Takanori Endo

First Performance Results of Ce:GAGG Scintillation Crystals With Silicon Photomultipliers

A new single-crystal Cerium doped Gd₃Al₂Ga₃ O₁₂₃Al₂Ga₃ O₁₂ (GAGG) scintillation crystal with high luminosity, high density and relatively fast decay time has successfully been grown. We report on the first performance results of the new GAGG scintillation crystal read out with silicon photomultipliers (SiPM) from Hamamatsu (MPPC) and FBK. The best energy resolution (511 keV peak of Ge-68) of 7.9% was attained with GAGG coupled to MPPC and 9.0% with the FBK SiPM after correcting for non-linearity. On the other hand, the best coincidence resolving time (FWHM) of polished 3 × 3 × 5 mm³ and 3 × 3 × 20mm³crystals were 464 ±12 ps and 577 ±22 ps for GAGG crystals compared to 179 ±8 ps and 214 ±6 ps for LYSO crystals respectively with MPPCs. The rise time of GAGG was measured to be 200 ps (75%) and 6 ns (25%) while the decay time was 140 ns (92%), 500 ns (7.7%) 6000 ns (0.3%).

Crystal Growth and Scintillation Properties of Ce Doped

Ce1%, 2% and 3% doped Gd3(Ga,Al)5O12 (GAGG) single crystals were grown by the Cz method. Luminescence and scintillation properties were measured. Light yield change along the growth direction and effects of Ce concentration on scintillation properties in Ce:GAGG were studied. Ce3+ 5d-4f emission within 520-530 nm was observed in the Ce:GAGG crystals. The Ce1%:GAGG sample with 3×3×1 mm size showed the highest light yield of 46000 photon/MeV. The energy resolution was 7.8%@662 keV. With increasing solidification fraction, the LY were decreased. It is proposed that the increase of Ga concentration along the growth direction is the main cause of the decrease of LY. The scintillation decay times were accelerated with increasing Ce concentration in the Ce:GAGG crystals. The scintillation decay times were 92.0 ns, 79.1 ns and 68.3 ns in the Ce1, 2 and 3% GAGG, respectively.

{\rm Gd}_{3}({\rm Ga},{\rm Al})_{5}{\rm O}_{12}

Due to the accident of Fukushima Daiichi Nuclear Power Plant, some areas were contaminated by released radioisotopes (mainly 137Cs and 134Cs). Effective decontamination is demanded to encourage evacuated people to return. This paper proposes a new survey system using an unmanned helicopter equipped with a Compton camera for localizing radionuclides. As a prototype, 32 Ce:Gd3(Al,Ga)5O12 (GAGG) crystals were coupled to 16 silicon photomultipliers and 16 avalanched photodiodes as the scatterer and absorber, respectively. A new Dynamic Time-over-Threshold (dToT) method was applied to convert CR-RC shaping signals to digital signals for multi-channel spectra and coincidence acquisition. The system was designed to work in two modes: one is Compton-camera mode (CCM) which obtains the radiation distribution maps through Compton imaging using hovering flights, while the other one is Gamma-camera mode (GCM) which maps the radiation distribution via measured coincidence events using programmed flights. For point source in CCM, an intrinsic efficiency of 1.68% with a combined standard uncertainty of 0.04% and an angular resolution of about 14° (FWHM, full width at half maximum) was achieved. In GCM, a spatial resolution of about 11 cm (FWHM) was obtained when detecting area is 11.2 cm away from the detector, while it was about 28 cm (FWHM) in single detector mode (SDM). Promising results were obtained in field in Fukushima.

Single Crystals

Ce doped Gd3Al2Ga3O12 (Ce:GAGG) is a newly developed single-crystal scintillator which has a large light output and longer emission light wavelength. The longer wavelength of the scintillation photons will produce a larger signal when coupled to typical silicon photomultiplier (Si-PM) as the quantum efficiency of semiconductor based photodetector is generally higher for light with longer wavelength. A block detector with higher spatial resolution may thus be realized by combining Ce:GAGG with Si-PM arrays. To achieve the highest possible spatial resolution for PET and SPECT detectors, we developed an ultrahigh resolution block detector using 0.4 mm × 0.4 mm × 5 mm Ce:GAGG pixels assembled to form a 24 × 24 matrix that is coupled to an Si-PM array and evaluated the performance. All Ce:GAGG pixels were separated in the 2-dimensional position histograms for Cs-137 (662 keV) gamma photons with an average peak-to-valley (P/V) ratio of 2.4. The energy resolution was 21.6% FWHM for Cs-137 (662 keV) and 23.8% for Co-57 (122 keV) gamma photons. Since Ce:GAGG does not contain naturally occurring radioisotope (Lu), beta-gamma true coincidences can be avoided and randoms are reduced when used for PET detectors. Furthermore, this property, together with its high light output and good intrinsic energy resolution, make the scintillator suited for SPECT detectors. An ultrahigh resolution PET/SPECT hybrid system might be an interesting application using Ce:GAGG/Si-PM block detectors.

A prototype of aerial radiation monitoring system using an unmanned helicopter mounting a GAGG scintillator Compton camera

A novel digital PET scanner based on Time over Threshold method is developed. The positron emission tomography (PET) is composed of 144channel Ce:Gd3Al2Ga3O12 (GAGG)-Avaranche photodiode (APD) detector arrays individually coupled with custom designed Time over Threshold (ToT) application-specific integrated circuit (ASIC) to realize the high count rate and good spatial resolution. Such an imaging system provides a simple front-end circuit and flexible digital signal processing like multiplexing such as a pulse train method. The measured energy resolution of the detector system was 6.7% for the 511 keV peak, and 4.25 ns time resolution was measured with a single detector module. The measured spatial resolution for a point source was 1.37 mm FWHM for our initial data with a columnar 22Na source.

