Radiological Quantities And Units: A “Real World” Perspective

Introduction

The subject of quantities and units used in radiation protection is an important, complex, controversial, and sometimes even an emotional one. While the United Stales maintains, for the most part, use of the historical (special) units, the rest of the world has converted to SI units. While the author has his own opinion of which system is better. or at least easier to use, both are presented and discussed here with an emphasis on real world use of quantities and units rather than the theory behind them.

Radioactivity

Radioactivity is a quantity used primarily to measure an amount of  radioactive material. It is expressed in terms of the rate of decay of a radioactive material. The special unit of radioactivity is the curie (1 Ci  2.22 X 1012 dpm) and the SI unit is the becquerel (1 Bq 1 dps). Since both of these units are defined in terms of the number of atoms decaying per unit time, use of disintegrations per second (dps) and disintegrations per minute (dpm), along with metric multiples and sub-units (µCi, GBq, etc.) are also acceptable.

The curie, abbreviated Ci, was originally based on the amount of radon in equilibrium with I g of 226Ra (i.e., 1g,  226Ra 1 Ci). As scientists’ ability to accurately measure decay events improved, the “definition” of the curie kept changing- not a good thing. Finally it was decided that the curie would be defined as that quantity of radioactive material in which exactly 3.7 x 1010 atoms were decaying each second (dps), without reference to the activity of 1 g of  226Ra . Since, for most radiation protection work, the curie is a rather large amount of activity, various metric prefixes are used to express smaller quantities, e.g., mCi ,µCi, nCi, and pCi.

It is important to keep in mind what the quantity radioactivity is not. Radioactivity quantifies how many atoms undergo decay per unit time, not how many particles and/or photons are emitted during this decay. For many isotopes, the number of disintegrations will vary greatly from the number of radiation quanta emitted.

As an example, let’s examine 60CO: for each atom of 60CO that decays, one beta (0.318 MeV Emax) and two gammas (1173.2 and 1332.5 keV) are emitted. This means that in one second, one curie of 60CO will have approximately 3.7 x 1010 atoms decay, while about 3 times as many (1.11 X 1010 ) radiation quanta are emitted.

The becquerel, abbreviated Bq, is the SI unit defined for radioactivity, The becquerel is defined as 1 dps. Since the Bq is such a small amount of activity, metric quantities such as MBq, TBq, or GBq are often used. It follows then that 1 Ci equals exactly 3.7 x 1010 Bq, and that 1 Bq is approximately equal to 2.7 x 10-11 Ci (not a very easy or convenient conversion!)

Radiation

The three common quantities used to measure ionizing radiation are exposure, (X), absorbed dose, (D), and dose equivalent, (H). These quantities are defined differently, have different applications and uses, and should not be used interchangeably by the radiation protection professional.

Exposure is a quantity used to measure the ability of photons ( x- and γ-gamma-rays) to ionize air. Since it is defined in these terms, it’s correct use is only for x- and y-radiation in air. Because of limitations, i.e., exposure applies only to photons in air and the unit used to quantify exposure is defined for photon energies <3 MeV, exposure is only a valid quantity when dealing with x- and γ-surveys. RSOs rarely encounter photons with energies >3 MeV, so this limitation is not usually a consideration.

The special unit of exposure is the roentgen, abbreviated R. The roentgen was adopted in 1928 and is now defined as that amount of x- or gamma-radiation that will produce a charge of 2.58x 10-4 C kg-4 in air. In the SI system, the unit for exposure is the coulomb per kilogram (C kg-1). No special name has been given to this unit.

The concept of Absorbed Dose was introduced in 1953 by the International Commission on Radiation Units and Measurements (ICRU) and is defined in terms of energy absorbed per unit mass. Absorbed dose, therefore, is valid for any type of ionizing radiation and any absorber material. The special unit defined for absorbed dose is the rad, which has no abbreviation, where 1 rad = 100 erg g-1.  The historical reason for the choice of 100 erg g-1 is that, under conditions of charged particle equilibrium, a specific energy absorption of 100 erg g-1 results from the exposure of a small volume of soft tissue to 1 roentgen. The SI unit defined for absorbed dose is the J kg-1, , named the gray, abbreviated Gy.

Unit conversion shows that 100 erg g-1  “” 10-2 J kg-1. It then follows that I Gy = 100 rad.

Many radiation protection professionals use exposure and absorbed dose interchangeably, i.e., 1R = 1 rad. While this is certainly close, it is not quite true. An exposure of 1 R will deposit 87.8 ergs g-1 in air and 95 ergs g-1 in muscle. This means that 1 R to air is 0.88 rad, and that 1 R to muscle tissue is 0.95 Rad. If one is taking a certification exam, this relationship may be important. For routine surveys and records, however, a conversion of 1 R = 1 Rad has proven to be acceptable to the NRC and to Agreement States. Remember, the meter you are reading only has a calibration accuracy of ± 10%, and the difference between Rand rad falls within that range.

The same absorbed dose from different types of radiation causes differing amounts of biological damage; Dose Equivalent is the quantity defined to account for this difference in biological effectiveness. Dose equivalent is defined as the product of the absorbed dose D, a dimensionless quality factor Q, and the product of all other modifying factors N, therefore H = DQN. [A joint task group of the International Commission on Radiological Protection (lCRP) and ICRU recommended in 1986 that N be deleted from this calculation] Therefore, H = DQ.

