Primer of Ionizing Radiation
   isotopes have same atomic number (Z) = # protons
                          different atomic mass (A) = Z + N (# protons + # neutrons)
          [nuclides are isotopes differing in energy level: don't confuse with nucleotides]
   radioisotopes (radionuclides) are unstable:
         nucleus & electron shell are energetcially unbalanced
         nucleus undergoes radioactive decay:
              spontaneous release of energy and/or mass as particles or waveforms

Particles
    alpha & beta emitters (³²P, ³⁵S, ¹⁴C, ³H, ¹³¹I)  [read as "P 32" etc.]
        alpha particle: nucleus ejects He nucleus (2 protons + 2 neutrons)
        beta particle: neutron decays to proton + e^- (electron)
                           or, proton decays to neutron + e⁺ (positron)
            [alpha & beta emission may be accompanied by release of gamma photon: ¹³⁷Cs]

 Waveforms
     gamma emitters (¹²⁵I)
         radioactive ¹²⁵I is an isotope of stable (non-radioactive) ¹²⁷I
           125 = 53 protons + 72 neutrons (versus 127 = 53 Z + 74 N) (see periodic table)
          Decay occurs as electron capture: e^- + proton neutron + gamma photon

      Planck's Equation predicts energy content:
           E = h /    where E = energy,   = wavelength, h = Planck's constant
               shorter wavelength radiation more energetic radiation
           Energy: UV (ultraviolet) radiation  <  X-rays  <  gamma rays < cosmic rays

 Neutron activation: N bombardment renders materials radioactive
        "Criticality Incidents" &"Nuclear Excursions"
             Louis Slotin (1910 -1946), Canadian physicist at Los Alamos
             Los Alamos (Dec 1958) & Tokaimura (Sept1999) accidents
         Enhanced Radiation Weapons: "Neutron Bombs"

"Fallout"
        fission & fusion weapons introduce radioisotopes into the environment & food chain
            Ex:  Chernobyl accident (April 1986): >1 Ci ¹³⁷Cs / km²    added dose ~1 mSv / yr

      direct effects: formation of thymine dimers  (T~T)
           covalent linkage of adjacent T T bases causes errors in replication
               UV irradiation causes skin cancer
                   photoreactivation or excision repair reverse damage
                   xeroderma pigmentosum is a genetic disease caused by a repair defect

           cross-linking - different DNA molecules covalently joined
                                       H-bonds covalent bonds

           dsDNA chromosome breaks
                 non-homologues join end-to-end to form dicentric chromosomes
                 1920s radium watch dial painters ingested  ²²⁶Ra [high-energy alpha]

       indirect effects: oxidative damage
            Radiolysis of H₂0 produces free radicals:
                H₂O         H + OH             [hydroxy radical]
                HO + OH H₂O₂                [hydrogen peroxide]
                H₂O₂        H  +  HO₂^-      [superoxide radical]

             oxidation of bases modifies pairing rules
                  8-oxo-7-hydro-deoxyguanosine (GO)
                 dG GO  by oxidation, pairs with A   transversion

             Prevention & repair of oxidative damage
                superoxide dismutase (SOD): HO₂^- + H H₂O₂
                                                       catalase: H₂O₂ H₂O

Half-life
     physical - Time before 1/2 of radioactivity lost by physical decay
     biological - Time before 1/2 of material eliminated from body metabolically
     body burden - Amount of material that stays in body permanently
           critical organ depends on isotope
           ²³⁹Pu   - Plutonium: calcium analog, "bone-seeker"
         ^131,125I   - Radioiodine: used in tests of thyroid function as "thyroid-seeker"
                  ³H  - Tritium: enters "body water"
 
Dosimetry of ionizing radiation
     Measures of mass
         curie (Ci) = 1 gm of radium (²²⁶Ra)  = 3.7 x 10¹⁰ dps
               [dps = disintegration per second = 1 becquerel (Bq)]

     Measures of dose
          How much radiation strikes target?
               Measure this with a Geiger-Muller tube ("Geiger counter")
                     1 Gray (Gy) = 100 Roentgen (R) = 1 J / kg
                           Ex.: typical X-Ray series = 2.2 mGy = 220 mR

          How much radiation is absorbed by body ?
               Depends on radiation type & target
                    1 Gy X-Rays delivers 0.1 rad (radiation-absorbed dose)

           Biological effect depends on nature of radiation
                    1 Gy X-Rays delivers 1 Sievert (Sv)
                       or,  10mSv = 1 rem [roentgen-equivalent (in) man - effect dose]

direct incorporation into chromosomes  high rbe