Measurement of the fluence response of the GSI neutron ball in high-energy neutron fields produced by 500 AMeV and 800 AMeV deuterons

Experiments were performed in Cave C of GSI (Gesellschaft für Schwerionenforschung) using the LAND (Large Area Neutron Detector) in combination with the deflection magnet ALADIN (A LArge DIpol magNet) in front of the LAND where charged particles and neutrons can be separated. This arrangement is used to create high-energetic neutron fields by irradiation of a thick lead target (5 cm) with deuteron beams with the energies of 500 or 800 MeV per nucleon. In break-up reactions the neutron is separated from the proton which is deflected in the magnetic field of the ALADIN. The produced neutron radiation, which has a pronounced peak at the nucleon energy, is used to measure the fluence response of the GSI neutron ball. A thermoluminescence (TL) based spherical neutron dosemeter was developed for the area monitoring for the quantity H(10) at high-energy accelerators. In the same experiment, the spectral neutron fluence Phi(E) is measured with the LAND in the energy range from 100 MeV to 1 GeV. The measured fluence responses are compared with results of FLUKA calculations and the corresponding fluence-to-dose conversion coefficients. The measured dosemeter responses are too high in comparison to the calculated ones (up to approximately 50%), the dosemeter reading gives dose values which are too high by a factor of 1.1-2.2 related to the corresponding fluence-to-dose conversion factors.     

Neutron dosimetry with TL albedo dosemeters at high energy accelerators

The GSF-Personal Monitoring Service uses the TLD albedo dosemeter as standard neutron personal dosemeter. Due to its low sensitivity for fast neutrons however, it is generally not recommended for workplaces at high-energy accelerators. Test measurements with the albedo dosemeter were performed at the accelerator laboratories of GSI in Darmstadt and DESY in Hamburg to reconsider this hypothesis. It revealed that the albedo dosemeter can also be used as personal dosemeter at these workplaces, because at all measurement locations a significant part of neutrons with lower energies could be found, which were produced by scattering at walls or the ground.     

Estimation of bremsstrahlung photon energy in the environment of high-energy electron accelerator using CaSO4:Dy based TL dosemeter

High-energy bremsstrahlung X rays constitute the major radiation hazard to working personnel around the high-energy electron accelerators. Thermoluminescent (TL) dosemeter system based on CaSO4:Dy Teflon disc used in the routine individual monitoring was used to estimate the bremsstruhlung photon energy at different locations of the experimental hall of 450-MeV synchrotron accelerator. The response of TL discs under different filter regions of the dosemeter system undergo change with photon energy due to the lack of build up and interaction of photon in the metal filters. This change in the response of the discs used to estimate the energy of the bremsstrauhlung photon in conjunction with suitable calibration curve generated using known photon energy from medical linear accelerator. The photon energies estimated were in the range 1-4 MeV, depending on the locations.     

Neutron dose measurements with the GSI ball at high-energy accelerators

A moderator-type neutron monitor containing pairs of TLD 600/700 elements (Harshaw) modified with the addition of a lead layer (GSI ball) for the measurement of the ambient dose equivalent from neutrons at medium- and high-energy accelerators, is introduced in this work. Measurements were performed with the Gesellschaft für Schwerionenforschung (GSI) ball as well as with conventional polyethylene (PE) spheres at the high-energy accelerator SPS at European Organization for Nuclear Research [CERN (CERF)] and in Cave A of the heavy-ion synchrotron SIS at GSI. The measured dose values are compared with dose values derived from calculated neutron spectra folded with dose conversion coefficients. The estimated reading of the spheres calculated by means of the response functions and the neutron spectra is also included in the comparison. The analysis of the measurements shows that the PE/Pb sphere gives an improved estimate on the ambient dose equivalent of the neutron radiation transmitted through shielding of medium- and high-energy accelerators.     

