AProposalfora 2ndPREXRun -...

A Proposal for a 2nd PREX Run

Kent Paschke University of Virginia

PAC 38August 26, 2011

1− 4 sin2 θW � 1

A =GFQ2

2√2πα

�1− 4 sin2 θW − FN(Q2)

FP(Q2)

�

PAC 38 -‐ PREX-‐II ProposalKent Paschke

Weak Charge Distribution of Heavy NucleiNuclear theory predicts a neutron

“skin” on heavy nuclei

208Pb

Neutron distribuBon is not accessible to the charge-‐sensiBve photon.

γ

�

MEM = 4παQ2

Fp Q2( )

�

MPVNC = GF

21− 4sin2θW( )Fp Q2( ) − Fn Q2( )[ ]

Q2 ~ 0.01 GeV2 APV ~ 0.6 ppmRate ~1 GHz5o scattering angle

PREX (Pb-Radius EXperiment)

proton weak charge << neutron weak charge

for the spin-0 208Pb nucleus

5.6 5.65 5.7 5.75 5.8

rn in

208Pb (fm)

0.25

0.26

0.27

0.28

0.29

0.3

Fn(Q

2)/

N

Skyrme

covariant mesoncovariant point coupling


A crucial calibration point for nuclear theory

( R.J. Furnstahl )

…and measuring RN pins down the symmetry energy

24 26 28 30 32 34 36 38 40 42 44

symmetry energy a4 (MeV)

0.05

0.1

0.15

0.2

0.25

0.3

0.35

r n!

rp (

fm)

24 26 28 30 32 34 36 38 40 42 44


0.05

0.1

0.15

0.2

0.25

0.3

0.35

r n!

rp (

fm)

Skyrme

relativistic mesonrelativistic point coupling

24 26 28 30 32 34 36 38 40 42 44


0.05

0.1

0.15

0.2

0.25

0.3

0.35

r n!

rp (

fm)

The single measurement of Fn translates to a measurement of Rn via mean-field nuclear models

Rn calibrates the EOS of neutron rich matter - provides an important calibration point for nuclear theory and description of neutron stars


Integrating in the High Resolution Spectrometers

PREx Collaboration Jefferson Lab Hall A!

"

#

$

Experimental Setup• Std. Hall A HRS Spects. with detector huts well shielded against bkgds.• Running dual, symmetric arms cancels out Atrans and other systematics• Use septum magnet to bend 5o to 12.5o

• Upgraded polarimetry (non-inv. Compton! 1%, Inv. Moller! 1%)• 0.5mm thick Lead in between two 0.15mm Diamond targets (! 1"1in2)with cryogenically cooled frame; used fast rastered beam• Quartz Cerenkov detectors with 18-bit integrating ADCs

Dustin McNulty, PANIC11, Massachusetts Institute of Technology, Cambridge, MA, July 24 - 29, 2011 11

Very clean separaBon ofelasBc events by HRS opBcs

no PID needed; detector sees only elas1c events

p (GeV/c)

Carbon Ground

State

detector footprint

Analog integra1on of everything that hits the detector

A [ppm]-800 -600 -400 -200 0 200 400 600 800

1

10

210

310

A, Pb/D #1, Run 4714µDither Corrected Asymmetry, I = 70

= 170.9 ppm!

FIGURE 1. Distribution of the asymmetries for a typical run at 70µA. Beam-relatednoise has been subtracted using the standard “dither correction” method. The width of171 ppm is consistent with counting statistics.

24

30 Hz

30 Hz, 70 µA

1 GHz


Summary of 2010 Run

Ara

w (p

pm)

Slug ~ 1 day

APV = -0.593 +/- 0.051, dof=308, χ2=1.00

OUT / IN, L/R from “slow” spin reversals to cancel systematicsCorrections

• beam asymmetry correction (48 ppb) • charge normalization (114 ppb) • backgrounds (-1.6%)• acceptance averaging (1.5%)• beam polarization (11%)

Q2 = 0.00906 GeV2

ppm 9.2% 2.0%

à Statistics limited ( 9% )à Systematic error goal achieved ! (2%)

Lost Time Issues:• vacuum system failure• radiation damage to sensitive electronics•Helicity-flip Pockels cell translation•Septum / HRS tune

BPM 4ax

mic

rons

points not sign corrected. Average, with sign correction: 2.3 nm

Position differences small (<4 nm)

