New measurement of the angular acceptance of the Antares
Optical Module
M.Anghinolfi, H.Costantini, K.Fratini, D.Piombo and M.Taiuti
Istituto Nazionale di Fisica Nucleare
Sezione di Genova
Via Dodecaneso 33, I16148 GENOVA, IT
Abstract
The Cherenkov light from cosmic muons crossing a water tank has been measured with an Antares
optical module (OM) placed at a distance of about 0.5 m from the muon traks tagged by two plastic
scintillator slabs . The OM was located in a special support which allows rotations around the axis
of the PMT (  ) and variations of the angle of the PMT axis with respect to the vertical direction .
With this apparatus we were able to measure the angular acceptance of the OM as a function of the
angle C between the PMT axis and the Cerenkov light as well as the dependence on  . An
estimate of the absolute number of the photoelectrons was also obtained, based on the
one
photoelectron peak amplitude.
1.Introduction
The Antares neutrino telescope, in its almost final configuration, has already provided a wealth of
data
and their preliminary analysis shows that most reconstructed tracks corresponds to the
atmospheric down going muons with a smaller contribution from the up-going muons generated by
neutrinos.
Though these events are not of astrophysical interest, they can be efficiently used to calibrate the
telescope; in particular the measurement of their angular dependence can be compared to the
MonteCarlo expectations and any possible difference can be useful to understand the behaviour of
the detector. The same can be applied to the coincidence rate between two Optical Modules (OM)
of the same storey or between two adjacent storeys , due either to muons or to 40K decays.
Early results showed that the MC underestimated the measured flux for almost vertical down going
muons. The Cherenkov light produced by these events impinges the PMTs in Antares at ~ 90° with
respect to its axis: given the fact that in Antares the PMTs are pointing dowards, at an angle of 1350
to the vertical, the PMT angular acceptance at these large angles is critical for an accurate
determination of the muon flux close to the vertical.
Previous results from the experimental set up known as ‘Gamelle’ installed in Saclay [1,2] showed a
still sizable angular acceptance at angles larger than 90° with respect to the PMT axis.
For a deeper understanding of the PMT angular acceptance that, as we have seen, affects the
measured atmospheric muon rate, we have repeated this measurement in Genova using a similar
experimental apparatus.
This note describes the apparatus, the results obtained and a comparison with the mentioned Saclay
data.
2.The experimental set up
The apparatus is a simplified version of Gamelle. Above and below a water tank, 150 cm in
diameter and 170 cm high, we have located two plastic scintillators 21 cm long and 18 cm large at
a vertical separation of ~ 200 cm to trigger on cosmic ray muons. The Cherenkov light from these
tracks was then measured by the OM located inside the tank. The two plastic scintillators were
aligned along the vertical axys and located outside the water tank and shielded by 5 cm lead
bricks. The scintillator thickness was 1 cm and the energy loss of MIPs crossing these slabs was
characterised by a typical Landau distribution. The threshold was set well below the peak in the
pulse-height spectrum. In order to minimize the reflection of Cherenkov photons, the inner part of
the tank was shielded by a black tissue which resulted to have the best absorption coefficient when
compared to a black neoprene adhesive layer or black paint. This comparison was obtained using a
light source and measuring with a photodiode the light scattered by the different samples.
The OM was fixed on a support (see fig.1) which was able to change the angle of the PMT axys
with respect to the vertical (  , azimuthal angle) and to rotate the OM around the PMT axis
(  angle.). The rotation was driven by an hydraulic system connected to the exterior of the water
tank: we were able therefore to change the position of the OM from the outside without the
necessity to switch off the PMT during the data acquisition.
Due to the quite large dimension of the support and the limited depth of the water we were able to
vary
 from 90 ° (PMT horizontal ) up to 175° (PMT facing down), the surface of the OM being
not completely illuminated by the Cherenkov light for smaller values of  . When  =90° the
mean direction of the Cherenkov photons formed an angle C = 48° (90°-42°) with respect to the
PMT axys while for   C = 133°
Fig.1: The optical module on its support
The read out of the system was very simple: the anode signal from the PMT (Antares serial number
0355) passed through a passive attenuator (0-12 dB) and was then directly sent to a CAMAC
Lecroy 2249W ADC converter ( full scale: 512 pCoul in 1024 channels), gated by the coincidence
of the two plastic scintillators. The light produced by the muon was in general seen by a portion of
the PMT photocatode area but in the case of PMT facing down (  =175°) many tracks did not
produce any detectable photon. However, due to our read out system where the anode signal did
not go through any discriminator, these events were included in the computation of the average
number of detected photons at a given angle.
In these conditions we could measure ~ 100 almost vertical muon tracks/hour while a typical
measurement consisted of  E =400 events, sufficient to provide an uncertainty on the mean value
of the measured spectrum of the order of few percent.
In fact the histogram of the charge collected at the PMT anode shows a gaussian distribution
where the average value M is related to the number of collected photoelectrons  pe and to the
standard deviation

