University of Bucharest [604358]

University of Bucharest
PhD Thesis
Advanced detection systems for
cosmic rays investigation
Author:
Alexandru BalaceanuSupervisors:
Acad. Prof. Dr. Nicolae
Victor ZAMFIR
This thesis fulls the requirements for the degree of PhD of Physics
Faculty, Bucharest University
It was carried out in
Nuclear Physics Department
of the National Institute for Physics and Nuclear
Engineering-Horia Hulubei, Bucharest, Romania

Declarat ie
Subsemnatul Alexandru B al aceanu, declar pe propria r aspundere c a lucrarea de
fat  a este rezultatul muncii mele, pe baza cercet arilor mele  si pe baza informat iilor
obt inute din surse care au fost citate  si indicate, conform normelor etice, ^ n note
 si ^ n bibliograe. Declar c a nu am folosit ^ n mod tacit sau ilegal munca altora
si c a nici o parte din tez a nu ^ ncalc a drepturile de proprietate intelectual a ale
altcuiva, persoan a zic a sau juridic a. Declar c a lucrarea nu a mai fost prezentat a
sub aceast a form a vreunei alte institut ii de ^ nv at  am^ ant ^ n vederea obt inerii unui
grad sau titlu  stiint ic ori didactic.
Semn atura candidat: [anonimizat] :
Data:
Semn aturile conducatorilor lucr arii :
ii

UNIVERSITY OF BUCHAREST
Abstract
Faculty of Physics
Nuclear Physics Department
PhD
Advanced detection systems for cosmic rays investigation
by Alexandru Balaceanu

Acknowledgements
iv

Contents
Declaration of Authorship ii
Abstract iii
Acknowledgements iv
1 WILLI-AIR Experiment 1
1.1 Short description . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.2 The method of delayed coincidences . . . . . . . . . . . . . . . . . . 2
1.3 The electromagnetic calorimeter WILLI . . . . . . . . . . . . . . . . 4
1.3.1 Previous measurements and results . . . . . . . . . . . . . . 8
1.4 The mini-array . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
1.4.1 Upgrade to 24 stations for the mini array . . . . . . . . . . 13
1.5 WILLI-AIR experiment . . . . . . . . . . . . . . . . . . . . . . . . . 15
1.6 Monte Carlo simulations . . . . . . . . . . . . . . . . . . . . . . . . 19
List of Figures 21
A Abbreviations 31
Bibliography 33
v

Chapter 1
WILLI-AIR Experiment
Historically, the muon investigations at IFIN-HH started with the WILLI detec-
tor, a calorimeter developed for muon charge ratio and muon
ux measurements
using the method of delayed coincidences, as respect to magnetic spectrometers,
which are aected at low muon energies by systematic uncertainties. The rst con-
guration of the detector was designed as being composed of 20 layers of plastic
scintillators vertically positioned, in between being placed layers of lead, a thick
slab of lead being positioned on top for ltering other particles than muons. The
concept evolved in time, the nal conguration of the detector losing its lead lay-
ers, excepting the top slab, 16 detection layers being vertically positioned, with the
top layer placed at 1 m distance from the rest in order to narrow the solid angle
and other 4 vertical detection layers on the lateral, as anticoincidence counters.
The mechanical mount was built in order for WILLI to be able to rotate both
azimuth (360) and zenith (60).
Later, an array of 12 eld stations, named EAS array, was added to the experi-
ment, in order to measure the muon charge ratio for only the secondary muons
of individual Extensive Air Showers. After several tests, the results validated the
concept, but also indicted that the array is not big enough and that the method
of synchronizing WILLI calorimeter with the array is not very eective, the acqui-
sition time required to obtain a valid muon charge ratio value for a xed azimuth
and zenith direction being of 140 days.
This lead to the upgrade of the array with another 12 stations, extending the
covered area to 1600 m2, but also to a new design for the front end electronics
1

