Performance of a cylindrical tissue-equivalent proportional counter for use in neutron monitoring

Tissue-equivalent proportional counters (TEPC) allow the measurements of the absorbed dose and the ambient dose equivalent for neutron fields. A device based on this approach, called NAUSICAA((1,2)), has already been developed by IRSN to be used in high energy neutron fields for space applications. The response of this detector underestimates significantly the dose equivalent at low energies (several hundred keV) which represent the major component of neutron fields at workplaces in the nuclear industry. A counter with a similar geometry (cylindrical detector) and a lower gas pressure was studied in order to simulate a 1 microm biological site. In 2003, the performance of the device was further improved by adding a small amount of 3He to the tissue-equivalent gas (propane based) in order to increase the response for the lower energies of neutrons. Three amplification circuits were used to cover lineal energy range from 10(-1) to 10(4) keV microm(-1). Tests were performed in monoenergetic neutron and source fields. This paper presents the experimental results obtained with this change.     

A practical design of a cylindrical tissue-equivalent proportional counter for microdosimetry

A 12 mm tissue-equivalent proportional counter (TEPC) was designed, constructed and successfully tested. This detector achieves features common to others TEPCs, but it does not use field shaping electrodes and it is able to work at higher bias voltages which makes it capable of measuring the whole range of energy deposition events for gamma rays. The following approach was used to design the detector: first of all the use of a cylindrical shape detector featured as simple as possible but keeping its performance as well, the next point is to avoid the use of field shaping tubes and the last one is to make the preamplifier small and as close as possible to the detector. Its construction is based on a cylindrical proportional counter with A-150 tissue-equivalent material as cathode, TE-methane gas as a proportional gas and a 20 microns diameter wire as anode.     

Shielding experiment of heavy-ion produced neutrons using a tissue-equivalent proportional counter

A shielding experiment was performed at the HIMAC (Heavy Ion Medical Accelerator in Chiba), of National Institute of Radiological Sciences (NIRS), to measure neutron dose using a spherical TEPC (tissue-equivalent proportional counter) of 12.55 cm inner diameter. Neutrons are produced from a 5 cm thick stopping length Cu target bombarded by 400 MeV/nucleon C6+ ions and penetrate concrete or iron shields of various thicknesses at 0 degree to the beam direction. From this shielding experiment. y-distribution, mean lineal energy, absorbed dose, dose equivalent and mean-quality factor were obtained behind the shield as a function of shield thickness. The neutron dose attenuation lengths were also obtained as 126 g cm(-2) for concrete and 211 g cm(-2) for iron. The measured results were compared with the calculated results using the MARS Monte Carlo code.     

Measurement of the neutron fluence and dose spectra using an extended bonner sphere and a tissue-equivalent proportional counter

A conventional Bonner Sphere (BS) set consisting of six polyethylene spheres was modified to enhance its response to a high-energy neutron by putting a lead shell inside a polyethylene moderator. The response matrix of an extended BS was calculated using the MCNPX code and calibrated using a 252Cf neutron source. In order to survey the unknown photon and neutron mixed field, a spherical tissue equivalent proportional counter (TEPC) was constructed and assembled as a portable measurement system. The extended BS and the self-constructed TEPC were employed to determine the dosimetric quantities of the neutron field produced from the thick lead target bombarded by the 2.5 GeV electron beam of Pohang Accelerator Laboratory (PAL) and the neutron calibration field of Korea Atomic Energy Research Institute (KAERI).     

