Other Accelerator Applications

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Other Accelerator Applications
Fernando Sannibale
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• Pulsed Neutron Sources

• Medical Accelerators

• Other Applications

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• NEUTRONS HAVE NO ELECTRICAL CHARGE
• Neutron cross sections for interaction with
  materials are small. Therefore, neutrons are
  highly penetrating, nondestructive probes.
  Neutron interactions are known with great
  accuracy. Therefore, neutron scattering data are
  more easily interpreted than comparable photon or
  electron scattering data.

• NEUTRONS ARE SCATTERED BY NUCLEI, NOT BY
  ELECTRONS
• Neutrons "see" both light and heavy atoms and can
  distinguish neighboring atoms in the periodic
  table.
• Neutrons can distinguish isotopes of the same
  element, enabling isotopic substitution and
  contrast variation.
• The large difference in neutron scattering cross
  sections for H and D facilitates studies of
  biological samples.

• NEUTRONS ARE SPIN 1/2 PARTICLES
• Spin polarized beams can be produced.
• The neutrons magnetic moment can probe
  microscopic magnetic structure and magnetic
  fluctuations.

• NEUTRON WAVELENGTH IS WIDELY TUNABLE
• Neutrons provide structural information over nine
  orders of magnitude (10-5 to 104 Å), making them
  useful probes for distance scales ranging from
  the wavefunction of atomic hydrogen to those of
  macromolecules.

• NEUTRON ENERGIES ARE COMPARABLE TO EXCITATIONS IN
  SOLIDS AND LIQUIDS
• Neutrons provide energetic information on
  materials for transitions from the nanoelectron
  volt to electron volts.

NEUTRON BEAMS HAVE LOW BRILLIANCE AND DIFFICULT
TO PRODUCE!
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• Evolution of the performance of reactors and
  pulsed spallation sources. In recent years,
  dramatic improvements in accelerator technology
  have made it possible to design and construct a
  source to produce very intense neutron pulses
  (updated from Neutron Scattering, K. Skold and D.
  L. Price, eds., Academic Press, 1986).

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Pulsed neutron sources are mainly based on the
spallation process
High energy protons collide in a target with high
Z nuclei and generate a number of secondary
particles (including neutrons) that interact with
other nuclei triggering a chain generation that
amplifies the number of neutrons
A spallation source is an inherently safe way to
produce neutrons because the neutron production
stops when the proton beam is turned off. It also
produces fewer hazardous materials with respect
to a nuclear reactor.
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The backbone of the facility is the proton
accelerator complex
Proton Beam Parameters P beam on target
1.44MW I beam aver. 1.44mA Beam
energy 1 GeV Pulse length on target
700 ns Proton on target 1.5
1014 Linac duty factor 6 Rep.
rate 60Hz Linac macropulse length
1ms
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• Negatively charged hydrogen ions (H- ) are
  produced by an ion source over 1 ms long pulses
  at 60 Hz rep. rate.

• The ions are injected into a several stage
  linear accelerator, which accelerates them up to
  the (very high) energy of 1 GeV.

• The H- are then injected into a ring and during
  the injection process they go through a foil,
  which strips off the electrons converting the
  ions into protons. The stripping process is not
  Hamiltonian (Liouville theorem does not apply)
  and allows for accumulation in the phase space.
  The 1 ms macropulse from the linac fills about
  1000 turns of the ring (T 1ms). A single
  bucket RF system (1MHz) maintains a bunch length
  of 700 ns. The bunch is then extracted in a
  single shot.

• The extracted proton pulse strikes a 1 m3 liquid
  mercury target. The corresponding pulse of
  neutrons freed by the spallation process is then
  slowed down in a moderator and guided through
  beam lines to the experimental areas.

• Once there, neutrons of different energies can
  be used in a wide variety of experiments. The
  whole cycle is repeated 60 times per second.

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RFQ Radio Frequency Quadrupole (accelerates and
focuses simultaneously)
DTL Drift Tube Linac (Wideroe-Alvarez)
CCL Coupled Cell Linac (coupled pillbox-like
structures)
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In the front end, the first part of the linac,
the ion source produces negative hydrogen (H- )
ions that are formed into a pulsed beam and
accelerated to an energy of 2.5 million electron
volts (MeV).

• The SNS front end consists of
• volume-production, cesium enhanced, RF-driven,
  H- ion source
• an electrostatic low energy beam transferline
  (LEBT)
• a 4-vane RFQ that accelerates the 65-keV beam
  from the ion source to 2.5 MeV
• beam-chopping systems
• and a beam-transport, rebunching, and matching
  section (MEBT)

Built by LBNL
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• Volume-production, cesium enhanced, RF heated
  (2 MHz), 28 mA peak, 1 ms, 60 Hz H- ion
  source
• LEBT electrostatic low energy beam line Final
  energy 65 kV.

