Water Cherenkov Detector Array at the University of
Puebla to study cosmic rays
J. Cotzomi, E.Moreno, T. Murrieta, B. Palma, E. Pérez, H. Salazar and L. Villaseñor†
Facultad de Ciencias Físico-Matemáticas, BUAP, Puebla Pue., 72570, México
† On leave of absence from Instituto de Física y Matemáticas, University of Michoacan, Morelia Mich., 58040,
México
Abstract
We describe the design and performance of a hybrid extensive air shower detector array built on the Campus of the
University of Puebla (19oN, 90oW, 800 g/cm2) to measure the energy, arrival direction and composition of primary cosmic
rays with energies around 1PeV, i.e., around the knee of the cosmic ray spectrum. The array consists of 3 water Cherenkov
detectors of 1.86m2 cross section and 12 liquid scintillator detectors of 1 m2 distributed in a square grid with a detector
spacing of 20 m over an area of 4000 m2 We discuss the calibration and stability of the array for both sets of detectors and
report on preliminary measurements and reconstruction of the lateral distributions for the electromagnetic and muonic
components of extensive air showers. We also discuss how the hybrid character of the array can be used to measure mass
composition of the primary cosmic rays by estimating the relative contents of muons with respect to the EM component of
extensive air showers. This facility is also used to train students interested in the field of cosmic rays.
Key words: Cherenkov light; scintillation detector; extensive air showers
1. Introduction
The hybrid extensive air shower detector array at UAP (EAS-UAP) was designed to measure the lateral
distribution of the EM and muonic components of secondary particles for EAS in the energy region of
1014-1016 eV. These measurements allow a determination of the arrival direction, energy and mass
composition of the primary cosmic ray for energies around the “knee” of the spectrum where the
spectral index  in the power law representation of the energy spectrum, dE/dx α E-, steepens from
2.7 to 3 at E = 3x1015 eV. One of the main experimental problems in cosmic ray physics is the
measurement of the primary composition at energies around and higher than this structural feature,
known as the "knee" (1), whose nature remains a puzzle despite the fact that it was discovered 45 years
ago (2). Most theories consider it of astrophysical origin and relate it to the breakdown of the accelation
mechanisms of the possible sources or to a leakage during propagation of cosmic rays in the magnetic
fields within our galaxy; in particular, these theories lead to the prediction of a primary composition
richer in heavy elements around the knee related, to the decrease of galactic confinement of cosmic
rays with increasing particle energy. The main tool to study the composition of primary cosmic rays
with ground detector arrays is by measuring the ratio of the muonic to the electromagnetic component
of EAS; in fact, Monte Carlo simulations show that heavier primaries give rise to a bigger muon/EM
ratio compared to lighter primaries of the same energy (3). In fact, evidence for such variations has
been reported recently (4). Alternatively, there are scenarios where a change in the hadronic interaction
at the knee energy gives rise to new heavy particles (5) which produce, upon decay, muons of higher
energies than those produced by normal hadrons. It is therefore important to achieve a good
determination of the ratio of the muonic to the electromagnetic components along with the muon
energies involved for primary cosmic rays around the knee.
In this paper we report on preliminary results obtained by continuous operation over one year of
the first stage of the EAS-UAP array, consisting of 12 liquid scintillator detectors and 3 water
Cherenkov detectors out of the 18 and 6 planned, respectively. Throughout the operation period we
have made use of the hybrid scintillator-Cherenkov nature of our array to obtain complementary data to
measure the arrival direction, energy and primary mass composition of cosmic rays. The special
location of the EAS-UAP array (2200 m above sea level and all the facilites coming from the Campus
of the University of Puebla) make it a valuable apparatus for the long term study of cosmic rays and at
the same time an important training center for physics students interested in getting a first class
education in Mexico in the field of cosmic rays.
2. Experimental setup
EAS-UAP (19oN, 90oW, 800 g/cm2) will consist of 6 water Cherenkov and 18 liquid scintillator
detectors distributed on a square grid with spacing of 20m, as shown in Fig. 1.
Fig 1. EAS-UAP array located on the Campus of the University of Puebla. Stars represent Cherenkov detectors filled with
2230 l of water and the cylinders represent scintillation detectors filled with 130 l of liquid scintillator.
