**5. Conclusions**

136 Radioisotopes – Applications in Physical Sciences

gamma spectrum of a soil sample. The gamma pulse was delayed with respect to the alpha pulse because an analog amplifier with a shaping constant of 1μs was employed to reduce electronic noise and obtain a better resolution in the germanium detector. Electronic noise reduction in the germanium chain could also be achieved using digital

Fig. 6. Am-241 alpha/gamma coincident pulses (above) and coincident gamma spectrum

(below) using liquid scintillation and a low energy germanium detector.

procedures.

Analytical procedures used to determine actinides in environmental samples were reviewed. The standard procedure using PIPS detectors is the most often used. It allows very small MDA to be reached due to the high resolution and very low backgrounds of these detectors. However, the analysis time is long (several days) and impossible to apply when many samples need to be processed or it is necessary to know the results in record time. LS spectrometry overcomes this inconvenience but at the expense of a poor resolution which makes it difficult to eliminate some interferences and thus determine quantitatively certain radionuclides. The LS analysis time is very short and improvements could be made to increase the light emission from the cocktail, reducing quenching and improving energy resolution. Future photodiode detectors could also help to improve energy resolution. A digital system is the natural way to implement many of these new developments due to its advantages in treating the digitized pulses. The MDA is ultimately dependent on the background of the technique. In our case it was possible to reduce the background by eliminating electronic signals which did not correspond with the correct pulse shape and also random electronic noise using standard digital procedures. Discrimination between different types of particles could be implemented by measuring the pulse light decay time without the need to resort to analogue electronic equipment.

Coincidence experiments could be easily set up using digital cards with two or more channels. The coincidence time could be determined with resolution times between 20 nsec and 1 nsec for cards of 50 Ms/sec to 1 Gs/sec sampling rates. Two-parameter studies using coincidence and 2-D diagrams can help to determine certain types of actinides. The technique is usually applied with analogue electronic equipment to determine radio-xenons resulting from nuclear subterranean tests but the setup presents more difficulties than when using digital equipment. In the case of actinide identification the technique could be applied to eliminate interferences between radioisotopes of similar alpha energies but different gamma emissions.

In some cases, for instance, the analysis of certain plutonium isotopes (Pu-239 and Pu-240), the alpha energies are similar and also equal the X-ray energy emissions. In this case conversion electron spectrometry with PIPs detectors using the same electro-deposited samples prepared for alpha spectrometry could help in obtaining a quantitative determination of both radioisotopes.

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X-ray spectrometry could also be used to determine the total activity of the different radioisotopes of an element by using a high-resolution germanium detector in coincidence with LS spectrometry. The coincidence experiment is necessary to eliminate the background of the germanium detector and improve the MDA limit. Si (PIN) detectors, cooled by the Peltier effect, could offer a simple alternative if detectors of large area and small electronic noise are built in the future. Modern Si (PIN) detectors can reach energy resolutions of 125 eV at 5 keV with very low backgrounds but at the cost of a low counting efficiency because of their small detector area.
