Reference Neutron Fields Research Articles

TWO NEW SINGLE-EXPOSURE, MULTI-DETECTOR NEUTRON SPECTROMETERS FOR RADIATION PROTECTION APPLICATIONS IN WORKPLACE MONITORING.

This communication describes two new instruments, based on multiple active thermal neutron detectors arranged within a single moderator, that permit to unfold the neutron spectrum (from thermal to hundreds of MeV) and to determine the corresponding integral quantities with only one exposure. This makes them especially advantageous for neutron field characterisation and workplace monitoring in neutron-producing facilities. One of the devices has spherical geometry and nearly isotropic response, the other one has cylindrical symmetry and it is only sensitive to neutrons incident along the cylinder axis. In both cases, active detectors have been specifically developed looking for the criteria of miniaturisation, high sensitivity, linear response and good photon rejection. The calculated response matrix has been validated by experimental irradiations in neutron reference fields with a global uncertainty of 3%. The measurements performed in realistic neutron fields permitted to determine the neutron spectra and the integral quantities, in particular H*(10).

Measurement and calculation of fast neutron and gamma spectra in well defined cores in LR-0 reactor

A well-defined neutron spectrum is essential for many types of experimental topics and is also important for both calibration and testing of spectrometric and dosimetric detectors. Provided it is well described, such a spectrum can also be employed as a reference neutron field that is suitable for validating selected cross sections. The present paper aims to compare calculations and measurements of such a well-defined spectra in geometrically similar cores of the LR-0 reactor with fuel containing slightly different enrichments (2%, 3.3% and 3.6%). The common feature to all cores is a centrally located dry channel which can be used for the insertion of studied materials. The calculation of neutron and gamma spectra was realized with the MCNP6 code using ENDF/B-VII.0, JEFF-3.1, JENDL-3.3, ROSFOND-2010 and CENDL-3.1 nuclear data libraries. Only minor differences in neutron and gamma spectra were found in the comparison of the presented reactor cores with different fuel enrichments. One exception is the gamma spectrum in the higher energy region (above 8MeV), where more pronounced variations could be observed.

INVESTIGATION OF THE EXTENDED RANGE REM-COUNTER SMARTREM-LINUS IN REFERENCE AND WORKPLACE FIELDS EXPECTED AROUND HIGH-ENERGY ACCELERATORS.

Radiation survey instrumentation is adequate for the use around high-energy accelerators if capable to measure the dose arising from neutrons with energies ranging from thermal up to a few gigaelectronvolts. The SmartREM-LINUS is a commercial extended range rem-counter, consisting of a central (3)He-proportional counter surrounded by a spherical moderator made of borated polyethylene with an internal shield made of lead. The dose rate indicated by the SmartREM-LINUS was investigated for two different irradiation conditions. The linearity and the angular dependence of the indicated dose rate were investigated using reference neutron fields produced by (241)AmBe and (252)Cf. Additional measurements were performed in two different workplace fields with a component of neutrons with energies >20 MeV, namely the CERN-EU high-energy reference field and near the beam dump of the SwissFEL injector test facility. The measured dose rates were compared to a commercial rem-counter (WENDI2) and the results of Monte Carlo simulations.

Using a Tandem Pelletron accelerator to produce a thermal neutron beam for detector testing purposes

Active thermal neutron detectors are used in a wide range of measuring devices in medicine, industry and research. For many applications, the long-term stability of these devices is crucial, so that very well controlled neutron fields are needed to perform calibrations and repeatability tests. A way to achieve such reference neutron fields, relying on a 3MV Tandem Pelletron accelerator available at the CNA (Seville, Spain), is reported here. This paper shows thermal neutron field production and reproducibility characteristics over few days.