The three water Cherenkov detectors are located in the same positions as the liquid scintillator
detectors t1, t5 and t7. Each of the 12 scintillator detectors consists of a cylindrical light-tight container
of 1 m2 cross section made of polyethylene filled up to a height of 13 cm with liquid scintillator
(Bicron BC-517H); each detector has one 5" photomultiplier (PMT, EMI model 9030A) located inside
along the axis of the cylinder facing down with the photo-cathode placed 70 cm above the surface of
the liquid scintillator. The three water Cherenkov detectors consist of cylindrical tanks made of
polyethylene with an inner diameter of 1.54 m and a height of 1.30 m filled up to a height of 1.2 m with
2230 l of purified water. They have a single 8" PMT (Electron Tubes model 9353K) located at the top
of the level water along the cylinder axis. All the inner surfaces of the three Cherenkov tanks were
optically sealed and covered with a material called Tyvek which reflects light in a highly diffusive way
in the relevant wavelength range of 300-500 nm. The trigger signal is formed by the “AND”
coincidence (LeCroy NIM coincidence unit model 465) of the signals from the three water Cherenkov
detectors discriminated (LeCroyNIM discriminator modules Octal 623B and Quad 821) using a
threshold of -30 mV. The data acquisition system consists of a set of 3 four-channel digital
oscilloscopes (Tektronix model TDS 224 and 2014) that digitize the signals from 9 of the 12 PMTs of
the 12 liquid scintillator detectors and the 3 PMTs from the 3 water Cherenkov detectors. The system is
controlled by a PC running a custom-made acquisition program.
3. Results and discussion
Monitoring of all detectors is essential for the correct analysis and operation of the EAS array.
A method based mainly on single-particle rates provided by a CAMAC scaler (CAEN 16 ch scaler
model C257) was used as a simple first estimator of the operation stability. The typical dispersion
rms/mean was lower than 3% for the scintillator detectors and lower than 2% for the Cherenkov
detectors. More complex monitoring can be done by using bi-dimensional correlation plots, i.e.,
amplitud vs. risetime, or, as shown in Fig. 2 for one of the water Cherenkov detectors,
Charge/Amplitude vs risetime 10%-90%. In fact these correlation plots can be used to separate many of
the physical processes occurring inside the water Cherenkov detectors (6).
Fig 2. Correlation plot of Charge/Amplitude vs risetime 10%-90% for one of the Cherenkov detectors; charge, amplitude
and risetime have been scaled to the corresponding values for Vertical Equivalent Muons (VEM). The region corresponding
to isolated electrons is clearly visible at Q/A < 0.5 and risetime 10%-90% < 0.5.
Calibration of the detectors allows us to convert the electronic signals measured in each detector
into the number of particles in the EAS reaching the array and finally into the energy of the primary
cosmic rays. We consistently use the natural flux of background muons and electrons to calibrate our
detectors. For the location of the EAS-UAP, muons are the dominant contribution to the flux of
secondary cosmic rays for energies above 100 MeV with a flux of about 85 m-2s-1sr-1 and a mean
energy of 4 GeV; at lower energies, up to 100 MeV, electrons, positrons and photons are also an
important component; electrons coming from muon decays with energies up to 53 MeV dominate the
flux of EM particles for detectors placed outside, while knock-on electrons are another important
contribution for detectors placed inside or close to buildings. We used dedicated calibration runs to
obtain the spectra of charge depositions for secondary cosmic ray events triggered by a simple
amplitude threshold, as shown in Fig. 3. The plot on the left is the PMT charge for one of the 12 liquid
scintillator detectors and the plot on the right for one of the three water Cherenkov detectors. Liquid
scintillator detectors are more sensitive to low energy particles (dE/dx is proportional to the inverse of
the squared velocity for charged particles), while Cherenkov detectors are more sensitive to relativistic
muons and electrons. While a 4 GeV muon can fully cross the detector with a dE/dx of around 2
MeV/cm, a 10 MeV electron has a range of about 5 cm in a water-like liquid.
Fig 3. Spectrum of charge depositions scaled by the charge of Vertical Equivalent Muons (VEM). Left. Calibration
histogram for a water Cherenkov detector filled with purified water up to a height of 1.2 m; the shaded histogram
corresponds to electrons selected by requiring Q/A < 0.5 and risetime 10%-90% < 0.5, as shown in Fig. 2. The first peak of
the non-shaded histogram is dominated by corner-clipping muons, low-energy muons and electrons from muon decays; the
second peak is dominated by muons crossing the whole depth of the the detector in all directions. Right: Spectrum of PMT
charge deposition for one of the scintillator detectors filled with liquid scintillator up to a height of 13 cm.
In the case of the water Cherenkov detectors the signals of crossing muons are proportional to
their geometric path leghts. The second peak in the non-shaded histogram of the plot on the left of Fig.
