PAC38, Newport News, August 22-26, 2011

PAC38, Newport News, August 22-26, 2011

PEPPoPEPPo a Proof-of-Principle Experimenta Proof-of-Principle Experiment

Polarized Electrons for Polarized PositronsPolarized Electrons for Polarized Positrons

A. Adeyemi4, P. Aguilera1, H. Areti1, M. Baylac2, G. Bosson2, A. Camsonne1,

L. Cardman1, S. Covrig1, C. Cuevas1, O. Dadoun3, J. Dumas1,2, E. Fanchini2,

T. Forest5, E. Forman1, A. Freyberger1, S. Golge6, J. Grames1, P. Guèye4,

C. Hyde7, Y. Kim1,5, J.-F. Muraz2, M. Marton2, M. Poelker1, S. Setiniyaz5, J.-S. Réal2, R. Suleiman1, A. Variola3, E. Voutier2

1 Jefferson Lab, Newport News, VA, USA2 LPSC, Grenoble, France

3 LAL, Orsay, France4 Hampton University, Hampton, VA, USA

5 Idaho State University & IAC, Pocatello, ID, USA6 North Carolina University, Durham, NC, USA

7 Old Dominion University, Norfolk, VA, USA

Newport News, August 22-26, 2011

This proposal (PR12-11-105) aims at a demonstration of a new technique for producing polarized positrons, and constitutes a first step towards the development of a polarized positron source for Physics experiments at JLab, and other potential applications.

Objectives

It follows PAC35’s enthusiastic endorsement of LOI-10-010 which noted that

“Any accelerator facility, like JLab, using polarized electrons for its physics program would like an intense beam of polarized positrons. This Letter marks a proof of principle experiment that should become a full

proposal.”

2/17

Why polarized positrons at JLab ?

The investigation of the partonic structure of nucleons and nuclei via Generalized Parton Distributions (GPDs) is a major goal of the JLab and

JLab12 experimental programs.

Parton Imaging

M. Burkardt, PRD 62 (2000) 071503 M.Diehl, EPJC 25 (2002) 223


• The 4 quark-helicity conserving GPDs (Hq, H

q, E

q,

Eq) of the nucleon are extracted from the DVCS

reaction off proton and neutron targets, combining experimental observables from polarized beam and targets.

• These observables involve different linear and bilinear combinations of Compton form factors whose real and imaginary parts are linked to GPDs.

• Other reactions (DVMP, TCS…) are expected to bring additional information.

A large set of different observables is required to unravel the partonic structure of hadronic matter and

extract unambiguously the flavored nucleon GPDs.

~ ~

3/17

(e,e′) Differential Cross Section

M. Diehl at the CLAS12 European Workshop, Genova, February 25-28, 2009

INTlINTlDVCSlDVCSBHeP0 σ~Pσeσ~Pσσσ



γNNγ Ae

γNNγ Am

INTDVCSBH00 σσσσ

INTDVCS00 σ~2σ~2σσ

Electron observables

INT0000 σ2σσ

INT0000 σ~4σσσσ

Electron & positron observables INTlINTlDVCSlDVCSBHl

eP0

ePS σPσ~eσPσ~σPSσσ

Polarized electrons and positrons provide the separation of the different reaction amplitudes of the (e,e') cross section allowing for an experimentally complete determination of the GPDs.

4/17


Two Photons Physics

The availability of a 100 nA e+ beam (unpolarized or polarized) would permit

an order of magnitude improvement in the statistical accuracy of e+e-

comparisons.

Following the very first measurements of polarization transfer observables in electron elastic scattering, the validity of the 1 exchange approximation of the electromagnetic interaction has

been questioned.

P.A.M. Guichon, M. Vanderhaeghen, PRL 91 (2003) 142303

J. Guttmann, N. Kivel, M. Meziane, M. Vanderhaeghen, arXiv:hep-ph:1012.0564

The precision of Coulomb corrections in the study of the EMC effect at JLab energies is also a concern.

5/17

e+ p

) /

(e- p

)

Storage Ring Scheme

Polarized positrons have been obtained in high energy storage ring taking advantage of the Sokolov-Ternov effect which leads

to positrons polarized parallel to the magnetic field.

Why PEPPo ?


5

3

2

22e

γ

ρ

e

cm

35

8τ

Polarization builds up exponentially with a time constant characteristic of the energy and the curvature of the positrons

~ 20 mn @ HERA

6/17Not compatible with CW beam delivery at JLab.

Fixed Target Schemes

The principle of polarization transfer from circular photons to longitudinal positrons has been demonstrated in the context of the ILC project.

T. Omori et al, PRL 96 (2006) 114801

G. Alexander et al, PRL 100 (2008) 210801

P(e+) = 73 ± 15 ± 19 %

Newport News, August 22-26, 2011 7/17

Why PEPPo ?

Compton Backscattering Undulator

1.3 GeV

Require independent ~GeV to multi-GeV electron beam.

Polarized Bremsstrahlung

E.G. Bessonov, A.A. Mikhailichenko, EPAC (1996) A.P. Potylitsin, NIM A398 (1997) 395

Sustainable polarized electron intensities up to 4 mA have been demonstrated from a superlattice photocathode.

R. Suleiman et al., PAC’11, New York (NJ, USA), March 28 – April 1, 2011

PEPPo intends to demonstrate the feasibility of using bremsstrahlung of polarized electrons for the production of

polarized positrons.


e- → → e+

Why PEPPo ?

The PEPPo experiment @ JLab

Experiment Layout

PEPPo is proposed to run at the injector, taking advantage of the existing experimental capabilities that uniquely define with precision the CEBAF

polarized electron beam at creation.


