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

在10 kV/cm的液氩中，α粒子的电离产额约为初始产生电荷的10%。这是由于离子和电子在电离密度大的α轨道中的复合。在液态Ar中掺杂光电离分子，它们将被复合产生的闪烁光子电离，电离产额增加。光电离量子效率对丙二烯为60%，对三甲胺为20%。类似的效果也见于液体石蜡。在液相中，三乙胺和三乙胺的量子效率均为80%。为了探讨液体Ar和液体Ar中量子效率差异的原因，我们考虑了闪烁光的波长与掺杂在液体中的分子电离势，特别是与过剩能量的关系，液体Ar中闪烁光子的波长为130 nm，液体Ar中闪烁光子的波长为175 nm。在液氩中掺杂10 ppm ~ 200 ppm的Na_2SO_4时，波长从130 nm移动到175 nm。在此基础上，我们测量了光电离的量子效率，并研究了它与闪烁光子波长的关系。结果表明，随着掺杂浓度的增加，即过剩能量的减少，三甲胺在Xe掺杂液氩中的量子效率降低。因此，不能用过剩能量来解释液态Ar和液态Ar中量子效率的差异。相反，笼子效应是最可靠的候选原因。

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.