Cold Neutron Research Facility Research Articles

Growth and Challenges of the HANARO Neutron Beam Facility

Six years have passed since Young Jin Kim and I reported on the HANARO neutron beam facility for Neutron News [1]. The most notable is that the Cold Neutron Research Facility (CNRF) project ended a...

Thermo-siphon mock-up test for the cold neutron source of HANARO

Due to the national demand for a cold neutron beam utilization, a cold neutron research facility project has been carried out since July 2003 to install a cold neutron source (CNS) in HANARO. The CNS adopts a two-phase thermo-siphon of liquid hydrogen as a working fluid to remove a heat load. The CNS consists of an in-pool assembly (IPA) and other components including a hydrogen buffer tank. The liquid level in the moderator cell is measured by a gamma-ray densitometer under a normal cold operation, and the cool down time to fill the required liquid hydrogen into the moderator cell is numerically simulated and compared with the experimental data from a thermo-siphon mock-up test. The self-regulating characteristic of the two-phase thermo-siphon loop is also confirmed by a comparison of a model simulation’s results, which use a thermodynamic model, with the experiment’s results. From these results, the HANARO-CNS is found to maintain the required conditions for a stable cold neutron production against a heat load variation along with the reactor’s power. Furthermore, the thermodynamic behavior in the IPA is observed to determine whether or not the integrity of the IPA is maintained under an abnormal condition of the helium refrigeration system.

OPPORTUNITIES AND CHALLENGES OF NEUTRON SCIENCE AND TECHNOLOGY IN KOREA

Neutron science and technology, the utilization of neutron beams for a wide variety of scientific and engineering research ranging from materials and life science to industrial applications, has been one of the key elements of modem science and technology. Currently, the neutron science and technology in Korea is in rapid growth with the operation of the 30 MW High-flux Advanced Neutron Application Reactor (HANARO) at the Korea Atomic Energy Research Institute, which is one of the most powerful nuclear research reactors in the world. Furthermore, a state of the art HANARO cold neutron research facility, which will open a new era for the neutron science and technology in Korea, is expected to become available in 2010. In this paper, the progress of neutron science and technology in Korea is reviewed and its unprecedented new opportunities and challenges in coming years are presented.

Current situation of the cold neutron research facility project at HANARO

Current situation of the cold neutron research facility project at HANARO

Current status of the 40 m small-angle neutron scattering instrument development at the HANARO research reactor

The cold neutron research facility project at the Korean HANARO research reactor was launched in 2003. A state-of-the-art small-angle neutron scattering instrument was selected as a top-priority instrument. An analytical calculation was used to obtain the optimum instrument design and the appropriate range of scattering vectors of the instrument. Then, in order to optimize the instrument layout and to enhance the instrument's performance, a Monte Carlo simulation was performed. The Q range [where Q is the magnitude of the scattering vector given by (4π/λ)sin(θ/2), θ is the scattering angle and λ is the wavelength] of the instrument will extend from 0.0008 to 1.0 Å−1. The instrument layout was optimized based on simulation results. The maximum flux at the sample position can reach about 5.5 × 10 7 n cm−2 s−1.

Design of 40M SANS instrument at HANARO, Korea

The HANARO cold-neutron research facility project was launched on July 1, 2003. A state of the art SANS instrument was selected as a top-priority instrument by an instrument selection committee, which consisted of domestic users and HANARO personnel. An instrument development team and an international and domestic instrument advisory team were formulated. At the end position of the CG1B guide, 40M SANS will be installed in the cold-neutron guide hall. The guide and the instrument simulation were performed using Vitess software and the optimum conceptual design was completed based on the simulation results and the international advisory team reviews. The 100×100 cm 2 area detector will be installed and the sample-to-detector distance will vary from 1 to 20 m. The refractive focusing optics and the transmission polarizer/spin flipper setup is planned for installation. The Q range of the 40M SANS instrument will cover from 0.0008 to 1.0 Å −1 when considering the refractive focusing optics. In this paper, the simulation results and the design of the new 40M SANS instrument are presented.

