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Radiation Safety
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 Radiation Sign

The objective of this training is to provide the learner with a general knowledge of radiation theory as it applies to radioactive material used by USDA. Radiation use may be divided into three categories: sealed sources of radioactive material, unsealed sources or radioactive material, and machine produced:

General Radiation Information

Radiation is energy travelling through space, its most familar form is sunshine. Living things have evolved in an environment which has significant level of ionising radiation. Thus background radiation is what which is naturally and inevitably present in our environment.

Radiation artwork
The following video is a supplement to the training session.

 ARS Radiation Safety in Quicktime
RunningTime: 22 minutes

Radiation Source Pie Chart

Cosmic Radiation

High energy particles and photons from the sun and other sources outside the earth's atmosphere

  • Atmosphere provides sheilding from cosmic radiation
  • An increase in altitude results in an increase in exposure
  • 26 mrem/year at sea level. 50 mrem/year in Denver, Colorado
  • 0.5 mrem/hour at 39,000 feet

Terrestrial Radiation

Radiation from radioactive materials occuring naturally in the earth's crust

  • Lowest on the Atlantic coast at 16 mrem/year
  • Highest on the eastern slopes of the Rockies at 63 mrem/year
  • About 30 mrem/year in the remainder of the U.S.

Internal Radiation

Radiation from radioactive materials incorporated in the human body

  • Primarily Carbon14 and Potassium40
  • Total dose of 39 mrem/year, mainly due to Potassium40

Inhaled Radiation

Primarily Radon and its progeny

  • Radon is released from the soil as Radium226 decays
  • Radium is part of Uranium238 decay chain
  • Level vary widely from area to area
  • Average dose is 200 mrem/year
  • May be enhanced by poor ventilation or the use of Uranium containing building materials

Properties of Radiation

Radiation Atom

Radioactivity is the natural property of certain nuclides to spontaneously emit energy, in the form of ionizing radiation, in an attempt to become more stable.

Ionizing radiation has the ability to remove electrons from atoms, creating ions. The result of ionization is the product of negatively charged free electrons and positively charged ionized atoms.

There are 4 types of ionizing radiation:

  1. Alpha Particles
    • Alpha particles are highly charged (+2), heavy (4 amu), and very energetic (range 4-6 MeV)
    • Alpha particles are not very penetrating and are not an external hazard. However if inhaled or ingested, they are a significant internal hazard.
    • Alpha decay is common among nuclides with a high atomic number.

  2. Beta Particles
    • Beta particles have a charge of -1, and a small mass (1/1836 amu).
    • Low energy betas are not external radiation hazards and may require no shielding
      • Hydrogen3
      • Carbon14
      • Sulfur35
    • Higher energy betas are external hazards and are effectively shielded by 1/4 thick clear plastic laboratory shields
      • Phosphorus32
      • Phosphorus33
      • Calcium45

  3. Neutrons
    • Classified by energy
    • Thermal neutrons - same kinetic energy as gas molecules in the same environment
    • A concern at the nuclear reactor and with soil moisture probes
    • Emission of neutrons accompanies the splitting of Uranium and Plutonium nuclei

  4. Gamma Rays
    • Unlike alphas and betas, gammas cannot be entirely shielded
    • Goal of shielding is to reduce the dose rate due to those gamma rays, to a level which is as low as reasonably achievable
    • Survey meters must be available in laboratories utilizing radionuclides having a specific gamma ray emission of more than 0.1 R/hr at one meter greater. These are listed in Survey & Monitoring portion.
    • Examples of gamma-emitting nuclides
      • Iodine125
      • Iodine131
      • Zinc64
      • Cesium137
      • Cobalt60

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 Radiation Level

Alpha particles have a low penetrability and can be shielded by a piece of paper

Beta particles have a higher penetrability and are usually shielded with Plexiglas or aluminum

Gamma rays have the highest penetrability of the three, and are shielded with thick concrete or lead

Radiation Units

Two types of units are used for radiation:

  1. Units of Activity = amount of radiation emitted by source
  2. Units of Exposure (dose) = amount of radiation absorbed or deposited in a specific material by source

Units of Activity

Unit of activity of radiation is the Curie, abbreviated Ci. Most laboratories use only millicurie (mCi or .001 Ci) or microcurie (uCi or .000001 Ci)

Curie is an amount of radioactive material emitting 2.22 X 1012 disintegrations (particle or photons) per minute (DPM). Activities can be measured with an appropriate radiation detection equipmentsuch as a Geiger Counter. These instruments detect a percentage of the disintigrations and diplay in counts per minute (CPM)

Units of Exposure

The rad and rem are the two main radiation units used when assessing radiation exposure.

The rad (radiation absorbed dose) refers to the energy deposition by any type of radiation in any type of material.

The rem (radiation equivalent man) is the unit of human exposure and is a dose equivalent. It takes into account the biological effectiveness of differnt types of radiation. To account for these differences, a unit called a quality factor (QF) is used in conjuction with the radiation absorbed dose in order to dtermine the dose equivalent in rem:

rem dose = rad x QF x other modifying factors

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Radioactive Decay

As radiation energy is emitted, the material becomes less radioactive over time, decaying exponentially. The equation to calculate radioactive decay is:



A = Current amount of radioactivity

A0 = Original amount of radioactivity

e = base natural log (approximately 2.718)

k = the decay constant =0.693/t1/2 (where t1/2 = half life)

t = the amount of time elasped from A0 to A

It is important to be careful of the units used for the time. Days, hours, and years must not be mised into the calculation.

Or you can use this Radiation Decay Calculator

Half-Life and Decay

Each radioactive nuclide has its own uniqe characteristic pattern of decay, based on:

  • Types (alpha, beta, etc.) and energies of the emission involved
  • Rate of decay, or half-life

A radionuclide's half-life is the amount of time it takes for on-half of the radioactive atoms present to disintegrate or decay

Radioisotopes Half-Life Table




12.26 years


5720 years


14.3 days


23 days


88 days


3.1x105 years


12.4 hours


165 days


45.1 days


243.6 days


18.7 days


28.1 years


450 days


60 days


30.23 years

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Effect of Radiation Exposure

Radiation causes ionizations in the molecules of living cells. These ionizations result in the removal of electrons from the atoms, forming ions or charged atoms. The ions formed can go on to react with other atoms in the cell causing damage.

At low doses, such as what we receive every day from background radiation, the cells repair the damage rapidly. Most cells that die are of little consequence, the body can just replace them. Cells changed permanently may go on to produce abnormal cells when they divide. In the right circumstances, these cells may become cancerous. This is the origin of our increased risk in cancer, as a result of radiation exposure.

Risk comparision show that radiation is a small risk when compared to risks we take every day. Radiation exposure is not a mysterious source of cancer, but a well-understood phenomenon, better understood than most other cancer-causing agent to which we are exposed.

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