Degradation Of Time Resolution Research Articles

Time Resolution Measurements of EJ-232Q With Single- and Dual-Sided Readouts

In view of its importance to applications in high-energy physics, nuclear security, and time-of-flight positron emission tomography (TOF-PET), the time resolution of 0.5% benzophenone-quenched EJ-232Q plastic scintillators of various lengths was studied using a fast waveform digitizer and fast photomultiplier tubes (PMTs). The scintillator sizes were 5 mm $\times 5$ mm $\times10$ , 25, 50, 75, or 100 mm. First, the timing measurements were performed by using a single PMT reading out one end. The degradation in time resolution with increasing scintillator length was studied. The irradiation geometry was also observed to have a nonnegligible impact on the time resolution due to the variation of depth-of-interaction (DOI) positions, especially for the scintillators with a high aspect ratio. Second, two PMTs were employed for a dual-sided readout. This readout together with the mean time method was demonstrated to be an effective approach to improve the timing performance since the DOI-related effect was considerably mitigated. A time resolution of 79 ± 1 ps (full-width at half-maximum) has been achieved for the $5 \times 5 \times 10$ mm3 scintillator. The $5\times 5\,\,\times100$ mm3 scintillator can still provide a time resolution as good as 172 ± 1 ps. The energy range used for each was between 340 and 450 keVee. In addition, the time information from individual PMTs in a dual-sided readout was able to show a time resolution almost identical to that determined with the mean-time-based technique after correcting for the DOI variation for the 100-mm-long scintillator.

Pile-up correction algorithm for high count rate gamma ray spectroscopy

In nuclear applications related to radiation energy measurement, digital pulse processing for fast sampling facility has great advantages compared to the traditional analog pulse processing. A distortion in energy spectrum and degradation in timing resolution take place when pulse pile-up occurs. This in turn leads to degradations of the signals used in applications such as nuclear medicine. Pile-up pulses are commonly removed in most algorithms if they are detected, but in this case, getting a clean spectrum becomes a challenge due to data loss. A proposed pile-up detection and recovery algorithm based on fitting the preamplifier data and signal extrapolation is presented. The extrapolated pulse is used for recovering (extracting) the piled-up pulse to estimate its parameters. A smoothed first derivative algorithm is used for peak tracking inside the detected events. To evaluate the proposed pile-up recovery algorithm theoretically and experimentally, it is applied on both simulated input signals generated with different time displacements between the piled-up events, and experimental preamplifier radiation signals. The obtained results prove that the proposed algorithm achieves better utilization of power and can be used for high count rate (high activity elements) spectroscopy systems.

Application of bi-Gaussian S-transform in high-resolution seismic time-frequency analysis

The S-transform is one of the most widely used methods of time-frequency analysis. It combines the respective advantages of the short-time Fourier transform and wavelet transforms with scale-dependent resolution using Gaussian windows, scaled inversely with frequency. One of the problems with the traditional symmetric Gaussian window is the degradation of time resolution in the time-frequency spectrum due to the long front taper. We have studied the performance of an improved S-transform with an asymmetric bi-Gaussian window. The asymmetric bi-Gaussian window can obtain an increased time resolution in the front direction. The increased time resolution can make event picking high resolution, which will facilitate an improved time-frequency characterization for oil and gas trap prediction. We have applied the slightly modified bi-Gaussian S-transform to a synthetic trace, a 2D seismic section, and a 3D seismic cube to indicate the superior performance of the bi-Gaussian S-transform in analyzing nonstationary signal components, hydrocarbon reservoir predictions, and paleochannels delineations with an obviously higher resolution.

