[Gate-users] how to creat beams in main-write-phs.mac

[????]

hi ,
   I do simulation of Linac, i do not know to deal with the command in the main-write-phs.mac,  the following are 
 
/gate/source/addSource                mybeam gps

/gate/source/mybeam/gps/particle      e-

/gate/source/mybeam/gps/pos/type      Beam

/gate/source/mybeam/gps/pos/rot1      0 1 0

/gate/source/mybeam/gps/pos/rot2      1 0 0

/gate/source/mybeam/gps/pos/shape     Circle

/gate/source/mybeam/gps/pos/centre    0. 0. 0.6 mm

/gate/source/mybeam/gps/pos/sigma_x   1.274 mm

/gate/source/mybeam/gps/pos/sigma_y   1.274 mm 

/gate/source/mybeam/gps/ang/rot1      1 0 0

/gate/source/mybeam/gps/ang/rot2      0 1 0

/gate/source/mybeam/gps/ang/type      beam1d

/gate/source/mybeam/gps/ang/sigma_r   0 deg

/gate/source/mybeam/gps/ene/type      Gauss

/gate/source/mybeam/gps/ene/mono      6.7 MeV

/gate/source/mybeam/gps/ene/sigma     0.077 MeV

/gate/source/mybeam/gps/direction     0 0 -1



 any one can give me a explaination about the pos/rot1 ,/pos/centre and/pos/sigma  ?



Thank you in advance.
Best regards.