| X-ray phase contrast imaging has drawn much research interest in detecting bio-medical samples composed of low elements, which just makes up for the shortcomings of the traditional X-ray absorption imaging. However, there are many difficulties for the X-ray phase contrast imaging's application on clinical diagnosis, the main issue is that phase contrast imaging's source must be interference and has high energy. Recently, the most commonly used light is still using synchrotron radiation facility. But this its volume is too big and its power is high, and it is not suitable for clinical diagnosis. A cold cathode X-ray tube was proposed for X-ray phase contrast imaging application because of its small volume, fast response, and good coherence.For the application on phase contrast imaging, we focused on the optimization of electron emission, the design and simulation of focusing structure of the electron beam, and the X-ray imaging test.CNTs were transferred to the stainless steel by screen printing and carbon fibers was introduced in the middle layer between CNTs and the substrate. The emission current of the lmm CNTs was 0.3mA when cathode voltage was 1.6KV, while the current was only 0.1mA without carbon fibers. The emission stability test without any cooling measure shows that the emitting current can keep on 0.3mA within 0.5h. The result shows carbon fibers in middle layer can enhanced the adhesion of CNTs on the substrate, reduced the contact resistance between CNTs and the substrate, and increase the current.In order to make the electric field more uniform and improve the emission current, we improved the gate mesh in two ways. One side is covered the copper grid with carbon film. The experimental results show that the carbon film can increase the current and reduce the turn-on voltage. It can be considered that the carbon film made the electric field uniform, increased the effective emitting area and current. The other side is deposited MgO on the surfaces of the stainless mesh, the experimental results show MgO can generate secondary electrons,the electrons accumulated on the gate and increased the field between gate and cathode, then the emission current increased.For the application of micro focus CNTs X-ray sources, the structure and parameters of focusing electrode was simulated and optimized with the cathode voltage (Vc) of OKV, the gate voltage (Vg) of 2kV and anode voltage (Va) of 35kV. In the optimized focusing structure, a small focus around 50um was obtained and the electron transmittance on the grid was 50% with the Vg of 2kV and Va of 35kV.Finally, the imaging experiments were performed using sources of the CNTs X-ray tube designed by ourselves and the electron beam of FESEM. The image of the metal and rats can be observed clearly with the current of the CNTs X-ray tube keeping above 100uA, while the rats' muscle and hair cannot seen since no focusing structures were considered in the tube. Then the plastic rope's image was taken by the electron beam of FESEM, the plastic rope can be seen clearly when the focus is smaller than 250um. It shows the X-ray phase contrast image can be taken when the current is big enough and the focus size is small enough. These results provided the basis for the simulation of the focusing structure. |
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