Answer to Question #11334 Submitted to "Ask the Experts"

Category: Instrumentation and Measurements — Personnel Monitoring (PM)

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

Q

I am working in a dosimetry laboratory. Recently we received the Panasonic Model UD 809 neutron dosimeter. I need information on how to use this dosimeter.

A

The Panasonic Model UD 809 neutron dosimeter has been used at numerous facilities for interpreting personal doses from neutrons. As you know, the dosimeter is an albedo dosimeter, which relies on the scatter and reflection of neutrons in the body to reduce their energies to near thermal so that the dosimeter elements will yield a favorable response. Keep in mind that for albedo dosimeters to respond properly they must be worn very close to the surface of the main trunk of the body. Some facilities use elastic chest bands to ensure that the dosimeter does not move significantly away from the body.

In the 809 dosimeter there are four elements; all four are lithium tetraborate (Li2B4O7), the first being a beta/gamma-sensitive element enriched in lithium-7 (7Li) and boron-11 (11B), and the remaining three neutron-sensitive elements being enriched in 6Li and 10B, these latter two chemicals having large thermal neutron capture cross sections. Each of the three neutron-sensitive elements use differing filtration materials (front and back of each element) to provide different responses to incident neutrons from a given source. Here is a link to a table of the properties of all of Panasonic's 800 series dosimeters.

In evaluating the neutron dose, the user subtracts the response on the first element from each of the other three elements to obtain a net effect on each element due to neutrons. All elements are not always used by specific facilities in evaluating the neutron dose, although sometimes the intent is to be able to make an estimation of the respective contributions from thermal, epithermal, and fast neutrons, using all the element data. Unfortunately, as is the case for all neutron albedo dosimeters, the element responses may vary quite dramatically with neutron energy so that specific calibration data may be required for each source of interest, leading to dose algorithms that are suitable for use in particular facilities for particular sources.

Various individuals have carried out studies to evaluate expected energy responses of the dosimeter. One approach is to do theoretical calculations, using simulation codes to predict the response of the dosimeter. One such example is provided by F. Cummings in a 2010 paper published in Health Physics titled "The Neutron Energy Response of the Panasonic Model 809 Personnel Dosimeter" (Health Phys 98(4):584–590; 2010). This paper is available online through the Members Only section of the Health Physics Society website.

The Cummings paper used the MCNP-X Monte Carlo transport code to generate energy response results for monoenergetic neutrons from 10-8 megaelectronvolts (MeV) to 20 MeV. It then went further to evaluate energy-spectrum-averaged dose conversion factors (picosieverts [pSv] of dose per unit neutron fluence) for a number of polyenergetic sources of interest. In this approach the determined neutron dose was evaluated from the difference of element 4 and element 1 of the UD 809 dosimeter divided by the appropriate neutron calibration factor.

I expect that you will have to perform some testing and evaluation to determine an acceptable approach to using the UD 809 dosimeter at your facility. Good luck.

George Chabot, PhD

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