ICARUS T600 Report to the XXXV meeting of the
LNGS Scientific Committee
LNGS, 14-15 April 2011
(The ICARUS Collaboration)
1. Introduction.
In the last report presented at the XXXIV LNGS Scientific committee meeting, a
detailed description of the ICARUS T600 start-up operation was given. Detector
commissioning including cryogenic operations, DAQ optimization and trigger activation, LAr
purity measurements, tuning of the analysis tools, was discussed. First data taking with the
CNGS beam and cosmic muons was reported.
Starting from the 1st of October 2010 the T600 is fully operational for data taking.
CNGS neutrino events have been collected with high live time until 22 nd of November
(CNGS stop). LAr purity is continuously measured with cosmic muons. The self-triggering
capabilities (based on internal PMT’s signals) allow recording of atmospheric neutrino
events, in the GeV energy range. The cryo-power plant has been completed (12 cryo-coolers
units). Maintenance of the purification/recirculation systems has been successfully performed.
The software for event visualization and reconstruction has been commissioned,
including automatic procedures for event scanning and initial classification, based on the
actual T600 running conditions.
2. Cryogenics.
The whole cryogenic plant has been operating for one year since the commissioning
start. It has demonstrated to be safe, stable, reliable and functional.
The liquid argon purification and recirculation plant has been continuously and regularly
running on both half-modules for months (except for short periods for pump maintenance)
thus allowing increasing the electron lifetime in both half-modules.
In March 2011 a new reliable vacuum pumping system dedicated to insulation panels
and instrumented with pressure sensors to be monitored by SCADA was installed and put into
operation. This system will keep stable the vacuum level inside the insulation panels thus
avoiding their pressure increase that will have as a consequence an increase of the heat loss
through them.
In the last several months, we have been using on average about 8/9 Stirling cryo-cooler
units out of 10. As a consequence two extra units were ordered with the aim of reaching a
wider safety margin on the required cold power; they were delivered and mounted on March
2011 and are at present fully operational.
All the necessary maintenance of the cryogenic (LN2) and purification (LAr) plant was
completed before the CNGS neutrino beam physics run started on March 18, 2011. Besides, a
set of various spare parts and new equipments for the cryogenic plant were ordered at the end
of 2010 with the aim to reduce dead time in case of fault and anomalies and are now available
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During winter an emergency situation (short circuit in the dedicated ICARUS power
cabinet) impacted on the operation of the T600 cryogenics. The safety procedure developed at
LNGS for the ICARUS detector allowed keeping the plant safely under control until the
electric power was re-established. However this event pointed out that additional
implementation on the cryogenic plant itself, external infrastructures (such us electrical power
supply) and detector facility could be envisaged. In particular a second line to feed the cryocooler system is under construction. The possibility to implement LN2 storage in
underground was studied too.
Under AirLiquide responsibility an important upgrade of the gravity-driven nitrogen
circuit is ongoing in order to allow the GAr re-condensation during emergency situations such
as total lack of power supply in the tunnel.
3. Trigger system.
The ICARUS-T600 scientific program addresses the neutrino physics with the CNGS
beam from CERN, cosmic neutrinos and proton-decay search. The trigger system relies on
the scintillation light signals provided by the internal PMTs and on the SPS proton extraction
time for the CNGS beam.
The trigger set-up is based on a controller crate, hosting a FPGA-board for signals
processing, interfaced to a PC in the Control Room for data communication and parameter
setting. It can handle different trigger sources, like signals from internal photomultipliers,
CNGS proton extraction time and test pulses for electronic channel calibration. Moreover it
provides the absolute time stamp for the recorded events and the opening of the CNGS proton
spill gate. The handling by the FPGA controller of the different input signals, of the absolute
timing generated by CNGS atomic clock, of the beam extraction early warning packets from
CERN and the distribution of the trigger to the electronic crates is shown in Figure 1.
Figure 1. Schematics of the trigger system implemented for the ICARUS T600 experiment.
