Lecture-3

  Radiation Quantities and units

 

Exposure

—When a volume of air is irradiated with x-rays or with gamma rays, the interaction that occurs between the radiation and neutral atoms in the air results in some electrons being librated from those air atoms as they are ionized.

—Consequently, the ionized air can function as a conductor and carry electricity (why?).

—As the intensity of x-rays exposure of the air volume increases, the number of electron-ion pairs produced also increases.  

—Thus, the amount of radiation responsible for the ionization of a well-defined volume of air may be determined by measuring the number of electron-ion pairs or charged particles in that volume of air

—This radiation ionization in the air is termed exposure.

—Radiation “Exposure” is a measure of the ability of radiation to ionize air.

—Conventional Units Roentgen (R) 

—SI unit: Coulomb/Kilogram (C/Kg)

   Absorbed dose

    —As ionizing radiation passes through an object, some of energy of that radiation is transferred to that medium.

—It is actually absorbed by the object and stays within it.

—Absorbed dose (D): is defined as the amount of energy per unit mass absorbed by irradiated object.

—This absorbed energy is responsible for any biologic damage resulting from the tissue being exposed to radiation.

—D = ∆E/∆m

—SI unit; Gray (Gy)

—Conventional unit: J/Kg

 

—Anatomic structures in the body posses different absorption properties; some structures can absorb more radiant energy than others.

—The amount of energy absorbed by a structures depends on:

Ø The atomic number (Z) of the tissue composing the structure.

ØThe mass density of the tissue (measured in kg/m3).

ØThe energy of incident photon.

Equivalent dose

—The different in biological effectiveness must be taken into account if we wish to add doses of different radiations to obtain the total biologically effective dose.

—To do this we must multiply the absorbed dose of each type of radiation by a radiation weighting factor (WR) which reflects the ability of particular type of radiation to cause damage.

—The quantity obtained when absorbed dose is multiplied  by WR is known as equivalent dose.

— Equivalent dose = D.WR

—Unit:  Sievert.

weighting factors

 

—Equal absorbed dose of different types of radiation produce different amounts of biological damage in the body.

—For example, 1 Gy absorbed dose of fast neutrons causes more biological damage than 1 Gy absorbed dose of x ray.

—The concept of dose equivalence takes this biologic impact into consideration by using specific modifying, or Quality factor (Q), to adjust the absorbed dose value.

— Quality factor (Q), or weighting factor (W), is an adjustment multiplier that  was used in the calculation of dose equivalence  to specify the ability of a dose of any type of  ionizing radiation to cause biologic damage.

 

—For practical purposes of assessing and regulating the hazards of ionizing radiation to workers and the general population, weighting factors (previously called quality factors, Q) are used.

—Weighting factors are dimensionless multiplicative factors used to convert physical dose (Gy) to equivalent dose (Sv) ; i.e., to place biological effects from exposure to different types of radiation on a common scale.

—A weighting factor is not an Relative Biological Effectiveness RBE.

Effective dose

—Different organs and tissue have differing sensitivities to deal with the very common situation in which the body is not uniform exposed.

—Effective dose (E), is summing the equivalent doses to all tissues and organs of the body, multiplied by a weighting factor WT for each tissue or organs.

—E = ∑T HT.WT

—Where HT is the equivalent dose in tissue T,

—Unit, Sieverts (Sv). 

     Dose Rate

—The dose rate is a measure of how fast a radiation dose is being received. Knowing the dose rate, allows the dose to be calculated for a period of time.

— Dose rate = dose/time

—Example:

   If the dose rate is found to be 0.8rem/hour, then a person working in this field for two hours. How much dose he received?

   1.6 rem

Units of Radioactivity

—The original unit for measuring the amount of radioactivity was the curie (Ci)

—The becquerel (Bq) measures the activity of the radioactive source, meaning the number of atoms which, within a particular time frame, transform and emit radiation.

—1 curie = 3.7x1010 radioactive decays per second

—In the International System of Units (SI) the curie has been replaced by the becquerel (Bq), where

—1 becquerel = 1 radioactive decay per second = 2.703x10-11 Ci.

—The System International of units (SI system) uses the unit of becquerel (Bq) as its unit of radioactivity. One curie is 37 billion Bq.

Unit of Radiation Dose

 

Radiation “Dose” is a measure of amount of energy deposited (eg, in tissue) by radiation

—Conventional Unit:  rad:

   1 rad = amount of radiation that deposits 100 ergs of energy in 1 gram of irradiated material

—Standard International (SI) unit: Gray (Gy):

  100 Rads = 1 Gy

  1 Rad = 1/100 Gray = 1 Centigray

 Units of Biological Effect

  Equal amounts of energy deposited by different types of radiation (photons, alpha particles, neutrons, etc) cause different amounts of damage and thus different  amounts of biological effect.  Units of bio-effect are derived from units of dose by applying a “Quality Factor” QF, which depends on the type of radiation.

— Conventional Unit:  rem (radiation equivalent-man)

—1 rem = 1 rad x QF

— Standard Internation (SI) unit:  Sievert (Sv)

—1 Sievert = 1 Gray x QF

   —100 rem = 1 Sievert

Summary of Radiation Units

1 gray (Gy) = 100 rad

1 rad = 10 milligray (mGy)

1 sievert (Sv) = 1,000 millisieverts (mSv) = 1,000,000 microsieverts (μSv)

1 sievert = 100 rem

1 becquerel (Bq) = 1 count per second (cps)

1 curie = 37,000,000,000 becquerel = 37 Gigabecquerels (GBq)