SLAC Research Articles

Ionizing photon interactions modulate the optical properties of crystals with femtosecond scale temporal resolution

In this work, we continue our study of a new method for the detection of ionizing radiation with the potential for a dramatic improvement in coincidence time resolution (CTR) for time-of-flight positron emission tomography (ToF-PET) using the modulation of a material’s optical properties instead of the scintillation mechanism. Our previous work has shown that for non-scintillation materials such as bismuth silicon oxide (BSO) and cadmium telluride (CdTe), their refractive index can be modulated by annihilation photon interactions. The ultrafast nature of this process however remains unexplored. The ionizing radiation-induced charge carriers alter the local band structure in these materials, thus changing the complex refractive index. This mechanism is routinely used at the linac coherent light source (LCLS) facility of the SLAC National Accelerator Laboratory to measure x-ray pulse arrival times with femtosecond scale resolution for photon energies between 0.5 and 10 keV. The method described here follows that example by using a frequency chirped visible continuum pulse to provide a monotonic wavelength-to-time mapping by which one can measure the time-dependent refractive index modulation. In addition, we describe an interference-based measurement setup that allows for significantly improved sensitivity while preserving a timing precision of approximately 10 fs (σ) when measuring the arrival time of below 10 keV x-ray pulses with yttrium aluminum garnet (YAG) crystal. The method is presented in the context of ToF-PET application with further discussions on the potential CTR achievable if a similar detection concept is adopted for detecting 511 keV photons. Semi-empirical analysis indicates that the predicted CTR achievable is on the order of 1 ps (FWHM).

Virtual Diagnostic Suite for Electron Beam Prediction and Control at FACET-II

We discuss the implementation of a suite of virtual diagnostics at the FACET-II facility currently under commissioning at SLAC National Accelerator Laboratory. The diagnostics will be used for the prediction of the longitudinal phase space along the linac, spectral reconstruction of the bunch profile, and non-destructive inference of transverse beam quality (emittance) while using edge radiation at the injector dogleg and bunch compressor locations. These measurements will be folded into adaptive feedbacks and Machine Learning (ML)-based reinforcement learning controls to improve the stability and optimize the performance of the machine for different experimental configurations. In this paper we describe each of these diagnostics with expected measurement results that are based on simulation data and discuss progress towards implementation in regular operations.

Selecting XFEL single-particle snapshots by geometric machine learning.

A promising new route for structural biology is single-particle imaging with an X-ray Free-Electron Laser (XFEL). This method has the advantage that the samples do not require crystallization and can be examined at room temperature. However, high-resolution structures can only be obtained from a sufficiently large number of diffraction patterns of individual molecules, so-called single particles. Here, we present a method that allows for efficient identification of single particles in very large XFEL datasets, operates at low signal levels, and is tolerant to background. This method uses supervised Geometric Machine Learning (GML) to extract low-dimensional feature vectors from a training dataset, fuse test datasets into the feature space of training datasets, and separate the data into binary distributions of “single particles” and “non-single particles.” As a proof of principle, we tested simulated and experimental datasets of the Coliphage PR772 virus. We created a training dataset and classified three types of test datasets: First, a noise-free simulated test dataset, which gave near perfect separation. Second, simulated test datasets that were modified to reflect different levels of photon counts and background noise. These modified datasets were used to quantify the predictive limits of our approach. Third, an experimental dataset collected at the Stanford Linear Accelerator Center. The single-particle identification for this experimental dataset was compared with previously published results and it was found that GML covers a wide photon-count range, outperforming other single-particle identification methods. Moreover, a major advantage of GML is its ability to retrieve single particles in the presence of structural variability.

A high efficiency and low vibration liquid nitrogen cooling system for silicon crystal based x-ray optics.

A compact low-cost cryocooling system has been designed, constructed, and tested at SLAC National Accelerator Laboratory. The cooling power is provided by natural convection and phase change of the liquid nitrogen. The initial application was to cool silicon crystal optics to the sub-100 K range. A silicon crystal of dimension (width × depth × height) 50 × 50 × 30 mm3 has been used with an electric heater on the top surface in this prototyping test. This system can effectively provide more than 80 W of cooling power to the optics with a consumption of liquid nitrogen less than 2.1 l/h. The vibration of the silicon crystal was monitored during the tests with added electric heater power on the crystal. The vibration of the silicon crystal due to liquid nitrogen boiling is negligible.

