Hall C Support Structure and Shield House Mini-Review 28 May 2008

1. Shielding Requirements for SHMS Structure Tanja Horn Hall C Support Structure and Shield House Mini-Review 28 May 2008

2. Hall C Radiation Sources • 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 Electron beam Target Beam dump Beam line

3. Radiation Types • Scattered electrons • Produce radiation • bremsstrahlung is the dominant process except at very low energy • Lose energy through collisions with atomic electrons • The probability for interaction is large • 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

4. HMS Shielding Model 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°

5. SHMS Shielding Issues • Experience shows that a shield house design like the HMS is a good solution, but the SHMS has additional requirements

6. SHMS Layout Electron beam

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

8. SHMS Shielding Requirements • Scattered electrons • Produced when primary electron beam hits the target • Produce radiation • bremsstrahlung is the dominant process except at very low energy • Neutral particles: photons and neutrons • Produced from electron interactions at the target or as electrons traverse material • Require special shielding considerations • Moderation/Attenuation and capture of the resulting lower energy particles • Neutral particle interactions can produce additional particles • Selection and ordering of absorbing materials is important • Hadrons: protons, pions • Hadronic cross sections are small • 1m of concrete almost fully stops 1 GeV protons

9. Scattered Electrons • Energy loss dominated by radiation (bremsstrahlung) • Effectiveness of material thickness: radiation length • the distance over which electron energy is reduced by 1/e For compounds, where wj and Xj are fraction by weight and radiation length of the jth element • 200 cm of concrete (=20 X0) almost fully stops all scattered electrons • A lead layer placed after the concrete absorbs produced photons

10. Neutral Particles: Photons • Three principal interactions: Lead • The total cross section is small at energies of 1-2 MeV • Photoelectric effect is large at low energies • Pair production dominates at higher energies Photoelectric effect Pair production • First step: attenuate photons from the difficult 1-2 MeV region Compton Probability per unit length for the interaction • Concrete followed by a high Z material (e.g. lead) effectively attenuates photons, absorbing the low energy ones.

11. Neutral Particles: Neutrons • Total probability for neutrons to interact Hydrogen • Capture cross section is large only at very low energies 1 MeV fast neutrons • Important first step: moderation • Slow down fast neutrons through elastic scattering • Light elements, e.g. hydrogen preferable since the energy loss per collision is large

12. Neutron Moderation MCNP: A General Monte Carlo N-particle Transport Code 1 MeV neutron point source concrete Neutron 1 MeV N/N0 Add 1cm of boron • MCNP shows that 100 cm of concrete fully thermalizes 1 MeV neutrons. All remaining neutrons are captured by an additional boron layer. • In reality, higher energy external neutrons and neutrons are produced in the concrete by electrons • to moderate these a thicker concrete wall is needed

13. Neutron Transmission: >1 MeV GEANT4 simulation Attenuation Natural lead Neutron 1-10 MeV N/N0 Iron CH2 Concrete Thickness (m) • GEANT4 also suggests that concrete stops the majority of the fast neutrons

14. Neutron Capture • Capture of low energy (thermal) neutrons after moderation • Capture cross section very high for some elements, e.g. boron Thermal neutrons Boron • Capture produces photons through two relevant reaction channels (n,γ) produces high energy photons, but small cross section • Boron efficiently captures low energy neutrons, but needs to be followed by a high Z material (lead) to absorb the produced capture and additional contribution from Compton scattered photons

15. Neutron Capture at Higher Energies Lead Boron Boron Lead B10 abundance ~20%, so true N/N0 is larger • Lead has no effect on neutrons except at high energy • But lead absorbs photons – the photoelectric effect is still 50% 500 keV • Boron remains a relatively efficient neutron absorber up to the MeV region

16. SHMS Shielding Optimization shield wall 2 5 6 1 3 4

17. Optimization – Front Wall (1) • 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 • The outgoing particle spectrum is soft (<10 MeV)

18. Addition of Lead and Boron to Front Wall • 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 • 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 5 cm 5 cm 200 cm concrete lead boron

19. Optimization – 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

20. Effect of Beam Side shielding cut • Current cut section does not contribute significantly to the background rate Cut away section for beam line • Background rate increases rapidly as the cut section increases 30 cm

21. Optimization – Intermediate Wall (3) Normalized to the rate without intermediate wall for SHMS at 8.5° (electronics at 25°) 3 • Charged particles are largely stopped by the outer walls of the shield house • Optimal configuration for the intermediate wall: 80-100cm of concrete

22. Optimization – other walls (4) • Top, bottom, back, far side Nominal configuration 4 • The nominal 64cm of concrete is sufficient, but one may add 3mm of lead, preceded by 2mm of boron, to absorb low energy photons and thermal neutrons – the back wall being the main priority

23. SHMS Back Configuration • 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. Detector PMTs

24. SHMS Back Configuration • 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 SHMS at 5.5°

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

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

27. SHMS Back Shielding: (5) and (6) • Background rates comparable for both shielding options • Adding thin plug provides more efficient shielding from low-energy background • Depends on spectrometer angle

28. Summary • The SHMS shield hut wall thicknesses have been optimized to provide proper shielding for the detectors • The separate electronics hut provides for even better radiation shielding • Shielding Configuration • Concrete moderates/attenuates particles • Low energy (thermal) neutrons are absorbed in a boron layer • Low energy and 0.5 MeV capture photons are absorbed in lead • With the proposed SHMS shield hut design, the rates at 5.5° are: • 0.7 of the design goal (HMS at 20°) in the detector hut • <0.5 in the electronics hut