# Distance covered when 22 MeV gamma rays travel through air

## Summary:

How long can 22 MeV gamma rays travel through air before they turns harmless
Both alpha and beta radiations can only travel short distances through air as they're not as penetrating as gamma radiations. How long gamma radiations with 22 MeV energy can travel in air? Is it meters, kilometers, miles, etc.

Is there a difference between a lower energy gamma ray and a higher energy gamma ray when it comes to the maximum distance covered in air?

## Answers and Replies

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mfb
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Alpha and beta lose their energy in many individual interactions, while photons will keep traveling until they convert to something else in a single interaction, so gamma radiation doesn't have a clear range like the other two. The intensity decays exponentially with the radiation length as constant (about 300 meters in air at sea level density) - and typically it also goes down with the inverse distance squared because it spreads out.
That means it can travel over a kilometer. This doesn't depend much on the energy, as long as it is significantly over the threshold for pair production (1 MeV).

Secondary radiation might be a concern: The photon will most likely produce a high energy electron and positron pair, both of them will typically lead to more photons via bremsstrahlung, and they travel through the air again.

mfb
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Pair production grows slower than I expected in air, looks like 22 MeV is about the place where both effects are equally important (graph).
I'm more used to photons at the very right edge of that plot and beyond.

Alpha and beta lose their energy in many individual interactions, while photons will keep traveling until they convert to something else in a single interaction, so gamma radiation doesn't have a clear range like the other two. The intensity decays exponentially with the radiation length as constant (about 300 meters in air at sea level density) - and typically it also goes down with the inverse distance squared because it spreads out.
That means it can travel over a kilometer. This doesn't depend much on the energy, as long as it is significantly over the threshold for pair production (1 MeV).

Secondary radiation might be a concern: The photon will most likely produce a high energy electron and positron pair, both of them will typically lead to more photons via bremsstrahlung, and they travel through the air again.
Thank you, does the same applies to 22 MeV bremsstrahlung too? If 22 MeV gamma rays produced by pair production travel over a max distance of over a kilometer, what is the max distance that 22 MeV gamma rays generated by bremsstrahlung can travel?

mfb
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We were discussing the processes that stop the photon (or convert it to something else).

The origin of the photon is irrelevant. A 22 MeV photon is a 22 MeV photon.

There is no maximal distance, as discussed before.

wonderingchicken
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@wonderingchicken , this, like many of your posts, is oddly specific. 22? Not 20 or 25?

It would probably help you get better answers if you told us what this is all about.

@wonderingchicken , this, like many of your posts, is oddly specific. 22? Not 20 or 25?

It would probably help you get better answers if you told us what this is all about.
If you see here in this link HERMES III especially in pg. 39-40, after reading all the responses, that means 22 MeV bremsstrahlung rays can actually go for longer distances around several meters more if there are no shieldings. In order to be safe from radiation, I used to think that shielding isn't necessary and we just only have to go farther from the source, and I thought the 22 MeV bremsstrahlung in the link I give to you only travel to several feet long without shieldings prior asking the question. Gamma radiations are indeed scary. I underestimated about how long the distance radiation can travel.

Anyway, I actually don't have any background in engineering and even physics. I just thought radiation only travel to short distances such as around 100-200 feet long before it turns harmless.

Vanadium 50
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Again, why this oddly specific focus on Hermes-III?

Again, why this oddly specific focus on Hermes-III?
Because HERMES III is the most powerful gamma simulator that I familiar of. If we can simply move farther from HERMES III in order to be safe from the radiation that it produced, that means we are safe from other gamma simulators (most gamma ray simulators are less powerful than HERMES III) and also nuclear explosions in hypothetical situations.

mfb
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Every stronger source of gamma rays has shielding for that reason. Usually concrete, steel, lead, or a combination of these. The range in air is irrelevant, you are safe if you are behind the shielding.

Distance in air still lowers the dose simply from the radiation spreading out over a larger area.

wonderingchicken
Every stronger source of gamma rays has shielding for that reason. Usually concrete, steel, lead, or a combination of these. The range in air is irrelevant, you are safe if you are behind the shielding.

