Answer to Question #13273 Submitted to "Ask the Experts"

What is calibration factor for devices that provide neutron dose rates from counts per second? I am observing two instruments with ³He detectors that are giving different dose rate values. Is it due to different dose rate calibration factors?

I cannot provide a definitive answer to your question regarding the calibration factor because there are a variety of neutron measuring instruments available, and their dose sensitivities vary depending on design. However, we can briefly discuss the appropriate calibration quantity to be used and how some typical instruments might respond and attempt to postulate possible causes for differences in readings among different instruments.

Generally, per recommendations of the International Commission on Radiation Units and Measurements (ICRU), calibrations of instruments used in health physics dose (rate) measurements are done in terms of the quantity ambient dose equivalent, a specific operational quantity often recommended for use as a surrogate for effective dose, which is not directly measurable with available instrumentation. The ambient dose equivalent is symbolized H*(d), where d is measured in mm, and the value of H*(10) refers to the equivalent dose (rate) at a depth of 10 mm in soft tissue equivalent material; this 10 mm value is used to estimate the effective dose (rate). In the ICRU model, the value is determined when the radiation field is assumed to be aligned (the field is made unidirectional) and expanded with respect to a suitable phantom, ICRU having recommended a 30 cm diameter sphere of tissue equivalent material, and the dose at the 10 mm depth is evaluated along a radius opposing the direction of the aligned field. You can read more about ambient dose equivalent by typing "ambient dose" into the search box on this HPS ATE web page or by searching the internet.

The response of any instrument will depend on its particular design. Instruments of the same design and model would be expected to respond similarly to the same radiation field under the same conditions. As an example, I have looked up specifications for a couple of arbitrarily selected ³He-based detectors. One includes a spherical moderator of polyethylene in which the ³He gas tube is contained. The one I reviewed is a Ludlum model 42-31, which uses a 22.9 cm diameter spherical moderator and is described as having a response sensitivity of 10,000 counts per mSv per hr (cpm per mSv h^-1) for neutrons produced by an ²⁴¹Am-Be neutron source. A second detector offered by the same manufacturer is the Model 42-38 WENDI-2, which contains the ³He tube within a cylindrical moderator, measuring 22.9 cm x 21.2 cm. The response sensitivity claimed for this detector is 45,000 cpm per mSv h^-1 for neutrons produced by an ²⁴¹Am-Be neutron source. The difference in response between the two instruments simply reinforces the importance of instrument design. Naturally, both instruments could be calibrated to provide legitimate estimates of ambient dose equivalent.

You have noted that you have observed results for two instruments that show different dose rates, presumably for the same neutron radiation field. If the calibrations for the two instruments were performed differently, either calibrating to different dose quantities or using significantly different neutron radiation fields, such factors could account for differences in readings. Depending on the particular instrument, I have also seen instances in which the internal discriminator level was set incorrectly, allowing for some pulses from gamma radiation interactions in the ³He tube to be recorded in radiation fields that contained both neutrons and gamma radiation. We should also keep in mind that the ³He detector responds primarily to thermal neutrons. In order to account for dose from intermediate energy and fast neutrons, the ³He tube is surrounded by moderating material, sometimes with other materials added so as to attempt to force the dose response of the instrument to follow an acceptably close approximation to the effective dose curve as a function of neutron energy, most often over the range from about thermal energy (0.025 eV) to 10 or 12 MeV, although some instruments extend response into the GeV region. The experimental response curves are never perfect and, depending on design, deviations from true dose rates may be significantly greater at some energies than at others, and instruments of different designs will not show the same deviations. Such differences can lead to measurements that may differ in the same radiation fields depending on the energy characteristics of the fields. Of course, such differences would only apply to instruments of different designs. For instruments of the same model or design, the dose rate results should be comparable within statistical uncertainty unless some other influencing factor, as discussed earlier, might be effective.

I hope this has been helpful.

George Chabot, PhD, CHP