Project number: P05/03
Final scientific report
Project Title: OPERA trigger background
LNGS Access period: February 2003 – July 2003
Group Leader: D. Autiero: IPN Lyon (France)
Researchers: D. Autiero, C. Heritier, E. Baussan, J. Marteau: : IPN Lyon (France)
Delivered access (filled by TARI administration): 40 man-days
Final scientific report (filled by group leader):
A prototype of the OPERA target tracker (TT), consisting in two scintillator modules of 167x167 cm2, providing two orthogonal views, was brought in July to the Gran Sasso underground laboratory (Hall B) in order to take measurements of the environmental background under various conditions. Each target tracker module was consisting of 64 scintillator strips with embedded wave length shifting (WLS) fibres. The WLS fibres of each module were readout form one side only with one photomultiplier Hamamatsu M64. The two target tracker modules were hold in vertical position by a structure capable as well of supporting two lead walls covering completely the surface of the scintillator modules. The two TT modules were placed in between the two lead walls, which had a thickness of 5 cm each. The lead walls were covered with iron plates of 2 mm thickness in order to shield the TT modules from the radioactivity coming from the lead itself. The overall set-up was representative of a small fraction of the OPERA target-tracker and wall structure with a total weight of about 4 tonnes. Each one of the two M64 photomultipliers was connected to an OPERA self-triggering DAQ Ethernet card. An external NIM logic was set up in order to measure coincidences among the triggers generated by the two cards.
In order to disentangle the various contributions to background rate, the spectra of the singles and the coincidences were measured under various conditions:
a) The thermal emission of the two photomultiplier was measured before connecting them to the scintillator modules
b) The singles and coincidences were measured for the two modules without the presence of the lead shielding provided by the two walls
c) The TT modules were sandwiched in between the two lead walls and the singles and coincidences spectra were measured again
d) The surface of the lead walls in contact with the TT modules was covered by a thin layer of radiopure lead of the type which will be used for the production of the OPERA bricks (it would not have been possible at the moment, given the large quantity of lead, to make the entire walls out of radiopure lead. The simulations show that due to the lead self-shielding the contribution to the background rate due to the lead radioactivity is limited to a thin external layer.
e) The radon concentration in the air was measured independently during data-taking
The average measured rates per photomultiplier channel were:
0.5 Hz singles for a) ;
31 Hz singles, 1.6 Hz coincidences for b);
16 Hz singles, 0.25Hz coincidences for c);
no appreciable differences among c) and d).
The setting up and the measurements took in total 10 days. The quality of the data taken allow to perform a detailed study of the various components of the background: thermal emission, radioactivity from the rock, radon, radioactivity from the lead etc ... The measurements taken thanks to the TARI contract are therefore considered completed and successful.
The data are now being compared to a detailed simulation that takes into account the response of the scintillator and environmental radioactivity measurements taken in the past in the underground lab.
Once the simulation will have been carefully cross-checked with the data taken in all these configurations, it will be possible to use it in order to predict the background in all the regions of the OPERA TT. These will be generally more shielded with respect to the radioactivity coming from the rock than the prototype of this test due to the presence of several lead walls or because the region in question will be in a inner part of a TT plane. The rates measured during this test represent an upper limit to the background rates which will be measured in OPERA. This final simulation will allow to optimise the data-flow in the DAQ system, to tune the trigger strategy in OPERA and to understand the background for some physics topics like the sensitivity in the detection of supernova neutrinos. The results of the measurements we took are also very relevant in order to understand the procedure for the calibration of the TT modules by using the environmental radioactivity.
A future set of measurements on another topic can be foreseen with a completely different set-up integrating emulsions and electronic detectors in order to measure the capability of detecting rare signals in presence of the environmental background. For these new measurements we also consider the possibility for a TARI contract in 2004.
Publications planned or in preparation (filled by group leader):
1) OPERA internal note on: the various contributions of the environmental radioactivity on the TT background rates, consequences on the DAQ structure and trigger strategy optimisation, TT modules calibration procedure with the environmental radioactivity
2) Paper on the detection of supernova neutrinos with the OPERA detector
Evaluation by USP (October 2003)
The aim of this project was to test a prototype of the OPERA experiment target tracker in the environmental conditions of the underground lab. Most of the objectives have been achieved.
