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DOE OSTI2021

Reverse-Engineering Strain in Nanocrystallites by Tracking Trimerons

Although strain underpins the behavior of many transition-oxide-based magnetic nanomaterials, it is elusive to quantify. Since the formation of orbital molecules is sensitive to strain, a metal–insulator transition should be a window into nanocrystallite strain. Using three sizes of differently strained Fe 3 O 4 polycrystalline nanorods, the impact of strain on the Verwey transition and the associated formation and dissolution processes of quasiparticle trimerons is tracked. In 40 and 50 nm long nanorods, increasing isotropic strain results in Verwey transitions going from T V ≈ 60 K to 20 K. By contrast, 700 nm long nanorods with uniaxial strain along the (110) direction have T V ≈ 150 K—the highest value reported thus far. A metal–insulator transition, like T V in Fe 3 O 4 , can be used to determine the effective strain within nanocrystallites, thus providing new insights into nanoparticle properties and nanomagnetism.

36 MATERIALS SCIENCE↗

DOE OSTI2023

Broadband Photoconductive Sampling in Gallium Phosphide

Abstract Direct measurements of the electric field of light enable new observations of light–matter interactions. In the near‐infrared and visible spectral ranges, this typically relies on techniques that exploit nonlinearities in gases or solids, which limits their sensitivity. Here, a method for the detection of broadband near‐infrared fields spanning more than one octave from 110 to 220 THz based on linear absorption in a semiconductor is demonstrated. This technique, which avoids complex vacuum setups and works under ambient conditions, employs linear photoconductive sampling (LPS) in gallium phosphide. Simulations reveal that the response function of LPS is concerned with the intensity envelope of the gate field, in contrast to electro‐optic sampling, relaxing the stringent temporal requirements on the gate pulse.

36 MATERIALS SCIENCE↗

DOE OSTI2021

Computing the shape gradient of stellarator coil complexity with respect to the plasma boundary

Coil complexity is a critical consideration in stellarator design. The traditional two-step optimization approach, in which the plasma boundary is optimized for physics properties and the coils are subsequently optimized to be consistent with this boundary, can result in plasma shapes which cannot be produced with sufficiently simple coils. To address this challenge, we propose a method to incorporate considerations of coil complexity in the optimization of the plasma boundary. Coil complexity metrics are computed from the current potential solution obtained with the REGCOIL code (Landreman, Nucl. Fusion , vol. 57, 2017, 046003). While such metrics have previously been included in derivative-free fixed-boundary optimization (Drevlak et al. , Nucl. Fusion , vol. 59, 2018, 016010), we compute the local sensitivity of these metrics with respect to perturbations of the plasma boundary using the shape gradient (Landreman & Paul, Nucl. Fusion , vol. 58, 2018, 076023). We extend REGCOIL to compute derivatives of these metrics with respect to parameters describing the plasma boundary. In keeping with previous research on winding surface optimization (Paul et al. , Nucl. Fusion , vol. 58, 2018, 076015), the shape derivatives are computed with a discrete adjoint method. In contrast with the previous work, derivatives are computed with respect to the plasma surface parameters rather than the winding surface parameters. To further reduce the resolution required to compute the shape gradient, we present a more efficient representation of the plasma surface which uses a single Fourier series to describe the radial distance from a coordinate axis and a spectrally condensed poloidal angle. This representation is advantageous over the standard cylindrical representation used in the VMEC code (Hirshman & Whitson, Phys. Fluids , vol. 26, 1983, pp. 3553–3568), as it provides a uniquely defined poloidal angle, eliminating a null space in the optimization of the plasma surface. In comparison with previous spectral condensation methods (Hirshman & Breslau, Phys. Plasmas , vol. 5, 1998, p. 2664), the modified poloidal angle is obtained algebraically rather than through the solution of a nonlinear optimization problem. The resulting shape gradient highlights features of the plasma boundary that are consistent with simple coils and can be used to couple coil and fixed-boundary optimization.

