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Status of the ICARUS experiment
Nuclear Physics B (Proc. Suppl.) 217 (2011) 186–188 www.elsevier.com/locate/npbps
Status of the ICARUS experiment P. R. Sala for the ICARUS collaborationa a
INFN Milano, via Celoria 16, 20133 Milano, Italy
The ICARUS T600 detector is in operation at the Gran Sasso underground laboratory. High quality cosmic ray and CNGS beam events are being collected and analyzed.
1. Introduction
2. The T600 detector
The T600 detector, presently in operation at the LNGS INFN underground laboratory, is the first large-mass application of a powerful detection technique: the Liquid Argon Time Projection Chamber (LAr TPC)[1], first proposed to INFN in 1985[2]. This technique allows to perform simultaneously calorimetry and track reconstruction, both with excellent granularity and resolution, on a large mass scale. Therefore, it is the perfect choice for the present and future generation of neutrino and rare event detectors. After a long R&D, the viability of large mass detectors was already demonstrated on surface in 2001[3]. Finally, the T600 fulfilled the safety and technical requirements of the LNGS laboratory. Its operation underground seals the maturity of the LAr TPC technique. T600 offers also some interesting physics. Neutrino interactions and neutral current discrimination will be studied thanks to the ≈ 1200 νμ CC events and 400 NC events per year from the the CNGS[4] beam. ντ events will be searched for with kinematical criteria. A search for sterile ν in LSND parameter space will be possible looking for an excess of e-like CC events at E>10GeV. The T600 is also collecting simultaneously self triggered events, in particular about 100 events/year of atmospheric CC interactions and 300 from solar neutrino interactions. Supernova ν are also detectable, with, for instance, about 200 events from an explosion at 10kpc. Finally, the T600 allows for zero background proton decay searches with 3 · 1032 nucleons
The ICARUS-T600 detector [3] is composed by two identical half-modules, with internal dimensions 3.6 × 3.9 × 19.9 m3 each. Externally the cryostats are surrounded by a common thermal insulation layer. The detector layout is completed by a cryogenic plant made of a liquid Nitrogen cooling circuit. Moreover, each semi-module is equipped with two gas and one liquid recirculation and purification systems. Each half-module houses two Time Projection Chambers (TPC) separated by a common cathode and two arrays of photo-multipliers. Each TPC is made of three parallel planes of wires, 3 mm apart, with 3 mm wire pitch, at 00 and ±600 with respect to the horizontal direction. The signals coming from each wire are independently digitized every 400 ns. The maximum drift path, i.e.the distance between the cathode and the wire planes, is 1.5 m and the nominal drift field 500 V/cm, corresponding to an electron drift velocity of about 1.5mm/μs. Globally 74 photomultipliers coated with TPB wavelength-shifter to detect the UV scintillation light are directly installed in LAr behind the wire planes of TPCs[5].
0920-5632/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysbps.2011.04.096
3. Detector commissioning and first events LAr purity is essential in order to avoid the attachment of drifting electrons by electronegative impurities. A concentration lower than 0.1 ppb (part per 109 ) O2 is required for the T600 1.5 m drift distance. Therefore, the detector is evacuated to a pressure of the order of 10−4 mbar before filling for a proper out-gassing. The cryostats
P.R. Sala / Nuclear Physics B (Proc. Suppl.) 217 (2011) 186–188
187
Figure 1. The first CNGS neutrino interaction in T600.