Chapter 2. WILLI-AIR xperiment 2
and for the data acquisition system, both WILLI and the new array, named AIR,
being integrated and functioning as a single detection system.
1.1 Short description
The WILLI-AIR (Weakly Ionization Lead Lepton Interaction for Air-shower Inves-
tigations in Romania) experiment is developed in order to investigate the cosmic
ray muons with energies bellow 1 GeV.The goal of the experiment is to measure
the ratio particle/antiparticle for an extended air shower. Among the available
information, the ratio between positive to negative muons is of special interest
because of dierent implications in astroparticle physics.
These particles are the result of cosmic particles interactions in the external layers
of the atmosphere. After the primary collision the resulted charged particles are

ying to the earth, colliding new atoms and so developing what is called air shower.
This experiment is focused on measuring muons using WILLI caloremeter, and all
charged particles or photons using the scintillators array. The detection system
contain a number of 48 individual detectors and one electromagnetic calorimeter
with 20 layers. The role of WILLI is to determinate the muon charge ratio using
the delayed coincidence method.
The experiment is investigating the charged particles that arriving at the ground
level and produced by primary particles with initial energy up to the value of
1015eV. The detection area is small in comparison with the experiments such as
CASCADE GRANDE[27] or Pierre Auger Observatory[28].
1.2 The method of delayed coincidences
The ratio between the number of positive muons and negative muons is an im-
portant quantity, called muon charge ration and is a signicant quantity for the
hadrons production in EAS[29]. It can have an great interest for the primary mass
composition[30]. The idea of the experimental approach is to distinguish +and
, according to the diered behaviour for each of them, in interaction with mat-
ter. As dierence between them, the negative muon can be captured by the atoms

Chapter 2. WILLI-AIR xperiment 3
Figure 1.1: The CAD geometry of the WILLI-EAS experiment .
and form an muonic atom, while the positive muon lifetime is constant [31]. This
bounding is in
uencing the lifetime[32]. With a mass about 200 times larger
than the electron, the corresponding Bohr radius is smaller and it will overlap
with the nucleus wave functions. In this scenario the muon can be captured by
the nucleus or it can decay. The lifetime is decreased, in correlation to the atomic
number of the host atom (table 1.1). It can be seen the mean lifetime for negative
muons after the atomic capture, while the mean lifetime for the positive muons
is constant. The representation is for predominant materials utilised in detector
structure.
Table 1.1: Mean lifetime for selected materials. [33]
Element Atomic Number Z Lifetime of +(s)Lifetime of (s)
Free Decay 0 2.2 2.2
Carbon 6 2.2 2.0
Aluminium 13 2.2 0.88
Iron 26 2.2 0.22
Lead 82 2.2 0.08
The method utilised by the presented experiment is based on negative muons
behaviour. They are traversing materials and can be captured inside atoms by
replacing one electron and thus forming muonic atoms. By comparing the dierent
lifetimes of these excited atoms with the lifetime of positive muons it is possible
to obtain the ratio between the positive and the negative muons. The mixed
atmospheric
ux contains both positive and negative muons. In the interaction
with dierent materials inside the detector, the decay law contains a superposition
of several decay laws[29]

Chapter 2. WILLI-AIR xperiment 4
dN
dt=N+c01
0exp
t
0
+N+mX
j=1cj1
jexp
t
j
(1.1)
where
N+;N- number of +;interacting inside the detector. m – number of materials
in the detector. c0- detection eciency for +in all materials. cj- detection
eciency for in material j.0- mean lifetime of +(2.197s).j- mean
lifetime ofin material j.
The mean lifetime jis known from experimental measurements [33]. Another
parameter of interest is c0andcjand can be determined by Monte-Carlo simu-
lations. These constants express the stopping power of dierent materials. The
remain parameters N+andNare of interest. These two are expressed in the
total muon number(Equation 1.2) and muon charge ratio(Equation 1.3).
N0=N++N (1.2)
N0=N++N (1.3)
By replacing Equation 1.2 and Equation 1.3 in Eguation 1.1, we nd:
dN
dt=N0
R+ 1"
Rc01
0exp
t
0
+mX
j=1cj1
jexp
t
j#
(1.4)
This function is utilised for the data analysis. The results of the t strongly
depends on the calculus of the eciencies c0andcj.
1.3 The electromagnetic calorimeter WILLI
The Weakly Ionizing Lepton Lead Interaction detector (WILLI [34]) is installed
in Bucharest at the National Institute for Nuclear Physics and Engineering Horia
Hulubei.

