"Development and Neutronic Validation of pelletized Cold and Very Cold Moderators for Pulsed Neutron Sources" Phase II Final report
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36 MATERIALS SCIENCE↗
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Improved Acquisition and Reconstruction for Wavelength-Resolved Neutron Tomography
Wavelength-resolved neutron tomography (WRNT) is an emerging technique for characterizing samples relevant to the materials sciences in 3D. WRNT studies can be carried out at beam lines in spallation neutron or reactor-based user facilities. Because of the limited availability of experimental time, potential imperfections in the neutron source, or constraints placed on the acquisition time by the type of sample, the data can be extremely noisy resulting in tomographic reconstructions with significant artifacts when standard reconstruction algorithms are used. Furthermore, making a full tomographic measurement even with a low signal-to-noise ratio can take several days, resulting in a long wait time before the user can receive feedback from the experiment when traditional acquisition protocols are used. In this paper, we propose an interlaced scanning technique and combine it with a model-based image reconstruction algorithm to produce high-quality WRNT reconstructions concurrent with the measurements being made. The interlaced scan is designed to acquire data so that successive measurements are more diverse in contrast to typical sequential scanning protocols. The model-based reconstruction algorithm combines a data-fidelity term with a regularization term to formulate the wavelength-resolved reconstruction as minimizing a high-dimensional cost-function. Using an experimental dataset of a magnetite sample acquired over a span of about two days, we demonstrate that our technique can produce high-quality reconstructions even during the experiment compared to traditional acquisition and reconstruction techniques. In summary, the combination of the proposed acquisition strategy with an advanced reconstruction algorithm provides a novel guideline for designing WRNT systems at user facilities.
47 OTHER INSTRUMENTATION↗
Demagnetization of Nd 2 Fe 14 B, Pr 2 Fe 14 B, and Sm 2 Co 17 Permanent Magnets in Spallation Irradiation Fields
Prompted by the need for radiation-resistant permanent magnets for insertion devices (IDs) of high-brilliance next-generation synchrotrons such as the National Synchrotron Light Source II, the demagnetization of Nd 2 Fe 14 B and Pr 2 Fe 14 B was studied after exposure to a mixed irradiating field. Degradation and damage of the permanent magnetic material by components of electromagnetic showers induced in magnets by intense high-energy electron beams will alter the magnetic field structure of the IDs. Plate-like Nd 2 Fe 14 B magnets were irradiated to 1.8 Grad dose and were evaluated against Pr 2 Fe 14 B magnets irradiated to a lower dose of 20 Mrad. In addition, annular Sm 2 Co 17 and Nd 2 Fe 14 B magnets integrated within a ferrofluidic feedthrough (FFFT) rotary seal were also irradiated to dose levels of 2 Grad for Sm 2 Co 17 and 20 Mrad for Nd 2 Fe 14 B. Post-irradiation measurements of the magnetic intensity revealed that severe demagnetization exceeding 85% occurs in Nd 2 Fe 14 B magnets after only 50 Mrad dose and over 87% for Pr 2 Fe 14 B after 10 Mrad dose. The annular-shaped Sm 2 Co 17 magnets of the FFFTs were almost insensitive to irradiation up to a dose of 2 Grad. Annular-shaped Nd 2 Fe 14 B magnets also showed resistance to demagnetization, a direct consequence of the annular shape which is characterized by the removal of the stronger demagnetizing field present at the center of a disk-like magnet. As a result, the sensitivity of boron-based permanent magnets to neutron energy (thermal versus fast) was also assessed via specifically designed experiments and discussed.
43 PARTICLE ACCELERATORS↗
Calculations of cosmogenic nuclide production rates in the Earth's atmosphere and their inventories
The production rates of cosmogenic isotopes in the Earth's atmosphere and their resulting terrestrial abundances have been calculated, taking into account both geomagnetic and solar-modulatory effects. The local interstellar flux was assumed to be that of Garcia-Munoz, et al. Solar modulation was accounted for using the heliocentric potential model and expressed in terms of the Deep River neutron monitor count rates. The geomagnetic field was presented by vertical cutoffs calculated by Shea and Smart and the non-vertical cutoffs calculated using ANGRI. The local interstellar particle flux was first modulated using the heliocentric potential field. The modulated cosmic-ray fluxes reaching the earth's orbit then interacted with the geomagnetic field as though it were a high-pass filter. The interaction of the cosmic radiation with the Earth's atmosphere was calculated utilizing the Bolztmann transport equation. Spallation cross sections for isotope production were calculated using the formalism of Silberberg and Tsao and other cross sections were taken from standard sources. Inventories were calculated by accounting from the variation in solar modulation and geomagnetic field strength with time. Results for many isotope, including C-14, Be-7 and Be-10 are in generally good agreement with existing data. The C-14 inventory, for instance, amounts to 1.75/sq cm(e)/s, in excellent agreement with direct estimates.
