Change in Baseline Configuration for ILC Muon Spoilers/PPS Wall
Reduction of 18m muon spoilers to 5m and elimination of 9m muon spoilers 08 Sep 2006 1. Requester’s Contact Information. BDS area leaders: Deepa Angal-Kalinin, Andrei Seryi, Hitoshi Yamamoto 2. Summary of change request. The current baseline configuration for the BDS specifies one 18 m and one 9 m magnetized muon spoiler for each branch of the BDS for a total of four 18 m and four 9 m muon spoilers. The 18 m muon spoilers are located 350 m from the IR and the 9 m muon spoilers are located farther upstream at 625 m from the IR. The function of the muon spoilers is the following: They reduce the muon flux striking the detector, which is generated primarily by collimation of halo particles in the betatron and energy collimation sections of the beam delivery. With 18 m and 9 m spoilers per beamline, the number of muons reaching the IP is less than ten muons per 200 bunches, in the assumption that 0.1% of the beam population is collimated. They reduce the muon dose rate to humans who are accessing the IR area while the beam is delivered to the upstream tune-up dump or to the other IR. They protect the occupants of the IRs in the “worst case accident” when all beam containment devices have failed.
Considering that: The estimation of 0.1% beam halo population is conservative and all simulations predict much less The detector time projection chamber can tolerate a higher muon flux than 10muons/200 bunches The minimum muon wall required for personnel protection is a single 5 m wall The cost of long muon spoilers is substantial and dominated by material cost and thus approximately proportional to the muon wall length We are proposing to reduce all four 18 m muon spoilers to 5 m and eliminate the four 9 m muon spoilers completely. The caverns specified for all eight muon spoilers would be retained at their present dimensions and locations to accommodate future upgrade of the muon spoilers from 5 m to 18 m or the addition of the 9 m muon spoilers, if much higher halo population and flux of muons is measured during ILC operation.
Reduction of the 18 m muon spoilers to 5 m and the elimination of the 9 m muon spoilers would result in a significant cost reduction to the muon spoiler/PPS subsystem. Additionally, the installation time of the muon spoilers would be reduced as well. With one 5 m muon spoiler per beam line, the expected rate of muons in the detector TPC (time projection chamber) is 384 muons per 200 bunch crossing (at 500 GeV CM, and with 0.1% of the beam collimated, and assuming 2.5m radius TPC) [Keller] which corresponds to roughly 0.15% occupancy of the TPC. The Machine Detector Interface [MDI] panel have considered this and concluded that it should be manageable. The risks involved with reducing the length of the muon walls appear manageable. If the measured muon flux downstream of the muon spoilers exceeds the predictions from simulation, the muon spoilers could be upgraded to reduce the muon flux to acceptable levels, or small doughnut type spoilers could be installed near local sources. Such an upgrade would require a shutdown of about three months. 3. Replacement text for relevant parts of Baseline Configuration Document. The following text should be inserted at the end of the second paragraph of the BCD section “Reduction of background fluxes at detectors. Muons”: “At the initial installation of the ILC, only the shortened 5 m muon walls will be installed in place of the 18 m wall, and the 9 m wall will not be installed. The 5 m wall also serves as a Personal Protection System, reducing the muon dose rate to people who are accessing the IR area while beam is delivered to the upstream tune-up dump or other IR. The caverns for all muon spoilers will be built with full dimensions, allowing future upgrade of the muon spoilers from 5 m to 18 m or the addition of the 9 m muon spoilers, if much higher halo population and flux of muons would be measured during ILC operation. It is estimated that such an upgrade would require a shutdown of about three months.” 4. Classification of Change Request. We believe the change request should be designated as “Class 1” in the CCB classification. The cost implications (a reduction) are significant but will not exceed $100M. If the beam halo is much higher than anticipated, or if the muon transmissibility is much higher than predicted in simulations, there could be a noticeable impact on technical issues for the detector, safety issues in the IR hall region and operational issues in general (discussed below in section 6), which however could be mitigated by upgrades of the muon walls. There is no impact on other ILC systems. 5. Detailed description and reasons for the change request. The current baseline configuration for the BDS specifies one 18 m and one 9 m muon spoiler for each branch of the BDS. The 18 m muon spoilers are located 350 m from the
IR and the 9 m muon spoilers are located upstream of the 18 m muon spoilers at 625 m from the IR. The muon spoiler consists of large plates of steel joined together to form a continuous block which fills the tunnel, extending past the 5 m tunnel bore by 0.6 m in width. The steel is magnetized by means of an excitation coil which is wound around a central core defined by vertical slots cut through the steel plates [Design]. In order for the muon spoiler to function as a PPS device, certain criteria must be met. SLAC Radiation Rules require a dose rate of ≤0.05 mRem/hr in occupied areas, with an integrated dose limit of <100 mRem/yr for non-radiation workers [IR_Radiation_criteria]. We believe this dose rate criterion is conservative, for example the limits for public on a site assumed in TDR were ≤150mRem/yr [TDR]. Simulation of the muon dose rate to humans within IR2 due to all muon sources in the IR1 beamline for a 20 