Physics↗

DOE OSTI2026

Strain and Defect-Tailored Magnetotransport in NiCo 2 O 4 Thin Films and Freestanding Membranes

Magnetic spinel NiCo 2 O 4 is promising for developing spintronic applications due to its high magnetic Curie temperature, high spin polarization, fast spin dynamics, and strain-tunable magnetic anisotropy, while its electronic and magnetic properties depend sensitively on epitaxial strain and disorder. Here, in this study, we use epitaxial NiCo 2 O 4 thin films and freestanding NiCo 2 O 4 membranes as model systems to reveal the complex interplay of strain and defects in determining the metallicity and magnetotransport properties of the ferrimagnetic spinel. NiCo 2 O 4 on perovskite substrates and NiCo 2 O 4 membranes exhibit insulating behaviors and spin canting, in sharp contrast to the metallic NiCo 2 O 4 films on spinel substrates that possess strong perpendicular magnetic anisotropy. Anisotropic magnetoresistance studies provide critical information about disorder-induced spin scattering and strain-induced tetragonal magnetocrystalline anisotropy, which is corroborated by comprehensive electron microscopy characterizations. Our study presents a promising venue for designing flexible magnetic memory, sensor, and spintronic applications.

36 MATERIALS SCIENCE↗

DOE OSTI2020

Broken time-reversal symmetry in the topological superconductor UPt 3

Topological properties of materials are of fundamental as well as practical importance. Of particular interest are unconventional superconductors that break time-reversal symmetry, for which the superconducting state is protected topologically and vortices can host Majorana fermions with potential use in quantum computing. However, in striking contrast to the unconventional A phase of superfluid 3 He where chiral symmetry was directly observed, identification of broken time-reversal symmetry of the superconducting order parameter, a key component of chiral symmetry, has presented a challenge in bulk materials. The two leading candidates for bulk chiral superconductors are UPt 3 and Sr 2 RuO 4 , although evidence for broken time-reversal symmetry comes largely from surface-sensitive measurements. A long-sought demonstration of broken time-reversal symmetry in bulk Sr 2 RuO 4 is the observation of edge currents, which has so far not been successful. The situation for UPt 3 is not much better. In this work, we use vortices to probe the superconducting state in ultraclean crystals of UPt 3 . Using small-angle neutron scattering, a strictly bulk probe, we demonstrate that the vortices possess an internal degree of freedom in one of its three superconducting phases, providing direct evidence for bulk broken time-reversal symmetry in this material.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2019

Tunneling noise and defects in exfoliated hexagonal boron nitride

Hexagonal boron nitride (hBN) has become a mainstay as an insulating barrier in stackable nanoelectronics because of its large bandgap and chemical stability. At mono- and bilayer thicknesses, hBN can function as a tunnel barrier for electronic spectroscopy measurements. Noise spectroscopy is of particular interest, as noise can be a sensitive probe for electronic correlations not detectable by first-moment current measurements. In addition to the expected Johnson-Nyquist thermal noise and nonequilibrium shot noise, low frequency (<100 kHz) noise measurements in Au/hBN/Au tunneling structures as a function of temperature and bias reveal the presence of thermally excited dynamic defects, as manifested through a flicker noise contribution at high bias that freezes out as temperature is decreased. In contrast, broad-band high frequency (~250MHz – 580MHz) measurements on the same device show shot noise with no flicker noise contribution. The presence of the flicker noise through multiple fabrication approaches and processing treatments suggests that the fluctuators are in the hBN layer itself. Device-to-device variation and the approximate 1/ f dependence of the flicker noise constrain the fluctuator density to on the order of a few per square micron.

36 MATERIALS SCIENCE↗

DOE OSTI2016

Cross-sectional scanning tunneling microscopy of antiphase boundaries in epitaxially grown GaP layers on Si(001)

In a fundamental cross-sectional scanning tunneling microscopy investigation on epitaxially grown GaP layers on a Si(001) substrate, differently oriented antiphase boundaries are studied. They can be identified by a specific contrast and by surface step edges starting/ending at the position of an antiphase boundary. Moreover, a change in the atomic position of P and Ga atoms along the direction of growth is observed in agreement with the structure model of antiphase boundaries in the GaP lattice. This investigation opens the perspective to reveal the orientation and position of the antiphase boundaries at the atomic scale due to the excellent surface sensitivity of this method.

