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02582a78 | 1 | \section{Introduction} |
2 | ||
3 | % It shows very briefly how can we proceed to execute | |
4 | %some basic things like the event generation, reconstruction and data | |
5 | %analysis. For more details on how AliRoot works and how the experimental | |
6 | %data can be analyzed, I strongly recommend to read the AliRoot primer~\cite{ALIROOT:doc} | |
7 | %(the present document is not a substitute of the ``AliRoot primer'' | |
8 | %but an introduction for beginners), and also the AliEn pages~\cite{ALIEN:alien} | |
9 | %or go directly to the code in~\cite{ALIROOT:svn}. | |
10 | %For detailed information on the EMCAL code you can go to the EMCAL offline pages (work in progress)~\cite{EMCAL:doc}. | |
11 | ||
12 | %I will assume that you have already AliRoot and its environment properly | |
13 | %installed in your machine, if not, look at \cite{ALIROOT:doc} or to the offline installation page \cite{ALIROOT:install}. | |
14 | %If you type ``cd \$ALICE\_ROOT'', you will find all the AliRoot | |
15 | %code. In the directories ``\$ALICE\_ROOT/macros'' and ``\$ALICE\_ROOT/EMCAL/macros'', you will find different examples of macros. All the EMCal | |
16 | %code is in ``\$ALICE\_ROOT/EMCAL'', in case you want to study | |
17 | %thoroughly how the simulation, reconstruction and analysis software | |
18 | %code of the calorimeter work. | |
19 | ||
20 | %If we want to generate some data (realistic physics data or just single | |
21 | %particle to study the performance of the ALICE detectors) or read | |
22 | %real experimental data, and analyze it with AliRoot, we have to pass | |
23 | %through three steps: \textbf{simulation}, \textbf{reconstruction} | |
24 | %and \textbf{analysis}. This steps are explained in the next sections. | |
25 | ||
486ebd44 | 26 | This document is addressed to those who want to work with the EMCal data. It explains the different steps to have the data taken ready to be analyzed: geometrical description of the detector, how to get the calibration, how works the simulation and reconstruction of the data and how to access the analysis objects, ESDs and AODs.\\ |
02582a78 | 27 | |
486ebd44 | 28 | For introduction on the detectors, the TDR's of the EMCal and DCal can be found in ~\cite{EMCalTDR,DCalTDR}. |
29 | For a fast introduction on the code and how it works you can have a look to the EMCal for beginners guide \cite{EMCAL:beginners}. Some other interesting references are the AliRoot primer~\cite{ALIROOT:doc}, the offline AliRoot page \cite{ALIROOT}, and the installation page from Dario Berzano \cite{ALIROOT:berzano}.\\ | |
30 | ||
31 | In general, a lot of information can be found in many twikis collected in the main EMCal twiki page that can be found here~\cite{EMCAL:MainTwiki} and also is interesting to point to the main EMCal offline twiki that can be found here~\cite{EMCAL:OffTwiki}. | |
32 | ||
33 | \subsection{Mechanical description of the EMCAL} | |
34 | (Federico Ronchetti)\\ | |
02582a78 | 35 | |
b1f776f8 | 36 | The chosen technology is a layered Pb-scintillator sampling calorimeter with a longitudinal pitch of 1.44 mm Pb and 1.76 mm scintillator with longitudinal |
486ebd44 | 37 | Wavelength Shifting Fiber (WLS) light collection, see the EMCal TDR~\cite{EMCalTDR}. The full detector spans $\eta$ = -0.7 to $\eta$= 0.7 with an azimuthal acceptance of $\Delta\phi~107^\circ$ |
b1f776f8 | 38 | and is segmented into 12,288 towers, each of which is approximately projective in $\eta$ and $\phi$ to the interaction vertex. |
486ebd44 | 39 | The towers are grouped into super modules of two types: full size which span $\Delta\phi=20^\circ$ (24 ($\phi$) $\times$48 ($\eta$) towers) and 1/3 size which span $\Delta\phi$ = 6.67$^\circ$ (8 ($\phi$) $\times$48 ($\eta$) towers). |
40 | There are 10 full size and 2, 1/3-size super modules in the full detector acceptance (Fig. \ref{fig:emcal-full}). \\ | |
02582a78 | 41 | |
b1f776f8 | 42 | \begin{figure}[ht] |
43 | \begin{center} | |
4095af25 | 44 | \includegraphics[width=1.0\textwidth]{figures/emcal-full.pdf} |
b1f776f8 | 45 | \end{center} |
486ebd44 | 46 | \caption{\label{fig:emcal-full} Azimuthal view from the A-side (opposite to the di-muon arm) of the full EMCal as installed into the ALICE detector. |
b1f776f8 | 47 | The two 1/3-size super-modules are visible at ~9 o'clock position.} |
48 | \end{figure} | |
02582a78 | 49 | |
b1f776f8 | 50 | |
