Bremsstrahlung Signals Research Articles

Solution of the inverse problem definition surfaces at electron beam additive manufacturing

In this work, the inverse problem of surface restoration by the X-ray bremsstrahlung signal is posed in relation to the process of bead surfacing in additive manufacturing, and its solution is given using the iterative restoration method. Numerical implementation is carried out on the example of recovery of three types of symmetrical objects: Gaussian surface, hemisphere, cylinder. The comparison showed a good agreement between the reconstructed surfaces and the given ones.

Bremsstrahlung X-rays as a non-invasive tool for ion beam monitoring

Non-invasive beam monitoring tools are required to guarantee the delivered dose during ion irradiation. Measuring bremsstrahlung X-rays emitted directly from the irradiated medium is a promising approach. This work presents the feasibility of the method for different incident particles with different energies. A PMMA target was bombarded with helium ions of 68 MeV and protons beams of 30, 40, and 50 MeV. The bremsstrahlung signal, measured with a SDD, was in the magnitude of 104 X-rays/nC, indicating the significant sensitivity of the method. A good agreement was found (<6%) between the experimental and simulation bremsstrahlung spectra. For a thick target, the bremsstrahlung yield saturates and is directly proportional to the beam energy and the deposited dose at the surface. In addition, the bremsstrahlung spectrum shape contains information about the beam energy. Therefore, the bremsstrahlung signal gives access to the deposited dose and beam energy with precision depending on the experimental uncertainties.

Development of a predictive capability of short-pulse laser-driven broadband x-ray radiography

High intensity, short-pulse laser interaction with a solid metal target produces broadband hard x-rays potentially for various applications of x-ray radiography. Here experimental benchmarking of numerical modelling for short-pulse laser-driven broadband x-ray radiography is presented. Angular dependent x-ray spectra are first calculated with a hybrid particle-in-cell code, Large Scale Plasma (LSP), using fast electron parameters inferred from an analysis of measured bremsstrahlung signals. Subsequently, a calculated x-ray spectrum in the direction of radiography is used in photon transport calculations using a Monte Carlo code, Particle and Heavy Ion Transport code System (PHITS), to simulate a radiographic image including a modelled 3D test object, an x-ray attenuation filter and an image plate detector. Simulated radiographic images are compared with measurements obtained in an experiment using a 50-TW Leopard short-pulse laser at the University of Nevada Reno. Results show that simulations reproduce the experimental images well for three different attenuation filters (plastic, aluminium, and brass), while 1D transmission profiles for the plastic and aluminium filters are quantitatively in good agreement. The modelling approach established in this work could be used as a predictive tool to simulate radiographic images of complex 3D solid objects at any arbitrary angular position or to optimize experimental components such as the source spectrum, x-ray attenuation filters and a detector type depending on a radiographic object without carrying out radiographic experiments.

Development of broadband x-ray radiography for diagnosing magnetically driven cylindrically compressed matter

Experiments and modeling of x-ray radiography of millimeter diameter solid Al wires with laser-produced broadband x rays are reported. Experiments were performed using the 50-TW Leopard short-pulse laser in a laser and pulsed power chamber at the Nevada Terawatt Facility. To characterize broadband x rays and demonstrate a radiographic capability, bremsstrahlung, escaping electrons, and radiograph images of Al wires were simultaneously measured. The angularly resolved x-ray spectra are modeled by comparing measured bremsstrahlung signals in the range between 10 and ∼500 keV with hybrid particle-in-cell simulations. Transmission of Al wires from the radiograph images is further simulated with a Monte Carlo code. The measured transmission profiles of Al wires with three different diameters agree with calculations when a simulated x-ray spectrum composed of line emissions and bremsstrahlung is used with a source size of 600 ± 200 μm. Transmission calculations with only 22 keV Ag Kα or an exponential x-ray spectrum do not reproduce the measurement, suggesting that the accurate determination of an x-ray source spectrum, as well as the inclusion of the photon sensitivity of the detector, is critical in transmission calculations to infer the density of an object. The laser-based broadband x-ray radiography that was developed has been successfully implemented in a pulsed power chamber for future laser-pulsed-power coupled experiments.