42 ENGINEERING↗

NASA NTRS2012

The Atacama Cosmology Telescope: Relation Between Galaxy Cluster Optical Richness and Sunyaev-Zel'dovich Effect

We present the measured Sunyaev-Zel'dovich (SZ) flux from 474 optically-selected MaxBCG clusters that fall within the Atacama Cosmology Telescope (ACT) Equatorial survey region. The ACT Equatorial region used in this analysis covers 510 square degrees and overlaps Stripe 82 of the Sloan Digital Sky Survey. We also present the measured SZ flux stacked on 52 X-ray-selected MCXC clusters that fall within the ACT Equatorial region and an ACT Southern survey region covering 455 square degrees. We find that the measured SZ flux from the X-ray-selected clusters is consistent with expectations. However, we find that the measured SZ flux from the optically-selected clusters is both significantly lower than expectations and lower than the recovered SZ flux measured by the Planck satellite. Since we find a lower recovered SZ signal than Planck, we investigate the possibility that there is a significant offset between the optically-selected brightest cluster galaxies (BCGs) and the SZ centers, to which ACT is more sensitive due to its finer resolution. Such offsets can arise due to either an intrinsic physical separation between the BCG and the center of the gas concentration or from misidentification of the cluster BCG. We find that the entire discrepancy for both ACT and Planck can be explained by assuming that the BCGs are offset from the SZ maxima with a uniform random distribution between 0 and 1.5 Mpc. In contrast, the physical separation between BCGs and X-ray peaks for an X-ray-selected subsample of MaxBCG clusters shows a much narrower distribution that peaks within 0.2 Mpc. We conclude that while offsets between BCGs and SZ peaks may be an important component in explaining the discrepancy, it is likely that a combination of factors is responsible for the ACT and Planck measurements. Several effects that can lower the SZ signal equally for both ACT and Planck, but not explain the difference in measured signals, include a larger percentage of false detections in the MaxBCG sample, a lower normalization of the mass-richness relation, radio or infrared galaxy contamination of the SZ flux, and a low intrinsic SZ signal. In the latter two cases, the effects would need to be preferentially more significant in the optically-selected MaxBCG sample than in the MCXC X-ray sample.

Sehgal, Neelima↗

NASA NTRS2017

Shaped pupil coronagraph design improvements for the WFIRST Coronagraph Instrument

NASA's WFIRST Coronagraph Instrument (CGI) is planned to image and characterize exoplanets at high contrast. The CGI operating modes are the hybrid Lyot coronagraph for planet detection and inner debris disk imaging, a shaped pupil coronagraph for planet characterization, and another shaped pupil for outer disk imaging. Early CGI designs focused on overcoming the di raction of the large pupil obscurations. Now that those designs have been proven to work in the lab, the design emphasis has been on achieving higher throughput and lower sensitivities to jitter and polarization aberrations. Here we present those improvements and the integration of science yield modeling into the spectrographic shaped pupil design process.

Riggs, A J Eldorado↗

DOE OSTI2018

UAVs for Radiological Monitoring: Determining Ideal Operational Parameters From GEANT4 Simulations - 18025

Owing in no small part to the enormous advances in radiation detector technology (size, sensitivity and spectral-resolution), there has been an increased drive and requirement to accurately map both the spatial extent and magnitude of radiological contamination at the greatest possible spatial resolutions. To undertake this, a range of different methods have been devised and include the use of cars and human, but also planes (and more recently) unmanned aerial vehicles (UAVs). However, critical to the remote sensing using such UAVs in contrast to ground-based sensing, is (i) the selection of the ideal operational parameters to attain the best on-ground resolution in addition to, (ii) understanding the correct normalisation procedures and other influences on results that surveys at these reduced altitudes might present. Resulting from earlier works, through employing combined modelling and experimental data, the optimal height for a UAV survey using the more realistic 'area' radioactive source term (opposed to an infinitely-small 'point-source') is determined to be approximately 5 m - suitably maximizing cross detector counts. At greater altitudes than this, the influence of variations in neighboring topology / topography and nearby structures are observed to be negligible. However, at this and further-reduced altitudes (where optimal count-rates exists), small height-related variations are observed to result, and impact considerably on the complexity of the data normalisation. Whilst not occurring as a tool to entirely replace ground-based surveys, UAV systems are able to perform in challenging or dangerous environments, inadvisable or problematic to send humans. (authors)

