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The safety of radioactivity in Medicine

  • #1
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Homework Statement:

the isotope 99Tc decays to another isotope
Explain why this decay of 99Tc will not cause much damage in the body?

Relevant Equations:

Knowledge of Radioactive decays
The question also considered 99m-Tc which has a 6h of half-life. And 99Tc has about 2.2E5 years. My argue was, “the half life of 99Tc is greater than the lifespan of a person. So ionization energy that is released has minimal effect to the person compared to Tc-99m which decays (and releases energy) every 6 hours.

Is this statement correct and enough?
 

Answers and Replies

  • #2
scottdave
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The question also considered 99m-Tc which has a 6h of half-life. And 99Tc has about 2.2E5 years. My argue was, “the half life of 99Tc is greater than the lifespan of a person. So ionization energy that is released has minimal effect to the person compared to Tc-99m which decays (and releases energy) every 6 hours.

Is this statement correct and enough?
Which types of decay cause tissue damage? I think this should be addressed in your answer. Also, what does it decay to? Does that decay further?
 
  • #3
gleem
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My argue was, “the half life of 99Tc is greater than the lifespan of a person. So ionization energy that is released has minimal effect to the person compared to Tc-99m which decays (and releases energy) every 6 hours.
You should also elaborate on why this is so.
 
  • #4
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You should also elaborate on why this is so.
That’s is what I am not sure about. The only way I can think is because Tc99m decays many times thus releases more energy. But I am not sure. When I get a similar question where they ask it between two elements with (or isotopes) how can I evaluate that?
 
  • #5
gleem
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Problem Statement
the isotope 99Tc decays to another isotope
Explain why this decay of 99Tc will not cause much damage in the body?
Is this the actual statement of the problem or is it your version? It should have read , The nuclear isomer Tc-99m decay to the ground state which is also unstable. Why does the Tc99 decay not cause much damage.

The half life of Tc99 decay being greater than that of Tc99m by itself does not prove anything.

The half life of an isotope compared to its daughter is only part of the explanation. The dose (producing the damage) a person receives from an internal radioisotope depends directly on the half life to be sure as well as the amount of energy deposited in the body from each decay.

Dose = 1.44 τ½D0

where D0 is the initial dose rate.

The initial dose rate depends depends on the type and energy of the radiation emitted and how much of that radiation is absorbed. For example a portion of high energy gamma rays escape the body while most beta particles do not depositing all their energy within the body. Another factor that contributes to the dose is determined by the isotopes biological half life how long it takes the body to excrete one half of the the isotope.

Actual magnitudes of half-lives and decay modes must be evaluated.
 
  • #6
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Okay it makes sense now. The question actually has a long paragraph and a couple more questions but briefly it is:
“Technetium is a substance that reach chemically about the same as calcium (of which bones consist mainly), but is radioactive. Tc is used in the excited state, Tc-99m. This excited state decays to the ground state Tc99 while emitting a gamma radiation.”
~~~~
“Technetium-99m emits gamma radiation and has a half-life of 6.0 hours. In this process, technetium-99m decays to the normal technetium-99”
After this last line,
A) give the reaction equation for this decay
B) explain why this decay of Tc99 will not cause much damage in the body

For A my answer was correct : beta-1 decay = 43Tc-99 -> 44Ru-99 + -1 beta
For B, since there is a beta decay shouldn’t there be a greater damage on the body? Like you said because beta decay releases all its energy to the body and the first statement already says that Tc99 is radioactive to the bones. How am I supposed to explain that it does no damage?
 
  • #7
gleem
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You correctly identified half life as an important factor. Why is this important in this case?
 
  • #8
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You correctly identified half life as an important factor. Why is this important in this case?
In that case indicating the half life is enough. Thanks!
 
  • #9
gleem
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In that case indicating the half life is enough. Thanks!
And noting that the half life of Tc99 is 3 billion time greater than Tc99m
 
  • #10
SammyS
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Okay it makes sense now. The question actually has a long paragraph and a couple more questions but briefly it is:
...
~~~~
“Technetium-99m emits gamma radiation and has a half-life of 6.0 hours. In this process, technetium-99m decays to the normal technetium-99”
After this last line,
A) give the reaction equation for this decay
B) explain why this decay of Tc99 will not cause much damage in the body

For A my answer was correct : beta-1 decay = 43Tc-99 -> 44Ru-99 + -1 beta
For B, since there is a beta decay shouldn’t there be a greater damage on the body? Like you said because beta decay releases all its energy to the body and the first statement already says that Tc99 is radioactive to the bones. How am I supposed to explain that it does no damage?
For Tc-99m to Tc99:
What is the energy of the emitted gamma rays? Compare with diagnostic X-rays.
Is there any other decay mode? - a mode other than the one producing a gamma?

For Tc-99 to Ru-99:
What is the energy of an emitted beta? How harmful?

What is the ratio of the number Tc-99m to Tc99 decays to the number of Tc-99 to Ru-99 decays over some fixed time interval - such as 1 hour?

Another issue:
What is the biological half-life of the compound in which the Tc-99m is delivered to the patient?
 
Last edited:
  • #11
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For Tc-99m to Tc99:
What is the energy of the emitted gamma rays? Compare with diagnostic X-rays.
Is there any other decay mode? - a mode other than the one producing a gamma?

For Tc-99 to Ru-99:
What is the energy of an emitted beta? How harmful?

What is the ratio of the number Tc-99m to Tc99 decays to the number of Tc-99 to Ru-99 decays over some fixed time interval - such as 1 hour?
Well Tc99 decays to another isotope by releasing a beta particle. Now if we only consider this decay ( and ignore the tc99m part which I wanted to compare to make sense in my head) the only appropriate answer should be the half life right.?
Beta decay is known to be harmful when it penetrates to the body. by considering that this decay happens 220000 years later, so, the significant release in energy doesn’t affect the body because the person wouldn’t be alive to be affected.

When I also think Tc-99m decay, gamma ray has a high penetration so it doesn’t cause a relatively significant damage to the body. Yes it’s halflife is about 6 hours but the gamma ray is just passes as if there were nothing.

This is my argument, would that be right? If we think theoretically. Because I don’t have the values given to find the activity or the number of particles and since it says explain...
 
  • #12
gleem
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To make things even clearer Tc-99m and Tc-99 have the same biological half life, about 1 day. Tc99m gives have its dose to the patient in 6 hrs while Tc-99 can give half its dose to the patients body in 211,000 year if it remained in the patient that long. in about 10 day there is virtually no Tc99 of any type left.
 

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