Advanced Radioactive Isotop E-Laboratory
The Advanced Radioactive IsotopE Laboratory (ARIEL) is a flagship project at the TRIUMF nuclear physics Laboratory in Vancouver, Canada.[1] [2] [3] Phase-I of the project involved the installation of an electron linac that was commissioned and declared operational in 2013. [4] [5] Phase-II involves the installation of a pair of On-Line Isotope Mass Separator-type target stations; ARIEL Electron Target East (AETE) accepting electrons up to 35 MeV from the linac built in phase-I, and ARIEL Proton Target West (APTW) accepting 500 MeV protons from the Triumf main cyclotron.
ARIEL Electron Target East (AETE)
Principles of operation
The AETE Target Station is designed to cause nuclear reactions in target material by the process of photofission. The electron driver beam (up to 100 kW of up to 35 MeV electrons) is converted to gamma radiation by interaction with matter via Bremstrahlung.[6] Electrons of this energy are stopped in a material thickness on the order of millimeters, as quantified by the Bragg peak, resulting in extreme power deposition densities in the target material. The gamma radiation produced has a spectrum of energy bounded at the upper end by the energy of the incident electron beam. Target materials are stimulated via giant dipole resonance to gamma radiation above a certain threshold. A peculiar result of response is that radiation with energy below the threshold contribute negligibly to the number of fissions but contribute substantially to the heat deposited in the target material. The fission rate can be increased by increasing the driver beam power up to the limit of the heat dissipation capacity of the target. The Alto photofission facility at IPN Orsay had been operating photofission targets for several years before ARIEL. The AETE Target Station is designed to overcome the driver beam power limit by physically separating the process of converting the driver beam electrons into gamma radiation from the stimulation of GDR in the target material. An assembly referred to as a photo-converter consisting of a millimeter thick layer of dense (i.e. high-Z) material, typically tantalum or tungsten, to rapidly decelerate the incident the driver electrons producing high energy gamma radiation, followed by a light (low-Z) backing material, typically aluminium, to absorb remaining primary electrons which on balance will have lower energy while minimizing the absorption of high energy gammas. The result is a cone of gamma radiation of useful energy spectrum while filtering out all primary electrons. This method significantly reduces the ratio of heat deposited in the target per gamma in the useful energy range. While the Alto photofission target operates in the range of 10's or 100's of Watts of primary beam, AETE is being designed to accept up to 100kW of primary beam power.
Design
The AETE target station is designed for remote target exchanges on a three-week rotating schedule. To meet this schedule requires that the target be exchanged within 24 hours of end of beam (EOB). The residual activity of the spent target is sufficiently high to necessitate a fully remote target exchange operation. This is achieved by full remote control of the operation of the target station together with a remotely operated crane. The crane installed for this purpose is a 20 tonne overhead bridge crane with a rotating stage on the hook block. All motions of the crane, including North-South Bridge motion, East-West Trolley motion, raise and lower drum rotation, are fully redundant to ensure that operators can recover from any single point failure occurring with a radioactive load on the hook. The rotation stage is not redundant, and safe landing locations are designed to allow for a target to be landed in any rotational orientation.
Target
The target material is tailored to the specific species to be extracted in that particular operational run to achieve the desired yields.[7] Typical materials include uranium carbide (UCx), silicon carbide, beryllium, and tantalum. The target material is placed inside a cylindrical target container approximately 40 mm (1.6 in) in diameter and 60 mm (2.4 in) long. The target container is made of a refractory metal such as tantalum that maintains structural integrity at the operational temperature of 2,000 °C (3,600 °F). The target container is mounted to a water cooled plate via high current leads capable of delivering 2400 A of Ohmic heating, and enclosed in a vacuum tight vessel, referred to as the target vessel. The paradigm of replacing the ISOL target in-situ is copied from the ISOLDE method of operation. The entire target vessel is disconnected from the Target Station and discarded following a run of approximately 3 weeks. However, the high residual activity of the target vessel necessitates a cool-down period of several years before being chemically processed and transported to Chalk River Ontario for disposal. The species created in as a result of the nuclear reactions diffuse through the target material and desorb from the surface into the vacuum all around the target. The target material and container are kept at high temperature to mitigate the "freezing out" of or deposition of the newly created species on cold surfaces. At low beam powers the target container is wrapped in heat shields (Mo and / or Ta foils) and heated by passing a current through the target, while at high driver beam power the target is operated without a foil heat shield.
