Intrinsic Position Resolution Research Articles

Effects of magnetic fields on HPGe tracking detectors

We present a study of magnetic fields effects on the position resolution and energy response of hyper-pure germanium detectors. Our results provide realistic estimates of the potential impact on the resolving power of tracking-arrays from (fringe) magnetic fields present when operating together with large spectrometers.By solving the equations of motion for the electron and holes in the presence of both electric and magnetic fields, we analyzed the drift trajectories of the charge carriers to determine the deviations in the positions at the end point of the trajectories, as well as changes in drift lengths affecting the energy resolution and peak shift due to trapping. Our results show that the major effect is in the deviation of the transverse (to the electric field direction) position and suggest that, if no corrective action is taken in the pulse-shape and tracking data analysis procedures, a field strength ≳ 0.1 T will start to impact the intrinsic position resolution of 2 mm (RMS). At fields above ∼1 T, the degradation of the energy response becomes observable.

Development of a plastic scintillation detector for muon scattering imaging

To develop a device of large-area cosmic ray imaging in complex measurement environments, we design a muon scattering imaging detector composed of triangular scintillators, with a new type of photodetector (SiPM) as readout. A 500 mm × 500 mm imaging detector composed of multiple scintillator bars is presented in this paper. GEANT4 simulation shows that its intrinsic position resolution can be improved by the incidence angle and air gap corrections. To optimize its imaging performance, the reconstruction and correction methods of hit position are discussed in detail and compared with the experimental results.

Single-photon image sensor at room temperature with only four anodes

A two-dimensional tetra-lateral position-sensitive cap-resistive-layer silicon photomultiplier (CRL-SiPM) is employed as a single-photon imaging sensor at room temperature. The CRL-SiPM has only four anodes and one back cathode with an active area of 2.77 mm × 2.77 mm and a micro avalanche photodiode cell pitch of ~10 μm. It achieved a 150 μm-linewidth-resolved single-photon image, which is near the device's theoretical intrinsic single-photon position resolution limit of ~115 μm. With increasing light intensity, the position resolution of the device improved. When the mean photoelectron numbers were 1.34 and 41.56, the corresponding intrinsic position resolutions were 101.6 and 14.08 μm, respectively. Finally, owing to the single photoelectron imaging capability of the CRL-SiPM, we achieved the lensless imaging of dark counting sites, i.e., the locations of thermal carriers inducing avalanche breakdown of the device.

Spatial resolution properties of krypton-based mixtures using a 100 μm thick Gas Electron Multiplier

A gaseous imaging detector based on a cascade of two non-standard gas electron multipliers 100 μm thick GEM) was operated in five different krypton-based mixtures and the achievable position resolution was determined. The micropattern gaseous detector (MPGD) has an active area of 10 × 10cm2. The GEM foils are twice as thick as a standard GEM, being more robust to electric discharges and less prone to sparking-caused damage. Krypton, being the noble gas with the lowest value of intrinsic position resolution in the range 14– 34 keV, is a good candidate for imaging applications based on gaseous detectors. Our measurements have confirmed that krypton-CO2 mixtures showed an improvement in position resolution above 18 keV and up to 30 keV relatively to Ar-CO2 mixtures. The MTF value at 10% in the energy range 22–24 keV was evaluated, being 0.5876(342)lp/cm for Ar-CO2 (70:30) and a maximum of 3.2702(15)lp/cm for the Kr-CO2 (90:10) mixture.

Pressure effects on the X-ray intrinsic position resolution in noble gases and mixtures

A study of the effect of gas pressure in the position resolution of an interacting X or γ-ray photon in a gas medium is performed. The intrinsic position resolution for pure noble gases (Argon and Xenon) and their mixtures with CO2 and CH4 was calculated for several gas pressures (1–10 bar) and for photon energies between 1 and 60 keV, being possible to establish a linear relation between the intrinsic position resolution and the inverse of the gas pressure in the indicated energy range, as intuitively expected. We show how, at high pressures and low photoelectron energies, this intrinsic 1/P scaling is modified due to the diffusion of the primary ionization in the direction perpendicular to the electric field. In order to evaluate the quality of the method here described, a comparison between the available experimental data and microscopic simulations is presented in this work and discussed. In the majority of cases, a good agreement is observed. The conditions to achieve position resolutions down to 10 μm in a realistic detector are shown and discussed.

