Crystal Block Research Articles

Sonic crystal acoustic switch device

This study reports a wave-controlled sonic crystal switch device that exhibits a destructive interference-based wave to wave reverse switching effect. By applying control waves, this acoustic device, composed of a two-dimensional square lattice sonic crystal block, reduces acoustic wave transmission from input to output. The finite difference time domain simulation and experimental results confirm the wave-to-wave reverse switching effect at the peak frequencies of the second band. The proposed sonic crystal switch prototype provides a contrast rate of 86% at 11.3 kHz frequency. This wave-to-wave switching effect is useful for controlling wave propagation for smart structure applications.

The X'tal cube PET detector with a monolithic crystal processed by the 3D sub-surface laser engraving technique: Performance comparison with glued crystal elements

The X'tal cube is a depth-of-interaction (DOI)-PET detector which is aimed at obtaining isotropic resolution by effective readout of scintillation photons from six sides of the crystal block. The X'tal cube is composed of a 3D crystal block with isotropic segments. Each face of the 3D crystal block is covered with a 4×4 array of multi-pixel photon counters (MPPCs). Previously, in order to fabricate the 3D crystal block efficiently and precisely, we applied a sub-surface laser engraving technique to a monolithic crystal block instead of gluing segmented small crystals. A dense arrangement of multiple micro-cracks carved by the laser beam works efficiently as a scattering wall for the scintillation photons. The X'tal cube with the laser-processed block showed excellent performance with respect to crystal identification and energy resolution. In this work, for characteristics comparison between the laser-processed block and the conventional segmented array block, we made the laser-processed block and two types of segmented array blocks, one with air gaps and the other with glued segmented small crystals. All crystal blocks had 3D grids of 2mm pitch. The 4×4 MPPC arrays were optically coupled to each surface of the crystal block. When performance was evaluated using a uniform irradiation of 511keV, we found that the X'tal cubes with the laser-processed block could easily achieve 2mm3 uniform crystal identification. Also, the average energy resolution of each 3D grid was 11.1±0.7%. On the other hand, the glued segmented array block had a pinched distribution and crystals could not be separated clearly. The segmented array block with air gaps had satisfactory crystal identification performance; however, the laser-processed block had higher crystal identification performance. Also, the energy resolution of the laser-processed block was better than for the segmented array blocks. In summary, we found the laser-processed X'tal cube had excellent crystal identification performance and high energy performance due to its effective dense arrangement of micro-cracks.

The influence of temperature on water treeing in polyethylene

The influence of temperature on water treeing in polyethylene (PE) is investigated in this paper. Low-density polyethylene (LDPE) and peroxide cross-linked polyethylene (XLPE) are chosen as the test materials. The liquid needle electrode method is used for water tree development at 20, 40, 60 and 80 °C, and a metallographic microscope is used to observe water tree morphology. The sizes and initiation rate of water trees are measured. The results obtained from the water treeing experiments indicate that water treeing is significantly influenced by temperature. The initiation rates of water trees decrease first, and then increase as temperature rises. The sizes of water trees also exhibit an increasing trend with ascending temperature. The experiments conducted on LDPE and XLPE demonstrate similar results. However, XLPE is found to have better anti water treeing performance than LDPE. Mechanical tensile test results show that the mechanical properties of the two materials, such as elastic modulus, tensile strength, tensile breaking stress and tensile yield stress, drop significantly as temperature increases. The results of the crystallinity determination and crystal morphology observation show that the crystal blocks of XLPE are smaller and denser compared with those of LDPE, which are lager and relatively sparser. Based on the analysis, XLPE is found to have better anti water treeing performance than LDPE because of cross-linking limitation to the formation of large crystal blocks and the enhancement of the mechanical properties of the material. Moreover, the failure of the mechanical properties of the materials, pore expansion and the accelerated movement of moisture and ions are the reasons for the acceleration of water treeing at high temperatures.

