6.2 Development of a Highly Sensitive New Element Quantification Method of Multi-Element, Simultaneous Analysis with a Thousand Times Better Resolution than gamma-ray Detection


Fig. 6-4 Principle of multiple gamma-ray detection method

It is known that most radioactive nuclei emit multiple coincident gamma-rays. The gamma-rays are detected with an array of multiple gamma-ray detectors, and such events are accumulated on two-dimensional matrix with axes proportional to gamma-ray energies. The energy resolution is 1/1,000 for both axes and thus the energy resolution on the matrix becomes 1/1,000,000 (= 1/1,000 x 1/1,000). Furthermore, the background level is reduced to only a few counts so that very weak gamma-ray peaks can be observed.


Fig. 6-5 An example of gamma-ray detector array

An anti-Compton gamma-ray spectrometer with a BGO Compton suppressor and germanium detector achieves low background and high resolution. A gamma-ray detector array, GEMINI, constructed in tandem accelerator facility in the Tokai Establishment consists of 12 such spectrometers in spherical symmetry.


Fig. 6-6 An example of a two-dimensional matrix

A standard rock sample (JP-1) was activated with JRR-4 neutrons and a two dimensional matrix was obtained after
gamma-ray measurement with GEMINI. 152Eu at a concentration of 4 ppb has been identified as the smallest component.



Widely used neutron activation analysis utilizes a single detector to detect gamma-rays from a neutron-activated sample. The energy resolution is approximately 1/1,000 for high-resolution germanium detectors. The radioactive nuclei emit more than ten gamma-rays on average, so that if the sample includes many elements, the number of gamma-rays exceeds one thousand, making it difficult to resolve all the gamma-rays.
By detecting multiple coincident gamma-rays emitted simultaneously from many radioactive nuclides and by analyzing a g-g two-dimensional matrix, we could improve the energy resolution by a factor of one thousand. This indicates that one can completely separate as many nuclides as there are present in a sample (even for 2,700 nuclides so far found in nature).
The multiple gamma-ray detection method has been used for nuclear structure study at high excitation energy. We applied it for the first time to neutron activation analysis and demonstrated that 27 elements can be simultaneously quantified without chemical separation, for standard rock samples, JB-1a and JP-1, issued by the Geological Survey of Japan. It is also shown that 49 elements can be quantified simultaneously by this method. Furthermore, the background level can be greatly suppressed and very weak gamma-rays can be detected: a high-sensitivity of ppb (10-9) order could be achieved. This new technology is now expected to contribute to various fields, such as environmental, medical, space and earth sciences, and exploitation of natural resources etc.


Reference
Y. Hatsukawa et al., Application of Multidimensional Spectrum Analysis for Analytical Chemistry, Am. Inst. Phys., CP 495, 429 (1999).

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