First measurement of spinexchange collision cross section in the low temperature region

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1 First measurement of spinexchange collision cross section in the low temperature region Marco Capiluppi Universita degli studi di Ferrara

2 Outline o HERMES target o Spin exchange collision measurement o Conclusion

3 e ± storage ring (27 GeV) e ± self polarized by Sokolov- Ternov (P ~ 40 %) HERMES fixed target experiment

4 Polarized Gas Target sextupoles + e beam storage cell magnet coils iron yoke storage cell RFT nozzle RFT chopper H 1 chopper detector sextupole detector ABS B TGA BRP

5 sextupoles + e beam storage cell magnet coils iron yoke storage cell RFT nozzle RFT chopper H 1 chopper detector sextupole detector ABS B TGA BRP

6 Atomic beam source Atoms with m j =+½ focused in sextupole magnets. RF transitions select HFS. Injection for data taking: P z P z - 2 3

7 sextupoles + e beam storage cell magnet coils iron yoke storage cell RFT nozzle RFT chopper H 1 chopper detector sextupole detector ABS B TGA BRP

8 The storage cell DENSITY increase No dilution factor Recombination Depolarization

9 sextupoles + e beam storage cell magnet coils iron yoke storage cell RFT nozzle RFT chopper H 1 chopper detector sextupole detector ABS B TGA BRP

10 Target Gas Analyzer TGA system CEM ABS injection tube 1st baffle chopper QMS storage cell sample tube extension tube 30 o 7 o 2nd baffle ionizing volume BRP Atomic fraction (α TGA ) φa φ + φ a m

11 sextupoles + e beam storage cell magnet coils iron yoke storage cell RFT nozzle RFT chopper H 1 chopper detector sextupole detector ABS B TGA BRP

12 Brait Rabi Polarimeter r/mm extension tube sample tube SFT beam blocker MFT sexp. magn. syst. detector cell beam shutter chopper z/mm Hyperfine states selection Atoms detection n 1 n 2 n 3 n 4 P BRP a = n 1 + n 4 -(n 2 + n 3 ) { { P + z P - z

13 Target polarization P T = total target polarization T [ α P + (1 P ] P = α ) 0 r a α r α 0 =atomic fraction in absence of recombination α r =atomic fraction surviving recombination P a = polarization of atoms P m = polarization of recombined molecules m Relation to measured quantities: Sampling corrections α r = c a α r TGA P a = c P P a BRP

14 P T 2003 P T 2004 P T Target Polarization in time (days since October 1st 2003) cell warm up time (days since January 1st 2004) 1 First Cell Second Cell beam dump st stable period 2nd stable period time (days since January 1st 2005) Smooth performance P T = ± Cell damaged by a warming up of the line First cell damaged by a beam dump target running

15 Spin Exchange Measurement o Introduction o Measurement with the HERMES target o Results o Conclusion

16 Introduction Universe fill by hydrogen atoms Determination of the distribution of Hydrogen in the space In case of collision switch from the triplet to the singlet configuration Emission of radio Waves (1.42 GHz)

18 The process Σ i m F = constant

19 Theory

21 Measurement with HERMES target P z+ injected ( 1>, 4>) spin-exchange collision 4> 2> ( P z ) P z - injected ( 2>, 3>) spin-exchange collisions 2> 4> ( P z ) spin exchange collisions cause depolarization of the sample

22 The spin-exchange cross section formula σ se = L d 2 Pse (1 + x n (1 2 P 2 se ) ) L d diffusion length Monte Carlo n average density internal note INDEX P se depolarization by spin-exchange x = B/B c H measured Absolute detemination of the Target density for the transverse H running ( ) measured

23 Total cross section (10-16 cm 2 ) K K K K First set 21.8± ± ± ±2.3 Second set 19.0± ± ± ±1.9 Third set 20.4±2.3

25 Conclusions o first measurement in the low temperature region; o novel method to measure the spin exchange collision cross section presented; o good agreement with the only other existing experimental data; o experimental data in agreement with Allison s calculation;

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