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗

DOE OSTI2016

Small-Angle Neutron Scattering Measurements of the δ Deuteride Phase in Zircaloy 4

Hydrogen pickup by Zr-based LWR fuel cladding during service leads to the formation of circumferential hydride platelet particles embedded within the Zr matrix. The precipitation of the hydride phase occurs preferentially on the water-side due to the temperature gradient within the clad wall. The non-uniform hydride morphology is known as a rim structure. Significant tensile hoop stresses within the clad wall develop during the fuel lifetime due to fission gas release and pellet-clad mechanical interaction. One potential post-service cladding failure mechanism is delayed hydride cracking (DHC) initiated by hydride reorientation under this tensile hoop stress. As a consequence, hydride reorientation and DHC have been extensively studied over the last few decades to understand the interdependencies of tensile stress, temperature, hydride morphology, and hydrogen concentration. Much of the most recent work in this area has taken advantage of 3. generation synchrotron X-ray sources to perform in situ measurements of hydride phase stability. Neutron scattering based techniques offer unique advantages in the study of hydrogen in metals (typically with deuterium substituted for hydrogen) but are under utilized in the study of hydrides in Zr. We present small-angle neutron scattering (SANS) measurements of the deuteride phase in Zircaloy 4 (Zy4) here. The SANS measurements presented here demonstrate the sensitivity of the technique to deuteride phase particles embedded in a Zy4 matrix. Quantifiable net scattering at 50 wppm effective hydrogen concentration is derived from a subtraction of a reference sample. This demonstrates that the strong χ phase Porod scattering can be effectively eliminated, revealing the response from the deuteride phase. A similar experimental protocol using small-angle X-ray scattering (SAXS) would likely fail due to poor contrast between the hydride phase and the Zr matrix. Stated in different terms, the SAXS response would be dominated by SPP Porod scattering with unresolvable net scattering from hydride particles. (authors)

36 MATERIALS SCIENCE↗

DOE OSTI2019

Surface-Enhanced Raman Scattering (SERS) Studies of Disc-on-Pillar (DOP) Arrays: Contrasting Enhancement Factor with Analytical Performance

The use of nanomachining methods capable of reproducible construction of nano-arrayed devices have revolutionized the field of plasmonic sensing by the introduction of a diversity of rationally engineered designs. Significant strides have been made to fabricate plasmonic platforms with tailored interparticle gaps to improve their performance for surface-enhanced Raman scattering (SERS) applications. Over time, a dichotomy has emerged in the implementation of SERS for analytical applications, the construction of substrates, optimization of interparticle spacing as a means to optimize electromagnetic field enhancement at the localized surface plasmon level, and the substrate sensitivity over extended areas to achieve quantitative performance. Here, we assessed the enhancement factor of plasmonic Ag/SiO 2 /Si disc-on-pillar (DOP) arrays of variable pitch with its analytical performance for quantitative applications. Experimental data were compared with those from finite-difference time-domain (FDTD) simulations used in the optimization of the array dimensions. A self-assembled monolayer (SAM) of benzenethiol rendered highly reproducible signals (RSD ~4–10%) and SERS substrate enhancement factor (SSEF) values in the orders of 10 6 –10 8 for all pitches. Furthermore, spectra corresponding to rhodamine 6G (R6G) and 4-aminobenzoic acid demonstrated the advantages of using the more densely packed DOP arrays with a 160 nm pitch (gap= 40 nm) for quantitation in spite of the strongest SSEF was attained for a pitch of 520 nm corresponding to a 400 nm gap.

