PET Course KFSH&RC Salam rahma cylotron targetry - operation & maintenance

1. Cyclotron Targetry:
Operation & Maintenance
Salam Rahma
Technical Specialist, Cyclotron Section
Cyclotron and Radiopharmaceuticals Department
1

2. TARGET TYPES
The target material may be either gas,
liquid or solid. Targets are, consequently,
designed to accommodate the material
being irradiated. The design of the target
will also depend upon whether the target
is placed inside (internal) or outside
(external) the cyclotron.
2

3. Internal Targets
• The real advantage of these targets at present
is that the target may be built to exactly match
the beam curvature and, therefore, spread the
power of the beam over the maximum area
and increase the amount of beam current that
may be applied to the target.
3

4. External Targets
Solid targets
Because the density of solids is typically
higher than that of liquids or gases, the
path length of the beam is shorter, and the
target somewhat smaller. The solid can be
in the form of a foil or a powder. If the
solid is a good heat conductor, then the
beam can be aligned to be perpendicular
to the solid target.
4

5. OVERPRESSURE AND SEALED
CONDITIONS
The water target is usually run in one of two conditions. The first is with an
overpressure of an inert gas such as helium and the second is with the target
sealed such that the pressure inside the target is determined by the pressure
that builds up in the target during irradiation. One limitation of pressurization
with an inert gas in the target is that an initial amount of non-condensable gas
exists, which mixes with the liquid and vapour during irradiation. Even a small
component of non-condensable gas produces a dramatic decrease in heat
transfer at a condensing surface
The other method is to fill the target volume completely with water and then
either pressurize the target with additional water, or seal it off completely and
let it attain its own equilibrium pressure with the beam on. This method is
intended to better utilize the effective heat transfer area in the condensing
region of the target volume by eliminating the presence of non-condensable
gas.
5

6. A typical solid powder target
6

7. Photograph of the solid powder target
7

8. Liquid Targets
In the case of liquids, targets have similar
dimensions to those of solid targets, since the
target material occupies a specific volume
unless the liquid volatilizes. The difference is
that the liquid is typically added and removed
from the target while it is in place on the
cyclotron.
8

9. Water Targets
Water targets are the most commonly used targets
for the production of PET radionuclides, with 18F
being the most popular radionuclide by far. The other
water target commonly used is for the production of
13N. Since the main difference in these two targets is
whether the water is enriched in 18O or not, the
problems and considerations are very similar. One
major difference is that the production of 13N is
much more forgiving in the choice of target body
materials than is the production of F18.
9

10. A typical liquid target for the
production of 18F from 18O water.
10

11. High Power Target Body
11

12. Gas targets
Gas targets are widely used and are usually
some type of cylinder to hold the gas under
pressure, with a thin beam entry foil usually
referred to as a window.
The design and construction of cyclotron
targets is a multidisplinary field. Engineering,
physics and chemistry each play an important
part in the way cyclotron targets are
constructed.
12

13. 13

14. Gas target used for production of 123I
from 124Xe.
14

15. SUGGESTED MAINTENANCE SCHEDULE
FOR GAS TARGETS
Frequency Procedure
Weekly Visually check for damaged tubing or fittings
Leak check the system
Check front foil for signs of damage
Test plastics for radiation damage
Track target yields to establish trends
Monthly
15

16. Diagram of beam spot on the front
entrance window of a gas target.
16

17. 13C powder water slurry target
One development in targetry has been the use
of mixed phase targets to alter the chemical
form of the radionuclide. One example of this
is the 13C powder water slurry target
developed by CTI, Inc. (Knoxville, TN).
In this target, the slurry of 13C powder in
water is irradiated in the cyclotron beam to
produce 13NH3.
17

18. Carbon-13 slurry target
18

19. Physical characteristic of target
• Interactions of charged particles with
matter;
• Stopping power and ranges;
• Energy straggling;
• Small angle multiple scattering.
19