• The filter magnets remove electrons from the
  plasma that could ionize the H- that are mailnly
  created in the Cs collar.

• The dumping magnets and electrode remove the
  electrons that are generated in the Cs collar
  region (100 times the number of H-).

• The action of the dumping system deflects also
  the ions. The plasma chamber must be misaligned
  respect to the LEBT for compensating for this
  effect.

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The longitudinal component of the RF field
accelerates the ion beam while the transverse
part creates a quadrupole-like focusing geometry.
Output energy measured by Time Of Flight in MEBT
2.45MeV (the ions are not relativistic yet)
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• The linac accelerates the H- beam from 2.5 to
  1 GeV.
• The linac is a superposition of normal conducting
  and superconducting radio-frequency cavities that
  accelerate the beam and a magnetic lattice that
  provides focusing and steering.
• Three different types of accelerators are used.
  The first two, the drift-tube linac and the
  coupled-cavity linac are made of copper, operate
  at room temperature, and accelerate the beam to
  about 200 MeV. The remainder of the acceleration
  is accomplished by superconducting niobium
  cavities. These cavities are cooled with liquid
  helium to an operating temperature of 2 K. The
  superconducting accelerator allows to contain the
  AC power requirements to an acceptable level.
• Diagnostic elements provide information about the
  beam current, shape, and timing, as well as other
  information necessary to ensure that the beam is
  suitable for injection into the accumulator ring
  and to allow the high-power beam to be controlled
  safely.

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• The SNS accumulator ring converts the H- ions
  into protons, accumulate all the protons of the
  long linac pulse in a relatively short bunch and
  shoots it at the mercury target 60 times a second.

• The intense H- beam from the linac must be
  shortened more than 1000 times to produce the
  700 ns short bunch of neutrons needed for optimal
  neutron-scattering research. To accomplish this
  goal, the H- pulse from the linac is wrapped into
  the ring through a stripper foil that strips the
  electrons from the negatively charged hydrogen
  ions to produce the protons that circulate in the
  ring. Approximately 1000 turns are accumulated,
  and then all these protons are kicked out at
  once, and delivered to the target.

• The use of the "stripping" technique for the
  injection has a dual benefit
• Being the stripping a non-Hamiltonian phenomenon
  Liouville theorem does not apply and accumulation
  in the phase space is allowed.
• Such an injection scheme does not require pulsed
  element

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• In order to efficiently dissipate the large
  amount of energy that the 1 GeV proton beam
  deposits in a shot in the spallation target, a
  liquid mercury target was used rather than a more
  "traditional" solid target such as tantalum or
  tungsten.

• SNS is the first scientific facility to use pure
  mercury as a target for a proton beam. The volume
  of the target is 1 m3 ( 18 tons).
• Mercury was chosen for the target for several
  reasons
• (1) it is not damaged by radiation, as are
  solids
• (2) it has a high atomic number, making it a
  source of numerous neutrons
• (3), because it is liquid at room temperature, it
  is better able than a solid target to dissipate
  the large, rapid rise in temperature and
  withstand the shock effects arising from the
  rapid high-energy pulses.

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The neutrons coming out of the target must be
turned into low-energy (thermal) neutrons
suitable for researchthat is, they must be
moderated to room temperature or colder. A
thermal neutron, for instance, with a temperature
of 300 K (corresponding to E 26 meV) travels
with a speed of 2200 m/s, and has a De Broglie
wavelength of ? 0.17 nm.
The neutrons emerging from the target are slowed
down by passing them through a moderator cells
filled with water (to produce room-temperature
neutrons) or through containers of liquid
hydrogen at a temperature of 20 K (to produce
cold neutrons). These moderators are located
above and below the target. Cold neutrons are
especially useful for research on polymers and
proteins.
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Each high-energy proton that hits a high Z target
nucleus generates between 20 and 30 neutrons by
spallation
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Accelerators in medical science are used as a

• Diagnostic tool. For instance
• Radiology
• Angiography

• Therapeutic tool. For instance
• Radiotherapy
• Proton and ion therapy
• Boron-Neutron capture therapy

