Today's lesson is a basic guide to the two most important quantities that we will be discussing; Radiation Dose and Radiation Dose Rate. To keep this post a manageable length, I will only write about the units used to describe these quantities and won't go into how to measure them or what the effects of various doses are. (BTW, while I will mention various specific doses and dose rates for the purposes of illustration below, you can ignore the actual numbers for the time being.)
Before we get into discussing the units involved it is necessary to have a clear idea of what the terms dose and dose rate actually mean. The distinction is fairly straight-forward but somehow media reports manage to misuse these terms much more often than they get them right. Since, in the event of a nuclear war (or other accident), your life might depend on understanding this, you can't afford to be imprecise in your thinking.
By dose, we mean how much damage has been done to you by radiation.
Usually, a dose will be related to a specific incident (e.g. "the CAT Scan gave you a dose of 8 mSv") or a period of time (e.g. "the allowed dose for members of public is 1 mSv per year").
By dose rate, we mean how fast damage is being done to you by radiation. A Geiger counter measures dose rate; the faster it clicks, the higher the dose rate.
Dose rate is used to describe the radiation field in a place (e.g. "the dose rate due to cosmic radiation in an aircraft in flight may be up to 4 μSv/hr").
Radiation Dose
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| American Civil Defence dosimeters from the Cold War. You clip one of these to your person and can view your radiation dose by looking through the tube at a light source. You would see something like the image below. The fine vertical line marks the dose you have received and moves (hopefully very gradually) to the right over time. [source] |
Technically, the correct, and most commonly encountered unit for radiation dose is the Sievert, however there are a number of different related units which you should be aware of. They are related to the Sievert as follows:
1 Sv (Sievert)
= 1 Gy (Gray)
= 100 R (Roentgen)
= 100 REM (Roentgen Equivalent in Man)
= 100 RAD (Radiation Absorbed Dose)
To convert between these units, it is helpful to recognise that all units which start with "R" are 100x smaller than those which don't. Visualise an "R" unit as a centimetre and a "non-R" unit as a metre.
Grays are more often used in a medical context, REMs and RADs mostly appear in older Cold War era documents and Roentgens are often seen on Geiger counters, however, you will see all of these units used frequently enough that you will need to be able to convert from one to another.
Standard SI prefixes are used with these quantities. E.g.:
mR (milliRoentgens) - thousandths of a Roentgen
μSv (microSieverts) - millionths of a Sievert
Occasionally you will see odd-ball units e.g.:
cGy (centiGrays - not to be confused with Centigrade) - hundredths of a Gray (i.e. effectively the same as Roentgen)
There's nothing special about units like these, they follow the same logic, although you might want to double-check you're multiplying by the correct power of ten when converting to more familiar units.
At this point, someone with a bit of technical knowledge will have their hands twitching over the keyboard, desperate to inform me that I've got it all wrong: These units don't mean the same thing at all and moreover the conversion from Roentgens to Sieverts - in so far as these incompatible units can be equated at all - is more like 114:1.
To which I reply that in the context of waving around a Geiger counter and using it to work out the dose you're receiving - which is what this post is about - these equivalences are all perfectly good. Most emergency response agencies, including the FEMA (the United States Federal Emergency Management Agency) decree that for their purposes 100 R = 1 Sv. The other uncertainties related to dose measurement vastly outweigh the small error that such an approximation gives.
Radiation dose is different to the dose of a chemical substance in that it can apply to either the entire body, or a portion of the body. The same units are used for both situations so it's sometimes necessary to explicitly state whether we are talking about whole-body dose or dose to some specific body part. This can make a big difference; 20 Sv of whole body radiation will kill you for certain, whereas 20 Sv to the skin of a hand will give you nasty radiation burns to that hand, but will otherwise have comparatively little effect. In fact, you will probably even get to keep your hand if the type of radiation involved does not penetrate much past the skin. You could think of it as roughy equivalent to dipping your hand into water at 100°C versus heating your entire body to 100°C.
I have read some reports which use Grays for whole body dose and Sieverts for dose to an extremity, but this is not the actual meaning of these units and you should definitely not rely on this.
Some parts of the body are more vulnerable to radiation than others (the eyes, bone marrow and gonads are particularly sensitive), and clearly you can get a very complicated situation where different parts of a person's body can be exposed to different levels of radiation. It's possible to determine an equivalent whole body dose in this situation, but this requires a lot of data/calculation and isn't relevant to us.
Fortunately (if that word can be used in this context) the survivor of a nuclear war has things simpler. You'll mainly be worried about gamma radiation from fallout that will be scattered fairly evenly about the place. Your body will be receiving a uniform dose of radiation. Therefore, what you read on your Geiger counter will be what you are experiencing in terms of full-body dose.
The main situation where a nuclear war survivor might encounter the concept of dose to a specific organ - as opposed to a whole-body dose - would be in the case of iodine-131 which strongly concentrates in the thyroid glands. I will cover Iodine-131 in another post.
