SUPERCONDUCTING RF (SRF) SYSTEMS
Peter McIntosh (STFC)
Accelerator-driven Production of Medical Isotopes
8th – 9th December 2011
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Outline
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Medical Isotope Generation
The Photo-Fission Process
Accelerators and Isotope Production
Superconducting RF Technology
SRF Accelerator Solutions for 99Mo Production
Conclusions
Medical Isotopes
• 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.
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• 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.
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.
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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.
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Photo-Fission Process
E-Linac
Electron
Photon
Converter
238U
Target
Ion
Source
Mass
Separator
Neutron
High energy
photon
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Users
Fission
fragments
Neutron
Neutron
Photo-fission of 238U was proposed by W. T. Diamond
(Chalk River) in 1999 as an alternative production method
for RIB.
RIB
Fission
fragments
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 highpower 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.
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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 highquality 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.
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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:
2
Pc 
V acc
2R
Q
Qo
• 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.
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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
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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:
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–
–
–
–
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.
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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.
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ARIEL E-Linac Parameters
Parameter
Value
Bunch Charge
16 pC
Bunch Repetition Rate
0.650 GHz
Frequency
1.3 GHz
Average Current
10 mA
Energy
50 MeV
Beam Power
0.5 MW
Duty Factor
100 %
Bunch Properties
Injected
Ejected
Normalised Emittance (µm)
-30
-100
Longitudinal Emittance (eV.ns)
-20
-30
Bunch Length (FW), inject (ps)
-170
-30
<1 keV
<1 %
Energy Spread (FW)
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}
}
The requirement:
50 MeV × 10 mA
= 0.5 MW beam
power
Not critical; beam
dumped on target
ARIEL E-Linac Layout
Gun 100 keV, 650 MHz
Buncher
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
Solenoids
Capture Cavities
Solenoid
9-cell Cavities
• 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
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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).
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Staged Implementation
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E-Linac High Power RF Delivery
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ARIEL Injector Cryomodule
Cryo-insert with 4K phase separator.
2K JT valve & HX not shown.
Insulating
vacuum
77K thermal
shield
Input
coupler
Strong back
Cavity
string
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Warm/cold
transition
Main Linac Cryomodule
Cold Mass
Scissor Tuner
Input Couplers
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Integrated
Cryomodule
HOM Mitigation
• 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.
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• Use SS rings or other damping
material to damp dipole to
1e7 Ω.
• Back-up solution – use HOM
couplers.
E-Linac Implementation
ERL
Upgrade
HPRF
RLA
Upgrade
SRF
Cryomodules
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Cryoplant
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!
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