Positron Detector Research Articles

Optimization study of a new β +-γ coincidence positron annihilation lifetime spectrometer using Geant4

Positron annihilation lifetime (PAL) spectroscopy is a unique method for characterizing atomic-scale defects and ultramicropores in materials. The conventional PAL spectrometer adopts the γ-γ coincidence principle, and its performance, especially the coincidence counting rate (CCR), can hardly be further increased. Another coincidence principle, β +-γ coincidence, has the potential to simultaneously improve the CCR and coincidence time resolution (CTR) of PAL spectrometers. However, early β +-γ coincidence PAL spectrometers have not been widely applied due to the considerable room for improvement in their performance. In this work, we proposed a new β +-γ coincidence PAL spectrometer utilizing silicon photomultiplier (SiPM) array as the positron detector and conducted a comprehensive optimization of its structure with the aim of achieving a breakthrough in performance. The effects of start signal threshold and structure parameters on its CTR, CCR, and proportion of source contribution (P SC) were studied using Geant4. The simulation results show that, with a 68Ge positron source of 30 μCi, the optimized β +-γ coincidence PAL spectrometer can achieve an extremely high CCR exceeding 10000 counts per second (cps) and an outstanding CTR below 160 picoseconds (ps) while maintaining a low P SC below 12%. This study provides valuable guidance for constructing high-performance β +-γ coincidence PAL spectrometers.

Imaging low-energy positron beams in real-time with unprecedented resolution

Particle beams focused to micrometer-sized spots play a crucial role in forefront research using low-energy positrons. Their expedient and wide application, however, requires highly-resolved, fast beam diagnostics. We have developed two different methods to modify a commercial imaging sensor to make it sensitive to low-energy positrons. The first method consists in removing the micro-lens array and Bayer filter from the sensor surface and depositing a phosphor layer in their place. This procedure results in a detector capable of imaging positron beams with energies down to a few tens of eV, or an intensity as low as 35e+/s/mm2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${35}\\,{\\hbox {e}^+/\\hbox {s/mm}^{2}}$$\\end{document} when the beam energy exceeds 10 keV\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\\hbox {keV}}$$\\end{document}. The second approach omits the phosphor deposition; with the resulting device we succeeded in detecting single positrons with energies upwards of 6keV\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${6}\\,{\\hbox {keV}}$$\\end{document} and efficiency up to 93%. The achieved spatial resolution of 0.97 μm\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\upmu \\hbox {m}$$\\end{document} is unprecedented for real-time positron detectors.

Slow positrons from a magnetic bottle

A magnetic bottle device with optimized sizes for high-efficiency positron moderation was manufactured and attached to a positron detection system based on two gamma detectors operating in coincidence to detect positron annihilation events. The detection system’s performance was tested with a 22Na calibrated source. Titanium foils with a natural isotope mixture that were 2-μm-thick were proton irradiated to produce 48V. Experiments with a stack of up to four 48V foils showed that the magnetic bottle trap achieved a 10-fold increase in the standard positron moderator efficiency of tungsten in transmission mode. However, the measured moderation efficiency of ∼0.4% was found to be in huge discrepancy with the value of 1.8% obtained by Gerchow et al. for a non-optimized trap. One possible reason for this discrepancy was pointed out, namely that Gerchow et al mainly detected moderated positrons but almost no fast/unmoderated positrons, which escape the magnetic bottle trap and should also reach the positron detector used. Our experimental and simulation results showed that the ratio of moderated to fast/unmoderated positrons that reach our detection system is about 1:7.

Development and test of a next-generation positron detector

This report is focused on the development and test of a TDC (time to digital converter) and amplifier circuit for a state-of-the-art positron detector with position sensitivity. A newly developed one chip amplifier-shaper-discriminator (ASD) circuit with 16 channels input and a high-resolution TDC with a timing resolution on the order of picoseconds were tested. Silicon photomultipliers (SiPMs) were employed to give electric pulses for the optical input coming from a pulsed laser. We have successfully controlled the threshold level of the ASD circuit and measured the time structure of the SiPMs, as well as their multiple photon response from the pulse height histogram and the time over the threshold (TOT) histogram measured by the sub-nano second TDC. This is a technical basis for the next generation detector system with the positron tracking capability.