Chapter 2. WILLI-AIR xperiment 5
The active area of the WILLI experiment is composed by 20 scintillator plates as is
shown in the left part of Figure 1.2. Each plate is a NE114 plastic type scintillator
read out by 2 Philips XP2081B PMT's.
The scintillator plates (sketched in yellow) of 90 x 90 x 3 cm3in size are housed
in an aluminium box as it can be seen in Figure1.2 – right side. Each layer has
a conguration with two counters (sketched in blue) which are placed in opposite
corners. The light is collected by four wave length shifters (sketched in green),
two for each Photomultiplier [29, 35].
The relative arrangement of the detectors is presented in Figure 1.2 – left side. A
number of 16 layers are mounted on top of the each other and 4 boxes (the so
called anti-counters [36]) that track the muons leaving the detection system are
situated on side (sketched in red).
Figure 1.2: CAD drawing for the detector structure; Left: arrangement of the
detector layers; Rigth: the structure of one detection layer: the scintillator (in
yellow) and the four wave length collector and shifters (in green).
A closer view for the mechanical part and detector arrangement inside the elec-
tromagnetic calorimeter can be seen in 1.2 left side. On the top are situated two
detections layers with the main purpose of triggering an also will deliver the time
and energy deposit information, represented with grey. Next layer is composed
of 100 mm of lead and 12 mm os iron to decrease the energy of the muons, re-
spectively to increase the energy capability of the detector. On the bottom side
a number of 16 layers are stacked one on top of the each other, each read out
by two photomultipliers on two opposite corners. The structure of each layer is

Chapter 2. WILLI-AIR xperiment 6
representated in 1.2 left side. The plastic scintillator have a thickness of 30mm
and contains four wave guiders and shifters on the sides, two for each PMT.
On the top part of the electromagnetic calorimeter is mounted a layer of absorber
consisting of 12 mm of steel and 100 mm of lead as shown in the left part of Figure
1.3.
Starting from the top, with green is represented the rst two detection layers,
used for triggering. Before these is represented a section of absorption material
composed by 12 mm of Iron and 100 mm of Lead. After the dense material are
situated 16 detection module, one on top of the each other [34].
Figure 1.3: Left: CAD drawing for mechanical structure; Rigth: Schetch
of the WILLI detector with the representation of the detection layers and the
rotation axes.
The actual conguration is correlated to the last known conguration for mea-
surements [37] to improve the trigger condition and to have a good information
about the muon natural
ux. In this scenario all the muons that are coming from
the top will be measured, even they are stopped in the absorber material or not.
Each active layers contains a plastic scintillator of 900 x 900 mm2, mounted inside
an Aluminium box. The size of the walls thickness for this coverage is 10 mm,
except the lid which is only 1mm.
The absorber is used to shift up the energy range (the interval 0.3 – 0.5 GeV goes
to 0.4 – 0.6 GeV) of the muons where they are stopped inside the detector. The
corresponding plot can be seen in Figure 1.4 in the upper canvas.

Chapter 2. WILLI-AIR xperiment 7
Figure 1.4: Momentum, zenith and azimuth angle distributions of accepted
events for rotatable WILLI [38] [39] .
The gure shows the acceptance of muons for the energy, the zenith and azimuth
incident angles WILLI detector in the presented mechanical structure utilising a
rotatable conguration, utilising and not using a 10 cm Pb absorber above the
detector top lid.
As a consequence of the interaction with Earths magnetic eld, +andare
de
ected in opposite directions. This eect is noticeable on muons arriving from
East or West direction, where the muon charge ratio will shift to higher values for
West or lower values for East.
This represents the formal East-West eect. The investigation of East-West eect
in the muon charge ratio was antecedently reported [40]. For energies larger than
1 GeV for the incident muons, the in
uence of Earth magnetic eld is smaller with
increasing muon energies.

