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3 CREATING GEANT HITS

In AgSTAR user does not need to write a detector specific routine to create GEANT hit structure and to fill it with a useful information. Instead, a geant statements with the HITS operator, called in a block describing a sensitive volume, is used to produce a relevant GEANT hit definitions and to steer their filling at the tracking time. This statement generates all necessary GEANT calls (see GSDET and GSDETH routines, HIT 100) with their parameters as follows:

The set name is defined by the first 4 letters of the module name;
The detector name is the name of the geant block ;
Following the GSTAR standard, IDTYPE is taken as the STAR detector number;
The name argument of the HITS operator, (hit address) is the name of the volume used to identify the hit, usually the sensitive detector itself. The AgSTAR interface finds the path to the selected volume using generic names of all higher level volumes and builds the NAMESV array. It also defines the number of branchings and the number of bits required at all levels (NBITSV array) to uniquely describe the path to each instance of the selected volume;
For memory allocation defaults values of NHWI, NWDI = 1000 are used.

The definition part of the HITS operator contains a list of information quantities, measurements, which should be saved in each GEANT hit, and their packing in one of the form

For a measurement, or bin are mandatory, option and limits are optional. At present the following variables are known as measurements to the AgSTAR interface (the track point means here the middle point of the track segment producing the hit in the sensitive volume):

xx,yy,zz - track point coordinates in Master Reference System (MRS);
x,y,z - local cartesian coordinates of the track point;
- local cylindrical (or polar) coordinates of this point;
Cx, Cy, Cz - local direction cosines of the track segment;
Px,Py,Pz - components of the track momentum in MRS;
Ct - cosine of the angle between the track segment and the radius pointing to its center;
TDR - closest approach of the track segment to the local z-axis;
STEP - the length of track segment producing the hit;
SLENG - track length from the vertex;
ELOS - the energy lost at this step;
BIRK - registered energy loss, corrected by Birk's law (see PHYS 337);
TOF - time of flight for this hit;
ETOT - particle energy in the current point;
LGAM - of the particle Lorentz factor;
ETA - pseudorapidity of the track point.

An integer number, following a measurement variable, is interpreted as

- the number of bits for packing the variable values. 0 means that the value is a cumulative sum, occupying a full computer word. Due to the GEANT limitation 0 can be used only in last elements of the HITS statement.

If a measurement variable is followed by a real number or an expression, it is interpreted as the packing bin size, and the number of bits, required for packing, will be calculated by the AgSTAR interface.

If the user does not provide the limits explicitly, min and/or max are determined by the AgSTAR interface using the volume dimensions. This works fine in local coordinates, but should be used with caution in global MRS coordinated.

Example: HITS VRAD xx:16:H(-25,25) yy:16:(-25,25) zz:32:(-1000,1000),
px:16:(-100,100) py:16:(-100,100) pz:16:(-100,100),
Slen:16:(0,1.e4) Tof:16:(0,1.e-6) Step:16:(0,100),
SHTN:16: Eloss:32:(0,1)

Possible measurement options are listed below. If an option is not mentioned, the alternative default solution is used. Most of the options acts on the whole list of measurements, except for option R.

C - calorimeter type HITS, i.e. comulative values for all tracks are added together. By default comulative values are added for each KINE track separately.
H - interprete the rest of the non-comulative hit elements as measured values rather than pseudo-volumes. By default any non-comulative hit elements are concidered as pseudo-volumes, is stored in the volume part of GEANT hits and used to distinguish between defferent comulative channels
X - calculate local coordinates even if they are not required by the list of hit elements. They may be used in a user defined hit element.
S - Single step option, i.e. a single hit will be produced for several sequential steps made by the same particle in a sensitive volume, until the particle exits it or a STEMAX limit is reached. By default a separate his is produce for each step performed by GEANT.
R - bin rounding up to 2 decimal digits is done. Default is the exact value to fit the required bin or value.

3.1 Non-standard HIT elements

In a number of cases user may need in hits a specific value, unknown to the AgSTAR interface . There are two possibilities to provide such a value to AgSTAR interface :

to hang a rotine YYYYHIT(pointer,hit) to be executed for any hit referencing a YYYY measurement.

Example:

So called shower track number is calculated for a SHTN element in a GSTAR specific routine SHTNHIT. It is used in most of the STAR tracking detectors to provide in their hits the secondary particle (i.e produced by GEANT) track number (see the previous example of the VPD hits).

to hang a routine XXXXSTEP(pointer,hit) to be executed in a specific sensitive block XXXX using USER hit element. This is especially usefull when user wants to control and in certain cases to correct the normal values, calculated by the AgSTAR interface (i.e. instead of a linear track center approximation to use the parabolic one, like it is done in TPADSTEP routine of the TPCEGEO module).

These subroutines should be described as EXTERNAL in the module header. Their first integer input argument pointer is the address of the hit descriptor array (10 words, real) in the GCBANK memory and they should return in the second real argument hit the measurement.

Next: 4 DIGITISATION Up: Advanced Geometry Interface for Previous: 2 GEOMETRY DESCRIPTION

William J. Llope

Thu Aug 15 17:05:49 EDT 1996