For every CNGS cycle two proton spills, lasting 10.5 µs each, separated by 50 ms, are
extracted from the SPS machine. An "early warning" packet is sent from CERN to LNGS via
Ethernet 80 ms before the first proton spill extraction, allowing to open two ~ 60 µs gates in
correspondence to the predicted extraction times. A ~ 35 µs precision has been measured, due
to a jitter in the extraction time prediction.
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The most accurate timing inside the controller is realized by a 40 MHz counter, reset
every ms by a synchronization signal containing absolute time information, generated by the
master clock unit of the LNGS external Laboratories (GPS unit and 10 MHz atomic clock)
synchronized to CERN SPS accelerator clock.
The discrimination thresholds for the PMT sum signals have been set at a threshold
around 90 phe and 110 phe for the West and East half-module respectively, during a 60 µs
spill gate in coincidence with each CNGS extraction. Therefore the CNGS-type trigger is
generated when a signal from the internal PMTs of a TPC chamber is present within the
CNGS gate. As a result about 80 events per day are recorded with a trigger rate of about 1
mHz. The analysis of the recorded neutrino interactions in LAr, shows a synchronization
between ICARUS and the actual proton extraction time as written in the CNGS-database,
sufficient to reconstruct the 10.5 µs width of the two proton spills (Figure 2). The residual 2.4
ms delay is in agreement with the neutrino time of flight (2.44 ms) taking into account the
timing signal propagation delay to Hall B (~ 44 µs).
Figure 2. Time distributions of the recorded neutrino interactions in the T600 with respect to the CNGS spill time.
Top: according to event classification; bottom: according to first and second CNGS spill.
The trigger for cosmic events requires a low discrimination threshold to maximize the
low energy event detection. An efficient reduction of the spurious signals is provided
exploiting the coincidence of the PMTs sum signals of the two adjacent chambers in the same
module, relying on the 50 % cathode transparency.
A trigger rate of about 12 mHz per cryostat has been achieved leading to about 83
cosmic event rate per hour collected on the full T600 (only 6 % of the events are classified as
empty by a visual scanning), to be compared with the 160 events/hours predicted by Monte
Carlo. The present scheme of the PMT’s HV biasing and signal read-out does not allow full
collection of the scintillation light produced by ionizing events due to a signal distortion
introduced by the propagation on the 33 HV/signal cable and a mismatch of the termination
impedance.
Therefore, at the end of the 2010 CNGS campaign, a major effort to improve the PMT
readout system was started aiming at improving the cosmic ray data taking efficiency. The
full HV biasing system has been re-designed integrating in each PMT channel a custom-made
low-noise fast amplifier thus increasing by at least a factor two the recorded light signals. The
new system has been successfully qualified on few PMT’s and is presently under installation
on the T600. This PMT system update should help recovering the missing low energy fraction
of the cosmic event rate.
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As reported at the previous LNGS Scientific Committee, in addition to the PMT based
trigger, a new algorithm (DR-slw) to detect the local Region of Interest (ROI) of each event,
avoiding the full acquisition of the detector, was implemented in a new chip called
SuperDaedalus. Both Induction and Collection boards will be equipped with the chip aiming
at triggering on isolated low energy events using directly the wire signals [B. Baibussinov et
al., JInst 5:P12006 (2010)]. About 30 circuits are already installed on the T600 Collection
views boards and are now under test in parallel to normal data taking.
In the meantime, the proposed DR-slw algorithm can be tested in the software version
as second level trigger in order to select the CNGS events triggered only with the CNGS early
warning signal. In such a way, an additional independent trigger system for CNGS events is
being realized with the aim of directly qualifying the performance of the forthcoming new
SuperDaedalus hardware system.
Preliminary tests show the same detection efficiency of the trigger based on PMT’s, with
an empty event rejection exceeding a factor 1000. In Figure 3 the first 2011 CNGS event
acquired with such software filter is shown.
Figure 3. Example of a quasi-elastic muon-neutrino CC event from the CNGS beam, recorded with the early
warning CNGS signal and selected by DR-slw novel software algorithm. Top: collection view; bottom: induction2
view, The PMT induced signal is also visible.