A custom high-stability pulse generator for the test of a novel in-situ magnetic sensor developed to detect radiation damage in undulators

High-performance analog-to-digital converters (ADC) have been widely applied in many areas of science. For instance, magnetic field measurements based on the Faraday’s induction law require high-precision voltmeters to measure induced voltages. In the context of free-electron lasers technology, the Magnetic Measurements Team at SLAC National Accelerator Laboratory proposed a novel in-situ radiation damage detection system (RDDS) for detecting small field variations in undulators. The system measures the flux change in a flexible printed-circuit coil attached to the magnet array during the undulator gap movement. The gap movement changes the magnetic field, which induces a voltage signal in the coil that is measured and integrated with an ADC. Although the system is capable of detecting relative flux changes better than 100ppm, drift in the ADC’s gain or offset can cause apparent changes in the relative flux. This paper describes the first attempt to developed a high-precision verification circuit to perform ADC testing in the framework of the novel RDDS. The circuit generates a reference voltage pulse with a voltage–time integral relative precision better than 50ppm for a few hundreds of mVs — the typical order of magnitude measured with the RDDS. The circuit’s design combines a fast and precise switch with a low-noise voltage reference. Long-term measurements allowed statistical analysis and showed that averaging the voltage–time integral of ten pulses gives the required 50ppm stability. Moreover, reproducibility tests confirmed that the circuit’s output is invariable under small power supply instabilities and equipment shutdown. Instruments and applications designed to quantify the magnetic field by integrating voltage signals may use the pulse generator proposed in this paper for verification purposes.

Time-dependent effects in melting and phase change for laser-shocked iron

Using the LCLS facility at the SLAC National Accelerator Laboratory, we have observed X-ray scattering from iron compressed with laser driven shocks to Earth-core like pressures above 400GPa. The data shows shots where melting is incomplete and we observe hexagonal close packed (hcp) crystal structure at shock compressed densities up to 14.0 gcm-3 but no evidence of a double-hexagonal close packed (dhcp) crystal. The observation of a crystalline structure at these densities, where shock heating is expected to be in excess of the equilibrium melt temperature, may indicate superheating of the solid. These results are important for equation of state modelling at high strain rates relevant for impact scenarios and laser-driven shock wave experiments.

Search for lepton-flavor-violating decays D0→X0e±μ∓

We present a search for seven lepton-flavor-violating neutral charm decays of the type $D^{0}\rightarrow X^{0} e^{\pm} \mu^{\mp}$, where $X^{0}$ represents a $\pi^{0}$, $K^{0}_{\rm S}$, $\bar{K^{*0}}$, $\rho^{0}$, $\phi$, $\omega$, or $\eta$ meson. The analysis is based on $468$ fb$^{-1}$ of $e^+e^-$ annihilation data collected at or close to the $\Upsilon(4S)$ resonance with the BaBar detector at the SLAC National Accelerator Laboratory. No significant signals are observed, and we establish 90\% confidence level upper limits on the branching fractions in the range $(5.0 - 22.5)\times 10^{-7}$. The limits are between one and two orders of magnitude more stringent than previous measurements.

Evaluation of Skyshine from an Accelerator Facility: Dependence on Distance and Angle.

Neutron skyshine from Linac Coherent Light Source II 4 GeV electron beam operation at SLAC National Accelerator Laboratory can contribute to prompt radiation exposure to the public at distances far beyond the accelerator tunnel housing. One of the shielding design requirements at SLAC is that the annual dose to a member of the public is no more than 0.05 mSv y. This study uses Monte Carlo code FLUKA to simulate the generation of neutrons from 4 GeV electron beam losses on a thick copper target inside a generalized geometry of the Linac Coherent Light Source II Beam Transport Hall accelerator tunnel section. The effective dose from neutron skyshine was characterized as a function of both distance from the tunnel wall (up to 1 km away) and angle relative to the beam direction (between 0° and 180°). This new methodology for evaluating neutron skyshine dose is applicable to high-energy GeV-range electron accelerator facilities both at SLAC and elsewhere.

RF design and experimental test of a quadrupole-free X-band TM01 mode launcher

In this work, we present a low-power RF characterization of a novel TM <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">01</inf> X-band mode launcher for the new generation of high-brightness RF photo-injectors. The proposed structure employs a fourfold symmetry in order to minimize both the dipole and the quadrupole fields that could cause an emittance growth in the early stages of the acceleration process. A "cold" model aluminum structure, comprising two mode launchers connected by a central 62 mm-long circular waveguide, was fabricated by milling aluminum blocks, assembled and measured in back-to-back configuration. The low-power RF test, performed at the Istituto Nazionale di Fisica Nucleare Laboratori Nazionali del Sud (INFN-LNS), validated both the numerical simulations and the quality of the fabrication. An oxygen-free high-conductivity copper version of the device is being manufactured for high-power and ultra-high-vacuum tests that are planned to be conducted at the Stanford Linear Accelerator Center (SLAC).

