[1211] | 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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