Implementation of Monte Carlo based PET verification of ion beam delivery into a TPS

ObjectiveTo develop and implement an ion beam teletherapy verification system based upon simulations of induced radioactivity and comparison with patient positron emission tomography (PET) scans. MethodsA Monte Carlo algorithm was integrated into an in-house built treatment, information, management, and planning system (TIMPS®) to calculate distributions of positron emitters induced by ion beams. These distributions are processed to simulate PET images taking into account positron diffusion, radioactive decay, biological washout, and detector blurring. Additional tools were developed to register simulated PET images with planning CT images and perform comparisons of measured and simulated PET images. ResultsProduction cross-sections were generated for various isotopes and materials. Treatment plans for several different body sites were used to verify the functioning and utility of the new tools for both proton and carbon ion beams. ConclusionsSeveral tools have been developed to simulate PET scans of patients irradiated with ion beams and compare the simulated images to measurements. After commissioning and implementation into the clinic, the appropriate sizes for margins that account for uncertainties may be investigated.

Monte Carlo Simulations of a Non-Invasive Positron Detector to Measure the Arterial Input Function for Dynamic PET

This work presents Monte Carlo (MC) study of a novel non-invasive positron detector, hereinafter called NID, designed to measure the arterial input function (AIF) through the wrist of a patient for use with dynamic positron emission tomography (PET). The goal of the study was to optimize a previously developed NID prototype, to determine its efficiency and ability to distinguish between the arterial and venous portions of the signal escaping a wrist phantom. A user code based on the Geant4 MC toolkit was developed to model the NID and a wrist phantom. The scintillator based detectors were modelled as 64 polystyrene cylinders, 0.97 mm in diameter, 10 cm long and capped with cylindrical photomultiplier tubes. The scintillator fibers were arranged in a single band around a 64.13 mm polyethylene cylinder representing a model of the wrist. Two cylinders, 2.30 mm in diameter were placed 6 mm apart, 2 mm below the surface of the wrist phantom representing the radial artery and vein. Two simulations were performed by placing 100 million decay events of oxygen-15 (15O) or fluorine-18 (18F), randomly distributed between the artery and vein. Visible wavelength photon tracking was enabled, and Photomultiplier tubes were simulated to collect the visible photons. Deposited energy per event and location of energy deposition were calculated. A python algorithm was used to analyse the results. The arterial signal produced a 5.28 mm and 10.32 mm FWHM for 15O and 18F respectively. The algorithm could determine the correct location of interaction 98% of the time. The NID can resolve the arterial and venous signal and is thus suitable for determining the AIF for dynamic PET.

Mulan: a part-per-million measurement of the muon lifetime and determination of the Fermi constant

The part-per-million measurement of the positive muon lifetime and determination of the Fermi constant by the MuLan experiment at the Paul Scherrer Institute is reviewed. The experiment used an innovative, time-structured, surface muon beam and a near-4\piπ, finely-segmented, plastic scintillator positron detector. Two in-vacuum muon stopping targets were used: a ferromagnetic foil with a large internal magnetic field, and a quartz crystal in a moderate external magnetic field. The experiment acquired a dataset of 1.6 \times 10^{12}1.6×1012 positive muon decays and obtained a muon lifetime \tau_{\mu} = 2\, 196\, 980.3(2.2)τμ=2196980.3(2.2)~ps (1.0~ppm) and Fermi constant G_F = 1.166\, 378\, 7(6) \times 10^{-5}F=1.1663787(6)×10−5 GeV^{-2}−2 (0.5~ppm). The thirty-fold improvement in \tau_{\mu}τμ has proven valuable for precision measurements in nuclear muon capture and the commensurate improvement in G_FF has proven valuable for precision tests of the standard model.

Cross-validation of a non-invasive positron detector to measure the arterial input function for pharmacokinetic modelling in dynamic positron emission tomography

Kinetic modeling of positron emission tomography (PET) data can assess index rate of uptake, metabolism and predict disease progression more accurately than conventional static PET. However, it requires knowledge of the time-course of the arterial blood radioactivity concentration, called the arterial input function (AIF). The gold standard to acquire the AIF is by invasive means. The purpose of this study was to validate a previously developed dual readout scintillating fiber-based non-invasive positron detector, hereinafter called non-invasive detector (NID), developed to determine the AIF for dynamic PET measured from the human radial artery. The NID consisted of a 3 m long plastic scintillating fiber with each end coupled to a 5 m long transmission fiber followed by a silicon photomultiplier. The scintillating fiber was enclosed inside the grooves of a plastic cylindrical shell. Two sets of experiments were performed to test the NID against a previously validated microfluidic positron detector. A closed-loop microfluidic system combined with a wrist phantom was used. During the first experiment, the three PET radioisotopes 18F, 11C and 68Ga were tested. After optimizing the detector, a second series of tests were performed using only 18F and 11C. The maximum pulse amplitude to electronic noise ratio was 52 obtained with 11C. Linear regressions showed a linear relation between the two detectors. These preliminary results show that the NID can accurately detect positrons from a patient’s wrist and has the potential to non-invasively measure the AIF during a dynamic PET scan. The accuracy of these measurements needs to be determined.

