The high energy nature of TJNAF's electron beam has the potential to produce high levels of radiation when the beam is on as well as the ability to produce radioactive material in the systems and surrounding structure of the beam enclosure.

9.1.1 Prompt Radiation

Radiation resulting from the accelerator beam or the interaction of the beam with matter is called prompt (or direct) radiation. Prompt radiation is produced only when the beam is operating. Prompt radiation consists mainly of high energy x-rays (gamma rays), and neutron radiation.

Prompt radiation within the beam enclosure is the most intense radiation present at TJNAF. Direct exposure to a particle beam or secondary radiation 'shower' could result in a potentially dangerous or lethal dose of radiation. (Dose rates in the enclosure may exceed 10⁶ rad/hr). Interlocked access points provide a fail-safe barrier against direct beam exposure during operation. Additionally, the accelerator tunnel is well shielded by its underground design. There are a few locations where radiation levels can be elevated due to the proximity of the beam enclosure. These include:

• Above unshielded penetrations in the accelerator service buildings
• Roof of operating end station
• Accessible shielded labyrinths in the vicinity of the beam enclosure

Always consult the Radiation Control Group, Operational Safety Procedure, or Operations Crew Chief for requirements for working in these areas.

9.1.2 Residual Radioactivity

In addition to direct radiation, the interaction of TJNAF's high energy beam with matter can also cause the formation of radioactive materials. This process is often referred to as "activation", and the radiation itself as "residual radiation". Activated materials emit mostly gamma and beta radiation. This radioactive material emits radiation for some time following operation of the beam. Many of these materials are short-lived and become stable within days or weeks of activation. Others require years for decay to stable isotopes. Exposure to activated materials is a major contributor to worker dose at accelerators such as TJNAF. Activated material usually decays by beta/gamma emission.

It is important to remember that activation can occur in any material subjected to TJNAF's activating primary radiation field.

Important sources of activation include:

Beam dumps and stops, magnets, beam-lines and beam-line components.

Targets, detectors, and other experimental equipment.

Other materials which may become activated are lubricants, cooling water, and air that is contained in spaces within the beam enclosure. Closed cooling systems associated with beam dumps are subject to a build up of activation products that can present a radiation and contamination hazard during maintenance activities on these systems. Buildings or rooms which house cooling system components for high-power beam dumps may require special entry controls during beam operations.

9.1.3 Survey Requirements

Because of the potential for elevated dose rates in the beam enclosure, a radiation survey must be performed upon each entry following beam operations. The purpose of this survey is to identify significant activation or contamination sources so that appropriate radiological controls can be established.

Any tools, equipment, components, or structural material present in the beam enclosure is subject to becoming radioactive and must be monitored prior to removal from the enclosure by the RCG or an Assigned Radiation Monitor.

Only RCG personnel may approve the release of such material from control.

9.1.4 Activation and Contamination

It is important to understand the difference between "activated material" and "contaminated material". Contaminated materials are items which either have removable surface contamination or contamination which is fixed in the surface but may be removed by abrasion or chemicals. Radioactive liquids are also a source of contamination. Activated materials are thought of as items which are "contaminated in depth or volume" but do not have easily removable surface contamination.

When handled properly, activated materials normally do not present a loose contamination hazard. They are usually controlled based on the external radiation dose rate. However, activated materials can become a source of contamination during activities such as:

grinding or filing



burning or welding



machining, cutting or drilling

ANY such modification to radioactive material, including beamline components and the structural components of the enclosure, requires RCG concurrence.

Contamination can also occur due to the activation of materials which are inherently transferable, such as dust, rust, lubricants, and liquids. Items and systems which may be sources of contamination include:

• beam dump and component cooling water (LCW) and filters or ion exchange media
  
  
• surface coatings (dust, rust, epoxy, etc.) on beamline components
  
  
• vacuum pump oil used in the beam enclosure
  
  
• tunnel and end station air handling and dehumidification equipment
  
  
• any item which has been directly irradiated by the beam, such as an experimental target

Always obtain RCG approval before working with any of these materials or systems.

There are other sources of radiation at TJNAF in addition to accelerator generated sources. Generally, they don't add significantly to personnel exposures.

Small test sources or x-ray generators are used in experimental detector setups and for instrument calibrations and checks. These sources are controlled by a source custodian. You must be specifically approved to use these sources by the Radiation Control Group and the appropriate source custodian.

The radio-frequency (RF) cavities used to accelerate TJNAF's beam produce x-rays when operated, therefore they may only be operated in shielded, interlocked enclosures. These cavities are tested in shielded enclosures in the Test Lab.

Some equipment such as klystron tubes, high voltage power supplies and other high energy electrical equipment may also produce x-rays when operated.