20. Nuclide
Half-life
(min)
Decay mode
Maximum energy
(MeV)
Mean
energy
(MeV)
Max. range
(mm)
Max. SA
(theoretical)
(Ci/mmol)a
C-11 20.4 100% b+ 0.96 0.386 4.1 9 220
N-13 9.98 100% b+ 1.19 0.492 5.4 18 900
O-15 2.03 100% b+ 1.7 0.735 8.0 91 730
F-18 109.8 97% b+ 0.69 0.250 2.4 1 710
Cu-62 9.74 99.7% b+ 2.93 1.314 14.3 19 310
Ga-68 68.0 89% b+ 1.9 0.829 9.0 2 766
Br-75 96.0 75.5% b+ 1.74 0.750 8.2 1 960
Rb-82 1.25 95.5% b+ 3.36 1.5 16.5 150 400
I-122 3.62 75.8% b+ 3.12 1.4 15.3 51 950
I-124 6019.2 23.3% b+ 2.13 0.8 10.2 31
a 1 Ci = 37 GBq.
COMMON PET RADIONUCLIDES AND THEIR
DECAY CHARACTERISTICS
20

21. Radiation damage
Another problem in cyclotron targetry is the
degradation of certain materials with radiation
exposure. Gamma and beta radiation have little
effect on metals, but they break chemical bonds
and prevent bond recombination of organic
compounds. This is particularly true of plastics,
some of which are quite radiation resistant while
others degrade under prolonged exposure. For a
given gamma or neutron flux, the degree of
degradation observed depends on the type of
chemical bonding present.
21

22. SUSCEPTIBILITY OF MATERIALS TO
RADIATION DAMAGE
Material Radiation resistance Tolerance level (kGy)
Teflon Poor (becomes powdery) 5
Polyethelyene Good 1 000
Polypropylene Fair 50
PEEK (polyethyletherketone) Excellent 10 000
Nylon (aliphatic or
amorphous)
Fair 50
Polycarbonate Good 1 000
Polystyrene Excellent 10 000
22

23. Heat transfer
The energy lost when charged particles pass
through the target medium is dissipated in
the form of heat. One of the most challenging
problems in the design of cyclotron targets is
finding methods to remove this heat from the
target during irradiation. The heat generated
in the target often has several detrimental
effects. A few of these, such as reduction of
target density, chemical reactions occurring in
the target material or products.
23

24. Heat transfer by radiation
The heat loss by radiation is usually minor, except for target foils or
solid targets with low thermal conductivity when high beam currents
are used. It is usually assumed in these cases that the heat emitted by
the surface is all absorbed in the beamline or target, and very little is
radiated back to the target. The radiative heat loss is the easiest term
to estimate. If the thermal radiation emitted from a body is thought of
as a photon gas, then it is possible to show from thermodynamics that
the energy density of the radiation is given by:
Qrad = AsT4
where
Qradis the heat emitted as radiation (W);
Ais the area of the surface (cm2);
sis the Stefan–Boltzmann constant; and
Tis the absolute temperature (K).
The Stefan–Boltzmann constant has a value of 9.66 ¥ 10–14 W·cm–
2·K–4.
24

25. HEAT LOSS BY RADIATIONTemperature (°C) Temperature (°K) Qrad (W)
100 373 0.0001
200 473 0.0023
300 573 0.0117
400 673 0.0371
500 773 0.0906
600 873 0.1878
700 973 0.3479
800 1073 0.5935
900 1173 0.9507
1000 1273 1.449
1100 1373 2.122
1200 1473 3.000
1300 1573 4.138
1400 1673 7.336
1500 1773 9.496 25

26. HEAT TRANSFER PARAMETERS FOR
HELIUM
Temperature
b(K)
Thermal
conductivity,
k (mW·cm–1·K–1)c
Temperature
(K)d
Viscosity,
m (cP)e
200 1.151 273 0.01861
300 1.499 293 0.01941
350 1.646 373 0.02281
400 1.795 473 0.02672
450 1.947 523 0.02853
26

27. 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 aluminium are very good
thermal conductors. The thickness of the foil will
also determine the amount of heat that can be
removed by this process, as is evident from
examining the equation for heat transfer by
conduction.
27

