When ionizing radiation interacts with the human body, it gives its energy to the body tissues. The amount of energy absorbed per unit weight of the organ or tissue is called absorbed dose and is expressed in units of gray (Gy). One gray dose is equivalent to one joule radiation energy absorbed per kilogram of organ or tissue weight. Rad is the old and still used unit of absorbed dose. One gray is equivalent to 100 rads.
1 Gy = 100 rads
Equal doses of all types of ionizing radiation are not equally harmful. Alpha particles produce greater harm than do beta particles, gamma rays and x rays for a given absorbed dose. To account for this difference, radiation dose is expressed as equivalent dose in units of sievert (Sv). The dose in Sv is equal to "absorbed dose" multiplied by a "radiation weighting factor" (WR - see Table 2 below). Prior to 1990, this weighting factor was referred to as Quality Factor (QF).
| Table 2 Recommended Radiation Weighting Factors | |
|---|---|
| Type and energy range | Radiation weighting factor, WR |
| Gamma rays and x rays | 1 |
| Beta particles | 1 |
| Neutrons, energy < 10 keV > 10 keV to 100 keV > 100 keV to 2 MeV > 2 MeV to 20 MeV > 20 MeV | 5 10 20 10 5 |
| Alpha particles | 20 |
Equivalent dose is often referred to simply as "dose" in every day use of radiation terminology. The old unit of "dose equivalent" or "dose" was rem.
Dose in Sv = Absorbed Dose in Gy x radiation weighting factor (WR)
Dose in rem = Dose in rad x QF
1 Sv = 100 rem
1 rem = 10 mSv (millisievert = one thousandth of a sievert)
1 Gy air dose equivalent to 0.7 Sv tissue dose (UNSEAR 1988 Report p.57)
1 R (roentgen) exposure is approximately equivalent to 10 mSv tissue dose
No comments:
Post a Comment