Bringed by the fission (breaking of the atom kernel), she is make principally by three type of radiations : Alpha, Beta and Gamma. Some atoms have instability in the time and can be break spontaneously with period of a fraction of second to billions of years. When the atom kernel break, he will explode to new smallers atoms ans can emit high energy photons,the Gamma rays; and can emit some particles, the Alpha and Beta.
   

   This particle is make by a piece of atom kernel, 2 neutrons and 2 protons (equivalent of helium kernel) ejected at high speed when an atom is break. This particle have a low prenetration in the physical matter because she have a  high mass. She can be easily in collision with the physical matter because she have a big mass and high volume. She make a high ionization and strong damages. Imagine a bowl in pins. Radioactive matter emitting Alpha particles inside human body is highly dangerous.
   

    This particle is make by an electron or a positon (electron with positive charge) ejected at high speed when the kernel of an atom is break. This particle have a low mass and can penetrate in the matter more easily than Alpha.
   

   High energy photons (electromagnetic wave with very high frequency) emit when an atom is break. He have a high penetration inside the matter. In industry, Gamma sources are used for radiography matter with high density and detect microfissures, for irrady food for sterilization (not residual radiations).
   

   Photon of medium energy (electromagnetic wave with very high frequency) emit by some high tension devices. He have a high penetration inside the matter, but less than Gamma rays. H rays are usualy used to radiography organic matter.
   

   The cumul of dose is generally measured in Sievert or in Rem. In the radioactivity, you should considerate two factors. The quantity (cumul of dose) of radioactivity received and the power of the radiations received (dose rate). You can compare this to the power of an electric device as a heater (the watts) and at his consumption per month (the Kw/h). His consumption vary with the time of using. More the consumption per month is high, more damages on your bank account are high (your health capital).
   

   Number of atom disintegration per second in the physical matter.  1 Becquerel = 1 disintegration per second.
   

   Measuring unit of Gamma radiations. 
1 Roentgen = 0,93 Rad
   

   Measuring unit of ionising radiations.
   

   Measuring unit of ionising radiations.
1 Gray = 100 Rads.
   

   Measuring unit of radiations absorded by human body.  Integrate biological dammages of different type of radiations. For Gamma and Beta rays, 1 Rem = 1 Rad ; for Alpha, 1 Rem = 0,05 Rad (coefficient x20). For Geiger counters, the cumulative dose if generaly gived for Gamma irradiation, because it is not possible to know easily the type of radiation you have receive and how you have receive them. 
   

   Measuring unit of ionising radiations absorbed by human body.
1 Sievert = 100 Rems.
   

   Measuring unit equal to the radioactivity generated by a gram of Radium per second. It is equal to 37 billions of Becquerels (disintegrations per second).
   

   Some atoms have instability in the time and can be break spontaneously with period of a fraction of second to billions of years. In radioactivity, we call period the time for the physical matter to decrease to half the radioactivity. For the Radium the period is 1600 years, for the Colbat 60 the period is 5 years, for the Radon the period is 4 day.