Well next discuss the used units to express the quantity and, strength of radiation and radioactivity.

　　

Units of radiation

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○Gray

This is a unit of the quantity of radiation.
It expressed with the sign, Gy.
It was definded by Lewis Harold Gray, a British physicist.
1Gy expresses the dose when there are 1J of energy absorption occuring per kg of theweight of an object.
Gray is not a unit showing the radiation dose the radioactive material is emitting,
it is a unit showing the radiation dose which the ofject which is receiving radiation received.
However, sieverts differ from this in being used only for radiation doses which are being received by living things,
while grays also used for the doses of radiation which in animate ofjects are receiving.

○Sievert

This is a unit of the quantity of radiation most often heard in the news now.
It's expressed with the sign, Sv.
It was defind by Rolf Maximilien Sievert, a physicist of Sweden.
When a human body receives radiation, it is used together with Gray as an index showing the received radiation dose.
The difference is the influence of the kind of radiation and also the emphasis on helth risks from receiving radiation.
In brief, Gray is the unit which shows the quantity of energy which a substance being bathed in radiation.
In contrast, Sieverts are units which measure the influence or radiation on a human body.
In this way, the two measurement sytems differ greatly.

　　

Unit of radioactivity

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○Becquerel

This is a unit of the strength of radioactivity.
It's expressed with the sign, Bq.
It was definded by Henry Becquerel, a French physicist.
1Bq means one nucleus collapses in 1 second.
For example,
in the situation, "200-Bq radioactivity were detected from 1 kg of apples."
This means that the radioactive material contained in 1 kg of the apple is giving off radiation 200 times in 1 second.
With the value of this becquerel, it can be understood what depth of radioactive material is contained in the apple in question.
Therefore, we can know the degree of safety of water or food.