Question

Suppose that an 85.0gram laboratory animal ingested 10.0mg of a substance that contained 2.50% by mass Pu-239, an alpha emitter with a half-life of 24,110 years.

1) What is the animal's initial radiation exposure in Curies?

2) If all of the energy from the emitted alpha particles is absorbed by the animal's tissues, and if the energy of each emission is 7.77x10E-12J what is the dose in rads to the animal in the first 4.5 hours following the ingestion of the radioactive material?

3) Assuming a biological effectiveness factor of 20, what is the 4.5-hour dose in rems?

Answer 1

1) Actually to determine accurately, initial radiation dose up to 3.2 Gy, which is lethal dose for dogs.

1 Gy = 1 joule/kilogram - a physical quantity.

2) The radiation dose to the animal in the first 4.5 hours and following the ingestion of the radioactive material is 4.5 Gy of acute gamma radiation.

3) 1 Sv = 100 rems

The symbol "Sv" means sievert. It is a derived unit of ionizing radiation dose in the International System of Units (SI). It is a measure of the health effect of low levels of ionizing radiation on the human body.

To obtain the equivalent dose for a mix of radiation types and energies, a sum is taken over all types of radiation energy dose.

where

H_T is the equivalent dose absorbed by tissue T

D_T,R is the absorbed dose in tissue T by radiation type R

W_R is the radiation weighting factor defined by regulation

H_T = 20 x 4.5

H_T = 90 Sv

An absorbed dose of 1 Gy by alpha particles will lead to an equivalent dose of 20 Sv.

This may appear to lead to a paradox, as this would suggest that the energy of the incident radiation field in joules has increased by a factor of 20, thereby violating the laws of Conservation of energy. However this is not the case, the sievert is used only to convey the fact that the biological effect of absorbing a gray of alpha particles would result in a 20 fold increase in the amount of biological effects that one would observe by absorbing a gray of x-rays. It is this biological component that is being expressed when using sieverts rather than the actual physical energy delivered by the incident absorbed radiation.