Hall C Radiation Sources - PowerPoint PPT Presentation

1 / 31
About This Presentation
Title:

Hall C Radiation Sources

Description:

Neutron Moderation. MCNP shows that 100 cm of concrete fully thermalizes 1 MeV neutrons. ... Capture of low energy (thermal) neutrons after moderation ... – PowerPoint PPT presentation

Number of Views:42
Avg rating:3.0/5.0
Slides: 32
Provided by: davega2
Category:

less

Transcript and Presenter's Notes

Title: Hall C Radiation Sources


1
Hall C Radiation Sources
placeholder
2
Challenges
  • 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

3
SHMS Shielding Issues
4
SHMS Shielding Issues
  • Experience shows that a shield house design like
    the HMS is a good solution, but the SHMS has
    additional requirements

5
Hall C Radiation Sources
6
Proposed Design
2
6
5
1
3
4
7
Scattered Electrons
  • Dominated by energy loss through 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
200cm of concrete20X0
8
Neutral Particles Photons
  • Three principal interactions

Lead
  • Measure of material effectiveness attenuation

Photoelectric effect
Probability per unit length for the interaction
Pair production
  • Use concrete to attenuate photons
  • High Z materials afterwards, e.g. lead, to absorb
    low energy photons

9
Neutral Particles Neutrons
  • Total probability for neutrons to interact

Hydrogen
  • Capture cross section is large only at very low
    energies
  • 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

10
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

11
Neutron Transmission gt1 MeV
Natural lead
Iron
CH2
Concrete
Thickness (m)
  • GEANT4 also suggests that concrete stops the
    majority of low energy neutrons

12
Neutron Capture
  • Capture of low energy (thermal) neutrons after
    moderation
  • Capture cross section very high for some
    elements, e.g. boron
  • Capture photons
  • Need high Z material like lead to absorb these
  • Additional contribution from Compton Scattering
    photons

Thermal neutrons
Boron
Two relevant reaction channels
(n,?) produces high energy photons, but small
cross section
13
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

14
Optimization
2
6
5
1
3
4
15
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 (lt10 MeV)

16
Addition of Lead and Boron to Front Wall
  • Radiation damage assumption photons lt100 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
  • Adding lead reduces the low energy photon flux
    and absorbs capture ?s

lead
17
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

18
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

19
Optimization Intermediate Wall (3)
Normalized to maximum possible ratio at SHMS
angle 25
3
Mechanical design minimum thickness
  • Charged particles are largely stopped by the
    outer walls of the shield house
  • Optimal configuration for the intermediate wall
    80-100cm of concrete

20
Optimization other walls (4)
  • Top, bottom, back, far side

Nominal configuration
4
  • Nominal configuration of 64cm of concrete is
    sufficient, but may add 3mm of lead, possibly
    preceded by 2mm of boron, to absorb low energy
    photons and thermal neutrons

21
SHMS Back Configuration (5)
  • 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.

To beam dump
Calorimeter
Cerenkov
22
SHMS Back Configuration (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

SHMS at 5.5 deg
23
SHMS Back Shielding Configuration (5)
  • Introduce a concrete wall to shield from the dump
  • Example shielding during the G0 experiment

GEANT3 Hall C top view
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

24
SHMS Back Shielding Configuration (6)
  • 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

Plug
Shield wall
To beam dump
Calorimeter
Cerenkov
25
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

26
Summary
Hut wall thicknesses have been optimized to
provide proper shielding for detectors. Special
electronics hut provides for even better
radiation shielding. Concrete moderates/attenuate
s particles, low-energy neutrons then absorbed in
layer of boron, low-energy photons 0.5 MeV
produced photons abosrbed in lead. With present
hut design, rates for SHMS at 5.5 degrees are 1)
0.87 of design goal (HMS at 20 degrees) in
detector hut 2) x.xx in electronics hut
27
Construction Material Alternatives
  • Replace boron by polyethylene
  • Polyethylene is a good moderator since consists
    of mostly hydrogen, BUT not much effect on
    thermal neutrons
  • Does not protect electronics more efficiently
    than boron

Thermal neutrons
Thermal neutrons
Boron
Hydrogen
28
Construction Material Mechanical Aspects
  • Attach boron and lead to the concrete shield
    walls rebar uni-struts

concrete
boron
lead
  • ¼ Aluminum plate
  • Provides support for lead weight

29
Proposed SHMS Side View
30
Construction Material Concrete
  • Typically a cheap building material
  • Radiation lengths and densities vary depending on
    the aggregates used
  • Radiation length for Portland cement is 9.2cm,
    density is 2.5 g/cm3

31
Construction Materials
  • Relative costs from construction site there,
    concrete is actually different
Write a Comment
User Comments (0)
About PowerShow.com