[Halld-offline] [EXTERNAL] proposed change to ccdb GEOMETRY tables

[Richard Jones]

Colleagues,

As a side-effect of recent studies of kaons in hdgeant4, I came across an
oddity in our materials tables that we use for simulation. Both hdgeant and
hdgeant4 use the same Materials_HDDS.xml file for this. Therein you will
find an entry for Vacuum that is characterized as an element with Z=1,
density=1e-12. Although this seemed to make sense at the time, in
retrospect it is a mistake.  Some physics libraries keep just one entry for
each element, so this forces a choice for Z=1, to use the Vacuum
description or Hydrogen (liquid density). In recent tests with FLUKA
(hdgeant with HADR 4) the code that propagates low-energy neutrons in
through the detector noticed this inconsistency and threw a fatal error,
but there may be other portions of the library that fail silently.

I propose the change that Vacuum be described as a mixture (doesn't matter
what, but I chose a 50-50 mixture of nitrogen and helium) so it does not
collide with hydrogen. This is a change to a small section in
Material_HDDS.xml which is recorded on a run-dependent basis in ccdb. Here
is the full history of that record in ccdb.

/> ls /GEOMETRY/Material_HDDS.xml
/GEOMETRY/Material_HDDS.xml
(ID)   (Created)              (Modified)              (variation)     (run
range)      (comments)
 227238  2019-10-12 17-44-23    2019-10-12 17-44-23     mc_generic
 30000L-inf      fix garbage in hdr
 201973  2019-03-27 21-24-59    2019-03-27 21-24-59     default
60921L-69999L   PrimEx Phase  I
 186382  2019-01-23 15-29-13    2019-01-23 15-29-13     default
60000L-inf      introducing the DIRC
 186367  2019-01-23 09-49-05    2019-01-23 09-49-05     mc_generic
 30000L-inf      eliminate overlaps
 132189  2018-06-01 09-33-19    2018-06-01 09-33-19     default
9999L-9999L     Copied from HDDS, v3
 93740  2017-11-20 17-02-24    2017-11-20 17-02-24     default
10000L-29999L   Copied from HDDS, v3
 93716  2017-11-20 16-58-38    2017-11-20 16-58-38     default
30000L-inf      Copied from HDDS, v3
 93692  2017-11-20 16-56-28    2017-11-20 16-56-28     mc_generic
 0L-inf          Copied from HDDS, v3
 79710  2017-06-07 18-29-21    2017-06-07 18-29-21     mc_generic
 0L-inf          Copied from HDDS, v3
 78214  2017-06-01 08-48-01    2017-06-01 08-48-01     default
30000L-39999L   Fall 2016 survey
 40960  2017-03-16 18-38-23    2017-03-16 18-38-23     default
10000L-39999L   Copied from HDDS

I propose updates to mc_generic (30000-inf), and default (30000-inf,
30000-39999, and 60000-inf), each one customized so that only the Vacuum
section is touched. There is no "mc" variation in ccdb for this table, so
it falls over to default. Sasha can decide if he wants this update
propagated to default (60921-69999) for the Primex run period.

-Richard Jones
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