Ionizing Radiation Safety

I would like to share the basic radiation safety knowledge which I learnt during research in City University of Hong Kong in 1996 and attended the training course under Occupational Safety & Health Council (OSHC) in 2003.

The photo was taken from TVB news (unit is mSv). The unit is explained below.

Ionization Radiation Hazard and Protection

Units and terminology used in radiation

A) Activity (Curie (Ci) and Becquerel (Bq))

The Becquerel is the amount of radioactive material in which, on average, 1 atom disintegrates per second. (1 Ci = 3.7x1010 Bq)

B) Exposure (the roentgen (R))

This exposure is defined only for X-rays or γ-rays. The SI unit is Coulomb per kilogram (C/kg) where 1 R = 2.58x10-4 C/kg of air.

C) Absorbed dose (D) (rad and gray)

The absorbed dose is defined as the quotient dE/dm, where dE is the mean energy imparted by the ionizing radiation to the medium of mass dm. The SI unit is J/kg and is given the name gray (Gy), i.e. 1 Gy = 1 J/kg of the material. The old unit was rad where 1 rad = 0.01 J/kg = 0.01 Gy.

D) Dose equivalent H (rem and Sievert)

A weighting factor (Q), called the quality factor, is introduced which represents the relative amount of damage likely to be caused to living cells by the same amount of different radiations.

Q = 1 (X-ray, Gamma-ray and electrons)

Q=10 (other particles carrying a single electric charge)

Q=20 (alpha particles and particles carrying more than one unit of electric charge)

Q= 2 to 20 (neutrons (depending on energy))

The absorbed dose is then multiplied by Q to give the dose equivalent H.

H = D x Q x N

Where N is the product of all modifying factors

The SI unit of dose equivalent is still J/kg (if N = 1) and but is given the name Sievert (Sv), i.e., 1 Sv = 1 J/kg of the material. The old unit was rem where 1 rem = 0.01 J/kg = 0.01 Sv.

Another description using Radiation Weighting Factor (WR). H = D x WR

WR=1 (X-ray, Gamma-ray and β Particles)

WR=5 (Neutrons <10kev>20MeV)

WR=10 (Neutrons >10keV to 100keV and >2MeV to 20MeV)

WR=20 (Neutrons >100keV to 2MeV)

WR=20 (α Particles)

E) Effective Dose (E)

The quantity obtained by multiplying the equivalent doses to various tissues and organs by the Tissue Weighting Factor (WT) appropriate to each and summing the products. E = H x WT

F) Hierarchy of Dose Quantities

Biological Effects of Radiation

Effects can be classified as acute or chronic. Acute effects result from a single high dose exposure. Chronic effects are due to continuous exposure to relatively low dosage over long periods of time.

For Acute effects: (Dose in Sv: Clinical effects)

0.05-0.25 Sv: Threshold of detectable effects

0.25-1 Sv: Detectable temporary blood changes

1-2 Sv: Blood changes but expect 100% recovery

4.5 + 1 Sv (LD3050) : Lethal Dose. Wide range of symptoms. Expect 50% of persons receiving such dose to die within 30 days.

10 Sv: Radiation sickness; blood changes in 2 days

100 Sv: Death within a few hours

For Chronic effects: (Dose in mSv/day: Clinical effects)

0.01 mSV/day: Approximately background

0.1 mSv/day: Approximately max permissible dose rate for occupational workers

0.3 mSv/day: Decrease in lymphocytes

10-100 mSv/day: Illness within 6 weeks, death in 3 months

Hong Kong background radiation is about 2 mSv/year.

Reference:

Radiation Ordinance, Cap 303 of Laws of Hong Kong (1982) - http://www.hklii.org/hk/legis/en/ord/303/

Radiation (Control of Radioactive Substances) Regulation - http://www.hklii.org/hk/legis/en/reg/303A/

Radiation (Control of Irradiating Apparatus) Regulation - http://www.hklii.org/hk/legis/en/reg/303B/

Certificate of Competence Courses - (RA)Certificate of Competence in Ionising Radiation Protection in OSHC (http://www.oshc.org.hk/)

Program for Workshop on Laboratory Safety organized by City University of Hong Kong in 1996