evacuation started on January 2010 reaching on April the residual vacuum pressure in the detector 4.5 10−5 mbar and 3.8 10−5 mbar in West cryostat and East cryostat. These values correspond to residual vacuum losses of 0.06 and 0.04 mbar l/s respectively mainly due to out-gassing. Sensors for the measurement of the thermal insulation and lateral panel deformations allowed to measure the thermal tightness on the panels, in agreement with the expectations. Cooling down of the detector was achieved at an average rate of about 1 K/hour by filling with ultra-pure Argon gas. Finally cryostats were filled at a rate of ∼ 1m3 /hour/cryostat with 47 trucks in about 2 weeks. The commercially available liquid Argon was purified at the cryostat input by means of commercial filters. During the whole period the four gaseous re-circulation systems were operating at maximum speed. The liquid recirculation became available on both cryostats, at a speed of ∼ 1 m3 /hour/cryostat, one month after the filling completion. On May 27th , the readout was activated on half of the detector. Cosmic muon tracks and neutrino interactions were immediately detected and visualized, as shown
in fig1. The data quality is amazingly good for a start-up condition. Residual small issues, like for instance the wrong wire polarization that affects the vertex area in fig.1, are being looked after. The average electronic noise was measured to be well within expectations on practically all the 54000 channels: 1500 electrons rms to be compared with ∼ 15000 free electrons produced by m.i.p. over 3 mm (S/N ≥ 10). Muon tracks are used to evaluate electron lifetime in real time. The evolution of LAr purity in the two cryostats is shown in fig2. The residual impurity concentration can be described with a simple model as N (t) = kτR + [N (t = 0) − kτR ] exp(−t/τR ) where τR is the time needed to recirculate a full detector volume and k is the total impurity leak and degassing rate and N (t = 0) corresponds to the initial level of impurities. The free electron lifetime can be expressed as τele [ms] = 0.3/N [ppb], Fitting the data with this model yields a recirculation time of 6 days, in agreement with the nominal pump speed, and leak rates of 7 and 20 ppt/day O2 equivalent impurity concentration in the West and East half-modules respectively. The presently attained purities correspond to free
188
P.R. Sala / Nuclear Physics B (Proc. Suppl.) 217 (2011) 186–188
Figure 3. Example of ν interaction. Top: collection view, bottom: 3D reconstruction 4. Conclusions
Figure 2. Evolution of LAr purity in the West (top) and East (bottom) T600 cryostats
electron charge attenuation for 1.5 m of 17% and 31% respectively. The initial data taking has been performed with a PMT based trigger, exploiting either PMT sum signals or two and three-fold PMT coincidences. Optimization of the trigger configuration is ongoing. Synchronization with the CNGS beam time stamp is progressing, and a CNGS gate will be implemented soon. The reconstruction software is also in the commissioning phase on real data. An example of 3D event reconstruction is shown in fig3 for a ν interaction with small (≈ 850 MeV) energy deposition.
The ICARUS T600 detector has started data taking in the Gran Sasso underground laboratory. Bubble-chamber like events from cosmic rays and from the CNGS beam are being collected, thus demonstrating that the technology is mature for a full physics exploitation. REFERENCES 1. C. Rubbia, CERN-EP/77-08 (1977). 2. F. Arneodo et al.[ICARUS Collab.], LNGS P28/01, LNGS-EXP 13/89 add. 1/01; LNGSEXP 13/89 add. 2/01. 3. S. Amerio et al. [ICARUS Collab.], Nucl. Instr. and Methods A527 (2004) 329. 4. G. Acquistapace et al., CERN-98-02, INFN/AE-98/05 (1998); CERN-SL/99034(DI), INFN/AE-99/05 Addendum. 5. A. Ankowski et al. [ICARUS Collab.] Nucl. Instr. and Meth. A556 (2005) 146.
Status of the ICARUS experiment P. R. Sala for the ICARUS collaborationa a
INFN Milano, via Celoria 16, 20133 Milano, Italy
The ICARUS T600 detector is in operation at the Gran Sasso underground laboratory. High quality cosmic ray and CNGS beam events are being collected and analyzed.
1. Introduction
2. The T600 detector
The T600 detector, presently in operation at the LNGS INFN underground laboratory, is the first large-mass application of a powerful detection technique: the Liquid Argon Time Projection Chamber (LAr TPC)[1], first proposed to INFN in 1985[2]. This technique allows to perform simultaneously calorimetry and track reconstruction, both with excellent granularity and resolution, on a large mass scale. Therefore, it is the perfect choice for the present and future generation of neutrino and rare event detectors. After a long R&D, the viability of large mass detectors was already demonstrated on surface in 2001[3]. Finally, the T600 fulfilled the safety and technical requirements of the LNGS laboratory. Its operation underground seals the maturity of the LAr TPC technique. T600 offers also some interesting physics. Neutrino interactions and neutral current discrimination will be studied thanks to the ≈ 1200 νμ CC events and 400 NC events per year from the the CNGS[4] beam. ντ events will be searched for with kinematical criteria. A search for sterile ν in LSND parameter space will be possible looking for an excess of e-like CC events at E>10GeV. The T600 is also collecting simultaneously self triggered events, in particular about 100 events/year of atmospheric CC interactions and 300 from solar neutrino interactions. Supernova ν are also detectable, with, for instance, about 200 events from an explosion at 10kpc. Finally, the T600 allows for zero background proton decay searches with 3 · 1032 nucleons
The ICARUS-T600 detector [3] is composed by two identical half-modules, with internal dimensions 3.6 × 3.9 × 19.9 m3 each. Externally the cryostats are surrounded by a common thermal insulation layer. The detector layout is completed by a cryogenic plant made of a liquid Nitrogen cooling circuit. Moreover, each semi-module is equipped with two gas and one liquid recirculation and purification systems. Each half-module houses two Time Projection Chambers (TPC) separated by a common cathode and two arrays of photo-multipliers. Each TPC is made of three parallel planes of wires, 3 mm apart, with 3 mm wire pitch, at 00 and ±600 with respect to the horizontal direction. The signals coming from each wire are independently digitized every 400 ns. The maximum drift path, i.e.the distance between the cathode and the wire planes, is 1.5 m and the nominal drift field 500 V/cm, corresponding to an electron drift velocity of about 1.5mm/μs. Globally 74 photomultipliers coated with TPB wavelength-shifter to detect the UV scintillation light are directly installed in LAr behind the wire planes of TPCs[5].
