It is important to obtain reliable data regarding W-values and photoabsorption cross sections of rare gas atoms for clarification of fundamental interaction between photon and material. The W-value is defined as average energy required to produce an electron-ion pair in a gas system by ionizing radiation. However, there are limited number of data reported about these interaction constants in the soft X-ray region. In this report we present precise data on W-values and photoabsorption cross sections of rare gases in the sub-keV X-ray region using a multi-electrode ion chamber [1 - 4].
Synchrotron radiation from the TERAS electron storage ring in AIST was dispersed using a Grasshopper monochromator. Thin filters and low energy operation of the ring were used for improving spectral purity. Monochromatic soft X-rays entered the ion chamber, which includes 4 electrodes with different lengths for collecting photoions. The photoion-current (i) collected with one of the electrodes from the 1st order light is given by the following equation.
i = enIexp(- dpσ)(1- exp(- Lpσ)). (1)
In this equation, I, e, n, d, L, p, and σ denotes the incident photon intensity, the elementary charge, the number of electrons totally produced from a photon in the chamber, the length of insensitive region in front of the electrode, the electrode length, the gas density, and photoabsorption cross section, respectively. We have obtained the photoabsorption cross section by fitting Eq.(1) to the photocurrents of the 4 electrodes measured with several gas densities, which include contributions from the first order light, stray and higher orders.
In obtaining the W-values, these photoion currents were measured as a function of the gas density. The photon W-value, W_p, is given with
W_p=E_p/N (2)
In this equation, N denotes the number of electrons totally produced in the sufficiently voluminous gas system and E_p is the photon energy. The number of electrons at a gas density is the summation of the initial ionization and the secondary ionization effects in the chamber.
n(p)=γ+δ(p) (3)
In eq.(3), γ denotes the number of electrons ejected from the atom absorbing a photon, and δ is the number of electrons secondarily produced through the collision between ambient atoms and emitted electrons. Photon W-values have been determined from ion currents at sufficiently low and high gas densities in the ion chamber.
The present results are being discussed at the conference.

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