# How do I determine the half life of an alpha particle?

1. Mar 5, 2013

### Galactonaut

I am fairly new to nuclear chemistry and I was just curious as how to find the half life of a helium nucleus. Would it be the same as finding the half life of 1 neutron x2 and 1 proton x2? Or is this the completely wrong way to do this. Also would the half life be the same as a helium atom or completely different? This isn't for any homework assignment, I am just curious.

2. Mar 5, 2013

### mpv_plate

Helium nucleus is stable, also helium atom is stable. Stability of atom is given by stability of its nucleus.

Standalone neutron half life is 15 minutes and it is not related to half life of various nuclei. There are effects in nucleus which "stabilize" the neutron.

3. Mar 5, 2013

### snorkack

Not completely. There are many unstable atoms with a stable nucleus, starting with beryllium 7, and a few stable atoms with unstable nucleus.
It is not unrelated, but the half-lives of various nuclei are not given by neutron lifetime. Rather, neutron is one nucleus of many, and the rules giving nucleus lifetime apply to free neutrons among others.

4. Mar 5, 2013

### Bill_K

I can't think of one, could you give an example?

5. Mar 5, 2013

### Staff: Mentor

All nuclei where electron capture is the only possible decay process, but I think those are quite rare.
I am interested in an example here, too.

The helium 4 nucleus is stable if the proton is stable. If it is not, the individual nucleons in the nucleus can decay, and the nucleus has a lifetime comparable to the lifetime of protons (more than 10^30 years). The electrons are not relevant here.

6. Mar 5, 2013

### snorkack

Dysprosium 163.

Not sure whether others exist or whether Dy-163 is the only one.

7. Mar 6, 2013

### ZapperZ

Staff Emeritus
One isn't "many".

What makes Dy163 an "unstable atom with a stable nucleus"? What is the characteristic of such a thing?

Zz.

8. Mar 6, 2013

### snorkack

No, but "a few" includes one.
Vice versa. It is a "stable atom with unstable nucleus".

Beryllium 7 is an "unstable atom with stable nucleus". Its nucleus is stable because it has no way to decay in the absence of available electrons. The atom is unstable because if an electron is around the nucleus, it is captured.

9. Mar 6, 2013

### Staff: Mentor

Nice, I thought of that option, but I did not know that such a nucleus exists.

They $\beta^-$ decay energy is not sufficient to produce a free electron. If the inner shells are free, on the other hand, the nucleus can do a $\beta^-$ decay and produce a bound electron. Therefore, the atom is stable, but the nucleus (without electrons) is not.

10. Mar 6, 2013