Kent Paschke PAC 38 -‐ PREX-‐II Proposal

Error Source Absolute (ppm) RelaBve ( % )

Polariza(on (1) 0.0071 1.1

Beam Asymmetries (2) 0.0072 1.1

Detector Linearity 0.0071 1.1

BCM Linearity 0.0010 0.2

Resca;ering 0.0001 0.0

Transverse Polariza(on 0.0012 0.2

Q2 (1) 0.0028 0.4

Target Thickness 0.0005 0.112C Asymmetry (2) 0.0025 0.4

Inelas(c States 0.0000 0.0

TOTAL 0.0130 2.0

(1) Normalization Correction applied(2) Nonzero correction (the rest were negligible)

Systematic Errors Met GoalsPREX-I controlled systematic errors at

the level required for full precision!


PREX-I ResultFirst electroweak observation of the neutron skin of a heavy nucleus (CL = 95%)

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()@(1Coulomb corrections

about 30%uncertainty <1%

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HebelerSteinerTamiiTsang

PREX-IIproposal


PREX could provide an electroweak complement to Rn predictions from a wide range of physical situations and model dependencies

Recent Rn predictions:

Hebeler et al. Chiral EFT calculation of neutron matter. Correlation of pressure with neutron skin by Brown. Three-neutron forces!

Steiner et al. X-Ray n-star mass and radii observation + Brown correlation. (Ozel et al finds softer EOS, would suggest smaller Rn).

Tamii et al. Measurement of electric dipole polarizability of 208Pb + model correlation with neutron skin.

Tsang et al. Isospin diffusion in heavy ion collisions, with Brown correlation and quantum molecular dynamics transport model.

Recent Rn Predic7ons Can Be Tested By PREX at Full Precision

δ(APV)/APV ~ 3%δ(Rn)/Rn ~ 1%

These can be tested with


What we’ve learned for a 2nd RunWhat Worked:

Problems to resolve: • Lead Target • Septum Vacuum system• Radiation damage in Hall

26 p.e.’s

New SeptumWe now know how to tune it to optimize FOM

AT false asymmetryAT is small (<1 ppm Pb, <10 ppm C) and Afalse will be small if PT is minimized

HRS TuneWe have a tune and good first-guess optics matrix for a tune optimized for the small detectors

Fast Helicity FlippingWe know how to control false asymmetries and monitor performance

Beam Modulation SystemFast beam kicks cancel low frequency noise and improve precision of beam position corrections

Injector Spin ManipulationSecond Wein and solenoid are calibrated and used for helicity reversal. Important cancellation for systematic beam asymmetries from the polarized source.

Lead TargetSurvival > 1 wk, 15 C

New DetectorsSuitable energy resolution achieved for 1 GeV electrons. <5% precision loss.

Polarimetry at low energyMoller at 1.3%, Integrating Compton at 1.2%


Figure of Merit

0.004 0.006 0.008 0.01 0.012

310

Rates w/ I=730 septum

Rates w/ I=775 septum (future)

PREX 1-HRS Rates (MHz) vs Qsq (GeV^2)

Future RunE = 925 to 1000 MeV

2010 Run

370 MHz

740 MHz

Q^2 (GeV^2)

MHz

FIGURE 13. Simulated rates in one HRS versus Q2for two assumptions about septum

current setting. For I=729 Amps (2010 run) the minimum scattering angle was 4.58 degrees

and was not optimal. For I=775 Amps (suggested future run point) the min angle will be

4.35 degrees. By putting the septum there and reducing the energy to the range 925 to

1000 MeV, we expect to double the rate.

36

2 GHz (both arms), 122 ppm / 30 Hz

At lower Q2, PB*APV = 0.5 ppm

15.2 ppb / 25 days= 3.0% of 500 ppb


Target Thickness and Uniformity

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*+,-./011234567289:

4;<02!"Target #3

Melted after >1week production

!

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4:;02!Target #1

Never melted, or even degraded. Last 4 days at 70 uA

Target uniformity degraded by beam damage.

Created noise due to raster. Raster synched for lissajous period matching helicity flip, recovered width

Conclusion: Target non-uniformity is NOT a problem

Targets with thin diamond backing (4.5 % background) degraded fastest.

Thick diamond (8%) ran well and did not melt at 70 uA.

