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Category: Medical and Dental Equipment/Shielding — Shielding

The following question was answered by an expert in the appropriate field:

Q

A survey of a linear accelerator room turned up a small part of the modulator room (equipment for running the accelerator) where the instantaneous dose rate could be measured as high as 700 milliroentgens per hour (mR h-1). Are there recommendations or regulations that would require this room to be interlocked (rather than merely locked)?

A

Unfortunately, this question provides no details regarding the linear accelerator type (medical, industrial, etc.), radiation type (electron beam, photon beam, etc.), energy, beam-on time, occupancy factor, dose rate fall-off with distance, size of the beam area, etc. Therefore, I will have to provide a very general response.

Assuming that you are measuring photons with energies less than a few megaelectronvolts (MeV), an exposure rate of 700 mR h-1 corresponds to a dose rate of 6.72 millisievert per hour (mSv h-1) in tissue. First, check your state regulations to see if there are any limits or requirements for instantaneous dose rates. Then consider that typically the dose limits used in shielding design are as follows:

• Annual effective dose is less than 1 mSv per year (mSv y-1) in uncontrolled areas after use of the occupancy factor (T).
• Annual effective dose is less than 5 mSv y-1 for controlled areas, which requires an assumption of T = 1.
• Maximum dose to an uncontrolled area is less than 20 microsieverts (µSv), which requires an assumption of T = 1.

Let me go through a couple scenarios, assuming that the modulator room is a controlled area. You will see clearly that beam-on time is an important parameter.

• If the beam is on continuously, the maximum effective dose in one hour is 6.7 mSv. This is clearly not acceptable for controlled (or uncontrolled) areas as the dose in one hour exceeds the annual effective dose limit.
• If the beam is on for only four minutes in one hour, then the maximum dose in one hour is about 0.5 mSv. The annual dose limit will be exceeded if the individual is exposed for more than about 10 hours per year.

Interlocking the room is akin to treating only the symptoms and not the root cause. I suspect that there is a thin spot in the shielding, resulting in this high instantaneous dose rate. The shielding should be evaluated carefully, and additional shielding should be added to reduce the dose rate to acceptable levels. This step will also help you find additional hot spots that could have been missed during the radiation survey.

In radiation protection, engineered solutions should be the first line of defense. Interlocking should be considered if, and only if, it is extremely difficult to retrofit the shielding. Locking the room should be the last option, as it will involve administrative procedures. If you measured 6.72 mSv h-1, a portion of the room may need to be labeled "High Radiation Area," the room may require warning signs and barriers, and you may need to enforce strict access conditions (wearing a personnel dosimeter, performing a radiation survey, etc.) during entry. All of this depends also on how the dose rate falls off as one moves away from the area.

Nisy Elizabeth Ipe, PhD, CHP

Ask the Experts is posting answers using only SI (the International System of Units) in accordance with international practice. To convert these to traditional units we have prepared a conversion table. You can also view a diagram to help put the radiation information presented in this question and answer in perspective. Explanations of radiation terms can be found here.
Answer posted on 12 April 2016. The information posted on this web page is intended as general reference information only. Specific facts and circumstances may affect the applicability of concepts, materials, and information described herein. The information provided is not a substitute for professional advice and should not be relied upon in the absence of such professional advice. To the best of our knowledge, answers are correct at the time they are posted. Be advised that over time, requirements could change, new data could be made available, and Internet links could change, affecting the correctness of the answers. Answers are the professional opinions of the expert responding to each question; they do not necessarily represent the position of the Health Physics Society.