Unpolished Crystals Research Articles

Improving timing performance of double-ended readout in TOF-PET detectors

Scintillation crystals of 20 mm length or longer are needed for clinical time-of-flight positron emission tomography (TOF-PET) to ensure effective detection efficiency for gamma photons. However, the use of long crystals would deteriorate the key performance of TOF-PET detectors, time and spatial resolution, because of the variations in the travel times of the photons in crystals and the effects of parallax errors. In this work, we studied double-ended readout TOF-PET detectors based on coupling a long scintillation crystal to SiPMs at both ends for correcting the depth-dependent effects to improve the coincidence time resolution (CTR). In particular, we focused our attention to analyze timing performance using different correction methods, including trigger times of the individual photodetectors at both ends of the crystal, the simple average of the trigger times, and the weighted average based on the inverse variances of the depth-dependent corrected trigger times. For a 3 mm × 3 mm × 25 mm unpolished lutetium fine silicate (LFS) crystal with double-ended readout and practical head-on irradiation, a CTR of 246 ps FWHM can be achieved using depth-dependent timing-correction and weighted average time method compared to 280 ps FWHM using the conventional simple average time method and 393 ps FWHM using the conventional single-ended readout. The results show that the depth-dependent timing-correction and weighted average time method in double-ended readout can effectively correct for the trigger time variations in TOF-PET detector utilizing long unpolished crystals, resulting in an improvement in the CTR of as much as 37% compared to single-ended readout.

A depth-encoding PET detector that uses light sharing and single-ended readout with silicon photomultipliers

Detectors with depth-encoding capability and good timing resolution are required to develop high-performance whole-body or total-body PET scanners. In this work, depth-encoding PET detectors that use light sharing between two discrete crystals and single-ended readout with silicon photomultipliers (SiPMs) were manufactured and evaluated. The detectors consisted of two unpolished 3 × 3 × 20 mm3 LYSO crystals with different coupling materials between them and were read out by Hamamatsu 3 × 3 mm2 SiPMs with one-to-one coupling. The ratio of the energy of one SiPM to the total energy of two SiPMs was used to measure the depth of interaction (DOI). Detectors with different coupling materials in-between the crystals were measured in the singles mode in an effort to obtain detectors that can provide good DOI resolution. The DOI resolution and energy resolution of three types of detector were measured and the timing resolution was measured for the detector with the best DOI and energy resolution. The optimum detector, with 5 mm optical glue, a 9 mm triangular ESR and a 6 mm rectangular ESR in-between the unpolished crystals, provides a DOI resolution of 2.65 mm, an energy resolution of 10.0% and a timing resolution of 427 ps for events of E > 400 keV. The detectors simultaneously provide good DOI and timing resolution, and show great promise for the development of high-performance whole-body and total-body PET scanners.

Effects of surface orientation, fluid chemistry and mechanical polishing on the variability of dolomite dissolution rates

Recent studies of carbonate surface reactivity have underscored the fundamental variability of dissolution rates and the heterogeneous distribution of the reaction over the mineral surface due to the inhomogeneous distribution of surface energy. Dolomite dissolution rates relative to different cleavage planes (r-planes) and surfaces cut approximately perpendicular to the c-axis (c-planes) were studied at 50°C as a function of pH (3.4≤pH≤9.0) and solution composition by vertical scanning interferometry (VSI) and atomic force microscopy (AFM), with the aim of providing an estimate of the intrinsic rate variation of dolomite single crystals and describing the surface reaction distribution and the rate controlling mechanisms. Surface normal retreat rates measured under acidic conditions increased linearly with time and were not visibly affected by the parallel increase of surface roughness. Mean total dissolution rates of r-planes decreased by over 200 times from pH 3.4 to pH 9.0 and CO32–-rich solutions, whereas corresponding rate variations spanned over 3 orders of magnitude when also c-plane rate distributions were included in the analysis. At acid to near neutral pH, c-planes dissolved∼three times faster than the adjoining r-planes but slower at basic pH and high total carbon concentration, displaying a distinctive morphologic evolution in these two regimes. The comparison of polished and unpolished crystals showed that polished cleavage planes dissolved about three times faster than the unpolished counterpart at near neutral to basic conditions, whereas no significant difference in reactivity was observed at pH<5.Although experimental data and observations indicate a tendency of dolomite faces to reach a low-energy topography over the course of the reaction, the evolution of the entire crystal morphology depends also on the reactivity of edge and corner regions, whose contribution to measured rates is not generally taken into account by laboratory experiments. The study of time-dependent mineral morphology and reactivity requires an integrated approach of kinetic modeling and experimentation, where measured rate variance and observed reaction mechanisms represent fundamental parameters for the improvement of geochemical models in predicting long-term reaction rates in a wide range of environmental conditions.

Study of the Anomalous Sorption Behavior of CO2 into Poly(methyl methacrylate) Films in the Vicinity of the Critical Pressure and Temperature Using a Quartz Crystal Microbalance (QCM)

The anomalous solubility maximum of CO2 in polymer thin films in the vicinity of the critical temperature and pressure has not yet been clearly understood when the quartz crystal microbalance (QCM) technique has been used to determine the micromass change. In this study, the adsorption of CO2 on the surface of bare polished and unpolished crystals at different pressures and temperatures was investigated using the QCM technique to illustrate why a plot of the true frequency shift as a function of temperature and pressure can intuitively exhibit the adsorption behavior of CO2 on bare crystals. The sorption of CO2 into a PMMA film at different temperatures, pressures, and PMMA film thicknesses was also investigated. An accurate solubility for CO2 in the PMMA film could be obtained by an improved data correction method from the linear relation between the true frequency shift and the polymer film mass, and the anomalous solubility maximum could be corrected by this method. The mechanism of nonabsorbed CO2 transitorily staying in the interspace between the PMMA film and the crystal surface can be explained by the morphology change of the PMMA film. The assumption of "passerby CO2" was satisfactorily confirmed to explain the anomalous sorption behavior of CO2 into PMMA films in the vicinity of the CO2 critical temperature and pressure, and this assumption could be valid for other CO2-polymer thin film systems.

