The main purpose of the PCA is to remove the effect of phototube walk. The SNO phototubes produce pulses with a rise time of around 5ns and a fall time of around 15ns. The time at which the discriminator fires with respect to the peak of a pulse depends on the pulse size, smaller pulses fire later than large ones. The walk can be seen in a plot of ECA time against QHS in counts above pedestal for an external asynchronously triggered laserball run. The recorded ECA time falls by about 5ns over the first 40 ADC counts above pedestal before reaching an asymptotic value. The PCA must also remove the relative time offsets of the PMTs resulting from different cable delays. Removing the walk and the relative time offsets can be achieved by recording the ECA time against QHS counts above pedestal for a laserball run.
The walk fitter, on a single pass over the laserball data, chooses a set of
time/charge points and records them in a titles banks for later use in
calibrating other data. User control is via the TPCA 3 or 4 (depending on which
version is being run) bank in the file pca_data.dat. An example of a
command file which runs the walk calibration is pca_walk.cmd. This
command file need to be modified in a similar way
to the gain. The validity ranges should be adjusted to the desired values.
If the ECA constants are not in the database then they must be loaded through
titles (CAL does not need to be called in the processor list). Similarly if
the DQXX banks are not in the database they should also be loaded through
titles. In addition to this the source of the laserball data should be
specified, see _LRBP_MODE in section 26.3.3.
The wpca titles banks are written to the file on stream PCA 1 and the lrbp bank is written to the file on stream PCA 2. Both these files names can be automatically generated by putting AUTO in place of the file name and both are compulsory. The optional fit file should be specified on PCA 3 and the optional raw data file on PCA 4.
The fit file contains two ntuples, one is an interpolation point level ntuple and one is a channel level ntuple. The raw data ntuple contains column wise ntuples for every card containing some times and charges for every channel in that card. As the raw data column wise ntuples have been compressed there is a minimum and maximum charge and time specified in TPCA 3. Only the first few hundred hits get recorded for each channel.
For walk version 2, the charge bin level ntuple, id=1000, contains:
lcn Logical Channel Number
crate Crate Number
card Card Number
chan Channel Number
bin Bin Number
qbn Bin Charge
num Number of bin entries
med Median bin time
ave Average bin time
rms Bin time rms
muse Bin Gradient
While the channel level ntuple, id=1001 contains:
lcn Logical Channel Number
crate Crate Number
card Card Number
chan Channel Number
online 0=Offline 1=Online
type 1=Normal PMT (see ccc_type.inc)
stat 0=Fine 1=Offline 2=Zero occ 3=Low occ 4=Low ECA
qual 0=Not fitted 1=Fine 2=No points 3=Failed cut 4=Failed rms
tpeak The overall time peak
ntypical Typical number of hits a PMT received
ntot Total Number of Hits
ncal Number of time/charge calibrated hits
nused Number of hits used in the interpolation
rmstot The total channel rms
tstep The largest significant step up in time
fout The fraction of hits that are early or low charge
flate The fraction of hits that are late
hiqint The high charge fit intercept
hiqgra The high charge fit gradient