neutron dosimetry ii
TRANSCRIPT
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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 twodosimeters having different values ofB/A, Eq. (2)can then be applied to each one and solvedsimultaneously to obtainD andDn, so long asBandA have known values
The best dosimeter pair is a TE-plastic ionchamber containing TE gas (for whichB/A 1) tomeasure the total n + dose, and anonhydrogenous dosimeter having as little neutronsensitivity 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
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Gamma-Ray Dosimeters with Relatively
Low Neutron Sensitivity (B
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Non-hydrogenous Ion Chambers
Graphite-walled ion chambers through which CO2gas is flowed at 1 atm have the advantage of being
low in atomic number, thus avoiding overresponsefor low-energy rays due to the photoelectric
effect
However, the discrimination against neutrons is
only moderate, withB/A 0.30 at 15 MeV for a0.3 cm3 cylindrical chamber, decreasing gradually
as the neutron energy is increased
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Non-hydrogenous Ion Chambers
Somewhat better neutron discrimination canbe achieved with a magnesium chamber
containing argon, because of the decrease inthe energy transferred to the heavier nuclei
by neutron elastic scattering
For a 2.4-cm3 spherical Mg-Ar chamber theB/A value for 14.8-MeV neutrons is about
0.17
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Thermoluminescent Dosimeters
7LiF (TLD-700) and CaF2:Mn TLDs both haveB/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 aB/A valuethat 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 beemployed as an indirect fast-neutron dosimeter bycoupling it with a large moderating mass, forexample, by wearing it in a personnel badge on thebody
The incident fast neutrons become thermalized bymultiple elastic collisions in the body and some ofthem diffuse back out to the dosimeter
This is called an albedo dosimeterbecause itsreading depends on the ability of the body toreflect the thermalized neutrons
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Thermoluminescent Dosimeters
Since 6LiF and LiF (containing natural lithiumwith 7% 6Li content) both are sensitive to rays
also, it is usually necessary to provide a secondTLD in the dosimeter package that is insensitive to
thermal neutrons
Both dosimeters in the pair require -ray
calibration, as their -ray sensitivities are seldomidentical
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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 showan extra TLD glow peak at about 250300C, produced by the thermal-neutron dose
deposited by the secondary -particle andthe triton
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X-Ray Film
Nuclear-track emulsions are thick enough to allowfast neutrons to scatter protons elastically, and toallow them to spend their energy internally inproducing chemically developable tracks
An x-ray film has an emulsion thickness of 25mg/cm2, which is comparable to the range of a 1-MeV proton
If the film is sandwiched between Pb foils to keepout protons from the films surroundings,B/A canbe reduced to even lower levels than thoseexhibited by 7LiF
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Miniature G-M Counters
A miniature stainless-steel G-M counterwith a high-Z filter to flatten the energy
dependence of the -ray response has beenfound to have the lowestB/A ratio of any
known -ray dosimeter: approximately 0.02
for 15-MeV neutrons, decreasing graduallywith decreasing neutron energy
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Calibration of a Tissue-Equivalent
Ion Chamber for n + Dosimetry The -ray calibration factorA is first obtained
from a 60Co -ray beam for which the free-spaceexposure rate is known
The absorbed dose at the center of an equilibriumsphere of tissue, 0.52 g/cm2 in radius, for a free-space exposureX(C/kg) at the same location, isgiven (in grays) by
tissTCPE
eneq
airair
( ) (3)c
WD K A X
e
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Calibration of a Tissue-Equivalent
Ion Chamber for n + Dosimetrywhere 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 absorptioncoefficients for tissue/air, 0.0293/0.0266 =1.102
tiss
en air/
air
/W e
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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 theTE ion chamber when it is given the same -
irradiation that depositsD (Gy) in the
tissue sphere, then
37.1 GyD X
TE
TE (C/Gy)Q
AD
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Calibration of a Tissue-Equivalent
Ion Chamber for n + Dosimetry The absorbed doseD in muscle tissue can
be related to the dose (D )TE in the TE
plastic chamber wall under TCPEconditions by
TETCPE
enTE TE TE TE
TE
tissTE TE
Q Q D Q
A D DD D
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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
TE
TE
TE / /g g
Q V
D W e S
TE
enTE TE
tiss / /g g
VA
W e S
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Calibration of a Tissue-Equivalent
Ion Chamber for n + Dosimetry The neutron calibration factorBTE for the
TE ion chamber can next be expressed in a
form similar to that ofATE:
CPETETE TE TE TE
TE tiss
TE TE
n n n n
n
n n n n
Q Q D QB F
D D D D
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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
TE
TE
TE / /
n
nn n gg
Q V
D W e S
TE
TE TEtiss
/ /n
n n gg
VB F
W e S
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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
TEtiss
TE
en TEtissTE TE
/ /
/ /
g g
n
n g n g
W e SBFA W e S
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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 inthe gas and wall have no effect
These elements have practically identical en/values over the wide range of-ray energies wherethe Compton effect dominates
Therefore, for the TE-gas-filled TE-plasticchamber:
tiss
en TE/
TE
tissTE
/
/
g
n
n g
W eBF
A W e
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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 energyfor A-150 plastic and ICRU muscle
The reciprocal of the W -ratio has been
computed as a function of neutron energyby Goodman and Coyne for methane-based
TE gas
TE
tissnF
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Calibration of the Low-Neutron-
Sensitivity Dosimeter In principle one could use
to calculateB/A for a graphite-CO2 or Mg-Ar ionchamber to be employed in the paired-dosimetermethod
The resultingB/A value so obtained is seldomaccurate enough to be useful, especially where the-ray content is fairly low
TEtiss
TE en
TEtiss
TE TE
/ /
/ /
g g
n
n g n g
W e SBF
A W e S
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Calibration of the Low-Neutron-
Sensitivity Dosimeter The most practical approach to determiningB/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 escapesfrom 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 doseDn
The low-neutron-sensitivity dosimeter (x) forwhich the value of (B/A)x is to be determined isgiven an identical irradiation, yielding the readingQx
Bx is simply equal to Qx/Dn, assumingD to bezero
Ax for that dosimeter is obtained from a60Co -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 isneeded to purify the beam adequately, orhow much of the -ray contamination mayhave come from elsewhere than the beam
portGamma 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 openbeam, 7.6-cm Pb, and a steel plug 66 cm
long filling the entire bore hole
The six measurements and responseequations 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
elsewheremostly H-capture rays emitted fromthe face of the Benelex shield
Dn is the open-beam neutron dose, is that withthe Pb filter, and that with the plug in place
aD
sD
nDn
D
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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 thetissue-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-Mcounters 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 themeasurement 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