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Health Physics > External Personnel Dosimetry > Flashcards

External Personnel Dosimetry Flashcards

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1
Q

What are characteristics for the ideal personnel dosimeter?

A
  • Small size
  • Range from 10 mrem to 1000 rem
  • Re-readable
  • Linear accuracy
  • Negligible fading
  • Energy independent
  • Not affected by light/heat/humidity
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2
Q

What is the basis for the desired range of 10 mrem to 1,000 rem for the ideal personnel dosimeter?

A

Lower Limit ⇒ Current radiation control regulations do not require reporting a work dose of less than 10% of the annual limits. This translates to about 10 mrem/week.

Upper Limit ⇒ 1,000 rem is determined from the maximum surviavable dose for humans under acute exposure conditions and with the availability of medical care.

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3
Q

What are the programs that must accredit those organizations that process dosimetry results issued to workers on an NRC or agreement state license?

A
  • NVLAP ⇒ National Voluntary Laboratory Accreditation Program
  • DOELAP ⇒ Department of Energy Laboraty Accreditation Program
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4
Q

Define

Shallow Dose Equivalent

A

The dose equivalent, measured in rem, at the respective depth of 0.007 cm in a rectangular slab of soft tissue with a density of 1 g cm-3 and a size of 30 cm x 30 cm x 15 cm.

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5
Q

Define

Deep-Dose Equivalent

A

The dose equivalent, measured in rem, at the respective depth of 1 cm in a rectangular slab of soft tissue with a density of 1 g cm-3 and a size of 30 cm x 30 cm x 15 cm.

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6
Q

Photographic Dosimetry

What filter gives the maximum discrimination between beta rays and photons?

A

Teflon

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7
Q

Photographic Dosimetry

How do film badges record thermal neutron exposures?

A
  • In special neutron track film, the emulsion is made about three times thicker than the beta gamma X-ray film.
  • The grain size is reduced to about 0.3 micron.
  • When exposed to thermal neutrons, nitrogen nuclei in the emulsion can capture a neutron which produces a proton and a C-14 nucleus.
  • The ejected proton has high stopping power and travels on a short distance in the emulsion.
  • This series of grains in a row appears after development and is termed a “track.”
  • By counting the number of tracks in a unit area, the thermal neutron dose is determined.
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8
Q

Photographic Dosimetry

How do film badges record fast neutron exposure?

A
  • Neutrons with energies over 0.5 MeV will elastically scatter from hydrogen nuclei in the emulsion.
  • The recoiling hydrogen then is capable of producing a track which can be seen microscopically after the film procesing.
  • Exposure ranges from 200 mrem to 250 rem.
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9
Q

Thermoluminescence Dosimetry

What is the principle of thermoluminescence?

A
  1. Energized electrons detach from the atoms
  2. The electrons move somewhat freely around inside the phsophor crystal.
  3. Many of the electrons will become trapped at a luminescence center which are impurity atoms added to the phosphor during manufacture.
  4. When the phosphor is heated, the thermal energy causes the electrons to escape from the traps and return to their ground state.
  5. In dropping from a higher energy to a lower one, the energy difference is given off in the form of a light photon.
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10
Q

Thermoluminescence Dosimetry

Conduction vs. Valence Band

A
  • When the electron is bound to a phosphor atom it is said to be in the valence band.
  • When it becomes free it goes into the conduction band.
  • The energy gap between the valence and conduction bands is called the forbidden energy gap.
  • The trapping centers are located within the forbidden energy gap.
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11
Q

Thermoluminescence Dosimetry

What is the relationship between temperature required to release trapped electrons and fading?

A
  • The higher the temperature required to dump a trap, the less the fading.
  • Fading is often measured by the fading half-life, the time it takes to lose half of the stored dose information.
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12
Q

Thermoluminescence Dosimetry

What does a typical glow curve look like for LiF?

A
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13
Q

Thermoluminescence Dosimetry

Graph Thermoluminescent Intensity (log) vs. Exposure (log)

A
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14
Q

Thermoluminescence Dosimetry

Advantages and Disadvantages of LiF TLD

A

Advantages

  • LiF is almost tissue equivalent (8.1)
  • Small size
  • Reusable
  • Excellent resistance to enivironment
  • Can distinguish different radiation types using differ Li isotopes

Disadvantages

  • Reading process clears signal, so exposure cannot be reread
  • No permanent record (like a film badge)
  • Dust on detector will glow when heated and be recorded by the phototube as a false positive
  • TLDs are sensitive to ultraviolet light and must be sealed in a light tight badge
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15
Q

Thermoluminescence Dosimetry

How do you measure neutrons using LiF?

