The TOTEM Gas System

Total Cross Section, Elastic Scattering and Diffraction Dissociation at the LHC EDMS No.: TOTEM-DI-ER-0001 TOTEM Experiment document No.: IN-2000/02 The TOTEM Gas System M. Bozzo (1), M. Castoldi (2), A. Morelli (1) (1) Universita` di Genova and INFN Genova (2) CERN European Organization for Nuclear Research Abstract This document describes the gas systems proposed for the TOTEM experiment at the LHC. In the report a separate chapter is devoted to each complete detector gas system; this includes outline design drawings and description of modules. Modules of standard design will be employed as far as possible, in order to minimise design overheads and long term support costs. In the last chapter an exercise is made to evaluate the cost of the system using as base price the costs of the different components as they appear in a similar report devoted to CMS. Geneva, 9 November, 2000

TOTEM IN-2000/02 1 Table of Contents 1. INTRODUCTION...2 2. THE CSC GAS SYSTEM...3 2.1 Introduction...3 2.2 The gas system...4 2.2.1 The SGX Building...4 2.2.2 The US cavern...5 2.2.3 The UX cavern...5 2.3 Gas Purity...6 3. THE RPC GAS SYSTEM...8 3.1 Introduction...8 3.2 Gas system...8 3.2.1 The SGX Building...8 3.2.2 The US cavern...9 3.2.3 The UX cavern...9 4. MOVING THE END CAPS DURING THE SHUT-DOWN...11 5. COST ESTIMATE...12 5.1 Gas running costs...12 5.2 Piping cost estimation...12 5.3 Gas system cost estimate...13 5.4 Gas Analysis...14 6. CONCLUSIONS AND COMMENTS...14 ACKNOWLEDGEMENT...14 REFERENCES:...14

2 TOTEM IN-2000/02 1. INTRODUCTION The Research Board has approved the TOTEM experiment on the 26 th of November 1999 to measure Elastic and Total Cross Section in the early phase of LHC. For TOTEM the accelerator needs to be tuned in a mode completely different than that required by the other experiments, but the few specific data taking periods will be short in time. The Collaboration has asked to be able to repeat the measurement after the data analysis of the first run is completed. The inelastic detectors will be inserted in regions of angular acceptance that have been identified inside the CMS experiment area in IP5. The experiment is made up of three separated parts: the roman pots, 150-200 m from the Interaction Point (already in the tunnel) that will need no gas, and two telescopes, Telescope 1 (T1) and Telescope 2 (T2), installed in the End Caps of CMS and inside the Rotating Shielding (Figure 1) where CSC and RPC detectors will be installed. Figure 1 - The schematic diagram of the TOTEM inelastic telescopes installed inside CMS. One side shows the detector in running conditions, the other during installation. TOTEM operation is foreseen only for the first couple of years of LHC, since the operation of the detectors will become impossible as soon as the machine luminosity will increase to a level of few 10 33 sec 1 also because of the radiation dose received. Each TOTEM telescope is composed of 5 planes of CSC and 2 double planes of RPC, detectors that need specific gas mixtures for their operation: this note will outline the requirements and the choices made by the collaboration. TOTEM must be able to operate during the very first days of commissioning and setting up of the LHC, most likely in a way independent from CMS.

TOTEM IN-2000/02 3 TOTEM then needs a completely separate gas system: whenever possible and without putting limits on the possibility of TOTEM to operate independently, some part of the system may be shared with CMS. The functional modules of the TOTEM gas system will be located in the SGX gas building, in the US cavern and in the UX cavern (see Figure 2). To simplify the comprehension of the system the three locations will be analysed separately for each type of detector. Figure 2 - Schematic drawing of the TOTEM CSC and RPC gas system. On this drawing only one endcap is shown, the gas piping for the two endcaps is split in the US area. 2. THE CSC GAS SYSTEM 2.1 Introduction The basic function of the gas system is to mix the three gas components in appropriate proportions and to distribute the clean gas mixture to the individual planes of the CSC detectors installed in the two separate telescopes T1 and T2 at a pressure of ~2 mbar above atmospheric pressure. For a detector system of the TOTEM size a closed loop circulation system is not necessary. The operating gas is a non-flammable mixture of Ar-CO 2 -CF 4 (40%-50%-10%). Even if this composition has been studied in numerous test measurements and has shown that it is suitable for CSC operation in the LHC, the ratio between Ar/CO 2 in the mixture may however be modified in the future.

