This work presents results on the study of mechanisms and performance of negative ion drift and gas gain in gas-filled radiation detectors with electronegative fill gases. Negative ions drift with slow drift velocity and the lowest possible (thermal limit) diffusion, which leads to relaxed requirements for readout electronics without sacrificing accuracy. Slow drift velocity and low diffusion are the bases for this work because these characteristics are highly desirable for space-based detectors and for low pressure Time Projection Chambers (TPCs) used to make direction sensitive searches for Galactic Dark Matter. The present work led to two major successes: (1) Nitromethane (CH3NO2) was discovered as a new electronegative fill gas for TPCs. Nitromethane anions drift even slower (mobility = 0.032 m2·T/V ·s) than the only other known capture agent, carbon disulfide (CS2, mobility = 0.036 m2·T/V ·s), and the measured longitudinal diffusion remains at the thermodynamic (lower) limit for fields up to 7 V/cmT. Nitromethane is of particular interest for X-ray photoelectric polarimeters
in the 2-10 keV energy range because of its low atomic number (Z). (2) Using a Diethorn plot, the mechanism which initiates electron avalanches in electronegative gas mixtures was accounted for. The Diethorn plot parameter Emin, is the minimum field needed to start avalanche gain in a proportional counter. Electronegative gases were found to
have extremely large pressure-reduced starting fields (10-50 times larger
than electron gases) which are themselves approximately independent
of pressure. This can only be accounted for by a collisional ionization
mechanism leading to the release of electrons from the ions and subsequent development of gain through a normal Townsend avalanche.
The collision cross-section (σc) calculated from drift velocity data allows an estimate of Emin to be made which agrees with the experimental
findings (5 - 25 % difference).
The light output from Townsend avalanches has been proposed as an
alternative readout mechanism for TPCs. The light output from negative
ion avalanches was investigated using a Negative Ion Drift Chamber (NIDC)
with a Gaseous Electron Multiplier (GEM)-like device as the amplification
structure, viewed through a quartz window by a photomultiplier tube. This study allowed an upper limit of light output to be assigned to several negative
ion gas mixtures.
Finally, a Micromegas NIDC was assembled to test the performance of
negative ion mixtures in this gain device. The detector was stable and
operated at gas gains of ∼ 104
in an electron gas mixture (argon-isobutane).
However, with the two negative ion gases studied, CS2 and CH3NO2, high
voltage breakdown and microphonic noise developed before appreciable gain
was obtained. / Physics
| Identifer | oai:union.ndltd.org:TEMPLE/oai:scholarshare.temple.edu:20.500.12613/1106 |
| Date | January 2009 |
| Creators | Dion, Michael P. |
| Contributors | Martoff, Charles Jeffrey, Riseborough, Peter, Tao, R. (Rongjia), Meziani, Zein-Eddine, Strongin, Daniel R. |
| Publisher | Temple University. Libraries |
| Source Sets | Temple University |
| Language | English |
| Detected Language | English |
| Type | Thesis/Dissertation, Text |
| Format | 172 pages |
| Rights | IN COPYRIGHT- This Rights Statement can be used for an Item that is in copyright. Using this statement implies that the organization making this Item available has determined that the Item is in copyright and either is the rights-holder, has obtained permission from the rights-holder(s) to make their Work(s) available, or makes the Item available under an exception or limitation to copyright (including Fair Use) that entitles it to make the Item available., http://rightsstatements.org/vocab/InC/1.0/ |
| Relation | http://dx.doi.org/10.34944/dspace/1088, Theses and Dissertations |
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