1. Pair Spectrometer Design Optimization Pair Spectrometer Design Optimization A. Somov, Jefferson Lab GlueX Collaboration Meeting September 9 2009

2. Outline  Asymmetric-energy design of Pair Spectrometer based on scintillator counters  Symmetric-energy design. Instrumentation of Pair Spectrometer with microstrip/GEM detectors - detector size - energy resolution - detector pitch size - rate - selection of PS magnet and length of the vacuum chamber Pair Spectrometer Design Optimization, Hall-D Collaboration Meeting, September 9 2010 2

3. Main components:  Thin target (10 -3 radiation length)  Dipole magnet - 1.8 m long - 1.6 T in the gap.  1.5 m long vacuum chamber  Hodocope counters: - 24 FSF counters (positrons) all tilted by 15  - 6 WSF counters (electrons) tilted individually by 6 - 16  Current Design of Pair Spectrometer  Proposed by H.Hakobyan  Based on a few readout channels ( scintillator counters ) Pair Spectrometer Design Optimization, Hall-D Collaboration Meeting, September 9 2010 3

4. Energy Resolution FSF counters WSF counters FSF counters: - 24 counters cover the energy range from 3 GeV to 4 GeV - counter’s width 0.61 – 1.11 cm (  ~ 12 MeV ) WSFcounters: 6 counters: 3.25, 4.25, 5.25, 6.25, 7.25, 8.25 GeV counter width 0.2 – 1.36 cm (  ~ 17 MeV ) Converter thickness L = 5x10 -3 X 0 Resolution at the photon end-point energy region is about 50 MeV - dominated by the beam spot size of about 4 mm at the converter Reconstructed photons Pair Spectrometer Design Optimization, Hall-D Collaboration Meeting, September 9 2010 4

5. Energy Spectrum Pair Spectrometer Design Optimization, Hall-D Collaboration Meeting, September 9 2010 5 Photon energy spectrum 5  10 -3 X 0 converter high-luminosity runs 24 x 6 = 144 energy bins

6. Current Design Summary - Precise measurement of the photon flux stability with the required accuracy of about 1% - Determination of the photon polarization with the accuracy of a few % Disadvantages: small counter acceptance ( less than 0.1% ), spacial separation of the counters - potential difficulties with the counter acceptance determination relatively poor energy resolution of reconstructed photons at the end-point region - calibration of the tagger microscopes using Pair Spectrometer require huge magnet (BNL 30D72 magnet, weight 64 tons, gap size 6  - has to be reduced ) Study feasibility of the alternative symmetric-energy design Pair Spectrometer Design Optimization, Hall-D Collaboration Meeting, September 9 2010 6

7. Detector Size Detector width as a function of the magnetic field Study energy resolutions/rates of the microstrip detectors: - use ‘default’ lengths of the vacuum chamber and the magnet - lower magnetic ffield to 1.1 T ( 1.98 T m ) detector width ~26 cm - symmetric-energy range 3 - 6 GeV for both e  arms Pair Spectrometer Design Optimization, Hall-D Collaboration Meeting, September 9 2010 7

8. Energy Resolution  Initial spread of the polar angle distribution of e  in photon pair production  Multiple scattering of e  in the converter  Photon beam spot size/emittance at the converter  Pitch size of the detectors Pair Spectrometer Design Optimization, Hall-D Collaboration Meeting, September 9 2010 8

9. Initial Angular Spread of e  in Pair Production Pair Spectrometer Design Optimization, Hall-D Collaboration Meeting, September 9 2010 9 r.m.s.  GAUSS

10. Energy Resolution: Initial Angular Spread & Multiple Scattering Optimization of Tagger Magnet Optics, Collaboration Meeting, JLab 10 Pair production Multiple Scattering ( 2  10 -3 X 0 )  GAUSS r.m.s. Correlation between e + and e – angles Pair production angle dominates energy uncertainties up to ~ 5  10 -3 X 0

11. Energy Resolution: Beam Spot Size Pair Spectrometer Design Optimization, Hall-D Collaboration Meeting, September 9 2010 11 Relative energy shift of reconstructed photons as a function of the fractional electron energy E e / E  Pencil beam photons shifted by  X = 2.5 mm at the converter  E  ~ 0

12. Overall Energy Resolution Pair Spectrometer Design Optimization, Hall-D Collaboration Meeting, September 9 2010 12 | E e / E  | < 0.02 Full Geant detector simulation 3.4 mm primary collimator 10 -3 X 0 converter

13. Pair Spectrometer Rates Pair Spectrometer Design Optimization, Hall-D Collaboration Meeting, September 9 2010 13 Background random coincidence rate: - estimated to be on the level of 1% - design of the tapered collimators should be optimized All e + e - coincidences

14. BNL Magnet 18D36 Pair Spectrometer Design Optimization, Hall-D Collaboration Meeting, September 9 2010 14  Several magnets available at BNL  Maximum field 2.2 T with 6  gap size ( 2200 A - 2 T, linear up to 1900 A) - reduce the gap size, operate at 1.5 T ( ~ 1.4 Tm ) - acceptance: E e > 0.9 GeV  20 tons, can be split in half  The same magnet is used in Hall-B ( 6  gap size ) - ‘reliable’ operation, mapped the field. Will try to find the calibration stand for us.

15. Resolution for Different Vacuum Chamber Length Pair Spectrometer Design Optimization, Hall-D Collaboration Meeting, September 9 2010 15 Detector width: 14 cm Pitch size: 400  m GEM (?) ( 350 x 2 = 700 channels) Energy resolution: ~10 MeV for 6 GeV e  ( 20 MeV for  )

16. Summary Pair Spectrometer Design Optimization, Hall-D Collaboration Meeting, September 9 2010 16 We performed a study of an alternative symmetric-energy instrumentation of the Pair Spectrometer which has several advantages compared with the default instrumentation: - better energy resolution - possibility to reduce energy uncertainties associated with the beam spot size - the acceptance of the microstrip detectors is expected to be easier to calibrate - possible to use smaller magnet (18D36 magnet is about 3 times lighter than 30D72)