Can alpha radiation make other materials radioactive?

  • #1
ErikSwan
2
0
Most general sources that describe the basics of ionizing radiation assert that the only type of radiation that can directly make other materials radioactive is neutron radiation (via "neutron activation").

Googling "does radiation make things radioactive?" produces a few examples, for instance, from the Nuclear Regulatory Commission:
Of the five types of ionizing radiation discussed here, neutrons are the only one that can make objects radioactive.

Or from this "Ask the Experts" answer from the Health Physics Society:
For most radiations that people, especially laypeople, encounter, which would include x rays, gamma radiation, beta particles, and alpha particles, the exposure of the people or objects to the radiation does not produce any radioactivity within them.

[...]

There is one radiation, well-known but less common than those mentioned above, that is capable of inducing radioactivity in an irradiated person or object; that is neutron radiation. The reason that neutrons are effective in that regard and other radiations are not is because radioactivity is a property of the nucleus of an atom, and the common x, gamma, beta, and alpha radiations interact with the electrons of atoms, but not within the nuclei, whereas neutrons are able to penetrate the electron cloud around an atom and be absorbed by the nucleus, changing the nuclear configuration and, in some cases, making it unstable against radioactive decay. For example, if a person is exposed to significant neutron radiation, one of the most notable radioactive products that we would expect to be produced would be 24Na (sodium-24), produced when stable 23Na captures a neutron. Sodium is a relatively abundant element in the body and is readily activated to the 24Na product. Similarly, many other materials, especially metals, are subject to neutron activation.

However, I am reading The Making of the Atomic Bomb by Richard Rhodes and just came across the description of the experiment (p. 201) which won Irène and Frédéric Joliot-Curie the 1935 Nobel Prize in Chemistry, in which they discovered that aluminum bombarded with alpha particles produced a radioactive isotope of phosphorus (30P):
$$^{27}\text{Al} + \alpha \rightarrow ^{30}\text{P} + \text{n}$$
The 30P is unstable and decays to 30Si via positron emission with a half life of about 3 minutes.

Although a neutron is emitted here (which could conceivably go on to cause neutron activation), from the description it seems like it is the capture of the alpha particle by the aluminum which directly causes the production of the unstable (radioactive) 30P.

This seems to imply that if I had an alpha source and some aluminum foil, the aluminum foil would become somewhat naturally radioactive from exposure to the alpha source because it actually now contains a small amount of radioactive 30P.

Are the assertions made by the NRC and HPS that non-neutron radiation (specifically alpha radiation in this case) cannot induce radioactivity simply wrong? Or is there some subtlety that I'm missing?

Even the Wikipedia article on Induced radioactivity explains how it was discovered by the Joliot-Curies with their alpha particle + aluminum experiment but then goes on to explain that neutron activation is the main form of induced radioactivity, with photodisintegration (via high-energy gamma rays) as a less common form, with no mention at all of alpha-particle induced radioactivity.

So, what am I missing?

Thanks!
 
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  • #2
ErikSwan said:
Are the assertions made by the NRC and HPS that non-neutron radiation (specifically alpha radiation in this case) cannot induce radioactivity simply wrong? Or is there some subtlety that I'm missing?
I think it's more a case of the 'rule' applying 99% of the time and not bothering to worry everyone about the 1% they'll never encounter.
 
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  • #3
Aluminum isn't exactly some rare element, though... Is it the case that only a very small fraction of the aluminum atoms will capture an alpha particle, and/or that the positrons emitted from the 30P are very low energy, so the induced radioactivity is negligible?

From my understanding, exposing aluminum to an alpha source will make the aluminum at least slightly radioactive. I'm trying to understand why from a practical standpoint that doesn't seem to be of any concern, so much so that alpha-induced radioactivity is never really mentioned.
 
  • #4
That I can't answer. Perhaps only certain alpha sources cause this effect?
 
  • #5
"Radioactive" is a very broad term. The degree to which it should be concerning depends upon the details, particularly the type and strength of the the emitted radiation and the chemical and biologic activity of the resultant. The question is sort of like asking "are chemicals dangerous?" The answer is maybe (the devil is in the detail). A positron emitted inside a conductive foil is probably not a big deal but I am no expert.
But the statement that "only" neutrons can induce radioactivity is overly broad.
 
  • #6
Popularizations cut some corners, and different ones cut different corners. You are not going to get a consistent and accurate picture by stitching together what you read in different popularizations.

I did some checking, and out of ~300 or so stable nuclei, the number that make acceptable targets for this reaction (alpha in, neutron out) appears to be 3. Which is about 2 more than I would have guessed. There are probably several more, but these are themselves radioactive, so the reaction is as likely to decrease the radiation as increase it.

Furthermore, at 5 MeV (virtually al;l alphas are at 4-5 MeV) this reaction doesn't occur often - only 1% of the time or less. So this process happens for 1% of the nuclei 1% of the time - so the statement is 99.99% correct in some sense. For a popularization, that's not horrible.

It's also true that this probability falls like a stone with decreasing alpha energy, and alphas lose all their energy in a few mm of air. So if you actually wanted to do this, you would alloy your aluminum with radium or whatever which would make it far, far, far more radioactive than your aluminum daughters.

Finally, "radioactive" is a matter of degree. Look hard enough and you will find it. There is tritium everywhere. There is radon everywhere. Thorium and uranium everywhere. Carbon-14 everywhere. Need I go on?
 
  • #7
ErikSwan said:
Most general sources that describe the basics of ionizing radiation assert that the only type of radiation that can directly make other materials radioactive is neutron radiation (via "neutron activation").

Googling "does radiation make things radioactive?" produces a few examples, for instance, from the Nuclear Regulatory Commission:Or from this "Ask the Experts" answer from the Health Physics Society:However, I am reading The Making of the Atomic Bomb by Richard Rhodes and just came across the description of the experiment (p. 201) which won Irène and Frédéric Joliot-Curie the 1935 Nobel Prize in Chemistry, in which they discovered that aluminum bombarded with alpha particles produced a radioactive isotope of phosphorus (30P):
$$^{27}\text{Al} + \alpha \rightarrow ^{30}\text{P} + \text{n}$$
The 30P is unstable and decays to 30Si via positron emission with a half life of about 3 minutes.

Although a neutron is emitted here (which could conceivably go on to cause neutron activation), from the description it seems like it is the capture of the alpha particle by the aluminum which directly causes the production of the unstable (radioactive) 30P.

This seems to imply that if I had an alpha source and some aluminum foil, the aluminum foil would become somewhat naturally radioactive from exposure to the alpha source because it actually now contains a small amount of radioactive 30P.

Are the assertions made by the NRC and HPS that non-neutron radiation (specifically alpha radiation in this case) cannot induce radioactivity simply wrong? Or is there some subtlety that I'm missing?

Even the Wikipedia article on Induced radioactivity explains how it was discovered by the Joliot-Curies with their alpha particle + aluminum experiment but then goes on to explain that neutron activation is the main form of induced radioactivity, with photodisintegration (via high-energy gamma rays) as a less common form, with no mention at all of alpha-particle induced radioactivity.

So, what am I missing?

Thanks!
The (alpha,n) reactions are the biggest source of background in low background laboratories, and a hot topic of research. The cross section of the reaction is extremely low, and it is challenging to measure. See this conference slides for further details.
 

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