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Target Foil Characteristics
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Target Foils
•
•
In order to separate the target
material from the vacuum of the
cyclotron, a thin metal foil is often
used on the front of a cyclotron
target.
This metal foil will attenuate the
beam and therefore thin is better.
On the other hand, the foil must
be strong enough to withstand the
pressure differential between the
cyclotron vacuum and the target
material.
Contents
• Thermal Conductivity
• Tensile Strength
• Chemical Reactivity
• Energy Loss in the Foil
• Activation of Foils
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Thermal Conductivity
Radiopharmaceutical
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Target Foils
Contents
Thermal Conductivity
Tensile Strength
Chemical Reactivity
Energy Loss in the Foil
Activation of Foils
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•
The thermal conductivity of the foil will determine the rate at
which heat will be removed from the foil. If the foil is also
cooled by either forced or free convection on the front surface
(not in a vacuum), the heat deposited by the beam will be
removed by a combination of these two processes.
Foil materials such as aluminum are very good thermal
conductors. The thickness of the foil will also determine the
amount of heat which can be removed by this process as is
evident from examining the equation for heat transfer by
conduction. A list of some common foil materials and the
thermal conductivity for each is given in the Table on the next
page.
Thermal Conductivity
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Physical and Thermal Properties of some Foil Materials
Material
density
(g/cm3)
Melt. Pt.
(°C)
Tensile
St.
(kpsi)
Thermal Cond. dE/dx
(watt/cm-°K)
(MeV/g/cm2)
Contents
Carbon
2.2
>3000
---
2.51
41.08
Thermal Conductivity
Aluminum
2.71
660
30
2.37
33.96
Titanium
4.5
1668
120
0.31
29.77
316
Stainless
8.02
1427
120
0.29
28.91
Havar
8.3
1493
250
0.17
28.6
Nickel
8.9
1453
120
0.91
28.53
Tantalum
16.6
2996
70
0.53
18.57
Tungsten
19.3
3387
500
1.8
18.42
Platinum
21.4
1769
20
0.72
18.3
Niobium
8.57
2477
40
0.54
Target Foils
Tensile Strength
Chemical Reactivity
Energy Loss in the Foil
Activation of Foils
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Thermal Conductivity
Radiopharmaceutical
Production
Target Foils
Contents
Thermal Conductivity
Tensile Strength
•
As an illustration of the effects that convective cooling can have
on the temperature of a foil, a simulation has been carried out and
is presented in the Table. Increasing the film coefficient (h)
decreases the temperature of the foil so that it can withstand
higher beam currents. Havar was chosen as an example because
the thermal conductivity is low which means that convective
cooling must be the primary means of heat removal. (See the
section on heat transfer) The blanks in the table means the
temperature was above the melting point of Havar at 1493°C.
Chemical Reactivity
Energy Loss in the Foil
Activation of Foils
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beam
current
(µA)
Foil Temperature (°C)
Power
density
(watts/cm2)
h=0
h=0.01
h=0.03
h=0.06
20
15.3
---
1114
484
240
40
30.6
---
---
936
491
60
45.8
---
---
1331
735
80
61.1
---
---
---
973
100
76.4
---
---
---
1199
Tensile Strength
Radiopharmaceutical
Production
•
Target Foils
Another important parameter is the tensile strength of the foil.
The stress placed on a circular membrane in a clamping flange
with radiused edges is given by the relation:
 P Ea 

2
h


  0.25 
Contents
Thermal Conductivity
•
Tensile Strength
Chemical Reactivity
Energy Loss in the Foil
Activation of Foils
•
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2
2
1/ 3
– where
φ = stress placed on the membrane
–
P = pressure (psi)
–
E = Young's Modulus (psi)
–
a = radius of the foil (cm.)
–
h = thickness of the foil (cm.)
If the stress on the foil exceeds the tensile strength of the foil,
then the foil will burst. This will usually occur in the center of
the foil since this is where the maximum stress occurs on a well
clamped foil (i.e. a clamping flange whose edges have been
radiused). Some values for the tensile strength of some
common foil materials are given in the table on the Physical and
Thermal Properties of some Foil Materials
Tensile Strength and Temperature
Radiopharmaceutical
Production
Target Foils
Contents
Thermal Conductivity
Tensile Strength
Chemical Reactivity
Energy Loss in the Foil
Activation of Foils
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•
The temperature dependence of the yield strength can be quite
different depending on the material. The yield strength versus
stress curves for several materials is given in the figure below. It
can be clearly seen that for most materials, the yield strength
decreases rapidly with increasing temperature. This is not the
case however with certain types of stainless steels where the yield
strength increases slightly before decreasing with increasing
temperature. Thus, the pressure in the target and the
temperature during irradiation will determine the thickness of the
foil which will be necessary to withstand the stress.
Chemical Reactivity
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Target Foils
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Contents
Thermal Conductivity
Tensile Strength
Chemical Reactivity
•
Energy Loss in the Foil
Activation of Foils
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•
The next important characteristic of the foil is the chemical
reactivity. This depends on the target material. In nitrogen
targets, the foil is often aluminum since this material is
chemically inert to the nitrogen gas and to the carbon-11
products produced.
Aluminum cannot be used in a target for the production of
fluorine-18 from oxygen-18 water since the fluorine interacts
with the aluminum and it is very difficult to remove the fluorine18 from the target.
An aluminum target can be used for gaseous fluorine-18
production since the surface can be made non-reactive by
exposure to fluorine gas at low concentrations.
It is necessary to consider the chemical combination of the foil
material with the target material not only at room temperature
but also at elevated temperatures since this is often the
situation inside the target. Each target must be considered on a
case by case basis and there are no rules other than those of
chemistry.
Energy Loss in the Foil
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Target Foils
•
•
Contents
Thermal Conductivity
Tensile Strength
Chemical Reactivity
Energy Loss in the Foil
Activation of Foils
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•
The energy loss in the foil is another consideration, since this
will have an impact on the beam energy incident on the target
material and also on the heat which is deposited in the foil.
The energy degradation relates to the stopping power of the
material as was calculated in the section on physics. The ideal
is to have a foil as thin as possible to withstand the pressure in
the target so that the minimum amount of energy is deposited in
the foil.
An exception to this rule comes up when it is necessary to
reduce the beam energy in order to have the energy incident on
the target material at an optimum energy with respect to the
cross-section of the desired nuclear reaction.
Activation of Foils
Radiopharmaceutical
Production
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Target Foils
Contents
Thermal Conductivity
•
Tensile Strength
Chemical Reactivity
Energy Loss in the Foil
Activation of Foils
•
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Another consideration is the radioactivation of the target foils,
since this will often determine how radioactive the target will be.
All target foils need to be replaced at fairly frequent intervals
and this can result in a radiation dose to the person working on
the target.
Aluminum is often the material of choice in this regard because
there are very few long lived activities formed in the foil. Nickel
alloys and steels, which must be used for chemical inertness in
certain situations, are perhaps the worst commonly used
materials with respect to activation since these metals often
have several long-lived activities associated with them.
One of the most common foils used for cyclotron targets is
Havar and it has many activation products. A gamma spectrum
is shown on the next slide.
Activation of Foils
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Gamma spectrum from a Havar foil after 80 hours of irradiation
Target Foils
O’Donnell et al Appl Radiat Isot, 2004
Contents
Thermal Conductivity
Tensile Strength
Chemical Reactivity
Energy Loss in the Foil
Activation of Foils
Dose rates to the skin from handling these foils can be more than
20 mSv/hr Vivek Manickam, et al Health Phys.96(Supplement 1):S37–S42; 2009
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