Experimental test of a newly developed single-moderator, multi-detector, directional neutron spectrometer in reference monochromatic fields from 144 keV to 16.5 MeV

A new directional neutron spectrometer called CYSP (CYlindrical SPectrometer) was developed within the NESCOFI@BTF (2011–2013) collaboration. The device, composed by seven active thermal neutron detectors located along the axis of a cylindrical moderator, was designed to simultaneously respond from the thermal domain up to hundreds of MeV neutrons. The new spectrometer condenses the performance of the Bonner Sphere Spectrometer in a single moderator; thus requiring only one exposure to determine the whole spectrum. The CYSP response matrix, determined with MCNP, has been experimentally evaluated with monochromatic reference neutron fields from 144keV to 16.5MeV, plus a 252Cf source, available at NPL (Teddington, UK). The results of the experiment confirmed the correctness of the response matrix within an overall uncertainty of ±2.5%. The new active spectrometer CYSP offers an innovative option for real-time monitoring of directional neutron fields as those produced in neutron beam-lines.

SPECTRUM WEIGHTED RESPONSES OF SEVERAL DETECTORS IN MIXED FIELDS OF FAST AND THERMAL NEUTRONS

The spectrum weighted responses of various detectors were calculated to provide guidance on the proper selection and use of survey instruments on the basis of their energy response characteristics on the neutron fields. To yield the spectrum weighted response, 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 (TRS-403). The reference neutron fields of 21 kinds with 2 spectra groups with different proportions of thermal and fast neutrons have been produced using neutrons from the 241Am-Be sources held in a graphite pile, a bare 241Am-Be source, and a DT neutron generator. Fluence-average energy (Eave) varied from 3.8 MeV to 16.9 MeV, and the ambient-dose-equivalent rate [H*(10)/h] varied from 0.99 to 16.5 mSv/h.

EUROMET Project 822 'Comparison of neutron fluence measurements for neutron energies of 15.5 MeV, 16 MeV, 17 MeV and 19 MeV' (EUROMET.RI(III)-S2)

Neutron reference fields in the energy range above 15 MeV are produced using the T(d,n)4He reaction and solid-state Ti(T) targets. The potential presence of non-monoenergetic neutrons from parasitic (d,n) reactions induced in the target materials makes this energy range particularly challenging for reference instruments which cannot discriminate the neutron energy, such as long counters or activation foils. Therefore, a supplementary comparison was carried out in the energy range from 15.5 MeV to 19 MeV involving reference instruments of different sensitivities to non-monoenergetic neutrons. The spectral distributions of the neutron beams were characterized by time-of-flight measurements and by Monte Carlo simulations. Main text.To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/.The final report has been peer-reviewed and approved for publication by the CCRI, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).

Review of neutron calibration facilities and monitoring techniques: new needs for emerging fields

Neutron calibration facilities and monitoring techniques have been developed since the middle of the 20th century to support research and nuclear power energy development. The technical areas needing reference neutron fields and related instruments were mainly cross section measurements, radiation protection, dosimetry and fission reactors, with energy ranging from a few millielectronvolts to about 20 MeV. The reference neutron fields and calibration techniques developed for these purposes will be presented in this paper. However, in recent years, emerging fields have brought new needs for calibration facilities and monitoring techniques. These new challenges for neutron metrology will be exposed with their technical difficulties.

Simulation of neutron production using MCNPX+MCUNED

In standard MCNPX, the production of neutrons by ions cannot be modelled efficiently. The MCUNED patch applied to MCNPX 2.7.0 allows to model the production of neutrons by light ions down to energies of a few kiloelectron volts. This is crucial for the simulation of neutron reference fields. The influence of target properties, such as the diffusion of reactive isotopes into the target backing or the effect of energy and angular straggling, can be studied efficiently. In this work, MCNPX/MCUNED calculations are compared with results obtained with the TARGET code for simulating neutron production. Furthermore, MCUNED incorporates more effective variance reduction techniques and a coincidence counting tally. This allows the simulation of a TCAP experiment being developed at PTB. In this experiment, 14.7-MeV neutrons will be produced by the reaction T(d,n)(4)He. The neutron fluence is determined by counting alpha particles, independently of the reaction cross section.

