Chapter 3: Unit 6. Units of Radioactivity

Units of Radioactivity

We already know that radioactivity is not detectable by our senses. We cannot hear it, feel it or smell it. Then how do we know it is there? Alpha, beta, gamma, positron all have a property, we can use to detect them: when these rays interact with matter, they usually knock electrons out of the electron cloud surrounding an atom and creates positively charged ions from the neutral atom. That is why, we sometimes call them ionizing radiation.

Ionizing radiation is characterized by two physical measurements 1) intensity, which the number of particles of photons emerging per unit time and 2) the energy of each particle or photon emitted.

Effect:

The exposure of a substance to radioactivity is measured with the Rad (radiation absorbed dose) or the rem (radiation equivalent for man).

Both  natural and human made sources of low level radiation exist such as 1) radon seepage in buildings, 2) rocks and soils 3) minerals in our body 4) cosmic radiation, 5) medical X-rays, 6) nuclear medicine.

Intensity:

Instrument such as Geiger Muller counter and the proportional counter contain a gas such as helium or argon. When radiation ionizes this gas, the instrument registers this fact by indicating that an electric field is passing between two electrodes.

Radiation in a sample is measured by the number of disintegration per second. most often using the curie(Ci)=3.7*10¹⁰ disintegration per second. This is radiation of very high intensity. Becquerel is another unit: 1 Bq= 1 disintegration. Bequerel is SI unit of radiation.

We can also define millicurie  and microcurie.

1 mCi= 3.7*10 dps and 1 μCi= 3.7 *10⁴ dps

The intensity of any radiation decreases with the square of the distance from the source. If the distance from the radiation source doubles, radiation decreases by a factor of four.

Energy:

Roentgen: measure of energy delivered by a radiation source. 1 roentgen= 2.58*10^-4 C of ions per kilogram.

The exposure of a substance to radioactivity is measured with the Rad (radiation absorbed dose) or the rem (radiation equivalent for man). Sievert is the SI unit. 1 Sv= 100 rem.

Both  natural and human made sources of low level radiation exist such as 1) radon seepage in buildings, 2) rocks and soils 3) minerals in our body 4) cosmic radiation, 5) medical X-rays, 6) nuclear medicine.

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