Lecture 6

• • • •

Opening Packages of Radioactive Materials Records of Radioactive Materials Uses Radioactive Waste Disposal Units used in Radiation Safety and Radiation Biology

Measure of

Radiation Units

Quantity Amount of radioactive material Ionization in air Absorbed energy per mass Activity Exposure Absorbed Dose Absorbed dose weighted by type of radiation Dose Equivalent

*Note: For most types of radiation

Unit curie (Ci) roentgen (R) Rad (mRad) Rem (mRem )

1 R



1 rad = 1 rem

In some cases, SI units have replaced conventional units.

Background Radiation

360 millirem per year

Sources of Average Radiation Dose to the U.S. Population

Internal, 39 10% Radon, 200 55% Terrestrial, 28 8% Cosmic, 27 8% Nuclear Medicine, 14 4% Medical x-rays, 39 11% Consumer Products, 10 3% Other, 3 0.8% Source: BEIR V Report, 1990

Radiation Exposures at AMH

• • The average occupational radiation exposure to all personnel, who do not handle radioactive material or perform invasive radiology/cardiology procedures, is less than 100 millirem per year.

For radiation workers, the federal and state annual radiation exposure limits are set to safe levels (negligible risk of biological effects). The annual whole body radiation exposure limit is 5000 millirem.

Radiation Exposure

• • • • • Roentgens still used.

One Roentgen = 2.58 Coulomb/KG of air Therefore, Roentgen is measured as Q/m Q/m = Charge/Mass Useful in the range of photon energies used in radiological sciences.

Radiation Exposure Continued,

• Devices expressly designed to measure exposure are called air ionization chambers.

Exposure

• • •

Must be able to assess the intensity of radiation in an environment.

Devices can measure the magnitude of electrical charge produced in a detector.

Radiation Exposure means the quantity of electrical charged produced per unit mass of air in a detector

Exposure

• • • • •

Mathematical Definition: X = Q/m where X = Exposure Q m = charge (in Coulombs) = mass of air

Units used for Exposure

• • •

Roentgen: 2.58 x 10**-4 C/kg.

Still widely used.

Most survey meters have scales in units of R, and in cpm

What is a “Dose” of Radiation

?

• • • • • When radiation’s energy is deposited into our body’s tissues, that is a dose of radiation.

The more energy deposited into the body, the higher the dose.

Rad

is a unit of measure for radiation dose.

Small doses expressed in

mrem = 1/1000 rem

.

Rem

&

R

(Roentgens) are similar units that are often equated to the Rad.

Radiation Absorbed Dose (RAD)

• • Absorbed dose is measured as E/m Where E=energy, m = mass of absorber material.

• • 1 joule = 1.0 x 10 7 ergs 1 RAD = 100 ergs of energy deposited /gram of absorbing material.

• One Rad = 1.0 x 10 5 ergs/kilogram .

Radiation Absorbed Dose (Gray)

• • • • • One Gray = 1 Joule/kg 1 joule = 1.0 x 10 7 ergs And one Rad = 1.0 x 10 5 ergs/kilogram.

Therefore, one Gray = 100 rads.

1 rad = 0.01 Gray

Absorbed Dose Measurements

•

We want to be able to quantify the amount of damage to a unit mass of tissue.

• • • • •

RAD: Radiation Absorbed Dose SI Unit: Gray 1 Gray = 100 Rads 1 Gray = 1 joule/kg of tissue. 1 rad = 100 ergs/gm.

Dose Equivalent

• Different biological effects results from the same dose of different types of radiation.

• • DE = DQN Where, D = absorbed dose in Grays or rads, Q = quality factor for the type of radiation, N = product of all other modifying factors that apply in a given situation.

Equivalent to What?

The dose of one type of radiation that produces the same amount of biological damage as the dose of a reference radiation which produces the

equivalent

amount of damage.

Effective Dose Equivalent

• • • H E = Σw i x H i Where: H E = Effective dose equivalent, Σ w i x H i Means “the sum of” the product of individual dose equivalents and weighting factor for the particular organ.

Effective Dose Equivalent

• The effective dose equivalent for the whole body is the sum of dose-equivalents for various organs in the body weighted to account for different sensitivities of the organs to radiation. It includes the dose from radiation sources internal and/or external to the body. The effective dose equivalent is usually expressed in units of millirem (mrem).

Q Factor

• • • • Dose is influenced by LET: ionization per unit path length. LET measured as keV/mm or micron.

