Proton induced radiation damage studies on plastic scintillators for the Tile calorimeter of the atlas detector

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Introduction 
Plastic scintillators are organic materials which exhibit the effect of 
luminescence when interacting with ionising radiation. Incident radiation causes 
electronic excitations within the scintillator material which then relax back to 
their ground state through the fluorescence process where emission of light 
occurs
[1]. Since the amount of light emitted is dependent on the energy of the 
exciting particle, scintillators can be utilized for particle identification and hence 
play an important role in the field of detector physics.
In addition to detection of ionising particles, plastic scintillators can be used for 
neutron identification since neutrons can elastically scatter with hydrogen, 
resulting in proton recoils. Plastic scintillators have been used for a vast number 
of applications. They are employed in radiation dosimetry in medical physics and 
in cosmic detection systems like the Alpha Magnetic Spectrometer (AMS) in the 
field of astrophysics, but it is their applications to high energy detector system s 
that are of interest to this study.
1.1.
 
Motivation 
The ATLAS detector (A Torroidal LHC Apparatus), is a multipurpose detector 
involved in the search for new particles through the reconstruction of high energy 
proton-proton (p-p) collisions generated at the Large Hadron Collider (LHC) of 
CERN. During the data taking period of 2009-2012 (Run1), the LHC generated 
p-p collisions at energies of up to
√𝑠 = 7 TeV
, and in 2012, ATLAS announced 
the discovery of a boson consistent with that of the Higg’s boson. This discovery 
lends support to the idea of the existence of a Higg’s field which plays a crucial 
role in explaining how particles gain mass
. 
[2]. The Tile calorimeter of ATLAS 
contains tiles of plastic scintillators which provide the key detection mechanism 
for detecting hadronic jets and showers of quarks and gluons that result from the 
proton-proton collisions.

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Plastic scintillators are ideal for use in the Tile calorimeter since their properties 
of high light output and high optical transmission ensure that good resolution in 
measurements can be achieved. Their fast rise and decay times are ideal since 
fast timing responses are required [1]. Furthermore, plastic scintillators are easier 
to manufacture as compared to inorganic crystals which require special growi ng 
methods that can prove to be costly. As a result, plastic scintillators are more cost 
effective for covering large detector areas [3].
The main drawback of plastic scintillators however, is their susceptibility to 
radiation damage. When charged particles pass through a scintillator, they 
dissipate their energy to the scintillator molecules via ionization losses. This 
generates a large number of electronic excitations along the path of the particle
which may fluoresce to emit light. Ionization may also lead to the formation of 
ions and free radicals which can affect both the structural and optical properties 
of the scintillators. Any processes occurring that reduce the intensity of 
fluorescence emission are regarded as quenching processes.
Radiation damage may lead to additional quenching of the scintillation light 
emitted by a scintillator [1]. This introduces an error into the data acquired by 
the detector, and if not accounted for correctly, can compromise the credibility 
of the detector accuracy. As the amount of radiation exposure is increased, these 
light losses may become more significant. Therefore, plastic scintillators that are 
employed need to exhibit a certain degree of tolerance against radiation damage .
Presently, the LHC is undertaking several upgrades in order to broaden the scope 
of physics that can be studied. In 2015, Run2 began with the first beams of 13 
TeV being reached. In addition, the LHC plans to increase the luminosity 
(number of proton collisions per bunch crossing) by a factor of 10 beyond its 
design value after 2022. 
These upgrades will significantly impact the radiation environment that 
scintillators are exposed to. Several new collider and detection systems are also 
presently under discussion with plans to commence within the next 20 ye ars. 
Some of these plan to run at much higher energy and luminosities [4].

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As such, a need has arisen for a study into how sustai nable current available 
scintillators used in high energy physics detectors are. Furthermore, the Tile 
Calorimeter has implemented a series of upgrades in order to ensure that the 
detector performance can be sustained for several years to come. Part of phase 
two of this upgrade will be implemented in 2018 where scintillators from the Gap 
region of the Tile Cal will be replaced with more radiation tolerant plastics.
This study, therefore, investigates the radiation damage undergone by 
polystyrene and polyvinyl toluene based plastic scintillators after exposure to 
proton irradiation. Proton irradiation was decided upon since the Tile calorimeter 
interacts with hadrons. In addition, the proton carries charge and would therefore 
account for the coulomb interactions that occur in collisions betwee n charged 
particles. The facilities for proton irradiation were readily available for the study, 
i.e. the 6 MV Tandem accelerator of iThemba LABS.
Scintillators obtained from Eljen Technologies, Saint Gobain Crysta ls as well as 
those presently used by the Tile Calorimeter of ATLAS have been investigated.
This study forms part of a larger investigative effort that will be used for choosing 
the scintillator replacement candidate for the 2018 upgrade of the Tile 
Calorimeter.