Radiation

First go to these sites and read a quick overview of ionization radiation and associated health hazards.

Now some details:

Gamma rays and X-rays.
Gamma and X rays are very high frequency EM radiation. They overlap in the EM spectrum. The chief difference is that X-rays are produced in machines and gamma rays are from natural radioactivity. Gamma radiation is produced in nucleus of radioactive atoms during decay and their energy depends on the atomic element which generated them. They are stopped by several inches of lead shielding.

Alpha particles
Alpha particles consist of 2 protons and 2 neutrons (that is the same as a helium nucleus). They are a very heavy particle and are stopped by 4” of air or a sheet of paper. Alpha particles typically come from the nucleus of a radioactive atom. They are an inhalation hazard when a radioactive element is inhaled and the alpha particle is released very close to the respiratory tract tissue.

Beta particles
Beta particles have the same mass and charge as electrons. They have a wide range of energies, some can penetrate and burn skin. They are stopped by a half-inch of aluminum.

Neutrons
Neutrons are electrically neutral and hard to measure. They have highly variable energies, many can penetrate deeply and damage tissue.

There you have the five main types of ionizing radiation.

Here are the basic quantities used to measure all five types:
Exposure (Roentgens)
Absorbed Dose (RAD)
Dose Equivalence (REM)
Activity (Curie)

We'll talk about these in turn.
Exposure
Exposure is the ability of a beam of X or gamma rays to ionize air. The Roentgen (R) is the unit of exposure
1 R = 2.58 * 10^-4 coulombs/kg. Having defined R this way, we will use it to describe radiation of types other than gamma rays.

Absorbed Dose
The absorbed dose is the energy taken from a beam and absorbed at some point.
The RAD, Roentgen Absorbed Dose, is the unit of absorbed dose. For a beam of gamma rays being absorbed in air, 1 R = 1 RAD. (The more modern units for absorbed dose are the Gray (Gy). 1 Gray = 100 RADs, but RAD is still quite common so I'll be using that.)

Dose Equivalent
To estimate biological effects, you need to know the type of radiation, its energy, the impacted area, and the tissue type.
These factors are summed up in the REM, roentgen equivalent man. (Again, the Sievert (Sv) is a more modern unit of dose equivalence. 1 Sievert = 100 REMs, but I'll stick with REMs because the REM is still the common unit for occupational exposures.) REM = Dose*Q*N, where the dose in roentgens, Q is a quality factor, [which equals 1 for X- and gamma rays, 10 for alpha and neutrons, and 2 for beta particles] and N is modifying factor, frequently 1.

Unfortunately, much is known about the health effects of radiation. Early radiation workers reported injuries, cancer, and deaths before 1910. Safe doses are known from the use of radiation for routine diagnostic and therapeutic applications, and medical treatments that use high doses. High uncontrolled exposure in radium watch dial workers and Japanese atom bomb survivors gave the world more data.

Activity
The last metric we will discuss is the Curie, which is 3.7 x 10¹⁰ disintegration / second, which is a lot of radiation, so smaller units: milli-curies, micro-curies, pico-curies are used. A pico-curie is 10^-12 curies. Activity is chiefly used to describe natural sources of radioactivity. For example radiography (x-ray) sources used in inspection. Four pico-curies/L is the standard environmental limit due to radioactive dust or gas.

The activity of natural sources decrease exponentially. The rate of decrease is described by the decay constant "k" or the related term "half-life." [The relation is k = 0.693 / half-life ]

For example, if the initial activity of this lump of radioactivium was 500 curies and it decreases to 250 curies in one minute, it would be 125 curies (half of 250) at the end of the next minute. What would it be at the end of three minutes . Half-lives of common interest have an enormous range, from microseconds to billions of years.

NEXT

Module 06 Index

ENVE 649 Homepage