Correlated Fluctuations between Luminescence and Ionization in Liquid Xenon

Abstract

(EXO Collaboration)The ionization of liquefied noble gases by radiation is known to be accompanied by fluctuations much larger than predicted by Poisson statistics. We have studied the fluctuations of both scintillation and ionization in liquid xenon and have measured, for the first time, a strong anti-correlation between the two at a microscopic level, with coefficient −0.80 < ρep < −0.60. This provides direct experimental evidence that electron-ion recombination is partially responsible for the anomalously large fluctuations and at the same time allows substantial improvement of calorimetric energy resolution.The measurement of ionizing radiation in a liquiefied noble gas 1 such as Ar, Kr, or Xe can be characterized by two parameters: W e , the mean energy required to create a free electron-ion pair and the Fano factor, F e , which parameterizes the fluctuations in the number of ion pairs.2 The Fano factor is defined bywhere σ 2 e is the variance of the charge expressed in units of the electron charge, e, and N e is the number of ion pairs. F e = 1 corresponds to Poisson statistics. Fano originally predicted that the charge fluctuations would be sub-Poissonian, F e < 1, because the individual ion-pair creation processes are not independent once the additional constraint E = N e W e is included, where E is the energy deposited by the incident particle. The Fano factor for gaseous noble elements is relatively well understood, where for example the theory for argon predicts F e = 0.16 while experiment yields F e = 0.20. 3The Fano factor for the liquid phase has been far more difficult to understand, even though the values of W e are similar to those of the gas. The pioneering theoretical work of Doke 4 in 1976 predicted F e ≈ 0.05 for liquid Xe (LXe). Experiments, however, showed F e > 20, that is, a variance 20 times worse than Poissonian and 400 times worse than predicted by Fano's original argument. This discrepancy has not only raised important issues for the understanding of the phenomena of energy loss and conversion, but it is also of interest for experimental physics since this large Fano factor limits the resolution of calorimeters used in nuclear and particle physics.Luminescence light provides a second process, complementary to ionization, with which to study energy loss and conversion phenomena. The properties of luminescence light emitted by ionizing radiation (scintillation) in liquid Ar, Kr, and Xe detectors have been extensively studied and have been exploited in calorimetry 1,5 . As described elsewhere 6 , scintillation photons are produced by the relaxation of a xenon excimer, Xe * 2 → 2Xe + γ. The excimer is formed in two ways; 1) direct excitation by an energetic particle, 2) recombination of an electron with a xenon molecular ion, Xe7 . The electron-ion recombination rate depends upon the ionization density and the applied electric field. The quantities W p and F p can be defined in analogy with the ionization case, where W p is the mean energy absorbed per emitted photon and F p parameterize...