**9. Other radiation protection issues**

A number of radiation protection issues associated with TeV energy muon colliders will challenge accelerator health physicists. The issues related to large neutrino effective dose values and effective neutrino dosimetry were previously noted. Before construction of a muon collider, thorough studies will be performed to define the accelerator's radiation footprint. These studies will: (1) define muon collider shielding requirements; (2) assess induced activity within the facility and the environment (e.g., air, water, and soil), including the extent of groundwater activation; (3) assess radiation streaming through facility penetrations (e.g., ventilation ducts and access points); (4) assess various accident scenarios such as loss of power or beam misdirection; and (5) assess the various pathways for liquid and airborne releases of radioactive material. Facility waste generation and decommissioning are other areas that will require evaluation.

In addition to the aforementioned radiation protection issues, the TeV energy neutrino beam will create new issues. Radiation protection concerns unique to muon colliders have been reported by Autin et al. (1999), Bevelacqua (2004), Johnson et al. (1998), Mokhov & Cossairt (1998), and Mokhov et al. (2000). These authors suggest that above about 1.5 TeV, the neutrino induced secondary radiation will pose a significant hazard even at distances on the order of tens to hundreds of kilometers. The neutrino radiation hazard presents both a physical as well as political challenge (King, 1999a).

These issues also complicate the process for locating a suitable site for a TeV energy muon collider. There are a number of potential solutions to reduce the neutrino effective dose associated with a muon collider. These include using radiation boundaries or fenced-off areas to denote areas with elevated effective dose values. Building the collider on elevated ground or at an isolated area would also minimize human exposure. Effective dose reduction measures are also available for specific muon collider configurations.

In a linear muon collider operating at the higher TeV energies, dose reduction is achieved by locating the interaction region above the earth's surface. In a circular muon collider, dose reduction is achieved by minimizing the straight sections in the ring, burying the collider deep underground to increase the distance before the neutrino beam exits the ground, and orienting the collider ring to take advantage of natural topographical features.

Orders of magnitude reductions in the neutrino effective dose are required for the muon colliders noted in this chapter (See Tables 3, 5, and 7) to meet current regulations for public exposures (ICRP, 2007). Some of the possible effective dose reduction solutions may be difficult to implement for the TeV energy muon colliders. The most feasible options for locating and operating the highest TeV energy muon collider are to either use (1) an isolated location where no one is exposed to the neutrino radiation before it exits into the atmosphere as a result of the earth's curvature, or (2) a linear muon collider

However, the negative pion dominantly decays into a muon and antimuon neutrino, and

*e*

Subsequently, the muon decays following Eq. 1. Eq. 30 then yields:

the neutral pion decays primarily into photons.

assumed.

 

 

 

neutrinos contributing to the tau collider to muon collider effective doses:

*N*

of generation j neutrinos emitted from decay mode i, and *ni*

antineutrinos which are 1.0 and 0.5, respectively.

number of muon neutrinos emitted in a muon decay, and *n*

effective doses from a tau collider and muon collider is given by:

3

1 1

*i j*

*e*

<sup>0</sup> (25.51%) 

 

 

The net result of the decay is that multiple neutrinos are produced from the tau and subsequent decay of particles. The factor ξ described in subsequent discussion incorporates the effects of the multiple tau decay modes and their effects on the neutrino effective dose.

Subsequent discussion assumes no annihilation of particles and antiparticles in the beam produced by the tau decay products. In addition, the narrow beam approximation is

The neutrino dose from tau decays is determined by comparing the number of neutrinos emitted from an equal number of tau and muon decays. ξ defines the ratio of the number of

*e*

   

*<sup>j</sup>* is the number of generation

*<sup>e</sup>* is the number of

(33)

(32)

*i i j i j*

*Y an bn*

*an bn* 

In the numerator of Eq. 32, i labels the various decay modes of the tau, N is the number of tau decay modes, Yi is the branching fraction of the ith tau decay mode, ni(υj) is the number

j antineutrinos emitted from decay mode i. In the denominator of Eq. (32), n(υμ) is the

antielectron neutrinos emitted in a muon decay. The j sum counts the three neutrino generations, and a and b are the cross-section factors of King (1999a) for neutrinos and

The ratio of tau neutrino to muon neutrino effective doses is obtained by utilizing the value of ξ and the calculated ratio of tau and muon neutrino cross-sections (β) (Jeong & Reno, 2010). The discussion is applicable to circular and linear muon and tau colliders. For equivalent accelerator operating conditions (e.g., beam energy and number of beam particle decays) and receptor conditions (e.g., distance and ambient conditions), the ratio of neutrino

> *H E*

*H E* 

*E*

 

(29)

(30)

 

 (31)

constructed such that the individual muon beams collide in air well above the earth's surface.

For Option 1, the accelerator could either be constructed at an elevated location or at an isolated area. The area will need to be large, perhaps having a site boundary with a diameter greater than 100 km (King, 1999a). This size requirement restricts the available locations, and would normally require that the facility have access to the resources of an existing accelerator facility such as CERN or Fermilab. Alternatively, the facility could be located in an isolated area and scientific personnel relocated to that area with the establishment of a self-sufficient site. The final decision regarding facility location will involve funding and political considerations that are part of new facility development, licensing, and construction.

Option 2 would be technically feasible, and could be located at a smaller site. However, design considerations for both Options 1 and 2 would need to address a number of potential radiation issues associated with accelerator operation (Bevelacqua, 2008, 2009, and 2010a) that could lead to significant, unanticipated radiation levels in controlled as well as uncontrolled areas. Radiation protection issues include beam alignment errors, design errors, unauthorized changes, activation sources, and control of miscellaneous radiation sources (Bevelacqua, 2008, 2009, 2010a). These operational issues require close control because they have the potential to produce large and unanticipated effective dose values.

Beam alignment errors could direct the beam in unanticipated directions. Given the long range of the muon effective dose profile, these errors could have a significant impact on licensing and accident analysis. Beam alignment errors are caused by a variety of factors including power failures, maintenance errors, and magnet failures. Both human errors and mechanical failures lead to beam alignment issues.

Changes in the beam energy or beam current, that exceed the authorized operating envelope, lead to elevated fluence rates, the creation of unanticipated particles, or the creation of particles with higher energy than anticipated. Changes to beam parameters must be carefully evaluated for their impact on the radiation environment of the facility.

The control of secondary radiation sources, radio-frequency equipment, high-voltage power supplies, and other experimental equipment merits special attention. These sources of radiation are more difficult to control than the primary or scattered accelerator radiation because health physicists may not be aware of their existence, the experimenters may not be aware of the hazard, or the radiation source is at least partially masked by the accelerator's radiation output. These miscellaneous radiation sources will include x-rays as well as other types of radiation.
