A Study On The Characteristic Of Cosmic-induced Gamma Radiation Field And Average Atomic Number On Lunar Surface

The characteristic of cosmic-induced gamma radiation field on lunar surface is not only the basis for researching the radiation environment,but also significant for studying the material composition.In this thesis,the generation mechanism of cosmicinduced gamma radiation field and the characteristics of energy distribution are set as fundamental theory to discuss the characteristic and physical mechanism of cosmicinduced gamma ray spectrum,in which the energy ranges from 0 to 10 MeV.Through the the Monte Carlo simulation,the physical model of cosmic-induced gamma ray,leaking from lunar surface,is constructed to acquire the original gamma spectrum.The forward model,based on flux of positron annihilation radiation,to calculating the average atomic number is established.The key parameters,such as the flux ratio in the different sequences,the mass attenuation coefficient of different kinds of lunabase rocks,the relative detection efficiencies of CE1-GRS and so on,is acquired by Monte Carlo simulating.The map of average atomic number on lunar surface is drew,relying on CE1-GRS data,and interpreted in combination with geological information.The main conclusions are listed below:?1?The data,originating from lunar samples,such as high alumina basalt,KREEP,anorthosite,dunite,low titanium basalt and high titaninum basalt,is set as the foundation to simulate the cosmic-induced gamma radiation field.The information of original gamma spectrum of these 6 kinds of lunar rocks,at various densities,is acquired.The continuous cosmic-induced gamma ray spectrum is consist of two parts: one is the characteristic spectral sequences,concluding: ¹⁶O?n,n?.2.71 MeV,3.85 MeV,6.13 MeV,6.92 MeV,7.12MeV?,²⁴Mg?n,n?.1.37MeV?,²⁷Al?n,n?.1.01 MeV,2.21 MeV,3.00MeV?,²⁸Si?n,n?.1.78 MeV,2.84 MeV,6.88MeV?,²⁸Si?n,?.3.54 MeV,4.93 MeV,6.88MeV?,⁴⁰Ca?n,n??.1.61MeV?,⁴⁰Ca?n,n?.3.74 MeV,3.90MeV?,⁴⁸Ti?n,?.1.38 MeV,6.42 MeV,6.76MeV?,⁵⁶Fe?n,n?.0.847MeV?,⁵⁶Fe?n,?.6.02 MeV,7.28 MeV,7.65 MeV?,etc.The other is a continuous spectrum,in which the flux are farther more at low energy.This phenomenon is affected by both photoelectric and Compton effect.The peak-tovalley ratio approaches 15 at 0.511 MeV,which is named as positron annihilation radiation.?2?The forward model,based on flux of positron annihilation radiation,to calculating the average atomic number is established.The directly proportional relationship between the fluxes of positron annihilation radiation and the average atomic number is proved.It is negative to the product of the flux of the gamma ray generating positron,the lunar media mass attcnuation coefficients and the natural logarithm of gamma ray energy.The forward modeling results are positive to the lunar media average atomic numbers.The correlation coefficient is 0.98.?3?A new methord to rebuild the baseline of CE1-GRS spectrum in virtue of its original spectrum simulated by MCNPX.These four methords,such as SNIP,FFT,AirPLS and AIMA,are utilized for deduction and extraction of characteristic peaks.The scheme of baseline subtraction is optimized to fetch the 12 characteristic peaks on the whole spectrum.The results are 59% with the relative error less than 10%,33% between 10% and 20% and 8% between 20% and 30%.?4?Furthermore,the average atomic numbers in 75°N⁸0°N are in less variation,which relative standard deviation is only 2.91%.So a new methord,based on the fiducial value in the stable composition circle,is proposed to leveling the global data of average atomic number.The average atomic mass,where the Apollo 11,Apollo 12,Apollo 11 17,Lunar 16,Lunar 20 and Lunar 24 land,are used to verify the average atomic number by least square method.It presents obviously positive correlation with a ratio coefficient of 1.875 and the correlation coefficient is 0.72.?5?The regions with high value of average atomic number are consistent with the distribution of mare,especially in the region of 0⁴0°W,30°S¹5°N and 50°W⁷0°W,50°N⁶5°N.The value even reaches 13.2 in the Sinus Aestuum?10.9°N,8.8°W?,which is at the edge between the Mare Vaporum and the Mare Insularum.The middle value,surrounded by the irregular oval rings,are in the middle of West Oceanus Procellarum and Mare Frigoris.The South Pole-Aitken Terrane and the transition zone from mare to highland are also in middle value.Except those,the main regions on lunar surface are coved with low average atoic number media.There is a great difference in average atomic number among the Mare Basalt Terrane,the Feldspathic High-lands Terrane and the South Pole-Aitken Terrane.The average atomic number presents the unique feature in sequence stratigraphy.It also refers to the era of geological structure reformation oppositely.The average value in the South Pole-Aitken basin are almost less than 11.8,which is farther less than that in the Oceanus Procellarum?from 12.0 to 12.5?.It is supposed that the South Pole-Aitken basin is far older than the Oceanus Procellarum.