1. Results with the RPC system of OPERA and perspectives
Mini-Workshop: "LNGS results on the neutrino velocity topic“
LNGS Seminars
March 28th 2011
Results with the RPC system of OPERA and perspectives
A. Bertolin (INFN-Padova)
On behalf of the OPERA collaboration

2. RPC1 RPC2 Time measurements with the OPERA RPC detectors TT
22, RPC0, + 22, RPC1, planes of Resistive Plate Chambers
each plane is measuring the x - y coordinates (copper readout strips) and the
time stamp of the first signal (x or y) reaching the readout
goal: evaluate all delays in the detection process

3. Delay due to signal formation in the RPC gas mixture
hardware setup: test RPC + PMT in lab.
avalanche signal too small
“discriminate” only the streamer

4. working point: fully efficient detectors
Delay due to signal formation in the RPC gas mixture (cont.)
7 kV
7 kV
working point: fully efficient detectors
delay due to signal formation: 30 ± 5 ns

5. injected triangular pulse
Delay due to signal propagation in RPC readout strips
method: inject a triangular pulse at strip end point, the readout flat is detached from the strip starting point, a reflected pulse is measured at the strip end point
digitized scope output
injected triangular pulse
reflected pulse
strip delay: 4.82 ± 0.17 ns/m
(taking into account different strips, triangular wave frequency …)

6. Delay due to signal propagation in flat cables
distribution of the hor. and ver. flat delays as implemented in the geometry:
delay (ns)
delay (ns)
send a squared wave through a BNC cable, delay between pulse and reflection: ns
attach the same BNC cable to the lowest horizontal flat, flat cable detached from the other side, measure the delay between pulse and reflection: 310 ns
= / 2 ns = 79.2 (± 1) ns
idem for the second horizontal flat from the top
d = / 2 ns = 42.7 (± 1) ns

7. Readout electronic delays: schematic layout
ctrl board
mezzanine:
DAQ time stamp
CLK fanout
extender
PPS reset (0.6 sec) + CLK signals
default path
FEB board
PPmS
known delay: function generator, used for a fixed delay part, + lemo cables
principle: the PPmS signal is sent to the FEB board with a known delay
by comparing the DAQ time stamp and the known delay one gets the overall time delay of the default path
Master Clock

8. Delay due to signal propagation through the FEB board
delay from FEB input flat connector to the mezzanine input pin (where the signal is time stamped)
scope value: 124 ns
this value has to be corrected for the extra cables used for this measurement
delay from the FEB input flat connector to the mezzanine input pin = ± 1 ns
FEB polarity, FEB channel to channel dependence: variations seen within ± 1 ns

9. Delay due to PPS reset (+ CLK) propagation
cyan: signal sent to the FEB input, KNOWN delay with respect to the OPERA Master Clock
light green: PPS reset signal at the mezzanine input pin
taking into account the extra cables and delays used for this measurement
delay due to PPS (+ CLK) propagation = ns
the 5 peaks structure of the light green signal corresponds to the ± 25 ns jitter ( ) of the OPERA Master Clock (see G. Sirri’s talk)

10. Delay internal to the mezzanine
in the conditions just presented
total delay between the (first of the 5) PPS and the trigger signal = ns
if the internal mezzanine delay was 0 ns the DAQ output should be ns
in these conditions the DAQ goes from 25 to 26 counts, inflection point of the quantization curve, i.e. 260 ns (first of the 5 pulses)
the difference, 48.0 ± 2 ns, should be equal to the time needed to reset the DAQ scale and time stamp the FEB pulse
first PPS at 260 ns
first PPS at 250 ns

11. RPC1 RPC2 Principle of the time measurement: mean time at track start
take the average time of the RPC muon track hits in successive planes,
each one scaled to the leftmost hit RPC plane
RPC1
RPC2
hits are individually corrected for:
propagation in the RPC readout strips and flat cables
processing in the FEB boards
delays due to the OPERA DAQ system (down to the Hall C Master Clock)
drift of the OPERA Master Clock (see G. Sirri’s talk)

12. Cross check on the quality of the (relative) RPC time alignment
mean t SM2
mean t SM1
the time difference use the extrapolated time at the A1 reference point (end point of the CERN – LNGS base line, zA1 = cm)

13. are working hard to have it ready for the next BB
Independent time stamp system for the RPC
goal: RPC time stamp independent from the OPERA Master Clock
a VME PPmS receiver designed for Nautilus (LNF electronics workshop: R. Lenci, M. Beretta, A. Balla) is available:
1) PPmS decoding
2) event time stamp
3) PPmS repetition
some modifications will be needed to cope with the light yield available underground (about 2 mW)
inputs: 9 XPC/RPC planes (per magnet) with Timing Boards, needed to trigger the High Precision Tracker
are working hard to have it ready for the next BB

14. Conclusions
presented a summary of all the RPC delays
presented an RPC timing algorithm where all delays are computed on an event by event basis
achieved a 4 ns time resolution
2011 BB data have been analyzed, the result … will be addressed in M. Sioli’s talk …
an RPC time stamp independent from the OPERA Master Clock is being setup for the next BB run, 1 ns time resolution