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Visual observation of boiling in high power liquid target
J. L. Peeples, M. H. Stokely, M. C. Poorman, M. Magerl, and B. W. Wieland
Citation: AIP Conference Proceedings 1509, 76 (2012);
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Published by the American Institute of Physics
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150 µA 18F target and beam port upgrade for the IBA 18/9 cyclotron
AIP Conference Proceedings 1509, 71 (2012); 10.1063/1.4773943
Visual Observation of Boiling in High Power
Liquid Target
J.L. Peeplesa, M.H. Stokelya, M.C. Poormana, M. Magerlb and B.W.
Bruce Technologies Inc., 1939 Evans Rd. Cary, NC 27513
IBA Molecular, 801 Forestwood Dr. Romeoville, IL 60446
Abstract. A top pressurized, batch style, 3.15 mL total volume (2.5 mL fill volume) water target
with transparent viewing windows was operated on an IBA 18/9 cyclotron at 18 MeV proton
energy and beam power up to 1.1 kW. Video recordings documented bubble formation and
transport, and blue light from de-excitation of water molecules produced images of proton beam
stopping geometry including location of the Bragg peak.
Keywords: Fluorine-18, Batch boiling target, Cyclotron targets.
PACS: 44, 47.
Batch boiling water targets are commonly used to produce 18F by proton
bombardment of 18O-enriched water, through the 18O p, n18 F reaction. These targets
operate under boiling conditions in the target irradiation chamber, but the distribution
of vapor in the chamber under steady-state operating conditions is unknown.
Recently, there has been an increased interest in modeling the thermal processes which
drive target performance of boiling water targets [1-5]. Prior experiments performed
using a vertical sight tube provided information about the total average void in the
target with no indication of void distribution or behavior. Thermal performance of
batch targets was directly correlated to average void, and performance models were
developed [1]. This led to new targets with enhanced production capabilities [6-7].
Current Bruce Technologies batch targets operate at 28 bar (400 psi) with heat
inputs of 1 to 3 kW and fill volumes of 2 to 4 mL. Prior publications by other
researchers used visualization targets to document measurements at disparate
pressures, power levels, and fill volumes [8-9]. This visualization target features
realistic dimensions and was operated at high pressures and beam currents consistent
with recently deployed targets used for routine production.
Heselius, et. al. (1989)
Hong, et. al. (2011)
This Work (2012)
Typical Batch Target
TABLE 1. Summary of visualization targets.
Fill Vol.
Operating Pressure
Incident Energy
0.3-0.6 mL
50 psi
11.6-14.7 MeV
4.5 mL
400 psi
30 MeV
2.5 mL
70-300 psi
18 MeV
2-4 mL
400 psi
11-26 MeV
14th International Workshop on Targetry and Target Chemistry
AIP Conf. Proc. 1509, 76-80 (2012); doi: 10.1063/1.4773944
© 2012 American Institute of Physics 978-0-7354-1127-2/$0.00
Power Level
58-220 W
300-600 W
0-1.1 kW
1-3 kW
The visualization target featured an aluminum body with a 0.005 inch integral
aluminum window and two viewing windows made of optically clear sapphire
(Al2O3). The chamber dimensions were 14 x 15 x 15 mm, for a 3.15 mL maximum
volume. Due to the excellent physical and thermal properties of the sapphire viewing
windows and the use of well-optimized water cooling, the target can be operated at
typical production beam powers and operating pressures with a realistic chamber
FIGURE 1. (a) Isometric view of the solid model for the visualization target. (b) Camera view
through the sapphire window.
During irradiation, the proton beam excites the water molecules, producing visible
blue light emissions during de-excitation. These light emissions can be recorded using
a video camera in dark ambient conditions and used to observe the position of the
Bragg peak and detect beam penetration. With good ambient lighting, the width of the
Bragg peak and natural circulation effects are clearly visible. When a strong backlight
is employed, it is possible to see and record bubble formation and transport in the
target chamber. Use of no lighting is best for observation of the particle range,
including any beam penetration.
FIGURE 2. Sample images from the visualization experiment illustrate visible features for (a) ambient
and backlight, (b) ambient light and (c) no light conditions during low power operation. Sample images
for higher power operation are shown for (d) ambient and backlight, (e) ambient light and (f) no light.
Thermal performance of batch production targets was previously correlated to
average void to develop performance models [1]. The visualization target was
modeled using a finite element code to simulate heat conduction in the solid parts and
fluid dynamics in the cooling water system. The model can be used to predict the total
average void in the target which corresponds to a total heat input, or beam power, and
compared to the experimental results at the same beam current and pressure.
