ILS_target

01/10/17 Li Target for BNCT 1
LS Neutron Source:LS Neutron Source:
June 2009June 2009
Carl Willis
The Ohio State University
In conjunction with
Linac Systems, LLC
Albuquerque, New Mexico (www.linac.com)

AgendaAgenda
1. Activation of treatment station components: new calculations
2. Gamma radiation doses, prompt and delayed: new calculations
3. Reference slides: dimensions and masses of treatment station
components
4. HEBT concept discussion
5. Target replacement discussion
6. Other progress

Target GeometryTarget Geometry

Target Geometry (ctd.)Target Geometry (ctd.)

Target HeatTarget Heat
ExchangerExchanger
Coaxial conical ducts
guide coolant over the
channelized target heat
exchanger surface,
limiting interference to
the neutron field and
preserving neutronically-
advantageous azimuthal
symmetry.

Lithium oven for PVD coating of targetLithium oven for PVD coating of target
Oven components
assembled.
Oven components disassembled.
(Left to right): chamber; pedestal with
700W heater and iron crucible;
collimator

Anti-blistering SubstrateAnti-blistering Substrate
• The problem: hydrogen gas is implanted in the target
substrate at the rate of about 0.007 sccm / mA.
1. The hydrogen concentration quickly increases to the point that partial
pressure of dissolved H in copper exceed the strength of the metal.
2. The metal blisters as the gas escapes.
3. The blistered surface cannot conduct heat effectively.
• The proposed solution: palladium (alloy) substrate
– Pd exhibits BOTH high solubility AND high permeability for hydrogen
– Pd easily chemically plated on Cu, chemically compatible with Li
– Pd capable of absorbing some hydrogen without significant mechanical
deformation (and much more hydrogen allowing for some deformation)
– The Li-Pd solid solutions are known to have high solubility for H
– Pd is nearly inert w / respect to (p,X) nuclear reactions at 2.5 MeV and
neutron capture

Anti-blistering Substrate (2)Anti-blistering Substrate (2)
The required palladium
substrate only needs to
be thick enough to stop
~1.8 MeV protons (TPd
about 20 µm in beam
direction). The total
quantity used weighs
about 1 g. The Pd can
be commercially and
inexpensively
electroplated.

Anti-blistering Substrate (3)Anti-blistering Substrate (3)
Completed Pd electroplating on
copper target surface.
Freshly-plated Pd must be treated
with boiling water to remove
hydrogen incorporated in the
plating process.

Neutronics Design PhilosophyNeutronics Design Philosophy
• Aim for the highest epithermal flux possible while meeting beam
quality criteria.
– We can accurately model contributions to beam quality (e.g. neutron spectra,
secondary gamma yields), but factors that can potentially prevent attainment of
theoretical source yield will only be quantified through experiment. Leave room to
accommodate yield deficits.
• Use reliable, readily-available materials for prototype construction.
– Design basis consisting of PTFE moderator, lead reflector, light water coolant.
– MgF2 moderators can be implemented if / when this material becomes available, and if
earlier results encourage the investment.
• Evaluate design choices using beam assessment parameters that are in
wide use and directly comparable with data from other installations.
– Use in-air flux-based figures-of-merit to guide computational design.
– Conduct experimental verification and testing with simple physical phantoms.

Neutronics ComponentsNeutronics Components
Components
1. Target
2. Moderator
3. Reflector / Delimiter
4. Thermal neutron shield
5. Gamma shield
Geometric Design Variables
• Moderator thickness (Tm)
• Moderator truncation (Tt)
• Reflector thickness (Tr)

Neutronic component dimensionsNeutronic component dimensions
(All values in mm)

Neutronics component massesNeutronics component masses
R0: 519 kg
R1: 1046 kg
R2: 752 kg
R3: 1001 kg
R4: 630 kg
R5: 134 kg
M: 48 kg

Downstream beamlineDownstream beamline
1. Backstreaming neutron trap
2. Vacuum port (standard CF part)
3. Reducer (standard CF part)
4. Beamline nipple (standard CF part)
5. Bellows
6. Wilson seal (custom)
7. ISO-80 pneumatic gate valve
8. Neutron reflector

Backstreaming neutron trapBackstreaming neutron trap
This custom HEBT component surrounds the beam near its focus downstream of the
expanding quadrupole magnet. Borated water absorbs neutrons traveling in the
beamline (and heat from proton beam halo intercepted by the constriction). A CT is
placed in an evacuated recess to measure beam current.