Design and Construction of Large Size Micromegas Chambers for the ATLAS Upgrade of the Muon Spectrometer

Large area Micromegas detectors will be employed for the first time in high-energy physics experiments. A total surface area of about $150~\text {m}^{2}$ of the forward regions (pseudo-rapidity coverage, $1.3 \,\, { ) of the Muon Spectrometer of the ATLAS detector at LHC will be equipped with 8-layer Micromegas modules. Each module extends over a surface from 2 to $3~\text {m}^{2}$ for a total active area of $1200~\text {m}^{2}$ . Together with the small strip Thin Gap Chambers they will compose the two New Small Wheels (NSW), which will replace the innermost stations of the ATLAS endcap muon tracking system in the 2018/19 shutdown. In order to achieve a 15% transverse momentum resolution for 1 TeV muons, in addition to an excellent intrinsic position resolution, the mechanical precision of each plane of the assembled module must be 30 microns along the precision coordinate and 80 microns perpendicular to the chamber. All readout planes are segmented into strips with a pitch of 450 microns for a total of 8180 strips per plane and more than 2 million channels for the NSW. In two of the four planes the strips are inclined by ±1.5° and provide a measurement of the second coordinate. The design and construction procedure of the Micromegas modules are presented, as well as the design for the assembly of modules onto the NSW. Emphasis is given on the methods developed to achieve the challenging mechanical precision. Measurements of deformation on chamber prototypes as a function of thermal gradients, gas over-pressure and internal stress (mesh tension and module fixation on supports) are also shown and compared to simulation. These tests were essential in the development of the final design in order to minimize the effects of deformations. During installation and operation all deformations and relative misalignments will be monitored through an optical alignment system and compensated in the tracking software.

Position resolution limits in pure noble gaseous detectors for X-ray energies from 1 to 60 keV

The calculated position resolutions for X-ray photons (1–60 keV) in pure noble gases at atmospheric pressure are presented. In this work we show the influence of the atomic shells and the detector dimensions on the intrinsic position resolution of the used noble gas. The calculated results were obtained by using a new software tool, Degrad, and compared to the available experimental data.

Development of the Pixel OR SOI detector for high energy physics experiments

A Silicon-On-Insulator (SOI) Technology is suitable for vertex detectors for high energy physics experiments since complex functions can be fabricated on the SOI wafer with a small amount of material thanks to the monolithic structure. We developed a new sensor processing scheme “PIXOR(PIXel OR)” for pixel detectors using a LAPIS 0.20μm SOI process.An analog signal from each pixelated sensor is divided into two dimensional directions, and 2n signal channels from a small n by n pixel matrix are ORed as n column and n row channels, then the signals are processed by a readout circuit in each small matrix. This PIXOR scheme reduces the number of readout channels and avoids a deterioration of intrinsic position resolution due to large circuit area, that was a common issue for monolithic detectors. This feature allows high resolution, low occupancy and on-sensor signal processing at the same time. We present the successful results of the PIXOR readout scheme using a first prototype.

Study of spatial resolution properties of a glass RPC

A prototyping glass RPC with excellent signal-to-noise ratio was constructed and tested. Detection efficiencies for cosmic rays of about 95% were obtained for both avalanche and streamer modes of operation. A simple method to measure the signal charge profile is developed, which is consistent with other methods such as direct optical observation. A narrow profile for avalanche signal mode is obtained, which may lead to an intrinsic spatial resolution less than 1.0 mm FWHM. If a delay-line or charge division technique is used to determine the centroid of the signal charge distribution, it is important to reduce the electronics noise and the fluctuations of the delay-line (or resistor) units in order to reach the ultimate intrinsic position resolution.