1


pe

on this value by
In our conditions  pe


50 on the average; the error on the average value M,  is

E and

1

 7 E
 1 % for the fixed
 E =400
3.The measurement.
The aim of the measurement was to collect at different  the charge spectra (CS) and to evaluate
its average mean M which is proportional to the number of Cherenkov photons reaching the
photocatode area . For  =90° ( C = 48°) and  =130° ( C = 88°) we have also investigated
the dependence of M for different  .
In order to properly adjust the voltage setting of the PMT and to estimate the systematic
uncertainties some preliminary evaluation were performed. To investigate possible saturation
effects as described in [1] we have measured as a function of the pilot voltage PV:
1) the average value M of the CS at  =90° where the number of the collected photons (i.e. the
anode amplitude) is maximum,
2) the amplitude of the one single photoelectron (SPE) peak,
3) the signal from a blue diode matched to a  =420 nm filter.
The diode was triggered by a very short positive pulse and its light was driven to the surface of the
OM by an optic fibre; the amplitude of the pulse was set in order to give, at the anode output, a
pulse similar to a muon event. For the SPE signal, due to the small amplitude, we do not expect any
saturation effect. Therefore any relative deviation of the CS or LED spectra from the SPE can be an
indication of this effect. In fig. 2 we represent the result of this measurement: up to the highest
values of PV we do not observe any substantial deviation from the SPE behaviour : as a final value
we set for our measurements PV = 2.3 V corresponding to a power supply of 800+ 2.3*400 =
1720V.
We have then determined the uncertainty on  in our rotation device. To do this we have placed
the optical fibre from the blue LED close to the OM, in a direction perpendicular to its surface. We
have then performed many measurements of the PMT signal amplitude from the LED as a function
of the angle   as shown in fig 3, where the peak at  ~ 50° is due to the gel-air dioptre. From
the plot we can estimate an uncertainty  ≈1° to be added to a systematic error of ≈1° on the
absolute value.
Fig.2 : the position of the SPE, LED and charge peaks as a function of the pilot voltage
Fig.3 : the amplitude of the LED signal as a function of the angle for two different measurements
The uncertainty due to a possible dependence of the angular acceptance on  is expected to be
very weak; however, the photocatode is deposited from two or more evaporation sources and some
non uniformities especially on the borders of the PMT may be expected. The results obtained as a
function of  in an angular range 120° wide are shown in fig.4 for the two values of C = 48°
and 88°
Fig.4 : the acceptance at two  for different 
We may observe a very weak modulation at C = 88° but within errors we may assume a
uniform behaviour. The spectra where collected in less than a week. During that period the stability
of the apparatus was checked by measuring the position of the SPE peak as well as the mean M of
the Cherenkov spectrum at  =90°. In a five day period the SPE peak position did not show any
trend compatible with a variation of the PMT gain. On the contrary, as shown in fig. 4, the value
of M indicates a decrease up to 10%: this effect can be explained by the degradation of the water
properties during the 5 days period , most likely due to the presence of the black tissue in the tank
walls. A correction to account for this drift was applied to the measured spectra, assuming a linear
regression.
Fig.5: check of the stability of the apparatus: the position of the SPE and charge peaks during a 5
days period.
4.The data
Some of the charge spectra collected at different  are reported in fig.6. Down to C = 100 ° the
spectra show a gaussian shape while at more extreme angles the distribution is the sum of the 1,2
