Radiation Safety

General Procedures and Access to Accelerator Areas

Considerations of safety to personnel and minimization of risk to the accelerator have led to the following procedures, which will be adhered to at all times:

1. When the accelerator is operating there will be a qualified operator and a second person in the Control Room, Computer Room Accelerator Room or Target Room at all times. The only exception is for a short period (not exceeding 20 minutes) when one of the two will be away from the accelerator area. During this period the remaining person will stay in the Control Room. There will be a person experienced in the "start up" and "shut down" of the source on duty at all times.

2. Radiation dosimeters have to be worn every time a person enters the Tandem Hall, Linac Hall or Target Rooms, whether beam is present or not.

3.a. For beams with Z<4 (H,He,Li): Access to the Tandem Hall, the Linac Hall or/and the Target Rooms is not allowed while a beam with Z<4 is present in the area. Before a beam with Z<4 is allowed into an area, all persons have to leave the respective areas. The operator in charge has to verify that no person remains in the area before the doors are closed and the door interlock alarms in the control room are activated. If a door is opened while the beam is present, the interlock system will automatically interrupt the beam at the LE end of the Tandem. The operator in charge will investigate the reason the doors were opened. Only after verifying that nobody is present in the protected area, the operator will close the affected door and re-activate the beam.

3.b. For beams with Z>=4: Access to the Tandem Hall, the Linac Hall and/or the Target rooms is only allowed while the Gamma and Neutron dose rates in the area are below 2.5 mrem/h. Before access is granted to a given area, the radiation levels will be verified at the installed area radiation monitors and with portable gamma and neutron radiation monitors, which are placed near the location where a person works. If radiation levels are above these limits, access to the areas is prohibited, all persons have to leave the affected areas and the operator in charge has to verify that no person remains in the areas before the doors are closed and the door interlock alarm in the control room is activated. The same rules as in 3.a apply in the case of access doors being opened.

4. In all cases, the Tandem HE shield door should be closed if a beam is being accelerated, independent of the radiation levels.

5. If the Tandem is being conditioned and/or levels of X-rays above 2.5 mRem/h are observed on the Tandem Area x-ray monitor, the Tandem shield door has to be closed, the LE Shield door has to be locked and the door alarm has to be activated in the control room.

6. Any time a beam delivery is initiated into the Tandem, the Linac Hall or the Target rooms, the operator will announce the intent to inject beam into a new area through the laboratory intercom before injecting it.

More detailed procedures addressing specific areas of the laboratory are listed below.

Sources of Radiation around the Tandem

In the Tandem area the sources of radiation are the Tandem itself, the accelerated ion beam, activated material and the ion sources. In the target room there is the ion beam and the possibility of activated material.

Ion Sources

The high voltages involved with ion sources result in some x-rays during operation. For example, 2 mREM/h is typically measured near the injection magnet when the sputter source is operating at 120 kV (However, as with all high voltage equipment, a greater hazard by far is the possibility of electrocution).

Tandem

The terminal and acceleration tubes near the terminal can be a strong source of x-rays, depending on the operating conditions (terminal voltage, beam current, etc.). Radiation levels at the tank wall near the terminal of 100 mREM/h are not uncommon. During Conditioning and whenever the radiation doses exceed 2.5 mRem/h, the Tandem access doors have to be closed and secured through the interlock alarm panel in the control room.

Accelerated Ion Beam

The main factors on which the level of radiation resulting from the accelerated ion beam depend have been mentioned above. As regards to the Tandem, little nuclear radiation (as opposed to x-rays) is to be expected at the Low Energy end – except for proton, deuteron and 3He, 4He beams. Significant levels of γ-rays and neutrons are to be generally expected at the High Energy end of the Tandem. And at various places along the beam path into the target room and to the experimenters target. Again, in general, higher currents, higher energies and lighter projectiles all tend to produce more radiation. Usually the strongest sources will be the entrance and exit slits of the 90◦ magnet.

Procedures

1. Do not use the route past the H.E. end of the Tandem to access the Linac hall when there is a beam from the Tandem. If beam is being run in the Linac area, use the route through the target room 1. If beam is being run in the Target room 1, enter and exit the Linac through the door in the hallway.
2. If radiation at the entrance to the H.E. end of the Tandem exceeds 2.5 mREM/h, the radiation door will be closed to prevent access.
3. If radiation at the entrance to the Target Room 1 area exceeds 2.5 mREM/h, the radiation door will be closed to prevent access.

Sources of Ionizing Radiation In the Linac

There are three distinct sources of radiation in the Linac area: the resonators, the accelerated ion beam, and any material (slits, etc.) which has been activated by bombardment with the ion beam.

Resonators

The superconducting resonators, which make up the Linac, produce x-rays when operating. The production of x-rays increases sharply with the R.F. field level at which the resonators are operating. For example, with a portable Geiger counter placed against the cryostat wall, the radiation from resonators running at 2 MV/m or less was found to be barely detectable (<0.04 mREM/h), while a resonator running at 2.25 MV/m gave 7 mREM/h measured against the cryostat wall.