3 corresponds to the mean PMT charge deposited by muons crossing the detector in arbitrary
directions, while the first peak corresponds to corner-clipping muons and also EM particles that
produce smaller signals. For water Cherenkov detectors, we performed a control experiment where two
scintillation hodoscopes were placed above and below the detector to allow us to determine the position
of the charge peak for muons crossing the detector vertically. We call this paramenter a vertical
equivalent muon (VEM). Our measurements indicate that the position of the second peak in the charge
distribution of the plot on the right of Fig. 3 is about 1.05 VEM. A selection of low energy electrons by
requiring Q/A < 0.5 and risetime 10%-90% < 0.5, as shown in Fig. 2, gives a signal ratio muon:electron
of 1:0.045, i.e., in agreement with our espectations for electrons with energies around 10 MeV for the
geometrical path lengths of 120 cm for vertical muons and about 5 cm for electrons . The plot on the
right of Fig.3 corresponds to a calibration run for one of the liquid scintillation detectors. The shaded
histogram gives the distribution of the PMT charge for penetrating muons triggered by a scintillator
paddle placed below the scintillation detector and shielded with a 2 cm steel slab. The first peak of the
non-shaded histogram corresponds to electrons and corner-clipping muons while the second peak is
dominated by crossing muons. The ratio of the MPVs of these two peaks is about 3.3, i.e., in rough
agreement with the fact that crossing muons deposit around 26 Mev of energy in 13 cm of liquid while
low energy electrons deposit all of their energy, i..e., around 10 MeV.
In consequence, we have a reliable way of converting the charge deposited in each detector into
a number of equivalent particles, therefore we can determine the number of equivalent muons and lowenergy electrons in each detector and for each EAS using the single-particle charge spectrum discussed
above. The core position, lateral distribution function and total number of shower electromagnetic
particles Ne and muonic component N  , are reconstructed from a fit of the the NKG expression given
by (S,r)=K(S)(r/R0)S-2(1+r/R0)S-4.5 (7), where S is the shower age, r the distance of the detector to the
shower core and R0 is the Molliere radius (90 m for an altitude of 2200 m a.s.l.). We have measured the
zenithal angular distribution (8); we found that it can be fitted by the function Acos8θ sinθ using the
scintillation and the Cherenkov detectors in independent ways; this distribution is in good agreement
with the literature.
Finally, the shower energy is obtained by using the relation Ne(E0)=99.8 E01.15 , where E0 is the
energy of the primary cosmic ray expressed in TeV (9). Fig. 4 shows the measured electromagnetic and
muonic particle densities in terms of VEM’s) and the fitted lateral distribution for one example of a
vertical showers in the plot on the left; while the plot on the right shows a comparision of the
electromagnetic particle density with the muon density multiplied by a factor of 10. The reconstructed
energy for this event was 1.8x1014 eV. Analysis in progress, along with detailed simulation studies will
allow us to report on the determination of the mass composition for those and other events on an event
by event basis.
Fig 4. Left. Lateral Distribution from Cherenkov and scintillator detectors for a single vertical EAS as
detected with the EAS-UAP array. The units on the vertical axis for the density are VEM m-2. Right.
Separation into the muonic and the electromagnetic component; for a better comparison, the muonic
component is scaled by factor of 10. The units on the vertical axis for the number of particles are muon
m-2 and electrons m-2 for the muonic and electromagnetic lateral distributions, respectively.
4. Conclusion
A first analysis of the data collected with the EAS-UAP allows us to conclude that the detector array
shows good stability. We have implemented a method of calibrating the detectors to perform the
conversion of the charge deposited in each detector to vertical equivalent muons and low-energy
equivalent electrons. This array has the potential to allow us to determine the lateral distribution
funcion for both the electromagnetic and the muonic components with the possibility to determine not
only the energy and the arrival direction, but also the mass composition of the primary cosmic ray on a
event by event basis for energies around 1015 eV. In the near future we will increase the number of
water Cherenkov detectors to improve the muonic lateral distribution determination and the number of
scintillator detectors to improve our aperture.
References
1. S.P. Swordy et al., Astroparticle Physics, Volume 18 (2002) 129-150.
2. G.V. Kulikov and G.V. Khristiansen, Sov. Phys. JETP, 41 (1959) 8.
3. B. Alessandro, et al., Proc.27th ICRC, 1 (2001) 124-127.
4. KASCADE Collaboration (Klages H. O. et al.), Nucl. Phys. B (Proc. Suppl.), 52
(1997) 92
5. A.A. Petrukhin, Proc. XIth Rencontres de Blois "Frontiers of Matter" (The Gioi Publ., Vietnam,
(2001) 401.
6. H. Salazar and L. Villaseñor, Separation of cosmic-ray components in a single water Cherenkov
detector, submitted to these proceedings.
7. J. Nishimura, Handbuch der Physik XLVI/2, (1967) 1.
8. H. Salazar, O. Martínez, E. Moreno, J. Cotzomi, L. Villaseñor, O. Saavedra, Nuclear Physics B
(Proc. Suppl.) 122 (2003) 251-254.
9. M. Aglietta et al., Phys. Lett. B, 337 (1994) 376-382.