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Projected performance of NPDGamma and new systematics introduced by SM polarizer. Christopher Crawford University of Kentucky NPDGamma Collaboration Meeting 2008-02-01. Outline. McStas simulations of FnPB and Smpol optimization and final design projected performance at the SNS
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Projected performance of NPDGamma and new systematics introduced by SM polarizer Christopher Crawford University of Kentucky NPDGamma Collaboration Meeting 2008-02-01
Outline • McStas simulations of FnPB and Smpol • optimization and final design • projected performance at the SNS • new systematics associated with the SMpol
Transmission of SMpol T=30.3% P=96.2% N=2.6£1010 n/s FOM = 12.0% without Gd undercoating: T=30.8% P=91.8% N=2.4£1010 n/s FOM = 11.0% m=3.0, n=45, r=14.8 m, l=40 cm, d=0.3 mm
Beam profile before and after SMpol before vertical horizontal after
Sensitivity of NPDG to A at SNS • Gain in the figure of merit at the SNS: • 12 x brighter at the end of the SNS guide • 4 x gain by new SM polarizer • 7 x longer running time • A ~ 1.1x10-8 in 107 s at the SNS • Higher duty factor at SNS • Can’t account for factor of 10 reduction in LANSCE data
Average deflected angle vs. wavelength deflected angle (mrad) wavelength (Ang)
Systematics – summary of original proposal • L/R asymmetries + detector mixing – alignment • npd PC • np elastic • Mott-Schwinger • H2 spin rotation • PC systematic effects – minimize • dB - Stern Gerlach steering • npd - Compton analyzing power • PV background asymetries – measure • n decay • ndt • n+6Li • material activation • instrumental • drifts in efficiency, beam, etc – +--+-++- • detector asym, ped; beam fluct. – detector + helicity asym.
L/R mixing • Can measure L/R and U/D asymmetries • estimate magnitude of mixing • use table motion data to correct for alignment • non-geometrical mixing, due to detector efficiencies • if one diagonal contributesmore to the statistics,U/D and L/R asymmetriesget mixed • solution: pair detectorstop/bottom or left/rightinstead of diagonal efficient inefficient
Supermirror Polarizer Systematics • SMpol: • position-dependent polarization, intensity • wavelength-dependent polarization, intensity • wavelength-dependent bending of beam • vibration and thermal drifts • gamma radiation • RFSF and holding field • wavelength and position-dependent efficiency • Stern-Gerlach steering • target and detectors • position / solid angle effects • L/R and U/D mixing • Compton scattering / analyzing power • 2nd order effects from combinations of above? • for any point in SMpol phase space, direction, polarization, lambda are fixed • RFSF will cancel out false asymmetries – only dilution of asymmetry • only position in detector, not direction of neutrons relevant • worse systematic: 60 Hz vibration – randomize groups of spin sequences • radiation: increase background, plus fluctuations (before RFSF)
List of Systematics from Seppo • Systematic effect related to beam: • Changes in beam – we are sensitive since the reflective polarizer • Beam intensity fluctuations – should average out • Beam position – beam gravity point fluctuations or phase space fluctuations caused by bender guide section. Any of these changes will lead to left right asymmetry • Beta delayed neutrons – less than 10-4 fraction in beam, • not a problem in the NPDGamma • Temperature changes in target hall causes changes in the guide beam, • slow drift, effect cannot be large • will be averaged out by the eight-step spin sequence.
List of Systematics from Seppo • Systematic effect related to Bender SM polarizer (BSMP) • Reflection angle depends on neutron energy this means that after the neutrons have reflected from the bender the beam gravity point on the LH2 target has left-right dependence on neutron energies. • How large this left right asymmetry effect is, you should see from your beam runs. How to correct this depends on the size of the effect. • Mechanical vibration of the BSMP • Typical mechanical vibration frequency is sub Hertz, 0.x Hz • the 60Hz operation should average over this effect • Temperature change in cave – cave temperature should be stable +/- 5 C • Reflection angle will change since the holder of the lamellas will change • Slow and small process – this drift is zeroed by 8-step spin sequence • BSMP location respect to beam guide will change because the support is distorted by temperature change. • Again cannot be big effect if the support is properly done.
List of Systematics from Seppo • Systematic effect related to Bender SM polarizer (BSMP) • Polarization flatness – polarization as a function of neutron energy. • This we will learn from the manufacturer and we need to measure also this in situ. • Most probably not a problem in the NPDGamma • Polarization across the beam varies since the coating is not perfect. • This we need to measure in situ. Most probably not a problem in the NPDGamma. • The technical magnetization loop doesn’t have a flat top which means that the magnetization depends on the holding field. • The magnetization is sensitive to changes or fluctuations in holding field. • We need to learn this from manufacturer. • Relaxation of the remanent magnetism in magnetic material (relaxation of magnetic domains) after a change in the holding field or reversal of the polarization direction • This can be issue since the FP14 is a spin echo instrument and they need to demagnetize the steel structures once – twice per day with significant field pulse that then can little change the remanent field which then slowly decays to stable magnetization. We need to learn more about this. • Effect of long term and short-term gamma-ray radiation from the BSMP. • Radiation from n-10B dies out fast. Ti, Ni, and Fe (coating materials) have long term decay components? • The frame of BSMP will be activated. But collimation should keep all these components out of sight of the detector. Mostly all these are slow drifts.
