1 | \section[Photoabsorption cross section at low energies] |
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2 | {Photoabsorption Cross Section at Low Energies} \label{secsandia} |
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3 | |
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4 | \subsection{Method} |
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5 | |
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6 | The photoabsorption cross section, $\sigma_{\gamma}(\omega)$, where $\omega$ |
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7 | is the photon energy, is used in Geant4 for the description of the |
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8 | photo-electric effect, X-ray transportation and ionization effects in very |
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9 | thin absorbers. As mentioned in the discussion of photoabsorption |
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10 | ionization (see section \ref{secpai}), it is convenient to represent the |
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11 | cross section as a polynomial in $\omega^{-1}$ |
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12 | \cite{sandia.bigg} : |
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13 | \begin{equation} |
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14 | \sigma_{\gamma}(\omega) = \sum_{k=1}^{4}a_{k}^{(i)}\omega^{-k} . |
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15 | \end{equation} \\ |
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16 | |
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17 | \noindent |
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18 | Using cross sections from the original Sandia data tables, calculations of |
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19 | primary ionization and energy loss distributions produced by relativistic |
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20 | charged particles in gaseous detectors show clear disagreement with |
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21 | experimental data, especially for gas mixtures which include xenon. \\ |
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22 | |
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23 | \noindent |
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24 | Therefore a special investigation was performed \cite{sandia.grich} by |
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25 | fitting the coefficients $a_{k}^{(i)}$ to modern data from synchrotron |
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26 | radiation experiments in the energy range of $ 10 - 50 \ eV$. The fits |
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27 | were performed for elements typically used in detector gas mixtures: |
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28 | hydrogen, fluorine, carbon, nitrogen and oxygen. Parameters for these |
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29 | elements were extracted from data on molecular gases such as $N_2$, $O_2$, |
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30 | $CO_2$, $CH_4$, and $CF_4$ \cite{sandia.lee73, sandia.lee77}. Parameters |
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31 | for the noble gases were found using data given in the tables |
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32 | \cite{sandia.marr, sandia.west}. |
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33 | |
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34 | \subsection{Status of this document} |
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35 | 18.11.98 created by V. Grichine \\ |
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36 | 10.05.02 re-written by D.H. Wright \\ |
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37 | |
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38 | \begin{latexonly} |
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39 | |
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40 | \begin{thebibliography}{99} |
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41 | \bibitem{sandia.bigg} Biggs F., and Lighthill R., |
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42 | {Preprint Sandia Laboratory, SAND 87-0070} (1990) |
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43 | \bibitem{sandia.grich} Grichine V.M., Kostin A.P., Kotelnikov S.K. et al., |
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44 | {Bulletin of the Lebedev Institute no. 2-3, 34} (1994). |
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45 | \bibitem{sandia.lee73} Lee L.C. et al., |
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46 | {J.Q.S.R.T., v. 13, p. 1023} (1973). |
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47 | \bibitem{sandia.lee77} Lee L.C. et al., |
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48 | {Journ. of Chem. Phys., v. 67, p. 1237} (1977). |
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49 | \bibitem{sandia.marr} G.V. Marr and J.B. West, |
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50 | {Atom. Data Nucl. Data Tabl., v. 18, p. 497} (1976). |
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51 | \bibitem{sandia.west} J.B. West and J. Morton, |
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52 | {Atom. Data Nucl. Data Tabl., v. 30, p. 253} (1980). |
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53 | \end{thebibliography} |
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54 | |
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55 | \end{latexonly} |
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56 | |
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57 | \begin{htmlonly} |
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58 | |
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59 | \subsection{Bibliography} |
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60 | |
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61 | \begin{enumerate} |
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62 | \item Biggs F., and Lighthill R., |
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63 | {Preprint Sandia Laboratory, SAND 87-0070} (1990) |
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64 | \item Grichine V.M., Kostin A.P., Kotelnikov S.K. et al., |
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65 | {Bulletin of the Lebedev Institute no. 2-3, 34} (1994). |
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66 | \item Lee L.C. et al., |
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67 | {J.Q.S.R.T., v. 13, p. 1023} (1973). |
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68 | \item Lee L.C. et al., |
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69 | {Journ. of Chem. Phys., v. 67, p. 1237} (1977). |
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70 | \item G.V. Marr and J.B. West, |
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71 | {Atom. Data Nucl. Data Tabl., v. 18, p. 497} (1976). |
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72 | \item J.B. West and J. Morton, |
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73 | {Atom. Data Nucl. Data Tabl., v. 30, p. 253} (1980). |
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74 | \end{enumerate} |
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75 | |
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76 | \end{htmlonly} |
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77 | |
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