FIGURE 3. (a) Average void fraction is predicted for a given heat input. (b) Average void fraction is
predicted for a given beam current for 18 MeV incident energy.
The visualization target was operated as high as 1.1 kW on an 18/9 cyclotron.
Beam collimation to 10 mm diameter, use of an extension on the beam port, and
cyclotron tuning resulted in a maximum available current on target of 65A. The
target was operated in a top pressurized configuration with a fill volume of 2.5 mL and
a helium vapor space. Stable natural convection, bubble diameter and transport as a
function of operating pressure, and particle range for a variety of currents and
pressures were observed and recorded.
A stable circular natural convection current was visible in the target even before the
onset of boiling. This phenomenon was most visible by using ambient room lighting
only. For higher beam currents, bubbles formed in both the region of the Bragg peak
and near the surface of the beam window and rapidly traveled upward due to
buoyancy forces. For this reason, the average void fraction in the target chamber, and
the proton range as a direct result, increased with height. The natural convection
current was still highly visible during the boiling process. The size and distribution of
bubbles was most visible by using both ambient room lighting and a bright diffuse
backlight. The average bubble diameter decreased with increased pressure.
FIGURE 4. The decrease in bubble diameter due to increased pressure is shown for operation at (a)
40A and 72.5psi, (b) 50A and 130psi, (c) 50A and 200psi and (d) 50A and 300psi.
Range of the protons as a function of height in the target chamber, as well as any
beam penetration due to either under filling or excessive voiding, was clearly visible
by viewing the light produced by the water de-excitation with dark ambient
FIGURE 5. Beam penetration through the top of the target chamber, at the 10 mm beam collimation
height, is visible in dark ambient conditions due to (a) target under fill at 5A and 215psi and (b)
excessive voiding at 55A and 72.5psi with 2.5 mL fill volume.
Infrequent disruption of the helium bubble was observed at high beam current.
During a disruption event, the helium bubble would descend, collapse, disperse
through the chamber, and then rapidly recollect and rise to the top of the chamber.
FIGURE 6. Images captured during helium disruption events show (a) the helium bubble beginning to
descend in dark ambient conditions, (b) the dispersed helium bubble with ambient and backlighting and
(c) the helium bubble recollecting and beginning to rise with ambient and backlighting.
A water target with a fill volume of 2.5 mL was successfully operated at up to 1.1
kW on an IBA 18/9 cyclotron. An aggressive, circular natural convection current was
visible even before the onset of boiling. Increasing the operating pressure resulted in
smaller bubble diameter. Beam penetration through the helium bubble at lower fill
level was observed, as was infrequent disruption of the helium bubble at high beam
Experiments are in progress to determine the effect of bottom pressurization on
target dynamics. Future experiments may address the effects of beam energy and
distribution. New visualization targets with modified chamber dimensions, which
reflect current trends for higher power targets, will be constructed and tested.
This work was supported by NIH/NCRR/NIBIB grant 1RC3RR0307493-01.
1. J. L. Peeples, M. H. Stokely and J. M. Doster, Appl. Radiat. Isot. 69, 1349-1354 (2011).
2. J. Peeples, M. Stokely and J.M. Doster, WTTC12 Proceedings, Seattle, WA, 2008, pp. 29-30.
3. J. L. Peeples, “Design and Testing of Thermosyphon Batch Targets for Production of F”, Ph.D.
Thesis, North Carolina State University, 2008.
4. J. L. Peeples, “Design and Optimization of Thermosyphon Batch Targets for Production of F”,
Master’s Thesis. North Carolina State University, Raleigh, North Carolina, 2006.
5. J. Cornelius, M. Humphrey, J. M. Doster and B. Wieland, WTTC11 Proceedings, Cambridge, UK,
2006, pp. 18-19.
6. M. Stokely, J. Peeples, J. M. Doster, G. Bida and B. Wieland, WTTC12 Proceedings, Seattle, WA,
2008, pp. 19-21.
7. M. H. Stokely, T. M. Stewart and B. W. Wieland, WTTC13 Proceedings, Roskilde, Denmark, 2010.
8. S. Heselius, D. J. Schlyer and A. P. Wolf, Appl. Radiat. Isot. 40(8), 663-669 (1989).
9. B. H. Hong, T. G. Yang, I. S. Jung, Y. S. Park and H. H. Cho, Nucl. Inst. Meth. Phys. Res. A 655,
103-107 (2011).
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