Removable target sectionRemovable target section
During target replacement,
these components are extracted
together. When activity has
decayed sufficiently to allow
direct handling, the reflector
sections D and E may be
salvaged for reuse. Total mass
is 70 kg.
A: ISO-80 aluminium gate valve
B: Flats for rotating target into bayonet lock
C: Inert gas port (used during lithium replacement)
D: Upper lead reflector
E: Lower lead reflector
F: Target heat exchanger

Neutronics Components (ctd).Neutronics Components (ctd).
• Target
– 7
Li(p,n) at 2.5 MeV produces 8.47E+14 n s-1
A-1
with a flux-weighted mean energy of
~330 keV, maximum energy of ~800 keV.
• Moderator
– Downscatters fast neutrons into epithermal (0.5 eV – 10 keV) band useful for BNCT.
– Ideally does not downscatter epithermal neutrons.
• Reflector / Delimiter
– A low-lethargy / high density material that returns neutrons leaking from the sides of
the moderator, improving flux intensity in the treatment port
• Thermal neutron shield
– A thin composite that removes low-energy neutrons via 6
Li(n,a) before they contribute
to patient healthy-tissue dose.
• Gamma shield
– Thin layer of high-Z material to filter (p,p’g) and (n,g) gammas produced in target and
moderator.

In-Air Flux Quality Parameters [1]In-Air Flux Quality Parameters [1]
The following in-air quantities have been chosen to guide optimization
studies on the ILS Treatment Station because they are established in
the field, easy to calculate, and can be compared to other facilities.
• Epithermal flux (φe) and fluence (Φe)
– 0.5 eV < En < 10 keV
– IAEA recommended minimum value: 1.0E+09 n cm-2
s-1
.
• Fast dose ratio (Df / Φe)
– Ratio of fast neutron (En > 10 keV) absorbed dose in healthy tissue to
epithermal fluence; lower values desirable.
– IAEA recommended maximum value: 2E-11 cGy cm2
n-1
• Gamma dose ratio (Dg / Φe)
– Ratio of gamma absorbed dose to epithermal fluence.
– IAEA recommended maximum value: 2E-11 cGy cm2
n-1

In-Air Flux Quality Parameters [2]In-Air Flux Quality Parameters [2]
• Thermal flux ratio (φthermal / φe)
– Ratio of thermal neutron flux to epithermal flux; lower values desirable.
– IAEA recommended maximum value of 0.05
• Epithermal quality parameter (Φe / [rbeDf + rbeDg])
– Ratio of epithermal fluence to RBE-weighted absorbed doses from gamma
rays and fast neutrons.
– Higher values desirable.
– A minimum acceptable value, derived from advice offered by Harling, is
3.6E+09 n cm-2
RBEcGy-1
.
• 1-10 keV quality parameter (Φ1-10 keV / [rbeDf + rbeDg])
– As above, except only considers epithermal flux in the 1-10 keV band.
– 1-10 keV neutrons are known to have the highest therapeutic gain for mid-
brain BNCT treatment.

Design Study SequenceDesign Study Sequence
1. Moderator material and thickness [Slides 7-17]
– MgF2, PTFE (Teflon™), physical mixtures of Mg metal (or Al metal) and
PTFE
2. Proton beam energy [Slides 18-21]
– 2.8 MeV vs. 2.5 MeV
3. Reflector material and reflector thickness (Tr) [Slides 22-24]
– Lead vs. bismuth
4. Gamma shield thickness (Tg) [Slides 16-18]
5. Moderator truncation (Tt) [Slides 19-22]
– What is the tradeoff between radial uniformity in moderator thickness vs.
more moderator material?

Moderator Materials [1]Moderator Materials [1]
Desirable moderator materials have the following:
• Large scattering cross-sections (elastic or inelastic) above 10 keV
• Small cross-sections over the epithermal band (0.5 eV – 10 keV)
• Small capture cross-sections at all energies
• High atom density
Fluorine (19
F) meets these criteria exceptionally well. Substances with a
high 19
F atom density and low capture cross-section include:
PTFE (Teflon™) SF6 (liquid or solid) MgF2
AlF3 (Fluental™) Perfluorocarbon liquids PTFE / metal mixtures
We considered MgF2 previously because of its superior atom density and
consequent good performance, but it is impractical (difficult to produce in
large solid pieces).

