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

Swiss Neutronics remanent SM coating

Optimization of SMpol

Transmission of SMpol

m=3.0, n=45, r=14.8 m, l=40 cm, d=0.3 mm T=30.3% P=96.2% N=2.6

£ 10 10 n/s FOM = 12.0% without Gd undercoating: T=30.8% P=91.8% N=2.4

£ 10 10 n/s FOM = 11.0%

Neutron spectrum

Beam profile before and after SMpol

before horizontal vertical 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 10 7 s at the SNS Higher duty factor at SNS Can’t account for factor of 10 reduction in LANSCE data

Trajectory profile of neutrons

Reflectivity profile w/o Gd undercoat

QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture.

Reflectivity profile w/o Gd undercoat

Reflectivity profile with Gd undercoat

Position – velocity profile

Position – velocity profile

Polarization profile vs. position

Polarization profile vs. position

Average deflected angle vs. wavelength

wavelength (Ang)

Systematics – summary of original proposal

    • • • • L/R asymmetries + detector mixing npd  PC np elastic Mott-Schwinger H 2 spin rotation • • PC systematic effects  dB - Stern Gerlach steering npd  - Compton analyzing power • • • • PV background asymetries n decay ndt  n+ 6 Li material activation • • instrumental drifts in efficiency, beam, etc detector asym, ped; beam fluct.

– alignment – minimize – measure – +--+-++ – 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 contributes more to the statistics, U/D and L/R asymmetries get mixed solution: pair detectors top/bottom or left/right instead 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 10 B 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.