4 TOTEM IN-2000/02 Table 1 - Basic parameters of the TOTEM CSC gas system Gas volume ~ 400 l Concentration ratio Ar-CO 2 -CF 4 40-50-10 Chamber relative pressure Leak rate of the whole system Maximum flow rate Gas Flow rate at operating conditions 1-3 mbar Undetectable 200 l/h 70 l/h Some basic parameters of the CSC gas circulation are listed in Table 1. The total detector volume for one side is approximately 200 litres; the detailed list of detectors and their volume can be found in table 3. During normal running CSC gas should be circulated with a flow such as to allow the exchange of one gas volume in the detectors once every 6 hour. The expected circulation flow rate is quite low, amounting to 35 l/h (or 0.5 l/min) for one side. The expected total gas flow at operating conditions is about 60 to 70 l/h, which is 30-40% of the maximum flow allowing variations by nearly a factor three up or down. 6-8 full volume changes with fresh gas at maximum flow will be needed to obtain operating conditions, leading to a start-up time of ½ a day 1. 2.2 The gas system The gas system proposed can be subdivide in functional modules, which are designed as far as possible uniformly for all the gas systems of LHC experiments. However it is worth reminding here that the component sizes and ranges should be adapted to meet the specific requirements of the small TOTEM CSC system. The functional modules of the CSC gas system and the location are schematically outlined in Figure 2 and listed in Table 2. Module Table 2 - Functional modules of the CSC gas system Situated in Primary Gas Supplies Mixer Distribution, Pressure Control and Vent Chamber Distribution Systems SGX Building SGX Building USC55 Cavern UXC55 Cavern 2.2.1 The SGX Building The SGX building hosts the gas input for the mixer, and the buffer. In the mixer the flows of component gases are metered by mass flow controllers, which have an absolute precision of ± 1% over a year, and have a medium term stability of ± 0.3% in constant conditions. The medium term stability in constant flow conditions is better than 0.1%: absolute stability will depend on the absolute precision of the analysing instrument. 1 The maximum value of the flow available will allow a full volume exchange with fresh gas in less than 2 h.

TOTEM IN-2000/02 5 Flows are monitored by a process control computer, which continually calculates the mixture percentages supplied to the system. The process computer will be most likely part of the control system of the experiments installed in the experimental area and will also communicate with the TOTEM computer system. The functions and the actions to be assigned to the process control computer will be discussed in detail and should be properly adapted to the size of TOTEM. The gases are mixed in the buffer and piped to the US cavern through a Stainless Steel pipe of 12 mm diameter. 2.2.2 The US cavern The US cavern is permanently accessible and hosts the primary chamber pressure regulation, the flow regulation and the gas analysis instruments. The chambers inlet pressure regulation is done independently for each of the two TOTEM sides allowing global flow adjustments for the detectors on one side even during physics periods. The gas is fed to, and returns from the experiment via four Stainless steel pipes of 12 mm diameter. These follow the path of the services that reach the UX cavern on each side and then the cable chain to the top of HF to a Totem Experiment Rack. The Rack will house electronics and gas connections both for T1 and T2. Ten compressed air lines are foreseen to remotely actuate pneumatic valves in the distribution panel. Similarly two Stainless steel pipes of 12 mm diameter return the gas from each side to a backpressure regulator (a pump) located in the US cavern, that keeps the pressure at a value suitable to maintain the detector at 2 mbar above the atmospheric pressure. 2.2.3 The UX cavern Due to the particular location of T2 (inside the rotating shielding) the distribution of gas for the two telescopes (T1 and T2) on the same side of the cavern will be done from the same rack. Therefore two Experiment Racks (one on each cavern side) are installed on the top of HF. The patch panel can be easily disconnected in case of TOTEM removal. A gas distribution set is also installed in the Experiment Rack. Due to the high residual magnetic field (up to 0.1 Tesla) in the rack location, pneumatic or manual valves have to be foreseen. Each telescope is composed of 5 planes of CSC and each plane is fed independently. This last requirement, dictated by the fact that it is not desirable to flush too many detectors in series, will also allow speeding up the installation/removal operations since all the connections will be made at the Experiment Rack. Considering now that the telescopes is split into two halves for installation reasons, the gas from the Experiment Rack has to be fed to 10 half planes for each telescope T1 and T2. In T1 each of the 10 CSC half planes is made of 3 CSC detectors fed in series by one of the 10 gas lines. A schematic view of the two telescopes with indication of the detector s planes and distribution pipes is shown in Figure 3. Each line to the detector will be a short flexible pipe with a self-sealing quickconnector, both for the inlet and the outlet line. This will allow gas channels to be individually disconnected from the circulation loop for flushing with inert gas or exhausting to direct vent. If necessary every supply and return line will be equipped with remotely read flow meters allowing a direct comparison between inlet and outlet flows. The adjustment of individual channel flows will be done with a simple (manual) needle valve at the inlet. Accurate pressure regulation is needed for a proper working of the CSC, their mechanics being

