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/*! @page  geometry The Detector Construction

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\section geo_list Description of the geometry
The \lemu detector consists of the last part of the spectrometer, from the trigger detector to the sample cryostat.

The top volume is the laboratory volume (World), which is a cubic air volume (2mx2mx4m). 
\image html det2.gif XZ projection view of the detector in the World volume. 
*
\subsection geo_submethod The contruction methods
For clarity reasons, the different parts of the detector are built by different methods. This allows a better control of the geometry and a better modularity. We also had to build different vacuum volumes because it is not possible to assign more than one field per volume. Hence, in LEMuSRDetectorConstruction.cc, on can see that the detector construction method is call other methods to construct different parts of the detector according to the following repartition:
* - <tt>lemuTrigger_Detector()</tt>: builds the carbon foil, the vacuum volume, the steel cylinder and flanges around it and three vacuum volumes to introduce the electric fields. \image html trigger.gif Trigger detector carbon foil, electric field regions and  vacuum chamber.
* - <tt>lemuCGate()</tt>: builds the compensation gate steel tube and the vacuum volume.
* - <tt>lemuLinse3()</tt>: builds the third lens, the vacuum volume and the tube around it. The electric field of the third lens is assigned to the vacuum volume. \im det_L3.gif Third lens and chamber.
* - <tt>lemuMCP2()</tt>: builds the gate valve tube and flanges, the cryostat chamber and flange and a vacuum volume for the gate valve and the sample cryostat region. Indeed, we have and overlap of the electric fields (ring anode, sample cryostat) and magnetic field (coils) in this region. For this volume we built a class for superimposing electric fields and a class for combining electric and magnetic fields. 
* - <tt>lemuANODE()</tt>: builds the ring anode.\im det_mcpv.gif Cryostat chamber and Gate Valve chamber.
* - <tt>lemuSCINT()</tt>: builds the inner and outer scintillators.
* - <tt>lemuMCPdet()</tt>: builds the MCP2 detector. \im det_mcp_scint.gif MCP2 detector and scintillators.
* - <tt>lemuCryo()</tt>: builds the sample cryostat. \im det_cryo.gif Sample Cryostat.
*.
The electromagnetic fields of the trigger and the third lense are build in their respective contruction methods. For the MCP2 region, the field is build by the method <tt>BuildAnodeField()</tt>, which will construct the electromagnetic field according to the user settings. In this region, it is possible to enable or disable the magnetic field, the electric field at the sample and the electric field of the ring anode. Two special classes, LEMuSRElFieldMix and LEMuSRElMagField, were specialy implemented in order to handle the superposition of two electric fields for the first one, and the superposition of an electric field and a magnetic field for the second one.

\subsection geo_pvolumes List of the geometry elements
Here is a list of the different physical volume and their specifications:
*- pv_LABO
*
* MCP and Gate Valve Chambers
*- pv_MCPV:
*- pv_MCPS:
*- pv_F100:
*- pv_F160:
*- pv_F200:
*- pv_GATV:
*- pv_GATS:
*
* RING ANODE
*- pv_RA_E
*- pv_RA_M
*- pv_RA_G
*- pv_RAV
*
* MCP2 Detector
*- pv_DMPC
*- pv_MCPA
*- pv_MCPM
*- pv_MCPM2
*- pv_ANVA
*- pv_MCSR
*- pv_MCSR
*- pv_MCVR
*- pv_MCSS
*
* CRYOSTAT
*- pv_SAH1
*- pv_SAH2
*- pv_SAH3
*- pv_SAPH
*- pv_COFI
*- pv_CRY1
*- pv_CRY2
*- pv_CRY3
*- pv_CRY4
*- pv_CRSH
*- pv_CRSH2
*- pv_Guard1
*- pv_Guard2
*
* Third Lense and Chamber
*- pv_L3VA
*- pv_L3ST
*- pv_L3F1
*- pv_L3F2
*- pv_L3GP1
*- pv_L3GP2
*- pv_L3GP3
*- pv_L3GP4
*- pv_L3GP5
*- pv_L3GP6
*- pv_L3GP7
*- pv_L3GP8
*- pv_L3HP
*- pv_L3HP4
*- pv_L3HP5
*- pv_L3HP7
*- pv_L3HP8
*
* Compensation Gate Chamber
*- pv_CGateV
*- pv_CGate
*- pv_CGateF1
*- pv_CGateF2
*
*Trigger Detector and Chamber
*- pv_Trigger
*- pv_TriggerV
*- pv_TriggerF1
*- pv_TriggerF2
*- pv_CFOIL
*- pv_TriggerB
*- pv_TriggerB2
*- pv_TriggerB3
*
* Inner and Outer Scintillators
*- pv_SCISt
*- pv_SCISb
*- pv_SCISr
*- pv_SCISl
*- pv_SCOSt
*- pv_SCOSb
*- pv_SCOSr
*- pv_SCOSl
*.








*.
The list of all the volumes and their densities and masses can be accessesd during the simulation by entering the commands 
*
* <tt>vis/ASCIITree/verbose x</tt> to set the level of details (\f$ 0 < x < 15 \f$ )
*
* <tt>vis/drawTree</tt>
*










*/