Characterization of fast electron divergence and energy spectrum from modeling of angularly resolved bremsstrahlung measurements

Characteristics of fast electrons generated in an intense laser-solid target interaction are studied by modeling angularly resolved bremsstrahlung measurements with a hybrid Particle-In-Cell code, Large Scale Plasmas. The experiment was performed using the 50 TW Leopard laser at the Nevada Terawatt Facility. A 100 μm thick Cu foil was irradiated by the 15 J, 0.35 ps laser at a peak laser intensity of 2 × 1019 W/cm2. Bremsstrahlung produced by transport and recirculation of the fast electrons in the foil was measured with two differential filter-stack x-ray spectrometers at 22° and 40° from the laser axis. The two-spectrometer signals simultaneously fit by varying single slope temperatures (Thot) and divergence angles (θ) enable for determining Thot and θ to be 1.1 ± 0.3 MeV and 15° ± 8°, respectively. The Thot inferred from the bremsstrahlung signals agrees with that from an escaped electron measurement, suggesting that that the bremsstrahlung is predominantly produced by the transport of the high energy fast electrons in the first pass.

Zeff profile measurement system using a Thomson polychromator

To measure the effective ion charge Zeff profile, most plasma machines are equipped with a bremsstrahlung measurement system as a filter-scope diagnostic device. In Korean Superconducting Tokamak Advanced Research (KSTAR) machine, a new type of bremsstrahlung measurement system, called hybrid-type polychromator system, was developed and tested at a single position during the 10th KSTAR plasma campaign [1]. The main idea of this hybrid-type polychromator system is to combine a polychromator system, which can measure the Thomson scattering signal, with a bremsstrahlung signal measurement system. The advantages of this hybrid-type polychromator system include the reduction of diagnostic vacuum windows in the Tokamak or Stellarator, and the measurement of the Zeff parameter with electron temperature (Te) and electron density (ne) at a same position. In this study, we improve last year's hybrid-type polychromator system. A major improvement is the change in the structure of the optical system. This allows measuring the bremsstrahlung signals more clearly, and four hybrid-type polychromators were equipped to measure the Zeff profile preliminarily. In this paper, we explain the structure of the hybrid-type polychromator and the system's measurement results.

Sharp gamma-ray spectral features from scalar dark matter annihilations

The search for sharp features in the gamma-ray spectrum is a promising approach to identify a signal from dark matter annihilations over the astrophysical backgrounds. In this paper we investigate the generation of gamma-ray lines and internal bremsstrahlung signals in a toy model where the dark matter particle is a real scalar that couples to a lepton and an exotic fermion via a Yukawa coupling. We show that the Fermi-LAT and H.E.S.S. searches for line-like spectral features severely constrain regions of the parameter space where the scalar dark matter is thermally produced. Finally, we also discuss the complementarity of the searches for sharp spectral features with other indirect dark matter searches, as well as with direct and collider searches.

Characterization of the fast electrons distribution produced in a high intensity laser target interaction

Experiments on the Titan laser (∼150 J, 0.7 ps, 2 × 1020 W cm−2) at the Lawrence Livermore National Laboratory were carried out in order to study the properties of fast electrons produced by high-intensity, short pulse laser interacting with matter under conditions relevant to Fast Ignition. Bremsstrahlung x-rays produced by these fast electrons were measured by a set of compact filter-stack based x-ray detectors placed at three angles with respect to the target. The measured bremsstrahlung signal allows a characterization of the fast electron beam spectrum, conversion efficiency of laser energy into fast electron kinetic energy and angular distribution. A Monte Carlo code Integrated Tiger Series was used to model the bremsstrahlung signal and infer a laser to fast electron conversion efficiency of 30%, an electron slope temperature of about 2.2 MeV, and a mean divergence angle of 39°. Simulations were also performed with the hybrid transport code ZUMA which includes fields in the target. In this case, a conversion efficiency of laser energy to fast electron energy of 34% and a slope temperature between 1.5 MeV and 4 MeV depending on the angle between the target normal direction and the measuring spectrometer are found. The observed temperature of the bremsstrahlung spectrum, and therefore the inferred electron spectrum are found to be angle dependent.