High-energy response of passive dosemeters in use at LANL

The high-energy neutron response of three passive dosemeters in use at the Los Alamos National Laboratory (LANL) has been investigated using metrology-grade fields. The dosemeters include the LANL Model 8823 TLD badge and the LANL PN3 track etch device. Both are dosemeters of record at LANL. The third device was the Personal Neutron Dosemeter (PND), a superheated emulsion device, manufactured by Bubble Technology Industries, Inc. (BTI). The response of the three dosemeters at neutron energies exceeding 10 MeV was assessed with monoenergetic neutrons at the Physikalisch-Technische Bundesanstalt facility (14.8 and 19 MeV). For the sake of completeness, data collected at lower energies are also included in this study. High-energy quasi-monoenergetic beams produced by the cyclotron facilities at the Université Catholique de Louvain (UCL) and the The Svedberg Laboratory (TSL) were also utilised as part of this study. These measurements were made to better understand and help interpret dosemeter readings obtained by workers at high-energy accelerators, such as the 800 MeV spallation neutron source facility located at the Los Alamos Neutron Science Center (LANSCE).     

Measurement of the spectral fluence rate of reference neutron sources with a liquid scintillation detector

Reference neutron sources such as (241)AmBe(alpha,n) and (252)Cf are commonly used to calibrate neutron detectors for radiation protection purposes. The calibration factors of these detectors depend on the spectral distribution of the neutron fluence from the source. Differences between the spectral fluence of the neutron source and the ISO-recommended reference spectra might be caused by the properties of the individual source. The spectral neutron fluence rates of different reference neutron sources used at PTB were measured with a liquid scintillation detector (NE213), using maximum entropy unfolding and a new, experimentally determined detector response matrix. The detector response matrix was determined by means of the time-of-flight technique at a pulsed neutron source with a broad energy distribution realised at the PTB accelerator facility. The results of the measurements of the reference sources are compared with the ISO-recommended reference spectra. For the PTB (241)AmBe(alpha,n) reference source, the spectral neutron fluence was determined by means of a high-resolution (3)He semiconductor sandwich spectrometer in 1982. These measurements were the basis for the ISO recommendations. The current measurements confirm the high-energy part (E(n) > 2 MeV) of this spectrum and demonstrate the suitability of this new method for high-resolution spectrometry of broad neutron spectra.     

Electronic personal neutron dosemeters for high energies: measurements, new developments and further needs

The results of measurements with neutron energies up to 60 MeV are shown for the personal neutron dosemeters Thermo Electron EPD-N2, ALOKA PDM-313 and the PTB prototype dosemeter DOS-2002. All dosemeters show dose equivalent responses that are about a factor of 10, too high at 60 MeV. A new prototype dosemeter-called DOS-2005-consisting of a detector with a thin effective layer of 6 microm has been set up at PTB. The dose equivalent response of this dosemeter and that of the newly developed dosemeter SAPHYDOSE-N was measured up to 19 MeV. Both dosemeters indicate a more flat response at high neutron energies. Further needs-optimisations, measurements and calculations-for use at high-energy accelerators and in space are discussed.     

Prediction analysis of dose equivalent responses of neutron dosemeters used at a MOX fuel facility

To predict how accurately neutron dosemeters can measure the neutron dose equivalent (rate) in MOX fuel fabrication facility work environments, the dose equivalent responses of neutron dosemeters were calculated by the spectral folding method. The dosemeters selected included two types of personal dosemeter, namely a thermoluminescent albedo neutron dosemeter and an electronic neutron dosemeter, three moderator-based neutron survey meters, and one special instrument called an H(p)(10) monitor. The calculations revealed the energy dependences of the responses expected within the entire range of neutron spectral variations observed in neutron fields at workplaces.     

