4.THE ELECTROMAGNETIC CALORIMETER

Overview and requirements

The physics process that imposes the strictest performance requirements on the electromagnetic calorimeter is the intermediate mass Higgs () decaying into two photons. Thus the Di-photon mass resolution is the benchmark against which the performance of the electromagnetic calorimeter (ECAL) should be measured.

To detect properly the two-photon decay mode of a Higgs boson in the intermediate mass region, the stochastic term (a), constant term (b) and noise term (c) of the energy resolution usually parametrized as , have to be small. A small a-term of is readily obtained with homogeneous calorimeters. A low is difficult to reach for any type of calorimeter. With a crystal type calorimeter, one can expect to achieve required level of performance as was demonstrated by the physics result of several past and present experiments (Crystal Ball, L3, etc.). Also, fine granularity and good hermiticity are relatively easy to achieve. However, very high luminosity foreseen at LHC bring additional severe constraints on the detector, such as speed of crystal response and of electronics and resistance to radiation of all its components. Also direct measurement of the photonís direction is required to associate unambiguously photon to its primary vertex. Hence a preshower detector is needed for high luminosity running. Endcap preshower will be also required for low luminosity runs in order to improve single rejection.

The CMS collaboration decided to construct a precision electromagnetic calorimeter consisting of 110 000 lead tungstate (PWO4 or PWO) crystals, initially investigated and proposed to CMS by groups of physicists from IHEP, Protvino, Russia and from INP, Minsk, Belarus. The decision was followed by extensive research and development program carried out by the physics community. Several fast scintillating crystals (NaBi(WO4)2 BaF2, CeF3, undoped CsI and PbWO4) were thoroughly investigated for this physics motivation. Lead tungstate was chosen by the CMS experiment because of its high density, acceptable cost, fast decay time and the possibility of crystals production in industry. Since 1993 extensive test beam studies in frame of CMS have been performed on by RDMS ECAL groups from IHEP, Protvino, INR, Moscow and INP, Minsk.

The main directions of RDMS CMS ECAL activity are:

  • study of crystal properties and organization of its production,

  • certification of PWO parameters,

  • beam tests,

  • monitoring system,

  • scientific/technical design of ECAL Endcap,

  • participation in ECAL Preshower detector.


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4.1. PWO CRYSTALS


Properties of PWO crystals and production

Radiation hardness

The RDMS institutions lead practically all investigations concerning crystal production. In 1996 RDMS efforts were mainly focused on improvement of the low dose radiation hardness of the PWO crystals at different dose rate of irradiation. For this purpose a special activity was organized with the main goal of express test of radiation hardness of the every new batch of crystals, analysis of the data obtained and development of recommendations for producer. The technique for radiation tests was developed by INR (Moscow). The tests were performed at Dubna Microtron beam with energy up to 25 MeV, LINAC MIFI with energy range of 24-27 MeV and Dubna neutron pulse source IBR-2. The technique developed allows accurate measurements of variation of the crystal parameters under different type of radiation and doses rate.

Based on this experimental data phenomenological model to explain PWO crystal behaviour in a radiation fields was developed by INP (Minsk). The hard efforts of the researchers improve the crystal radiation hardness (see Fig. 4.1). The origin and kinetics of the radiation damage were set.

Yield of the crystals

This approach allows to understand better the parameters influencing the reproducibility of crystal properties. It also permits to work at an early stage of the project on the factors influencing the yield at every step of the production. This way allowed to solve problem of acceptable crystal yield. The dynamic of growth of the yield of crystals is shown in Fig.4.1.

Afterglow

The yield of crystals with fine optical transmission was stabilized. More than 80 % of crystals satisfy to CMS requirements. A systematic investigation of the origin of the afterglow led to the conclusion that a Mo impurity strongly influences on the afterglow. As a result of research the afterglow has been suppressed. The specification on contamination of Mo in raw material and the technology of cleaning of raw material. Also the technology of the minimization Pb losses during crystal growing is developed.

Crystal Specification

It is currently planned in CMS to order crystals from 3 producers. The crystals will be systematically checked at the production plants and again at the regional centers. Producers and regional centers will be equipped with Automatic Crystal COntrol System (ACCOS), in which precise dimensions (5 micron), planarity and angles together with optical transmission, decay time and light yield of 20 crystals will be measured in 5 hours and the data transferred to CRISTAL system (Concurrent Repository and Information System for Tracking Assembly Lifecycles), which provide information and workflow management. The data will be recorded in a object-oriented database accessible from any laboratory in the world via WWW. Institutions from Protvino and Minsk are now participating in design of these systems which should be operational in the seven plants and laboratories in 1998.