486ebd44 | 51 | In 2014, DCal extension was installed \cite{DCalTDR}, consisting of 6 super modules, with 2/3 the acceptance of an EMCal super-module (same acceptance in $\phi$ smaller in $\eta$ where 0.22$<|\eta|<$0.7, 24($\phi$)$\times$32 ($\eta$) towers) and 2 super-modules with 1/3 acceptance like for the EMCal. This extension covers 67 degrees in azimuth and same coverage in $\eta$ as the EMCal if one considers PHOS, although there is a gap of approx. 0.09 pseudo-rapidity units between PHOS and DCal super-modules, see Fig.~\ref{fig:dcal}. DCal is located opposite to EMCal, in azimuth, in the ALICE coordinate system EMCal is located at 80$<\phi <$187 degrees and DCal at 260$< \phi <$ 327 degrees. The total number of towers in DCal is 5,376.\\ |
52 | ||
53 | ||
54 | \begin{figure}[ht] | |
55 | \begin{center} | |
56 | \includegraphics[width=0.5\textwidth]{figures/DCalPHOS.jpg} | |
57 | \end{center} | |
58 | \caption{\label{fig:dcal} Cartoon of the DCal+PHOS configuration in super-modules.} | |
59 | \end{figure} | |
b1f776f8 | 60 | |
61 | ||
486ebd44 | 62 | The super module is the basic structural units of the calorimeter. |
63 | These are the units handled as the detector is moved below ground and rigged during installation. | |
64 | Fig.~\ref{fig:emc-sm} | |
b1f776f8 | 65 | %shows a super module with its external mechanical structure stripped away to illustrate the stacking of modules within the super module. |
486ebd44 | 66 | shows a full size super module with $12\times24$ modules configured as 24 strip modules of 12 modules each. DCal super modules contain $12\times16$ modules, 16 strip modules of 12 modules each. |
67 | The supporting mechanical structure of the super module hides the stacking into a nearly projective geometry which can be inferred by the different tilt | |
68 | of the strip modules from lower $\eta$ to higher $\eta$. %going from the left to the right part of the picture. | |
69 | The electronics integration pathways are also visible. | |
b1f776f8 | 70 | Each full size super module is assembled from $12 \times 24$ = 288 modules arranged in 24 strip modules of 12 modules each. |
71 | ||
72 | \begin{figure}[ht] | |
73 | \begin{center} | |
4095af25 | 74 | \includegraphics[width=1.0\textwidth]{figures/emc-sm.jpg} |
b1f776f8 | 75 | \end{center} |
76 | \caption{\label{fig:emc-sm} View of one EMCal super-module during the installation into the ALICE detector. The cradle holds the 24 strip modules into a mechanically rigid unit. | |
77 | Each strip module holds 12 unit modules. On the right side the two electronics crates are visible. } | |
78 | \end{figure} | |
79 | ||
80 | Each module has a rectangular cross section in the $\phi$ direction and a trapezoidal cross section in the $\eta$ direction with a full taper of 1.5$^\circ$. | |
81 | The resultant assembly of stacked strip modules is approximately projective with an average angle of incidence of less than 2$^\circ$ in $\eta$ and | |
82 | less than 5$^\circ$ in $\phi$. An assembled strip module is shown in Fig. \ref{fig:strip}. | |
83 | ||
84 | \begin{figure}[ht] | |
85 | \begin{center} | |
3fd21830 | 86 | \includegraphics[width=0.5\textwidth]{figures/strip-module.pdf} |
b1f776f8 | 87 | \end{center} |
88 | \caption{\label{fig:strip} View of a fully assembled strip module. The photo shows the APD+CSP package and copper shielding monunted light guide fixture. On the right part | |
89 | of the photo the LED UV optical fiber distribution system is visible. Each strip module, is cabled via 3 T-Cards visible in the center of the assembly.} | |
90 | \end{figure} | |
91 | ||
486ebd44 | 92 | The smallest building block of the calorimeter is the individual module illustrated in Fig. \ref{fig:module}. |
b1f776f8 | 93 | Each individual module contains $2\times 2$ = 4 towers built up from 77 alternating layers of 1.44 mm Pb and 1.76 mm polystyrene, |
94 | injection molded scintillator. White, acid free, bond paper serves as a diffuse reflector on the scintillator | |
95 | surfaces while the scintillator edges are treated with TiO2 loaded reflector to provide tower to tower optical | |
96 | isolation and improve the transverse optical uniformity within a single tower. | |
97 | The Pb-scintillator stack in a module is secured in place by the static friction between individual | |
98 | layers under the overall load of ~350 kg. The module is closed by a skin of 150 $\mu$m thick stainless | |
99 | steel screwed by flanges on all four transverse surfaces to corresponding front and rear aluminum plates. | |
100 | This thin stainless skin is the only inert material between the active tower volumes. | |
101 | The internal pressure in the module is stabilized against thermal effects, | |
102 | mechanical relaxation and long term flow of the Pb and/or polystyrene by a customized | |
103 | array of 5 non-linear spring sets (Bellville washers) per module. | |
104 | In this way, each module is a self supporting unit with a stable mechanical lifetime of | |
105 | more than 20 years when held from its back surface in any orientation as when mounted in a strip module. | |