The special unit defined for dose equivalent is the rem (“Roentgen Equivalent Man”) and has no abbreviation. The rem is used to express dose equivalent when the absorbed dose is in rad. The SI unit defined for dose equivalent is the Sievert, abbreviated Sv. The sievert is used to express dose equivalent when the absorbed dose is in gray. Since 1 Gy = 100 rad , it follows that 1 Sv = 100 rem.

The Quality Factor is a dimensionless number used to account for the difference in the biological effectiveness of different types of radiation. It is only valid for use within the range of doses of concern in radiation protection activities for the biological effects of principal concern, i.e., cancer induction and severe genetic defects. Q is only to be used at low absorbed doses in the range of the annual radiation protection limits, and not for high doses, such as those that might be encountered in accidents.

For example, note that in Title IO of the Code of Federal Regulations Part 20 (10 CFR 20) the NRC defines a radiation area and a high radiation area in terms of dose equivalent, but that a very high radiation area is defined in terms of absorbed dose.  Recommended quality factors, and their sources, are given in Table I .

Editor’s note: NCRP Report No. 91, Recommendations on Limits For Exposure to Ionizing Radiation (NCRP, 1987) has been replaced by NCRP Report No. 116, Limitation of Exposure to ionizing Radiation (NCRP, 1993). However, the new recommendations have not been incorporated into the regulations as yet. A future article will discuss new limits recommended in NCRP 116.

Records

In the U.S ., use of the special units for record keeping is required by both 10 CFR 20 and 10 CFR 835. (10 CFR 835 applies only to DOE facilities.) When recording “dose rates” on surveys, be careful to use the correct units! Misuse of these units is a very common mistake by many radiation protection personnel. For example, when using an air-filled ion chamber, the survey results should normally be recorded in mR h-1, R h-1, etc., unless correctly converted to units of absorbed dose or dose equivalent. When using other types of detectors, such as G-M, results should generally be recorded in units of absorbed dose rate such as mrad h-1, or converted to dose equivalent rate units such as mrem h-1. For those cases when use of SI units is permitted or mandated, units of absorbed dose rate, such as cGy h-1 , or dose equivalent rate, such as cSv h-1 are common.

In general, the units to record on a survey should be correct for the quantity, and should usually be what is displayed on the meter face. Also. keep in mind that in some instances, such as the revised DOT regulations and 10 CFR § 20.2101 (b), SI units may be mandated. In this case. only Gy or Sv is correct. If a survey report states “exposure rate,” then mR h-1, µR -1, etc. are correct. Units of mrad or mrad h-1 should be used for “absorbed dose” and “absorbed dose rate” respectively; whereas, units of mrem or mrem h-1 should be used for “dose equivalent” and “dose equivalent rate” respectively. One confounding note: 10 CFR § 20.2101 (a) requires licensees to use units (including their multiples and subdivisions) of rem and rad; yet, many regulations have so far not enforced the rule to this level of detail, and continue to accept multiples and subdivisions of the roentgen unit.

A very common mistake observed by the author on many survey records, texts, etc., is use of the unit “mr” or “”mr h-1.” Remember that there is no abbreviation for rad or rem, and that survey records, as legal documents, may be used to help defend lawsuits that are becoming all too common. Would you want to go into court and explain why your survey records use mr h-1 when “mr” has no meaning in health physics? I would not want to be placed in this uncomfortable situation.

One final example is in order here. One of the most grievous misuses of radiation quantities and units observed by the author is this statement in a textbook: “An exposure dose of 41 r was estimated for one physicist.” I’m not sure what this is supposed to mean; was it an exposure of 41 R, a dose of 41 rad, or something else altogether?

Summary

Use of correct quantities and units for radioactivity and radiation is something for which every radiation protection professional should strive. Besides the fundamental quantities of radioactivity, exposure, absorbed dose, and dose equivalent, there are other derived quantities in use, especially the different dose equivalents. These other quantities include shallow, deep, and lens of the eye dose equivalent, committed and committed effective dose equivalent. and equivalent dose. These, along with weighting factors, are not reviewed here but will be covered in a future article.

References

National Council on Radiation Protection, NCRP Report No. 82, SI Units in Radiation Protection and Measurements, 1985.  National Council on Radiation Protection, NCRP Report No. 91, Recommendations on Limits  for Exposure to Ionizing Radiation, 1987. International Commission on Radiation Units and Measurements, ICRU Report 33, Radiation Quantities and Units, 1980. International Commission on Radiation Units and Measurements, ICRU Report  51, Quantities and Units in Radiation Protection Dosimetry, 1993. Nuclear Regulatory Commission, 10 CFR 20, Standards for Protection Against Radiation; rlfli11 Rule. U.So Department of Energy, 10 CFR  835, Occupational Radiation Protection; Final Rule.

About the Author

Steven D. Rima is a partner in DATUM Enterprises, a firm specializing in providing health physics training services and materials  and radiation safety consulting services. He has been registered with the National Registry of Radiation Protection Technologists (NRRP1) since 1985 and previously served on the NRRPT Panel of Examiners and Board of Directors. He has been certified In comprehensive health physics by the American Board of Health Physics (ASHP) since 1993. He is the author of the textbook Health Physics Fundamentals and Frontiers, to be published in late 1996. Article originally published in RSO Magazine, 1996. Steven D. Rima 12 Ardilla Road Tijeras, NM 87059-7412 sdrima@aol.com

 

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