Finite Detector Size Research Articles

Novel Spectrometer Designs for Laser-Driven Ion Acceleration

We propose novel spectrometer designs that aim to enhance the measured spectral range of ions on a finite-sized detector. In contrast to the traditional devices that use a uniform magnetic field, in which the deflection of particles increases inversely proportional to their momentum, in a gradient magnetic field, the deflection of particles will decrease due to the reduction of the magnetic field along their propagation. In this way, low-energy ions can reach the detector because they are deflected less, compared to the uniform field case. By utilizing a gradient magnetic field, the non-linear dispersion of ions in a homogeneous magnetic field approaches nearly linear dispersion behavior. Nonetheless, the dispersion of low-energy ions, using a dipole field, remains unnecessarily high. In this article, we discuss the employed methodology and present simulation results of the spectrometer with an extended ion spectral range, focusing on the minimum detectable energy (energy dynamic range) and energy resolution.

Probing the spatial dimensions of nanoscale patterns with Rutherford backscattering spectrometry

We realize the quantitative determination of the spatial dimensions of nanopatterns using Rutherford backscattering spectrometry (RBS) with a mm-sized ion beam. The methodology is based on the effects of the nanopatterns on the RBS signal from the substrate. We demonstrate the approach with the study of a periodic pattern of 35 nm wide and 75 nm high SiO2 lines which have a periodicity of 90 nm. We analyse the relation between the features in the experimental spectra and the properties of the nanostructures. Current voxel-based simulation software allows to reproduce many of the experimental observations well. However, we found that the present simulation software inadequately incorporates the effect of the angular spread due to the finite detector size. We propose a method to model the effect more accurately and through this we demonstrate a dramatically improved agreement with the experimental spectra.

The Root-Cause Analysis on Failed Patient-Specific Measurements of Pencil-Beam-Scanning Protons

The purpose of this study is to present the root-cause analysis on failed patient-specific quality assurance (QA) measurements of pencil-beam-scanning (PBS) protons; referred to as PBS-QA measurement.A criterion to fail a PBS-QA measurement is having a < 95% passing-rate in a 3.0%-3.0mm Gamma-index analysis. The 2D Gamma-index analysis toolkit was clinically used to obtain the passing-rate. A 2D detection array with finite-size ionization-chamber was utilized.A total 2488 PBS-QA measurements were catalogued. The percentage of measurements for the sites of Head/Neck, Breast, Prostate, and Other are 53.3%, 22.7%, 10.5%, and 13.5% respectively. The percentage of measurements with a passing rate of 100%-to-95%, 94%-to-89%, and < 89% were 93.6%, 5.6%, and 0.8% respectively. The percentage of failed measurements with a < 95% passing rate was 10.9%. 8.1% became acceptable by either re-measurement or re-analysis to remove the setup or user errors. A feature of > 3% per-mm dose gradient along depth of beam path was observed on the failed measurements. To study the effect of depth dose gradient, a 3D Gamma-index analysis toolkit was used. 43.1% of measurements improved using this 3D toolkit. A feature among measurements that remained sub-optimal was a sharp > 3% per-mm lateral dose gradient that may not be well handled by using the detector size of 5.0mm in-diameter. An analysis of the sampling of finite-size detectors using 1D error function showed a large dose deviation at locations of low doses in a valley between two plateaus. User error, large depth dose gradient, and the effect of detector-size are identified.With an understanding and mitigation of these root causes, the goals of patient-specific QA, specifically detecting actual deviation of beam delivery or identifying limitations of dose calculation algorithm in used treatment planning system, can be directly related to the failure of PBS-QA measurements.

Useful relations for the analysis of stellar scintillation at the entrance pupil of a telescope.

The development of new techniques for characterizing atmospheric optical turbulence (OT) has become an active topic of research again in recent years. In order to facilitate these studies, we reconsidered known theoretical results and obtained some new practically useful conclusions. We introduce a dimensionless Fresnel filter, which allows us to approximate a polychromatic weighting function (WF) by a monochromatic one with a typical precision of several percent. A so-called dimensionless WF can be easily scaled for a receiving aperture of any size. For the case of a circular aperture and monochromatic radiation, an analytical expression for the WF was found. The WFs for a square aperture and for a circular aperture match with relative difference less than 0.01 if the circular aperture diameter is 1.15 times larger than the square aperture side. A linear digital filter can be applied to the scintillation signal from an image detector. As an example of digital filtering, we considered the power law filter ∝f5/3 with the WF being constant in a wide range of altitudes. We discuss the main limitations of this approach for measuring OT integral: finite pixel size, aliasing, and finite image detector size.

