Annual Report 2011 / 2012 - E21 - Technische UniversitÃ¤t MÃ¼nchen

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22 E21 Annual Report 2011/2012
Chapter 2. Nuclear and Fundamental Physics 23 β-Emitter Bremsstrahlung in γ-Spectroscopy B. Rohrmoser 1 1 Zentrale Technisch-Wissenschaftliche Betriebseinheit Radiochemie RCM, Technische Universität München, D-85748 Garching, Germany γ-spectroscopy is a successfully applied method for decades for the identification and quantification of gamma-emitting nuclides. In the characterization of radioactive waste packages it does not consider, however, any information on β- emitting nuclides yet. But there is the phenomenon of charged particle radiation named Bremsstrahlung, which may be detected in γ-spectra too. This offers a way for the non-destructive characterization of β-emitters in radioactive waste packages by separating the contribution of the β-emitters in the measured spectrum from the remaining part, mainly produced by γ-emitters. It has already been proven that this separation is theoretically possible [1]. In a well defined geometry three different measurements on some β- and γ-emitters have been performed. First, a β-Emitter was measured together with a γ-emitter. Then the two emitter types have been measured separately. The combined spectrum is equal to the sum of the individual spectra, if the same measurement parameters are used. Thus, the individual β- and γ-spectrum can be calculated from the combined spectrum by subtracting one of the individual spectra. For real radioactive waste packages it is not possible to measure this three spectra separately. Monte-Carlo simulations of these experiments showed that the spectra are extremely sensitive to changes in the geometry of the experimental setup and thus to the composition of a waste matrix. Experiments have been performed to investigate this sensitivity in more detail. Some results are shown here for the γ-emitter 137 Cs. In Fig. 1 the smoothed differences in the γ-spectra between measurement positions with sampledetector-distances (SDD) of 15 cm and 20 cm relative to a SSD of 10 cm are shown, respectively. Figure 1: Smoothed difference spectra of 137 Cs. Black curve shows the difference in count rate of SDD of 20 cm H(max) minus SDD 10 cm H(min). Red shows SDD of 15 cm H(mid) minus SDD 10 cm. The backscatter peak at about 200 keV is caused by the 661 keV γ-line of 137 Cs and has its origin from gamma rays being Compton-scattered in the material surrounding the detector system. At the position of the peak the total count rate of the 20 cm SDD measurement is almost two times bigger then the count rate of the 10 cm SDD measurement, if both spectra are normalized to the net peak area of the 661 keV peak. This shows, that the backscattering of γ-emitters has a great influence on the shape of the “background” between the γ-lines of a γ-spectrum. But this is the region where the Bremsstrahlung-information is hidden and must be taken into account when the Bremsstrahlung-spectrum is to be extracted from a measured spectrum. To investigate the dependence of a pure β-emitter on the geometry a 90 Sr-source has been analyzed. This nuclide is always in radioactive equilibrium with its daughter 90 Y, being a pure β-emitter too. Both β-emitters show a continuous energy distribution without characteristic lines. In Fig. 2 the measured 90 Sr/ 90 Y spectra for three different SDDs are shown. Figure 2: Spectra of 90 Sr/ 90 Y for three SDDs. The count rates are normalized to E avg of 90 Sr at 195.6 keV. A difference in the three spectra is noticeable around the maximum at about 37 keV. Where a higher SDD means less count rate. The relation seems to be approximately 1/r here, but further investigation is needed to affirm this. This “attenuation” occurs only up to about 80 keV. In waste packages this isn’t noticeable because of the occurrence of x-ray lines in this region. Transforming the spectra into each other via I = I 0 e −(µ/ρ)ρx with the mass attenuation coefficient µ/ρ of air[2] does not work here. Maybe the answer is found in a convolution of the spectrum. Nevertheless the spectra of the different SDDs show qualitatively the same shape compared to each other, up to the highest detected counts at about 1800 keV (not shown in Fig 2) after background correction. The investigations show, that the measured spectra of pure β-emitters seem to be nearly insensitive on geometric effects. This is in strict contrast to the spectra of γ- emitters. Thus, if theγ-spectrum can be well simulated a relatively clear-cut and therefore identifiable Bremsstrahlungsspectrum remains after subtraction from the measured spectrum. References [1] B. Rohrmoser conference proceedings, KONTEC 2011. [2] http://www.nist.gov/pml/data/xraycoef/index.cfm
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Page 75 and 76: Chapter 6. Activities 69 Seminar
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Chapter 6. Activities 73 E21 Member
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Chapter 6. Activities 75 Alumni Nam
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Annual Report 2011 / 2012 - E21 - Technische UniversitÃ¤t MÃ¼nchen

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