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Basic Radiation - Radiation Protection Section


									Science Teacher Workshop
     “Get a Half- Life”

NC Chapter - Health Physics Society
NCSU – Nuclear Engineering Dept
American Nuclear Society
Basics of Radiation
n   This is a revision of the talk presented
    at the NCSTA on 11/12/04.
n   The threshold value on the Radiation
    Bioeffects slide(slide 23) is 100 Rem for
    deterministic effects.
            Theories and Models
§   Theory: tentative explanation of observed
    phenomena; never proven; represents most
    logical explanation based on currently available
    evidence; becomes stronger as more
    supporting evidence is gathered; provides a
    context for predictions
§   Model: useful way of describing and explaining
    interrelationships of ideas; can be mental,
    physical, and/or verbal representation of an
    idea; represent what we know about an idea or
    concept; under constant change as new data
    are obtained

[NC Science Standard Course of Study & Grade Level Competencies]
            Models of Atoms
     Bohr                   Quantum Mechanical
     Model                 Model of Electron Orbits
                           1s, 2s

            +       -

 Proton                    2d

Electron orbits nucleus
~1015 times per second
    What Is “Radiation”?
         Ionizing                    Non-Ionizing
  ionizes [strips electrons        many other modes of
   from] atoms; includes:          interaction; includes:

Particulate                   Electromagnetic
- etc.
      “Radioactive” vs.

   Source                       Irradiated
  [radioactive                   Material
material or X-ray
      “Nuclide” vs. “Radionuclide”
§   Nuclide - general term referring to any known
    isotope, whether stable (about 290) or unstable
    (about 2200), of any chemical element
§   Radionuclide - a radioactive nuclide
                                 - Shleien (1998), pp. G12 & G17
Nuclide Designation:

  A = the atomic mass (no. protons + no. neutrons)
  Z = the atomic number (number of protons)
  X = the symbol for the chemical element
Note: Physical & Chemical properties depend only on Z!
          Why Some Atoms Decay:
          Nuclear Forces and Stability
  Beryllium             Forces acting on
 Atom [6Be]               6
                            Be nucleus
                            n           force
                        p       p
                        n p n          neutron

nucleus                             electrostatic
           electrons                  repulsion
                     The Curve of
                     Nuclear Stability
                     Unstable – too           N=Z
Z (number protons)

                     many protons

                                                       N ≈ 1.5 Z

                                      Unstable – too
                                      many neutrons

                      N (number of neutrons)
                     Chart of the
                     Unstable – too
Z (number protons)

                     many protons

                                                  Unstable – too
                                                  many neutrons

                              N (number of neutrons)
                      [Brookhaven National Laboratories -]
          Principal Types of
          Ionizing Radiation
PARTICULATE                          ELECTROMAGNETIC
Alpha (a) – helium nucleus Gamma (g) - photon
- from heavy nuclei [Z>82] X-ray (X) – photon
Beta (b) – electron
<250 keV max: "Low Energy
>250 keV max: "High
Energy Beta"
Neutron (n) – uncharged
Note: It is customary to categorize radionuclides according to
the type of radiation emitted.
           What About X-rays?
high speed
          target nucleus
          [e.g. tungsten]
                            “hole” at lower orbital


                                          electron falls into
                                             lower orbital
                             Penetrating Abilities of
                             Various Radiations
                   4                            Paper [or dead layer of skin]
                       2 a ++
                                  ALPHA               Plastic
Radiation Source

                                                              Lead or Concrete
                   0          -               igh
                       -1 b       BETA      h
                              GAMMA & X-RAYS
                  All Together Now:
     C ® 14N + b (low energy; 156 keV max)
     [T½=5730 y]

32        32
     P®        S+b (high E; 1,156 keV max)[T½=14.3

     Co ® 60Ni + b (high E; 318 keV max) + g's
     (1.17 MeV & 1.33 MeV) [T½ = 5.27 y]

     U ® 234Th + α’s (4.15 MeV & 4.2 MeV) + g (50
     keV) [T½ = 4.5 x 109 yr]
                   Radioactive Decay
                   & Half-Life [T½]
     1.0 __ Ao
          l               Decay Constant [λ] = 0.693/T½
              __          A(t) = Aoe-λt = Aoe-(0.693/T½)t
A(t)/Ao       _
                                   = Ao(½)t/T½
      ½       _
                              Negative Exponential Decay Curve
     ¼        _
              __               l
              _                        l
          0                                     l      l
              0     1T½ 2T½ 3T½       4T½     5T½     6T½
    Specific Activity
    [activity per unit mass or volume]

n   Inversely proportional to Half-Life
    n   Long half-life → low specific activity
    n   Short half-life → high specific activity
n   Pure Phosphorous 32 (T½ = 15 days)
    n   Sp. Act. = 286,000 Ci/g
n   Pure Carbon-14 (T½ = 5730 years)
    n   Sp. Act. = 4.5 Ci/g
                 Radioactivity Units
Activity – Amount of radioactive material
curie (Ci): 3.7x1010 disintegration/second               1934

