Charge breeding of radioactive isotopes at the CARIBU facility with an electron beam ion source

The CAlifornium Rare Isotope Breeder Upgrade (CARIBU)¹ provides radioactive beams to the Argonne Tandem Linac Accelerator System (ATLAS), see Fig. 1. Fission fragments are produced by a ∼1 Ci ²⁵²Cf fission source located inside a large-volume RF/DC helium gas catcher.² The fragments are thermalized and rapidly extracted at up to 50 kV forming a low-energy beam of 1+ or 2+ ions. The isotope of interest is selected via a high-resolution (1:20 000) magnetic separator. These components are housed on a high voltage platform capable of being biased to 200 kV [Fig. 2(a)]. The beam is then transported to an in-room experimental area, a remote stopped beam experimental area, or the EBIS charge breeder [Fig. 2(b)], where the beam is charge bred for subsequent acceleration in the ATLAS linac.

CARIBU began operations in 2011, and originally the fission fragments were charge bred with an ECR ion source.³ While its charge breeding efficiency and high charge state production were at the forefront of ECR charge breeding, its overall performance as a part of the accelerator system was hindered by the pervasive stable background present in ECR ion sources.⁴ Attempts were made to reduce the background level through surface conditioning as well as judicious selection of A/Q ratios, but the radioactive ion beam (RIB) component typically accounted for <3% of the total beam measured at target. The decision was made to replace the ECR with an EBIS charge breeder for the express purpose of lowering the stable background component present in the radioactive beam. In general, an EBIS has a lower level of stable background than an ECR and exhibits improved charge breeding efficiency and faster breeding time.^5,6 As opposed to the CW nature of the ECR, an EBIS is typically operated as a pulsed device. This requires the beam to be cooled and bunched before injection into the charge breeder in order to achieve optimum efficiency. Extracted beam pulses often require modification to limit the instantaneous rate on target.

The EBIS-CB (Fig. 3) was designed in collaboration with Brookhaven National Laboratory and utilizes many of the technologies developed for the RHIC TestEBIS.⁷ Several parameters with regard to the electron gun, potential distribution in the ion trap region, electron collector, and injection/extraction systems were modified from those used for the TestEBIS with the goals of a shortened breeding time, higher transverse ion acceptance, and higher breeding efficiency.^8,9 The EBIS is pumped with turbomolecular and cryogenic pumps installed at either end of the trap, as well as NEG strips installed along the length of the trap, resulting in a typical trap operating pressure of ∼6 × 10⁻¹¹ Torr. The EBIS was commissioned off-line in 2015 using ¹³³Cs⁺ from a surface ionization source. An electrostatic steerer after the source produced a beam pulse of 40 µs, and a set of apertures were used to adjust the beam intensity and emittance. An absolute breeding efficiency of 20% into ¹³³Cs²⁸⁺ was achieved with an electron beam density of 385 A/cm², 28 ms breeding time, 10⁷ ions/pulse (without preparation in a cooler/buncher), and a 10 Hz repetition rate.¹⁰ 

With the off-line commissioning goals complete, the EBIS-CB replaced the ECRCB at the ATLAS front end in October 2015. The EBIS-CB and all of its support systems are housed on a 5.7 × 10.6 m, non-magnetic, high voltage platform capable of 200 kV operation [Fig. 2(b)]. This platform is connected to the CARIBU platform via a low-energy transport line and accelerator tube column. The accelerator tube column enables the EBIS platform to be biased independently of the CARIBU platform thus allowing EBIS-CB and CARIBU development tasks to be pursued in parallel. During RIB charge breeding operation, the accelerator tube column is shorted and the CARIBU and EBIS-CB high voltage platforms are biased to a common potential with a single high voltage power supply. The EBIS-CB itself is capable of being biased 50 kV above that of the main high voltage platform thus decelerating the incoming 1+ or 2+ beam for capture in the ion source. Installation of the EBIS-CB was completed in April 2016, and the first beam of ¹⁶O produced from residual gas was extracted and accelerated in the linac in May 2016. After completion of the RIB low-energy transport line from CARIBU, the first charge bred radioactive beam of ¹⁴²Cs was produced in September 2016.

A radioactive beam breeding cycle starts with a DC ion beam being extracted from the CARIBU gas catcher, typically at 36 kV and either 1+ or 2+ charge state. The beam has a typical energy spread of 1 eV and an emittance of 3π mm mrad. The good beam quality allows for 1:20 000 resolution with the isobar separator, but due to cooling water fluctuations the system more typically runs at 1:14 000. After mass selection and while the previous beam bunch is undergoing charge breeding, the ions are accumulated in the Radio Frequency Quadrupole Cooler/Buncher (RFQ-C/B). The RFQ-C/B collects up to 10⁶ ions/bunch and ejects them at ∼3 kV in a 2 µs wide pulse with a transmission efficiency between 50% and 70%. This step was not necessary with the ECRCB, but the efficiency of an EBIS-CB in a pulsed injection mode has been demonstrated to be significantly higher than in the continuous over-the-barrier injection mode.^11,12 The ions are then transported to a Multi-Reflection Time of Flight device (MR-ToF) which has a design resolution of 1:100 000 with 50% transmission.¹³ This device has not yet been used in conjunction with EBIS charge breeding. The beam then enters a two-step elevator which due to space constraints is located above the MR-ToF. The ion energy is increased to ∼23 keV for transport through the fully electrostatic beamline, which returns to the main beamline level for delivery to the EBIS-CB. The relative positions of the various devices are shown in Fig. 4.