Testing a newly developed single-sphere neutron spectrometer in reference monochromatic fields from 147 keV to 14.8 MeV

A new neutron spectrometer, designed to simultaneously respond from the thermal domain up to hundreds of MeV neutrons, was designed and built in the framework of the INFN project NESCOFI@BTF. It has been called SP2 (SPherical SPectrometer) and it consists of 31 thermal neutron detectors embedded in a 25cm diameter polyethylene sphere with an internal 1cm thick lead shell. The new spectrometer shows similar performance as the Bonner sphere spectrometer, but has the notable advantage of requiring only one exposure to determine the whole spectrum. The SP2 response matrix, previously calculated with MCNP, has been experimentally evaluated with monochromatic reference neutron fields from 147keV to 14.8MeV at PTB Braunschweig. As suitable thermal neutron detectors, Dysprosium activation foils were adopted at this stage. The results of the experiment confirmed the correctness of the response matrix within an overall uncertainty of ±3%. The next phase of the NESCOFI@BTF project will be the replacement of passive detectors with active counters, thus leading to a real-time spectrometric monitor that is expected to significantly innovate the neutron control task in neutron-producing facilities, such as the beam-lines for industrial irradiation or condensed matter studies.

Experimental study for improving the angle dependence of the response of PADC-based personal neutron dosemeters

Abstract The large angle dependence of the H p (10) response in PADC-based personal neutron dosemeters constitutes a serious concern in operational radiation protection dosimetry. For planar dosemeters, the typical H p (10) response falls by half or more when the incidence angle changes from 0° to 60°. To reduce this source of systematic uncertainty, configurations based on multiple detectors at different angles have been developed, but their complex geometries constitute an important obstacle to the implementation in routine services. This works proposes a simplified configuration, based on two orthogonal PADC detectors, which is suitable for the implementation in the routine service of INFN-LNF (Frascati). This system was tested, using a ISO slab standard phantom, in the following reference neutron fields: 241 Am–Be and 252Cf(D2O) available at ENEA-Bologna, and 1.2 MeV, 5 MeV and 14.8 MeV mono-chromatic beams available at PTB Braunschweig. Incidence angles of 0°, ±15°, ±30°, ±45° and ±60° were chosen. The sum of the track density in the PADC parallel to the phantom and that in the detector normal to the phantom face, was regarded as “dosemeter reading”. On this basis the Hp(10) response was calculated for different energies and incidence angles. As expected, the angular response of the two-orthogonal-element dosemeter is highly improved with respect to that of a single planar PADC.

Reference neutron fields of the KIGAM for the neutron energy range between 144 keV and 2.5 MeV

The Korea Institute of Geoscience and Mineral Resources (KIGAM) is constructing a reference neutron field facility as a national project. Neutron fields consist of mono-energetic sources of 144 keV, 250 keV, 565 keV, and 2.5 MeV have a fluence range from 102 neutrons/cm2/sec to 103 neutrons/cm2/sec. The systems for the reference neutron fields, such as a duo-plasmatron ion source, a 4-MHz beam bunching system, a neutron chamber, an irradiation room, a neutron time-of-flight (n-TOF) system, a long-counter, and a sample moving system, were designed and fabricated. The neutron energies of the reference neutron fields and their spreads were observed by using the n-TOF system. The neutron fluence was measured by using a long-counter for energies below 1 MeV and a proton-recoil counter for 2.5 MeV. The long-counter efficiency was calibrated by the Japan Atomic Energy Agency (JAEA) which had a traceability of mono-energetic neutron sources to both Japanese and international standards. The efficiency of the proton-recoil counter was obtained by using a calculation with detailed construction information.

Photon contributions from the 252Cf and 241Am–Be neutron sources at the PSI Calibration Laboratory

Abstract At the accredited PSI Calibration Laboratory neutron reference fields traceable to the national standards of the Physikalisch-Technische Bundesanstalt (PTB) in Germany are available for the calibration of ambient and personal dose equivalent (rate) meters and passive dosimeters. The photon contribution to the ambient dose equivalent in the neutron fields of the 252Cf and 241Am–Be sources was measured using various photon dose rate meters and active and passive dosimeters. Measuring photons from a neutron source usually involves considerable uncertainties due to the presence of neutron induced photons in the room, due to a non-zero neutron sensitivity of the photon detector, and last but not least due to the energy response of the photon detectors. Therefore eight independent detectors and methods were used to obtain a reliable estimate for the photon contribution of the two sources as an average of the individual methods. For the 241Am–Be source a photon contribution of approximately 4.9% was determined and for the 252Cf source a contribution of 3.6%.

