Answer to Question #944 Submitted to "Ask the Experts"

Please give details about radiation safety in a particle accelerator facility—such as air contamination, stack release, water contamination (tritium level monitoring), etc.

Answering this question in any degree of detail would require writing a treatise much too long for this forum. There is a great diversity in size and type of accelerators, so particular radiation safety concerns may vary greatly from one facility to another. Your question points to radiation hazards from activation products, which is one of several radiation safety concerns. I will give a quick general overview and recommend references where one can find details for specific cases. In my opinion, a radiation safety program at an accelerator facility needs to deal with the following aspects:

I. RADIATION HAZARDS

1. Prompt Radiation - Generated by intentional and accidental beam losses in beam line components. Can be very intense, but is present only when the accelerator is on. Behind shielding, usually only photons and neutrons are of concern, but muons are also present at high-energy (~GeV) accelerators. Radiation scattered through air or "skyshine" (for example, due to a weaker shield on the accelerator roof) may cause radiation at remote occupied areas which are not in direct sight of the accelerator. Skyshine may contribute to the dose to the public beyond the boundary of the accelerator site. Large national accelerator labs use dozens of radiation detectors to monitor skyshine at the boundary.
    
2. Induced Activity - Generated mostly in the highly irradiated beam-line components, cooling water of these components, but also air (i.e., dust and aerosols), shielding materials, ground, and groundwater. Activation in accelerator enclosures usually requires a "cooling" waiting period after shutdown before access by personnel is allowed. Preliminary survey may be necessary. Activation in beam-line components may be minimized by a judicious choice of materials (those with low activation cross sections in a specific radiation environment). Air activation may require adequate ventilation and stack monitoring. Besides radioactive gases, ozone and other noxious gases may be generated. Sampling of air, ground, and groundwater is usually performed at large facilities.

II. RADIATION SAFETY SYSTEMS

1. Access Control - Barriers, gates, and interlocks designed to keep people away from radiation.
    
2. Radiation Control - Beam containment interlocks and devices, and shielding. This one is designed to keep radiation away from people. These systems also help to mitigate electrical hazards, which often present the greatest risk at accelerator facilities. The extent and degree of redundancy in the safety systems should be commensurate with the risks. It is very desirable that components of safety systems be reliable and failsafe.

III. RADIATION MONITORING

As at other facilities, one must take care of personnel and area monitoring. Problems specific to accelerator environments may be:

1. Pulsed fields—that may cause severe dead-time problems in certain instruments.
    
2. Radiofrequency interference (near klystrons and such).
    
3. Great variability in the radiation field depending on location, requiring careful determination of calibration factors.
    
4. Many detectors are inadequate for use in high energy fields.

IV. RADIATION WASTE MANAGEMENT/DISPOSAL

Radioactive waste consists mostly of old beam-line components and shielding blocks. Characterization may often provide technical challenges.

V. OPERATIONAL AND MANAGEMENT ISSUES

Similar to other, nonaccelerator facilities: Establishment of a radiation safety program, policies and procedures, training specific to the facility, etc. Specifically, many facilities develop a "Beam Permit" procedure, describing prerunning and running conditions for their accelerators. Other very important procedures are those related to interlock implementation, tests, and bypass conditions. As mentioned earlier, specific problems may vary greatly. A research low-energy proton machine that routinely uses tritium targets will need to develop and implement procedures for safe ³H handling and have a ³H monitoring system in place. On the other hand, ³H is of no concern at an electron linac used for radiation therapy. A great wealth of specific details can be found in these publications:

• NCRP Report No. 51: "Radiation Protection Design Guidelines for 0.1-100 MeV Particle Accelerator Facilities"
   
• IAEA Report No. 188: "Radiological Safety Aspects of the Operation of Electron Linear Accelerators," International Atomic Energy Agency, Vienna, 1979
   
• IAEA Report No. 283: "Radiological Safety Aspects of the Operation of Proton Accelerators," International Atomic Energy Agency, Vienna, 1988
   
• SLAC Report 327: "Health Physics Manual of Good Practices for Accelerator Facilities," Stanford Linear Accelerator Center, 1988
   
• A.H. Sullivan: "A Guide to Radiation and Radioactivity Levels Near High Energy Particle Accelerators"

Vashek Vylet
Duke University