Production and Extraction of [10C]-CO2 From Proton
Bombardment of Molten 10B2O3
M.J. Schueller1, R.J. Nickles2, A.D. Roberts2, M. Jensen3
1
Brookhaven National Lab, Upton, NY, 11973
University of Wisconsin, Madison, WI, 53706
3
Risoe National Laboratory, Roskilde, Denmark
2
Abstract. This work describes the production of 10C (t½ = 19 s) from an enriched 10B2O3 target using a CTI RDS-112 11
MeV proton cyclotron. Proton beam heating is used to raise the target to a molten state (~ 1300 °C), enabling the
activity to diffuse to the surface of the melt. An infrared thermocouple monitors the melt temperature. Helium sweep
gas then transports the activity to flow-through chemistry processing for human inhalation of 10CO2 for blood flow
imaging with Positron Emission Tomography. The temperature-related diffusion of activity out of the white-hot molten
glass target is discussed.
INTRODUCTION
TARGET DESIGN
There is a demand for short lived radiotracers for
performing basic psychological research in Positron
Emission Tomography (PET). Shorter half lives can
reduce the time needed between subsequent
administrations of activity, allowing more scans per
session. Furthermore, the very short half life of
Carbon-10 labeled carbon dioxide (t1/2 = 19.2 s) makes
steady state imaging an attractive proposition.
However, this short half life also has drawbacks for
production and processing.
Target Material
All targetry discussed in this work was performed
on the CTI RDS-112 cyclotron at the University of
Wisconsin. After passing through the dual windows,
the proton energy is 10.2 MeV.
Carbon-10 is produced through the 10B(p,n)10C
reaction. Thick target measurements of 99.75%
enriched 10B powder show a yield of 8.8-10.8 mCi/uA
for 11 MeV protons, which is in agreement with
published values of 8.6-10.0 mCi/uA (1). The main
contaminant is 11C from the 11B(p,n)11C reaction with a
thick target yield of 93 mCi/uA. As this cannot be
removed chemically, isotopic purity of the 10B target
material is of primary importance.
This paper describes a target and chemical
processing system designed to produce a continuous
supply of [10C]-CO2 for PET imaging studies. The
behavior of carbon atoms diffusing out of the molten
glass 10B2O3 target will be discussed at some length.
The diffusion of activity through the target material
is a very strong function of temperature, yielding
insight into the behavior of the radiotracer as it exits
the molten target. This behavior demands operating
the target near its upper temperature limit in order to
produce an acceptable dose of [10C]-CO2 at the
scanner, 60 meters away.
Equally as important is the ability of the carbon
product to diffuse out of the target material. A variety
of boron bearing compounds, at natural enrichment,
were irradiated, then placed inside a quartz tube inside
a furnace and heated while being swept by a stream of
helium. A collimated NaI detector monitored the
radioactivity present in the source as it was heated,
similar to published methods (2). Boron powder,
CP680, Application of Accelerators in Research and Industry: 17th Int'l. Conference, edited by J. L. Duggan and I. L. Morgan
© 2003 American Institute of Physics 0-7354-0149-7/03/$20.00
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The aluminum target body holds an infrared
thermocouple (IRt/c) which monitors the temperature
of the melt. The IRt/c is an Exergen (4) IRt/c.3AMF,
which looks at a 3mm diameter spot located 70mm
from the face of the thermocouple. The “Hi-E” model
was chosen for looking at a glass object, as its spectral
range is from 2 to 20 microns. The IRt/c peers
through a germanium window, located above the B2O3
melt and optically coated for transmission of IR light
(> 94% from 7-13 µm, >98% from 7.5 –10 µm). The
helium sweep gas is fed from near this window to keep
the window clean, and exits at the rear of the target.
boron carbide, boron nitride and di-aluminum boride
showed no evidence of releasing the 11C, and the
samples showed a 20 minute half life regardless of
temperature.
Boron oxide (B2O3) begins to release activity upon
heating to ~250 °C. Both 11C and 13N are released,
leaving some trapped activity with a 10 minute half
life. Repeating this experiment with 99.61% enriched
10
B2O3 to nearly eliminate the 11C component, and
waiting for the 10C to decay away, shows that 60% of
the 13N produced leaves the B2O3 material, with the
remaining 40% of the 13N activity remaining trapped
in the target even under continued heating.
