Radioactive Nuclei: Stability & Effects of Size

[jimmy p]

What properties of nuclei make them radioactive? I know it is mainly to do with stability of the nucleus, but are there any other reasons? Why are elements with smaller nuclei less radioactive than elements with larger nuclei?

 

[chroot]

This is a complex question. The simplest answers are these:

1) Very large nuclei can undergo alpha decay. Alpha particles are just helium nuclei -- two protons, two neutrons. You can think conceptually of a large nucleus as having an alpha particle "swimming around" inside it. Every now and then, the alpha particle finds itself far enough from the rest of the nucleus that it can tunnel through the potential barrier and escape. Large nuclei are less tightly bound than small nuclei -- the nucleons on the outside of a large nucleus feel less strong force than the nucleons in the middle of a small nucleus. Note that this is also just a concept: nucleons are quantum-mechanical particles and don't really exist "in the middle" or "on the surface" of a nucleus -- at least, not for long. On the other hand, you can calculate the probability of an alpha particle escaping a large nucleus using quantum-mechanical statistics.

2) Even small nuclei with too many neutrons can undergo beta decay. Beta particles are just electrons. They result from the transmutation of a neutron into a proton. Free neutrons are unstable, and undergo beta decay with a half-life of 15 minutes. Neutrons in a nucleus can survive indefinitely, if the neutron:proton ratio is within a certain stability band. If there are too many neutrons, however, one can decay, turning into a proton and releasing an electron.

3) Gamma emission can result when an excited nucleus (large nuclei can have many modes of excitation) transfers energy to an inner-shell electron. The electron is ejected with very high energy from the shell, and can radiate several times before returning to the atom (or another nearby atom).

- Warren

 

[jimmy p]

OK, thanks but what is are the causes of this decay?

 

[ranyart]

Originally posted by jimmy p
What properties of nuclei make them radioactive? I know it is mainly to do with stability of the nucleus, but are there any other reasons? Why are elements with smaller nuclei less radioactive than elements with larger nuclei?

One can take a viewpoint that the smaller condensed nucleus has less degree's of freedom. The basis of all nucleus is the Proton, a single fragment of Three-dimensional space. The bigger the nuclei the more space can allow collisions and degree's of freedom for movements.

A single Proton moves in relation to its self, just with its inner vibrational signal, lots of Protons and NEUTRONS rattle and shake with regards to each other. If one holds a single marble firmly in your hand, shake it around what do you hear? nothing its stable.

Now put a number of marbles into a your hand, shake hand? you can hear them rattle.

The vibrational contact is enough for energy to be passed back and forth as Photons..electrons throw their momentum into the surrounding space.

 

[jimmy p]

Oh cool, that's really great. And i like the analogy with marbles! makes a lot of sense now. Thanx to both of you guys!