Biological Effects of Radiation. Dose and Dose Equivalent.


Are bananas radiologically harmful?
Are bananas radiologically harmful?

Here’s the rules. When handling IR (ionising radiation)

keep exposure short

get as far away as you can

get behind something dense

If exposed to ionising radiation, macromolecules which rely on precise conformations are damaged, change shape and don’t work. DNA and other nucleic acids can’t repair themselves and replicate nonsense proteins. Irradiating water produces highly reactive free radicals which have biological implications since water is a universal solvent.

Absorbed dose D is defined as the energy absorbed per unit mass or tissue or absorber, so D = E/m of irradiated material in J/kg

where D is 1J/kg = 1 GRAY (Gy) or the old unit, the rad, 100 rad = 1Gy

Damage H produced is in “dose equivalents” since the ionising damage is dependent on radiation type. Q is a quality factor , a dimensionless integer, 1 for betas and gammas, 20 for alpha particles.

Screen Shot 2014-04-19 at 9.16.15 PMH is measured in SIEVERT (Sv) – also J/kg,  where 1Sv = 100 rem – the old unit.

A dose of 0.01mSv is received from

  • an average year of TV watching
  • an airline flight from New York to San Francisco
  • a year living next door to a normally operating nuclear power plant

Maximum permissible dose is a dose level applied to workers in the radiation industries, including hospitals, and is about 50mSv per year from a variety of background sources, chest X-rays and so on. Workers are regularly monitored in hazardous environments.

The loss in life expectancy from a 0.01mSv dose is about 1.2 minutes, equivalent to crossing the street three times or three puffs on a cigarette. Eating a banana contributes 1 uSv (microsievert)

A dose of 5Sv is huge – causing massive tissue breakdown, consequent internal bleeding, and death within about six weeks. Some Hiroshima victims may have received as much as 100Sv. It was said that if you survived the first three weeks, you might just pull through.

Look at Q1 and 2 on P 714

Sometimes, an alternative unit, “exposure”, E, can be used. If you are sheltering behind something dense, your dose is less than if you were “exposed”. The unit for exposure is charge-dependent and is in C/kg.  Put simply, we can calculate that D = 34E in J/kg ( one ion requires 34eV to be produced in air)

We can say without explanation that Screen Shot 2014-04-19 at 9.43.41 PM

where f is a quality factor – a dimensionless integer dependent either on photon energy or the material receiving the dose, and sometimes both.