1. SUPERCONDUCTING RF (SRF) SYSTEMS Peter McIntosh (STFC) Accelerator-driven Production of Medical Isotopes 8 th – 9 th December 2011 1

2. Outline Medical Isotope Generation The Photo-Fission Process Accelerators and Isotope Production Superconducting RF Technology SRF Accelerator Solutions for 99 Mo Production Conclusions 2

3. 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. 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. Medical Isotopes 3

4. 99m Tc 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 99 Mo) that decays to a shorter-lived daughter (6-hour 99m Tc). Specialized reactors produce the 99 Mo as a fission fragment for transport to end-use sites, where clinicians “milk” the 99m Tc daughter from the generator as needed. 4

5. 99 Mo Production (a) Neutron fission of 235 U (present-day reactor technique). (b) Neutron-capture process on 98 Mo. (c) Photo-neutron process on 100 Mo. (d) Photo-fission of 238 U. 5

6. Photo-Fission Process E-Linac Electron Photon Converter 238 U Target Ion Source Mass Separator RIB Users Photo-fission of 238 U was proposed by W. T. Diamond (Chalk River) in 1999 as an alternative production method for RIB. Fission fragments Fission fragments Neutron High energy photon 6

7. Isotope sources are produced using either reactors or accelerators of varying sizes. 99 Mo is a fission fragment that is today typically produced using Highly Enriched Uranium (HEU) reactors. 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. HEU Reactors and Accelerators 7

8. 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 99 Mo, and also have sufficient power and flexibility to supply a steady stream of research isotopes. 8

9. 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 (V acc ) 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 (P c ) required is dominated by the bare quality factor Q ₀, which is typically 10 5 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. 9

10. Neutron Amplifier Target for Isotope Production in a 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 10

11. 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. 67 Cu, which requires similar energies to 99 Mo. 11

12. 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. 12

13. ARIEL E-Linac Parameters ParameterValue Bunch Charge16 pC Bunch Repetition Rate0.650 GHz Frequency1.3 GHz Average Current10 mA Energy50 MeV Beam Power0.5 MW Duty Factor100 % The requirement: 50 MeV × 10 mA = 0.5 MW beam power Bunch PropertiesInjectedEjected Normalised Emittance (µm) -30  -100  Longitudinal Emittance (eV.ns) -20  -30  Bunch Length (FW), inject (ps)-170-30 Energy Spread (FW)<1 keV<1 % } Not critical; beam dumped on target } 13

14. ARIEL E-Linac Layout 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 Gun 100 keV, 650 MHz Buncher Solenoids Capture Cavities Solenoid 9-cell Cavities Main Linac Two cryomodules Two cavities/module, 10 MV/m, Q=10 10 10 mA, 40 MeV gain ≤400 kW beam power Injector 10 MV/m, Q=10 10 10 mA, 5-10 MeV gain ≤100 kW beam power 14

15. MW Class SRF Linac The present E-linac design concept, based on 1.3 GHz Superconducting RF technology (SRF) in CW operation, offers flexibility, possibility for expansion to other applications (Free Electron Laser, Energy Recovery Linac). 15

16. Staged Implementation 16

17. E-Linac High Power RF Delivery 17

18. ARIEL Injector Cryomodule Cryo-insert with 4K phase separator. 2K JT valve & HX not shown. Insulating vacuum Input coupler Cavity string Warm/cold transition Strong back 77K thermal shield 18

19. Main Linac Cryomodule Scissor Tuner Input Couplers Cold Mass Integrated Cryomodule 19

20. 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 < 10 7 ohm. Modelling 9-cell cavity found TE 111 trapped dipole mode at 2.56 GHz with Rd = 3e7Ω. Asymmetric end cells & beam pipes push the mode towards the tuner end. Use SS rings or other damping material to damp dipole to 1e7 Ω. Back-up solution – use HOM couplers. HOM Mitigation 20

21. E-Linac Implementation Gun & LEBT Cryoplant HPRF SRF Cryomodules ERL Upgrade RLA Upgrade 21

22. 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 99 Mo production are now becoming a reality using proven accelerator technology. Funding agency priority is now being demonstrated! 22