Radiological Principles

þ LET Concept
 - Alpha, Neutrons - High LET  
 - Electrons, Photons - Low 
 - LET LET Correlated with Hazard? 
 - High-LET radiation has a greater biological effect than low-LET
radiation. Relative Biological Effectiveness (RBE) is a quantitation of
this biological effectiveness. Low-LET radiations are assigned an RBE of
1, while High-LET radiation radiations have a RBE greater than 1, with
values as high as 20. 
 - Clinical relevance of High-LET radiation is significant in cases where
neutron generation, a criticality excursion, or a nuclear detonation
occurs. All of these events generate High-LET neutrons. A criticality
excursion is a special event in which an uncontrolled chain fission
reaction occurs when a certain amount of nuclear material exceeds a
threshold called critical mass. This results in generation of heat,
neutron and gamma radiation, along with fission gas. Criticality
excursions usually occur in research or industry accidents where fissile
material exceeds critical mass in an "unsafe" geometry. 

þ Half-Life
- Physical half-life is the the time required for a radionuclide to lose
  half of its material by radioactive decay. 
- Biological half-life is the time required for a biological system to
  eliminate by natural processes, half of the amount of substance
  (radionuclide) that entered it. 

þ Units
 - The Curie (Ci) is the traditional / conventional unit of activity is
   defined as that amount of radioactivity contained in 1 gram of pure
   radium and is equal to almost 2 trillion atomic transformations per
   minute or 37 Billion atomic transformations per second. The Becquerel
   (Bq) is the SI unit of activity and equal to one atomic transformation
   per second. Consequently, there is a difference in magnitude between the
   two units of approximately 10^10!
 - R (Roentgen) 	exposure unit	1 exposure unit = 3881 R 
 - rad (radiation absorbed dose) 1 rad = 0.01 Gy Gy (Gray) 1 Gy=100rad 
 - rem (radiation equivalent man) 1 rem = 0.01 Sv Sv (Sievert) 1 Sv= 100 rem  
 - The units R, Rad, and Rem should be considered about the same for
   clinical purposes! The SI units of Gy and Sv may also be considered
   approximately equivalent for clinical purposes! 
 - If a technical survey nerd speaks to you in "counts per minute," convert 
   to mR/h by dividing by 2500 for estimating mR / hr (a working approximation 
   for medical providers only) 

þ Exposures
 - Chest x-ray 10 mrem 0.1 mSv 
 - CT Abdomen 1000 mrem 10 mSv 
 - 10 CFR 20 by the Nuclear Regulatory Commission (NRC) specifies
   radiation dose limits for pregnancy, the public, and occupational
   workers. Medical radiation exposures do not count towards these exposure
   limits. 
Pregnant woman 0.5 rem (entire pregnancy) 5 mSv (entire pregnancy) 
Public 0.5 rem / yr 5 mSv / yr 
Occupational Worker (dose equivalent)' 5 rem / yr 0.05 Sv / yr 

[DOE] Guidelines for Control of Emergency Exposures 
Dose Limit 
(Total Effective Dose Equivalent) 	activity Performed	 Conditions 
5 rem (0.05 Sv) 			All
10 rem (0.10 Sv) 			Protecting major property Only on a voluntary basis 
where lower dose limit not practicable 
25 rem (0.25 Sv) 			Lifesaving or protection 
					of large populations