A preliminary reactor design for the center for neutron research

The Center for Neutron Research is a new, national experimental facility being planned by Oak Ridge National Laboratory to meet the need for an intense steady-state source of neutrons and for associated experimental space and equipment. This article describes a preliminary design for a reactor to serve as the neutron source. The design offers an unprecedented thermal neutron flux for scattering experiments (5×10¹⁹ to 10×10¹⁹ neutrons m⁻² s⁻¹) as well as capabilities for isotope production and engineering materials irradiation experiments that surpass those at present research reactors.     

Results of nDOSE and HiDOSE Experiments for Dosimetric Evaluation During STS-134 Mission

HiDOSE (Heavy ion DOSimetry Experiment) and nDOSE (neutron DOSimetry Experiment) experiments conducted as a part of BIOKIS (Biokon in Space) payload were designed to measure the dose equivalent due to charged particles and to neutron field, on the entire energy range, during STS-134 mission. Given the complexity of the radiation field in space environment, dose measurements should be considered an asset of any space mission, and for this reason HiDOSE and nDOSE experiments represent an important contribution to the radiation environment assessment during this mission, a short duration flight. The results of these experiments, obtained using Thermo Luminescence Dosimeters (TLDs) to evaluate the charged particles dosimetry and neutron bubbles dosimeters and stack bismuth track dosimeters for neutron dosimetry, indicate that the dose equivalent rate due to space radiation exposure during the STS-134 mission is in accordance with the results obtained from long duration flights.     

Direct nn-Scattering Measurement With the Pulsed Reactor YAGUAR

Although crucial for resolving the issue of charge symmetry in the nuclear force, direct measurement of nn-scattering by colliding free neutrons has never been performed. At present the Russian pulsed reactor YAGUAR is the best neutron source for performing such a measurement. It has a through channel where the neutron moderator is installed. The neutrons are counted by a neutron detector located 12 m from the reactor. In preliminary experiments an instantaneous value of 1.1 × 10¹⁸/cm²s was obtained for the thermal neutron flux density. The experiment will be performed by the DIANNA Collaboration as International Science & Technology Center (ISTC) project No. 2286.     

Detection of primary and secondary cosmic ray particles aboard the ISS using SSNTD stacks

To study the radiation environment inside the International Space Station, solid state nuclear track detector stacks were used. Within the BRADOS experiments, Phase 1, seven stacks were exposed at different locations of the Russian segment 'Zvezda' for 248 days in 2001. It was supposed that the radiation field inside the ISS was composed from primary cosmic ray particles penetrating the wall of the ISS and secondaries, mainly neutrons induced by primaries in the wall and other structural materials surrounding the detectors. Based on the calibration made by utilising the high energy neutron reference field CERF at CERN (Geneva, Switzerland), the tracks induced by neutrons were separated from those induced by primary particles. Thus, the stacks, on one hand, provided the secondary neutron ambient dose equivalent. On the other hand, from the analysis of the rest of the tracks, the linear energy transfer spectra were computed and the flux and the dose of the primary particles were determined as shown in this paper.     

Bubble detector investigations in China

Investigation on bubble detectors started in China in 1989. Five types of bubble detectors have been developed, with LET thresholds ranging from 0.05 to 6.04 MeV mg(-1) cm(2) at 25 degrees C. The neutron response of bubble detectors made with freon-12 has been investigated with mono-energetic neutrons from 20 keV to 19 MeV. Its effective threshold energy for neutron detection is approximately 100 keV at 28 degrees C. The response above this threshold is approximately 1.5 x 10(-4) (bubble cm(-2))/(n cm(-2)). Bubble detectors are unique not only for neutron dosimetry but also for monitoring and identifying high-energy heavy ions such as cosmic radiation in the space. High-energy heavy ion tracks in large size bubble detectors have been investigated in cooperation with scientists in Japan. The key parameter behind the thresholds of bubble detectors for track registration is the critical rate of energy loss. Three approaches to identify high-energy heavy ions with bubble detectors are suggested.     

