Primer of Ionizing Radiation
   isotopes have same atomic number (Z) = # protons
                       different atomic mass (A) = Z + N (# protons + # neutrons)
            Ex.: ¹²⁵I is an isotope of ¹²⁷I
                   125 = 53 protons + 72 neutrons (versus 127 A = 53 Z + 74 N) (see periodic table)
          [nuclides: isotopes differing in energy level: nucleotides]
   radioisotopes (radionuclides) are unstable:
         nucleus & electron shell are energetically 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)

 Waveforms
     gamma emitters (¹³⁷Cs)
          [Beta-decay to ¹³⁷Ba, then] electron capture: proton + e^- 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
               Ex.:  long-wave UV B in "black lights" is safer than short-wave UV A in tanning beds

 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 (Sept 1999) accidents
         Enhanced Radiation Weapons: "Neutron Bombs"

"Fallout"
        fission & fusion weapons introduce radioisotopes into the environment & food chain
              Neutron activation of atmospheric elements

        Nuclear reactor accidents release short- & long-lived radioisotopes             
               Ex:  Chernobyl (April 1986) & Fukushima Daiichi (March 2011) release ¹³⁷Cs  & ¹³¹I

      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
                 radium watch dial painters (1920s) 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
                 Ex.:  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 ( t_1/2)
     physical - Time to lose 1/2 of radioactivity by physical decay (T_p)
                        amount remaining = 0.5^t
                        Ex.: 10 half-lives leaves 1 / 1024 ~ 0.1%

            ¹³¹I:      t_1/2 = 8 days
            ¹³⁷Cs:  t_1/2 = 30.2 yrs

     biological - Time to eliminate 1/2 of material from body metabolically (T_b)
           ¹³¹I:      t_1/2 = 138 days
           ¹³⁷Cs:   t_1/2 =   70 days

     effective - combined physical & biological decay loss ( 1 / T_e  =  1/ T_p  +  1/T_b )
                        then T_e = (T_p)(T_b)/ (T_b + T_p)
                        HOMEWORK: Prove the formula; calculate T_e for ¹³¹I and ¹³⁷Cs

     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
               1 dps = 1 disintegration per second = 1 becquerel (Bq)
               1 PetaBq (PBq) = 10¹⁵ Bq = 27 kCi  [Table of SI Units]
                    Ex.: Acute Polonium (²¹⁰Po) poisoning as assassination tool

     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

     Public Health concerns arise from large-scale release
                Ex.: Hiroshima (Aug 1945) explosion (14 kt TNT) released 59 x 10¹² J (59 TJ) =  8×10²⁴ Bq = 8 YottaBq

                Ex:  Chernobyl accident (April 1986) released ~85 PBq ¹³⁷Cs & 1,760 PBq ¹³¹I
                              Environmental Contamination < 1500 kBq / m²
                              Exposure of Reactor crew 
                                                                 
                Ex:  Fukushima Daiichi accident (March 2011) released ~15 PBq ¹³⁷Cs & 500 PBq ¹³¹I

    Measures of effect
      How much radiation is absorbed by body ?
                     Depends on radiation type & target
                    1 Gy X-Rays delivers 100 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]

little effect [except: risk of cataracts]

direct incorporation into chromosomal DNA high rbe