I am not sure if I have got the question correctly because I do not understand the term sensitivity in the specific context of the question.

The quantity of interest for a measurement device for ambient ionising radiation is the dose rate (H*(10) per time or Hp(10) per time, in units of rad/h or Sv/h). However, measuring the dose rate of neutron radiation is quite sophisticated because of the requirement of moderating the neutrons. Therefore, simple neutron measurement devices only give the neutron count rate, which can be used to determine the presence of neutron radiation in general.

Please be aware that rad/h (unit rad/h) and nv/cps (neutron flux in units of cps) are two different representations. H*(10) / rad/h (with a dot on top of the H for the time derivative) would correspond to nv/cps.

The idea to mention the gamma radiation sensititvity in Rad/h is applied when neutron tubes are used in a combined neutron and gamma radiation field.

Neutron tubes, typically He3 proportional counter tubes, are somewhat sensitive to gamma radiation as well. If the gamma field is high, then the tube may falsely start to count the gamma's as neutron pulses. The Rad/h indication means that the tube is insensitive for gamma radiation field till that declared value.

This gamma radiation seen by a tube is typically less of energy than a neutron pulse that would be detected by the counter. Some solutions exist to make a tube less gamma sensitive. Suppliers can use special coating, specific less sensitive gazes or a tube base preamplifier with an LLD potentiometer to cut of that low energy part to obtain a neutron tube that is less sensitive to gamma radiation.

@Ratty agreed. However is it a universal convention or varies from company to company. If universal then IEC ISO recommendations etc. Must be there.

Dear Manoj,

Neutron detection efficiency and gamma-ray sensitivity are reported differently depending on the organization conducting the measurement. There are presently many methods used, some briefly described in paper we published a few years ago.

D.S. McGregor, J.K. Shultis, “Reporting detection efficiency for semiconductor neutron detectors: A need for a standard,” Nucl. Instrum. Meth., 632 (2011) pp. 167-174 (available on RG).

The reason for the various methods is most likely a consequence of limited resources. Institutions without a convenient nuclear reactor with a thermalized neutron beamport must use an alternative neutron source, usually a moderated AmBe or Cf-252 source.

Unfortunately, such a neutron source can have a complicated spectrum. Hence, for consistency, a “gold standard” detector is used as a witness to the calibration. These standard witness detectors are usually calibrated at a reliable laboratory, such as NIST, and kept as a witness standard for that type of detector at the company or laboratory.

Vendors will measure the count rate of a detector under test in their system, commonly a HDPE–lined box with a Cf-252 source or AmBe source nearby, and compare the result to the calibration detector. The result is reported as number of counts observed per known flux unit (cps/nv), where nv is determined by the witness detector.

As for gamma-ray sensitivity, the problem arises in that gamma rays can interact in a neutron detector through three main processes, those being photoelectric absorption, Compton scattering, and pair-production. In all three cases, it is unlikely that all of the energy will be deposited in the detector; hence ionization is dependent upon the initial electron energy, the range, and construction materials. This is especially true for gas-filled detectors. Hence, most vendors will report the following: equivalent setting of the lower discriminator, counts observed, gamma-ray source type, and intrinsic exposure (mR/H). The exposure is used because, as written before, it is unlikely that a gamma-ray event will result in total energy absorption.

In recent times, the Pacific Northwest National Laboratory has promoted a method of comparing gamma-ray rejection, or “gamma absolute rejection ratio for neutrons” (GARRn). The idea is to use a portable neutron source (Cf-252) to measure the detector neutron sensitivity, another gamma-ray source (Co-60 or Ir-192)  to measure the gamma-ray sensitivity, and finally a combined measurement with both. GARRn is defined as

GARRn = єabs γn/ єabs n

This measurement method is used in some circles, but has yet to enjoy universal acceptance. You can read more about it in

R.T. Kouzes, et al., “Neutron Detector Gamma Insensitivity Criteria,” PNNL-18903, Oct. 28, 2009.

Sincerely,

Douglas

The monitor you have in mind is perhaps meant for mixed (n, 𝞬) monitoring where the neutron flux is measured in units of counts/sec per n/cm2/sec and the gamma dose rate in rads/hr. This implies there are separate built-in detectors for monitoring of neutrons (insensitive to gamma rays) and gamma rays (insensitive to neutrons). Examples of the former detector are BF3/3He proportional counter (for thermal neutrons) or BF3/3He proportional counter (surrounded by a suitable moderator like paraffin wax or polythene) for intermediate/fast neutrons. Examples of the latter are energy compensated (from about 60 keV – 2 MeV) GM detectors covered with perforated filters. Note that the neutron counters as well as the GM counters mentioned above measure basically the count rate (cps). In the case of GM counters with filters as above, the instrument can be calibrated in R/hr. or Rads/hr. or mSv/hr. (exposure rate, dose rate or ambient dose equivalent rate) using a Cs-137 source, since the radiation weighting factor is nearly equal to 1. Also, there is a linear relation between cps and dose rate when the GM counters are covered by these specially-designed perforated filters. For neutrons, there is no such simple linear relation between cps and dose (equivalent) rate; while we can measure separately the flux due to thermal/slow neutrons with a bare detector as above, for high energy neutrons, the flux can be approximately estimated by surrounding the detector with a cylindrical moderator as above to thermalize the fast neutrons. However, to convert these cps values to ambient dose equivalent rate, one has to know the approximate spectrum of neutrons. Values are available in the literature to convert neutron flux to dose equivalent rate for various Quality/Radiation weighting factors (see for example: USNRC 10 CFR 20) for a standard 30 cm diameter TE phantom. Usually, these instruments are calibrated with pure neutron sources like Am-Be with known spectrum. For arriving at an approximate value of QF (in the absence of spectrum information), one can roughly assume the average energy of the neutron spectrum or assume a value of 10 in the absence of spectral information. Again, please note the ratio of flux to dose rate varies markedly with neutron energy and that one has to use these instruments in scatter-free environment.

There are instruments available in the market to estimate the neutron ambient dose equivalent rate directly in mrem/hr. (the so-called REM counters); these have negligible gamma sensitivity for low gamma dose rates. These are adaptations of “long counters” where there is a complex system of moderator assembly surrounding the detector to tailor its response for various neutron energies, taking into account their quality factors. These should be used in a particular orientation to the direction of radiation. Also, there is a special sensitive neutron survey instrument using a TE multiplication ionization chamber, based on Bragg-Gray principle, to estimate the total dose in rads for a mixed (n, 𝞬) neutron field (e.g.,  from Cf-252 source) but this requires a suitable multiplication factor for the neutron/gamma spectrum to arrive at the total dose equivalent rate  

The literature on neutron monitoring is very extensive with various techniques and with several types of instruments being marketed by companies. The wide variation of the radiation weighting factor (from 2 to 11) with energy makes the measurement of dose equivalent complex.

Above is only a cursory view of neutron field monitoring. Nevertheless, I hope the above discussion will be useful.