Jun Mok Ha

A vacuum-sealed miniature X-ray tube based on carbon nanotube field emitters

A vacuum-sealed miniature X-ray tube based on a carbon nanotube field-emission electron source has been demonstrated. The diameter of the X-ray tube is 10 mm; the total length of the tube is 50 mm, and no external vacuum pump is required for the operation. The maximum tube voltage reaches up to 70 kV, and the X-ray tube generates intense X-rays with the air kerma strength of 108 Gy·cm2 min−1. In addition, X-rays produced from the miniature X-ray tube have a comparatively uniform spatial dose distribution.

Enhancement of antireflection property of silicon using nanostructured surface combined with a polymer deposition

Silicon (Si) nanostructures that exhibit a significantly low reflectance in ultraviolet (UV) and visible light wavelength regions are fabricated using a hydrogen etching process. The fabricated Si nanostructures have aperiodic subwavelength structures with pyramid-like morphologies. The detailed morphologies of the nanostructures can be controlled by changing the etching condition. The nanostructured Si exhibited much more reduced reflectance than a flat Si surface: an average reflectance of the nanostructured Si was approximately 6.8% in visible light region and a slight high reflectance of approximately 17% in UV region. The reflectance was further reduced in both UV and visible light region through the deposition of a poly(dimethylsiloxane) layer with a rough surface on the Si nanostructure: the reflectance can be decreased down to 2.5%. The enhancement of the antireflection properties was analyzed with a finite difference time domain simulation method.

Highly stable carbon nanotube field emitters on small metal tips against electrical arcing

Carbon nanotube (CNT) field emitters that exhibit extremely high stability against high-voltage arcing have been demonstrated. The CNT emitters were fabricated on a sharp copper tip substrate that produces a high electric field. A metal mixture composed of silver, copper, and indium micro- and nanoparticles was used as a binder to attach CNTs to the substrate. Due to the strong adhesion of the metal mixture, CNTs were not detached from the substrate even after many intense arcing events. Through electrical conditioning of the as-prepared CNT emitters, vertically standing CNTs with almost the same heights were formed on the substrate surface and most of loosely bound impurities were removed from the substrate. Consequently, no arcing was observed during the normal operation of the CNT emitters and the emission current remained constant even after intentionally inducing arcing at current densities up to 70 mA/cm2.

Synthesis of flake-like graphene from nickel-coated polyacrylonitrile polymer

Graphene can be synthesized from polyacrylonitrile (PAN) polymer through pyrolysis. A metal catalyst such as nickel (Ni) is required for the conversion of the polymer to graphene. The metal catalysts can be placed either atop or underneath the polymer precursor. We observed that spatially non-uniform and disconnected graphene was fabricated when PAN film coated with a Ni layer was pyrolyzed, resulting in flake-like graphene. Formation of the flake-like graphene is attributed to the dewetting of the Ni layer coated on the PAN film. Dewetting phenomenon can be reduced by decreasing the pyrolysis temperature, and hence, more uniform graphene could be prepared. The effects of Ni coating thickness and the pyrolysis temperature on the fabricated graphene have been experimentally analyzed.

Small-sized flat-tip CNT emitters for miniaturized x-ray tubes

Small tip-type CNT emitters with the diameter of 0.8 mm were fabricated for miniaturized X-ray tubes. The CNT emitters were prepared by dropping CNTs and silver nanoparticles on a flat surface of a W metal tip followed by annealing at 800°C for 2h under vacuum. The CNT emitters exhibit good field emission properties with the threshold electric field of 1.15 V/µm and the field enhancement factor of 12,050. CNTs were well attached to a flat W tip surface without coating on the side plane of the tip, and thus beam divergence could be minimized. Consequently, a miniaturized X-ray tube with the inner diameter of 5mm was successfully demonstrated using the tip-type CNT emitter.

Behavior of characteristic X-rays from a partial-transmission-type X-ray target

The angular distribution of characteristic X-rays using a partial-transmission tungsten target was analyzed. Twenty four tallies were modeled to cover a 360° envelope around the target. The Monte Carlo N-Particle (MCNP5) simulation results revealed that the characteristic X-ray flux is not always isotropic around the target. Rather, the flux mainly depends on the target thickness and the energy of the incident electron beam. A multi-energy photon generator is proposed to emit high-energy characteristic X-rays, where the target acts as a filter for the low-energy characteristic X-rays.