Potential for reducing the numbers of SiPM readout surfaces of laser-processed X’tal cube PET detectors

We are developing a three-dimensional (3D) position-sensitive detector with isotropic spatial resolution, the X’tal cube. Originally, our design consisted of a crystal block for which all six surfaces were covered with arrays of multi-pixel photon counters (MPPCs). In this paper, we examined the feasibility of reducing the number of surfaces on which a MPPC array must be connected with the aim of reducing the complexity of the system. We evaluated two kinds of laser-processed X’tal cubes of 3 mm and 2 mm pitch segments while varying the numbers of the 4 × 4 MPPC arrays down to two surfaces. The sub-surface laser engraving technique was used to fabricate 3D grids into a monolithic crystal block. The 3D flood histograms were obtained by the Anger-type calculation. Two figures of merit, peak-to-valley ratios and distance-to-width ratios, were used to evaluate crystal identification performance. Clear separation was obtained even in the 2-surface configuration for the 3 mm X’tal cube, and the average peak-to-valley ratios and the distance-to-width ratios were 6.7 and 2.6, respectively. Meanwhile, in the 2 mm X’tal cube, the 6-surface configuration could separate all crystals and even the 2-surface case could also, but the flood histograms were relatively shrunk in the 2-surface case, especially on planes parallel to the sensitive surfaces. However, the minimum peak-to-valley ratio did not fall below 3.9. We concluded that reducing the numbers of MPPC readout surfaces was feasible for both the 3 mm and the 2 mm X’tal cubes.

Combining Surface Treatments With Shallow Slots to Improve the Spatial Resolution Performance of Continuous, Thick LYSO Detectors for PET

Positron emission tomography (PET) detectors based on continuous scintillation crystals can achieve very good performance and have a number of practical advantages compared to detectors based on a pixelated array of crystals. Our goal is to develop a thick continuous detector with high energy and spatial resolution, along with high γ-photon capture efficiency. We examine the performance of two crystal blocks: a 46 × 46 × 14 mm3 and a 48 × 48 × 25 mm3 block of LYSO (Lutetium Yttrium Orthosilicate). Using Maximum Likelihood (ML) positioning based upon the light response function (LRF) in the 14 mm thick crystal, we measure a spatial resolution of 3 mm in the central region of the crystal with degradation near the edges due to reflections off the crystal sides. We also show that we can match the spatial resolution achieved using a 14 mm thick crystal by using a 25 mm thick crystal with slots cut into the gamma entrance surface to narrow the LRF. We also find that we can improve the spatial resolution performance near the detector edges by reducing the reflectivity of the crystal sides, albeit with some loss in energy resolution.

Development of a Dual-Sided Readout DOI-PET Module Using Large-Area Monolithic MPPC-Arrays

We are proposing a novel design for a module with depth of interaction (DOI) capability for gamma rays by measuring the pulse-height ratio of double-sided Multi-Pixel Photon Counters (MPPCs) coupled at both ends of a scintillation crystal block. Thanks to newly developed monolithic MPPC arrays consisting of 4 × 4 channels with a three-side buttable package, the module is very thin and compact, thereby enabling less dead space between each module when arranged into a fully designed gantry. To demonstrate our concept of a DOI measuring technique, we first made a 1-D crystal array consisting of five Ce-doped Gd3Al2Ga3O12 (Ce:GAGG) cubic crystals measuring 3×3×3 mm3 in size, separated by a layer of air approximately 10 μm-thick. When the light signals output from both ends are read with the 3×3 mm2 MPPCs, the position of each crystal is clearly distinguished. The same measurements were also made using Ce-doped (Lu,Y)2(SiO4)O (Ce:LYSO), achieving a similarly good separation. We then fabricated thin Ce:GAGG 2-D crystal arrays consisting of two types: [A] 4 × 4 matrix of 3×3×3 mm3 pixels, and [B] 10 t 10 matrix of 0.8×0.8×5 mm3 pixels, with each pixel divided by a BaSO4 reflector 0.2 mm-thick. Then four arrays are laid on top of each other facing the DOI direction through a layer of air 10 μm-thick. We demonstrated that the 3-D position of each Ce:GAGG pixel is clearly distinguished in both the 2-D and DOI directions for type A and B when illuminated by 662 keV gamma rays. Average energy resolutions of 9.8 ± 0.8% and 11.8 ± 1.3% were obtained for types A and B, respectively. These results suggest that our proposed method is simple and offers promise in achieving both excellent spatial and energy resolutions for future medical imaging, particularly in positron emission tomography (PET).

