1. SHMS Optics and Background Studies Tanja Horn Hall C Summer Meeting 5 August 2008

2. SHMS Experiment Requirements Charged particle detection with momenta up to the beam energy (11 GeV) at forward angles down to 5.5° even with HMS at small angles Well understood acceptance function to perform L/T separations High luminosity to measure small cross sections also requires well-shielded detectors ExperimentTargetSHMS Angles (deg) Momentum (GeV) Fpi128-cm5.5-13.02.261-8.070 Measurement of R in SIDIS 15-cm5.5-20.05.40-5.80 Pion Factorization8-cm5.5-23.02.42-8.52 x>115-cm8.0-16.04.80-10.60 g2, (A 1 N )40-cm11-15.5, (5.5-30.0)2.25-7.50 G Ep 30-cm15.7-25.04.03-8.35

3. SHMS/HMS in Hall C SHMS: dQQQD HMS: QQQD SHMS scattering angle range: 5.5 to about 40 degrees SHMS can reach 5.5° with HMS at 12.5°

4. SHMS Layout Electron beam

5. SHMS Acceptance Solid angle >4.5 msr for all angles Using a SHMS MC similar to the one used for the HMS Optics model will be updated to SHMS2008 this Fall, but expect no significant changes 50-cm target viewed at 90 degrees Vertical: ± 35 mradHorizontal: ± 65 mrad

6. SHMS detector size summary Nominal target length and angle set by approved experiments –40cm target, 40deg Scattering chamber can accommodate 50cm targets DetectorZ (cm) Xsize (cm) Ysize (cm) NG Cerenkov-310 to -607080 DC1-407580 DC2+408590 HG Cer+70 to +250115100 Calorimeter+280 to +360130120 Values are given for the back of the detectors Beam envelope at selected detector locations

7. SHMS resolution Δp/p (%)Δφ (radians)Δθ (radians) -10% +22% Spec’d Resolution 2x Spec’d Resolution & MCS Experimentp (GeV) Δp/p (%)Δθ (rad)Δφ (rad) Pion Form Factor2.2-8.12x10 -3 1.5x10 -3 Transition Form Factors* 1.0-8.51x10 -3 1.0x10 -3

8. SHMS Detectors and Shielding Calorimeter PMTs Due to space requirement of the SHMS detector stack cannot have a uniform back concrete wall Need window to access calorimeter PMTs for maintenance etc.

9. Hall C Radiation Sources TargetBeam dump Beam line Electron beam Radiation is produced by interactions of the beam with material in the hall There are three main sources of radiation in Hall C: Target, beam line, and beam dump

10. Radiation Types Scattered electrons Produce radiation bremsstrahlung is the dominant process except at very low energy Neutral particles: photons and neutrons Have a higher penetration power than charged particles Are attenuated in intensity as traverse matter, but have no continuous energy loss Thickness of attenuating material vs. penetrating power Photons interact primarily with electrons surrounding atoms Neutrons interact with nuclei Hadrons: protons, pions Hadronic cross sections are small 1m of concrete almost fully stops 1 GeV protons

11. HMS Shielding as Example HMS shield house Target The HMS shielding design provides good shielding for the detectors The shielding of the electronics is sufficient down to angles of 20° (F1TDCs!)

12. SHMS Shielding Issues Experience shows that a shield house design like the HMS is a good solution, but the SHMS has additional requirements Detectors Space requirements at beam side at forward angles Design of the back of the hut accounting for length of the detector stack Electronics Increased sensitivity of new SHMS electronics Separate Electronics Room

13. Proposed SHMS Shielding Design 1 2 3 4 5 6 100 cm concrete Detector Hut Electronics Hut 200 cm concrete 63.5 cm concrete 90 cm concrete 5 cm boron 5 cm lead 400x400x800cm 20 cm 50 cm Electron beam shield wall

14. Front Wall (1) The outgoing particle spectrum is soft (<10 MeV) Take electronics in the HMS at 20° as a relative starting point Recent F1 TDC problems seem to dominate at lower angles Full Hall C GEANT simulation (includes walls, roof, floor, beam line components) suggests optimal front shielding thickness of 2 m

15. Addition of Lead and Boron to Front Wall 2 m of concrete reduce the total background flux for SHMS at 5.5° to half of HMS at 20° Boron eliminates the thermal neutron background, BUT produces 0.48 MeV capture γ’s Adding lead reduces the low energy photon flux and absorbs capture γ’s lead concrete boron 200 cm5 cm Radiation damage assumption: photons <100 keV will not significantly contribute to dislocations in the lattice of electronics components, while neutrons will cause damage down to thermal energies

16. Beam Side Wall (2) Beam side wall constraint is 107 cm total Given by clearance between detector stack and side wall Optimal configuration: 90 cm concrete + 5 cm boron + 5 cm lead layer Boron works like concrete, but in addition captures low energy neutrons

17. SHMS Back Shielding Configuration (5) Introduce a concrete wall to shield from the dump Example: shielding during the G0 experiment Hall C top view beam HMS, 20° Shield wall Adding the shield wall has the largest effect at forward angles Reduces the rate at 5.5° by about a factor of two

18. SHMS Back Shielding Configuration (6) Drawback: limits the maximum spectrometer angle to 35° 5°/0.5 m GEANT3: Hall C top view target beam HMS, 20° SHMS electronic hut SHMS detector hut Plug 50cm 20cm To beam dump Cerenkov Calorimeter Shield wall Length (m)Max. Scattering Angle (deg) 2427 2335 22.539.5 Add a concrete plug of 20-50cm thickness Suppresses low-energy background flux further to an acceptable level

19. SHMS Shielding Summary The separate electronics hut provides for even better radiation shielding The SHMS shield hut wall thicknesses have been optimized to provide proper shielding for the detectors

21. SHMS Design Parameters Using a SHMS MC similar to the one used for the HMS Optics model will be updated to SHMS2008 this Fall, but expect no significant changes

22. SHMS Back Configuration SHMS at 5.5° Rates without additional shielding from radiation from the beam dump At 20°, SHMS rates are comparable to those for HMS At forward angles, the SHMS rates are about factor of two higher Hall C top view

23. SHMS Back Shielding: (5) and (6) ConfigurationBackground Flux at forward angles (norm) No shield wall, no plug 1.9 Shield wall, 20-50cm plug 0.7 Background rates comparable for both shielding options Adding thin plug provides more efficient shielding from low-energy background Depends on spectrometer angle