Neutron Dosimeter Research Articles

SINGLE ANODE TRIPLE GEM TISSUE EQUIVALENT PROPORTIONAL COUNTER AS THE BASIS FOR A PERSONAL NEUTRON DOSIMETER.

This paper reports on a tissue-equivalent proportional counter (TEPC) based on a triple gas electron multiplier structure, with a single pad readout, as a basis for a personal neutron dosimeter. Its dosimetric response was studied using the 252Cf neutron source at the Health Physics Generator Facility of the Canadian Nuclear Laboratories. Measured lineal energy spectra were found to be in agreement with numerical simulations performed with Monte Carlo N-Particle eXtended (MCNPX). Both simulations and measurements showed that the mean pathlength of secondary charged particles in the TEPC gas was best represented by the thickness of the drift region of the device. It was determined that the Cauchy Theorem, used to calculate the mean chord length in spherical and cylindrical TEPCs, overestimated the simulated mean chord length by nearly a factor of two. Important operational characteristics of the device were investigated, including gas gain, sensitivity and dosimetric response, as functions of tissue-equivalent gas pressure. This work demonstrates that the proposed design can serve as the basis for a personal neutron dosimeter device, which would satisfy the angular dosimetric response criteria of the personal dosimeter standard IEC61526.

Secondary radiation doses in IMRT techniques

Introduction The probability for developing the secondary cancer related to low peripheral doses during teleradiotherapy is nowadays of increasing interest. Among the components of the secondary dose photoneutrons and activated radionuclides should be discussed. Purpose Investigation of the secondary neutron and induced gamma radiation doses during intensity modulated radiation therapy employing Step&Shoot, sliding window and VMAT techniques. Materials and methods Elekta Synergy linacs with Agility MLC, working in 10 MV and 15 MV modes were used. Six plans with a 15 Gy fractional dose to PTV in mediastinum were studied and realized at anthropomorphic male phantom. Neutron flux was measured by the activation of indium 115 In foils. Neutron dosemeter LB 123N was also used. The induced gamma-activity of linac head was investigated using high-purity germanium spectrometer. Doses were assessed using fluence-to dose conversion coefficients. Results The dominant activation dose comes from 28 Al, 56 Mn and 187 W isotopes and may reach 2 μSv/h 10 cm from the linac head. Neutron flux densities are of the order of 10 3 –10 4 cm −2 s −1 for slow and 10 4 –10 5 cm −2 s −1 for fast neutrons. Additional doses connected with neutrons are ∼0.5 mSv/MU for 10 MV beam and within the range of 0.8–1.7 mSv/MU for 15 MV beam. Variation of neutron doses across the patient body is within 13%, for studied plans. Conclusion The neutron flux can be even 3.5 times lower whereas gamma-ray doses 5 times lower for 10 MV than for 15 MV beams in IMRT techniques. Disclosure No conflict of interest.

Monte Carlo based investigation of a universal two-component albedo neutron dosimeter in a deep geological disposal system for high-level nuclear waste

Albedo neutron dosimeter utilizes the property of the human body to moderate incident high energy neutrons, which are scattered and reflected as thermal and low energy neutrons that can be measured by thermoluminescence detector (TLD) with high sensitivity. The current study is devoted to the assessment of possible utilization of the albedo neutron dosimeter as a personal monitoring device in a deep geological disposal system for high-level nuclear waste. When a nuclear waste package is transported to its final disposal position in a deep geological repository, such as in a horizontal emplacement drift in a rock salt mine, the question about the appropriate calibration factor of the albedo neutron dosimeter arises. Neutrons backscattered by the surrounding host rock layers might contribute significantly to the composition of the radiation field and in turn influence the response of the neutron detector. To account for different radiation field compositions, a field calibration technique providing field-specific calibration factors was applied in the current study. The field calibration of an albedo neutron dosimeter in a deep geological repository was performed with Monte Carlo method. The radiation field was simulated by a shielding cask loaded with a 252Cf neutron source that is placed in a horizontal drift of a rock salt mine. Calibration factors were calculated at various distances to the shielding cask. In the investigated case, the neutron radiation field in a geological repository can be assigned to the calibration factor N1, which is recommended for reactors and accelerators with heavy shielding.

Albedo Neutron Dosimetry in a Deep Geological Disposal Repository for High-Level Nuclear Waste.

