Classification of particle accelerators

The great diversity of particle accelerators does not facilitate the application of a single classification system or criteria for the location and layout of facilities. A number of criteria could be used to classify particle accelerators. They can be classified according to the technology by which acceleration is achieved (e.g. power source, acceleration path geometry), the application or use of the accelerators (e.g. industrial, medical), the maximum energy, the maximum beam current, and the time structure of the beam delivery [26]. Tables 3 and 4 demonstrate two different criteria used to classify particle accelerators.

TABLE 3. ACCELERATOR PRODUCED RADIATION CLASSIFIED BY ROUTINE APPLICATION

(reproduced from Ref. [26] with permission courtesy of National Council on Radiation Protection and Measurements)

Description Electron Proton Ion Photon Neutron

Radiotherapy * * * * *

Industrial radiography n.a. n.a. n.a. * *

Analysis of materials n.a. n.a. n.a. n.a. n.a.

Activation analysis n.a. * * n.a. *

Microscopy, electron, ion or positron * * * * n.a.

X ray fluorescence analysis * * * *

Geological well-logging * n.a. * * *

Neutron scattering n.a. * n.a. n.a. *

Synchrotron light sources * n.a. n.a. * n.a.

FEL * n.a. n.a. n.a. n.a.

Ion separation n.a. n.a. * n.a. n.a.

Archaeological investigations * n.a. * n.a. n.a.

Ion implantation, wear assessment n.a. n.a. * n.a. n.a.

Surface conditioning, roughening * n.a. n.a. * n.a.

Radioisotope production n.a. * * n.a. *

Radiation processing * n.a. n.a. * n.a.

Radiation sterilization * n.a. n.a. * n.a.

Research and training n.a. n.a. n.a. n.a. n.a.

Nuclear structure physics * * * * *

Neutron physics * * * * *

Atomic and solid state physics * * * * *

Biology, chemistry * * * * *

Radiation effects on materials * * * * *

Particle physics * * * n.a. *

Note: * — existing or applicable; n.a. — not applicable.

The energy ranges referenced in Classes 1–4 of the particle accelerators in Table 4 are provided solely for generic purposes. These energy ranges do not account for the differences that will be found between hadron and electron accelerators or between accelerators that have been used for different applications. In general, electron accelerators will exhibit much lower levels of activation than hadron accelerators. For this reason, the suggested threshold for Class 2 accelerators is 10 MeV for proton and 30 MeV for electron machines. No activation can be expected in electron machines up to roughly 8 MeV. The classification given in Table 4 will be used in the rest of this publication.

Another possible classification could make use of the expected amounts and extent of activated waste resulting from decommissioning. According to this criterion, the first category would be applied to accelerators for which activation is confined to the target. The second category would be applied when activation includes the target and its immediate surroundings (e.g. treatment head and collimator). The third category would refer to the activation of the entire accelerator machine. The fourth category would also include neighbouring infrastructure and services (e.g. shielding, building structure, soil). In complex facilities, categorization might vary between different areas of the facility.

3.3.2. Specifics of Class 1–4 particle accelerators

Many of the components of particle accelerators are common across all the categories of such accelerators.

The components and their functions are listed in Table 5. The reader is advised to consult Ref. [27] for a full description of accelerator parts, operational techniques and so on.

Three potential categories of radioactivity can be encountered in particle accelerators:

(a) Activation. The principal components that become activated are those that can directly interact with the beam (i.e. targets, dumps, screens, collimators, jaws, kickers and septa, antiparticle and neutron sources). Other components likely to become activated are in positions where beam loss occurs (i.e. magnets and narrow apertures).

(b) Contamination. Contamination can arise from activated oil or cooling water leaks or from the spread of activated materials from damaged targets and the beamline windows. Contamination can also arise from TABLE 4. ACCELERATORS CLASSIFIED BY ENERGY RANGE

(reproduced from Ref. [14])

Class Energy range^a Type of accelerator Examples

Class 1 Low energy (2–30 MeV)

Electron linacs and electrostatic accelerators

● Radiotherapy linacs

● Van de Graaff, tandem accelerators, Pelletron, with a potential lower than 10 MV

Class 2 Medium energy (10–100 MeV)

Proton, H^- or multiple particle cyclotrons and linacs

● Cyclotrons for PET/single photon emission computed tomography (SPECT) radionuclide production

● Cyclotrons for neutron sources

● Other accelerators (linacs and cyclotrons) in physics research (injectors)

● Wide range of interdisciplinary research with light sources

a For ions: kinetic energy per nucleon.

TABLE 5. TYPICAL COMPONENTS PRESENT IN ACCELERATOR ENCLOSURES AND THEIR FUNCTIONS

Component Function Main material

(secondary material^a)

Source or gun Producing charged particles Steel, Cu (alumina, NEG^b)

Chopper Imparting a pulsed beam structure Fe, Cu, W, misc.

Vacuum chamber Providing vacuum environment for the particle beam path Steel, Al Vacuum pumping system,

vacuum valves

Generating and maintaining vacuum Steel

Radiofrequency cavities Accelerating beam Cu, Nb

Radiofrequency wave guides Conducting radiofrequency power to radiofrequency cavities Cu, Al, Cu/Al Magnetic systems Bending, focusing or defocusing beam; limiting energy

spread

Cu, Fe (steel, Al)

Electrostatic and magnetic kickers and septa

Switching beam direction Cu, Fe (ferrites)

Cryogenic systems Cooling superconducting magnets and accelerator cavities (for Class 3 and 4 accelerators)

Steel, Al

(liquid N and He) Cooling systems (pipes and

ion exchange resin beds)

Cooling and maintaining de-ionized water Steel, Cu, resin, water (steel, Al)

Collimators, jaws Restricting aperture at beamline transitions W/Cu alloy, Pb Masks, heat shields Absorbing the heat from synchrotron radiation Cu (steel, Al)

Wigglers, undulators Producing synchroton light Cu, Fe (Co, Ni, Fe, rare earths) Stoppers, beam dumps Absorbing or terminating the beam Steel, Cu, Al, water, graphite Targets, beamline windows Modifying beam composition by interaction with target

material; used either for structural or experimental purposes;

can be either internal or external to the vacuum system

Variety of materials; Be, Al alloys and C for windows

Screens, scanning wires, Faraday cups

Performing beam diagnostics Cu, steel, misc.

Permanent magnets Preventing accidental beam transport into optical lines at synchrotron facilities

Fe, Co, Ni, Al, Sm, rare earths

Dedicated antiparticle sources, neutron sources

Creating secondary beams, used in specific facilities; created by bombarding solid targets with the primary beam

W, Hg, misc.

Note: misc. — miscellaneous.

a Only typical materials are listed. A wide variety of materials may be used for specific applications (e.g. photocathode materials for electron guns, insulators, coatings).

b Non-evaporable getter of impurities in vacuum vessel.

accident scenarios, from sputtering or evaporation after beam interaction with materials, or from radioisotope production (including deposition from radioisotope beams or accelerator produced radionuclides).

(c) Radioactive substances not generated by accelerator operation. Such substances include calibration sources in experimental equipment and other components (e.g. depleted uranium shielding, detectors).

The presence and quantity of radioactivity that will require consideration as part of planning for decommissioning will be different for each of the four categories of particle accelerator and will typically be higher for hadron accelerators than for electron accelerators.