The ion source and operations groups at the 88 Inch Cyclotron are developing a high-current titanium beam to be used for the production of superheavy elements. In recent years, the 88 Inch Cyclotron delivered over two particle microamps (particle current is electrical current divided by beam charge state) of 48Ca using the superconducting ECR ion source VENUS. High currents of this beam were essential for reasonable superheavy element production rates, and the mass measurements of produced particles using these beams received significant press . A promising path toward the production of undiscovered superheavy elements requires switching from 48Ca to 50Ti with similar beam currents.
Acceleration of 50Ti should be similar to 48Ca, however the beam production by the source is considerably more difficult. Ion beams are extracted from a plasma in VENUS, and beams can be made of anything that can be added to the plasma without destroying it. High-current metal beams are often produced by heating the metal in a small oven until it sublimates gaseous material into the plasma. Fig 1. shows the resistive “boat” ovens that were used for the recent titanium beam production. Inexpensive blank “boats” (a) are modified with holes for mounting and a wire support to secure the titanium metal (b). Electric currents run through the oven and titanium. VENUS’ high magnetic fields lead to strong Lorentz forces on these objects and can distort the oven. A contorted oven used in these recent runs in VENUS is shown in (c).
The required temperatures for sublimation are much higher for titanium (~1700 ℃ vs. ~600 ℃ for calcium), which introduces engineering difficulties. More importantly, titanium not captured by VENUS’ plasma coats vacuum walls and reacts extremely strongly with other gasses used in the plasma leading to source instabilities.
During a development run in March focusing on oven optimization and improved cyclotron efficiency, over 1.4 particle microamps of 48Ti were extracted from the cyclotron at energies sufficient for superheavy element production—a result that appears to significantly lead other laboratories trying to do the same. Natural titanium was used for these tests (74% 48Ti) as isolated 50Ti is considerably more expensive. It is expected that both beam current and material consumption efficiency will increase when using refined titanium.
This work was led by Janilee Benitez with Jaime Duran, Damon Todd, and Dan Xie from the ion source group at the 88-inch Cyclotron.
 J.M. Gates, G.K. Pang, J.L. Pore, et al., “First direct measurements of superheavy-element mass numbers.” Physical Review Letters 121, 222501 (2018). [DOI: 10.1103/PhysRevLett.121.222501]