Seventh User Workshop on High-Power Lasers at the Linac Coherent Light Source

We report on a seventh annual workshop in a series focused on science realized by the combination of hard X-ray free electron lasers with high power optical lasers, hosted at the SLAC National Accelerator Laboratory in Menlo Park, CA. Members from the user community of the Matter in Extreme Conditions (MEC) endstation of the Linac Coherent Light Source (LCLS) and other scientists met with local scientists to discuss developments at LCLS and MEC and related facilities, including experimental results and future plans.

Crystal Structure of CYP2C9*8 Genetic Variant Bound to an Anti‐Hypertensive Drug Losartan.

Cytochrome P450 (CYP) enzymes are one of the major xenobiotic metabolizing enzymes with increasing importance in pharmacogenetics, and more recently to pharmacogenomics. The human CYP2C9 is involved in the metabolism of over 15% of clinical drugs that include warfarin, losartan, tolbutamide, etc. More than eighty genetic variations have been identified in CYP2C9, and many of these have demonstrated significantly reduced activity compared with the wild‐type (WT) enzyme. The CYP2C9*8 allele, prevalent among African‐American population with a frequency of around 0.06, is associated with altered clearance of several drug substrates of CYP2C9. The *8 represents an amino acid variation from arginine to histidine at position 150 (R150H). The R150H variant was generated using CYP2C9 WT construct by site‐directed mutagenesis, and the enzyme was expressed in E. coli followed by protein purification and crystallization. The CYP2C9*8 was crystallized in the presence of the drug substrate losartan and the structure was determined using X‐ray crystallography at 2.3 Å resolution. The R150H, found on the surface of the protein on D‐helix that is distal from the active site, illustrates minimal effect on the overall conformation of the protein compared to the WT. Despite subtle changes in the structure itself, there were clear differences in the binding of losartan compared to the previously solved CYP2C9 WT complex. One molecule of losartan was bound in the active site and one on the surface, consistent to that observed in the WT complex. However, unlike the WT complex, the losartan in the access channel was not observed in the *8 complex. The region near the access channel was more compact than the WT enzyme. Furthermore, the losartan turn‐over rates measured using enzymatic assays differed significantly, with the variant demonstrating marked reduction in activity than the WT enzyme. Together, our findings from multiple techniques suggest an alternate mechanism may be involved in reduced activity of this variant located on the surface that leads to differences in binding of losartan near the active site. The results yield insights into the role of simultaneous binding of multiple substrate molecules, orientation of important amino acid side chains and altered hydroxylation profile of losartan with this variant.Support or Funding InformationThe X‐ray data collection at the Stanford Synchrotron Radiation Lightsource (SSRL) was done remotely. Use of the SSRL, SLAC National Accelerator Laboratory, is supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences under contract no. DE‐AC02076SF00515. The SSRL Structural Molecular Biology Program is supported by the DOE Office of Biological and Environmental Research, and by the NIH, NIGMS (P41GM103393). The research was funded by the start‐up funds from the Albany College of Pharmacy and Health Sciences.

Structural defect leads to human severe (Class I) loss of function in glucose‐6‐phosphate dehydrogenase

Oxidative stress caused by infection, medication, food, and imbalance of metabolic cycles damages DNA and organelles, which may lead to cancer, blood disorders, and other serious diseases. Glucose‐6‐phosphate dehydrogenase (G6PD) is the rate‐limiting enzyme in the pentose phosphate pathway, which is essential for nucleotide, fatty acid, cholesterol, and hormone synthesis. In addition to those roles, G6PD reduces NADP+ to NADPH, which is crucial for reducing reactive oxygen species. Dysfunction of G6PD increases susceptibility to oxidative stress, especially in erythrocyte due to the lack of mitochondria, which is another source of NADPH. Interestingly, 400 million people worldwide have mutations on the g6pd gene, and the World Health Organization (WHO) has classified more than 160 missense mutations into five classes. The Class I G6PD deficiency is the most severe form; patients have less than 10% enzymatic activity of wild‐type G6PD and suffer from chronic non‐spherocytic hemolytic anemia. Here, we provide a new insight into the loss of activity in G6PD deficiency, based on the structure of the Class I pathogenic mutants of G6PD. We will discuss the structural basis of the Class I mutants of G6PD and its implications to various symptoms in G6PD deficiency.Support or Funding InformationNational Institutes of Health, R01 grant, HD084422; Japan Society for the Promotion of Science KAKENHI, Grant‐in Aid for Research Activity Start‐up, JP19K23713; University of Tsukuba; Stanford University; SLAC National Accelerator Laboratory