• 1 Ci = a lot of of activity [based on 1 g radium]
• adult human has ~0.1 microcurie (µCi) 14C              1906

            becquerel (Bq): 1 disintegration/second
            • 1 Bq = tiny amount of activity [SI unit]
1852-1908                                  14
            • adult human has ~3,700 Bq         C

                1 µCi = 37 kBq = 2.22x106 dpm
          Interactions of
          Radiation with Matter
§   Ionization: ejection of orbiting electrons from
    the atom [Gollnick (1994), p. 51]
§   Excitation: raising of orbital electrons to higher
    energy levels within the atom [Gollnick
    (1994), p. 51]
§   Activation: the process of making a material
    radioactive by bombardment with neutrons,
    protons, or other nuclear radiation [Shleien
    (1998), G-1]
           Units of Radiation Dose
    Quantity                  Unit             Applicability
Exposure:             Old: roentgen [R] =    Obsolete but still
ionization per unit   2.58x10-4 C/kgair      on many direct
mass air; only for    (C = coulomb)          reading
gamma & X-ray         SI: no SI unit         instruments;
                                                  1 R » 1 rad
Absorbed Dose:        Old: rad=100 erg/g     Short term dose
energy absorbed       SI: gray [Gy] 1 J/kg   effects; generally if
per unit mass          1 Gy = 100 rad        dose>100rad(1 Gy)
Dose Equivalent:      Old: roentgen equiv.   Used for latent
absorbed dose ´       man (mammal)           effects (e.g. cancer,
weighting factor      [rem]=rad´Q            genetic effects); if
based on rad. type    SI: sievert            dose<100 rad(1Gy)
 U.S NRC Quality Factors (Q)

Type of radiation                                      Q
X-, gamma, or beta                                      1
Alpha particles, multiple-charged                      20
particles, fission fragments & heavy
particles of unknown charge
Neutrons of unknown energy                             10
High-energy protons                                    10
- from 10 CFR §20.1004; weighting factors from other
  organizations ( e.g. ICRP, NCRP, ICRU) may differ
        Radiation Interaction: Main
        Chemical Effects in Tissue
§   Primary reactions [within ~10-10 seconds of passage
    of ionizing radiation] - Water molecule dissociates
    into free radicals:
        H2O ® H + OH
§   Secondary reactions [subsequent 10-5 seconds]
        H + H ® H2 (gas)
        H + OH ® H2O (water)
        OH + OH ® H2O2 (hydrogen peroxide)
        from Gollnick (1994)
         Radiation Bioeffects
    DETERMINISTIC                       STOCHASTIC
§   Chronic vs Acute               §   Probability of occurrence
§   Severity increases with            increases with radiation
    radiation dose                     dose
§   Threshold ~ 100 Rem            §   Threshold ~10 rem, but
§   Dose & dose rate                   regulatory models
    dependent                          assume no threshold
Examples:                          Examples:
§   Cataract induction
                                   §   Cancer Induction
§   Epilation
                                   §   Genetic Mutations
§   Erythema
                                   §   Developmental
§   Blood changes                      Abnormalities
    - NCRP 138 (2001), p. 28; HPS (1995) “Risk Assessment”
        Radiation Effects
Health Effect                 Organ        Dose (rad)
Blood cell depression      Bone Marrow         50
Reversible skin effects        Skin           200
Permanent sterility           Ovaries      250 – 600
Vomiting                         GI           300
Temporary hair loss             Skin       300 – 500
Permanent sterility            Testis         350
Skin erythema                   Skin       500 – 600

 - [Acute, low LET dose] NCRP 138 (2001)
                 Stochastic Effects
§   Cancer: incidence begins to increase in populations
    acutely exposed to >10 rem [0.1 Sv], continues to
    increase with increasing dose. –BEIR V, 1990
§   Genetic Effects: ³ 100 rad of low-dose rate, low LET
    radiation needed to double the incidence of genetic
    defects in humans. -BEIR V, 1990; no human hereditary
    effects seen at gonadal doses <0.5 Gy (50 rad) –NCRP
    138 (2001)
§   In Utero Irradiation: developmental & other effects
    begin to increase at ~10 rem - NCRP 138 (2001)
§   Conclusion: “…assessments of radiological risk [should]
    be limited to dose estimates near and above 10 rem.” –
    HPS Position Statement (1995)
Annual Dose to Member of the
U.S. Population [NCRP 93 (1987)]
                     Natural     (mrem)
                     Radon          200
                     Cosmic          27
                     -external       28
                     -internal       39
                     Artificial (mrem)
                     -Diag. X-rays  39
                     -Nuc. Med.     14
                     -Consumer Pro. 10
                     -Other         ~1
                     TOTAL          ~360
Fundamentals Radiation Protection

§   TIME -- Limit time near source
    Dose = (dose rate)x(time)

§   DISTANCE -- Stay away
    Inverse square law: D2=D1(d1)²/(d2)²

§   SHIELDING – block radiation

    Precautions & monitoring

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