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Radiation Damage in Polarized Ammonia Solids

Radiation Damage in Polarized Ammonia Solids. Karl J. Slifer UNIVERSITY OF VIRGINIA on behalf of the UVA Polarized Target Group. XII th International Workshop on Polarized Sources, Targets and Polarimetry Brookhaven National Laboratory Sept. 13, 2007. Outline.

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Radiation Damage in Polarized Ammonia Solids

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  1. Radiation Damage in Polarized Ammonia Solids Karl J. Slifer UNIVERSITY OF VIRGINIA on behalf of the UVA Polarized Target Group XIIth International Workshop on Polarized Sources, Targets and Polarimetry Brookhaven National Laboratory Sept. 13, 2007

  2. Outline • Overview of Target Operation • General Characteristics of Polarized Ammonia Solids • Rate of polarization decay with accumulated charge • Target Annealing to recover polarization • Optimal Microwave Pumping Frequency • Frequency Modulation • Summary

  3. History

  4. Spokespersons NH3: M. Battaglieri, A. Deur, R. De Vita, M. Ripani (Contact) ND3: A. Deur(Contact), G. Dodge, K. Slifer PhD. Students K. Adhikari, H. Kang, K. Kovacs EG4 Ran in 2006 Measurement of g1 at low Q2 0.015<Q2<0.5 GeV2 Test of ChPT as Q2 0 Measured Absolute XS differences Goal : Extended GDH Sum Rule Proton Deuteron

  5. Spokespersons NH3: M. Battaglieri, A. Deur, R. De Vita, M. Ripani (Contact) ND3: A. Deur(Contact), G. Dodge, K. Slifer PhD. Students K. Adhikari, H. Kang, K. Kovacs EG4 Ran in 2006 Measurement of g1 at low Q2 0.015<Q2<0.5 GeV2 Test of ChPT as Q2 0 Measured Absolute XS differences Goal : Extended GDH Sum Rule Proton Deuteron See talk later in this session

  6. Polarized Target 5 T field Split Helmholtz pair superconduct. magnet 1K 4He evaporation refrigerator Cooling power: about 1 W Microwave Power 1W at 140 GHz to pump electrons Insulated cryostat 85 L Liquid He resevoir 57 L Liquid N shield (300K BB shield) SLAC/JLab Hall C configuration

  7. CLAS Cryogenic Service Module Target Pumps R1 Beam Pipe BPM R2 Beam Direction R3 Electromagnetic calorimeters Cerenkov TOF Scintillaotrs Large angle calorimeters Polarized Target in CLAS

  8. Hall B Target

  9. Polarized Target in CLAS

  10. Target Insert Short Carbon Long Carbon Short NH3 Long NH3 Empty

  11. Target Material Ammonia as polarized target material Raw frozen Ammonia irradiated to create paramagnetic centers needed for DNP Dose : 1017 e-/cm2from small electron accelerator. Target material under liquid Argon at 87K. Creates NH2 (ND2) radicals in the material. High luminosity : 1035 cm-2s-1 Excellent resistance to radiation damage. NH3 polarizes to >90% (5T/1K) after initial irradiation. ND3 polarizes to less than 20% initially. Further radiation @ 1K increases the max ND3 polarization significantly. 40-45% (5T/1K) after an additional cold dose of about 1016 e-/cm2.

  12. General Characteristics E143 data NH3 NH3: Annealing restores polarization to approximately previous max. Accumulated Charge (1014 e-) ND3 ND3: low temp in-situ irradiation pushes polarization to greater than 40% Crabb and Meyer Annu. Rev. Nucl. Part. Sci. 47 (1997), p. 67

  13. General Characteristics ND3 GEN01 P.M. McKee NIM A526 (2004) 60-64

  14. General Characteristics ND3 ND3 GEN01 P.M. McKee NIM A526 (2004) 60-64

  15. General Characteristics E155 NH3 P.M. McKee NIM A526 (2004) 60-64

  16. General Characteristics Rate of decay increases with dose E155 NH3 NH3 P.M. McKee NIM A526 (2004) 60-64

  17. Annealing Radiation Damage = creation of Atomic Hydrogen (or Deuterium) which don’t participate in DNP, but do provide additional relaxation mech. Annealing: Reduce the concentration of unwanted radicals in the material by warming the material (greater than 80K for atleast 20 mins) Restores maximum achievable polarization to previous/higher value. Process can be repeated many times. But: Anneal at too high T and can destroy the paramagnetic centers needed for DNP EG4 Followed the GEN01 recipe 90K for approx 1 hour with good results

  18. EG4 Deuteron ND3 Initial polarization of 20% Max Polariz. increased to greater than 40% after additional cold dose of 20 x 1015 e-/cm2 Accumulated Charge (1015 e-/cm2) ND3 can withstand up to 100 x 1015 e-/cm2

  19. EG4 Proton Max Polarization increases with Anneal, But not quite to its previous maximum NH3 Accumulated Charge (1015 e-/cm2) Accumulated Charge (1015 e-/cm2)

  20. Optimal Microwave Frequency Initial separation of the two freq is about 200 MHz. These optimal frequencies drift with accumulated dose. Variation differs slightly for +-. + : Almost linear - : Inverted exponential Negative Positive P.M. McKee NIM A526 (2004) 60-64

  21. Optimal Microwave Frequency EG4 polarized ND3 only in positive state Optimum frequency varied by about 100 MHz over each anneal cycle.

  22. Frequency Modulation Frequency modulation of microwave frequency previously demonstrated to have dramatic effect on maximum achievable polarization. Deuterated Butanol (SMC) FM turned on Crabb and Meyer Annu. Rev. Nucl. Part. Sci. 47 (1997) 67.

  23. Frequency Modulation NH3 ND3 FM turned on FM turned on (1kHz FM 5Vpp) Few percent improvement found during EG4. Note: sinusoidal BG in proton signal from overzealous cryo PID loop.

  24. EG4 Proton Work in Progress Still analyzing EG4 proton polarimetry

  25. EG4 Proton Work in Progress PbPt analyses Still analyzing EG4 proton polarimetry Issue: Cross talk of two Polarized NH3 cells during the run.

  26. Summary • UVA Group : more than 15 yrs worth of experimental data on NH3 and ND3 • NH3 : polarizes to >90% with warm irradiation of 1017 e-/cm2 at 87 K • ND3 : Further cold irradiation crucial to obtaining large polarizations • NH3 decay rate increases strongly with dose • ND3 decay rate not very sensitive to dose • Annealing : 90-100K for 60 mins. Tanneal increasing slightly with dose • New batch of polarized target runs in 2008 JLab Hall C • SANE : proton A1,g2 2.5<Q2<6.5 GeV2 • Semi-SANE : proton and deuteron semi-inclusive long. spin asymmetries • WAC : Wide Angle Compton Scattering • g1d/F1d : deuteron spin structure • Also, conditionally approved experiment with first time installation in Hall Longitudinal-Transverse Spin Polarizability

  27. Hall B Refridgerator

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