RIB Target Systems

In general, many radioactive nuclei of interest to the research programs at the HRIBF have short lifetimes, are only produced in small quantities and undergo substantial losses during release, transport and ionization in the target/ ion source. Production of usable intensities of RIBs, therefore, is usually preceded by the development of a target/ ion source technique which optimizes release of the radioactive species from the target material, transport to ionization region and efficient ionization of the specific chemical species. This page presents a few highlights of some new target techniques which have been or shortly will be employed on the RIB injector.

Early investigations into ¹⁷ F production through the ¹⁶ O(d,n)¹⁷F reaction were conducted with fibrous targets of Al₂O₃ and the maximum ¹⁷ F ^- yield measured leaving the RIB platform was 5x10⁴ ions/s, an insufficient intensity for RIB experiments. To solve this problem we have designed several new target systems which are based on a technique of forcing a reactive, electropositive transport vapor through a fibrous metal oxide network which is considerably more refractory than Al₂O₃. The table below shows several oxides which are commercially available in fiber form (4-6 um dia.) and packed to densities of 3-10% of the pure material generally in the form of a weave or felt.  See for example, Zircar Products Inc.

The maximum operating temperature of the target material is set by the characteristic vapor pressure curve for that material since the closely coupled ion source cannot tolerate a vapor loads greater than 10^-4 Torr.  Since, the major RIB loss processes; diffusion, desorption and effusion generally depend exponentially on temperature, we expected the gains made by increased operating temperature and increased production (more production beam on target) to far out weight the effect of shorter production ranges due to increased stopping power of the higher Z, more refractory materials: Hf, Zr, Y, Ce versus Al.  HfO₂ is the most refractory of these materials and the microstructure of HfO2 target is shown here.  The following target system designs are based on this material and have been used or are being developed for use at the HRIBF.

• This figure shows the initial configuration of the HfO2 target system which was tested on the RIB injector. The target system features a high temperature BN transport vapor oven which was used with Al vapor and a W-Re target material reservoir heater capable of sustained operation at 2300 C. Initial tests of this system have yielded ¹⁷F^- beams of 2x10⁷ ions/s measured leaving the RIB platform, a significant improvement over the best yield of 5x10⁴ ions/s from Al₂O₃. This system was employed in the first HRIBF astrophysical investigation of ¹⁷F.

• This figure shows a sketch of a high power HfO2 target  and associated elongated heater which has been designed and is presently being constructed. This design increases the effective radiating surface area of the target by a factor of ~20 over the present design (shown above). This is accomplished by accepting a wider beam and inclining the target material 85 degrees with respect to the beam axis.  Widening the beam will be accomplished either by rastering, diffusing or defocusing the present ORIC beam.

 
• A second design under development increases still further the effective radiating surface of the target by longitudinally spacing the HfO₂ disks thereby allowing direct radiative heat transfer from the interior of the target material to the relatively cool enclosure walls.  By uniformly spacing the  HfO₂ disks this effective area can be increased by a factor of ~50 over the present design.  This figure shows the concept of a longitudinally distributed target system which employs the same elongated target heater shown above.  Here, the disks are supported with a thin W rod which intercepts only a few percent of the beam while drawing several hundred watts of beam heat directly from the hottest portion of the target to an external sink.

 

A simple non re-circulating liquid Ge target system was employed for ⁶⁹ As and ⁶⁷Ga produced through the ⁷⁰Ge(p,2n)⁶⁹As and ⁷⁰Ge(p,a)⁶⁷Ga reactions.  Approximately 1x10⁶ ions/s of ⁶⁹As and 1x10⁵ ion/s of ⁶⁷Ga ions/s were delivered to an experimental target. In the near future, several experiments are planned which require roughly an order of magnitude more intensity. To achieve these goals several new liquid target systems have been designed and are currently being tested:

• A liquid target system has been developed which utilizes a highly pervious, low density structure to support the liquid material by virtue of the cohesive force. Metallic or carbide materials are used to suspend um coatings of the liquid onto a fiber or foam structure thereby dramatically decreasing the effective diffusion length from mm to um for a given target thickness. This format also has the effect of increasing the surface area of the target by a factor of 10²-10³ over traditional liquid targets. Specifically, liquid Ge has been shown to readily wick in a uniform fashion onto fibers of Mo or SiC when contained by a graphite reservoir. This highly pervious liquid target system target system is shown here along with a cooled re-circulating vapor trap and a secondary target material reservoir. The vapor trap has been added to the system to suppress unwanted target material vapors which should be more prevelent in this design compared with the traditional dense liquid targets for a given operating temperature.  The secondary reservoir can be heated through the same temperature range as the primary target reservoir and serves to extend the operational lifetime of the system by replenishing lost target material. In general terms this target system maybe operated with other liquid and non liquid target materials allowing in situ vapor deposition of variable thickness (um) coatings on small scale structures such as graphite fibers. At present, this system is being tested at our on-line test facility.

• A re-circulating, high power liquid target system has also been developed which inclines a thin layer of liquid target material at an 85 degree incidence angle with respect to the production beam. This has the effect of increasing the total beam current tolerated by the target since the actual effective radiating area is increased by a factor of ~5 due to the incline. This system has been designed and built and awaits testing. Both liquid target systems employ the cooled re-circulating baffle which allows the target material to operate at temperatures hotter than would be allowed by the material's vapor pressure without the baffle.

• A UC₂ target has been developed at the HRIBF which is based on chemically vapor depositing thin um coatings of UC₂ on RVC fiber matrixes. The target material is able to withstand high temperatures (2000 C) while maintaining highly pervious structure without sintering. The microstructure of the UC2 target material is shown in this figure. The results of RIB yield experiments with this material are shown on the yields page.