Target Module
The infrastructure immediately surrounding the Target is referred to as the target station front end. The front end is suspended below a mass of steel approximately 2 m (6.6 ft) in thickness, referred to as the shield plug. The effect of the shielding is to drastically reduce the intensity of the radiation to levels that allow for conventional vacuum pumps, lubricants and polymers to be used. The entire assembly of front end, shield plug, and equipment above is referred to as the target module, and weighs just under 20 tonnes. This module is designed to be periodically lifted from the installed location using a remotely operated overhead bridge crane and transported to a hot cell facility (HCF) where skilled operators can remotely repair and replace components as needed. The paradigm of periodically transporting the front end to the HCF for periodic maintenance is copied from the ISAC target stations, and differs from the ISOLDE target stations.
Remotely handleable beamline segments
The beamline equipment in the approximately 6 m (20 ft) immediately downstream of the target is also mounted to shielding comprising assemblies referred to as the RIB modules. As with the target module, this allows for periodic hot cell maintenance of components. A notable exception is the so-called pre-separator, which is a magnetic bender that is floor mounted due to its size.
ARIEL Proton Target West (APTW)
Principles of operation
The APTW Target Station is designed to cause nuclear reactions in target material via spallation, fragmentation, and fission. A proton driver beam originating from the TRIUMF main cyclotron passes directly through the target with a fraction of the beam interacting with the nuclei of the target material. Most of the driver beam energy passes through the target material and is stopped downstream of the target in a separate assembly called the beam dump. The beam dump is essentially a water cooled mass of copper that stops the beam and removes the associated energy. In addition to stopping the beam, the APTW beam dump assembly is also designed to optionally host a second, symbiotic target, referred to as a medical target.
The proton driver beam generated in the TRIUMF main cyclotron is transported to the APTW target station via a beamline called "beamline 4 north" (BL4N). The continuous vacuum of this beamline is separated from the vacuum in which the target is located by a physical window made of millimeter scale thickness of aluminum, which is essentially transparent to 500 MeV protons. The quality of the vacuum surrounding the target is intrinsically dirty by comparison because of the desorption of species originating within the target itself. The proton window prevents contamination originating from the target from migrating upstream and contaminating BL4N and the Cyclotron.
APTW Symbiotic Medical Target
The medical target is designed to accommodate up to 1.5kg of target material, such as thorium, that can be inserted and removed from the beam dump module via a pneumatic tube that connects the beam dump to a specially designed hot cell in the HCF. The medical target and delivery system is designed to allow for a target exchange without interrupting service to the APTW primary target. The medical target system is called symbiotic because it makes use of APTW beam that is otherwise wasted, and the medical target HCF infrastructure is also used for some processing of APTW spent targets.
APTW Target Module
The APTW target module is designed to be interchangeable with the AETE target module with relatively quick series of modifications. This allows for fewer spare parts to be kept on-hand, reducing operating costs and reducing design time by sharing components between the two stations.
Target Storage Vault
Targets that have been removed from service are remotely placed in the Target storage vault for decay until the dose rate from the target while in the shielded transport vessel. The length of decay required is dependent on the specifics of the target and the duration of irradiation. Typically Targets from APTW are stored for two to three and a half years, while AETE targets are stored for one to two years. The vault is designed with capacity to store 90 targets.
References
- ↑ Dilling, Jens; Krucken, Reiner; Merminga, Lia (January 2014). "ARIEL overview". Hyperfine Interact. 225 (1–3): 253–262. Bibcode:2014HyInt.225..253D. doi:10.1007/s10751-013-0906-6. ISSN 1572-9540.
- ↑ Marchetto, Marco; Baartman, Rick; Laxdal, Robert (January 2014). "ARIEL front end". Hyperfine Interactions. 225 (1–3): 275–282. Bibcode:2014HyInt.225..275M. doi:10.1007/s10751-013-0908-4.
- ↑ Ames, Friedhelm; et al. (September 2011). "ARIEL:TRIUMF's Advanced Rare IsotopE Laboratory". Particle accelerator. Proceedings, 2nd International Conference. IPAC 2011, San Sebastian, Spain, September 4-9, 2011. C110904. pp. 1917–1919. Retrieved April 8, 2019.
- ↑ Koscielniak, Shane (January 2014). "ARIEL e-linac". Hyperfine Interactions. 225 (1–3): 263–273. doi:10.1007/s10751-013-0907-5.
- ↑ Kolb, Philip; Laxdal, Robert; et al. (July 2011). "HOM Cavity Design for the TRIUMF eLINAC" (PDF). Proceedings of SRF20141. MOPO047. pp. 203–205.
- ↑ Diamond, William (11 August 1999). "A radioactive ion beam facility using photofission". Nuclear Instruments and Methods in Physics Research A. 432 (2–3): 471–482. Bibcode:1999NIMPA.432..471D. doi:10.1016/S0168-9002(99)00492-1.
- ↑ "ISAC Yield Database | TRIUMF : Canada's National Laboratory for Particle and Nuclear Physics". ISAC Yields Database. TRIUMF. Retrieved May 29, 2019.
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