Neutron Survey Instruments Research Articles

Understanding the Radiation Soaking Effect in Neutron Survey Instruments.

The drift in response of neutron survey meters that employ moderated 3He and BF3 proportional counters has been reported and investigated by instrument calibration laboratories for over 20 y. When within a constant radiation rate field of high intensity, dose rate readings of an instrument may steadily increase with a rate proportional to the field intensity before reaching a relatively stable plateau. This phenomenon, commonly referred to as the radiation soaking effect, has been attributed to the accumulation of charge on insulating surfaces inside the detector. This paper provides results of a study specifically conducted to better understand the effect and, hence, to possibly identify mitigation techniques. The magnitude of the effect in different instruments tested under the same conditions has been assessed, and a reason for the detector response variance has been clearly identified. In addition to measures used in calibration practice to compensate for the radiation soaking effect, a potential method to return an instrument sensitivity to its baseline level is also discussed.

The effects of a revised operational dose quantity on the response characteristics of neutron survey instruments

The ICRU is considering revising the definition of ambient dose equivalent. This paper investigates the impacts of the proposed change on four designs of neutron survey instrument, the GNU, HSREM, LB6411 and Studsvik 2202D, in terms of their respective energy dependences of response and their performances in realistic workplace fields. In some circumstances the current designs of instrument still produce acceptable characteristics, but in general they may need to be re-optimized to better match the requirements of the new operational quantity; to that end, a simple retrofit solution for the GNU is demonstrated. The performance criteria against which instruments are judged may also need to be revised to reflect the proposed change.

Beyond Californium-A Neutron Generator Alternative for Dosimetry and Instrument Calibration in the U.S.

Evaluations of neutron survey instruments, area monitors, and personal dosimeters rely on reference neutron radiations, which have evolved from the heavy reliance on (α,n) sources to a shared reliance on (α,n) and the spontaneous fission neutrons of californium-252 (Cf). Capable of producing high dose equivalent rates from an almost point source geometry, the characteristics of Cf are generally more favorable when compared to the use of (α,n) and (γ,n) sources or reactor-produced reference neutron radiations. Californium-252 is typically used in two standardized configurations: unmoderated, to yield a fission energy spectrum; or with the capsule placed within a heavy-water moderating sphere to produce a softened spectrum that is generally considered more appropriate for evaluating devices used in nuclear power plant work environments. The U.S. Department of Energy Cf Loan/Lease Program, a longtime origin of affordable Cf sources for research, testing and calibration, was terminated in 2009. Since then, high-activity sources have become increasingly cost-prohibitive for laboratories that formerly benefited from that program. Neutron generators, based on the D-T and D-D fusion reactions, have become economically competitive with Cf and are recognized internationally as important calibration and test standards. Researchers from the National Institute of Standards and Technology and the Pacific Northwest National Laboratory are jointly considering the practicality and technical challenges of implementing neutron generators as calibration standards in the U.S. This article reviews the characteristics of isotope-based neutron sources, possible isotope alternatives to Cf, and the rationale behind the increasing favor of electronically generated neutron options. The evaluation of a D-T system at PNNL has revealed characteristics that must be considered in adapting generators to the task of calibration and testing where accurate determination of a dosimetric quantity is necessary. Finally, concepts are presented for modifying the generated neutron spectra to achieve particular targeted spectra, simulating Cf or workplace environments.

Improved Bonner sphere neutron spectrometry measurements for the nuclear industry

A novel, two-stage approach has been developed for producing the a priori spectrum for Bonner sphere unfolding in a case where neutrons are produced by spontaneous fission and (α,n) reactions, e.g. in UF6. The code SOURCES 4C is first used to obtain the energy spectrum of the neutrons inside the material, which is then fed into a MCNP model of the entire geometry to derive the neutron spectrum at the location of the Bonner sphere. Using this as the a priori spectrum produces a much more detailed unfolded Bonner sphere spectrum retaining fine structure from the calculation that would not be present if a simple estimated spectrum had been used as the a priori spectrum. This is illustrated using a Bonner sphere measurement of the neutron energy spectrum produced by a 48Y cylinder of UF6. From the unfolded spectrum an estimate has been made of the neutron ambient dose equivalent, i.e. the quantity which a neutron survey instrument should measure. The difference in the ambient dose equivalent of the unfolded spectrum is over 10% when using the novel approach instead of using a simpler estimate consisting of a single high energy peak, 1/E continuum, and thermal peak.

Calibration of the GNU and HSREM neutron survey instruments

Two innovative designs of neutron survey instrument have recently been developed to estimate ambient dose equivalent in the workplace: the GNU has an improved energy-independence of response in the meV to TeV range; the HSREM is a comparatively lightweight device covering the meV to 10MeV range. Both designs offer good detection sensitivity, allowing measurements to be made efficiently and thereby minimizing the exposure to their users. Prototypes of both devices have been constructed and exposed to sets of well-characterized reference fields: the resulting measured responses are presented and discussed here, compared against comprehensive Monte Carlo data.

Neutron dose equivalent rates in the vicinity of waste containers containing TRU

The Transuranic Waste Processing Center is charged with treating, packaging and shipping TRU waste for disposal in a safe and efficient manner. The nature of TRU waste, i.e. it contains radionuclides which spontaneous fission and emit alpha particles which can lead to the production of (α,n) neutrons, can lead to neutron dose equivalent rates of radiation protection concern. Neutron dosimetry measurements were performed at the TWPC facility in locations where neutron dose equivalent rates of significance had been observed using neutron survey instruments. These measurements provide an assessment of the dosimetric properties of neutron fields that are deemed to be typical at the TWPC. The neutron measurements were performed at four locations using a Bonner sphere spectrometer which provides a low resolution spectrum that yields a good estimate of the neutron dose equivalent rate and the fluence rate. A tissue equivalent proportional counter dose equivalent measurement was performed at one location. Based on the Bonner sphere measurements, over 90% of the ambient dose equivalent is delivered between approximately 100 keV and 7.4 MeV.