Cold neutron research facility at the Budapest Neutron Centre

The new cold neutron research facility has been routinely operated at the Budapest Neutron Centre (BNC) since February 2001. This ensemble of equipment consists of a liquid hydrogen cold neutron source, an optimised supermirror neutron guide system and a set of experimental stations located in the neutron guide hall.

Developments in neutron beam devices and an advanced cold source for the NIST research reactor

The last 5 yr has been a period of steady growth in instrument capabilities and utilization at the National Institute of Standards and Technology Center for Neutron Research. Since the installation of the liquid hydrogen cold source in 1995, all of the instruments originally planned for the Cold Neutron Research Facility have been completed and made available to users, and three new thermal neutron instruments have been installed. Currently, an advanced cold source is being fabricated that will better couple the reactor core and the existing network of neutron guides. Many improvements are also being made in neutron optics to enhance the beam characteristics of certain instruments. For example, optical filters will be installed that will increase the fluxes at the two 30-m SANS instruments by as much as two. Sets of MgF 2 biconcave lenses have been developed for SANS that have demonstrated a significant improvement in resolution over conventional pinhole collimation. The recently commissioned high-flux backscattering spectrometer incorporates a converging guide, a large spherically focusing monochromator and analyzer, and a novel phase space transform chopper, to achieve very high intensity while maintaining excellent energy resolution. Finally, a prototype low background, doubly focusing neutron monochromator is nearing completion that will be the heart of a new cold neutron spectrometer, as well as two new thermal neutron triple axis spectrometers.

On the utilization of the ETRR-2 neutron beams in research and applications

The cold neutron research facility (CNRF) at the Egyptian Atomic Energy Authority Research Reactor (ETRR-2) is now coming on line with the first five experimental stations operational. This report provides a short description of the facility.

Statistical planning for a neutron lifetime experiment using magnetically trapped neutrons

At the NIST Cold Neutron Research Facility, a two-stage experiment will be repeated many times. During the first stage of each run cycle, ultracold neutrons will be produced and confined in a magnetic trap filled with superfluid 4He. Ultracold neutrons will result when cold (8.9 Å) neutrons undergo inelastic scattering in the superfluid helium. After filling the trap to a desired level, decay events will be recorded as a function of time. Detection is possible because the charged particles created by neutron decay generate detectable scintillations in the helium. In addition to neutron decay events, background events will be recorded. I model the background as a stationary Poisson process. By Monte Carlo methods, I study the performance of two nonlinear algorithms for estimating the mean lifetime of the neutron. In one method, the event time data are summarized as a histogram where the bin widths vary. I select the time endpoints of the bins so that the expected number of counts per bin contributed by the decay process is approximately constant. In the second method, the lifetime is estimated from the complete sequence of event times. The histogram method yields a less variable estimate than does the complete data estimation method. The allocation of time between the fill and decay stages affects the precision of the estimate. To get the optimal time allocation, I minimize the asymptotic variance of the estimated mean lifetime (estimated from the pooled histogram data from all cycles) given knowledge of the rate at which neutrons enter the trap and parameters which characterize the background. The mean lifetime estimate is biased. I observe bias reduction when estimating the lifetime from data pooled from many cycles (rather than averaging estimates from each of the cycles).

Cold neutron prompt gamma-ray activation analysis at NIST — Recent developments

The cold neutron capture prompt γ-ray activation analysis (CNPGAA) spectrometer located in the Cold Neutron Research Facility (CNRF) at NIST has proven useful for the analysis of hydrogen and other elements in a wide variety of materials. Modifications of the instrument and the CNRF have resulted in improved measurement capabilities for PGAA. The addition of an atmosphere-controlled sample chamber and Compton suppression have reduced γ-ray background and increased signal-to-noise ratio. More recent revisions are expected to yield still further improvement in analytical capabilities. Replacement of the D2O ice cold source with a liquid H2 moderator is expected to yield a 5–10 fold increase in neutron capture rate, and improved neutron and γ-ray shielding will result in further reduction of the background. Other modifications to the instrument allow easier sample mounting and more precise positioning of samples in the neutron beam. Significant improvements in detection limits and analytical accuracy are expected.

