The Naked Scientists

The Naked Scientists Forum

Author Topic: How quickly does radiation risk decay with distance?  (Read 3770 times)

Kylara Martin

  • Guest
Kylara Martin  asked the Naked Scientists:
   
Dear Chris,

So much news has focused on the nuclear crisis in Japan, it has become clear that the general public and reporters in particular do not understand how radiation works.  In particular, the difference between gamma rays and radiogenic particles such as iodine and cesium seems to have been lost.  Could you maybe do a brief article or scrapbook entry on the various routes of radiation exposure associated with an event such as this?  I feel that such an explanation could make clear how rapidly the dangers decrease as you move away from ground zero.

At the very least, students might be interested in a discussion of the sources of harmless radiation that we are all exposed to everyday (like the potassium in bananas and other food) and how the levels produced by these sources compare to man-made sources and the background levels from the sun.

I am a geophysics graduate student in the US and use your articles and scrapbook entries to send to family and friends when they ask me questions I can't answer.

Best wishes and keep up the good work!
Kylara Martin

What do you think?
« Last Edit: 27/03/2011 09:30:02 by _system »


 

Offline Bored chemist

  • Neilep Level Member
  • ******
  • Posts: 8669
  • Thanked: 42 times
    • View Profile
How quickly does radiation risk decay with distance?
« Reply #1 on: 27/03/2011 16:58:21 »
The subject is very complicated. Here's the simplified version.
There are lots of different sorts of radiation. The biggies are alpha, beta gamma and neutron.

Alpha particles are stopped by a few inches of air. They don't travel far in any solid material, but if they are produced in the body by radioactive material that has been inhaled, swallowed or has got into a cut or something then they do quite a lot of damage. The range is only a few cells or so of tissue, but they deposti all their energy in those few cells so they can cause a fair bit of harm.

Beta particles have a range that's rather bigger than the alphas. They can travel up to a few metres in air but they are stopped by a fairly thin layer of anything denser. A few cm of water or a few mm of metal.
Again, unless they are produced inside the body  they don't generally do a lot of harm.

Gammas don't have as clearly defined a range. In principle their range is infinite, but they are "degraded" as they travel through anything but a vacuum. The rate at which they are destroyed depends on their energy and on the material they travel through. On the whole, things with high atomic numbers (lead is the classic example) shield against gammas rather better than things with low atomic numbers. The effect varies roughly as z squared. So lead (atomic number 92 has, on a weight for weight basis roughly 4 times the gamma shielding  of palladium (z=46).
Since air has very little effect on them it makes sense to think about gammas as having an inverse square law  variation with distance, so, if you are twice as far away then the dose you get is a quarter ans much and if you are ten times further away, the dose drops by a factor of a hundred.
On the other hand, if you are considering shielding from them then it's easier to treat them as having a "halving thickness" so, if a lead plate 1mm think drops the dose by a factor of 2 for some energy of gamma rays then 2 mm will drop it by a factor of 4 and 10 mm will reduce it by 1024 fold.

Neutrons are more awkward.
For a start they interact particularly strongly with hydrogen- and most of the atoms in a person are hydrogen, so that's important.
Their interaction with other materials is harder to predict. Some materials gobble them up very enthusiastically but some are less affected. Lead does a rather poor job of blocking neutrons. Other materials like boron or cadmium are used instead.


Contamination is a different problem. It relies on actual bits of material travelling from the source to the thing that's contaminated. So. a plastic bag (which doesn't let anything through) will stop contamination. If there's no real barrier then eventually, all the radioactive material will be dispersed evenly across the whole world. On the other hand, it may take a while for things to travel far and a lot of the radiation will decay away.

Like I said, it's complicated.
 

Offline CliffordK

  • Neilep Level Member
  • ******
  • Posts: 6321
  • Thanked: 3 times
  • Site Moderator
    • View Profile
How quickly does radiation risk decay with distance?
« Reply #2 on: 27/03/2011 21:32:48 »
If there's no real barrier then eventually, all the radioactive material will be dispersed evenly across the whole world. On the other hand, it may take a while for things to travel far and a lot of the radiation will decay away.
Depending on the source material, it may or may not decay away quickly.

Some radioactive materials decay in less than a second.
Others a few days, or a few years.
Others, in thousands of years.

The ocean, however, is a very big sink.  Undoubtedly any radiation washing from Japan to North America will be so dilute that it will be undetectable.  Some elements and isotopes formed in nuclear reactors are not naturally occurring on Earth.  So, they could potentially be detected across the Pacific, or into the Atlantic.  However, the concentration would be low enough to have no biologic impact away from Japan.

There is a risk, however, of long-term soil contamination and groundwater contamination in Japan.  This would be localized to near the reactor site, although ground water can move a bit.

Potentially nearby fish, shellfish, and reefs would be at risk with the reefs potentially being able to sequester radioactive minerals for an extended period of time.

Risks are as above...  ingestion or inhalation of materials that undergo alpha/beta decay, and external exposure mediated by distance for other types of radiation.

Radium, Carbon-14, and other radioactive materials are found throughout our environment.  Small doses of radiation are generally considered harmless, and damage can be repaired with our cellular repair mechanisms, or through cell death and replacement.
 

The Naked Scientists Forum

How quickly does radiation risk decay with distance?
« Reply #2 on: 27/03/2011 21:32:48 »

 

SMF 2.0.10 | SMF © 2015, Simple Machines
SMFAds for Free Forums