6 TOTEM IN-2000/02 able to stand larger overpressure: an overpressure safety valve should be foreseen. The typical detector operating pressure is near 1 mbar above local atmosphere and the pressure drop in the detector return pipe must be properly considered. The flow metering technology must be simple, reliable and inexpensive. Small mass flow meters (hot-wire type) which can be mounted on a PC board are currently being studied for this purpose. Figure 3 - Schematic layout of the two telescopes: the inset shows how three detectors in one plane of T1 are connected in series for gas circulation. It is foreseen to sample the chamber output gas for gas analysis. The analyser will be in the UX cavern area and will receive the sampled gas by a dedicated < 6 mm diameter Stainless steel pipe and a remote controlled manifold for the selection of the channel to analyse. The volumes and foreseen gas flow rates for the CSC of each telescope are summarised in Table 3 and the pipe sizes and lengths are listed in Table 4. 2.3 Gas Purity Although CSC detectors are very robust and insensitive to air, a purifier system might be necessary to limit and stabilise oxygen and water contamination in the gas (in the case of usage of lower purity grade Ar or CO 2 ).

TOTEM IN-2000/02 7 For example a set of purifier cartridges, of size appropriate to the TOTEM flow, with molecular sieve (3Å) to remove water vapour and activated copper as reducing agent for oxygen removal might be inserted after the mixer stage. Humidity and oxygen meter must be available to measure the impurity concentrations before and after the purifier. Table 3 - Detector volumes and gas flowrates for CSC (one side)* Detector Number Volume (l) Vol plane (l) Tot volume (l) N. of gas channels Gas chan. volume (l) Channel flowrates (l/h) Gas flow (*) (l/h) T1/C/1 6 4.9 29.1 2.0 14.6 2.4 4.9 T1/C/2 6 5.5 33.0 2.0 16.5 2.8 5.5 T1/C/3 6 6.1 36.6 185.8 2.0 18.3 3.0 6.1 T1/C/4 6 6.8 41.0 2.0 20.5 3.4 6.8 T1/C/5 6 7.7 46.0 2.0 23.0 3.8 7.7 T2/C/1-5 2 1.2 2.5 12.5 2.0 1.2 0.2 0.4 Totals 40 det. 198.3 198.3 20.0 33.0 (*) For a volume change every 6 h. Table 4 - Listing of gas pipes in the TOTEM CSC distribution system. The nominal flow corresponds to 1 volume gas replacement in 6h. Pipe Description No. Inner Pipe Pipe Norm. Max. Sect. No. of Pipes Pipe Diam. Material Length [m] Flow [lt/h] Flow [m 3 /h] [mm] 1 SGX to UXC55 1 12 SS 350 70 <0,3 2a US to Rack side 1 2 12 SS 80 35 2b US to Rack side 2 2 12 SS 80 35 3a 3b 3c 3d Rack side 1 to Chambers T1 Rack side 1 to Chambers T2 Rack side 2 to Chambers T1 Rack side 2 to Chambers T2 20 8 Rilsan 10 2.4 3.8 20 8 Rilsan 10 0.2 20 8 Rilsan 10 2.4-3.8 20 8 Rilsan 10 0.2 4a Sense side 1 to US 1 <6 SS 80 Traces

