PHYSICS 345

Introduction

Radiation Safety

The first experiment(s)

Radiation Safety

• Radiation safety is your responsibility!

• Guiding principle: ALARA

As Low As Reasonably Achievable

Types of Radiation

• Photons (x-rays, gamma rays) causing ionization

• Beta (e-, e+)

• Alpha particles (He nuclei)

• Other charged ions (e.g., protons)

• Neutrons

Radiation Characteristics

• Photons: – most penetrating– interaction mechanisms (all produce energetic

electrons) -• Photoelectric absorption

• Compton scattering

• Pair production

– Electrons can produce subsequent ionizations– Photon sources are nearly always sealed.

Radiation Characteristics

• Betas (e.g., < 2 MeV): – Do not penetrate deeply into tissue– interaction mechanisms -

• Ionization of the medium

• Scattering may produce photons (e.g., bremsstrahlung)

– Electrons can produce subsequent ionizations– Sources may be exposed; avoid contact with

exposed source

Radiation Characteristics

• Alphas (e.g., < 10 MeV): – Do not penetrate dead skin cells on tissue surface– interaction mechanisms -

• Ionization of the medium - lose energy rapidly (dE/dx)

– Electrons can produce subsequent ionizations– Small external hazard; very large internal hazard

if radioactive material is inhaled or ingested.– Sources are usually exposed; avoid contact!

Radiation Characteristics

• Charged ions (e.g., > 10 MeV): – May penetrate deeply into tissue.– interaction mechanisms -

• Ionization of the medium - lose energy rapidly (dE/dx)

– Electrons can produce subsequent ionizations– May be significant external hazard– Sources are usually accelerators.– (These are not present in this laboratory.)

Radiation Characteristics• Neutrons:

– May penetrate deeply into tissue.

– interaction mechanisms -• Nuclear interaction - only; very disruptive to material.

• Charged particles from interaction can produce considerable ionization of material.

– One of the most serious external radiation hazards.

– QF is a significant function of the neutron energy

– (You should not be exposed to neutrons in this laboratory.)

Biological Consequences:Assume Chronic Exposure

• Ionization in biological cell - – Change DNA structure =>

• Mutations• Cancers

– Alter cell activity => cell fails to perform intended biological function.

– Cell dies; may be replaced by other cells– Cell repairs damage; no lasting consequences.

• Damage to cell nucleus is most severe

Radiation Safety Units: Exposure = ∆x

• R = Roentgen (unit for photons - only)

1R = 2.58 x 10-4 coulombs/(kg of dry air)

+V

Dry Air

Photon

ion pairs created

collected electrons

Radiation Safety Units: Absorbed Dose = ∆D

• Rad = a measure of energy deposited in any material by any radiation.

• 1 Rad = 100 erg/gm

= 107 joules/kg

• 1 Rad ≈ 1 R --– if radiation is photons– if material is mammalian tissue

Radiation Safety Units: Absorbed Dose Equivalent= ∆DE• REM = ∆D • QF1 • QF2 • QF3 • …

• 1 REM = 1 Rad • QF1 • QF2 • QF3 • …

• QF1 = 1 for photons, betas

> 1 for alpha (absorbed)

> 1 for high energy ions

• QF2 = distribution factor, e.g.,

> 1 for eyes, bone marrow, gonads, …

> 1 for internal absorption of material

Radiation Safety Units: Rates

• ∆x/∆t = exposure rate (R/hr)• ∆D/∆t = absorbed dose rate (Rad/hr)• ∆DE/∆t = absorbed dose equivalent rate (REM/hr)

… and fractions thereof, e.g., – mR/hr, – mRad/hr, – mREM/hr, – etc...

Safety GuidelinesU.S. N.R.C.

• For radiation workers (voluntary employees)

e.g., hospital technicians, physicists, radio-

chemists, etc., the recommended “safe” chronic doses are --

• 5 REM/yr is maximum

• 1.25 REM/qtr (13 weeks - maximum average)

• 100 mREM/wk (maximum average)

• 2.5 mREM/hr (maximum average)

Guidelines...

• “Safe” => body will repair minimal damage

=> probability of consequential harm

is small.

• For non-radiation workers, the limits are smaller, and especially so for -– Pregnant women, children (< 18 yrs), – students, general public

Chronic Exposures

• For chronic exposures, it is total absorbed dose (not the instantaneous absorbed dose rate) which is important.

• Example: The measured dose rate at a location is 20 mREM/hr. If you work at that location for 20 minutes, what is the total absorbed dose?

Averaging Dose Rates

• These dose rates are assumed not to be acute. (e.g., a dose rate of 10 R/hr is acute!)– Acute doses: radiation accidents, patients

undergoing radiation therapy, ...

• Dose Averaging (example): If you absorb 300 mREM in 1 week, you should remove yourself from exposure for at least 2 weeks to bring the average to no more than 100 mREM/wk.

Assignment

• Find “acceptable” absorbed dose limits for -– Students– General public– Source: Title 10 CFR Part 20

• (Code of Federal Regulations)• In library, and, • Online: www.nrc.gov/NRC/CFR/

• Read (scan) the posting in laboratory for your rights and responsibilities.

Reducing Exposure

• You can reduce your exposure to radiation from a source if you --

• Increase your distance from the source

• Decrease your time of exposure to the source

• Increase the shielding between you and source

… but you need to know what the exposure

rate is...

Radiation Safety Measurements

• Select the correct instrument: type of radiation and range of dose rate.

• Measure at the location you will be working.

• Photons and betas --– Geiger-Mueller (GM) counter

• Detects ions from photon interactions (e.g., photoelectric absorptions and/or Compton scatters on gas ions in GM tube)

Geiger-Mueller Tube

• Filled with a “counting gas” (e.g., argon-ethane mixture) - ion multiplication saturates

+V

G-M gas

Photon

ion pairs created

collected electrons

Geiger-Mueller Counter

• Counter must be calibrated in mR/hr on all ranges on the meter (you can/should check the date of the last calibration).

• GM tube may detect any ionizing radiation which will penetrate tube enclosure.

• Calibration is for photons only.

• GM tube must be handled with care; tubes can (and do) break, and they cost $$$.

Neutron Detectors

• Must detect neutron nuclear interaction and produce ionization in detector.

• Counter must be calibrated in mREM/hr on all ranges on the meter (you can/should check the date of the last calibration)

• Calibration is for neutrons only.

• Instrument must be handled with care; they can (and do) break.

Personnel Monitoring• Before you work in a radiation environment (near a

source of radiation) you must take a radiation survey - and record it.

• Take and record a background reading first.

• Record reading where you will be working - not very near the source where you will not be working.

• Evaluate whether the environment is “safe” for you to continue. Ask for help if needed!!

Personnel Monitoring• At all times while you work in the laboratory you

must wear a radiation monitoring badge.

• Be sure to return your badge to the rack when leaving at the conclusion of laboratory work.

• You may check your radiation badge report.

• You should always sign-in upon entering the laboratory; sign-out when leaving.

• You must abide by the Rules and Regulations for the Nuclear Physics Laboratories - posted.

Complacency...

• Familiarity breeds carelessness!!

• Pay attention to all radiation warning signs.– Radioactive materials => potential hazard if

materials are handled.– Radiation hazard => exercise caution, measure

radiation exposures, ask for advice.

• You are responsible for your own safety!