Bonner Multisphere Spectrometer Research Articles

New method for unfolding neutron source spectra

For solving the Fredholm equation of the first kind, the domain can be discretized in M spheres and N energy bins using a Bonner multisphere spectrometer, in which the neutron flux is to be deduced from the response matrix Rij(E), and the count rate obtained for each sphere Ci. For this study we use an approach that introduces certain neutron energy indeterminations, which are difficult to account for if some absolute calculations are required to be performed. For M different from N, a problem arises after obtaining the deconvolution of this equation, because the extracted solution is not unique, even if a host neutron energy spectrum is used. We propose a different method for extracting neutron energy spectra from the count rate obtained using a Bonner spectrometer. The method consists of partitioning the response matrix into a number of matrices such that M is equal to N, and depends on the response matrix and the number of spheres under consideration. Thus, the overall solution of the Fredholm equation can be divided and applied to N/M segments with M equations, and M unknowns, which can be solved in every energy segment for the neutron energy spectra. With this, the neutron spectra can be recovered by combining successive energy intervals as will be thoroughly described in the paper.

Neutron background measurements at China Jinping underground laboratory with a Bonner multi-sphere spectrometer

The neutron background spectrum from thermal neutron to 20MeV fast neutron was measured at the first experimental hall of China Jinping underground laboratory with a Bonner multi-sphere spectrometer. The measurement system was validated by a 252Cf source and inconformity was corrected. Due to micro charge discharge, the dataset was screened and background from the steel of the detectors was estimated by MC simulation. Based on genetic algorithm we obtained the energy distribution of the neutron and the total flux of neutron was (2.69±1.02) ×10−5cm−2s−1.

Development of the Graphite-Moderated Neutron Calibration Fields Using <sup>241</sup>Am-Be Sources in JAEA-FRS

Background: The moderated neutron calibration fields using 241Am-Be sources and a graphite moderator have been constructed at the Facility of Radiation Standard (FRS) in the Japan Atomic Energy Agency (JAEA). Materials and Methods: The neutron spectra of the fields were evaluated by the Monte-Carlo calculations and measurements using the Bonner Multi-sphere Spectrometer. Results and Discussion: The fields have continuous neutron spectra from several MeV to thermal neutron energy, with fluence-averaged energies of 0.84 MeV and 0.60 MeV. Reference values of fluence rates and ambient/personal dose equivalent rates were determined from neutron spectra by measurements. Conclusion: Currently, the fields are available for calibration or performance test of neutron measuring instruments.

Calibration of Spherical Neutron Measuring Devices with a 252Cf Source in the Neutron Calibration Room of the KAERI

ISO recommends that the neutron scattering correction procedures, such as the shadow-cone method, the polynomial fit method and the semi-empirical method, be used for a calibration of neutron measuring devices. This study compared three methods using the Bonner Multisphere Spectrometer (BMS) and neutron survey meters based on a 252Cf source in the neutron calibration room of the Korea Atomic Energy Research Institute (KAERI). As for the results, the average correction factors and calibration factors of the shadow-cone method compared with the semi-empirical method using the BMS and neutron survey meters. The results were consistent to within ± 0.7 % and ± 2.6 % respectively. These averaged values for the three methods by the BMS and the neutron survey meters were then varied within ± 5 %, respectively. It is then noted that the results based on the BMS and the neutron survey meters agreed well with each other. From the results, it is suggested that the semi-empirical method be useful for the neutron calibration room of the KAERI.

Long-term measurements of cosmic ray neutrons by means of a Bonner spectrometer at mountain altitudes - first results

A Bonner multi-sphere spectrometer has been installed in 2005 at the Environmental Research Station 'Schneefernerhaus' (2660 m above sea level) on the Zugspitze mountain, Germany, to measure the energy spectrum of cosmic-ray neutrons at high altitudes continuously. The system can be used to investigate small temporal variations in the cosmic radiation intensity. For example, measurements were done during periods of 2 Forbush decreases of the cosmic radiation intensity in July and September 2005, respectively. The results were compared with those obtained by using neutron monitors, and neutron fluence spectra measured during these events are presented and discussed.

Measurement of Neutron Flux in a PWR Nuclear Power Plant in Korea

Neutron energy spectra were measured in a 590 MWe PWR nuclear power plant, Kori unit 1 in Korea to estimate the radiation dose for worker to be exposed from neutron during emergent repairing work. Fifteen measuring points in a containment building were selected referring to beta/gamma ray fluxes surveyed by GM counter and neutron fluxes by rem-meter. Neutron fluxes were measured by a Bonner multi-sphere spectrometer and the obtained data were unfolded using BUNKI computer code. From them dose equivalents for worker to be exposed at the points were estimated. To convert the dose equivalents into equivalent doses, a computer code was developed and used. The estimated equivalent dose rates at the measuring points were between 0.004mSv/h and 2.650 mSv/h.

Bonner sphere neutron spectrometry at spent fuel casks

For transport and interim storage of spent fuel elements from power reactors and vitrified highly active waste (HAW) from reprocessing, various types of casks are used. The radiation exposure of the personnel during transportation and storage of these casks is caused by mixed photon–neutron fields and, frequently, the neutron dose is predominant. In operational radiation protection, survey meters and even personal dosemeters with imperfect energy dependence of the dose-equivalent response are used, i.e. the fluence response of the devices does not match the fluence-to-dose equivalent conversion function. In order to achieve more accurate dosimetric information and to investigate the performance of dosemeters, spectrometric investigations of the neutron fields are necessary. Therefore, fluence spectra and dose rates were measured by means of a simple portable Bonner multisphere spectrometer (BSS). The paper describes briefly the experimental set-up and evaluation procedure. Measured spectra for different locations, types of casks and inventory are discussed. The spectra provide a basis to determine dose rates and other integral quantities with higher accuracy and for choosing suitable area monitors, respectively, to establish correction factors applied to the dosemeter reading.

A Method for Correcting the Calibration Factor Used in the TLD Dose Calculation Algorithm

The method is described for estimating calibration factors used in the TLD neutron dose calculation algorithm in order to assess the personal neutron dose equivalent to radiation workers in a nuclear power plant in accordance with ICRP 60 recommendations. Neutron spectra were measured at several locations inside the reactor containment building of Youngkwang Unit 4 in Korea by using a Bonner multisphere spectrometer (BMS) system. Based on the fractional distribution of measured neutron fluence, four locations were selected for in situ TLI) calibration. TL responses for the four selected locations were calculated from the measured spectra and the reported fit response function of TLD-600. TL responses were also measured with Harshaw type 8806 albedo dosemeters mounted on the water phantom, and compared with the calculated TL responses. From the responses measured with Harshaw 8806 TLDs, thermal neutron fluence was evaluated, and used to adjust the neutron spectrum obtained with BMS. TL responses calculated for the adjusted neutron spectra showed an excellent consistency with the measured TL responses within 15% difference. Neutron calibration factors were calculated for the measured neutron spectra and the D 2 O-moderated 252 Cf spectrum, and used to calculate correction factors which ranged from 2.38 to 11.18 The correction actor estimated in this way for the known neutron spectrum at an area can he conveniently used to calculate the personal dose equivalent at the area from the calibration factor obtained for a calibration neutron spectrum.