Neutron Interactions and Dosimetry II

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Neutron Interactions and Dosimetry II

• Paired Dosimeters
• Calibration of the Low-Neutron-Sensitivity
  Dosimeter

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Separate Measurement of Neutron and ?-Ray Dose
Components by Paired Dosimeters

• If a mixed n ? field is measured by means of
  two dosimeters having different values of B/A,
  Eq. (2) can then be applied to each one and
  solved simultaneously to obtain D? and Dn, so
  long as B and A have known values
• The best dosimeter pair is a TE-plastic ion
  chamber containing TE gas (for which B/A ? 1) to
  measure the total n ? dose, and a
  nonhydrogenous dosimeter having as little neutron
  sensitivity as possible to measure the ? dose
• Ideally this dosimeter should measure only ?-rays

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Dosimeters with Comparable Neutron and ?-Ray
Sensitivities (B/A ? 1)

• A-150 TE plastic ion chambers (B/A ? 1)
• Rossi TE proportional counter (B/A ? 1)
• Tissue-equivalent plastic calorimeters (B/A ? 1)
• Aqueous chemical dosimeters (B lt A)
• Organic or plastic scintillators (B lt A)

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Neutron Dosimeters Insensitive to ?-Rays (A B)

• Activation of metal foils (A ? 0)
• Fission foils (A 0)
• Etchable plastic foils (A ? 0)
• Damage to silicon diodes (A ? 0)
• Hurst proportional counter (A ? 0)
• Rem meters
• Long counters
• Bubble detectors

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Gamma-Ray Dosimeters with Relatively Low Neutron
Sensitivity (B lt A)

• There are no known dosimeters for which B 0
  while A ? 0
• The primary means available for minimizing the
  value of B is the avoidance of hydrogen in a
  dosimeter, including its CPE buildup layer, since
  elastic scattering of H nuclei accounts for most
  of the absorbed dose in the interaction of fast
  neutrons in tissue and other hydrogenous media

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Non-hydrogenous Ion Chambers

• Graphite-walled ion chambers through which CO2
  gas is flowed at 1 atm have the advantage of
  being low in atomic number, thus avoiding
  overresponse for low-energy ? rays due to the
  photoelectric effect
• However, the discrimination against neutrons is
  only moderate, with B/A ? 0.30 at 15 MeV for a
  0.3 cm3 cylindrical chamber, decreasing gradually
  as the neutron energy is increased

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Non-hydrogenous Ion Chambers

• Somewhat better neutron discrimination can be
  achieved with a magnesium chamber containing
  argon, because of the decrease in the energy
  transferred to the heavier nuclei by neutron
  elastic scattering
• For a 2.4-cm3 spherical Mg-Ar chamber the B/A
  value for 14.8-MeV neutrons is about 0.17

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Thermoluminescent Dosimeters

• 7LiF (TLD-700) and CaF2Mn TLDs both have B/A
  values comparable to that of the Mg-Ar ion
  chamber
• Thus either of these TLDs can be employed as the
  neutron-insensitive member of the
  paired-dosimeter method
• 7LiF, at least, has been shown to have a B/A
  value that is nearly proportional to the energy
  of the fast neutrons below 15 MeV

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Thermoluminescent Dosimeters

• A LiF (TLD-100) or 6LiF (TLD-600) TLD can be
  employed as an indirect fast-neutron dosimeter by
  coupling it with a large moderating mass, for
  example, by wearing it in a personnel badge on
  the body
• The incident fast neutrons become thermalized by
  multiple elastic collisions in the body and some
  of them diffuse back out to the dosimeter
• This is called an albedo dosimeter because its
  reading depends on the ability of the body to
  reflect the thermalized neutrons

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Thermoluminescent Dosimeters