36 MATERIALS SCIENCE↗

DOE OSTI2023

Ag Alloying in Cu 2– y Ag y Ba(Ge,Sn)Se 4 Films and Photovoltaic Devices

Trigonal Cu 2 BaGe 1–x Sn x Se 4 (CBGTSe) has recently gained interest as a potential photovoltaic absorber to target mitigation of antisite defect formation in Cu 2 ZnSn(S,Se) 4 . This study examines partial substitution of Cu by Ag as a potential approach to tune the properties of Ag-incorporated CBGTSe in the following aspects: 1) phase stability and crystal structure as a function of Ag content; 2) film morphology and grain structure; 3) charge carrier properties; 4) band positions; and 5) charge carrier kinetics and recombination. Up to 20% of Cu can be substituted by Ag in CBGTSe, while above 20% a phase mixture appears. Increasing Ag content induces larger average grain size and reduced hole carrier densities. In contrast, photoelectron spectroscopy and photoluminescence measurements reveal negligible impact of Ag substitution on ionization potential (≈5.4 eV) and electron affinity (≈3.7 eV). Also, Ag content offers negligible impact on carrier lifetimes (few ns). Consistent with these fundamental properties, solar cells based on two different Ag/(Ag + Cu) ratios (≈0% and ≈20%) show comparable power conversion efficiencies (≈2.7–2.8%). Finally, these results indicate that CBGTSe films and solar cells may be less sensitive to Ag substitution compared to Cu 2 ZnSn(S,Se) 4 , at least at the current level of absorber and device optimization.

14 SOLAR ENERGY↗

DOE OSTI2025

Glacial‐Interglacial and Millennial‐Scale Changes in Nitrous Oxide Emissions Pathways and Source Regions

During the transition from the Last Glacial Maximum (LGM) to the Holocene, the atmospheric N 2 O mole fraction increased by 80 nmol mol −1 . Using ice core measurements of N 2 O isotopomer ratios, we show that this increase was driven by increases in both nitrification and denitrification, with the relative partitioning between both production pathways depending on the assumed isotopic end-member source signatures. Similarly, we also attribute a 35 nmol mol −1 N 2 O mole fraction increase during the Heinrich Stadial 4/Dansgaard Oeschger 8 (HS4/DO8) millennial-scale event to increases in both N 2 O production pathways. In contrast, the 25 nmol mol −1 N 2 O mole fraction decrease during the Younger Dryas was driven almost exclusively by a decrease in nitrification. The deglacial and HS4/DO8 increases in N 2 O production occurred in both marine and terrestrial environments, with the terrestrial source responding faster to warming by about two centuries. Constraints on changes in nitrification and denitrification emissions are robust and consistent with previous studies showing the sensitivity of N 2 O emissions to abrupt Northern Hemisphere warming. This study demonstrates for the first time the importance of both denitrification and nitrification pathways in driving source changes. Absolute emissions are more uncertain due to uncertainty about source isotopomer signatures. For instance, the contribution of denitrification to emissions at the LGM shifts from (65 ± 10) % to (91 ± 6) % when factoring in isotope enrichment due to partial reduction of N 2 O to N 2 during denitrification. Reducing uncertainty in source signatures will increase the power of ice core N 2 O isotope records in deducing environmental change.

54 ENVIRONMENTAL SCIENCES↗

DOE OSTI2021

Crystal structure of silver pentazolates AgN 5 and AgN 6

Silver pentazolate, a high energy density compound containing the cyclo-N 5 – anion, has recently been synthesized under ambient conditions. However, due to high sensitivity to irradiation, its crystal structure has not been determined. In this work, silver–nitrogen crystalline compounds under ambient conditions and at high pressures, up to 100 GPa, are predicted and characterized by performing first-principles evolutionary crystal structure searching with variable stoichiometry. It is found that newly discovered AgN 5 and AgN 6 are the only thermodynamically stable silver–nitrogen compounds at pressures between 42 and 80 GPa. In contrast to AgN 5 , the pentazolate AgN 6 compound contains N 2 diatomic molecules in addition to cyclo-N 5 – . These AgN 5 and AgN 6 crystals are metastable under ambient conditions with positive formation enthalpies of 54.95 kJ mol –1 and 46.24 kJ mol –1 , respectively. Here, the underlying cause of the stability of cyclo-N 5 – silver pentazolates is the enhanced aromaticity enabled by the charge transfer from silver atoms to nitrogen rings. To aid in the experimental identification of these materials, calculated Raman spectra are reported at ambient pressure: the frequencies of N 5 – vibrational modes of AgN 5 are in good agreement with those measured in the experiment.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2025

Modeled sensitivity of multi-MA accelerator performance to electrode contaminant inventory

