Basic knowledge of radioactivity / Q & A

A:The Becquerel is a unit for measuring the capacity of a radioactive subsistence to emit radiation (“activity”).
On the other hand, the Sievert is a unit for measuring the effect of radiation to human body (“dose”).   

        Becquerel(Bq): Radioactive substances undergo what is called “radioactive decay.”
Each time a radioactive subsistence decays, one alpha ray or one beta ray is emitted from its atomic nucleus.
Gamma rays are also emitted in many cases. One Becquerel (1Bq) indicates that one radioactive decay and emission is occurring each second.

        Sievert(Sv): The influence of radiation differs depending upon the type of radiation (alpha, beta. gamma).
The effect of the same amount of radiation can differ depending on the part of the body which is exposed.
The Sievert is a unit for indicating damage to humans (“dose”), which takes the different possible effects of radiation into account.   

A:The period of time in which the capacity of a radioactive subsistence to emit radiation becomes halved,
due to its natural radioactive decay, is called its “half-life.”   
For example, the half-life of Iodine131 is 8 days.
After 16 days (two half-lives), its radioactivity becomes 25% of what it riginally was, not 0%.
The half- life of Cesium 134 is 2.06 years. Therefore, by March, 2013, the amount of Cesium 134 in the environment spread
by the Fukushima accident had been naturally reduced by about half, making it harder to detect.  


Radioactive nuclide
Radioactive nuclide Emitted radiation  Half-life

Iodine 131 

Beta ray, Gamma ray

8 days       

Cesium 134

Beta ray, Gamma ray

2.06 years

Cesium 137

Beta ray, Gamma ray

30.2 years

Strontium 90

Beta ray 

28.8 years

Plutonium 239

Alpha ray

24,100 years

A:External exposure means a person is exposed to radiation outside of the body, by a radioactive source that is outside.

Internal exposure means a person is exposed to radiation inside of the body; the radiation source is inside the body,
as can happen when radioactive materials are inhaled or ingested (through breathing or eating).

A:The radiative rays emitted by radioactive  substances include alpha rays, beta rays, gamma rays, x rays and neutron rays.
Please refer to the question on half-life above to see the type of radiation emitted by various radioactive  substances.

  Alpha rays and beta rays are easily stopped by skin, clothes, or other simple protection when the exposure is external,
but they should not be considered “weak.”
Rather, they can have a strongly damaging effect when they are accumulated inside the body (internal exposure)
because the absorbed dose of energy from them can be very big.


A:Different types of radioactive substances are likely to accumulate in different internal organs.
A substance which has a long “half-life” continues to emit radiation inside the body for a long time.
As a result of this radiation, internal organs might be damaged and DNA might be affected.

A.  In the past, the unit “Curies” was used to measure radioactivity.
  After the accident at the Chernobyl nuclear power plant, the Becquerel became more common.
  One Curie is equivalent to 37 billion Becquerel. Similar, in most places the “Rad” was replaced by the “Gray.”

  However, the Rad is still used to describe radiation the workplace today. Refer to the website for the table of units.