Chapter 2. WILLI-AIR xperiment 8
1.3.1 Previous measurements and results
On many years of data acquisition WILLI detector published results on dierent
papers. Some of respective results are underlined in next paragraph. For observing
the azimuth dependence of the charge ratio of atmospheric muons, a sequence of
measurements [38] has been achieved on four azimuth directions of incidence of
the atmospheric muons: North, East, South, West, (N, E, S, W) for muons with
inclined incidence, mean value at 35and mean incident energy 0.5 GeV/c. The
primary data reported in [39] have been re-analysed and the results display that
CORSIKA simulations, based on DPMJET model reproduce approximate well the
azimuthal variation on the East-West eect as observed by WILLI (see Figure 1.5).
Figure 1.5: The azimuthal variation of the muon charge ratio measured with
WILLI compared with CORSIKA (DPMJET model) simulation [41] .
The muon
ux measured using WILLI detector was compared with Monte-Carlo
simulations implemented with CORSIKA using DPMJET model in order to study
the azimuthal dependence of the muon
ux for muons with a mean zenith angle
of 35.
The plot presents the muons measured statistics with WILLI for small polar angles,
compared to other experiments[41]. The WILLI determination show a smooth
reduction of the charge ratio nearly lower energy region, this could be expected
due to geomagnetic cut-o. CAPRICE observation reported a similar slope for the

Chapter 2. WILLI-AIR xperiment 9
data, but with smaller values, for New Mexico, at approximately same geomagnetic
cut-o as in Bucharest.
Figure 1.6 presents the measured data with WILLI for vertical muons compared to
other experiments. The plot represents the muon charge ratio determined using
WILLI detector for muons having small polar angles (vertical direction of inci-
dence) and data obtained in CAPRICE experiment, which has used a magnetic
spectrometer.
Figure 1.6: Muon charge ratio WILLI compared with results from other ex-
periments using electromagnetic spectrometers
There are several empirical approximations describing the
uxes by analytical
expressions like power-law distributions. one of them is the approach of Judge
and Nash [42] uses as input the production spectra of parent pions and kaons and
calculates the
ux resulting from pion and kaon decay by:
D(E;) =A
W
E


H
E
cos +H(1.5)
Dk(E;) =Ak
W
E

k
Hk
Ek
cos +Hk(1.6)
The spectra of pions and kaons is given by the relation:

Chapter 2. WILLI-AIR xperiment 10
N(p;k) =A;k
p2:97
;k (1.7)
where the factors AandAkhave the values: A= 0:373 andAk= 1. Based on
this aspects a study of the in
uence of the kaons spectra on the muon
ux was
performed by reducting the value of Akto 0.373 [38], [43], [44] . The results is
plotted in Figure 1.7 and shows the muon
ux measured using WILLI detector
and BESS experiment compared with CORSIKA simulation.
Figure 1.7: The muon
ux data compared with Monte Carlo simulations and
semi-analytical formulae for Hirosima and Bucharest [38], [43], [44] .

Chapter 2. WILLI-AIR xperiment 11
1.4 The mini-array
In order to discriminate if the muons measured by WILLI detector are produced
in EAS, an array was designed. The eld stations system, is measured dier-
ent aspects of the cascades produced through interactions of cosmic rays with air
molecules, in coincidence with WILLI calorimeter, forming the WILLI-EAS de-
tector system .An array of 12 scintillators is utilised to measure charged particle
arrival time and to determine the position for the shower core. The respective
array which compose the detection stations, placed at about 50 m from WILLI,
as in Figure 1.8.
Figure 1.8: Schematic view with conguration of the experiment for 12 station.
Each station is composed of two pices of 0.5m2plastic scintillator read-out by
one photomultiplier each. Covering an area of around 800 m2, with constant
pitch between the eld detectors, it was designed to measure EAS produced by
cosmic rays with energies between 1013eV to 1015eV. The design for one station
is presented in Figure 1.9. The mecanichal structure is simplied in order to
evidentiate the main components. It is sketched the scintillator material with
yellow, the wave shifter with red and with black the photomultipliers. Insithe
the detector box the preamplier is mounted. Also inside each photomultiplier
mechanical support is included a custom PCB for distributing the HV line to the
dynodes.

