ICARUS T600: low energy electrons

1. ICARUS T600:low energy electrons R. Dolfini, A. Gigli Berzolari, F. Mauri, A. Menegolli, A. Rappoldi, G.L. Raselli ICARUS Collaboration Meeting

2. A search for low energy electrons (E < 5 MeV) in T300 test run data has been performed in order to get information on the spectral shape and on the rate of the background • Sources can be: • Cosmic rays • Environmental radioactivity • Contaminants of detector materials • Electrons produced through: • Compton effect • Pair production • b decay of 39Ar ? For this kind of event no zero-time information is available: a distortion of the spectrum is expected, as well as an underestimation of the energy ICARUS Collaboration Meeting

3. Signals were searched in clean regions of Collection view (Right Chamber) not crossed by muons or electromagnetic showers A dedicate software finds and reconstructs all the hits in the selected Collection regions, wire by wire: electrons with few hundred keV of kinetic energy are well discriminated from the electronic noise: Low energy electron with 103 ADC count area (~ 370 keV energy) ICARUS Collaboration Meeting

4. Requests of hit finding: • Minimum signal area of 60 ADC counts • Minimum peak high of 5.5 ADC counts over baseline • - Track composed by a maximum of 8 wires A sample of 3336 selected electron candidates has been found over 283681 scanned wires Most of events hits just one wire ICARUS Collaboration Meeting

5. peak @ 140 ADC counts Measured electron energy spectrum: a small peak at ~ 140 ADC counts is evident ICARUS Collaboration Meeting

6. Assuming that the energy calibration factor, obtained with MIP muons is roughly usable for these small energy electrons, the energy of the peak is in the region of photoelectric peak, produced by photons of the positron annihilation (511 keV) • Positrons come from pair production due to: • Photons from cosmic rays • Radioactive decays of the detector material contaminants • (like the 2.6 MeV photons of 232Th or the 2.2 MeV of 238U) The observed peak seems to be not smeared by the the drift electron mean life, which would deform the spectrum if the electrons were produced equally inside the liquid argon volume This indicates that these photoelectrons are produced at the same drift time: maybe close to the wire planes? ICARUS Collaboration Meeting

7. Analysis: • Just the electrons reconstructed on one wire only were used. • All the electrons of the same trigger within a 10 cm radius sphere were • associated in one cluster. • A fit with a Gauss function plus a 3rd degree polynomial was performed • on the distribution of 2337 clusters. The best fit gives the following energy calibration parameter: E(keV) = (3.59 ± 0.25) · ADC(counts) ICARUS Collaboration Meeting

8. The peak is not a Compton edge, in this case the energy would be ~ 340 keV, or the calibration parameter too different (~40% lower) from that measured with MIP muons ICARUS Collaboration Meeting

9. To properly evaluate the trigger rate, the efficiency of the hit finding as a function of the electron energy has been evaluated simulating with FLUKA-ICARUS electrons of five different energies below 1 MeV: Preliminary Efficiency > 90% above 700 keV Fake hits: No more than 2% ICARUS Collaboration Meeting

10. A calculation: how many photoelectric events are expected from the wall contaminants? A measurement of the radioactive activity of the Al honeycomb walls has been accomplished in 2003, giving the following results: We assume that the 511 keV photoelectrons due to detector contaminants arise from the following mechanisms (cross sections come from NIST): Al honeycomb 32 cm 1. 238U (7.4 Bq/Kg) and 232Th (1.8 Bq/Kg) of the honeycomb walls produce photons (2.2 and 2.6 MeV respectively) in their decays;   2. half of the photons enter the LAr volume and produce (e+,e-) pairs; e- e+ 3. the positron travels few mm of LAr before to annihilate in two 511 keV photons…  wires 4. …which interact with an Ar nucleus, producing a 511 keV photoelectron (0.56% with respect the Compton interaction in Ar). photoelectron ICARUS Collaboration Meeting

11. 57 triggers were analysed, corresponding to an observation time of 0.114 s; • the expected rate of photoelectrons, detected in the fiducial volume, • produced by positron annihilations in the first 30 cm is 103 Hz; • it corresponds to 8 photoelectrons (103*0.114*0.68=8) expected from the • wall contaminants in the detection time of 0.114 seconds with a detection • efficiency of 68%; • these 8 photoelectrons are part of the sample of 138 electrons found in • the 511 keV peak; • it results that the photoelectrons produced by the honeycomb contaminants • are 6%. of the total measured, being the residual 94% due to the cosmic rays. ICARUS Collaboration Meeting

12. Trigger rate for T300 module Hit finding efficiency assumed equal to one. Detection efficiency, that is 1 beyond 700 keV, is taken into account. Expectation at underground laboratory, where the cosmic contribution is cut assuming the ratio 8/138 constant at all energies. ICARUS Collaboration Meeting

13. Conclusions • A search for electrons with energy below 5 MeV in T300 test run data has been performed; • in the electron spectrum is clearly visible a peak, interpreted as a photoelectric peak; • in this assumption an energy calibration parameter similar to the one measured with MIP muons has been obtained: C = 3.59 ± 0.25 keV/ADC; • the photoelectric events are 138, of which a calculation shows that only ~ 8 are due to detector wall contaminants; • this, together with the reconstruction efficiency, allows to foresee the trigger rate underground, where the contribution of cosmic rays is negligible. ICARUS Collaboration Meeting