The IRt/c was calibrated according to manufacturer
specifications by replacing the B2O3 target material
with a natB2O3-coated cartridge heater. IRt/c output
was then measured against a thermocouple embedded
in the B2O3 glass, and a correction function was
determined. This allows continuous measurement of
the target temperature without exposing a
thermocouple to the corrosive environment of molten
B2O3.
As a source of 10B2O3 (3) was found at 99.61%
enrichment, the decision was made to use a boron
oxide target. Continuous production requires keeping
the target at high temperature (1200-1300°C) while
sweeping with helium to remove the activity for
processing. To facilitate the diffusion of activity out
of the target, a slanted target design was used, with the
10
B2O3 sitting on a molybdenum mesh. The slope was
approximately 15 degrees from horizontal, allowing
the target to be thick to the beam but be very thin to
carbon atoms diffusing to the surface.
TARGET GAS PROCESSING
Because B2O3 begins to melt at 450 °C, and
because at lower temperatures the 11C diffuses out of
the target over a period of several minutes, the target
must be heated to the point where the B2O3 is a
viscous, molten glass.
Table 1 describes the various isotopes produced
in the target, based on analysis of the raw target gas.
The gas stream passes through a Tewson NOx trap (5)
and through a CuO furnace at 700 °C to convert CO to
CO2. Radiochemical purity of the gas stream is
measured by gas chromatography, using a Porapak-Q
column to separate CO2 from N2 and CO, then using a
Molecular Sieve 5Å column to split the N2 and CO
peaks. The NOx fraction was calculated from well
counter measurements of the NOx trap. The 10C was
released as 85% CO2, 15% CO. After processing, no
CO was detected in the gas stream. The Fluorine-18
produced in the target stayed there.
The target was constructed with a water-cooled
aluminum body, which held a stainless steel support
structure to hold the molybdenum mesh. The final
target design is shown below in Figure 1. Most of the
cooling of the melt was done by helium conducting
heat across the 0.1 mm gap between the stainless
support and the aluminum target body.
Radionuclidic purity was measured by decay
analysis of samples placed in a Capintec CRC15R
dose calibrator, interrogated by RS-232 port and
logged every 10 seconds. 10C to 11C ratios were
measured by trapping the gas stream on soda lime after
converting the CO to CO2. 10C to 13N ratios were
measured by using sealed 60ml samples of the gas
stream.
With a flow rate of 450 sccm of helium, activity
entering the gas stream in the target was delivered to
the scanner 10 seconds later, traveling a path of some
60 m.
FIGURE 1. Final target design. The stainless steel
inner section holds the B2O3 target on a slanted
molybdenum mesh. The cone shows the sensitive
volume of the IR thermocouple.
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TABLE 1. Radioisotopes produced by 11 MeV protons on 99.61% enriched 10B2O3
Isotope
Half Life
EOSB Yield at this
Chemical Forms
Target Abundance
10
CO2 (85%)
C
19.2 sec
2.9 mCi/uA
CO (15%)
11
C
20 min
0.11 mCi/uA
CO, CO2
13
N
10 min
5 mCi/uA
unknown ( 2 mCi/uA)
0.17 mCi/uA
NOx (0.2 mCi/uA)
N2 (2.8 mCi/uA)
F-, OF
18
F
109 min
If beam current and thick target yield are known,
production is easily calculated. During irradiation, the
following data are acquired, on time-synchronized
computers:
The diffusion of activity out of the target melt is
assumed to follow the Arrhenius temperature
relationship,
(1)
In the vault, the neutron production rate, gamma
ray production rate, and IRt/c output are measured. In
the chemistry lab, a NaI detector is placed by a loop in
the gas line, giving excellent temporal resolution (< 1
ml in the loop, 450 sccm of helium), and the total CO2
production is measured on a sodalime trap placed in a
Capintec dose calibrator with an RS-232 interface.
Defining the mean travel distance as
x0 = D τ
(2)
where τ is the mean transit time to the surface.
Defining the extraction as the atoms which travel
distance x0 before they decay,
ε = e − λτ
ln(ε ) = − λτ = − λ
By acquiring data rapidly (10 Hz) on the vault and
chemistry computers, establishing a stable beam
condition, and turning the ion source off to create an
abrupt change, the transit time from the target surface
to the outlet of the chemistry system can be accurately
measured. As the target cools by over 200 ºC in the
first half second, this method does indeed produce a
sharp step function in the activity production. This 10
second transit time corresponds to a 30% decay in 10C
activity during transport and processing.
