Heedong Kang

Scintillation Characteristics of the SrCl

The aim of this study is to develop scintillation crystals for the study of neutrinoless beta+/EC decay process. The 84Sr isotope is one of the potential candidates for the neutrinoless beta+/EC decay process. None of the scintillation crystals containing a strontium element up till now were used for the study of neutrinoless beta+/EC decay process. In general our crystal growth program include studies of starting material preparation, growth procedures and characterization of grown crystals. We grew a SrCl2 single crystal by using the Czochralski method. The cylindrical shape of the SrCl2 crystal was cut into the dimensions of Oslash2 times 1.5 cm3. The scintillation properties were studied by using various gamma ray sources and an alpha source. The scintillation properties such as energy resolution, light output, linearity and decay time were presented. In particular, the alpha/beta light ratio and possibility of a pulse shape discrimination between alpha and gamma quanta using a 210Po alpha source were studied.

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Avalanche photodiode (APD) is a kind of photodiode that internally amplifies the photocurrent by an avalanche effect. The quantum efficiency of the APD is about 80% which is five times higher than that of typical photomultiplier tube (PMT). A 16 mm diameter of beveled-edge large area avalanche photodiode (LAAPD) made by Advanced Photonics was used for the CsI(Tl) scintillator characterization. A CsI(Tl) crystal was attached to LAAPD and energy spectra were measured with various γ-ray sources. Measurements of energy spectra of the CsI(Tl) crystal to optimize the energy resolutions were performed with different shaping time constants by shaping amplifier and with different gains by high voltage variation. The absolute light yield and energy resolution of CsI(Tl) crystals were measured with the LAAPD for 662 keV γ-rays from a 137Cs source. The LAAPD was calibrated with X-ray of 55Fe source for the calibration of the number of e-h pairs per channel. The absolute light yield of newly developed SrCl2 scintillation crystal is determined by this method. This study shows that it is possible to use the LAAPD for the characterization of newly developed scintillation crystal and for the photo-sensor of scintillation detector.

Single Crystal for the Neutrinoless

A silicon sensor is very attractive material for medical imaging due to its intrinsic high resolution and fast readout. Silicon sensors of various types were fabricated on a 380 μm-thick n-type, float zone, 5 in. wafer which has high resistivity, <100> orientation, and is double-sided polished. We manufactured not only DC- and AC-coupled strip sensors but also pixel sensors and RC chips. The RC chips were fabricated for the purpose of using signal readout of the DC-coupled strip sensors. Electrical characteristics of the fabricated silicon sensors, leakage current and capacitance as a function of reverse bias voltage, were measured. Coupling capacitors and biasing resistors in the AC-coupled strip sensors were made by separating implantation from metallization strips with a thin oxide layer and made of polysilicons, respectively. The capacitance and resistance of each channel in the AC-coupled sensor were measured and compared with target values. The signal-to-noise ratio of the fabricated large pixel sensor was also measured by using a 90Sr radioactive source.

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A portable data acquisition (DAQ) system with embedded microcontroller was developed for radiation detection and environment monitoring purpose. The developed DAQ system consists of an ATMEGA128 of high performance, low-power 8-bit AVR microcontroller made by ATMEL and a AD7746 24-bit capacitance to digital converter. A software based Universal Serial Bus (USB) and a RS-232 serial port is used for the communication between the developed system with a main computer. The developed DAQ system has 8-channels analog to digital converter (ADC) with 10-bit resolution. The ADC has singled ended mode, differential mode using inside amplifier and it has maximum signals sampling rate 50-Kbit per second. It has discriminator capability with an analog comparator and signal counter capability with a 16-bit timer/counter. It can also measure precious capacitance, temperature and voltage with the functionality of the AD7746. The developed embedded DAQ system was tested with scintillation crystal irradiated with in radioactive sources. This embedded DAQ system is simple but portable and useful for radiation and environment monitoring.

/EC Decay Search

New scintillation crystal of a Ba0.20Sr0.80Cl2 grown by the Czochralski technique is reported. The grown crystal was cut into a size of 10×10×10 mm3. The lattice parameter of the grown crystal is a = b = c = 7.041 Å and the crystal structure is cubic. The luminescence and scintillation properties of the Ba0.20Sr0.80Cl2 crystal were studied. The luminescent characteristics were measured with ultraviolet (UV) light and X-ray source to study the emission spectra of the crystal. Scintillation properties of the crystal such as pulse height spectra, energy resolution, α/β ratio, light output, proportionality curve, and fluorescence decay time were measured with the various radioactive γ-ray sources and α-source at room temperature. The light output of the crystal was compared with that of the CsI(Tl) crystal and the absolute light output was measured with avalanche photodiode (APD). While the light output of the crystal was less than that of the CsI(Tl), the energy resolution was approximately 6.9% for 137Cs 662 keV γ-ray with PMT which is similar to that of the CsI(Tl) crystal for 137Cs γ-rays.