Enhancement effects on excitonic photoluminescence intensity originating from misaligned crystal blocks and polycrystalline grains in a ZnO wafer

We have systematically investigated a relation between excitonic photoluminescence intensity and crystal quality in a (0001)-oriented ZnO wafer. We visualize the crystal quality of a whole wafer using a circular polariscopic measurement and a reflection-type X-ray topograph measurement. The reflection-type X-ray topograph exhibits regions of grain-like patterns that result from internal strains. The circular polariscopic map shows that the internal strains induce local stresses. The θ-2θ X-ray diffraction pattern indicates the presence of misaligned crystal blocks and polycrystalline grains. We have measured photoluminescence spectra and found that the presence of misaligned crystal blocks and polycrystalline grains leads to enhancement of the excitonic photoluminescence intensity. The present phenomenon is attributed to the suppression of exciton diffusion caused by the grain and domain boundaries that connect with the grain-like patterns in the X-ray topograph.

A new polymer electrolyte based on a discotic liquid crystal triblock copolymer

A discotic liquid crystal triblock copolymer consisting of a central main chain triphenylene-based liquid crystal block capped at both ends by blocks of poly(ethylene oxide) (PEO) (MW=2000gmol−1) has been doped with lithium perchlorate in an EO:Li 6:1 ratio. The polymer electrolyte exhibits a phase separated morphology consisting of a columnar hexagonal liquid crystal phase and PEO-rich regions. The polymer electrolyte forms self-supporting, solid-like films. The ionic conductivity on initial heating of the sample is very low below ca. 60°C but increases rapidly above this temperature. This is attributed to the melting of crystalline PEO-rich regions. Crystallisation is suppressed on cooling, and subsequent heating cycles exhibit higher conductivities but still less than those measured for the corresponding lithium perchlorate complex in poly(ethylene glycol) (MW=2000gmol−1). Instead the triblock copolymer mimics the behaviour of high molecular weight poly(ethylene oxide) (MW=300,000gmol−1). This is attributed, in part, to the anchoring of the short PEG chains to the liquid crystal block which prevents their diffusion through the sample. Temperature and pressure variations in ion mobility indicate that the ion transport mechanism in the new material is closely related to that in the conventional PEO-based electrolyte, opening up the possibility of engineering enhanced conductivities in future.