Characterization of scintillating fibers for use as positron detector in positron emission tomography

PurposeManual and automatic blood sampling at different time intervals is considered the gold standard to determine the arterial input function (AIF) in dynamic positron emission tomography (PET). However, blood sampling is characterized by poor time resolution and is an invasive procedure. The aim of this study was to characterize the scintillating fibers used to develop a non-invasive positron detector. MethodsThe detector consists of a scintillating fiber coupled at each end to transmission fiber-optic cables that are connected to photo multiplier tubes in a dual readout setup. The detector is designed to be wrapped around the wrist of the patient undergoing dynamic PET. The attenuation length and bending losses were measured with excitation from gamma radiation (137Cs) and ultraviolet (UV) light. The response to positron-emitting radio-tracers was evaluated with 18F and 11C. ResultsThe attenuation length for a 3.0 m and 1.5 m long scintillating fiber both coincides with the attenuation length given by the manufacturer when excited with the 137Cs source, but not with the UV source due to the differences in scintillation mechanisms. The bending losses are smaller than the measurement uncertainty for the 137Cs source irradiation, and increase when the bending radius decrease for the UV source irradiation. The signal-to-noise ratio for 18F and 11C solutions are 1.98 and 22.54 respectively. The measured decay constant of 11C agrees with its characteristic value. ConclusionThe performed measurements in the dual readout configuration suggest that scintillating fibers may be suitable to determine the AIF non-invasively.

The new versatile general purpose surface-muon instrument (GPS) based on silicon photomultipliers for μSR measurements on a continuous-wave beam

We report on the design and commissioning of a new spectrometer for muon-spin relaxation/rotation studies installed at the Swiss Muon Source (SμS) of the Paul Scherrer Institute (PSI, Switzerland). This new instrument is essentially a new design and replaces the old general-purpose surface-muon (GPS) instrument that has been for long the workhorse of the μSR user facility at PSI. By making use of muon and positron detectors made of plastic scintillators read out by silicon photomultipliers, a time resolution of the complete instrument of about 160 ps (standard deviation) could be achieved. In addition, the absence of light guides, which are needed in traditionally built μSR instrument to deliver the scintillation light to photomultiplier tubes located outside magnetic fields applied, allowed us to design a compact instrument with a detector set covering an increased solid angle compared with the old GPS.

A Prototype High-Resolution Small-Animal PET Scanner Dedicated to Mouse Brain Imaging.

We developed a prototype small-animal PET scanner based on depth-encoding detectors using dual-ended readout of small scintillator elements to produce high and uniform spatial resolution suitable for imaging the mouse brain. The scanner consists of 16 tapered dual-ended-readout detectors arranged in a 61-mm-diameter ring. The axial field of view (FOV) is 7 mm, and the transaxial FOV is 30 mm. The scintillator arrays consist of 14 × 14 lutetium oxyorthosilicate elements, with a crystal size of 0.43 × 0.43 mm at the front end and 0.80 × 0.43 mm at the back end, and the crystal elements are 13 mm long. The arrays are read out by 8 × 8 mm and 13 × 8 mm position-sensitive avalanche photodiodes (PSAPDs) placed at opposite ends of the array. Standard nuclear-instrumentation-module electronics and a custom-designed multiplexer are used for signal processing. The detector performance was measured, and all but the crystals at the very edge could be clearly resolved. The average intrinsic spatial resolution in the axial direction was 0.61 mm. A depth-of-interaction resolution of 1.7 mm was achieved. The sensitivity of the scanner at the center of the FOV was 1.02% for a lower energy threshold of 150 keV and 0.68% for a lower energy threshold of 250 keV. The spatial resolution within a FOV that can accommodate the entire mouse brain was approximately 0.6 mm using a 3-dimensional maximum-likelihood expectation maximization reconstruction. Images of a hot-rod microphantom showed that rods with a diameter of as low as 0.5 mm could be resolved. The first in vivo studies were performed using (18)F-fluoride and confirmed that a 0.6-mm resolution can be achieved in the mouse head in vivo. Brain imaging studies with (18)F-FDG were also performed. We developed a prototype PET scanner that can achieve a spatial resolution approaching the physical limits of a small-bore PET scanner set by positron range and detector interaction. We plan to add more detector rings to extend the axial FOV of the scanner and increase sensitivity.

