Neutron Calibration Fields Research Articles

Parameter of Neutron Calibration Field in Viet Nam

Parameter of Neutron Calibration Field in Viet Nam

Determination of the neutron scattering contribution in a neutron calibration field based on MNSR

Recently, the idea of using the radiation field of the Isfahan MNSR neutron radiography beamline has been proposed for the calibration of neutron detectors and dosimeters. Examining the contribution of scattered neutrons in the neutron calibration field (NCF) is one of the key challenges in the calibration process. The aim of this study is determination and quantification the scattering contribution in the proposed NCF at the Isfahan MNSR based on Monte-Carlo calculation and experiments using the Semi-Empirical Method (SEM), Reduced-Fitting Method (RFM), and Generalized Fit Method (GFM). There is good agreement between the results, which shows that the characterization approach of the NCF is reliable. The maximum contribution of scattered neutrons with increasing distance up to 80 cm from the beamline outlet was obtained less than 14%, which meets the recommended range of ISO-8529.

Impact of the newly proposed ICRU/ICRP quantities on neutron calibration fields and extended range neutron rem-counters

In July 2017, the International Commission on Radiation Units and Measurements (ICRU) and the International Commission on Radiological Protection (ICRP) proposed the introduction of new operational quantities for external radiation exposure, with the aim of improving coherence between protection quantities and operational quantities within the system of radiological protection. A change in operational quantities will impact both instrumentation and reference radiation fields used for their calibration. This paper evaluates the potential impact of the new quantity ambient dose, H*, meant to replace ambient dose equivalent, H*(10), on two neutron reference fields, the Am-Be source and the CERF high-energy workplace field, and on the response of two models of extended-range neutron rem counters (LINUS and LUPIN). The conclusions are that calibration procedures should in general not be affected and that changes should only be expected in calibration coefficients. Considering the acceptable measurement uncertainties for operational radiation protection, for the extended-range rem counters changes in their design would not be required for measurements outside particle accelerators shielding and for aircrew dosimetry. One can expect that this type of instrument can still be calibrated with Am-Be source neutrons and employed in neutron fields with energy distributions spanning several decades. For uses in radiation fields with very peculiar neutron energy distribution, a specific workplace field calibration may instead be required.

Evaluation of gamma contribution in a neutron calibration field of [formula omitted] source

The paper presents the evaluation of gamma contribution in a neutron calibration field of a 241Am−Be source. The characterization of gamma flux spectra and gamma ambient dose equivalent rates has been performed using a portable NaI(Tl) gamma spectrometer. For obtaining the gamma ambient dose equivalent rates from the measured spectra, two approaches were applied based on the applications of the G(E) function and the ICRP 74 conversion factors, respectively. Comparison of the gamma ambient dose equivalent rates obtained from the two approaches shows an agreement within 5%. The gamma contribution in the neutron calibration field is evaluated about 2.3–3.3% of the total neutron ambient dose equivalent rate in the distance range of 100–250cm.

Characterization of a neutron calibration field with 241[formula omitted] source using Bonner sphere spectrometer

This paper presents the characterization of a neutron calibration field of a 241Am−Be source using Bonner sphere spectrometer. The characterization of the neutron field in terms of neutron ambient dose equivalent, H*(10), rates and neutron flux spectra was performed using generalized-fit and semi-empirical fit methods together with an unfolding technique using the MAXED code. Monte Carlo simulations using MCNP5 have also been conducted to qualify the characterization process. Comparison of the neutron fluxes and the H*(10) rates obtained from the various methods shows that the components of the neutron fluxes are in good agreement with a discrepancy of about 2% while the discrepancy of the direct H*(10) rates is within 3%. This good agreement among the methods indicates the reliability of the characterization process for practical calibration of neutron measuring devices. The results also suggest that the calibration should be conducted at a distance less than 200cm to ensure the contribution of the scattered component in the reading of the device less than that of the direct component by 40%.

