Specialty Chemistry Forums > Nuclear Chemistry and Radiochemistry Forum
why does n/p ratio inrease?
Grejak:
--- Quote from: sapta on August 05, 2005, 03:10:47 PM ---yes but shud it be that way?it shud have tried to bring down the ratio.
--- End quote ---
I think your question it more specific than you want it to be. You asked why alpha decay in specific increases the n/p ratio in a nucleus. That is simple number manipulation and has been sufficiently answered multiple times. No matter how many times you ask the question, alpha decay will always increase the n/p ratio in 238U. Not due to the stability of the nucleus or any other factors, simply due to the definition of what alpha decay is. That cannot be changed.
So what can be changed? Well, the decay mode does not always have to be alpha, and different decay modes will change the n/p ratio in different ways. Perhaps your question is why uranium undergoes alpha decay instead of one of the other decay modes. That is a much more difficult question.
sapta:
--- Quote from: Grejak on August 05, 2005, 03:32:24 PM ---
So what can be changed? Well, the decay mode does not always have to be alpha, and different decay modes will change the n/p ratio in different ways. Perhaps your question is why uranium undergoes alpha decay instead of one of the other decay modes. That is a much more difficult question.
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yes yes that's my question.never thought u were misunderstanding what i wanted ::)my poor English :-[
Grejak:
--- Quote from: sapta on August 05, 2005, 03:43:00 PM ---yes yes that's my question.never thought u were misunderstanding what i wanted ::)my poor English :-[
--- End quote ---
There are several possible ways for radioactive nuclides to decay, most notably alpha decay, beta decay, electron capture and spontaneous fission: Alpha decay involved the emission of a stripped 4He nucleus from the atom. Beta decay involves the emission of an electron or a positron (along with neutrinos) from the nucleus. During B+ decay a proton is converted into a neutron and the reverse is true for B- decay. Electron capture is the process by which an orbital electron is absorbed in the nucleus, destroys a nuclear proton and creates a neutron. Spontaneous fission involves the splitting of a nucleus into two or more smaller nuclei.
In order to simplify the 238U problem, let’s ignore the spontaneous fission decay mode and focus only on why 238U decays via alpha emission instead of beta decay or electron capture.
The Q value of a reaction determines whether or not a reaction is thermodynamically stable or not. In the case of nuclear reactions, the Q value is simply the mass of the reactants minus the mass of the products (the reverse of most of chemistry). A positive Q value implies that the reaction is exothermic and that over time the reaction will tend towards the products. The reverse is true for a negative Q value. In this case, the reaction is endothermic and over time will favor the reactants.
The values below are the Q values for each reaction:
Alpha decay: 4.27 MeV
B- decay: -0.147
B+ decay: -4.483
Electron Capture: -3.461
As you can see, the only decay mode that will release energy is alpha decay. As the other modes all require energy, 238U will not decay by them.
On a fascinating side note, the Table of Radioactive Isotopes lists 238U as having a bb decay, which I assume means double beta decay. :)
sapta:
i get it :)
--- Quote from: Grejak on August 05, 2005, 10:09:50 PM ---
On a fascinating side note, the Table of Radioactive Isotopes lists 238U as having a bb decay, which I assume means double beta decay. :)
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interesting;what's double beta decay?(1 beta emission would create a new nuclide)
Elgon:
Double beta decay is the simultaneous conversion of 2 neutrons into 2 protons under emission of 2 electrons and 2 anti neutrinos. The important part is that both conversions happen at the same time and not successively.
This type of decay has been postulated, and there are several large scale experiments planned to examine it. But as far as I know in the case of uranium there has only been a limit determined so far. One way of determining the rate for the double beta decay of uranium would be to measure the amount of plutonium 238 that is produced in a sample of uranium 238 over time. Unfortunately you have to make sure that your uranium does not contain any traces of plutonium from other sources. And this is very, very difficult.
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