In proton therapy treatment, proton residual energy after transmission through the treatment target may be determined by measuring sub-relativistic transmitted proton time-of-flight velocity and hence the residual energy. We have begun developing this method by conducting proton beam tests using Large Area Picosecond Photon Detectors (LAPPDs) which we have been developing for High Energy and Nuclear Physics Applications. LAPPDs are 20cm x 20cm area Micro Channel Plate Photomultiplier Tubes (MCP-PMTs) with millimeter-scale spatial resolution, good quantum efficiency and outstanding timing resolution of ≤70 picoseconds rms for single photoelectrons. We have constructed a time-of-flight telescope using a pair of LAPPDs at 10 cm separation, and have carried out our first tests of this telescope at the Massachusetts General Hospital's Francis Burr Proton Therapy Center. Treatment protons are sub-relativistic, so precise timing resolution can be combined with paired imaging detectors in a compact configuration while still yielding high accuracy in proton residual energy measurements through proton velocity determination from nearly monoenergetic protons. This can be done either for proton bunches or for individual protons. Tests were performed both in "ionization mode" using only the Microchannel Plates to detect the proton bunch structure and also in "photodetection mode" using nanosecond-decay-time quenched plastic scintillators to excite the photocathode within each of the paired LAPPDs. Data acquisition was performed using a remotely operated oscilloscope in our first beam test, and using 5Gsps DRS4 Evaluation Board waveform digitizers in our second test, in each case reading out both ends of single microstrips from among the 30 within an LAPPD. First results for this method and future plans are presented.
Atomic layer deposition (ALD) has enabled the development of a new technology for fabricating microchannel plates (MCPs) with improved performance that offer transformative benefits to a wide variety of applications. Incom uses a “hollow-core” process for fabricating glass capillary array (GCA) plates consisting of millions of micrometer-sized glass microchannels fused together in a regular pattern. The resistive and secondary electron emissive (SEE) functions necessary for electron amplification are applied to the GCA microchannels by ALD, which – in contrast to conventional MCP manufacturing– enables independent tuning of both resistance and SEE to maximize and customize MCP performance.
Incom is currently developing MCPs that operate at cryogenic temperatures and across wide temperature ranges. The resistive layers in both, conventional and ALD-MCPs, exhibit semiconductor-like behavior and therefore a negative thermal coefficient of resistance (TCR): when the MCP is cooled, the resistance increases, and when heated, the resistance drops. Consequently, the resistance of each MCP must be tailored for the intended operating temperature. This sensitivity to temperature changes presents a challenge for many terrestrial and space based applications.
The resistivity of the ALD-nanocomposite material can be tuned over a wide range. The material’s (thermo-) electrical properties depend on film thickness, composition, nanostructure, and the chemical nature of the dielectric and metal components. We show how the structure-property relationships developed in this work can be used to design MCPs that operate reliably at cryogenic temperatures. We also present data on how the resistive material’s TCR characteristics can be improved to enable MCPs operating across wider temperature ranges than currently possible.
Microchannel plates have been made by combining glass capillary substrates with thin films. The films impart the resistance and secondary electron emission (SEE) properties of the MCP. This approach permits separate choices for the type of glass, the MCP resistance and the SEE material. For example, the glass may be chosen to provide mechanical strength, a high open area ratio, or a low potassium-40 concentration to minimize dark rates. The resistive film composition may be tuned to provide the desired resistance, depending on the power budget and anticipated count rate. Finally, the SEE material may be chosen by balancing requirements for gain, long term stability of gain with extracted charge, and tolerance to air exposure.
Microchannel plates have been fabricated by Incom Inc., in collaboration with Argonne National Laboratory and UC Berkeley. Glass substrates with microchannel diameters of 10 and 20 microns have been used, typically with a length to diameter ratio of 60:1. Thin films for resistance and SEE are applied using Atomic Layer Deposition (ALD). The ALD technique provides a film with uniform thickness throughout the high aspect ratio microchannels. MCPs have been made in sizes up to 8”x8”. This three-component method for manufacturing MCPs also makes non-planar, curved MCPs possible.
Life testing results will be presented for 10 and 20 micron, 60:1 l/d ratio MCPs, with an aluminum oxide SEE film and two types of glass substrates. Results will include measurements of resistance, dark count rates, gain, and pulse height distributions as a function of extracted charge.
We report pilot production and advanced development performance results achieved for Large Area Picosecond
Photodetectors (LAPPD). The LAPPD is a microchannel plate (MCP) based photodetector, capable of imaging with
single-photon sensitivity at high spatial and temporal resolutions in a hermetic package with an active area of 400 square
centimeters. In December 2015, Incom Inc. completed installation of equipment and facilities for demonstration of
early stage pilot production of LAPPD. Initial fabrication trials commenced in January 2016. The “baseline” LAPPD
employs an all-glass hermetic package with top and bottom plates and sidewalls made of borosilicate float glass. Signals
are generated by a bi-alkali Na2KSb photocathode and amplified with a stacked chevron pair of “next generation” MCPs
produced by applying resistive and emissive atomic layer deposition coatings to borosilicate glass capillary array (GCA)
substrates. Signals are collected on RF strip-line anodes applied to the bottom plates which exit the detector via pinfree
hermetic seals under the side walls. Prior tests show that LAPPDs have electron gains greater than 107, submillimeter
space resolution for large pulses and several mm for single photons, time resolutions of 50 picoseconds for
single photons, predicted resolution of less than 5 picoseconds for large pulses, high stability versus charge extraction,
and good uniformity. LAPPD performance results for product produced during the first half of 2016 will be reviewed.
