Search

The size of a hard Kα x-ray source ($${\mathrm{E}}_{{\rm{K}}_{\rm{\alpha }}}$$ E K α = 17.48 keV) produced by a high intensity femtosecond laser interacting with a solid molybdenum target is experimentally investigated for a wide range of laser intensity (I ~ 1017–2.8 × 1019 W/cm2) and for four values of the temporal contrast ratio (6.7 × 107 10). Results point out the size enlargement of the x-ray source with the increase of laser intensity and with the deterioration of temporal contrast. It amounts up to sixteen times the laser spot size at the highest laser intensity and for the lowest temporal contrast ratio. Using hydrodynamic simulations, we evaluate the density scale length of the pre-plasma L/λ just before the main pulse peak. This allows us to show that a direct correlation with the laser absorption mechanisms is not relevant to explain the large size broadening. By varying the thickness of the molybdenum target down to 4 µm, the impact of hot electron scattering inside the solid is also proved irrelevant to explain the evolution of both the x-ray source size and the Kα photon number. We deduce that the most probable mechanism yielding to the broadening of the source size is linked to the creation of surface electromagnetic fields which confine the hot electrons at the solid surface. This assumption is supported by dedicated experiments where the evolution of the size enlargement of the x-ray source is carefully studied as a function of the laser focal spot size for the highest contrast ratio.

Cone-beam computed tomography (CBCT), a modern technique with many applications, is becoming increasingly important and essential in many areas of economics and life. CBCT equipment has been commercialized in both hardware and software. However, having a device system suitable for a specific group of objects will be the best. This work used Monte Carlo simulation to optimize the design of a Cone-beam Computed Tomography (CBCT) for lightweight and small-size casts. Factors affecting image quality such as radiation-source, flat-panel detector, and geometrical distance were investigated. Simulation results indicate that an X-ray generator with a focal spot size of 4 × 4 $${\upmu \mathrm{m}}^{2}$$ , high voltage of 240 kV, and a radiation intensity of $${10}^{13}$$ photon/s with a flat panel CsI (Tl) detector of 0.3 mm thickness and a pixel size of 0.1–0.2 mm, is suitable for a CBCT to inspect small objects in industry. The Monte Carlo model was validated against experiments and to evaluate some characteristics of the existing system.

Plasma 4(4), 670 - 680 (2021). doi:10.3390/plasma4040034, Published by MDPI, Basel

Purpose: Evaluate Acuros® XB dose calculation accuracy following TRS-483 recommendations in small static fields for flattened and un-flattened 6 MV X-ray beams. Methods: Field output factors were measured following TRS-483 recommendations using four radiation detectors. Two sets of field output factors were measured. One set was used to configure the beam model into Acuros® XB down to a jaw-defined field size of 1.0 cm × 1.0 cm. The second set was used to evaluate the differences between calculated and measured field output factors for MLC-fields down to a field size of 0.5 cm × 0.5 cm. Results: Acuros® XB showed an accuracy within 1.5% down to an MLC-field of 1.0 cm × 1.0 cm, for a focal spot size of 1.0 and 0.0 mm in the cross and in-plane directions. For an MLC-field of 0.5 cm × 0.5 cm, an agreement was found within 3% between calculated and measured field output factors. These results were addressed by optimizing the focal spot size to minimize the differences between calculated and measured dose profiles. Conclusions: By optimizing the focal spot size, Acuros® XB showed an acceptable agreement within 3% down to an MLC-field of 0.5 cm × 0.5 cm. The results of this work suggest that if static and modulated delivery of very small targets is planned, then a field output factor table down to a field size of 1.0 cm is required in the beam configuration model.

