Answer to Question #10452 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

We need to do shallow, eye, and deep-dose measurements. I understand that what has to be done is an external absorber must be placed over the window of the survey meter to get a window density of 7,300 and 1,000 mg cm2 for each of the measurements.

I would appreciate your help regarding the absorber. What material is used, how do we determine the dimensions of the absorber, is the size of the absorber dependent of the make/model of the survey meter, and how do we confirm the absorber is correct, etc.?

We have a Ludlum Model 12-4 count rate meter with a Ludlum Model 42-31 detector for neutron measurements and an NDS ND-2500 gamma meter.

The manufacturer of the gamma gauge says that thermoluminescent dosimeters or film badges must be used but this does not tie up with the requirements of the ANSI/HPS N43.8-2008.

A

As you have noted, the shallow, eye, and deep-dose measurements are to be made below respective tissue-equivalent thicknesses of 7 mg cm-2, 300 mg cm-2, and 1,000 mg cm-2. When a portable instrument is used to make such measurements the sensitive volume of the detector must be covered, on at least the surface of radiation incidence, by a reasonably tissue-equivalent material of the thickness to yield the desired thickness for each of the three dose quantities to be measured.

Many commonly used detectors have thin entrance windows that constitute one flat surface of a cylindrically shaped detector. Such windows commonly range from about somewhat less than 1 mg cm-2 to about 2.5 mg cm-2. Some instruments with thin windows, especially some of the popular ionization chambers, have sufficient tissue-equivalent material (often polyethylene terephthalate, trade name Mylar™) added outside the window to produce a total thickness of about 7 mg cm-2 to meet requirements for the measurement of shallow dose (rate). If you have a thin-window detector you can add sufficient material to make up the required thickness when added to the window thickness.

Appropriate near-tissue equivalent materials include polyethylene, polyethylene terephthalate, polymethyl methacrylate (trade names Lucite™, Plexiglas™, Perspex™), and polystyrene. These are fairly common plastics; other plastics are also suitable, but I would avoid plastics such as polyvinyl chloride and teflon that include elements with atomic numbers greater than that of oxygen). The densities of these common plastics is generally within about 20 percent of 1.0, with variances likely on the high side. Even paper is suitable as a tissue simulant, but it has the disadvantage that, depending on the type of paper and the temperature and humidity conditions, the paper may absorb significant water, thus changing its density thickness somewhat and also sometimes diminishing its ability to maintain a reasonably rigid configuration. The material that is added over the window of the detector should be large enough in area to at least completely cover the area of the entrance window of the detector. On a temporary basis the material can often be held in place using common adhesive tape.

The thin window of the detector in common ionization chambers is often available on the side (bottom) of the instrument opposite the readout panel and the instrument case is often in the shape of a normal rectangular parallelipeped. In such cases it may be convenient to simply cut out a piece of plastic equal in rectangular dimensions to the bottom of the instrument case and put it in place over the window. The density thickness of the added material is equal to its linear thickness, cm, times the material density. Thus, a 0.318 cm thickness of plastic with a density of 1 g cm-3 would have a density thickness of 318 mg cm-2. It may be necessary to use several layers to make up the required thickness, depending on what materials are available and what your fabrication abilities are. Sheet Mylar, commonly available with or without an aluminized face, the latter often used for windows in counting systems and portable instruments, can be obtained in small thicknesses, ranging from <1 mg cm-2 to at least 3 mg cm-2 and is useful for making up the thickness for shallow dose measurements and for small adjustments to other thicknesses. Note that in the aluminzed type the aluminum layer is so thin as to present no significant effect to influence the tissue equivalence of the Mylar™. Other polyester sheet materials may also be available under different trade names.

The gamma meter you have, the NDS ND-2500, does not appear to have a thin window. The manufacturer quotes a wall thickness of 30 mg cm-2. If this value is true, the instrument would not be suitable for the shallow dose measurement since the thickness exceeds the 7 mg cm-2. (I suspect that the 30 mg cm-2 probably refers to the wall of the GM tube itself, and the walls of the instrument case would add considerably to this, although I did not find the manufacturer’s further description for the instrument case.) Also, the ANSI N43.8 standard that is referenced specifies in Section 7.3.1 that the standard instrument used for measurements of gamma, x, and beta radiations shall be an air ionization chamber with an energy response within +15 percent from 15 keV to 1.3 MeV. The ND-2500 gamma instrument is a Geiger-Mueller-based instrument. The standard further states that if instruments other than the recommended ion chamber are to be used for the measurements, they should be corrected for response against such an ion chamber. This may further limit the applicability of the gamma meter you cite.

The measurements to be made at the variable depths refer only to the gamma, x-ray, and beta measurements if such are to be made. The neutron instruments to which you refer are not subject to the same depth dose methods since the instruments require heavy moderating assemblies around the sensitive detector to yield the desired dose equivalent response.

I hope this answers most of your questions and that you are able to make the required measurements.

George Chabot, PhD

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