Obrien, K.↗
Relativistic Be-7 - A probe of cosmic-ray acceleration
The cosmic-ray Be-7/Be ratio was observed to be 0.6 below 500 MeV/nucleon, as expected from spallation reactions, but then to drop sharply to 0.4 by 1500 MeV/nucleon. This measurement suggests that primary cosmic rays traverse some material at their sources, producing Be-7 which picks up electrons and decays by K capture during or after the acceleration to relativistic energies. All cosmic rays then enter interstellar space, where further pickup is negligible.
Buffington, A.↗
Deuterium and He-3 in cosmic rays
Observation of a large flux of antiprotons in cosmic rays prompted many to postulate new ideas relating to the origin and propagation of cosmic rays in the Galaxy, within the framework of the secondary hypothesis. Under this hypothesis, cosmic rays traverse a large amount of matter either in the source region or in the interstellar space. As a result, large amounts of deuterium and He-3 are also produced as a consequence of spallation of helium and heavier nuclei. In this paper, the spectra of these isotopes are derived, using various models for the propagation of cosmic rays and compare with the existing observations.
Stephens, S. A.↗
Oak Ridge Hosted Second Graduate Course on Neutron Scattering Applications in Structural Biology
No abstract prepared.
60 APPLIED LIFE SCIENCES↗
Muon Muon Collider: Feasibility Study
A feasibility study is presented of a 2 + 2 TeV muon collider with a luminosity of L = 10{sup 35} cm{sup -2}s{sup -1}. The resulting design is not optimized for performance, and certainly not for cost; however, it does suffice - we believe - to allow us to make a credible case, that a muon collider is a serious possibility for particle physics and, therefore, worthy of R and D support so that the reality of, and interest in, a muon collider can be better assayed. The goal of this support would be to completely assess the physics potential and to evaluate the cost and development of the necessary technology. The muon collider complex consists of components which first produce copious pions, then capture the pions and the resulting muons from their decay; this is followed by an ionization cooling channel to reduce the longitudinal and transverse emittance of the muon beam. The next stage is to accelerate the muons and, finally, inject them into a collider ring wich has a small beta function at the colliding point. This is the first attempt at a point design and it will require further study and optimization. Experimental work will be needed to verify the validity of diverse crucial elements in the design. Muons because of their large mass compared to an electron, do not produce significant synchrotron radiation. As a result there is negligible beamstrahlung and high energy collisions are not limited by this phenomena. In addition, muons can be accelerated in circular devices which will be considerably smaller than two full-energy linacs as required in an e{sup +} - e{sup -} collider. A hadron collider would require a CM energy 5 to 10 times higher than 4 TeV to have an equivalent energy reach. Since the accelerator size is limited by the strength of bending magnets, the hadron collider for the same physics reach would have to be much larger than the muon collider. In addition, muon collisions should be cleaner than hadron collisions. There are many detailed particle reactions which are open to a muon collider and the physics of such reactions - what one learns and the necessary luminosity to see interesting events - are described in detail. Most of the physics accesible to an e{sup +} - e{sup -} collider could be studied in a muon collider. In addition the production of Higgs bosons in the s-channel will allow the measurement of Higgs masses and total widths to high precision; likewise, t{bar t} and W{sup +}W{sup -} threshold studies would yield m{sub t} and m{sub w} to great accuracy. These reactions are at low center of mass energy (if the MSSM is correct) and the luminosity and {Delta}p/p of the beams required for these measurements is detailed in the Physics Chapter. On the other hand, at 2 + 2 TeV, a luminosity of L {approx} 10{sup 35} cm{sup -2}s{sup -1} is desirable for studies such as, the scattering of longitudinal W bosons or the production of heavy scalar particles. Not explored in this work, but worth noting, are the opportunities for muon-proton and muon-heavy ion collisions as well as the enormous richness of such a facility for fixed target physics provided by the intense beams of neutrinos, muons, pions, kaons, antiprotons and spallation neutrons. To see all the interesting physics described herein requires a careful study of the operation of a detector in the very large background. Three sources of background have been identified. The first is from any halo accompanying the muon beams in the collider ring. Very carefully prepared beams will have to be injected and maintained. The second is due to the fact that on average 35% of the muon energy appears in its decay electron. The energy of the electron subsequently is converted into EM showers either from the synchrotron radiation they emit in the collider magnetic field or from direct collision with the surrounding material. The decays that occur as the beams traverse the low beta insert are of particular concern for detector backgrounds. A third source of background is e{sup +} - e{sup -} pair creation from {mu}{sup +} - {mu}{sup -} interaction. Studies of how to shield the detector and reduce the background are addressed in the Detector Chapter. Polarization of the muons allows many very interesting measurements which are discussed in the Physics Chapter. Unlike the electron collider in which the electron beam is highly polarized and the positron beam unpolarized, both muon beams may be partially polarized. It is necessary to select forward moving muons from the pion's decay and thus reduce the available number of muons and hence the luminosity. The necessary machine technology needed to achieve such a collider is discussed in the Option Chapter; at the moment it is not part of our point design, although such capability would almost certainly be incorporated into an actual device.