4. LAr purity measurement.
Electron lifetime is measured directly by the charge signal attenuation in Collection view
along through-going clean muon tracks, i.e. a track without evident associated delta-rays, that
extend at least 50 wires and 1200 time samples in both Collection and Induction 2 views
(Figure 4). Only hits at less than 3 mm distance from the reconstructed track (linear fit) are
retained. For each selected hit, the pulse area, proportional to the collected charge, is
calculated over the signal baseline. At present the associated uncertainty is 3% given by the
pre-amplifier gain uniformity. However, the precision of the lifetime measurement is
dominated by the Landau fluctuations in the 10-20 % range depending on the effective
sampling pitch determined by the track direction.
To further improve the measurement precision the following "truncation" method is
adopted: 10% of the hit signals, with the largest energy deposition of the Landau tail of dE/dx
distribution, are rejected as well as 10% of smallest charge values with the worse signal to
noise ratio. The RMS of the residual quasi-gaussian charge distribution, 14% on average, is
assigned as effective uncertainty on single hit charge measurement. A precise and systematic
measurement of the electron lifetime in LAr was initiated soon after the beginning of the
liquid recirculation in the two half-modules (Figure 5). During winter operation, the purity
reached and exceeded the 6 ms value in both cryostats. This corresponds to a maximum free
electron yield attenuation of 14 %, at the maximum drift distance of 1.5 m. Pump
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maintenance required some stops of the LAr recirculation lasting several days, resulting in a
sudden degradation of the purity, which however never went below 1 ms. The different
asymptotic values of the LAr purity reached, after each restarting of the recirculation system,
are probably due to a different equilibrium value of the effective recirculation speed.
Figure 4. Top: Example of muon track crossing a 1.5 m full drift volume. Bottom: Signals recorded on single
collection wires as a function of the drift time. The electronic noise is 1.5 ADC counts r.m.s (1500 electrons
equivalent), the pulse height is above 20 ADC counts (20000 electrons). The T = 0 mark is estimated by the
induction of the PMT signals on Collection wire plane. The charge attenuation along the track allows a precise
event-by-event measurement of the LAr purity.
Figure 5. Free electron lifetime evolution in the West (left) and East (right) cryostats as a function of the elapsed
time for the first 200 days of operation of the T600 detector. For details see the text.
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The evolution of the residual impurity concentration can be described with a simple
model as dN(t)/dt = N / τR + kL + kD exp(t / τD) where τR is the time needed to recirculate a full
detector volume, kL is the total impurity leak rate and kD is the internal residual degassing rate
assumed to vanish with a time constant τD. Uniform distribution of the impurities throughout
the detector volume is also assumed, as experimentally supported by the lifetime
measurement with muon tracks in different regions of the TPC's.
The free electron lifetime can be expressed as τele[ms] ~ 0.3 / N[ppb], when the concept
of Oxygen equivalent impurities is introduced. Fitting the data with this model yields a
recirculation time of 6 days, in agreement with the nominal pump speed, and extremely low
leak rates (< few ppt/day O2 equivalent of impurity concentration) in both half-modules.
Internal initial degassing rate seemed to be higher in the East module, resulting in a slightly
worse purification rate.
A complementary technique to measure the argon cryostat purity has been developed at
the end of 2010 with the attempt to better estimate and clarify the different behavior in the
purity evolution in the two modules, by means of direct methods as an alternative
measurement with respect the indirect technique coming from the ionization tracks’ analysis.
The approach consists in the analysis of the gas phase from both cryostats by means of a
commercial mass spectrometer that allows reaching an ultimate sensitivity of the order of 1
ppm for all atoms-molecules with mass less then 100 amu contaminating the argon fluid.
The gas phase is expected to be more polluted than the liquid phase because impurities,
entering the detector volume, rapidly diffuse with a density gradient depending mainly on the
temperature and only partially on the Argon density. Hence the ppb level in liquid
corresponds roughly to ppm level in gas. Moreover most impurities came from external leaks
or from material degassing, located mainly in the detector gas phase. Analysis of the gas
phase impurities composition allows also a better understanding of the pollution source and of
the overall detector purity evolution.
A series of analyses on samples of argon gas showed a total amount of contaminants
below 1 ppm in both cryostats. The level of nitrogen pollution is extremely low (below 0.5
ppm), thus compatible with the level measured during the filling of the detector (200 ppb).