Albedo neutron dosemeter is the German official personal neutron dosemeter in mixed radiation fields where neutrons contribute to personal dose. In deep geological repositories for high-level nuclear waste, where neutrons can dominate the radiation field, it is of interest to investigate the performance of albedo neutron dosemeter in such facilities. In this study, the deep geological repository is represented by a shielding cask loaded with spent nuclear fuel placed inside a rock salt emplacement drift. Due to the backscattering of neutrons in the drift, issues concerning calibration of the dosemeter arise. Field-specific calibration of the albedo neutron dosemeter was hence performed with Monte Carlo simulations. In order to assess the applicability of the albedo neutron dosemeter in a deep geological repository over a long time scale, spent nuclear fuel with different ages of 50, 100 and 500 years were investigated. It was found out, that the neutron radiation field in a deep geological repository can be assigned to the application area 'N1' of the albedo neutron dosemeter, which is typical in reactors and accelerators with heavy shielding.

Cosmogenic effects on Cu isotopes in IVB iron meteorites

We measured Cu isotope compositions of 12 out of the 14 known IVB iron meteorites. Our results show that IVB iron meteorites display a very large range of δ65Cu values (−5.84‰<δ65Cu<−0.24‰; defined as per mil deviation of the 65Cu/63Cu ratio from the NIST-976 standard). These Cu isotopic data display clear correlations with W, Pt, and Os isotope ratios, which are very sensitive to secondary neutron capture due to galactic cosmic ray (GCR) irradiation. This demonstrates that δ65Cu in IVB irons is majorly modified by neutron capture by the reaction 62Ni(n,γ)63Ni followed by beta decay to 63Cu. Using correlations with Pt and Os neutron dosimeters, we calculated a pre-exposure δ65Cu of −0.3±0.8‰ (95% conf.) of IVB irons that agrees well with the Cu isotopic compositions of other iron meteorite groups and falls within the range of chondrites. This shows that the volatile depletion of the IVB parent body is not due to evaporation that should have enriched IVB irons in the heavy Cu isotopes.

Use of borated polyethylene to improve low energy response of a prompt gamma based neutron dosimeter

The feasibility of using a combined sample of borated polyethylene and normal polyethylene to estimate neutron ambient dose equivalent from measured prompt gamma emissions is investigated theoretically to demonstrate improvements in low energy neutron dose response compared to only polyethylene. Monte Carlo simulations have been carried out using the FLUKA code to calculate the response of boron, hydrogen and carbon prompt gamma emissions to mono energetic neutrons. The weighted least square method is employed to arrive at the best linear combination of these responses that approximates the ICRP fluence to dose conversion coefficients well in the energy range of 10−8MeV to 14MeV. The configuration of the combined system is optimized through FLUKA simulations. The proposed method is validated theoretically with five different workplace neutron spectra with satisfactory outcome.

USE OF A SIMPLE THERMALISED NEUTRON FIELD FOR QUALITY ACCEPTANCE OF WHOLE BODY TLDS.

The individual monitoring service of Public Health England (PHE) uses Harshaw™ whole-body and extremity thermoluminescent dosemeters (TLDs) with high-sensitivity lithium fluoride LiF:Mg,Cu,P, together with Harshaw 8800™ automated readers. The neutron-insensitive, (6)Li-depleted variety of TLD material is used by PHE because the service provides separate neutron and photon dosemeters. The neutron dosemeters are not sensitive to photons and vice versa Since insensitivity to neutrons is a supply requirement for TLDs, there is a need to test every new (annual) consignment for this. Because it is thermal neutrons that produce a response in (6)Li TLDs, a thermal field is needed. To this end, PHE has adopted the simple approach of sandwiching the TLDs between two ISO water-filled slab phantoms. In this arrangement, the fast neutrons from an Am-Be source are effectively thermalised. Details of the method are given, together with the results of supporting MCNP calculations and some typical results.

Performance of Large Neutron Detectors Containing Lithium-Gadolinium-Borate Scintillator

This paper describes the development and testing of a neutron counter, spectrometer, and dosimeter that is compact, efficient, and accurate. The 1.3 liter detector head for this instrument is a composite detector with an organic scintillator containing uniformly distributed <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> Li <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">nat</sup> Gd <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</sup> B <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">9</sub> : Ce (LGB:Ce) microcrystals. Moderating neutrons that have slowed sufficiently in the plastic, capture in one of the Lithium-6, Boron-10, or Gadolinium-157/155 atoms in the LGB:Ce scintillator, which then releases the capture energy in a characteristic cerium emission pulse. The measured captured pulses indicate the presence of neutrons. When a scintillating fluor is present in the plastic, the light pulse resulting from the neutron moderating in the plastic is paired with the LGB:Ce capture pulse to identify the energy of the neutron. This data was used to assist in the unfolding of the neutron spectra. The unfolded spectra were then validated with known spectra, at both neutron emitting isotopes and fission/accelerator facilities, Los Alamos Neutron Science Center (LANSCE), Edwards Accelerator Laboratory (EAL) at Ohio University. The instrument can measure neutrons and their spectra over the range between 0.8 MeV and 150 MeV with an average uncertainty in neutron rate of only ±8%.

