\section[Photoabsorption cross section at low energies]
{Photoabsorption Cross Section at Low Energies} \label{secsandia}

\subsection{Method}

The photoabsorption cross section, $\sigma_{\gamma}(\omega)$, where $\omega$ 
is the photon energy, is used in Geant4 for the description of the
photo-electric effect, X-ray transportation and ionization effects in very 
thin absorbers.  As mentioned in the discussion of photoabsorption 
ionization (see section \ref{secpai}), it is convenient to represent the 
cross section as a polynomial in $\omega^{-1}$ 
\cite{sandia.bigg} :
\begin{equation}
\sigma_{\gamma}(\omega) = \sum_{k=1}^{4}a_{k}^{(i)}\omega^{-k} . 
\end{equation} \\

\noindent
Using cross sections from the original Sandia data tables, calculations of 
primary ionization and energy loss distributions produced by relativistic 
charged particles in gaseous detectors show clear disagreement with 
experimental data, especially for gas mixtures which include xenon. \\

\noindent
Therefore a special investigation was performed \cite{sandia.grich} by 
fitting the coefficients $a_{k}^{(i)}$ to modern data from synchrotron 
radiation experiments in the energy range of $ 10 - 50 \ eV$.  The fits
were performed for elements typically used in detector gas mixtures:
hydrogen, fluorine, carbon, nitrogen and oxygen.  Parameters for these 
elements were extracted from data on molecular gases such as $N_2$, $O_2$, 
$CO_2$, $CH_4$, and $CF_4$ \cite{sandia.lee73, sandia.lee77}.  Parameters 
for the noble gases were found using data given in the tables 
\cite{sandia.marr, sandia.west}.

\subsection{Status of  this document}
18.11.98 created by V. Grichine \\
10.05.02 re-written by D.H. Wright \\

\begin{latexonly}

\begin{thebibliography}{99}
\bibitem{sandia.bigg} Biggs F., and Lighthill R.,
{Preprint Sandia Laboratory, SAND 87-0070} (1990)
\bibitem{sandia.grich} Grichine V.M., Kostin A.P., Kotelnikov S.K. et al.,
{Bulletin of the Lebedev Institute no. 2-3, 34} (1994).
\bibitem{sandia.lee73} Lee L.C. et al.,
{J.Q.S.R.T., v. 13, p. 1023} (1973).
\bibitem{sandia.lee77} Lee L.C. et al.,
{Journ. of Chem. Phys., v. 67, p. 1237} (1977).
\bibitem{sandia.marr} G.V. Marr and J.B. West,
{Atom. Data Nucl. Data Tabl., v. 18, p. 497} (1976).
\bibitem{sandia.west} J.B. West and J. Morton,
{Atom. Data Nucl. Data Tabl., v. 30, p. 253} (1980).
\end{thebibliography}

\end{latexonly}

\begin{htmlonly}

\subsection{Bibliography}

\begin{enumerate}
\item Biggs F., and Lighthill R.,
{Preprint Sandia Laboratory, SAND 87-0070} (1990)
\item Grichine V.M., Kostin A.P., Kotelnikov S.K. et al.,
{Bulletin of the Lebedev Institute no. 2-3, 34} (1994).
\item Lee L.C. et al.,
{J.Q.S.R.T., v. 13, p. 1023} (1973).
\item Lee L.C. et al.,
{Journ. of Chem. Phys., v. 67, p. 1237} (1977).
\item G.V. Marr and J.B. West,
{Atom. Data Nucl. Data Tabl., v. 18, p. 497} (1976).
\item J.B. West and J. Morton,
{Atom. Data Nucl. Data Tabl., v. 30, p. 253} (1980).
\end{enumerate}

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