Yong Kyun Kim

Comparative Study of a CsI and a ZnSe(Te/O) Scintillation Detector’s Properties for a Gamma-ray Measurement

A ZnSe crystal based on a II -VI compound semiconductor has various physical properties: electroluminescent, photoelectric, luminescent and scintillation. Activated ZnSe crystals are highly-efficient scintillators, and they are already being applied to the detecting units of a X-ray introscopy and a dosimetric system. ZnSe-based scintillators have a high absolute light output, and their radiation spectra matches well with the Si-photodiode spectral sensitivity. The present study was performed by using a polished ZnSe and CsI(Tl). ZnSe is a low-density crystal(5.42 g/cm3). The emission wavelength of ZnSe(Te) is 610 nm and ZnSe(O) is 592 nm. We have fabricated 10×10×1 mm and 10×10×2 mm ZnSe crystals in which the activators were doped with tellurium and oxygen. ZnSe and CsI crystals were mounted on a S3590-08 HAMAMATSU PIN photodiode and an R3479 HAMAMATSU PMT. Teflon tape as a reflector for the PIN photodiode and PMT. Gamma-ray spectrum measurements were performed by using 241Am, 57Co, 133Ba and 137Cs radio isotopes. We have compared the measured spectra of ZnSe and CsI under the same conditions.

Properties of ZnSe:Te,O Crystals Grown by Bridgman-Stockbarger Method

Zinc selenide crystals were grown in graphite crucibles by the Bridgman–Stockbarger method in a vertical compression furnace under argon pressure of 5×106 Pa. From the absorption spectra, the band gap energies of the ZnSe single crystals were calculated by a linear fitting process. The maximum wavelength of the emission spectrum of the ZnSe:Te,O scintillator was 630 nm, which was well matched with the response wavelength of the Si photodiode. The energy resolution of the ZnSe:Te,O scintillator was 11.9% when it was exposed to 137Cs γ-ray. Its size was 10×10×1 mm3. The afterglow level of the ZnSe:Te,O scintillator after 5 ms was 0.023%. The relative light output of the ZnSe:Te,O scintillator was 2.167 times higher than CsI:Tl. The luminescence decay time of the ZnSe:Te,O scintillator has two exponential components with 27 and 84 γs time constant.

Influence of the Annealing Process for the Metal Contacts of the SiC Semiconductor Radiation Detector

We have studied the radiation response of a prototype SiC radiation detector by using a 6H-SiC wafer. Metal contacts on the surface of the SiC samples were fabricated by using a thermal evaporator in a vacuum condition. Among the SiC samples, several samples were heated by a Rapid Temperature Annealing(RTA) device for 10 minutes at 300°C. The metal contacts on the annealed and non-annealed samples were scanned by using AFM(Atomic Force Microscope) before and after an annealing process. The current-voltage characteristics of the SiC detectors were measured by parameter analyzer and the radiation response was evaluated by 238Pu with 5.5 MeV α-ray at room temperature and a atmospheric pressure. After annealing process, the surface roughness and the current-voltage characteristics decreased. The Schottky barrier heights of non-annealed and annealed SiC samples were determined as 0.638 eV and 0.688 eV, respectively. Also radiation response spectra of the annealed and non-annealed detectors were similar.We have studied the radiation response of a prototype SiC radiation detector by using a 6H-SiC wafer. Metal contacts on the surface of the SiC samples were fabricated by using a thermal evaporator in a vacuum condition. Among the SiC samples, several samples were heated by a Rapid Temperature Annealing(RTA) device for 10 minutes at 300°C. The metal contacts on the annealed and non-annealed samples were scanned by using AFM(Atomic Force Microscope) before and after an annealing process. The current-voltage characteristics of the SiC detectors were measured by parameter analyzer and the radiation response was evaluated by 238Pu with 5.5 MeV α-ray at room temperature and a atmospheric pressure. After annealing process, the surface roughness and the current-voltage characteristics decreased. The Schottky barrier heights of non-annealed and annealed SiC samples were determined as 0.638 eV and 0.688 eV, respectively. Also radiation response spectra of the annealed and non-annealed detectors were similar.

Growth and Characteristics of Gd2SiO5 Crystal Doped Ce3

Gadolinium oxyorthsilicate doped with cerium, Gd2SiO5:Ce (GSO:Ce), is a scintillator crystal discovered in 1983, which has excellent properties, high light yield, fast decay time, and good radiation hardness. The Gd2SiO5 (GSO) and GSO:Ce (0.5 mole%) single crystals were grown by Czochralski method under argon atmosphere. After the growth, thermal treatment of the GSO crystals was carried out at 1780K for 3 hours under argon atmosphere. The maximum wavelength of the emission spectrum of the GSO:Ce scintillator was 430 nm. The energy resolution of the GSO:Ce scintillator was 9.1 % with PMT, when it was exposed to 137Cs γ-ray. The luminescence decay time of the GSO:Ce scintillator has two exponential components with 68.7 ns and 754 ns time constant.Gadolinium oxyorthsilicate doped with cerium, Gd2SiO5:Ce (GSO:Ce), is a scintillator crystal discovered in 1983, which has excellent properties, high light yield, fast decay time, and good radiation hardness. The Gd2SiO5 (GSO) and GSO:Ce (0.5 mole%) single crystals were grown by Czochralski method under argon atmosphere. After the growth, thermal treatment of the GSO crystals was carried out at 1780K for 3 hours under argon atmosphere. The maximum wavelength of the emission spectrum of the GSO:Ce scintillator was 430 nm. The energy resolution of the GSO:Ce scintillator was 9.1 % with PMT, when it was exposed to 137Cs γ-ray. The luminescence decay time of the GSO:Ce scintillator has two exponential components with 68.7 ns and 754 ns time constant.

Development of a High Pressure Xe Ionization Chamber for Environmental Radiation Spectroscopy

A High Pressure Xenon ionization chamber is a promising radiation detector for environmental radiation measurement due to its radiation hardness, its physical rigidity, and its capability of operation at a high temperature up to about 170 °C. A cylindrical high pressure xenon ionization chamber, which was configured with a shielding mesh to improve its energy resolution, was designed on the basis of an electron transfer simulation code (EGSnrc) to extract an optimal density of Xe gas and a thickness of the chamber wall. An electron drift simulation code, Garfield, which was coupled with a Maxwell electric filed calculator, was also employed for the electron drift simulations due to the geometry of the shielding mesh. Shielding inefficiency was also calculated. A spherical ionization chamber was also designed and fabricated to monitor environmental radiation. A noble gas system was constructed to create a noble gas with a high purity and to inject the noble gas up to 60 atm. The combination of an oxygen absorbent (Oxisorb), a molecular sieve, and a high temperature getter can minimize the electro-negative impurities, such as the O2 and N2 gas, to below about several ppb levels. Preliminary tests such as leakage currents, saturation currents, and gas leak test were performed. The performance of the two fabricated ionization chambers at a low dose rate was tested by using a conventional shadow technique with a NIST certified 33.52 MBq 226Ra source in the calibration room at KAERI.