2:30 PM - 2:45 PM
[MAG33-04] Development of Microarea Imaging Method for Isotopes on Cross-section of Morten Particles Collected in Hiroshima
Keywords:The ”Black Rain” caused by A-bomb explosion, Radioactive fallout, Isotope analysis, Laser resonance ionization, High resolutin isotope imaging
Introduction
Spherical small particles collected from a beach in Hiroshima are thought to through a high temperature melting process, and they are fall-out related to "Black Rain" by the bomb. It is difficult to deduce the origin of the molten particles because it has been long time after the bomb. We are trying to get the evidence of the origin of the molten particles using small area mass spectrometry, called secondary ion mass spectrometry (SIMS). It is possible to analyze isotope ratio in mass spectrometry. However, the precision of isotope ratio analysis degradates when isobaric interferences occur. We have developed a new imaging mass spectrometry using resonance ionization of sputtered atoms with wavelength tunable lasers. We call this technique as R-SNMS. In this study, the applicability of R-SNMS to analyze isotope ratio in the molten particles was evaluated.
Experimental
The Time-of-Flight SIMS (TOF-SIMS) apparatus was developed by the authors. One of the features of this instrument is a high lateral resolution down to 40 nm, and ability of detection of all elements and isotopes simultaneously. The R-SNMS is the another operation mode where tunable lasers are added to ionize an aimed element by tuning the wavelength of the lasers identical to internal excitation energy and auto-ionization process. Therefore, R-SNMS is not affected by isobaric interference. And this time, we added another laser, femto-second (fs) laser to enhance the ionization yield while maintaining element-selective ionization and detection. The sample particle was collected at a beach in Hiroshima. Particle were used after magnet separation, and cross-sectioned with an ion milling apparatus. Then the particles were introduced into the SIMS/SNMS instrument.
Results and Discussion
These particles were classified into two types; silica-based and Fe-oxide-base particles. Both of them were several hundreds micrometers in diameter, and inhomogeneous distribution of some elements were observed. This means that these particle were generated after melting at high temperature and then cooled at a certain rate. This kind of thermal history is one of the things to consider, but there are many origins such as iron manufacturing, shipbuilding, other industries and urban activities. Therefore we investigate isotope ratio abnormality due to thermal neutron. According to the estimate, 57Fe/56Fe increased to 0.04 at maximum. We measured the ratio using commercial Fe, and it was 0.0243, where the ratio of natural abundance is 0.0231. There was a difference, but the SNMS result was closer to that of natural abundance. In case of SIMS, the ratio was 0.0514. This is due to isobaric interference with hydrogen attached Fe ions. In the next, the molten particle was analyzed. The result was 0.0280. This value is slightly larger than that of natual abundance. But this difference is significant because the dispersion was smaller than the difference.
Conclusion
It was shown that R-SNMS can reveal isotope ratio with high precision. We are going to analyze many particles and consider other isotope ratio abnormality.
Spherical small particles collected from a beach in Hiroshima are thought to through a high temperature melting process, and they are fall-out related to "Black Rain" by the bomb. It is difficult to deduce the origin of the molten particles because it has been long time after the bomb. We are trying to get the evidence of the origin of the molten particles using small area mass spectrometry, called secondary ion mass spectrometry (SIMS). It is possible to analyze isotope ratio in mass spectrometry. However, the precision of isotope ratio analysis degradates when isobaric interferences occur. We have developed a new imaging mass spectrometry using resonance ionization of sputtered atoms with wavelength tunable lasers. We call this technique as R-SNMS. In this study, the applicability of R-SNMS to analyze isotope ratio in the molten particles was evaluated.
Experimental
The Time-of-Flight SIMS (TOF-SIMS) apparatus was developed by the authors. One of the features of this instrument is a high lateral resolution down to 40 nm, and ability of detection of all elements and isotopes simultaneously. The R-SNMS is the another operation mode where tunable lasers are added to ionize an aimed element by tuning the wavelength of the lasers identical to internal excitation energy and auto-ionization process. Therefore, R-SNMS is not affected by isobaric interference. And this time, we added another laser, femto-second (fs) laser to enhance the ionization yield while maintaining element-selective ionization and detection. The sample particle was collected at a beach in Hiroshima. Particle were used after magnet separation, and cross-sectioned with an ion milling apparatus. Then the particles were introduced into the SIMS/SNMS instrument.
Results and Discussion
These particles were classified into two types; silica-based and Fe-oxide-base particles. Both of them were several hundreds micrometers in diameter, and inhomogeneous distribution of some elements were observed. This means that these particle were generated after melting at high temperature and then cooled at a certain rate. This kind of thermal history is one of the things to consider, but there are many origins such as iron manufacturing, shipbuilding, other industries and urban activities. Therefore we investigate isotope ratio abnormality due to thermal neutron. According to the estimate, 57Fe/56Fe increased to 0.04 at maximum. We measured the ratio using commercial Fe, and it was 0.0243, where the ratio of natural abundance is 0.0231. There was a difference, but the SNMS result was closer to that of natural abundance. In case of SIMS, the ratio was 0.0514. This is due to isobaric interference with hydrogen attached Fe ions. In the next, the molten particle was analyzed. The result was 0.0280. This value is slightly larger than that of natual abundance. But this difference is significant because the dispersion was smaller than the difference.
Conclusion
It was shown that R-SNMS can reveal isotope ratio with high precision. We are going to analyze many particles and consider other isotope ratio abnormality.