RBE: Relative Biological Effectiveness, determines the dose equivalent. RBE defined: The ratio of the absorbed dose that produces the same damage as the reference dose.

Biological Effectiveness

• • •

Not all types of radiation produce the same level of damage. Higher LET, such as charged particles, will produce greater damage along a path length/ amount of energy imparted to tissue.

A quality factor (RBE) relative biological effectiveness, is applied to measurements to account for this.

Biological Effectiveness

• • • •

REM: Roentgen Equivalent Man.

REM takes dose measured in rads and multiplies by the QF to obtain the dose equivalent. Equivalent to what?

Gamma and beta radiation are assigned 1.

Therefore, alpha, with QF of 20, is equivalent to 20 times the damage from the same dose of gamma radiation.

RBE Examples

• • If 20 RADs of x-rays produce the same biological damage as one rad of neutrons, the RBE is 20. The quality Factor for x-rays of this type is therefore 20.

QF is really the only modifying factor used in practice. Therefore N in the above equation can be set to 1, giving:DE = DxQ

Units of Radioactivity

• • • • • •

Curie (Ci): 3.7 x 10 10 dps milliCurie (mCi): 3.7 x 10 7 dps microCurie (uCi): 3.7 x 10 4 dps Bequerel (Bq): 1 dps kiloBequerel (kBq): 1.0 x 10 3 dps megaBequerel (kBq): 1.0 x 10 6 dps

Half-Life

1.00

• Half-life is the 0.80

amount of time needed for the 0.60

activity to reach one half of the original 0.40

amount.

f 1 2 t T 1/2 0.20

0.00

0 20 f e l t l

One half-life Two half-lives

40 Days 60 T 1/2 80

0.007

100

Some Other Terms

• • Flux: # of neutrons, photons, etc, passing through one cm 2 /instant of time • Fluence: # of neutrons, photons, etc, that passed through one cm 2 over a period of time.

Cross Section: a probability of interaction, and thus transmutation after target bombardment.

Measurement of Radioactivity

• • • Gas filled detectors.

Radiation ionizes gas molecules. The free electrons are attracted to the anode, positive charges to the cathode.

This exists because there is a potential difference across the electrodes.

Detecting and Measuring Radiation •

Instruments:

Verify presence of radiation or locate contamination - GM Survey Meter (Geiger counter) - NaI Survey Meter (Sodium iodide detector) •

Personal Dosimeters:

Measure doses (radiation exposure) to staff – Radiation Badge: Luxel / Film / TLD

Typical background is 0.03 mR/hr or 100 cpm Battery check Range selector GM pancake probe NaI probe

Review of Sample Dosimetry Report

• • • • • • • • Series code “F” Participant Number (Part. No.) Dosimeter (Badge) type – “chest” vs. “collar” 3 Categories of Exposure / Exposure Limits (Deep/Eye/Shallow) Wear Period Quarterly exposures Year to date Lifetime

Regions

• • • Recombination: If the voltage is to low, the ions recombine before reaching the electrodes.

Ionization (Ion Chambers) Size of pulse directly related to energy deposited. Thus, can measure exposure in Roentgens.

Proportional Basically the same as Ion except stronger signal. Probably better in lower radiation areas where you still want exposure measurements.

Regions

• Geiger-Mueller (The Geiger counter) Very sensitive to small amounts of radiation. Best calibrated for cpm and to locate radiation. • Use ion chamber for exposure (hence dose)

Scintillation Detectors.

• The most sensitive to radiation. Calibrated in cpm.

• • Can detect very small amounts.

The type of survey meter that is used at landfills

Multichannel Analyzers

• • • • It should be easy to differentiate between the types of radiation if there exists a relationship between energy deposited in a crystal, and size of pulse.

Therefore, one can discriminate between various energy levels.

Very useful for the identification of types of radiation.

In imaging, isolates the energy used for imaging, such as Tc-99m

Image of the Week Neurological PET We will now examine the effects of various external stimuli on the PET scan image. The first stimulation is auditory, i.e. the subject will listen to some music.

Notice the increased activity in the PET image containing the auditory cortex. Note that nonverbal stimuli (music) predominantly activates the nondominant (right) hemisphere. Simultaneous stimulation with language and music would cause a more bilateral activation of the auditory cortex.

The subject now must perform a "thinking" task, Notice the increased activity in the stimulated brain PET image (arrowhead). This region of increased activity corresponds to the frontal cortex