Summary: target design is PROVEN to be sufficientalthough maybe limited... maybe only 1 week per target


Lead / Diamond Target

Diamond

LEAD

Lead (0.5 mm) sandwiched by diamond (0.15 mm)Liquid He cooling (30 Watts)

Other ideas for improving lead/diamond thermal

contact will be explored, but there is a preference

for a proven design

The ladder has space for at least 10 targets. If

targets last only 1 week, this is more than enough


scattering chamber

HRS-L

HRS-Rcollimator

Septum Magnet collimator

Tungsten beamline collimator

sieve box

5o Septum to augment the HRS

to electric form factor of the proton. The averageinclusive analyzing power for carbon, with angularacceptance from 5! to 20! (solid squares) and 5! to70! (solid circles), is shown in Fig. 17.

4. Beamline

The instrumentation along the beamline (shownin Fig. 18) consists of various elements necessary

to transport the electron beam onto the target andinto the dump, and to measure simultaneously therelevant properties of the beam. The resolutionand accuracy requirements are such that specialattention is paid to the control and determinationof the beam energy, current and polarization, andalso to the position, direction, size and stability ofthe beam at the Hall A target location.

Table 2 lists all the beam parameters monitoredalong the beamline and the associated instrumen-tation. In nearly all cases, two or more indepen-dent methods are available to determine andmonitor the various parameters in order toprovide confidence in the absolute measurementsand redundancy if any of the instrumentationshould fail during a run. The beam parameterslisted in Table 2 are determined and monitored ata level which in most cases meets or exceeds theneeds of the approved experiments. The variouselements along the beamline are listed anddescribed in detail in the Hall A OperationsManual [33].

4.1. Basic beamline

The beam entrance channel consists of 63:5 mminner diameter stainless steel tubing connectedwith conflat flanges. Through magnets the innerdiameter of the tubing is restricted to 25:4 mm:

ARTICLE IN PRESS

Proton Momentum [GeV/c]

0.5 1 1.5 2 2.5 3A

naly

zin

g P

ow

er

0

0.1

0.2

0.3

0.4

0.5

0.6

20°!c"!5°

70°!c"!5°

p-Carbon Inclusive Analyzing Powers

Fig. 17. The average analyzing power of the carbon analyzer asa function of proton momentum. Solid squares are averagesover scattering angles from 5! to 20! and the solid circles forthose from 5! to 70!:

EP

RASTER

BCM

COMPTONPOLARIMETER

MOLLERPOLARIMETER

BPM

Q1

Q2

DIPOLE

Q3

SHIELD HOUSE

BEAMDUMP

ACCESSRAMP

Fig. 18. Schematic layout of Hall A, indicating the location of the Compton and the M^ller polarimeters, the raster, the EP energymeasurement system, the beam current monitors (BCM) and the beam position monitors (BPM) upstream of the target. Also indicatedare the locations of the components of one of the high-resolution spectrometers (Q1, Q2, dipole, Q3 and shield house) and of the beamdump and the truck access ramp.

J. Alcorn et al. / Nuclear Instruments and Methods in Physics Research A 522 (2004) 294–346316

SEPTUM

TARGET


Vacuum Chamber, Sieve Box

Tungsten Plug:~2kW, will require water cooling

sieve box

septum

Target

beam

PREX-I: O-ring here at ~34.5 mrad

Redesign area for all metal seals


Radiation Level

to electric form factor of the proton. The averageinclusive analyzing power for carbon, with angularacceptance from 5! to 20! (solid squares) and 5! to70! (solid circles), is shown in Fig. 17.

4. Beamline

The instrumentation along the beamline (shownin Fig. 18) consists of various elements necessary

to transport the electron beam onto the target andinto the dump, and to measure simultaneously therelevant properties of the beam. The resolutionand accuracy requirements are such that specialattention is paid to the control and determinationof the beam energy, current and polarization, andalso to the position, direction, size and stability ofthe beam at the Hall A target location.

Table 2 lists all the beam parameters monitoredalong the beamline and the associated instrumen-tation. In nearly all cases, two or more indepen-dent methods are available to determine andmonitor the various parameters in order toprovide confidence in the absolute measurementsand redundancy if any of the instrumentationshould fail during a run. The beam parameterslisted in Table 2 are determined and monitored ata level which in most cases meets or exceeds theneeds of the approved experiments. The variouselements along the beamline are listed anddescribed in detail in the Hall A OperationsManual [33].