Weak signal extraction using matrix decomposition, with application to ultra high energy neutrino detection

In radio-based physics experiments, sensitive analysis techniques are often required to extract signals at or below the level of noise. For a recent experiment at the SLAC National Accelerator Laboratory to test a radar-based detection scheme for high energy neutrino cascades, such a sensitive analysis was employed to dig down into a spurious background and extract a putative signal. In this technique, the backgrounds are decomposed into an orthonormal basis, into which individual data vectors (signal + background) can be expanded. This expansion is a filter that can extract signals with amplitudes ∼1 % of the background. This analysis technique is particularly useful for applications when the exact signal characteristics (spectral content, duration) are not known. In this proceeding we briefly present the results of this analysis in the context of test-beam experiment 576 (T576) at SLAC.

Visibility challenges for Asian scientists

Nine researchers, editors and science communicators share their views about the barriers that Asian scientists encounter in publishing their work and becoming more visible on the international level. Shravan Hanasoge has been a faculty member in the Department of Astronomy and Astrophysics at the Tata Institute of Fundamental Research, Mumbai, since 2013. He received his Ph.D. at Stanford University and then pursued a postdoctoral fellowship at the Max Planck Institute for Solar System Research and the Department of Geosciences, Princeton University. Noriaki Horiuchi is a senior editor at Nature Photonics, based in the Springer Nature Tokyo office. He obtained a Ph.D. from Universite Joseph Fourier, Grenoble, and then pursued postdoctoral research at Toyota Technological Institute and RIKEN. He joined Nature Photonics in 2009. Congcong Huang is a team manager and senior editor at Nature Communications, based in the Springer Nature Shanghai office. She obtained a Ph.D. from the University of Cambridge, UK, then pursued postdoctoral research at Stanford University and SLAC National Accelerator Laboratory in the USA. She joined Nature Communications in 2012. Hepeng Jia is a science communication researcher at the School of Communication, Soochow University in China and a science journalist, writing for the Nature journals and Chemical & Engineering News. He has a Ph.D. in science communication from Cornell University. Na Young Kim is an associate professor at the Institute for Quantum Computing, University of Waterloo, Canada, leading her Quantum Innovation laboratory. She has a Ph.D. from Stanford University and has also worked in industry for Apple Inc. Mio Murao is a professor at the Department of Physics, School of Science, The University of Tokyo, Japan. She has a Ph.D. from Ochanomizu University in Japan, then pursued postdoctoral research at Harvard University, Imperial College London and RIKEN before joining the University of Tokyo as a faculty member. Minah Seo is principal research scientist at the Sensor System Research Center, Korea Institute of Science and Technology, South Korea. She has a Ph.D. from Seoul National University, then pursued postdoctoral research at Los Alamos National Laboratory, USA. Rebecca Tan is the Editor in Chief of Asian Scientist Magazine, a science and technology magazine that highlights research and development news stories from Asia to a global audience. She has a Ph.D. from the National University of Singapore. Jens Wilkinson is communications officer at the International Affairs Division in RIKEN, Japan. He has been working in Japan for over 15 years.

Observation of Radar Echoes from High-Energy Particle Cascades.

We report the observation of radar echoes from the ionization trails of high-energy particle cascades. Data were taken at the SLAC National Accelerator Laboratory, where the full electron beam (∼10^{9} e^{-} at ∼10 GeV/e^{-}) was directed into a plastic target to simulate an ultrahigh-energy neutrino interaction. The target was interrogated with radio waves, and coherent radio reflections from the cascades were detected with properties consistent with theoretical expectations. This is the first definitive observation of radar echoes from high-energy particle cascades, which may lead to a viable neutrino detection technology for energies ≳10^{16} eV.