An accelerator-based neutron microbeam system for studies of radiation effects

A novel neutron microbeam is being developed at the Radiological Research Accelerator Facility (RARAF) of Columbia University. The RARAF microbeam facility has been used for studies of radiation bystander effects in mammalian cells for many years. Now a prototype neutron microbeam is being developed that can be used for bystander effect studies. The neutron microbeam design here is based on the existing charged particle microbeam technology at the RARAF. The principle of the neutron microbeam is to use the proton beam with a micrometre-sized diameter impinging on a very thin lithium fluoride target system. From the kinematics of the ?Li(p,n)?Be reaction near the threshold of 1.881 MeV, the neutron beam is confined within a narrow, forward solid angle. Calculations show that the neutron spot using a target with a 17-μm thick gold backing foil will be <20 μm in diameter for cells attached to a 3.8-μm thick propylene-bottomed cell dish in contact with the target backing. The neutron flux will roughly be 2000 per second based on the current beam setup at the RARAF singleton accelerator. The dose rate will be about 200 mGy min?1. The principle of this neutron microbeam system has been preliminarily tested at the RARAF using a collimated proton beam. The imaging of the neutron beam was performed using novel fluorescent nuclear track detector technology based on Mg-doped luminescent aluminum oxide single crystals and confocal laser scanning fluorescent microscopy.     

Measurement of lineal-energy distributions for neutrons of 8 keV to 65 MeV by using a tissue-equivalent proportional counter

The lineal-energy spectra for monoenergetic and quasi-monoenergetic neutrons of 8 keV to 65 MeV were obtained using a tissue-equivalent proportional counter (TEPC). The frequency-mean lineal energy, the dose-average lineal energy and mean quality factor were estimated from the measured data. The neutron absorbed doses obtained with this TEPC were compared with the kerma coefticient for A-150 plastic defined by ICRP 26 and the mean quality factors were compared with the data of ICRP 74. respectively. These comparisons indicated good agreement between them.     

A tissue equivalent proportional counter filled with a mixture of tissue equivalent gas and 3He for neutron monitoring

Tissue equivalent proportional counters (TEPC) allow the measurement of the ambient dose equivalent H(*)(10) in mixed fields. IRSN has been studying the design and the response of a TEPC in terms of neutron H(*)(10). First, a cylindrical counter was filled with propane gas at a low pressure. H(*)(10) measured in monoenergetic neutron fields underestimated the reference (>50%) at low energies (< 500 keV). A small amount of (3)He was then added to the gas in order to increase the response. The underestimation observed decreased but the results (> 40%) were not totally complying with the objectives (< 20%). Finally the choice was made to improve the analysis of the microdosimetric spectra y.d(y) in order to identify the energy of the incident neutrons. The analysis allows a better estimate of H(*)(10). The aim of this article is to describe the TEPC and the effect of these methods of optimisation.     

Fabrication of a multiwire tissue equivalent proportional counter and its use in 14 MeV neutron dosimetry

Fabrication and performance details of a multiwire tissue equivalent planar proportional chamber are presented. Together with an AlAr dual chamber, it is then used to evaluate the fast neutron and gamma ray tissue-absorbed dose at one point of the mixed neutron-gamma field produced by a 14 MeV neutron generator. Comparison of the result with that of the Monte Carlo computer simulation of the neutron-chamber interaction shows a good agreement. Further improvement in the accuracy can be obtained by increasing the accuracy of the evaluation of the relative neutron sensitivity of the dual AlAr chamber.     

A method for measuring tissue-equivalent dose using a pin diode and activation foil in epithermal neutron beams with EN < 100 keV

Silicon (Si) pin diodes can be used for neutron dosimetry by observing the change in forward bias voltage caused by neutron induced displacement damage in the diode junction. Pin diode energy response depends on Si displacement damage KERMA (K(Si)). It is hypothesised that tissue-equivalent (TE) neutron dose could be expressed as a linear combination of K(Si) and foil activation terms. Monte Carlo simulations (MCNP) of parallel monoenergetic neutron beams incident on a cylindrical TE phantom were used to calculate TE dose, K(Si) and Au, Cu and Mn foil activations along the central axis of the phantom. For spectra with neutron energies <100 keV, it is possible to estimate the TE kerma based on silicon damage kerma and Cu or Mn foil measurements. More accurate estimates are possible for spectra where the maximum neutron energy does not exceed 30 keV.     

