On 11st, March, a M9.0 earthquake strike east Japan around 3pm. this caused malfunction of the nuclear plant, Fukushima Daiichi.

i am not going to talk about the malfunction, but on the radioactivity.

The radioactivity is caused by nucleus decay into a more stable state. like Carbon-14 will decay to Carbon-13 by beta decay. there are 3 common types of radioactivities: alpha – a Helium nucleus, beta – a electron and gamma – high energy light.

the radioactivity was measured by Bq ( Becquerel ), which is 1 decay per second. for a typical radioactivity substance, there may be 10 billion decay per second. think about how many atom in 1 mole of matter.

each radiation carry some energy, for alpha decay, energy is about 5MeV, beta decay is 1 MeV, and for gamma decay, it is around 0.1 to 1 MeV. 1 MeV is about 1.60217653 \times 10^{-13} Joule. for boiling a water to increase 1 degree, it take 2400Joule. you can see how small it is.

1 more thing to consider is, not every radiation will be absorbed. some will just pass through and leave and no effect. the chance for being absorbed is proportion to the density of the material, or for simplicity, how much weight of the material.

for easy understanding, we multiple these 3 factors together, to give us how much energy absorbed per kg. and we call it Gy (Gray), the absolute does.  and we can have Gy per hour, Gy per second, depends on the time scale.

Thus, for 1Gy, an alpha source, typical 5MeV per decay and a total absorption, it takes 6241 billion hits. for 10 billion decay a second, it takes 10 mins for this does.

in other system, people use rad for radioactivity, for example in the game fallout. which is 1 Gy = 100 rad.

Here is a list of different does and the health effect

  • 0.25 Gy or less  –  no effect
  • 0.50 Gy  – temporary reduction of while blood cells
  • 1 Gy  –  feel nausea, vomit
  • 1.5 Gy – 50%  more chance to have cancer, cataracts or leukemia
  • 2 Gy – 5% people will die
  • 4 Gy – 50% people will die in 30 days
  • 6 Gy – 90% people will die in 14 days
  • 7 Gy – all die

However, different part of our body have different does limit. so, for 1Gy radiation, our eye will be blinded, but our skin has no effect. therefore, we introduce another does, called Effective does Sv ( Seivert ). it is produced by a weight factor.

Gy \times w = Sv

different part of out body has different weight factor, for lower weight factor, the more easy the tissue will get hurt.

here is the list of weight factor

  • whole body  – 1
  • skin — 0.01
  • lung — 0.12
  • liver — 0.05
  • breast — 0.05
  • stomach — 0.12
  • gonads – 2

we see that the weight factor for whole body is 1, therefore, in general we use Gy or Sv in equal way.

And we also observed that, when we eat some radioactive matter inside, the limit does will be decreased by 8 times in out stomach. or to say, the radioactive matter is 8 times dangerous than it is at outside. the reason is, when the matter is inside our body, we will absorb all energy.

a typical does is 3.75mSv per year. which means we received 3.75mSv in a year, 0.4uSv per hours.

for radioactive effect, there are 3 types of effect: acute ( short term ), late ( long term) and genetic effect ( permanent ) .

  • short term – Erythema, hair loss, etc
  • long term – Cateracts, Leukemia or cancer, etc

and the effects can be divided into 2 groups – deterministic and stochastic

  • deterministic mean the chance of having those effect is increase with the radiation increased.
  • stochastic is similar to deterministic, but there is a threshold. below the threshold there is no effect. – Cancer and genetic effect are belong to this group.