What defines the kind of decay an isotope would go for?

[magdi_gamal]

first, is my initial understanding correct that each isotope go for only a particular kind of decay?

and if so, why? What makes uranium-238 go only for an alpha decay? while uranium 235 go for a fission decay?

 

[magdi_gamal]

and why does uranium-236 decay faster than uranium-235?

 

[jtbell]

first, is my initial understanding correct that each isotope go for only a particular kind of decay?

No, it's possible for an isotope to decay in more than one way. For example, radon-221 can decay either via alpha decay to polonium-217 (about 22% of the time), or via beta decay to francium-221 (about 78% of the time).

Whether a particular mode of decay is possible for a particular isotope, depends on the masses of the decay products.

If the sum of the masses of the decay products is less than the mass of the initial nucleus, then the decay is possible. The "disappearing" mass appears as the kinetic energy of the products.

If the sum of the masses of the decay products is greater than the mass of the initial nucleus, then the decay is impossible as a spontaneous process; energy needs to be supplied from outside in order to create the extra mass.

What makes uranium-238 go only for an alpha decay? while uranium 235 go for a fission decay?

Spontaneous fission is possible in both U-235 and U-238.

http://en.wikipedia.org/wiki/Spontaneous_fission

 

[magdi_gamal]

Thanks a lot jtbell, it's clear now.