In this study we compare the performance of moderators ranging in
thickness from 14 – 65 cm, constructed from the following materials:
• MgF2
• Mixture of PTFE with 20%(at.) Mg metal
• Mixture of PTFE with 40%(at.) Mg metal
• Mixture of PTFE with 20%(at.) Al metal (one data point)
This study will answer the following questions:
• For each moderator material, what range of thicknesses present an
acceptable epithermal flux AND acceptable beam quality?
• What is the effect of diluting PTFE with elements having elastic
scattering resonances complimentary to 19
F? Do mixtures of PTFE with
Al or Mg usefully outperform PTFE in beam quality?
• How do PTFE and mixtures of PTFE and Mg compare with “the best”
known BNCT moderator, MgF2?

Fast Dose Ratio vs. Moderator Thickness (Lower is Better)
0.0E+00
2.0E-11
4.0E-11
6.0E-11
8.0E-11
1.0E-10
1.2E-10
1.4E-10
1.6E-10
1.8E-10
2.0E-10
10 20 30 40 50 60 70
Thickness (cm)
FastNeut.Abs.Dose/Epitherm.FluxRatio(cGycm
2
n
-1
)
Pure Teflon
20% Mg
40% Mg
MgF2
IAEA 1223 Recommended
IAEA 1223 Reported Range

Gamma Dose Ratio vs. Moderator Thickness (Lower is Better)
0.E+00
1.E-11
2.E-11
3.E-11
4.E-11
5.E-11
6.E-11
10 20 30 40 50 60 70
Thickness (cm)
GammaAbs.Dose/Epitherm.FluxRatio(cGycm
2
n
-1
)
Pure Teflon
20% Mg
40% Mg
MgF2
IAEA 1223 Recommended Target

Thermal Ratio vs. Moderator Thickness (Lower is Better)
0.E+00
1.E-03
2.E-03
3.E-03
4.E-03
5.E-03
6.E-03
7.E-03
8.E-03
10 20 30 40 50 60 70
Thickness (cm)
Therm.Flux/Epitherm.FluxRatio
Pure Teflon
20% Mg
40% Mg
MgF2

Φe/Dfg vs. Moderator Thickness
0.0E+00
2.0E+09
4.0E+09
6.0E+09
8.0E+09
1.0E+10
1.2E+10
1.4E+10
1.6E+10
1.8E+10
10 20 30 40 50 60 70
Thickness (cm)
Φe/Dfg(ncm
-2
RBEcGy
-1
)
Pure Teflon
20% Mg
40% Mg
MgF2
Target Value from Harling, Kononov

Φ(1keV - 10keV)/Dfg vs. Moderator Thickness
0.0E+00
5.0E+08
1.0E+09
1.5E+09
2.0E+09
2.5E+09
3.0E+09
3.5E+09
10 20 30 40 50 60 70
Thickness (cm)
Φ(1keV-10keV)/Dfg(ncm
-2
RBEcGy
-1
)
Pure Teflon
20% Mg
40% Mg
MgF2

Φe/Dfg vs. Epithermal Flux
0.E+00
1.E+09
2.E+09
3.E+09
4.E+09
5.E+09
6.E+09
7.E+09
8.E+09
9.E+09
1.0E+01 5.0E+08 1.0E+09 1.5E+09 2.0E+09 2.5E+09 3.0E+09 3.5E+09 4.0E+09
Epithermal Flux (n cm
-2
s
-1
)
Φe/Dfg(ncm
-2
RBEcGy
-1
)
Pure Teflon
20% Mg
40% Mg
MgF2
IAEA 1223 Recommended Flux
Harling Recommended Quality

Some conclusions:
Slide 11 (Epithermal flux vs. material, thickness)
• Acceptable flux intensity is achieved for all materials with moderator
thicknesses below about 50 cm.
• Flux decreases as atom density increases.
• The MgF2 curve is most closely matched by pure Teflon.
Slide 12 (Fast dose ratio vs. material, thickness)
• Fast neutron doses below the IAEA recommended maximum can only
be obtained with thick MgF2 moderators. All other materials require more
than 50 cm, which results in unacceptable epithermal flux intensity.
Slide 13 (Gamma dose ratio vs. material, thickness)
• Only MgF2 can limit the gamma dose ratio below the IAEA recommended
maximum.

