Purpose of the Radiation Safety Handbook
Fundamentals of Radiation Safety
Background Radiation
Common Units
Time, Distance Shielding


The University of Colorado Radiation Safety Handbook (RSH) is intended to be a user’s guide for anyone working with radioactive materials and/or radiation.  Radioactive materials and radiation include unsealed sources, sealed sources, x-rays, and lasers.  The RSH is required by the University’s Radioactive Materials License.  This chapter will address basic radiation safety concepts including some background information for reference use.  It will also address the University’s Radioactive Materials License, regulations, and inspections.


Radioactivity is defined as the spontaneous emission of radiation, generally alpha or beta particles, often accompanied by gamma rays, from the nucleus of an unstable atom.  Radiation may be particles [alpha, beta, neutron] or photons [gamma, x-ray] emitted from an unstable radioactive atom as a result of radioactive decay.  All of these types of radiation are represented at the University of Colorado.  Each type of radiation has unique safety considerations and handling techniques that will be discussed in this chapter.

Background Radiation

Radiation is part of everyday life.  There are many sources of natural “background” radiation, both external and internal.  External radiation sources include cosmic (beyond the Earth’s atmosphere) and terrestrial (the Earth’s contribution) radiation.  Internal radiation sources include carbon (14C), potassium (40K), numerous other minerals which make up bones and soft tissues, and radon deposited in the lungs through inhalation.  The average radiation dose from exposure to natural and man-made background radiation in the United States is approximately 360 mrem per year (see common units below and the glossary for an explanation of these units).  As a rough estimate or rule of thumb, this value doubles for each mile of elevation gain.  Therefore, living in Boulder, Denver, or Colorado Springs increases the average background dose to approximately 500-600 mrem per year.  The increase is due to a higher contribution from cosmic radiation at higher altitudes and terrestrial radiation, specifically the transuranic elements. 

Common Units

There are many units used to describe activity, dose, dose equivalent, and exposure.  In the United States, conventional units are still being used, although the complete conversion to the Systeme International (SI) units may happen in the future.  At the present time, SI units are used in addition to conventional units on packages and radiation sources.  At the University of Colorado, subdivisions of units such as millicuries (mCi) and millirem (mrem) are used, but both SI and conventional units are acceptable.

The conventional unit for radioactivity is the Curie (Ci).  One Curie is equal to 3.7 x 1010  nuclear disintegrations or decays per second.  Other conventional units include the rad for dose and the rem for dose equivalent.  Rad is an acronym for Radiation Absorbed Dose and rem is an acronym for Roentgen Equivalent Man.  Dose equivalent was developed in an effort to incorporate biology into the physics of radiation exposure.  Not all forms of radiation produce the same biological effect.  For example, 1 rad of beta radiation is not equivalent to 1 rad of neutron radiation.  However, 1 rem of beta radiation is equivalent to 1 rem of neutron radiation.  Exposure is defined only for gamma or x-rays in air, not tissue.  Roentgen (R) is the unit of exposure.  Many radiation survey meters use units of milliroentgen (mR).  Please refer to the Glossary for additional clarification of these terms.

The SI unit of radioactivity is the Becquerel (Bq) which is equivalent to one disintegration or decay per second.  The SI unit of dose is a Gray (Gy) and the unit of dose equivalent is a Sievert (Sv), both of which are equivalent to 1 Joule per kilogram by definition.  As mentioned previously, not all forms of radiation produce the same biological effect.  For example, 1 Gray (Gy) of beta radiation is not equivalent to 1 Gray (Gy) of neutron radiation.  However, 1 Sievert (Sv) of beta radiation is equivalent to 1 Sievert (Sv) of neutron radiation.

Conventional Units
Systeme International (SI) Units
1 Ci = 3.7 x 1010 disintegrations / second
1 Bq = 1 disintegration / second
1 Ci = 1000 mCi
1 Bq = 2.7027 x 10-11 Curies
1 mCi = 1000 uCi
1 kBq = 1,000 Bq = 2.7027 x 10-8 Ci = 2.7027 x 10-5 mCi
1 mCi = 37,000 kBq = 37 MBq
1 MBq = 1000 kBq = 1,000,000 Bq = 0.027027 mCi
1 Rad = 100 ergs / gram = 0.01 J / kg
1 Gy = 1 J / kg = 100 Rad
1 Rad = 0.01 Gy
1 Gy = 100 centigray (cGy)

1 cGy = 1 Rad
Dose Equivalent
Dose Equivalent
1 Rem = 1000 mrem
1 Sv = 1 J / kg = 100 Rem
1 Rem = 0.01 Sv
1 mSv = 0.1 Rem = 100 mrem
1 mrem = 0.01 mSv


A common acronym used in radiation safety is ALARA, which stands for As Low As Reasonably Achievable.  The ALARA philosophy attempts to incorporate physical, social, and economic factors in reducing doses to individuals.  The University of Colorado has an ALARA program which is reviewed each year to evaluate efforts at keeping doses and exposures ALARA.  It is the responsibility of each radiation worker to keep the dose to themselves and the people around them ALARA.  Refer to Appendix A for a copy of the University’s ALARA program.

Time, Distance, Shielding

Radiation doses may be reduced by taking advantage of time, distance, and shielding.  By reducing the time spent working with radioactive materials and/or radiation producing machines, the dose received from the radiation is reduced.  Increasing the distance from a point source also will reduce the dose because the intensity of radiation decreases at approximately 1/d2, where d is the distance from the source.  For example, if the distance (d) is doubled, the intensity is reduced to ¼ (d2) of the original intensity.  This is also known as the Inverse Square Law.  Shielding can be very effective in reducing the dose received.  There are different types of shielding for different types of radiation.  Use caution when selecting shielding to reduce the radiation dose.  The dose may actually increase by selecting the wrong shielding.  Verify radiation levels with a survey meter to ensure that appropriate and/or enough shielding has been used.  Health Physics normally does not provide shielding to researchers.   However, staff members are available to answer general shielding questions and discuss the efficacy of shielding materials.

Remember:  Time, Distance, and Shielding
Doses can be minimized by taking advantage of the following simple methods:
1.    Reduce the amount of time spent near the radioactive material/source
2.    Increase the distance from the source
3.    Use appropriate shielding whenever possible

Table of Contents

Foreward Laboratory Licensing