28. The choice of materials for the target
body and entrance foil
The choice of materials for the target body and
entrance foil will depend not only on their strength and
chemical stability but also on their thermal properties.
In the following sections, we will explore some basic
principles in heat transfer as they relate to accelerator
targets.
From the first law of thermodynamics we can write:
E = Q – W
where
E is internal energy;
Q is heat; and
W is work done.
28

29. Target body materials
There are five important characteristics
for target body materials. These are:
(1) Thermal conductivity;
(2) Chemical reactivity;
(3) Activation;
(4) Ease of machining/manufacture;
(5) Mechanical strength.
29

30. PHYSICAL AND THERMAL PROPERTIES
OF SOME TARGET BODY MATERIALS
Thermal
conductivity
(W·m–1
·°C–1
)
Tensile
strength
(klbf/in2
)a
Chemical
reactivity
Nuclear
activation
Ease of
machining
Material
Aluminium 2.37 Good p negligible
d–24
Na
Good 110
Aluminium 3003 2.63 Good 130
Aluminium 6061 1.80 Good 115
p–65
Zn, 62
Zn, 64
Cu
d–65
Zn
p–57
Ni, 55
Co
d–64
Cu
As for nickel
p, d–107
Cd, 109
Cd
p–48
V
d–49
V
p–56
Co, 55
Co
d–57
Co, 56
Co
As for copper
Copper 4.03 Fair Excellent 344
Nickel 0.91 Excellent Good 175
Monel 0.32 Excellent Good 112
Silver 4.27 Good Good 170
Titanium 0.31 Excellent Good 300
Stainless steel 0.29 Good Good 860
Brass 2.01 Fair 250
Niobium 0.54 Excellent Good 300
a
1 lbf/in2
= 6.895 kPa. 30

31. TARGET BODY MATERIALS
Several common materials are used for
fabrication of target bodies. The ideal materials
should be strong, able to withstand high
pressures, be chemically inert under gas plasma
conditions, and have a good thermal
conductivity. Six target body materials constitute
99% of the gas and liquid target bodies in routine
use. These are aluminium, titanium, nickel,
niobium, tantalum and silver.
31

32. TARGET BODY PROPERTIES
Property Aluminium Titanium Nickel Niobium Tantalum Silver
Thermalconductivity
(W·m–1
·K–1
)
Melting point (ºC)
Chemicalinertness
Heatcapacity(J·mol–1
·K–1
)
167 21.9 90.9 53.7 57 429
582
Fair
24.2
1725
Good
24.9
1453
Fair
26.1
2410
Excellent
24.6
3290
Excellent
25.3
961
Good
25.4
32

33. Aluminum
The more commonly used alloy of aluminium,
which is used in the fabrication of target bodies, is
aluminium 6061-T6.
In general, aluminium 6061 combines relatively
high strength, good workability and high resistance
to corrosion. It has excellent joining characteristics
and is widely available. The 6061-T8 and 6061-T9
alloys offer better chipping characteristics over the
6061-T6 alloy, but for target fabrication the T6 type
is the most common.
33

34. Cleaning Aluminium
Normally, aluminium needs no cleaning when
used as a target body. If it becomes discolored
or corroded, it can be cleaned with mild
abrasive and water. Targets used for the
production of high SA 11C should not be
exposed to organic solvents except under
unusual circumstances. In such a case, the
surface must be rinsed with distilled water
several times and then dried with dry nitrogen
before use.
34

35. ENERGY OF PROTONS ON ALUMINIUM
(DENSITY = 2.71 g/cm3)
Foil thickness (cm) Ein (MeV) Eout (MeV) ΔE (MeV
0.0125 19.0
17.5
16.5
13.0
11.0
10.0
19.0
17.5
16.5
13.0
11.0
10.0
19.0
17.5
16.5
13.0
11.0
10.0
18.8
17.3
16.3
12.75
10.7
9.7
18.65
17.1
16.1
12.5
10.45
9.4
18.3
16.75
15.7
12.0
9.85
8.75
0.2
0.2
0.2
0.25
0.3
0.3
0.35
0.4
0.4
0.5
0.55
0.6
0.7
0.75
0.8
1.0
1.15
1.25
0.0250
0.0500
Ein = energy input; Eout = energy output; ΔE = energy difference. 35

