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Ionising radiation

Radio-active decay:

  • radioactive material is composed of atoms that are radioactive which means their nuclei are unstable in an high energy state & when they decay to lose this excess energy, this appears as ionising radiation which, in general, may be either alpha, beta or gamma rays
  • a person or object exposed to radiation does not make them radioactive
  • food and medical equipment is frequently exposed to gamma radiation to sterilise it, but they do not pose a radiation risk.
  • a person or object contaminated by radioactive material whether on the skin or ingested, does become radioactive, although the risk to others is very small if adequate precautions are taken
  • it is generally advised that patients undergoing nuclear med scans (during which they are purposely “contaminated” with radioactive material) should not be in close contact with pregnant women for prolonged periods until the material is excreted - usually within 48hrs.

3 categories of ionising radiation:

charged particles:

alpha rays:

  • 2 protons + 2 neutrons (ie. Helium nuclei)
  • +ve charged from elements greater than lead in atomic number 
  • RBE 10-20;
  • absorbed by the outer layers of the skin, a sheet of paper, etc

beta rays: 

  • electrons (-ve charged); 
  • RBE=1;
  • absorbed by thin slices of low density material such as aluminium or plastic

other ionising particles:

  • fast protons: RBE 5-10;
  • deuterons: RBE 10-15;
  • heavy ions/fission fragments: RBE 20-200!!;

electromagnetic radiation (photons):  

gamma rays:

  • RBE=1;
  • absorbed by significant thicknesses of dense material such as lead or concrete

uncharged particles:


  • RBE 1-10;
  • absorbed by hydrogenous materials such as water & paraffin


Sievert (Sv)

  • the SI unit of effective dose = 1 joule of energy absorbed per kg body mass = 100 REM 
  • ie. 1 mREM = 0.01mSv
  • takes into account partial-body irradiation & the particular type of radiation that results in an absorbed dose
  • CXR = 0.02mSv
  • average annual background radiation = 2-3 mSv
  • average annual additional exposure for general public = 1mSv
  • average annual additional occupation radiation for radiographers = 1 mSv
  • max. allowable occupational radiation = 50 mSv in any one yr or 100mSv over 5yrs
  • max. allowable occupational radiation exposure to pregnant women is 0.5mSv/month

other measures of radiation:

  • Becquerel (Bq)
    • radiation decay activity of a substance and 1Bq equates to 1 decay per second
    • the activity of a given amount of a radioactive substance can be estimated by:
      • number of moles x Avogadro constant x ln(2) / decay half life in seconds
  • Roentgen
    • = amount of X-rays or gamma-rays required to produce 2.08 x 10^9 ion-pairs per cc of dry air at STP;
    • = 83.3 ergs/g of air (1 erg = 10-7 J) ~ 93 ergs/g of water or biologic tissue;
    • this cumbersome definition was derived by X-ray workers & was related to the current that could be drawn from ionisation chambers of a particular volume;
    • this measurement of exposure ignores the excitations produced by radiation but roughly agrees with rads;
  • rad
    • = radiation absorbed dose = 0.01 gray (Gy);
    • = amount of X-rays or gamma-rays required to deposit 100 ergs/g in the particular absorber (eg. tissue);
    • this is measured by the heat given off;
    • eg. a PA CXR = 0.15mGy; an AP Cx Spine XR = 0.95mGy; an AP AXR = 3.0mGy (ie. 20 CXR's!!)
  • RBE
    • = relative biological effectiveness (gamma-rays=1);
    • takes into account radiation other than gamma-rays & X-rays by comparing that radiation's degree of damage to gamma-rays & is related to the particles' linear energy transfer rate & ? oxygen enhancement ratio;
  • REM
    • = roentgen equivalent in man;
    • = dose in rads RBE;

Average annual radiation exposures (mSv/yr)


  • cosmic = 0.35
    • varies with latitude;
  • soil = 0.70
    • varies with location (1000 in India/Brazil);
  • body K40 = 0.20
  • body Ra226 = 0.05
  • air Rn222 = 0.50
    • highly variable;


Total average radiation exposures

  • total average = 4.82
    • probable range for majority population is 200-600 (ie. 2-6mSv/yr)
radiation_ionising.txt · Last modified: 2019/10/25 23:59 by

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