There are three primary radiation types associated with radioactivity. Radioactivity is the spontaneous emission of energy from the nucleus of certain elements, most notably uranium. There are three forms of energy associated with radioactivity; alpha, beta and gamma radiation.
The classifications were originally made according to the penetrating power of the radiation, see figure 2
Alpha Particle Radiation were found to be the nuclei of helium atoms, two protons and two neutrons bound together. Alpha rays have a net positive charge. Alpha particles have the weak penetrating ability; a couple of inches of air or a few sheets of paper can effectively block them.
While alpha particles are stopped by a piece of paper, the situation changes once alpha radiation is inside the body, then the effects are more harmful than beta or gamma radiation. (See "Q" factor in testing food for radioactivity.) So materials that radiate Alpha particles are harmful if ingested or inhaled. But are not dangerous when external to the human body.
Beta Radiation were found to be electrons, identical to the electrons found in atoms. Beta rays have a net negative charge. Beta rays have a greater penetrating power than Alpha rays and can penetrate 3mm of aluminum.
Gamma Radiation and x-rays are high-energy electromagnetic radiation (high energy photons). This classification of radiation has the greatest penetrating power. High energy gamma rays are able to pass through several centimeters of lead and still be detected on the other side. Gamma rays are produced naturally from the decay of some radioactive materials. X-rays on the other hand, is man-made radiation used in medicine and dentistry. While x-rays are man made electromagnetic radiation their frequency is so high that the radiation is also ionizing.
Natural Background Nuclear Radiation:
Nuclear radiation is a normal part of our life on planet Earth. We are bombarded with nuclear radiation every day. Background radiation, from natural sources on earth and cosmic rays will cause the Geiger counter to click randomly a number of times every minute. In my corner of the world I have a background radiation that triggers the counter 22-34 times a minute. When performing radiation checks to see if a material is radioactive or contaminated with radioactive material, this background radiation count is usually deducted from the reading to evaluate if a material is radioactive.
What Geiger Counters Do Not Detect:
Geiger counters do not detect cell phone radiation. Do not detect radio frequency (RF) or electromagnetic field (EMF) radiation. EMF radiation is emitted from power transformers and other types of power electrical inductors. They cannot detect microwave radiation from a microwave oven. Nor can they detect neutrons.
Images Scientific Instruments has a free online radiation penetration calculator you can use. https://www.imagesco.com/geiger/cal/index.html