The electronic pulses generated in the phototubes at each end of a
scintillator cell must be reconstructed through a combination of electronic
hardware and data aquisition software. The electronics serves the primary
purpose of defining a valid event and its type. There are six main events
processed by the electronics: 1) neutron-neutron (NN), 2) neutron-proton
(NP), 3) proton-proton (PP), 4) single neutron hits (SINGLE), 5) cosmic
rays (COSMIC), and 6) proton monitor events. The calibration events, PP
and COSMIC were described above. For the data events, NN, NP, and NSING
the electronics will determine their validity based on which combination
of the 6 planes experienced a hit. The NN and NP have already been
described. The NSING is a single hit in either the `analyzer' or the
`catcher' without a hit in the charged particle veto paddles. The NSING
data are
generally only used during set-up when a high count rate is desired, since
it is useless for polarimetry (which requires detection of the second
scattering), and its count rate is heavily suppressed during the actual
experiment. The proton monitor signal was mentioned in section
and
adds an uncorrelated data stream pertaining to the beam quality to the data
from the neutron polarimeter.
The signals from the phototubes are split off twice before being used in
the electronics as described above. A diagram is shown in figure
.
Figure: Front-end electronics for a single neutron cell (CFD is a constant
fraction discriminator).
One of these signals is attenuated and
sent to a FERA ADC which will integrate the pulse-height information from
the phototube. The other signal is sent, via a constant fraction
discriminator (CFD), to a FERA TDC which produces timing information by
producing a constant current during a window delineated by a common start
and the arrival of the signal from the CFD. The signal produced by the TDC
is sent to another FERA ADC. When the electronics determine a valid event
has occurred it triggers a FERA driver to read the FERA ADC modules via a
front bus. The FERA driver is then read on an event-by-event basis by VME
hardware with an MC68040 Motorola microprocessor. This bypasses the
typical use of the CAMAC memory buffer. The data is then transferred,
using standard TCP/IP protocol, to a back-end computer, a VAX station
4000-90. This VAX station runs XSYS, which is IUCF's standard data
acquisition and analysis software, to record and analyze the data. Using
this set up the INPOL facility can take data at 
events/s with a
live time (percent of time gathering data that is processed) of
%, with a 10% fraction analyzed. This represents
approximately an order of magnitude improvement over the speed at which the
detector acquired data when it was at Los Alamos (NTOF).
Fraction analyzed is the
fraction of data fully analyzed online. The remainder of the raw data is
written to disk and tape for later offline processing.