New μSR spectrometer at J-PARC MUSE based on Kalliope detectors

We developed a new positron detector system called Kalliope, which is based on multi-pixel avalanch photo-diode (m-APD), application specific integrated circuit (ASIC), field programmable gated array (FPGA) and ethernet-based SiTCP data transfer technology. We have manufactured a general-purpose spectrometer for muon spin relaxation (μSR) measurements, employing 40 Kalliope units (1280 channels of scintillators) installed in a 0.4 T longitudinal-field magnet. The spectrometer has been placed at D1 experimental area of J- PARC Muon Science Establishment (MUSE). Since February of 2014, the spectrometer has been used for the user programs of MUSE after a short commissioning period of one week. The data accumulation rate of the new spectrometer is 180 million positron events per hour (after taking the coincidence of two scintillators of telescopes) from a 20×20 mm sample for double-pulsed incoming muons.

Muonium production target for the muon g-2/EDM experiment at J-PARC

There is more than three standard-deviations discrepancy between measurement and theoretical prediction of the muon anomalous magnetic moment. We are going to measure the precision value of muon g−2 and search for physics beyond standard model. In addition, we can search for muon EDM which violates CP symmetry. CP violation in charged lepton sector is currently not found. We are developing the “Ultra Cold Muon Beam” instead of tertiary muon beam with electric focusing. Ultra cold muon is realized by laser ionization of muonium (bound state of a muon and an electron) from the production target. Increase of muonium yield is essential for our experimental goal; 0.1ppm statistical precision. Muonium production experiment at J-PARC MLF MUSE is planned in 2012 autumn. In this paper, we discuss the development of muonium production target and positron detector for the study.

A Monte Carlo investigation of the spatial resolution performance of a small-animal PET scanner designed for mouse brain imaging studies

Our laboratory has developed PET detectors with depth-encoding accuracy of ∼2 mm based on finely pixelated crystals with a tapered geometry, readout at both ends with position-sensitive avalanche photodiodes (PSAPDs). These detectors are currently being used in our laboratory to build a one-ring high resolution PET scanner for mouse brain imaging studies. Due to the inactive areas around the PSAPDs, large gaps exist between the detector modules which can degrade the image spatial resolution obtained using analytical reconstruction with filtered backprojection (FBP). In this work, the Geant4-based GATE Monte Carlo package was used to assist in determining whether gantry rotation was necessary and to assess the expected spatial resolution of the system. The following factors were investigated: rotating vs. static gantry modes with and without compensation of missing data using the discrete cosine transform (DCT) method, two levels of depth-encoding, and positron annihilation effects for 18F. Our results indicate that while the static scanner produces poor quality FBP images with streak and ring artifacts, the image quality was greatly improved after compensation of missing data. The simulation indicates that the expected FWHM system spatial resolution is 0.70 ± 0.05 mm, which approaches the predicted limit of 0.5 mm FWHM due to positron range, photon non-colinearity and physical detector element size effects. We conclude that excellent reconstructed resolution without gantry rotation is possible even using FBP if the gaps are appropriately handled and that this design can approach the resolution limits set by positron annihilation physics.

Novel System for Potential Nondestructive Material Inspection Using Positron Annihilation Lifetime Spectroscopy

A new positron annihilation lifetime spectrometer consisting of a start γ-ray detector, a stop γ-ray detector, a digital oscilloscope, and a positron detector, which is a plastic scintillator coupled to a photomultiplier tube, is described. A 22Na source is placed between the positron detector and a sample to be studied. γ-ray signals related to positrons annihilating in the positron detector are rejected by anti-coincidence processing. Comparison of the positron lifetime spectrum of a steel strip collected with the new system with that collected with a conventional system using two specimens sandwiching the 22Na source shows that accurate positron lifetime measurements are possible with the new system. The new system does not require cutting of the samples and is potentially applicable to truly nondestructive onsite inspection of various materials such as those used in nuclear power plants, aircraft and cars, etc., by positron annihilation lifetime spectroscopy (PALS).