LNGS Access period: February 2003 – July 2003
Group Leader: D. Autiero: IPN Lyon (France)
Researchers: D. Autiero, C. Heritier, E. Baussan, J. Marteau: : IPN Lyon (France)
Delivered access (filled by TARI administration): 40 man-days
Final scientific report (filled by group leader):
A prototype of the OPERA target tracker (TT), consisting in two scintillator modules of 167x167 cm2, providing two orthogonal views, was brought in July to the Gran Sasso underground laboratory (Hall B) in order to take measurements of the environmental background under various conditions. Each target tracker module was consisting of 64 scintillator strips with embedded wave length shifting (WLS) fibres. The WLS fibres of each module were readout form one side only with one photomultiplier Hamamatsu M64. The two target tracker modules were hold in vertical position by a structure capable as well of supporting two lead walls covering completely the surface of the scintillator modules. The two TT modules were placed in between the two lead walls, which had a thickness of 5 cm each. The lead walls were covered with iron plates of 2 mm thickness in order to shield the TT modules from the radioactivity coming from the lead itself. The overall set-up was representative of a small fraction of the OPERA target-tracker and wall structure with a total weight of about 4 tonnes. Each one of the two M64 photomultipliers was connected to an OPERA self-triggering DAQ Ethernet card. An external NIM logic was set up in order to measure coincidences among the triggers generated by the two cards.
In order to disentangle the various contributions to background rate, the spectra of the singles and the coincidences were measured under various conditions:
a) The thermal emission of the two photomultiplier was measured before connecting them to the scintillator modules
b) The singles and coincidences were measured for the two modules without the presence of the lead shielding provided by the two walls
c) The TT modules were sandwiched in between the two lead walls and the singles and coincidences spectra were measured again
d) The surface of the lead walls in contact with the TT modules was covered by a thin layer of radiopure lead of the type which will be used for the production of the OPERA bricks (it would not have been possible at the moment, given the large quantity of lead, to make the entire walls out of radiopure lead. The simulations show that due to the lead self-shielding the contribution to the background rate due to the lead radioactivity is limited to a thin external layer.
e) The radon concentration in the air was measured independently during data-taking
The average measured rates per photomultiplier channel were:
0.5 Hz singles for a) ;
31 Hz singles, 1.6 Hz coincidences for b);
16 Hz singles, 0.25Hz coincidences for c);
no appreciable differences among c) and d).
The setting up and the measurements took in total 10 days. The quality of the data taken allow to perform a detailed study of the various components of the background: thermal emission, radioactivity from the rock, radon, radioactivity from the lead etc ... The measurements taken thanks to the TARI contract are therefore considered completed and successful.
The data are now being compared to a detailed simulation that takes into account the response of the scintillator and environmental radioactivity measurements taken in the past in the underground lab.
Once the simulation will have been carefully cross-checked with the data taken in all these configurations, it will be possible to use it in order to predict the background in all the regions of the OPERA TT. These will be generally more shielded with respect to the radioactivity coming from the rock than the prototype of this test due to the presence of several lead walls or because the region in question will be in a inner part of a TT plane. The rates measured during this test represent an upper limit to the background rates which will be measured in OPERA. This final simulation will allow to optimise the data-flow in the DAQ system, to tune the trigger strategy in OPERA and to understand the background for some physics topics like the sensitivity in the detection of supernova neutrinos. The results of the measurements we took are also very relevant in order to understand the procedure for the calibration of the TT modules by using the environmental radioactivity.
A future set of measurements on another topic can be foreseen with a completely different set-up integrating emulsions and electronic detectors in order to measure the capability of detecting rare signals in presence of the environmental background. For these new measurements we also consider the possibility for a TARI contract in 2004.
Publications planned or in preparation (filled by group leader):
1) OPERA internal note on: the various contributions of the environmental radioactivity on the TT background rates, consequences on the DAQ structure and trigger strategy optimisation, TT modules calibration procedure with the environmental radioactivity
2) Paper on the detection of supernova neutrinos with the OPERA detector
Evaluation by USP (October 2003)
The aim of this project was to test a prototype of the OPERA experiment target tracker in the environmental conditions of the underground lab. Most of the objectives have been achieved.