Scintillation properties of and proton-induced effect on CdWO4 crystals

The scintillation properties of CdWO4 (CWO) crystals have been studied to develop an X-ray scanner for a cargo container. The CWO crystal is composed of higher effective-Z-number materials, and it has a high density, therefore, a high detection efficiency for X-rays and γ-rays is expected. We selected various CWO crystals made in Russia (Novosibirsk) and China for comparison. The X-ray luminescence and the scintillation properties were studied with various sizes (4 × 4 × 30 mm3, 4 × 7 × 30 mm3 and 4 × 12 × 30 mm3) for polished and unpolished crystals. The transmittance of the CWO crystals was found to be 75% in the visible range. The X-ray emission spectrum of CWO crystals showed a peak at 480 nm. The scintillation properties, like the decay time, the energy resolution and the light yield, were measured and compared by using gamma source excitation. All samples exhibited similar decay times, with a value of 12 µs. The light yield and the energy resolution of the Russian samples were found to be significantly better than those of the Chinese samples. The proton response and radiation hardness of the CWO crystal were studied by using the 45-MeV proton beam from the MC-50 cyclotron at the KIRAMS (Korea Institute of Radiological and Medical Science).

Investigation of Depth of Interaction Encoding for a Pixelated LSO Array With a Single Multi-Channel PMT

A new approach to depth of interaction (DOI) encoding for a pixelated LSO array using a single multi-channel PMT was investigated. In this method the DOI information was estimated by taking advantage of optical crosstalk between LSO elements and examining the standard deviation (spread) of signals on all channels of the PMT. Unpolished and polished 6×6 LSO arrays with a crystal size of 1.3×1.3×20 mm(3) were evaluated on a Hamamatsu H7546 64-channel PMT. The arrays were placed on the center of the PMT and the central 16 channels of the PMT were individually read out and digitized. For the unpolished array, all crystals were resolved in the flood histogram. An average DOI resolution of 8 mm was obtained. The energy resolution was ∼25% after the signal amplitude was corrected using the measured DOI information. For the polished array, the flood histogram was superior to the unpolished array, however no DOI information could be measured. Using unpolished crystals, this method could be a practical way to achieve limited DOI information in PET detectors. The standard deviation of all PMT channels can be readily obtained using a resistor network. Only five signals (four signals to determine the x-y position and one signal measuring the standard deviation) need to be digitized, and this method only requires a single photon detector to read out the array. Unlike phoswich detectors, the method does not require segmenting the scintillator array into layers. The measured DOI resolution was much worse than that obtained with the dual-ended readout method, however, it was similar to that obtained with a two-layer phoswich detector.

Gold thin-film electrodes: an EQCM study of the influence of chromium and titanium adhesion layers on the response

Abstract The effect of the adhesion layer between gold films and quartz on the response of an electrochemical quartz crystal microbalance (EQCM) has been investigated in 0.1 M HClO4 and in 0.1 M phosphate buffer pH 7.4. Cyclic voltammetry (CV) was applied in an anodic potential range, with polished and unpolished crystals, provided with a Cr or Ti adhesion layer. Crystals with Cr undercoatings showed—in HClO4 and in buffer—oxidation currents and frequency shifts due to dissolution of Cr in a potential range in which gold oxidation also takes place. This led to loss of resonance of the crystal and/or to an increase in surface roughness. The deleterious effects of Cr are most pronounced with unpolished crystals. Crystals with Ti undercoatings neither showed a sign of oxidation or dissolution of adhesion metal, nor a significant increase in surface roughness upon repeated cycling, probably due to passivation of Ti by a thin oxide film, and its resistance to dissolution. The small positive frequency shifts observed upon repeated cycling in stirred solutions of HClO4 and phosphate buffer, with unpolished crystals in particular, are most likely due to dissolution of gold. The oxygen chemisorption on gold in HClO4 and in buffer, and the enhanced dissolution of gold in the presence of small concentrations (0.5 mM) of chloride in HClO4, could be very well studied with Ti-coated crystals. For applications in the anodic potential range (extending into the region of oxygen chemisorption on gold), crystals with Cr adhesion layers should not be used.

Chemical polishing of LSO crystals to increase light output

Surface treatment of scintillators is critical for light collection from narrow rectangular crystals. The authors investigated chemical polishing which is less labor intensive and expensive than hand and machine polishing. They used phosphoric acid to chemically polish LSO crystals at 110/spl deg/C, 150/spl deg/C and 190/spl deg/C, and compared these with unpolished and mechanically polished crystals. Groups of five 2/spl times/2/spl times/10 mm crystals were etched for different times at each temperature. Weight loss, light output and energy resolution were measured as a function of treatment time and temperature. The authors found that chemical polishing can increase light output by 250% relative to unpolished crystals and by 16% relative to mechanically polished crystals. The energy resolution was relatively independent of surface treatment, with values of between 14 and 16%. The rate of loss of LSO was 0.02%/min at 110/spl deg/C, 0.1%/min at 150/spl deg/C and 0.36%/min at 190/spl deg/C. Maximum light output occurred when 5-10% of the LSO was removed. The crystals were also imaged using scanning electron microscopy and the surface roughness quantitatively assessed by using a profilometer. These measurements helped to clarify the effect of acid polishing on the surface of the scintillator. In summary, chemical polishing appears to be a convenient and effective method for improving light output from small LSO crystals.