Health Physics

What is 10 CFR 20?

Part 20 of Title 10 in the Code of Federal Regulations. It outlines the standards for protection against radiation including

1. General Provisions

2. Radiation protection programs

3. occupational dose limits

4. Radiation Dose Limits for Individual Members of the Public

5. Radiological Criteria for License Termination

6. Surveys and Monitoring

7. Control of Exposure From External Sources in Restricted Areas

8. Respiratory Protection and Controls to Restrict Internal Exposure in Restricted Areas

9. Storage and Control of Licensed Material

10. Precautionary Procedures

11. Waste Disposal

12. Records

13. Reports

14. Exemptions and Additional Requirements

15. Enforcement

Discuss activity and relationship to dose

the rate of disintegration (transformation) or decay of radioactive material. The units of activity are the curie (Ci) and the becquerel (Bq). 1 ci=3.7×10^10 bq.

Dose rate is determined by the activity of the sample and the distance; if you have a higher activity sample, you get more disintegrations per time reaching you.

Activity can be viewed as the intensity of the radiation source, more formally known as the number of nuclei that decay per unit time in a sample.

It, along with distance and energy, can be related to dose rate through the following equation. The more activity there is, the higher the dose

Define absorbed dose and LET

Absorbed dose is the energy imparted by ionizing radiation per unit mass of irradiated material. Units are Grays (Gy), which is 1 joule deposited in 1 kg of material, or RAD (radiation absorbed dose) which is equal to .01 gray (equivalent to 100 ergs per gram)

LET stands for linear energy transfer. Average amount of energy lost per unit track length by individual particles or photons while passing through an absorbing medium.

Essentially due to different types of radiation acting differently on different materials, it’s how much energy is deposited over a given area or volume

Define Dose rate, units, and what determines it

Dose Rate is the amount of energy absorbed per unit mass of material per unit time, or the quantity of radiation absorbed per unit of time/strength of radiation field.

Mrem/hr is most common unit, but any absorbed dose unit per time works.

Determined by activity and distance

Define Dose equivalent, units, and list the quality factors

The product of the absorbed dose in tissue, quality factor, and all other necessary modifying factors at the location of interest. It is meant to account for different radiation types, and is measured in units of Seiverts (Sv), which is gray*quality factor or REMs (roetgen equivelent mammal) which is RAD*QF

The quality factors are:

• Alpha = 20

• Fast n = 11

• n = 10 (energy unknown)

• thermal n = 2

• beta = 1

• x-ray = 1

Define effective dose equivalent, units, and list weighting factors

The product of the absorbed dose in tissue, quality factor, and all other necessary modifying factors. The units of dose equivalent are the rem and sievert (Sv).

Weighting factors:

• Rest of body = .3

• Gonads = .25

• breast = .15 (rapid cell division)

• lungs = .12 (high surface area)

• Red bone marrow = .12

• Thyroid = .03 (has I-131)

• Bone surface = .03 (has Sr-90 and Po-239)

Discuss ALARA: what does it stand for and 3 pillars. For each pillar, describe how it is connected to dose rate and give an example for our facility.

As low as reasonably achievable. The three pillars are time, distance, and shielding.

Time:

Total dose = Dose rate × Time

Switching off during jobs to minimize individual time

know procedure, prep, check meters before, mis en place for rabbit runs

don’t rush

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Distance:

Inverse square law shows us that more distance means less dose because some of the radiation will interact with other things/lose energy before getting further…The equation treats both source and detector as point sources so is an approximation, and radiation propagating through a vaccum. We use tongs to keep things further from ourselves, we don’t spend excessive time in the ram storage closet, we rope off certain areas when radiation is present in them

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Shielding:

Effective shielding means that the radiation is blocked, at least partially, as seen by the shielding equation. This means fewer radiation particles per unit time are reaching the area beyond the shielding, lowering dose.

Alpha can be blocked by pretty much anything: paper, dead skin cells, etc. If concerned, use things with high atomic number so there are more electrons to interact with alpha

Betas need to have carbon based shielding because with higher atomic number elements they can undergo bremsstrahlung, which causes secondary radiation that goes further through the shield. Also lead can cause backscattering.

Gammas penetrate further than charged particles so they need more shielding. However, they don’t cause secondary radiation. Use high density, high atomic number elements like lead.

Neutron shielding: because neutrons have more complex interactions with various particles, the shielding is also more complex. It consists of a moderator to slow the neutrons like water, a neutron poison/absorber with a high cross section like boron, and a lead shield to block any secondary gamma radiation that might be caused by the neutrons.

What types of shielding do we have around the facility?

