ICARUS T600 Status Report

LNGS_SC, 21 October, 2010

(The ICARUS COLLABORATION)

Introduction

The last report was presented on May 6, 2010.

Here we remind the main activities during the first months of 2010, after the cryostats evacuation phase started on January 9th:

The detector’s vacuum was broken on April 14th and the volume was filled with ultra-pure Argon gas at a slight overpressure.
The liquid Nitrogen cool-down of the ICARUS walls started on April 16th and proceeded smoothly, reaching the LAr temperature (90K) on April 23rd.
The filling phase with ultra-pure LAr started immediately after (April 29th) at a rate of ~2 m3/hour.

On May 18th both modules were completely full. On May 27th HV and wire biasing and PMT’s were turned on the West Cryostat. At 12.24 the first muon crossing track was recorded.

On May 28th at 19.54 the first CNGS neutrino interaction was observed.

In the following, a summary of the accomplished and on-going activities is presented in more details.

On June 1st also the East cryostat was turned on, after a repair of the HV interconnection, and became operational.

Cryogenic plant and Laboratory infrastructures

The ICARUS cryogenic and liquefaction systems were put in operation and completed tested during the first months of 2010.

The laboratory SCADA (Supervisory Control and Data Acquisition) is correctly interfaced with the Air Liquid control system, based on PLCs, and the complete monitoring of the cryogenics is implemented in the Icarus Control Room.

The 10 Stirling units needed earlier maintenance. The foreseen time interval between interventions was 6000h, but maintenance was necessary after 3000h due to the use of not appropriate gaskets by manufacturer.

The refurbishing of all units has been made by Stirling specialists. Actually 8 Stirling machines, out of 10, are required for normal operation, that means about 32kW of cold power required to assure detector operation.

One of the two LAr recirculation pumps went out of order at the beginning of September in the West cryostat. They were dismounted for repairing. Wrong parts sent in US to verify the reason of the failure.The missing pump in the west cryostat caused purity loss. Lifetime decreased to 1 ms. The recirculation restarted last week before end of September. Purity is expected to recover rapidly.

Fig. 1 – Lifetime measurement

Electron lifetime was evaluated (Fig. 1) via attenuation measurement of long track cosmic ray muons. Each point refers to about ~35 tracks.The gray band correspond to recirculation pump malfunctioning.

Complete LAr recirculation requires about 6 days. Estimated residual leaks in LAr: < 10 ppt/day O2 equivalent (~ 3 x 10-3 g/day of O2). Probably some impurities trapped in East cryostat are gradually released and are being purified. Design target of 3ms reached, with relevant improvement of the Pavia test run result (1.8ms). The lifetime of 3ms implies less than 30% signal attenuation on 1.5m drift. The lifetime of 3ms implies a 30% signal attenuation on 1.5m drift. However the estimated asymptotic value of the electron lifetime is higher than 5 ms in both cryostat. (less that 18 % of signal attenuation at the maximum drift of 1.5 m).

Contentious with ALIS

On the 29 July Icarus-INFN delegation met ALIS executives to clarify some key issues:

Correction of the vacuum leak of a row of bottom supports;
Proposal for the improvement of the whole coibentation of the cold vessels to approximate the design figures;
Proposal for the modification of the thermo-siphon operation. This function should assure safe operation of the system in case of long back-out. Presently there is a heat entrance from top that should be corrected.

ALIS has presented on the 15 September a document with proposals that the experiment and LNGS will evaluate by the end of October.

PMT alignment

Response of PMTs has been equalized for each chamber. Actual gains and thresholds evaluated in units of photoelectrons. The total signal from all the PMTs of each chamber is used to generate the trigger to the DAQ. Thresholds are set to get about 10 mHz for each chamber, close to the expected cosmic rate, Fig. 2.

Fig. 2 - Comparison between trigger frequencies as a function of the threshold.

The integral spectrum of the PMT sum signal in the 2R chamber has been measured with a multichannel analyzer both for cosmic rays alone and in coincidence with the CNGS gate (Fig. 3).

Fig. 3 - Integral trigger rate with CNGS gate (blu curve), and without (red curve).

An independent trigger has been set at lower threshold set in coincidence with the CNGS gate to acquire CNGS events with very high efficiency (including muons from the upstream rock (blue curve in figure 3).

Trigger and CNGSspill gate

The trigger system performs three key functions:

Handling of different trigger sources (internal PMTs, external scintillators, test pulse);
Setting timestamp for events;
Generation of CNGS gate;
Communication with DAQ.

The set-up is composed of a Nuclear Instrument crate, interfaced to a PC in the Control Room, hosting a controller for data communication, parameter setting, and an FPGA-board for signals processing.

Initially gate for tagging CNGS neutrino events has been implemented via software: each time an Early Warning packet arrives from CERN, two gates, each one lasting 30 ms and separated by 50 ms, are generated.The absolute time is derived from broadcasted NTP(Network Time Protocol) information (~2 ms resolution).In Fig. 4 time distribution of events of CNGS neutrinos interacting either in the upstream rocks or in the T600 is shown.

Presently, since mid of September, we use absolute time information distributed in the internal lab via optic fiber. This signal has 100 ns resolution. This information is used to generate a gate signal at time defined by Early Warning packet from CERN.

Figure 5 shows the CNGS interaction time distribution with respect to the hardware gate obtained with the present set up. The presently obtained resolution is ~12µs. Work is in progress to improve time resolution.

