Geometry of the MPS wire chambers

Data structures

The geometry of the MPS wire chambers (beam PWCs and hodoscopes, TPX1..3, DM1..7, TDX4) is described by the space_t structure defined in mpsX.h.

The space_t record contains following fields:

ncmax --- (unknown)
ntot --- number of chambers in the chamber array
nact --- (unknown)
nr --- number of regions
region[] --- array of region_t recordsd describing the chamber regions
chamber[] --- array of chamber_t records describing each chamber

All the chambers are subdivided into regions-

region 0 --- upstream area- the beam PWCs and hodoscopes
region 1 --- inside the magnet- TPXes and drift chambers
region 2-9 --- not used by experiment E852

Each region is described by a region_t record:

number --- number of planes in the region
begin --- index of the first chamber on the chamber_t array

The information about each chamber is stored in chamber_t records. Most useful fields of that record are described here, for full information, see the include file.

idtype --- type of the chamber, consists of a 3-digit number: XXYY, where:
- XX --- is the chamber type
- YY --- is the measurement type: XMEASURE, YMEASURE, UMEASURE, VMEASURE and PHIMEASURE
For example, chamber 401 is an X-measureing beam PWC.
ofset --- chamber offset (position) (see below for more info)
dcos --- unit vector in the direction perpendicular to the wires
unv --- unit vector normal to the chamber plane
cell --- wire spacing
driftv --- drift velocity (for drift chambers only. Undefined for PWCs)
ncells --- number of wires
dnorm --- distance for the chamber plane to the point (0,0,0)
aname --- chamber name
*nhits --- number of hits on the chamber
(*hits)[] --- array of hits (Filled by the FillCoors subroutine)
misc->efficiency --- chamber efficiency
misc->t0 --- drift parameters
misc->d0 --- drift parameters
misc->nexclude --- number of disabled wires
misc->exclude[] --- list of disabled wires

Loading the chamber parameters

Use the following code sequence to load the chamber parameters:


	#include <param.h>
	#include <mpsX.h>
	#include <space.h>

	space_t space; // the space structure to be loaded
	int runNo;     // run number

	int ret;       // error code

	// Load the database file

	ret = param_loadDatabase("space",runNo);

	// load the chamber parameters

	ret = space_getParameters(&space);

Loading the chamber hits

Use the following code sequence to load chamber hits from an event into the space_t structure (chamber[]->hits[]):


	#include <param.h>
	#include <mpsX.h>
	#include <space.h>

	space_t space; // the space structure to be loaded
	coors_t coors; // the hits buffer

	int ret;       // error code

	itape_header_t *event;  // the event


	// Fill the chamber hits from the event

	InitCoors(&space,&coors);
	FillCoors(event,&space,&coors);

Chamber geometry

The section decribes the 3-D chamber geometry.

To define a plane in a 3 dimentional space, it is sufficient to specify it's normal vector (unv) and the distance from the plane to the origin of coordinates (dnorm). To define the positions of the cells (wires) on the plane, it is sufficient to specify either direction of the wires or a vector orthogonal to the wires (dcos).

How to use the drift information

Assuming:

chm --- the chamber structure
iwire --- wire number (ocunting from 1)
tdc --- drift as measured by a TDC

The drift distance shall be calculated as:



	chamber_t chm;   // chamber record
	int iwire;       // wire number (counting from 1)
	int tdc;         // drift time (tdc)

	double dd;       // drift distance

	dd = chm.d0 + chm.driftv * (tdc - chm.misc->t0[iwire-1]);

Notes: the tdc value can be positive and negative, the drift velocity (driftv) is positive for the MPS drift chamber and negative for TDX4.

CO 1995-June-12