sPHENIX HCal discussion

[pinkenburg <pinkenburg AT bnl.gov>]

    Dear Xiaochun et al.
    
    point taken - in this case the material should be implemented in
    PHG4Reco::DefineMaterials(). Then it's available when the hcals are
    build. But don't use a G4_XX name - I would like to keep those
    reserved for actual G4 defined materials. Uniplast (or
    Uniplast_scintillator) should be just fine.
    
    Who will do this?
    
    Thanks
    Chris

On 9/21/2022 11:55 AM, Xiaochun He wrote:

        Dear Chris and all,
      
        
      
      
        My preference is the following, if we decide to make changes to
        the default parameter:

1. Define new material, maybe called it as G4_POLYSTYRENE_Uniplast and update the constants.
2. This new material can then be referenced in gdml files.

Changing optical parameters in a gdml file is a tricking business to me which may require tedious tests and ultimately further delay our work. Do we know which subsystems use this material type? 

Best,

Xiaochun

Xiaochun He

Regents Professor

Department of Physics & Astronomy

Georgia State University

Atlanta, GA 30303

Email: xhe AT gsu.edu

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From: sPHENIX-HCal-l <sphenix-hcal-l-bounces AT lists.bnl.gov> on behalf of pinkenburg via sPHENIX-HCal-l <sphenix-hcal-l AT lists.bnl.gov>
Sent: Wednesday, September 21, 2022 10:05 AM
To: sPHENIX-HCal-l <sphenix-hcal-l AT lists.bnl.gov>
Subject: [Sphenix-hcal-l] Birks constant for G4_POLYSTYRENE

 

Hi folks,

let's put this discussion here to sort out how to move forward.

Here is the composition of the uniplast scintillators we use for the hcal:

1. Polystyrene     98.46%
2. Paratephenil    1.5%
3. POPOP            0.04%

based on this Polysytrene as material is good enough. G4 has polystyrene among its predefined materials, G4_POLYSTYRENE should be the way to go (and the gdml files for the hcals have been changed accordingly). It is one of the very few materials where the Birks constant is defined with a value of 0.07943mm/MeV. based on M.Hirschberg et al., IEEE Trans. Nuc. Sci. 39 (1992) 511

In our hcal test beam analysis we found that a Birks constant of 0.2mm/MeV best describes the data. There is nothing wrong with this - Makoto Asai, one of the G4 gurus told me that the Birks constant is a bit of a free parameter one can use to adjust simulations to data.

As you can see there are also dependencies on the physics list (and with that on the G4 version) which is different now than what was used here (FTFP_BERT instead of QGSP_BERT_HP) but we have known this for years and I don't see anyone spending time to get into this now (and we are about 7 months behind schedule when it comes to the sims).

We can change the Birks constant for G4_POLYSTYRENE easily but the problem is that this is a global change which will affect everything using G4_POLYSTYRENE. Just grepping I see that G4_POLYSTYRENE is used in the zdc and the emc test beam sim (not sure about the cemc, that's kind of opaque to me). Since the Birks constant is used to adjust the sims to test beam data our value is likely very specific to the outer hcal - I wouldn't even be sure it applies to the inner hcal which is now Al, not steel as it was in the test beam. But certainly for other detectors it'll be off.

My proposal would be to introduce some hcal polystyrene as a new material and set the Birks constant for this material and leave the G4_POLYSTYRENE as is. This looks to me like the safest option so we do not confuse ourselves when using G4_POLYSTYRENE somewhere else. This would mean changing the material in the gdml files and define it there (or add it to PHG4Reco::DefineMaterials()) and then add the Birks constant where it is currently set as part of the hcal detector construction (I am not sure if this can be added to the material definition in PHG4Reco::DefineMaterials(), there is a funny initialization order in G4 which might prevent this)

Chris

-- 
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Christopher H. Pinkenburg	;    pinkenburg AT bnl.gov
				;    http://www.phenix.bnl.gov/~pinkenbu

Brookhaven National Laboratory	;    phone: (631) 344-5692
Physics Department Bldg 510 C	;    fax:   (631) 344-3253
Upton, NY 11973-5000

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-- 
*************************************************************

Christopher H. Pinkenburg	;    pinkenburg AT bnl.gov
				;    http://www.phenix.bnl.gov/~pinkenbu

Brookhaven National Laboratory	;    phone: (631) 344-5692
Physics Department Bldg 510 C	;    fax:   (631) 344-3253
Upton, NY 11973-5000

*************************************************************