for gamma radiation 1rad=1rem, but for alpha radiation 1 rad=20rem. Why is there a difference?
Alpha radiation is more damaging per unit of energy deposited in living tissue than is beta or gamma radiation. The "quality" factor of twenty used for alpha radiation is somewhat arbitray and has changed over the years. A factor of ten is used for neutrons.
The difference in the relationship between radiation dose units (rad) and dose equivalent units (rem) for gamma and alpha radiation arises from the different levels of ionizing potential and biological effects of these two types of radiation.
1. Dose Units:
- The rad (radiation absorbed dose) measures the amount of energy deposited (absorbed) by any type of radiation in a particular material, such as human tissue. It is defined as 1 rad = 1 joule of energy absorbed per kilogram of tissue.
- The rem (roentgen equivalent in man) is a dose equivalent unit that takes into account the effectiveness of different types of ionizing radiation in causing biological damage. It allows for a more accurate estimation of the potential harm to human tissue.
2. Gamma Radiation:
- Gamma radiation consists of high-energy photons and is highly penetrating.
- Gamma rays have a relatively low ionizing potential because they carry less charge compared to alpha particles.
- Therefore, the biological effectiveness of gamma radiation is considered to be 1 rad = 1 rem. This means that 1 rad of gamma radiation is expected to cause the same amount of biological damage as 1 rem of gamma radiation.
3. Alpha Radiation:
- Alpha radiation consists of larger, highly charged particles called alpha particles, which are helium nuclei.
- Alpha particles have a much greater ionizing potential than gamma radiation due to their larger size and multiple positive charges.
- When alpha particles interact with human tissue, they deposit a higher amount of energy per unit of distance (as compared to gamma radiation).
- As a result, the biological effectiveness of alpha radiation is considered to be approximately 20 times higher than gamma radiation. Hence, 1 rad of alpha radiation is equivalent to 20 rem.
In summary, the difference in the dose equivalent for gamma and alpha radiation (1 rad = 1 rem for gamma, 1 rad = 20 rem for alpha) accounts for the higher biological effectiveness and ionizing potential of alpha particles compared to gamma rays.