36. Titanium
Titanium has several advantageous characteristics. It is
lightweight, strong and corrosion resistant. The alloys
have good tensile strengths ranging from 210–1380
MPa (30 000 to 200 000 psi) similar to those found in
some steel alloys. Titanium has a corrosion resistance
similar to that of platinum, but with a density that is
only 56% of that of steel.
There are many alloys of titanium that can be used in
the construction of cyclotron targets. However,
commercially pure, grade 1 titanium is the most
commonly used.
36

37. Cleaning Titanium
Titanium can be cleaned effectively with nitric acid alone or
in combination with hydrochloric acid. Nitric acid alone is
also an excellent way to passivate the titanium surface. Acid
cleaning of titanium surfaces to remove deposits is
sometimes necessary. Acid cleaning cycles can be used
provided proper inhibitors are present. Ferric ion, as FeCl3,
is very effective as an inhibitor for titanium in acid solutions.
For instance, as little as 0.1% (by weight) FeCl3 will inhibit
corrosion of titanium by hydrochloric acid. At ambient
temperatures, a solution of 25% (by weight) of HCl can be
safely used on titanium, with FeCl3 used as an inhibitor.
37

38. Nickel
Nickel is a relatively hard, malleable and ductile metal which
is a fairly good conductor of heat and electricity. The surface
of nickel can be passivated with nitric acid, but it will dissolve
in other weak acids. There are many alloys of nickel with a
wide variety of different properties. One of the most widely
used is stainless steel. There are some target bodies made of
stainless steel in use. Target bodies made of other nickel
alloys are common, particularly with corrosive gases such as
the halogens. For example, monel is an alloy of nickel and
copper (e.g.ﾠ70% nickel, 30% copper, with traces of other
elements) that has good resistance to corrosion by dilute
fluorine gas.
38

39. Cleaning Nickel
Nickel targets can be washed with a mild
detergent in warm water and rinsed thoroughly
and air dried. For stubborn stains, one can make a
paste of baking soda and water or alcohol, cover
the item with the paste and allow the paste to dry,
run under warm water and buff dry with a mild
abrasive pad. Commercial silver cleaning polish
can be used on silver and nickel, if the baking soda
paste does not remove the tarnish.
39

40. Niobium
Niobium is a component of some stainless steels
and also alloys with non-ferrous metals. These
alloys have good strength and other properties. The
metal has a low capture cross-section for thermal
neutrons and so finds use in some applications in
the nuclear industry and in cyclotron targetry.
niobium a good choice for low temperature targets
(such as water), but perhaps not the best choice for
higher temperature gas targets.
40

41. Cleaning Niobium
To properly clean niobium, the following steps
are recommended:
• Degrease;
• Immerse in commercial alkaline cleanser for 5–10
min;
• Rinse with water;
• Immerse in 35–40% HNO3 for 2–5 min at room
temperature;
• Rinse with tap water and follow by a rinse with
distilled water;
• Force air dry.
41

42. Tantalum
Tantalum is one of the refractory metals that offers a valuable
combination of properties. It can be handled easily at room
temperature. Its strength at elevated temperature is low
compared with tungsten and molybdenum. Tantalum’s corrosion
resistance is unusually good in most commercial combinations of
acids. It has several unique properties that have made it essential
to certain applications, making it well worth the high cost. It offers
approximately the same corrosion resistance to most acids and
caustics as glass. In addition, it can be fabricated by bending, roll
forming and welding with relative ease, by personnel experienced
with the metal.
42

43. Cleaning Tantalum
Tantalum can be cleaned with steel grit in a bead
blaster and then rinsed with hot hydrochloric acid to
remove traces of iron from the steel beads. The
tantalum surface is inert to hydrochloric acid even at
elevated temperatures. Tantalum surfaces can be
cleaned with hot chromic acid solution (a saturated
solution of potassium chromate in hot concentrated
sulphuric acid). After this treatment, the tantalum
surface should be very thoroughly rinsed with distilled
water to remove any traces of the cleaning solution.
43

