GEOM110 Positioning a volume inside its mother

CALL GSPOS (CHNAME,NR,CHMOTH,X,Y,Z,IROT,CHONLY) Places a copy of a volume previously defined by a call to GSVOLU inside its mother volume CHMOTH.

CHNAME

( CHARACTER*4) name of the volume being positioned;

NR

( INTEGER) copy number of the volume CHNAME being positioned;

CHMOTH

( CHARACTER*4) name of the volume in which copy NR of CHNAME is positioned;

X

( REAL) x position of the volume in the mother reference system;

Y

( REAL) y position of the volume in the mother reference system;

Z

( REAL) z position of the volume in the mother reference system;

IROT

( INTEGER) rotation matrix number describing the orientation of the volume relative to the coordinate system of the mother;

CHONLY

( CHARACTER*4) flag to indicate whether a point found to be in this volume may also be in other volumes which are not direct descendants of it -- possible values are ONLY and MANY.

The following points are important for a correct use of GSPOS:

• the position and rotation with which a volume is positioned are relative to the mother reference system, that is to the reference system of the volume which contains the copy being positioned. To see how the reference system is defined for the GEANT shapes, see [GEOM050];
• GSPOS can be called several times with the same name, to place multiple copies of the same volume either in the same mother volume or in different ones. The copy number parameter NR is user defined, and it allows the different copies of the same volume to be distinguished. The user is free to assign any valid integer to this parameter. If two volumes are positioned with the same copy number, it will be impossible to decide in which one a particle is during tracking. If the same volume is positioned twice inside the same mother with the same copy number, the parameters of the second call to GSPOS will override the first ones for that copy;
• all the copies of a volume are identical: in particular they contain all the same daughters recursively -- conversely, positioning a volume inside a mother will insert it in all the copies of that mother;
• the volumes mentioned in the call must be already defined by a call to GSVOLU;
• no volume can be positioned in a divided mother;
• in case of positioning without rotation, IROT should be set to 0 -- note that this is faster than defining a unit matrix;
• the first volume, containing all the apparatus, cannot be positioned;
• if the CHONLY flag is equal to ONLY, the volume being positioned should not overlap with any other volume except his mother and his daughters and should not extend beyond the limits of its mother. In this case the search routines assume that, when a point is found in this volume, the only further searching required is in its contents. Other volumes at the same or higher levels or in separate branches will not be looked at. The first volume must be ONLY;
• if the CHONLY flag is set to MANY, a point found in this volume can also be in another volume at the same or higher level or in a different branch of the tree. To decide where a point is the following algorithm is applied:
  1. the tree is scanned upward until an ONLY mother with positioned contents (i.e. non-divided) is found;
  2. all the branches of the tree descending from this volume are scanned to find other volumes where the point may be;
  3. a winner is selected among the candidates according to the following rules:
     • if one of the candidates is ONLY, the point is declared in this volume. If more than one candidate is ONLY there is an error in the geometry definition;
     • amongst several MANY candidates, the one at the deepest level is chosen;
     • amongst several MANY candidates at the same level, the first one found is chosen;
  Although a considerable effort has been put in making sure that the MANY option works properly and caters for all the possible cases, still it introduces a considerable complication in the tracking and performance can be rather poor.

  Users should resort to this only when it is not possible to describe the experimental setup with ONLY volumes.

The data structure is so designed that, in any case, the parameters of a volume and the information about the number and position of its daughters are stored only once. This allows the creation of very complex geometries without correspondingly large storage requirements.

GEOM120