Monitoring of the ANTARES optical module efficiencies using 40K decays in sea water

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PoS(ICRC2017)1058
Monitoring of the ANTARES optical module
efficiencies using 40Kdecays in sea water
I. Salvadori∗and V. Kulikovskiy
Aix Marseille Univ, CNRS/IN2P3, CPPM, Marseille, France
E-mail: salvadori@cppm.in2p3.fr,kulikovskiy@cppm.in2p3.fr
On behalf of the ANTARES Collaboration
Using the data collected by the ANTARES neutrino telescope from 2009 to 2016, the optical
module (OM) efficiencies have been determined through the so called 40K method. The results
have been computed on a 6-day basis, after applying selection cuts in order to provide reliable
time-dependent OM efficiencies for most of the individual OMs. The results show an impressive
stability over time, as well as the benefit of the high voltage tuning (HVT), which is a dedicated
procedure aimed to keep efficiencies at their best.
35th International Cosmic Ray Conference - ICRC2017-
10-20 July, 2017
Bexco, Busan, Korea
∗Speaker.
c
Copyright owned by the author(s) under the terms of the Creative Commons
Attribution-NonCommercial-NoDerivatives 4.0 International License (CC BY-NC-ND 4.0). http://pos.sissa.it/

PoS(ICRC2017)1058
ANTARES OM Efficiencies I. Salvadori
1. Introduction
40K is the most abundant radioactive isotope in sea water. Its Cherenkov light spectrum is equal
to the one produced by muons detected by the ANTARES neutrino telescope [1]. It constitutes the
principal source of background light. However, 40 K is as well an important calibration tool. In
ANTARES, the optical modules (OMs) are arranged in groups of three (storey) and, if a 40 K decays
near a storey, its Cherenkov light can be recorded by two OMs simultaneously. Such coincidences
are dominated by 40K, therefore the measured rates can be used to tune the overall OM efficiency
in a detailed GEANT4 [5] simulation of the OM. This simulation provides valuable input for the
global detector simulation.
The document is organized as follows: in Section 2a brief description of the ANTARES
neutrino telescope is given; the 40K method for the computation of the OM efficiencies is described
in Section 3; in Section 4the used data set is presented; the fitting procedure together with the
quality cuts applied are explained in Section 5, while the results of the analysis are presented in
Section 6. In Section 7a brief description of GEANT4 dedicated simulations for the overall OM
efficiency is given. Conclusions and an outlook to the next generation of neutrino telescopes in the
Mediterranean Sea are given in Section 8.
2. The ANTARES neutrino telescope
The ANTARES neutrino telescope was deployed in the Mediterranean Sea, 40 km from the
coast of Toulon (France), at a depth of around 2.4 km. It was completed in 2008. The main
goal of ANTARES is, at high energies, the study of energetic astrophysical objects. However, at
lower energies, neutrino oscillations can be measured by analyzing distortions in the energy/angular
spectrum of upward-going atmospheric neutrinos.
Figure 1: Schematic representation of the ANTARES neutrino telescope [2].
ANTARES is composed of 12 detection lines, each one equipped with 25 floors of 3 optical
modules. Each OM holds a photomultiplier tube (PMT). The horizontal spacing among the lines
is around 60 m, while the vertical spacing between the storeys is around 15 m (see Figure 1). The
OMs in a storey are arranged in such a way that the axis of the PMTs points 45◦downwards (see
Figure 2). The photomultipliers are 10-inch tubes from Hamamatsu. The relative positions of all
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ANTARES OM Efficiencies I. Salvadori
OMs in the detector are monitored in real time by a dedicated positioning system.
When the distribution of the fitted charge of all PMTs in the detector shows either a broadening
or a shift with respect to its nominal value, resulting in losses of efficiency and trigger bias, a
dedicated procedure of high voltage tuning (HVT) is performed. The aim of this operation is to
reset the effective threshold to its canonical value.
Figure 2: Schematic representation of an ANTARES storey [3]. The spheres stand for the OMs, which
contain one PMT each, facing 45◦downwards.
3. The 40KMethod
The main decay channels of 40K are βdecay and electron capture:
40K→40 Ca +e−+νe(89.3%)
40K+e−→40 Ar∗+νe(10.7%)
→40Ar +γ
The free electron produced in the first decay channel induces Cherenkov light emission when travel-
ing in water; fast electrons with subsequent Cherenkov light emission are also produced by Comp-
ton scattering of the photon produced by the excited Argon nuclei.