The electron beam is established and reaches a steady state 4 ms before the low charge state ions are injected into the EBIS-CB. The potential barrier on the electron collector side of the trap is then lowered allowing the ions to enter the main trap volume. The bias voltage applied to the EBIS-CB combined with the trap potential serves to decelerate the ions into the trap volume. Before the ions complete a roundtrip within the trap, the barrier is raised thus providing axial confinement. Provided there is good overlap with the electron beam, the low charge state ions are ionized in a stepwise fashion further trapping them in the potential well created by the electron beam which provides radial confinement. After some time, the collector side barrier can be lowered to allow the highly charged ions to escape producing a 10 µs beam pulse. However, such a pulse results in high instantaneous rates and excessive detector dead time. Techniques have been developed to slowly lower the barrier potential resulting in extracted pulse lengths of 80 ms.^11,14 A trap-over-barrier extraction scheme has been implemented at ATLAS in which the potential of the trap electrodes is slowly ramped using an empirically tuned arbitrary function generator. Once the energy of the individually trapped ions is high enough to overcome the collector side barrier potential, the ions spill out of the trap and are extracted from the EBIS-CB as shown in Fig. 5(a). This scheme enables control of the pulse width and results in a reduced energy spread of the extracted ion beam. Pulse widths of 3 ms have been achieved using this technique. When the ramping cycle is complete, the electron beam is terminated and the collector side barrier is lowered clearing the trap volume of any residual ions.

Stable ¹³³Cs beam with an intensity of 10-40 epA is used as a pilot beam to establish the accelerator tune. The entire system is then scaled to the A/Q of the radioactive beam species. The scale factor can be as great as 15%, but is more typically <2%. Standard Thermionics Faraday cups with −300 V DC suppression coupled with a Keithley 6517B electrometer are used for detection of the low-intensity cesium. For the radioactive species, microchannel plates (MCP) are used for beam energy and timing in the CARIBU and EBIS-CB areas. Throughout the entire accelerator, silicon barrier detectors (SBD) (300 mm² active area, 1000 μm depletion depth) with an aluminum cover foil are used for beta decay detection. The SBD stations are of identical configuration permitting accurate measure of the beam transmission efficiency. After each linac section, there are SBDs with gold scattering foils used for beam energy and composition measurements (examples are shown in Fig. 8). The target station is equipped with a SBD for beta detection as well as a gas ionization chamber (GIC) used for particle identification and timing spectra of the beam (Fig. 5). The particle ID spectra are 2D histograms of the energy loss vs. residual energy signals from the two sections inside the GIC. The timing spectra are created using a start trigger from the EBIS-CB and a stop trigger from the particle induced energy signal in the first section of the GIC.

After the EBIS-CB was installed at the ATLAS front end, it was commissioned with ¹³³Cs produced by a surface ionization source. The experimental setup was identical to the offline setup in that the beam intensity and emittance was controlled with a set of apertures as well as the ion source heater.¹⁰ An electrostatic steerer was used to pulse the ¹³³Cs⁺ producing a beam pulse with a width of 40 µs, a typical charge of 1.6 pC/pulse, and a rate of 10 Hz. The EBIS-CB operational parameters during commissioning are shown in Table I. The online commissioning differed from the offline parameters in that the breeding time was 23 ms and the trap-over-barrier scheme was used for ion extraction resulting in an extracted pulse width of 1 ms. A spectrum of the extracted beam accompanied by the residual contaminants consisting mainly of hydrogen, helium, carbon, nitrogen, and oxygen is shown in Fig. 6. An absolute single charge state efficiency of 21.6% into ¹³³Cs²⁷⁺ was achieved with an absolute global efficiency (all visible charge states) of 95% (a plot of the full charge state distribution is shown in Fig. 7). This beam was accelerated in the linac to an energy of 454 MeV and served as the baseline tune for subsequent radioactive beams.

The ¹³³Cs tune served as a baseline for the radioactive beam commissioning both in terms of the accelerator tune as well as the EBIS-CB configuration. And while many of the EBIS-CB parameters were optimized for each of the radioactive ion species studied, the significant changes from the stable ¹³³Cs tune were the breeding time, the injection/extraction steering, and the collector side barrier timing. In all cases, the radioactive beams entered the EBIS-CB with a pulse width of ∼10 µs, and the same EBIS-CB operating parameters as shown in Table I were used for all of the radioactive species. The trap-over-barrier scheme was used for ion extraction resulting in an extracted pulse width of 1 ms as observed on the GIC. The breeding efficiency improved with the slow trap-over-barrier extraction scheme. For ¹³³Cs²⁷⁺, the absolute efficiency increased from 16.5% for fast extraction to 20.8% for slow extraction. The overall absolute breeding efficiency improved from 81% to 93%.

