Energy calibration of the Hall B bremsstrahlung tagging

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

Tagger energy variations q The first observation of the nonlinearities in the tagger energy spectrum in the search for pentaquarks in the g 2 a data (S. Stepanyan et al. ). Empirical corrections were derived using the exclusive reaction gd’pp+p-(n). q Later similar results have been obtained by M. Williams et al. from the analysis of g 1 c data (higher statistics, full focal plane). q 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 07 -29 -05 Prim. Ex collaboration meeting

Experimental measurements q 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. q 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. q Data are taken during the g 10 run. Pair spectrometer was equipped with microstrip detectors for better position determination of e+ and e-, and thus better energy resolution. q DAQ configuration: tagger + PS with microstrip detectors. q New EPICS control of the PS magnet. Automated procedure for scans. q Total of 10 scans, 115 stettings of PS field. q Measurements at field settings close to the end point energy were conducted to set the absolute energy scale. 07 -29 -05 Prim. Ex collaboration meeting

Experimental setup MS n e+ e- n Single counters of PS 1 on each side. PS 2 – full plane. PS 1 & PS 2 n n MS – microstrip detectors from photon polarimeter: 2 X and 2 Y planes, 50 mm pitch. Each pair of (XY) planes cover 20 x 20 mm 2 area. Distance between two X planes was 450+/-0. 05 mm, centered on the beam within ~1 mm. Field in the center of the PS dipole magnet was measured with few x 10 -3 precision. 07 -29 -05 Prim. Ex collaboration meeting

Analysis of hits in the microstrip detectors 9 8 7 6 5 4 3 2 1 0 1 2 X 2 Y 2 3 4 Plane # # of non-adjacent hits 1 2 3 # of hits X 1 Y 1 X 2 1 50 mm n For adjacent hits the position is calculated as a weighted average using the ADC values. 07 -29 -05 Prim. Ex collaboration meeting 2 3 # of hits

Tagger energy corrections 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. 07 -29 -05 Prim. Ex collaboration meeting

Determination of the effective field length q Integral is calculated using trajectories simulated from the target center to the center of the microstrip X-plane. q Trajectories are simulated using Runge. Kutta-Nystroem method (ray-tracing program from B. Mecking). q TOSCA generated field distribution (from A. Glamazdin). 07 -29 -05 Prim. Ex collaboration meeting

Correction for the finite detector size, G(DX) DX is the distance between e+e. Ec uses Leff from above 07 -29 -05 In the ideal case of accurate knowledge of the field distribution photon energy is reconstructed with accurace much betetr than 0. 1%. Prim. Ex collaboration meeting

Difference between measured and TOSCA maps Integral is taken along the Z axis for different transverse positions X. X=18. 7 cm; Y=0 cm X=13. 5 cm; Y=0 cm X=8. 6 cm; Y=0 cm X=0 cm; Y=0 cm q Shape of the dependence at X=0; Y=0 was used to correct Ec. q The maximum variations of the r for a single X is used as an error for F(B), +/ -0. 05. 07 -29 -05 Prim. Ex collaboration meeting

End point energy measurement q q 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. q EB for this measurements was 3. 7765 Ge. V (from accelerator and Hall A beam energy measurements). q Ee. p. is defined as a mid point of the falling edge of the e+e- coincidence rate, and is 3. 784 Ge. V. 07 -29 -05 Prim. Ex collaboration meeting

Final corrections 07 -29 -05 Prim. Ex collaboration meeting

After correcting for swapped cables 07 -29 -05 Prim. Ex collaboration meeting

Final results, reaction gd’p. K+K-(n) Simulations G 10, 3375 A 07 -29 -05 Prim. Ex collaboration meeting

Summary n n n 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+ecoincidence rate close to the “end point” energy. 07 -29 -05 Prim. Ex collaboration meeting

Photon energy calibration MS e+ e. Pair Spectrometer SC 1 & SC 2 115 settings of PS field Single scintillator counter SC 1 plane on each side. n Full SC 2 plane. n MS – microstrip detectors : X and Y planes, 50 mm pitch. Reaction gd’p. K+K-(n) MM of the neutron 07 -29 -05 n Prim. Ex collaboration meeting

Measured PS dipole field 07 -29 -05 Prim. Ex collaboration meeting