EDMS No: Revision: Pages: Date: Addendum to IT-3036/EP/CMS

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1 EDMS No: Revision: Pages: Date: CMS-IZ-CI-0003 draft Page 1 of EDMS No: Revision: Pages: Date: Addendum to IT-3036/EP/CMS Technical Specification for Supply and Installation of Gas Pipes on the CMS Yoke Structure Derogation Request Abstract The CMS experiment requests a partial derogation for cables with respect to ionizing radiation resistance as described in IS 23. For all areas of the CMS experiment exposed to a dose rate of less than 200 Gy/10 y, no specific radiation requirements and testing should beabstract necessary. For all areas above this value, the calculated value for the dose rate This Addendum (multiplied to the by Tender an adequate Documents uncertainty provides the factor) Contractor should with be the basis additional for the and radiation updated information requirement on the as Technical in IS 41. Specifications. It does not replace the previous documents of IT-3036/EP/CMS. Part of this Addendum is a CD-ROM with drawings. These drawings represent CERN s best knowledge of the geometry of the CMS Detector. Nevertheless CERN cannot guarantee or made liable for the complete correctness of these drawings with respect of the actually build detector. Therefore these drawings are purely for education and information of the Contractor. The only reference is thus the built CMS detector, to which the Contractor has free access during working hours. Author Author Checked Checked Approved Christoph Schäfer Christoph Schäfer Ron Pintus Alain Hervé Mika Huhtinen 1

2 2 1 Radiation Environment at CMS It has been recognized from the early days of LHC design that the challenges from the hostile radiation environment at such a high-luminosity hadron collider will be unprecedented. At the LHC peak luminosity of cm 2 s 1 about inelastic protonproton collisions per second will take place. Each with an average multiplicity of about 125. The decay particles emitted from these collisions are mostly emitted into the central rapidity region. Almost half of the emitted particles have η < 3 and 75% fall in the range η < 5. However, these 75% of particles carry only 5% of the total energy. According to present estimates the central part of the CMS detector will absorb about 120 GeV per event. The two Forward Hadron Calorimeters (HF) together absorb about 620 GeV and each TAS about 2.3 TeV, i.e. 4.6 TeV in total. About 8.5 TeV per event escape into the LHC ring. These particles from the pp-interaction are responsible for a huge radiation background which leads to radiation damage of detectors and cables, but also poses important radiation safety issues. The predictive simulations of this background have been performed with the FLUKA simulation code and in some instances cross-checked with the MARS package. The input for all calculations is a 10 year operation of the CMS detector which is equivalent to an integrated luminosity of 500 fb 1, i.e s at LHC peak luminosity of cm 2 s 1. Figure 1.1 shows the overall ionizing radiation level inside the CMS experiment. Due to the symmetry of the CMS detector, only one quarter is shown. As can be seen from this figure, the overall radiation level inside and around the CMS detector is low, with the exception of the area along the beam pipe. Outside the forward region and the inner part of the detector (Tracker and Electromagnetic Calorimeter) the dose rate is below 200 Gy/10y. Thus about 90% of all cables of the CMS experiment are in a non-radiation or at least very low radiation environment.

3 E E E E E E E E E E E E-12 Figure 1.1: Dose rate (Gy/10 y) for the CMS experiment, as calculated with FLUKA. [1] Except for sensitive detector layers, like the silicon of the inner Tracker, the FLUKA predictions have been obtained for averaged materials. In the particular case of cables in a neutron environment, this leads to a systematic underestimate in the plastic insulations because the (n,p) reactions creating slow, heavily ionizing protons, are not properly concentrated in the plastic shell. Depending on the ratio of neutrons and ionizing particles, this effect can vary from negligible to about a factor of 5 in the worst case. This factor of 5 appears only metal/plastic sandwiches in well shielded regions, where the dose is low. The doses for cable channels have been calculated in average materials, where hydrogen is present, but only with a smeared distribution. In such smeared material the effect will be much less than the worst case factor of 5. To cover for this systematic effect, we apply another uncertainty factor of up to 3 on top of the standard factor of 3, generally to be added to all radiation estimates. Rounded up, the total uncertainty factor we apply to the cable specifications can be up to 10. Table 1.1 gives the predicted dose rates in some parts of CMS. The minimum and maximum values indicate a possible variation as a function of spatial position within the region concerned. For instance for the Pixel, Si-tracker, the EE and the 53 o crack the dose decreases with increasing radius. In the EB the dose increases with the z-coordinate. In areas with low dose and neutron dominance the uncertainty factor of 10 has been applied, based on the reasoning explained before. Inside of the Tracker

