Photoabsorption Cross Section at Low Energies

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 7.9), it is convenient to represent the cross section as a polynomial in $\omega^{-1}$ [1] :

$$\sigma_{\gamma}(\omega) = \sum_{k=1}^{4}a_{k}^{(i)}\omega^{-k} .$$ (7.74)

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.

Therefore a special investigation was performed [2] 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 $CF_4$ [3,4]. Parameters for the noble gases were found using data given in the tables [5,6].

Status of this document

18.11.98 created by V. Grichine
10.05.02 re-written by D.H. Wright

Bibliography

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