On-line Ion Sources

• A high efficiency Electron-Beam-Plasma Ion Source ( EBPIS) is employed for production of positive RIBs which is used in conjunction with a charge exchange cell to form negative ions for injection into the tandem accelerator. The ion source design is based on the ISOLDE plasma source which has been adapted to our system. The operation of this source is straightforward: several hundred amperes of heating current flows through a Ta tube which connects the RIB target to the ion source. This tube achieves nominal operating temperatures of ~1800 C which is needed to transport the radioactive nuclei to the ion source once they are released from the target material. Once current passes through the transport tube it is directed across a high resistance emission surface which operates nominally at 2100 C. Electrons emitted from this surface are then accelerated into a biased ionization chamber where a plasma is formed and electron impact ionization takes place. The ion beam is extracted from this plasma across a ~150V acceleration gap through an opening of a few mm and then accelerated to platform potential through a set of accelerating apertures. The source emittance is on the order of 4 p mm mrad at 40kV and the ionization efficiency curves for stable Xe and Kr are shown here as a function of emission current.

• A direct Negative ion source for radioactive fluorine and other halogen species has been developed at the HRIBF. The design is similar to the EBPIS in that a vapor transfer line is also directly heated to high temperatures by a current of several hundred amperes. This current also heats a positive Cs surface ionizer with is either a porous W frit or a spiral of W foil. The positive Cs ions are then accelerated through a high transmission Mo mesh and strike the surface of a cooler Ta cone which collects both Cs and nuclear reaction products which have effused from the target material. The Ta cone is also biasable and held at a less negative potential than the Cs acceleration grid so that negative ions formed though the sputter ejection process from the cone will be electrostatically expelled from the ion source. Shown here is an ionization efficiency curve for stable F^- produced from injected SF₆ gas as shown as a function of source heating current.

• A small volume, all permanent magnet, high efficiency electron cyclotron resonance (ECR) source has also been designed and is under construction for the generation of positive ion beams. This source should efficiently dissociate and ionize gaseous molecules such as CO, O₂, NO, etc.

• A 'batch mode' negative ion source for use on the RIB injector has also been designed, fabricated and is currently being assembled.  Multiple RIB production targets are located on a wheel which is irradiated for a period of time with the ORIC beam.  Nuclear reaction products formed in collisions between the incident beam and the target material collect very close to the surface of the target. Once a sufficient number of reaction products have collected in the target, the wheel is rotated into a  conventional Cs sputter source and negative RIB species are directly produced for injection into the tandem accelerator.  As RIBs are being generated and transported through the ion optical system, the ORIC beam irradiates the next target sample and a automated batch process is achieved.  This technique is ideal for long-lived activities such as ⁵⁶Ni and ¹⁸F.

Send questions and comments to Robert Welton