x 2 mrad crossing angle layout predicted a muon dose rate of 0.04 mRem/hr at 1 TeV CM and <0.01 mRem/hr at 500 GeV CM for a 5 m magnetized steel wall [Keller]. A preliminary estimate for the 14 x 14 mrad layout shows the dose rate in IR2 when beam is being delivered to IR1 to be even less. [Keller,082906_traj_field.pdf], shows trajectories of muons reaching the IR Hall from two different sources in the IR1 beam line. From this example it is seen that no muons have reached the IR2 detector region, which would be separated from IR1 by a wall or fence between the two IP’s. For a “worst case” accident where all beam containment devices have failed and the beam hits stoppers in the IR1 line while IR1 is occupied, a simulation has shown that the muon dose rate is <1 Rem/hr, well below the 25 Rem/hr limit for this extreme situation [IR_Radiation_criteria]. The 18 m and 9 m length requirement for the muon spoilers was based on a study of machine related backgrounds in the proposed SiD detector at ILC [Mokhov, Keller]. To simulate the machine generated muon flux at the detector it was assumed, from SLC experience, that a halo population of 0.1% would be lost in the collimators upstream. Simulations showed that a 9 m and 18 m magnetic steel walls would reduce the muon flux at the detector a few thousand times [Mokhov]. However, the assumption of a 0.1% halo population is considered conservative and simulations predict the halo population to be about three orders of magnitude smaller [Brinkmann], although evaluation of all possible sources of halo is difficult. Such simulations are being redone now within the EuroTeV program. Additionally, the current design for the muon spoiler eliminates the 2 cm gap between spoiler halves provided for the beam pipe, further decreasing the muon flux seen at the detector. If it is determined that the muon flux in the IR hall exceeds the acceptable levels with a 5 m muon spoiler then upgrade to a longer wall will be required. In order to minimize the impact to availability, an upgrade strategy has been devised which will allow the upgrade to be accomplished during a typical scheduled maintenance shutdown of 3 months. The current estimate for the upgrade from 5 m to 18 m assumes a schedule of three shifts, seven days a week. To successfully complete the upgrade on schedule, several requirements must be satisfied. Priority use of both the IR shaft cranes and the IR bridge crane during the material transport phases will be crucial as this is the limiting factor at the current IR crane capacity specification of 1.2 kton. Using a single 1.2 kton
crane would extend the duration of the installation schedule from 56 days to 85 days. In the event that the 1.2 kton cranes are no longer available, either the 400 ton or 80 ton movable cranes in the surface assembly building could be used for this procedure (the capacity of movable cranes is determined by the details of the on-surface detector assembly, which is presently under study). With the efficiency of the 400 ton crane being nearly equal to that of the 1.2 kton crane, we expect the installation period to be the same as the single 1.2 kton crane scenario of 85 days. Using an 80 ton crane would extend the Transport Materials Surface to IR Phase by 5 days, however it would not impact the duration of the installation period. Details of the upgrade scenarios and installation scheduling are given in the supporting materials [Upgrade].
6. Assessment of the Impacts of Change. The main impact of the change in configuration will be an 81% reduction in the cost of the muon spoiler system. If the beam halo or the muon transmissibility to the IP is much higher than anticipated, there could be a noticeable impact on the detector, safety procedures in the IR hall region and operational issues in general. The impact on the detector due to increased muon flux, operation with undesirably high occupancy in a TPC may be required. The impact on the safety system due to excessive muon flux in the IR hall would make occupation by humans impossible while the beam is being delivered to the other IR. It should still be possible to occupy either IR region when the beam is on the tuneup dump. The impact on operation due to more complicated tuning of the machine, where optimization of machine settings or collimation gaps may result in significant changes of muon background in the detector. These issues can be addressed, first by finding the sources of muons and installing local shielding or smaller muon spoilers, and if needed making upgrade of the muon walls to a full configuration. Finally, should an upgrade of the muon spoilers be necessary, the downtime (three months) required to perform this upgrade may impact availability if it can not be scheduled during a periodic summer maintenance shutdown.
7. Supporting Materials. The following files are giving more information about the CCR [Brinkmann] Estimation of the beam halo in TESLA, “brinkmann.pdf”, http://acfahep.kek.jp/BDIR2000/proceedings/brinkmann.pdf [Design] Jin-Young Jung, et al, Muon Spoiler Design without Water Cooling, “muon_spoiler_single_coil_pack_no_water_cooling.ppt” J.Amann, Solid Edge drawings of muon spoiler, “Muon Spoiler Drawing.doc” [Keller] L.Keller, considerations of the effects of muon walls on PPS and background, “Keller_ILCMuonDoseRates.ppt” “2006_Aug_shield_wall.ppt” “082906_traj_field.pdf” [Upgrade] J.Amann, “Muon Spoiler Upgrade Scenario Estimation of Installation Time.doc” [MDI] Minutes of the MDI panel meeting, “Revised MDI meeting minutes (81506).txt” [Mokhov] N.Mokhov et al, Machine-Related Backgrounds in the SiD Detector at ILC, “SiD_bkg_mokov.pdf” [TDR] TESLA TDR, Section 8-3, Radiation safety, “tdr_section_8_3.pdf” [IR_Radiation_criteria] Design Guidelines for ILC Beam Containment and Interaction Region Shielding, “ILC-rad-req-rev2.doc”