Veličina fokusne točke površina je na izvoru zračenja iz kojeg se zračenje emitira. Poznavanje veličine fokusne točke i njezinih značajki ključno je za izračun geometrijske neoštrine i provodi se na nekoliko različitih načina, od kojih je devet spomenuto u ovom radu. Ideja na kojoj se temelji zadatak ovog rada bila je provjeriti i istražiti mogućnosti karakterizacije fokusne točke u smislu procjenjivanja veličine fokusne točke izračunate prethodno određenom geometrijskom neoštrinom. Geometrijska neoštrina predstavlja gubitak oštrine slike te se teži da njena vrijednost bude što je manje moguća, a samim time da i veličina fokusne točke bude što manja. U sklopu ovog rada provedene su tri metode, pri čemu je prva metoda bila ona koju se želi provjeriti, dok su druge dvije metode služile kao kontrola prvoj. Za metode dupleks i pinhole napravljene su četiri različite geometrijske postavke snimanja, dok su za treću metodu u kojoj se koristio ERESCO generator zračenja, napravljene dvije. U geometrijskim postavima su se na određene udaljenosti od slikovne ploče (filma) postavljali objekt i izvor kako bi se dobilo više izračuna u svrhu procjene veličine fokusne točke. Prva metoda koja koristi dvostruke žičane indikatore rabila se za izračun geometrijske neoštrine, nakon čega je bilo moguće procijeniti veličinu fokusne točke. Manjom modifikacijom ove metode dobiveni rezultati bili su precizniji od onih propisanih normom. A focal spot size is an area on the source of radiation from which radiation is emitted. Determination of the focal spot size, as well as her characteristics, is crucial for determination of geometric unsharpness and it is done in several different ways, from which nine are mentioned in this paper. The idea on which this paper is based on was to check and explore the possibilities of characterization of focal spot in terms of estimating the size of the focal spot calculated by previously determined geometric unsharpness. Geometric unsharpness represents loss of image sharpness and the aim is to make its value as small as possible, and therefore also minimalize the focal spot size. As part of this work three methods were used, whereby the first one was the one that wants to be implemented, while the other two methods were used as supervision for the first method. For duplex and pinhole methods, four different geometric setups were done, while for the third one that uses ERESCO x-ray generator, only two were made. In those setups, distances from an object to a source and from the object to a film were being set at the certain values to get more calculations for assessment purposes. The primary method, that uses duplex indicators, was used to calculate geometric unsharpnes, after which it was possible to assess the value of the focal spot size. With small modification of this method, received results were more precise than those prescribed by the norm.

The problem of overcoming the diffraction limit does not have an unambiguously advantageous solution because of the competing nature of different beams’ parameters, such as the focal spot size, energy efficiency, and sidelobe level. The possibility to overcome the diffraction limit with suppressed sidelobes out of the near-field zone using superoscillating functions was investigated in detail. Superoscillation is a phenomenon in which a superposition of harmonic functions contains higher spatial frequencies than any of the terms in the superposition. Two types of superoscillating one-dimensional signals were considered, and simulation of their propagation in the near diffraction zone based on plane waves expansion was performed. A comparative numerical study showed the possibility of overcoming the diffraction limit with a reduced level of sidelobes at a certain distance outside the zone of evanescent waves.

Purpose To assess the task-based performance of images obtained under different focal spot size and acquisition mode on a dual-energy CT scanner. Methods Axial CT image series of the Catphan phantom were obtained using a tube focus at different sizes. Acquisitions were performed in standard single-energy, high resolution (HR) and dual-energy modes. Images were reconstructed using conventional and high definition (HD) kernels. Task-based transfer function at the 50% level (TTF50%) for teflon, delrin, low density polyethylene (LDPE) and acrylic, as well as image noise and noise texture, were assessed across all focal spots and acquisition modes using Noise Power Spectrum (NPS) analysis. A non-prewhitening mathematical observer model was used to calculate detectability index ( d NPW ′ ). Results TTF50% degraded with increasing focal spot size. TTF50% ranged from 0.67 mm−1 for teflon to 0.25 mm−1 for acrylic. For standard kernel, image noise and NPS-determined average spatial frequency were 8.3 HU and 0.29 mm−1, respectively in single-energy, 12.0 HU and 0.37 mm−1 in HR, and 7.9 HU and 0.26 mm−1 in dual-energy mode. For standard kernel, d NPW ′ was 61 in single-energy and HR mode and reduced to 56 in dual-energy mode. Conclusions The task-based image quality assessment metrics have shown that spatial resolution is higher for higher image contrast materials and detectability is higher in the standard single-energy mode compared to HR and dual-energy mode. The results of the current study provide CT operators the required knowledge to characterize their CT system towards the optimization of its clinical performance.