Optimized dark matter searches in deep observations of Segue 1 with MAGIC

We present the results of stereoscopic observations of the satellite galaxy Segue 1 with the MAGIC Telescopes, carried out between 2011 and 2013. With almost 160 hours of good-quality data, this is the deepest observational campaign on any dwarf galaxy performed so far in the very high energy range of the electromagnetic spectrum. We search this large data sample for signals of dark matter particles in the mass range between 100 GeV and 20 TeV. For this we use the full likelihood analysis method, which provides optimal sensitivity to characteristic gamma-ray spectral features, like those expected from dark matter annihilation or decay. In particular, we focus our search on gamma-rays produced from different final state Standard Model particles, annihilation with internal bremsstrahlung, monochromatic lines and box-shaped signals. Our results represent the most stringent constraints to the annihilation cross-section or decay lifetime obtained from observations of satellite galaxies, for masses above few hundred GeV. In particular, our strongest limit (95% confidence level) corresponds to a ∼ 500 GeV dark matter particle annihilating into τ+τ−, and is of order ⟨σannv⟩ ≃ 1.2 × 10−24 cm3 s−1 — a factor ∼ 40 above the ⟨σannv⟩ ≃ thermal value.

Scalar dark matter models with significant internal bremsstrahlung

There has been interest recently on particle physics models that may give rise to sharp gamma ray spectral features from dark matter annihilation. Because dark matter is supposed to be electrically neutral, it is challenging to build weakly interacting massive particle models that may accommodate both a large cross section into gamma rays at, say, the Galactic center, and the right dark matter abundance. In this work, we consider the gamma ray signatures of a class of scalar dark matter models that interact with Standard Model dominantly through heavy vector-like fermions (the vector-like portal). We focus on a real scalar singlet S annihilating into lepton-antilepton pairs. Because this two-body final-state annihilation channel is d-wave suppressed in the chiral limit, σff̄v∝v4, we show that virtual internal bremsstrahlung emission of a gamma ray gives a large correction, both today and at the time of freeze-out. For the sake of comparison, we confront this scenario to the familiar case of a Majorana singlet annihilating into light lepton-antilepton pairs, and show that the virtual internal bremsstrahlung signal may be enhanced by a factor of (up to) two orders of magnitude. We discuss the scope and possible generalizations of the model.

Lower hybrid current drive efficiency in tokamaks and wave scattering by density fluctuations at the plasma edge

The turbulence in the scrape-off layer (SOL) plasma of FTU is characterized in order to assess its effect on the current drive efficiency of the lower hybrid (LH) waves. Amplitude, frequency and perpendicular wave vector of the fluctuations are measured for a variety of the main plasma conditions in front of the LH antenna together with the temperature and density in the SOL and used as inputs for the linear scattering theory of the LH waves developed many years ago. This theoretical model can account for both the frequency spectral broadening of the LH pump and the variations of the driven current, inferred by the perpendicular fast electron bremsstrahlung signals. The fraction of the LH power that is then deduced to be effective for current drive appears to be well related to the calculated optical thickness τ of the SOL. It drops as low as 40% as τ increases, consistent with the model prediction. Possible ways to control the SOL optical depth are investigated and a clear relation of the fluctuation level with the collisionality is found.

The observational signature of the first H ii regions

We use three-dimensional smoothed particle hydrodynamics simulations together with a dynamical ray-tracing scheme to investigate the build-up of the first H ii regions around massive Population III stars in minihaloes. We trace the highly anisotropic breakout of the ionising radiation into the intergalactic medium, allowing us to predict the resulting recombination radiation with greatly increased realism. Our simulations, together with Press-Schechter type arguments, allow us to predict the Population III contribution to the radio background at 100 MHz via bremsstrahlung and 21 cm emission. We find a global bremsstrahlung signal of around 1 mK, and a combined 21 cm signature which is an order of magnitude larger. Both might be within reach of the planned Square Kilometer Array experiment, although detection of the free-free emission is only marginal. The imprint of the first stars on the cosmic radio background might provide us with one of the few diagnostics to test the otherwise elusive minihalo star formation site.