106 | ||
486ebd44 | 107 | %\begin{figure}[ht] |
108 | %\begin{center} | |
109 | %\end{center} | |
110 | %\caption{\label{fig:module} | |
111 | % | |
112 | %} | |
113 | %\end{figure} | |
114 | ||
115 | ||
b1f776f8 | 116 | \begin{figure}[ht] |
117 | \begin{center} | |
486ebd44 | 118 | \includegraphics[width=0.45\textwidth]{figures/open-module.jpg} |
119 | \includegraphics[width=0.32\textwidth]{figures/emc-module.jpg} | |
b1f776f8 | 120 | \end{center} |
486ebd44 | 121 | \caption{\label{fig:module} The first 1.5$^\circ$ tapered module of the EMCal generation II prototype produced in EU shown: Left: Internal Pb-scintillator sandwich of EMCal modules. Right: The module's internal compression is maintained by a set of 5 Bellville washers (non linear springs) acting between the top and bottom containment |
b1f776f8 | 122 | Al plates to prevent the delamination of the internal Pb-scintillator sandwich. |
123 | } | |
124 | \end{figure} | |
125 | ||
486ebd44 | 126 | |
b1f776f8 | 127 | All modules in the calorimeter are mechanically and dimensionally identical. The front face dimensions of the |
128 | towers are $6\times6$ $cm^2$ resulting in individual tower acceptance of $\Delta\eta\times\Delta\phi=0.014\times0.014$ at $\eta$=0. | |
129 | The EMCal design incorporates a moderate detector average active | |
486ebd44 | 130 | volume density of ~5.68 g/cm$^3$ which results from a 1:1.22 Pb to scintillator ratio by volume shown in Fig.~\ref{fig:module}. |
b1f776f8 | 131 | This results in a compact detector consistent with the EMCal integration volume at the chosen detector thickness of 20.1 radiation lengths. |
132 | %Practical considerations, including the total assembly labor cost, | |
133 | %suggest reducing the total number of Pb/scintillator layers thus decreasing the sampling frequency. | |
134 | ||
135 | As described above, the super module is the basic building block of the calorimeter. | |
136 | Starting with 288 individual modules which are rather compact and heavy, the main engineering task is to create a super module structure which is rigid, | |
137 | with small deflections in any orientation yet does not require extensive, heavy external stiffening components that would reduce the volume available for the active detector. | |
138 | The solution adopted for the ALICE EMCal is to develop a super module crate which functions not as a box for the individual modules | |
139 | but rather an integrated structure in which the individual elements contribute to the overall stiffness. | |
140 | The super module crate is effectively a large I-beam in which the flanges are the long sides of the crate and the 24 rows of strip modules together. | |
141 | This configuration gives to the super module good stiffness for both the 9 o'clock and 10 o'clock locations. For the 12 o'clock location, | |
142 | the I-beam structure of the super module is augmented by a 1 mm thick stainless steel forward sheet (traction loaded), | |
143 | which controls the bending moment tending to open the crate main sides, and helps to limit deflection of strip modules. | |
144 | Ridges are provided on the interior surfaces of the crate to allow precision alignment of the strip modules at the correct angle. | |
145 | The stiffness given by this I-beam concept allows the use of non-magnetic light alloys for main parts of the super module crate. | |
146 | Parts of the super module crate will be made mainly from laminated 2024 aluminum alloy plates. | |
147 | The two main sides (flanges of the I-beam) of the crate will be assembled from 2 plates, 25 mm and 25 mm thick, | |
148 | bolted together and arranged so as to approximately follow the taper of the 20 degree sector boundary. Each of the 24 rows of a super module contain 12 modules as described above. | |
149 | Each of the modules is attached to a transverse beam by 3.4 mm diameter stainless steel screws. | |
150 | The 12 modules and the transverse beam form a strip module. The strip module is 1440 mm long, 120 mm wide, 410 mm thick. | |
151 | The total weight of the strip module is approximately 300 kg and like module, it is a self supporting unit. | |
152 | The transverse beam, which is the structural part of the strip module, is made from cast aluminum alloy | |
153 | with individual cavities along its length where the fibers emerging from towers are allowed to converge. | |
154 | The casting process is well suited to forming these cavities and the overall structure, saving considerable raw material and machining time. | |
155 | ||
156 | In addition to functioning as a convenient structural unit which offers no interference with the active volume of the detector | |