Argon-filled proportional counter with a small xenon additive for X-ray spectroscopy

X-ray spectrometric performance of an argon-filled proportional counter containing a 2 mol % xenon additive have been studied. The energy resolution was checked by measuring the X-ray fluorescence and by detecting monochromatic radiation from a ²⁴¹Am radionuclide. The small addition of xenon results in a reduction in the working voltage from 1500 to 800 V, improvement of the resolution, and a more than twofold increase in the efficiency of detection for photons with energies above the absorption K-edge of xenon.     

Measurement and simulation of lineal energy distribution at the CERN high energy facility with a tissue equivalent proportional counter

The response of a tissue equivalent proportional counter (TEPC) in a mixed radiation field with a neutron energy distribution similar to the radiation field at commercial flight altitudes has been studied. The measurements have been done at the CERN-EU High-Energy Reference Field (CERF) facility where a well-characterised radiation field is available for intercomparison. The TEPC instrument used by the ARC Seibersdorf Research is filled with pure propane gas at low pressure and can be used to determine the lineal energy distribution of the energy deposition in a mass of gas equivalent to a 2 mum diameter volume of unit density tissue, of similar size to the nuclei of biological cells. The linearity of the detector response was checked both in term of dose and dose rate. The effect of dead-time has been corrected. The influence of the detector exposure location and orientation in the radiation field on the dose distribution was also studied as a function of the total dose. The microdosimetric distribution of the absorbed dose as a function of the lineal energy has been obtained and compared with the same distribution simulated with the FLUKA Monte Carlo transport code. The dose equivalent was calculated by folding this distribution with the quality factor as a function of linear energy transfer. The comparison between the measured and simulated distributions show that they are in good agreement. As a result of this study the detector is well characterised, thanks also to the numerical simulations the instrument response is well understood, and it's currently being used onboard the aircrafts to evaluate the dose to aircraft crew caused by cosmic radiation.     

A multi-function materials facility for a Spallation neutron source

A neutron beam instrument is described which will allow the simultaneous study of the microstructural, crystalline phase, internal stress, defect, local order, texture, diffusional and vibrational properties of materials. The penetration of neutrons permits all these properties to be studied in a bulk specimen in situ during a heat treatment of chemical reaction as a function of time. The possibility of using narrow incident and scattered neutron beams allows the simultaneous monitoring of these properties as a function of position across the sample. Only by using the polychromatic neutron beam from a pulsed source, can all these properties be studied at the same time and at the same point on the specimen. The instrument is designed for installation on the Spallation Neutron Source recently commissioned at the Rutherford Appleton Laboratory. Its performance is evaluated for a series of key experiments in applied science and shown to permit a complete neutron examination of the sample within minutes.     

Analysis of the effect of structural materials in a wall-less tissue-equivalent proportional counter irradiated by 290 MeV u(-1) carbon beam

Effects of structural materials in a wall-less tissue-equivalent proportional counter were evaluated based on the calculation of energy deposits by EGS5 and the measurement of lineal energy distributions using 290 MeV u(-1) carbon beams. It is found that the correction of measured data based on simulation is necessary for understanding the energy deposition spectra in the homogeneous condition in tissues.     

Procedures for neutron scattering corrections in a calibration facility with a non-symmetric set-up

When calibrating neutron monitors, it is important to correct for scattering effects. In general, the correction factors depend on the type of source and monitor used, and on the configuration of the calibration room. These correction factors have been determined for the specific non-symmetric configuration of the calibration room used at SCK.CEN by means of the different analytical techniques recommended by the ISO. These results are compared with each other and also with the outcome of numerical calculations performed using the Tripoli-3 and MCNP 4B code.     

