**1. Introduction**

90 Particle Physics

Matsunaga, H. (2009). Grid Computing for High Energy Physics in Japan. *Journal of Korean* 

Lepton accelerators incorporate electron, muon, and tau beams. First generation lepton machines, electron accelerators, are basic research tools and their radiation characteristics are well established. A second generation muon machine presents additional research possibilities as well as new health physics challenges. Third generation tau accelerators are currently theoretical abstractions and little development has been forthcoming. Although this chapter focuses on muon colliders and their unique radiation characteristics, initial scoping calculatons for tau colliders are presented.

Neutrinos are electrically neutral particles, interact solely through the weak interaction, and have very small interaction cross sections (Particle Data Group 2010). They are present in the natural radiation environment due to cosmic rays, solar and terrestrial sources, and are produced during fission reactor and accelerator operations. From a health physics perspective these neutrino sources produce effective doses that are inconsequential. Although this will remain true for a number of years, planned muon accelerators or colliders will produce copious quantities of TeV energy neutrinos. In the TeV energy region, the health physics consequences of neutrinos can no longer be ignored. Upon operation of these accelerators, neutrino detection and the determination of neutrino effective doses will no longer be academic exercises, but will become practical health physics issues.

In a muon collider, neutrinos are produced when muons decay. The neutrino effective dose arises from neutrino interactions that produce showers or cascades of particles (e.g., neutrons, protons, pions, and muons). It is the particle showers that produce the dominant contribution to the neutrino effective dose (Bevelacqua, 2004).

Concerns for consequential neutrino effective doses have been previously postulated. Collar (1996) presented a hypothesis that the final stages of stellar collapse could produce neutrino effective doses that are sufficiently large to lead to the extinction of some species on earth. This concern has been challenged (Cossairt et al., 1997; Cossairt & Marshall, 1997), but the potential concern for large neutrino effective doses, on the order of hundreds of mSv/y or greater, remains, particularly for the planned muon colliders that will become operational in the next few decades of the 21st Century (Autin et al., 1999; Bevelacqua, 2004; Geer, 2010; King, 1999a; Kuno, 2009; and Zisman, 2011).

As background for muon colliders, an overview of the radiation environment at an electron accelerator is presented. This overview provides a foundation for a discussion of the characteristics of muon decays and the resultant neutrino effective doses. The characteristics

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

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

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

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.

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

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

However, the synchrotron radiation differs from the bremsstrahlung spectrum.

bremsstrahlung and synchrotron radiation induce an electromagnetic cascade.

beam particles are defected by magnetic fields.

Bevelacqua (2008, 2009, 2010a).

beam particles.

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