8 TOTEM IN-2000/02 4b Sense side 2 to US 1 <6 SS 80 traces 3. THE RPC GAS SYSTEM The RPC chambers in the TOTEM detectors are used for their superior timing capabilities and are necessary for triggering and background rejection. Double planes of RPC will be installed at the front and at the back of both telescopes T1 and T2. 3.1 Introduction The RPC operate with a non-flammable gas mixture whose characteristics are summarised in Table 5. The total gas volume is approximately 60 litres; a detailed list of detectors and their volumes is presented in Table 7. The basic function of the gas system is to mix the two components i-c 4 H 10 and C 2 H 2 F 4 in appropriate proportions (95.5%/4.5%) and to distribute the fresh gas mixture to the individual detectors installed in the two separate telescopes T1 and T2 keeping the pressure at ~2 mbar above atmospheric pressure. The very small detector volume does not justify a closed-loop circulation system. Gas volume Table 5 - RPC gas specifications. ~ 60 l Concentration ratio C 2 H 2 F 4 /i-c 4 H 10 (95.5 ± 0.3):(4.5 ± 0.3) Tolerable contamination: O 2 < 1% Tolerable contamination: H 2 O < 1% Relative pressure in the detector Maximum flow rate Gas Flow rate at operating conditions < 3 mbar 30 l/h 10 l/h The expected gas flow at operating conditions is about 10 l/h. This rate is 30-40% of the maximum allowing variations of the gas flow by nearly a factor three up or down. The flow at running conditions implies a full volume exchange with fresh gas in 4-6 h. The maximum value of the flow available will allow a full volume exchange with fresh gas in less than 2 h. To obtain operating conditions one needs 6-8 full volume changes with fresh gas leading then to a start-up time of ½ a day. 3.2 Gas system The system is based on the assembly of functional modules, whose sizes and ranges have to be adapted to meet the specific requirements of the TOTEM RPC system. The functional modules of the RPC gas system are listed in Table 6 and schematically outlined in Figure 2. 3.2.1 The SGX Building The SGX building hosts the gas input for the mixer and the buffer.

TOTEM IN-2000/02 9 In the mixer the flows of component gases are metered by mass flow controllers, which have an absolute precision of 0.3% in constant conditions. The medium-term stability in constant flow conditions is better than 0.1% and the absolute stability will depend on the absolute precision of the analysing instrument. A process control computer, which continually calculates and adjusts the mixture percentages supplied to the system, monitors the flow. Module Table 6 - Modules for the RPC gas system. Primary Gas Supplies Mixer Distribution, Pressure Control and Vent Chamber Distribution Systems Situated in SGX Building SGX Building USC55 Cavern UXC55 Cavern Isobutane, one of the components of the gas mixture is flammable, while the gas mixture itself is not considered flammable. Safety requires that the mixture be monitored continuously, that he gas flow must stop automatically if the i-c 4 H 10 fraction increases beyond the flammability limit (T ci = 5.75%) 2 and that an interlock signal must be sent to the experiment HV power supplies. After the mixer and the buffer the gas is piped to the US cavern through a Stainless Steel pipe of 12 mm diameter. 3.2.2 The US cavern The US cavern is permanently accessible and hosts the chamber pressure regulation, the flow regulation and the sampling analysis instruments. The chambers inlet pressure regulation is done independently for each of the two TOTEM sides allowing global flow adjustments for the detectors on one side even during physics periods. The gas is fed to, and returns from the experiment via four stainless steel pipes of 12 mm diameter and routed with the HF detector services to reach the UX cavern on different sides. Ten compressed air lines are foreseen to remotely actuate pneumatic valves in the distribution panel. 3.2.3 The UX cavern Also the RPC gas system distribution panel will be housed in the Experiment Rack placed in the UX cavern on the top of each HF. Fresh gas arrives from (and returns to) the US cavern to each side of the UX cavern through the HF cable chain. The two pipes located in the cable chain (one input and one output) terminate on a patch panel available on each Experiment Rack. This patch panel can be easily disconnected in case of TOTEM removal. The panel on the Experiment Racks houses the gas distribution control in the UX cavern. A pair of RPC detector modules will be installed in T1 (and T2) as first and last plane of the telescopes. Each plane is split into two parts due to installation requirements. One gas line supplies two adjacent chambers and another collects the used gas, leading to 4 2 The concentration of a flammable gas in a mixture for which the mixture is not flammable in air.