Afterglow by Christopher Ries

Christopher Ries, (1952- ), was raised on a farm in the rural Columbus, Ohio, area. He received his BFA from The Ohio State University in Columbus in 1975 and his MFA from the University of Wisconsin in Madison in 1978, where he served as an assistant to the founder of the American studio glass movement.1Christopher Ries. http://en.wikipedia.org/wiki/Christopher_Ries. Accessed May 9, 2012.Google Scholar, 2Biography Ries website. http://www.christopherries.com/biography.htm. Accessed May 9, 2012.Google Scholar His initial studies delved into glassblowing as an art medium, and he maintained a glassblowing studio for several summers after graduation.1Christopher Ries. http://en.wikipedia.org/wiki/Christopher_Ries. Accessed May 9, 2012.Google Scholar With the aid of the products from a company called Schott Optical Glass, which notes it markets 115 different glass formulas, Ries discovered his defining medium and technique to create a unique, specialized, and new form of glasswork–sculpture.2Biography Ries website. http://www.christopherries.com/biography.htm. Accessed May 9, 2012.Google Scholar, 3Schott Optical website. http://www.us.schott.com/advanced_optics/english/our_products/materials/optical_glass.html. Accessed May 11, 2012.Google Scholar Starting with massive blocks of glass crystal, often weighing as much as 3000 lbs, Ries carves and polishes each piece into highly reflective, refractive, and prismatic forms.4Art Glass Ries website. http://www.christopherries.com/artglass.htm. Accessed April 9, 2012.Google Scholar As with all of Ries' creations, Afterglow contains an internal life of its own. The external boundaries of the glassworks' edges frame a highly dynamic microcosm of its surrounding environment. Each moment a newly introduced object becomes temporarily imprisoned within Afterglow. The structure of the sculpture defined by these lines grants it solidity. Its shape imitates a falling tear or a modern-day award given in honor of excellence. The interior reflections are as volatile as any in a butterfly effect5Glossary definition: Butterfly effect. http://www.pbs.org/faithandreason/physgloss/bfly-body.html. Accessed May 11, 2012.Google Scholar outcome. Each of the 4 images reveal a completely new view of Afterglow in kaleidoscope fashion, depending on the angle, light, or passerby that was present when each view of Afterglow was photographed. If a viewer gets tired of the current image, he needs only to turn his head or squint his eye and a surprising change takes place. Afterglow, sculpted in 2006, is a gift from Elizabeth M. Ross and is located on the 14th floor of the Gonda Building, in Rochester, Minnesota.View Large Image Figure ViewerDownload (PPT)

Evaluation of High Density Pixellated Crystal Blocks With SiPM Readout as Candidates for PET/MR Detectors in a Small Animal PET Insert

Arrays of silicon photo-multipliers (SiPMs) are good candidates for the readout of detectors in PET MR inserts due to their high packing density, efficiency, low bias voltage and insensitivity to magnetic fields. In this study we report the readout performance of SensL SiPM arrays in terms of their ability to resolve all elements of pixellated lutetium oxyorthosilicate (LYSO) crystals their energy resolution, and coincidence response function. Two SensL SPMarray-4 were used as light sensors. Two L YSO crystal blocks consisting of a arrays of 8 × 8 1.2 mm × 1.2 mm × 6.0 mm crystals on the lower layer and 7 × 7 1.2 mm × 1.2 mm × 4.0 mm crystals on the upper layer (which is offset by Yz the crystal width) were mounted on the SensL arrays. A second study was performed with single layer arrays of 8 × 8 1.2 mm × 1.2 mm × 10.0 mm crystals. All of the crystals in the dual layer block were easily identified with a peak to valley ratio of 7.2 while the single layer blocks had a peak to valley ratio of 11.2. The crystal dots were more uniformly spaced in the dual layer arrays suggesting that the inter-layer connection affords some useful light sharing. The average energy resolution for all 113 crystals was 16.3 ± 2.3%. The coincidence response to a 0.25 mm Na-22 source in plastic was 0.97 ± 0.12 mm for normally incident gamma rays with the detectors 120 mm apart. These results indicate that it should be possible to achieve a spatial resolution of about one millimetre near the center of the field of view.

