Answer to Question #8437 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

With the goal to perform occupational exposures to neutrons from Am:Be sources widely used in nuclear density gauges, I bought a new instrument created by ICX Radiation and sold by Thermo Scientific called Interceptor (detector for gamma/neutron). The problem I have is the neutron channel display is presented only in count rate (cps) where gamma channel is presented in effective dose. How can I convert those cps into effective dose to compare with occupational limits?

A

Although I have not used the “Interceptor” detector that you describe, it appears, from the supplier’s specifications and description, to be primarily an instrument for gamma-radiation assessment, including nuclide identification with allowance for gamma-dose-rate estimation. The inclusion of a 3He detector allows for the incidental detection of neutrons but does not seem to be suited to dose measurements. The detector does not appear to contain a significant amount of neutron moderator material to enhance thermal neutron production from incident fast neutrons so its response is presumably due to whatever thermal neutrons might be present in the environment plus a possible direct response to fast neutrons that might be present. Note that in the specifications available from the manufacturer, the neutron-induced count rate per unit neutron fluence rate is given as 1.2 cps/nv (i.e., 1.2 counts per second per n cm-2 s-1). I have done a hand calculation and found that this value would apply to the expected response when thermal neutrons are incident on the detector.

The major interaction that produces a detectable pulse is the 3He(n, p)3H reaction in which the recoiling proton and triton (3H) produce the measurable pulse. Unfortunately, the reaction cross section for the reaction varies markedly with neutron energy, decreasing from about 5,300 barns at thermal energies to about one barn at 1 MeV and about 0.2 barns at 10 MeV. Additionally, as the neutron energy increases into the MeV region, a secondary interaction in which fast neutrons scatter elastically from 3He nuclei becomes progressively important, and pulses produced by the scattered nuclei produce a signal that adds to the observed count rate. The end result is that, when dealing with uncharacterized neutron fields, the count rate cannot readily be correlated with a neutron dose rate unless the detector has been properly designed to provide enhanced sensitivity to fast neutrons and equipped with electronics that allow for pulse shape discrimination to separate the pulses from the (n,p) reaction from the 3He recoil nuclei induced pulses.

If the only neutron sources of concern are Am-Be sources, and most of the exposure of individuals involved in their use comes from the fast neutrons directly from the sources, you may be able to conduct a simple calibration in which the detector is located at a relatively close distance from an Am-Be source of known neutron output. The low response of the detector to fast neutrons may preclude against this for low and moderate neutron source strengths. A simple example should demonstrate the problem.

The neutron fluence rate (n cm-2 s-1) can be calculated from the quotient of the neutron emission rate by 4pr2, where r is the distance between the source center and the center of the 3He detector. (This assumes that the contribution of room return from scattered neutrons is insignificant.) If you are abiding by U.S. Nuclear Regulatory Commission or U.S. Agreement state regulations, the fluence-to-dose conversion factors are as given in Table 1004(b).2 of paragraph 20.1004 of 10 CFR 20. For Am-Be neutrons, the table would confirm a fluence rate of about 6.7 n cm-2 s-1 to produce a dose equivalent rate of 0.01 mSv per hour, and such a conversion factor may be used to obtain the dose equivalent rate at the detector location so that the relationship between count rate and dose rate may be established. The weakness in this method is that the count rate from fast neutrons might be quite low. For example, for an Am-Be source containing 2 Ci of 241Am (a fairly common size for calibration purposes), the neutron emission rate would be about 3.5 x 106 n s-1; at a rather close distance of one foot from the source, the fast neutron fluence rate would be about 300 n cm-2 s-1. The average neutron energy from the source is about 4.5 MeV, and the (n,p) cross section at this energy is about 0.4 barns, about 1.3 x 104 times smaller than the cross section at thermal energies, and the expected detector count rate, given no significant neutron moderation, would be on the order of 2.8 x 10-2 cps, a rate that might be too low to measure accurately with the detector in question (some additional counts might be observed from the 3He recoil events). To complicate things more, you would have to demonstrate during the calibration that the reading you obtained was not being affected by thermal neutrons produced by scattering in the room. Even a relatively small thermal neutron fluence rate may heavily weight the detector response, and this could lead to gross misinterpretation of neutron dose rate.

In summary, the detector you describe does not appear to be capable of providing a direct assessment of neutron dose rate. It might have some restricted applications for which you may be able to establish a correlation between count rate and dose rate, but the detector is not generally well-suited to neutron dose measurements. The manufacturer you cite does offer other instruments that are better suited for neutron dose measurements—e.g., the Wendi-2 (FHT 7562); this instrument (and others like it from other manufacturers) uses a large-volume polyethylene moderator to slow fast neutrons for detection by the 3He detector and yield an acceptable dose response over a wide energy range. They suffer from the associated drawback of being physically large in size and very heavy, typically on the order of 20 pounds.

I hope you find that you are able to use the instrument you purchased for some of your measurement needs. Good luck.

George Chabot, PhD, CHP

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