Chapter 2. WILLI-AIR xperiment 12
Figure 1.9: Cad drawing view for one station, containing the scintillator de-
tector(yellow), optical guides(green) and the PMT's
The energy calibration for one scintillator sistem of the presented experimental
set-up conguration is represented in Figure 1.10. It is performed by comparing
the measured energy deposit spectrum of the minimum ionizing particles with
simulated energy deposit using GEANT 3.
Figure 1.10: The energy calibration for one scintillator plate of the previous
conguration, made by comparing the measured energy deposit spectrum of the
minimum ionizing particles with GEANT 3.21 simulated one [45].

Chapter 2. WILLI-AIR xperiment 13
1.4.1 Upgrade to 24 stations for the mini array
Each stations contains two pices of 0.5m2scintillator read-out by one photomul-
tiplier each. The surface of the experiment is correlated with the energy of the
primary particle and covers around 1600m2, with a pitch of 10 m between stations
on both directions .
SHOWREC [46] is a computer software especially developed for the KASCADE
Grande experiment. It is a procedure for fast reconstruction of the shower pa-
rameters based on the deposited energy by particles in the array detectors. It was
built mainly to reconstruct from the measured energy deposition in the array scin-
tillators the number of hitting charged particles and their lateral distributions[47].
A custom version of SHOWREC was implemented, named EAS-rec, for the shower
reconstruction of the parameters of the EAS array from IFIN-HH [41]. The pro-
gram contains the parametrisation of the deposited energy in the WILLI-EAS
array detectors for dierent particles (muons, electrons and photons) at dierent
energies and dierent angular direction. The program perform the reconstruc-
tion of the detector response using a parametrisation of the energy deposit in the
scintillator plates from the mini-array.
Utilising the modied version of the SHOWREC program, the reconstruction of
the simulated shower is implemented. The reconstruction for the extended air
shower for a xed position of the center of the cascade is presented in Figure 1.11
using an array of 24 stations.
The shower cores for 250 proton initiated showers in the same position and with
the incident zenith angles of 20 degree and an initial energy of 1016eV were
reconstructed. The position is aproximately in the center of the detectors array,
respectiveli X = 20 and Y = 0. The quality of the reconstruction is given by the
dierence between the reconstructed and the true core position.
The quality for the reconstruction for the real conguration of the mini-arrayin
in comparisson with a number of 12 stations is presented in Figure 1.12. As it
expected the number of detectors is in
uencing the reconstruction.

Chapter 2. WILLI-AIR xperiment 14
Figure 1.11: The reconstruction of the EAS center given by the array for a
xed position of the center of the cascade [48].
Figure 1.12: Dierence between the true and the reconstructed center of the
EAS for the two congurations of the array [48].

Chapter 2. WILLI-AIR xperiment 15
1.5 WILLI-AIR experiment
The idea to build a WILLI-AIR detection system is to combine WILLI detector
with a of 48 scintillators array in order to measure muon charge ratio and the muon
density in correlation with the air shower. The concept of experimental approach
for the current experiment came out from the need of studies of the azimuthal and
radial variation of the charge ratio of the muon density in EAS.
The in
uence of the geomagnetic eld and the separation of +andincrease
with the path length of the muon trajectories in the atmosphere. Hence the az-
imuthalRvariation gets more pronounced with increasing distances from the
shower core, with the threshold of observed muon energies since muons of higher
energies branch from earlier generations, and with the zenith angle of EAS inci-
dence. This is displayed in Figure 1.13.
Figure 1.13: The azimuthal variation of the EAS muon charge ratio of the
muon density of proton induces EAS incident with dierent zenith angles from
North with the primary energy of 1015eV, observed at radial distance o 45-50 m
[32].
The experiment can deliver data more ecient if the detectors pitch is smaller.
The mini-array is increased from 12 to 24 stations and each station contains two
detectors pieces of 0.5 m2as it can be seen in Figure 1.14. As the result a number
of 48 detectors are situated around the calorimeter(yellow square in the map).

