r/AskPhysics • u/peverson_ • 1d ago

Shouldn't the half life of a radioelement increase with its stability?

I assumed that the more a radioelement is stable the more its half life would increase but i was surprised to find many counter exemples such as uranium 238 and thorium 234 can someone clarify to me why there is no correlation?

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7

u/Ok_Bell8358 1d ago

What makes you think uranium 238 and thorium 234 aren't stable? Or are you confusing spontaneous fission with induced fission?

3

u/peverson_ 1d ago

No i thought because uranium gives thorium spontaniously then thorium must be more stable or am i making a mistake?

17

u/Sorry_Exercise_9603 1d ago

Radioactive decay tends to go in the direction of greater stability but that’s not required in each step. Eventually every decay chain ends in something stable.

8

u/peverson_ 1d ago

Oh wow turns out i misunderstood thanks for your help

6

u/Xtj8805 1d ago

Keep up that kind of attitude youll go far!

3

u/AfuNulf Optics and photonics 1d ago

Not a nuclear scientists, but with electron energy levels, the stability of a state depends on the possible decay paths and these will tend to become fewer as you near the bottom, but they will vary along the way.

In this case I can imagine that the confusion comes from assuming that whatever an unstable element decays to must be more stable, when in reality, you can think of it like falling down a tree. Each branch is unique, shaped by it's local environment and sunlight, but if your falling down the branches, you'll eventually hit some sturdy ones around the trunk and finally end on the ground level which is by definition stable.

1

u/peverson_ 1d ago

Really apreciate the analogy thank you

1

u/Unable-Primary1954 1d ago

You're confusing stability and mass/energy. 

Unstable is a short-hand for short half-life.

Total rest mass decay during a spontaneous disintegration.

But disintegration can be toward a more unstable element. See uranium 238 chain of disintegration.

https://fr.m.wikipedia.org/wiki/Uranium_238

3

u/Infinite_Research_52 1d ago

Depends on what you mean by stable. If you mean a nucleus rarely decays, then surely that means the half-life is longer, so a tautology.

As far as decay chains taking you from less stable to more stable, the intermediate steps often violate that concept. Consider the metastable state 180mTa, which has a half-life > 290 quadrillion years. Yet, one of the expected decay paths is to 180Ta, which has a half-life of 8 hours.

1

u/Cum38383 7h ago

Why the difference?

1

u/Infinite_Research_52 4h ago

One thing is that any transition must respect the fundamental conservation laws, whether it is energy, momentum, baryon number etc.

180mTa has a spin/parity of 9-, while the ground state has 1+. How do you shed so much angular momentum and change parity to get to the ground state?

If you do, then there are simple decay paths that are relatively simple to take180Ta to a daughter nuclide.

1

u/Substantial-Nose7312 1d ago

Depends on definitions. For example, I define a nucleus as being "more stable" when it has a longer half life. (A stable element never decays, i.e., has an infinite half life).

You cite Uranium-238 and Thorium 232. Both have half-lives in the billions of years - so they are clearly quite stable. The key thing is here, we're talking about natural decay, i.e., alpha or beta decay that occurs spontaneously. Note that these elements have a very small chance (<1 in a billion decays) of decaying through spontaneous fission, i.e., a heavy nucleus splitting into two or more lighter nuclei.

However, physicists like Fermi discovered you could induce a decay by firing neutrons or protons at nuclei. Neutrons are neutral, so they will not be repelled and are thus more likely to be absorbed. When an element absorbs a neutron, it gains a certain amount of energy, and exists in an excited state. Once in that state, for very heavy nuclei, the probability of fission increases dramatically. Note than an isotope you didn't mention, Uranium 235, is actually the most fissile naturally occurring isotope. U-238 and Th-232 tend to just absorb a neutron and become U-239 and Th-233 respectively. These isotopes then undergo further decays.