Thermal neutron equivalent dose assessment around the KFUPM neutron source storage area using NTDs. King Fahd University of Petroleum and Minerals

Area passive neutron dosemeters based on nuclear track detectors (NTDs) have been used for 13 days to assess accumulated low doses of thermal neutrons around neutron source storage area of the King Fahd University of Petroleum and Minerals (KFUPM). Moreover, the aim of this study is to check the effectiveness of shielding of the storage area. NTDs were mounted with the boron converter on their surface as one compressed unit. The converter is a lithium tetraborate (Li2B4O7) layer for thermal neutron detection via 10B(n,alpha)7Li and 6Li(n,alpha)3H nuclear reactions. The area passive dosemeters were installed on 26 different locations around the source storage area and adjacent rooms. The calibration factor for NTD-based area passive neutron dosemeters was found to be 8.3 alpha tracks x cm(-2) x microSv(-1) using active snoopy neutron dosemeters in the KFUPM neutron irradiation facility. The results show the variation of accumulated dose with locations around the storage area. The range of dose rates varied from as low as 40 nSvx h(-1) up to 11 microSv x h(-1). The study indicates that the area passive neutron dosemeter was able to detect accumulated doses as low as 40 nSv x h(-1), which could not be detected with the available active neutron dosemeters. The results of the study also indicate that an additional shielding is required to bring the dose rates down to background level. The present investigation suggests extending this study to find the contribution of doses from fast neutrons around the neutron source storage area using NTDs through proton recoil. The significance of this passive technique is that it is highly sensitive and does not require any electronics or power supplies, as is the case in active systems.     

Evaluation of solid state nuclear track detector stacks exposed on the international space station

The aim of the study was to investigate the contribution of secondary neutrons to the total dose inside the International Space Station (ISS). For this purpose solid-state nuclear track detector (SSNTD) stacks were used. Each stack consisted of three CR-39 sheets. The first and second sheets were separated by a Ti plate, and the second and third sheets sandwiched a Lexan polycarbonate foil. The neutron and proton responses of each sheet were studied through MC calculations and experimentally, utilising monoenergetic protons. Seven stacks were exposed in 2001 for 249 days at different locations of the Russian segment 'Zvezda'. The total storage time before and after the exposure onboard was estimated to be seven months. Another eight stacks were exposed at the CERF high-energy neutron field for calibration purposes. The CR-39 detectors were evaluated in four steps: after 2, 6, 12 and 20 h etching in 6 N NaOH at 70 degrees C (VB = 1.34 microm h(-1)). All the individual tracks were investigated and recorded using an image analyser. The stacks provided the averaged neutron ambient dose equivalent (H*) between 200 keV and 20 MeV, and the values varied from 39 to 73 microSv d(-1), depending on the location. The Lexan detectors were used to detect the dose originating from high-charge and high-energy (HZE) particles. These results will be published elsewhere.     

Review of bubble detector response characteristics and results from space

A passive neutron-bubble dosemeter (BD), developed by Bubble Technology Industries, has been used for space applications. Both the bubble detector-personal neutron dosemeter and bubble detector spectrometer have been studied at ground-based facilities in order to characterise their response due to neutrons, heavy ion particles and protons. This technology was first used during the Canadian-Russian collaboration aboard the Russian satellite BION-9, and subsequently on other space missions, including later BION satellites, the space transportation system, Russian MIR space station and International Space Station. This paper provides an overview of the experiments that have been performed for both ground-based and space studies in an effort to characterise the response of these detectors to various particle types in low earth orbit and presents results from the various space investigations.     

The Fundamental Neutron Physics Beamline at the Spallation Neutron Source

The Spallation Neutron Source (SNS), currently under construction at Oak Ridge National Laboratory with an anticipated start-up in early 2006, will provide the most intense pulsed beams of cold neutrons in the world. At a projected power of 1.4 MW, the time averaged fluxes and fluences of the SNS will approach those of high flux reactors. One of the flight paths on the cold, coupled moderator will be devoted to fundamental neutron physics. The fundamental neutron physics beamline is anticipated to include two beam-lines; a broad band cold beam, and a monochromatic beam of 0.89 nm neutrons for ultracold neutron (UCN) experiments. The fundamental neutron physics beamline will be operated as a user facility with experiment selection based on a peer reviewed proposal process. An initial program of five experiments in neutron decay, hadronic weak interaction and time reversal symmetry violation have been proposed.     

