**2. Electron-positron colliders**

Although this chapter addresses the neutrino effective dose from a muon collider, it is illustrative to provide a summary of the effects of other radiation types within a lepton collider (Bevelacqua, 2008, 2009, 2010a). These radiation fields are illustrated by considering an electron-positron collider. The radiation field within the muon collider facility are similar to those described in this section for electron-positron colliders.

An electron-positron collider accelerates electrons and positrons in circular rings before colliding the individual beams. There are a number of electron-positron colliders that have operated, are currently operating, or are being planned. These include the Large Electron Positron (LEP) Collider, and other machines summarized in the Review of Particle Properties (Particle Data Group, 2010). A new electron-positron machine, the International Linear Collider, is under design and is addressed from a health physics perspective in Bevelacqua (2008).

From an experimental physics perspective, electron-positron colliders have a number of advantages when compared to hadron colliders. First the collision results are less complex in terms of the particles produced, because electrons and positrons are fundamental particles without underlying structure or features. Hadrons are composed of quarks, but the electron and positrons have no such substructures. Therefore, the lepton's final state interactions are less complex than the structures that are produced from the interaction of the hadron's quarks. Particle interaction complexity is not the only advantage of electronpositron colliders.

The lepton colliders are also capable of achieving larger luminosities than hadron colliders. In addition, an order of magnitude less energy is required in electron-positron machines vice hadron colliders to achieve similar experimental results. For example, an electronpositron collider with a center-of-mass energy of 2 TeV is roughly equivalent to a 20 TeV center-of-mass energy hadron collider. In spite of these advantages, electron-positron collider health physics concerns exist (Bevelacqua; 2008, 2009, 2010a).

Electron-positron colliders produce more bremsstrahlung than hadron colliders. This bremsstrahlung production serves to limit the upper energies achieved by circular electronpositron colliders. In addition, electric power requirements rapidly increase with increasing energy unless beam power recovery mechanisms are developed and implemented.

The bremsstrahlung produced in a circular electron-positron collider is a fundamental concern that can only be decreased by increasing the circumference of the machine. The logical conclusion is to use an accelerator with an infinite radius (i.e., a linear collider). This is most easily achieved by replacing the dual beams in a circular collider with colliding beams from two linear colliders.

92 Particle Physics

of muon accelerators are addressed in this chapter and models for calculating the neutrino effective dose at a muon collider are provided. The radiological impacts of muon colliders and how basic dose reduction principles are affected by the underlying physics inherent in weak interaction processes are also discussed. Finally, a brief discussion of the neutrino

Although this chapter addresses the neutrino effective dose from a muon collider, it is illustrative to provide a summary of the effects of other radiation types within a lepton collider (Bevelacqua, 2008, 2009, 2010a). These radiation fields are illustrated by considering an electron-positron collider. The radiation field within the muon collider facility are similar

An electron-positron collider accelerates electrons and positrons in circular rings before colliding the individual beams. There are a number of electron-positron colliders that have operated, are currently operating, or are being planned. These include the Large Electron Positron (LEP) Collider, and other machines summarized in the Review of Particle Properties (Particle Data Group, 2010). A new electron-positron machine, the International Linear Collider, is under design and is addressed from a health physics perspective in

From an experimental physics perspective, electron-positron colliders have a number of advantages when compared to hadron colliders. First the collision results are less complex in terms of the particles produced, because electrons and positrons are fundamental particles without underlying structure or features. Hadrons are composed of quarks, but the electron and positrons have no such substructures. Therefore, the lepton's final state interactions are less complex than the structures that are produced from the interaction of the hadron's quarks. Particle interaction complexity is not the only advantage of electron-

The lepton colliders are also capable of achieving larger luminosities than hadron colliders. In addition, an order of magnitude less energy is required in electron-positron machines vice hadron colliders to achieve similar experimental results. For example, an electronpositron collider with a center-of-mass energy of 2 TeV is roughly equivalent to a 20 TeV center-of-mass energy hadron collider. In spite of these advantages, electron-positron

