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 N/sec at 1.4 MW. 89 days data to disk for LH and Al

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

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.

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

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  Asymmetry = -1.9± Gericke, Bowman, and Johnson published A n,elastic = A ,L-R ~ Contradiction! Theory is wrong.

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 ~ 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, , is negligible compared to the M-S asymmetry. Agrees with extrapolation from 10 MeV, A~ E 3/2. (.01x /2 = ). The L-R asymmetry in ~ dominates, but is too small for us to measure.

Aluminum neutron capture,  cascade, and  decay

Yields from n+p−>d+ , prompt Al, and  -delayed Al  ’s

Prompt Al  ’s A PV = -2± (measured in LANL run), 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  ’s 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 we aim for 5% fractional uncertainty in subtraction.

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.

Detector  signal after beam off from LANL run Most delayed  ’s come from Al 28 Al half life =2.32 min

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

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

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

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

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

Al bremsstrahlung asymmetry estimate Conclusion: Delayed asymmetry is negligible

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