Introduction of a thermal response to the DSTL PADC personal neutron dosemeter

The response of the Defence Science and Technology Laboratory (DSTL) PADC personal neutron dosemeter is strongly dependent upon neutron energy, with a range of 300-500 tracks per cm2 per mSv for energies between 1 and 5 MeV. Below 1 MeV the response drops off sharply. This lack of sensitivity is undesirable when the dosemeter is employed with the softened fission spectra encountered in the workplace. In order to incorporate a thermal response, a polypropylene converter doped with LiF has been placed directly in front of the PADC elements. Tritons produced in the thermal neutron reaction 6Li (n,t)alpha at 2.7 MeV will then penetrate the PADC, leaving a trail of damage. The reaction rate within the converter has been calculated using MCNP for thermal neutrons and a range of higher energies, while transport of the tritons is modelled using the SRIM/TRIM package to determine the resultant track density and depth distribution. The modelling and experimental work have demonstrated that a concentration of 0.2% natural lithium by weight results in a track density in a thermal field comparable with that produced per unit personal dose equivalent by neutrons greater than 1 MeV in the standard dosemeter. Additional MCNP modelling has demonstrated that the dosemeters' albedo response to intermediate energy neutrons can be enhanced considerably by placing a boron-doped shield in front of the converter and increasing its lithium concentration.     

An accelerator-based epithermal neutron beam design for BNCT and dosimetric evaluation using a voxel head phantom

The beam shaping assembly design has been investigated in order to improve the epithermal neutron beam for accelerator-based boron neutron capture therapy in intensity and quality, and dosimetric evaluation for the beams has been performed using both mathematical and voxel head phantoms with MCNP runs. The neutron source was assumed to be produced from a conventional 2.5 MeV proton accelerator with a thick (7)Li target. The results indicate that it is possible to enhance epithermal neutron flux remarkably as well as to embody a good spectrum shaping to epithermal neutrons only with the proper combination of moderator and reflector. It is also found that a larger number of thermal neutrons can reach deeply into the brain and, therefore, can reduce considerably the treatment time for brain tumours. Consequently, the epithermal neutron beams designed in this study can treat more effectively deep-seated brain tumours.     

Development of neutron-monitor detector using liquid organic scintillator coupled with 6Li + ZnS(Ag) Sheet

A phoswitch-type detector has been developed for monitoring neutron doses in high-energy accelerator facilities. The detector is composed of a liquid organic scintillator (BC501A) coupled with ZnS(Ag) sheets doped with 6Li. The dose from neutrons with energies above 1 MeV is evaluated from the light output spectrum of the BC501A by applying the G-function, which relates the spectrum to the neutron dose directly. The dose from lower energy neutrons, on the other hand, is estimated from the number of scintillations emitted from the ZnS(Ag) sheets. Characteristics of the phoswitch-type detector were studied experimentally in some neutron fields. It was found from the experiments that the detector has an excellent property of pulse-shape discrimination between the scintillations of BC501A and the ZnS(Ag) sheets. The experimental results also indicate that the detector is capable of reproducing doses from thermal neutrons as well as neutrons with energies from one to several tens of megaelectronvolts (MeV).     

A nuclear physics program and experimental needs

This paper reviews work on the properties of the neutron, of strong interactions of neutrons, and of weak interactions of neutrons, identifying how and where the experiments can be improved, and the needs that these suggest for a new high flux reactor.     

Investigation of radiation fields outside the Sub-critical Assembly in Dubna

The radiation fields outside the planned experimental Sub-critical Assembly in Dubna (SAD) have been studied in order to provide a basis for the design of the concrete shielding that cover the reactor core. The effective doses around the reactor, induced by leakage of neutrons and photons through the shielding, have been determined for a shielding thickness varying from 100 to 200 cm. It was shown that the neutron flux and the effective dose is higher above the shielding than at the side of it, owing to the higher fraction of high-energy spallation neutrons emitted in the direction of the incident beam protons. At the top, the effective dose was found to be -150 microSv s(-1) for a concrete thickness of 100 cm, while -2.5 microSv s(-1) for a concrete thickness of 200 cm. It was also shown that the high-energy neutrons (> 10 MeV), which are created in the proton-induced spallation interactions in the target, contribute for the major part of the effective doses outside the reactor.     

High-fidelity MCNP modeling of a D-T neutron generator for active interrogation of special nuclear material