43 PARTICLE ACCELERATORS↗
Time Correlation Measurements of 252Cf Source Driven Fission Chains in Bare HEU Metal Assemblies
Passive multiplicity measurements of fissile metal assemblies are well established for measuring the multiplication, fissile content, and detection efficiency. In plutonium metal assemblies, most of the neutrons are unambiguously observed from fission chains because of the high spontaneous fission rate. However, uranium metal assemblies have low spontaneous fission rate such that the observed neutrons are a mixture of fission chain neutrons and cosmic-ray spallation neutrons. As a result, the multiplication, fissile content, and detection efficiency are difficult to measure without accurate quantification of the single neutron events unrelated to the fission chains. However, studying fission chains is simplified by knowing when each chain starts, which can be accomplished using a time-tagged active source such as a 252 Cf ionization chamber or a deuterium-tritium (D-T) neutron generator with an alpha detector. This paper presents time correlation measurements that have been performed with up to five 18 kg highly enriched uranium (HEU) storage castings and a 252 Cf ionization chamber placed near the center of each assembly. The measurements included liquid scintillators with pulse shape discrimination operating on the timescale of fission chains and 3 He proportional counters whose traditional use in multiplicity counting is well understood within a point-kinetics framework. Neutron time correlation distributions have been measured to characterize each assembly and are compared with the equivalent distributions collected passively.
252-Cf Ionization Chamber↗
Harvesting Isotopically Pure Ac-225 from Ra-225 Produced in Thorium Target Spallation Reaction
A method for isolating Ra-225 from linear accelerator irradiated Th metal chemical processing waste streams has been established. Separating Ra-225 from the complex matrix following proton irradiation of Th metal provides a source of isotopically pure Ac-225 free of the Ac-227 impurity that is present in accelerator-produced Ac-225. Ra isotopes are first separated from the irradiated Th target via cation exchange methods. Importantly, the Ra-containing fraction is cleaned from residual citrate via dilute nitric acid washes (0.5 M) on AG50W-X8 resin. The mixed Ra isotopes are then further purified from Ba-140 impurity using a Sr resin column. Ra elutes through the resin with successive washes of 2.5 M HNO 3 while Ba-140 is retained. Ac-225 is then harvested by loading the Ra content onto an AG50W-X8 cation exchange column where the Ra-225 parent is eluted with 2.5 M HNO 3 before the Ac-225 daughter is eluted with 8 M HNO 3 .
07 ISOTOPE AND RADIATION SOURCES↗
Gamma-ray line emission above 8 MeV
The spectra of energetic nuclei within astrophysical sources may be determined by studying gamma-ray line emissions above 8 MeV. Excited states which can be produced by inelastic scattering, charge exchange, and spallation reactions in the abundant nuclear species were considered in order to identify nuclear lines which may contribute to the gamma spectrum. The cross sections for production of most high-energy states are sparsely measured. Those which were determined are comparable to the cross section for production of the 15.11 MeV level in C-12 with few exceptions. The branching ratios for gamma-ray and particle emission are, however, better known. Of those states considered, 44 measured branching ratios greater than 40% for emission of a gamma-ray with energy above 8 MeV. For twelve more states the branching ratios, although not yet determined, are expected to be small. The gamma-ray emission from other individual nuclear states is not likely to be as great as that that for the 15.11 MeV state in C-12.
