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

• Solid angle >4.5 msr for all angles

• 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

50-cm target viewed at 90 degrees

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)

-10% +22%

Spec’d Resolution

2x Spec’d Resolution & MCS

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

Calorimeter PMTs

• 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.

Hall C Radiation Sources

Target Beam dump

Beam line

Electron beam

• 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

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

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° (F1TDCs!)

SHMS Shielding Issues

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

Detectors

Space requirements at beam side at forward angles

Design of the back of the hut accounting for length of the detector stack

Electronics

Increased sensitivity of new SHMS electronics

Separate Electronics

Room

Proposed SHMS Shielding Design

1

2

3

4

5

6

100 cm concrete

Detector Hut

Electronics Hut

200 cm concrete

63.5 cm concrete

63.5 cm concrete

90 cm concrete

5 cm boron

5 cm lead

400x400x800cm

20 cm

50 cm

Electron beam

shield wall

Front Wall (1)

• The outgoing particle spectrum is soft (<10 MeV)

• 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

Addition of Lead and Boron to Front Wall

• 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

leadconcreteboron

200 cm 5 cm 5 cm

• 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

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)

• Introduce a concrete wall to shield from the dump

• Example: shielding during the G0 experiment

Hall C top view

beam

HMS, 20°

Shield wall

• 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)

• Drawback: limits the maximum spectrometer angle to 35°

• 5°/0.5 m

GEANT3: Hall C top view

target

beam

HMS, 20°

SHMS electronic hut

SHMS detector hut

Plug

50cm

20cm

To beam dump

Cerenkov Calorimeter

Shield wall

Length (m) Max. Scattering Angle (deg)

24 27

23 35

22.5 39.5

• Add a concrete plug of 20-50cm thickness

• Suppresses low-energy background flux further to an acceptable level

SHMS Shielding Summary

• The separate electronics hut provides for even better radiation shielding

• The SHMS shield hut wall thicknesses have been optimized to provide proper shielding for the detectors

SHMS Design Parameters

• 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

SHMS at 5.5°

• 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

Hall C top view

SHMS Back Shielding: (5) and (6)

Configuration Background Flux at forward angles

(norm)

No shield wall, no plug

1.9

Shield wall, 20-50cm plug

0.7

• Background rates comparable for both shielding options

• Adding thin plug provides more efficient shielding from low-energy background

• Depends on spectrometer angle