Other
Scientific paper
Dec 1952
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1952phrv...88..987c&link_type=abstract
Physical Review, vol. 88, Issue 5, pp. 987-998
Other
10
Scientific paper
The multiplication curves and the pulse shapes have been studied in a counter filled with pure argon and argon plus carbon dioxide mixtures at pressures ranging from 150 to 1000 mm Hg. The pulse shapes in argon in the high proportional zone and up to voltages corresponding to the corona threshold are explained by assuming a production in the Townsend avalanche of photons hard enough to extract electrons from the cathode. These photoelectrons are found to be produced during a time Tf0 of the order of a few μsec when at 150 mm Hg pressure, and during a smaller time when at higher pressures. The analysis of the features of the excited levels of argon suggests the four lower excited levels to be responsible for the photoelectric effect. These photons are produced in collisions leading to destruction of these levels, and are not resonance photons. The Tf0 values deduced from our measurements are in accordance with those obtained by extrapolating the Molnar values for the first metastable levels at the pressures used. This photoelectric process has been found to be the main one responsible for sustaining corona discharge, which starts at multiplication values of about 200. If carbon dioxide concentrations varying from 10-4 to 5×10-3 are added to argon, one obtains an increase of NS, the threshold multiplication, which is linear versus the carbon dioxide percentage. This fact is readily explained by assuming the quenching of argon excited levels by collisions of the second kind on carbon dioxide molecules. From the experimental results one may deduce a value of the quenching cross section which is about 600×10-16 cm2 in a first approximation. Further studies concern the space charge effect at the photoelectric process divergence threshold, which allows the formation of a very narrow Geiger zone at carbon dioxide concentrations over 5×10-4. No processes other than the photoelectric one have been found to be efficient in counters filled with argon plus carbon dioxide mixtures up to a concentration of 5×10-3. At higher percentages the photoelectric process is considerably reduced; there has been found to be effective in the discharge build-up a process by which electrons are released in the gas body. The counter behaves as a regular fast counter. The effect of mercury vapor in the discharge in pure argon is also described; it is accounted for by the well-known Penning process of mercury ionization by collision on excited argon atoms. This process causes the discharge to start at extremely low multiplication values.
Colli Laura
Facchini Ugo
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