PURPOSE The purpose of this study was to quantify location and direction-dependent variations in the 3D modulation transfer function (MTF) of a high-resolution CT scanner with selectable focal spot sizes and resolution modes. METHODS The Aquilion Precision CT scanner (Canon Medical Systems) has selectable 0.25 mm or 0.5 mm detectors (by binning) in both the axial (x-y) and detector array width (z) directions. For the x-y and z orientations, detectors are configured (x-y) = 0.5 mm/(z) = 0.5 mm for normal resolution (NR), 0.25/0.5 mm for high resolution (HR), and 0.25/0.25 mm for super high resolution (SHR). Six focal spots (FS1-FS6) range in size from 0.4 (x-y) × 0.5 mm (z) for FS1 to 1.6 × 1.4 mm for FS6. Phantoms fabricated from spherical objects were positioned at radial distances of 0, 4.0, 7.5, 11.0, 14.5, and 18.5 cm. Axial and helical acquisitions were utilized and reconstructed using filtered back projection with the FC18 "Body," FC30 "Bone," and FC81 "Bone Sharp" kernels. The reconstructions were used to measure a 1D slice of the 3D MTF by oversampling the 3D ESF in the axial plane [MTF(fr ); φ = 0°)], 45° out of the axial plane [MTF(fr ); φ = 45°)], in the longitudinal direction [MTF(fr ); φ = 80°)], and along the radial and azimuthal directions within the axial plane. RESULTS The MTF(fr ); φ = 45°) drops to 10% (f10 ) at 1.20, 1.45, and 2.06 mm-1 for NR, HR, and SHR, respectively, for a helical acquisition with FS1, FC30, and r = 4 cm from the isocenter. The MTF(fr ); φ = 45°) includes contributions of both the axial-plane MTF (f10 = 1.10, 2.04, and 2.01 mm-1 ) and the longitudinal MTF (f10 = 1.17, 1.18, and 1.82 mm-1 ) for the NR, HR, and SHR modes, respectively. For SHR, the axial scan mode showed a 15-25% improvement over helical mode in the longitudinal resolution. Helical pitch, ranging from 0.569 to 1.381, did not appreciably affect the 3D resolution (

Purpose Stationary computed tomography (s-CT) conceptually offers several advantages over existing rotating gantry-based CT. Over the last 40 yr, s-CT has been investigated using different technological approaches. We are developing a s-CT system specifically for head/brain imaging using carbon nanotube (CNT)-based field emission x-ray source array technology. The noncircular geometry requires different assessment approaches as compared to circular geometries. The purpose of the present study is to investigate whether the CNT source array meets the requirements for stationary head CT (s-HCT). Methods Multiple prototype CNT x-ray source arrays were manufactured based on the system requirements obtained from simulation. Source characterization was performed using a benchtop setup consisting of an x-ray source array with 45 distributed focal spots, each operating at 120 kVp, and an electronic control system (ECS) for high speed control of the x-ray output from individual focal spots. Due to the forward-angled geometry of the linear anode, the projected focal spot shape is expected to vary at wide angle views. A pinhole method was implemented to determine the effective focal spot size (FSS) in the imaging plane at a range of angular viewpoints with a flat panel detector. The output spectrum and half value layer (HVL) were also evaluated for a range of viewing angles to characterize the beam quality across the fan-beam. Dosimetry was performed on a simulated scan to evaluate total exposure. Results The prototype CNT x-ray source array demonstrated adequate specifications for a s-HCT imaging machine. The source array was operated at 120 kVp with long-term stability over a full year of regular laboratory use. Multiple cathode current measurements were used to confirm submicrosecond accuracy with regards to exposure time and subsequently dose control. All 45 focal spots were measured with an average value of 1.26 (±0.04) mm × 1.21 (±0.03) mm (equivalent to IEC 1,0). The x-ray spectrum was found to be appropriately filtered based on sources used in existing rotary CT systems. A stable and reliable output of 0.04 mAs per emitter and a resulting dose of 0.015 mGy per projection were observed over several months of rigorous phantom imaging. Dose per projection was regulated by the ECS and measured with ±0.5% tolerance. Conclusions The CNT x-ray source array was found to meet the requirements for the proposed stationary head CT scanner, with regard to FSS, beam quality, and dose precision. The remaining challenges are related to the overall system design of a nonrotating CT scanner with distributed sources. The next phase of the project will incorporate multiple CNT source arrays with multirow detectors in a proof-of-concept study and analysis of a fully functional s-HCT system.