The root cause analysis on failed patient-specific measurements of pencil beam scanning protons using a 2D detection array with finite size ionization chambers.

The aim of this report is to present the root cause analysis on failed patient‐specific quality assurance (QA) measurements of pencil beam scanning (PBS) protons; referred to as PBS‐QA measurement. A criterion to fail a PBS‐QA measurement is having a <95% passing rate in a 3.0%‐3.0 mm gamma index analysis. Clinically, we use a two‐dimensional (2D) gamma index analysis to obtain the passing rate. The IBA MatriXX PT 2D detection array with finite size ionization chamber was utilized. A total of 2488 measurements performed in our PBS beamline were cataloged. The percentage of measurements for the sites of head/neck, breast, prostate, and other are 53.3%, 22.7%, 10.5%, and 13.5%, respectively. The measurements with a passing rate of 100 to >94%, 94 to >88%, and <88% were 93.6%, 5.6%, and 0.8%, respectively. The percentage of failed measurements with a <95% passing rate was 10.9%. After removed the user errors of either re‐measurement or re‐analysis, 8.1% became acceptable. We observed a feature of >3% per mm dose gradient with respect to depth on the failed measurements. We utilized a 2D/three‐dimensional (3D) gamma index analysis toolkit to investigate the effect of depth dose gradient. By utilizing this 3D toolkit, 43.1% of the failed measurements were improved. A feature among measurements that remained sub‐optimal after re‐analysis was a sharp >3% per mm lateral dose gradient that may not be well handled using the detector size of 5.0 mm in‐diameter. An analysis of the sampling of finite size detectors using one‐dimensional (1D) error function showed a large dose deviation at locations of low‐dose areas between two high‐dose plateaus. User error, large depth dose gradient, and the effect of detector size are identified as root causes. With the mitigation of the root causes, the goals of patient‐specific QA, specifically detecting actual deviation of beam delivery or identifying limitations of the dose calculation algorithm of the treatment planning system, can be directly related to failure of the PBS‐QA measurements.

Searching for lepton number violating Λ baryon decays mediated by a GeV-scale Majorana neutrino with LHCb

We consider the lepton-number-violating processes in $\Lambda_b$/$\Lambda_c$ decays mediated by on-shell GeV-scale sterile neutrino. We calculate the branching ratio for the following processes: $\Lambda_b^{0} \rightarrow \mathcal{B}^+ \mu^{-}\mu^- \pi^+$, where $\mathcal{B}^+$ is $\Lambda_c^+$ or proton, and $\Lambda_c^{+} \rightarrow \Lambda \mu^{+}\mu^+ \pi^-$, as function of the mass of the sterile neutrino $m_N$ and the heavy-light mixing coefficient of the extended Pontecorvo-Maki-Nakagawa-Sakata (PMNS) matrix $|U_{\mu N}|^2$. The effect of finite detector size is included in our calculation. After comparing the theoretical effective branching ratio with the expected experimental ability of LHCb, we give the sensitivity upper bounds on $|U_{\mu N}|^2$ on the ($|U_{\mu N}|^2$-$m_N$) plane. These channels give comparable results with the bounds given by different search strategies (e.g., at Belle, NuTeV, DELPHI and BEBC) in the mass region $m_N \simeq$ 0.25 GeV - 4.5 GeV, and the limits are stronger in the mass range 2 GeV $<m_N<$ 4.5 GeV.

Unruh effect and information flow

We study memory effects as information backflow for an accelerating two-level detector weakly interacting with a scalar field in the Minkowski vacuum. This is the framework of the well-known Unruh effect: the detector behaves as if it were in a thermal bath with a temperature proportional to its acceleration. Here we show that if we relax the usual assumption of an eternally uniformly accelerating system, and we instead consider the more realistic case in which a finite-size detector starts accelerating at a certain time, information backflow may appear in the dynamics. Our results demonstrate the existence of a connection between the trajectory of the detector in Minkowski space and the behavior of information flow. This allows us to inspect the Unruh effect under a new light, making use of the latest developments in quantum information theory and open quantum systems.