• Since 6LiF and LiF (containing natural lithium
  with 7 6Li content) both are sensitive to ? rays
  also, it is usually necessary to provide a second
  TLD in the dosimeter package that is insensitive
  to thermal neutrons
• Both dosimeters in the pair require ?-ray
  calibration, as their ?-ray sensitivities are
  seldom identical

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Thermoluminescent Dosimeters

• An alternative to using 7LiF as a separate ?-ray
  dosimeter in the albedo package is offered by the
  fact that LiF and 6LiF show an extra TLD glow
  peak at about 250 300 C, produced by the
  thermal-neutron dose deposited by the secondary
  ?-particle and the triton

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X-Ray Film

• Nuclear-track emulsions are thick enough to allow
  fast neutrons to scatter protons elastically, and
  to allow them to spend their energy internally in
  producing chemically developable tracks
• An x-ray film has an emulsion thickness of 2 5
  mg/cm2, which is comparable to the range of a
  1-MeV proton
• If the film is sandwiched between Pb foils to
  keep out protons from the films surroundings,
  B/A can be reduced to even lower levels than
  those exhibited by 7LiF

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Miniature G-M Counters

• A miniature stainless-steel G-M counter with a
  high-Z filter to flatten the energy dependence of
  the ?-ray response has been found to have the
  lowest B/A ratio of any known ?-ray dosimeter
  approximately 0.02 for 15-MeV neutrons,
  decreasing gradually with decreasing neutron
  energy

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Calibration of a Tissue-Equivalent Ion Chamber
for n ? Dosimetry

• The ?-ray calibration factor A is first obtained
  from a 60Co ?-ray beam for which the free-space
  exposure rate is known
• The absorbed dose at the center of an equilibrium
  sphere of tissue, 0.52 g/cm2 in radius, for a
  free-space exposure X (C/kg) at the same
  location, is given (in grays) by

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Calibration of a Tissue-Equivalent Ion Chamber
for n ? Dosimetry

• where ? ? 1.003,
• Aeq attenuation of photons in
  penetrating to the center of the tissue sphere ?
  0.988,
• 33.97 J/C, and
• the ratio of mass energy
  absorption coefficients for tissue/air,
  0.0293/0.0266 1.102

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Calibration of a Tissue-Equivalent Ion Chamber
for n ? Dosimetry

• Eq. (3) thus reduces to
• If (Q?)TE is the charge (C) produced in the TE
  ion chamber when it is given the same
  ?-irradiation that deposits D? (Gy) in the tissue
  sphere, then

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Calibration of a Tissue-Equivalent Ion Chamber
for n ? Dosimetry

• The absorbed dose D? in muscle tissue can be
  related to the dose (D? )TE in the TE plastic
  chamber wall under TCPE conditions by

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Calibration of a Tissue-Equivalent Ion Chamber
for n ? Dosimetry

• The B-G relation, assumed to be valid here,
  allows one to write
• Substituting gives

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Calibration of a Tissue-Equivalent Ion Chamber
for n ? Dosimetry

• The neutron calibration factor BTE for the TE ion
  chamber can next be expressed in a form similar
  to that of ATE

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Calibration of a Tissue-Equivalent Ion Chamber
for n ? Dosimetry

• Applying the B-G relation to the neutron case
• Now substituting gives

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Calibration of a Tissue-Equivalent Ion Chamber
for n ? Dosimetry

• (B/A)TE for the TE chamber is the ratio
• The compositions of the TE gas and TE-plastic
  wall are sufficiently similar that the stopping
  power ratios are both close to unity, as is their
  ratio

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Calibration of a Tissue-Equivalent Ion Chamber
for n ? Dosimetry

• The ratio is also nearly unity,
  since differences in the carbon and oxygen
  content in the gas and wall have no effect
• These elements have practically identical ?en/?
  values over the wide range of ?-ray energies
  where the Compton effect dominates
• Therefore, for the TE-gas-filled TE-plastic
  chamber

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Calibration of a Tissue-Equivalent Ion Chamber
for n ? Dosimetry

