Conclusion
Radionuclide X-ray fluorescence (RXRF) analysis takes up an important place among other nuclear analytical methods. This nondestructive method enables to identify a great number of elements of the periodic table on the basis of their characteristic X rays excited by radionuclide sources. It predestinates this method for its usage for the check-up of composition and purity of materials and recently also for the monitoring of pollutants in various parts of environment.
At the Department of Nuclear Physics of FMFI, CU, which is a prominent workplace in the sphere of monitoring of radioactivity in the environment and which trains specialists in this field, X-ray fluorescence method has not been worked out and widely used so far. Hence, the aim of my work was to participate in building experimental set-up for RXRF method, optimise its parameters and show the possibilities of its application for the analysis of environmental samples as well as metal materials.
Within the framework of optimisation of experimental parameters chiefly the influence of voltage applied across the Si(Li) detector and of shaping time constant of the linear amplifier on energy resolution of our spectrometer was studied. We determined 300 Vas optimum voltage of operation of the Si(Li) spectrometer and 12 s as convenient shaping time constant of the linear amplifier. Moreover, in this section of work we presented that relative energy resolution worsens rapidly for energies of X rays below 15 keV and improves only slowly for energies above 15 keV. In addition, we compared Si(Li) and HPGe detectors belonging to the Department of Nuclear Physics, FMFI, CU in terms of their detection efficiency and energy resolution for 23,174 keV of characteristic X rays of Cd. Relative energy resolution of the Si(Li) detector is about 1,8 % while for the HPGe detector it increases up to 5 % . On the contrary, the HPGe detector reaches approximately 64 times higher detection efficiency than the Si(Li) detector. As a result of these tests, it is more convenient to use the Si(Li) detector for measurements, for which a high energy resolution is a need. Conversely, if energy resolution is not a decisive factor, but rather a good detection efficiency is required, it is more advantageous to use the HPGe detector.
In the part devoted to metal analysis about 60 different pure metals and metal objects were tested. The capabilities of identification of particular metals and determination of composition of various metal objects were studied using our spectrometer. It turned out that rather light pure metals of Z 22 – 50 we can identify according to Ki-lines using gamma radiation source 241Am of 59,5 keV energy within 200 sof measurement. Heavier pure metals of Z > 70 are possible to be identified according to Li-lines within time of 1000 – 5000 s.
Possibilities of the usage of RXRF method and our spectrometer for polycomponent express qualitative analysis of metal materials were studied mainly on the set of coins and several “gold” objects. It was found out that measured element composition of Slovak coins dating from 1993 – 1996 was in agreement with the composition of these alloys declared by National Bank of Slovakia. Furthermore also tests of golden objects were successful. The presence of gold was detected even in objects, where gold was not revealed by the standard jewellery method. Likewise the differences in composition of gold objects were found out by means of RXRF analysis as demonstrated by our results. This method was found very convenient for express analysis of purity of gold objects. The ratio of L / L lines was constant for most of the gold objects.
In the part devoted to RXRF usage for the determination of water composition we analysed 2 types of waters: mineral waters (5 samples) and drinking-waters (9 samples). In mineral waters all elements of Z 20, declared by producers at labels of bottles, were identified by our spectrometer. In addition also the presence of strontium was observed. Using 55Fe source for excitation of Ca (K) X rays the efficiency of Ca identification in mineral waters was approximately 2,5 times higher, when compared to 241Am source. On the other hand the advantage of the usage of 241Am source for Ca detection is that the dependence of counts under Ca (K) peak on the ratio of mCa/ mt is linear in the whole range of mCa/ mt (mCa is mass of Ca in the analysed sample, mt is the total mass of the sample). For analyses carried out with 55Fe source this dependence is linear only for mCa/ mt >0,3 using saturated samples.
Further also the dependence of counts rate on mass and areal mass of samples was investigated for K X rays of iodine in potassium iodide (KI) using 241Am source. The main result of this part of work is the discovery, that the total detection efficiency of the detector can be evaluated out of the measured dependence for the actuated K X rays. This is the factor, which is experimentally difficult to determine with X-ray fluorescence analysis.
Outcomes of the analysis of drinking-waters confirmed that the applied experimental set-up used for RXRF analysis is sensitive enough also for the identification of metals in these waters. In general, using samples prepared only from the volume of 1 ℓ the time of one measurement did not exceed 24 hour span.
In the last part of this thesis we dealt with the usage of RXRF analysis for the identification of metals in solid substances taken up on filters after their exposure to several thousands m3 of filtered atmospheric air. Altogether there were analysed 20 sets of filters from different periods of the year 2001. After 1-3 day measurement of a sample, elements like Fe, Cu, Zn, Pb and Cd could be identified on filters. A high correlation was found out between count rates under Cu (K); Cd (K) peaks and masses of solid matters caught on filters.
We suppose that results of this thesis could be used also in pedagogical process by the training of specialists in nuclear and environmental physics within the framework of laboratory lessons. It could illustrate processes taking place in atomic shells and their usage for analytical purposes. This work can also stimulate interest in the study of interaction processes of different types of radiation with atoms as well as encourage further development of applications of radionuclide X-ray fluorescence analysis.
1