Neutron calibration field at Institute for Nuclear Science and Technology

In this study, a neutron calibration field of a 241Am-Be standard source at the Institute for Nuclear Science and Technology is characterized in terms of neutron spectral fluxes and neutron ambient dose equivalent ( ) rates based on ISO 8529 recommended generalized fit method together with measurements using a Bonner Sphere Spectrometer and MAXED unfolding techniques. The total, direct and scattered neutron components as well as rates and neutron spectral fluxes are separated from each other. The direct rates and neutron spectral fluxes in the free field as functions of distances from the source are also theoretically calculated based on the neutron source strength. The comparison between experimental data and theoretical calculations of the rates and neutron spectral fluxes shows good agreement within 3%, this has confirmed the reliability of the field characterization process and its applicability in the practical calibration works for neutron survey meters.

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.

Neutron Calibration Field of a Bare 252Cf Source in Vietnam

Abstract This paper presents the establishment and characterization of a neutron calibration field using a bare 252 Cf source of low neutron source strength in Vietnam. The characterization of the field in terms of neutron flux spectra and neutron ambient dose equivalent rates were performed by Monte Carlo simulations using the MCNP5 code. The anisotropy effect of the source was also investigated. The neutron ambient dose equivalent rates at three reference distances of 75, 125, and 150 cm from the source were calculated and compared with the measurements using the Aloka TPS-451C neutron survey meters. The discrepancy between the calculated and measured values is found to be about 10%. To separate the scattered and the direct components from the total neutron flux spectra, an in-house shadow cone of 10% borated polyethylene was used. The shielding efficiency of the shadow cone was estimated using the MCNP5 code. The results confirmed that the shielding efficiency of the shadow cone is acceptable.

Characterization of neutron calibration fields at the TINT's 50 Ci americium-241/beryllium neutron irradiator

Reliable measurement of neutron radiation is important for monitoring and protection in workplace where neutrons are present. Although Thailand has been familiar with applications of neutron sources and neutron beams for many decades, there is no calibration facility dedicated to neutron measuring devices available in the country. Recently, Thailand Institute of Nuclear Technology (TINT) has set up a multi-purpose irradiation facility equipped with a 50 Ci americium-241/beryllium neutron irradiator. The facility is planned to be used for research, nuclear analytical techniques and, among other applications, calibration of neutron measuring devices. In this work, the neutron calibration fields were investigated in terms of neutron energy spectra and dose equivalent rates using Monte Carlo simulations, an in-house developed neutron spectrometer and commercial survey meters. The characterized neutron fields can generate neutron dose equivalent rates ranging from 156 μSv/h to 3.5 mSv/h with nearly 100% of dose contributed by neutrons of energies larger than 0.01 MeV. The gamma contamination was less than 4.2-7.5% depending on the irradiation configuration. It is possible to use the described neutron fields for calibration test and routine quality assurance of neutron dose rate meters and passive dosemeters commonly used in radiation protection dosimetry.

Development of portable long counter with two different moderator materials

A portable, light-weight long counter (LC) with small dimensions was developed. This LC consists of a (3)He thermal neutron counter, a cylindrical moderator and outer shields. It was designed to have an almost flat response in a neutron energy range of 0.4 eV to 5 MeV. The portable LC has a radius of 11 cm and a length of 39 cm. Its weight was successfully reduced to 15 kg. Polystyrene was employed instead of polyethylene for the front part of the moderator in order to increase the sensitivity to low-energy neutrons. The response function calculated using the MCNP code was consistent with the results of experiments using monoenergetic neutron calibration fields.