Status of the LCLS-II Cryogenic Distribution System

The LINAC Coherent Light Source II (LCLS-II) located at SLAC National Accelerator Laboratory (SLAC) in Menlo Park, CA, is a U.S. Department of Energy project tasked to design and build a world-class x-ray free-electron laser facility for scientific research. The Linac has superconducting radio frequency cryomodules that are connected to the cryogenic plant by the Cryogenic Distribution System (CDS), which consists of distribution boxes with heat exchangers and reliefs, feed caps, end caps, and surface, vertical, and bypass transfer lines. The CDS components were designed and built to specification by industry. The components have been delivered and their installation at SLAC will be discussed. The as-built relief system design will be presented, showing minimization of relief inlet pressure drops while meeting capacity requirements. The relieving flow pressure drops along the lengths of the CDS to centrally located reliefs at the distribution box were analysed to satisfy Pressure Vessel and Process Piping Code criteria to ensure relief performance. The sub-atmospheric 2 K circuit relieving approach will be discussed, which includes a three-way diverter isolation valve suitable for sub-atmospheric service. The component anchoring load design approach and installation into concrete floor will be discussed. This addresses loading, including seismic loading, along the component’s load path to floor anchoring system.

Search for Rare or Forbidden Decays of the D^{0} Meson.

We present a search for nine lepton-number-violating and three lepton-flavor-violating neutral charm decays of the type D^{0}→h^{'-}h^{-}ℓ^{'+}ℓ^{+} and D^{0}→h^{'-}h^{+}ℓ^{'±}ℓ^{∓}, where h and h^{'} represent a K or π meson and ℓ and ℓ^{'} an electron or muon. The analysis is based on 468 fb^{-1} of e^{+}e^{-} annihilation data collected at or close to the ϒ(4S) resonance with the BABAR detector at the SLAC National Accelerator Laboratory. No significant signal is observed for any of the twelve modes, and we establish 90%confidence level upper limits on the branching fractions in the range (1.0-30.6)×10^{-7}. The limits are between 1 and 3 orders of magnitude more stringent than previous measurements.

Laser-ionized, beam-driven, underdense, passive thin plasma lens

We present a laser-ionized, beam-driven, passive thin plasma lens that operates in the nonlinear blowout regime. This thin plasma lens provides axisymmetric focusing for relativistic electron beams at strengths unobtainable by magnetic devices. It is tunable, compact, and it imparts little to no spherical aberrations. The combination of these features make it more attractive than other types of plasma lenses for highly divergent beams. A case study is built on beam matching into a plasma wakefield accelerator at SLAC National Accelerator Laboratory’s FACET-II facility. Detailed simulations show that a thin plasma lens formed by laser ionization of a gas jet reduces the electron beam’s waist beta function to half of the minimum value achievable by the FACET-II final focus magnets alone.

Sidney David Drell (September 13, 1926–December 21, 2016): A Biographical Memoir

Sidney David Drell, professor emeritus at Stanford University and senior fellow at the Hoover Institution, died shortly after his 90th birthday in Palo Alto, California. In a career spanning nearly 70 years, Sid—as he was universally known—achieved prominence as a theoretical physicist, public servant, and humanitarian. Sid contributed incisively to our understanding of the electromagnetic properties of matter. He created the theory group at the Stanford Linear Accelerator Center (SLAC) and led it through the most creative period in elementary particle physics. The Drell-Yan mechanism is the process through which many particles of the Standard Model, including the famous Higgs boson, were discovered. Sid advised Presidents and Cabinet Members on matters ranging from nuclear weapons to intelligence, speaking truth to power but with keen insight for offering politically effective advice. His special friendships with Wolfgang (Pief) Panofsky, Andrei Sakharov, and George Shultz highlighted his work at the interface between science and human affairs. He advocated widely for the intellectual freedom of scientists and in his later years campaigned tirelessly to rid the world of nuclear weapons.

Suggestion of coherent radio reflections from an electron-beam induced particle cascade

Testbeam experiment 576 (T576) at the SLAC National Accelerator Laboratory sought to make the first measurement of coherent radio reflections from the ionization produced in the wake of a high-energy particle shower. The >10 GeV electron beam at SLAC End Station A was directed into a large high-density polyethylene target to produce a shower analogous to that produced by an EeV neutrino interaction in ice. Continuous wave (CW) radio was transmitted into the target, and receiving antennas monitored for reflection of the transmitted signal from the ionization left in the wake of the shower. We detail the experiment and report on preliminary hints of a signal consistent with a radio reflection at a significance of 2.36$\sigma$. We recommend another test-beam measurement in order to verify the signal.