University of Bucharest [604359]

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 About cosmic rays 1
1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.2 Extensive air showers . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.3 The muons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.3.1 Muons
ux . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.3.2 Muon charge ratio . . . . . . . . . . . . . . . . . . . . . . . 8
List of Figures 9
A Abbreviations 15
Bibliography 17
v

Chapter 1
About cosmic rays
1.1 Introduction
The upper Earth atmosphere is constantly bombarded by particles either of solar
origin from the solar wind or from the outer solar system. All these particles
are designated as cosmic rays or astroparticles. They consist of mostly protons
(85 %), alpha particles ( 12 %) and heavier nuclei up to iron.
Discovered in 1912 [1] cosmic rays have been measured extensively in the hope
of answering fundamental questions in physics and astronomy, their sources and
propagation mechanisms being a subject of intense research. During the last years
signicant progress has been made and a more accurate picture of cosmic-ray
observations has emerged.
The energy spectrum of primary particles extends from 1 GeV to 1020eV (100
EeV) as shown in Figure 1.1. The primary
ux varies from 1 particle per m2per
second to 1 particle per km2per century at the highest energies [2].
The low energy part of the spectrum, up to 1014eV is available through direct
measurements employing satellites and has been investigated by experiments such
as ATIC [3], PROTON [4], and RUNJOB [5].
The high energy part of the spectrum, above 1014eV, is accessible only through
air showers measurements. In this region direct detection is not an option any
more as any detector would be saturated at these energies and the detection area
of a satellite is much too small to provide measurements at the low
ux associated
1

Chapter 1. Introduction to cosmic ray 2
Figure 1.1: Energy spectrum of primary cosmic rays. The
ux is multiplied
by E2:5in order to emphasize the discontinuities in the slope. The axis at the
top indicates the equivalent center-of-momentum energy if the cosmic rays were
protons[6].
with high energy astroparticles. This part of the spectrum can be divided in two
energy regions: galactic cosmic rays up to energies of about 1017to 1018eV and
the extragalactic component at higher energies.
The slope of the energy spectrum presents a small change, from E2:7toE3:1,
near 1015eV. This region of the cosmic spectrum is known as the "knee". The
origin of the knee in cosmic rays spectrum represented a problem in astroparticle
physics. Dierent models were used to investigate the origin of this part of the
spectrum [7].
Galactic cosmic rays, i.e. particles accelerated within our galaxy, reach ener-
gies of 1018eV, while more energetic particles are of extragalatic origin. Around
61019eV the Greisen-Zatsepin-Kuzmin (GZK) cut-o appears due to pion
production in the interaction of protons with the cosmic microwave background
(CMB) [8].

Chapter 1. Introduction to cosmic ray 3
The main topics in cosmic ray studies are related to astronomy and fundamen-
tal physics. From the astronomy point of view nding the acceleration sources of
these astroparticles would open the window to multi-messenger astronomy as these
sources can be noticed using dierent observables, from electromagnetic waves
(radio, optical or X-ray astronomy) to particles (charged nuclei or neutrinos). Is-
sues in fundamental physics addressed by cosmic rays studies concern acceleration
mechanisms that are not well understood at the highest energies, primary spec-
trum and primary mass composition studies [9] and particle interactions that at
high energies (within the development of an extensive air shower) disagree in some
points with simulations that are an extrapolation of known data from the Large
Hadron Collider (LHC) particle accelerator [10].
1.2 Extensive air showers
The development of an extensive air shower is depicted in Figure 1.2.
Figure 1.2: Schematic view of the EAS development[11].
As a primary nucleus arrives at the top of the atmosphere it will interact with one
of the constituent element typically in the rst few kilometers. At high energies
the average interaction length for a nucleus is N= 80g=cm2[11] while a heavy
nucleus will interact after only a few g/cm2.

Chapter 1. Introduction to cosmic ray 4
In this rst interaction a few very energetic particles are generated and they
will subsequently interact in the atmosphere generating other secondary particles.
Thus at each step in the shower formation process the number of particles will
grow and the average energy will decrease. This rapidly developing phenomenon
is called an extensive air shower (EAS). It can spread over kilometres in both
length and transversely and contain billion of secondary particles.
The shower size, meaning the number of particles, and the energy transferred to
the secondaries will reach a maximum at a specic atmospheric depth (X maxas
shown in Figure 1.2). This maximum depends on the nature and energy of the
primary particle and on the type of the interactions it will undergo.
The three main components of an EAS are: the muonic component, the hadronic
component and the electromagnetic component, presented in Figure 1.3.
Figure 1.3: Particle components of an extensive air shower [11].
The ratio between these components is not equal. Most of the secondary particles
are pions an kaons that can decay into muons and neutrinos. Muons represent the
most penetrating component of the air showers and they easily reach the ground
level even when the other components do not [2].
The electromagnetic component, made of electrons, positrons and gammas orig-
inating from neutral pions, is by far the most abundant in an air shower, but it

Chapter 1. Introduction to cosmic ray 5
quickly dies out and usually does not reach the ground level intact. One of the con-
tributions of the electromagnetic component appears in-
ight and it is represented
by the
uorescence and radio emissions [12].
The produced secondary radiation represents a specic part for the natural back-
ground. It can be detected at ground level with dedicated equipment and interacts
with material and also living things. For some experiments that require very low
background such as low activity isotopic measurements [13] shielding from cosmic
rays is a must.
1.3 The muons
The muon component is the most abundant component that reaches ground level,
i.e. detector level. This type of lepton delivers an important information in com-
parison with the other particles. It can reveal clues on high energy processes in
the rst interaction of the cosmic particle and further to the primary energy. Due
to their small energy loss per each interaction, a relative big number of muons
that arrives at ground level are produced close to the primary interaction.
Cosmic ray muons are produced in decays of hadrons within the EAS:
NCR+NAIR=(KK) (1.1)
+=++(mean lifetime : 26ns) (1.2)
=+ (mean lifetime : 26ns) (1.3)
K+=++(mean lifetime : 12ns) (1.4)
K=+ (mean lifetime : 12ns) (1.5)
The muons are unstable and decay with a mean life time of 2.2 s:
=e+ e+ (1.6)
+=e++e+  (1.7)