Capillary neutron optics for prompt-gamma activation analysis

A neutron lens has been constructed to focus cold neutrons from the exit of a58Ni neutron guide, which delivers a beam to the Prompt-Gamma Activation Analysis (PGAA) station at the NIST Cold Neutron Research Facility. The lens compresses a neutron beam of cross section 50 mm× 45 mm onto a focal spot of diameter 0.53 mm (fwhm) wich an average gain of 80 in neutron current density. PGAA measurements have been performed to demonstrate the enhanced sensitivity and detection limits for various elements and the spatial resolution in one transverse dimension. For the two test particles (a gadolinium glass bead and cadmium metal of sizes less than 0.5 mm), the gain in the γ-count rate with the lens is a factor of 60, and the detection limit is improved by a factor of 20. The system can be used for two-dimensional mapping of samples on a sub-millimeter scale to complement other analytical techniques such as neutron depth profiling (NDP).

Neutron Research Community

The News & Comment article “U.S. neutron scientists settle for less” by Andrew Lawler ([9 Aug., p. 728][1]) provides a sobering assessment of the current status of neutron facilities in the United States and prospects for new ones. I take issue, however, with the description of the neutron research community as “small and fractious.” The “fractious” aspects of our community (as Lawler amply demonstrates) are mainly within the U.S. Department of Energy and its competing national laboratories, not among neutron researchers, the large majority of whom come from universities and industries. Nor is our community “small.” Well over 1000 physicists, chemists, materials scientists, and engineers participate in research in the United States every year using neutron beams, and this number has doubled in the last decade. As an example, researchers using the reactor at the National Institute of Standards and Technology, which has recently developed the only internationally competitive cold neutron research facility (CNRF) in the United States, have tripled since the opening of the CNRF in 1990. Moreover, these numbers do not reflect broad and critical U.S. needs for isotopes and irradiation facilities applied to medicine and technology, which also require modern neutron sources. Unfortunately, our community has not benefited from development of a totally new neutron source in this country for more than 25 years, and we are continuing to fall behind our international competitors, a point clearly documented by Lawler. We must find a way to meet the increasing neutron research needs of modern U.S. science and technology. Even in the face of tight budgets, we in our field must unite to make a coherent case and set priorities for critically needed investments for a new neutron source and for upgrades to existing neutron facilities. This effort must be less concerned with the needs of any particular national laboratory than with providing the best capabilities for the nation. Failure to succeed in this mission could lead to a loss of any chance for the United States to be competitive in vital neutron measurements, nuclear medicine, and related technologies, as we move into the next century. [1]: /lookup/doi/10.1126/science.273.5276.728

Modeling detector response for neutron depth profiling

In Neutron Depth Profiling (NDP), inferences about the concentration profile of an element in a material are based on the energy spectrum of charged particles emitted due to specific nuclear reactions. The detector response function relates the depth of emission to the expected energy spectrum of the emitted particles. Here, the detector response function is modeled for arbitrary source and detector geometries based on a model for the stopping power of the material, energy straggling, multiple scattering and random detector measurement error. At the NIST Cold Neutron Research Facility, a NDP spectrum was collected for a diamond-like carbon (DLC) sample doped with boron. A vertical slit was placed in front of the detector for collimation. Based on the computed detector response function, a model for the depth profile of boron is fit to the observed NDP spectrum. The contribution of straggling to overall variability was increased by multiplying the Bohr Model prediction by a ramp factor. The adjustable parameter in the ramp was selected to give the best agreement between the fitted profile and the expected shape of the profile. The expected shape is determined from experimental process control measurements.

Cold neutron prompt gamma activation analysis at nist: A progress report

An instrument for prompt gamma-ray activation analysis is now in operation at the NIST Cold Neutron Research Facility (CNRF). The cold neutron beam is relatively free of contamination by fast neutrons and reactor gamma rays, and the neutron fluence rate is 1.5·108 cm−2·s−1 (thermal equivalent). As a result of a compact target-detector geometry the sensitivity is better by a factor of as much as seven than that obtained with an existing thermal instrument, and hydrogen background is a factor of 50 lower. We have applied this instrument to multielement analysis of the Allende meteorite and other materials.