Recent advances in the development of LAPPD will also be reviewed, as the baseline design is adapted to meet the
requirements for a wide range of emerging application. These include a novel ceramic package design, ALD coated
MCPs optimized to have a low temperature coefficient of resistance (TCR) and further advances to adapt the LAPPD
for cryogenic applications using Liquid Argon (LAr). These developments will meet the needs for DOE-supported RD
for the Deep Underground Neutrino Experiment (DUNE), nuclear physics applications such as EIC, medical, homeland
security and astronomical applications for direct and indirect photon detection.
Spectral computed tomography (SCT) generates better image quality than conventional computed tomography (CT). It has overcome several limitations for imaging atherosclerotic plaque. However, the literature evaluating the performance of SCT based on objective image assessment is very limited for the task of discriminating plaques. We developed a numerical-observer method and used it to assess performance on discrimination vulnerable-plaque features and compared the performance among multienergy CT (MECT), dual-energy CT (DECT), and conventional CT methods. Our numerical observer was designed to incorporate all spectral information and comprised two-processing stages. First, each energy-window domain was preprocessed by a set of localized channelized Hotelling observers (CHO). In this step, the spectral image in each energy bin was decorrelated using localized prewhitening and matched filtering with a set of Laguerre–Gaussian channel functions. Second, the series of the intermediate scores computed from all the CHOs were integrated by a Hotelling observer with an additional prewhitening and matched filter. The overall signal-to-noise ratio (SNR) and the area under the receiver operating characteristic curve (AUC) were obtained, yielding an overall discrimination performance metric. The performance of our new observer was evaluated for the particular binary classification task of differentiating between alternative plaque characterizations in carotid arteries. A clinically realistic model of signal variability was also included in our simulation of the discrimination tasks. The inclusion of signal variation is a key to applying the proposed observer method to spectral CT data. Hence, the task-based approaches based on the signal-known-exactly/background-known-exactly (SKE/BKE) framework and the clinical-relevant signal-known-statistically/background-known-exactly (SKS/BKE) framework were applied for analytical computation of figures of merit (FOM). Simulated data of a carotid-atherosclerosis patient were used to validate our methods. We used an extended cardiac-torso anthropomorphic digital phantom and three simulated plaque types (i.e., calcified plaque, fatty-mixed plaque, and iodine-mixed blood). The images were reconstructed using a standard filtered backprojection (FBP) algorithm for all the acquisition methods and were applied to perform two different discrimination tasks of: (1) calcified plaque versus fatty-mixed plaque and (2) calcified plaque versus iodine-mixed blood. MECT outperformed DECT and conventional CT systems for all cases of the SKE/BKE and SKS/BKE tasks (all p<0.01). On average of signal variability, MECT yielded the SNR improvements over other acquisition methods in the range of 46.8% to 65.3% (all p<0.01) for FBP-Ramp images and 53.2% to 67.7% (all p<0.01) for FBP-Hanning images for both identification tasks. This proposed numerical observer combined with our signal variability framework is promising for assessing material characterization obtained through the additional energy-dependent attenuation information of SCT. These methods can be further extended to other clinical tasks such as kidney or urinary stone identification applications.
In this work, we propose a novel spectral computed tomography (CT) approach that combines a conventional CT
scanner with a Ross spectrometer to obtain quasi-monoenergetic measurements. The Ross spectrometer, which is a
generalization of a Ross filter pair, is a set of balanced K-edge filters whose thicknesses are such that the transmitted
spectra through any two filters are nearly identical except in the energy band between their respective K-edges. The
proposed approach is based on these specially designed filters, which are used to synthesize a set of quasi-monoenergetic
sinograms whose reconstruction yields energy-dependent attenuation coefficient (μE) images. In this way, we are able
to collect data using conventional CT data acquisition electronics, then to synthesize spectral CT datasets with highly
stable, rate-independent energy bin boundaries. This approach avoids the chromatic distortion due to event pile-up
which can cause difficulties with single photon spectrometry-based methods. To validate our Ross Spectrometer CT
concept, we performed phantom studies and acquired data with a balanced filter set consisting of thin foils of silver, tin,
cerium, dysprosium and tungsten. For each energy bin, a synthesized quasi-monoenergetic CT image was reconstructed
using the filtered back projection (FBP) algorithm operating on the logarithmic ratio of corresponding energy-resolved
intensity and blank sinogram pairs. The reconstructed attenuation coefficients showed satisfactorily good agreement
with NIST reference values of μE for water. The proposed spectral CT technique is potentially feasible and holds
promise to provide a more accurate and cost-effective alternative to single-photon counting spectral CT techniques.