[Phys-l] converting radioactivity level to exposure dose ... becquerel to gray to sievert

[John Denker <jsd@av8n.com>]

Some good stuff can be found at:

  http://www-diva.eng.cam.ac.uk/energy/parksteaching/4m16/health_physics.pdf

Among other things, there is the formula for converting from activity to dose.

I quote:

> It has been shown that the dose depends on the level of activity and the energy of the
> source. The following expression can be used for calculating the dose from a gamma source.
>
>        D =   1.6e-13 M Σ E t
>            ------------------                    [1]
>               4 π R^2 ρ
> where
> D = dose in Gy
> M = activity in Bq
> Σ = absorption cross section (= 3 m^-1 for gamma rays in human tissue)
> E = gamma energy (total) in MeV
> t = exposure time in seconds
> R = distance from source in metres
> ρ = density of tissue (= 1000 kg/m^3)

There are additional weighting factors to convert the calorimetric dose (in
grays) to the effective dose (in sieverts).

I disapprove of the inelegant and unenlightened treatment of the units
in equation [1] ... but an inelegant formula is better than no formula.

====================

Another take on the Bq / Sv connection can be found here:
  http://rogerarm.freeuk.com/rogerarm/Pages/UnitsOfRadiation.htm

although it seems more of a historical curiosity, rather than something
that can be conveniently applied to practical problems.

=================================================


I get tired of reading garbage like the Oehmen article:
  http://www.businessinsider.com/japan-reactors-pose-no-risk-2011-3
i.e. articles that use made-up facts to reach absurd conclusions.

With rare exceptions, I don't like to discuss misconceptions in front of
students, especially at the introductory level, because
 a) It is not a good use of time;  there are just too many misconceptions, and
 b) It runs the risk of reinforcing the misconception, rather than dispelling it.

HOWEVER there are exceptions, and the Oehmen article may be one.  Here's
why:
 1) All of us on this list strive to get students to develop their critical
  thinking skills, and to develop some respect for science and scientists.

  As a corollary, this requires learning to identify unscientific and
  anti-scientific thinking.

 2a) Business Insider and tens of thousands of other outlets featured the 
  Oehmen article.

 2b) Rush Limbaugh cited the BI article, quoted key conclusions from it, and
  referred to Oehmen as a "nuclear expert".  
     http://www.rushlimbaugh.com/home/daily/site_031411/content/01125104.guest.html

 2c) Similarly, Jim Cramer referred to it as "best piece on the nuke issue"
     http://twitter.com/#!/jimcramer/status/47043010080411649

 3) I don't see why anybody pays any attention to the sources mentioned above,
  but the fact is, millions of people do.  Apparently nobody bothered to do
  even the most superficial checking.  Nobody noticed that many of the key
  details were made up, and the units don't even make sense.

 4) Students conclude that the "experts" are idiots.

This garbage has got to stop.  Bad things happen in a society where it is
acceptable to just make up facts to support whatever preconceived ideas
somebody wants to tout.  You can't run a democracy on made-up facts.

Constructive suggestions:

A) Students need to know how low the standards are.  Just because a story
 has gone viral doesn't mean it makes any sense.  Anybody who did even 
 a little bit of checking would have known that Oehmen was not a "nuclear 
 expert" and that many of the key facts in the article were made up.

B) Students in the introductory course may not have the skills necessary
 to figure that out on their own, but they need to know that the course
 is an important step along the path to that goal.

 That is:  Sometimes you have to figure stuff out on your own.  This is
 easier than you might think.  Sometimes you can smell a rat even if you
 can't precisely measure the rat.  That is, even if you don't know the 
 exact right answer, you can figure out that the Oehmen article is not 
 legit.  It is dripping with rat-stink.

C) Students need to understand that appeal to authority is unscientific
 and is very very weak evidence.  A huge amount of weak evidence cannot
 ever outweigh even a single bit of strong evidence.  However ... if you
 are going to rely on authority even a little bit, you need to select
 your authorities carefully.  _Reputation_ is a big part of this.  Almost
 the only case where you can trust an authority is if/when the authority
 has a good reputation, and an incentive to protect that reputation.
   http://www.av8n.com/physics/authority.htm

 At some point students need to realize that they can't believe anything
 that Cramer says, or anything Limbaugh says, or anything they see posted
 on the ClimateSanity blog ... even if there are tens of thousands of
 copies posted in various corners of the web.  There is such a thing as
 a bad reputation.  Don't pay attention to people who have a track record
 of saying things that cannot possibly be true.

D) You don't want to go toooo far in the other direction.  If you disbelieve
 everything, you'll have a very hard time getting through life.

 So, a big part of life involves learning how to learn.  One key rule is 
 to check each new fact as it comes in.  Check to see how it fits in with 
 other facts thats that you know.  
  a) This helps you to remember the true facts, since the more connections 
   a fact has, the easier it is to remember, and
  b) This helps you to weed out the bogus facts, i.e. the ones that 
   cannot possibly be consistent with other stuff you know.

 Actually there is a sliding scale, such that good facts have more than
 99% supporting evidence with less than 1% conflicting evidence, while 
 bogus facts have 1% supporting evidence and 99% conflicting evidence 
 ... and there are lots of possibilities in between.

=========

The earthquake and tsunami were one month ago today.  I observance of the
date, there are a number of relatively analytical review articles in the
Japanese press ... available in English translation.  I don't believe
everything I read there, but still it's waaay better than anything I've
seen in the US press.  We are still nowhere near knowing the physics 
behind what is going on in the damaged reactors.  Quite possibly TEPCO
doesn't know, and if/whatever they do know, they're not saying.

  http://www.yomiuri.co.jp/dy/national/T110410001935.htm
  http://e.nikkei.com/e/fr/tnks/Nni20110411D11JF641.htm
  http://www.asahi.com/english/TKY201104090153.html
  http://www.asahi.com/english/TKY201104090150.html
  http://www3.nhk.or.jp/daily/english/12_05.html