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University of Southern California

Rare Gas Ionization Cell
The rare gas ionization cell provides the absolute integral solar flux in the ionization region of the working gas (lambda <= 575A for Neon). The cell is operated in the optically thick mode. The neon gas is periodically introduced and exhausted through an open (windowless) aperture in order to avoid any time dependent sensitivity changes associated with an accumulation of contaminant gas and/or time dependent window transmission. The absolute flux is proportional to the extrapolated current at zero density. Since the quantum yield is one for this instrument, and since no window is utilized, the rare gas cell used is a radiometric absolute detector. A similar cell (double ionization cell) was used by the National Institute of Standards and Technology (NIST) as the standard reference detector for VUV radiation prior to the recently adopted stadard of the SURF II electron synchotron storage ring.
The electron-ion pairs formed in the gas cell by the absorption of ionizing EUV photons are collected and the ion current measured using a highly stable electrometer. Knowledge of the absolute gas density is not required since the cell is optically thick in the EUV. Aside from intrinsic interest, the absolute integral flux obtained is used to normalize the spectral data obtained by a solar EUV spectrometer.
The rare gas cell is appx. 130 cm. in length and completely absorbs all photons shortward of the ionization limit of the working gas. The diameter of the cell is 5 cm. The length and diameter of the cell were selected to provide an optically thick cell for EUV photons, and optically thin cell for the secondary electrons produced in the photoionization process. The power consumption amounts to appx. 5 watts during continuous operation with a 12% duty cycle (28 V @ 1.5A peak power required to open the gas valve). The gas consumption rate is appx. 10e-5 moles/s during operation.


    website last updated 5/31/2007