I saw in an article that when a boron-lined proportional counter is used at high voltage for a period of time, its internal CO2 will be rapidly consumed as a quencher, resulting in a rapid decrease in output pulse amplitude and causing the counter tube to fail.
Therefore, I want to simulate the changes in pulse waveform caused by CO2 consumption using Garfield++. I speculate that the reduction of CO2 not only has a certain impact on the gas amplification factor (which can be simulated by generating Gasfiles), but also leads to the occurrence of space charge effects near the anode wire (due to the lack of CO2 quenching, Ar
deexcitation produces a large number of photons, causing photoelectric effects).
My question is, if I want to consider the effect of space charge on the electric field near the anode wire, is there any example or method that can provide a reference?
I would propose you dig more into literature because I would not trust such bold statements by looking at a single article only. How is this CO2 consumed exactly? I am quite puzzled how this article passed the scrutiny of the referees.
Space-charge effects are being included in garfield, but this is a long process and need much more development. For simulating space-charge effects I would consider you try to implement a hydrodynamic simulation in software such as comsol.
The article only states that CO2 acts as a quencher to absorb photons and dissociate. And after adding a new sensitive gas to the counter after receiving a high neutron injection, its plateau curve returned to normal.
By reducing the content of CO2 in the Ar/CO2 mixture and lowering the total pressure, I generated a new Gasfile and conducted a particle injection simulation. I observed that the amplitude of the pulse actually increased a lot.
The article is very imprecise, it does not show a figure of the detector, does not discuss whether this detector is operated in sealed mode or has a constant flush of gas (i guess sealed) and it does not give a reference for more details on either the detector or the mode of operation.
It is known that if you increase the Ar content and reduce CO2 that you have higher signals. it is also known that if you reduce the pressure you increase the mean free path and hence electron energy, and also this results in higher gain, so higher pulse height.
So CO2 dissociates after absorbing a UV photon with an energy of 5-7eV, so this means that one of the double C-O bounds is broken. So instead of 1 molecule of CO2 you now have O and one CO. The total pressure remains the same. If you want to see how less quenching this situation is, you have to make a new gasmix with Ar,CO2,CO and O. I would gues that CO can still absorb one more time a UV photon to break the last C-O double bound. Single oxygen is unfortunately not available in magboltz, and I do not know how to handle this, but maybe you can treat this for now as a 2nd order effect?
Can you do a back of the envelope calculation? At 0.25 bar you have approximately 1.5 10E23 gas molecules, so that would be 2.25E22 CO2 molecules. (You can do the exact calculation in garfield++ with MediumGas::GetNumberDensity). So reducing by 10% CO2 means that you would “consume” or destroy 2.25E21 CO2 molecules. Do you expect so many photon absorptions with E>5.5eV (check number of excitations of Ar with E> 5.5eV) or do you expect so many electron attachments to CO2 that then lead up to dissociation (DEA)?
