Superconducting RF (SRF) Systems Peter McIntosh (STFC) Accelerator-driven Production of Medical Isotopes 8th – 9th December 2011
Outline • Medical Isotope Generation • The Photo-Fission Process • Accelerators and Isotope Production • Superconducting RF Technology • SRF Accelerator Solutions for 99Mo Production • Conclusions
Medical Isotopes • Positron Emission Tomography (PET) scans, produce detailed maps of active areas in the brain and other organs. Technetium-99m has become the workhorse of diagnostic nuclear medicine: • with over 50,000 procedures performed each day in the U.S. • Radioisotopes have become a vital tool for scientific research and industry: • applications in medicine, biology, physics, chemistry, agriculture, national security and environmental and materials science. • The ability to attach radionuclide to a pharmaceutical agent for transport to the desired site is key to its effectiveness for medical applications.
99mTc Production • The half-life of an isotope must be long enough to allow transport from production sites to end-use locations without excessive loss, and short enough to minimize the unwanted radiation dose to the patient after the procedure is complete. • The use of “generators,” involves a longer-lived parent (2.75-day 99Mo) that decays to a shorter-lived daughter (6-hour 99mTc). • Specialized reactors produce the 99Mo as a fission fragment for transport to end-use sites, where clinicians “milk” the 99mTc daughter from the generator as needed.
99Mo Production (a) Neutron fission of 235U (present-day reactor technique). (b) Neutron-capture process on 98Mo. (c) Photo-neutron process on 100Mo. (d) Photo-fission of 238U.
Photo-Fission Process Neutron Fission fragments High energy photon Neutron Fission fragments Neutron Photo-fission of 238U was proposed by W. T. Diamond (Chalk River) in 1999 as an alternative production method for RIB.
HEU Reactors and Accelerators • Specific isotopes from accelerator induced nuclear reactions, using appropriate beam species and energies: • Typically 10 – 100s MeV protons • Major advances in high-power particle accelerators and high-power target technologies offer significant advantages for isotope production via spallation-induced neutron irradiation. • Isotope sources are produced using either reactors or accelerators of varying sizes. • 99Mo is a fission fragment that is today typically produced using Highly Enriched Uranium (HEU) reactors.
SRF Accelerators • Superconducting RF (SRF) accelerator technology is capable of 20x beam power required, which using rapid-switching techniques could distribute beam simultaneously to many target stations. • Electron beam accelerator technologies show promise for radionuclide production, either via photo-nuclear reactions with MW-class beams, or using Compton backscatter of high-intensity laser beams with high-quality light source beams. • A high-power accelerator facility could provide a reliable source of neutron produced isotopes such as 99Mo, and also have sufficient power and flexibility to supply a steady stream of research isotopes.
Superconductivity Benefits • Both Normal-conducting RF (NRF) and SRF accelerating structures can produce high gradients (10s of MeV/m), but only SRF technology can sustain high duty operation. • The power dissipated for CW operation in a NRF structure is potentially enormous, but not so for SRF cavities. • RF power required to generate an accelerating voltage (Vacc) is defined by: • The ratio of shunt resistance to quality factor (R/Q), which depends on cavity geometry, is not vastly different between NRF and SRF cases; and so the cavity power (Pc) required is dominated by the bare quality factor Q₀, which is typically 105 times larger for SRF. • Overall, SRF reduces the wall plug power compared to NRF, by a factor ~200, reducing power consumption from MWs to kWs.
Neutron Amplifier Target for Isotope Production ina SRF Based Accelerator Facility at ANL • The SRF linac uses energy recovery linac (ERL) technology, whereby the electron beam is transmitted through the target and is recollected and re-injected into the accelerating structure. • The recollected beam transfers beam power to the injected electron beam, and reduces the amount of RF power required to accelerate the electrons to their final energy. • ~10 x less RF power compared to non-ERL linac technology. • The SRF linac is only ~ 2 meters long making it compact in comparison to existing NC technology. The energy recovery efficiency is >95%. The depleted beam power is dumped into a low energy beam stop, which is physically separate from the target. Courtesy J Nolan, ANL
Accelerator Flexibility • The beam energy, target thickness, and recovered power can be optimized for maximum isotope yield. • The target has a vacuum loadlock, which can be removed from the target chamber and a new target installed without breaking vacuum or stopping the linac operation. • The activated target requires robotic control, but this is a well established technology. • System has a number of advantages over conventional linac technology: • more compact, • higher current for increased yields, • improved thermal management of waste beam power, • a continuous target feed, • and multiple target capability. • The linac can also be configured for other isotope production, e.g. 67Cu, which requires similar energies to 99Mo.
ARIEL E-Linac at TRIUMF • Advanced Rare IsotopE Laboratory as part of the ISAC facility at TRIUMF. • New complementary electron linac (E-linac) driver for photo-fission. • New proton beamline. • New target stations and front end. • Staged installation proposed.
ARIEL E-Linac Parameters } The requirement: 50 MeV × 10 mA = 0.5 MW beam power } Not critical; beam dumped on target
ARIEL E-Linac Layout Injector 10 MV/m, Q=1010 10 mA, 5-10 MeV gain ≤100 kW beam power Main Linac Two cryomodules Two cavities/module, 10 MV/m, Q=1010 10 mA, 40 MeV gain ≤400 kW beam power Gun 100 keV, 650 MHz • Division into injector & main linacs allows: • Possible expansion for: • Energy Recovery Linac (ERL) – e.g. 10 mA, 80 MeV • Recirculating Linear Accelerator (RLA) – e.g. 2 mA, 160 MeV Buncher Solenoids Capture Cavities Solenoid 9-cell Cavities
MW Class SRF Linac The present E-linac design concept, based on 1.3 GHz Superconducting RF technology (SRF) in CW operation, offersflexibility, possibility for expansion to other applications (Free Electron Laser, Energy Recovery Linac).
Staged Implementation
E-Linac High Power RF Delivery
ARIEL Injector Cryomodule Cryo-insert with 4K phase separator. 2K JT valve & HX not shown. Insulating vacuum 77K thermal shield Input coupler Strong back Warm/cold transition Cavity string
Main LinacCryomodule Cold Mass Scissor Tuner Input Couplers Integrated Cryomodule
HOM Mitigation • Use SS rings or other damping material to damp dipole to 1e7 Ω. • Back-up solution – use HOM couplers. • RF generator drives fundamental mode, E-beam drives all modes, leaves wakefield, leading to beam break up (BBU) instability. Analysis defines the criteria of (Rd/Q)⋅QL < 107 ohm. • Modelling 9-cell cavity found TE111 trapped dipole mode at 2.56 GHz with Rd = 3e7Ω. • Asymmetric end cells & beam pipes push the mode towards the tuner end.
E-Linac Implementation ERL Upgrade HPRF RLA Upgrade Cryoplant SRF Cryomodules Gun & LEBT
Conclusions • High power accelerators are a viable alternative to conventional HEU reactors. • SRF linac technology provides significant power advantages compared to conventional linac technology: • Order of magnitude higher if ERL techniques are employed • Significant capital and operational cost savings • SRF accelerators also provide wide flexibility in terms of beam power, allowing optimisation for a variety of isotope species and effective yield. • SRF linac solutions for 99Mo production are now becoming a reality using proven accelerator technology. • Funding agency priority is now being demonstrated!
