Research Progress Of High Resolution Cold Neutron Imaging Detector

As a useful non-destructive imaging technology,cold neutron radiography is widely used in materials science,chemistry,biology and condensed matter physics for investigating materials such as polymers,metals,ceramics,magnetic materials,porous media,fluids and gels,and biological molecules.To resolve the details of samples with a micron resolution in the said researches,the research and development of high resolution cold neutron imaging detectors have become a hot topic recently.In this paper,the key components of proposed high resolution cold neutron imaging detector have been studied.These research work and results are organized into four sections:1.Two high resolution cold neutron imaging systems consisted of a high resolution phosphor screen and a CCD were designed and fabricated.The first one uses a microscopic optical system with large NA objective lenses(NA = 0.5@10,0.75@20 and 0.85@40);the second one is based on a large aperture lens(f = 0.75).2.A Gd-doped neutron collimator has been simulated with a Geant4-based simulation program.The simulation results show that: 1)the collimator absorbs about 94.8% of incident neutrons;2)the collimator can effectively improve the L/D of incident neutron beam;3)the collimator can reduce the divergence of neutron exit angle.4)the results of cold neutron imaging of the leaf show that the neutron collimator can improve the spatial resolution of the neutron imaging system and the Gd distribution in the collimator is not uniform.3.A procedure for making high-resolution cold neutron imaging phosphor screen has been invented.The process has the characteristics of low cost,simple equipment and convenient operation.It is able to efficiently fill phosphors into a capillary glass array or a microporous structure.The main steps include: the phosphor grinding,the particle size filtering,the filling liquid mixture preparation,the two filling processes.A phosphor screen with a 10-μm capillary glass array made with this method has a resolution of 230μm@CTF=0.03.The phorsphor system for the scintillation light may be the main reason to degrade its resolution.In order to diversify the development of the phosphor screen,a 100-um co-doped Tb3+/Ce3+ and Gd flicker glass fiber faceplate has been developed and studied.The imaging test with cold neutrons shows that it has a resolution of 250μm@CTF=0.02.The lower resolution may be caused by the longer trackes of secondary particles(electrons,X-rays and low energy gammas)produced by the neutron absorption reaction in the glass and the non-total reflection of the capillary.4.The single-photon calibration of a low-gain PMT has been done with the fall-time discrimination method.The calibrated PMT was then used to measure the absolute light yields of the newly-developed scintillation glasses with gammas.