The A2 recoil nucleon polarimeter

Daniel Watts

University of Edinburgh, UK

Why nucleon polarimetry?

Would add a unique capability to the MAMI setup – Valuable compliment to circularly and linearly polarised photon beams and polarised target systems.

Observables from recoil group will allow MAMI to make the first complete measurement in pion/eta photoproduction

(Nucleon polarimetry proposal approved by last MAMI-ELSA PAC)

Double-polarisation in pseudo-scalar meson photoproduction

Polarisation of

target recoil

Observable

Nucleon Scattering and polarisation

Analysing power of scatterer

Polar angle distributionfor unpolarised nucleons

x and y (transverse) components of nucleon polarisation

Number of nucleons scattered In the direction

n() =no(){1+A()[Pycos()–Pxsin()]

Simulate p()p channel – realistic beam/target & detector parameters

New routines written for GEANT3 – introduced modulation of for hadronic interactions (take A=1)

All other processes left in. e.g. coulomb scattering, nuclear de-excitations …

Explore possible designs for polarimeter

New GEANT simulations incorporating polarimetry

0

p/

p

x p defines plane

Reconstructed Phi in incident nucleon frame (deg)

E(MeV)

A =

(+

- - ) /

(++

- )

Design 1: Graphite at CB exit

~32% reduction in Apol ~ 2.4% (E=0.3-0.6 GeV, scat>20)

(cm) > 130

Graphitescatterer

TAPS

Target

CB skirt

Analyser efficiency(7cm graphite)

Yie

ld (

a.u

)

COM (Deg)

Yie

ld

(a.u

)

Design 2: Graphite in CB tunnel

~45% reduction in Apol ~ 3% (E=0.3-0.6 GeV, scat>15)

(cm) > 130

Graphitescatterer

Target

Analyser efficiency(7cm graphite)

Reconstructed Phi in incident nucleon frame (deg)

A =

(+

-- ) /

(+-

- ) Yie

ld (

a.u

)

Yie

ld (

a.u

)

E(MeV)COM (Deg)

Design 3: Graphite Near Target

~46% reduction in Apol ~ 3 % (E=0.3-0.6GeV, scat>15)

Graphitescatterer

Target

Analyser efficiency(7cm graphite)

(cm) > 90

Reconstructed Phi in incident nucleon frame (deg)

A =

(+

-- ) /

(+-

- ) Yie

ld (

a.u

)

Yie

ld (

a.u

)

COM (Deg) E(MeV)

• ~35% dilution of analysing power

• Acceptance X%

• If proves worth can move more upstream to greatly increase acceptance

Design 4: Graphite Near Target+ subsequent CB detection!!

(cm) > 60o

~53% reduction in Apol ~ 2.6% (E=300-600 MeV, scat>20)

Graphitescatterer

Target

Analyser efficiency(7cm graphite)

Reconstructed Phi in incident nucleon frame (deg)

A =

(+

-- ) /

(+-

- )

Yie

ld (

a.u

)

Yie

ld (

a.u

)

COM (Deg) E(MeV)

Test polarimeter

• Polarimeter with adjustable thickness and hole diameter

• Will fit in “orange pipe” used in PID tests

• Polarimeter presently being machined in Edinburgh

• Ready for use in tests from late Oct

Tracker detector(s)

First polarimetry measurements on proton target do not need tracker – BUT tracker necessary for neutron target measurements (Fermi motion)

Need to finalise polarimeter design before can finalise tracker design – need test beam time

Tracker Possibilities - Si detectors on face(s) of graphite - Wire chambers - Scintillating fibre

Money already available – Edinburgh £120k GWU £50k

Also Mainz, UCLA , …

Conclusion

Simulations give good indication that we can start testing nucleon polarimeter (and getting first data!) now.

Test polarimeter module ready this month - need test beamtime with prototype to move the project forward

Forward angle tracker pre-requisite to allow neutron target measurements in the longer term

• ~35% dilution of analysing power

• Acceptance X%

• If proves worth can move more upstream to greatly increase acceptance

Reconstructed Phi in incident nucleon frame

Egamma (MeV)

A =

(+

-- ) /

(+-

- )

GraphiteCB tunnel

Design 4: Graphite Near Target+ subsequent CB detection!!

