Dimo Yosifov, Igor Sekachev
TRIUMF, Canada’s National Laboratory for Particle and Nuclear Physics, Vancouver, British Columbia, V6T 2A3, Canada
Abstract: The Isotope Separator and Accelerator (ISAC) facility at TRIUMF has been in full operation since 1999. The ISAC East and West targets use proton beam from the cyclotron to produce various radioactive isotopes, which are then ionized and extracted. The ions are then passed through the mass separator and selected ions are transferred to the low energy experiments or injected into an RFQ accelerator. The accompanying radioactive contamination from the production of radioactive ions requires a complex vacuum system. The main target vacuum space consists of two semi-separate (primary and secondary) volumes pumped by turbo-molecular pumps. The primary volume uses four pumps while the secondary volume uses two pumps. Two hermetic rotary vane pumps are used as backing pumps. The nominal vacuum in both volumes is about 1.33E-4Pa (1.0E-6 Torr). The pressure is monitored by two cold cathode and two hot filament ion gauges. The cold cathode gauges are used to interlock the system, which is critical during the bake out of the target and beam production. The exhaust gas from the vacuum pumps can be radioactive. Three gas storage tanks (decay tanks) are used for temporary storage of the radioactive exhaust and its controlled release to the atmosphere. Gas-specie insensitive membrane gauges are used for monitoring the pressure in the storage tanks. This paper describes the details of the ISAC target vacuum system as well as some procedures related to the handling of the exhaust gas with traces of radioactive contamination produced by the targets.
TRIUMF’s ISAC (Isotope Separator and Accelerator) uses the Isotope Separation-On-Line (ISOL) technique to produce radioactive ion beams. The ISOL system is based on a primary production beam and consists of a target/ion source, a mass separator, and a separated beam transport system. These systems act together to produce radioactive ion beams for an accelerator or for the low-energy experimental areas.
The TRIUMF 500 MeV cyclotron delivers proton beam used as primary beam for target system. Beam line 2A (Fig.1) transports the beam to one of two target stations. The target stations contain proton beam monitoring devices, production target and ion source, a beam dump, and front-end heavy ion-beam optics. By bombarding thick production targets, ISAC has ability to produce radioactive beams of unprecedented intensity. The radioactive species coming from the target or ion source are ionized, extracted, and mass separated, forming high quality beams of isotopes. The isotopes can be delivered directly to low energy experimental facilities, or they can be accelerated.
Fig.1. Schematic of cyclotron, beam line 2A, targets and mass separators line.
The ISAC II upgrade will extend the range of radioactive nuclei that can be accelerated from a maximum mass number of A=30 (ISAC I) to mass A= 150, and the maximum energy from the present 1.5MeV/A to 6.5 MeV/A for the entire mass range and as high as 15 MeV/A for light nuclei. The accelerated mass range will be increased by means of a Charged-State Booster, which is capable of efficiently increasing the charged state of 1+ ions from the target to n+.
2. Targets Vacuum System.
Fig 2. West Target Station.
ISAC targets are located in two target stations (East and West). Each Target Station consists of five modules listed and shown in Fig. 2.
The Target Module, First Exit Module, and the Second Exit Module consist of three main parts (Fig 3). The top part (Service Cap) contains all vacuum feed-troughs, pumping and service ports. The Entrance Module and the Beam Dump Module do not have Service caps. The middle part is the radiation shield plug (common for all modules), which is a block of steel. The bottom part is the Containment Box housing the Target and Ion Source (in the Target Modules) or the front-end heavy ion beam optics (in the Exit Modules). Although installed in one target station tank, the modules separate the main vacuum space into two semi-separated vacuum volumes. The vacuum volume enclosed by the Containment Box forms the primary vacuum volume. The vacuum volume between the containment box and the target station tank is the secondary vacuum volume. The Containment Box is used to keep the radioactive contamination in the vicinity of the target and while the target module is moved out of the target station for maintenance and target replacement, it prevents radioactive contamination of the Target Maintenance Hall.
Fig 3. Target module.
Presently ISAC operates three target modules and one ECR Module (one at a time can be used). Another target module is under construction. Any of the modules can be installed into West or East Target Station.
Fig.4. West Target Station vacuum system (Primary, Secondary and Decay Storage Tanks are shared by both East and West stations).
The gas removed from the primary and secondary vacuum volumes is temporarily stored in the primary and secondary operating storage tanks respectively (Fig. 4). The tanks are located in a concrete bunker to reduce local radiation fields. When the beam production schedule permits, gas is transferred from the operating storage tanks to the decay storage tank. After decay, when the radioactivity of the gas has sufficiently decreased, it is exhausted to the atmosphere through the ISAC nuclear exhaust filtering system.