4.1. Basic beamline

The beam entrance channel consists of 63:5 mminner diameter stainless steel tubing connectedwith conflat flanges. Through magnets the innerdiameter of the tubing is restricted to 25:4 mm:

ARTICLE IN PRESS

Proton Momentum [GeV/c]

0.5 1 1.5 2 2.5 3A

na

lyzin

g P

ow

er

0

0.1

0.2

0.3

0.4

0.5

0.6

20°!c"!5°

70°!c"!5°

p-Carbon Inclusive Analyzing Powers

Fig. 17. The average analyzing power of the carbon analyzer asa function of proton momentum. Solid squares are averagesover scattering angles from 5! to 20! and the solid circles forthose from 5! to 70!:

EP

RASTER

BCM

COMPTONPOLARIMETER

MOLLERPOLARIMETER

BPM

Q1

Q2

DIPOLE

Q3

SHIELD HOUSE

BEAMDUMP

ACCESSRAMP

Fig. 18. Schematic layout of Hall A, indicating the location of the Compton and the M^ller polarimeters, the raster, the EP energymeasurement system, the beam current monitors (BCM) and the beam position monitors (BPM) upstream of the target. Also indicatedare the locations of the components of one of the high-resolution spectrometers (Q1, Q2, dipole, Q3 and shield house) and of the beamdump and the truck access ramp.

J. Alcorn et al. / Nuclear Instruments and Methods in Physics Research A 522 (2004) 294–346316

Compton photon detector (~35 meters upstream of target, in alcove) saw about half its signal from radiation in the hall!

Heavy neutron load in hall, also EM?- HRS magnet power supply/controls on platform- cryo electronics monitors at Q1s - Corruption in the shielded detector hut

Strategy: create neutrons in one place, and keep them there.

Not much space for shielding between HRS

SEPTUM

TARGET

z=6m z=22m


Relative Silicon Damage vs. Neutron Energy

100 keV

10 MeV


PREX-I: Sources of Radiation

Even with W plug, significant spray into beam pipe and hall

photon neutron

dumptunnel

targetarea

Photoproduction dominates, not spallation: • soft energy spectrum• quadratic in target thickness, not linear

Neutrons from the target are NOT the problem - the 10% target radiation length is

GEANT/FLUKA studies by J. Mammei and L. Zana


Poly Shield Neutron Stopping Power“simple” geometry: target + collimator in spherical poly shield of various thickness

0-1 MeV

1-50 MeV


PREX-II: Shielding Strategy

neck down W plug, (cover 0.7-3o) so whatever gets past the plug gets to the dump

30cm thick Polyethelene hut to moderate neutrons

Still “just” a concept, but should be robust. Gaps matter only for solid angle


PREX-II: Shielding StrategyPREX-II: tighter collimation. whatever gets past the plug

gets to the dumpPREX-IPolyethelene to

moderate neutrons


PREX-II: Sources of Radiation

neut

rons

per

inci

dent

e-

neut

rons

per

inci

dent

e-

phot

ons

per i

ncid

ent e

-

end of hall

targetcollim

atorend of septum

beampipe

target

collimator

septum beampipe


Neutron rates in “sensitive regions”At z=6m 0.1-1

MeV1-10 MeV

>10 Mev

PREX-I 0.454 0.343 0.038

PREX-II collimator 0.735 0.332 0.011

PREX-II collimator

+Shield0.044 0.019 0.003

At z=22m 0.1-1 MeV

1-10 MeV

>10 Mev

PREX-I 0.080 0.080 0.008

PREX-II collimator 0.055 0.030 0.001

PREX-II collimator

+Shield0.013 0.008 0.0005

Energy Spectrum at z=6m

Integrated rates, above the floor,

1m<r<10m

Factor of ~10 improvement


2nd Run can reach physics goal

Pb

Many experimental questions have been answered in the first run• Lead sandwich target• Precision polarimetry at 1 GeV• HRS optics optimization• Source performance• Transverse Asymmetry• Detectors• Beam Corrections

Primary technical issues for a full run have been addressed• vacuum vessel design is underway• target ladder can provide enough

ammunition for the run• Radiation in the hall down by 10x after

improved collimation + shielding

PREX-‐I was the most precise measurement of electron-‐nuclear scaSering asymmetry• 2σ observaBon of neutron skin in heavy nucleus• Success in experimental technique: systemaBc errors can be controlled

Full precision in 25 addiBonal (PAC) days: 3% on APV, 1% on Rn

CompaBble with 12GeV-‐era restart in FY14 (1 GeV required)

AProposalfora 2ndPREXRun -...

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