Conclusions, continued:
Slide 14 (Thermal flux ratio vs. material, thickness)
• The thermal flux is lower than the IAEA recommended maximum by an
order of magnitude for all materials and thicknesses.
• Good performance is attributed to lithiated thermal shield
• Teflon is the “worst” performer, probably because the carbon content
makes it a high-lethargy material.
Slides 15-16 (Quality parameters)
• For a given thickness, the highest-quality fluxes are produced with MgF2
moderators.
• In Slide 16, we can see that quality maxima occur, but at the necessary
thicknesses, epithermal flux would be too low.
• The maximum quality values for Teflon / Mg mixtures appear to be
slightly higher than that for pure Teflon.
• Among the thinner (and more reasonable) moderators, Teflon produces
the highest beam quality after MgF2.

Conclusions, continued:
Slide 17 (Quality vs. epithermal flux)
• This plot is the best way to visualize the flux / quality tradeoff.
• Points lying in the upper right quadrant of the recommended flux / quality
axes are considered acceptable.
• MgF2 provides the widest range of acceptable thicknesses: 30-50 cm.
Next comes Teflon, whose acceptable range lies from 34-44 cm. The
Mg / Teflon mixtures are poorer.
Recommendation: Various factors, with which we have little experience,
may degrade target yield significantly. On the other hand, for a given
yield, we expect the MCNPX radiation transport calculations to be very
accurate. Thus, we should choose the material and thickness that result
in the widest margin of acceptable epithermal flux while achieving the
recommended quality. On the basis of this philosophy and the
results of the study, I recommend a 34-35 cm pure Teflon
moderator.

Proton Energy [1]Proton Energy [1]
Question: Can a 2.8 MeV target produce treatment beams with higher
epithermal flux and / or higher quality than a 2.5 MeV target operating at
the same power level (50 kW)?
Some advantages and disadvantages of using 2.8 MeV proton energy:
• 145% neutron yield vs. 2.5 MeV for the same power level
• Higher average neutron energy
• Higher yield of (p,p’g) gamma rays
• Longer / more expensive linac
In this study we compare epithermal flux intensity and beam quality
obtained with 2.5 and 2.8 MeV targets and Teflon moderators of varying
thickness. The MATLab-generated (p,n) source models are derived from
the cross-section data of Liskien and Paulsen.

Useful Flux vs. Moderator Thickness, 50 kW Beams
0.E+00
1.E+09
2.E+09
3.E+09
4.E+09
5.E+09
6.E+09
7.E+09
8.E+09
9.E+09
1.E+10
10 20 30 40 50 60 70
Thickness (cm)
UsefulFlux,>1eV(ncm
-2
s
-1
)
2.5 MeV
2.8 MeV

Φe/Dfg vs. Moderator Thickness, 50 kW Beams
0.0E+00
2.0E+09
4.0E+09
6.0E+09
8.0E+09
1.0E+10
1.2E+10
10 20 30 40 50 60 70
Thickness (cm)
Φe/Dfg(ncm
-2
RBEcGy
-1
)
Teflon Moderators, 2.5 MeV

Φ(1keV - 10keV)/Dfg vs. Moderator Thickness
0.0E+00
5.0E+08
1.0E+09
1.5E+09
2.0E+09
2.5E+09
10 20 30 40 50 60 70
Thickness (cm)
Φ(1keV-10keV)/Dfg(ncm
-2
RBEcGy
-1
)

Φe/Dfg vs. Epithermal Flux, 50 kW Beams
0.E+00
1.E+09
2.E+09
3.E+09
4.E+09
5.E+09
6.E+09
7.E+09
8.E+09
9.E+09
1.0E+01 5.0E+08 1.0E+09 1.5E+09 2.0E+09 2.5E+09 3.0E+09 3.5E+09 4.0E+09
Epithermal Flux (n cm
-2
s
-1
)
Φe/Dfg(ncm
-2
RBEcGy
-1
)
Harling Recommended Quality