Development of a virtual Zeff diagnostic for the W7-X stellarator

For the W7-X stellarator a diagnostic system for measurement of local Z(eff) values from the visible bremsstrahlung continuum is foreseen. The method is based on passive, absolute measurement of the bremsstrahlung intensity along several lines of sight. In order to eliminate the spurious impact of other radiation sources of different spectral intensity distribution than the bremsstrahlung, like the line radiation, it is intended to spectrally resolve the detected radiation, e.g., by use of microspectrometers. The visible bremsstrahlung background can be extracted by making use of the 1/lambda(2) dependence of its intensity (expressed in unit Wm(3) sr nm) in the high temperature plasmas by using Bayesian data analysis techniques. In a second step, the local values of Z(eff) as a function of the effective plasma radius are derived by inversion, using different regularization methods, of the line-integrated bremsstrahlung signals with the knowledge of the magnetic flux surfaces. Inversion of the full model based on statistical methods allows taking into account all conceivable uncertainties accompanying Z(eff) measurement and provides uncertainties of the local Z(eff) values and valuable information on other uncertain parameters of the model. In this paper we show the first steps in developing a virtual Z(eff) diagnostic for W7-X which allows the optimization of the statistical model as well as of the future diagnostic setup.

Time-resolved x-ray pinhole camera with grazing incidence mirror to eliminate bremsstrahlung noise signal on Z

An on-axis time-resolved x-ray pinhole camera has been used on the 20MA 100ns driver Z to image the converging shock wave in dynamic Hohlraum experiments and to image pellet hot spots in inertial confinement fusion implosions. This instrument is susceptible to detecting significant amounts of pinch bremsstrahlung radiation with energies at hundreds of keV and yields of roughly 1kJ. Quite often the bremsstrahlung noise signals have overwhelmed the desired x-ray images. In an effort to eliminate this large source of noise we have incorporated a 6° gold-coated grazing incidence mirror into the time-resolved x-ray pinhole camera system. The mirror reflects soft x rays at energies under 2keV but does not reflect bremsstrahlung radiation at hundreds of keV. We will present data from the instrument without the mirror showing large amounts of bremsstrahlung noise contamination and data with the mirror in the system showing greatly reduced noise levels.

Application of visible bremsstrahlung to a density monitor in steady state fusion reactor

The measurement of electron density is still important in a steady state fusion reactor in the future in addition to neutron and magnetic measurements in order to control the fusion output and to sustain stable plasma discharges. In the present toroidal fusion devices, FIR interferometer and Thomson scattering diagnostics have been used for the electron density measurement as an active diagnostic method. However, it is very difficult to operate such instruments continuously over a year. The use of visible bremsstrahlung as the passive diagnostics is, then, proposed as the electron density monitor instead of such active diagnostics. Since the effective charge, Z eff, is considered to be much less than 2 in the high-density fusion reactor, the bremsstrahlung intensity, which is proportional to the square of electron density, becomes a typical indicator of the electron density. In order to check the feasibility, the line-averaged density and the square root of visible bremsstrahlung signals were compared using two kinds of high-density LHD discharges in a range of (5–10) × 10 13 cm −3, such as H 2 gas puff and repetitively injected H 2 ice pellets discharges. A ratio of the square root of the visible bremsstrahlung signal to the density was a constant in both discharges. The visible bremsstrahlung profile was also compared with the density profile. The results obtained in the present study strongly suggested the use of the visible bremsstrahlung to the density monitor in the future reactor diagnostics.

A new method on recycling coefficient measurement using impurity pellet injection in a large helical device

Recycling coefficients of carbon, aluminum, and titanium were evaluated using a new technique combining impurity pellet injection with high-spatial resolution bremsstrahlung measurement in hydrogen and helium plasmas on the large helical device. The recycling coefficient of impurities was investigated by measuring absolute intensities with the visible bremsstrahlung array. The time evolution of the bremsstrahlung signals was modeled by an impurity transport code adjusting the diffusion coefficient, convective velocity, and recycling coefficient. As a result, a finite value of the recycling coefficient was required in the case of carbon, whereas aluminum and titanium were explained as nonrecycled particles. It was also clarified that the recycling coefficient of carbon had a larger value in hydrogen plasmas (R=0.5–0.65) than in helium plasmas (R=0–0.2), suggesting the formation of hydrogen molecules.

Experimental Study of Conventional and Multipass Thomson Scattering Diagnostic Systems

A low-cost, simple and effective multipass structure was built and tested for the first time in a theta-pinch device, the compact torus TC-1. A detailed analysis of the gain in the scattered signal produced by the multipass system is presented. The use of a multipass geometry for Thomson scattering experiments in theta-pinch devices or in more dense plasmas as well as in plasmas with higher Z eff is recommended for increasing the relationship between scattered and bremsstrahlung signals.

