Determination of cerium stable isotopes by photon, neutron and proton activation analysis

Full text: Cerium is the second element in the lanthanide group of metals. Its occurrence in nature is rare and two of its radioactive isotopes 144Ce (t1/2∼ 284 days) and 141Ce (t1/2 = 32.5 days) are important fission products. In cases of accidental ingestion of these isotopes information concerning their behavior throughout the human body, especially the fractional amount absorbed into the systemic circulation from the gastro-intestinal tract, is important, since required for dosimetric evaluation. Biokinetic information can be obtained directly on humans by using stable isotopes as tracers of the element under investigation, that is, the stable isotopes are substitutes for radioactive isotopes. In the design of tracer kinetic investigations, where the amounts of stable isotopes administered have to be maintained at physiological levels, the performances of the analytical techniques are of great importance. The possibility of using NAA, PAA and CPAA for the determination of two isotopes of cerium in blood plasma samples, as previously done for gadolinium, has been tested. By considering proton activation at different incident energies of the projectiles, preliminary analyses were undertaken to identify convenient reaction channels of high cross sections leading to the production of radionuclides of proper characteristics for the off line measurements. A possible candidate reaction is 142Ce(p,n)142Pr, while for the reactions occurring at higher energy a complexity in the quantitative analysis is evident due to induced interfering reaction. Indeed, the possible reaction channels have the disadvantages of low cross section values and/or low gamma emission probability. By using PAA136Ce can be determined in plasma in the order of dozens of ppb, after a cooling time of about 12 h after EOB, while the sensitivity for other candidate isotopes (140Ce, 142Ce) is not as high as this isotope. By using NAA significant interference, caused by radionuclides produced in the biological matrix, are evident. However, a cooling time of at least one week, allows a sufficient reduction of the matrix background to obtain a high sensitivity for the determination of 140Ce, that is most cerium abundant isotope in the natural abundance. This fact confirms the good performance NAA technique in quantitative elemental determination of Ce in the case of a natural composition whereas, in the case of a design of investigation for biokinetic studies based on double tracer technique which make use of two Ce isotopes as tracers, one given orally and a second intravenously, a combination of techniques using different projectiles has to be considered