Comparison of single and dual layer detector blocks for pre-clinical MRI–PET

Dual or multi-layer crystal blocks have been proposed to minimise the radial blurring effect in PET scanners with small ring diameters. We measured two potential PET detector blocks' performance in a configuration which would allow 16 blocks in a ring which could be inserted in a small animal 7T MRI scanner. Two crystal sizes, 1.60×1.60mm2 and 1.20×1.20mm2, were investigated. Single layer blocks had 10 or 12mm deep crystals, the dual layer blocks had 4mm deep crystals on the top layer and 6mm deep crystals on the bottom layer. The crystals in the dual layer blocks are offset by ½ of the crystal pitch to allow for purely geometric crystal identification. Both were read out with SensL 4×4 SiPM arrays. The software identifies 64 crystals in the single layer and either 85 or 113 crystals in the dual layer array, (either 49 or 64 in the lower layers and 36 or 49 in the upper layers). All the crystals were clearly visible in the crystal identification images and their resolvability indexes (average FWHM/crystal separation) were shown to range from 0.29 for the best single layer block to 0.33 for the densest dual layer block. The best coincidence response FWHM was 0.95mm for the densest block at the centre of the field. This degraded to 1.83mm at a simulated radial offset of 16mm from the centre, while the single layer crystals blurred this result to 3.4mm. The energy resolution was 16.4±2.2% averaged over the 113 crystals of the densest block.

Intrinsic spatial resolution evaluation of the X’tal cube PET detector based on a 3D crystal block segmented by laser processing

The X'tal cube is a depth-of-interaction (DOI)-PET detector which is aimed at obtaining isotropic resolution by effective readout of scintillation photons from the six sides of a crystal block. The X'tal cube is composed of the 3D crystal block with isotropic resolution and arrays of multi-pixel photon counters (MPPCs). In this study, to fabricate the 3D crystal block efficiently and precisely, we applied a sub-surface laser engraving (SSLE) technique to a monolithic crystal block instead of gluing segmented small crystals. The SSLE technique provided micro-crack walls which carve a groove into a monolithic scintillator block. Using the fabricated X'tal cube, we evaluated its intrinsic spatial resolution to show a proof of concept of isotropic resolution. The 3D grids of 2 mm pitch were fabricated into an 18 × 18 × 18 mm(3) monolithic lutetium yttrium orthosilicate (LYSO) crystal by the SSLE technique. 4 × 4 MPPCs were optically coupled to each surface of the crystal block. The X'tal cube was uniformly irradiated by (22)Na gamma rays, and all of the 3D grids on the 3D position histogram were separated clearly by an Anger-type calculation from the 96-channel MPPC signals. Response functions of the X'tal cube were measured by scanning with a (22)Na point source. The gamma-ray beam with a 1.0 mm slit was scanned in 0.25 mm steps by positioning of the X'tal cube at vertical and 45° incident angles. The average FWHM resolution at both incident angles was 2.1 mm. Therefore, we confirmed the isotropic spatial resolution performance of the X'tal cube.

Time delays in heavy-ion-induced fission of medium-Znuclei, measured by crystal blocking

Time delays in fission induced by bombardment of Mo with 170- and 180-MeV ${}^{32}$S, 225- and 240-MeV ${}^{48}$Ti, and 300-MeV ${}^{58}$Ni have been measured by observation of crystal blocking of fission fragments. In contrast to earlier measurements with a W target, the results are consistent with fission of a compound nucleus in competition with mainly neutron emission. Most of the fissions happen on a time scale much shorter than attoseconds but there is a significant component of fission with much longer lifetimes. The measurements are reproduced with a standard statistical model, including a Kramers correction to fission widths from the viscosity of hot nuclear matter. These new results support the interpretation of our earlier measurements with a W target, which indicate that there is a transition in heavy-ion-induced fission at large atomic number and mass, from multichance fission in the standard Bohr-Wheeler picture to fission without formation of a compound nucleus. The process is slowed down by nuclear viscosity, with measured delays of order attoseconds.

