Understanding Sieverts and Their Impact on Human Health

[Petr Mugver]

I searched "sieverts" here and with google, but I didn't find what I wanted.

So, in theese days after Fukushima we see on tv a lot about chernobyl, Hiroshima etc.
Journalists go around in theese places with a little counter that displays sieverts.
Now, sieverts measure equivalent energy per unit mass. The problem for me is not the "equivalent", but the fact it is not an instantaneous quantity, but a cumulative quantity.

So, for example, if I weigh 80 Kg and stay in a place with 10^-3 Sv for 30 s, hoh much energy do I absorb?

 

[tiny-tim]

Hi Petr!

Journalists go around in theese places with a little counter that displays sieverts.
Now, sieverts measure equivalent energy per unit mass. The problem for me is not the "equivalent", but the fact it is not an instantaneous quantity, but a cumulative quantity.

So, for example, if I weigh 80 Kg and stay in a place with 10^-3 Sv for 30 s, hoh much energy do I absorb?

If your counter shows 10^-3 Sv, it doesn't matter how long that took to register, the energy absorbed is 10^-3 joules per kilogram …

1 Sv = 1 J/kg, see http://en.wikipedia.org/wiki/Sievert#Definition"

 

[vanesch]

I searched "sieverts" here and with google, but I didn't find what I wanted.

So, in theese days after Fukushima we see on tv a lot about chernobyl, Hiroshima etc.
Journalists go around in theese places with a little counter that displays sieverts.

Well, usually a counter displays Sieverts PER TIME UNIT, most often per hour.
The most practical unit is microsievert per hour. It is what most counters with a calibration display.

Sometimes, this is converted in milli Sieverts per year. This is because "dose limits" are usually specified in millisieverts per year, so this is a good way to compare.

As there are 8760 hours in a year, we have:

1 microSievert per hour = 8.76 milliSievert per year.

Or 1 milliSievert per year = 0.12 microsievert per hour.

To give you an idea, the AVERAGE world background radiation is of the order of 2.4 millisievert per year, which comes down to something like 0.3 microsievert per hour. However, this background is seriously depending on the place where you are.

(there are some exceptional places where you take 260 mSv per year, but these are not so very common).

The yearly dose limits for nuclear activities on the public are 1 mSv / year, and for radiation workers, 20 mSv/year.

Now, be careful: a special event can raise *temporarily* the dose rate. For instance, if after a certain event, the local dose rate is something like 5 microsievert per hour, which would correspond to something like 45 millisievert per year and hence more than twice the dose rate allowed for radiation workers, if this dose rate diminishes over time, or if you don't stay there for a year, then 5 microsievert per hour is by far not as bad as 45 millisievert in a year of course.

So one should be careful with expressions like "the hourly dose rate is 10 times the one radiation workers can take" or something. It is only if that dose rate is constant, and if the exposed person will remain there for a year, that this is correct.

 

[Petr Mugver]

Thanks Vanesch, the "per unit time" part that was missing is exactly what I didn't understand. The serious problem here, but we go beyond physics to go to politics, is the disinformation we get from tv. For instance in theese days in Italy we have a referendum on nuclear energy, and I wonder how we can go vote in a conscious way if even I (quite familiar with physics) don't understand what they tell me to convince me to vote for one side or the other.

 

[Drakkith]

Unfortunently, the average person is NOT going to make any effort to understand how radiation works. They rely on what is told to them to make their decisions. Since you cannot explain how it works because they do not have a basic understanding of physics, then you have to drastically dumb it down and leave out many fundamental parts of the issue.

 

[Petr Mugver]

Well, usually a counter displays Sieverts PER TIME UNIT, most often per hour.

Now, be careful: a special event can raise *temporarily* the dose rate. For instance, if after a certain event, the local dose rate is something like 5 microsievert per hour, which would correspond to something like 45 millisievert per year and hence more than twice the dose rate allowed for radiation workers, if this dose rate diminishes over time, or if you don't stay there for a year, then 5 microsievert per hour is by far not as bad as 45 millisievert in a year of course.

Just to be sure that I understand it correctly, you say that the counters mentioned above measure Q(t) = dE/dmdt, where E is the equivalent energy. So a person of mass M that stays there from t_1 to t_2 should absorb the amount of equivalent energy:

$$E = M\int_{t_1}^{t_2} Q(t)dt$$

Right?

 

[vanesch]

Just to be sure that I understand it correctly, you say that the counters mentioned above measure Q(t) = dE/dmdt, where E is the equivalent energy. So a person of mass M that stays there from t_1 to t_2 should absorb the amount of equivalent energy:

$$E = M\int_{t_1}^{t_2} Q(t)dt$$

Right?

Absolutely.

 

[vanesch]

Thanks Vanesch, the "per unit time" part that was missing is exactly what I didn't understand. The serious problem here, but we go beyond physics to go to politics, is the disinformation we get from tv. For instance in theese days in Italy we have a referendum on nuclear energy, and I wonder how we can go vote in a conscious way if even I (quite familiar with physics) don't understand what they tell me to convince me to vote for one side or the other.

I know, I'm also sad about that. What's even wrong is to do this NOW. The time is not right to have a calm debate about the pro and contra arguments of nuclear energy when there is an exceptional situation.

This is like organizing a referendum on capital punishment just after a serial killer has been at work or something.

It is not so much that being pro or contra nuclear power by itself is "good" or "bad", it is that one should have clearly defined goals (lowest risk, or cheapest power, or best economical performance, ... ), and have as much objective information of the different energy plans as possible, and then see what goals are best met with what plan.

I use to say that no nuclear power is better than nuclear power but that that is not the question: the question is what you replace it with (realistically). But that coal power is far worse than nuclear power. And that no power is also worse than nuclear power.
It is in that light that one should make a choice.
Of course I would also prefer that my power comes from a few wind mills and a few solar panels. The whole debate is whether that is technically, economically and industrially achievable in the next few decades.

This would be an honest referendum. But this is not the way the real political world works (or even, how most people think).

However, we digress from pure physics.

An advice for people interested in energy questions is the book (freely available) by McKay:

http://www.withouthotair.com/"

It contains basic physics and estimations of what several (renewable or not) sources can deliver, and what certain consumption patterns will need. It is oriented on the UK case, but the physics is simple, well explained and to the point.