10 TOTEM IN-2000/02 independent short flexible pipes (Rilsan 3 ) 8 mm diameter gas channels with a self-sealing quick-connector for T1 (and 4 for T2) on each side. Details about volumes and flows in the RPC can be found in Table 7. Detector label Table 7 - Detector volumes and gas flowrates for RPC (one side, normal flow) number volume (l) vol/plane (l) tot volume 1 side (l) n. of gas channels gas chan. volume (l/h) channel flowrates (l/h) gas flow (l/h) T1/R/1.1 2 1.6 3.3 1 (r) 3.3 0.5 0.5 T1/R/1.2 2 1.6 3.3 30.6 1 (l) 3.3 0.5 0.5 T1/R/5.1 2 6.0 12.0 1 (r) 12.0 2.0 2.0 T1/R/5.2 2 6.0 12.0 1 l(l) 12.0 2.0 2.0 T2/R/1.1 (1.2,5.1,5.2) 2 0.12 0.25 0.98 1 (4 ch. Tot) 0.25 0.04 0.04 16 31.4 31.4 8 5.2 Table 8 - Listing of gas pipes in the TOTEM RPC gas system Pipe Description No. of Inner Pipe Pipe Norm. Max. Flow Sect. Pipes Diam. Material Length Flow [l/h] No. [mm] [m] [lt/h] 1 SGX to UXC55 1 12 SS 350 10 50 2a US to Rack side 1 2 12 SS 80 5 2b US to Rack side 2 2 12 SS 80 5 3a 3b 3c 3d Rack side 1 to Chambers T1 Rack side 1 to Chambers T2 Rack side 2 to Chambers T1 Rack side 2 to Chambers T2 8 8 Rilsan 10 0.5-2.0 8 8 Rilsan 10 0.02 8 8 Rilsan 10 0.5-2.0 8 8 Rilsan 10 0.02 4a Sense side 1 to US 1 <6 SS 80 traces 4b Sense side 2 to US 1 <6 SS 80 traces 3 To indicate a flexible material. Radiation resistance has to be checked.

TOTEM IN-2000/02 11 Every supply and return line has a remotely read flowmeters allowing a direct comparison between inlet and outlet flows. Manual adjustment of individual flows is obtained with a needle valve at the inlet. Accurate pressure regulation is needed for a proper working of the RPC, and it is independent for the two sides. The pressure control will be such that at nominal flow rate, the pressure inside the chambers is between 1 and 3 mbar. The mechanical stability of the chamber is sufficient to stand the full hydrostatic pressure of about 5 mbar for correct filling or purging of the chambers. The schematic layout of the gas distribution system to the chambers is similar to the one for the CSC and can be seen in Figure 3. The flow metering technology must be simple, reliable and inexpensive. It is foreseen to sample the chambers output gas for every channel from within the distribution panel and send the sample on a dedicated sample line for gas analysis. Several detector groups can share gas analysis instruments, if necessary. Table 8 lists the gas pipes for the TOTEM RPC gas system, the normal flow corresponds to 1 volume gas replacement every 6 hours. 4. MOVING THE END CAPS DURING THE SHUT-DOWN During the yearly running it may become necessary to have an intervention opening the End Caps: in this event the TOTEM telescope must be removed and the time added to the operation must be kept to a minimum. The location of the gas connection in the Experiment Rack on the detector platform allows the individual telescopes to be easily removed simply disconnecting the gas lines at the patch panel. During the shutdown the TOTEM telescopes and platform containing the gas distribution system will be removed from the cavern. Only the pipes from the patch panel will be disconnected. No gas pick-up will be needed elsewhere in the cavern.

12 TOTEM IN-2000/02 5. COST ESTIMATE This is a fairly precise and detailed description of the TOTEM gas system. An estimation of the total cost is attempted here with prices extracted from the CMS Gas System proposal in which systems similar to ours (even if of much larger sizes) have been evaluated [1]. 5.1 Gas running costs The running costs have been calculated assuming that a period of 8 months will be equivalent to one year of running at LHC. Details can be found in Table 9. Argon and Carbon dioxide prices from [1] seem quite low, considering the purity necessary for our CSC. However a tenfold increase in price would mean only a 30% of the total value, which is probably the incertitude to give to this specific evaluation. The total cost for the TOTEM gases at CERN bulk prices amount to less than 16 kchf/year. Table 9 - Gas consumption and cost per primary gas component for the TOTEM experiment (both CSC and RPC). Name Price/m 3 (CHF) Fraction Component quantity (l/h) Amount per 8 months (m 3 ) Mix flow (l/h) Mol. weight Density at T=20 0 Cost of 1 m 3 of mixt Cost of components per 8 months Ar 1.80 40% 26.40 154.28 66.00 46.79 1.92 32.3 4983.24 CO 2 2.01 50% 33.00 192.85 6229.06 CF 4 305.99 10% 6.60 38.57 1245.81 C 2 H 2 F 4 57.34 95.5% 9.74 58.06 15.00 40.77 4.11 55.6 3228.14 ic 4 H 10 18.54 4.5% 0.46 2.74 152.34 15838.59 5.2 Piping cost estimation The cost of piping is based on the assumption of a totally independent gas distribution system without recirculation loop as described in paragraphs 2 and 3. Table 10 gives the evaluation of the piping costs for the pipes listed in detail in Table 4 and 8. Stainless Steel pipes have been evaluated at 30.- CHF/m. The pipes to the detector include a section of 10 m of flexible stainless steel at 50.- CHF/m The last row indicates a rough evaluation of the price for the flow meters to be installed in the gas distribution rack. Detector piping gives the cost of Rilsan pipes (1.2 CHF/m) to/from the individual detectors planes and includes self-locking quick-connectors and all other kind of accessories.

