Decay Method and Time of Nucleus

[Dragonetti]

Hi,
I have a question regarding the types of decay and the time of decay for an atomic nucleus.

Do all nuclei have a set decay time and method, (by method I mean Alpha, Beta, Gamma decay). i.e. does Uranium only decay via beta decay over a set time?

Or can an nucleus decay by any of the decay methods and by any amount of time?

Thanks
Dominic

 

[kloptok]

Theoretically I think all nuclei can decay in all kinds of ways. However the probabilities are usually much, much larger for a specific type of decay, which type depends on the specific nucleus we're talking about. For example, a very heavy nucleus almost exclusively decays by alpha decay or fission whereas this is more uncommon for lighter nuclei. A neutron- or proton-rich nucleus can decay by beta decay to optimize its proton-neutron ratio. In short, the type of preferred decay depends on the mass and proton/neutron numbers of the nucleus.

I think the decay time is largely determined by the type of interaction governing the decay, i.e. strong force (dominant over em force I think) for alpha decay, weak force for beta and em force for gamma decay. Now, I'm not sure but I think alpha decay is fastest, followed by beta decay and last gamma. (Please correct me if this is wrong).

 

[mathman]

Decay rate is dependent on the nucleus involved, not just the type of decay. U-238 decay is alpha, with a very long half life.

 

[kloptok]

I think we're discussing different times here. I may have been unclear, but I was referring to the time of the decay itself, not the half-life, i.e. the time from start of decay until the decay is finished.

What type of time was your original question about, Dragonetti? If you are talking about half-life then yes, a specific nucleus has a set half-life for a certain decay. For Uranium, the preferred decays will be different for different isotopes. 235-U for example, which is the fuel used in fission reactors, is fissile and fissions spontaneusly (right? I'm a bit uncertain as to whether you have to use thermal neutrons or if they just increase the reaction rate?) whereas 238-U mainly decays by beta decay.

So I think the answer to your questions (if I've interpreted them correctly) would be that, no a nucleus does not have one set decay type, it can in general decay in more than one way. The half-life for a given type of decay in a nucleus would be the same though (although I'm assuming you know that decay is a random process, we can not say definitely that all nuclei in a sample will decay after a certain time).

Do all nuclei have a set decay time and method, (by method I mean Alpha, Beta, Gamma decay). i.e. does Uranium only decay via beta decay over a set time?

Or can an nucleus decay by any of the decay methods and by any amount of time?

 

[Drakkith]

I think we're discussing different times here. I may have been unclear, but I was referring to the time of the decay itself, not the half-life, i.e. the time from start of decay until the decay is finished.

I believe the decay effectively happens instantly.

 

[kloptok]

Instantaneously in the context of seconds and minutes yes, but there must take some time for the decay process to happen. Anyway, I really don't know much about this and I don't think it was the question intended by the thread starter. Therefore I'm as of now officially out of this discussion.

A suggestion for you Dragonetti is to look at the Wiki article on 'Radioactive decav'. I skimmed through it and it seems like you can read about why certain nucei have a preference towards certain types of decay.