Proton Energy [2]Proton Energy [2]
Slide 22 shows epithermal flux attainable in the treatment port with 2.5 /
2.8 MeV protons and moderators of various thicknesses.
• 2.8 MeV protons have higher neutron yield (no surprise).
Slides 23-24 show in-air beam quality figures.
• 2.8 MeV protons produce lower-quality beams. Because of the larger proportion
of higher energy neutrons generated, this is no surprise.
Slide 25 shows a plot of beam quality against epithermal flux. This is the
best way to visualize the tradeoff between these variables.
• For a given beam quality, the 2.5 MeV target produces greater epithermal flux.
• At the lowest acceptable beam quality, the 2.8 MeV target can only produce
1.2E+09 n cm-2 s-1, barely above the IAEA threshold of acceptable flux intensity-
leaving no room for error, target degradation, etc.
• Given the poorer performance and higher cost associated with 2.8 MeV vs.
2.5 MeV, we conclude that the higher proton energy is not recommended.

Reflector Thickness, TReflector Thickness, Trr [1][1]
Having established that a 2.5 MeV target with Teflon moderator in the range
of 34-35 cm is an optimal design, we examine the impact of reflector material and
thickness on epithermal flux. The original OSU design specified a crystalline CaF2
reflector, but this material is impractical. We consider its substitution with cast
lead (Pb) housed in aluminum shells.
Some general observations about the reflector:
• The ideal reflector is infinitely thick. However…
• Material cost of the reflector can be expected to rise in proportion to (Tr)2
• Successive incremental increases in Tr result in logarithmically-
diminishing increases in epithermal flux intensity.
This study will answer (or at least inform the debate over) the following questions:
• For a lead reflector, what is the relationship between Tr and flux?
• What is a reasonable choice for Tr?
• Might substitution of Bi for Pb have any benefits?

Epithermal Flux vs. Reflector Thickness, 20 mA Beam
0.0E+00
5.0E+08
1.0E+09
1.5E+09
2.0E+09
2.5E+09
15 20 25 30 35 40 45 50
Thickness (cm)
EpithermalFlux,0.5eV-10keV(ncm-2
s-1
)
34 cm Teflon

Φ e/Dfg vs. Reflector Thickness
0.0E+00
5.0E+08
1.0E+09
1.5E+09
2.0E+09
2.5E+09
3.0E+09
3.5E+09
4.0E+09
4.5E+09
15 20 25 30 35 40 45 50
Thickness (cm)
Φe/Dfg(ncm-2
RBEcGy-1
)
34 cm Teflon

Reflector Thickness [2]Reflector Thickness [2]
Slide 28 (Epithermal flux intensity)
• As the radius of the reflector increases from 16 cm toward infinity,
epithermal flux in the treatment port rises by about 40%.
Slide 29 (Epithermal beam quality)
• As the radius of the reflector increases from 16 cm toward infinity, beam
quality improves by about 20%: the reflector improves the fast dose
ratio by forcing some fast neutrons, otherwise lost to leakage, to pass
through the moderator twice en route to the patient.
• The reflector must have at least a 24 cm radius in order to meet the
beam quality recommendation.
Obvious issues omitted from this discussion are the cost of lead and the
difficulty of mounting, moving or fabricating large pieces. We can
conclude that a proper reflector will comprise at least several thousand
kg of lead and will be fabricated in multiple interlocking pieces.

Gamma Shield [1]Gamma Shield [1]
Neutron scattering and capture in the target and moderator materials
results in secondary gamma radiation that contributes to patient dose.
From Slide 11, we can see that our design exceeds the IAEA
recommendation for the ratio of gamma dose to epithermal neutrons.
This study will answer the following questions:
• What is the energy spectrum of the radiation contributing to the gamma
dose? What interactions give rise to this radiation?
• Can gamma dose be meaningfully reduced by interposing a lead shield
between the moderator and patient?
• What thickness (Tg) of lead is required to reduce our external gamma
dose below the IAEA recommended gamma dose ratio?
The gamma shield geometry is illustrated in Slide 4 with thickness Tg.