44. Silver
Silver metal has several very useful characteristics
for the construction of targets. It has the highest
thermal and electrical conductivity of all the
metals although cold working the silver will
reduce the conductivity. It is quite easily
machined, being only a little harder than gold. It
does not react with air, water or many acids under
normal conditions. It does tarnish with exposure
to very active oxidizing species such as ozone. This
reaction can be seen in water targets after high
current irradiations. 44

45. Silver Cleaning
Washing soda (Na2CO3) can be used to clean large pieces of
silver. For targets, one can combine the soda with ethyl or
isopropyl alcohol to form a paste, dip a clean, damp sponge
into the paste and rub it onto the silver, then scrub with
cotton swabs until something approaching a mirror finish is
restored to the silver, and finish by rinsing the silver with hot
distilled water in an ultrasonic bath, followed by drying; the
longer the paste is left on the silver, the more tarnish will be
removed. If the silver is being used for cleaning a water target
for the production of 18F, toothpaste, which is a common
method for cleaning, should be avoided because the fluoride
that is commonly in the toothpaste will greatly decrease the
SA of the 18F.
45

46. Target Cleaning
Targets get dirty
Clean after 5000 µAhr
46

47. TARGET WINDOW MATERIALS
The materials used for target windows need to be
very strong in thin sheets and maintain their strength
at elevated temperatures. The materials used most
frequently for target windows are Havar, aluminium,
niobium and titanium. The last three are also used as
target bodies, and, therefore, most of the
characteristics are listed in the previous section. One
important characteristic that is critical to foils is the
yield strength as a function of temperatur
47

48. Target window foil materials
One of the most important components of any
target system is the foil through which the beam
enters the target material. This component is
sometimes absent in solid targets, but is usually
required in both liquid and gaseous targets. There
are several important parameters in the choice of a
foil. Often, the best choice with regard to one
parameter will not be the best choice with regard to
another parameter, so compromises are often
necessary.
48

49. The important parameters in the
choice of a foil
• Its thermal conductivity;
• Its tensile strength;
• Its chemical reactivity (inertness);
• The energy degradation properties to which it is subject;
• Radioactive activation;
• Its melting point.
Each of these parameters interacts with the others in subtle
ways. For example, the stopping power will determine the
amount of power deposited in the foil, which in combination
with the thermal conductivity will set the temperature. The
temperature will have an effect on the yield strength of the
foil and may affect the chemical reactivity of the foil.
49

50. Chemical reactivity
The next important characteristic of a foil is its
chemical reactivity. This depends on the target
material. In nitrogen targets, the foil is often
aluminium, since this material is chemically inert to
nitrogen gas and to the 11C products produced.
Aluminium cannot be used in a target for
production of 18F from 18O in water, since fluorine
interacts with aluminium and it is very difficult to
remove 18F from the target. An aluminium target
can be used for gaseous 18F production, since the
surface can be made non-reactive by exposure to
fluorine. 50

51. A list of some common foil materials
Melting
point
(°C)
Tensile
strength
(klbf/in2
)a
Thermal
conductivity
(W·cm–1
·K–1
)
Density
(g/cm3
)
dE/dx
(MeV·g–1
·cm–2
)
Material
Carbon
Aluminium
Titanium
Stainless
steel
Havar
Nickel
Tantalum
Tungsten
Platinum
Niobium
2.2
2.71
4.5
>3000
660
1668
—
30
120
2.51
2.37
0.31
41.08
33.96
29.77
8.02 1427 120 0.29 28.91
8.3
8.9
16.6
19.3
21.4
8.57
1493
1453
2996
3387
1769
2477
250
120
70
500
20
40
0.17
0.91
0.53
1.8
0.72
0.54
28.6
28.53
18.57
18.42
18.3
—
a
1 lbf/in2
= 6.895 kPa.
51

