Target Foil Characteristics STOP 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 b ID: 479172
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Slide1
Radiopharmaceutical Production
Target Foil Characteristics
STOPSlide2
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.
ContentsThermal ConductivityTensile Strength
Chemical Reactivity
Energy Loss in the Foil
Activation of Foils
STOPSlide3
Thermal Conductivity
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.Slide4
Thermal Conductivity
Physical and Thermal Properties of some Foil Materials
Material
density
(g/cm3)
Melt. Pt.
(°C)
Tensile St.
(
kpsi
)
Thermal Cond.
(watt/cm-°K)
dE/dx
(
MeV
/g/cm
2
)
Carbon
2.2
>3000
---
2.51
41.08
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
0.54Slide5
Thermal Conductivity
beam current
(µA)
Power density
(watts/cm
2
)
Foil Temperature (°C)
h=0
h=0.01
h=0.03
h=0.06
20
15.3
---
1114
4842404030.6------9364916045.8------13317358061.1---------97310076.4---------1199
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
.Slide6
Tensile Strength
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: 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 MaterialsSlide7
Tensile Strength and Temperature
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. Slide8
Chemical Reactivity
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 fluorine-18 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.
Slide9
Energy Loss in the Foil
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.Slide10
Activation of Foils
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.Slide11
Activation of Foils
Gamma spectrum from a
Havar foil after 80 hours of irradiation
O’Donnell et al
Appl
Radiat Isot, 2004
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; 2009Slide12
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