Changes to Irradiation Conditions of VVER-1000 Surveillance Specimens Resulting from Fuel Assemblies with Greater Fuel Height

The goal of the work was to obtain experimental data on the influence of newtype fuel assemblies with higher fuel rods on the irradiation conditions of surveillance specimens installed on the baffe of VVER-1000. For this purpose, two surveillance sets with container assemblies of the same design irradiated in reactors with different fuel assemblies in the core were investigated. Measurements of neutron dosimeters from these sets and retrospective measurements of 54 Mn activity accumulated in each irradiated specimen allow a detailed distribution of the fast neutron flux in the containers to be obtained. Neutron calculations have been done using 3D discrete ordinate code KATRIN. On the basis of the obtained results, a change of the lead factor due to newtype fuel assemblies was evaluated for all types of VVER-1000 container assemblies.

Experimental evaluation of neutron dose in radiotherapy patients: Which dose?

The evaluation of peripheral dose has become a relevant issue recently, in particular, the contribution of secondary neutrons. However, after the revision of the Recommendations of the International Commission on Radiological Protection, there has been a lack of experimental procedure for its evaluation. Specifically, the problem comes from the replacement of organ dose equivalent by the organ-equivalent dose, being the latter "immeasurable" by definition. Therefore, dose equivalent has to be still used although it needs the calculation of the radiation quality factor Q, which depends on the unrestricted linear energy transfer, for the specific neutron irradiation conditions. On the other hand, equivalent dose is computed through the radiation weighting factor wR, which can be easily calculated using the continuous function provided by the recommendations. The aim of the paper is to compare the dose equivalent evaluated following the definition, that is, using Q, with the values obtained by replacing the quality factor with wR. Dose equivalents were estimated in selected points inside a phantom. Two types of medical environments were chosen for the irradiations: a photon- and a proton-therapy facility. For the estimation of dose equivalent, a poly-allyl-diglicol-carbonate-based neutron dosimeter was used for neutron fluence measurements and, additionally, Monte Carlo simulations were performed to obtain the energy spectrum of the fluence in each point. The main contribution to dose equivalent comes from neutrons with energy higher than 0.1 MeV, even when they represent the smallest contribution in fluence. For this range of energy, the radiation quality factor and the radiation weighting factor are approximately equal. Then, dose equivalents evaluated using both factors are compatible, with differences below 12%. Quality factor can be replaced by the radiation weighting factor in the evaluation of dose equivalent in radiotherapy environments simplifying the practical procedure.

DESIGN DEVELOPMENT OF A PASSIVE NEUTRON DOSEMETER FOR THE USE AT HIGH-ENERGY ACCELERATORS.

For the radiation survey at intermediate and high-energy accelerators, there is a need for a neutron dosemeter which provides reliable readings of the neutron dose in a wide energy range for continuous and pulsed radiation. The objective of this development is to find a dosemeter that fulfils the necessary requirements and can be reliably used to prove that the radiation levels in areas around accelerators are in accordance with the limits of the respective radiation protection legislation. A simple layout with small dimensions and light weight as well as the usage of common materials to lower the production costs is to be achieved.

PRACTICAL METROLOGICAL ASPECTS OF NEUTRON PERSONAL DOSIMETRY.

This article addresses practical issues as being faced in the process of testing, verification and calibration of passive and active personal neutron dosemeters. Namely, a limited choice of neutron sources and geometric limitations occurring when ISO water phantom is used. The requirements of international standards for personal neutron dosemeters and the possibilities to fulfil them are discussed briefly.

FIELD CORRECTION FACTORS FOR PERSONAL NEUTRON DOSEMETERS.