Sensitivity of lepton number violating meson decays in different experiments

We study the discovery prospect of different three body lepton number violating~(LNV) meson decays $M_{1}^{-}\to\ell_{1}^{-}\ell_{2}^{-}M_{2}^{+}$ in the framework of right handed~(RH) neutrino extended Standard Model~(SM). We consider a number of ongoing experiments, such as, NA62 and LHCb at CERN, Belle II at SuperKEK, as well as at the proposed future experiments, SHiP, MATHUSLA and FCC-ee. The RH Majorana neutrino $N$ mediating these meson decays provides a resonant enhancement of the rates, if the mass of $N$ lies in the range $(100\, \text{MeV}-6\, \text{GeV})$. We consider the effect of parent mesons velocity, as well as, the effect of finite detector size. Using the expected upper limits on the number of events for the LNV decay modes, $M_{1}^{-} \to\ell_1^{-}\ell_2^{-}\pi^{+}$~($M_{1}=B, B_c,D, D_{s}\,\text{and}\,K$), we analyze the sensitivity reach of the mixing angles $|V_{e N}|^{2}$, $|V_{\mu N}|^{2}$, $|V_{\tau N}|^{2}$, $|V_{e N}V_{\mu N}|$, $|V_{e N}V_{\tau N}|$ and $|V_{\mu N}V_{\tau N}|$ as a function of heavy neutrino mass $M_{N}$. We show that, inclusion of parent meson velocity can account to a large difference for active-sterile mixing, specially for $D$, $D_s$ meson decay at SHiP and $K$ meson decay at NA62. Taking into account the velocity of the $D_s$ meson, the future beam dump experiment SHiP can probe $|V_{eN}|^2 \sim 10^{-9}$. For RH neutrino mass in between 2 - 5 GeV, MATHUSLA can provide best sensitivity reach of active-sterile mixings.

Patient-specific quality assurance for proton depth dose distribution using a multi-layer ionization chamber in a single-ring wobbling method.

The use of a multi-layer ionization chamber, Zebra, in patient-specific quality assurance (QA) for proton depth dose distributions in a single-ring wobbling method is investigated. The depth dose distributions measured using Zebra are compared with those calculated using the treatment planning system (TPS), XiO-M, and measured using an ionization chamber with a motorized water phantom system. Because the TPS only provides point doses, the average doses are calculated using in-house software. The detector size-corrected depth dose distributions are obtained by determining the average of the dose distributions from the TPS over a cylindrical region similar to the size of the Zebra detectors. The calculated depth dose distributions from the cases with a simple compensator shape are in good agreement with those obtained from the TPS without performing volume averaging; however, a 15% difference was shown when compared with those from the cases with a complex compensator shape. Then, the measurements are compared with the detector size-corrected depth dose distributions, showing an improved agreement within 3% for the highly steep dose gradient regions. Although there are some field size limitations, the Zebra system is a useful device for the fast measurement of patient-specific QA for depth dose distributions in wobbled proton beams. However, careful consideration is required for complex dose distribution fields, because the measurements obtained using Zebra cannot be directly compared to the depth dose distributions from the TPS owing to the finite detector size of the Zebra chamber.

Precision and informational limits in inelastic optical spectroscopy

Using Fisher information and the Cramér-Rao lower bound, we analyse fundamental precision limits in the determination of spectral parameters in inelastic optical scattering. General analytic formulae are derived which account for the instrument response functions of the dispersive element and relay optics found in practical Raman and Brillouin spectrometers. Limiting cases of dispersion and diffraction limited spectrometers, corresponding to measurement of Lorentzian and Voigt lineshapes respectively, are discussed in detail allowing optimal configurations to be identified. Effects of defocus, spherical aberration, detector pixelation and a finite detector size are also considered.

A Unified Method for Digital Super-Resolution and Restoration in Infrared Microscopy Imaging

Infrared (IR) imaging systems are known to have a range of sensor and optical limitations that result in degraded imagery. Fixed pattern noise (FPN), resulting from pixel-to-pixel response nonuniformity, is a dominant source of error that manifests in collected imagery through the appearance of temporally and spatially correlated noise patterns that are mixed with each image. Furthermore, finite detector size coupled with imperfect system optics can introduce blurring effects and aliasing, ultimately reducing resolution in acquired images. Here, we propose a unified method to reduce FPN and recover high-frequency image content in IR microscopy images. The proposed method uses regularized nonlocal means to highlight spatial features in the scene while maintaining fine textural image details. We derive an iterative optimization method based upon a gradient descent minimization strategy that applies a Wiener deconvolution in each iteration to estimate the blur artifacts. The method is implemented within an embedded mid-wave IR imaging system for microscopy applications. We demonstrate a reduction in FPN and blurring artifacts, achieving improved image resolution in the reconstructed images that are apparent in recovered details on scene objects.