Determination of neutron fluence in 1.2 and 2.5 MeV mono-energetic neutron calibration fields at FRS / JAEA

*A highly efficient silicon semiconductor detector with a polyethylene convertor (CH2-SSD) was developed and the neutron fluences were precisely evaluated with it at 1.2 and 2.5 MeV mono-energetic neutron calibration fields at the Facility of Radiation Standard (FRS) in the Japan Atomic Energy Agency (JAEA) using an accelerator. The CH2-SSD was designed to have a higher detection efficiency than that of the conventional one in order to measure the neutron fluencies at the calibration point of more than 100 cm from the target. The NRESP-ANT and PHITS codes were used to calculate the efficiency of the CH2-SSD and its uncertainties. The maximum neutron fluence rates at 100 cm from the target were evaluated to be 1.0×10 3 and 2.0×10 3 cm -2 s -1 in the 1.2 and 2.5 MeV fields, respectively. They are high enough to calibrate dosemeters

Measurement of photon dose rates in moderated neutron calibration fields

The authors performed a complementary study to determine the proportion of the contaminating photons present in a moderated neutron field produced by a 252 Cf neutron source surrounded by moderators. The photon dose rates were measured with three types of instruments based on different detection principles, and the results were compared with those obtained from Monte Carlo calculation. The contaminant photon dose rates observed from each instrument were in good agreement, and the measurement and calculation results also agreed well.

Neutron fluence monitoring system in mono-energetic neutron fields at FRS / JAEA

Mono-energetic neutron calibration fields were developed from 8 keV to 19 MeV using an accelerator at the Facility of Radiation Standards (FRS). Two kinds of neutron detectors are used for monitoring the neutron fluence at a calibration point. One is the Long Counter placed at the 60 degree angle with respect to the beam line except for the 14.8 and 19 MeV fields where the angle is 90 degree. The other is the 110deg-monitor installed in a collimator at 110 degree angle with respect to the beam line. These two monitors were designed to have flat response to the neutrons from a few keV to a few MeV. The neutron fluence at a calibration point can be evaluated from the counts of the Long Counter with corrections of scattered neutrons. The neutron energy is sensitive to the beam energy especially for the neutrons below a few hundreds of keV. It was found that the beam energy was able to be monitored using the count ratio of two neutron monitors.

Fast neutron calibration fields constructed using a proton accelerator of KIRAMS and spectrumweighted response of various neutron detectors

This paper presents the dosimetric quantities, realistic neutron calibration fields and spectrum-weighted response (SWR) of several neutron-measuring devices to provide guidance to health physicists and dosimetry specialists on the proper selection and use of survey meters in neutron fields of varying energy spectra. To yield the SWR, the detector response functions of 17 neutron-measuring devices were numerically folded with each of the produced calibration neutron spectra through the in-house developed software 'K-SWR'. The detectors' response functions were taken from the IAEA Technical Reports Series No. 403. Eight kinds of calibration neutron fields with high energy and high dose rates were produced using a proton accelerator (MC50 cyclotron) of Korea Institute of Radiological and Medical Sciences (KIRAMS). The fluence-average energy (Eave) varied from 3.8 to 16.9 MeV, and the ambient-dose-equivalent rate [H(*)(10) h(-1)] varied from 0.99 to 16.5 mSv h(-1).

Development of neutron-sensitive glass dosimeter containing isotopically enriched boron

Abstract Neutron-sensitive glass dosimeters were successfully made from reagents of NaPO 3 , Al(PO 3 ) 3 , enriched 10 or 11 B 2 O 3 and AgCl. The glass dosimeters had good RPL characteristics: satisfactory linearity of 0.01–500 mGy and low variation in sensitivity. The RPL characteristics for gamma-rays were compatible with those of a commercial glass dosimeter. A pair of the 10 or 11 B-containing glass dosimeters was effectively used for the thermal neutron dosimetry. In 565 keV and 5.0 MeV monoenergetic neutron calibration fields, albedo neutrons from the ISO water slab phantom were successfully detected with the glass dosimeter pairs.

Development of the Quasi-monoenergetic Neutron Calibration Fields of Several Tens of MeV at TIARA

Development of three calibration fields for 45, 60 and 75 MeV neutrons has been in progress at Takasaki Ion Accelerators for Advanced Radiation Application (TIARA) of JAEA-Takasaki. Achievement of the neutron calibration fields requires establishment of both measurement of neutron fluence and its monitoring technique. In order to measure the neutron fluence of main energy peak especially at a reference point on low fluence condition, a proton recoil counter telescope with high efficiency has been developed. The design of relatively large telescope components brought a high detection efficiency at the reference point with wide irradiation area at a long distance. Effective and precise measurement of the neutron fluence could be performed with uncertainties less than 6.5 % (standard deviation). For detecting neutrons without scattering near the target, a transmission type neutron fluence monitor has been newly developed to measure the neutron fluence directly at the collimator exit. Its performance such as counting rates was tested for various beam intensities.