Development of the X'tal Cube: A 3D Position-Sensitive Radiation Detector With All-Surface MPPC Readout

We have developed a new three-dimensional (3D) position sensitive radiation detector, called the X'tal cube. The X'tal cube is composed of a scintillation crystal block and a number of multi-pixel photon counters (MPPCs) which are coupled on all six sides of the block. The block is segmented into cubes and no reflector is used between the segments. Scintillation light originating in a crystal segment accordingly propagates three-dimensionally along the alignment of the crystal segments and is efficiently detected by the MPPCs. The X'tal cube could be used as the detector element of a PET system, for instance. We constructed two prototypes of the X'tal cube and evaluated their performance using gamma-ray sources. The crystal block of each prototype is composed of a 3D array of Lu2xGd2(1-x)SiO5: Ce (LGSO, x = 0.9) crystal segments. Each crystal volume is 3.0 mm × 3.0 mm × 3.0 mm. MPPCs of a 3.0 mm × 3.0 mm active area are coupled to each surface of the crystal block. In this work, we show that all crystal segments are identified by a simple Anger-type calculation performed on the MPPC signals for both prototypes. The X'tal cube provides high spatial resolution in three dimensions regardless of the incident angle of the radiation.

CHIRALITY-ASYMMETRY MACROSCOPIC FORCE INDUCED BY AXION BETWEEN THE α-QUARTZ AND COPPER BLOCK

A macroscopic force induced by the effective potential suggested by Moody and Wilczek between α-quartz crystal and copper block is studied in detail. The matrix elements in the force formula are analyzed according to representations of the non-symmorphic space group P 3121 of α-quartz crystal. The asymmetry distribution of valence electrons and the selection rules do not cancel the force. We conclude that there exists a CP-violating macroscopic force. According to the limit of the production of the axion coupling constants gs gp/ℏc ≤ 10-26 at the Compton wavelength λ = 10-3 m, the macroscopic force between a 0.08 × 0.08 × 0.02 m3 block of α-quartz and a 0.08 × 0.08× 0.01 m3 copper block with a separation being 0.5 × 10-3 m in between, is estimated at less than 3.09 × 10-26 N.

Luminescent Magneto-Optical Photonic Crystals

We compare luminescent properties of several Er-doped garnet films as building blocks in all-garnet heteroepitaxial magneto-optical photonic crystals: La3Ga5O12, Gd3Ga5O12, Y3Fe5O12, Bi3Fe5O12, and Bi2.97Er0.03Fe4Al0.5Ga0.5O12. Er substituents on the dodecahedral lattice sites do not decrease giant Faraday rotation in Bi3Fe5O12 garnet; meanwhile providing intense room temperature C-band photoluminescence (PL). Fe3+ ion works as a sensitizer for Er resulting in fivefold PL enhancement in iron garnets compared to gallium ones. PL lifetime in gallium garnets is in millisecond range reaching 6 ms in Gd2.9Er0.1Ga5O12. The first luminescent one-dimensional heteroepitaxial all-garnet magneto-optical (MO) photonic crystal was composed from diamagnetic Sm3Ga5O12 and MO-active Bi2.97Er0.03Al0.5Ga0.5O12 garnet layers by rf-magnetron sputtering on Gd3Ga5O12(111) substrate. Substitution of ferric ions by aluminum and gallium improved transparency and induced perpendicular anisotropy in pure Bi3Fe5O12. Photonic crystals owned a record high magneto-optical quality and a latching type (magnetic remnant) Faraday rotation (FR). At the resonance wavelength 775 nm, specific FR θF = − 14.1 deg/μm and MO-quality factor Q = 99.3 deg represent the highest MO performance achieved so far. Long-lived near-IR luminescence in Er substituted gallium and iron garnet layers used both as Bragg mirrors and microcavities promises magneto-optical photonic crystals to become an active lasing medium.

Morphology study of a series of azobenzene-containing side-on liquid crystalline triblock copolymers

A series of azobenzene containing side-on liquid crystalline ABA triblock copolymers were investigated. This triblock series possesses the same central liquid crystal block B and various lengths of the amorphous block A. Transmission electron microscopy (TEM), small angle X-rays and neutron scattering (SAXS and SANS) were used to study their morphologies. After annealing the samples over weeks at a temperature within the nematic temperature range of block B, different morphologies (disordered, lamellar, perforated layer and hexagonal cylinder) were observed by TEM. The alignment behavior of these azo triblock copolymers in the magnetic field for artificial muscle application, as well as the phase period and the order-disorder transition (ODT) were studied in situ by SANS.

