Lack of stability after radiation

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    Radiation Stability
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Discussion Overview

The discussion revolves around the stability of uranium and thorium isotopes following radioactive decay, specifically focusing on the mechanisms and implications of alpha decay versus beta decay. Participants explore the neutron-to-proton ratio and its influence on stability, as well as the nature of decay chains in radioactive elements.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation

Main Points Raised

  • One participant notes that uranium (U) has a neutron-to-proton (n/p) ratio of 1.59, suggesting it is radioactive and emits alpha rays to gain stability, but questions why this leads to thorium (Th) with a higher n/p ratio of 1.6, implying more instability.
  • Another participant clarifies that the decay process does not stop at thorium, as Th-234 is also radioactive and continues to decay until reaching lead-206, the stable end product.
  • Several participants express confusion about why uranium emits alpha rays if it leads to greater instability, suggesting that beta decay might be a more effective means of achieving stability.
  • One participant explains the differences between alpha, beta, and gamma emissions, noting that alpha decay reduces both protons and neutrons, while beta decay alters the proton count without changing the atomic weight.
  • Another participant argues that uranium's decay chain exists due to its instability, and outlines reasons for atomic decay, including excessive neutron count, insufficient neutron count, or an oversized nucleus.
  • It is mentioned that the mode of decay is determined by the specific cause of instability in the nucleus, with large nuclei like uranium typically undergoing alpha decay.

Areas of Agreement / Disagreement

Participants express differing views on the implications of alpha decay and the stability of thorium compared to uranium. There is no consensus on why uranium specifically emits alpha rays instead of beta rays, and the discussion remains unresolved regarding the optimal decay pathway for achieving stability.

Contextual Notes

Some participants' arguments rely on assumptions about the decay process and the definitions of stability and instability, which may not be universally agreed upon. The discussion also touches on the complexities of decay chains and the conditions under which different types of decay occur.

americast
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U = Th + alpha rays
92p 90p
146n 144n

I have heard that if n/p ratio exceeds 1.56, the substance becomes radioactive. Now, for Uranium, n/p ratio is 1.59. It gives out alpha rays in order to gain stability and in turn forms Thorium. n/p of thorium is 1.6. The ratio increases. This means Thorium is more unstable than Uranium. Then why on Earth does Uranium give off alpha rays? It only leads to more instability.

Thanx in advance...
 
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You are assuming the process stops when U turns into Th. It does not. Th-234 is itself radioactive with a very short half-life of about 24 days. The decay process continues, producing various other radioactive elements, until the final element, lead, is reached. Lead-206 is the end product of the decay of U-238.

For more information, see the Radium-series (or Uranium-series) for the complete decay chain in the following article:

http://en.wikipedia.org/wiki/Decay_chains
 
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SteamKing said:
You are assuming the process stops when U turns into Th. It does not. Th-234 is itself radioactive with a very short half-life of about 24 days. The decay process continues, producing various other radioactive elements, until the final element, lead, is reached. Lead-206 is the end product of the decay of U-238.

For more information, see the Radium-series (or Uranium-series) for the complete decay chain in the following article:

http://en.wikipedia.org/wiki/Decay_chains

Thanx you @SteamKing. My concept is clear now. But one silly question I would like to ask you, why are alpha rays given out at all when it leads to more instability? I mean, if, in a nucleus, n decreases by 2 and p decreases by 2, n/p ratio will always increase. Therefore it makes no sense to give off alpha rays. Some other rays would have been better ;P

Thanx again...
 
There are three common types of radioactive decay: alpha emission, beta emission, and gamma emission.

Gamma emission does not alter the number of protons or neutrons in the nucleus, so the chemical properties of the element are unaffected.

Alpha emission reduces the number of protons by 2 and the atomic weight by 4, therefore the element undergoing decay changes chemical properties.

Beta emission increases the number of protons by 1 and reduces the number of neutrons by 1, thus leaving the atomic weight unchanged. The element changes chemical properties also due to this radiation.

Like I said earlier, radioactive elements go through a series of different changes until they reach atomic stability. It can't be done (usually) in one fell swoop.

For more on radioactive decay:

http://en.wikipedia.org/wiki/Radioactive_decay
 
Thanx again...
I understand what you say. But I still wonder why uranium gives off alpha rays. I mean, these alpha rays will only lead to more instability. So why not give off beta rays nstead...? What makes it so necessary for uranium to give off alpha ray? Would not beta rays be better? It would have at least reduced the ratio, isn't it?

Thanx...
 
Like I explained, if U-238 gave off beta rays, the atomic number goes up by 1, so U-238 would become Neptunium-238 (half-life about 2 days). A further hypothetical beta emission would produce plutonium-238 (half-life about 88 years).

The naturally occurring elements all have atomic numbers of 92 and below. As of the present, no stable elements with atomic numbers greater than 92 are known to exist. Some isotopes of elements with atomic numbers > 92 have relatively long half-lives, but eventually all undergo radioactive decay such that the atomic number declines and they wind up as lighter, more stable elements. There are several other decay series (like the radium series) which lay out this decay progression.