A field-dependent correction factor can be obtained by comparing the readings of two albedo neutron dosemeters fixed in opposite directions on a polyethylene sphere to the H*(10) reading as determined with a thermal neutron detector in the centre of the same sphere. The work shows that the field calibration technique as used for albedo neutron dosemeters can be generalised for all kind of dosemeters, since H*(10) is a conservative estimate of the sum of the personal dose equivalents Hp(10) in two opposite directions. This result is drawn from reference values as determined by spectrometers within the EVIDOS project at workplace of nuclear installations in Europe. More accurate field-dependent correction factors can be achieved by the analysis of several personal dosimeters on a phantom, but reliable angular responses of these dosemeters need to be taken into account.

STABILITY OF THE NEUTRON DOSE DETERMINATION ALGORITHM FOR PERSONAL NEUTRON DOSEMETERS AT DIFFERENT RADON GAS EXPOSURES.

Since 2008 the Paul Scherrer Institute (PSI) has been using a microscope-based automatic scanning system for assessing personal neutron doses with a dosemeter based on PADC. This scanning system, known as TASLImage, includes a comprehensive characterisation of tracks. The distributions of several specific track characteristics such as size, shape and optical density are compared with a reference set to discriminate tracks of alpha particles and non-track background. Due to the dosemeter design at PSI, it is anticipated that radon should not significantly contribute to the creation of additional tracks in the PADC detector. The present study tests the stability of the neutron dose determination algorithm of the personal neutron dosemeter system in operation at PSI at different radon gas exposures.

GENERALISATION OF RADIATOR DESIGN TECHNIQUES FOR PERSONAL NEUTRON DOSEMETERS BY UNFOLDING METHOD.

A novel technique for designing a radiator suitable for personal neutron dosemeter based on plastic track detector was discussed. A multi-layer structure has been proposed in the previous report, where the thicknesses of plural polyethylene (PE) layers and insensitive ones were determined by iterative calculations of double integral. In order to arrange this procedure and make it more systematic, unfolding calculation has been employed to estimate an ideal radiator containing an arbitrary hydrogen concentration. In the second step, realistic materials replaced it with consideration of minimisation of the layer number and commercial availability. A radiator consisting of three layers of PE, Upilex and Kapton sheets was finally designed, for which a deviation in the energy dependence between 0.1 and 20 MeV could be controlled within 18 %. An applicability of fluorescent nuclear track detector element has also been discussed.

PALLADIUM ISOTOPIC EVIDENCE FOR NUCLEOSYNTHETIC AND COSMOGENIC ISOTOPE ANOMALIES IN IVB IRON METEORITES

The origin of ubiquitous nucleosynthetic isotope anomalies in meteorites may represent spatial and/or temporal heterogeneity in the sources that supplied material to the nascent solar nebula, or enhancement by chemical processing. For elements beyond the Fe peak, deficits in s-process isotopes have been reported in some (e.g., Mo, Ru, W) but not all refractory elements studied (e.g., Os) that, among the iron meteorites, are most pronounced in IVB iron meteorites. Palladium is a non-refractory element in the same mass region as Mo and Ru. In this study, we report the first precise Pd isotopic abundances from IVB irons to test the mechanisms proposed for the origin of isotope anomalies. First, this study determined the existence of a cosmogenic neutron dosimeter from the reaction 103Rh(n, beta-)104Pd in the form of excess 104Pd, correlated with excess 192Pt, in IVB irons. Second, all IVB irons show a deficit of the s-process only isotope 104Pd (\varepsilon 104Pd = -0.48 ± 0.24), an excess of the r-only isotope 110Pd (\varepsilon 110Pd = +0.46 ± 0.12), and no resolvable anomaly in the p-process 102Pd (\varepsilon 102Pd = +1 ± 1). The magnitude of the Pd isotope anomaly is about half that predicted from a uniform depletion of the s-process yields from the correlated isotope anomalies of refractory Mo and Ru. The discrepancy is best understood as the result of nebular processing of the less refractory Pd, implying that all the observed nucleosynthetic anomalies in meteorites are likely to be isotopic relicts. The Mo-Ru-Pd isotope systematics do not support enhanced rates of the 22Ne(alpha,n)25Mg neutron source for the solar system s-process.

