Section 3 Dosimetry

1
Radiation Dosimetry

• Chapter 7
• Section 3 Dosimetry

2
Objectives

• Know
• The terms roentgen, rem, sievert, gray and rad
• How to convert standard units to SI units
• The difference between exposure and absorbed dose
• How to calculate a dose equivalent if given an
  absorbed dose
• How to use the rules of thumb to estimate a dose
  a distance from a source of radiation
• Understand the term relative biological
  effectiveness

3
Dosimetry Vs Shielding

• Dosimetry
• The measurement or calculation of energy
  deposition to material by radiation
• Shielding
• The reduction of radiation by use of absorbing
  materials
• The material used to reduce radiation fields

4
Units and Terminology

• Two systems of measurement
• Conventional
• roentgen
• rad
• rem
• SI (Systeme Internationale)
• exposure
• gray
• sievert
• You get to learn both!

5
1st Definition Exposure

• Only applies to X or ? radiation in air
• SI units
• 1 X unit 1 C kg-1 of air
• Conventional units
• 1 Roentgen (R) 2.5 x 10-4 C kg-1 (air)
• Based on the ionization of air

6
Exposure, continued

• One of the earliest radiation measurements
• Easy to build equipment that responds to
  ionization of air by radiation
• Common terms used in describing exposure
• 1000 mR 1 R, 1 µR 10-6 R, etc
• Also describe exposure rate
• . Indicates it is a rate

7
2nd Definition Absorbed Dose

• Energy deposition in material by radiation (i.e.,
  energy per gram)
• Conventional units
• rad
• 100 ergs of energy per 1 gm of material
• 1 rad 100 ergs/gm 0.01 j/kg
• Systeme Internationale
• Gray (abbreviated Gy)
• 1 joule/kg

8
Absorbed Dose, continued

• 1 Gy 1 J/kg 100 rad
• Used for any and all types of ionizing radiation
• Neutrons, gammas, alpha particles, x-rays
• Internal and external exposure.
• Is a simple physical fact the amount of energy
  deposited in a unit mass of material.
• Absorbed dose rate is given in rad/s, Gy per
  hr,...etc.

9
3rd Definition Relative Biological Effectiveness

• Radiation damage depends on
• Type of material
• Type of radiation
• Relative biological effectiveness (RBE) is a
  measure of these differences in living tissue
• If it takes 20 rad of x-rays to cause the same
  effect as 1 rad of neutrons, the RBE for neutrons
  would be 20

10
RBE, Continued

• RBE is experimentally determined
• Wide range of measured values
• Variety of endpoints (cell death, reproduction
  rate.)
• Many values are not whole numbers
• Too complex to be used in regulations
• Simplified and appears as
• Quality factor (Q) in old system and for NRC
• Radiation weighting factor (wR) in Systeme
  Internationale, and most recent dose calculation
  method

11
Accounting for Biological Effects

• Important Note
• The techniques used to calculate biological dose
  have changed substantially in the last 30 years
• The terminology used to describe biological dose
  has not
• The resultant dose predictions have changed,
  sometimes a lot, sometimes not at all
• Sorry

12
Biological Effects, continued

• Dose equivalent
• Modifies absorbed dose by a modifying factor
• Accounts for varying biological impacts of
  different radiations
• Conventional system
• Rem (Roentgen equivalent man)
• Systeme Internationale
• Sievert (Sv)

13
Dose Equivalent

• Calculated by multiplying absorbed dose by the
  appropriate radiation weighting factor
• H wR D(gy) Sv
• Where D absorbed dose in gy
• If D is in rad, dose equivalent is in rem
• Example values of weighting factors shown on next
  page
• Note the term quality factor is used by the NRC
  instead of WR

14
ICRP-60 Values for WR
The values of Q used by the NRC are slightly
different.
15
SoBiological Dose Today

• Measure
• Exposure (X or roentgen) or
• Absorbed dose (rad, Gy)
• Calculate
• Dose equivalent for each effected organ (GyWR)
• Result expressed in rem, Sv

16
Rules of Thumb

• Some rules of thumb from the old system
• 1 R exposure (? or x-rays in air) ?
• 1 rad dose to tissue
• 1 rem
• 0.01 Sv
• 10 mGy
• This is true because the quality factor (or
  radiation weighting factor) for ? and x-rays is
  equal to 1

17
Rule of Thumb 1

• Some equations to memorize
• For estimating dose rate from a gamma point
  source where the distance is in feet, and the
  source strength is in Ci, and the energy of the
  gamma is expressed in MeV, then

18
Rule of Thumb 2

• For beta radiation, the equation is similar,
  where the maximum energy of the beta radiation is
  used.

19
Rules of Thumb, continued

• It is important to note that if a nuclide decays
  by multiple emissions (betas or gammas) that they
  have to be accounted for in the calculation. You
  can estimate the dose from each separately and
  sum the total.