Electron-positron colliders produce more bremsstrahlung than hadron colliders. This bremsstrahlung production serves to limit the upper energies achieved by circular electronpositron colliders. In addition, electric power requirements rapidly increase with increasing

The bremsstrahlung produced in a circular electron-positron collider is a fundamental concern that can only be decreased by increasing the circumference of the machine. The logical conclusion is to use an accelerator with an infinite radius (i.e., a linear collider). This is most easily achieved by replacing the dual beams in a circular collider with colliding

energy unless beam power recovery mechanisms are developed and implemented.

collider health physics concerns exist (Bevelacqua; 2008, 2009, 2010a).

effective doses anticipated at a third generation tau collider are provided.

to those described in this section for electron-positron colliders.

**2. Electron-positron colliders** 

Bevelacqua (2008).

positron colliders.

beams from two linear colliders.

The electron and positron beams produce a variety of radiation types that are derived from the direct beam and its interactions. Secondary radiation is produced from bremsstrahlung when beam particles strike accelerator components and from synchrotron radiation when beam particles are defected by magnetic fields.

Bremsstrahlung has a number of health physics consequences. These health physics issues include (NCRP 144, 2003): (1) electromagnetic cascade radiation containing high-energy photons, electrons, and positrons, (2) high-energy radiation including neutrons, pions, muons, and other hadrons, (3) activation of accelerator structures and components, (4) activation of air, cooling water, and soil, and (5) ozone and oxides of nitrogen produced in the air. Synchrotron radiation also has health physics consequences including: (1) electromagnetic cascade radiation, (2) photons, (3) neutrons, (4) activation of accelerator structures and components, (5) activation of air, cooling water, and soil, and (6) ozone and oxides of nitrogen produced in the air. These secondary radiation categories and their health physics consequences are addressed in more detail in subsequent discussion and in Bevelacqua (2008, 2009, 2010a).

The primary electron (positron) beams are contained within beam tubes, and secondary radiation is produced when the primary particles exit the beam tube either by design or accident. When electrons (positrons) exit the beam tube they strike accelerator components such as the beam tube structure, vacuum components, collimators, or structural members. When this occurs, the beam particle decelerates and radiates photons through the process of bremsstrahlung. The high-energy, bremsstrahlung photons produce electron-positron pairs that lead to additional bremsstrahlung. This process repeats itself, and produces an electromagnetic shower or cascade that contains numerous particles and a spectrum of photons having energies up to the kinetic energy of the initial beam particles.

A second category of secondary radiation occurs when the beam particles traverse the accelerator's magnetic fields. The magnetic field produces a force that alters the particle's trajectory. It also changes the particle's velocity and leads to the emission of photon radiation. This process is known as synchrotron radiation. Synchrotron radiation is related to bremsstrahlung because a change in velocity or acceleration is involved in both processes. However, the synchrotron radiation differs from the bremsstrahlung spectrum.

With bremsstrahlung, the photon energy extends from zero up to the energy of the beam particle. However, synchrotron radiation is governed by the configuration and strength of the magnetic field. Therefore, the synchrotron spectrum is machine specific. For example, CERN's decommissioned Large Electron-Positron collider had a synchrotron spectrum that extended from the range of visible light to a maximum intensity that occurred in the range of a few hundred keV (Bevelacqua, 2008). The synchrotron radiation intensity rapidly decreases from its peak value as the photon energy increases above a few MeV. Both bremsstrahlung and synchrotron radiation induce an electromagnetic cascade.

