1. ICARUS T600: low energy electrons
R. Dolfini, A. Gigli Berzolari, F. Mauri, A. Menegolli, A. Rappoldi, G.L. Raselli
28/06/2005
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
28/06/2005
ICARUS Collaboration Meeting

3. not crossed by muons or electromagnetic showers
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)
28/06/2005
ICARUS Collaboration Meeting

4. Most of events hits just one wire
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 scanned wires
Most of events hits just one wire
28/06/2005
ICARUS Collaboration Meeting

5. ICARUS Collaboration Meeting
140 ADC counts
Measured electron energy spectrum: a small peak at ~ 140 ADC counts is evident
28/06/2005
ICARUS Collaboration Meeting

6. This indicates that these photoelectrons are produced
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?
28/06/2005
ICARUS Collaboration Meeting

7. Just the electrons reconstructed on one wire only were used.
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)
28/06/2005
ICARUS Collaboration Meeting

8. ICARUS Collaboration Meeting
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
28/06/2005
ICARUS Collaboration Meeting

9. ICARUS Collaboration Meeting
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%
28/06/2005
ICARUS Collaboration Meeting

10. A calculation: how many photoelectric events
are expected from the wall contaminants?
We assume that the 511 keV photoelectrons due to detector contaminants arise from the following mechanisms (cross sections come from NIST):
A measurement of the radioactive activity of the Al honeycomb walls has been accomplished in 2003, giving the following results:
Contaminant
Bq/Kg
238U
7.4
232Th
1.8
Al honeycomb
32 cm
U (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
28/06/2005
ICARUS Collaboration Meeting

11. ICARUS Collaboration Meeting
57 triggers were analysed, corresponding to an observation time of 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 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.
28/06/2005
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.
28/06/2005
ICARUS Collaboration Meeting

13. ICARUS Collaboration Meeting
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.
28/06/2005
ICARUS Collaboration Meeting