Kye-Ryung Kim

Field-induced cathodic oxidation of low-energy Ar-ion-bombarded silicon by AFM

Local oxidation by atomic force microscopy (AFM) was studied on a 3keV Argon (Ar)-ion-bombarded silicon (Si) (100) substrate. Giant oxide features higher than 100nm were patterned by applying positive voltages to the tip with respect to the substrate. To analyze the growth rate of oxide features, we used the power-of-time law model. The growth rate of oxide features on an Ar-ion beam-bombarded silicon surface was increased approximately 1.8-fold compared to a common silicon surface. Furthermore, we obtained that the heights of oxide features increased as the exposure time to the tip decreased and the scan area increased.

MFM and gas adsorption isotherm analysis of proton beam irradiated multi-walled carbon nanotubes

To enhance the gas adsorption properties and modify the physical properties of carbon nanotubes, multi-walled carbon nanotubes (MWCNTs) were irradiated by high-energy proton beams, and the physical properties including morphology and local surface structure were investigated by using a transmission electron microscope (TEM), magnetic force microscope (MFM) and a gas adsorption isotherm apparatus which can deeply probe the fine structure of surface. Interestingly, clearer MFM images were obtained from the proton irradiated samples which supports that carbon exhibits magnetism under proton bombardments, although the intrinsic magnetic property is not understood. The layering properties of argon on MWCNTs were measured from 59 to 69 K and the interaction of argon on the surface was analyzed. The calculated values of isosteric heat of adsorption demonstrated that higher interaction of gas molecules with surface is found from the proton irradiated MWCNTs. This result strongly supports that the local surface modification, partial defects, for example, were created due to the external high energy impacts. Our results are worthy to note that gas adsorption technique can provide the fine atomic resolution which beyond the one of TEM and MFM.

Proton beam energy measurement using semiconductor detectors at the 45MeV test beam line of PEFP

For the pilot and feasibility studies on the development of beam utilization technologies of PEFP (proton engineering frontier project), the test beam line was installed at the MC-50 cyclotron of KIRAMS (Korea institute of radiological and medical sciences) [1-2]. The energy measurement of proton beam with high accuracy is very important for the some experiments such as radiation hardness test of semiconductor devices for the spacecraft, detector development and test for the nuclear physics, etc. Energy measurement of 35 MeV and 45 MeV proton beam using a 5 mm thick Si(Li) in air was performed. The energy was controlled by Al degrader in the range of 0.02 mm~6.2 mm. The measured value was compared to the results of code simulation using SRIM 2003.

The Operation Experience at KOMAC

A 100-MeV proton linac at the KOMAC (Korea Multipurpose Accelerator Complex) is composed of a 50-keV microwave ion source, a 3-MeV four-vane-type RFQ, a 100-MeV DTL and 10 target stations for proton irradiation on samples from many application fields. The linac was commissioned in 2013 and the user service started in July 2013 with delivering proton beam to two target stations: one for a 20-MeV beam and the other for a 100MeV beam. In 2015, the linac has been operated more than 2,800 hours with an availability of greater than 89 %. The unscheduled downtime was about 73 hours, mainly due to troubles of ion source arcing and failures of pulsed high voltage power system. More than 2,100 samples from various fields such as materials science, bio-life, nano technology and nuclear science, were treated in 2015. Currently, a new target station for radioisotope production is under commissioning and a new target station for low flux irradiation experiments is being installed. Operational experiences of the 100-MeV linac during the past 3 years will be presented in the workshop.

Proton Energy Straggling Measurement in Metal Foils by Using a Si(Li) Detector

Measurements of energy loss and transmitted energy straggling were performed using a Si(Li) detector for 28.6 MeV protons incident on thin foil targets of Al, Cu and Au in air. The straggling widths were studied as functions of the target thickness and the target species. The initial proton beam energy from the MC-50 cyclotron was 45 MeV and it was decreased to 28.6 MeV by passing it through a 2-mm-thick Al window, 34.5 cm of air, a 2-mm-thick Al degrader and 144.5 cm of air. This 28.6 MeV proton beam was incident on Al, Cu and Au foils of various thicknesses installed in front of a Si(Li) detector. As a result, the energy loss strongly depended on the target thickness and species and the transmitted energy straggling was not related to the target species. Compared with the calculated results based on a simple Bohrs expression, the measured values agreed with the calculated ones in the low energy loss region less than 60 % of the incident proton beam energy. In the high-loss region, there was big mismatch of more than 25 %.

A Performance and an Imaging Result of Silicon Strip Detector

Since capacitively coupled readout has an advantage of shielding the readout electronics from sensor leakage current, we fabricated an AC-coupled single-sided silicon strip sensor on 5-in. 380 μm-thick wafer and its active area is 3.2 × 3.2 cm2. It has 64 implanted and readout strips with a strip pitch of 500 μm. Two silicon strip sensors were mounted perpendicularly one another to get two-dimensional position information with a 5 mm space gap. Two low noise analog ASICs, VA1 chips, were used for signal readout of the strip sensor. The single-sided silicon strip detector (SSSD) consists of the two AC-coupled single-sided strip sensors and readout electronics. The SSSD was housed in an Al light-tight box. A CsI(Tl) scintillation crystal and a 2-in. photomultiplier tube were used to trigger signal events. The data acquisition systems were based on a 64 MHz FADC and control softwares for the PC-Linux platform. Experiments were performed at room temperature and silicon sensors were fully depleted. The SNRs were measured to be 29.1 ± 2.2 and 26.4 ± 0.3, respectively, for a 45 MeV proton beam and a 90Sr radioactive source. Imaging tests were performed using a Pb phantom with a 90Sr radioactive source and a 45 MeV proton beam, respectively. From the two-dimensional hit distribution, the phantom images were obtained and the position resolution was estimated to be about 240 μm.