FNTD Neutron Personal Dosimeter Test Technology Research

As neutrons are widely used in the nuclear industry,energy production,scientific research and other fields,the risk to be exposed to neutron radiation is increasing.Neutrons have high energy deposition in the human body and produce harmful biological effects,so estimating the doses of neutron radiation received by people is important for the purpose of radiation protection.However,accurate evaluating the neutron doses received is rather complex,and have not yet been covered by a neutron individual dosimeter,which ranges from thermal neutron(0.01 eV)to high-energy neutron(20MeV).Passive neutron detectors have the advantage of small size,no power supply,high reliability and convenient wearing,so they can be used as a legal neutron personal dosimeter.Therefore,optimizing or designing a new type of passive neutron dosimeter to became an important research focus in the field of neutron radiation protection.Conventional passive neutron detectors include:plastic track detector(CR-39),Thermoluminescence detection(TLD),and Optically Stimulated Luminescence detection(OSL).Each of these detectors have advantages and disadvantages,and researchers have long been searching for a neutron dosimeter that overcomes the current passive neutron detector restrictions,such as the wider of Linear Energy Transfer(LET)sensitive range of heavy ion,without chemical treatment,to be able to use automatic equipment reading for many times,repeated use,etc.Landauer Inc developed a fluorescent nuclear track detector combining ?-Al2O3:C,Mg single crystal materials with a confocal scanning microscope.This device showed excellent performances in detecting protons,heavy charged ions and neutron,and can accurately track the three-dimensional path of ions,with a resolution that achieves the optical diffraction limit.This device can replace the conventional passive neutron detectors.In this paper,we studied the preparation process of a ?-Al2O3:C,Mg single crystal,and prepared for the first time in China a large-sized ?-Al2O3:C,Mg single crystal(30× 70mm).We then studied the actual effect of C on the formation of F-type color in crystals.Using a Monte Carlo simulation software(Geant4),we designed the overall structure of the neutron dose meter(FNTD),performed a simulation study for neutron dose and energy response in FNTD,and used the least square method to study the neutron H*(10)and total fluence of flattening in a wide energy range(0.01 eV?20 MeV).In Chapter 3 and chapter 4 we study the preparation,performance and color heart formation mechanism of the ?-Al2O3:C,Mg single crystal.This is the main part of the paper.For the first time,the Czochralski method of graphite resistance heating was used to grow a large-size ?-Al2O3:C,Mg single crystal(30× 70 mm).The method uses graphite resistance to heat,and the crystal grows with Al2O3,MgO and graphite powder as initial growth materials.The single crystal diffraction test indicates that the growth crystal belongs to the trigonal crystal system R-3c,which is the ?-Al2O3:C,Mg single crystal.The GDMS test showed that the element content of the doped element C and Mg was well controlled in the growth process,with the C content being 2768ppm and the Mg content being 17ppm,and no other impurities were introduced.Absorption spectrum test showed that absorption peaks appeared at 206 nm,232 nm and 256 nm.Applying Smacula 's formula for F center concentration we find 3.81 ×1016cm-3,while the two absorption position of F+ center concentration are 4.1 ×1015cm-3 and 1.46×1016cm-3.The excitation-emission spectra also indicate that the crystal contains a fluorescence center at(43 5/510nm),which is characteristic of F22-(2Mg)color heart.The study of the actual effect of C on the formation of color center in the crystal,the annealing process of ?-Al2O3:C,Mg single crystal was performed in air,H2 and vacuum.Single crystal diffraction shows that after annealing in air,H2 and vacuum,the single crystal structure did not change,still belonging to the R-3c space group of crystal structure.In addition,also crystal cell parameters and volume did not change,as well as the initial growth of crystals.GDMS test shows that after the annealing treatment,the concentration of the doped C and Mg were higher than in the initial crystals.Annealing therefore accelerated the diffusion of impurity elements,makeing the distribution more uniform.Absorption spectra showed that the absorption peak abundantly decreased and the fluorescence center was destroyed.Combined with the fluorescence test,the oxygen vacancy in the crystal was eliminated by annealing at high temperature.According to the three academic controversy on the effect of doped C((?)C2+ replace Al3+,(?)C4-replace O2-and(?)to form the atmosphere rather than doping.We tend to the third point of view,that is that the C forming fluorescent center in the process of crystal growth is not the main function of doping,while this lies in the formation of the reductive atmosphere conducive to form a large number of oxygen vacancy in crystal.To exclude the possibility that the atom into the crystal lattice is from the annealing atmosphere,we annealed the crystal in 10-4 Pa vacuum at 1300?.The the light absorption,fluorescence and GDMS testing results are consistent with annealing in air or H2.This suggests that the disappearance of the oxygen vacancy is caused by C elements diffusion.The innovation and most difficule part of this paper is in the fifth chapter.By Monte Carlo simulation(Geant4)we designed the overall structure,material type and element content of FNTD.In order to improve the response to neutron as much as possible in a wide energy range(0.01 eV?20MeV),we combined the back scattering part in the detector structure.The material is made by contains 10B polyethylene and the whole size is 4.5×7×1cm3.We therefore studied in our simulations the energy and dose response of FNTD in a wide energy range(0.01 eV ?20MeV),useing 30×30×15cm of water tank to replace the human body.The FNTD dosimeter was placed at the center of the tank surface,and we set up 30 points monoenergetic neutron with flux density of 0.933 ×1017cm-2.The results showed that the energy response of the low-energy segment was higher(less than 10eV)with 6LiF conversion material,with the highest response of 100um thickness.We found that using(CH2)n conversion material has high energy response to high energy segments(higher than 1MeV).The backscatter structure that used 6LiF conversion material has higher energy response to middle energy segments.The ambient dose equivalent(H*(10))was used to evaluate the radiation dose of the human body in the neutron radiation field,and the relationship between the response value with the neutron individual dose was established.The dose response value was in accordance with the energy response.We find that 6LiF conversion materials have higher dose response to neutron with energy below 10eV,that(CH2)n conversion materials have higher dose response to neutron with energy higher than 1MeV and that the back scattering structure using 6LiF conversion material have higher dose response for low-energy neutron,but lower dose response for high-energy neutron.This is because the value of neutron fluence dose conversion coefficient u is higher at high energy range,almost 40 times compared with the low energy range.The neutron H*(10)and total neutron flux measurement of FNTD are optimized by using the least square method.Optimized H*(10)response constraint are between 0.30?1.4,of which 0.01 eV?70 keV and 4?14 MeV is relatively flat,constraints in 0.8?1.4,accounted to eight out of nine energy level in wide energy range(0.01 eV?20MeV),total fluence response constraints between 0.89?1.1 in wide energy range.The simulation results show that the FNTD dosimeter can properly measure the neutron dose and total fluence between the wide energy range(0.01 eV?20MeV).In the next step,we will further optimize the growth method of ?-Al2O3:C,Mg single crystal,and prepare dosimeter products for irradiation experiments.The FNTD dosimeter has great potential to develop into a new generation of passive neutron dosimeter.