157 | and forming the web of the I-beam structure of the super module, the transverse beam of the strip module provides protection for the fibers, | |
158 | a structural mount for the light guide, APD and charge sensitive preamplifier and a light tight enclosure for these elements. | |
159 | ||
160 | ||
486ebd44 | 161 | \subsection{Functional description of the EMCAL} |
b1f776f8 | 162 | |
486ebd44 | 163 | %**** need some additional info on PE APDs ********* |
b1f776f8 | 164 | %EMCAL basic units are cells/towers (Pb-scintillator sandwich of about 70 layers). We have 12 SuperModules (4 in 2010, 10 in 2011-2012) composed of 24 (phi direction) x 48 (eta direction) cells (except last 2 SuperModules made of 8 cells in phi direction). |
165 | ||
166 | Particles traversing the calorimeter, in particular photons and electrons, will deposit energy in different towers. | |
167 | The EMCAL reconstruction measures such energy per tower, forms clusters of cells produced by a given particle, | |
168 | and if possible matches them with particles detected by the tracking detectors in front of EMCAL (charged particles). | |
169 | ||
170 | Scintillation photons produced in each tower are captured by an array of 36 Kuraray, | |
171 | Y-11, double clad, WLS fibers that run longitudinally through the Pb/scintillator stack. | |
172 | Each fiber terminates in an aluminized mirror at the front face end of the module and is integrated into a polished, | |
173 | circular group of 36 at the photo sensor end at the back of the module. | |
174 | The fiber bundles are pre-fabricated and inserted into the towers after the module mechanical assembly is completed. | |
175 | The 36 individual fibers are packed into a circular array 6.8 mm in diameter and held in place inside a custom injection | |
176 | molded grommet by Bicron BC-600 optical cement. An optical quality finish is applied to the assembled bundle using a diamond polishing machine. | |
177 | At the other end of the bundle, individual fibers are similarly polished and mirrored with a sputtered coat | |
178 | of aluminum thick enough to ensure the protection of the inner mirror. | |
179 | The response of the Al-coated fiber is considerably flatter with an overall increase in efficiency in the range of about 25\% in the vicinity of shower maximum | |
180 | (i.e. the location of the highest energy deposition for an electromagnetic shower). | |
181 | This number accounts for material immediately in front of the detector; which ranges between 0.4 and 0.8 radiation lengths, | |
182 | and assumes 5.5 - 6.0 radiation lengths for shower maximum for 10 GeV photons. | |
183 | At this depth in the detector, the mirrored fiber response is very uniform does not contribute to the non-linearity of the detector as a whole. | |
184 | ||
185 | Other factors which can significantly impact the electromagnetic performance of the calorimeter, | |
186 | include scintillator edge treatment and the density of the wavelength shifting fiber readout pattern and the material | |
187 | chosen for the interlayer diffuse reflector. For scintillator edge treatment and fiber density, advantage was taken from the | |
188 | extensive studies made by the LHCb collaboration for their ECAL. In particular, a diffuse reflector edge treatment was adopted, | |
189 | such as that obtained with Bicron Titanium Dioxide loaded white paint (BC622A) with a total fiber density of about one fiber per $cm^2$. | |
190 | In the case of the interlayer diffuse reflector, a white, acid free, bond paper was used in place of the Teflon based commercial TYVEK. | |
191 | While TYVEK produces slightly better surface reflectivity, its coefficient of friction is too low to permit its use in this | |
192 | design where the module's mechanical stability depends somewhat on the interlayer friction. | |
193 | ||
194 | The 6.8 mm diameter fiber bundle from a given tower connects to the APD through a short light guide/diffuser | |
195 | with a square cross section of 7 mm $\times$ 7 mm that tapers slowly down to 4.5 mm $\times$ 4.5 mm as it mates (glued) | |
196 | to the 5 mm $\times$ 5 mm active area of the photo sensor. | |
197 | The 4 pre-fabricated fiber bundles are inserted into the | |
198 | towers of a single module. | |
199 | ||
486ebd44 | 200 | The selected photo sensor is the Hamamatsu S8664-55 Avalanche Photo Diode. |
201 | % ************ | |
b1f776f8 | 202 | This photodiode has a peak spectral response at a wavelength of 585 nm compared to an emission peak of 476 nm for the Y-11 fibers. |
203 | However, both the spectral response and the quantum efficiency of the APD are quite broad with the latter dropping from the maximum by only ~5\% at the WLS fiber emission peak. | |
204 | At this wavelength, the manufacturer's specification gives a quantum efficiency of 80\%. |