A

6LiF element is comprised of 95.6% 6Li and 4.4% 7Li

7LiF element is comprised of 0.07% 6Li and 99.93% 7Li

6Li has a large cross section for capturing thermal neutrons.
6Li + 1n → 3H + 4He + 4.8 MeV

The large energy release is locally deposited as both of the reaction products have very high stopping power.

The dose from 6LiF is a combined neutron and gamma exposure, therefore neutron exposure is calculated by subtracting the 7LiF exposure from the 6LiF exposure.

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16
Q

Thermoluminescence Dosimetry

What is the standard composition of TLD-100, TLD-600, and TLD-700?

A

TLD-100 ⇒ 7.5% 6Li and 92.5% 7Li

TLD-600 ⇒ 95.6% 6Li and 4.4% 7Li

TLD-700 ⇒ 0.07% 6Li and 99.93% 7Li

17
Q

OSL Dosimetry

What is the principle of optically stimulated luminescence (OSL)?

A
  • Incoming radiation deposits energy, which free electrons, which become trapped in electron traps.
  • Aluminum oxide is doped with a carbon impurity to produce stable electron traps that appear in the forbidden energy gap inside the cyrstalline structure.
  • The reading process involves exposing the phosphor to a short series of ten pulses of green laser light (compared to heat for thermoluminescence).
  • After each pulse, blue light is emitted and recorded by a photomultiplier tube (PMT).
  • The total intensity recorded by the PMT represents the exposure.
18
Q

OSL Dosimetry

Advantages and Disadvantages of Optically Stimulated Luminescence

A

Advantages

  • Most sensitive (1 mrem to 1,000 rem)
  • Energy range from 5 keV to 40 MeV
  • Complete reanalysis if necessary
  • Not affected by heat or humidity

Disadvantages

  • More expensive than film or TLD
19
Q

OSL Dosimetry

What is the main phosphor used?

A
  • Aluminum Oxide (Al2O3).
  • Because the atomic number of aluminum oxide is about 11, the photon energy response would be expected to show a small over-response for low photon energies.
20
Q

OSL Dosimetry

Graph the response of Al2O3 OSL phosphor

A
21
Q

How often is NVLAP accreditation required?

A

Every 2 years

22
Q

Criticality Dosimetry

How does a criticality badge operate?

A

Uses the principle of neutron activation to obtain the needed information.

23
Q

Criticality Dosimetry

What is a resonance activation detector

A

Will capture neutrons only within a narrow energy range. The cross section is minimal for other energies.

24
Q

Criticality Dosimetry

What is a threshold activation detector?

A

Will capture neutrons only if they exceed some minimum energy.

25
Q

Criticality Dosimetry

Compare resonance and threshold activation detectors

A
26
Q

Criticality Dosimetry

How do neutron badges have double resonance?

A
  • Thermal neutrons plus slow neutrons.
  • By combining two foils in a badge holder, one shielded by cadmium, the two different neutron energies can be distinguished by subtraction of the daughter activities.
  • By subtracting the daughter activities of adjacent threshold detectors, the neutrons in some energy interval are recorded.
27
Q

At what tissue depth is skin dose evaluated?

A

0.007 cm (7 mg cm-2)

28
Q

At what tissue depth is deep dose equivalent evaluated?

A

1 cm (1,000 mg cm-2)

29
Q

A whole-body TLD (with filters for skin and eye dose) was worn on the chest under the coveralls during a radiation incident. Worker was also wearing a facepiece.

List 4 factors to consider when comparing the TLD dose to the calculated dose.

A
  • Correction of the TLD skin dose response to account for attenuation of beta particles by the coveralls.
  • Correction of the TLD eye dose response to account for attenuation of beta particles by the coveralls and facepiece.
  • Correction of the TLD skin dose response by the factor of (dTLD/dskin)2 to correct for the difference in distance.
  • Correction of the TLD eye dose response by the factor of (dTLD/deye)2 to correct for the difference in distance.

Health Physics (17 decks)

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