Recent activities on neutron standardization in Japan

NMIJ provides national standards on neutron fluence as well as neutron emission rate in Japan. The neutron fluence standards are established for thermal neutrons, monoenergetic neutrons at seven energies from 24 keV to 14.8 MeV, radionuclide-source neutrons from 252 Cf and 241 Am-Be. The neutron emission standards are provided for these radionuclide sources. These are basically prepared in accordance with ISO 8529-1. Worldwide consistency of the standards has been supported by participating in key comparisons organized by BIPM. NMIJ issues calibration certificates accepted worldwide within the range specified in our calibration and measurement capabilities listed in the BIPM web page. The development of the 19 MeV neutron field is in progress to complete standardization in 2011. The radionuclide-source neutron fluence standards were extended in 2008 so that calibrations are also available in terms of ambient/personal dose equivalent. The Japan calibration service system has launched to improve the traceability of neutron measurement in Japan. A new thermal neutron calibration field is developing with a guided beam at the research reactor JRR-3M of JAEA. Aiming at the establishment of the 20-100 MeV high-energy neutron standard, comparison studies are currently being conducted between the cyclotron facilities TIARA of JAEA and CYRIC of the Tohoku University. -1 ) in Japan. We report the present status and recent activities of these neutron standards.

Design of a graphite-moderated 241Am–Li neutron field to simulate reactor spectra

Abstract A neutron calibration field using 241Am–Li sources and a moderator was designed to simulate the neutron fields found outside a reactor. The moderating assembly selected for the design calculation consists of a cube of graphite blocks with dimensions of 50 cm by 50 cm by 50 cm, in which the 241Am–Li sources are placed. Monte Carlo calculations revealed the optimal depth of the source to be 15 cm. This moderated neutron source can be used to provide a test field that has a large number of intermediate energy neutrons with a small portion of MeV component.

A new neutron monitor with silver activation

Abstract A moderator-type neutron monitor has been developed, which registers delayed beta rays from neutron-induced silver activation and which is able to measure dose equivalent in pulsed fields with peak dose rates of several thousand Sv h−1. The monitor uses four silicon diodes in the centre of a polyethylene moderator, 30 cm in diameter. Two of the diodes are covered by natural silver foils and two of them by tin foils. The latter are used to subtract photon-induced pulses. For registering signals, a pulse height threshold is set at 662 keV, which minimizes the effect of 137Cs and lower energy radiation and – in addition – enhances the detection of beta rays from the shorter half-life silver isotope 110Ag (25 s) as compared to the longer half-life isotope 108Ag (144 s). The results of measurements in neutron and photon calibration fields, of MCNPX neutron response calculations and of first measurements in a high-intensity pulsed field at the PSI accelerator are shown.

A new simulated neutron workplace field at PIAF

Abstract The range of neutron energies encountered at workplaces extends from 10 −2 eV to 10 7 eV or even higher neutron energies. The monoenergetic neutron calibration fields cover the neutron energy range from 10 4 eV to 10 7 eV. Hence calibrations in so-called realistic fields with a broad spectral distribution similar to those at workplaces are still essential for radiation protection equipment. This is the reason why PTB has developed a simulated workplace field. The field is produced using a proton beam on a thick Li or Be target installed in a moderating sphere which produces the intermediate and thermal part of the spectrum. Different target materials and compositions and different constructions of the target were investigated with special focus on the long-term stability of the neutron yield, as well as the stability of the spectral neutron fluence of the primary neutron spectrum. The spectral distribution outside the moderator sphere was measured using the PTB Bonner sphere spectrometer NEMUS and calculated using MCNPX.