07-29-05PrimEx collaboration meeting Energy calibration of the Hall B bremsstrahlung tagging system using magnetic pair spectrometer S. Stepanyan (JLAB)

07-29-05PrimEx collaboration

meeting

Energy calibration of the Hall B bremsstrahlung tagging system

using magnetic pair spectrometer

S. Stepanyan (JLAB)


meeting

Tagger energy variations

The first observation of the nonlinearities in the tagger energy spectrum in the search for pentaquarks in the g2a data (S. Stepanyan et al.). Empirical corrections were derived using the exclusive reaction d’p+-(n).

Later similar results have been obtained by M. Williams et al. from the analysis of g1c data (higher statistics, full focal plane).

These was explained by the effects of gravitational sag and various possible misalignmets of the tagger focal plane (D. Sober et al.).

S. Stepanyan CLAS Analysis Note 03-105

D. Sober et al., CLAS-NOTE 2004-019


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Experimental measurements

The tagged photon energy spectrum was measured in coincidence with e+e- pairs detected in the pair spectrometer at several different values of the PS dipole field.

The average ratio of the photon energies, reconstructed in PS and defined by the tagger for the E-counter “i” is defined as relative energy correction for that E-counter.

Ci=Ec/Ei.

Data are taken during the g10 run. Pair spectrometer was equipped with microstrip detectors for better position determination of e+ and e-, and thus better energy resolution.

DAQ configuration: tagger + PS with microstrip detectors.

New EPICS control of the PS magnet. Automated procedure for scans.

Total of 10 scans, 115 stettings of PS field.

Measurements at field settings close to the end point energy were conducted to set the absolute energy scale.


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Experimental setup

e+

e-

MS

PS1 & PS2 MS – microstrip detectors from photon polarimeter: 2X and 2Y planes, 50mm pitch.

Single counters of PS1 on each side.

PS2 – full plane.

Each pair of (XY) planes cover 20x20 mm2 area.

Distance between two X planes was 450+/-0.05mm, centered on the beam within ~1mm.

Field in the center of the PS dipole magnet was measured with few x10-3 precision.


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Analysis of hits in the microstrip detectors

50m

For adjacent hits the position is calculated as a weighted average using the ADC values.

1 2 3 4 Plane #

9876543210 -

# o

f h

its

X1 Y1 X2 Y2

1 2 3# of hits

X1

Y1

X2

# o

f n

on

-ad

jacen

t h

its

1

2 3

1 2

3 1

2 3


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Tagger energy corrections

sizesdetectorfiniteaccountto)()(2997.0 XGXGrrBEeeCC

effCeff

peff

LBBdlasdefinedL

x

lLr

:

;12

2

""countertaggerofenergytheis; iEEE

EC i

i

Ci

Bc - field value at the center of the magnet,

lp - distance from the center of the magnet to the MS plane

x - distance between hit position and the beam center.


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Determination of the effective field length

Ceff B

BdlL

Integral is calculated using trajectories simulated from the target center to the center of the microstrip X-plane.

Trajectories are simulated using Runge-Kutta-Nystroem method (ray-tracing program from B. Mecking).

TOSCA generated field distribution (from A. Glamazdin).


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Correction for the finite detector size, G(X)

X is the distance between e+e-

Ec uses Leff from above

In the ideal case of accurate knowledge of the field distribution photon energy is reconstructed with accurace much betetr than 0.1%.


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Difference between measured and TOSCA maps

Integral is taken along the Z axis for different transverse positions X.

X=13.5cm; Y=0cmX=8.6cm; Y=0cmX=0cm; Y=0cm

Shape of the dependence at X=0;Y=0 was used to correct Ec.

The maximum variations of the r for a single X is used as an error for F(B), +/-0.05.

dlB

dlBBF

T

M

C )(

X=18.7cm; Y=0cm


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End point energy measurement

Normalized yield of e+e- as a function of Ec. For 4 different acceptances of e+e- detection the ratio of beam

energy to the defined “end point” energy was within 0.1%.

Ee.p.9985.0Correction

..

pe

B

E

E

EB for this measurements was 3.7765 GeV (from accelerator and Hall A beam energy measurements).

Ee.p. is defined as a mid point of the falling edge of the e+e- coincidence rate, and is 3.784 GeV.


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Final corrections

..

)()(2997.0

pe

B

i

CeeCi E

E

E

BGXFrrBC


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After correcting for swapped cables


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Final results, reaction d’pK+K-(n)

G10, 3375 A

Simulations


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Summary

The tagger energy corrections were derived from the measurements of the tagged photon spectrum in coincidence with e+e- pairs in the PS at several different values of the PS dipole field.

For calculation of the effective field length, and the correction for the finite detector sizes TOSCA generated field distributions (maps) were used.

Calculated energy was corrected for the difference between the generated and the real field distributions.

Estimated error on obtained corrections ~0.1%.

Final energy scale is defined from measurements of the e+e- coincidence rate close to the “end point” energy.


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Photon energy calibration Single scintillator

counter SC1 plane on each side.

Full SC2 plane.

MS – microstrip detectors : X and Y planes, 50m pitch.

e+

e-

MS

SC1 & SC2

Pair Spectrometer

MeV2

115 settings of PS fieldReaction d’pK+K-(n)

MM

of

the

neu

tro

n


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Measured PS dipole field

Documents

07-29-05PrimEx collaboration meeting Energy calibration of the Hall B bremsstrahlung tagging system using magnetic pair spectrometer S. Stepanyan (JLAB)