Detection of water proximity to tryptophan residues in proteins by single photon radioluminescence

We recently developed a single photon radioluminescence (SPR) technique to measure submicroscopic distances in biological samples [Bicknese et al., and Shahrokh et al., Biophys. J., 63 (1992) 1256–1279]. SPR arises from the excitation of a fluorophore by the energy deposited from a slowing beta decay electron. The purpose of this study was to detect 3H 2O molecules near tryptophan residues in proteins by tryptophan SPR. To detect small SPR signals, a sample compartment with reflective ellipsoidal optics was constructed, and amplified signals from a cooled photomultiplier were resolved by pulse-height analysis. A Monte Carlo calculation was carried out to quantify the relationship between SPR signal and 3H 2O-tryptophan proximity. Measurements of tryptophan SPR were made on aqueous tryptophan; dissolved melittin (containing a single tryptophan); native and denatured aldolase; dissolved aldolase, monellin, and human serum albumin; and the integral membrane proteins CHIP28 (containing a putative aqueous pore) and MIP26 using 3H 2O or the aqueous-phase probe 3H-3- O-methylglucose (OMG). After subtraction of a Bremsstrahlung background signal, the SPR signal from aqueous tryptophan (cps · μCi −1 μmol −1 ± SE) was 8.6 ± 0.2 with 3H 2O and 7.8 ± 0.3 with 3HOMG ( n = 8). With 3H 2O as donor, the SPR signal (cps · μCi −1 μmol −1) was 9.0 ± 0.3 for monomeric melittin in low salt (tryptophan exposed) and 4.6 ± 0.8 ( n = 9) for tetrameric melittin in high salt (tryptophans buried away from aqueous solution). The ratio of SPR signal obtained for aldolase under denaturing conditions of 8 M urea (fluorophores exposed) versus non-denaturing buffer (fluorophores buried) was 1.53 ± 0.07 ( n = 6). Ratios of SPR signals normalized to fluorescence intensities for monellin, aldolase, and human serum albumin, relative to that for d-tryptophan, were 1.42, 1.09, and 1.04, indicating that the cross-section for excitation of fluorophores in proteins is greater than that for tryptophan in solution. For the CHIP28 and MIP26 proteins in membranes, the ratio of SPR signal obtained with 3H 2O versus 3HOMG was 1.35 ± 0.13 (CHIP28, n = 5) and 0.99 ± 0.02 (MIP26). These data are consistent with the existence of an aqueous channel through CHIP28 that excludes small solutes. We conclude that tryptophan radioluminescence in proteins is measurable and provides unique information about the presence of local aqueous compartments.

Evaluation of particle effects in bremsstrahlung and in quantitative energy-dispersive x-ray microanalysis

The magnitude and the spectral form of bremsstrahlung signal contains much information about the target and is significantly influenced by the surface irregularities. By computer modelling, it is possible to extract the information to perform more accurate quantitative x-ray microanalysis for specimens with special geometries. In this research, bremsstrahlung spectra obtained from electron beams striking at selected locations of spherical particles were investigated by Monte Carlo simulations and experimental observations. The effects of surface geometries and the x-ray path lengths in the specimens on the variation of bremsstrahlung spectra and peak-to-background ratios were studied.The Monte Carlo electron trajectory calculations were determined with a screened Rutherford cross-section and a plural scattering model. The continuum x-ray depth profile is computed using Kramer's bremmstrahlung cross-section. The cross-section for inner shell ionization proposed by Green and Cosslett were employed for calculating characteristic x-rays. Bethe stopping power was determined by using the Luo-Joy modification at low energies. The mass absorption coefficient was calculated using Heinrich expression. The detector efficiency was determined experimentally.

Limiter bias experiments in DIII-D

In DIII-D, H α emission was reduced by a factor of 2–3 both at the limiter and at the inner wall by negative limiter biasing (V lim ≈ − 300 V). In these discharges, the line-average electron densities were approximately the same with and without limiter bias, which suggests that τ p was improved by negative bias. Edge bremsstrahlung emission signals were reduced by a factor of 2. Repetitive bursts were observed in Hα signals and edge bremsstrahlung signals. Impurity levels increased with limiter bias, which induced major disruptions. No improvement of τ E was observed. These experimental data suggest that particle confinement characteristics of limiter biased discharges are close to those with divertor H-mode.