Chapter 1. Introduction to cosmic ray 6
When passing trough matter, due to the weakly interaction, the muons are not
only capable to pass the entire atmosphere but also to go deep underground. The

ux at sea level is about 100 muons s1m2sr1and represents approximately
80% of the total number of cosmic particles. From a theoretical point of view
numerous calculations have been done to determinate the atmospheric
ux for
muons and neutrinos [14{16].
Figure 1.4: Simulated dierential momentum spectrum of +andmulti-
plied by p2, together with corresponding
uxes measurements by the experiment
[17].
1.3.1 Muons
ux
The muon
ux is an important information for all the components of the nu-
clear physics, starting from cosmic rays eld, particle accelerators, neutrinos, dark
matter, solar activity characterization, etc.
There is an important number of underground laboratories across the world with
numerous ongoing experiments or experiments that already have ended, from neu-
trino detectors to dark matter searches, neutrino oscillations, proton disintegration
or low background spectroscopy.

Chapter 1. Introduction to cosmic ray 7
For all those kind of experiments, a good characterization of the radiation back-
ground, especially of the muon
ux, is extremely important. A selection of those

uxes for several important underground facilities is represented in Figure 1.5.
The water equivalent depth was computed from the muon
ux data using the
formula:
(X) =AX0
Xn
expX
X0(1.8)
Figure 1.5: The representation of
ux as function of mwe depth for the results
obtained at several underground experiments [18].
Muon
ux is
uctuating not only with depth, but also with altitude [18], as it was
recently put in evidence in Figure 1.5 with a mobile muon detector [13, 19, 20]
developed at IFIN-HH.
Based on angular muon
ux observation, a new technique recently emerged for
characterizing the content of big closed volumes, without opening them, like a

Chapter 1. Introduction to cosmic ray 8
scanner. It was already proving its utility in several applications, like volcanology
or pyramids characterization [21{25].
Recently, in IFIN-HH two experiments based on muon tomography are in progress.
One of them, NEMO, is developing a method of scanning ships at the entrance
of maritime or
uvial harbours, as it was identied those border crossing points
as vulnerable to nuclear material trac [26]. The other project that is being
implemented is developing a detector for scanning mines or tunnels in order to
identify hidden cavities or cracks that can put human lives in danger or can cause
important material loses.
1.3.2 Muon charge ratio
One important indication of the neutrino oscillations is the atmospheric neutrino
anomaly, the results obtained by several experiments, including Super-Kamiokande,
being in disagreement with theoretical expectation from the muonic to electronic
neutrinos ratio point of view.
For experiments that use water Cherenkov counters, like Super-Kamiokande, dis-
criminating between neutrinos and antineutrinos is a challenge, their experimental
data being required to be corrected with the ratio of the number of electronic neu-
trinos to the number of anti-electronic neutrinos, quantity that is correlated with
the muonic charge ratio. The ratio of the number of positive to that of negative
muons for vertical showers oers an important test parameter for the interaction
models regarding muon and neutrino production.
Muon charge ratio from inclined showers provides information on how the results
are aected by geomagnetic cut-o, the anisotropy of the primary cosmic rays
ux
being probed by muon charge ratio measurement in the azimuthal directions.
The muon charge ratio is the ratio between negative and positive muon, N=N+.
It depends on the ratio of kaons. In simulations the muon charge ratio depends
on how interactions are modeled. The muon charge ratio is a signicant quantity
wich re
ects important features of the hadronic meson production in cosmic ray
collision and can support to determine the primary mass composition. It is also
immediately obviously that the muon
ux in the atmosphere is strongly related
to the neutrino
ux and that the muon charge ratio provides relevant information
for neutrino anomaly:

List of Figures 9
R
+=

R(e=e) (1.9)
The atmospheric neutrino anomaly is observed by Super-Kamiokande and other
experiments that the ratio of muonic to electronic neutrinos is much smaller than
the teoretical predictions.
R(=e)observed=R(=e)predicted1 (1.10)
The decit is interpreted in terms of neutrino
avor oscillations.
Figure 1.6 presents the muon charge ratio measured in Tsukuba and simulated
using the Geant code. As it can be observed the muon charge ratio is slightly
dependent on muon energy up to 10 GeV=c . After this range the measurements
and simulation show a relative constant value for the ratio.
Figure 1.6: The simulated and experimental muon charge ratio at sea level in
Tsukuba[17].

List of Figures
1.1 Energy spectrum of primary cosmic rays. The
ux is multiplied by
E2:5in order to emphasize the discontinuities in the slope. The axis
at the top indicates the equivalent center-of-momentum energy if
the cosmic rays were protons[6]. . . . . . . . . . . . . . . . . . . . . 2
1.2 Schematic view of the EAS development[11]. . . . . . . . . . . . . . 3
1.3 Particle components of an extensive air shower [11]. . . . . . . . . . 4
1.4 Simulated dierential momentum spectrum of +andmulti-
plied by p2, together with corresponding
uxes measurements by
the experiment [17]. . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.5 The representation of
ux as function of mwe depth for the results
obtained at several underground experiments [18]. . . . . . . . . . . 7
1.6 The simulated and experimental muon charge ratio at sea level in
Tsukuba[17]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
11

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.
13

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