(cm) > 60

~50% reduction in Apol ~ X%

Reconstructed Phi in incident nucleon frame

Egamma (MeV)

A =

(+

-- ) /

(+-

- )

GraphiteCB tunnel

Design 1: Graphite at CB exit

~32% reduction in Apol ~ 2.4%

(cm) > 130

• ~35% dilution of analysing power

• Acceptance X%

• If proves worth can move more upstream to greatly increase acceptance

Reconstructed Phi in incident nucleon frame

Egamma (MeV)

A =

(+

-- ) /

(+-

- )

GraphiteCB tunnel

Design 3: Graphite Near Target

~46% reduction in Apol ~ X %

Reconstructed Phi in incident nucleon frame

Egamma (MeV)

A =

(+

-- ) /

(+-

- )

Design 2: Graphite in CB tunnel

GraphiteCB tunnel

~35% reduction in Apol ~ 3%

(cm) > 130

Nucleon polarimetry concept

Graphite sheet

TAPS

Crystal Ball

beam

Hydrogen target cell

Useful scattered eventSelect events with scattering angleslarger than ~10 degrees : arising from nuclear interaction

n() =no(){1+A()[Pycos()–Pxsin()]

Design 3 – 7cm Graphite 8cm from target

~30% dilution of analysing power

Acceptance X%

If proves worth can move more upstream to greatly increase acceptance

Reconstructed Phi in incident nucleon frame

Egamma (MeV)

A =

(+

-- ) /

(+-

- )

P

T

Previous experimental data – SAID database

Data for all CM breakup angles

Ox’ Cx’

Recent JLAB datanot in database

GEANT simulation of polarimeter

No GraphiteWith Graphite scatterer

• Simulation includes realisticsmearing of energy deposits due to experimental energy resolutionand proper cluster finding algorithms

• Finite target size and E resolution included

Angle between N(E,) and TAPS hit

• ~30% dilution of analysing power

• Acceptance X%

Reconstructed Phi in incident nucleon frame

Egamma (MeV)

A =

(+

-- ) /

(+-

- )

Design 1: Graphite in CB tunnel

GraphiteCB tunnel

CM) >~130o

E=150 MeVE=200Eg=300E=500E=750E=1000E=1500

Polarimeteracceptance

Nucleon angle in lab (deg)

Pio

n a

ngle

in C

M (

deg)

Kinematic acceptance of polarimeter

p()N

• More forward recoils than for pion production.

• Almost all recoils are incident on polarimeter up to ~0.8 GeV

Eg=720Eg=820Eg=920Eg=1520

Lab nucleon angle (degrees)

CM

a

ng

le (

deg

rees)

Polarimeter acceptance

Kinematic acceptance of polarimeter

p()N

MAID predictions and expected data accuracy - p()N

300 hrs MAMI B

500 hrs MAMI C

New GEANT simulations

Simulate New routines added to GEANT – introduced modulation for hadronic interactions (take A=1)

Simulated p(,p)0 data. Run through AcquRoot analysis. Accurate description of target size, beam properties, CB & mini TAPS. E=300-600 MeV

All other processes left in. Explore possible designs for polarimeter without

need for tracker

p

MAID predictions and expected data accuracy - p()N

300 hrs MAMI B

Full MAID

No P11(1440)

Cx’ – Extraction and expected accuracy

Plot difference in distributions for two helicity states (cut on region of with reasonable A())

Left with simple sin() Dependence. Extract Px

0 180 360

Photon energy (MeV)

Cx’

P=0.7, E=±25MeV, =130±10

~ 1 b/sr → Cx ~ 0.015

~ 0.1 b/sr → Cx ~0.05

Greatly improved data quality

-

Expected data accuracy

Common parameters:

Photon beam: 2.5x105 sec-1 MeV-1 Bin ±12.5 MeVTarget: 2.11023 nuclei / cm2

Meson: Bin ±10o

Polarimeter: 3% probability for a (detected) nuclear scatter Average analysing power ~0.4

Principles of nucleon polarimetry

Well established technique – relies on spin-orbit interaction in Nucleon-Nucleon interaction

Polarimeters - exploited nucleon or nuclear targets (2H, 4He, 12C, 28Si) – tended to use materials with well known analysing powers

pomme

A1 FPP

GEn Polarimeter

Kent state

Measure direction of nucleon before and after the scatterer with sufficient accuracy to determine an analysing reaction has taken place.

Polarimetry basics

For incident protons also have multiple (coulomb) scattering

scat=5-20o

scat

Scattered nucleon detection in TAPS

1 TAPS block ~ position resolution for hit TAPS~0.9m from scatterer

N

Straight through10o scatter20o scatter