Ⅱ-9. Instrumentation

RIKEN Accel. Prog. Rep. 47 (2014)

Development of SOI pixel sensor for environmental radiation
monitor†Y. Sekiguchi,∗1 Y. Arai ,∗2 H. Hamagaki,∗1 and T. Gunji ∗1
We have developed a prototype SOI pixel sensor,
called RADPIX, for a radiation monitor. This is basedon the silion-on-insulator (SOI) pixel CMOS technology and developed by Y. Arai et al.1) The RADPIX
is a monolithic pixel detector that consists of a thin
CMOS readout array (40 nm, 10 Ω), a buried oxidelayer (200 nm), and a thick high-registivity Si-sensor(260 µm, 700 Ω) vertically on a single chip. Figure 1
shows the schematic view of the Nested-Well structure
SOI detector (p-in-n type sensor). The buried N-well
suppresses the back gate eﬀect from the electric ﬁeld inthe sensor and reduces the cross-talk by isolating thesensor from the circuit.

Fig. 1. Schematic diagram of the nested-well SOI structure

RADPIX aims to count the rate of the radiation andto visualize the hit pattern in real time for environmental radiation. From the hit pattern, one can estimate
the radiative source such as beta-ray, γ-ray, or α-ray.
The sensor is a DC-coupled device and the sensor’s
capacitance is in a few tens of femtofarads. The typical leakage current for a 40 µm × 40µm pixel is of
the order of picoamperes at room temperature, whichsaturates the baseline within about 10 ms. Therefore,
this device needs to reduce the leakage current or compensate the current to be able to measure the environmental radiation, which is of the order of 10−2 Hz/cm2
in 1 µSv/h from 137 Cs. The Krummenacher feedback
scheme2) is used for long exposure, which has an individual leakage current compensation circuit. Figure
2 shows the schematic of RADPIX. We implemented
charge-sensitive preamplifers with diﬀerent gains (1fFand 5fF as feedback capacitors). Two inverter chopper comparators are implemented to generate a trigger
signal, and the latched output can be read from eachpixel. The analog signal is sampled by the 230 fF store
capacitance and is readout from each pixel. This analog information can be used for the pattern recognition∗1∗2Graduate School of Science, University of Tokyo
Institute of Particle and Nuclear studies, High Energy Accelerator Research Organization, KEK- 231 -and identiﬁcation of the radiative source.

Fig. 2. Schematics of RADPIX

Figure 3 shows the pixel-by-pixel gain variation for
two diﬀerent charge-sensitive preampliﬁers. This wasstudied by using test pulses. The average of thegain is 23 µV/electron (RMS∼1.4) and 94 µV/electron(RMS∼5.1) for the low gain type and high gain type,respectively. Position dependence is not found. We
also evaluated the performance of the comparator. Figure 4 shows the counts of comparator output as a function of comparator threshold. We observe a 5%-10%
fake hit rate even at high threshold. This is due to thelarge noise, which shows up when the digital signal isreadout. The reason of the large noise and the strategies for further improvement are under investigation.

Fig. 3. The pixel-by-pixel gain variation: low gain (left)
and high gain (right)

Fig. 4. Counts of comparator output as a function of comparator threshold for four typical pixels : low gain (left)
and high gain (right)

References1) Y. Arai, et al.; Nucl. Instr. and Meth. A636, 31 (2011).
2) F. Krummenacher; Nucl. Instrum. Methods A 305, 527532 (1991).