WAVELENGTH OF SCINTILLATION LIGHT FROM XENON-DOPED LIQUID ARGON AND ITS APPLICATION TO PHOTOIONIZATION DETECTORS

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Ionization yield for alpha particles in liquid Ar at 10kV/cm is about 10% of the initially prodeced charge. This is due to recombination of ions and electrons in alpha tracks where ionization density is large. If photoionization molecules are doped in liquid Ar, they will be ionized by scintillation photon from recombination and the ionization yield will increase. The quantum efficiencies of photoionization are 60% for allene and 20% for trimethylamine. The similar effect is seen in liquid Xe. The quantum efficiencies of trimethylamine and triethyl-amine are 80% in liquid Xe. To investigate the reason for this difference in the quantum efficiencies in liquid Ar and liquid Xe, we have considered the relation between the wavelength of scintillation light and the ionization potential of molecules doped in the liquids, especially the excess energy.The wavelength of sintillation photon in liquid Ar is 130nm and that in liquid Xe is 175nm. When Xe is doped in liquid argon, the wavelength shifts form 130nm to 175nm for the Xe concentration of 10 to 200ppm. Based on this fact, we have measured the quantum efficiencies of the photoionization and studied the relation to the wavelength of the scintillation photon. As a result, the quantum efficiency of trimethylamine in Xe-doped liquid Ar decreases with increase of the Xe concentration, that is, with decrease of the excess energy. Therefore, the difference in quantum efficiencies in liquid Ar and that in liquid Xe cannot be explained by the excess energy. Instead, the cage effect is the most reliable candidate for the reason.