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This week we have learned about alpha, beta, and gamma nuclear radiation and decay. We know that in beta decay a neutron is lost and a proton is gained, and that the daughter element has one more proton than its parent. For example, when Thorium-234 beta decays into Protactinium-234. This has aroused two questions: If the number of electrons is the same as that of the protons in normal atoms, does the new Pa-234 get a new electron in the electron cloud, and if so, where does it come from? Also what happens to the electrons that are left over when radioactive elements like Uranium-238 alpha decay? Does alpha decay somehow include a loss of 2 electrons so that the daughter element is not an ion?
Answer 1:

You have asked a very good question. It turns out that in the field of particle/nuclear physics certain assumptions have been made that as far as we can tell are true. These assumptions are that in reactions like you have mentioned above certain quantities are conserved such as energy, charge, momentum, and some other more confusing quantities such as baryon and lepton number.

So, if you take the beta decay reaction Th-234 -> Pa-234 where a neutron turns into a proton, is charge conserved in this process as is? If charge is not conserved then there must be another particle involved in the process that we are missing so that charge is conserved in the reaction. What must that other particle be?

If you remember the idea that charge must be conserved (and that you really only have protons, electrons, and neutrons to work with) you should be able to answer all your questions. Remember that atoms find a way to have zero net charge, if possible, due to the attraction between protons and electrons.

As an aside: protons and neutrons are part of a family called baryons and the number of baryons is conserved in any reaction. Electrons and neutrinos are called leptons and their number is also conserved. If you haven't heard of neutrinos yet you might want to take a few minutes to find out about them. They play a role in the reactions you asked about.


Answer 2:

Beta decay, as you said, effectively consists on a neutron decomposing into a proton and an electron (the beta particle), which gets ejected at high speed. Now, that beta particle is just as much an electron as any other, and will roam about, lose energy through collisions with other atoms, and eventually be attracted to, and fall into orbit about a positive ion.

The atom which decayed, as you observe, is now positively charged, and will attract electrons the same as any other ion would. These electrons could come from the "electron cloud" (in which case, the beta particlehas also joined the cloud) or it's evn possible (but perhaps not very likely) that the escaped beta particle will end up orbiting the same nucleus that created it!

Now let's look at the alpha-decay of U-238. After the decay, we have Th-236, with two extra electrons, and He-4, missing two electrons. Sometimes the outer shell electrons can be knocked out of place by the escaping He nucleus, but if they don't, we just have two charged ions floating around (okay, one of them at high speed!). Now you tell me,
what happens next?

Hope this helps...


Answer 3:

I will make a stab at this -- even though Physics has changed so much recently, an expert might need to correct me. In beta decay, the effective outcome is that a neutron decays into a proton, an electron, and a neutrino. It is rare, however, that the atom could retain this electron since it carries a significant amount of the released energy kinetically. i.e. It is traveling way too fast for the atom to retain it. You re correct in assuming that the resulting atom is an ion-- In general, the atom will steal an electron from a surrounding atom to which it is less strongly bound. (This of course still leaves a charge imbalance.) The ejected electron itself is likely to scatter other electrons from nearby atoms, creating even more ions. (This is why highly energetic nuclear radiation is called "ionizing").
There are several more mundane mechanisms for the charges to balance again, electrical conduction, plasma motion due to the field etc. Alpha emission is also capable of local ionization, but typically over a shorter distance because the alpha particle is more likely to scatter and lose energy.

If the radioactive sample is isolated (i.e. suspended in air for example) it will create ions in a cloud surrounding itself-- this effect has practical application. You might check out how a common smoke detector works-- There are also radioactive tipped lightning rods. Why do you think this is a good idea?

By the way: there is a nice MPG movie of beta decay at:

http://windows.ivv.nasa.gov/sun/Solar_interior/Nuclear_Reactions/Neutrinos/beta_decay.html


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