Why is dangerous radiation only dangerous within a certain distance of the radioactive source? (more in description)
I couldn't think of how to word what I wanted to know into a single question so that was the most concise I could be. My question stemmed from an earlier post regarding the nuclear explosions set off on earth since 1945 and how they affect radiation levels. The upshot seemed to be that there was little to no adverse effects on radiation levels despite over 2000 devices being detonated... I found this really hard to accept. What I want to clarify is why/how, the surely vast amounts of radioactivity caused during these explosions is contained to only the local area of detonation, or so greatly diluted during it's spread it becomes insignificant? This radiation takes thousands of years to become harmless, does it not? Surely during this time it has chance to be carried at the very LEAST 100 miles out of the desert testing sites to the population just on the wind?
Posted Jul 30, 2012, 12:29 PM UTC
I think you are confusing a few different ideas in regards to radiation.
Intensity from a point source decreases at rate of r^2 away from the source. For this purpose think of the radioactive source as a light bulb. The further you are away, the much less radiation you will be hit with.
Radiation itself does NOT have a half life; as it runs through the world it bumps into other things and will eventually be absorbed. For this reason the initial radiation blast of a bomb will be non existant in the 65 year timescale you are looking at.
Radioactive elements DO have a half life. Here is where a balancing act exists. Elements with short half-lifes would emit lots of radiation but only for a short time; elements with long half-lifes would emit low radiation but for a long time. Although transport of these radioactive materials is a concern, they are usually very heavy elements and not as prone to mass transport as some other elements. Additionally, more recent tests have been conducted underground/underwater where they are more contained by layers of clay/earth/water.
Hope that clears some things up.
Thanks for that, I think I have may been slightly tainted by Hollywood physics! Other than the contaminated material that is thrown into the air by the blast though.. is the strength of the radiation via the light bulb effect being deteriorated by coming into contact with matter along it's path or would it still decrease at the same rate travelling through a vacuum? If so where does the lost energy go?
It spreads with the surface of a sphere - if you were in space the energy would not disappear, it would simply become weaker and more spread out the further it went.
In atmosphere, random interactions with molecules limit the distance of ionizing radiation to an area normally smaller than the fireball of the nuclear weapon itself.
It is decreased even in a vacuum. Imagine a point source shooting photons strait out in every direction. If you are standing right next to it, a very large percentage of the total output will intersect you creating a large "dose". Move back even slightly and the amount of the sphere you "cover" is significantly diminished and many photons that previously intersected with you now pass by, lowering your "dose". This very precise relationship is the one described by the equation. Having matter to interact with further reduces the dose, though different types of radiation are diminished by different amounts. Alpha radiation for example is literally the nucleus to a helium atom (kicked out of a much larger nucleus) and can only travel inches through air before collisions slow it down and it picks up electrons to become an atom of non reactive helium! Thus, radiation that occurs by alpha emission is mostly dangerous if you ingest or inhale the emitter where it then decays in close proximity to cells that can be damaged by the fast moving particle before it interacts with anything else.
Ultimately, most energy ends up as heat, which is actually how nuclear reactors work - by catching all of the radiation in shielding and using the resulting heat to run turbines.
It gets deteriorated simply because there's less of it hitting you. For an analogy:
Take a solid sphere of clay that is 1m^3 in volume (r=0.62m). This represents the initial blast of radiation. So at 0.62m distance from the centre you go through 0.62m of clay.
Now expand that same volume of clay to a radius of 2m. You now only have to go through 0.52m of clay to reach the centre.
Expand that to a radius of 10m. You now have to go through only 8millimetres of clay to reach the centre.
So the radiation is like the clay. It is released in discrete quantities (like photons are released from light sources) with each release being represented by the initial 1m^3 of clay expanding. A constant volume (if you could call it that) of radiation is released, but the farther away you are (increased r), the more area that volume has to expand to cover, so less intensity (represented by thickness in the analogy) exists at any one point.
The decrease in intensity is really 4(pi)r^2. But r^2 is about right.
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Also, the factor of 1/(4*pi) is dependent on the units. For example, in Gaussian units, you don't need it.
I really love being corrected on this sub.
You are of course more correct; I mearly meant it was proportional, sorry for the ambiguity.
Radiation decreases differently from differently shaped sources. 1/r^2 is for point sources. In real life, true point sources (high dose radiation and small size) are rare to come by. Ground contamination, spherical, and cylindrical source (all of which exist) change the drop off differently.
No, 1/r^2 is for both point, sphere, and half-sphere sources. OP's question was directed towards blast radius's which for this purpose can be modeled using this rate.
See chapter 10 of Lamarsh, Introduction to nuclear engineering.
I dont have a copy, could you summarize?
Another point that might be interesting is that a point-like source of radiation, such as an accelerating electron, can produce a significantly non-1/r^2 radiation pattern when it moves (relativistically) fast. In synchrotrons and free-electron lasers, this effect is used to produce highly intense beams of X-rays from GeV electron bunches accelerating in magnetic lattices called wigglers and undulators; the divergence of the radiation emitted is more like tens of microradians around the beam axis rather than 4pi steradians.