Answer to Question #12750 Submitted to "Ask the Experts"

A neutron detector (752 SH-2) is used to search and locate neutron sources and its unit is counts per second. It is not suited for the determination of neutron dose rate. We want to calibrate it with a ²⁴¹Am-Be source. However, ISO 8592-3 deals with dosimeters for area and individual monitoring. Which standards can we perform and how is it applied?

I assume that your interest is in making dose (rate) measurements with the detector. The detector that you cite is a high pressure ³He detector. You do not state what kind of readout system you intend to use with the detector, and I'm not sure what the instrument characteristics are, how the detector will be incorporated into it, and what neutron fields you will be measuring after you have calibrated the instrument. If you are simply using the bare detector and using a scalar to accumulate counts, and not an instrument designed for dose measurements, you will be severely limited in terms of what kinds of meaningful measurements you can make. As you likely know, this detector is designed as a high-sensitivity detector intended for use in improving detection of neutron sources. It is not intended as a general use neutron dose-measuring detector unless it were incorporated into an appropriate instrument design.

If you are intending to use the bare tube, it could only be used for meaningful neutron dose measurements if it is calibrated under nearly exactly the same conditions under which it will be used. That means that if it is calibrated using a bare Am-Be source in a given room and at selected distances, its use would have to be for measuring neutron doses from bare Am-Be sources in the same room with the source at the same position used in calibration and dose points at pretty much the same distances as used for the calibrations. A major reason that such restrictions apply is because this detector has a very high response to thermal neutrons through the (n,p) reaction with ³He, but it will also respond to fast neutrons through recoil reactions with the ³He fill gas. Virtually all neutrons from a bare Am-Be source are fast, having an average energy of about 4.5 million electron volts (MeV).

The bare tube will respond to such neutrons but, at the same time, it will also respond to any thermal neutrons that are present. As soon as a source is placed in a closed environment, such as a typical room, some of the fast neutrons from the source will scatter from all available surfaces in the room, resulting in some thermalized neutrons reaching the detector. The thermal neutron absorption cross section of ³He is much higher than the fast neutron elastic scattering cross section so that even a very low thermal neutron population may produce a reaction rate in the detector that swamps that associated with elastic scatter pulses. The thermal neutron-to-fast neutron fluence ratio typically increases with increasing distance from the fast neutron source. This variable convolution of fast- and thermal-induced pulses makes dose interpretation very difficult unless the field conditions exactly mimic the calibration conditions. Additionally, the elastic scattering cross section varies with fast neutron energy so that differences in fast energy spectra between calibration and field conditions may result in erroneous dose estimations

That's enough said about the difficulties involved in using the bare detector. If you recognize such, or if you have other plans for using the detector, there are some sources that may be helpful regarding calibration for dose measurements. Chapter 6 of National Council of Radiation Protection and Measurements Report 112, "Calibration of Survey Instruments Used in Radiation Protection for the Assessment of Ionizing Radiation Fields and Radioactive Surface Contamination," 1991 deals with calibration of portable neutron instruments. One quantity commonly used and recommended by the ICRP for instrument calibration is the ambient dose equivalent. International Commission on Radiological Protection (ICRP) Publication 43, "Determination of Dose Equivalents from External Radiation Sources - Part 2," 1988, is a useful reference that shows how the various dose quantities, such as ambient dose equivalent and effective dose equivalent vary with neutron irradiation conditions; a partner reference is ICRP Publication 39, which is Part 1 under the same title. An International Atomic Energy Agency (IAEA) Technical Report No. 285, "Guidelines on Calibration of Neutron Measuring Devices," contains some information that may also be helpful. Also, IAEA Safety Report Series No. 16, "Calibration of Radiation Protection Monitoring Instruments," has a section on neutron instrument calibration that may be useful. None of these references are standards, but they are generally accepted as providing valid and acceptable technical information by the professional community of radiation protection specialists. Numerous other technical reports dealing with some aspects of the topic are accessible on the internet.

I hope this is helpful to you.

George Chabot, PhD, CHP