(3)
x02
D
(4)
Inverting equation 4, and combining with equation
1 yields
1
x2
ln( ) = λτ = λ 0 e + E a / kT
ε
D0
(5)
1
x2
E
ln[ln( )] = ln(λ 0 ) + a
ε
D0
kT
(6)
Pass over CuO => CO2
High 10B enrichment
High target temperature
Stays in target
Trapped by NOx trap
Stays in gas stream
Stays in target
the thick target yield (known), beam current on target
(routinely measured), and the temperature of the melt
(measured with IRt/c) can be determined.
EXTRACTION MEASUREMENT
D = D0e − E a / kT
Processing and
Removal
Desired Product
RESULTS AND CONCLUSIONS
The target temperature is nearly linear with beam
current. Despite efforts to make an even layer of 200250 mg of B2O3 glass on the molybdenum mesh, under
irradiation it melts, pulls itself into a smaller, thicker
mass at the center of the mesh, and then slowly
migrates down the slope of the mesh. Raising the
beam current above 15 microamps causes the target
While the target material is known to melt and flow
as a viscous fluid down the mesh, making x0 a
variable, the activation energy Ea can be measured if
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for O2, N2 and Ar (6). Between the unexpectedly low
value and the variability in what should be a constant
parameter, it is apparent that some mechanism other
than simple diffusion is operating in this target.
material to flow downward through the supporting
mesh, requiring total disassembly of the target.
Extraction efficiency is defined at the target
surface, and the Capintec measurements are decay
corrected for the 10 second delay between the target
and the well counter. In general, the targets had 20%
extraction efficiency near their upper temperature
limits, corresponding to a mean diffusion time of 45
seconds. This also means that virtually all of the 11C
contamination managed to diffuse into the gas stream
before it decayed, giving another reason to run the
target at as high a temperature as possible.
This is born out by two other facts. First, the target
material physically moves on its molybdenum mesh.
The thermocouple and the germanium window can be
removed, allowing an excellent view into the target
without moving the target substrate. Over time, the
glassy material moves as described above. Second, a
rapid ( 10 Hz) monitoring of the temperature readout
shows a definite fluctuation in the temperature, on a
time scale of 1-2 Hz. This occurs even when the
neutron production remains constant, is independent of
RF and magnet operation, and is assumed to be
bubbling of the molten glass.
Different assemblies of identical parts had different
performance characteristics. While in general the
targets had 20% extraction at 15 uA, one target had an
extraction of 24% at 15 uA, above which it melted.
Another target had a peak extraction of only 16%, but
it was at 18 uA. There was no discernable pattern to
this behavior.
In conclusion, [10C] CO2 can be produced on an 11
MeV proton cyclotron in quantities adequate for
performing cerebral blood flow imaging, albeit with
difficulty. This target illustrates some of the processes
and pitfalls found in harvesting short-lived isotopes
out of incandescent targets.
Diffusion Results
Equation 6 indicates that if the double logarithm of
the inverse extraction efficiency is plotted against
inverse temperature in Kelvins, the result should be a
straight line, the slope of which is a function of the
activation energy Ea. With the data logged as
described above, the results are as shown in Figure 2.
REFERENCES
1.
Alves F, Jensen M, Hensen JH, Nickles RJ, and Holm S
(2000): Determination of the excitation function for the
10
B(p,n)10C reaction with implications for the production
of [10C]CO2 for use as a PET tracer. J Appl Rad
Isotopes. 52(4):899-903.
2. Beyer GD and Pimentel-Gonzales, G (2000):
Physiochemical and radiochemical aspects of separation
of radioiodine from TeO2 targets. Radiochim Acta
88:175-178.
3. Eagle-Picher Industries, Inc. Qaupaw, OK.
4. Exergen Corporation, Watertown, MA.
5. Tewson TJ, Banks W, Franceschini MP, Hofftauir J. Int
J Appl Rad Isotopes 40 (1989), 765-8.
6. Shelby, JE (1996): Handbook of gas diffusion in
solids and melts. Materials Park, OH: ASM
International.
FIGURE 2.
Straight-line fit for temperatureextraction plot for one target assembly.
This plot is from the target with the highest peak
extraction efficiency, and indicates an enthalpy of
diffusion Ea/k of 690 R°K = 5.7 kJ/mol. Other targets
had lower values, down to 4.9 kJ/mol.
Values
reported in the literature for gas diffusion through
B2O3 range from 25 kJ/mol for helium to 100 kJ/mol
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