SHMS Optics and Background Studies
Tanja Horn
Hall C Summer Meeting
5 August 2008
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
Experiment
Target
SHMS Angles
(deg)
Momentum
(GeV)
Fpi12
8-cm
5.5-13 .0
2.261-8.070
Measurement of R in
SIDIS
15-cm
5.5-20.0
5.40-5.80
Pion Factorization
8-cm
5.5-23.0
2.42-8.52
x>1
15-cm
8.0-16.0
4.80-10.60
g2, (A1N)
40-cm
11-15.5, (5.5-30.0)
2.25-7.50
GEp
30-cm
15.7-25.0
4.03-8.35
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°
SHMS Layout
Electron beam
SHMS Acceptance
50-cm target viewed at 90 degrees
•
Using a SHMS MC similar to
the one used for the HMS
•
Solid angle >4.5 msr for all
angles
•
Optics model will be updated to
SHMS2008 this Fall, but expect
no significant changes
Vertical: ± 35 mrad
Horizontal: ± 65 mrad
SHMS detector size summary
•
Nominal target length and angle set by
approved experiments
– 40cm target, 40deg
•
Scattering chamber can accommodate
50cm targets
Detector
Z
(cm)
Xsize
(cm)
Ysize
(cm)
NG Cerenkov
-310 to -60
70
80
DC1
-40
75
80
DC2
+40
85
90
HG Cer
+70 to +250
115
100
Calorimeter
+280 to +360
130
120
Values are given for the back of the detectors
Beam envelope at selected detector
locations
SHMS resolution
Δp/p (%)
Δφ (radians)
Δθ (radians)
2x Spec’d Resolution & MCS
Spec’d Resolution
+22%
-10%
Experiment
p
(GeV)
Δp/p (%)
Δθ (rad)
Δφ (rad)
Pion Form Factor
2.2-8.1
2x10-3
1.5x10-3
1.5x10-3
Transition Form
Factors*
1.0-8.5
1x10-3
1.0x10-3
1.0x10-3
SHMS Detectors and Shielding
•
Due to space requirement of the SHMS
detector stack cannot have a uniform back
concrete wall
•
Calorimeter
PMTs
Need window to access calorimeter PMTs for
maintenance etc.
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
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
HMS Shielding as Example
•
•
The HMS shielding design provides good shielding for the detectors
The shielding of the electronics is sufficient down to angles of 20° (F1TDCs!)
HMS shield house
Target
SHMS Shielding Issues
•
Experience shows that a shield house design like the HMS is a good
solution, but the SHMS has additional requirements
Electronics
Increased sensitivity of new
SHMS electronics
Separate
Electronics
Room
Detectors
Space requirements at beam
side at forward angles
Design of the back of the hut
accounting for length of the
detector stack
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
5 cm lead
20 cm
6
50 cm
5 cm boron
5
Electron beam
2
90 cm concrete
400x400x800cm
shield wall
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)
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
200 cm
5 cm 5 cm
lead
concrete
boron
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
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
SHMS Back Shielding Configuration (6)
•
Add a concrete plug of 20-50cm
thickness
•
GEANT3: Hall C top view
Suppresses low-energy background
flux further to an acceptable level
SHMS electronic hut
To beam dump
Plug
Shield wall
20cm
target
beam
Cerenkov
•
Calorimeter
•
5°/0.5 m
HMS, 20°
50cm
Drawback: limits the maximum
spectrometer angle to 35°
SHMS detector hut
Length (m)
Max. Scattering Angle (deg)
24
27
23
35
22.5
39.5
SHMS Shielding 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
SHMS Design Parameters
Parameter
•
•
SHMS 2006 Design
Range of Central Momentum
2 to 11 GeV/c for all angles
Momentum Acceptance
-10% to +22%
Momentum Resolution
0.03-0.08%
Scattering Angle Range
5.5 to 40 degrees
SHMS/HMS angle
18 degrees
Solid Angle Acceptance
>4.5 msr for all angles
Horizontal Angle Resolution
0.5 - 1.2 mrad
Vertical Angle Resolution
0.3 - 1.1 mrad
Vertex Length Resolution
0.1 - 0.3 cm
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
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
SHMS at 5.5°
Hall C top view
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
Configuration
Background Flux
at forward angles
(norm)
No shield wall,
no plug
1.9
Shield wall,
20-50cm plug
0.7