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

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

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Hall c support structure and shield house mini review 28 may 2008 l.jpg

Shielding Requirements for SHMS Structure

Tanja Horn

Hall C Support Structure and Shield House Mini-Review

28 May 2008


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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


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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


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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°


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SHMS Shielding Issues

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


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SHMS Layout

Electron beam


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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


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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


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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


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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.


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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


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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


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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


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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


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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


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SHMS Shielding Optimization

shield wall

2

5

6

1

3

4


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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)


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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


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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


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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


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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


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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


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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


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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°


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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


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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


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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


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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


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