No contaminants with mass above 44 amu (where the spectrometer is extremely sensitive)
were found. Further measurements of samples of gases taken from different points of the
detector have been planned. Measurement with more sophisticated instruments with
sensitivity at the level of ppb are under study.
5. CNGS run during 2010.
As previously mentioned, the T600 detector in fully functional since October 1 st. In the
time interval from Oct. 1st to Nov. 22nd CNGS delivered 8  1018 pot. The detector lifetime in
the same period was up to 90%, allowing the collection of about 5.9  1018 pot (Figure 6).
From October 1st to October 27th the DAQ system included the multi event buffer
feature, with a total of 8 buffers, in addition to a 1 s veto to avoid triggering on PMT noise.
DAQ and trigger introduce an overall 7% dead time.
Since October 27th a new system, including a direct communication between the trigger
management and the DAQ has been installed, tested and has been running steadily. All trigger
information is stored into an online database. Initially to check every possible failure of the
system, only single buffer mode has been enabled. This means that after every trigger a veto
signal was set to block further triggers, until the building of the event is completed. The
related dead time resulted in a mean value of 19%. Introducing a multi event buffer
configuration helps reducing the dead time, since no veto signal is set until all the available
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buffers are full. This feature shows that with the available 8 buffers enabled, the dead time is
negligible.
Splitting the building procedure into 4 parallel streams, handled by different machines,
reduces the building time of a factor of 2. This will allow to further increase the trigger and
DAQ event collection rate above 0.5 Hz, opening the possibility to exploit the DR-slw
algorithm implemented in the SuperDaedalus chip.
Figure 6. Integrated proton on target delivered to CNGS in the 2010 campaign, in the period when the T600 was
fully operational (starting from 1st October 2010). The beam intensity recorded by T600 is also shown (blue).
The analysis of the data collected in the period October 1st - November 22nd with CNGStype trigger is proceeding smoothly. Out of the 5.9  1018 pot collected by ICARUS T600 in
this period, 4.54  1018 pot (~ 78%) have been fully classified by means of a visual scan. The
physical volume taken into account is 434 t, obtained from the instrumented active volume
(17.7 x 3.1 x 1.5 m3 for each TPC chamber) subtracting 20 cm in length direction (5 upstream
and 15 downstream) and 3 cm for each side in width and height directions. In this volume the
number of CC / NC interactions foreseen per pot is 2.6  10-17 / 0.87  10-17, accounting for
the whole energy range up to 100 GeV. This number – in good agreement with expectations –
has been corrected, accounting for 7% and 19% dead time in the two periods (Table 1).
Table 1. Number of collected interactions compared with number of interactions predicted ((2.6 ν CC + 0.86 ν
NC)  10-17/pot), in the whole energy range up to 100 GeV, corrected by fiducial volume and DAQ dead-time. In
“ν XC interaction type” photons or pieces of events, maybe remnants of ν interactions outside the active volume
entering the detector, are classified.
6. Events scanning, reconstruction analysis.
The software for event visualization and reconstruction is an evolution of what was
originally developed for the 2001 test. Developments were oriented both to add
functionalities for reconstruction, and to improve the interface with the user and data.
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The 3D track reconstruction starts from a 2D track finding algorithm based on an
automatic clustering over an angle-position matrix. A segment-based approach has been
developed for three-dimensional reconstruction. Both algorithms have been developed and
tested on Monte Carlo data and are now in the commissioning phase on real data.
The information from track reconstruction fed the algorithms for muon momentum
determination through multiple scattering, and for particle identification through neural
networks trained on the shape of the dE/dx energy behavior. A shower measurement
algorithm providing the reconstruction of shower geometry, initial dE/dx and total energy has
been developed on MC and is being validated with real data, in particular in view of the
identification and measurement of e CC events.
The initial effort at the beginning of the data taking has been put in the implementation
and operation of a method for the determination of LAr purity from muon tracks. During the
next years the cosmic muons will continue to be an important tool for the monitoring of the
detector performances, uniformity and stability.