Benchmarking shielding simulations for an accelerator-driven spallation neutron source

The shielding at an accelerator-driven spallation neutron facility plays a critical role in the performance of the neutron scattering instruments, the overall safety, and the total cost of the facility. Accurate simulation of shielding components is thus key for the design of upcoming facilities, such as the European Spallation Source (ESS), currently in construction in Lund, Sweden. In this paper, we present a comparative study between the measured and the simulated neutron background at the Swiss Spallation Neutron Source (SINQ), at the Paul Scherrer Institute (PSI), Villigen, Switzerland. The measurements were carried out at several positions along the SINQ monolith wall with the neutron dosimeter WENDI-2, which has a well-characterized response up to 5 GeV. The simulations were performed using the Monte-Carlo radiation transport code Geant4, and include a complete transport from the proton beam to the measurement locations in a single calculation. An agreement between measurements and simulations is about a factor of 2 for the points where the measured radiation dose is above the background level, which is a satisfactory result for such simulations spanning many energy regimes, different physics processes and transport through several meters of shielding materials. The neutrons contributing to the radiation field emanating from the monolith were confirmed to originate from neutrons with energies above 1 MeV in the target region. The current work validates Geant4 as being well suited for deep-shielding calculations at accelerator-based spallation sources. We also extrapolate what the simulated flux levels might imply for short (several tens of meters) instruments at ESS.

Optimizing the design of a neutron dosemeter with an extended energy range for high energy accelerators

Different versions of the design of a neutron dosemeter for high energy accelerators based on a fast scintillation lithium glass detector of thermal neutrons are considered. The detector is placed at the center of a polyethylene moderator shaped as a sphere, a cylinder, or a truncated cylinder with a lead insert used to increase the dosemeter sensitivity to neutrons with energies above 20 MeV. Calculations of the design parameters have been performed to optimize the angular and energy dependences of the response at energies ranging from the thermal energy to 1 GeV. The best results have been obtained for dosemeter versions comprising a boron filter along with the lead insert. In this case, the instrument has the lowest mass. The dosemeter has been developed for use in the radiation monitoring systems of the IHEP U-70 accelerator complex and other accelerators. It can also be used outside shields of reactors and other low-energy facilities.

Correlated cosmogenic W and Os isotopic variations in Carbo and implications for Hf–W chronology

An obstacle for establishing the chronology of iron meteorite formation using 182Hf–182W systematics (t1/2=8.9Myr) is to find proper neutron fluence monitors to correct for cosmic ray modification of W isotopic composition. Recent studies showed that siderophile elements such as Pt and Os could serve such a purpose. To test and calibrate these neutron dosimeters, the isotopic compositions of W and Os were measured in a slab of the IID iron meteorite Carbo. This slab has a well-characterized noble gas depth profile reflecting different degrees of shielding to cosmic rays. The results show that W and Os isotopic ratios correlate with distance from the pre-atmospheric center. Negative correlations, barely resolved within error, were found between ε190Os–ε189Os and ε186Os–ε189Os with slopes of −0.64±0.45 and −1.8(+1.9/−2.1), respectively. These Os isotope correlations broadly agree with model predictions for capture of secondary neutrons produced by cosmic ray irradiation and results reported previously for other groups of iron meteorites. Correlations were also found between ε182W–ε189Os (slope=1.02±0.37) and ε182W–ε190Os (slope=−1.38±0.58). Intercepts of these two correlations yield pre-exposure ε182W values of −3.32±0.51 and −3.62±0.23, respectively (weighted average ε182W=−3.57±0.21). This value relies on a large extrapolation leading to a large uncertainty but gives a metal–silicate segregation age of −0.5±2.4Myr after formation of the solar system. Combining the iron meteorite measurements with simulations of cosmogenic effects in iron meteorites, equations are presented to calculate and correct for cosmogenic effects on 182W using Os isotopes.

Present state of the art of a fast neutron dosimeter incorporating RPL detectors

The recently introduced radiophotoluminescent (RPL) detectors offer a unique combination of advantages for radiation monitoring that include rapid exploitation, stability to fading, reusability, and insensitivity to light, temperature and humidity. We look at the behavior of an RPL-based fast neutron dosimeter capable of measuring neutrons in an n-g field. The tested dosimeter consists of an ordered assembly of Al foil, RPL detector (I), Al foil, polyethylene converter, RPL detector (II) and Al foil encased in a polyethylene container. The difference between the two RPL configurationsrepresents the (n,p) protons and is related to the fast neutron dose. The dosimeter response is linear and shows an acceptable angular dependence. However the measured detection threshold for this dosimeter is too high for routine monitoring. This threshold could be lowered at to a more practicable value if next generation improvements in RPL detectors and the reader are applied. The main shortcomings we encountered are (i) a 1.7 µm thick dead layer at the front surface of the detectors that render them insensitive to a large fraction of the recoil protons and (ii) an intrinsic detector background that could be reduced if the reader were able to separate individual densely ionized zones created by the recoil protons from the gamma-ray signal.