Measures Relative to the Biological Effect of Radiation Exposure

There are four measures of radiation that radiographers will commonly encounter when addressing the biological effects of working with X-rays or Gamma rays. These measures are: Exposure, Dose, Dose Equivalent, and Dose Rate A short summary of these measures and their units will be followed by more in depth information below.

• Exposure:  Exposure is a measure of the strength of a radiation field at some point in air. This is the measure made by a survey meter. The most commonly used unit of exposure is the roentgen (R).
• Dose or Absorbed Dose:  Absorbed dose is the amount of energy that ionizing radiation imparts to a given mass of matter. In other words, the dose is the amount of radiation absorbed by and object. The SI unit for absorbed dose is the gray (Gy), but the “rad” (Radiation Absorbed Dose) is commonly used. 1 rad is equivalent to 0.01 Gy. Different materials that receive the same exposure may not absorb the same amount of radiation. In human tissue, one Roentgen of gamma radiation exposure results in about one rad of absorbed dose.
• Dose Equivalent:  The dose equivalent relates the absorbed dose to the biological effect of that dose. The absorbed dose of specific types of radiation is multiplied by a “quality factor” to arrive at the dose equivalent. The SI unit is the sievert (SV), but the rem is commonly used. Rem is an acronym for “roentgen equivalent in man.” One rem is equivalent to 0.01 SV. When exposed to X- or Gamma radiation, the quality factor is 1.
• Dose Rate: The dose rate is a measure of how fast a radiation dose is being received. Dose rate is usually presented in terms of R/hour, mR/hour, rem/hour, mrem/hour, etc.

For the types of radiation used in industrial radiography, one roentgen equals one rad and since the quality factor for x- and gamma rays is one, radiographers can consider the Roentgen, rad, and rem to be equal in value.

Exposure

Exposure is a measure of the strength of a radiation field at some point. It is a measure of the ionization of the molecules in a mass of air. It is usually defined as the amount of charge (i.e. the sum of all ions of the same sign) produced in a unit mass of air when the interacting photons are completely absorbed in that mass. The most commonly used unit of exposure is the Roentgen (R). Specifically, a Roentgen is the amount of photon energy required to produce 1.610 x 1012 ion pairs in one gram of dry air at 0°C. A radiation field of one Roentgen will deposit 2.58 x 10-4 coulombs of charge in one kilogram of dry air. The main advantage of this unit is that it is easy to directly measure with a survey meter. The main limitation is that it is only valid for deposition in air.

Dose or Absorbed Dose

Whereas exposure is defined for air, the absorbed dose is the amount of energy that ionizing radiation imparts to a given mass of matter. The absorbed dose is used to relate the amount of ionization that x-rays or gamma rays cause in air to the level of biological damage that would be caused in living tissue placed in the radiation field. The most commonly used unit for absorbed dose is the “rad” (Radiation Absorbed Dose). A rad is defined as a dose of 100 ergs of energy per gram of the given material. The SI unit for absorbed dose is the gray (Gy), which is defined as a dose of one joule per kilogram. Since one joule equals 107 ergs, and since one kilogram equals 1000 grams, 1 Gray equals 100 rads.

The size of the absorbed dose is dependent upon the the intensity (or activity) of the radiation source, the distance from the source to the irradiated material, and the time over which the material is irradiated. The activity of the source will determine the dose rate which can be expressed in rad/hr, mr/hr, mGy/sec, etc.

Dose Equivalent

When considering radiation interacting with living tissue, it is important to also consider the type of radiation. Although the biological effects of radiation are dependent upon the absorbed dose, some types of radiation produce greater effects than others for the same amount of energy imparted. For example, for equal absorbed doses, alpha particles may be 20 times as damaging as beta particles. In order to account for these variations when describing human health risks from radiation exposure, the quantity called “dose equivalent” is used. This is the absorbed dose multiplied by certain “quality” or “adjustment” factors indicative of the relative biological-damage potential of the particular type of radiation.

The quality factor (Q) is a factor used in radiation protection to weigh the absorbed dose with regard to its presumed biological effectiveness. Radiation with higher Q factors will cause greater damage to tissue. The rem is a term used to describe a special unit of dose equivalent. Rem is an abbreviation for roentgen equivalent in man. The SI unit is the sievert (SV); one rem is equivalent to 0.01 SV. Doses of radiation received by workers are recorded in rems, however, sieverts are being required as the industry transitions to the SI unit system. The table below presents the Q factors for several types of radiation.

Type of Radiation

Rad

Q Factor

Rem

X-Ray

1

1

1

Gamma Ray

1

1

1

Beta Particles

1

1

1

Thermal Neutrons

1

5

5

Fast Neutrons

1

10

10

Alpha Particles

1

20

20

Dose Rate

The dose rate is a measure of how fast a radiation dose is being received. Knowing the dose rate, allows the dose to be calculated for a period of time. Fore example, if the dose rate is found to be 0.8rem/hour, then a person working in this field for two hours would receive a 1.6rem dose.