Isomer tagging is done in Gammaspehere by placing a recoil stopper foil in the beamline between the forward BGO shield detectors (# 1,2,3,4,6). We then use the signals from these detectors to tag on isomeric decays in the deposited residues. We do not have germanium detectors in these modules in this tagging mode.
By switching on the isomer tag writeout flag, the EFF processors will write out the following data from the forward BGO shield detectors (# 1,2,3,4,6):
bbbb|bbbg|iiii|iiii b: bgo hit pat, g: ge bit, i: modID 0000|tttt|tttt|tttt t: bgo time 0000|eeee|eeee|eeee e: bgo energy(the g bit better be zero). I.e., three words per isomer module that fired is written out. These words are written out as 'fake' external data as:
0xff00 : external data flag 0x00nn nn: number of external data words to follow (3,6,9,12,16) bbbb|bbbg|iiii|iiii 0000|tttt|tttt|tttt 0000|eeee|eeee|eeee bbbb|bbbg|iiii|iiii 0000|tttt|tttt|tttt 0000|eeee|eeee|eeee . . .Note1: there is a data conflict if you want to use isomer data and also have regular external (FERA) data at the same time. At the moment we distinguis isomer data with the external data flag: 0xff00.
Note2: It makes little sense to write dirty germaniums and clean BGOs out if you have the isomer flag on.
Note3: The EFFs will identify the isomer modules and write then out in the format described above. The EFFs will furthermore subtract the isomer module BGO energies (>>4) from the summed BGO energy that the EFFs keep record of IF the BGO time is in the prompt time window set in the 'user/eff/eff.user' file with the variables:
bgo_sum_time_lo=1900 bgo_sum_time_hi=2110(shown here with typical limits). Thus if you are going to use isomer tagged data for KH extraction, you may want to carefully evealuate what these time limits should be. The EFFs also subtracts 1 from the 'bgo_time_count' which counts how many clean bgo modules there are in an event in the above prompt time interval.