3,.. photoelectron (PE) peaks with different amplitudes.
Fig.6: the charge spectra at two different angles of the OM
To allow a direct comparison to the results of the Monte Carlo simulation we have expressed our
amplitude spectra as a function of the number of p.e. using the conversion factor: 1 PE = 10.6+/0.7 pCoul
which was determined
measuring the SPE peak of the PMT after a proper
amplification.
The plot in fig. 7 shows our data at  =175° expressed in counts vs number of p.e . The data have
been fitted with different gaussians cantered at 1 , 2,3 p.e. and different amplitudes: though the
statistics is quite poor the agreement is satisfactory.
The already published values of the angular acceptance A [2] were normalized to give A =1 at the
maximum amplitude, around C =0. Unfortunately our data start from C =48 ° where the
value reported in [2] gives A=0.750 and our results
have been therefore normalized to this
number. Of course this procedure may introduce a systematic uncertainty but the substantial
agreement among data
[1,2]
and MonteCarlo simulation from P. Koojman indicates that in this
region its contribution is negligible.
In table 1 we report the summary of the data prior any possible finite geometry correction. This has
been evaluated using a MC simulation which will be described in a dedicated internal note but we
can already anticipate that the size of this effect is negligible.
Fig.7: the phoelectron distribution when the OM is facing down
C
cos( C )
A
error
133.00
123.00
108.00
93.000
78.000
63.000
48.000
48.000
133.00
123.00
108.00
93.000
48.000
48.000
48.000
133.00
123.00
48.000
-0.683
-0.545
-0.309
-0.052
0.208
0.454
0.669
0.669
-0.683
-0.545
-0.309
-0.052
0.669
0.669
0.669
-0.683
-0.545
0.669
0.037
0.074
0.192
0.318
0.466
0.615
0.750
0.734
0.037
0.080
0.201
0.311
0.734
0.756
0.757
0.031
0.074
0.750
0.002
0.002
0.003
0.006
0.008
0.008
0.010
0.010
0.001
0.002
0.003
0.007
0.012
0.012
0.012
0.002
0.002
0.012
Tab.1 : our experimental results
The results are also plotted in fig.8. Here the errors include the systematic which is the sum of the
uncertainty in the angle(1.5 %), in the normalization (2%) and in the correction of the drift of the
peaks due to the water degradation(1.5%)
Fig.8: our experimental values of the Antares OM angular acceptance
Our data span the range from -0.68<cos( C )<0.67. To extrapolate in the full range -1<
cos( C )<1 we have fitted the data using a polynomial up to the 4th power. Two sets of
coefficients have been obtained according to the cut off (i.e. A =0 for cos( C )< cut off) in the
angular acceptance at large angles: cos( C )= -0.8 (FIT1) and cos( C )= -1 ( FIT2). In both cases
we have also added the obvious condition A =1 at cos( C )= 1
2
3
4

A

A
x

A
x

A
x

A
x
0
1
2
3
4
In table 2 we report the coefficients of the fit A
where
cos( C )
A0
A1
A2
FIT1
0.349
0.547
0.063
-0.036
0.077
FIT2
0.350
0.554
0.053
-0.054
0.098
Tab.2: the coefficients of the 4th degree polynomials to fit our data
A3
A4
x=
5.Conclusion
The angular acceptance of the OM used in Antares was measured using the Cherenkov light from
cosmic muons crossing a water tank. Our results in the region near cos( C )~ 0 are significantly
higher (20-30%) than those of a previous measurements [2]
A simulation of the process based on a GEANT4 simulation will be described in a forthcoming
report. However we can already anticipate that for cos( C )< -0.3 our data show an enhancement
with respect to the simulation. This effect may be essentially ascribed to two reasons:
-an inadequate description of the OM inside the code or
-the contribution of light from showers which may occur simultaneously with the detected muon .
Both cases need a dedicated investigation that we are now undertaking. Therefore the data
presented here have to be used considering this remark.
REFERENCES
[1] ANTARES-OPMO-1998-001
[2] Nucl. Instr. and Methods A484 (2002) 369