A high background rejection method in a proportional counter using field-adjusting electrodes as guard cathodes

A single anode single large cell counter, with veto-counters around it, is described. To get uniform response of the pulse height and the energy resolution, the counter has field-adjusting electrodes at the end walls of the anode wire, and these electrodes are also used to reject the background events which are caused by cosmic rays passing through near the end walls of the anode.A rocket flight experiment showed that the combination of rise time discrimination and anti-coincidence method reduced the cosmic ray induced background events to a level of 1.2×10^?4 counts s^?1 cm^?2 keV^?1 in the energy range of 2–40 keV, which represented an overall background reduction of 98% in this energy range.     

GEANT4 used for neutron beam design of a neutron imaging facility at TRIGA reactor in Morocco

Neutron imaging has a broad scope of applications and has played a pivotal role in visualizing and quantifying hydrogenous masses in metallic matrices. The field continues to expand into new applications with the installation of new neutron imaging facilities.In this scope, a neutron imaging facility for computed tomography and real-time neutron radiography is currently being developed around 2.0MW TRIGA MARK-II reactor at Maamora Nuclear Research Center in Morocco (Reuscher et al., 1990 [1]; de Menezes et al., 2003 [2]; Deinert et al., 2005 [3]).The neutron imaging facility consists of neutron collimator, real-time neutron imaging system and imaging process systems. In order to reduce the gamma-ray content in the neutron beam, the tangential channel was selected. For power of 250 kW, the corresponding thermal neutron flux measured at the inlet of the tangential channel is around 3×10¹¹ ncm²/s.This facility will be based on a conical neutron collimator with two circular diaphragms with diameters of 4 and 2 cm corresponding to L/D-ratio of 165 and 325, respectively. These diaphragms' sizes allow reaching a compromise between good flux and efficient L/D-ratio. Convergent-divergent collimator geometry has been adopted.The beam line consists of a gamma filter, fast neutrons filter, neutron moderator, neutron and gamma shutters, biological shielding around the collimator and several stages of neutron collimator. Monte Carlo calculations by a fully 3D numerical code GEANT4 were used to design the neutron beam line (http://www.info.cern.ch/asd/geant4/geant4.html[4]).To enhance the neutron thermal beam in terms of quality, several materials, mainly bismuth (Bi) and sapphire (Al₂O₃) were examined as gamma and neutron filters respectively. The GEANT4 simulations showed that the gamma and epithermal and fast neutron could be filtered using the bismuth (Bi) and sapphire (Al₂O₃) filters, respectively.To get a good cadmium ratio, GEANT 4 simulations were used to define the design of the moderator in the inlet of the radiation channel. A graphite block of 22 cm thickness seems to be the optimal neutron moderator.The results showed that the combination of 5 cm of bismuth with 5 cm of sapphire permits the filtration of gamma-rays, epithermal neutrons as well as fast neutrons in a considerable way without affecting the neutron thermal flux.     

Report on the development of a low background proportional counter for monitoring plutonium in the lung

Techniques developed for use in X-ray astronomy detectors to reduce the internal background in proportional counters for the energy range 1–20 keV have been applied in the construction of a prototype low background proportional counter for monitoring plutonium in the lung. The most commonly used device at present is the phoswich which is mainly limited in its precision by the background count rate induced by the normal radioactivity of the subject being measured. A properly constructed proportional counter using the latest background rejection techniques should offer better rejection of these background events and improve the precision of the measurement. The design of such a counter is described together with preliminary results from the evaluation of a prototype. The limiting performance is shown to be due to radioactive contamination (tritium) of the counter and reduction of this to a low level gives a background lower than that of the phoswich, though not under operational conditions. Future improvements are described.     

Evaluation of a fast neutron coincidence counter for the measurements of uranium samples

A fast neutron coincidence counter using BC454/BGO phoswich detectors has been evaluated for the purpose of rapid verification measurements of uranium items. This counter uses custom electronics to identify and count coincidence neutrons in the presence of background radiation. Measurements of uranium standards were performed to evaluate the counter. This counter is successful in measuring uranium items but has a low efficiency that results in minimal improvement over current technology. An optimized counter can be built with better performance capabilities, but it is recommended that newer technologies be used instead.