Interaction of incident charged particles or photons with matter is the basis on which Gas Electron Multiplier detectors work. A gas mixture flows inside the detector, or is sealed in it; its components are chosen according to their ionization potential, ion transport characteristics and ageing properties.\\

In most cases, only the electromagnetic (Coulomb) interaction is used for detection purposes. Strong and weak interactions are by far less probable, and in any case they cannot be revealed in a GEM. Coulomb interactions result in excitation and ionization of the gas molecules, and in negligible part in bremsstrahlung, \v{C}erenkov and transition emission of radiation. In particular, noble gases do not present many energy dissipation modes: they can only be excited through photon absorption (and consequent emission). The absence of the many non-ionizing dissipation modes, which are typical of polyatomic molecules, is then the reason for which noble gases are chosen as main component in any gaseous detector. Ionization is indeed the main mode of interaction occurring in noble gases, so electron avalanche multiplication is eased. On the other hand, specific experimental requirements often oblige the use of compounds containing polyatomic molecules.\\

As already said, excited noble gases can only return to the ground state through emission of a photon. Argon, for example, releases photons with energy $E_{\gamma} \geq 11.6 eV$, that are themselves able to ionize copper atoms of the GEM anodes (whose ionization potential equals $W_i = 7.7 eV$). Argon positive ions drift instead towards the cathode, where they recombine extracting an electron and causing secondary photons or electrons to be emitted as energy balancement. Both these processes result in spurious avalanches; even with a moderate gain, detectors may therefore experience frequent discharges. In addition, spurious signals such those would affect the space and time resolution.\\

We thus need to add a percentage of so-called \textbf{quencher}s to the gas mixture. Hydrocarbons, alcohols and freons are examples of the most used quenchers. Their complex molecules feature a large number of non-radiative excitation modes and allow photon absorption in the range of energy of those emitted by argon. Elastic collisions and quencher dissociation into simpler molecules dissipate the energy in excess, ensuring stability of operation of the detector. Despite this, attention must be paid to the process through which polyatomic ionized molecules neutralize. In fact, if radicals use to combine together to form larger polymers, precipitate and accumulate inside the chamber, that might speed up its ageing process.\\

The expression obtained by Bethe and Bloch for the average differential energy loss due to Coulomb interactions is:
$$
	\dfrac{dE}{dx} = -\dfrac{2 \pi N z^2 e^4}{mc^2}\dfrac{Z}{A}\dfrac{\rho}{\beta^2} \left( \ln \dfrac{2mc^2 \beta^2 E_M}{I^2 \left( 1 - \beta^2 \right)} - 2 \beta^2 \right)
$$
where $N$ is the Avogadro number, $Z$, $A$, $\rho$ and $I$ are the atomic number, atomic mass, density and average ionization potential of the medium, $z$ and $\beta$ are the atomic number and speed in units of $c$ of the projectile \cite{sauli-pr}. $E_M$ represents the maximum transferable energy. \figurename~\vref{bethe-bloch} shows, as an example, the contributions of various processes to the total energy loss of muons $\mu^+$ in copper.\\
\begin{figure}
	\centering
	\subfigure[An example of $ - \dfrac{dE}{dx} $ of projectiles in a medium according to the Bethe-Bloch formula, from \cite{pdg}. Energy loss is plotted as a function of $\beta \gamma = \dfrac{p}{Mc}$]
	{\includegraphics[width=12cm]{Images/bethebloch}\label{bethe-bloch}}
	\vspace{2mm}
	\subfigure[Energy loss in air for different particles as a function of their energy, from \cite{sauli-pr}]
	{\includegraphics[width=6.6cm]{Images/dEdx}\label{dedx}}
	\caption{Energy loss due to Coulomb interaction of particles in media}
	\label{energyloss}
\end{figure}