Crannell, C. J.↗
R -process Element Cosmic Rays from Neutron Star Mergers
Neutron star mergers (NSMs) are one of the most plausible sources of r -process elements in the universe. Therefore, NSMs can also be a major source of ultra-heavy elements in cosmic rays. In this paper, we first estimate the contribution of r -process elements synthesized in NSMs to the ultra-heavy element cosmic rays (UHCRs) by calculating transport equations that take into account energy loss processes and spallations. We show that the flux of UHCRs accelerated by the NSMs themselves fluctuates by many orders of magnitude on a timescale of several million years and can overwhelm UHCRs accelerated by supernova remnants (SNRs) after an NSM takes place within a few kiloparsec from the solar system. Experiments with very long exposure times using meteorites as UHCR detectors can detect this fluctuation. As a consequence, we show that if NSMs are the primary source of UHCRs, future experiments using meteorites may be able to reveal the event history of NSMs in the solar vicinity. We also describe a possible difference in the abundance pattern and energy spectrum of UHCRs between NSM and SNR accelerations.
79 ASTRONOMY AND ASTROPHYSICS↗
Intense fusion neutron sources
The review describes physical principles underlying efficient production of free neutrons, up-to-date possibilities and prospects of creating fission and fusion neutron sources with intensities of 10{sup 15}-10{sup 21} neutrons/s, and schemes of production and application of neutrons in fusion-fission hybrid systems. The physical processes and parameters of high-temperature plasmas are considered at which optimal conditions for producing the largest number of fusion neutrons in systems with magnetic and inertial plasma confinement are achieved. The proposed plasma methods for neutron production are compared with other methods based on fusion reactions in nonplasma media, fission reactions, spallation, and muon catalysis. At present, intense neutron fluxes are mainly used in nanotechnology, biotechnology, material science, and military and fundamental research. In the near future (10-20 years), it will be possible to apply high-power neutron sources in fusion-fission hybrid systems for producing hydrogen, electric power, and technological heat, as well as for manufacturing synthetic nuclear fuel and closing the nuclear fuel cycle. Neutron sources with intensities approaching 10{sup 20} neutrons/s may radically change the structure of power industry and considerably influence the fundamental and applied science and innovation technologies. Along with utilizing the energy produced in fusion reactions, the achievement of such high neutron intensities may stimulate wide application of subcritical fast nuclear reactors controlled by neutron sources. Superpower neutron sources will allow one to solve many problems of neutron diagnostics, monitor nano-and biological objects, and carry out radiation testing and modification of volumetric properties of materials at the industrial level. Such sources will considerably (up to 100 times) improve the accuracy of neutron physics experiments and will provide a better understanding of the structure of matter, including that of the neutron itself.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Evidence for a secular variation in the C-13/C-12 ratio of carbon implanted in lunar soils
A curve of delta(C-13) vs delta(N-15) for lunar soils and breccias shows that the previously recorded 30% change in delta(N-15) is associated with a change in delta(C-13). The correlation represents concurrent changes in the isotope ratios of both elements at their source, and does not result from maturation effects or nonselective sample contamination. A computation of the relative production rates of C-13 and N-15 shows that spallation reactions in the sun could produce the observed ratio of the delta(C-13) to delta (N-15) variations.
Becker, R. H.↗
DIFFUSIVE NUCLEAR BURNING OF HELIUM ON NEUTRON STARS
Diffusive nuclear burning (DNB) of H by an underlying material capable of capturing protons can readily consume H from the surface of neutron stars (NSs) during their early cooling history. In the absence of subsequent accretion, it will be depleted from the photosphere. We now extend DNB to He, motivated by the recent observation by Ho and Heinke of a carbon atmosphere on the NS in the Cassiopeia A supernova remnant. We calculate the equilibrium structure of He on an underlying {alpha} capturing material, accounting for thermal, mass defect, and Coulomb corrections on the stratification of material with the same zeroth order {mu}{sub e} = A/Z. We show that Coulomb corrections dominate over thermal and mass defect corrections in the highly degenerate part of the envelope. We also show that the bulk of the He sits deep in the envelope rather than near the surface. Thus, even if the photospheric He abundance is low, the total He column could be substantially larger than the photospheric column, which may have implications for rapid surface evolution ({approx}1 yr timescales) of NSs. When nuclear reactions are taken into account, we find that for base temperatures {approx}>1.6 x 10{sup 8} K, He is readily captured onto C. As these high temperatures are present during the early stages of NS evolution, we expect that the primordial He is completely depleted from the NS surface like the case for primordial H. We also find that magnetic fields {approx}<10{sup 12} G do not affect our conclusions. Armed with the results of this work and our prior efforts, we expect that primordial H and He are depleted, and so any observed H or He on the surfaces of these NS must be due to subsequent accretion (with or without spallation). If this subsequent accretion can be prevented, the underlying mid-Z material would be exposed.