A compact and high sensitivity positron detector using dual-layer thin GSO scintillators for a small animal PET blood sampling system

For quantitative measurements of small animals such as mice or rats, a compact and high sensitivity continuous blood sampling detector is required because their blood sampling volume is limited. For this purpose we have developed and tested a new positron detector. The positron detector uses a pair of dual-layer thin gadolinium orthosilicate (GSO) scintillators with different decay times. The front layer detects the positron and the background gamma photons, and the back layer detects the background gamma photons. By subtracting the count rate of the latter from that of the former, the count rate of the positrons can be estimated. The GSO for the front layer has a Ce concentration of 1.5 mol% (decay time of 35 ns), and that for the back layer has a Ce concentration of 0.5 mol% (decay time of 60 ns). By using the pulse shape analysis, the count rate of these two GSOs can be discriminated. The thickness is 0.5 mm, which is thick enough to detect positrons while minimizing the detection of the background gamma photons. These two types of thin GSOs were optically coupled to each other and connected to a metal photomultiplier tube (PMT) through triangular light guides. The signal from the PMT was digitized by 100 MHz free-running A-D converters in the data acquisition system and digitally integrated at two different integration times for the pulse shape analysis. We obtained good separation of the pulse shape distributions of these two GSOs. The energy threshold level was decreased to 80 keV, increasing the sensitivity of the detector. The sensitivity of a small diameter plastic tube was 8.6% and 24% for the F-18 and C-11 positrons, respectively. The count rate performance was linear up to ∼50 kcps. The background counts from the gamma photons could be precisely corrected. The time–activity curve (TAC) of the rat artery blood was successfully obtained and showed a good correlation with that measured using a well counter. With these results, we confirmed that the developed blood sampling detector is promising for quantitative measurement for an animal positron emission tomography system.

Scintillating fibres for future spectrometers

Abstract The use of scintillating fibres as positron detectors in μ SR experiments is discussed. Experimental tests and numerical simulations show that 1 mm thick fibres represent an optimum choice between a sufficiently high signal level and a good spatial resolution. A comparison of decay positron detection efficiency with that of electrons from common 90Sr sources shows that the existing results with beta emitters could be transferred almost unvaried to fibres for use in μ SR spectrometers. Thanks to a quantum efficiency above 50% and to a continuously improving gain (currently at 1 × 10 6 ), the newly developed Geiger-mode avalanche photodiodes (G-APDs) represent the most promising choice as associated photon detectors.

Development of positron detector for μSR based on multi-pixel photon counter

In the pulsed muon facility (MUSE) being built as a part of J-PARC, muon beams with unprecedented intensity (∼10 6 μ +/pulse) will be delivered at its full operation. Because of the extreme instantaneous μ–e decay positron rates (∼10 4–10 5 e +/pulse), development of a highly segmented positron detection system is crucial for practical application of μSR. To this end, we have designed a new positron detector based on a multi-pixel photon counter (MPPC). The advantages of MPPC over conventional phototubes are its small size, low operation voltage, functionality under a high magnetic field, and low cost. The result of test experiment for the detector using a pulsed muon beam is reported.

Electron–positron colliders, detectors and experiments at BINP

An overview of electron–positron colliders and experiments on them currently performed and prepared at the Budker Institute of Nuclear Physics is presented. They include a new collider VEPP-2000 (an upgrade of the VEPP-2M collider) with CMD-3 and SND experiments, the VEPP-4M collider with the KEDR detector and the electron/positron injection complex.

Role of PET/CT in the evaluation of cervical cancer

PET imaging utilizes a dedicated camera system with multiple positron detector rings. PET/CT precisely aligns and combines metabolic PET mages with anatomical CT images, and is being increasingly preffered over PET scanning alone. FDG is the most widely used radiotracer in the management of cancer patients, and the prototypical PET/CT protocol used in other cancers can also be applied to the management of cervical carcinoma patients. The applications of PET/CT in cervical cancer patients include: assessing local tumor extension (information on metabolic tumor activity and possible endometrial involvement), evaluating pelvic nodal involvement (even in cases with negative CT or MRI studies), detection of distant metastases (PET/CT should be the first imaging technique used to evaluate extrapelvic disease before pelvic exenteration), radiation therapy planning (in patiens with PET scans positive for lymph nodes), identification of persistent/recurrent disease (especially in assessing response to neoadjuvant therapy and prognosis (with an inverse response–survival relationship.