Hi Dave, The reason I do not like binary gas combinations of Argon and CO2 or Argon CH4 in proportional chambers is that they are not Penning Mixtures of Argon. A Penning mixture of Argon has its companion, molecules with an ionization potential lower than that of the metastable energy states of Argon. The reason for this is that Argon has two metastable levels, one at ev and one at ev. A substantial fraction of the energy lost by a particle crossing the gas goes into creating metastable excitations of the Argon as well as ionization of the Argon. In a straw chamber, if the excited Argon cannot deexcite by colliding with its companion molecules, then they live long enough to diffuse to the outer wall where they deexcite by releasing an electron from the outer wall. A straw chamber is usually run at high gains for which single electrons produce large pulses. This means that the passage of a particle through the chamber is followed by a large number of after pulses, which, if the gain is high enough results in a continuous discharge. For lower gains, there are a large number of after pulses for each traversal of a particle. Now the numbers: Metastable energies of Argon: ev and ev Ionization potential of: CO ev: CH4(methane) 12.6 ev: C2H6(ethane) 11.5 ev: C3H8 (propane) 11.1 ev:C4H10(isobutane): Di Methyl Ether(DME) ev. From this, one sees that CO2 and CH4 are not Penning Mixtures with Argon, ethane is marginal, propane is good and isobutane is better. Even a little bit ~1 to 2 % of DME is very good, but there are two problems with DME. One is that quite often DME in a pressurized bottle is quite often not pure. Frequently it is used in the same bottles as are used for Freons. So you need to check the purity, and if not pure, purify it. Secondly, DME dissolves in plastic easily. If your straw chambers have walls of aluminized mylar, the DME will expand the length of the mylar, cause it to turn into a banana shape if the ends are held fixed. The walls will touch the wire and short out the chambers. If you use thin wall Aluminum instead of Aluminized Mylar for the walls, you are safer there, but have to solve the problem of supporting the wires with an insulating material that does not produce problems when surrounded by DME. You can use three component gasses with Argon to regulate the drift velocity and the diffusion characteristics as well as having a Penning mixture. Good Luck, Al

Hi Dave, The reason I do not like binary gas combinations of Argon and CO2 or Argon CH4 in proportional chambers is that they are not Penning Mixtures of Argon. A Penning mixture of Argon has its companion, molecules with an ionization potential lower than that of the metastable energy states of Argon. The reason for this is that Argon has two metastable levels, one at ev and one at ev. A substantial fraction of the energy lost by a particle crossing the gas goes into creating metastable excitations of the Argon as well as ionization of the Argon. In a straw chamber, if the excited Argon cannot deexcite by colliding with its companion molecules, then they live long enough to diffuse to the outer wall where they deexcite by releasing an electron from the outer wall. A straw chamber is usually run at high gains for which single electrons produce large pulses. This means that the passage of a particle through the chamber is followed by a large number of after pulses, which, if the gain is high enough results in a continuous discharge. For lower gains, there are a large number of after pulses for each traversal of a particle.

Now the numbers: Metastable energies of Argon: ev and ev Ionization potential of: CO ev: CH4(methane) 12.6 ev: C2H6(ethane) 11.5 ev: C3H8 (propane) 11.1 ev:C4H10(isobutane): Di Methyl Ether(DME) ev. From this, one sees that CO2 and CH4 are not Penning Mixtures with Argon, ethane is marginal, propane is good and isobutane is better. Even a little bit ~1 to 2 % of DME is very good, but there are two problems with DME. One is that quite often DME in a pressurized bottle is quite often not pure. Frequently it is used in the same bottles as are used for Freons. So you need to check the purity, and if not pure, purify it. Secondly, DME dissolves in plastic easily. If your straw chambers have walls of aluminized mylar, the DME will expand the length of the mylar, cause it to turn into a banana shape if the ends are held fixed. The walls will touch the wire and short out the chambers. If you use thin wall Aluminum instead of Aluminized Mylar for the walls, you are safer there, but have to solve the problem of supporting the wires with an insulating material that does not produce problems when surrounded by DME. You can use three component gasses with Argon to regulate the drift velocity and the diffusion characteristics as well as having a Penning mixture. Good Luck, Al