Effect of Electron-Beam Irradiation on Organic Semiconductor and Its Application for Transistor-Based Dosimeters

The effects of electron-beam irradiation on the organic semiconductor rubrene and its application as a dosimeter was investigated. Through the measurements of photoluminescence and the ultraviolet photoelectron spectroscopy, we found that electron-beam irradiation induces n-doping of rubrene. Additionally, we fabricated rubrene thin-film transistors with pristine and irradiated rubrene, and discovered that the decrease in transistor properties originated from the irradiation of rubrene and that the threshold voltages are shifted to the opposite directions as the irradiated layers. Finally, a highly sensitive and air-stable electron dosimeter was fabricated based on a rubrene transistor.

Controlled Fabrication of Nanoporous Oxide Layers on Zircaloy by Anodization

We have presented a mechanism to explain why the resulting oxide morphology becomes a porous or a tubular nanostructure when a zircaloy is electrochemically anodized. A porous zirconium oxide nanostructure is always formed at an initial anodization stage, but the degree of interpore dissolution determines whether the final morphology is nanoporous or nanotubular. The interpore dissolution rate can be tuned by changing the anodization parameters such as anodization time and water content in an electrolyte. Consequently, porous or tubular oxide nanostructures can be selectively fabricated on a zircaloy surface by controlling the parameters. Based on this mechanism, zirconium oxide layers with completely nanoporous, completely nanotubular, and intermediate morphologies between a nanoporous and a nanotubular structure were controllably fabricated.

Freestanding graphene nanosheets and large-area/patterned graphene nanofilms from indium-catalyzed graphite

We present a simple and convenient route to freestanding graphene nanosheets and large-area/patterned graphene nanofilms by thermal annealing of an indium (In) and graphite mixture. In particles mixed with graphite powder catalytically oxidize the graphite powder and produce carbon oxide gases. The carbon oxide gases are then catalytically graphitized to form graphene nanostructures on In. The morphologies of the graphene nanostructures can be controlled by the In geometry: vertically-standing graphene nanosheets are synthesized using In particles and a graphene nanofilm is grown on a thin In layer. In is then completely removed, and thus high-purity graphene is finally produced. The graphene nanosheets exhibited excellent field-emission performance due to high-density edges. In addition, the graphene nanofilms show electrical conductivities comparable to or better than that of reduced graphene oxide. Furthermore, a large-area or a patterned graphene nanofilm is synthesized using a uniform or a pre-patterned In layer, respectively.

SU‐E‐T‐79: Fabrication of Miniature X‐Ray Tube Using Carbon Nanotube Field Emitter for Electronic Brachytherapy

Purpose: A miniature x‐ray tube with the diameter of 10 mm has been fabricated using a carbon nanotube(CNT)field emitter, optimal design of electron optics and x‐ray target, and vacuum sealing of the x‐ray tube. The CNT emitter with nm∼μm scale and simpler cathode structure was used for miniaturization of x‐ray tube. A truncated conical shaped beryllium (Be) and thin tungsten (W) coating were adopted to emit 3‐dimensionally uniform x‐ray as an isotropic radiation source. Anode grounded high voltage (HV) connection and vacuum sealed x‐ray tube structure within 10 mm diameter are advantages for intra‐cavitary or skin surface electronic brachytherapy. Methods: The miniature x‐ray tube consisted with a tip‐shaped CNT emitter with the diameter of 0.25∼0.8 mm, a focusing electrode, a transmission‐type x‐ray target, getter film and a metalized HV isolation ceramic. The CNT emitter has been fabricated by coating and sintering of CNT and Silver nano particle mixture on the flat W tip. The miniature x‐ray tube is brazed in a 3×10−6 torr of vacuum furnace. The electron and x‐ray emission are calculated by EGN2 and MCNP5 program, and measure using a precision current meter (Fluke multimeter 189), a soft x‐ray chamber (PTW 34013), and x‐ray spectrometer (Amptek XR‐100t‐CdTe). Results: The CNT emitter emitted electron current over 1 mA at −9.8 kV/5 mm, and 0.1 mA was continued for over 3000 minutes with decrease ratio of −0.01 μA/min. The diode‐type miniature x‐ray tube is started to emit less than 5 Gy/min x‐ray at cm air depth from −20 kV to −50 kV following the gate to CNTcathode gap, and applying cathode voltage. Conclusions: The miniature x‐ray tube using CNTfield emitter has been demonstrated. The CNT based x‐ray tube can be more miniature, and is expected to apply various field of electronic brachytherapy, soon. This work was supported by the R&D Program of MKE/KEIT (10035553, Development of multi‐direction X‐ray emitting super‐miniature X‐ray tube for intra‐cavity imaging system) and the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (No. 2010‐0020387).