Partial Neutron Capture Cross Sections of Actinides using Cold Neutron Prompt Gamma Activation Analysis
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Beschreibung
Nuclear waste needs to be characterized for its safe handling and storage. In particular
long-lived actinides render the waste characterization challenging. The results described
in 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 was
acquired. Therefore, a sample design suitable for accurate and precise measurements of
prompt
-ray energies and partial cross sections of long-lived actinides at existing PGAA
facilities was presented. Using the developed sample design the fundamental prompt
-ray data on 237Np, 241Am and 242Pu were measured. The data were validated by repetitive
analysis 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 the
experiments. This is an improvement over older neutron activation based works at thermal
reactor neutron energies. 152 prompt
-rays of 237Np were identified, as well as
19 of 241Am, and 127 prompt
-rays of 242Pu. In all cases, both high and lower energetic
prompt
-rays were identified. The most intense line of 237Np was observed at
an 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 were
observed 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 first
reported quantifications on partial radiative capture cross sections for 237Np, 241Am and
242Pu measured simultaneously over the large energy range from 45 keV to 12 MeV. Detailed
uncertainty assessments were performed and the validity of the given uncertainties
was demonstrated. Compared to existing literature data on prompt
-ray energies and
emission probabilities the uncertainties of the data were improved.
In addition to the basic nuclear data necessary for PGAA, the thermal radiative neutron
capture cross sections of 237Np and of 241Am were determined from decay measurements
after neutron irradiation. The thermal radiative neutron capture cross section of 237Np
was determined as 0
c = 176.3(47) b. The thermal radiative neutron capture cross section
of 241Am was determined as 0
c = 667.7(312) b. The thermal radiative neutron capture
cross section of 242Pu was calculated as 0
c = 21.9(15) b using nuclear structure simulations
with the statistical decay code DICEBOX, constraint by the measured prompt
-ray
data. In the corresponding simulation the total radiative width of the capture state was
found to be 28(1) meV. Also, the neutron separation energies of 238Np and of 243Pu were
derived. 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. Detection
limits for PGAA at FRM II were calculated for 237Np as 0.056 µg, for 241Am as
0.017 µg and for 242Pu as 0.20 µg.
Autor*in
Christoph Genreith