Neutron dosimetry with 6LiF-rich TL sheet

A (6)LiF-rich thermoluminescent sheet-type dosemeter ((6)LiF-rich NTL sheet) was developed for neutron 2D dosimetry. The dosemeter utilises the (6)Li(n, alpha)(3)H reaction to detect thermal neutrons. Responses of the (6)LiF-rich NTL sheet to neutrons were measured at the neutron beam irradiation facility for BNCT in JRR-4 Research Reactor at the Japan Atomic Energy Research Institute. Placement of a multi-leaf collimator at the output port of the neutron (beam) irradiation facility, produced either stripe- or round-shaped neutron distributions; the spatial distribution was measured using the developed NTL sheets. Direct measurements of neutron attenuation in water were also carried out using the developed NTL sheet, submersed in a water phantom. In each experiment, NTL sheets having natural abundance (7.9%) of LiF, and (6)LiF-enriched NTL (18.94%) sheet were irradiated under the same conditions. The ratio of thermoluminescence intensities of the (6)LiF-rich NTL sheet to that of the normal NTL sheet was compared to a theoretically calculated value. The experimental measurements are shown to be in good agreement with the calculations.     

Construction of 144, 565 keV and 5.0 MeV monoenergetic neutron calibration fields at JAERI

Monoenergetic neutron calibration fields of 144, 565 keV and 5.0 MeV have been developed at the Facility of Radiation Standards of JAERI using a 4 MV Pelletron accelerator. The 7Li(p,n)7Be and 2H(d,n)3He reactions are employed for neutron production. The neutron energy was measured by the time-of-flight method with a liquid scintillation detector and calculated with the MCNP-ANT code. A long counter is employed as a neutron monitor because of the flat response. The monitor is set up where the influence of inscattered neutrons from devices and their supporting materials at a calibration point is as small as possible. The calibration coefficients from the monitor counts to the neutron fluence at a calibration point were obtained from the reference fluence measured with the transfer instrument of the primary standard laboratory (AIST), a 24.13 cm phi Bonner sphere counter. The traceability of the fields to AIST was established through the calibration.     

Neutron dose distribution at the GSI fragment separator

GSI is operating a facility for the production of rare isotopes. Nuclei are produced by fragmentation or fission of the impinging heavy ions with energies of approximately 1 GeV per nucleon. The major part of the primary beam and the produced nuclei is deposited in the components of the Fragment Separator (FRS) and generates neutron radiation. Thermoluminescence dosemeters (TLDs) (6LiF/7LiF pairs in PE spheres) were exposed in neutron fields produced by uranium beams with energies between 100 and 1000 MeV per nucleon during an irradiation period in the year 2002. Two-dimensional dose distributions are obtained using these TL measurements in combination with model calculations. The applied model describes the dose distribution as a superposition of dose patterns of 20 single sources equally distributed along the FRS. The single source distribution is based on a measured double differential neutron distribution for a 1 GeV per nucleon uranium beam.     

Two types of multi-moderator neutron spectrometers: Gamma-ray insensitive type and high-efficiency type

Two types of multi-moderator neutron spectrometers were developed; one is a gamma-ray insensitive type, and the other is a high-efficiency type. An indium activation detector is loaded in the former spectrometer, which can measure the photon-dominant pulsed neutron field such as in the primary photon beam of a high-energy medical electron accelerator. The latter, in which a ³He counter is loaded, is so sensitive that it can measure leakage neutrons from a well shielded facility or even the skyshine neutrons. The response functions of the spectrometers were measured by thermal and mono-energetic neutron standard fields, and were also calculated by the one-dimensional discrete ordinates transport code, ANISN. The measured and calculated responses showed generally good agreement. A benchmark measurement of ²⁵²Cf fission neutrons by using these two spectrometers agreed well with the calculated spectrum. The spectrometers were used in the measurements of neutrons produced by a medical electron accelerator and of skyshine neutrons from an intense 14 MeV neutron source facility.     

