Answer to Question #4511 Submitted to "Ask the Experts"

Category: Accelerators — Radiation fields

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

Q

I have been asked to do a survey of the exterior walls of a linear accelerator room. The room has thick concrete walls plus lead sheets. The levels of exposure I am getting with the Ludlum Model 3 survey meter (SM) with a 44-9 probe translate to equivalent dose rates between 1 and 3 µSv h-1 in the worst case (higher energy and pointing the beam to the wall). The SM was calibrated with a 137Cs and my beam nominal energy is 18 MV (mean ~5 MV). What kind of instrument is more adequate to use for this type of survey with a better energy response? For the secondary barriers, most of the reading comes from scattered photons.

A

If I understand the first part of your question correctly, photon radiation surveys have been performed outside the laminated primary barrier (consisting of lead and concrete) of a linear accelerator (Linac) room housing an 18 MV Linac. A Ludlum Model 3 survey meter with a 44-9 probe (which is a pancake Geiger Mueller [GM] probe) was used and you are wondering if there is an instrument with a better energy response that can be used. In response to your question, let me first state that although energy response is important, far more important is the pulsed nature of the Linac radiation which precludes the use of a GM counter as discussed below.

Pulsed radiation fields are encountered in the vicinity of electron Linacs (McCall and Ipe 1988). Most therapy Linacs are operated at repetition rates varying from 100 to 400 pulses per second with pulse widths of about 1 to 10 microseconds (AAPM 1986). The fraction of operating time (that is, pulse width x repetition rate) during which the beam is on is called the duty factor. For example, the duty factor for an electron Linac operating at 100 pulses per second with a pulse width of 10 µs is 0.001. These small duty factors impose severe limitations on the radiation detection instruments. The peak intensity is equal to the average intensity divided by the duty factor, which in this case will be 1,000 times higher than the average intensity. An instrument that normally responds well to the average dose rate spread out evenly in time will not be able to cope with such a high dose rate. The intense photon pulse usually overwhelms any active detector (that detects particle events electronically). Instruments which have long dead times, such as the GM and proportional counters, tend to become saturated in such fields and only count the repetition rate. Thus GM counters should not be used to perform radiation surveys outside exterior walls of the Linac room. Scintillation survey meters may become nonlinear at higher dose rates because photomultipliers cannot handle the high instantaneous currents.Ionization chambers are less influenced; however they must be operated with adequate voltage to  overcome recombination losses. There are several commercially available  ionization chambers that can be used for Linac radiation surveys.

In the second part of your question you ask if scattered photons dominate the dose outside the secondary barriers. Bear in mind that a Linac operating at 18 MV produces primary photons, leakage (from the Linac head) photons, and leakage neutrons (NCRP 1984). The primary photons can scatter from the patient and the walls. Thus the radiation sources that need to be considered outside the secondary barrier are leakage photons, scattered photons, and leakage neutrons. One can determine which source dominates only if the shielding material, thickness, Linac configuration, and room layout are known. Photon and neutron surveys should be performed outside the secondary barrier. Photon and neutron surveys should also be performed at the door or entryway and any penetrations in the shielding.
 
Typically if a Linac room is shielded adequately for photons with a hydrogenous material such as concrete, the shielding will be more than adequate for neutrons. However, for a primary laminated barrier, in addition to leakage neutrons, neutrons are also produced by interaction of the primary photon beam with the lead shielding in the primary barrier. Lead is not very effective in shielding neutrons, but only reduces the neutron energy. Thus both photon and neutron surveys must be performed outside the primary barrier since it is laminated. It appears that you have only performed photon radiation surveys outside the primary barrier.

Nisy Ipe, PhD, CHP

References

  • American Association of Physicists in Medicine. Neutron measurements around high energy radiotherapy machines. New York, NY: AAPM; AAPM Report No. 19; 1986.
     
  • McCall RC, Ipe NE. The response of survey meters to pulsed radiation fields. Proceedings of The Seventh International Congress of the International Radiation Protection Association. Sydney, Australia, April 1988. Radiation Protection Practice 1:133-136 [Also SLAC-PUB-4488].
     
  • National Council on Radiation Protection and Measurements. Neutron contamination from medical electron accelerators. Bethesda, MD: NCRP; NCRP Report No. 79; 1984. 
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