A small-angle neutron scattering (SANS) study of worm-like micelles under shear

The structure of a cationic worm-like cylindrical micelle was investigated by SANS (small-angle neutron scattering). Intensities from 0.1% by weight solutions in D2O, at rest and under shear, were measured on the NIST Cold Neutron Research facility 30-m spectrometer in the wave vector range 0.03⩽Q (nm1)⩽-2.0. Scattered intensity patterns from the solutions subjected to shears equal to or greater than 40s1 showed pronounced anisotropy, but such anisotropy could not be detected below this apparent threshold shear. The threshold was characterized by a relaxation time since anisotropy was detected only after several minutes of shearing. In contrast, the anisotropy was apparent immediately the shear was applied at the higher shears. The data were analyzed based on the assumption that the micelles behave as rigid rods. Estimates of the radii and length under shear are given. Polydispersity in rod length is discussed, and we argue that it contributes significantly to the scattering patterns.

Neutron focusing using capillary optics

Using the principle of multiple mirror reflection from smooth surfaces at small grazing angles, polycapillary fibers of narrow inner channels (diameter of a few micrometers) have been used to transport and bend slow neutron beams. Neutrons in the cold and thermal range have been focused to a small spot to produce significant gains ( ~ × 10) in intensity. We report studies of neutron transmission properties of individual capillaries, using cold and thermal neutrons at the CNRF (Cold Neutron Research Facility) of the NBSR (the 20 MW research reactor at the National Institute of Standards and Technology, USA). Several aspects of these properties are investigated, namely, the transmission as a function of bending curvature, of length of the fibers, and of composition of fiber materials. From these studies we obtain information on the critical angle of reflection, the reflectivity, and the roughness of the inner surface. We also report the characterization of the first prototype neutron lens constructed at the Kurchatov Institute in Moscow and show that a focal point of 1 mm has been achieved. Finally we discuss the design of a lens suitable for neutron absorption experiments at the end of a neutron guide.

Measuring hydrogen by cold-neutron prompt-gamma activation analysis

By irradiating with cold neutrons and avoiding hydrogenous materials of construction, we have developed a PGAA instrument at the Cold Neutron Research Facility at NIST with hydrogen detection limits in the microgram range in many materials. Quantities of 5–10 μg H/g are presently measurable in gram-sized samples of silicon or quartz, and of order 0.01 wt % can be quantitatively measured in complex silicate rocks.

Cold neutron prompt gamma activation analysis at NIST: A progress report

An instrument for prompt gamma-ray activation analysis is now in operation at the NIST Cold Neutron Research Facility (CNRF). The cold neutron beam is relatively free of contamination by fast neutrons and reactor gamma rays, and the neutron fluence rate is 1.5·108 cm−2·s−1 (thermal equivalent). As a result of a compact target-detector geometry the sensitivity is better by a factor of as much as seven than that obtained with an existing thermal instrument, and hydrogen background is a factor of 50 lower. We have applied this instrument to multielement analysis of the Allende meteorite and other materials.

Design of a High-Flux Backscattering Spectrometer for Ultra-High Resolution Inelastic Neutron Measurements

ABSTRACTWe discuss the design of a new backscattering spectrometer to be installed at the Cold Neutron Research Facility at the National Institute of Standards and Technology. Si (111) crystals cover both monochromator and analyzer which are spherically bent to a radius of curvature of ~ 2 m to focus the incident and scatterered neutron beams. The bending increases the intrinsic lattice gradient of Si beyond its Darwin limit, resulting in an energy resolution of ~ 0.75 μeV FWHM. The monochromator is Doppler-driven, allowing users access to a dynamic range of ±60 μeV. The elastic Q-range covers 0.15 to 1.8 Å-1. The most novel aspect of this design lies in the incorporation of a phase-space-transform chopper. This device rotates at 4700 rpm while neutrons are Bragg-diffracted from sets of pyrolytic graphite crystals affixed to its periphery. The process enhances the neutron flux at the backscattering energy of 2.08 meV, but at the expense of a larger horizontal divergence. Computer simulations indicate a resultant flux increase of order 3 should be obtained.