Above a kinetic energy of the order of magnitude of a few hundred $MeV$ every particle lies in the \textit{minimum ionization} region of the curve (\figurename~\vref{dedx}) and release on average $2 \dfrac{MeV \cdot cm^2}{g}$. At those energy levels, which is the most common situation in high-energy physics, the particles are called \textit{MIP}s.\\

A number of ionization clusters are created on the passage of a MIP through the drift gap of the detector. Drift and transfer fields drive the ejected electrons through the GEM foils, where they are accelerated and cause more ionizations. The induction field attracts the electron avalanche on the anode, where each drifting electron turns into a current signal of intensity $I = \dfrac{e}{{\Delta t}_{drift}}$ and duration ${ \Delta t}_{drift} = \dfrac{l_i}{v_{drift}}$, where $l_i$ is the thickness of the induction gap and $v_{drift}$ is the drift velocity of electrons ($v_{drift}$ depends on the voltage applied to the gaps). \\

The total signal induced by an incident particle is given by the superposition of the signals produced by the primary ionization electrons (the ones extracted directly by the MIP crossing the drift region) multiplied by the total gain factor. The signals coming from different primary electrons will be detected at different times, because the spacial distribution of the ionization clusters can be as wide as the drift gap. The time resolution of the detector is determined by the space distribution of each ionization cluster, which follow the Poisson law, with a sigma $ \sigma (t) \propto \dfrac{1}{n \cdot v_{drift}} $ (where $n$ is the average number of primary ionization clusters) \cite{3gem-fast}.\\

In order to optimise the time resolution of a detector it is therefore necessary to maximize the drift velocity, and the number of primary clusters, using gas mixtures with a large average atomic number. The gas inside the detector should allow a compromise between these two features and the minimum possible loss of efficiency as compared to a noble gas. A higher drift velocity may be obtained by either increasing the drift, transfer and induction fields or adding a percentage of polyatomic molecules (a full explanation of this topic can be found in \cite{sauli-pr}).\\
\begin{figure}
	\centering
	\includegraphics[width=12cm]{Images/drift_v}
	\caption{Drift velocity of electrons in several argon-isobutane mixtures}
	\label{vdrift}
\end{figure}

The measurements presented in this text were performed using two gas mixtures: Ar/CO\subscript{2} $70/30$ and Ar/CO\subscript{2}/CF\subscript{4} $60/20/20$, with CO\subscript{2} and CF\subscript{4} as quenchers. The choice of CF\subscript{4} for the last set of tests was driven by its good timing characteristics, already verified by the authors of \cite{3gem-fast}; moreover, with respect to methane and other hydrocarbons, it is safer. CF\subscript{4} is indeed non-flammable, non-toxic and non-corrosive either for metal or plastic materials. It might only be harmful if it comes into contact with hydrogen, as this would allow for the formation of hydrofluoric acid HF. Ar/CO\subscript{2} $70/30$ is instead the ``standard gas mixture'' in use at TOTEM and other experiments.\\

\begin{figure}
	\centering
	\subfigure[Electron drift velocity]
	{\includegraphics[width=6.6cm]{Images/drift_vel_gases}\label{drift-comp}}
	\hspace{2mm}
	\subfigure[Triple GEM gain]
	{\includegraphics[width=6.6cm]{Images/gain_gases}\label{gain-comp}}
	\caption{Properties of different gas mixtures. The measurements were done for a triple GEM detector with fields $E_d = E_t = 3 \frac{kV}{cm}$ \cite{3gem-fast}}
	\label{lit-gascomp}
\end{figure}
\figurename~\vref{lit-gascomp} shows the differences between Ar/CO\subscript{2} $70/30$ and Ar/CO\subscript{2}/CF\subscript{4} $60/20/20$ as found in literature; our measurements will be presented in \chaptername~\vref{ch-hvscan} for what concerns the relationship between GEM fields and gain, and in \chaptername~\vref{subsec-timing} I will show and discuss the timing tests we performed.