Energy resolved X-ray fluorescence imaging based on a micropattern gas detector

A simple system for energy resolved X-ray fluorescence imaging using a room temperature, 2-D sensitive Micro-Hole and Strip Plate (MHSP) operating in pure xenon is proposed. The Micro-Hole and Strip Plate is an electron multiplier with two stages of avalanche production, one of them in the holes and another one in the anode strips. The X-ray interaction via photoelectron absorption in the xenon produces a number of electrons proportional to the incoming X-ray energy. The electron cloud is, then, amplified in the two amplification stages, resulting in a charge pulse that is also proportional to the detected X-ray energy. The 2-D capability is achieved in the Micro-Hole and Strip Plate by using two orthogonal resistive lines, one connecting the anode strips on the bottom face of the Micro-Hole and Strip Plate and the other one connecting the strips structured on the Micro-Hole and Strip Plate top surface. This low cost detector has an active area of 28 × 28 mm 2, an intrinsic position resolution of σ∼ 125 µm, an energy resolution of about 825 eV (Full Width at Half Maximum) at 5.9 keV and a count rate capability as high as 0.5 MHz. Fluorescence images were obtained by irradiating the sample with X-rays and using a pinhole placed between the sample and the detector window. Elemental map discrimination for different samples, image amplification and detector parameters, are presented.

CdTe Strip Detector Characterization for High Resolution Small Animal PET

Excellent spatial resolution is a requirement for preclinical PET imaging. In order to achieve spatial resolution of significantly better than one millimeter, an appealing possibility is to employ direct detector materials, such as cadmium telluride (CdTe). Prototype thin orthogonal strip detectors have been developed for testing. They have dimensions of 20 mm by 20 mm and are 0.5 mm thick, and have strips of 0.5 mm pitch on one side and 2.5 mm pitch on the other. Results are presented for the energy resolution (3% at 511 keV), intrinsic position resolution (equal to the 0.5 mm strip pitch), and timing resolution (3 ns FWHM in coincidence with an LSO detector, 8 ns FWHM for coincidence of two CdTe detectors) of the detectors. A PET scanner design is proposed using blocks made of the CdTe strip detectors, oriented in the blocks with their thin edges toward the center of the scanner. Simulation results suggest that this scanner, using a threshold of 250 keV, would have a sensitivity of 3.4% for a point source at its center.

SPEIR: A Ge Compton camera

The SPEctroscopic Imager for γ - Rays (SPEIR) is a Compton scatter γ -ray imaging system of high efficiency and spectroscopic resolution with a 4- π field-of-view. The imaging system consists of double-sided segmented (DSSD) planar Ge detectors, an acquisition system instrumenting the detectors, and a set of data analysis methods. The analysis methods employ event reconstruction algorithms that increase the intrinsic position resolution and granularity of the detectors, and provide photon scattering information relevant for Compton imaging.

Large-size high-performance transparent amorphous silicon sensors for laser beam position detection

We present the measured performance of a new generation of semitransparent amorphous silicon position detectors. They have a large sensitive area (30×30 mm 2) and show good properties such as a high response (about 20 mA/W), an intrinsic position resolution better than 3 μm, a spatial-point reconstruction precision better than 10 μm, deflection angles smaller than 10 μrad and a transmission power in the visible and NIR higher than 70%.

Application of the microhole and strip plate detector for neutron detection

We introduce the microhole and strip plate (MHSP) detector as a micropattern detector for the detection of thermal and epithermal neutrons. Detection sensitivity is obtained by filling these detectors with /sup 3/He at high pressures. We propose the use of argon-xenon penning mixtures as the stopping gas as opposed to the usual carbon based stopping gases. These argon-xenon mixtures provide suitable gas gains for the high pressure/high resolution neutron detector applications. With these mixtures it is possible to obtain a sealed detector with only rare-gas filling which is simple to purify and not subject to ageing. An MHSP gas detector filled with a 3-bar argon/50-mbar xenon/6-bar helium mixture can achieve gains above 2/spl times/10/sup 3/. This mixture allows neutron detection efficiencies of about 70% at 1 /spl Aring/ for a 2.5-cm thick absorption region and intrinsic position resolution (full-width at half-maximum) of about 1.8 mm. The sensitivity to /spl gamma/-rays of the present mixture will be the same when compared to that of 2.6-bar CF/sub 4/.

