Run Time, Mott-Schwinger, Systematics, Run plan

David BowmanNPDGamma Collaboration Meeting

10/15/2010

Apparatus to measure A

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

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-RAsymmetry = -1.9±2.0 10-

7

• Gericke, Bowman, and Johnson published An,elastic =-41 10-6.

• A,L-R~-20 10-7.

• 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 ~ 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 ~ 1 10-8 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• 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 ’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.

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 offfrom 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 28Al g.s.

• Polarized neutron captures on J=5/2 27Al and forms a compound-nuclear state. J=2 or 3. Pol ~ .30

• Cascade ’s carry away angular momentum and depolarize 28Al. g.s. Pol ~ .15

• 28Al g.s. lives for 139 sec before it decays. The average polarization of neutrons is small 1/60/139 ~ 10-4.

• Spin-lattice relaxation further depolarizes the 28Al 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 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 delayed ‘s can constrain Al

asymmetry