Structural and Dielectric Properties of Glycine Phosphite Thin Films

Thin polycrystalline films of amino acid ferroelectrics glycine phosphite (GPI) have been prepared by evaporation method on NdGaO3(001) substrate with interdigital structure of electrodes deposited on the surface. Optical images of films obtained by means of polarized microscope in reflection mode have shown that the films consist of small single crystal blocks of several tens of microns in size. X-ray diffraction analysis proves that the non-polar c* axis in blocks coincides with the normal to the surface, and the polar axis b is oriented in the film plane. Temperature dependences of capacitance of GPI/NdGaO3 structures measured using interdigital structure of electrodes exhibit strong anomaly at ferroelectric phase transition temperature of glycine phosphite single crystals. Dielectric hysteresis loops in glycine phosphite films were measured by Sawyer-Tower method. The difference in frequency dependence of coercive field and remnant polarization in structures with GPI and previously studied betaine phosphite films is analyzed and attributed to considerable difference in dimensions of blocks in these structures.

Liquid Crystals in Novel Geometries Prepared by Microfluidics and Electrospinning

We describe two new techniques of preparing liquid crystal samples and discuss their potential for novel research and applications. Very thin polymer composite fibers functionalized by a liquid crystalline core are realized by coaxial electrospinning of a polymer solution surrounding the liquid crystal during the spinning process. The resulting fiber mats exhibit the special properties and responsiveness of the liquid crystal core, e.g. temperature dependent selective reflection when a short-pitch cholesteric is encapsulated. In the second approach an axisymmetric nested capillary microfluidics set-up is used to prepare liquid crystalline shells suspended in an aqueous continuous phase. The spherical geometry of the shell imposes specific defect configurations, the exact result depending on the prevailing liquid crystal phase, the director anchoring conditions at the inner and outer surfaces, and the homogeneity of the shell thickness. With planar director anchoring a variety of defect configurations are possible but for topological reasons the defects must always sum up to a total defect strength of s = +2. Homeotropic anchoring instead gives a defect-free shell, in contrast to a droplet with homeotropic boundary conditions, which must have a defect at its core. By varying the inner and outer fluids as well as the liquid crystal material and temperature, the defect configuration can be tuned in a way that makes the shells interesting e.g. as a versatile colloid crystal building block.

A SiPM-based isotropic-3D PET detector X'tal cube with a three-dimensional array of 1 mm3 crystals

We are developing a novel, general purpose isotropic-3D PET detector X'tal cube which has high spatial resolution in all three dimensions. The research challenge for this detector is implementing effective detection of scintillation photons by covering six faces of a segmented crystal block with silicon photomultipliers (SiPMs). In this paper, we developed the second prototype of the X'tal cube for a proof-of-concept. We aimed at realizing an ultimate detector with 1.0 mm3 cubic crystals, in contrast to our previous development using 3.0 mm3 cubic crystals. The crystal block was composed of a 16 × 16 × 16 array of lutetium gadolinium oxyorthosilicate (LGSO) crystals 0.993 × 0.993 × 0.993 mm3 in size. The crystals were optically glued together without inserting any reflector inside and 96 multi-pixel photon counters (MPPCs, S10931-50P, i.e. six faces each with a 4 × 4 array of MPPCs), each having a sensitive area of 3.0 × 3.0 mm2, were optically coupled to the surfaces of the crystal block. Almost all 4096 crystals were identified through Anger-type calculation due to the finely adjusted reflector sheets inserted between the crystal block and light guides. The reflector sheets, which formed a belt of 0.5 mm width, were placed to cover half of the crystals of the second rows from the edges in order to improve identification performance of the crystals near the edges. Energy resolution of 12.7% was obtained at 511 keV with almost uniform light output for all crystal segments thanks to the effective detection of the scintillation photons.