Answer to Question #11258 Submitted to "Ask the Experts"

Category: Instrumentation and Measurements — Surveys and Measurements (SM)

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

Q

I would like to determine the occupational dose received from a cobalt-60 (60Co) unit and a linear accelerator. Could you please provide me with the methodology to compare the dose from these two units?

I am considering using a Farmer dosimeter or a thermoluminescent dosimeter (TLD). Which one should I use to determine the dose received by the workers?

A

The question to your answer depends, at least in part, on the energy of the linear accelerator that you refer to. Because you mention a 60Co unit (I assume a medical therapy unit) and a Farmer dosimeter (I assume you are referring to a Farmer small volume ionization chamber), a device commonly used in the medical therapy area, I will assume that you are dealing with a medical accelerator. With this assumption it is safe to say that primary photon energies would not exceed about 20 MeV, with effective primary beam energies being noticeably lower than this but still potentially much higher than the approximately 1.25-MeV energy from 60Co.

We should first note that no occupational worker should be in a situation where he or she would be exposed to the primary photon radiation. Therefore, the implication of the higher energy photons for occupational dose measurement will depend somewhat on what angle-scattered gamma rays may reach the occupationally exposed worker. If a worker were exposed to photons scattered at relatively small angles, say less than 60o, then the scattered-photon energies could be significantly larger from the accelerator than from the 60Co unit. Because of the usual orientation of the treatment machine in the treatment room and the fact that workers are not allowed in the room while the machine (or a therapeutic 60Co unit) is being used, it is very unlikely that small-angle scattering will be significant. If this is the case, the expected result is that most scattered photons will have energies less than 1 MeV, whether they come from the accelerator or from the 60Co unit. For example, the highest-energy scattered photons that could result from 60o, 90o, and 120o scattering of 20 MeV photons, respectively, are 0.972 MeV, 0.498 MeV, and 0.335 MeV.

This reality has the associated consequence that any dosimeter or instrument that has the proper physical characteristics, most notably the proper thickness of material covering the active element to ensure that secondary charged particle equilibrium exists at the active element, to measure dose from 60Co will likely be adequate for the accelerator measurements as well. The requirement for proper tissue-equivalent thickness covering the dosimeter applies to both the TLD and the Farmer chamber. For personnel dosimetry the TLD covering thickness for deep-dose assessment is normally taken as 1,000 mg cm-2. Typical Farmer chambers often use a buildup cap for 60Co photons that is about 300–350 mg cm-2 in thickness, often made of polymethylmethacrylate (PMMA).

If the TLDs have been properly calibrated with the 60Co primary gamma rays, they should be useful for the dose measurements you have in mind for both the 60Co and the accelerator. Calibration of the TLDs, with their proper coverings, should be done on a phantom (usually a 30 cm x 30 cm x 15 cm deep PMMA slab). Delivered doses should range from roughly 0.1 cSv to 2 cSv. For the actual worker dose measurements the dosimeters will have to be worn by the workers (or placed in appropriate locations on a suitable phantom if area monitoring is to be used) for a long enough time to establish a reasonable estimate of the dose. The sensitivity of TLDs varies, depending on the particular phosphors being used. If you are using a typical TLD-100 material (LiF:Mg,Ti), the minimum measurable equivalent dose is usually about 0.10 mSv, and this will affect how long the devices must be in place. If you are trying to distinguish the dose produced by the 60Co from that produced by the accelerator, you will have to assign different dosimeters for use with each respective source and use the dosimeters only when the respective source is being used.

Regarding the Farmer chamber, a typical chamber used in therapeutic dose assessment is a small-volume chamber, often less than 1 cm3 in air volume, designed to measure relatively high dose rates in the primary beams of the irradiators, either 60Co or accelerators. As such, they would not be suitable for typical occupational dose rates, which would be too low to be meaningfully measured with such chambers even when used in an integrating mode. There are available numerous higher-volume ion chambers that would be useful for some such measurements. These include high-sensitivity, digital, high-pressure ion chambers with integrating capability that would be suited to dose assessments outside the shielded rooms where workers expectedly would be performing their duties while the irradiators are operating. Other varieties of integrating ion chambers may also be useful. You can find references to these by searching the Internet. The Health Physics Society maintains a Buyer's Guide list of manufacturers/providers who deal in instrumentation that can also be helpful.

I wish you well in your measurements.

George Chabot, PhD

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