Coming back to fElectricField units at TRestDetectorGas and TRestDetectorDriftVolume

RESTFramework RestDetectorLib
1

Hi @nkx,

I have a bit of confusion again with the units used at TRestDetectorDriftVolume and TRestDetectorGas.

Obviously, TRestDetectorDriftVolume is using standard REST units, i.e. V/mm. So, when I do pointer->SetElectricField(x) x is in V/mm and therefore it is directly assigned to fElectricField. The TRestDetectorDriftVolume::fElectricField is measured then in V/mm.

So, if I want to get the drift velocity at 10bar and 100V/cm, I believe I should do:

gas->SetPressure(10);
gas->SetElectricField(1000);

since I assume I must give the field in V/mm. Is this right?

If I look at the code, it is a bit misleading if the result will be returned in mm/us or cm/us. From the code I believe the result will be returned in cm/us, although the comment says //in standard unit mm/us.

Screenshot 2021-03-04 at 21.02.24
Screenshot 2021-03-04 at 21.02.242314×274 71.8 KB

If I use the standard PandaX-III gas mixture that I know well the result, and test this code:

TRestDetectorGas *gg = new TRestDetectorGas("server", "Xenon-TMA 1Pct 0.1-10E3Vcm");
gg->SetPressure(10);
gg->SetElectricField(1000);
gg->GetDriftVelocity()
(double) 11.425579

I believe I do not get the right result. But I do if I set

gg->SetElectricField(100);
root [10] gg->GetDriftVelocity()
(double) 1.8736602

That is, as if I would give the argument in V/cm and returned value in cm/us. But from the code I deduce the argument of SetElectricField should be in V/mm.

Where I am getting it wrong?

2

It’s right.

The code is also correct. TRestDetectorGas::GetDriftVelocity() returns in standard unit mm/us.

You should SetElectricField(100) to set 100V/mm, which is 1kV/cm as in PandaX-III gas mixture. If you don’t look into the code, it will be simple to comprehend that we always input a number in REST unit, and get the returned result in REST unit.

Indeed the code to multiply and divide the REST unit object is confusing: multiply means convert the value with REST unit to this unit. For example, 1*units("V/cm") = 10, which means 1V/mm = 10V/cm. On the other hand, divide means to strip the custom unit and make it become REST unit. 0.18/units("cm/us") = 1.8, which means 0.18cm/us is converted to 1.8mm/us in REST units.

3

Ok, then in PandaX-III 1kV/cm at 10bar is 1.87mm/us.

4

Where are the standard REST units defined? I’m sometimes also confused with units, so knowing this would be very helpful.

5

They are defined at the TRestSystemOfUnits header.

The default unit is the one that implements a conversion factor of 1. I.e. for distance it is mm because I read 1 at the third argument at the following line:

AddUnit(mm, REST_Units::Distance, 1.);
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