al odian's drift chamber gas information

7/28/2019 Al Odian's Drift Chamber Gas Information

1/3

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 11.55 ev and one at 11.72 ev.

A substantial fraction of the energy lost by a particle crossing the gas goes into creating metastableexcitations 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: 11.55 ev and 11.72 evIonization potential of: CO2 13.77 ev: CH4(methane) 12.6 ev: C2H6(ethane) 11.5 ev: C3H8 (propane)

11.1 ev:C4H10(isobutane): Di Methyl Ether(DME) 10.025 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 andshort 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


2/3

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 potentiallower than that of the metastable energy states of Argon. The

reason for this is that Argon has two metastable levels, one at

11.55 ev and one at 11.72 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 astraw 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.


3/3

Now the numbers: Metastable energies of Argon: 11.55 ev and 11.72

ev Ionization potential of: CO2 13.77 ev: CH4(methane) 12.6 ev:

C2H6(ethane) 11.5 ev: C3H8 (propane) 11.1 ev:C4H10(isobutane): Di

Methyl Ether(DME) 10.025 ev. From this, one sees that CO2 and CH4are 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 problemof 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

al odian's drift chamber gas information

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