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Electrostatic electron accelerators (30 - 100
keV). Electrons are accelerated from a thermionic
cathode and hit a metallic anode generating
x-rays by brehmsstrahlung (mainly) and by core
electrons ionization and recombination.
1 x-ray production efficiency most of the
beam energy goes in heat (cooling is a
technological issue)
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• Angiography is the name of a procedure that uses
  X-Rays to produce a picture (the "angiogram"). 
• This is an "invasive procedure, because it
  requires the injection into the patient of a
  substance that is radiopaque (absorbs X-Rays).
  This substance is commonly called a "Contrast
  Agent" or "Dye".
• Usually a very tiny tube, that has a special
  shape, is used to place the contrast into a
  particular artery or vein. While the artery or
  vein contains this radiopaque material, it will
  block the X-Rays, and will cast a shadow of the
  injected vessels onto the X-Ray film or
  fluoroscope.
• This image will reveal the shape of the artery,
  and can help to diagnose an obstruction,
  blockage, or narrowing ("stenosis")

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Radiation therapy (or radiotherapy) is the
medical use of ionizing radiation (x-rays) as
part of cancer treatment to control malignant
cells. The energy released by the radiation
breaks the chemical bond in molecules (DNA)
killing the cell.
Much higher x-ray doses are required for cancer
therapy than for radiography. As a consequence
the sources go from x-ray tubes to linacs
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When a fast charged particle moves through
matter, it ionizes particles and deposits energy
along its path. A peak occurs because the
interaction cross section increases as the
charged particle's energy decreases. The maximum
of this deposited energy (dose) is called the
Bragg Peak it occurs shortly before the particle
has lost all its energy and stops.
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In 1946, while observing the Bragg peak
structure, Bob Wilson realized the potential of
using proton and ion beams in cancer therapy and
the major advantages respect to the existing
schemes using photons and electrons.
The first treatments with a proton beam were
performed in 1954 when John Lawrence, Ernest
Lawrence's brother, treated patients at
Berkeley's 184-inch synchrocyclotron.
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• The depth of the Bragg peak can be tuned by
  changing the energy of the proton beam

• By focusing/collimating the proton beam
  transversely and controlling its energy it is
  possible to irradiate only the tumor minimizing
  the dose released on the healthy parts.

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• Gantries allows to irradiate tumor from all sides
  to further minimize damage to tissue surrounding
  tumor

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• Systematic studies of the biological efficiency
  of all ions from protons to uranium in cancer
  therapy showed that the radiation damage that
  carbon ions causes is repairable to a large
  extent in the entrance channel of the beam, and
  becomes irreparable only at the end of the beam's
  range (Bragg peak) where the tumor is located.

•   The crucial difference arises from the damage
  caused to cell DNA. Cancer cells and healthy
  cells alike die when their DNA sustains
  irreparable damage. In general, this means both
  DNA strands being broken since single strand
  breaks can frequently be repaired by the cell.
  Lighter particles such as protons, whilst
  depositing their energy in the Bragg peak, cause
  far fewer double-strand breaks than heavier ones
  like carbon. Moreover, the boundary between
  single and double-strand damage is particularly
  sharp with carbon which allows extremely precise
  targeting of the tumour.

•   Another advantage is that carbon ions do not
  scatter as much as lighter particles. This allows
  higher degrees of conformity to be achieved.
  Heavier ions, such as neon, were discounted
  because they tend to fragment. Carbon does
  fragment to some extent, but the fragmentation
  products include positron-emitting carbon 10 and
  11. Positron Emission Tomography, PET, then
  allows the radiotherapist to observe "live" the
  position of the beam in the patient with a
  resolution of 2.5 mm.

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• Boron Neutron Capture Therapy (BNCT) is a binary
  form of cancer therapy which uses a
  boron-containing compound that preferentially
  concentrates in tumor sites. The tumor site is
  then irradiated by a neutron beam.

• The neutrons in the beam interact with the boron
  in the tumor to cause the boron atom to split
  into an alpha particle and lithium nucleus. Both
  of these particles have a very short range (about
  one cellular diameter) and cause significant
  damage to the cell in which it is contained. In
  this way, damage is done to the tumor cell, while
  largely sparing healthy tissue.

• Unfortunately at the present time, the
  boron-containing compounds used in this technique
  are quite toxic. Present RD is dedicated to the
  localization of less toxic compounds.

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• Proton accelerators are also used for the
  production of unstable isotopes. Such unstable
  elements with a relative short decay time can be
  produced for synthesis into compounds used in
  oncology.

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• Linac based proton accelerators designed to
  replace large and demanding cyclotron systems for
  the production of positron emitting isotopes.
  Large amounts of fluorine-18, carbon-11,
  nitrogen-13, and oxygen-15 can be produced for
  synthesis into compounds used in oncology,
  cardiology, neurology, and molecular imaging.

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• Thanks to Dave Robin and Christoph Steier for
  sharing the viewgraphs of their course Charged
  Particle Sources and Beam Technology NEC 282
  at UCB.

• W. H. Scharf, "Biomedical Particle Accelerators",
  American Institute of Physics 1 edition (May 7,
  1997)

• The CERN Courier.

• The World Wide Web.