Significant particle-in-cell code development has enabled simulations of power flow in multi-MA accelerators to include the desorption of surface contaminants, their ionization into surface plasmas, and the impact of these plasmas on efficiency. The simulations base desorption on an Arrhenius equation, whose most significant unknown is the surface contaminant inventory. The sensitivity of power-flow simulations to this inventory is studied here using Sandia National Laboratories' Z accelerator with a 7-nH MagLIF load [Phys. Plasmas 17, 056303 (2010)]. Simulations are conducted in 3D cylindrical coordinates for the current-adder, or “convolute,” region of Z and in 2D for the final feed only. Simulated contaminant inventories are varied from 1 to 32 monolayers (MLs) in 2D, and 2 to 4 ML in 3D. The results reveal sensitivities to the local ratio of E/B⁠. The high B-field, low E-field region near the short-circuit load is insensitive to the contaminant inventory, where assumed values of 4–32 ML change the load current by ≤ 2%, and agree with experiment to within 2% at peak current. A 1-ML value is the outlier, increasing the load current by 5%, but still within measurement uncertainty. In contrast, the relatively higher E-field, lower B-field convolute region has slower contaminant desorption and higher-magnitude E-field penetration of the surface plasmas. The current loss in the convolute region does increase with contaminant inventory. The loss assuming 4 ML is 12% larger than for 2 ML, with 4 ML being the better match to experiment.

Arrhenius equation↗

DOE OSTI2016

Crystal structure and stability of gyrase–fluoroquinolone cleaved complexes from Mycobacterium tuberculosis

Mycobacterium tuberculosis (Mtb) infects one-third of the world’s population and in 2013 accounted for 1.5 million deaths. Fluoroquinolone antibacterials, which target DNA gyrase, are critical agents used to halt the progression from multidrug-resistant tuberculosis to extensively resistant disease; however, fluoroquinolone resistance is emerging and new ways to bypass resistance are required. To better explain known differences in fluoroquinolone action, the crystal structures of the WT Mtb DNA gyrase cleavage core and a fluoroquinolone-sensitized mutant were determined in complex with DNA and five fluoroquinolones. The structures, ranging from 2.4- to 2.6-Å resolution, show that the intrinsically low susceptibility of Mtb to fluoroquinolones correlates with a reduction in contacts to the water shell of an associated magnesium ion, which bridges fluoroquinolone–gyrase interactions. Surprisingly, the structural data revealed few differences in fluoroquinolone–enzyme contacts from drugs that have very different activities against Mtb. By contrast, a stability assay using purified components showed a clear relationship between ternary complex reversibility and inhibitory activities reported with cultured cells. Collectively, our data indicate that the stability of fluoroquinolone/DNA interactions is a major determinant of fluoroquinolone activity and that moieties that have been appended to the C7 position of different quinolone scaffolds do not take advantage of specific contacts that might be made with the enzyme. Lastly, these concepts point to new approaches for developing quinolone-class compounds that have increased potency against Mtb and the ability to overcome resistance.

59 BASIC BIOLOGICAL SCIENCES↗

DOE OSTI2015

Centrality dependence of particle production in p - Pb collisions at s NN = 5.02 TeV

We report measurements of the primary charged-particle pseudorapidity density and transverse momentum distributions in p-Pb collisions at √sNN=5.02TeV and investigate their correlation with experimental observables sensitive to the centrality of the collision. Centrality classes are defined by using different event-activity estimators, i.e., charged-particle multiplicities measured in three different pseudorapidity regions as well as the energy measured at beam rapidity (zero degree). The procedures to determine the centrality, quantified by the number of participants (N part ) or the number of nucleon-nucleon binary collisions (N coll ) are described. We show that, in contrast to Pb-Pb collisions, in p-Pb collisions large multiplicity fluctuations together with the small range of participants available generate a dynamical bias in centrality classes based on particle multiplicity. We propose to use the zero-degree energy, which we expect not to introduce a dynamical bias, as an alternative event-centrality estimator. Based on zero-degree energy-centrality classes, the N part dependence of particle production is studied. Under the assumption that the multiplicity measured in the Pb-going rapidity region scales with the number of Pb participants, an approximate independence of the multiplicity per participating nucleon measured at mid-rapidity of the number of participating nucleons is observed. Furthermore, at high-p T the p-Pb spectra are found to be consistent with the pp spectra scaled by N coll for all centrality classes. Our results represent valuable input for the study of the event-activity dependence of hard probes in p-Pb collisions and, hence, help to establish baselines for the interpretation of the Pb-Pb data.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