Purpose Microbeam radiation therapy is a preclinical concept in radiation oncology. It spares normal tissue more effectively than conventional radiation therapy at equal tumor control. The radiation field consists of peak regions with doses of several hundred gray, whereas doses between the peaks (valleys) are below the tissue tolerance level. Widths and distances of the beams are in the submillimeter range for microbeam radiation therapy. A similar alternative concept with beam widths and distances in the millimeter range is presented by minibeam radiation therapy. Although both methods were developed at large synchrotron facilities, compact alternative sources have been proposed recently. Methods and Materials A small-animal irradiator was fitted with a special 3-layered collimator that is used for preclinical research and produces microbeams of flexible width of up to 100 μm. Film dosimetry provided measurements of the dose distributions and was compared with Monte Carlo dose predictions. Moreover, the micronucleus assay in Chinese hamster CHO-K1 cells was used as a biological dosimeter. The focal spot size and beam emission angle of the x-ray tube were modified to optimize peak dose rate, peak-to-valley dose ratio (PVDR), beam shape, and field homogeneity. An equivalent collimator with slit widths of up to 500 μm produced minibeams and allowed for comparison of microbeam and minibeam field characteristics. Results The setup achieved peak entrance dose rates of 8 Gy/min and PVDRs >30 for microbeams. Agreement between Monte Carlo simulations and film dosimetry is generally better for larger beam widths; qualitative measurements validated Monte Carlo predicted results. A smaller focal spot enhances PVDRs and reduces beam penumbras but substantially reduces the dose rate. A reduction of the beam emission angle improves the PVDR, beam penumbras, and dose rate without impairing field homogeneity. Minibeams showed similar field characteristics compared with microbeams at the same ratio of beam width and distance but had better agreement with simulations. Conclusion The developed setup is already in use for in vitro experiments and soon for in vivo irradiations. Deviations between Monte Carlo simulations and film dosimetry are attributed to scattering at the collimator surface and manufacturing inaccuracies and are a matter of ongoing research.

The combination of brilliant liquid metal jet sources LMJS and polycapillary lenses yields substantial advantages in the excitation efficiency of traces of transition metals by micro X ray fluorescence spectroscopy amp; 956;XRF and confocal amp; 956;XRF in the laboratory. We show, that it is possible to especially adapt the parameters of polycapillary lenses to achieve an optimized performance in transmission and exit focal spot size. Furthermore, we demonstrate an energy dependent influence of the source spot size on the effective transmission of the lenses, which increases with decreasing field of view of the polycapillary lenses. A measurement on a mussel foot cross section using a amp; 956;XRF setup with a specially designed polycapillary lens in combination with the LMJS shows a significant increase of the excitation efficiency for vanadium. A 24 times higher net peak fluorescence intensity for V K radiation is achieved resulting in well defined and detailed vanadium structures compared to measurements performed with a state of the art amp; 956;XRF setup

Highlights • Line-focus X-ray tubes are suitable for clinical microbeam radiation therapy (MRT). • A modular high-voltage supply safely enables high electron beam powers. • An electron accelerator was designed to generate an eccentric focal spot. • We simulated a peak-to-valley dose ratio above 20 for single-field MRT. • Microbeam arc therapy spares healthy brain tissue compared to single-field MRT., Background and purpose Microbeam radiotherapy (MRT) is a preclinical concept in radiation oncology with arrays of alternating micrometer-wide high-dose peaks and low-dose valleys. Experiments demonstrated a superior normal tissue sparing at similar tumor control rates with MRT compared to conventional radiotherapy. Possible clinical applications are currently limited to large third-generation synchrotrons. Here, we investigated the line-focus X-ray tube as an alternative microbeam source. Materials and methods We developed a concept for a high-voltage supply and an electron source. In Monte Carlo simulations, we assessed the influence of X-ray spectrum, focal spot size, electron incidence angle, and photon emission angle on the microbeam dose distribution. We further assessed the dose distribution of microbeam arc therapy and suggested to interpret this complex dose distribution by equivalent uniform dose. Results An adapted modular multi-level converter can supply high-voltage powers in the megawatt range for a few seconds. The electron source with a thermionic cathode and a quadrupole can generate an eccentric, high-power electron beam of several 100 keV energy. Highest dose rates and peak-to-valley dose ratios (PVDRs) were achieved for an electron beam impinging perpendicular onto the target surface and a focal spot smaller than the microbeam cross-section. The line-focus X-ray tube simulations demonstrated PVDRs above 20. Conclusion The line-focus X-ray tube is a suitable compact source for clinical MRT. We demonstrated its technical feasibility based on state-of-the-art high-voltage and electron-beam technology. Microbeam arc therapy is an effective concept to increase the target-to-entrance dose ratio of orthovoltage microbeams.

