28/06/2005 ICARUS Collaboration Meeting 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 datahas 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- decay of 39Ar ?

For this kind of event no zero-time information is availableno 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

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 kineticenergy are well discriminated from the electronic noise:

Low energy electronwith 103 ADC count area

(~ 370 keV energy)

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A sample of 3336 selected electron candidates has been found over 283681 scanned wires

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

Most of events hits just one wire

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Measured electron energy spectrum: a small peak at ~ 140 ADC counts is evident

peak @ 140 ADC countspeak @ 140 ADC counts

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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 peakis in the region of photoelectric peak, produced by photons of the positron annihilation (511 keV511 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 deformdeform thespectrum if the electrons were produced equally insidethe liquid argon volume

This indicates that these photoelectrons are produced at the same drift time: maybe close to the wire planes?maybe close to the wire planes?

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0 100 200 300 400 500 600

0 keV 359 keV 718 keV 1077 keV 1436 keV 1795 keV 2154 keV

0

50

100

150

200

250

300

350

400

450

500

ADC counts

dN

/dA

DC

/ 1

0 A

DC

co

un

ts

low energy clustersfrom 3004 electrons

---------------------------------------------------------Chi squared = 3.9 / 5 dofFitted parameters: Standard deviations:bg-0 = 1371.727 ± 260.992bg-1 = -25.413 ± 5.685bg-2 = 0.160 ± 3.988e-2bg-3 = -3.319e-4 ± 9.121e-5

Am-1 = 51.1 ± 10.8xo-1 = 142.2 ± 2.2s-1 = 10.7 ± 2.7

Goodness of fit: = 0.95---------------------------------------------------------

photoelectric peak (511 keV)

2337 selected clusters(electrons within a 10 cm radius sphere are summed in one cluster)

Cal. factor = ( 3.59 ± 0.25 ) keV/ADC

electron multiplicity clusters1 18122 4103 944 165 46 1

138 events in the peak

Analysis:

- Just the electrons reconstructed on one wire onlyone wire only were used.- All the electrons of the same trigger within a 10 cm radius spherewithin 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 clusters2337 clusters.

The best fit gives the followingenergy calibration parameter:

E(keV) = (3.59 E(keV) = (3.59 ± 0.25) · ADC(counts)± 0.25) · ADC(counts)

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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

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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:

Efficiency Efficiency > 9> 90%0%above 700 keVabove 700 keV

Fake hits: No more than Fake hits: No more than 2%2%

Preliminary

Preliminary

28/06/2005 ICARUS Collaboration Meeting 10

A calculation: how many photoelectric events are expected from the wall contaminants?

Contaminant

Bq/Kg

238U 7.4232Th 1.8

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):

4. …which interact with an Ar nucleus, producing a 511 keV photoelectron (0.56% with respect the Compton interaction in Ar).

Al honeycomb

e-e+

photoelectron

wires

32 cm1. 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;

3. the positron travels few mm of LAr before to annihilate in two 511 keV photons…

28/06/2005 ICARUS Collaboration Meeting 11

• 57 triggers were analysed, corresponding to an observation time of 0.114 s0.114 s;

• the expected rate of photoelectrons, detected in the fiducial volume, produced by positron annihilations in the first 30 cm is 103 Hz103 Hz;

• it corresponds to 8 photoelectrons8 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%68%;

• these 8 photoelectrons are part of the sample of 138 electrons found in 138 electrons found in the 511 keV peakthe 511 keV peak;

• it results that the photoelectrons produced by the honeycomb contaminants are 6%. of the total measured6%. of the total measured, being the residual 94% due to the cosmic 94% due to the cosmic raysrays.

28/06/2005 ICARUS Collaboration Meeting 12

Trigger rate for T300 module

1. Hit finding efficiency assumed equal to one.

2. Detection efficiency, that is 1 beyond 700 keV, is taken into account.

3. 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

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/ADCC = 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.