Investigation of the stability of commercial neutron probes

At the Paul Scherrer Institute's Calibration Laboratory, neutron reference fields are provided for the calibration of ambient and personal dose equivalent (rate) metres and passive dosemeters. To ensure traceability to the standards of the Physikalisch-Technische Bundesanstalt (PTB) in Germany, the neutron fields are characterised by means of a PTB-calibrated Berthold LB6411 neutron probe which is used as a secondary standard. The LB6411 detector suffers from an unstable, increasing dose rate reading in the order of up to +5 % (according to the manufacturers, this is due to a charging effect in the (3)He proportional counter). In a calibration, this instability is usually corrected for based on the reading obtained with a test source. In this work, the instability was investigated by means of measurements under irradiation with ambient dose equivalent rates up to 24 mSv h(-1) for up to 20 h and compared with the behaviour of an LB6419 and a Thermo Wendi-2 probe. The reading of the instruments was found to reach a plateau, e.g. it becomes stable after ∼90 min during irradiation with 10 mSv h(-1) neutrons. The plateau is reached faster for higher dose rates. This supports the interpretation as a charging effect in the proportional counter. The effect could also be duplicated in an irradiation with photons from a (137)Cs source. The decay time of the accumulated charge was found to be very long, i.e. the instrument showed a stable reproducible reading for up to 6 h after the plateau was reached. From these observations, a conditioning procedure was derived which ensures a stable operation of the instrument after an irradiation of the instrument preceding its use in the reference measurements.

Calibration of PADC-based neutron area dosemeters in the neutron field produced in the treatment room of a medical LINAC

PADC-based nuclear track detectors have been widely used as convenient ambient dosemeters in many working places. However, due to the large energy dependence of their response in terms of ambient dose equivalent (H * (10)) and to the diversity of workplace fields in terms of energy distribution, the appro- priate calibration of these dosemeters is a delicate task. These are among the reasons why ISO has introduced the 12789 Series of Standards, where the simulated workplace neutron fields are introduced and their use to calibrate neutron dosemeters is recommended. This approach was applied in the present work to the UAB PADC-based nuclear track detectors. As a suitable workplace, the treatment room of a 15 MV Varian CLINAC DHX medical accelerator, located in the Ospedale S. Chiara (Pisa), was chosen. Here the neutron spectra in two points of tests (1.5 m and 2 m from the isocenter) were determined with the INFN-LNF Bonner Sphere Spectrometer equipped with Dysprosium activation foils (Dy-BSS), and the values of H *(10) were derived on this basis. The PADC dosemeters were exposed in these points. Their workplace specific H*(10) responses were determined and compared with those previously obtained in different simulated workplace or reference (ISO 8529) neutron fields.

Updating and Extending the IRDF-2002 Dosimetry Library

Abstract The International Reactor Dosimetry File (IRDF)-2002 released in 2004 by the IAEA (see http://www-nds.iaea.org/irdf2002/) contains cross-section data and corresponding uncertainties for 66 dosimetry reactions. New cross-section evaluations have become available recently that re-define some of these dosimetry reactions including: (1) high-fidelity evaluation work undertaken by one of the authors (KIZ); (2) evaluations from the US ENDF/B-VII.0 and candidate evaluations from the US ENDF/B-VII.1 libraries that cover reactions within the International Evaluation of Neutron Cross-Section Standards; (3) European JEFF3.1 library; and (4) Japanese JENDL-4.0 library. Additional high-threshold reactions not included in IRDF-2002 (e.g.,59Co(n,3n) and 209Bi(n,3n)) have been also evaluated to characterize higher-energy neutron fields. Overall, 37 new evaluations of dosimetry reactions have been assessed and intercomparisons made with integral measurements in reference neutron fields to determine whether they should be adopted to update and improve IRDF-2002. Benchmark calculations performed for newly evaluated reactions using the ENDF/B-VII.0 235U thermal fission and 252Cf spontaneous fission neutron spectra show that calculated integral cross sections exhibit improved agreement with evaluated experimental data when compared with the equivalent data from the IRDF-2002 library. Data inconsistencies or deficiencies of new evaluations have been identified for 63Cu(n,2n), 60Ni(n,p)60m+g Co, 55Mn(n,γ), and 232Th(n,f) reactions. Compared with IRDF-2002, the upper neutron energy boundary was formally increased from the actual maximum energy of typically 20 MeV up to 60 MeV by using the TENDL-2010 cross sections and covariance matrices. This extension would allow the updated IRDF library to be also used in fusion dosimetry applications. Uncertainties in the cross sections for all new evaluations are given in the form of relative covariance matrices. Newly evaluated excitation functions should be considered as suitable candidates in the preparation of an improved version of the IRDF that was planned to be released for testing in December 2011.

