Study On Energy Dependent Sensitivity Of Gated Framing Camera

The gated framing camera has been the core diagnostic equipment in ICF experiments,it plays a very important role in the fields of hohlraum physics, X-ray radiation transportation, radiation ablation, and so on. Recently,the framing camera using has been confused by many problems,such as the quantum efficiency is uncertainty,the energy sensitivity curve is unknown,the influencing factor of the response is undefined,and so on.These problems made the experimental result can't be accurately quantified.Thus,the study of the energy sensitivity of gated framing camera is very important to the broadband X-ray imaging,which is radiated by high-energy and high density plasma.After investigating the essential structure,the working principle and the primary parameters of gated framing camera in depth,a 2D theoretical model of the core component of framing camera,namely Au coated MCP's energy sensitivity is constructed,based on the research work of American Lawrence Livermore National Laboratory and Sandia National Laboratories. The calculational expression of the energy sensitivity is deduced,and the energy sensitivity curve is simulated using Mathematica.The theoretical research is done in many aspects,such as the photoelectric efficiency of the photocathode,multichannel effect,photoelectric effect of lead layer on the microchannel inwall,the gain of a single microchannel,and so on.For a given photocathode material and incident photon energy,the photoelectric efficiency is directly proportional to the mass photoionization cross-section and mass density of the cathode,as well as the incident photon energy.The X-ray attenuation length and the average escape depth of the secondary electrons in the photoncathode influence the photoelectron quantum of photocathode too.When the incident photon energy is high,the X-ray may penetrate the MCP microchannel wall and enter into the next channel,so as to produce photoelectrons.Researching the influence of the multichannel effect to Au coated MCP energy sensitivity, the crux of the matter is the X-ray absorption coefficient of MCP material and the position where the photoelectrons yielded.After penetrating the microchannel wall,the X-ray may bormbard the lead layer of next microchannel wall,and the photoelectrons produced by lead's photoemission may be multiplication too.So the calculation should consider the computation of the photoelectron quantum of lead layer and the electron gain caused by that, as well as the from the gain of a single channel.A microchannel can be regarded as discrete dynodes,the number of dynodes may be dissimilar because of the different position of the photoelectrons yielded. All in all,the gain is decided by the ratio of length to diameter of MCP and the distance between the position of the photoelectrons yielded to the output surface of secondary electrons.From the simulative curve based on the theoretical expressions,a conclusion can be gained that the energy sensitivity of Au coated MCP involved many parameters of Au and Pb,such as density,Fermi vector,mass photoionization cross-section and secondary electron escape depth,as well as the X-ray attenuation length and the X-ray absorption coefficient of the MCP material.In order to find out the energy dependent characteristic of Au coated MCP at ultraviolet band,and also to explore the calibration experiment condition of Au coated MCP's energy sensitivity,an experiment for measuring the energy sensitivity of Au coated MCP detector at ultraviolet band was taked.Analyzing the experimental result,some conclusion can be drawn.When the distance between electron collector and output surface of MCP is only 0.5mm,then 9V is enough to collect all the secondary electrons.The dark current is in the range of pico-ampere,and randomly fluctuate at 25pA.The MCP detector with Au photocathode will response a little even when the incident photon energy is lower than 4.9eV(the work fuction of Au) but higher than 4.0eV.There is a peak in the spectral response curve of Au photocathode at 5.72eV.Analyzing this phenomenon,it is found that the peak is cased by the selective photoeffect of Au.Based on the experiment of measuring the energy dependent characteristic at ultraviolet band,the calibration experiment of three Au coated MCP detectors at X-ray band(2.1~6keV) was accomplished,using the monochromatic X-ray of 3B3 experimental station of BSRF. The result shows that:â‘ As the theoretic shows,the energy sensitivity of Au coated MCP will attenuate as the incident photon energy increase.But there are several sharp peaks in the energy sensitivity curve because of the absorption edge of some elements.â‘¡The energy sensitivity will be lower as the incident angle becomes larger and the gain smaller.The X-ray normal incident to the MCP surface,so the incidence angle equals to the channel bias angle of MCP.Thus,smaller the channel bias angle,higher the energy sensitivity of MCP detector.â‘¢If the Au film goes into the microchannel too long,the multiplication effect of the lead layer to secondary electrons will be weaken.Simultaneously,the X-ray absorption edge will be much visible than Pb,so as to cover up the absorption edge of Pb.â‘£The cathode quantum efficiency will be higher as Au film of MCP input surface is thicker.However,the channel orifice will be plug up if the Au film is too thick.Thus,choose an optimal cathode structure and thickness will make the photoelectric conversion efficiency of the photocathode to be optimization.Combining the theoretical and experimental analyse,framing camera's energy sensitivity is related to the factors which include absorption coefficient of the photocathode, structural parameter of cathode material, X-ray absorption coefficient of the MCP substrate material,secondary electron emission capability,etc.In addition,reducing the MCP channel axis angle and increasing the thickness of Au film properly may improve the energy sensitivity of the framing camera too.