Chapter 2. WILLI-AIR xperiment 16
The simulations has been performed and has shown that a larger array allow the
possibility to measure showers produced by primaries with higher energy up to
1016eV and with larger muon component [32, 49]..
Near these stations is placed the electromagnetic calorimeter(WILLI), rebuild us-
ing the old experiment WILLI using a new fast and integrated electronics. The
window of the detector is 0.8m2and will have 20 layers separated by iron and
lead absorbers. The absorbers are used to decelerate the cosmic muons and to
capture the negative muons. The main concerning on the experimental part, is
the time correlation between the calorimeter and the stations, due to a relative
long distance between them. Dierent tests are performed in order choose the best
solution for the transmission lines. This choice is in
uencing the signals proper-
ties during the propagation inside the lines and it is mandatory to be without
deforming their leading edge.
Figure 1.14: Schematic view with the actual conguration of the experiment
The main observables determined by direct measurements in such experiments are
the position of the shower core, the time arrival and the anisotropy of the shower,
the lateral distribution of the charged particles and the muon arrival times. The
behaviour of the muon charge ratio in extensive air showers has still not been
investigated. Such lack of information lead us to the idea to develop the WILLI-
AIR experiment, by building a mini-array for the detection of the air shower, that
would operate in connection with the WILLI detector. Such experimental studies

Chapter 2. WILLI-AIR xperiment 17
could provide detailed information on the shower development under the in
uence
of the geomagnetic eld and probably also on hadronic interaction.
The detection system WILLI-AIR can bring:
-better precision for muon charge ratio
-new data about muon component in EAS, info correlated with the mass of the
primary cosmic rays
-new data about muon charge ratio in EAS
The main focus on the experimental part, is the correlation between the calorime-
ter and the array stations. A relative long distance between them, made the
development of the new boards for the front end electronics, a challenge. Properly
has to be the transmission lines of the signals which has to let the fast signals to
propagate without to deform their specications and this taking into account that
the detected particles are minimum ionizing and as a result the signals are smalls.
One objective is to upgrade the front-end electronics and DAQ system. The entire
system is now correlated, in terms of time, by implementing a new data acquisition
system that is merging all the 88 detection channels.
The time resolution of the presented system can be under 100ps and a rate ca-
pability of 1-2 kHz should be proper to make the system to discriminate between
dierent charged particle inside the detector such as e positive and negative muons
.
In the last years there were developed very fast front end electronics and fast acqui-
sition systems which can deliver a time resolution under 20 ps. Such improvement
is delivering a good time measurement for the particles that are arriving into the
detectors. By example a goot time resolution can deliver information about the
incident angle of the air-shower, by comparison the arrival time in each station.
For the rst tests it was choosen a VME crate (V 1718 produced by CAEN [50])
to process and to operate the time information coming from TDS's ( V 1925) with
25 ps for the time resolution and ADC (CAEN V785) for the energy information.
For a better management and customization a DAQ, based on RIO4 processor
with a fast board for trigering, TRIVA and VULOM4B, is choosed.

Chapter 2. WILLI-AIR xperiment 18
Figure 1.15: Picture with the real detector, Front-END and DAQ
Although the data acquisition system is called Multi-Branch System (MBS) it is
in fact scalable from small single-crate CAMAC or VME systems, single branch
systems with multiple intelligent controllers to large hierarchical structured (multi-
layered) multi-branch systems. The data bus between crate controllers and event
builders can be memory mapped like VME bus, dierential VSB bus and PVIC
(CES), or transfer oriented like Ethernet [51].
The Multi-Branch System (MBS)[51] runs under the operating system Lynx OS
(v2.5), a realtime UNIX system. Almost all of the software is written in GNU C.
The tcsh shell must be used in order to run the system. The software runs on the
GSI developed CAMAC processor board CVC, on the ELTEC E6 and E7 VME
processors and on VME PowerPC platforms (CES RIO4)[52].
By using plactic scintillator and fast photomultipliers, a rise time arrownd few
ns for the raw signals from the anode can be achived. The time resolution of
the entire system(detector, front-end electronics and DAQ) should be under 100
ps. This result can be obtained after the walk correction algoritm and for detector

