The mission of Nuclear Services is to provide high quality analytical and irradiation services utilizing the irradiation and user facilities associated with the PULSTAR Reactor.
Facilities and Capabilities Include:
The reactor and experimental facilities are available for use by domestic and international academic, corporate, and government users. The NRP Nuclear Services Center facilitates all utilization of the reactor by external parties and serves as the primary point of contact for users.
The Positron Intense Beam Facility (PIBF) includes an Intense Positron Source (IPS) installed in beamport #6 of the PULSTAR Reactor, a Pulsed Positron Annihilation Spectrometer (e+-PAS), and a Positronium Positron Annihilation Spectrometer (Ps-PAS). The physical properties of the positron enables its application as a unique and nondestructive probe for materials characterization, especially the detection of the atomic-level open spaces, e.g. vacancies, free volumes, and voids, in condensed matter. More specifically, the positron annihilation events are measured in the time and energy domains by positron annihilation lifetime spectroscopy (PALS) and Doppler broadening spectroscopy (DBS) to obtain information directly related to the properties of these open spaces. Examples of these measurements include depth-profiled PAS on inhomogeneous vacancy, pore structure, and morphology that are intrinsic to materials, such as polymers, nanocomposites, thin films, membranes, and/or introduced by specific procedures. Additionally, in-situ measurement using the positron as a probe during thermal, mechanical, electrical, and adsorption processes is another major advantage of the PAS techniques.
Positron Annihilation Spectroscopies; PALS and DBS
The positron – the antimatter of the electron – has been used as a probe to study the defect properties of materials for several decades. Due to its positive charge, positrons can naturally diffuse to and be trapped in vacancies and open spaces of materials before they finally annihilate with electrons, and emit photons with total energy of 1.022 MeV – a complete conversion of mass to energy following Einstein’s famous equation, E=mC2. From the specific information in the energy and time domains of the e+-e- annihilation photons, characteristics of vacancy and defect can be extracted of the target materials. Positron annihilation spectroscopies are non-destructive, quantitative with unique sensitivities, and complementary to traditional intrusive and microscopic techniques. Additionally, instead of using positrons emitted from radioactive sources with broad energy distributions, by generating a mono-energetic positron beam, one can control the penetrating power of the positrons and examine depth dependent effects in thin films.
Positron Annihilation Lifetime Spectroscopy (PALS) measures the decay lifetime of positrons upon their first injection into the materials of interest. Immediately after their implantation, the positrons lose most of their kinetic energy by scattering with the host atoms and thermalize rapidly. The following diffusion process may result in the trapping of the positrons in defects and voids before their eventual annihilation with the environmental electrons. The positron annihilation lifetime is directly related to the electron density, which is sensitive to the concentration and type of vacancy defect and defect clusters.
In semiconductors and insulators, positrons can also form positronium, a meta-stable bound state of a positron and an electron. The lifetime of the positronium triplet state (spin of 1), also called ortho-positronium, is found to be directly related to the size of the voids where it annihilates, and is not sensitive to the material itself. This unique feature has found a wide variety of applications in characterizing nano-voids and free volumes in polymers and thin films. Therefore, PALS can also refer to positronium annihilation lifetime spectroscopy.
Another positron spectroscopic technique, Doppler Broadening Spectroscopy (DBS) of the annihilation irradiation, examines the energy spread of the 511 keV back-to-back annihilation gamma due to the momentum of the annihilating electron when the annihilation occurs. Since the electron energy levels vary in the vicinity of ion cores and vacancies, DBS provides complementary information of the vacancy type and density in the energy domain.
Traditional PALS and DBS directly using 22Na as a typical positron source, which emits high energy positrons with a wide energy spread, and is only suitable for studying thick (millimeter range) materials. The advent of mono-energetic positron beams with controllable positron implantation depth (see Fig. 3.3.4) enabled the study of thin films, surfaces, interfaces, and other depth-dependent localized/heterogeneous properties. In particular, beam-based positron spectroscopies have demonstrated extensive utility in characterizing very thin films (tens to hundreds of nanometers) on thick substrates. Intense positron sources (IPS), like the one at the NCSU PULSTAR reactor, would tremendously facilitate the use of PALS and DBS techniques in materials defect/vacancy characterizations, especially in data extensive and/or time restraining situations.
Embraced by the microelectronics industry as a standard characterization technique for low-k dielectrics, positron annihilation spectroscopy technique is sensitive to both the size and concentration of voids in solids and has found applications in the analysis of vacancies, nano-voids, and morphology of materials ranging from metals, semiconductors to insulators and composites. As an example of its application, PALS has been used to study the vacancy and vacancy clusters in nuclear graphite.
PAS can address important questions in a variety of polymers, such as PEMs and OPVs for energy applications, where free volumes play critical roles. For example, organic semiconductors are promising candidates for low-cost, flexible electronics that can be deposited by a simple printing process. The nanostructure of the OPV materials is believed to have profound impact on the final device performance. However, their exact correlation is still topics of debate. Since PALS is sensitive to the intermolecular packing, it could provide a straightforward and quantitative view on this matter and may even lead to a rapid route to predict the performance of organic semiconductors.
