Radiation Levels in ALICE

1. Radiation Levels in ALICE Andreas Morsch Meeting on ALICE Radiation Tolerance 30/8/2004

2. Order of Magnitude of the Problem • The ALICE design parameters together with running plans (collision systems, luminosity, running time) determine the order of magnitude of the radiation load. • Event related background which is highest in central PbPb • mid-rapidity (0.01 – 100) cm-2 • muon spectrometer < 0.1 cm-2 • determines granularity, distance to IP and shielding • However, 6 1014 particles are produced in PbPb but 4 1015 in all planned collisions (mainly from pp and ArAr) • 2 1014 particles are produced in beam-gas collisions inside the ALICE experiments (IP +/- 20 m) • 8 1014 particles enter as beam-halo

3. Order of Magnitude of the Problem • The number of charges produced at mid-rapidity in 10 years corresponds to a dose of ~0.1 – 1000 Gy in the mid-rapidity detectors. • The highest hadron fluence is dominated by primary hadrons (~1012 cm-2 in SPD) only at higher radii neutrons from secondary interactions dominate. • In the Muon Spectrometer the design particle density of 0.1 cm-2 in central PbPb extrapolated to the complete running scenario corresponds to a dose of ~1 Gy. The hadron fluence is dominated by neutrons. Assuming that the ratio n/charged particles is ~100 the highest hadron fluences are of the order of 1011 cm-2.

4. Order of Magnitude of the Problem • Electronic racks are located up to 5m from the beam line and shielded by 1-2 m of concrete • Dose and hadron (neutron) fluence are expected to be 3-5 orders of magnitude smaller than close to the beam-pipe • 10-4 - 10-2 Gy • 106 - 108 n/cm2

5. Simulations • Detailed results can only be obtained from simulations using transport codes • Input primary particles simulated with HIJING, Pythia, DPMJET and boundary source for beam halo • Transport code: FLUKA • Scaling of results performed for 10 years running scenario

6. 10 years Running Scenario

7. Beam-Gas Interaction • ALICE zone (IP +/- 20 m) • Assume H2-equivalent gas-pressure of 2 1013 molecules/m3 • Most pessimistic 2 1014 molecules/m3 • ALICE requirement 3 1012 molecules/m3 • Beam Halo • Use boundary source provided by Protvino • Assumes pessimistic gas pressure estimate for LSS approximately valid for the running in phase.

8. Detector Locations

9. Rack Locations

10. Doses and Fluences • Dose = absorbed energy / mass • [J/kg] = Gy • Fluence = track length / volume • [cm / cm3] = 1/cm2

11. 1 MeV n-equivalent Fluence

12. 1 MeV n-equivalent Fluence

13. New since last presentation • ALICE Internal Note final • Re-evaluation of beam-halo contribution • HIP rates have been evaluated for Pb-Pb collisions, but will be (~5 x) higher in high-luminosity Ar-Ar • A priory only Muon Spectrometer can participate • Which detectors/racks are concerned ? • SPD, ...

14. Doses in Mid-Rapidity Detectors SPD1: Primary neutron fluence important

15. Neutron Fluences in Mid-Rapidity Detectors

16. Hadron Fluences in Mid-Rapidity Detectors

17. HIP Rates Highly Ionizing Particles are produces by fragments from inelastic h-A interactions and can be related to the fluence of hadrons with Ekin > 20 MeV. Mika Huhtinen, CMS Note 2002/11 100 Mip

18. HIP Rates for PbPb MB Compare CMS/ALICE: Luminosity 1034/1027 = 107 Cross-Section 0.08/8 = 10-2 Multiplicity 8/2000 = 4e-3 Total = 400 @r = 22 cm CMS: 2.8MHz cm-2 ALICE 6.5 kHz cm-2✓

19. Doses in Forward Detectors

20. Hadron Fluence in Forward Detectors

21. Radiation Load at Rack Positions

22. LHC Gas Pressures and ALICE • Residual gas pressures in LHC are important since ALICE will run at low pp luminosity (0.1 – 3) 1030 cm-2 s-1 and nominal intensity. • Previous estimates give • Up to 1 MHz/cm2 hadron flux inside beam pipe from beam halo • 12 kHz/m beam-gas interaction rate inside the experimental region

23. New Gas Pressure Estimates • A. Rossi and N. Hilleret, LHC PR 676 (2003) • Changes with respect to previous calculations • Parameters of characteristics of TiZrV NEG coating • Ion induced desorption using the “multi-gas” model • Beam pumping deriving from gas ionisation and subsequent implantation or sticking to the surface • Electron induced desorption is negligible according to Operation Scenario • The photon flux on the wall coming from synchrotron radiation is uniformly distributed along the section considered. It is believed that this overestimates the flux.

24. Operating Scenarios • In order to achieve the nominal parameters the unbaked surface of the cold vacuum system must be conditioned to lower secondary electron emission and avoid electron multiplication. It is foreseen to have three operation periods: • Machine start-up • Conditioning Period • After machine conditioning

25. Operating Scenario • Machine Start-Up • Operation with beam current below multipacting threshold = 1/3 of nominal intensity • Conditioning • During conditioning period the beam current is gradually increased to maintain electron multipacting conditions. The electron bombardment will lower the secondary electron emission (electron conditioning). • After Conditioning • Operation at nominal beam-current • Electron multipacting assumed to be neglebible • Materials show memory-effect . Only short reconditioning needed after shut-down and venting.

27. Gas pressure at IP2 • LHC Project Note in preparation (A. Rossi) • Main conclusion: the density will be lower 1013 molecules/m3, because the beam pipe is NEG coated. Adriana has measured the electron cloud activity with NEG at the SPS and no such activity has been observed.

28. Conclusions • The radiation load in ALICE detectors and electronic racks has been studied. Contributions from interactions at the IP and beam-gas interactions have been taken into account. • Collisions for physics are the dominant source of radiation load. However, for more pessimistic assumptions on residual gas pressure the beam-gas contribution could be of equal order of magnitude. • Highest doses (several kGy) are reached in the inner SPD layer and at the inner radii of forward detectors (FMS, V0, T0). • Hadron fluences up to 4 1012 cm-2 (SPD1). • The highest doses in the electronic racks are in the 10 mGy range with n-fluences up to 109 cm-2.

29. Conclusions • New calculations for residual gas pressure at IP and in the LSS give ~ one order of magnitude lower values. Reason: • No measurable electron cloud activity from NEG coated surfaces at the SPS • Cold surfaces will be conditioned (“scrubbing”) according using an operation scenario that avoids electron cloud activity.