Chapter 2. WILLI-AIR xperiment 19
shape that can avoid position dependance. Souch as plastic bars scintillators read-
out at both ends to have the info about the position of the hit. The time resolution
is acceptable for this type of measurements. A rate capability of a 1kHz should be
more then enough to make the system to write to tape all the particles interacting
into the detectors.
1.6 Monte Carlo simulations
To determinate the detector eciency and for a proper energetic calibration sim-
ulations are performed using Geant4 software [53].
This is using Monte Carlo methods to study the interaction of muons inside the
sensitive volume in the utilised detectors.
For simulations the mechanical structure of the electromagnetic calorimeter is
reduced to the geometry represented in Figure 1.16. In the left side of the drawing
the layers for anti-coincidence are cut in order to have a better view to the detectors
stack.
Figure 1.16: The geometry reconstruction in GEANT4 fot the WILLI
caloremeter. In left picture can be seen te inner layers and in the right side
the anti-coincidence detectors are situated on the sides
The detectors response to the vertical muons interaction with the active material
was simulated for muons with kinetic energies in the range of 100 MeV – 1 GeV.

Chapter 2. WILLI-AIR xperiment 20
The energy deposited in the electromagnetic calorimeter by the detected muons is
presented in Figures 1.17, 1.18, 1.19, 1.20, 1.21, 1.22. In red the energy spectra of
positive muons is represented while in blue the one corresponding to the negative
ones.
For muons having an initial energy between 300 and 500 MeV that reach the active
medium of the electromagnetic calorimeter, the energy deposited in the 20 layers
of the detector is represented in Figures 1.17, 1.18, 1.19. The 1 through 16 layers
are stacked on top of each other while the last 4 are situated laterally. The latter
will detect the particles that have left the detector through the side parts.
One can observe the number of the layer up to which the randomly generated
(above the detector over a surface of 1 m2) muons enter. To determine this number,
in Figures 1.17, 1.18, 1.19 only the energies directly deposited by the muons inside
the scintillators were selected and represented without taking into account the
contribution from the secondary particles that were generated either when the
muons disintegrated or from other interactive mechanisms that may occur.
It can be seen that muons with kinetic energies lower than 0.2 GeV have a higher
probability to be stopped in the upper layers of the detector, which in practice act
as a trigger. Muons that have a higher than 0.4 GeV energy pass through all the
horizontal detection layers. Because of this and in order to maximize the energy
deposited by muons that can disintegrate in the calorimeter it was necessary to
increase the thickness of the lead layer above the detector.
It can be seen that muons with kinetical energies lower than 0.2 GeV have a higher
probability to be stopped in the upper layers of the detector, which in practice act
as a trigger. Muons that have a higher than 0.4 GeV energy pass through all the
horizontal detection layers. Because of this and in order to maximize the energy
deposited by muons that can disintegrate in the calorimeter it was necessary to
increase the thickness of the lead layer above the detector.
The energy deposited by all charged particles in all the 20 detection layers is
represented in Figures 1.20, 1.21, 1.22. The incident particles are vertical muons
with an initial energy between 200 and 400 MeV. The presented energy spectra
contain both the energy deposited by the muons in the scintillators and the energy
deposited by the secondary particles resulted from the muons interaction with the
material.

List of Figures 21
It can be observed that depending on the energy of the incident muons the energy
deposited inside the lower detector layers is considerably larger than the energy
deposited in the upper layers. This is the result of the muons disintegrating inside
the calorimeter. It can also be noticed that higher levels of energy are deposited
in the layers of anti-coincidence than those deposited by the muons. This is due
to the secondary particles resulted from the disintegration of the muons. In some
cases, this eect can be confused with an anti-coincidence event, the dierence
between it and a valid event can be made from a time analysis.