TOTEM IN-2000/02 13 The unit cost of PEEK will be larger, but not such as to change the order of magnitude of the present evaluation. Table 10 - Evaluation of costs for piping and detector cabling. Pipes Number Length Dia. Unit cost CSC CHF RPC CHF Total cost SG to US 2 350 12 10'500 1 10'500 1 10'500 21'000 US to Det. 8 80 12 2'600 4 10'400 4 10'400 20'800 Sense 4 80 <6 2'600 2 5'200 2 5'200 10'400 Det. Piping 112 10 8 25 80 2'000 32 800 2'800 Det. Flowm. 56 450 40 18'000 16 7'200 25'200 46'100 34'100 79'200 5.3 Gas system cost estimate The total cost of the TOTEM gas system has been estimate considering the same modules as CMS, which are tailor-made for very large systems. Therefore it can be considered a conservative estimation, whose interest is to obtain an order of magnitude and to allow a comparison of the relative weight of each item for a totally independent system. The usage of an automatic system of control system is certainly necessary, and here it is assumed that one system will be sufficient to keep under control both gases (price should not depend on system size). The total cost for the CSC gas system is shown in Table 11. Table 11 Total cost for the TOTEM gas system Item Cost Mixer 46'000 Electrical control 36'000 (one for both gas mixtures) Piping SG-UXC-USC 26'100 Detector piping 2'000 Flowmeters and panel 18'000 TOTAL 128'100 Table 12 Total cost for the TOTEM RPC gas system. Item Cost Mixer 46'000 IR analysis 18'000 Piping SG-USC-UXC 26'100

14 TOTEM IN-2000/02 Detector piping 800 Flowmeters and panel 7'200 TOTAL 98'100 The RPC gas system cost is estimated including an independent gas mixer system equipped with an IR analyser to guarantee the non-flammable properties of the mix at all times. However we feel that the IR safety analysis might be available from the other analogous system saving on the cost. The total cost for the RPC gas system is shown in Table 12. Having made the hypotheses outlined above the total for the TOTEM gas system amounts to 222'200 CHF. 5.4 Gas Analysis Instruments that are required to monitor the quality of the gas are listed below together with an indicative price: O 2 meter 6 500 CHF H 2 O meter 8 500 CHF Micro Gas Chromatograph 35 000 CHF We believe that it will be possible to share most of the instruments with the CMS and Gas Working Group groups. 6. CONCLUSIONS AND COMMENTS This note describes the gas system for the TOTEM detector following the style of [1]. Since very small gas volumes are concerned, the flow of gas is such that neither a recirculation nor a recuperation system will be required. The cost of the gas distributed in one year is less than 10% of the total cost of the gas system. TOTEM with the help of the Gas Working Group and of the Safety Group is studying cheaper solutions to obtain and distribute gas mixture to the detectors, while retaining the possibility to have its detectors operationally independent from the CMS schedule. ACKNOWLEDGEMENT We would like to thank F.Hahn, C.Schaefer and D.McFarlane for the invaluable suggestions that helped us during the preparation of this note. REFERENCES: [1]: M. Bosteels, H. Breuker, R.C.A. Brown, I. Crotty, P. Giacomelli, C.R. Gregory, F. Hahn, S. Haider, R. Lindner, R. Loveless, C.W. Nuttall, D. Pandoulas, D. Peach, V. Perelygine, H. Reithler, M. Treichel, CMS Gas System Proposal, CMS IN 1999/018, 10 May 1999.