Introduction
Batch-style water targets used for F-18 production are known to operate under boiling conditions in the target irradiation chamber, but the distribution of vapor under steady-state conditions was previously unknown. Thermal performance of batch targets has been correlated to average void in the target [1], but the simplified assumptions of such models do not represent the true non-uniform boiling behavior. Visualization targets can be used to observe boiling inside of a target during operation [2–5].
Commercial BTI targets operate at 28–35 bar (400–500 psi) with heat inputs of 0.5 to 3 kW and fill volumes of 1 to 4 mL. Recently, a visualization target featuring two transparent viewing windows was used to observe boiling conditions for realistic operating beam power, target pressure, and fill volume [4]. The same methodology has been applied to three additional visualization targets to examine the effect of target geometry on observed boiling phenomena.
Material and Methods
The original visualization target featured an aluminum body with a 0.127 mm (0.005 inch) integral aluminum beam window and two viewing windows made of optically clear sapphire (Al2O3). It was operated on an IBA 18/9 cyclotron with 18 MeV protons at beam power up to 1.1 kW, for pressures of 5 to 21 bar (70 to 300 psi), and a fill volume of 2.5 mL.
All of the new designs featured a wider chamber to allow for higher beam transmission and an increased chamber height, consistent with cur-rent trends in high power targets. One target featured a reduced chamber depth, and another had a ramp in the back of the chamber to reduce fill volume. Target pressure was limited to a maximum of 14 bar (200 psi) due to the larger diameter beam window.
A video camera was used to record the boiling conditions observed for each target under several lighting conditions. During irradiation, the proton beam excites the water molecules, producing visible blue light emissions during de-excitation. These light emissions provide a good indication of beam distribution and penetration depth. A strong backlight can be used to produce clearer images of bubbles generated during boiling.
Results and Conclusion
Proton range and visible blue light emissions were recorded in dark ambient conditions. The width of the Bragg peak and natural circulation in the bulk fluid were visible with good ambient lighting. Size and distribution of vapor bubbles could be observed by using a strong backlight. The beam current was increased gradually to determine the thermal limit for each target for several fill volumes and pressures.
Two thermal limits were observed which resulted in some beam penetration in the top region of the beam. For lower fill volumes, steam ac-cumulates in or around the helium overpressure bubble, causing the helium bubble to move into the upper region of the beam. For higher fill volumes, beam penetration occurs due to excessive voiding, when bubbles produced in the beam region cannot rise quickly enough out of the path of the beam.
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa.de:bsz:d120-qucosa-166357 |
| Date | 19 May 2015 |
| Creators | Peeples, J., Stokely, M., Poorman, M., Magerl, M., Wieland, B. |
| Contributors | BTI Targetry LLC,, Helmholtz-Zentrum Dresden - Rossendorf, |
| Publisher | Helmholtz-Zentrum Dresden - Rossendorf |
| Source Sets | Hochschulschriftenserver (HSSS) der SLUB Dresden |
| Language | English |
| Detected Language | English |
| Type | doc-type:conferenceObject |
| Format | application/pdf |
| Source | WTTC15 |
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