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Run Time, Mott-Schwinger, Systematics, Run plan

Run Time, Mott-Schwinger, Systematics, Run plan. David Bowman NPDGamma Collaboration Meeting 10/15/2010. Apparatus to measure A . Run Time Estimate: Number of Neutrons from guide. 1.46 10 11 N/sec at 1.4 MW. 89 days data to disk for LH and Al. Systematic Uncertainties.

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Run Time, Mott-Schwinger, Systematics, Run plan

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  1. Run Time, Mott-Schwinger, Systematics, Run plan David Bowman NPDGamma Collaboration Meeting 10/15/2010

  2. Apparatus to measure A

  3. Run Time Estimate: Number of Neutrons from guide 1.46 1011 N/sec at 1.4 MW. 89 days data to disk for LH and Al

  4. Systematic Uncertainties Our policy has been to make systematic uncertainties < 1/10 of goal statistical uncertainty, 10-9

  5. Mott-Schwinger • Interaction of the neutron magnetic moment with the motional magnetic field of the target creates a spin-orbit interaction and a parity-allowed L-R analyzing power in elastic scattering.

  6. Strategy for Statistics and Systematics • All indications are that the apparatus will operate at neutron statistics • Measure n flux • Measure rate of detected gammas • Measure asymmetry uncertainty • Compare • Measure Al fraction in data • Compare with detector transport model

  7. Mott-Schwinger Neutron Elastic-Scattering Asymmetry. L-R mixes with U-D if the detector and the magnetic field are misaligned • LANL run gave L-R g Asymmetry = -1.9±2.0 10-7 • Gericke, Bowman, and Johnson published An,elastic =-41 10-6. • Ag,L-R~-20 10-7. • Contradiction! • Theory is wrong.

  8. Mott-Schwinger Analyzing Power • The new calculations used the method of phase shifts - plane-vanilla approach. • The MS spin-orbit interaction leads to a L-R asymmetry ~ 10-9 at 10 meV. • In addition to the M-S interaction, GBJ considered the n-p spin-orbit interaction. We made an error in transforming from the n-p system to the n-molecule system. • The L-R asymmetry from the corrected n-p spin-orbit force, 1 10-16, is negligible compared to the M-S asymmetry. Agrees with extrapolation from 10 MeV, A~ E3/2. (.01x10-9 3/2 = 3 10-16). • The L-R asymmetry in ~ 110-8 dominates, but is too small for us to measure.

  9. Aluminum neutron capture, g cascade, and b decay

  10. Yields from n+p−>d+g, prompt Al, and b-delayed Al g’s

  11. Prompt Al g’s • APV = -2±3 10-7 (measured in LANL run), 1.3 10-7 RMS (theory, Gericke et al.) • Estimate that 3% of the neutrons capture on Al and 15% of the prompt signal comes from Al • False asymmetry from prompt g ’s 2 10-8 . • We must measure this false asymmetry in Al runs and subtract from LH asymmetry. Optimal time fraction for Al runs is 15% and an additional 15% to improve LH statistics. • If the Al asymmetry is 2. 10-8 we aim for 5% fractional uncertainty in subtraction.

  12. What knowledge is required for correction? • Fraction of neutrons capture in Al and LH • Geometry differences for the Al in Al runs and Al in LH runs. • n-H scattering dominates transport in LH runs • Al runs: no neutrons capture on side walls • LH runs: many neutrons capture on side walls • We must apply Monte-Carlo corrections. We need experimental constraints.

  13. Detector g signal after beam offfrom LANL run 28 Al half life =2.32 min Most delayed g’s come from Al

  14. Strategy for subtracting b delayed background In situ measurement of pedestals reduces neutron rate by 12%. The pedestals come from electronic and b-delayed g signals. They depend on the irradiation history of the target.

  15. Strategy for Al prompt/delayed measurements • Interrupt beam and measure the decay of activated Al in Al runs. Determine the ratio of prompt to activation gammas. (Knowledge of the irradiation history is required.) Expect 7.7/1.8 • Interrupt beam in LH runs. Activation tail gives the amount of prompt Al signal in LH runs. • The Monte-Carlo model is needed to calculate the difference in between the LH and Al because the neutron transport changes

  16. Strategy for LH Running • Run-time estimate gives 89 days for production • LH data • Al data • Pedestals are included in LH and Al • Auxiliary runs are not included ~ 30 days (wag) • Prompt/Delayed gamma runs • Detector angle runs (A. source or B. neutrons) • Neutron flux and monitor calibration runs

  17. Scenario for b decay of 28Al g.s. • Polarized neutron captures on J=5/2 27Al and forms a compound-nuclear state. J=2 or 3. Pol ~ .30 • Cascade g’s carry away angular momentum and depolarize 28Al. g.s. Pol ~ .15 • 28Al g.s. lives for 139 sec before it b decays. The average polarization of neutrons is small 1/60/139 ~ 10-4. • Spin-lattice relaxation further depolarizes the 28Al g.s. • The b decay energy is inefficiently converted to g energy • Multiple scattering washes out the b direction.

  18. Delayed-neutron asymmetry reduction vs. spin-lattice relax. time

  19. Al bremsstrahlung asymmetry estimate Conclusion: Delayed asymmetry is negligible

  20. (Preliminary) Conclusions concerning systematic uncertainties • 90 days of data + 30 days for auxiliary experiments are needed for 10-8 uncertainty • Mott-Schwinger asymmetry is small and understood • The dominant source of systematic uncertainty is the subtraction of the prompt Al asymmetry • Observation of b delayed g‘s can constrain Al asymmetry

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