Gamma Flux Spectra, Teflon Moderators
1.E-07
1.E-06
1.E-05
1.E-04
1.E-03
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Energy (MeV)
Flux(γ/cm
2
/s/MeV)
16 cm
34 cm
54 cm

Gamma Dose Ratio vs. Gamma Shield Thickness
0.0E+00
5.0E-12
1.0E-11
1.5E-11
2.0E-11
2.5E-11
3.0E-11
0 0.5 1 1.5 2 2.5 3
Thickness (cm)
GammaAbs.Dose/Epitherm.FluxRatio(cGycm2
n-1
)
34 cm Teflon

Φ e/Dfg vs. Gamma Shield Thickness
0.0E+00
5.0E+08
1.0E+09
1.5E+09
2.0E+09
2.5E+09
3.0E+09
3.5E+09
4.0E+09
4.5E+09
0 0.5 1 1.5 2 2.5 3
Thickness (cm)
Φe/Dfg(ncm-2
RBEcGy-1
)
34 cm Teflon

Epithermal Flux vs. Gamma Shield Thickness, 20 mA Beam
0.0E+00
2.0E+08
4.0E+08
6.0E+08
8.0E+08
1.0E+09
1.2E+09
1.4E+09
1.6E+09
1.8E+09
2.0E+09
0 0.5 1 1.5 2 2.5 3
Thickness (cm)
s-1
)
34 cm Teflon

Gamma Shield [2]Gamma Shield [2]
Slide 32 (Gamma spectrum)
• The bulk of the gamma spectrum in the treatment port lies below 500
keV. Peaks arising from 19
F inelastic scattering and from annihilation
radiation can be discerned.
•
Slide 33 (Gamma dose ratio)
• A lead shield plate of about 0.75 cm will attenuate the gamma radiation
sufficiently to bring the gamma dose ratio below the IAEA target value.
Slide 34 (Epithermal beam quality)
• Quality shows an improving trend as the shield plate is thickened.
Slide 35 (Epithermal flux intensity)
• As expected, some loss in flux accompanies the addition of the shield
plate. Losses amount to about 13% per cm thickness.
Conclusion: Adding ~1 cm of lead shielding is justified because it
decreases the gamma dose ratio below the IAEA recommended value.

Moderator Truncation [1]Moderator Truncation [1]
Our studies so far have considered moderators of uniform thickness, i.e.
the downstream side of the moderator is conical in reflection of the
conical target recess in the upstream side. But how important is the “tip”
of this moderator cone? Relatively few neutrons interact with it because
the amount of material is small, but it adds distance between the patient’s
head and the target with the concomitant geometric loss of flux intensity.
This study will examine the following:
• What effect does truncating the conical moderator tip have on epithermal
flux and beam quality?
• How does the conical moderator shape influence flux and quality
uniformity throughout the treatment port?
The gamma shield geometry is illustrated in Slide 4 with thickness Tg.

Epithermal Flux vs. Truncation Thickness, 20 mA Beam
0.0E+00
5.0E+08
1.0E+09
1.5E+09
2.0E+09
2.5E+09
3.0E+09
-12 -9 -6 -3 0 3 6 9 12
Distance from Axis (cm)
s-1
)
5 cm
10 cm
15 cm
20 cm
Radial uniformity of treatment port flux for truncated 35 cm Teflon moderators

Φ (1keV - 10keV)/Dfg vs. Truncation Thickness, 20 mA Beam
0.0E+00
2.0E+08
4.0E+08
6.0E+08
8.0E+08
1.0E+09
1.2E+09
1.4E+09
1.6E+09
1.8E+09
2.0E+09
-12 -9 -6 -3 0 3 6 9 12
Distance fromAxis (cm)
s-1
)
5 cm
10 cm
15 cm
20 cm
Uniformity of treatment port flux quality for truncated 35 cm Teflon moderators

Φ (1keV - 10keV)/Dfg vs. Truncated Thickness
0.0E+00
2.0E+08
4.0E+08
6.0E+08
8.0E+08
1.0E+09
1.2E+09
1.4E+09
1.6E+09
0 2 4 6 8 10 12 14 16 18 20
Thickness (cm)
Φ(1keV-10keV)/Dfg(ncm-2
RBEcGy-1
)
35 cm Teflon

Moderator Truncation [2]Moderator Truncation [2]
Slides 38-39 show the impact on epithermal flux intensity and quality at
various radial positions in the treatment port, and the effect thereupon
caused by truncating the moderator.
• Uniformity of intensity and quality appears to be minimally affected by
truncation.
Slide 40 shows how epithermal beam quality is impacted by truncation.
• Quality starts to drop off significantly beyond about 12 cm of truncation.
Slide 41 revisits the quality vs. flux plot of Slide 17, adding data
representing truncated 35 cm Teflon moderators (purple trace).
• Truncation dramatically improves the flux intensity, and at less than
about 15 cm, minimally degrades the beam quality.
Conclusion: Truncating the conical tip on the basic moderator design
results in significant improvements to flux intensity and quality.

