Radiation Measurements

Curie or Becquerel

Unit for measuring radioactivity

• 1 Ci = 3.7×10¹⁰ Bq

• 1 mCi = 1×10^-3 Ci

• 1 MBq = 1×10⁶ Bq

• 1 mCi = 37 MBq

Roentgen or Coulomb/kg

Measurement of Exposure

(1 R = 2.58×10^-4 C/kg)

Rad or Gray

Measurement of Absorbed Dose

(1 Rad = .01 Gy)

Roentgen Equivalent Man or Sievert

Measurement of Dose Equivalent

(1 Rem = .001 Sv)

Radioactivity

The rate of energy emitted as radiation, as atoms disintegrate/decay over some amount of time (eg seconds or minutes). Measured in Ci or Bq.

• 1 Ci = 3.7×10¹⁰ dps, or 2.2×10¹² dpm

• 1 Bq = 1 dps, or 60 dpm

Exposure

Amount of electrical charge (coulomb) in some amount of air (eg. kg) due to ionizing radiation from the interaction with gamma or x-rays. Measured in R or C/kg.

• 1 R = 2.58×10⁴ C/kg of air

Absorbed Dose

The amount of energy that is absorbed by some substance (or organism!) per unit mass (eg. kg or g). This absorbed energy (ionizing radiation) is deposited by radioactive sources. Measured in Rad or Gy.

• 1 Rad = 100 ergs of energy per gram of absorbing material

• 1 Rad = 0.01 Gy

• 1 Gy = 100 Rads

Dose Equivalent

Biologic harm imparted onto tissue from ionizing radiation given off by some radioactive source material. A combined measurement of the type of radiation absorbed by tissue, and the resulting medical effects. Measured in Rem or Sv.

• 1 Rem = 0.01 Sv or 10 mSv

• 1 Sv = 100 Rem

Equation for Dose Equivalent

H = D x QF

• H = dose equivalent (Rem)

• D = absorbed dose (Rad)

• QF = Quality factor (dependent on type of radiation)

True/False: Exposure, measured in R, is applicable to all types of radiation.

False! The Roentgen only applies to gamma and x-rays, not to particulate radiation like beta decay or neutron emission.

True/False: Absorbed Dose, measured in Rad, applies to only certain materials and types of radiation.

False! Unlike the Roentgen (R), the Radiation Absorbed Dose (Rad) can apply to any form of radioactive decay (gamma, beta, etc.). Likewise, while the R only applies to radiation in the air, the Rad is applicable to any material it interacts with.

Occupational Dose

The dose received by those who work with radioactive materials, and are routinely exposed to ionizing radiation. The NRC’s safe limit for such workers is set at 5000 mrem (50 mSv) per year.

True/False: The Occupational Dose does not apply to radiation received outside of a job site.

True! Specifically, Occupational Dose excludes:

• Natural background radiation (eg UV radiation on a sunny day)

• Individual medical exposures (eg the dental x-ray at your yearly appointment)

• Exposure to someone who has been administered radioactive material and released (eg a relative who recieved I-131 therapy for hyperthyroidism)

• Exposure due to voluntary participation in medical research

• Any other exposure encountered as a member of the public (look, one time I ate uranium yellow cake. I had to skip lunch that day and was like, the tiredest I’d ever been in my life)

Tracking exposure on a job site

If a worker is likely to receive more than 10% of the NRC occupational dose limit in 1 year, they must wear some type of dosimeter badge to “log” the radiation exposure.

Thermal Luminescent Dosimeters (TLD)

A dosimeter badge that contains Lithium Fluoride chips that absorb energy given off by ionizing radiation to which they are exposed. When heated, the chips release the absorbed energy as light.

Whole Body Badge

Thermal Luminescent badge worn from the neck to waist, where a worker will receive the most exposure to ionizing radiation.

Ring Badge

Thermal Luminescent badge worn by those who handle radiopharmaceuticals. Should be worn on the dominant hand, with the label facing inward towards the palm.

TLD badge function

In a TLD badge, one or more of the Lithium Fluoride chips is partially covered by a filter to distinguish between “non-penetrating” doses (skin/shallow), and “penetrating doses” (whole body/deep). The chips are inside of a light-tight holder, but when heated, they produce light. The brighter the light, the greater the dose was absorbed.

True/False: TLD Badges can measure any form of radiation exposure

False! TLD badges measure beta, gamma, and x-rays. But those the ones you need to worry about, anyway.

True/False: A ring badge only contains one Lithium Fluoride chip

True! I mean look at it, it’s tiny

Optically Stimulated Luminescence Dosimeter

A dosimeter badge that contains a thin layer of Aluminum Oxide or Beryllium Oxide that absorbs energy given off by ionizing radiation to which it is exposed. When subjected to optical stimulation, it emits a light signal proportional to the radiation absorbed.

True/False: OSL badges are “one and done”

False! OSL badges can be re-read, unlike TSL badges

Deep Dose Equivalent (DDE)

Dose equivalent at tissue depth of 1 cm. Applies to external whole-body exposure.

• 5 rem, or 50 mSv

Shallow Dose Equivalent (SDE)

Dose equivalent at tissue depth of 0.007 cm. Applies to external exposure of skin or extremity.

Committed Dose Equivalent (CDE)

Dose to some specific organ or tissue. Applicable to internal exposures.

