Tadashi Orita

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.

Development of a Compton Camera based on digital SiPMs and GAGG crystals

In this work the feasibility of digital silicon Photomultipliers (dSiPM) coupled to GAGG (Gadolinium Aluminium Gallium Garnet) scintillator was tested as a Compton camera system for environmental radiation survey applications. The dSiPM is a newly developed photon counting device by Philips which digitizes the photon counts already on the cell level. One sensor tile consists of 4 × 4 dies with 2 × 2 pixels each. Every pixel contains 3200 cells. Two arrays of 8 × 8 GAGG crystals (crystal size is 3.2 × 3.87 × 8 mm3) are individually coupled to the pixel matrix of two dSiPMs tiles. The scatter detector and the absorbing detector are stacked at 5 cm distance to build the Compton camera system. Data was acquired using a coincidence time window of 3 ns. For the two detectors in coincidence, the measured energy resolution is 9.7 % FWHM@511 keV and coincidence time resolution is ~ 740 ps FWHM. The first image of a 22Na point source located at 25 cm from the Compton Camera was successfully reconstructed using filtered back projection (FBP).

Dynamic time-over-threshold method for multi-channel APD based gamma-ray detectors

Recently pixelated detectors with many channels are being applied to gamma-ray imaging application owing to advanced manufacturing technology. Pulse height information is important for gamma-ray measurement since we can avoid unnecessary events like scattered gamma-rays. However, as the number of channels increases, we need more electronics for processing each channels signal. As a result, its circuit size and power consumption cause problems. The Time-over-Threshold (ToT) [1] method is an effective signal processing method for those problems. However, ToT suffers from a poor linearity and its dynamic range is limited. We have proposed a new ToT method named dynamic Time-over-Threshold method (dToT) [2]. The dToT method has a dynamically changing threshold. Its threshold voltage starts sweeping after the detection of input signal. Both the sweeping pattern and the input pulse waveform affect the linearity of the system. Then we have proposed a new signal processing system with dToT and CR-RC shaping and showed a much better linearity than that of a conventional ToT. We have implemented a test circuit with GAGG scintillator and avalanche photodiode and measured pulse height spectra for a 137CS and a 22Na source, which showed good linearity. Encouraged by the test, we have designed a new ASIC for the multi-channel dToT system and measured spectra of 22Na.

Ultra linear dynamic Time-over-Threshold with a simple CR-RC filtering

The Time over Threshold (ToT) method [1] is an effective signal processing method for increasing integrity and decreasing power consumption. However ToT suffers from a poor linearity and its dynamic range is limited. We have proposed a new dynamic Time-over-Threshold (dToT) [2] which converts the pulse height into the time width with a dynamically changing threshold voltage. Its linearity is better than the conventional ToT with a constant threshold voltage, around 3%. This paper describes a new method to achieve further linearity improvement. We propose a combination of a CR-RC filter circuit and a dToT with an RC integrating circuit. In the case that the time constants of the CR-RC filter circuit, the dToT circuit, and the delay time are all the same, the linearity of such a system is much better than ToT (INL:0.1%, theoretically) and the system can still be very simple and the wider dynamic range is achieved. We also analyze its characteristics by changing time-constant parameters and have made a prototype circuit.

Application of dynamic time-over-threshold method to pixellated CdTe detector

We have designed, fabricated and tested a 64channel ohmic-type CdTe detector for the gamma camera application with aconventional PHA (Pulse Height Analysis) circuit dynamic ToT based circuit. The pixel size of the detector is 2mm × 2mm and it consists of 8×8 arrays. The thickness of detectors is 1mm. Our group has been proposing the energy resolving Time over Threshold method with a dynamically changing threshold (dynamic ToT). Although the ToT is suitable for the readout circuits for multi-pixel detectors because of the binary readout and circuit simplicity, the linearity of the ToT versus the input charge is rather poor. The dynamic ToT method greatly improves the linearity and dynamic range. We have tested 64channel CdTe detectors with a PHA method and dynamic ToT method and confirmed that dynamic ToT method is usable as a energy resolving system, and more suitable as a multi-channel individual readout system.