Title: Hall C Radiation Sources
1Hall C Radiation Sources
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2Challenges
- 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
3SHMS Shielding Issues
4SHMS Shielding Issues
- Experience shows that a shield house design like
the HMS is a good solution, but the SHMS has
additional requirements
5Hall C Radiation Sources
6Proposed Design
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7Scattered 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
8Neutral 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
9Neutral 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
10Neutron 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
11Neutron Transmission gt1 MeV
Natural lead
Iron
CH2
Concrete
Thickness (m)
- GEANT4 also suggests that concrete stops the
majority of low energy neutrons
12Neutron 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
14Optimization
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15Optimization 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)
16Addition 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
17Optimization 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
18Effect 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
19Optimization 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
20Optimization 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
21SHMS 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
22SHMS 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
23SHMS 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
24SHMS 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
25SHMS 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
26Summary
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
27Construction 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
28Construction 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
29Proposed SHMS Side View
30Construction 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
31Construction Materials
- Relative costs from construction site there,
concrete is actually different