A prototype MR insertable brain PET using tileable GAPD arrays

The aim of this study was to develop a prototype magnetic resonance (MR)-compatible positron emission tomography (PET) that can be inserted into a MR imager and that allows simultaneous PET and MR imaging of the human brain. This paper reports the initial results of the authors' prototype brain PET system operating within a 3-T magnetic resonance imaging (MRI) system using newly developed Geiger-mode avalanche photodiode (GAPD)-based PET detectors, long flexible flat cables, position decoder circuit with high multiplexing ratio, and digital signal processing with field programmable gate array-based analog to digital converter boards. A brain PET with 72 detector modules arranged in a ring was constructed and mounted in a 3-T MRI. Each PET module was composed of cerium-doped lutetium yttrium orthosilicate (LYSO) crystals coupled to a tileable GAPD. The GAPD output charge signals were transferred to preamplifiers using 3 m long flat cables. The LYSO and GAPD were located inside the MR bore and all electronics were positioned outside the MR bore. The PET detector performance was investigated both outside and inside the MRI, and MR image quality was evaluated with and without the PET system. The performance of the PET detector when operated inside the MRI during MR image acquisition showed no significant change in energy resolution and count rates, except for a slight degradation in timing resolution with an increase from 4.2 to 4.6 ns. Simultaneous PET/MR images of a hot-rod and Hoffman brain phantom were acquired in a 3-T MRI. Rods down to a diameter of 3.5 mm were resolved in the hot-rod PET image. The activity distribution patterns between the white and gray matter in the Hoffman brain phantom were well imaged. The hot-rod and Hoffman brain phantoms on the simultaneously acquired MR images obtained with standard sequences were observed without any noticeable artifacts, although MR image quality requires some improvement. These results demonstrate that the simultaneous acquisition of PET and MR images is feasible using the MR insertable PET developed in this study.

Optimizing timing performance of silicon photomultiplier-based scintillation detectors

Precise timing resolution is crucial for applications requiring photon time-of-flight (ToF) information such as ToF positron emission tomography (PET). Silicon photomultipliers (SiPM) for PET, with their high output capacitance, are known to require custom preamplifiers to optimize timing performance. In this paper, we describe simple alternative front-end electronics based on a commercial low-noise RF preamplifier and methods that have been implemented to achieve excellent timing resolution. Two radiation detectors with L(Y)SO scintillators coupled to Hamamatsu SiPMs (MPPC S10362–33-050C) and front-end electronics based on an RF amplifier (MAR-3SM+), typically used for wireless applications that require minimal additional circuitry, have been fabricated. These detectors were used to detect annihilation photons from a Ge-68 source and the output signals were subsequently digitized by a high speed oscilloscope for offline processing. A coincident resolving time (CRT) of 147 ± 3 ps FWHM and 186 ± 3 ps FWHM with 3 × 3 × 5 mm3 and with 3 × 3 × 20 mm3 LYSO crystal elements were measured, respectively. With smaller 2 × 2 × 3 mm3 LSO crystals, a CRT of 125 ± 2 ps FWHM was achieved with slight improvement to 121 ± 3 ps at a lower temperature (15° C). Finally, with the 20 mm length crystals, a degradation of timing resolution was observed for annihilation photon interactions that occur close to the photosensor compared to shallow depth-of-interaction (DOI). We conclude that commercial RF amplifiers optimized for noise, besides their ease of use, can produce excellent timing resolution comparable to best reported values acquired with custom readout electronics. On the other hand, as timing performance degrades with increasing photon DOI, a head-on detector configuration will produce better CRT than a side-irradiated setup for longer crystals.

Time properties of multigap RPC with glass electrodes operated in streamer and avalanchemodes

Abstract We tested the multigap RPC operated in avalanche and streamer modes and investigated the various characteristics with the variety of the detector structure such as the number of gaps and the thickness of the glass electrodes. We found that the detection efficiency seems to be improved by multigap structure and depend on the thickness of the electrodes. No significant degradation of time resolution was found even with the number of gaps increased.

