Niobium Sputtered Havar Foils for FDG Production
R Johnson1, J Wilson, C Backhouse2, B Der, C Doerkson and S McQuarrie
Edmonton PET Centre, 2 University of Alberta, Edmonton, AB, 1 Advanced Cyclotron Systems,
Richmond, BC Canada
Havar Limitations
Introduction
The Edmonton PET Centre currently supplies FDG to hospitals as far away as 1300 km. In order to meet this
commitment in a tight multi run schedule, the production of 18F had to be maximized, while preserving the
chemical environment within the water target so that the 18F could be subsequently used to provide FDG at
high and reproducible yields. Although our niobium target was able to produce > 275 GBq (in a one hour
irradiation) and operate with beam currents in excess of 100uA, subsequent FDG production yields were
compromised. An ensuing study determined that the source of the problem was due to excessive metal ions
coming from the Havar entrance foil and released into the water during irradiations that exceed 70uA.
Our improvement on the design of a high-yield cyclotron target is involves facile sputtering of selected
material onto traditional high strength metal foils used in cyclotron targets. We have recently successfully
tested this concept.
Ion Analysis on Irradiated
18
H2 O
(48 hrs)
Niobium Sputtered Foils - Cont.
Havar foils are still the foil of choice for high pressure target applications due to its incredible strength and
flexibility. However there are many limitations:
• high radioactivation with proton beam currents
• moderate heat conduction
• formation of water soluble contaminants leading to problems with
18F
reactivity
The advent of new high current, low pressure water targets has lessened the need for the strength but
increased the demand for non-reactivity in the very high temperature, caustic aqueous environment.
It has been our experience that we are reaching the temperature capability limits of Havar as observed
from pictures above.
Side 2 niobium sputtered foil
Sputtered Foils
Many alternate materials with desirable properties for beam current applications are lacking the
necessary strength to act as target foils.
Sputtering a thin layer of an inert material onto Havar combines the properties of strength and flexibility
with the advantage of non-reactivity. The sputtered metal is bonded very strongly to the Havar and
acts as a foil of the pure metal.
•
•
•
•
Havar foil
activated trace metals in the target
water arise from the constituent
metals of Havar.
Niobium foil
Only 93Mo observed with niobium foil.
(Scale is 1/10 of spectrum on left)
Vacuum chamber
++++++++++++++++
Havar Foils - Post Production
Substrate
Sputtered atoms
+
V
Ar plasma
_
Ar+
Ar
e-
Target
----------------
2042 uA hr
Side 1 (broad beam spot)
13200 uA hr
We found that the niobium sputtered foils produced more consistent
18F quality. Havar foils degraded over time which was not observed
with the niobium foils.
There is no indication that beam current limits have been reached
with the niobium sputtered foils whereas the Havar shows serious
breakdown under similar current conditions.
Niobium was chosen for our initial trial as it has:
excellent high temperature heat characteristics
inert to fluoride
very high m.p.
only radioactive by-product 93Mo t1/2 = 6.9 hr
Sputtering Process
Conclusions
Sputtering is a process for depositing a thin film
of a material (the target) onto a substrate.
1. A high voltage (~500V) is applied between target
and substrate to establish strong electric field.
2. Free electrons accelerated by electric field collide
with introduced Ar gas atoms, ionizing them into Ar+
and freeing more electrons. Heavy Ar+ ions are in
turn accelerated by the electric field towards the
target, causing atoms of the target material to be
sputtered (ejected) upon collision.
3. The sputtered target atoms get deposited on the
substrate, forming a thin film of the target material. A
vacuum environment is required for purity of the thin
film and a long mean free path of the sputtered
material.
4. In magnetron sputtering, a magnetic field is set up to
confine electrons to the region around the target in
order to improve sputtering efficiency and
performance
Further Steps
Further experiments can be run using a variety of different
materials other than niobium. Heat modeling can also be used to
provide theoretical prediction to the thermal performance of
sputtered foils before actual tests.
Heat Model
X-Y axis (cm)
Time duration: 2hours
Beam specifications:
16.5MeV, 100mA
Model Specifics: No water
cooling or evaporation of
water
Niobium Sputtered Foils – Post Production
Acknowledgements
569 uA hr
Side 2 (tight beam spot)
1223 uA hr
Side 1 niobium sputtered foil after 5585 uA hr
 National Institute for Nanotechnology
 Alberta Cancer Foundation