Gamma source modelingGamma source modeling
• MCNPX is not currently capable of creating gamma rays
from activation products. The following procedure
describes how delayed gamma radiation is modeled.
1. Model (n,g) occurrences in Mode N problem using the ENDF/B
absorption cross section tally multiplier (-2).
2. Calculate activation rate (= saturation activity) in each problem cell.
3. Adjust activities to account for mass differences between MCNPX
geometry and more accurate geometry. Currently only done for copper
parts.
4. Model sources of delayed gamma rays in Mode P problem based on
gamma ray energy and yield data from NNDC and activation yield in
each cell from #2. Tally photon doses, fluxes, etc.

Gamma source modeling [2]Gamma source modeling [2]
Al-28 Cu-64
Be-7 (n,g) Be-7* + Li-7(p,p’)
Cu-66
Mesh tallies comparing geometric distribution of photon fluxes
from various sources (color scale varies between drawings)

Gamma radioactivity and dosesGamma radioactivity and doses
Estimated induced activity (Al-28, Cu-64, Cu-66, Be-7) in the ILS
Treatment Station, and resulting doses 1.5m downstream from
operation at 20 mA. Dose estimates use ICRP-21 conversion factors
*Adjusted for differences between mass of real geometry and MCNPX modeled mass
Nuclide Saturation Activity Decay Modes Gamma Radiation Dose @ 1.5m
Be-7 458 Ci
(1.7 ·1013
Bq)
EC (100%) 0.48 MeV (10.4%) 46 µSv / hr
Cu-64 *29.7 Ci
(1.1 ·1012
Bq)
β- (39.0%)
EC (43.4%)
β+ (17.6%)
Ann. Rad. (35%)
1.35 MeV (0.48%) 14 µSv / hr
Cu-66 *6.75 Ci
(2.5 ·1011
Bq)
β- (100%) 1.04 MeV (9.2%)
0.83 MeV (0.2%)
12 µSv / hr
Al-28 15.3 Ci
(5.7 ·1011
Bq)
β- (100%) 1.78 MeV (100%) 1.4 mSv / hr

Prompt gamma radiation and dosesPrompt gamma radiation and doses
Prompt gamma radiation doses from 7
Li(p,p’), Be*, and (n,γ) in the ILS
Treatment Station are estimated using MCNPX and various external
data sources. Dose estimates use ICRP-21 conversion factors and
assume 20-mA beam.
Reaction Explanation Data Source Model Dose @ 1.5m
7
Li(p,p’) Proton inelastic
scattering in lithium
Lee et al. Monoenergetic,
isotropic 478-keV
photons
195 µSv / hr
Be-7* Excited-state (Jπ = ½)
Be-7
MatLab code
using Liskien-
Paulson cross-
section data
Monoenergetic,
isotropic 428-keV
photons
(n,γ) Capture and inelastic
scattering photons
MCNPX (n,γ) processes
explicitly modeled
in MCNPX
92.5 mSv / hr

Doses in operational scenarioDoses in operational scenario
We consider the proposed operational scenario of 8 hr / day, 5 day /
week with a treatment time of 1 hr and an infinite target lifetime. What
are the relative impacts of various sources of gamma radiation to (A)
patients, (B) care providers, and (C) maintenance personnel?
• Patient
– Essentially all (>98%) gamma dose results from prompt neutron-induced radiation
during treatment itself. Exposure from radioactivity during setup time is rather
unimportant.
• Care providers
– After treatment, need to be very careful of Al-28 dose (1.4-mSv / hr @ 1.5m)
– During setup, exposure to ~15 µSv / hr @1.5m, mostly from Be-7, some from Cu-64
– Unlike patient, care providers are exposed during setup / followup activities every day
• Maintenance workers
– Can expect to handle ~110 Ci of Be-7 and 7 Ci of Cu-64 in target replacement

