Daehee Lee

Scintillator-Based Electronic Personal Dosimeter for Mobile Application

A noble electronic personal dosimeter (EPD) can simultaneously measure the energy spectrum and the personal dose rate in radiation-exposed environment. This device is composed of a compact radiation sensor to detect gamma-ray, an integrated circuit of preamplifier and peak holder, and a software to calculate the personal dose from the measured spectrum, finally a smartphone application software to show the calculated personal dose in a mobile phone. The CsI(Tl)-coupled PIN diode is used as a compact spectroscopic radiation sensor to measure the energy spectrum for the radioisotope identification or the activity analysis. To optimally design the size of the compact radiation sensor to be used as an accessary of mobile personal devices, we determined a guideline such that the sensor must satisfy the international criteria of angular response as well as have the maximum value of a figure of merit which is a product of the geometric detection efficiency and the energy resolution. The energy spectrum must be converted to the personal dose or dose rate by a much simpler dose conversion algorithm, called a median bin approximation, without using a typical time-consuming deconvolution process typically used to identify the incident gamma energy. The accuracy of the algorithm depending on the gamma energy and gamma fluence was estimated by the difference rate. The average difference rate in the interested gamma energy ranging from 20 keV to 1.5 MeV was measured to be 17.3% experimentally using radioisotope check sources, and the difference rate becomes negligible over the fluence level of 103γ – ray/0.09 cm2. So, the possibility to devise the electronic personal dosimeter with a single spectroscopy sensor for the dual purpose, dosimetry and spectroscopy, was confirmed in this study.

CALCULATION OF GAMMA SPECTRA IN A PLASTIC SCINTILLATOR FOR ENERGY CALIBRATIONAND DOSE COMPUTATION

Plastic scintillation detectors have practical advantages in the field of dosimetry. Energy calibration of measured gamma spectra is important for dose computation, but it is not simple in the plastic scintillators because of their different characteristics and a finite resolution. In this study, the gamma spectra in a polystyrene scintillator were calculated for the energy calibration and dose computation. Based on the relationship between the energy resolution and estimated energy broadening effect in the calculated spectra, the gamma spectra were simply calculated without many iterations. The calculated spectra were in agreement with the calculation by an existing method and measurements.

A second-order sigma-delta pixel sensor for X-ray applications

X-ray sensor applications in general need a high resolution, a high SNR, a low FPN and a low power consumption. Digital pixels with an ADC integrated in each pixel can satisfy not only aforementioned requirements but also a fast frame rate. Nyquist ADCs, however, have a large area to be implemented and thus need a sophisticated circuit design in order to operate properly. In the other hand, a sigma-delta ADC does not require a complicated circuit design and consumes a small area. Therefore, a second-order sigma-delta ADCs in each X-ray log pixel is chosen due to its low power consumption, high resolution and high frame rate. The designed X-ray array has a dimension of 10×20 with a pixel size of 100μm and the measured power consumption of each sigma-delta pixel is less than 700nW.

Photon crosstalk in pixel array for x-ray imaging

A large-area X-ray CMOS image sensor (LXCIS) is widely used in mammography, non-destructive inspection, and animal CT. For LXCIS, in spite of weakness such as low spatial and energy resolution, a Indirect method using scintillator like CsI(Tl) or Gd2O2S is still well-used because of low cost and easy manufacture. A photo-diode for X-ray imaging has large area about 50 ~ 200 um as compared with vision image sensors. That is because X-ray has feature of straight and very small light emission of a scintillator. Moreover, notwithstanding several structure like columnar, the scintillator still emit a diffusible light. This diffusible light from scintillator can make spatial crosstalk in X-ray photodiode array because of a large incidence angle. Moreover, comparing with vision image sensors, X-ray sensor doesn’t have micro lens for gathering the photons to photo-diode. In this study, we simulated the affection of spatial crosstalk in X-ray sensor by comparing optical sensor. Additionally, the chip, which was fabricated in 0.18 um 1P5M process by Hynix in Korea, was tested to know the effect of spatial crosstalk by changing design parameters. From these works, we found out that spatial crosstalk is affected by pixel pitch, incident angle of photons, and micro lens on each pixels.

A modified a 3-Tr CMOS X-ray image sensor for low-distortion pixel output in source follower

This paper presents a low distortion source follower (LDSF) method in 3-Tr (Transistor) CMOS X-ray sensor. Most of the important parameters in source follower is a gate to source voltage (V_gs) difference. This additional circuit reduces fill factor of the photodiode from 86% to 78% in the 50 μm × 50 μm size. This circuit requires only a 1 P-mos and 1 N-mos. Simulation test represented that the LDSF method makes V_gs more constant. It means that V_gs is not easily affected by V_ds.

Study on the fast signal transfer for large-area X-ray image sensors

A large area X-ray CMOS image sensor (LXCIS) is a well-known imaging device for high speed and resolution. In design and fabrication process, we found several problems in making LXCIS, especially in signal transferring. A 3-transistor active pixel sensor (3T APS) in LXCIS has a long signal line about 16.896 cm as a worst case. This long signal line consists of metal and it has resistance and capacitance about 21.12 kΩ and 71.87 pF each. We have optimized 3T APSs transistors, applied boosting circuit, and designed a low parasitic resistance and capacitance. From our simulation result, we obtained a high speed operation, which ranges from 13.5 frame per second (FPS) to 18.6 FPS in 1536 × 3072 pixel arrays, and a high dynamic range by increasing maximum voltage of pixel output signal.

Performance comparison of CMOS-based photodiodes for high-resolution and high-sensitivity digital mammography

In order to develop a high-resolution and high-sensitivity digital mamographic detector, to use a commercially-available and well-developed CMOS image sensor (CIS) process can be a cost-effective way. However, in any commercial CIS process, several different types of n- or p-layers can be used so that various pn-junction structures could be formed depending on the choice of n- and p-layer combination. We performed a comparative analysis on the characteristics of three types of photodiodes formed on a high-resistivity p-type epitaxial wafer by applying three available n-layer processes in order to develop the high-sensitivity photodiode for a scintillator-based X-ray imaging detector. As a preliminar study, a small test-version CIS chip with an 80 × 80 pixel array of a 3-transistor active pixel sensor structure, 50 μm pitch and 80{%} fill factor was fabricated. The pixel area is subdivided into four 40 × 40 sub-arrays and 3 different types of photodides are designed for each sub-array by using n+, n− and n-well layers. All other components are designed to be identical for impartial comparison of the photodiodes only. Among 3 types, the n−/p-epi photodiode exhibited high charge-to-voltage gain (0.86 μV/e−), high quantum efficiency (49% at 532 nm wavelength) and low dark current (294 pA/cm2). The test CIS chip was coupled to a phosphor screen, Lanex Fine or Lanex Regular, both composed of Gd2O2S:Tb, and was tested using X-rays in a mammography setting. Among 6 cases, n−/p-epi photodiode coupled with the Lanex Regular also showed the highest sensitivity of 30.5 mV/mR.