52. TARGET FOIL CHARACTERISTICS
Property Aluminium Titanium Havar Niobium
Thermal conductivity(W·m–1
·K–1
)
Melting point (ºC)
Density(g/cm3
)
dE/dx (keV/µm)for 10 MeV protons
Tensile strength (25ºC), psi (MPa)
167
582
2.71
9.2
42 000
(290)
16.4
1725
4.5
13.5
63 000
(434)
14.7
1480
8.3
24.2
270 000
(1860)
53.7
2410
8.6
21.2
85 000
(585)
52

53. Havar
Havar is a cobalt base alloy that has a wide
variety of useful properties, the most useful of
which in cyclotron targetry is the very high
strength at high temperatures. Havar will retain
75% of its strength at room temperature up to
510°C. This property is particularly useful for
target foils and Havar is quite widely used in
this application. The endurance lifetime can be
maximized by heat treating the alloy at 540°C
after 80% cold work.
53

54. Activation of foils
Another consideration is the radioactivation of target foils,
since this will often determine how radioactive the target
will be. All target foils need to be replaced at fairly
frequent intervals, which can result in a radiation dose to
the person working on the target. Aluminium is often the
material of choice in this regard, because there are very
few long lived isotopes 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.
54

55. The system geometry
A very important aspect of radiative heat transfer is the system
geometry. When an object is heated as it is with an accelerator
beam, radiation is emitted in all directions, and only a fraction
of this will actually strike a surface where it can be absorbed
and removed. This fraction is called the shape factor, F. The
shape factor is defined as the fraction of thermal energy
leaving the surface of object 1 that reaches the surface of
object 2.
55

56. Schematic diagram of the flow
circulation patterns set up inside
the gas target
56

57. INTERNAL CIRCULATION AND HEAT
REMOVAL
In order to have a useful accelerator target for the production of a radionuclide, it
is necessary to effectively remove the heat generated by the beam. The three
modes of heat transfer that are active in gas targets are conduction, convection
and radiation. Radiation is only significant at high temperatures (>500ºC). Gases
and liquids usually transfer heat via convection and conduction. Heat transfer in
solids is somewhat simpler than in other media, since the heat usually flows
through the target matrix, mainly by conduction.
Once the heat has been transferred from the gas or liquid to the target body, it
will usually be removed by water flowing around the target. Most heat transfer
problems arise in the interfaces, where there are discontinuities in the heat
transfer, such as where the target material meets the target body or where the
target body meets the cooling water. The more efficient the design of the transfer
at these interfaces, the better the heat transfer will be and the less likely it is that
there will be problems with loss of target material or damage to the target during
the irradiation . 57

58. New target design with cooling fin.
58

59. Cylindrical targets
Cylindrical gas target for the production of 11C. The beam is incident through the
front foil with a water cooled grid and the rear foil is cooled by heat transfer from a
cooling water flow in the rear of the target. The rear foil is thin, to maximize the
heat transfer. The inner capacity of the cylinder is 78.5 mL.
59

60. Racetrack shaped target cavity for production of
18F from 18O enriched water.
60

61. WATER TARGET MATERIALS
Material ‘Quality’ Maintenance Conductivity Machinability Neutrons
Silver
Niobium
Tantalum
Good
Excellent
Excellent
Fair
Best
Good
Highest
Fair
Poor
Excellent
Fair
Fair
Fair
Fair
Excellent
61

62. Schematic diagram of water target design
taking into account the boiling during
irradiation.
Water
Typical
10 mm
Steam/water
Typical 80 mm
62

63. Heat transfer by conduction in four
types of metal water target.
6000
5000
Niobium
4000
Heat load
3000
2000
1000
0
0 0.2 0.4 0.6 0.8 1 1.2
Wall thickness (cm)
Heattransfer(W)
SIlv
Tita
Tan
er
nium
talum
63

64. ‘Keyhole’ target
One of the older designs for water targets is the keyhole design
shown in Fig. 3.4. It is similar to the racetrack target design, but
has a smaller area for condensation. This target design was
usually run completely full of water. The space above the main
target volume was for expansion and condensation of the water
boiling in the target.
These target designs could be run either with an inert gas
overpressure or closed off. This geometry has been utilized in
both traditional reflux and thermosyphon targets
64