A numerical simulation has been performed to study the effect of the mismatch of layer thicknesses of multilayer Laue lenses (MLLs) on X-ray focusing. The spatial distributions of X-ray intensities in the bulk of a lens have been calculated with different gradients of the variation of the period of a MLL. It has been shown that the focal length, as well as the focal spot size, decreases with an increase in the mismatch of layer thicknesses of the MLL. The calculated focal spot size are significantly different from the values reported in other works. A physical nature of the focusing by the MLL has been revealed. It is determined by the Bragg diffraction and is not related to the configuration of the Fresnel zone plate.

Phase contrast imaging (PCI) is used to extend X-ray computed nanotomography (nCT) technique for analyzing samples with a low X-ray contrast, such as polymeric structures or soft tissues. Although this technique is used in many variations at synchrotrons, along with the development of X-ray tubes and X-ray detectors, a PCI becomes available also for laboratory systems. This paper is focused on determining the conditions for propagation-based PCI in laboratory nCT systems based on three criteria. It is mostly employed in near-field imaging regime, which is quantified via the Fresnel number. X-rays must reach a certain degree of coherence to form edge enhancement. Finally, the setup of every computed tomography (CT) measurement has to avoid geometrical unsharpness due to the finite focal spot size. These conditions are evaluated and discussed in terms of different properties and settings of the CT machine. Propagation-based PCI is tested on a sample of carbon fibers reinforced polyethylene and the implementation of the Paganin phase retrieval algorithm on the CT data is shown.

This article presents a double-layer four-corner-fed slotted waveguide sparse array antenna designed for the near-field-focused applications in the X-band. Generally, it is challenging to realize the ideal phase distribution of the near-field focused array antenna, especially for the near-field focusing array antenna with a focal distance less than $5\lambda $ . We propose a four-corner-feed structure associated with appropriate waveguide dimensions to realize approximately ideal phase distribution for a focus at $1.5\lambda $ . The sparse array based on the particle swarm optimization is adopted to suppress the sidelobes and the grating lobes on the focal plane caused by the residual phase errors. A test antenna is designed at the center frequency of 10 GHz, and is fabricated by Direct Metal Laser Sintering technique for demonstration. It is verified by the experiment that we achieve satisfying near-field focusing effects, especially in terms of the focal spot size and sidelobe levels on the focal plane 45 mm away from the antenna aperture.

X-rays have the characteristics of high single photon energy, high frequency, and strong penetrating ability, are widely used in space communication, radiation imaging, medical diagnosis, non-destructive testing, and other fields. The development of X-rays has had a positive impact on broadening the use of the electromagnetic spectrum. As the core component of the X-ray application, high modulation rate, low power consumption, and excellent performance X-ray generators have been a hot research topic for decades. In this paper, a light-controlled pulsed X-ray source is modeled by simulation software. We perform simulation calculations and summary analysis on the effects of cathode structure, focus structure height, and metal anode voltage on the focal spot size, electron transit-time, and transit-time spread. At an operating voltage of 30KV, if the photocathode is 42mm in diameter, the focal spot diameter is about 1.5mm, the electron transit-time is 3.75ns, and the transit-time spread is 895ps when the focusing structure height is 6mm. And if the diameter of the photocathode is 12mm, the focal spot diameter is the smallest when the focusing pole height is 9mm, which is about 0.7mm, as for the time characteristics, the electron transit-time is 4.1ns, and the transit-time spread is 242ps. The results suggest that the light-controlled pulsed X-ray source is expected to be the next-generation source with easy modulation and short X-ray pulse properties.

High-energy, high-dose, microfocus X-ray computed tomography (HHM CT) is one of the most effective methods for high-resolution X-ray radiography inspection of high-density samples with fine structures. Minimizing the effective focal spot size of the X-ray source can significantly improve the spatial resolution and the quality of the sample images, which is critical and important for the performance of HHM CT. The objective of this study is to present a 9 MeV HHM CT prototype based on a high-average-current photo-injector in which X-rays with about 70μm focal spot size are produced via using tightly focused electron beams with 65/66μm beam size to hit an optimized tungsten target. In digital radiography (DR) experiment using this HHM CT, clear imaging of a standard 0.1 mm lead DR resolution phantom reveals a resolution of 6 lp/mm (line pairs per mm), while a 5 lp/mm resolution is obtained in CT mode using another resolution phantom made of 10 mm ferrum. Moreover, comparing with the common CT systems, a better turbine blade prototype image was obtained with this HHM CT system, which also indicates the promising application potentials of HHM CT in non-destructive inspection or testing for high-density fine-structure samples.