79 ASTRONOMY AND ASTROPHYSICS↗
A Quick Overview of the Extended Feynman Project
The low natural background rates and the penetrating nature of neutron radiation makes neutron detection a good method for quantifying and accounting for large amounts of special nuclear material capable of neutron induced fission and fission chains. Fission is one of the few natural processes that produces time-correlated neutrons, the others are spallation-type processes (e.g. (n,Xn), cosmic induced background, etc) which have low but measurable rates in common terrestrial material. The high rates of most transuranic spontaneous fission sources of even a gram or less usually swamp any cosmic induced background rate but even Kilogram quantities of natural uranium produce neutrons only on the same order as that of the cosmic generated background flux. However the primary characteristic of special nuclear material is its ability to fission and to support fission chains through neutron induced fission. This means that neutrons produced from fission are not produced randomly singly but rather in time-correlated bursts. Almost from the beginning of the atomic age, the measurement of time-correlated neutrons has been used to detect and quantify fission and fission processes. Feynman himself proposed a method, the grandfather of what is used today, to compare correlation rates of neutron flux in a fixed time to that which would be expected of a random neutron source. The larger variance (broader distribution) as compared to a random distribution is a tell-tale sign of correlated production and commonly used as signature of fission and more importantly fission chains, the broader the distribution (and its departure from a random distribution) the higher the multiplication. Feynman’s original method was used to determine the criticality of the first reactor and other critical or near critical systems. We at LLNL have been developing this concept over the past decade and a half and have employed it for the detection and assessment of nuclear materials. Our method for doing this involved generating a theoretical prediction of what we might expect to see, given a neutron source (spontaneous fission with a nubar, or alpha-n) and a possibly multiplying material (Plutonium or Uranium) and comparing it to a measured result and deducing from this what it is we are measuring. We first developed these methods for portable (low efficiency) thermal neutron detectors that were used predominantly for emergency response situations and we have applied some of these techniques with thermal neutron detectors to arms control verification and authentication, for example verifying that there are multiplying quantities of plutonium in an unknown object. In recent years we have been exploring the use of fast neutrons using liquid scintillator cells with Pulse Shape Discrimination (PSD) and in this project we developing both the theory of fast particle production from fission and fission chains and the use of fast particle detection methods primarily for arms control verification and authentication, but for other uses. We have found that fast detection has many advantages, primarily because the speed of the detection (nanoseconds) preserves the original time scale of the particles produced by fission (and more importantly from induced fission) but also because the fast time scales allow for considering much shorter measurement times (binning) that greatly reduce the pollution of random correlations.
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
Shielding design for the LANSCE/WNR neutron flight paths - 25334
The shielding surrounding the spallation target and the neutron flight paths at the Los Alamos Neutron Science Center Weapons Neutron Research facility is simulated using a detailed MCNPX Monte Carlo model to determine neutron and γ-ray backgrounds in the experimental areas. In some cases the shielding needs to be modified depending on the requirements of planned experiments. (authors)
07 ISOTOPE AND RADIATION SOURCES↗
Design of a transportable high efficiency fast neutron spectrometer
A transportable fast neutron detection system has been designed and constructed for measuring neutron energy spectra and flux ranging from tens to hundreds of MeV. The transportability of the spectrometer reduces the detector-related systematic bias between different neutron spectra and flux measurements, which allows for the comparison of measurements above or below ground. The spectrometer will measure neutron fluxes that are of prohibitively low intensity compared to the site-specific background rates targeted by other transportable fast neutron detection systems. To measure low intensity high-energy neutron fluxes, a conventional capture-gating technique is used for measuring neutron energies above 20 MeV and a novel multiplicity technique is used for measuring neutron energies above 100 MeV. The spectrometer is composed of two Gd containing plastic scintillator detectors arranged around a lead spallation target. To calibrate and characterize the position dependent response of the spectrometer, a Monte Carlo model was developed and used in conjunction with experimental data from gamma ray sources. Multiplicity event identification algorithms were developed and used with a Cf-252 neutron multiplicity source to validate the Monte Carlo model Gd concentration and secondary neutron capture efficiency. The validated Monte Carlo model was used to predict an effective area for the multiplicity and capture gating analyses. For incident neutron energies between 100 MeV and 1000 MeV with an isotropic angular distribution, the multiplicity analysis predicted an effective area of 500 cm 2 rising to 5000 cm 2 . For neutron energies above 20 MeV, the capture-gating analysis predicted an effective area between 1800 cm 2 and 2500 cm 2 . As a result, the multiplicity mode was found to be sensitive to the incident neutron angular distribution.
46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