PREPARATION AND CHARACTERIZATION OF SOURCES FOR ULTRA-HIGH RESOLUTION MICROCALORIMETER ALPHA SPECTROMETRY
Open Access
- Author:
- Rim, Jung Ho
- Graduate Program:
- Nuclear Engineering
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- None
- Committee Members:
- Kenan Unlu, Thesis Advisor/Co-Advisor
Kenan Unlu, Thesis Advisor/Co-Advisor
Stephen La Mont, Thesis Advisor/Co-Advisor
Jack Brenizer Jr., Thesis Advisor/Co-Advisor - Keywords:
- microcalorimeter
alpha spectrometry
fission track
SEM
actinide - Abstract:
- The determination of actinide isotopes in nuclear forensic samples is critical for assessing the provenance of nuclear material. Current techniques used for nuclear forensics to gain isotopic information are slow and labor intensive, requiring extensive radiochemical separations and purification prior to analysis by mass spectrometry. Due to this limitation, a new ultra-high resolution alpha particle detection system is being jointly developed by the Los Alamos National Laboratory (LANL) and the National Institute of Standards and Technology (NIST). This new system uses a microcalorimeter detector, and is capable of performing an isotopic analysis in fraction of time compared to old techniques. The preparation of the high-quality thin layer sample sources has been one of the major challenges associated with the routine used of this new detector system. This manuscript details the development and characterization of the new sample preparation methods required to produce uniform thin layer sources required to realize the full potential of microcalorimeter alpha spectrometry. Alpha particle emitting sources were prepared using an electrodeposition method optimized for preparing the 4.76 mm diameter sources required for the microcalorimeter alpha spectrometer at Los Alamos National Laboratory (LANL). The microcalorimeter is an ultra-high resolution detector has nearly an order of magnitude resolution improvement over the conventional silicon based alpha spectrometry, which facilitates the simultaneous determination of multiple isotopes which cannot be individually resolved using traditional silicon detector based alpha spectrometry methods. The resolution of the microcalorimeter, estimated from the full width at half maximum (FWHM), ranged from 1.00 to 2.5 keV, and represents a 75 to 88% improvement in resolution over a silicon alpha detector system. This improvement in resolution makes possible the determination of forensically important actinide signature radionuclides, for example Pu-239 and Pu-240, that cannot be resolved using silicon detector. The microcalorimeter resolution was, however, extremely sensitive to the quality of the sample. Samples which were contaminated with trace impurities produced degraded microcalorimeter spectra. Column ion-exchange chromatography was used to clean the stock actinide sample solution from any contaminants, and demonstrate improved resolution. The quality and uniformity of sample source surfaces was investigated using two surface characterization techniques; fission track analysis (FTA) and secondary electron microscopy (SEM). FTA was performed using a 1.0 MW TRIGA nuclear reactor at Penn State as a neutron source to induce fission reaction in U-233 samples. The results from fission track analysis showed that the actinide was distributed uniformly when electrodeposited on a titanium substrate. Based on the absence of any fission tracks forming star pattern, which indicates the agglomeration of actinides, and the number of individual fission tracks produced, clusters of actinides were less than approximately 100 nm in size. SEM images of electrodeposited sources showed the formation of approximately 100 nm size spherical particles on the surface.