For ¹⁵⁰Ce, the entire accelerator was tuned with 40 epA of ¹³³Cs²⁶⁺ achieving a transport efficiency of 73%. The accelerator was then scaled to ¹⁵⁰Ce²⁹⁺, and the radioactive beam was immediately observed on the target line beta decay detector. The tune was further optimized resulting in a transport efficiency (same locations as with ¹³³Cs) of 77%.

The charge breeding results for the stable and radioactive beams are summarized in Table II, and a summary of the complete charge state distributions of each ion species is shown in Fig. 7. There are direct comparisons for the EBIS-CB charge breeding efficiencies of ¹³³Cs and ¹¹⁰Tc with those achieved with the ECRCB. With that device, the efficiencies were 13% for the ¹³³Cs²⁷⁺ and 11.8% for ¹¹⁰Tc²²⁺ with a typical breeding time of 10 ms/charge state.⁴ In both cases, the charge bred radioactive beam had a significant stable background and accounted for <2% of the total beam extracted from the ECR.

The ubiquitous contamination of ECR ion sources is well known^15,16 and has complicated their use as RIB charge breeders due to the need for post-breeding separators¹⁷ or additional foil stripping to limit contaminants.¹⁸ An example of the stable background from the ANL ECRCB is shown in Fig. 8(a), a silicon barrier detector spectrum of a ¹⁴⁶Ba²⁸⁺ beam after acceleration in the ATLAS linac. The spectrum is dominated by mass 94, and the radioactive ¹⁴⁶Ba accounts for only 3% of the total beam intensity. In all cases, various A/Q ratios were examined to limit the level of stable background. The inability to adequately reduce the level of stable background through the use of surface cleaning techniques as well as plasma surface coating⁴ was the driving force for the EBIS-CB development.

Installation of the EBIS charge breeder has resulted in a substantial reduction in stable background. For the case of ¹⁴²Cs [Fig. 8(b)], the radioactive component now accounts for 76% of the beam incident on the silicon barrier detector with ¹²¹Sb being the dominant contaminant. It is also observed that the contaminant profile has changed. With the ECRCB, ¹²⁰Sn and ¹⁹⁸Hg contributed to the background load; however, ¹¹⁶Sn and ²⁰⁰Hg (both predicted contaminants for ¹⁴²Cs) are not observed in the EBIS-CB spectrum. It is also noted that the ¹⁴²Cs²⁷⁺ tune may not be the A/Q with the lowest background. The combination was chosen due to the availability of an existing accelerator tune. In addition to the silicon barrier detector, the ¹⁴²Cs beam was observed with the GIC. That spectrum also showed the RIB accounting for 80% of the total beam and provided additional atomic number resolution to reveal a ¹⁴²Ce contaminant. The GIC can provide a Z resolution of ∼2% for the expected CARIBU mass range, and a spectrum taken for ¹⁵⁰Ce [Fig. 5(b)] resolved peaks for ¹¹⁴Cd and ¹¹⁹Sn which are two atomic numbers apart.

Possible sources of the observed contaminants are still being investigated with potential sources being the IrCe cathode, the stainless steel drift tubes, and the copper electron collector. The EBIS-CB drift tube structure was assembled in a clean room and care was taken to limit the number of materials introduced into the source. After assembly, the source was baked at 450 °C and operates in a pressure regime of 6 × 10⁻¹¹ Torr. This eliminates a large fraction of trace atmospheric elements. There are also SAES St707 NEG strips installed along the length of the scaffold assembly to aid in pumping. The strips have a Constantan base material (55% Cu-45% Ni) compression coated with St707 powder consisting of Zr, V, and Fe. The electron beam transmission through the drift tube structure is 99% with small losses on the last drift tube possibly contributing the background. The drift tubes are constructed from 316 L stainless steel which contains amounts of phosphorous and molybdenum. Additionally, there were discharges occurring at the base of the drift tubes where the zirconia posts met the scaffold structure resulting in damage to several of the posts as well as deposition on select drift tubes. These posts have since been replaced with alumina posts and the areas of deposition were cleaned.

This work was supported by the U.S. Department of Energy, Office of Nuclear Physics, under Contract No. DE-AC02-06CH11357 and used resources of ANL’s ATLAS facility, an Office of Science User Facility. We express our gratitude to Dr. S. Kondrashev, Dr. J. Alessi, Dr. E. Beebe, and Dr. A. Pikin for their significant contributions during the design, assembly, and commissioning of the CARIBU EBIS-CB.