[Halld-offline] [EXTERNAL] Fwd: [New Logentry] changed HDDS geometry in ccdb to allow solenoid B-field inside BCAL, DIRC volumes

[Richard Jones]

changed HDDS geometry in ccdb to allow solenoid B-field inside BCAL, DIRC
volumes <https://logbooks.jlab.org/entry/3813727>

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Lognumber 3813727 <https://logbooks.jlab.org/entry/3813727>. Submitted by
jonesrt <https://logbooks.jlab.org/user/jonesrt> on Wed, 07/08/2020 - 21:23
<https://logbooks.jlab.org/entries?start_date=1594254191&end_date=1594261391&book=HDLOG>.

Logbooks: HDLOG <https://logbooks.jlab.org/book/hdlog>

I have made an update to ccdb that allows the magnetic field from the
solenoid to penetrate inside the volumes occupied by the BCAL and DIRC
detectors in the simulation geometry. Up until today, the interior of the
BCAL modules and the entire DIRC container volume were assigned to a null
magnetic field. This was originally motivated by the cost of tracking of
shower particles in a magnetic field in Geant3 (hdgeant) coupled with the
belief that it should not make a significant difference in electromagnetic
response. Things have changed.

   1. hdgeant4 has mostly replaced hdgeant as the simulation engine
   2. Geant4 does not show significant speed-up when simulating showers in
   the BCAL with the magnetic field disabled inside the lead sci-fi material
   3. We are interested in other things beside shower propagation in the
   BCAL, eg. muons from J/psi decays, punch-through particles like
   high-momentum pions, etc.
   4. The discontinuities in the magnetic field at the boundaries of the
   BCAL were causing problems in the tracking code of hdgeant4
   5. We should not be making uncontrolled approximations for the sake of
   minor gains in computation speed

This change was made to the master ccdb at 21:00 today.

-Richard Jones
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