As to why a given nucleus prefers a certain type of emission, that's beyond my level of knowledge.
 
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SteamKing said:
Like I explained, if U-238 gave off beta rays, the atomic number goes up by 1, so U-238 would become Neptunium-238 (half-life about 2 days). A further hypothetical beta emission would produce plutonium-238 (half-life about 88 years).

The naturally occurring elements all have atomic numbers of 92 and below. As of the present, no stable elements with atomic numbers greater than 92 are known to exist. Some isotopes of elements with atomic numbers > 92 have relatively long half-lives, but eventually all undergo radioactive decay such that the atomic number declines and they wind up as lighter, more stable elements. There are several other decay series (like the radium series) which lay out this decay progression.

As to why a given nucleus prefers a certain type of emission, that's beyond my level of knowledge.
@SteamKing, thanks a lot...!
This was exactly what I needed.
Thank you everyone at PF...
Thanx again...
 
Atoms have no "plan" like "I have to go to stable isotopes". If a decay is possible in terms of energy, it will happen after a while. If multiple decays are possible (usually alpha and beta for some heavy nuclei, sometimes fission), they will all occur with some relative frequencies.
The decay rates for beta decays are tricky to evaluate, for alpha decays there is a strong correlation between decay energy and lifetime. The more energy the decay releases, the shorter the lifetime (with some exceptions).
 
You are assuming the process stops when U turns into Th. It does not. Th-234 is itself radioactive with a very short half-life of about 24 days. The decay process continues, producing various other radioactive elements, until the final element, lead, is reached. Lead-206 is the end product of the decay of U-238.

For more information, see the Radium-series (or Uranium-series) for the complete decay chain in the following article:

http://en.wikipedia.org/wiki/Decay_chains

It is true that decay chains exists, but your argument is backwards and wrong. Uranium does not decay into thorium because the chain exists, instead the decay chain exists because uranium is unstable and it decays into thorium which is also unstable and decays into another unstable isotope. The series of decay will continue until you reach a stable isotope.

There are basically 3 reasons why atoms decay. The neutron to proton ration is too high for stability (they have too many neutrons), the neutron to proton ration is too low (they have too few neutrons), or they have too many nucleons in general (the nucleus it too big).

The mode of decay is determined by the cause of instability. Nuclei that have too many neutron typically under go beta minus decay. Here a neutron decays into an electron and a proton (the is also an electron anti-neutrino). This decrease the neutron to proton ration. Nuclei that have too few neutron typically under go beta plus decay. Here a proton decays into an positron and a neutron (there is also an electron neutrino) increasing the neutron to proton ratio. Finally large nuclei decay by alpha decay which decreases their size.

Uranium enters this third case. Its nucleus is too big for stability. Thus it will undergo alpha decay.
 
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the_wolfman said:
It is true that decay chains exists, but your argument is backwards and wrong. Uranium does not decay into thorium because the chain exists, instead the decay chain exists because uranium is unstable and it decays into thorium which is also unstable and decays into another unstable isotope. The series of decay will continue until you reach a stable isotope.

There are basically 3 reasons why atoms decay. The neutron to proton ration is too high for stability (they have too many neutrons), the neutron to proton ration is too low (they have too few neutrons), or they have too many nucleons in general (the nucleus it too big).

The mode of decay is determined by the cause of instability. Nuclei that have too many neutron typically under go beta minus decay. Here a neutron decays into an electron and a proton (the is also an electron anti-neutrino). This decrease the neutron to proton ration. Nuclei that have too few neutron typically under go beta plus decay. Here a proton decays into an positron and a neutron (there is also an electron neutrino) increasing the neutron to proton ratio. Finally large nuclei decay by alpha decay which decreases their size.

Uranium enters this third case. Its nucleus is too big for stability. Thus it will undergo alpha decay.

One possible reason of radiation of alpha rays, as many people are saying, is that the He++ ion thus released is very stable. Thus, at least one stable compund is formed out of the unstable radioactive element.

Thanx...
 
  • #11
americast said:
One possible reason of radiation of alpha rays, as many people are saying, is that the He++ ion thus released is very stable. Thus, at least one stable compund is formed out of the unstable radioactive element.

Thanx...
The alpha particle is tightly bound, certainly, but it is not the reason that it is released. Until Pb or Bi are reached, the much larger nucleus following alpha decay also decays, some by alpha, some by beta decay, and some by either.

http://hyperphysics.phy-astr.gsu.edu/hbase/nuclear/radser.html

For some heavier transuranic nuclei, spontaneous fission is also a possibility, and most fission products decay in a decay chain, some including neutron emission, which is the source of delayed neutrons in nuclear reactors.
 
  • #12
Yes, but it is very rare for any nucleus to release the also tightly bound nuclei like carbon 12 or oxygen 16.
 

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