The scintillation of GEMS coated with wavelength shifters

The optical readout of GEMs, presented a few years ago, is currently being considered for several detector projects. Although using suitable gaseous mixtures the GEM avalanches emit a large number of photons in the visible and/or NIR regions, many gaseous mixtures of great interest for detectors scintillate mainly in the UV, and the readout of these detectors is usually inefficient or very expensive. In this work we report on the operation of GEMs coated with a wavelength converter. We show that using a GEM with an optimal geometry the wavelength conversion process can be quite efficient and does not degrade the intrinsic position resolution of the detector, an usual drawback of the utilisation of wavelength shifters.

Scintillator and photodetector array optimization for functional breast imaging

Nuclear Medicine methods have been proposed as a means of imaging primary breast lesions and regional metastatic involvement based on tumor physiology. Recently, the positive predictive value of scintimammography using 99 Tc labelled SestaMIBI has been reported to be as high as 81%, with an associated negative predictive value of 97%. Visualization of small (o1 cm) lesions using scintimammography may be complicated, however, by the effects of overlying and underlying background uptake of MIBI in the breast soft tissue and the deterioration of lesion contrast with distance from the gamma camera. For these reasons dedicated compact gamma cameras have been proposed and successfully used. Nevertheless, the detection of very small tumors (o5–10 mm) is still very difficult. Many parameters affect the breast small tumors detection. The degree of pixellation of both the scintillator and photodetector is critical for the intrinsic position resolution of the detector and for the overall imaging performance. In this paper, we examine the basic imaging properties of systems using arrays of scintillators and pixellated photodetectors. The influence of readout systems is also taken into account. Simulations as well as preliminary experimental results are presented. r 2002 Published by Elsevier Science B.V.

Study of CMOS image sensors for laser beam position detection

We report on the study made on commercial CMOS image sensors in order to determine their feasibility for light beam position reconstruction. Measurements of the intrinsic position resolution, sensor photoresponse and uniformity were done. The effect of eventual background illumination was evaluated. The precision on the spatial point reconstruction was determined from linearity measurements. First results on gamma-ray radiation tolerance are presented.

Development of superconducting tunnel junction as an imaging radiation detector

Superconducting tunnel junctions (STJs) as X-ray detectors have been developed mainly aiming at high resolution spectrometers. We archived an energy resolution of 106 eV at 5.9 keV (FWHM) using an STJ developed at Nippon Steel Corporation with a cooled (∼ 100K) FET. Furthermore, series-connected STJs as an imaging radiation detector are developed. Both the pulse hight and the rise time of signals from 241Am α-particles irradiated on a series-connected STJ give a good position sensitivity, indicating the intrinsic position resolution less than 0.5 mm.

New improved methods of X-ray detection

We present CCDs (CCD = charge-coupled device) as high performance X-ray detectors. These silicon pixel detectors (whose concept was invented in 1970) are mainly used today for detection of visible light with digital cameras. Generally, X-rays are detected by inserting a phosphorescent screen and collecting the light obtained onto the CCD. However X-rays can also be detected directly with excellent background rejection capabilities, good energy resolution and intrinsic pixel-size position resolution but with mediocre efficiency in a relatively small energy range and little or no timing information. For this application a slow, low noise electronics is needed. We will discuss how these detectors work, their advantages and disadvantages as well as future possibilities and limitations.

Performance study of liquid xenon detector for PET

The coincidence time resolution and the intrinsic position resolution along the electron drift direction of a liquid xenon multiwire ionization chamber for PET were measured, being, respectively, 1.5 ns (fwhm) and 0.5 mm (fwhm). Data on the low temperature performance of a photomultiplier tube operating on the VUV are presented.