Development of the fast neutron standard using a Be({alpha},n) reaction at the National Metrology Institute of Japan

This paper describes the 8-MeV neutron field where the neutrons are generated in the (9)Be(alpha,n)(12)C reaction by bombardment of a beryllium target with a 2.4-MeV (4)He(+) beam from a Van de Graaff accelerator. The neutron field is being prepared for a new national standard on neutron fluence in Japan. Absolute measurement of the neutron fluence was taken using a proton recoil neutron detector, consisting of a silicon surface barrier detector with a polyethylene radiator. Neutron spectra were measured using a newly developed recoil proton spectrometer and a liquid organic scintillation detector. The gamma rays existing in the field were also characterised using a liquid organic scintillation detector. The ambient dose equivalents of the gamma rays were estimated to be <100 microSv at the neutron fluence of 10(7) neutrons cm(-2).     

Construction of monoenergetic neutron calibration fields using 45Sc(p, n)45Ti reaction at JAEA

The 8 and 27 keV monoenergetic neutron calibration fields have been developed by using (45)Sc(p, n)(45)Ti reaction. Protons from a 4-MV Pelletron accelerator are used to bombard a thin scandium target evaporated onto a platinum disc. The proton energies are finely adjusted to the resonance to generate the 8 and 27 keV neutrons by applying a high voltage to the target assemblies. The neutron energies were measured using the time-of-flight method with a lithium glass scintillation detector. The neutron fluences at a calibration point located at 50 cm from the target were evaluated using Bonner spheres. A long counter was placed at 2.2 m from the target and at 60 degrees to the direction of the proton beam in order to monitor the fluence at the calibration point. Fluence and dose equivalent rates at the calibration point are sufficient to calibrate many types of the neutron survey metres.     

Response of neutron detectors to high-energy mixed radiation fields

Radiation protection around CERN's high-energy accelerators represents a major challenge due to the presence of complex, mixed radiation fields. Behind thick shielding neutrons dominate and their energy ranges from fractions of eV to about 1 GeV. In this work the response of various portable detectors sensitive to neutrons was studied at CERN's High-Energy Reference Field Facility (CERF). The measurements were carried out with conventional rem counters, which usually cover neutron energies up to 20 MeV, the Thermo WENDI-2, which is specified to measure neutrons up to several GeV, and a tissue-equivalent proportional counter. The experimentally determined neutron dose equivalent results were compared with Monte Carlo (MC) simulations. Based on these studies field calibration factors can be determined, which result in a more reliable estimate of H(*)(10) in an unknown, but presumably similar high-energy field around an accelerator than a calibration factor determined in a radiation field of a reference neutron source.     

Calibration of personal dosemeters in mixed neutron-photon fields: some problems and their solution

In neutron reference radiation fields, the conventional true value of the personal dose equivalent, H(p)(10), is derived from the spectral neutron fluence and recommended conversion coefficients. This procedure requires the phantom on which the personal dosemeter is mounted to be irradiated with a broad and parallel beam. In many practical situations, the change of the neutron fluence and/or the energy distribution over the surface of the phantom may not be neglected. For a selection of typical irradiation conditions in neutron reference radiation fields, the influence of this effect has been analysed using numerical methods. A further problem, which is of relevance for the calibration of dosemeters measuring both the neutron and the photon component of mixed fields, is the 'double counting' of the dose equivalent due to neutron-induced photons. The relevance of this conceptual problem for calibrations in mixed-field dosimetry was analysed.     

Neutron measurements in a Varian 2,100C LINAC facility using a Bonner sphere system based on passive gold activation detectors

The use of high-energy linear electron accelerators (LINACs) for medical cancer treatments is widespread on an international scale. The associated bremsstrahlung X rays may produce neutrons as a result of subsequent photonuclear reactions with the different materials constituting the accelerator head. The generated neutron field is highly variable and depends strongly on the beam energy, on the accelerator shielding, on the flattering filter as well as on the movable collimators (jaws) design and on the irradiation field geometry. An estimate of this photoneutron component is, thus, of practical interest to quantify the radiological risk for the working staff and patients. Due to high frequency electromagnetic fields, and also to the presence of abundant leaked and scattered photons in these installations, measurements of the corresponding neutron fields by active dosemeters are extremely difficult. A modified version of the Bonner sphere system, based on passive gold activation detectors, has been used to perform neutron measurements at two points in a Varian 2,100C LINAC facility. A home-made unfolding procedure (CDM) has been utilised to determine the neutron spectra present at the measurement points. Results indicate that the giant dipole resonance process is the most adequate model to explain neutron production in the LINAC and that a thermal component is present at the measurement points.     

