Radiation Biology

Energy Transfer  Particles lose energy in matter.  Eventually energy loss is due to ionization.  Energy transferred describes the kinetic energy gained by charged particles.  Energy imparted is the energy lost by charged particles. 10 MeV particle energy energy transferred energy imparted

Kerma  Kerma is the energy transferred per unit mass. Kinetic Energy Released per unit MAssKinetic Energy Released per unit MAss  Radiative kerma is due to bremmstrahlung and annihilation.  Collision kerma subtracts the reradiated photons. Net energy transferred per massNet energy transferred per mass

Absorbed Dose  Absorbed dose or dose is the energy imparted per unit mass.  Like kerma dose is based on mean changes in energy.  Two units are used. 1 gray (Gy) = 1 J / kg1 gray (Gy) = 1 J / kg 1 rad = 100 erg / g (older)1 rad = 100 erg / g (older)

Lethality  Dose can be compared to physical effects.  Lethality refers to the likelihood that a dose will be fatal. Cell deathCell death Whole body death (see graph at right)Whole body death (see graph at right) Federation of American Scientists Dose (cGy) Lethality %

Exposure  Exposure is defined by the ionization produced by photons. Gammas and X-raysGammas and X-rays Charge per unit mass in airCharge per unit mass in air  The unit of exposure is the Roentgen (R). 1 R = 2.58 x C / kg1 R = 2.58 x C / kg Useful Conversion  Show that the original roentgen is equivalent to the modern one.  Look up constants: Density of air at STP is g / cm -3 1 esu = 3.34 x C  3.34 x C / x kg = 2.58 x C / kg

Radiation Factor  The effect of radiation on tissue depends on the linear energy transfer (LET). Higher LET is more damagingHigher LET is more damaging  Radiation has a weighting factor based on particle. Factor W R or QFactor W R or Q  In terms of LET LET L (keV /  m in water) < 10; W R = 1 10 – 100; W R = 0.32L – 2.2 > 100;  In terms of particle e, ,  ; W R = 1 n; W R = 5 – 20 p; W R = 5  ; W R = 20

Equivalent Dose  The equivalent dose is a measure that combines the type of radiation and dose.  Unit is Sievert (Sv) 1 Gy equivalent1 Gy equivalent  Older unit is rem Roentgen equivalent manRoentgen equivalent man 1 rad equivalent1 rad equivalent 100 rem = 1 Sv100 rem = 1 Sv  Natural doses Cosmics: 0.3 mSv / yr Soil: 0.2 mSv / yr Radon: 2 mSv / yr Total natural: 3 mSv / yr  Environmental hazards Flying at 12 km: 7  Sv / hr Chest x-ray: 0.1 mSv Mammogram: 1 mSv CT scan: 20 mSv