Optical design of low-cost polarimetric back-scatter sondes.

The optical design of a simple low-cost polarimetric balloon-borne backscatter sonde for detecting cloud thermodynamic phase is described. Simulations are performed to determine the best choice of polarization as well as the layout of the detectors with respect to the emitter. The variety of scattering angles of the detected photons, due to finite size detectors, has at least as great an effect as multiple scattering in determining the ratio of cross-polarized to co-polarized signal. The problem with aligned flat-plate ice crystals giving similar depolarization to spherical scatterers, noted in the context of cloud lidar, also occurs here even with a cloud of randomly oriented large plates. Nevertheless, we find that configurations of the instrument that can distinguish liquid droplets from a wide variety of ice crystal habits, including plates, are possible.

Shot-by-shot characterization of focused X-ray free electron laser pulses

X-ray free electron lasers (XFEL) provide intense and almost coherent X-ray pulses. They are used for various experiments investigating physical and chemical properties in materials and biological science because of their complete coherence, high intensity, and very short pulse width. In XFEL experiments, specimens are irradiated by XFEL pulses focused by mirror optics. The focused pulse is too intense to measure its coherence by placing an X-ray detector on the focal spot. Previously, a method was proposed for evaluating the coherence of focused pulses from the visibility of the diffraction intensity of colloidal particles by the speckle visibility spectroscopy (SVS). However, the visibility cannot be determined exactly because the diffraction intensity is integrated into each finite size detector pixel. Here, we propose a method to evaluate the coherence of each XFEL pulse by using SVS in combination with a theory for exact sampling of the diffraction pattern and a technique of multiplying the diffraction data by a Gaussian masks, which reduces the influence of data missing in small-angle regions due to the presence of a direct beamstop. We also introduce a method for characterizing the shot-by-shot size of each XFEL pulse by analysing the X-ray irradiated area.

Performance analysis of multiple-input multiple-output free-space optical systems with partially coherent Gaussian beams and finite-sized detectors

Multiple-input multiple-output (MIMO) techniques are employed in free-space optical (FSO) links to mitigate the degrading effects of atmospheric turbulence. We consider a MIMO FSO system, which consists of a radial laser array with partially coherent Gaussian beams at the transmitter and a detector array with Gaussian apertures at the receiver. The average power and the power correlation function at the finite-sized receiver apertures are formulated by using the extended Huygens–Fresnel principle in weak atmospheric turbulence. This let us further quantify the performance metrics such as the power scintillation index, the aperture averaging factor, and the average bit error rate (BER) as functions of system parameters. The derived power scintillation equation correctly reduces to the existing coherent and partially coherent Gaussian beam scintillation indices in the limiting cases. Using the performance metrics, we analyze the effect of various practical system parameters on the performance of a MIMO FSO system. Practical system parameters include the transmitter and receiver ring radius, number of beamlets, number of finite-aperture receivers, source size, degree of source coherence, receiver aperture radius, link distance, and the structure constant of atmosphere.

Correcting intensity loss errors in the absence of texture-free reference samples during pole figure measurement

Even with the use of X-ray polycapillary lenses, sample tilting during pole figure measurement results in a decrease in the recorded X-ray intensity. The magnitude of this error is affected by the sample size and/or the finite detector size. These errors can be typically corrected by measuring the intensity loss as a function of the tilt angle using a texture-free reference sample (ideally made of the same alloy as the investigated material). Since texture-free reference samples are not readily available for all alloys, the present study employs an empirical procedure to estimate the correction curve for a particular experimental configuration. It involves the use of real texture-free reference samples that pre-exist in any X-ray diffraction laboratory to first establish the empirical correlations between X-ray intensity, sample tilt and their Bragg angles and thereafter generate correction curves for any Bragg angle. It will be shown that the empirically corrected textures are in very good agreement with the experimentally corrected ones.

Image reconstruction with uncertainty quantification in photoacoustic tomography.

Photoacoustic tomography is a hybrid imaging method that combines optical contrast and ultrasound resolution. The goal of photoacoustic tomography is to resolve an initial pressure distribution from detected ultrasound waves generated within an object due to an illumination of a short light pulse. In this work, a Bayesian approach to photoacoustic tomography is described. The solution of the inverse problem is derived and computation of the point estimates for image reconstruction and uncertainty quantification is described. The approach is investigated with simulations in different detector geometries, including limited view setup, and with different detector properties such as ideal point-like detectors, finite size detectors, and detectors with a finite bandwidth. The results show that the Bayesian approach can be used to provide accurate estimates of the initial pressure distribution, as well as information about the uncertainty of the estimates.