A circularly polarized vortex beam possesses similar focusing properties as a radially polarized beam. This type of beam is highly valuable for developing optical manufacturing technology, microscopy, and particle manipulation. In this work, a left-hand circularly polarized terahertz (THz) vortex beam (CPTVB) is generated by utilizing a THz quarter wave plate and a spiral phase plate. Focusing properties of its longitudinal component Ez are detailedly discussed on the simulation and experiment. With reducing the F-number of the THz beam and comparing with a transverse component Ex of a general circularly polarized THz beam, the simulation results show that the focal spot size and intensity of its Ez component can reach 87 and 50% of Ex under a same focusing condition. In addition, the experimental results still demonstrate that the left-hand CPTVB can always maintain fine Ez focusing properties in a broad bandwidth, which manifest the feasibility of this class of THz beams.

The laser stealth dicing system is a unique wafer processing system to enable high-throughput and debris-free wafer dicing. In the laser stealth dicing system, a focused pulsed laser forms a modification layer and cracks inside a silicon wafer. During pulsed laser radiation and wafer scanning, cracks formed in the previous shot interfere with a focusing laser pulse on a defocus plane. As a result, a part of interacted laser beam scatters, generating back-side splash defects on the device layer. An asymmetric beam shaping of defocused spot becomes necessary to minimize the splash defects. At the same time, the focused spot should maintain sufficiently small focused spot size to generate a modified layer inside the silicon wafer. This paper presents a concept of an innovative focusing spot shaping with an asymmetrical pupil phase filter which is individually optimized for both defocused and focused spots. We optimized the phase filter to generate a threedimensionally- asymmetrical focused spot which deforms defocused spot with a near diffraction-limited focal spot size. The through-focus spot shaping technique is enable to minimize splash defects and improve the yield of the laser wafer dicing process.

It is shown, theoretically and numerically, that the distributions of the longitudinal energy flow for tightly focused light with circular and linear polarization are the same, and that the spot has circular symmetry. It is also shown that the longitudinal energy flows are equal for optical vortices with unit topological charge and with radial or azimuthal polarization. The focal spot has a minimum diameter (all other characteristics being equal), which is measured based on the intensity of an optical vortex with azimuthal polarization. The diameter of the focal spot calculated from the energy flow for light with circular or linear polarization is slightly larger (by a fraction of a percentage). The magnitude of the diameter based on the intensity plays a role in the interaction of light with matter, and the magnitude of the diameter based on the energy flux affects the resolution in optical microscopy which is crucial in sensorial applications.

Demands on high quality x-ray imaging techniques are required for early detection of abnormal tissues and quality control of mass production in the field of medical and industrial applications.Moreover, smaller focal spot size (FSS) of x-ray sources have been developed for many decades but still their structure and operational scheme are complicated. To optimize electron beam trajectory, at least two types of focusing electrodes are needed. We demonstrate under 0.5 mm FSS of cold cathode x-ray source triode type e-beam module with vertically aligned carbon nanotubes (VA-CNTs) without any additional focusing electrodes. In addition, gate hole offset effects were investigated to FSS.The authors anticipate that our simple but sophisticated gate structure will be beneficial for high quality x-ray imaging and cost effective in terms of massive production of cold cathode x-ray generators.

This Letter reports acoustic-resolution-photoacoustic microscopy (AR-PAM) based on a new optically transparent focused polyvinylidene fluoride (PVDF) transducer with a high acoustic numerical aperture (NA) of 0.64. Owing to the improved fabrication process, the new transducer has a much higher NA (0.64) than the previously reported low-NA transducer ( N A = 0.23 ). The acoustic center frequency and (pulse-echo) bandwidth are also increased to 36 and 44 MHz, respectively, which provides a 38 µm acoustic focal spot size and 210 µm acoustic depth of focus. For demonstration, AR-PAM was conducted on a twisted wire target in water and chicken breast tissue, and in vivo on a mouse tail. The imaging results show that high acoustic resolution and sensitivity can be achieved with a simple and compact setup to resolve the target at different depths. Such capabilities can be useful for the development of new AR-PAM systems for handheld, wearable, and even endoscopic imaging applications.

