The Overlay Option

The overlay function OVERLAY.FOR has been developed to allow detector noise to be used in the monte carlo instead of the poisson noise that is the default option. It was originally developed to use pulse global trigger data as its input but anything in an existing event bank will be used.

Overlay copies all the PMTs in the event PMT banks into the monte carlo PMT banks. If the PMT is calibrated this is all that overlay will do to it. However if the PMT is uncalibrated for whatever reason overlay will apply a crude calibration to if this option is enabled. This calibration should not be used if a true calibration is available, i.e. run CAL before MCO. After operation the event bank is then removed stop any further use of this bank. It will be replaced later in the operation of MCO by the trigger simulation.

To use overlay it is first suggested that the sample to overlay is first prepared. For example if using pulse global trigger (PGT) data, chose a run, then run the event filter and output only those event with the PGT trigger bit set. This zdab file will then be ready to use as input data. To use the overlay function the command file should be set up so that snoman reads in the sample file and calibrates it. The processor list should read:

$processor_list ' INP UPK CAL MCO FTT ANL END'

or variations on this theme.

To avoid reloading the database twice per event two further steps should be taken. The fist is to set the MC event time to lie within the run in which the sample data was taken, for example:

$initial_date 19980817 12150000.

Also the data type needs to be forced to monte carlo type using this switch:

$fake_data_type $mc

There are then two further switches. The first enables overlay and turns off poisson noise:

$enable_overlay $on

The second toggles between keeping uncalibrated tubes and applying a crude calibration, or discarding these tubes:

$overlay_keep_bad_cal $off or $on

The use of this option depends upon your purpose. If you are interested in Nhit spectrum then it suggested that you keep these tubes. However for the use of fitters and other analysis where PMT times are important it would be better to discard these tubes. Again the calibration used is crude and should be avoided if possible.