Abstracts of Poster Papers
A gridded ionization chamber has been used to study the maximum energy resolution achievable with liquid argon and liquid xenon. With its high density and atomic number, liquid xenon is a particularly interesting filling for a high resolution detector to be used in many different applications. Our specific interest is to develop a large volume high resolution imaging liquid xenon instrument for high energy gamma-ray astrophysics as well as to search for nuclear double beta decay of Xe-136 with much better sensitivity than existing instruments. The energy resolution of liquid argon or xenon ionization detectors is expected to be close to that achievable with Ge (Li) spectrometers, given the measured W values and the small Fano factors calculated by Doke. However, the best experimental results so far are nearly an order of magnitude worse than the theoretical values. In order to understand the reasons for this discrepancy, we have carried out several measurements with both liquids. The electric field dependence of conversion electrons has been measured up to 11 kV/cm with optimized grid geometry and liquid purity. In liquid argon, we obtain 2.7% fwhm for the resolution of the dominant 976 keV electron line in the Bi-207 spectrum. This value, the best reported so far in the literature, is still a factor of seven worse than the Fano limit. We find that our results can be explained if we take into account the additional statistical fluctuations associated with incomplete charge collection from delta-electron tracks produced in large number along the path of the primary ionizing particle. The strong recombination rate on these heavily ionizing delta-electrons is the limiting process to the ultimate energy resolution of noble liquid detectors, unless very high fields are used. Alternatively, one can increase the electron mobility.
KeywordsPulse Electric Field Sandia National Laboratory Breakdown Strength Liquid Argon Inertial Confinement Fusion
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