Sec. 10.3] GEIGER-MV LLER COUNTERS 307 



results in the direct ejection of free electrons which may also continue the 

 discharge. A third mechanism observed by Ramsey [5] and others occurs 

 when the initial avalanche is small. Photons emitted by those ions in the 

 avalanche which capture an electron before reaching the cathode can eject 

 photoelectrons which then maintain the discharge in what is observed to be a 

 series of diminishing pulses. 



Discharges in tubes containing only mono- or diatomic gases, consequently, 

 will, with certain exceptions, continue so long as the anode potential is 

 maintained. Quenching is accomplished in such tubes only by some external 

 device such as a high resistance, usually of the order of magnitude of 10 9 

 ohms, or by an electronic circuit which reduces the anode potential below the 

 threshold voltage after the initial part of the pulse and until the positive 

 ions are collected. These counters have revolving times of several times 

 10 -4 sec when appropriate external electronic quenching circuits are used, 

 but they may be as long as 10 -2 sec with simple resistance quenching. The 

 minimum resolving time is limited by the positive ion transit time. 



10.3. Self-quenching Counters. The addition of polyatomic gases such 

 as methane, alcohol, and amyl acetate to counter tubes alters the process in a 

 way to quench the discharge following the Townsend avalanche without the 

 use of external quenching circuits, and such counter tubes are called self- 

 quenching or "fast" counters. The resolving times of such counters is 

 usually of the order of 10~ 4 sec. 



The principal function of the polyatomic quenching gas is to prevent 

 further production of electrons following the completion of the initial Town- 

 send avalanche. This is accomplished mainly by reducing photoemission and 

 secondary electron emission from the cathode walls [4]. 



The effectiveness of polyatomic gases in absorbing photons is due to the 

 diffuse vibration-rotation interaction absorption bands present in the ultra- 

 violet region of their spectrum. Radiation emitted by excited argon, 

 helium, and most other permanent gases lies between 1,020 and 790 angstroms 

 and can, therefore, be absorbed by such molecules as methane and alcohol, 

 which have continuous or band absorption spectra blanketing this region. 

 In addition to absorbing radiation, it is equally important that the quenching- 

 gas molecules do not reemit the absorbed photons but have, instead, a strong 

 tendency toward releasing the excitation energy by photodecomposition. 

 This appears to be valid for all polyatomic gases exhibiting quenching 

 properties. 



Secondary electron emission from the cathode is effectively reduced by 

 two mechanisms. (1) The monatomic and diatomic ions formed in the 

 initial ionization process are prevented from reaching the wall by the process 

 of electron exchange with neutral polyatomic molecules with which they 

 collide with a frequency of roughly 10 2 to 10 3 times per centimeter of path. 