4 4 Sub-detector Maximum Minimum Uncertainty Factor Requirement Pixel Si-Tracker EE (TK side) EE (HE side) EB (HB side) SE (Patch Panel) EE (Patch Panel) EB (Patch Panel) o crack 0.3 < ME1/ < HF ROBOX HCAL ROBOX < None Other Muon < None UXC < None Table 1.1: Predicted dose rates (kgy for 500 fb 1 ) in various parts of CMS, the applied uncertainty factor and the resulting requirement of radiation resistance (kgy). the contribution from charged hadrons increases towards the IP and local effects due to neutrons in the plastic become less important. This justifies to decrease the uncertainty factors on the IP side of the ECAL. The uncertainty of charged hadron fluxes in the CMS Tracker is estimated to be a factor of 1.3 and that of neutron fluxes a factor of 2. Agian, to account for the neutrons although they are relatively much less important we use a factor of 5 for regions close to the IP side of the ECAL and for the Tracker itself a factor of 3. At the Pixel, where showering and neutron fluxes are negligible an uncertainty factor as low a 2 seems justified especially since most cables will run on the outer Pixel periphery with the minimum dose. In particular, the high 800 kgy value is for the 4 cm pixel layer, which will be serviced by Polyimide (e.g. Kapton c ) cables and is likely to be entirely replaced due to radiation damage before the 500 fb 1 are reached. 2 Radiation Properties of Plastics According to Figure 2.2, any relevant plastic material usually used is well suited to a dose of below 200 Gy. Since about 90% of the CMS cables are exposed to less than 200 Gy in 10 years, the minimal radiation resistance requirements stated in IS 23 of a dose of Gy and a dose rate of 1 Gy/s are not justified in this case. Nevertheless, since PTFE (e.g. TEFLON c ) has very poor radiation resistance, we will ban underground use of this material, with the exception of some very specific applications. In addition, PTFE contains fluorine and thus its use

5 5 requires anyhow a derogation. Figure 2.2: Compilation of radiation damage test data for plastic materials. [2]

6 6 3 Risks The main risk for cables exposed to ionizing radiation is that the plastic insulator material may eventually change its mechanical properties, i.e it disintegrates. Thus it is possible, that an electrical short circuit occurs which can lead to an electrical fire. In order to reduce that risk, only those cables shall be used which withstand the radiation environment they are installed in. The radiation level is calculated with the program FLUKA, taking into account the latest model of the CMS detector and its material composition. The risk of confusing cables and thus wrongly installing them into a higher radiation environment than foreseen is minimized because of their individual colour code and labeling. Each CMS sub detector has its distinct cable colour, e.g. green for the Tracker, grey for ECAL, yellow for HCAL and blue for the muon detector. When the CMS experiment is decommissioned at the end of its working lifetime, all cables will be disposed of. Re-utilization of the CMS cables will not be allowed. 4 Conclusion The CMS experiment requests a derogation for cables with respect to IS 23. Only the radiation resistance should be re-evaluated, all other requirements, especially fire resistance and smoke emissions, shall be strictly followed. In order to be consistent with IS 41, CMS wishes to take into account the predicted level of radiation for a given location to determine the necessary radiation resistance, i.e. the general limit of Gy shall be adjusted to the local dose. The calculated values for the radiation dose, which are the basis for this prediction, shall be multiplied by a factor of up to ten, to allow for any uncertainty. Taking into account the general properties of plastics, being radiation resistant to well over 200 Gy, no special radiation test for cables installed in very low radiation areas shall be foreseen. Apart from very specific applications, PTFE cables shall be banned in underground installations. 5 References * [1] Mika Huhtinen, Optimization of the CMS forward shielding, CMS Note 2000/068 [2] Marc Tavlet et al., Compilation of radiation damage test data, CERN 98-01