Fast and robust methods for interrogation of special nuclear material (SNM) are of interest to many agencies and institutions in the United States. It is well known that passive interrogation methods are typically sufficient for plutonium identification because of a relatively high neutron production rate from ²⁴⁰Pu [1]. On the other hand, identification of shielded uranium requires active methods using neutron or photon sources [2]. Deuterium-deuterium (2.45 MeV) and deuterium-tritium (14.1 MeV) neutron-generator sources have been previously tested and proven to be relatively reliable instruments for active interrogation of nuclear materials [3] and [4]. In addition, the newest generators of this type are small enough for applications requiring portable interrogation systems.Active interrogation techniques using high-energy neutrons are being investigated as a method to detect hidden SNM in shielded containers [4] and [5]. Due to the thickness of some containers, penetrating radiation such as high-energy neutrons can provide a potential means of probing shielded SNM. In an effort to develop the capability to assess the signal seen from various forms of shielded nuclear materials, the University of Michigan Neutron Science Laboratory’s D-T neutron generator and its shielding were accurately modeled in MCNP. The generator, while operating at nominal power, produces approximately 1×10¹⁰ neutrons/s, a source intensity which requires a large amount of shielding to minimize the dose rates around the generator. For this reason, the existing shielding completely encompasses the generator and does not include beam ports. Therefore, several MCNP simulations were performed to estimate the yield of uncollided 14.1-MeV neutrons from the generator for active interrogation experiments. Beam port diameters of 5, 10, 15, 20, and 25 cm were modeled to assess the resulting neutron fluxes. The neutron flux outside the beam ports was estimated to be approximately 2×10⁴ n/cm² s.     

Shielding evaluation of a medical linear accelerator vault in preparation for installing a high-dose rate 252Cf remote afterloader

In support of the effort to begin high-dose rate 252Cf brachytherapy treatments at Tufts-New England Medical Center, the shielding capabilities of a clinical accelerator vault against the neutron and photon emissions from a 1.124 mg 252Cf source were examined. Outside the clinical accelerator vault, the fast neutron dose equivalent rate was below the lower limit of detection of a CR-39 etched track detector and below 0.14 +/- 0.02 muSv h(-1) with a proportional counter, which is consistent, within the uncertainties, with natural background. The photon dose equivalent rate was also measured to be below background levels (0.1 muSv h(-1)) using an ionisation chamber and an optically stimulated luminescence dosemeter. A Monte Carlo simulation of neutron transport through the accelerator vault was performed to validate measured values and determine the thermal-energy to low-energy neutron component. Monte Carlo results showed that the dose equivalent rate from fast neutrons was reduced by a factor of 100,000 after attenuation through the vault wall, and the thermal-energy neutron dose equivalent rate would be an additional factor of 1000 below that of the fast neutrons. Based on these findings, the shielding installed in this facility is sufficient for the use of at least 5.0 mg of 252Cf.     

Total neutron yields from 100 MeV protons on Cu,Fe, and Th

The neutron yield per proton from thick targets of Fe, Cu and Th irradiated with 100 MeV protons was measured as 0.115, 0.145 and 0.53 respectively. The neutron yields were determined from the capture rate of thermal neutrons inside a water moderator surrounding the target. The neutron yields and flux distributions were also calculated with the intra nuclear-cascade-evaporation model code NMTC and found to be in reasonable agreement with the experimental ata.     

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.     

Properties of the flux line lattice in hysteretic type II superconductors. I. Neutron diffraction experiments

A detailed study of neutron rocking curves on flux line lattices was made in single-crystalline hysteretic type II superconductors with Ginzburg-Landau parameters between 1.0 and 2.5. Information was obtained about the symmetry and the quality of the flux line lattice as a function of crystal orientation, sample purity, and magnetic field. The flux line lattice is always observed to form a single crystal. The width of the rocking curves, which is a measure of the misorientation of flux line crystallites in the plane perpendicular to the external field, is found to decrease strongly with increasing field along the initial magnetization curve, and to be much smaller upon decreasing the field from aboveH _c2 . This width varied between 8 and 1.5°. The minimum size of a coherently scattering flux line crystallite was estimated to be about 2–3 µm. Along the initial magnetization nearH _c1 the flux is found to form flux line bundles of constant lattice spacing. Although large flux density gradients have been observed earlier with field probes nearH _c1 ^1,2 , the lattice parameter measured with neutrons remained constant in this field range.     

Measurement of neutron flux and beam divergence at the cold neutron guide system of the new Munich research reactor FRM-II

A sophisticated neutron guide system has been installed at the new Munich neutron source FRM-II to transport neutrons from the D₂ cold neutron source to several instruments, which are situated in a separate neutron guide hall. The guide system takes advantage of supermirror coatings and includes a worldwide unique “twisted” guide for a desired phase space transformation of the neutron beam. During the initial reactor commissioning in summer 2004, the integral and differential neutron flux as well as the distribution of beam divergence at the exit of two representative and the twisted neutron guide were measured using time-of-flight spectroscopy and gold-foil activation. The experimental results can be compared to extensive simulation calculations based on MCNP and McStas. The investigated guides fulfill the expectations of providing high neutron fluxes and reveal good quality with respect to the reflective coatings and the installation precision.