Before releasing the gas from the decay storage tank a gas sample is analyzed to determine if it is within the safety levels approved by the Canadian Nuclear Safety Council. Based on the results, the Health Physicist gives the authorization for release the gas. The exhaust gas is forced though charcoal and “HEPA” filters before release to atmosphere with its radioactivity level constantly monitored during the release.
The controls of the target station vacuum equipment are done via PLC based Experimental Physics Control System (EPICS). The system is multifunctional, upgradeable, allows active recording and archiving of the data collected from the vacuum gauges, and provides active personnel and equipment safety protection via interlocks.
There are four 1000 L/s turbo pumps on the primary vacuum volume and two 1000 L/s turbo pumps on the secondary vacuum volume in a Target Station. All turbo pumps servicing the target station are air-cooled. There are two solenoid valves, working in tandem as one, installed and used as baking valves for each turbo-pump. When removing a turbo-pump for service or repair, the backing line is disconnected between these valves, with one of the valves remaining attached to the turbo-pump and the other attached to the backing line. This is necessary in order to prevent the spread of radioactive contamination. The working parameters (temperature and power consumption) of the pumps are monitored for early identification of problems with the turbos or the vacuum in the target station. The turbo pumps’ performance is monitored on a weekly basis (when access to their controllers is allowed). In the case of a noticeable increase in the operating temperature and/or power consumption a scheduled replacement is arranged. During the last four years of operation three pumps out of twelve were replaced and repaired. Although they were radioactive no removable contamination was found.
Two hermetic rotary vane pumps are used as backing pumps. These pumps were chosen as their design allows a high level of tightness against leaks for the pumped gas. The pumps exhaust into evacuated operating storage tanks. This is necessary to prevent leaks of radioactive gas to atmosphere and to eliminate the health hazard to the personnel. The typical backing pressure of the pumps when the beam is running is about 1.5 Pa (1.1E-2 Torr). These pumps transfer the potentially radioactive gas out of the target stations. The pumps may become radioactive and their oil contaminated. The backing pumps are reliable and maintenance-free for about two calendar years. They are checked periodically for their oil level. The broken pumps are taken off operation. They are usually radioactive, and after being well packed, the pumps are stored in a radiation storage area. There they are periodically monitored for the decay of their radioactivity.
Ion gauges are used as high vacuum gauges for their fast response and reliability. They are nude Bayard-Alpert type with thoria-coated iridium filaments for long-life. The ion gauges are screen protected against electrostatic discharges generated by the beam production process. The nominal operating pressure is about 1.3E-5 Pa (1.0E-7 Torr).
Cold cathode gauges are used in locations where hot filament gauges can interfere with the main process. These are Inverted Magnetron type, placed entirely in vacuum and are screen protected against electrostatic discharges. The gauges are installed on a support which permits remote removal of the entire gauge for replacement. The cable connection is similarly modified for use with a remote manipulator arm. The operating pressure, measured by these gauges is between 1.3E-5 Pa to 1.3E-4 Pa (1.0E-7 Torr to 1.0E-6 Torr) at different locations within the vacuum space.
Conventional Convectron gauges and diaphragm manometers are used to monitor the pressure in the Target stations, Operating and Decay storage tanks. The diaphragm manometers are used for the range 67 Pa to 160 KPa (0.5–1200 Torr) at operating and decay storage tanks. They are gas-species insensitive, have a 316L stainless steel diaphragm, and offer high resolution.
The most common malfunction for the ion gauges is a burned filament. During the weekly maintenance day, the damaged gauge is removed and the filament replaced. The radiation-contaminated gauges require extra care.
Three types of vacuum seals are employed at the target stations: – Buna-N type for large rectangular ports, copper gaskets, and indium plated Helicoflex seals. These types of seals are found to be reliable and easy to replace.
Leaks in the Target Station Vacuum System occur infrequently. The leaks were related to radiation damaged Buna seals and small holes burned through plastic insulators by high voltage discharge. The leaks were located with Helium leak detector, protected from radioactivity and contamination by a Liquid Nitrogen trap. Also several leaks were located then repaired at Helicoflex seals for the connections of the cooling water lines of the Target Modules. These leaks were difficult to find and required removal of the target module from the Target station for a detailed leak check and repair. This is a very time consuming process and interrupts the scheduled beam production.
The ISAC facility at TRIUMF has been in full operation since 1999. The Targets Vacuum System of ISAC was operational, reliable, and serviceable. The down time due to failed vacuum components was minimal. Most of the activities related to troubleshooting and maintenance were done during the weekly maintenance day and two annual scheduled shutdowns.