Penetration of Radiation
Alpha particles can usually be stopped by a very thin barrier. Radioisotopes emitting alpha particles are usually not hazardous outside the body, but they can cause damage if ingested.
Betas (streams of electrons) can pass through a hand, but are usually stopped by a modest barrier such as a few millimeters of aluminum, or even a layer of clothing. As with alphas, beta particles are more hazardous if inhaled or ingested.
Gammas can be very penetrating and can pass through thick barriers. Several feet of concrete would be needed to stop some of the more energetic gammas. A natural gamma source found in the environment (and in the human body) is 40K, an isotope of potassium.
Neutrons are also very penetrating. Some elements, like hydrogen, capture and scatter neutrons. Water is commonly used as a neutron radiation shield.
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