What is the significance of nitrosamine contamination in medicines?

Topics for DTB review articles are selected by DTB's editorial board to provide concise overviews of medicines and other treatments to help patients get the best care. Articles include a summary of key points and a brief overview for patients. Articles may also have a series of multiple choice CME questions.

Forward modeling of Doppler velocity interferometer system for improved shockwave measurements.

When near-instantaneous shocks are recorded by using a Doppler velocity interferometer velocity interferometer system for any reflector (VISAR), they typically exceed the detector's ability to react and "skipped fringes" result, where its visibility briefly reduces. Traditionally, replacing skipped fringes required guesswork in analysis, which increased arrival time errors. Second, the use of long but velocity-sensitive interferometer delays with fast detectors which can resolve the delay has traditionally been avoided because of the fear of confusing the arrival time signal. However, shorter delays produce smaller fringe phase shifts per velocity and, thus, can decrease velocity precision. We realize that while some loss of fringe information occurs at shock events, this is often just a partial loss and the residual fringe information can still hold valuable information. We describe a forward model (FM) of the interferometer action and detector blurring that assists with VISAR fringe analysis at skipping events: (1) more precise shock arrival times, (2) arrival time precision not limited by long delays, and (3) improved ghost subtraction, which improves accuracy over a broad time region. We demonstrate the utility of a FM on National Ignition Facility and Laboratory for Laser Energetics Omega shots and discuss data process methods to increase the high time resolution of VISAR systems despite their limitations and complexities.

Benchmarking boron carbide equation of state using computation and experiment.

Boron carbide (B_{4}C) is of both fundamental scientific and practical interest due to its structural complexity and how it changes upon compression, as well as its many industrial uses and potential for use in inertial confinement fusion (ICF) and high-energy density physics experiments. We report the results of a comprehensive computational study of the equation of state (EOS) of B_{4}C in the liquid, warm dense matter, and plasma phases. Our calculations are cross-validated by comparisons with Hugoniot measurements up to 61 megabar from planar shock experiments performed at the National Ignition Facility (NIF). Our computational methods include path integral Monte Carlo, activity expansion, as well as all-electron Green's function Korringa-Kohn-Rostoker and molecular dynamics that are both based on density functional theory. We calculate the pressure-internal energy EOS of B_{4}C over a broad range of temperatures (∼6×10^{3}-5×10^{8} K) and densities (0.025-50 g/cm^{3}). We assess that the largest discrepancies between theoretical predictions are ≲5% near the compression maximum at 1-2×10^{6} K. This is the warm-dense state in which the K shell significantly ionizes and has posed grand challenges to theory and experiment. By comparing with different EOS models, we find a Purgatorio model (LEOS 2122) that agrees with our calculations. The maximum discrepancies in pressure between our first-principles predictions and LEOS 2122 are ∼18% and occur at temperatures between 6×10^{3}-2×10^{5} K, which we believe originate from differences in the ion thermal term and the cold curve that are modeled in LEOS 2122 in comparison with our first-principles calculations. To account for potential differences in the ion thermal term, we have developed three new equation-of-state models that are consistent with theoretical calculations and experiment. We apply these new models to 1D hydrodynamic simulations of a polar direct-drive NIF implosion, demonstrating that these new models are now available for future ICF design studies.

Ghost fringe removal techniques using Lissajous data presentation.

A VISAR (Velocity Interferometer System for Any Reflector) is a Doppler velocity interferometer which is an important optical diagnostic in shockwave experiments at the national laboratories, used to measure equation of state (EOS) of materials under extreme conditions. Unwanted reflection of laser light from target windows can produce an additional component to the VISAR fringe record that can distort and obscure the true velocity signal. Accurately removing this so-called ghost artifact component is essential for achieving high accuracy EOS measurements, especially when the true light signal is only weakly reflected from the shock front. Independent of the choice of algorithm for processing the raw data into a complex fringe signal, we have found it beneficial to plot this signal as a Lissajous and seek the proper center of this path, even under time varying intensity which can shift the perceived center. The ghost contribution is then solved by a simple translation in the complex plane that recenters the Lissajous path. For continuous velocity histories, we find that plotting the fringe magnitude vs nonfringing intensity and optimizing linearity is an invaluable tool for determining accurate ghost offsets. For discontinuous velocity histories, we have developed graphically inspired methods which relate the results of two VISARs having different velocity per fringe proportionalities or assumptions of constant fringe magnitude to find the ghost offset. The technique can also remove window reflection artifacts in generic interferometers, such as in the metrology of surfaces.

Holographic and time-resolving ability of pulse-pair two-dimensional velocity interferometry.

Previous velocity interferometers used at research laboratories for shock physics experiments measured target motion at a point or many points on a line on the target. Recently, a two-dimensional (2d) version (2d-velocity interferometer system for any reflector) has been demonstrated using a pair of ultrashort (3 ps) pulses for illumination, separated by 268 ps. We have discovered new abilities for this instrument, by treating the complex output image as a hologram. For data taken in an out of focus configuration, we can Fourier process to bring narrow features such as cracks into sharp focus, which are otherwise completely blurred. This solves a practical problem when using high numerical aperture optics having narrow depth of field to observe moving surface features such as cracks. Furthermore, theory predicts that the target appearance (position and reflectivity) at two separate moments in time are recorded by the main and conjugate images of the same hologram, and are partially separable during analysis for narrow features. Hence, for the cracks we bring into refocus, we can make a two-frame movie with a subnanosecond frame period. Longer and shorter frame periods are possible with different interferometer delays. Since the megapixel optical detectors we use have superior spatial resolution over electronic beam based framing cameras, this technology could be of great use in studying microscopic three-dimensional-behavior of targets at ultrafast times scales. Demonstrations on shocked silicon are shown.

Two-dimensional imaging velocity interferometry: data analysis techniques.

We describe data analysis procedures for an emerging interferometric technique for measuring target motion across a two-dimensional image at a moment in time, i.e., a snapshot 2D-VISAR. Conventional VISARs (velocity interferometer system for any reflector) are commonly used in shock physics to measure velocity history at a single point or many points across a line on target. These however are not recorded in two-dimensions and cannot be used with ultrashort pulsed illumination because the coherence length is smaller than the interferometer delay, preventing fringe formation. In our scheme, dual matched interferometers allow use of low- or incoherent illumination such as ultrashort laser pulses to freeze motion of target, allowing use of slow CCD imaging detectors. Quadrature phase recording and push-pull data analysis simultaneously produces an ordinary nonfringing reflectivity image and a fringing image. The latter is converted into a 2D-phase map which is proportional to target velocity. Example data on shocked crystalline [111] Si shows incipient features of ~50 µm scale.