Neutron-capture radiography has been applied to the mapping of natural boron in mouse histological sections. The method is based on the fact that the stable isotope boron-10 has an extremely large cross-section for the thermal-neutron reaction 10B(n, alpha)7Li. The local tissue boron concentrations were evaluated from the corresponding track densities of the nuclear reaction using an equation derived from a theoretical model of the system. The adjustable parameters of this equation were determined in two separate calibration experiments using standardized samples prepared by the addition of known amounts of boron. Interference from other nuclides engaged in nuclear reactions with thermal neutrons was also estimated. In the present experimental conditions the natural boron lower detection limit was 0.03 p.p.m. (fresh weight), and the spatial resolution was of the order of a few micrometers. Boron concentrations in mouse serum and urine were close to 0.22 and 0.57 micrograms ml-1, respectively. In the solid mouse tissues--liver, heart, brain, muscle and spleen--the concentration was usually low, ranging from 0.12 to 0.16 p.p.m. (fresh weight). They were significantly higher in the kidney, especially in the papilla (6.2 p.p.m. fresh weight). Apart from the kidney papilla, where boron was particularly concentrated in wall tubules, the boron distribution in most tissues appeared to be practically homogeneous. Natural boron, and even more so its enriched stable isotope 10B, appear as good candidates for molecular labelling using non-radioactive tracers.

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