Digitizing electronics for the EMI multi-wire proportional chambers. Page: 2 of 19
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DIGITIZING ELECTRONICS FOR THE EMI MULTI-WIRE
E. Binnall, F. Kirsten, K. Lee, C. Nunnally
Lawrence Berkeley Laboratory
University of California
A complete system of electronics for digitizing
pulses from a large array of multi-wire proportional
counters is described. The system is based on the
method of coupling the chamber pulses onto delay
lines developed by Perez-Mendez. All three wire
planes of each chamber are digitized, giving position-
al information for each avalanche in the chamber on
three coordinates, x, y and a redundant coordinate.
The system will be used with an array of 30
chambers each one meter square, which will serve as
an External Mon Identifier (EMI) for the 15-foot
diameter bubble chamber in the neutrino beam line at
NAL. Up to 16 events can be digitized in a 60 ps
A 15-foot diameter bubble chamber is being
built for the neutrino beam line at the National
Accelerator Center. Under a plan proposed by
Stevenson and Peterson,5 the bubble chamber will be
augmented by a layer of Multi-wire :roportional
Chambers (MWPC) surrounding a portion of the bubble
chamber periphery. The purpose of this layer is to
permit identification, external to the bubble chamber,
of muons involved in bubble chamber events. The
system of MWPCs is therefore referred to as an External
Muon Identifier (EMI).
Thi initial phase of this plan will have 30
square meters of MWPC, consisting of 30 individual
chambers each one meter on a side.2 Each chamber
will have 3 coordinate axes digitized--X, Y and U.
U is a redundant plane whose coordinate axis is
rotated 45' with respect to X. The location of a
charged particle passing through a chamber will be
digitized to a resolution of +5 m on each coordinate
axis. During a 60 us beam spTil, an average of 5
events is expected in each chamber.
The MWPC readout is based on the electromagnetic
delay lines developed by Perez-Mendez.3 For this
application, delay line digitization has some attract-
ive features compared to other digitizing means, such
as the logic channel-per-wire.4 First, it tends to
reduce the cost of electronics, since only seven
channels of electronics are necessary to digitize all
three coordinates of a chamber. Each channel is
reasonably inexpensive, in this case partly because
of the relaxed resolution requirements. Amplifier-
per-wire logic usually is attached only to one plane
of a chamber. In contrast, delay lines can be coupled
to all three planes, hence the number of chambers
required and the chamber cost is reduced.
*This work was done under the auspices of the U.S.
Atomic Energy Comnission.
Relationship of Electronics to the Chambers
Figure 1 shows a schematic view of a chamber and
its three delay lines, one each for X, Y and U coordin-
ates. The wires of the three planes are extended a
sufficient distance to establish a capacitive coupling
of each wire onto the delay line. An event (avalanche)
on a particular wire results in a voltage pulse Deing
introduced onto the delay line at that point. The
pulse then propagates to the ends of the delay line
at a velocity of the order of 0.2m/ns. Seven
digitizer channels are used with each MWPC: one on
each end of the three delay lines, plus one to record
the "prompt" (undelayed) times of the chamber events.
Each digitizer channel records the time at which it
receives each of up to 16 pulses during a beam burst.
Since there are 30 MWPCs, a total of 210 digitizer
channels is used. At the end of a beam burst, they
deliver all the data they have collected (up to
16x210-3360 words) during the burst.
A suitably designed computer program can analyze
the collected deta from each chamber to determine the
location and time of occurrence of each event within
that chamber. The Appendix gives some details of the
Components of the Digitizer Chain
The pulses at the delay-line outputs generally
have amplitudes of a few millivolts and rise-times of
the order of 100 ns. The delay lines have character-
istic impedances of about 1000 ohms. The preamplifier
is mounted physically adjacent to the delay line, and
amplifies the signals before they are transmitted from
the chambers to the discriminators on 50-ohm coaxial
cable. The discriminators develop timing pulses whose
leading edges are an accurate measure of the time-of-
arrival of the delay line pulse. The timing pulses
are delivered to the digitizers. They develop and
store binary numbers whose magnitudes are related to
The spatial resolution of the digitizing channels
depends on how well the times-of-arrival of the pulses
can be measured. This in turn is strongly dependent
on the signal/noise ratio at the timing discriminator.
The relationship is approximately:
where T3 - timing error (jitter) introduced by
the presence of the noise, seconds;
En " noise superimposed on the signal,
S - the slope of the signal waveform,
UIUIBTI OF lTftS DIICIUtiNT IS UNtUMITES
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Binnall, E.; Kirsten, F.; Lee, K. & Nunnally, C. Digitizing electronics for the EMI multi-wire proportional chambers., report, January 1, 1972; Berkeley, California. (digital.library.unt.edu/ark:/67531/metadc1033692/m1/2/: accessed September 19, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.