List of Figures 22
Figure 1.17: Energy deposit by positive and negative muons with an initial energy of 200 MeV in 16 layers of detection and 4
anticonicidence layers

List of Figures 23
Figure 1.18: Energy deposit by positive and negative muons with an initial energy of 300 MeV in 16 layers of detection and 4
anticonicidence layers

List of Figures 24
Figure 1.19: Energy deposit by positive and negative muons with an initial energy of 500 MeV in 16 layers of detection and 4
anticonicidence layers

List of Figures 25
Figure 1.20: Energy deposit by positive and negative muons with an initial energy of 200 MeV in 16 layers of detection and 4
anticompetitive layers

List of Figures 26
Figure 1.21: Energy deposit by positive and negative muons with an initial energy of 300 MeV in 16 layers of detection and 4
anticonicidence layers

List of Figures 27
Figure 1.22: Energy deposit by positive and negative muons with an initial energy of 400 MeV in 16 layers of detection and 4
anticonicidence layers

List of Figures
1.1 The CAD geometry of the WILLI-EAS experiment . . . . . . . . . . 3
1.2 CAD drawing for the detector structure; Left: arrangement of the
detector layers; Rigth: the structure of one detection layer: the scin-
tillator (in yellow) and the four wave length collector and shifters
(in green). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.3 Left: CAD drawing for mechanical structure; Rigth: Schetch of the
WILLI detector with the representation of the detection layers and
the rotation axes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.4 Momentum, zenith and azimuth angle distributions of accepted
events for rotatable WILLI [38] [39] . . . . . . . . . . . . . . . . . . 7
1.5 The azimuthal variation of the muon charge ratio measured with
WILLI compared with CORSIKA (DPMJET model) simulation
[41] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
1.6 Muon charge ratio WILLI compared with results from other exper-
iments using electromagnetic spectrometers . . . . . . . . . . . . . . 9
1.7 The muon
ux data compared with Monte Carlo simulations and
semi-analytical formulae for Hirosima and Bucharest [38], [43], [44] . 10
1.8 Schematic view with conguration of the experiment for 12 station. 11
1.9 Cad drawing view for one station, containing the scintillator detec-
tor(yellow), optical guides(green) and the PMT's . . . . . . . . . . 12
1.10 The energy calibration for one scintillator plate of the previous con-
guration, made by comparing the measured energy deposit spec-
trum of the minimum ionizing particles with GEANT 3.21 simulated
one [45]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
1.11 The reconstruction of the EAS center given by the array for a xed
position of the center of the cascade [48]. . . . . . . . . . . . . . . . 14
1.12 Dierence between the true and the reconstructed center of the EAS
for the two congurations of the array [48]. . . . . . . . . . . . . . . 14
1.13 The azimuthal variation of the EAS muon charge ratio of the muon
density of proton induces EAS incident with dierent zenith angles
from North with the primary energy of 1015eV, observed at radial
distance o 45-50 m [32]. . . . . . . . . . . . . . . . . . . . . . . . . 15
1.14 Schematic view with the actual conguration of the experiment . . 16
1.15 Picture with the real detector, Front-END and DAQ . . . . . . . . 18
29

List of Figures 30
1.16 The geometry reconstruction in GEANT4 fot the WILLI caloreme-
ter. In left picture can be seen te inner layers and in the right side
the anti-coincidence detectors are situated on the sides . . . . . . . 19
1.17 Energy deposit by positive and negative muons with an initial en-
ergy of 200 MeV in 16 layers of detection and 4 anticonicidence
layers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
1.18 Energy deposit by positive and negative muons with an initial en-
ergy of 300 MeV in 16 layers of detection and 4 anticonicidence
layers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
1.19 Energy deposit by positive and negative muons with an initial en-
ergy of 500 MeV in 16 layers of detection and 4 anticonicidence
layers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
1.20 Energy deposit by positive and negative muons with an initial en-
ergy of 200 MeV in 16 layers of detection and 4 anticompetitive
layers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
1.21 Energy deposit by positive and negative muons with an initial en-
ergy of 300 MeV in 16 layers of detection and 4 anticonicidence
layers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
1.22 Energy deposit by positive and negative muons with an initial en-
ergy of 400 MeV in 16 layers of detection and 4 anticonicidence
layers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

Appendix A
Abbreviations
EAS Extended Air Shower
ADC Amplitude-to-digital converter. Measures the maximum amplitude
of detector signals during a gate.
CFD Constant fraction discriminator.
DAQ Digital acquisition system.
NIM Nuclear Instrument Module.
Scaler Counts the number of pulses.
TDC Time-to-digital converter.
TOF Time-of-
ight.
VME VERSA module eurocard, a databus, commonly used in industry
for computing and control applications.
31

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