0920-5632/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysbps.2011.04.096
3. Detector commissioning and first events LAr purity is essential in order to avoid the attachment of drifting electrons by electronegative impurities. A concentration lower than 0.1 ppb (part per 109 ) O2 is required for the T600 1.5 m drift distance. Therefore, the detector is evacuated to a pressure of the order of 10−4 mbar before filling for a proper out-gassing. The cryostats
P.R. Sala / Nuclear Physics B (Proc. Suppl.) 217 (2011) 186–188
187
Figure 1. The first CNGS neutrino interaction in T600.
evacuation started on January 2010 reaching on April the residual vacuum pressure in the detector 4.5 10−5 mbar and 3.8 10−5 mbar in West cryostat and East cryostat. These values correspond to residual vacuum losses of 0.06 and 0.04 mbar l/s respectively mainly due to out-gassing. Sensors for the measurement of the thermal insulation and lateral panel deformations allowed to measure the thermal tightness on the panels, in agreement with the expectations. Cooling down of the detector was achieved at an average rate of about 1 K/hour by filling with ultra-pure Argon gas. Finally cryostats were filled at a rate of ∼ 1m3 /hour/cryostat with 47 trucks in about 2 weeks. The commercially available liquid Argon was purified at the cryostat input by means of commercial filters. During the whole period the four gaseous re-circulation systems were operating at maximum speed. The liquid recirculation became available on both cryostats, at a speed of ∼ 1 m3 /hour/cryostat, one month after the filling completion. On May 27th , the readout was activated on half of the detector. Cosmic muon tracks and neutrino interactions were immediately detected and visualized, as shown
in fig1. The data quality is amazingly good for a start-up condition. Residual small issues, like for instance the wrong wire polarization that affects the vertex area in fig.1, are being looked after. The average electronic noise was measured to be well within expectations on practically all the 54000 channels: 1500 electrons rms to be compared with ∼ 15000 free electrons produced by m.i.p. over 3 mm (S/N ≥ 10). Muon tracks are used to evaluate electron lifetime in real time. The evolution of LAr purity in the two cryostats is shown in fig2. The residual impurity concentration can be described with a simple model as N (t) = kτR + [N (t = 0) − kτR ] exp(−t/τR ) where τR is the time needed to recirculate a full detector volume and k is the total impurity leak and degassing rate and N (t = 0) corresponds to the initial level of impurities. The free electron lifetime can be expressed as τele [ms] = 0.3/N [ppb], Fitting the data with this model yields a recirculation time of 6 days, in agreement with the nominal pump speed, and leak rates of 7 and 20 ppt/day O2 equivalent impurity concentration in the West and East half-modules respectively. The presently attained purities correspond to free
188
P.R. Sala / Nuclear Physics B (Proc. Suppl.) 217 (2011) 186–188
Figure 3. Example of ν interaction. Top: collection view, bottom: 3D reconstruction 4. Conclusions
Figure 2. Evolution of LAr purity in the West (top) and East (bottom) T600 cryostats
electron charge attenuation for 1.5 m of 17% and 31% respectively. The initial data taking has been performed with a PMT based trigger, exploiting either PMT sum signals or two and three-fold PMT coincidences. Optimization of the trigger configuration is ongoing. Synchronization with the CNGS beam time stamp is progressing, and a CNGS gate will be implemented soon. The reconstruction software is also in the commissioning phase on real data. An example of 3D event reconstruction is shown in fig3 for a ν interaction with small (≈ 850 MeV) energy deposition.
The ICARUS T600 detector has started data taking in the Gran Sasso underground laboratory. Bubble-chamber like events from cosmic rays and from the CNGS beam are being collected, thus demonstrating that the technology is mature for a full physics exploitation. REFERENCES 1. C. Rubbia, CERN-EP/77-08 (1977). 2. F. Arneodo et al.[ICARUS Collab.], LNGS P28/01, LNGS-EXP 13/89 add. 1/01; LNGSEXP 13/89 add. 2/01. 3. S. Amerio et al. [ICARUS Collab.], Nucl. Instr. and Methods A527 (2004) 329. 4. G. Acquistapace et al., CERN-98-02, INFN/AE-98/05 (1998); CERN-SL/99034(DI), INFN/AE-99/05 Addendum. 5. A. Ankowski et al. [ICARUS Collab.] Nucl. Instr. and Meth. A556 (2005) 146.