# Half lives

1. Feb 16, 2008

### Denton

Ive been thinking about this for a while, what is the exact mechanism for radioactivity that results in there being a half life.

Say you have a single radioactive isotope of Uranium, this particle would ultimately emit an alpha particle or some other form of radiation and thereby quickly returning to a stable element. However this does not happen, we have huge half lives for uranium which I presumed was because when densely packed enough, the radiation emmited by one would then increase another and therefore it would take a long time for it to spread to the outside.

But if this were the case, we could just spread out nuclear material over a very large surface area and reduce its half life significantly. But this is incorrect by what ive heard that you can't reduce or change half lives.

can anyone fill me in on what im missing?

2. Feb 16, 2008

### malawi_glenn

The "exact" mechanism for radioactive nucleis is that they decay randomly. You cant say when a certain nuclei will decay, but if you have a large sample (of the order 10^6 and bigger), you can empirrically measure the half life, the time when half of the sample has dissapeared, and relate that to the decay constant $$\lambda$$, which is the AVERAGE probablilty that a particle decays each unit time. You can never extrapolate this to a small number of nuclei (ex. 1 nuclei), since the ultimate process is random, but on large scales we can find averages.

Like when you roll a dice, you can never predict what ONE single throw will yield. But if you roll the dice 1000times, then you can say that approx 160 will be 1, 160 will be 2 etc.
(However the thing is more complicated with a nuclei, but this example may enlighten the difference between one signle trial and a large collection of trials).

"Say you have a single radioactive isotope of Uranium, this particle would ultimately emit an alpha particle or some other form of radiation and thereby quickly returning to a stable element. However this does not happen, we have huge half lives for uranium which I presumed was because when densely packed enough, the radiation emmited by one would then increase another and therefore it would take a long time for it to spread to the outside."

The sencence "we have huge half lives for uranium which I presumed was because when densely packed enough, the radiation emmited by one would then increase another and therefore it would take a long time for it to spread to the outside."

Is wrong, the long half life of Uranium is due to its inertial protperties (size, shape, shell effects etc)

nope you cant change half lives. You can change the activity by decreasing the number of nuclei (N). ( Activity is : [itex] A = \lambda N [/tex] ). And by spreading a sample over a larger volume, the intensity (number of particles emitted per area) is decreasing, but the half life dont change (if you still have a "large" number of nucleis per unit area of course), since you have less radioactive particles om each area, and then you get less emitted particle per area too of course.

3. Feb 16, 2008

### ChrisLeslie

Therefore, uranium is not particulary radioactive by itself. A large mass of depleted U is required to produce a significant amount of emissions but practically no alpha particles will reach a handler unless it is vaporized and breathed in.

4. Feb 16, 2008

### Staff: Mentor

Isotopes of any element will have a range of half-lives. Some long, some short.

Here is a nice overview of the natural radioactive decay series.

Natural U is mostly U-238 (about 99.3%), with about 0.7% U-235. There will also be traces of U-234. U-238 has a half-life of ~4.5 billion years, while U-235 has a half-life of ~700 million yrs.

Another long-lived radioisotope is Th-232, which has a half-life of ~ 14 billion yrs.

The longer half-lived isotopes will survive long enough to be found in nature. The shorter the half-life, the smaller the amount found in nature.

Here is a useful resource for radionuclides - http://www.nndc.bnl.gov/chart/
Place cursor over the chart and left click on a location of interest, then click on the 1 under the Zoom (top right) for details of a nuclide and its neighbors.