• The value of the ratio is obtained from
  tables such as those in Appendix F, entered at
  the appropriate neutron energy for A-150 plastic
  and ICRU muscle
• The reciprocal of the W -ratio has been computed
  as a function of neutron energy by Goodman and
  Coyne for methane-based TE gas

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Calibration of the Low-Neutron-Sensitivity
Dosimeter

• In principle one could use
• to calculate B/A for a graphite-CO2 or Mg-Ar
  ion chamber to be employed in the
  paired-dosimeter method
• The resulting B/A value so obtained is seldom
  accurate enough to be useful, especially where
  the ?-ray content is fairly low

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Calibration of the Low-Neutron-Sensitivity
Dosimeter

• The most practical approach to determining B/A is
  an experimental one employing a narrow neutron
  beam of the desired spectrum
• The method makes use of a Pb filter to remove the
  ?-ray contamination from the beam, while passing
  most of the neutrons, which have a smaller
  attenuation coefficient
• Secondary radiation produced in the filter
  escapes from the narrow beam

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Calibration of the Low-Neutron-Sensitivity
Dosimeter

• A previously calibrated TE chamber is used to
  calibrate the beam in terms of neutron tissue
  dose Dn
• The low-neutron-sensitivity dosimeter (x) for
  which the value of (B/A)x is to be determined is
  given an identical irradiation, yielding the
  reading Qx
• Bx is simply equal to Qx/Dn, assuming D? to be
  zero
• Ax for that dosimeter is obtained from a 60Co
  ?-ray exposure

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Calibration of the Low-Neutron-Sensitivity
Dosimeter

• In practice one does not know the degree to which
  the beam is initially contaminated with ?
  radiation, how much Pb filtration is needed to
  purify the beam adequately, or how much of the
  ?-ray contamination may have come from elsewhere
  than the beam port
• Gamma rays from the face of the shield would, for
  example, not be removed by a beam filter

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Calibration of the Low-Neutron-Sensitivity
Dosimeter

• A solution to this problem was devised which uses
  the narrow-beam Pb-filtration method for
  determining (B/A)x
• The neutron beam was generated by 35-MeV
  deuterons on Be its average energy was 15 MeV
• It was collimated by a 2-cm hole through a large
  Benelex (pressed wood) shield, as shown in the
  following diagram

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Calibration of the Low-Neutron-Sensitivity
Dosimeter

• The dosimeters were a TE-plastic-TE-gas chamber
  and an air-filled graphite chamber
• The three beam filtrations chosen were open beam,
  7.6-cm Pb, and a steel plug 66 cm long filling
  the entire bore hole
• The six measurements and response equations are
  listed in the following table

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Calibration of the Low-Neutron-Sensitivity
Dosimeter

• was the absorbed dose at the measurement
  point in the open beam due to ? rays coming out
  of the beam port
• was the dose contributed by ? rays from
  elsewhere mostly H-capture ? rays emitted from
  the face of the Benelex shield
• Dn is the open-beam neutron dose, is that
  with the Pb filter, and that with the plug
  in place

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Calibration of the Low-Neutron-Sensitivity
Dosimeter

• This experimental approach to determining (B/A)x
  for a low-neutron-sensitivity dosimeter provides
  a value that is consistent with the (B/A)TE of
  the tissue-equivalent chamber with which it is
  compared, and is relevant to the neutron spectrum
  of the beam used
• The method works as well with TLDs, G-M counters
  or other nonhydrogenous dosimeters as it does
  with ion chambers

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Calibration of the Low-Neutron-Sensitivity
Dosimeter

• Narrow-beam geometry is required for this
  calibration procedure
• The beam must be narrow enough, and the
  measurement location distant enough from the
  filters, so that significant amounts of secondary
  radiation from the filters cannot reach the
  dosimeters
• The method therefore requires a collimatable beam
  of neutrons