Beam tests

The matrix 7x7 crystals with APD as photodetector was studied in the test beam in 1996. Main goal of these tests was crystal radiation hardness under a small dozes of irradiation. The production technology of the radiation hardness crystals in Bogoroditsk are developed simultaneously with progress in monitoring of variations of light yield during measurements. Was shown that the precision and stability level of 0.4% can be reached for signal losses due to radiation up to 20%. The monitoring precision was restricted by the temperature stability in prototype. Fig. 4.2 illustrates that the losses in transparency of crystal can be corrected practically without losses in energy resolutions. Nevertheless, the CMS requirements to radiation hardness allows signal losses only a few percents.

The lead tangstate heavy crystal calorimeter (160 crystals) was tested in a GAMS-type experiment in 70 GeV IHEP accelerator. Clean signals from - and -mesons were observed (see Fig. 4.3) in Di-photon mass spectra from charge exchange reaction at 25 GeV.


ECAL Endcap design

During two years some concepts of Endcap based on using PWO crystals up to value of pseudorapidity ~3 were developed in IHEP, Protvino. Obviously, the usage of APD as photodetector for such values of is impossible, because of a strong radiation damage. Development of the vacuum tetrodes and triods with diameter of 20 mm (fine mash technology) was started in St.Petersburg. The first tests shown promising results for characteristics of these photodetectors and their correspondence to CMS requirements.

This years it was decided and accepted by CMS ECAL management that the ECAL Endcaps will be build by the RDMS - UK collaboration. The distribution of responsibilities is reflected in the new Management structure of the ECAL CMS project.

Plans

It is assumed that the production of the crystals with characteristics corresponding to CMS requirements will be arranged in this year. Recent measurements in DAPNIA, Saclay irradiation facilities give the belief that problem of radiation hardness will be solved. In spite of progress in radiation hardness of crystals under small dozes of irradiation this problem remains to be the main object of investigations in 1997. The decision concerning the geometry and structure of ECAL Endcap will accepted before 1.06.1997. The Technical Design Report for ECAL Endcaps in preparation according to 1997 milestones and will be completed by December 1997. The test beam studies of the ECAL prototypes will be continued during this year.

Fig.4.4: RDMS group participation in the final stage of PWO technology development in 1996 - 97

Major tasks

One of the serious problem is the organization and start of PWO mass production in Bogoroditsk Techno-Chemical Plant. This Plant has 100 ovens to growth crystals. However, these setups are weared out. Reparation will required a large expenses. 50 000 crystals will be done during 5 years. It is very difficult task even all setups will work. To decrease the probability of fail in production and supplying of the crystals there is possible to increase twice number of setups in Bogoroditsk (without a large expenses) or to include Appatity plant (100 setups) in this activity.

The participation in creating of CMS setup has as high scientist so commercial profit for Russia.


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4.2. PRESHOWER

An important component of the electromagnetic calorimeter is the preshower detector, consisting of the silicon strip detectors with 2 mm pitch strips followed by a thin lead absorbers. The preshower detector will allow:

  • to measure the position of the incoming photons with accuracy 300 micrometers;

  • to measure the direction of the photons by using the information on electromagnetic shower position measured in the calorimeter. This feature is important at high luminosity running;

  • to verify that single photon candidate is not due to the overlapping of two closed photons from decay. This separation of single photons from decays is necessary to reduce the jet background for search of Higgs.


Main results in 1996

The elements of mechanical structure of the preshower detector are designed, including:

  • absorbers made of lead and inox;

  • radiators for cooling of the silicon detectors aiming to decrease the speed of the degradation of silicon detectors in a strong radiation field;

  • system for positioning of the silicon detectors.

A prototype of the preshower detector was designed and investigated at test beam H4 at CERN together with the prototype of the PWO-crystal electromagnetic calorimeter. It was demonstrated, that the position resolution is about 300 micrometers for the electron energy over 50 GeV. The energy resolution of the calorimeter measured with preshower in front deteriorates not more than 0,3 % in comparison with measurement without preshower for the energy of electron beam over 100 GeV. The energy resolution is shown in Fig. 4.5.


Future plans B

The prototype of preshower detector with the sensitive area of 24 by 24 cm2 will be manufactured. The prototype consists of:

  • 32 silicon detectors , the topology of the silicon detector is shown in Fig. 4.6;

  • two lead absorbers with mechanics for positioning of silicon detectors;

  • radiators for cooling system of silicon detectors.


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