Distance in air still lowers the dose simply from the radiation spreading out over a larger area.
According to Three principles for radiation safety, the greater the distance we are from the source the less the intensity of the radiation. So, in a hypothetical scenario, people in open spaces where there is no shielding can simply go farther from the source. But I agree with you that shielding is still necessary due to secondary radiation, like you said previously.

mfb
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That's what I mentioned in the second paragraph, but the intensity only drops with the inverse distance squared. Go to 10 times the distance and the dose you get goes down by a factor 100. Go to 100 times the distance and it goes down by another factor 100. That's sufficient in some but not all cases, and at some point larger distances become impractical.

That's sufficient in some but not all cases, and at some point larger distances become impractical.
Let me try to interpret your statement because I don't really get that as English isn't my first language. What render larger distances impractical are due to secondary radiations and fallout contamination. Isn't it? Because if I'm gonna say larger distances are impractical because photons, including gamma rays, can travel over virtually infinite distance, that is only true if we are talking about propagation of photons through vacuum. Is that correct?

mfb
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It's simply a matter of practicality. You want to run some experiments with gamma rays? With shielding you can sit a few meters away from your experiment. If needed you switch off the source, walk around the shielding, adjust something in your experiment, go back to your desk, and switch the source on again.
Without shielding you would need a car every time you do that.

For accelerator complexes you are typically not the only one using it. With shielding you can stop the radiation from getting into your experiment while the accelerator keeps running nearby. Without shielding that wouldn't work.
Imagine people would have to shut down the whole CERN accelerator complex whenever someone wants to exchange a broken cable somewhere. Completely impractical.

wonderingchicken
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fallout contamination
Is not a problem unless someone is detonating nuclear weapons near you. (If so, please ask them to stop)

wonderingchicken
Air does directly attenuate gamma rays... over range of hundreds of m.

This means that with air having scale height of 8 km, sea level ground is safe from cosmic gamma rays.
At a distance of a km or two from a weak nuclear detonation, like 15-20 kT of Hiroshima and Nagasaki, prompt gammas are a real issue, even compared to thermal radiation and impact. In case of bigger detonations, in MT range, visible radiation is transmitted much better than gamma rays and if you are far enough not to be burnt by light (several km), you also are safe from gamma rays.

wonderingchicken
Air does directly attenuate gamma rays... over range of hundreds of m.

This means that with air having scale height of 8 km, sea level ground is safe from cosmic gamma rays.
At a distance of a km or two from a weak nuclear detonation, like 15-20 kT of Hiroshima and Nagasaki, prompt gammas are a real issue, even compared to thermal radiation and impact. In case of bigger detonations, in MT range, visible radiation is transmitted much better than gamma rays and if you are far enough not to be burnt by light (several km), you also are safe from gamma rays.
The distance of how long gamma rays travelled, especially those from nuclear explosions, depend on the size of the fireball and the energy. Just saw a comparison between the destruction radius of each nuclear explosions throughout history, with the gamma rays from both Hiroshima and Nagasaki explosions were around 1.2-1.3 kilometers (0.7-0.8 mile) while Tsar Bomba which is the most powerful nuclear explosion so far were 3.1 kilometers (1.9 mile).

Thus, I think gamma rays produced by nuclear explosions, although only at around 7 MeV (I believe this is for the fission bombs only) which is lower than 22 MeV by the pulsed power accelerator HERMES III, travels over longer distance than the 22 MeV gamma rays produced by HERMES III. Is this correct?

mfb
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As noted (repeatedly), there is no sharp end to the range. If you start with a stronger source then of course you'll reach the same arbitrary intensity threshold at a larger distance.
while Tsar Bomba which is the most powerful nuclear explosion so far were 3.1 kilometers
Maybe, but the thermal radiation gives you third degree burns in a radius of 60 km.
3.1 km is still within the fireball radius of 4.5 km.

wonderingchicken
As noted (repeatedly), there is no sharp end to the range. If you start with a stronger source then of course you'll reach the same arbitrary intensity threshold at a larger distance.

Maybe, but the thermal radiation gives you third degree burns in a radius of 60 km.
3.1 km is still within the fireball radius of 4.5 km.
I'm sorry for keep forgetting that there is no sharp end to the range. If we're talking about which is the stronger source, do we have to see the electronvolt or kiloton/megaton, etc.? Because sometime I thought the HERMES III which generate 22 MeV gamma radiations is the stronger source but then I thought nuclear bombs are the stronger sources because of their destructive power around kiloton-megaton ranges.