Measurements with the new PHE neutron survey instrument

A novel design of survey instrument has been developed to accurately estimate ambient dose equivalent from neutrons with energies in the range from thermal to 20 MeV. The device features moderating and attenuating layers to ease measurement of fast and intermediate energy neutrons, combined with guides that channel low-energy neutrons to the single, central detector. A prototype of this device has been constructed and exposed to a set of calibration fields: the resulting measured responses are presented and discussed here, and compared against Monte Carlo data. A simple simulated workplace neutron field has also been developed to test the device.

Standardisation of water-moderated 241Am-Be neutron source using De Pangher neutron long counter: experimental and Monte Carlo modelling

A convenient neutron source is made for calibration of neutron survey instruments and personal dosimeters that are used in various nuclear installations such as fuel reprocessing, waste management, fuel fabrication and oil and well logging facilities, etc. This source consists of a bare (241)Am-Be neutron source placed at the centre of a 15-cm radius stainless steel spherical shell filled with distilled water. This paper describes the standardisation of the source at Bhabha Atomic Research Centre, using De Pangher neutron long counter both experimentally and using the Monte Carlo simulation. The ratio of neutron yield of water moderated to the bare (241)Am-Be neutron source was found to be 0.573. From the simulation, the neutron-fluence-weighted average energy of water-moderated (241)Am-Be source (fluence-weighted average energy of 2.25 MeV, dose-weighted average energy of 3.55 MeV) was found to be nearly the same as that of a (252)Cf source (fluence-weighted average energy of 2.1 MeV, dose-weighted average energy of 2.3 MeV). This source can be used for calibration in addition to (252)Cf, to study the variation in response of neutron monitoring instruments.

A new design of neutron survey instrument

Abstract A novel design of neutron survey instrument has been developed. The moderator has been modified via the use of “neutron guides”, which help thermal neutrons reach the central proportional counter. This innovation has allowed the variations in the energy dependence of ambient dose equivalent response to be reduced compared to prior single-detector designs, whilst maintaining a relatively light moderator and simple construction. In particular, the design has a relatively small over-response to neutrons with energies around 5 keV, when compared to prior designs. The final optimized design has been verified using MCNP5 calculations to ensure that the response is relatively independent of the energy and direction of the incident neutron. This has required the ends of the guides to be structured so that unidirectional and isotropic neutron fields have closely matched responses, as is necessary in the workplace. The reading of the instrument in workplace fields is calculated via folding and the suitability of the design for use in the workplace discussed.

Neutron area survey instrument measurements in the EVIDOS project

Neutron survey instruments have been exposed at all the measurement locations used in the EVIDOS project. These results have an important impact in the interpretation of the results from the project, since operationally the survey instrument will be used for an initial assessment of and routine monitoring of the ambient dose equivalent dose rate. Additionally, since the response of these instruments is in some cases very well characterised, their systematic deviations from the reference quantities provide an important verification of the determination of those quantities.

Modelling of neutron survey instrument performance and experimental validation of those calculated response data

Three moderator-type neutron survey instruments have been modelled for energy and angle dependence of the response, in greater detail than before. These response data have been verified by comparison with published experimental measurements and measurements made specifically for this project. Influences on the instrument response have also been investigated. These have included its mode-of-use and perturbations caused by variations in the instrument manufacture. The implications of these new response data have been assessed by an extensive programme of folding the responses with workplace energy distributions.

A New Design of Neutron Dose Equivalent Survey Instrument

The outline design and results of calculations of the response characteristics are given for a new design of neutron survey instrument. The novelty of the design is the use of differential attenuation by a thick boron layer together with moderation hy polyethylene or other (CH 2 ) n plastic layers and the linear combination of the signals from a central gas-filled detector and an array of outer solid state detectors. The proposed design has comparable mass and better sensitivity than current devices. much improved energy response characteristics, and additional spectrometric features.

A Neutron Survey Instrument Which Gives Information on the Low Energy Neutron Spectrum and Can be Used for Albedo Dosemeter Calibration

Abstract An instrument has been developed for surveying neutron environments. It contains two thermal neutron detecting elements which consist of touch proof surface barrier detectors in contact with thin layers of 6LiF. One detector is located at the centre of a 76 mm (3") diameter Polythene sphere. The readings of the two detectors give information on the thermal and intermediate components of the ambient dose equivalent, and can be combined to mirror the response of various albedo dosemeters. The instrument can also be used to scale pulses from a wide energy range neutron survey instrument, and hence be used to assess the proportion of the neutron dose equivalent detected by albedo dosimetry systems.

Method for Assessing Neutron Dose Equivalent Rate in Operational Health Physics

Many types of survey instruments for measuring exposure dose from neutrons and methods for assessing the dose equivalent rate have been developed in many countries. In JAERI, extensive efforts have been made for establishing the method to assess neutron dose equivalent.The main objectives of this report are to introduce the principle of measuring neutron dose equivalent rate based on an orthonormal expansion method and to describe its application for the operational health physics in JAERI.This report also describes an experience with the various types of neutron survey instruments which have been employed for the radiation protection in JAERI since 1958.