The present LUCIDgeometry is shown in Fig.59. It consists of two detectors, placed around the beam pipe symmetrically at about 17 m from the
ATLAS
IP. Each detector is formed by 16 PMT grouped by four and deployed in theATLAS
TASshield region, as shown in the upper part of Fig.59. The detecting medium is the quartz window of the PMTitself, which acts as a Cerenkov radiator. There are also 4 channels for which the radiator is a bundle of quartz fibres. These fibres are readout by PMT(one per bundle) placed on top of the ATLAS shielding (see the upper and middle part of Fig.59). This readout method has already been tested successfully for the old LUCID, and has the advantage that the readout PMTsit in an area with a low level of radiation. The detector is placedaround the LHC beam pipe in a supporting carbon fibre cylinder as shown in the middle part of Fig. 59, and inserted in the carbon fibre beam pipe supporting cone. All the 20 readoutPMTare inserted in mu-metal cylinder in order to shield them from stray magnetic fields. Details about the cross sectional view of the detector are shown in the lower part of Fig.59.
The beam pipe is in aluminium and the support cone is made of carbon fibres. Since the detector must withstand the high temperature reached by the beam pipe during the bake-out phase, a suitable cooling system for the LUCID detector has been provided.
The LUCID detector is actually composed of three different detectors (see Fig. 60). The main
Figure 60.Top Left:Hamamatsu R760PMTwith quartz window of10 mmdiameter.Bottom Left:Hamamatsu R760PMTmodified to have a quartz window of7 mmdiameter. Right: Bundles of fibres used in the Run 2 LUCID detector.
detection scheme is to measure the light produced by charged particles above the Cerenkov thresh- old crossing the windows of Hamamatsu R760PMTs (see upper part of Fig.60). One of the main design criteria was to keep the detector acceptance low because of the increased occupancy in Run 2. For this purpose, a Hamamatsu R760 PMT has been produced with a sensitive window reduced from 10 mmto7 mmwhich will significantly reduce the measured rate. This modification, which was specifically provided for LUCID, was produced by Hamamatsu by aluminising the PMT photo-cathode area.
An alternative readout method, already commissioned during Run 1, consists of measuring Cerenkov photons produced in bundles of long quartz fibres (Fig.60, lower part) readout by R760 PMTs placed far from the beam pipe, where the radiation level is reduced.
As far as the fibre readout is concerned, the Cerenkov light produced in the quartz fibres is guided by the fibres themselves to the R760PMTplaced in an area with low radiation level. There
are 4 bundles of fibres per side used for this purpose. The fibres are routed along the beam-pipe by aluminium pipes.
The
ATLAS
experiment will surely need luminosity monitors for the Phase-II period of operation. At the time of this document there is no detector design available. It is however likely that one of theATLAS
luminosity monitoring techniques for Phase-II will rely on some modified version of the present LUCID detector. The present Run 2 LUCID can be considered already as an R&D project for a Phase-II detector in itself. Of the three detection techniques implemented, the most promising as a baseline for Phase-II is certainly the one in which the Cerenkov light produced in the quartz fibres is guided by the fibres themselves to the R760 readout PMTplaced in an area with low radiation level. This version must be fully commissioned in the next years and, if proven to work in a reliable way, it is certainly the most appealing detection scheme since it decouples the heavily irradiated Cerenkov light detector, based on in very radiation hard quartz fibres from the readout devices, which can be placed in an area with low radiation level. The main item that must be commissioned in Run 2 in order to validate this detection scheme is the completely newLUCROD (LUCid ReadOut Device) card which performs, among other functions, the integral by bunch of the detected current, so providing the total delivered charge per bunch which is proportional to the instantaneous luminosity. This readout scheme is in principle free of the non-linearity of other commonly used signals over threshold counting algorithms, but depends critically on the detector stability. The monitoring of the fibres readout stability during Run 2 will therefore be crucial to validate the detection scheme based on quartz fibres.One of the main challenges to be understood and solved is the new positioning of theLHC VAX which is shown in Fig. 61. In the left part of this figure the present position of theVAXand in the right part its proposed position after LS3 are shown. The reason for this displacement is to reduce the need for people to enter a high radiation area in case some of the VAXequipment fails. The consequence is that the new positioning will conflict with the present location of LUCID. Both the cable routing and detector position will be affected by this change. It appears that the new constraint imposed on theLUCIDlocation in Phase-II will not represent an insurmountable problem and that solutions will be found.
Figure 61. TheTAScollimator area where theLUCIDdetector is deployed.The red arrows point to theVAX
Table 22.Summary of the costs of the existing
ATLAS
forward detectors, and the relevant physics goals for the Phase-II forward detectors.Detector Cost Estimate (kCHF) Physics Goals AFP 1,000 Hard Diffraction, CEP
ZDC 800 Heavy Ions runs
LUCID 500 Luminosity