Development of an Ultrahigh Resolution Block Detector Based on 0.4 mm Pixel Ce:GAGG Scintillators and a Silicon Photomultiplier Array

Pr:(Lu,Y)₃(Ga,Al)₅O₁₂ single crystals were grown by the micro-pulling down ( μ-PD) method. Luminescence and scintillation properties were measured. The substitution phenomenon in the Lu³⁺ sites with Y³⁺ and Al³⁺ sites with Ga³⁺ in garnet structure has been studied. Pr³⁺ 5d-4f emission within 300-400 nm accompanied by weak Pr³⁺ 4f-4f emission in 480-650 nm were observed in Ga 0-60 at.% substituted samples. Only Pr³⁺ 4f-4f emission was observed in Ga 80 at.% substituted sample. The light yield of Pr1%:Lu₂Y₁Ga₂Al₂O₁₂ sample was almost the same as that of Cz grown Pr:LuAG standard. Two-component scintillation decay of 17.9 ns (93%) and 68.0 ns (7%) were obtained using the PMT and digital oscilloscope TDS5032B. Slower decay components were reduced by Ga and Y substitution in LuAG structure.

Development of a Prototype Detector Using APD-Arrays Coupled With Pixelized Ce:GAGG Scintillator for High Resolution Radiation Imaging

ABSTRACT An improved light-weight Compton camera exhibiting low power consumption was developed to be mountable on an unmanned helicopter to detect cesium radiation hot spots and confirm the decontamination effect of cesium-affected areas. An increase in the Ce:Gd3(Al,Ga)5O12 scintillator array from 4 × 4 to 8 × 8 and expansion of the interlayer distance enhanced the detection efficiency and angular resolution, respectively. Measurements were performed over the Ukedo riverbed in Namie, Fukushima Prefecture (Japan). The helicopters flight path and speed were pre-programmed to lines interspaced by 5 and 10 m intervals and 1 m/s, respectively, facilitating measurements over areas of 65 × 60 m2 and 65 × 180 m2 at a height of 10 m for approximately 20 and 30 min, respectively. Results provided accurate ambient dose equivalent rate maps at a height of 1 m with an angular resolution corresponding to a position resolution of approximately 10 m from a height of 10 m. Hovering flights were executed over hot-spot areas for 10–20 min at a height of 5–20 m. Gamma-ray images of these hot spots were obtained using a reconstruction software. Comparison between position-shifted measurement results showed that the angular resolution coincided with that evaluated in the laboratory (approximately 10°).

Improvement of Scintillation Properties in Pr Doped

In this report 2inch size Ce:Gd<inf>3</inf>Al<inf>2</inf>Ga<inf>3</inf>O<inf>12</inf> (Ce:GAGG) single crystals were grown by the Czhocralski (Cz) method. Luminescence and scintillation properties were measured. In order to determine light yield the energy spectra were collected under 662 keV γ-ray excitation(¹³⁷Cs source) were detected by a APD S8664-55(Hamamatsu). The light yield of CeGAGG sample was calibrated from ⁵⁵Fe direct irradiation peak to APD. The light yield was around 46,000photon/MeV. Energy resolution was 4.9%@662keV by using 3×3×1mm size sample. The density of CeGAGG is theoretically 6.63 g/cm³. Further results will be reported in the presentation.

{\rm Lu}_{3}{\hbox {Al}}_{5}{\rm O}_{12}

We developed Time-over-Threshold based digital PET (TODPET2) tomograph using silicon photomultipliers (SiPM) arrays coupled with pixelized Ce:Gd3(Ga, Al)5O12 (Ce:GAGG) scintillators dedicated for non-invasive measurement of blood RI concentrations. The detector consists of 1.57 × 1.57 mm2 SiPM chips and 1.6 × 1.6 × 15 mm3 Ce:GAGG scintillators arranged on a 12 × 12 channel, both working as individual readout systems. After the development of the detector, we fabricated the PET gantry composed of 8 pieces of SiPM/Ce:GAGG detector array which signals were sent to the current-comparing type time-over-threshold (TOT) ASIC for individual readout of pixels. The PET scanner which we developed has 25 mm axial field-of-view (FOV) and 60 mm transaxial FOV. The spatial resolution reconstructed with maximum likelihood estimation method (MLEM) is 0.98 mm (FWHM) at the center of FOV. The sensitivity of the system is measured to be 1.31% using 22Na point source. Finally, timing response to changes in RI concentration was also measured using 5 mm diameter syringe injected with several concentrations of 18FDG.

Scintillator by Ga and Y Substitutions

Read out test of inorganic-organic hybrid scintillator; Pr:LuAG single crystal covered with plastic scintillator are demonstrated. The Pr:LuAG sample coated with bis-MSB+TB-PVD containing WLS showed the better light output and energy resolution than the Pr:LuAG itself. Light output was increased up to 55% and energy resolution was also improved to 6.1%@662keV using APD (Hamamats S8664–8220). In the case of MPPC (Hamamatsu S10362_33_050 3600pixel-type), light output was increased up to 35% and energy resolution was also improved to 8.8%@662keV. Decay curve of the Pr:LuAG sample coupled with the WLS was also measured and successfully modeled.