Partial Neutron Capture Cross Sections of Actinides using Cold Neutron Prompt Gamma Activation Analysis

摘要

Nuclear waste needs to be characterized for its safe handling and storage. In particularlong-lived actinides render the waste characterization challenging. The results describedin this thesis demonstrate that Prompt Gamma Neutron Activation Analysis (PGAA)with cold neutrons is a reliable tool for the non-destructive analysis of actinides.Nuclear data required for an accurate identification and quantification of actinides wasacquired. Therefore, a sample design suitable for accurate and precise measurements ofprompt -ray energies and partial cross sections of long-lived actinides at existing PGAAfacilities was presented. Using the developed sample design the fundamental prompt-ray data on 237Np, 241Am and 242Pu were measured. The data were validated by repetitiveanalysis of different samples at two individual irradiation and counting facilities –the BRR in Budapest and the FRM II in Garching near Munich. Employing cold neutrons,resonance neutron capture by low energetic resonances was avoided during theexperiments. This is an improvement over older neutron activation based works at thermalreactor neutron energies. 152 prompt -rays of 237Np were identified, as well as19 of 241Am, and 127 prompt -rays of 242Pu. In all cases, both high and lower energeticprompt -rays were identified. The most intense line of 237Np was observed atan energy of E = 182.82(10) keV associated with a partial capture cross section of = 22.06(39) b. The most intense prompt -ray lines of 241Am and of 242Pu wereobserved at E = 154.72(7) keV with = 72.80(252) b and E = 287.69(8) keV with = 7.07(12) b, respectively. The measurements described in this thesis provide the firstreported quantifications on partial radiative capture cross sections for 237Np, 241Am and242Pu measured simultaneously over the large energy range from 45 keV to 12 MeV. Detaileduncertainty assessments were performed and the validity of the given uncertaintieswas demonstrated. Compared to existing literature data on prompt -ray energies andemission probabilities the uncertainties of the data were improved.In addition to the basic nuclear data necessary for PGAA, the thermal radiative neutroncapture cross sections of 237Np and of 241Am were determined from decay measurementsafter neutron irradiation. The thermal radiative neutron capture cross section of 237Npwas determined as 0c = 176.3(47) b . The thermal radiative neutron capture cross sectionof 241Am was determined as 0c = 667.7(312) b. The thermal radiative neutron capturecross section of 242Pu was calculated as 0c = 21.9(15) b using nuclear structure simulationswith the statistical decay code DICEBOX, constraint by the measured prompt -raydata. In the corresponding simulation the total radiative width of the capture state wasfound to be 28(1) meV. Also, the neutron separation energies of 238Np and of 243Pu werederived. The neutron separation energy of 238Np was calculated as Sn = 5488.02(17) keV.The neutron separation energy of 243Pu was calculated as Sn = 5036.33(59) keV. Detectionlimits for PGAA at FRM II were calculated for 237Np as 0.056 μg, for 241Am as0.017 μg and for 242Pu as 0.20 μg