Radiation Basics

Updated August 11, 2010


The most prevalent isotopes worked with at MIT are:

Carbon - 14

Hydrogen - 3 (Tritium)

Iodine - 125

Phosphorus - 32

Phosphorus - 33

Sulfur - 35


When working with radiation, one question always comes up, "How radioactive is that?"  This is a loaded question with several answers.  Let's take a look at how it can be broken down below.

How many decays are there?

This is a great question to ask.  It directly relates to the half-life of the isotope and gives an accurate description of how many radioactive transformation are happening per unit time or simply, the activity.

When ordering or working with radioactive material, the most common units are the Curie (Ci) or (Bq).  A Bq is also one transformation (decay) per second (dps).

When detecting radioactive material with a portable instrument or a liquid scintillation counter, the most common unit is a count per minute (cpm) or decay per minute (dpm).  CPM is how many events the detector "sees".  When efficiency is taken into account, the unit DPM is used.  The DPM can also be used as a spin-off of the Bq since 1 Bq = 60 dpm.

What is the exposure?

This question will tell you how much ionizing radiation is in the air known as the exposure rate.  It is important to keep the Inverse Square Law in mind (see below).  Exposure is usually measured in Roentgen (R).

How much will I be exposed to?

This question will tell you how much energy is deposited into the body, referred to as radiation absorbed dose.  The common units are Gray (Gy) and rad.

How dangerous is this type of radiation?

This more accurately describes how radioactive the material is based on the radiation induced damage.  This question answers what damage is done to the body when the type of radiation is taken into account.  Not all radiation is created equal in terms of bodily damage.  Weighting factors (wr) have been given to each type of radiation converting radiation absorbed dose into dose equivalent.  Common units are Sievert (Sv) or rem.

Radiation Absorption

The alpha particle is essentially the nucleus of a Helium atom without the orbiting electrons.  Alpha particles only pose a significant risk when ingested or inhaled because they are only able to travel a few inches in air and can be stopped by a piece of paper.  Since they are such a large particle, they are also more damaging per particle than a beta or gamma as seen by giving it a weighting factor of 20.

The beta particle is an electron that can have either a negative or positive charge.  This distance it can travel in air is related to its energy.  A higher energy allows it to travel farther in the air.  Be sure to note, that even if a beta has a high energy 1.7 MeV like 32P, the average beta energy will be a third of that.  So on average, a beta particle will only travel a third of the maximum distance.  The best shielding material for beta radiation is plastic or Lucite shielding.  Do not use heavier shielding materials such as aluminum because the beta radiation will create x-rays from interactions with the aluminum, known as the bremsstrahlung effect.

Gamma rays, x-rays, and photons are the same particle.  The only difference is from where they originate.  The best way to shield them is with a high density material such as lead.

Radioactive Half-Life

Half-life is the amount of time needed for the radioactivity to reach one half of the original amount.