Topic 7 - Dose Limits

Radiation Protection

Review of Basic Concepts

• Absorbed dose
  • Physical quantity
  • D = dε/dm
  • dε is the mean energy imparted by ionizing radiation in mass m
  • erg/g or J/kg
• Dose equivalent (H)
  • Considers biological effectiveness of radiation
  • H = DQ or DW_R

Radiation Weighting Factors (Q, W_R)

• ICRP 1979
  • Alpha particles assigned value of 20
  • Beta particles (negatron/positron), 1
  • Gamma rays, 1
  • X-rays, 1
  • Other values…neutrons etc
• ICRP (later)
  • Some changes in neutrons, other radiations

Calculating Internal Dose

• Arbitrarily shaped container
  
• Distributed activity
• Calculate energy absorbed per unit mass
• Consider:
  • Energy per decay
  • Total activity
  • Mass of container (think: organ)
  • Fraction of energy emitted absorbed in container (called “target”)
  • “absorbed fraction”
    • φ
    • (Energy absorbed by target)/(energy emitted by source)

Absorbed fraction, φ

• Consider
  • Photons (“penetrating radiation”)
    • Fraction absorbed in target
    • Fraction escapes
  • Betas (“non-penetrating radiation”)
    • All energy absorbed in target
    • (exception- extremely small targets)
  • Alphas (non-penetrating)
    • All energy absorbed in target
    • (exception - even smaller target than for betas)

Generic Equation for Calculating Absorbed Dose Rate

• D    = absorbed dose rate, rad/h or Gy/s
• A    = activity, μCi or MBq
• n_i    = # of radiations of energy E emitted per transformation
• E    = energy per radiation (MeV/dis)
• Φ    = absorbed fraction
• m   = mass of target
• K    = proportionality constant
  • (rad-g/ μCi-hr-MeV or Gy-kg/MBq-s-MeV

Generic Equation for Calculating Total Absorbed Dose

• D  = absorbed dose rad or Gy
• Ã = cumulative activity, μCi-hr or MBq-s

Cumulative Activity, Ã

• Ã: integrated area under time-activity curve
  
• Units typically
  • Bq-s
  • μCi-hr

Evaluating Multiple Contaminated Objects

• Contaminated object irradiates
  • Itelf
  • Others
• How is dose calculated?

Calculating Total Dose

• Relationships to consider
• Object irradiating itself: φ(1¬1)
• Object being irradiated by others:
  • Source (S) and Target (T)
  • φ(1¬2)
  • φ(1¬3)
• Repeated for multiple targets and sources
• φ(2¬1), φ(2¬2), φ(3¬1), φ(3¬2),..etc

Target/Source Configurations

Calculating Total Dose

• Generic equation
• Expressed in many ways (see Cember, pp 201- )
• Parameters grouped
  • Physical
  • Biological
• Computers used to evaluate

Calculating Total Dose - ICRP 2

• Dose equivalent rate

Calculating Total Dose - ICRP 30

• Cumulative dose equivalent

ICRP 2 vs ICRP 30

• ICRP 30
  • Organ weighting factors used to derive effective dose eqiuvalent
    
• ICRP 2
  • Individual organ doses calculated
  • Organs assumed to be spheres
  • No irradiation of other organs

MIRD System

• Medical Internal Radiation Dose
• Simple equation (lots of lumped parameters)
• See Cember, pps 201 - …
  

Other Considerations

• Kinetic Models
  • Biological
  • GI Tract
  • Lung
  • Intake (back-calculating intake from measured data)
• Ã
• U_S
• λ_effective

Determination of CDE

Total Disintegrations, Total Dose

ICRP : Determination of the Committed Dose Equivalent,  H_T,50

• Total dose equivalent averaged throughout any tissue over the 50 y after an intake is the CDE:
  • M = mass of specified organ or tissue
  • i = radiation type
  • D_50,i = total absorbed dose during a period 50 y after intake of the radionuclide in the element dm of the specified organ or tissue
  • Q_i  = quality factor
  • N_i = product of other modifying factors (dose rate, fractionation, etc.)