Observation of d–t fusion gamma rays (invited)

Deuterium–tritium (DT) reaction rates of imploding capsules have historically been measured using neutron detectors. Temporal resolution is limited by the size of the detector and distance from the source to detector. The reaction rates can also be measured using the 16.7 MeV gamma ray, which is produced by the same DT reaction, but statistically far less often than the 14.1 MeV neutron. Cherenkov detectors detect gamma rays by converting the gamma rays to electrons, which in turn produce Cherenkov light and record this visible light using a fast optical detector. These detectors can be scaled to large volumes in order to increase detection efficiency with little degradation in time resolution, and placed well away from the source since gamma rays do not suffer velocity dispersion between the source and detector. Gas-based Cherenkov detectors can also discriminate against lower-energy photons produced in and around the target. A prototype gas Cherenkov detector has been built and tested for detector response at the Idaho State University electron linear accelerator (LINAC). Later tests at the University of Rochester’s OMEGA laser facility proved the feasibility of this diagnostic by successfully recording the 16.7 MeV gamma ray. A second version has now been built to demonstrate the temporal resolution.

The Bi-Gaussian S-Transform

The S-transform kernel is derived from the kernel of the Fourier transform through the introduction of a scalable, translating window. The width of the window is a function of inverse frequency. In effect the S-transform is a method of spectral localization, with some similarities to wavelet transforms but using the concept of frequency rather than the concept of scale. An important property of the S-transform is that it collapses into the Fourier transform when integrated over the time axis; this property requires the window to satisfy a normalizing condition. The window which has been used in the majority of previous S-transform research is the symmetric Gaussian window introduced by Stockwell, Mansinha, and Lowe [IEEE Trans. Signal Process., 44 (1996), pp. 998--1001]. One problem with the use of a Gaussian, however, is degradation of time resolution in the time-frequency spectrum due to the long front taper. In this paper, a bi-Gaussian window is introduced, constructed through the welding of two half Gaussian windows. The asymmetry of the bi-Gaussian introduces asymmetry in the resultant time-frequency spectrum, with time resolution better in the "front" direction, as compared with the "back" direction. The bi-Gaussian S-transform is better at resolving the sharp onset of events in a time series.

Test of a 3 m long, 4 × 4 cm 2 time-of-flight (TOF) scintillation counter using 38 × 38 mm 2 fine-mesh photomultipliers in magnetic fields up to 1.5 T

A prototype time-of-flight counter for the CDF detector was tested in magnetic fields up to 1.5 T using a test beam line of 2 GeV/ c, T-1, at the KEK-PS. The counter of a 3 m long scintillator bar (BC-408) of 4 × 4 cm 2 in cross section is viewed by a newly developed 38 × 38 mm 2 square-shaped fine-mesh photomultiplier tube at each end. In the first test the time resolution for a single photomultiplier after being corrected for the contribution from a reference counter was measured to be about 130 ps in a magnetic field of 1.2 T in the tube axial direction when the beam was perpendicularly incident at the middle of the counter, and it was found to be essentially constant in the magnetic field range below 1.2 T. This result implies that the prototype TOF counter with two fine-mesh photomultipliers can give a time resolution of about 100 ps in this field range. In the second test after the scintillator surface was damaged time resolutions were measured in magnetic fields up to 1.5 T. Data indicate a slight degradation in time resolution with increasing magnetic field above 1 T. The time resolution becomes poorer by about 20% at 1.5 T compared with those below 1 T.

Improved techniques in multiparameter coincidence experiments

The application of multiparameter techniques, such as position-sensitive detectors, to coincidence experiments has resulted in greatly improved collection efficiency over previously adopted single-channel methods. Often, however, the improvements made in quality of data are limited by an increase in the accidental count rate concomitant with this approach. This increase in background count rate results from the increased singles count rates in the detectors and the degradation in timing resolution due to the simultaneous detection of electrons over a wide energy range. The authors present two methods for reducing this background. Their combined effect was to reduce the accidental coincidence count rate by a factor of almost 20 in the coincidence measurement of (e, 2e) transitions in He in the autoionising region.

An improved positron lifetime spectrometer

A positron lifetime spectrometer incorporating digital stabilization allows for prolonged experiments without degradation of time resolution by electronic drifts. The consequent improvement in statistical precision increases the scope for realistic numerical analysis of complex types of spectra.