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

T=30.3%P=96.2%N=2.6£1010 n/sFOM = 12.0%

without Gd undercoating:

T=30.8%P=91.8%N=2.4£1010 n/sFOM = 11.0%

m=3.0, n=45, r=14.8 m, l=40 cm, d=0.3 mm

Neutron spectrum

Beam profile before and after SMpol

before

after

horizontal vertical

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

Trajectory profile of neutrons

Reflectivity profile w/o Gd undercoat

QuickTime™ and aTIFF (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

defl

ecte

d an

gle

(mra

d)

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.