Brain gamma absorbed dose rate vs. radial position
2.88
2.90
2.92
2.94
2.96
2.98
3.00
3.02
3.04
0 2 4 6 8 10 12 14
Radial distance from axis (cm)
Brainabsorbeddoserate(Gyhr
-1
)
Gamma absorbed dose rate calculated at a point
from ICRU 46 brain kerma factors and "in-air" flux
of prompt (n,g) photons. Delayed gamma sources
are ignored. Axial location is 6 cm downstream of
the gamma shield plate (approximate location of
brain center).

Neutron-induced gamma dose rate on treatment port axis
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
0 20 40 60 80 100 120 140 160 180 200
Distance Downstream of Treatment Port (cm)
Dose-EquivalentRate(Svhr
-1
)
Prompt neutron-induced gamma dose rate (ICRP-21) on
treatment port axis, as a function of distance downstream. A 20-
mA proton beam is assumed. This calculation does not include
the effects of neutron capture in the patient or treatment-room
surroundings, nor the shielding effects of the patient.

Proton-induced gamma dose rate on treatment port axis
0.0E+00
5.0E-04
1.0E-03
1.5E-03
2.0E-03
2.5E-03
3.0E-03
0 20 40 60 80 100 120 140 160 180 200
-1
)
Prompt proton-induced gamma dose rate (ICRP-21) on
treatment port axis, as a function of distance downstream. A 20-
mA proton beam is assumed. This calculation does not
consider shielding effects of the patient.

Al-28 gamma dose rate on treatment port axis
0.E+00
1.E-02
2.E-02
3.E-02
4.E-02
5.E-02
6.E-02
7.E-02
0 20 40 60 80 100 120 140 160 180 200
-1
)
Al-28 gamma dose rate (ICRP-21) on treatment port axis,
as a function of distance downstream. A 20-mA proton
beam and saturation activity are assumed. This
calculation does not consider shielding effects of the
patient, if present.

Be-7 gamma dose rate on treatment port axis
0.E+00
1.E-04
2.E-04
3.E-04
4.E-04
5.E-04
6.E-04
7.E-04
0 20 40 60 80 100 120 140 160 180 200
-1
)
Be-7 gamma dose rate (ICRP-21) on treatment port axis,

Cu-64 gamma dose rate on treatment port axis
0.0E+00
2.0E-05
4.0E-05
6.0E-05
8.0E-05
1.0E-04
1.2E-04
1.4E-04
1.6E-04
1.8E-04
2.0E-04
0 20 40 60 80 100 120 140 160 180 200
-1
)
Cu-64 gamma dose rate (ICRP-21) on treatment port axis,

Cu-66 gamma dose rate on treatment port axis
0.0E+00
2.0E-05
4.0E-05
6.0E-05
8.0E-05
1.0E-04
1.2E-04
1.4E-04
1.6E-04
1.8E-04
0 20 40 60 80 100 120 140 160 180 200
-1
)
Cu-66 gamma dose rate (ICRP-21) on treatment port axis,

Hydraulic TestingHydraulic Testing
The Linac Systems flow test circuit can
provide the entire range of anticipated
operating flow rates and pressures for
cooling the target, and includes automatic
computerized data collection.
Pump: 3 HP centrifugal, 208V / 3Φ
Pressure measurement: 0-5 bar
transducers on target inlet and outlet
Flow rate measurement: transit-time
ultrasonic flow meter
Water reservoir: 150 gallon tank
(enough water to absorb 50 kW from
target for ~10 minutes)

Prototype FabricationPrototype Fabrication
The target heat exchanger is machined from
OFE copper.
The manifold and its associated components
(the inner conical “flow separator” and outer
cover) are made from nickel-plated aluminum,
with some copper components joined to the
aluminum with low-temperature silver solder.
The manifold, flow separator, and cover are
joined by TIG welding.
Upon completion of the prototype target
assembly, the interior (coolant contact
surface) of the system is electroless-nickel-
plated to inhibit galvanic corrosion.
Manifold
Target
Flow separator
Outer cover

ILS_target

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