The application of PAS can also be extended to any two phase system where the interfacial behavior is interested, such as polymer nanocomposite, semicrystalline materials and highly porous materials. In some of these systems the electrical, chemical, and mechanical properties at the interface is crucial in determining the final performance of these systems. In model systems that are carefully made, PALS can be very useful in characterizing the confinement effect and even drawing critical correlation about the interfacial properties between thin films and nanocomposite. This can also be used as a tool to further study the nanocomposite during mechanical and thermal processes.
PAS can provide insight to damage initiation in polymers and epoxies due to physical relaxation, cyclic strain or fast impact. PAS may be used to evaluate fatigue in very early stages due to its sensitivity to nano-scale free volumes and may draw critical links between materials properties and chemical composition and/or cross-linking density. Works on polymeric fibers have shown direct correlation between the number of folding cycles and the varying degree of increase in the free volume voids detected by PALS. The damage only becomes obvious at much later stage using microscopy methods
Last but not the least, depth-profiled positron spectroscopies are useful in characterizing polymeric membranes for desalination or gas filtration where heterogeneous pore structure of multilayer films can be studied. The benefit of using positron as a probe in this type of studies is the fact that the activity and effect of the adsorbent can be monitored in-situ. PALS study on metal-organic framework (MOF) has directly revealed the evolution of nanostructure in MOF-5 during a monolayer adsorption of CO2 molecules.
Neutron diffraction data can be directly compared to theoretical models of crystal structure and composition, opening a wide variety of possible applications, such as:
New material crystal structure and composition: In the development of new materials, the correlation of structure, synthesis conditions and properties is essential.
Alloy atom location: The intensity of diffraction peaks is directly related to the atom species and fractional occupation of the crystal atom sites.
Solid – Solid phase transformation: Many materials change their crystal structure in response to changes in their environment (temperature and pressure). New crystal phases result in new diffraction peaks – the unmistakable signature of phase transformation.
Chemical and solid-state reaction pathways and kinetics: Many chemical and solid-state reactions can be monitored quantitatively through the intensity of the diffraction peaks corresponding to the various components.
Material stress – strain analysis: When crystalline materials are put under stress, the spacing between their atomic planes is changed. This results in a shift of the diffraction line positions. With neutron diffraction it is possible to probe material stress distributions in real engineering materials.
Magnetic ordering and magnetic structure determination.
The Neutron Imaging Facility (NIF) is located in beam port #5 of the PULSTAR Reactor and provides a powerful non-destructive imaging technique for the internal evaluation of materials or components. Neutron radiography involves the attenuation of a neutron beam by an object to be radiographed, and registration of the attenuation process (as an image) digitally or on film. It is similar in principle to X-ray radiography, and is complimentary in the nature of information supplied. The interactions of X-rays and neutrons with matter are fundamentally different, however, forming the basis of many unique applications using neutrons. While X-rays interact with the electron cloud surrounding the nucleus of an atom, neutrons interact with the nucleus itself.
Image Enhancement Techniques:
Examples of Neutron Imaging applications performed at NCSU include:
Coolant channel blockage in aircraft turbine blade castings
Water transport in operating PEM Fuel cell
Plant root growth in soil plaques (in-situ).
Spray pattern from fuel injector nozzle.
Additional applications may include:
Defects in silicon nitride (Si3N4) ceramics.
Imaging A/C refrigeration components for behavior of lubricating & cooling oils.
Detection of corrosion and entrapped moisture in mechanical structures.
Studying disbonding of carbon fiber composites (CFC's).
Measuring boron concentrations in shielding materials.
Measuring effectiveness of moisture repelling agents in building materials.
Imaging shock waves in gases.
Analysis of distribution of electrotransported hydrogen in palladium.
Quantitative evaluation of nuclear fuel pin structural features.
Imaging of wetting front instabilities in porous media.
Parameters for the NIF at NC State:
Neutron Flux = 3.2x106 to 3.7x107 n/cm2/sec
TNC ~ 70%
N/G = 4.43x104 to 1.34x106 cm-2mR-1
L/D = 100 to 150
Cd Ratio ~450
Scatter Content ~1.8%
The current aperture is a 1.57”×1.57” (1.7” effective diameter) square cross section opening in a BORAL plate, which yields an L/D ratio of ~150 at the 6.5 meter imaging plane. The resolution of the system is ~ 33 μm for conventional radiographic film. Measurements using ASTM standards show that the NIF achieves a beam quality classification of IA. Table 3.2.1 summarizes parameters associated with the NIF.
Instrumental Neutron Activation Analysis (INAA) is one of the most sensitive analytical techniques used for the quantitative multi-element analysis of major, minor, and trace elements in samples from almost every conceivable field of scientific or technical interest. The technique of INAA measures the total amount of an element present in sample matrices without regard to chemical or physical form and without any pre-treatment of the sample. For certain elements, INAA offers sensitivities that are superior to those possible by other techniques; on the order of parts per billion or better. In addition to the elemental and isotopic analysis of samples, INAA permits the analysis of non-radioactive tracers introduced into biological, chemical, and/or industrial processes for process identification and optimization.