In the “shadow” of the blue radiation shields, x-ray radiation is currently barely detectable. However, this is not the case during resonator conditioning, nor will it be the case as operating field levels of 3 MV/m are approached.

Procedure: A series of Geiger counters have been placed under the cryostats to detect x-rays from the resonators. When the resonators are excited and emitting x-rays, these Geiger counters cause red lights mounted on top of the cryostats to illuminate.

1. Whenever these lights are on, the walkway beside the cryostats, the tunnel under the cryostats and the tops and ends of the cryostats are out of bounds. Anyone needing to work in these areas with the resonators excited should use a Geiger counter to monitor the radiation level first.
2. Whenever working in the shadow of the radiation shield, but near a resonator operating at “high field”, or being conditioned, use a Geiger counter to monitor the radiation level. If in doubt, use a Geiger counter.

Sources of Radiation in the Experimental Areas

Accelerated Ion Beam

One source of radiation is the accelerated ion beam, or rather, the nuclear reactions produced when the beam strikes various objects. The effective sources are therefore the various slits, apertures, beam stops, magnets boxes along the path of the beam, and finally the experimenters target chamber. Because large fractions of the ion beam are usually rejected at the entrance and exit slits of the 90◦ magnet, these are usually the strongest sources. The intensity of radiation from a small source (such as a set of slits) fall off with distance according to the “inverse square law” – often the radiation level measured against a set of slits may be quite high, but is negligible a few feet away.

This beam-induced radiation consists mainly of fast neutrons and gamma-rays, and is properly detected with a neutron monitor and Geiger-counter respectively. Although often the radiation will be emitted roughly equally in all directions from the object being bombarded, in some cases the neutron radiation will be much stronger in the directions close to that of the ion beam, that is the neutrons are “forward peaked”. In general, because of their greater penetrating power, and because of their greater biological effectiveness, neutrons pose the greater hazard.

The amount of radiation produced depends on the type of beam, the beam energy, the beam current and on the material being bombarded, and so obviously varies greatly from experiment to experiment. In general, however, one can expect the radiation to increases as the beam energy increases and as the mass of the ion decreases. For example, the first Linac run used a ∼20 nA 29Si beam at 95 MeV. Except near slits, the beam induced radiation was undetectable using the Geiger counter and neutron monitor. By contrast, a high-current 6Li or proton beam could produce levels of around 100 mREM/h.

Procedures: A beacon on the switching magnets in either Target-room 1 or Target-room 2 will flash whenever there is an ion beam in the experimental hall (the beacon will come on when BS-1 or BS-2, respectively are open).

1. When a beam is being run in the Linac area, the doors to the hallway will be locked from the inside, independent of the radiation levels. All access will be through the control room and target room 1 and is to be controlled by the accelerator operator.
2. Once an experiment is running, a radiation survey will be carried out. If the radiation exceeds 2.5 mREM/h, access to the experimental area is prohibited, the access doors to the Tandem, Target Room 1 and the Linac Hall/Target Room 2 will be closed and the door interlock will be activated while the beam is delivered.

Activated Material

The third source of radiation is the radioactivity induced in material (slits, targets, etc.) that has been bombarded by an ion beam for a period of time. The radiation, mainly γ-rays and β-rays, can be detected with a Geiger counter. The amount of activity depends on the type, energy and currents of the bombarding beams, on the material being bombarded, and the length of time for which it was bombarded, and the length of time elapsed since bombardment.

Procedure: Use a Geiger counter before working on, or handling any device that has been bombarded by an accelerated ion beam. Be especially careful to contain any loose radioactive material. For proper disposal, contact Radiation Safety.

Source of Ionizing radiation from RESOLUT

The operation of the RESOLUT radioactive-beam facility creates a combination of radiation sources, which are also present in other experiments and beam lines. Since it operates at higher beam intensity than most experiments, heightened precautions are advised. Radiation sources include X-rays from the superconducting resonator. Neutron and Gamma-radiation from the area of the production target, Neutron and Gamma-radiation from the area near the focal plane, where the primary beam is separated from the secondary beam products. Beam line parts may be activated during extended operation.

Radiation Doses and Dosimetry

Radiation Dose: The statutory limit for “radiation workers” – anyone with a film badge, over 18, and not pregnant, is a dose of 1.25 REM per quarter. This is the dose you would receive if while you were at work, you were continuously exposed to a radiation level of 2.5 milliREM per hour throughout the 500 working hours of the quarter:

2.5 mREM/h * 500 h = 1.25 REM

However, there is no reason for anyone in the Tandem-Linac lab to receive even a small fraction of this dose. In fact, except under exceptional circumstances no one should need to receive a dose above the detection threshold of the film badge, namely 20 mRem (the rough equivalent of a chest x-ray) in any month. This dose corresponds to spending 25 minutes, everyday, in a radiation field of 2.5 mREM/h.