The forthcoming generation of multi-petawatt lasers opens the way to abundant pair production by the nonlinear Breit-Wheeler process, i.e., the decay of a photon into an electron-positron pair inside an intense laser field. In this paper we explore the optimal conditions for Breit-Wheeler pair production in the head-on collision of a laser pulse with gamma photons. The role of the laser peak intensity versus the focal spot size and shape is examined keeping a constant laser energy to match experimental constraints. A simple model for the soft-shower case, where most pairs originate from the decay of the initial gamma photons, is derived. This approach provides us with a semi-analytical model for more complex situations involving either Gaussian or Laguerre-Gauss (LG) laser beams. We then explore the influence of the order of the LG beams on pair creation. Finally we obtain the result that, above a given threshold, a larger spot size (or a higher order in the case of LG laser beams) is more favorable than a higher peak intensity. Our results match very well with three-dimensional particle-in-cell simulations and can be used to guide upcoming experimental campaigns., Comment: 34 pages, 11 figures, submitted to New J. Phys

Purpose To investigate the applicability of output correction factors reported in TRS-483 on 6-MV small-field detector-reading ratios using four solid-state detectors. Also, to investigate variations in 6-MV small-field output factors (OF) among nominally matched linear accelerators (linacs). Methods The TRS-483 Code of Practice (CoP) introduced and provided output correction factors to be applied to measured detector-reading ratios to obtain OFs for several small-field detectors. Detector readings for 0.5 cm × 0.5 cm to 8 cm × 8 cm fields were measured and normalized to that of 10 cm × 10 cm field giving the detector-reading ratios. Three silicon diodes, IBA PFD, IBA EFD (IBA, Schwarzenbruck, Germany), PTW T60017, and one microdiamond, PTW T60019 (PTW, Freiburg, Germany), were used. Output correction factors from the CoP were applied to measured detector-reading ratios. Measurements were performed on six Clinac and six TrueBeam linacs (Varian Medical Systems, Palo Alto, USA). An investigation of the relationship between the size of small fields and corresponding detector-reading ratio among the linacs was performed by measuring lateral dose profiles for 0.5 cm × 0.5 cm fields to determine the full width half maximum (FWHM). The relationship between the linacs' focal spot size and the small-field detector-reading ratio was investigated by measuring 10 cm × 10 cm lateral dose profiles and determining the penumbra width reflecting the focal spot size. Measurement geometry was as follows: gantry angle = 0°, collimator angle = 0°, source-to surface distance (SSD) = 90 cm, and depth in water = 10 cm. Results For a given linac and 0.5 cm × 0.5 cm field, the deviations in detector-reading ratios among the detectors were 9%-15% for the Clinacs and 4%-5% for the TrueBeams. Use of output correction factors reduced these deviations to 6%-12% and 3%-4%, respectively. For field sizes equal to or larger than 0.8 cm × 0.8 cm, the deviations were corrected to 1% using output correction factors for both Clinacs and TrueBeams. For a given detector and 0.5 cm × 0.5 cm field, the deviations in detector-reading ratios among the linacs were 11%-17% for the Clinacs and 5-6% for the TrueBeams. For 1 cm × 1 cm the deviations were 1%-2% for Clinacs and 1% for TrueBeams. For field sizes larger than 1 cm × 1 cm the deviations were within 1% for both Clinacs and TrueBeams. No relationship between FWHMs and detector-reading ratios for 0.5 cm × 0.5 cm was observed. For Clinacs, larger 10 cm × 10 cm penumbra width yielded lower 0.5 cm × 0.5 cm detector-reading ratio indicating an effect of the focal spot size. For TrueBeams, the spread of penumbra widths was lower compared to Clinacs and no similar relationship was observed. Conclusions Output correction factors from the TRS-483 CoP are not sufficient for accurate determination of OF for 0.5 cm × 0.5 cm fields but are applicable for 0.8 cm × 0.8 cm to 8 cm × 8 cm fields. Nominally matched Clinacs and TrueBeams show large differences in detector-reading ratios for fields smaller than 1 cm × 1 cm.