Study On Photodisintegrations And Nuclear Resonance Fluorescence Inspection Using Intense Quasi-monochromatic ? Source

In recent years,laser-Compton scattering(LCS)induced by the collision between laser pulses and relativistic electron bunches has become an important way of high-intensity ?-ray generation owing to the increase in laser power and the development of advanced accelerator technology.Compared with the traditional y-ray sources,LCS y-ray source is known for its unique characteristics,such as good directivity,quasi-monochromatic energy spectrum,variable energy,controllable polarization and high intensity.At present,LCS gamma sources have been or being developed in a number of internationally renowned institutions,such as SLEGS of Shanghai Institute of Applied Physics,HI?S of Triangle University,New-Subaru of Hyogo University,ELI-NP of the European Union.With the worldwide construction and operation of LCS y-ray sources,the related technologies,experimental measurements and applications have aroused intensive public interest.In astrophysical studies,LCS ?-rays can be used to measure photodisintegration reactions,which can provide necessary nuclear physical inputs for solving the mystery of p-nuclei nucleosynthesis.Meanwhile,LCS induced nuclear resonance fluorescence(NRF)that provides unique "fingerprints" for nuclear isotopes is an sensitive probe to the concentration and spatial distribution of isotopes,which is of great interest in the context of contraband inspections.Therefore,this thesis will focus on the theoretical analysis and Monte Carlo simulation for the applications of LCS y-ray source in two subjects:nuclear astrophysics studies and nondestructive inspection.Considering the compound and pre-equilibrium reaction mechanisms,the cross sections and astrophysical rates of(?,p)and(?,?)reactions for about 3000 target nuclei with 10 ?Z?100 ranging from stable to proton dripline nuclei are computed.We used two sets of optical model potentials(OMPs)in the calculations to investigate the sensitivity of photodisintegration rates on OMPs:the microscopic folding ones and phenomenological Woods-Saxon ones.The systematic comparisons show that the photodisintegration rates,especially in the(?,?)cases,are significantly affected by the OMPs.Consequently,it is important to better determine the OMP and thus constrain the photodisintegration rate uncertainties.Based on these theoretical results,we proposed measurements of six(?,p)and eight(?,?)reactions based on the Variable Energy Gamma system(VEGA)and the silicon strip array(ELISSA)at ELI-NP.Furthermore,Geant4 simulations on these(?,p)and(?,?)reactions are performed.Simultaneously satisfying the minimum detectable limit of the experimental yield and the particle identification of protons and ? particles,the minimum required energies of the measurements are estimated.It is shown that the direct measurements of these photonuclear reactions within the Gamow windows at the typical temperature of T9=2.5 for the p-process are fairly feasible and promising.The expected experimental results will be used to constrain the OMPs of the charged particles,which can eventually reduce the uncertainties of the reaction rates for the p-process nucleosynthesis.Another interesting application of photodisintegration experiment is the determination of the relative strengths of particle emissions(PEs)to excited states of the final nucleus,which may improve the reaction models used to predict reaction rates.We investigate for the first time PEs to excited and ground states in seventeen(?,p)and seventeen(?,?)reactions involving p-nuclei using ELI-NP.The cross sections of these(?,pi)and(?,?i)reaction branches are computed,where the energy level scheme is represented by the i-th excited states of the final nucleus(i=0 refers to the ground state).Realistic Geant4 simulations on these photodisintegration reactions are performed based on the ?-beam facility and the silicon strip array at ELI-NP.It is found that the detection of 96,98Ru(?,p1)reactions is feasible at E??Sp+ 2.5 MeV,since their PEs to first excited state are dominant(with high branching ratio of?95%).The predicted cross section ratios,?L0i/?L00,relative to the ground state transitions become significant at E?? Sp+2.5 MeV for eight(?,p)reactions(i.e.?L01/?L00 for those on 29Si,56Fe and 91Zr,?L02/?L00 for that on 119Sn,and ?L03/?L00 for those on 102Pd,106Cd,115Sn and 117Sn).The ?L01/?L00 for 123Te(?,?)and ?L02/?L00 for 125Te(?,?)are found to be detectable around E?? Sa+9.0 MeV,while the ?L01/?L00 for 87Sr(?,?)is notable when E?? S?+6.0 MeV.Accordingly,the cross-section ratios of ?L0irev/?L00rev for their reverse capture reactions are deduced in the framework of detailed balance.Such predictions could provide complementing results for charged particle capture reactions since the direct experiments can only proceed on nuclei in the ground state.This study suggests a novel methodology for measurements of nuclear photodisintegration leading to excited state of residual nuclei,which may help to constrain the astrophysical photodisintegration reaction ratesCombining NRF spectroscopy and the element(or isotope)ratio approach,we present a novel inspection method that can simultaneously reveal the elemental(or isotopic)composition of the illicit drugs.Monte Carlo simulations show that four NRF peaks from the nuclei 12C,14N and 160 can be detected with high significance of 7-24? using an induced photon beam flux of 1011.The ratio of 14N/12C and/or 16O/12C for illicit drugs inspected are then extracted using the element ratio approach.It is found that the present results of simulations are in good agreement with the theoretical calculations.The feasibility to detect the illicit drugs,inside the 15-mm-thick iron shielding,or surrounded by thin benign materials,is also discussed.It is indicated that,using the state-of-the-art y-ray source of high intensity and energy-tunability,the proposed method has a great potential for identifying drugs and explosives in a realistic measurement time.Combining scattering and transmission NRF spectroscopy(sNRF and tNRF),the feasibility to interrogate concealed SNM is studied.Monte Carlo simulations show that the isotope identification of 235,238U can be realized by observing their NRF signatures resulting from a one-dimensional scan using sNRF spectroscopy.The isotope ratio of 235U/238U/are further deduced according to the sNRF signatures,which agrees with theoretical value within uncertainty.After isotope identification and isotope ratio prediction,tNRF spectroscopy is employed to realize tomographic imaging of 235U.The reconstructed images show that the spatial distribution of 235U hidden in a iron rod can be clearly visualized.Moreover,the impact of the 23 5U density variation on the reconstructed attenuation factor in the image is further discussed.The feasibility of utilizing the proposed method to perform isotope-specific imaging for other SNMs,such as 239,240Pu and 237Np,is also investigated.It is suggested that the proposed method is able to screen SNMs concealed in dense materials effectively.Moreover,combining sNRF and emission computed tomography(ECT),simultaneously imaging on multiple SNM isotopes using a wide band y-ray beam in the MeV region was demonstrated.In the sNRF spectroscopy,235,238U in the measured object are excited by the induced photon beam,the resulting NRF y rays provides unique signature for the 235,238U isotopes.The NRF signatures were cooperated as inputs to reconstruct the tomographic image of 235U and 238U.The reconstructed images show that the spatial distribution of 235U and 238U hidden in an iron rod can be clearly visualized.