The net result of the electromagnetic cascade is the deposition of energy in materials that are penetrated. This energy includes both particles stopped in the material and photon absorption. The photons produce additional secondary radiation and particles (e.g., photoneutrons) that activate accelerator materials. These same mechanisms lead to effective doses when personnel are in the presence of this radiation. These secondary radiation types

To allow for massive neutrinos, the Standard Model must be modified and its assumptions altered. However, current experimental knowledge of neutrino properties does not permit the selection of a specific modification to the model. For example, it is not known if neutrino masses are to be interpreted as evidence of new, light, fermionic degrees of freedom (e.g., Dirac neutrinos), new, heavy, degrees of freedom (e.g., Majorana neutrinos), or whether a more complicated electroweak-symmetry-breaking interaction is present. However, the

Within the Standard Model, neutrino effective doses are determined from the muon decay

 *<sup>e</sup> e*

*<sup>e</sup> e*

 

The neutrino effective doses depend on the number of muon decays, and the subsequent production of neutrinos. Specific effective dose relationships are provided in subsequent

In a muon collider, muon decays arise principally from Eqs. 1 and 2 that produce neutrinos and antineutrinos. The neutrinos interact through a variety of complex processes. A neutrino interaction discussion is simplified by following the methodology of Cossairt et al. (1997) and defining four processes (A, B, C, and D) to describe neutrino interactions with matter. The deposition of energy into tissue defines the effective dose (Bevelacqua, 2009,

Process A involves neutrino scattering from atomic electrons. Electrons that recoil from elastic neutrino scattering deposit their energy in tissue and produce a neutrino effective dose. Process A occurs over a wide range of energy and the electron tissue interaction may

In Process B, neutrinos interact coherently with nuclei. This process is only effective for low neutrino energies where the neutrino wavelength is too long to resolve the individual nucleons within the nucleus. At higher energies, Processes C and D become more important. Process B leads to low-energy ions having large linear energy transfer values. These ions deposit their energy into tissue according to their ranges, which are typically << 1 cm. Although Process B is independent of the neutrino generation, the cross section for

Process C involves neutrino scattering from nucleons without shielding between the neutrinos and tissue. At energies below about 500 MeV, tissue dose is due to recoil nucleons. As the neutrino energy increases above about 0.5 GeV, secondary particle production increases. Eventually, these secondary particles produce particle showers or cascades in tissue. Process C is independent of the neutrino generation, affecting all three generations in

Process D is similar to Process C with the exception that the neutrinos are shielded before striking tissue. Neutrinos with energy greater than about 0.5 GeV, emerging from a layer of

(1)

(2)

 

Standard Model is sufficient for the purposes of this chapter.

**4. Neutrino interactions related to effective dose** 

neutrinos is about twice the antineutrino cross section (King 1999a).

involve multiple scattering of electrons.

processes:

discussion.

2010a).

the same manner.

are usually attenuated to insignificant levels by the concrete and earth shielding outside the accelerator tunnels containing the beam tubes.

From a health physics perspective, the energy loss of the circulating, accelerating electrons and positrons produces synchrotron radiation (photons). Given the mass of the electrons and positrons, their trajectories are easily altered. Therefore, synchrotron radiation is expected to be a large fraction of the available beam power. The synchrotron radiation requires shielding, and the extent of the shielding depends on the specific location within the accelerator facility.

The amount of synchrotron radiation depends on the specific design characteristics of the electron-positron collider. Dominant factors governing the production of synchrotron radiation are the beam power and radius of curvature of the accelerator ring. From a practical standpoint, radiation generated from the circulating electron and positron beams occurs within the unoccupied shielded ring and is not normally a health physics issue.

The dominant contributors to the radiation environment at an electron-positron facility include electromagnetic cascade showers, external bremsstrahlung, photoneutrons, muons, and synchrotron radiation. Muon pair production in the Coulomb field of a nucleus is possible above a photon energy of about 211 MeV. This process is analogous to electronpositron pair production, but the muon pair production cross-sections are smaller by a factor of about 40,000 due to the differences in electron (0.511 MeV) and muon (105.7 MeV) masses (Bevelacqua, 2008).

The dominant muon pair production process is coherent muon production. In coherent production, the target nucleus remains intact as it recoils from the photon interaction. In a few percent of the time, the nucleus breaks-up with the resultant emission of muons. Muons also result from the decay of photopions and photokaons. However, the number of muon decays in a conventional electron-positron collider is not sufficient to produce a neutrino effective dose concern. To understand the neutrino effective dose from a muon collider, it is necessary to understand neutrino physics and neutrino interactions.
