Nutzung von Bremsstrahlungsinformationen für die zerstörungsfreie Charakterisierung radioaktiver Abfälle
Projektleiter: Dr. Thomas Bücherl
Wissenschaftler: Dipl. Phys. Benjamin Rohrmoser
In Kooperation mit TU München, Physikdepartment, E 21, Prof. P. Böni
Das diesem Statusbericht zugrundeliegende Vorhaben wurde mit Mitteln des Bundesministeriums für Bildung und Forschung unter dem Förderkennzeichen 02S8669 gefördert.
Bremsstrahlung information for the non-destructive characterization of radioactive waste packages
Non-destructive techniques are the preferred methods for the characterization of radioactive waste packages. Compared to destructive methods it minimizes the radiation dose for the personal, the secondary radioactive waste production, and is less time consuming. In routine gamma-spectroscopy - applied successfully over decades - identification and quantification of gamma-emitting nuclides is possible. This method does not consider any information on beta-emitting nuclides embedded in the waste matrix. But there is the phenomenon of charged particle radiation called Bremsstrahlung, which may be detected in gamma scans, too. This possibility of an identification of beta-emitters is not considered in data evaluation at present. A feasibility study shall investigate, if the identification of beta-emitters in the gamma-spectra via their Bremsstrahlung is possible. First experiments have been started at laboratory dimensions. The first experiment consisted of three measurements with a high purity germanium detector (HPGe) detector. As samples 60Co, 133Ba, 137Cs and 241Am calibration standards were used as gamma-emitters and a 170Tm sample, produced at FRM II, as a Bremsstrahlung emitting nuclide. The selection of the latter was based on . First, the spectrum of 170Tm only was recorded. Then only the gamma emitters 60Co, 133Ba, 137Cs and 241Am were recorded. For the third measurement all samples were measured together. Fig. 1 shows the results as well as the difference of the third to the second measurement resulting in the same distribution as the measurement of only 170Tm.
Figure 1: Black: Spectrum of the gamma-emitters 60Co, 133Ba, 137Cs, 241Am; Red: Spectrum of the gamma-emitters 60Co, 133Ba, 137Cs, 241Am and 170Tm; Blue: Spectrum of 170Tm (identical to the difference of the red and black spectra (Green))
As a first result, it was demonstrated that in this simple set-up the extraction of the Bremsstrahlung-part from different gamma-emitters is possible. In a second experiment with 170Tm-samples of different activities the limits of the method were investigated, i.e. the minimum activity of the beta-emitter required for definite identification in dependence of the activities of the gamma-emitters being present. This value is defined as beta-to-gamma ratio. Figure 2 shows the spectra of 60Co (3.0·104 Bq) and 60Co together with 170Tm (5.7·107 Bq). In the spectrum of 60Co and 170Tm shoulders on the right hand side of the characteristic gamma-peaks of 60Co at 1173.3 and 1332.5 keV are noticeable, not being present in the spectrum of only 60Co. The only explanation is a contribution by 170Tm having an endpoint energy of about 970 keV. This becomes clear in figure 2, which also shows the difference of the two measured spectra. Up to about 700 keV the characteristics of the pure 170Tm is visible. Between 1100 keV and 1500 keV also summation effects take place, as well known in pure gamma-spectroscopy . The peaks at 1257 keV and 1418 keV, respectively, are the results of the summations of the two cobalt peaks with the only thulium peak at 84.3 keV. In figure 2, the pure 170Tm spectrum is added to the right side of the 1173.3 keV 60Co peak for better illustration. It indicates the trend of the 170Tm spectrum between 60 - 200 keV, thus reflecting the summation effect. These shoulders might be used for the determination of Bremsstrahlung-emitters in the future. In this case the beta-to-gamma-ratio has an extremely high value of about 2000.
Figure 2: Black: 60Co spectrum, Red: spectrum of 60Co with 170Tm. Green: Difference-spectrum of 60Co and 170T measured together minus the 60Co only spectrum. Blue: The energy range from 60 - 200 keV of the 170Tm spectrum is added to the right side of the 1173.3 keV 60Co peak for better illustration.
The experiment was repeated with 170Tm (2.8·106 Bq) and 137Cs (2.3·105 Bq). Here the beta-to-gamma- ratio is only about 12. Converted with the use of the their decay constants, the number of cesium-atoms was seven times bigger than the number of thulium-atoms. The problem here is, that the 661.7 keV Cs-peak occurs within the 170Tm-spectrum. However the same effect as discussed above for 60Co is still distinguishable, although less distinctive. The appearance of these summation effects may simplify the search for beta-emitters in radioactive waste matrices. Additional experiments as well as computer simulations will be performed to demonstrate the applicability of this method in practice. Special focus will be given on the influence of absorbing matrices surrounding the gamma- and beta-emitters.
 A. S. Dhaliwal, S. Amarjit Nucl. Instrum. and Methods B 198:32-36, 2002.
 G. Gilmore, J. Hemingway Practical Gamma-ray Spectroscopy, John Wiley and Sons, 1995.