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]

 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 wave length  more energetic radiation
           Energy: UV (ultraviolet) radiation  <  X-rays  <  Gamma Rays < Cosmic Rays

 Neutron activation: exposure of materials to neutron bombardment
       contamination versus activation
       "Neutron bombs" & "Nuclear Excursions":
             Los Alamos accident (December 1958)
             Tokaimura accident (September 1999)
        fission & fusion weapons introduce "Fallout" into the environment & food chain

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

       indirect effects: Oxidative damage -
            Radiolysis of H₂0 produces free radicals:
                H₂O H + OH               [hydroxy radical]
                HO + OH  H₂O₂          [hydrogen peroxide]
                HOOH  H  +  HO₂^-      [superoxide radical]
           oxidation of bases modifies pairing rules
                  8-oxo-7-hydro-deoxyguanosine (GO)
                 dG  GO  by oxidation, pairs with A   transversion
           single-strand  chromosome breaks - broken ends peroxidized
           double-strand  chromosome breaks -
                 non-homologues join end-to-end to form dicentric chromosomes
                ²²⁶Ra -  radium watch dial painters
          cross-linking - different DNA molecules covalently joined
                                    H-bonds converted to covalent bonds

           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  = 3.7 x 10¹⁰ dps
               [dps = disintegration per second = 1 becquerel (Bq)]

     Measures of dose
         roentgen (R)  - exposure dose
               How much radiation strikes target?
               Measure this with a Geiger-Muller tube ("Geiger counter")

         radiation-absorbed dose (rad) - absorption dose
               How much radiation is absorbed by body when exposed to 1 R ?
               Depends on target (e.g., skin versus gonads)
               [1 rad = 10 milligray (mGy)]

        roentgen-equivalent in man (rem) - effect dose
               What is biological effect of 1 R ?
                   [Average person is exposed to 1 millisievert (mSv) = 0.1 rem / year]
              Dose-response curve suggest linear relationship
               Depends on relative biological effect (rbe) of radiation
                     e.g., radionucleotides in nucleic acid experiments (³²P- & ³⁵S- dNTPs)
                             direct incorporation into chromosomes  high rbe
                Related to linear energy transfer (LET) = energy / path length
                     How much energy is released during passage through cell / body?
                     gamma & X-rays: (very) high energy over (very) long path length  low LET
                     alpha & beta particles: low energy over (very) short path length  high LET
                                                             high specific ionization over short path