65. Keyhole water target design. The
material of construction of this target
was silver
65

66. TARGET GEOMETRY (CONICAL VERSUS
CYLINDRICAL, LENGTH)
The basic geometries for gas targets are
cylindrical and conical. The cylindrical type is
the most common commercial design and the
simplest structure to manufacture. It has a
clean inner surface but can be damaged if the
beam spreads into the wall, due to scattering in
the foil or gas.
66

67. Conical design for 11C gas target.
67

68. Energy degradation
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 that is deposited in the foil. The
energy degradation relates to the stopping power of the
material. The ideal situation is to have a foil no thicker than is
necessary to withstand the pressure in the target, so that the
minimum amount of energy is deposited in the foil. An
exception to this rule is when it is necessary to reduce beam
energy in order to have the energy incident on the target
material at an optimum value with respect to the cross-section
of the desired nuclear reaction.
68

69. BEAM TRANSPORT
Extracted beams are often transported from the
accelerator to the target by the beam transport system.
This can consist of a series of dipole and quadrupole
magnets, magnetic lenses, collimators and beam
monitors, or, as in the case of many small cyclotrons
where the targets are attached directly to the machine, it
is just a beam collimator. The arrangement of magnets,
vacuum chambers and diagnostic instrumentation is
called a beamline. The beam transport system modifies
and monitors the beam to achieve the optimum beam
distribution inside the target volume.
69

70. Beam focusing
Beams leaving an accelerator can have unusual
shapes depending on the dynamics of the
acceleration and extraction, as well as on the
fringing field in cyclotrons. It is sometimes
desirable to focus the beam with quadrupole
magnets. This also allows some smoothing of
the beam in those accelerators where the
beam may have some ‘hot spots’, or spikes in
the intensity profile
70

71. Collimators
71

72. GRIDS — ADVANTAGES AND
DISADVANTAGES
One way to increase the pressure a target foil can
withstand is to support the foil with a grid. This is
especially important with low energy accelerators,
where a thinner foil means a higher energy on the
target and, therefore, a better production yield.
There are several designs for grids, including circular
holes and hexagonal arrays. The grids with circular
holes are limited to about 78% transmission, while
grids with a hexagonal design have a much higher
transmission.
72

73. Grid
73

74. GRID
74

75. Beam profile monitoring
Beamlines are often equipped with beam profile
monitors. These devices monitor the shape of the
beam, usually in two dimensions. They allow the
operator to adjust the beam shape with steering and
focusing magnets in order to obtain the ideal beam
shape on the target. There are many different
designs of beam scanners, from simple wires, which
can be inserted into the beam, to more sophisticated
scanners with a rotating wire attached to an
oscilloscope, which provide high resolution images
of the beam shape.
75

76. Wobbling and rastering
The purpose of ‘wobbling’ or rastering the beam is to
lower the power density on the target. This in turn
allows the target to be operated at higher beam
currents with the resulting higher yields of
radioisotope production. It is necessary to have a
beamline of some kind in order to take advantage of
this method of power dissipation. These dynamic
beam delivery systems (wobblers or scanners) are
developed to overcome the undesirable necessity of
scattering materials in the beam and, therefore, of
lowering the energy.
76

77. Melting point
The final item in the list of important foil
parameters is the melting point. The ideal foil
material would have a very high melting point
so that heating of the foil by the beam would
not be a problem. The most widely used foil is
aluminium, which has a low melting point. In
this case, the other attractive properties of
aluminium outweigh the disadvantage of a
low melting point.
77

78. Fine wire mesh used to determine the
beam profile
Support equipment
•Small volumes, high pressures and low contamination
•Much of the equipment comes from HPLC
•Capillary tubing
•Chromatography valves (up to 1000 psig)
•Pressure transducers
•Syringe pumps
•Diaphragm pumps
•HPLC pumps
78

79. Collimator melted and fused to target body
79

80. 80

81. Preliminary design of a solid thallium
target.
81

82. REFERENCES
IAEA RADIOISOTOPES AND
RADIOPHARMACEUTICALS SERIES No. 4
TECHNICAL REPORTS SERIES No. 465
82

83. Thank you for your
attention
83