In ANTARES, if a 40 K decays near a storey, its Cherenkov light can be recorded by two OMs
simultaneously: this is called a genuine coincidence. There exists also a background of random
coincidences, which happens when two hits by two different 40K decays appear to be close in time.
By plotting these signals as a function of the time differences between the two OMs, the shape is
that of a flat uniform background due to the random coincidences plus a Gaussian peak from the
genuine coincidences. An example of such distribution is shown in Figure 3.
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ANTARES OM Efficiencies I. Salvadori
/ ndf
2
χ 138.7222 / 116
p0 0.0125± 0.6345
p1 0.0794±0.5962 −
p2 0.0906± 3.7285
p3 0.0044± 3.3423
t [ns]∆
20−15−10−5−0 5 10 15 20
Rate per Bin [Hz]
3.2
3.3
3.4
3.5
3.6
3.7
3.8
3.9
4.0
4.1 / ndf
2
χ 138.7222 / 116
p0 0.0125± 0.6345
p1 0.0794±0.5962 −
p2 0.0906± 3.7285
p3 0.0044± 3.3423
0.004±p = 3.342
0.012±a = 0.634
0.079± = -0.596
0
t
0.091± = 3.728σ
Figure 3: Example of the detected hit time differences, from 40 K background, between two optical modules.
The histogram of the coincidence signal is fitted with a Gaussian distribution plus a uniform
one:
f(t) = p+a·exp(−(t−t0)2
2σ2)(3.1)
where pis the baseline, athe amplitude of the Gaussian peak due to genuine coincidences, σis
the peak width and t0the time offset. A mean value of σ∼4 ns is expected, due to the spatial
distribution of the 40K decays around the storey. The maximal travel distance for two photons
emitted at the same place and detected by two different OMs of the same floor is the sum of the
OMs distance (∼1 m) and the photocatode diameter (∼25 cm); considering the Cherenkov light
velocity of 0.22 m/ns a time difference of 5.6 ns is expected. By averaging over the whole space a
result compatible with 4 ns is found.
For perfectly calibrated OMs, t0would be expected at 0 ns. Deviations from the expected
value of t0are mainly due to imperfections in time calibration. This makes the 40K method also
a useful tool to cross check the time calibration. However in the following we concentrate on the
derivation of relative OM efficiencies from 40K data.
The fit parameters can then be used to estimate the rate of events corresponding to the peak
area, as:
R=a·σ·√2π
T·∆τ(3.2)
where ∆τis the bin length used for the histogram (in this work ∆τ=0.4 ns), and Tis the total
lifetime of the data set.
For each detector storey three coincidence rates are measured (R01,R12 and R20). These quan-
tities are directly related to the sensitivities of the three OMs (s0,s1and s2):
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Ri j =R∗sisj(3.3)
where R∗is the rate for two nominal OMs with sensitivities equal to 1. In this work a value of
R∗=15 Hz was used. It was obtained as an average detector coincidence rate at the beginning
of the analyzed data set. Solving the system of three equations the corresponding sensitivities are
derived:
s0=r1
R∗
R01R20
R12
,s1=r1
R∗
R12R01
R20
,s2=r1
R∗
R20R12
R01
(3.4)
When an OM is broken, only one coincidence histogram is filled, which is not enough to de-
termine the two efficiencies. In this case, equal sensitivities for the two working OMs are assumed,
namely:
si=sj=rRi j
R∗(3.5)
4. Data Set
Data collected from October 2009 to December 2016 have been used in this work. A dedicated
40K trigger was used during the data-taking. For this trigger, coincidence hits in adjacent OMs are
stored if they occur within a narrow time window of typically 50 ns. The trigger is applied with an
important down-scaling factor of 200 in order not to saturate the readout chain. Taking into account
this scaling factor, a total lifetime of 11 days has been analyzed. The runs have been collected in
groups of 6 calendar days, which corresponds to a lifetime of around 40 minutes for each data
point.
5. Procedure
The coincidence histograms are filled whenever the ∆tbetween hits is within a maximally
allowed time window, which for this work has been set to 90 ns, larger than the typical trigger time
window.
