Event Header Format

The event header consists of ten words of two bytes:

   word  hexmask   description

   0   0x80nn    total #words in event         
   1   0xmmnn    nn clean ge counts; mm is the MTM bit pattern at MAIN
   2   0xmmnn    dirty ge and bgo only counts, respectively
   3   0xffff    trigger time, ttH (16 bits)
   4   0x7fff    trigger time, ttM (15 bits)
   5   0xffff    trigger time, ttL (16 bits)
   6   0x0nnn    tac1(usec tac; next usec tic vs. master trigger)
   7   0x0nnn    tac2 (usually rf tac: next rf vs. pre-trigger)
   8   0xnnnn    sum of ge_low energies (within ge time window)
   9   0xnnnn    sum of BGO energies    (within BGO time window)

The first word marks the start of a header by having the highest bit set.

In the event stream, the order of the events are: first the clean germaniums, then follows the dirty germaniums (if they are read out) and finally any events where only the BGO detectors fired (if they are read out). The bits in the ModeFlags tells what is written to the data stream.

The summed energy values at the end of the header are generated from the low res ge signal and bgo signals compressed by a factor of four. To generate the summed energy signal, it is enough to add the "sum of ge_low values" and "sum of ge_low values" since the clean germaniums are already added to this sum.

The summed (and time gated) germanium and BGO energies are contracted by a factor of 4. Thus, if your lo res calibration was 4kev/ch (typical) and bgo energy calibration was 2.5 kev/ch (typical) then the calibration of your summed ge and BGO energy signals in the header will have the calibration

• GE: 16 Kev/ch (offset: 3.8 Kev)
• BGO: 10 keV/ch (offset small, but unknown)

update history:

• 98/10/22: the calibration parameters for the summed ge and BGO energy signals were found to be listed in wrong order. It should be ge: 16 Kev/ch and BGO 10 keV/ch.
• 98/4/10: the problem with garbage after the buffer header was fixed.
• 98/1/23: warning about garbage words between buffer header and first valid event header.
• 95/4/8: initial writeup

If you find errors in this GAMMASPHERE documentation; please send E-mail to Torben.