Roentgen equivalent, physical

Rep—Roentgen equivalent, physical A former unit of ionizing radiation equivalent to the dosage of 93 ergs absorbed per gram of tissue (93 erg/gm=0.93 rad). 

Roentgen equivalent man (rem) The amount of ionizing radiation of any type that produces the same damage to humans as 1 roentgen of radiation. One rem = 1 roentgen equivalent physical (rep)/relative biological effectiveness (RBE). In the latest nomenclature, 100 rem = 1 Sievert (Sv). 

Roentgen equivalent physical (rep) One rep is equivalent to the amount of ionizing radiation of any type that results in the absorption of energy of 93 ergs/g, and is approximately equal to 1 roentgen of X-radiation in soft tissue. 

Another commonly employed unit is the rep, which stands for roentgen equivalent, physical. This measures the energy absorbed by water or soft tissue. On the average, 1 rep is approximately 9.3 x 10 J per g of tissue, which is somewhat more than one roentgen. There is considerable variation of absorption in soft tissue, from 6 to 10 X 10" J per g, and bone absorbs about 100 x 10 J per g. Therefore, the rep is fairly satisfactory for soft tissue, but not for the whole body. 

While a curie is a measure of the physical amount, the roentgen is a measure of the amount of ionization produced by a specific material. It is the amount of x-ray or gamma radiation that produces 2 billion ionizations in 1 cm of dry air. A RAD is the radiation-absorbed dose (roughly equal to a roentgen). The radiation equivalent man (REM), also roughly equal to a roentgen, is a term for how much radiation has been absorbed, or the biological effect of the dose. 

In the old system of units, exposure was described as radiation absorbed dose rad), dose equivalent (rem), roentgen (R), or Curie (Ci). The official system of measurement, the International System of Units (SI), uses the Gray (Gy) and Sievert (Sv), where 1 Gy = 100 rad and 1 Sv = 100 rem. 1 Gy corresponds to 1 J of energy deposition in 1 kg of material. It should be noted that the Gray (and rad) are purely physical quantities reflecting the amount of absorbed radiation, while the Sievert (and rem) reflects the biological effect of the absorbed radiation and depends on the radiation type and energy. 

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