An investigation into the sensitivity of various albedo neutron dosimeters aimed at correcting the readings

The results of an experimental determination of the sensitivity of three types of individual neutron albedo dosimeters in neutron reference fields on the basis of radionuclide sources and at the top concrete shielding of the U-70 accelerator are presented. The results show that the ratios between the responses of the albedo dosimeters designed earlier at the Joint Institute for Nuclear Research (the albedo dosimeter (AD) and the multicomponent dosimeter (MD)) and the currently used DVGN-01 dosimeter are constant within 25% in a wide range of neutron energy. This fact makes it possible to use the results of measuring the AD and MD responses obtained earlier in neutron fields of nuclear-physical installations at the Joint Institute for Nuclear Research (JINR) for the correction of DVGN-01 dosimeter measurement results to apply it to personal radiation monitoring (PRM) at these installations. The correction factors for DVGN-01 measurement results are found and recommended to be used in PRM for most JINR installations.

Production and measurement of neutron reference fields : the AMANDE facility and MIMAC prototype as neutron detection system

The AMANDE facility produces mono-energetic neutron fields from 2 keV to 20 MeV with a metrological quality (Gressier et al. 2004). To be considered as a standard facility, fluence and energy distributions of neutron fields have to be mastered, i.e. measured by primary detector standards. For this purpose, a new gaseous Time Projection Chamber (μTPC) is being developed within the MIMAC project devoted to directional Dark Matter search (Billard et al. 2010). The μTPC is dedicated to measure neutron fields from 2 keV up to 1 MeV. The measurement strategy requires track reconstruction of recoiling nuclei down to a few keV, which can be achieved with low pressure gaseous detectors. Hence the energy must be measured precisely and the track of the recoiling nucleus must be 3D reconstructed. This is achieved with a low pressure micropattern gaseous detector (a pixelized bulk micromegas) equipped with a self triggered electronics able to perform the anode sampling at a 50 MHz frequency. Methods to measure fluence and neutron energy with the TPC have been described.

Monoenergetic fast neutron reference fields: II. Field characterization

Monoenergetic neutron reference fields are required for the calibration of neutron detectors and dosemeters for various applications ranging from nuclear physics and nuclear data measurements to radiation protection. In a series of two separate publications the metrological aspects of the production and measurement of fast neutrons are reviewed. In the first part, requirements for the nuclear reactions used to produce neutron fields as well as methods for target characterization and the general layout of reference facilities were discussed. This second part focuses on the most important techniques for field characterization and includes the determination of the neutron fluence as well as the spectral neutron distribution and the determination of the fluence of contaminating photons. The measurements are usually carried out relative to reference cross sections which are reviewed in a separate contribution, but for certain conditions ‘absolute’ methods for neutron measurements can be used which are directly traceable to the international system of units (SI).

Monoenergetic fast neutron reference fields: I. Neutron production

Monoenergetic neutron reference fields are required for the investigation of the energy-dependent response and the calibration of neutron detectors and dosemeters for various applications ranging from nuclear physics and nuclear data measurements to radiation protection. In a series of two separate publications the metrological aspects of the production and the measurement of fast neutrons are reviewed. In this first part, requirements for the nuclear reaction used to produce the neutron field as well as methods for target characterization and the general layout of reference facilities are discussed. The second part will focus on the most important techniques for field characterization. This includes the determination of the neutron fluence as well as the spectral neutron distribution and the determination of the fluence of contaminating photons.