Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: EvaH on 22/11/2018 09:51:20
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Gaither asks:
Why are there so many different ways to measure radiation? -Rads, Rem, curie, becquerel, roentgen, coulomb/kilogram, and sievert... (are there more?)
Are they each measuring some different type of emission? ie: X-rays, gamma rays, neutrons, alpha and beta particles..... (are there more?)
And how far does each type of radiation travel? IE: which can we detect through space?
What does each tell me? Or is it just feet vs meters?
What do you think?
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The preferred SI unit of radiation dose D is the gray. 1Gy is one joule of energy absorbed per kilogram of absorber. This simple concept is actually very difficult to realise in practice - we'll come back to that later!
Different types of radiation have different effects on biological materials in vitro, so we have a derived unit of "equivalent dose" Eeq called the sievert. For 200 kV x-radiation, 1 Gy = 1 Sv by definition, and the radiation weighting factor wR varies from 1 through 2 - 15 for neutrons (depending on the neutron energy) to 20 for alpha radiation. wR is very much a best guess or consensus value for alphas and exotic particles as the experiment is difficult to perform. Eeq = D x wR
Different bodily organs have different inherent radiosensitivity (rapidly dividing cells are more sensitive) and different functional sensitivity in vivo (damage to the skin or liver is more recoverable than to the pancreas, for instance) so we assign tissue weighting factors wT to the bits that have been irradiated, to calculate an effective dose Eeff = ∑D.wRwT summed over all organ doses. wT is assigned so that ∑wT= 1 for a human, and as the w values are dimensionless, Eeff is also expressed in sieverts.
The practical problem is that Eeff of 5 sievert is the dose that will kill 50% of the population within 30 days, and it's a very small amount of energy - just enough to raise your body temperature by about 0.001°C - so we have to devise more subtle means of assessing radiation dose - more later!
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Alan, would you happen to have a link to how much the natural background radiation has gone up the last hundred years?
Used to have one but it seems gone.
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I don't have a specific reference but I'd be surprised if the natural background doserate had increased. UK average background is about 2.5 millisievert /year, of which more than half comes from radon seeping out of the ground,and over the last 20 years or so we have become very enthusiastic about removing it from cellars and tunnels. Since all the terrestrial background comes from stuff disintegrating, you'd expect it to decrease (minutely) with time.
What has increased in the last 100 years is medical radiation, up from zero to 15% or more of your lifetime dose (excluding radiotherapy, which swamps everything else) and all industrial and fallout sources, which contribute a bit less than 1%.
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Back to units and stuff.
The easy way to measure ionising radiation is by measuring the current flow in air when exposed to radiation. The first unit of measurement was the roentgen, the amount of radiation that would release one electrostatic unit (3.3x 10-10 coulomb) of charge in 1 cubic centimeter of air. Multiplying by radiation and tissue weighting factors led to the concept of the rem (roentgen equivalent man). A unit of absorbed dose based on energy was the rad, equal to 0.01 gray. These legacy units are still in use in the USA and Russia, as there are thousands of legacy instruments still working and calibrated in the old centimeter-gram-second units. For x-rays, 1 rem = 1 rad and 1 roentgen ≈ 0.86 rem.
The SI unit coulomb/kilogram has replaced the roentgen, but the preferred concept of " air kerma" (kinetic energy released in matter) is used to describe doserate in a radiation beam (gray/second)
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And furthermore.....
Radioactivity is the disintegration of atomic nuclei, releasing energetic particles (neutrons, beta (electron) rays, alpha (helium nucleus) rays ) or gamma (electromagnetic) radiation. We measure the activity of a source by the number of disintegrations it undergoes in a second. 1 disintegration/second (averaged over time,since the process is random) = 1 becquerel (Bq). The legacy unit, the curie, is the number of disintegrations per second of 1 gram of radium, 3.7 x 1010 Bq.
All radiation travels through space indefinitely and in straight lines, except that charged particles (alpha, beta) are deflected by magnetic fields. The intensity (or dose rate) decreases with the inverse square of distance from a point source but is in principle always detectable. The odd one is the neutron which, although generally stable in an atomic nucleus, decays in free space with a half life of a few minutes to form a proton, an electron, and an antineutrino.
There's a whole zoo of other particles of various masses emitted by all sorts of nuclear and interparticle reactions, but the units of measurement and the general principle of rectilinear propagation apply to all.
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Yes, you're right. What I was thinking of though was a chart showing the 'natural background radiation', no matter if looked at as 'artificial' or natural. the way I see it everything we add becomes a part of it. That's the chart I'm missing.
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If you just google "radiation pie chart" you will get a whole load of information from different parts of the world. The only changes in the last 100 years, as I said above, are the addition of medical and industrial sources. These form a smaller percentage in the USA data than in, for instance, the UK charts, because although there is a greater use of medical radiation in the US, the truly natural background is 30% higher.
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My favorite unit is the banana equivalent dose (BED) https://en.wikipedia.org/wiki/Banana_equivalent_dose