First results on modeling of ITER infrared images

Infrared (IR) images of the ITER wide angle viewing system are modeled for the baseline plasma equilibrium and partially detached tungsten divertor, taking into account the three-dimensional structure of the first wall and the divertor. The modeling includes a comprehensive chain of calculations from the heat load specifications up to the synthetic, reflection-free IR images of the surface temperature, Tsurf. The effect of the optical blur due to finite IR detector size and diffraction/aberrations—approximated by a Gaussian filter—on the measured Tsurf is investigated. The optical blur characterized by σ = 0.7 pixel (approximately twice the diffraction limit) leads to underestimation of Tsurf,max on the inner vertical divertor target and near the upper X-point by <6% and <4%, respectively. This is within the required measurement accuracy of 10%. Larger underestimation of Tsurf,max (<12%) is observed on the outer vertical divertor target. The study demonstrates the importance of keeping the performance of the optical system as close as possible to the diffraction limit.

A phase retrieval algorithm based on three-dimensionally translated diffraction patterns

An iterative phase retrieval method is proposed that combines alternating projections and registration of three-dimensionally translated near-field diffraction patterns. This method allows to enhance resolution limited by a finite detector size and automatically stitches the assembled data while avoiding the need for a priori knowledge or scanning of the object as encountered in coherent diffraction imaging or ptychography.

Demonstrating continuous variable Einstein–Podolsky–Rosen steering in spite of finite experimental capabilities using Fano steering bounds

We show how one can demonstrate continuous-variable Einstein&#x2013;Podolsky&#x2013;Rosen (EPR) steering without needing to characterize entire measurement probability distributions. To do this, we develop a modified Fano inequality useful for discrete measurements of continuous variables and use it to bound the conditional uncertainties in continuous-variable entropic EPR-steering inequalities. With these bounds, we show how one can hedge against experimental limitations including a finite detector size, dead space between pixels, and any such factors that impose an incomplete sampling of the true measurement probability distribution. Furthermore, we use experimental data from the position and momentum statistics of entangled photon pairs in parametric downconversion to show that this method is sufficiently sensitive for practical use.

Measurement of bow tie profiles in CT scanners using a real-time dosimeter.

Several areas of computed tomography (CT) research require knowledge about the intensity profile of the x-ray fan beam that is introduced by a bow tie filter. This information is considered proprietary by CT manufacturers, so noninvasive measurement methods are required. One method using real-time dosimeters has been proposed in the literature. A commercially available dosimeter was used to apply that method, and analysis techniques were developed to extract fan beam profiles from measurements. A real-time ion chamber was placed near the periphery of an empty CT gantry and the dose rate versus time waveform was recorded as the x-ray source rotated about the isocenter. In contrast to previously proposed analysis methods that assumed a pointlike detector, the finite-size ion chamber received varying amounts of coverage by the collimated x-ray beam during rotation, precluding a simple relationship between the source intensity as a function of fan beam angle and measured intensity. A two-parameter model for measurement intensity was developed that included both effective collimation width and source-to-detector distance, which then was iteratively solved to minimize the error between duplicate measurements at corresponding fan beam angles, allowing determination of the fan beam profile from measured dose-rate waveforms. Measurements were performed on five different scanner systems while varying parameters such as collimation, kVp, and bow tie filters. On one system, direct measurements of the bow tie profile were collected for comparison with the real-time dosimeter technique. The data analysis method for a finite-size detector was found to produce a fan beam profile estimate with a relative error between duplicate measurement intensities of <5%. It was robust over a wide range of collimation widths (e.g., 1-40 mm), producing fan beam profiles that agreed with a relative error of 1%-5%. Comparison with a direct measurement technique on one system produced agreement with a relative error of 2%-6%. Fan beam profiles were found to differ for different filter types on a given system and between different vendors. A commercially available real-time dosimeter probe was found to be a convenient and accurate instrument for measuring fan beam profiles. An analysis method was developed that could handle a wide range of collimation widths by explicitly considering the finite width of the ion chamber. Relative errors in the profiles were found to be less than 5%. Measurements of five different clinical scanners demonstrate the variation in bow tie designs, indicating that generic bow tie models will not be adequate for CT system research.