Field calibration studies for ionisation chambers in mixed high-energy radiation fields

The monitoring of ambient doses at work places around high-energy accelerators is a challenging task due the complexity of the mixed stray radiation fields encountered. At CERN, mainly Centronics IG5 high-pressure ionisation chambers are used to monitor radiation exposure in mixed fields. The monitors are calibrated in the operational quantity ambient dose equivalent H*(10) using standard, source-generated photon- and neutron fields. However, the relationship between ionisation chamber reading and ambient dose equivalent in a mixed high-energy radiation field can only be assessed if the spectral response to every component and the field composition is known. Therefore, comprehensive studies were performed at the CERN-EU high-energy reference field facility where the spectral fluence for each particle type has been assessed with Monte Carlo simulations. Moreover, studies have been performed in an accessible controlled radiation area in the vicinity of a beam loss point of CERN's proton synchrotron. The comparison of measurements and calculations has shown reasonable agreement for most exposure conditions. The results indicate that conventionally calibrated ionisation chambers can give satisfactory response in terms of ambient dose equivalent in stray radiation fields at high-energy accelerators in many cases. These studies are one step towards establishing a method of 'field calibration' of radiation protection instruments in which Monte Carlo simulations will be used to establish a correct correlation between the response of specific detectors to a given high-energy radiation field.     

High-level dosimetry at the demagnetization experiments of permanent magnets

The measurements of high-energy and high dose mixed radiation from high-energy electron accelerator are carried out using a radiation damage monitor. It consists of two Radiation-Sensing Field-Effect Transistors (RADFETs) for total absorbed dose from mainly gamma ray and other charged particles and a Si PIN diode for neutron fluence. This is a part of the demagnetization study of rare earth permanent magnet irradiated by 2.5-GeV electron beam. The sensitivities of damage detectors are measured using 65-MeV quasi-monoenergic neutron, 14-MeV D-T neutron, (252)Cf neutron for Si PIN diode and (60)Co and (137)Cs gamma ray for RADFETs. Measured sensitivities are in acceptable range in the comparison of producer's proposed values. The dose and fluence measurements are carried out for the same target condition, Cu and Ta, as that for the demagnetization study. The 5 x 5 cm(2) cross-sectional and 5.5-cm-thick Pb target is also used for the general comparison with photoneutron yields. All measured dose and fluence are compared with the calculated results using the FLUKA code and agree well each other. The application of this kind of radiation damage monitor to high-level dosimetry at high-energy electron accelerator has been discussed.     

Characterization of Bonner sphere systems at monoenergetic and thermal neutron fields

The Institute for Radiological Protection and Nuclear Safety (IRSN) and the GFR, Universitat Autónoma de Barcelona (UAB) use Bonner spheres (BS) for neutron spectrometry at workplaces. The two systems, equipped with similar cylindrical 3He proportional counters, were simulated with the MCNP Monte-Carlo code to determine the response to neutrons of different energies for each polyethylene sphere. The BS systems were characterized at monoenergetic and thermal neutron fields. Measurements were performed at the Physikalisch-Technische Bundesanstalt (PTB) and at the National Physical Laboratory (NPL) standard laboratories, and with the newly characterized IRSN 'SIGMA' thermal neutron facility. The energy distribution of the reference neutron fluence was folded with the response functions for comparison purposes with the experimental data. In almost all cases related to monoenergetic neutrons, a good agreement between the experimental and the calculated count rates was found, and some discrepancies of a few per cent were observed in the thermal region.