• PPE can help against alphas

• Wooden sample holders to block betas

• Lead pig, many lead bricks in various places (ram storage closet, door stops, shielding lab supplies, etc) to help with beta shielding

• Water in pool

• Plastic bags in classroom, lab, grab bags

• Not typically thought of but walls around ram storage closet are actually shielding…

10 CFR 20 definitions of: SDE

Shallow-dose equivalent (HS), which applies to the external exposure of the skin or an extremity, is taken as the dose equivalent at a tissue depth of 0.007 centimeter (7 mg/cm2) averaged over an area of 1 square centimeter.

10 CFR 20 definitions of: DDE

Deep-dose equivalent (H_d), which applies to external whole-body exposure, is the dose equivalent at a tissue depth of 1 cm (1000 mg/cm2).

10 CFR 20 definitions of: LDE

The external exposure of the lens of the eye and is taken as the dose equivalent at a tissue depth of 0.3 cm

10 CFR 20 definitions of: CDE

Committed dose equivalent (HT,50) means the dose equivalent to organs or tissues of reference (T) that will be received from an intake of radioactive material by an individual during the 50-year period following the intake.

10 CFR 20 definitions of: CEDE

Committed effective dose equivalent (HE,50) is the sum of the products of the weighting factors applicable to each of the body organs or tissues that are irradiated and the committed dose equivalent to these organs or tissues (HE,50 = ∑wTHT,50).

10 CFR 20 definitions of: TEDE

Total Effective Dose Equivalent (TEDE) means the sum of the deep-dose equivalent (for external exposures) and the committed effective dose equivalent (for internal exposures).

(10 CFR 20) Define ALI, DAC, and DAC hours, how they relate, and give units

Annual limit on intake (ALI) means the derived limit for the amount of radioactive material taken into the body of an adult worker by inhalation or ingestion in a year. ALI is the smaller value of intake of a given radionuclide in a year by the reference man that would result in a committed effective dose equivalent of 5 rems (0.05 Sv) or a committed dose equivalent of 50 rems (0.5 Sv) to any individual organ or tissue. (ALI values for intake by ingestion and by inhalation of selected radionuclides are given in Table 1, Columns 1 and 2, of appendix B).

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Derived air concentration (DAC) means the concentration of a given radionuclide in air which, if breathed by the reference man for a working year of 2,000 hours under conditions of light work, results in an intake of one ALI. DAC values are given in Table 1, Column 3, of appendix B.

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Derived air concentration-hour (DAC-hour) is the product of the concentration of radioactive material in air (expressed as a fraction or multiple of the derived air concentration for each radionuclide) and the time of exposure to that radionuclide, in hours. A licensee may take 2,000 DAC-hours to represent one ALI, equivalent to a committed effective dose equivalent of 5 rems (0.05 Sv)

Explain and state all the 10 CFR 20 personnel dose limits and radiation areas. Are we ever allowed to exceed our dose limits?

Annual:

TEDE: 5 Rem

DDE+CDE to any individual organ or tissue other than the lens of eye = 50 REM

Eye: 15 REM

SDE: 50 REM to skin or any extremity

MOP: 0.1 rem, don’t put them in a place greater than .002 rem/hr

Embryo: 0.5 rem

Minor workers: 10% of everyone else

Discuss: why is it difficult to measure neutron dose?

Because of it’s variance in how it interacts with different particles, and because it can come in different energies, it is hard to take an accurate reading. Also most radiation detectors measure ionization, which since neutrons have no charge are difficult to detect

What is PSE, when/how/why do we do them?

Planned special exposures are not done at our facility. 

Generally they are for operations that would take an individual over their 10CFR20 dose limits in a non emergency situation when there is no other feasible solution. Allows one to exceed their annual dose limit, but cannot exceed 5 annual doses over the course of all PSE in a lifetime.

Procedure:

• licensee and employer must plan in writing

• others involved are informed of all information (procudure, expected dose, precautions, etc)

What is RWP, when/how/why do we do them?

Radiation work permit. Accomplished by filling out SOP 53 Appendix A, and having it reviewed by the director or associate director.

Required for operations w/out and SOP

Control room binder for archives of these

Discuss the biological effects of ionizing radiation (deterministic, stochastic, long term/short term, effects of high/low doses)

Direct Effect: Radiation interacts w/dna. Electrons ripped from atoms in strand, causes structural damage

Indirect effect: Ionization of water molecules in body, which forms free radicals (e^-) that damage dna, or react with other ions to form chemical compounds that damage dna like hydrogen peroxide

Genetic effects: passed down through dna. Generally not seen in humans

Teratogenic effects: affects fetal development

Somatic: manifest in exposed person

Deterministic: Acute/immediate or delayed effects occurring after passing a certain dose threshold (i.e. sunburn or cataracts)

Stochastic: Random process that cannot be predicted w/100% certainty (i.e leukemia)