To a first approximation, the damage done to you by a given dose of radiation is the same whether it's received over a long or a short period of time. In actual practice, a given dose does somewhat less damage if spread over a longer time period (there is a quantity known as the "dose and dose-rate effectiveness factor" - DDREF - which can be used to quantify this effect), but for our purposes we can ignore this effect.
Radiation Dose Rate
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| A Gamma-Scout Geiger counter showing a typical background level radiation dose rate. This a good quality consumer level Geiger counter which is quite popular, although fairly expensive for what it does. [source] |
Dose rate is given in units of dose per time. The time period used is almost always the hour, but strictly speaking could be any unit of time. Typical dose rates units are:
μSv/h - microSieverts per hour
mR/h - milliRoentgens per hour
During a nuclear war you may have to convert between dose rate and dose. E.g. your Geiger counter says the rate is X at a certain place and you have to work out what dose you will receive by spending Y time in that place.
Don't be intimidated by the unfamiliarity of the situation; the mathematics is exactly the same as planning a road trip: If you drive at 80 km/h for half an hour, you will have travelled 40 km, and if you enter a place where your Geiger counter says 80 μSv/hr and you stay there for half an hour, you receive 40 μSv (which BTW is pretty negligible under nuclear war conditions).
A common mistake you will see in news reports is to omit the "per hour" part when mentioning a dose rate. e.g. "The dose rate in the reactor room after the SL1 accident was in excess of 500 R". Rather than dismissing such reports as technically wrong, it's more helpful to just sigh quietly and add back the "per hour".
As mentioned earlier, dose rate is a measure of the radiation field of a place. It should not be really applied to an object, and this is another common mistake you will see.
The point here is that radiation follows the inverse square law*; that is to say if you have a lump of radioactive material, the radiation field it produces decreases proportional to the square of the distance from it; if you move from 1 m away to 2 m away, the intensity of the field will be reduced by a factor of 4.
[*=for the purists; I know this is not exactly true, but it is very close for the sort of high energy gamma radiation we are generally concerned with.]
So if you are talking about a single isolated source, you can't legitimately mention dose rate without also specifying how for away from it you are. If you view accounts of visitors to Chernobyl (there are quite a few on YouTube; it's a popular holiday destination for some), invariably someone will cram a hand-held Geiger counter right against a hot fragment lying on the ground and exclaim something like "wow, this fragment is 100 mSv/h**". Given that (if true) a radiation field of this magnitude will give you acute radiation poisoning in about an hour, this would seem to be scary and the person reckless, but in fact, the Geiger tube might only be 1 cm from the fragment; the person holding the counter might be more like 1 m away, at which point the field will be 10,000 times (1002) smaller. The hand holding the counter would of course be receiving a much higher dose, but in the 10 sec or so it would take film the video clip, this wouldn't be a big deal.
[** = I have heard this exact figure quoted, but in that situation, it is much more likely that it was actually 100 μSv/h. This is another thing to be wary of; sometimes people will confuse "milli" and "micro". Since the difference between the two is a factor of a thousand, it is rather important to get this right. The best thing is if possible to have some sort of idea what radiation level is to be expected in a given situation and if the reported level seems vastly low or high, suspect a confusion of units.]
With fallout (from a bomb and also from a reactor accident if you're far enough away that there aren't big fragments of the nuclear fuel lying on the ground), this effect is not so much of an issue. The stuff gets spread around evenly and the dose rate doesn't change much depending on your position.
Another one of those weird anomalies: You would think that if there was an even carpet of fallout on the ground, then the radiation field would reduce proportional to the square of distance above ground level. In fact, if you analyse the situation mathematically, you find that the field remains pretty constant until you get a significant height above the ground. In this situation, it is standard to measure the radiation field at a distance of 1 m above ground level.
Finally, you might be looking at Russian radiation monitoring sites on the web or you may have to use a Russian Geiger counter (fairly common on eBay). Here are some translations of relevant terms.
| Russian | English |
|---|---|
| Р | R |
| мР | mR |
| мкР | μR |
| мкР/ч | μR/h |
| Зв | Sv |
| мЗв | mSv |
| мкЗв | μSv |
| мкЗв/ч | μSv/h |
| Рентген | Roentgen |
| Зиверт | Sievert |
| час | hour |
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| Close up of the scale of a Soviet DP-5B Geiger counter showing mR/h above and R/h below. |
I think that's enough for one post.
To help cement concepts like this in your mind, it's often good to try to answer a couple of questions on the subject yourself, so have a go at these. This isn't an exam and the point isn't to get them right the first time but to go back and re-read if the answer isn't clear to you.
1. Does a Geiger counter measure radiation dose or dose rate?
2. Which is a higher radiation dose: 1 Sv or 1 R?
3. Find two things wrong with the following statement:
"The conglomeration of melted nuclear fuel known as the 'elephant's foot' at Chernobyl gives off 10,000 Roentgens of radiation".
4. You are visiting Chernobyl and decide to take a picnic in the Red Forest. Your trusty Geiger counter is showing readings of around 100 μSv/h. You spend half an hour there. What dose of radiation will you receive?
In the next post we will learn about what these units mean in practice.
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