Total Dose Equivalent

Sum of the DDE (external) and CDE (internal).

• 5 rem, or 50 mSv

NRC Occupational Dose Annual Limit for Whole Body

5 rem or 50 mSV

• Also referred to as Total Effective Dose Equivalent (TEDE)

NRC Occupational Dose Annual Limit for Any Organ

50 rem or 500 mSv

• Sum of Deep Dose Equivalent (DDE) + Committed Dose Equivalent (CDE)

NRC Occupational Dose Annual Limit for the Eye Lens

15 rem, or 150 mSv

• Also called Lens Dose Equivalent (LDE)

NRC Occupational Dose Annual Limit for Skin/Extremities

50 rem, or 500 mSv

• Also referred to as Shallow Dose Equivalent (SDE)

NRC Occupational Dose Annual Limit for Minors

10% of adult worker limits (eg 0.5 rem or 5mSv for TEDE, compared to 5 rem/50mSv for adults)

NRC Occupational Dose Annual Limit for the Public

0.1 rem, or 1 mSv

NRC Occupational Dose Annual Limit for Pregnant Workers

0.5 rem, or 5 mSv for duration of pregnancy

• Pregnancy must be declared in writing

NRC Dose Equivalent in any Unrestricted Area

2 mrem/hour, or 0.02 mSv/hour

ALARA

Reducing radiation exposure to workers “As Low As Reasonably Achievable”. Set at 10% of NRC limits, checked every 3 months by the site RSO.

ALARA Quarterly Limits for Whole Body

• Level I = 125 mrem (1.25 mSv)

• Level II = 375 mrem (3.75 mSv)

• Monthly = 42 mrem (0.42 mSv)

ALARA Quarterly Limits for Extremities

• Level I = 1250 mrem (12.5 mSv)

• Level II = 3750 mrem (37.5 mSv)

• Monthly = 420 mrem (4.2 mSv)

Methods of Radiation Protection

• Wear lab coat and monitor it before going to lunch or taking it home

• Glove up when handling radioactive materials and practice hand hygeine

• Don’t eat/drink/use cosmetics OR store these items in an area with radioactive materials

• Conduct weekly wipe tests to find contaminated areas

• Open vials of NaI-131 under a fume hood (it is volatile and gives off vapors) and perform Xenon ventilation studies in negative pressure rooms

Methods of Controlling External Exposure

• Time: don’t stick around in radiation areas, like the hot lab, and try to work as efficiently as possible

• Distance: Keep as far as possible from radiation sources, including patients after injection. You can also use tongs when calibrating doses.

Inverse Square Law

(I₁)(D_1²) = (I₂)(D_2²)

• I = Intensity of source/Exposure rate, D = Distance from source

Shielding

The use of various materials to prevent/reduce passage of ionizing radiation from a source to an area that needs to be protected.

• Plastic shielding is usually enough for Beta emitters

• Higher-energy Positron emitters require tungsten or lead shielding

How to use Shielding

• Syringe shields when drawing/administering doses

• Lead pigs when transporting those doses

• Lead bricks to surround the generator and the L-block where radiopharms. are stored

• Drawing doses and changing needles behind the L-block because it has a lead window, which reduces eye exposure

• Portable lead shields in rooms where you can’t move away from your radioactive patient while scanning. Rooms for PET patients must have substantial lead lining in the walls!

• Lead-lined walls and waste bins in storage areas

Half Value Layer (HVL)

The amount of lead (or other material) needed to reduce radiation exposure

• HVL of Lead against Tc 99m = 0.2 mm thick lead shielding

Shielding Equation

I = (I₀₎(e^^(-μx))

• I = Intensity after shielding, I₀ = before shielding

• e = base of natural log 2.718

• u = attenuation coefficient (0.693/HVL in cm)

• X = thickness of shield (cm)

^

Radioactive spill procedure

• Yap about it

• Isolate the area and cover the spill with absorbent paper to prevent spread

• Neb for contamination - shoes, hands, clothes. Remove the uncontaminated persons

• Remove contaminated clothes/shoes/etc and place in plastic bag

• Rinse contaminated body areas with warm water and wash with soap

• Yap to the RSO

• Wear PPE (coat, gloves, booties) to clean up with absorbent paper. Put the contaminated paper in the radioactive waste bin

• Check areas/persons/etc with the radiation survey meter

GM counter

Geiger-Muller counter. Used for detecting low-level radiation.

• Gas-filled

• Sensitive enough to detect levels of radiation close to background, but has high “dead time” making it not useful for high radiation areas

• Can check received packages, and detect contamination during daily surveys

• Test daily with the included Cs-137 source, and calibrate annually

Ionization Chamber

Used for detecting high-radiation areas.

• Gas-filled

• No “dead time” because it only collects one ion pair for every one radiation event, but less sensitive for low-level radiation areas

• Can be used to survey exposure rates in adjacent rooms and I-131 patients prior to release

• Calibrate annually

Well Counter

Used to test for small amounts of removable radioactivity.

• Contains an NaI crystal with a hole drilled into it to hold test tubes

• Used for wipe tests on recieved packages, weekly department wipe tests (for removable contaimination), semi-annual wipe tests on sealed sources, and to assay blood samples during DTPA renal scan (determine GFR)