Comments

• Q_i is a function of collision stopping power; varies along track of ionizing particle and may be different for each element mass.  However, due to other uncertainties constant values are given for each radiation type (and energy).
  • ICRP recommends N = 1.
  • Now called (ICRP 60) w_R radiation weighting factor.
  • Using constant Q_i values:
    
• Where  
  • Total absorbed dose during 50 y after intake averaged throughout the specified organ or tissue for each radiation type
  • ICRP 30 makes estimates of H_T,50 in a number of target organs from the activity in a given source organ

Dose Calculations, cont’d

• For each radiation type, i, the H_T,50,i in a target organ, T, resulting from a radionuclide j in a source organ S, is the product of two factors:
  • The total number of transformations of nuclide j in S over a period of 50 y after intake (U_s).
  • The energy absorbed per gram in T, modified by Q from radiation of type i per transformation of radionuclide j in S.
• This is known as the Specific Effective Energy
  • (MeV g^-1 per transformation)
  • (SEE(T ¬ S)_i).
    
    

Target/Source Calculations

• For each radiation of type i from radionuclide j
  
• And:
  
• Where
  • U_s is the number of transformations of j in S over the 50 years following intake of the radionuclide
  • 1.6 x 10^-13 is the number of joules in 1 MeV
  • SEE (T¬S)_i (in MeV g^-1 per transformation) is the specific effective energy for radiation typ I, suitably modified by quality factor, absorbed in T from each transformation in S
  • 10³ is the conversion factor from g^-1 to kg^-1

For all radiations by nuclide j

For progeny

• With progeny j^’ present

Mixtures of Radionuclides

• In cases with parent and progeny the organ dose is:

Multiple Sources

• Target T may be irradiated by radiations arising in multiple sources (S).  The total value of H₅₀ is then given by:

Specific Effective Energy (SEE)

• SEE (T¬S) = S SEE (T¬S)_i
• For any radionuclide j, SEE (T¬S)_j for target T and source S is given by:

• Y_i is the yield of radiations of type i per transformation of radionuclide j
• E_i (in MeV) is the average or unique energy of radiation i as appropriate
• AF(T ¬S)_i is the fraction of energy absorbed in T per emission of i in S
  • Alphas and electrons are completely absorbed in S
    • Exceptions are mineral bone and GI
  • Photon absorptions are given in ICRP 23
• Q_i is the quality factor appropriate to the radiation
• M_T is the mass of the target organs

More stuff

SEE Examples (ICRP)

Phantoms

Kinetic Models for Organs

Fig. 4.1 Mathematical model usually used to describe the kinetics of radionuclides in the body: exception to this model are noted in the metabolic data for individual elements.

Respiratory Model

Fig. 5.2. Mathematical model used to describe clearance from the respiratory system. The values for the removal half-times, T_a-1 and compartmental fractions, F_a-1 are given in the tabular portion of the figure for each of the three cleasses of retained materials. The values given for D_N-P, D_T-B and D_P (left column) are the regional depositions for an aerosol with an AMAD of 1 μm. The schematic drawing identifies the various clearance pathways from compartments a-i in the four respiratory regions, N-P, T-B, P and L.
n.a. = not applicable.

Fig. 5.1. Deposition of dust in the respiratory system. The percentage of activity or mass of an aerosol which is deposited in the N-P, T-B and P regions is given in relation to the Activity Median Aerodynamic Diameter (AMAD) of the aerosol distribution. The model is intended for use with aerosol distributions with AMADs between 0.2 an 10 μm and with geometric standard deviations of less than 4.5. Provisional estimates of deposition further extending the size range are given by the dashed lines. For an unusual distribution with an AMAD of greater than 20 μm, complete deposition in N-P can be assumed. The model does not apply to aerosols with of greater than 20 μm, complete deposition in N-P can be assumed. The model does not apply to aerosols with AMADs of less than 0.1 μm.

Particle Size Correction

Where D_N-P, etc are the deposition probabilities
in the respiratory region for a given AMAD

Stomach Model

Quick Methods