When you have slowed the neutrons down enough then they can combine with onether playdoh$^{235}$ to form playdoh$^{236}$ etc...