Calibration data from test pulses have also been analyzed, providing a database table for
wire signal equalization. A noise monitoring procedure, that automatically analyzes events as
they are collected, is an important support to hardware debugging and fixing. Tools
developed for this procedure are also implemented in the hit fitting stage, in order to exclude
noisy wires from the reconstruction. The track finding and PID algorithms are in the
commissioning phase, having been developed on MC events they are being checked on real
data.
Figure 7. Recorded events: parallel muons in three projections and a cosmic ray muon with associated
electromagnetic activities. The vertical dimension in all the images corresponds to 1.5 m drift.
In the initial phase of the commissioning a visual scanning of the collected events has
been performed. The output of this scan is used to validate automatic algorithm for event preselection and noise rejection. An automatic procedure for event scanning and initial
classification has been developed and included in the official reconstruction and visualization
code. A data sample of 11227 events collected in about 47 days during June and July has been
visually classified to identify different event topologies. About 90 % of the events are cosmic
muons, accompanied by sizeable electromagnetic activity in half of the cases. Some
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interactions involving multiple muons are shown in Figure 7, where the redundancy in the 3D
reconstruction provided by three independent stereo views, can also be appreciated.
As an example of the present reconstruction procedure, one CNGS beam related muon
neutrino event fully reconstructed is shown in Figure 8. The long muon track, about 14
meters, is impressive. As another example, a low energy neutrino interaction as seen in the
collection view is shown in Figure 9.
The available analysis tools allow a complete kinematical reconstruction of the neutrino
interactions as shown in the examples of Figures 8 and 9. All these tools are being integrated
into a global analysis framework, allowing the high level reconstruction of the event as a
whole and also to store and share partially analyzed events for further reconstruction by
different groups. A particularly relevant item will be the reconstruction of the kinematics in
the longitudinal and transverse projections, needed for the identification of CC candidates.
Another important point, which will be addressed, is the capability to identify and reconstruct
the neutral current interactions and distinguish them from the e CC events. This capability
identifies the LAr as the best candidate for the next generation experiments; therefore its
experimental demonstration is a milestone for the ICARUS T600 detector.
The same reconstruction and analysis tools are perfectly suitable also for the search for
proton decay events, which will highly profit from the trigger improvements foreseen in the
next future.
Figure 8. CNGS beam related muon neutrino event fully reconstructed. Primary vertex (A): very long leading 
(1), e.m. cascade reconstructed as a neutral pion(2), charged pion (3). Secondary vertex (B): the longest track (5)
is a  coming from stopping k (6). The  decay is also identified. All particle momenta are measured for a
complete kinematic reconstruction of the event.
Figure 9. Example of low energy atmospheric neutrino interaction candidate, recorded off CNGS beam spill gate
and reconstructed in 3D.
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7. Expectation and operation in 2011-2012.
ICARUS T600 detector smoothly started data taking on March 18th 2011 receiving the
CNGS neutrino beam operating in high intensity dedicated mode. The cryogenic and LAr
filtering systems are working well, allowing to reach about 6 ms of electron lifetime well
above the needs for a drift path of 1.5 m (1 ms).
The photomultiplier read-out system is presently under upgrade in view of a more
effective use in the trigger system for low energy events. The TPC front-end electronics is
undergoing a major upgrade, concerning the self-triggering capability, with the installation of
the novel Super-Daedalus chips. The implemented DR-slw algorithm, performing the event
filtering, is presently under tuning with the software version applied to CNGS data.
The analysis tools to fully reconstruct the event topology and kinematics are under
deployment with the real data, addressing in particular the main items of physics with the
CNGS beam, i.e.  search, e CC identification/measurement and NC rejection capability.
A consistent analysis framework unifying all the available tools and allowing storing and
sharing analysis results together with successive refinements by different groups is also being
finalized.
CNGS physics runs in 2011-2012 are expected to integrate as much as 1020 pot also
thanks to dedicated SPS periods at high intensity. For 1.1  1020 pot (including the data taken
in 2010) about 3000 beam related muon neutrino CC events are expected in the ICARUST600, including also 24 electron neutrino CC intrinsic beam associated events. About 80
atmospheric neutrino events per year will also be collected and analyzed.
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