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Overview of Radiation

Radiation is a topic the general public does not understand.  To understand the threat associated with a radiological dispersal device, such as a dirty bomb, a basic knowledge of radiation is required.  Knowing how to protect oneself from alpha or beta particles, gamma or x-rays, and neutrons is important to survive a radiological release – accidental or intentional. 


While regular exposure to minimal amounts of radiation is unavoidable because of its natural presence on earth, exposure to excessive amounts is avoidable by practicing the “time, distance, and shielding” principles.


According to the Nuclear Regulatory Commission (NRC), radioactivity is an atom which, “decays or disintegrates spontaneously, emitting radiation (NRC, 2005).”  It is important to remember that radiation comes from material that is emitting radioactive particles.  That is, a radiation-emitting substance must be present for a radioactive threat to be present.  Five types of radiation are worth mentioning – alpha particles, beta particles, neutrons, gamma rays, and x-rays.


Alpha Particles

Alpha particles (a) are relatively high in mass and energy yet move very slowly, penetrating only centimeters through air before being absorbed.  Very thin material can provide shielding from alpha particles such as skin, paper, clothing, or plastic sheeting.  Therefore, alpha radiation is not considered an external hazard – it is only a hazard to people if internalized.  If radioactive matter which is emitting alpha particles enters a person’s body through inhalation, ingestion or injection, the tissue lining those internal structures is not thick enough to block the alpha particles – they are an internal hazard.  Accordingly, the large amount of energy contained in those alpha particles will transfer to the cells they contact, and will result in damage.


Beta Particles

Beta particles (b-) are small in mass but travel very fast, capable of penetrating up to 10 feet through air.  Thick clothing, building walls, sheets of metal or plastic and vehicle structures will provide shielding from beta radiation.  Skin and normal street clothing are not thick enough to block beta particles, therefore they present an external hazard to humans.  Like alpha particles, ingesting or inhaling beta emitters will also cause internal damage, such as burns, tissue necrosis or DNA changes.


Gamma Rays and X-Rays

Gamma rays (g) and x-rays (x) are very similar, differing only in their origin.  Gamma particles are produced from the decay of the nucleus of an atom while x-rays are emitted from the electron cloud.  The high speed of these mass less rays allows them to travel for long distances through the air, at distances measured in miles.  Shielding against x-rays and gamma rays is with heavy materials, such as thick lead.  These rays will penetrate through clothing, skin and tissue, causing cellular damage.



Neutrons (n) are one of the components of an atom’s nucleus.  The evolution of a free neutron only occurs during nuclear fission, or atomic splitting.  The spontaneous release of a neutron in nature is a rare event, but can be induced if weaponized (CR Scientific).  When neutrons are liberated and travel through the air, they will induce radiation in whatever matter they encounter.  That is, by placing a neutron emitter next to an object, then removing the emitter, the object will become radioactive and begin emitting alpha or beta particles or gamma rays. 


This is different from the other types of radiation; neutrons are the only radioactive particles which can induce radiation.  Unfortunately for any living creature, water is one of the most efficient neutron absorbers.  Because of the high water content of animals, an attack using a neutron emitting device will permeate any metal structure but will be absorbed by the occupants, killing them and causing their corpses to become radioactive.