All the coincidence histograms have then been fitted accordingly to Equation 3.1, from −24
ns to +24 ns, and some quality cuts have been applied, to ensure stable and reliable input for the
subsequent efficiency calculation. The first cut on the number of entries of the histogram excludes
from the analysis all those cases for which the fit fails due to lack of statistics. Taking into account
the number of fitted parameters and the binning of the coincidence histograms, a χ2of around 116
is expected, thus, histograms with χ2>200 are excluded. Additional cuts on the amplitude value
and its uncertainty have been applied to ensure a clear signal above background. Furthermore,
expected values of the Gaussian mean and width are known, thus cuts on these parameters have
been applied, in order to avoid cases in which the fit falls outside the allowed regions.
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ANTARES OM Efficiencies I. Salvadori
6. Results
The histograms which passed the quality cuts are then used to compute the OM efficiency, as
described in Section 3. For each period analyzed, an average over all the non-zero efficiency OMs
is performed. Figure 4shows the global detector information on the OM efficiencies as a function
of time.
Year
Relative OM Efficiency
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
1.2
1.3
1.4
2010 2011 2012 2013 2014 2015 2016
Figure 4: Relative OM efficiency over all the detector as a function of time. The blue arrows indicate the
periods in which the HVT has been performed.
The values are normalized according to the first analyzed period. It can be seen that, despite the
expected drop in efficiency, due to the ageing of the OMs and the consequences of biofouling, the
ANTARES OMs show an impressive stability over time. An average decrease of the OM efficiency
by 20%, as observed from 2009 to 2016, leads to a drop of selected atmospheric neutrino events
of around 35%. However, a hypothetical astrophysical signal with a E−2flux would decrease only
of 15%. The effects of the HVT procedure, which is usually performed once or twice per year and
allows to recover the overall efficiency periodically, can be observed as well.
7. Detailed OM calibration with simulations and 40Krates
A detailed simulation is used in ANTARES to estimate the OM effective area and its depen-
dence on the photon incident angle and wavelength [4]. These estimations are the key ingredients
for the full detector simulation.
The light detection in OMs is modeled using the latest GEANT4 library [5]. A precise sim-
ulation of the photon interaction in the photocathode is performed, taking into account the optical
properties of bialkali photocathodes and using a dedicated algorithm [6]. The simulation accurately
reproduces the geometry of the OMs, including the glass sphere and the gel, which holds the PMT
in place.
The simulation is done till the photoelectron escapes to the PMT vacuum and further signal
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detection is calculated via collection efficiency, which is parameterized as a function of the photo-
electron production point on the photocathode. The shape of this function is obtained by comparing
PMT scans with lasers, and simulations. The absolute value of the function is tuned in order to re-
produce a 40K coincidence rate of 15 Hz. This is reached by setting the collection efficiency to
∼80% at the PMT center which is physically well motivated.
8. Conclusions and Outlook
Using data collected by the ANTARES neutrino telescope with a dedicated 40 K trigger, the
OM efficiencies have been computed until the end of 2016. The results show a good stability over
time. The 40K method can also be used to cross check the time calibration. The individual time
dependent OM efficiencies, as calculated with the procedure presented here, are used on all recent
ANTARES physics analyses.
The next generation of neutrino telescopes in the Mediterranean Sea is called KM3NeT [7].
It will be constituted by two main detectors, ARCA (Astroparticle Research with Cosmics in the
Abyss), in Sicily, devoted to high energy studies, and ORCA (Oscillation Research with Cosmics in
the Abyss), in France, optimised for GeV atmospheric neutrinos. The general detector layouts are
similar to the one of ANTARES, with a series of detection lines, each one equipped with floors of
digital optical modules (DOMs). The main difference is that each floor hosts 31 PMTs, instead of
three. This allows to collect not only double coincidences from 40K decays, but also multiple ones,
improving the technique to compute the DOMs efficiencies as well as to study and discriminate
background light.
References
[1] ANTARES Collaboration, ANTARES: the first undersea neutrino telescope,Nucl.Instrum.Meth. A656
11-38 (2011).
[2] http:://antares.in2p3.fr.
[3] ANTARES Collaboration, Measurement of the atmospheric muon flux with a 4 GeV threshold in the
ANTARES neutrino telescope,Astropart.Phys.,33, 86-90 (2010).
[4] C. M. F. Hugon, GEANT4 simulation of optical modules in neutrino telescopes,PoS(ICRC2015)1106.
[5] http://geant4.web.cern.ch/geant4/.
[6] D. Motta and S. Schönert, Optical properties of bialkali photocathodes,NIM A , 539 (2005) 217 - 235.
[7] KM3NeT Collaboration, Letter of Intent for KM3NeT 2.0,Journal of Physics G: Nuclear and Particle
Physics,43 (8) 084001 (2016).
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