Fig. 4 - Time distribution of events respect to software generated CNGS gate.

Fig. 5- Time distribution of events respect to hardware generated CNGS gate.

DAQ status and reconstruction software

The installation of the readout electronics racks on the T600 top was completed last year, together with the cabling of clock & trigger distribution. Each rack was commissioned with test pulses in order to identify any broken/malfunctioning channel. A suitable number of spare DAQ boards have been delivered. The event builder architecture was deployed including DAQ computers (10 server), storage (160 TB on disk and 100 TB on tape), networking (cabling, switches and fibres to control room & to external labs).

On May 18th, electronic racks were mechanically connected to feed-troughs, and Faraday cages closed in order to shield the electronics from external noise.
On May 20th, cathode HV suppliers of West cryostat were turned on without any problem: the -75 kV nominal voltage was reached, showing a stable current. East cryostat showed continuity problems in the HV feed-through. The problem was solved with an intervention on the 1st of June.
Signal from 19 over 21 internal photomultipliers in West cryostat (the remaining two are under investigation) is exploited to build up a prompt trigger signal. Electronics for PMTs’ signal discrimination and is under optimization.
On May 27th, nominal values were applied to wire biasing at (-220, 0, +280 V) on West cryostat without problems (low and stable current).
At 12.24 the first ionization track was recorded and visualized by DAQ; during the night the firsts horizontal muons crossing the cryostat West and pointing back to CERN were recorded (nu int. in upstream rock).
On May 28th at ~19.54 the first CNGS neutrino interaction was observed.
Muon track are presently used to evaluate electron lifetime in real time (present trigger rate: ~ 20 events/hour).

In the last two months the complete migration of DAQ readout software to the embedded real time OS version was implemented. This operation was required by newer VME cpu’s which replace dead ones.Also all the trigger subsystem information was integrated to provide direct access to the monitoring infrastructure/database.

Data are automatically transferred from the DAQ storage to the off-line farm in the external lab from where they can be distributed to the full collaboration.

The present reconstruction software is an evolution of the code developed and used to analyze the cosmic ray data collected with the T600 on surface in Pavia. Developments completed and being validated are:

Full coupling within the Root framework;
Full coupling with the mysql package, to handle on- and off-line databases;
Decoding of compressed data;
3D track reconstruction;
Automatic algorithms to reconstruct tracks and vertexes;
Automatic analysis of calibration runs;
Muon momentum reconstruction with multiple scattering;
LAr purity estimation with muon tracks;
Photo-multiplier signal analysis;
Reconstruction of the electromagnetic showers: shower axis, longitudinal and transverse profiles.

The reconstruction software tools are presently under final validation. Real data collected with the T600 (mainly crossing muons) are used to optimize the software with respect to the actual running conditions.

Event were distributed to the collaboration for scanning to collect a sample of events to be used as a reference for automatic reconstruction.

About 5,000 events collected in an initial phase before the complete PMT equalization were fully scanned, corresponding to a period of about 10 days livetime and 1.0 1018 pot x T600 equivalent.

In such a period 18 CC + 3 NC neutrino interactions were observed (to be compared with the expectation of 24 CC interactions with 100 % efficiency).

The selected events are being reconstructed, analyzed and used to qualify an automatic filter program which is progressing.

Fig. 5 – CNGS neutrino interaction (image is 1,4 m x 8 m)

Fig. 6 – Very low energy CNGS neutrino interaction. Fully reconstructed in 3D. Total visible energy 770 Mev.

Upgrades proposed for 2011

A new programmable system, based on the use of FPGA has been designe and tested in a LNL test facility (Icarino). The system is fully described in: B. Baibussinov et al., arXiv:1009.2262. Submitted to JINST.

It performs ROI recognition via filtering baseline low frequency noise averaging on a long sliding window, 128 samples long, and high frequency signal noise averaging on a short sliding window, 8 samples long.The principle is shown in Fig. 7.

Fig. 7 – 8 samples average reduces (a) high frequency oscillation, 128 samples average (b) represents baseline modulation, signal is extracted by comparison (c).

A digital signal (Peak) is extracted as it can be seen from Fig. 7 comparing the signals processed by the two windows. Threshold is programmable and also stretching of the discriminated signals to perform majority logic for inclined track where Peak signals from different wires are at different times.

The project of implementing this solution in the existing read out electronics has been preliminary approved. We plan to implement the new ROI processor before the next year run. The unit can also be used to self trigger the detector. In Fig. 8 a prototype is shown. The new unit is the large piggy-back board holding only a large FPGA.

Fig. 8 – The Icarus read-out digital board holding the new ROI detector implemente in FPGA.

Conclusions.

Cryogenic noble liquids and Argon “in primis” have recently regained a strong interest in the scientific community and the successful assembly and operation of the ICARUS-T600 LAr-TPC demonstrate that the technology is mature.

The wide physics potentials offered by high granularity imaging and extremely high resolution will be addressed already with the T600 detector:

proton decay, solar and supernova neutrinos;
long-baseline, high precision neutrino physics.

The T600 is presently taking data, smoothly reaching optimal working conditions. Data analysis is already on-going.

The ICARUS experiment at the Gran Sasso Laboratory is so far the most important milestone for this technology and acts as a full-scale test-bed located in a difficult underground environment.