LHCb 2004-ZZZ
CALO
19 April 2004
The monitoring system for the LHCb ECAL (update since Calorimeter EDR)
A.Arefiev (ITEP, Moscow, Russia)
Y.Gilitsky ( IHEP, Protvino, Russia )
A.Golutvin (ITEP, Moscow, Russia)
I.Korolko ( ITEP, Moscow, Russia )
E. Melnikov ( ITEP, Moscow, Russia )
1 Introduction
Precise monitoring of all detector components is obviously very important for all future LHC experiments. LHCb calorimeters are providing fast measurements used already on the very first trigger level for selecting events with high E
T
decay products of B mesons. Properties of the calorimeter scintillating plastic suffer from rather high radiation fields especially in the vicinity of beam pipe. On top of that the gain of phototubes is very sensitive to the variation of power supply voltage. The monitoring system of the LHCb electromagnetic calorimeter uses cheap commercial LEDs as a light sources. LED light is transported to all calorimeter cells with clear fibers. The stability of LEDs is in turn controlled with PiN diodes. All components of the ECAL monitoring system (LEDs, LED driver, PiN diodes, PiN diode readout) were described in details in the previous note [1]. This document describes the overall layout of these components within the experimental setup.
2 Light splitting and PiN diode control
The monitoring light signals will be initialized by a system of 536 light emission diodes (LED) fired by 184 independent trigger signals (92 for left and right ECAL parts). The light signal from each

LED will be distributed to the front side of the modules using system of the optical mixers (splitters) and clear optical transport fibers of 1,0 mm diameter. The light is transmitted from the transport fiber via an optical connector to another clear fiber of 1,2 mm diameter that penetrates each module cell and that is bundled in front of the PMT light mixer together with the other WLS fibers.
For the inner calorimeter modules LED light splitter to 7 fibers will be used – 6 transport fibers go to the modules front side and one fiber goes to a reference PIN diode. For middle and outer calorimeter modules LED light splitter to 17 fibers will be used – 16 transport fibers go to the modules front side and one fiber goes to a reference PIN diode. Several LEDs are controlled by a single PiN diode in order to minimize the total number of electronic channels used by the monitoring system. Figure 1 shows the groups of calorimeter cells flashed with a single LED for the outer and middle ECAL sections. The same picture also illustrates the groups of neighbour LEDs controlled with a single PiN diode. Most of PiN diodes in these two sections control 4 LEDs. The length of optical fiber bundles transporting the light from these LEDs to the calorimeter cells is different (~50cm for outer section and ~25cm for middle section). This difference will be compensated with clear fibers transporting the light to the reference PiN diode.
Figure 1 Light splitting scheme and LED grouping for outer and middle ECAL section.

The corresponding information for the inner ECAL section is presented in Figure 2. Almost all PiN diodes in this section control 6 LEDs. The difference in length of the corresponding optical fiber bundles is negligible (~12 cm) and therefore does not require special compensation. Twelve innermost cells located to the right side from the beam pipe are not equipped with phototubes and therefore do not require monitoring.
Figure 2 Light splitting scheme and LED grouping for inner ECAL section.
The whole monitoring system will use 128 PiN diodes. Table 1 presents the number of LEDs, PiN diodes and trigger signals for all sections of the LHCb calorimeter.
Table 1 Monitoring system components for one half of ECAL
Number of LEDs Number of PiN diodes Number of trigger signals
Outer section
Middle section
Inner section
84
56
128
28
14
22
42
28
22

3 Optical fibers distribution system
As a baseline solution for the mechanical layout of the monitoring system components including
LED’s with optical mixers (splitters), LED’s pulse drivers, reference PIN diodes and theirs readout, low voltage power supply we have chosen the full-width calorimeter bottom side and small region located in the center of the electronics platform on top of the calorimeter. The central part of the upper ECAL wall
(two 8 modules wide columns to the left and to the right from the calorimeter center) are serviced with two light distribution boxes located on top of the calorimeter, while the remaining calorimeter area is serviced with light distribution boxes located under the calorimeter supporting platform. There will be 8 light distribution boxes under the calorimeter – one box per 8 modules wide column. Table 2 summarizes the information about light distribution boxes for one half of the LHCb ECAL. Upper distribution box and two central boxes located under the calorimeter are 980 mm high and consist of two “floors” to handle large number of LEDs required for inner and middle ECAL sections. Two other boxes have only one
“floor” due to the smaller number of LEDs monitoring the outer ECAL section.
Table 2
Upper box
1 st
lower box
2 nd lower box
3 rd lower box
4 th
lower box
Light distribution boxes for one half of the calorimeter.
XYZ size (mm) Number of LEDs
735 x 980 x 90 80
735 x 980 x 120
520 x 980 x 120
520 x 500 x 120
520 x 500 x 120
80
56
26
26
Number of PiN diodes
15
15
16
9
9
All clear fibers lighten by a single LED, are grouped in a bundle, which is mechanically and optically protected with a heat shrinkable tube. All bundles from the light distribution boxes are passed through the thin gap between calorimeter and preshower supporting structures uniformly in one layer.
Reaching the calorimeter, bundles are grouped in vertical plastic trays attached to the calorimeter surface.
Plastic trays transport fiber bundles to the required calorimeter modules. Then individual fibers, protected with a smaller diameter heat shrinkable tubes, are connected to the corresponding ECAL cells.

4 Light distribution boxes
A schematic view of the light distribution box fragment is shown in Figure 5 for 6 LEDs monitoring 3x2 cells region of the inner ECAL section. The box has 2 layers (in Z direction). First layer is used for the arrangement of fiber bundles. The second layer hosts the main components of the monitoring system – LEDs, LED drivers, light mixers (splitters) and PiN diodes with their readout. All LEDs controlled with a single PiN diode are located together just in front of the controlling PiN diode. Fibers transporting the light from LEDs to PiN diodes are also placed in the second layer.
Figure 3 Fragment of the light distribution box (schematic view).

References
[1] Y.Gilitsky et al., LHCb2003-151 (internal note)