Applications of INAA
Samples analyzed via INAA at NC State typically fall into one of the four general catagories listed below. For informational purposes, sample plots including detection limit (MDL) data have been attached for select elements in certain sample matrices.
I. Agricultural, Biological and Environmental Samples:
Agricultural products, benthic organisms, biological tissues & fluids (blood, hair, nails, organs, urine), bone, fertilizer, fish (tissue and eggs), food products, forensic samples, plants, sediment, sludges, vegetation, water, wood.
II. Industrial Samples: Crude oil, filter media & resins, fly ash, plastics, synthetic fibers, textiles.
III. High Purtiy Matrix Samples: Carbon/graphite, ceramics, chemicals, metallic alloys, pharmaceuticals, refractory kiln bricks, semiconductor substrates and processing materials.
IV: Geologic Samples: Coal, ores, rocks, soils/sediments.
Advantages of using INAA for trace element analysis:
It is a multi-element technique capable of determining approximately 65 elements in many types of materials (see list of elements below*);
It is non-destructive and therefore, does not suffer from the errors associated with yield determinations;
It has very high sensitivities for most of the elements that can be determined by INAA - most detection limits range from ~0.05 to ~50 ppm (≤ 1 ppb for some high-purity materials)
It is highly precise and accurate;
It permits the analysis of samples ranging in volume from 0.1 ml to 20 ml, and in mass from ~0.001 gram to 10 grams depending on sample density.
Samples for INAA can be solids, liquids, gases, mixtures, and suspensions.
*Elements that may be analyzed via INAA include: Ag, Al, As, Au, Ba, Br, Ca, Cd, Ce, Cl, Co, Cr, Cs, Cu, Dy, Er, Eu, F, Fe, Ga, Gd, Ge, Hf, Hg, Ho, I, In, Ir, K, La, Lu, Mg, Mn, Mo, Na, Nb, Nd, Ni, Os, Pd, Pr, Pt, Rb, Re, Rh, Ru, Sb, Sc, Se, Si, Sm, Sn, Sr, Ta, Tb, Te, Th, Ti, Tm, U, V, W, Y, Yb, Zn, & Zr.
The irradiation of materials, samples, and nuclear instrumentation may be carried out in any one of the numerous penetrations adjacent to the PULSTAR reactor core. Existing penetrations range in inner diameter (ID) from 1.25 inches to 8 inches (with one radial port 12 inches square) and have varying neutron and gamma energy spectra and intensities.
Examples of irradiation projects include:
Irradiation of materials in high temperature environments.
Accelerated lifetime testing of ex-core neutron compensated ion chambers, fission chambers, and BF3 detectors.
Sensitivity evaluation of in-core (miniature) neutron fission chambers.
Measuring radiation damage in fiber optic stress sensors.
Radiation hardness testing of electronic nuclear instrumentation modules.
Transmutation doping of Silicon wafers.
Internet Reactor Laboratories (IRL) are available to external academic institutions who wish to utilize the PULSTAR in demonstrating nuclear reactor operations and kinetics for their students. This capability enriches academic programs at universities without research reactors of their own, and may be used to expand the educational opportunities for nuclear engineering students throughout the United States and internationally. IRL sessions are hosted from the PULSTAR reactor control room utilizing video conferencing and online reactor instrumentation and data acquisition systems.
IRL participants are able to interact with reactor facility personnel through direct video and audio communication links, and have the ability to direct remote control cameras in the control room. An online data acquisition system is utilized to provide real time visualization of the reactor operating parameters, and for collecting experimental data.
Examples of Internet Reactor Laboratory sessions that are available include:
Introduction to Reactor Plant Systems - Review functionality and schematics of reactor instrumentation and SCRAM logic systems, primary and secondary cooling loops, and radiation monitoring and confinement systems.
1/M Approach to Criticality - Use source range monitor count rate indication to plot projections for critical reactor control rod positions. Apply 1/M plot methodology to approach criticality conservatively.
Control Rod Calibration (See Fig. 3.5.3) - Measure the reactivity worth of a reactor control rod as a function of position. Withdraw the rod to place the reactor on a period and use the Inhour equation to determine the resulting reactivity addition. Plot differential and integral rod worth curves.
Power Defect Measurement - Bring the reactor critical at incremental power levels between 100W and 1 MW. Measure the control rod reactivity worth at each power level and calculate power defect as a function of reactor power.
Axial Flux Mapping of a Fuel Assembly - Utilize an in-core fission chamber to measure the flux distribution inside a fuel assembly. Plot flux distribution as a function of core height and determine the assembly average axial peaking factor.
Heat Balance Power Calibration - Operate the reactor at several power levels up to 1 MW. Allow the reactor cooling system to stabilize at each level and record cooling loop temperature values. Use Delta-T values across the core and heat exchanger to perform a heat balance and determine true reactor power.
Fabrication and Materials Processing Services
Nuclear Services Center, Nuclear Reactor Program has not received any reviews.
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