The Design And Investigation Of Field Emission Current Modulation In The Pulsed X-ray Imaging

X-rays have the characteristics of short wavelength,high frequency and high energy,and have strong penetrability.X-rays are widely used in materials analysis,medical inspection,industrial inspection and non-destructive testing because of the different ability of different objects to absorb X-rays.Compared with the traditional thermal cathode,the field emission cold cathode has significant advantages in miniaturization,rapid response,pulse modulation and stable operation of medical X-ray devices,and is expected to meet the requirements of low radiation dose and high image quality.In this paper,carbon nanotubes are used as cold cathode materials,and the field emission current modulation structure of pulsed X-ray imaging is studied and designed.Above all,cold cathodes of CNTs fabricated by screen-printed method and direct growth method have been prepared.The field emission performance is studied under vacuum chamber of 10^-4Pa.Compared with the cold cathode of CNTs prepared by direct growth method,the threshold field strength of the screen-printed CNTs is 1.13V/μm,which is only1/3 of the former.And the peak current density is 48.38 m A/cm²,which is about 1.57 times that of the former,its long-term work stability is good.Secondly,the paper focuses on the current modulation structure of the gate in the X-ray tube.In order to improve the electric field distribution characteristics between the cathode gate and improve the emission capability of the cathode,a COMSOL simulation model of the metal gate unit coated with the carbon film is established.The effect of carbon film thickness and mesh number on the field emission electric field of carbon film composite gate is studied.The vacuum chamber test results show that the 200 mesh grid covering 10～20nm thick carbon film has the largest increase in field emission performance,and the threshold field strength of the cathode is 1.53V/μm.At 1.5KV,the anode current can reach 412μA,and the electron transmittance is 50.9%.In order to improve the electron transmittance of the metal gate,the Mg O composite gate has also been studied.The CST simulation results show that the Mg O material has a large secondary electron emission coefficient,and is excited by electron emission from the cathode field to stimulate the emission of secondary electrons,which can compensate for the interception of the gate which causes the anode current to drop.A composite gate deposited 150 nm thick Mg O film is tested under the vacuum chamber,the threshold field strength of the cathode is 1.25 V/μm,the maximum anode current is 355μA at1.3 KV,and the electron transmittance is 58.5%.In addition,a CST simulation model of Hip-Hop metal gates has also been established.The effects of different etch angles and different gate thicknesses on current modulation characteristics are studied.The current modulation characteristics of the Hip-Hop metal gate deposited with the Mg O film are simulated and.analyzed.On this basis,the structure of the cathode gate assembly has been optimized,the assembled components are compact and the gap between the cathode and the gate can be controlled relatively accurate.The X-ray tube based on CNTs cold cathode and composite gate is fabricated and the field emission characteristics and imaging performance of the X-ray tube are tested.Finally,the paper proposes an eight-domain algorithm based on MATLAB to analyze and calculate the contrast of X-ray images of image intensifiers.Experiments show that the fabricated X-ray tube can reach a transmittance of 82.2%when working under the pulse gate voltage.static and dynamic object can be imaged.The obtained X-ray images are clear and the internal details are complete,and the contrast ratio can reach 72 and 52 respectively.