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

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    Decay Isotope
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Discussion Overview

The discussion revolves around the types of decay that isotopes undergo, specifically focusing on uranium isotopes and their decay modes. Participants explore why certain isotopes, like uranium-238 and uranium-235, are associated with specific decay processes, and whether isotopes can decay in multiple ways.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants propose that each isotope undergoes only a particular kind of decay.
  • Others argue that isotopes can decay in multiple ways, citing radon-221 as an example of an isotope that decays via both alpha and beta decay.
  • There is a question about the factors that determine the decay mode of an isotope, particularly the relationship between the masses of the decay products and the initial nucleus.
  • One participant notes that spontaneous fission is possible for both uranium-235 and uranium-238, challenging the idea that uranium-238 only undergoes alpha decay.
  • A question is raised regarding why uranium-236 decays faster than uranium-235, indicating a curiosity about the decay rates of different isotopes.

Areas of Agreement / Disagreement

Participants express differing views on whether isotopes are limited to a single decay mode, with some asserting that multiple decay pathways are possible. The discussion remains unresolved regarding the specific reasons behind the decay modes of uranium isotopes.

Contextual Notes

Participants mention that the possibility of decay modes depends on the masses of the decay products, but do not provide detailed mathematical steps or assumptions that underlie these claims.

magdi_gamal
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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?
 
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and why does uranium-236 decay faster than uranium-235?
 
magdi_gamal said:
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
 
Thanks a lot jtbell, it's clear now.
 

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