According to this online application NUKEMAP by Alex Wellerstein, the gamma radiations, I believe also along with other radiations produced by the explosion, are confined inside the fireball. I'm still not sure if that's hundred percent accurate.

mfb
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If we're talking about which is the stronger source, do we have to see the electronvolt or kiloton/megaton, etc.?
Both are relevant. "Stronger" generally means more photons (or more photons per time) in the same energy range.
Comparing sources with different energy spectrum can be complicated, but in this case it's easy: The nuclear weapon produces way more gamma rays.

For the highest photon energies (man-made) you need to go to the LHC, by the way. We produce photons above 2 TeV. Not many, but they are produced.
According to this online application NUKEMAP by Alex Wellerstein, the gamma radiations, I believe also along with other radiations produced by the explosion, are confined inside the fireball. I'm still not sure if that's hundred percent accurate.
It is accurate for big weapons as the fireball keeps growing a lot while the gamma rays still have their exponential decay.

wonderingchicken
The distance of how long gamma rays travelled, especially those from nuclear explosions, depend on the size of the fireball and the energy. Just saw a comparison between the destruction radius of each nuclear explosions throughout history, with the gamma rays from both Hiroshima and Nagasaki explosions were around 1.2-1.3 kilometers (0.7-0.8 mile) while Tsar Bomba which is the most powerful nuclear explosion so far were 3.1 kilometers (1.9 mile).
See, the comparison:
Czar Bomba at 50 MT was about 3300 times stronger than Hiroshima bomb at 15 kT.
There was some spreading - between 1,2 to 3,1 km, there would be about 6,7 times spreading.
But the rest, 500 times, was attenuation in 1900 m of air. Which means that gamma rays were halved in about 210 m of air.
Czar Bomba simply produced so much gamma rays that after 9 halvings (and first 6 halvings in the first 1200 m), the remaining fraction (1/30 000 of original) was still dangerous.
Hermes does not.

Assuming 200 m for halving in air still applies for Hermes, then 20 m from Hermes has 100 times less radiation than 2 m away (negligible attenuation, and 100x spreading). 200 m way has already 200 times less radiation than 20 m away (again 100x spreading, but now 2x attenuation also).

wonderingchicken
See, the comparison:
Czar Bomba at 50 MT was about 3300 times stronger than Hiroshima bomb at 15 kT.
There was some spreading - between 1,2 to 3,1 km, there would be about 6,7 times spreading.
But the rest, 500 times, was attenuation in 1900 m of air. Which means that gamma rays were halved in about 210 m of air.
Czar Bomba simply produced so much gamma rays that after 9 halvings (and first 6 halvings in the first 1200 m), the remaining fraction (1/30 000 of original) was still dangerous.
Hermes does not.

Assuming 200 m for halving in air still applies for Hermes, then 20 m from Hermes has 100 times less radiation than 2 m away (negligible attenuation, and 100x spreading). 200 m way has already 200 times less radiation than 20 m away (again 100x spreading, but now 2x attenuation also).
Interesting. I'm really sorry for being too curious and asking too much questions, but is that after considering the parameters of HERMES III (remember HERMES III is not a nuclear weapon but a pulsed-power linear accelerator that is used as a gamma radiation simulator at Sandia National Laboratories) which are the following :

Peak diode voltage 22 MeV,
peak diode current 700 kA,
total beam energy 370 kj,
power pulsewidth (FWHM) at 27 nanoseconds.

Dose and dose rate characteristics
Peak dose : at >100 kRads (Si),
peak dose rate : at >5 x 10^12 Rads (Si),
Risetime (10-90) : 12 nsec
Pulsewidth : 20 nsec (FWHM)

Exposure area : we are talking about how far gamma rays travel in air which means outdoor, the dose map of HERMES III in outdoor mode is in pg. 40 in the first link below. The outdoor test cell measures 66 feet (20 m) wide and 95 feet (28 meter) long.

Source : https://apps.dtic.mil/dtic/tr/fulltext/u2/a351472.pdf

https://www.osti.gov/biblio/6233581...-simulation-technology-laboratory-guide-users

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