Kenji Shimazoe

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

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.

Dynamic Time Over Threshold Method

The time over threshold (TOT) method has several advantages over direct pulse height analysis based on analog to digital converters (ADCs). A key advantage is the simplicity of the conversion circuit which leads to a high level of integration and a low power consumption. The TOT technique is well suited to build multi-channel readout systems for pixelated detectors as described in our previous work that also exploits the Pulse Width Modulation (PWM) method. The main limitation of the TOT technique is that the relation between the input charge to be measured and the width of the encoded pulse is strongly non-linear. Dynamic range limitation is also an issue. To address these aspects, we propose a new time over threshold conversion circuit where the threshold of the comparator is dynamically changed instead of being constant. We call this scheme the “dynamic TOT method”. We show that it improves linearity and dynamic range. It also shortens the duration of measured pulses leading to higher counting rates. We present a short analysis that explains how the ideal linear input charge to TOT transfer function can theoretically be obtained. We describe the results obtained with a test circuit built from discrete components and present several of the spectrums obtained with crystal detectors and a radioactive source. The proposed method can be used for applications like Positron Emission Tomography (PET) that require moderate energy resolution.

A PET detector prototype based on digital SiPMs and GAGG scintillators

Silicon Photomultipliers (SiPM) are interesting light sensors for Positron Emission Tomography (PET). The detector signal of analog SiPMs is the total charge of all fired cells. Energy and time information have to be determined with dedicated readout electronics. Philips Digital Photon Counting has developed a SiPM with added electronics on cell level delivering a digital value of the time stamp and number of fired cells. These so called Digital Photon Counters (DPC) are fully digital devices. In this study, the feasibility of using DPCs in combination with LYSO (Lutetium Yttrium Oxyorthosilicate) and GAGG (Gadolinium Aluminum Gallium Garnet) scintillators for PET is tested. Each DPC module has 64 channels with 3.2 × 3.8775 mm(2), comprising 3200 cells each. GAGG is a recently developed scintillator (Zeff = 54, 6.63 g cm(-3), 520 nm peak emission, 46 000 photons MeV(-1), 88 ns (92%) and 230 ns (8%) decay times, non-hygroscopic, chemically and mechanically stable). Individual crystals of 2 × 2 × 6 mm(3) were coupled onto each DPC pixel. LYSO coupled to the DPC results in a coincidence time resolution (CTR) of 171 ps FWHM and an energy resolution of 12.6% FWHM at 511 keV. Using GAGG, coincidence timing is 310 ps FWHM and energy resolution is 8.5% FWHM. A PET detector prototype with 2 DPCs equipped with a GAGG array matching the pixel size (3.2 × 3.8775 × 8 mm(3)) was assembled. To emulate a ring of 10 modules, objects are rotated in the field of view. CTR of the PET is 619 ps and energy resolution is 9.2% FWHM. The iterative MLEM reconstruction is based on system matrices calculated with an analytical detector response function model. A phantom with rods of different diameters filled with (18)F was used for tomographic tests.

Time over Threshold based digital animal PET (TODPET)

We have developed Time over Threshold (ToT) based Pr:LuAG-APD PET (TODPET) tomograph with a mixed signal front-end. The tomograph consists of 8 block detectors, each of which is composed of a 12 × 12 array of 2 × 2 x 10mm3 Pr:LuAG crystals individually coupled with 12 × 12 UV-enhanced APD arrays. The APDs are individually read out with a custom-designed Time over Threshold ASIC and FPGA readout system. Developed PET tomograph has the energy resolution of 10% and the time resolution of 4.2ns. The 1.76mm spatial resolution (FWHM) is achieved for the first result.

Novel front-end pulse processing scheme for PET system based on pulse width modulation and pulse train method

For the high-resolution PET system, architecture of multichannel front-end system is very important. We propose a novel front-end pulse processing scheme with pulse width modulation (PWM) for a PET system. This front-end can realize smart, low power dissipation, and multi-channel signal processing for the radiation detector system including PET system.

Novel Front-End Pulse Processing Scheme for PET System Based on Pulse Width Modulation and Pulse Train Method

The architecture of a multi-channel front-end system is important for realizing a high-resolution PET system. We propose a novel front-end pulse processing scheme with pulse width modulation (PWM) and pulse train method for PET systems. Each channel of the proposed system consists of a preamplifier, a shaping amplifier, a comparator, and a digital circuit that generates a pulse train for each event. The preamplifier-shaper-discriminator module first generates a trigger pulse with time-over-threshold (ToT), which contains the energy information. The trigger pulse is then processed through a digital circuit that adds subsequent pulses to form a pulse train. These additional pulses encode channel information, timing information, etc. The digital signal output of each channel can be connected by simple wired-OR logic, and the output is read in one transmission line. This multi-channel, low power consumption front-end scheme can acquire enough pulse height (energy) and position information to realize a PET system with a significantly smaller number of output pins in the front-end ASIC. The pulse width encoding also simplifies the digital processing system. We designed a new ASIC based on this concept. The proposed architecture can be applied to high-resolution PET systems with multi-channel ASICs.

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

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.

Multi-Level Time-Over-Threshold Method for Energy Resolving Multi-Channel Systems

Pixellated radiation detectors require an energy resolving multichannel system. The time-over-threshold (ToT) method provides an inexpensive way to energy-resolving signal processing. However, the conventional, single-threshold ToT method suffers from a small dynamic range and poor linearity. We have studied linearity in a ToT system for typical shaping filters and propose a new, multi-level, ToT scheme. This method provides higher linearity and wider dynamic range. We have analyzed the typical signal cases of triangular, CR-RC, and Semi-Gaussian filters, and achieved a linearity of less ≤3% in Integral Non-Linearity (INL).

Application of Time-over-Threshold readout method to Micro Strip Gas Chamber

Linearity problem has restrained the application of Time-over-Threshold (ToT) on Micro Strip Gas Chamber (MSGC). Here, we proposed an improved ToT method, Dynamic ToT, which could achieve better linearity in a large dynamic range. Both normal ToT and Dynamic ToT module has been assembled with discrete circuits and tested with Multi-Grid MSGC (M-MSGC). In our experiments, Dynamic ToT method has shown a significant linearity improvement, whose coefficient of determination (R-Squared) is ~ 0.94 over 10fC to 100 fC input charge. With a FPGA based DAQ system, the energy spectrum has been reconstructed. Utilizing Dynamic ToT readout, both 5.9 keV peak and argon escape peak could be clearly captured.

Field test around Fukushima Daiichi nuclear power plant site using improved Ce:Gd3(Al,Ga)5O12 scintillator Compton camera mounted on an unmanned helicopter

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°).