Polarization controls

Variable energy 2-7.5 MeV

Polarization measurement (1.5%)

Energy measurement (10-2)

PEPPo

9/17

Laser

Intensity controls

Production TargetT1 (0.1-1 mm W)



Beam position monitors

Viewers

Positron Selection Device

Dipoles

Solenoid

Viewer

Collimator

Corrector magnets

Solenoid

Faraday Cup

Annihilation detector

Beam Profiler

Viewers

Reconversion TargetT2 (2 mm W)

Polarized Analyzing Target

(7.5 cm Fe)

Compton Transmission Polarimeter

Corrector magnets

Quadrupoles

ELEGANT beam optics

PEPPo branch jonction

G4PEPPo experiment model


T1

T2

S1

S2

D

D

Pt

Pe-


Calorimeter

11/17

e+

e-

PEPPo

Principle of Operation

PEPPo is proposing to measure the polarization transfer from longitudinal electrons to longitudinal positrons in the 2-5 MeV/c

momentum range.

o Longitudinal e- (Pe-) produce elliptical whose circular (P) component is proportional to

Pe-.

o P transfers to e+ into longitudinal (Pe+) and

transverse (Pt) polarization components. On the average

Pt=0.

Ie = 1 µAT1 = 1 mm


Compton Transmission Polarimetry

12/17

Integrated Method Semi-Integrated Method

PEPPo polarimetry relies on the sensitivity of the Compton process to the polarization of the photons generated in the T2 target by the interaction of

incoming electrons/positrons.

eTeT APPA

EE

EEAT

jj

jjjT

NN

NNA

Data acquisition based on the JLab FADC250 allows implementation of 2 data taking modes.

Expected experimental asymmetries are small (1-8х10-3) and we will take advantage of the current JLab practices

for the control of helicity correlated systematics.


Detector non-linearities, polarized background PMT gain, Crystal response, pile-up


Proposed Measurements

Electron Measurements Positron Measurements

p(e-)(MeV/c)

AT

(10-

3)

A(e-)(%)

Pe-/Pe-

(%)

2 1.0 1.6 3.4

3 3.5 5.9 3.0

4 6.1 10.4 3.0

5 7.1 12.0 3.0

6 7.8 13.2 3.0

p(e+)(MeV/c)

T1

(mm)

A(e+)(%)

AT

(10-3)

P(e+

)(%)

Pe+/Pe+

(%)

21.0

4.61.6 50 5.9

0.1 1.3 40 16.2

31.0

8.74.0 67 4.1

0.1 3.5 58 5.7

41.0

12.36.4 75 3.9

0.1 6.2 73 4.3

51.0

13.87.7 81 4.8

0.1 7.5 79 5.2

Up to 4 different production (T1) and 2 reconversion (T2) targets configurations will be measured. Background data will be acquired for each configuration. Measurements will be done for + and - analyzing magnet polarities.

%3/PδP%1.5/PδP%85P TTeee

1 h/e- setting

2 h/e+ setting


Use measured polarization of the e- beam to calibrate the

polarimeter.


Beam Time Request

p(e-)(MeV/c)

I(e-)P(e-)(%)

Time(h)

Commissioning 2 – 6 1 nA – 1 µA ≥ 85 105

Mott Measurements

6 1 – 5 µA ≥ 85 10

e- Calibration 2 – 630 pA – 10

nA≥ 85 85

DetectorMeasurements

6.8 1 – 4 µA ≥ 85 36

e+ Measurements

6.8 1 – 4 µA ≥ 85 100

Total 336

The PEPPo experiment is asking for 2 weeks of beam time, including the commissioning of the new beam line, detector calibration, and

measurements of the polarization transfer to positrons.

PEPPo operation requires the injector and the PSS north linac locked.


The Collaboration is in a great position to finish PEPPo installation before the end of the 6MSD.


Instrumentation Status

The management of JLab Accelerator Division (Letter from A. Hutton in appendix) and Collaborating Institutions supported the development of

the PEPPo project up to its today status.Polarimeter calorimeter and table

PEPPo branch

S1 Solenoid

DD Spectrometer

Analyzing magnet

DAQ, Power Supplies….


Our pathway on going forward depends on the recommendation of PAC38.

Towards a CEBAF Polarized Positron Beam

Beyond PEPPo…

16/17Newport News, August 22-26, 2011

PEPPo will demonstrate the principle of a polarized positron source based on the bremsstrahlung of low energy (5-100 MeV) polarized electrons.

PEPPo will provide quantitative information that will further our understanding of polarization transfer in the pair creation process, and that will help to optimize the design of a polarized positron source for JLab physics.

Information will also be provided for potential applications in a low energy source for solid state physics.

o A natural “next step” after PEPPo would study the optimization of a positron collection system and a scheme to define the phase space of the positron beam for CEBAF acceleration.

R&D efforts developed within the context of PhDs told that e+ currents up to 1 µA and polarizations larger than 50% may be expected.

S. Golge, PhD, ODU, Norfolk (2010) J. Dumas, PhD, UJF, Grenoble (2011)

Details of future efforts will be guided by JLab management and the accelerator R&D required to complete a full design.

Summary

Proposal PR12-11-105

There is a strong interest from the User Community to use positrons for nuclear physics

experiments. The PEPPo Collaboration gathers a group of physicists willing to develop a polarized positron source.

The PEPPo experiment proposes to measure the polarization transfer from longitudinally polarized electrons to longitudinally polarized positrons as a proof-of-principle for a new technique for polarized positron source.


2 weeks of beam data taking at the injector are requested to complete PEPPo.

Possible customers : JLab12, SuperB, EIC, ENC, ILC(?), Solid State Physics…


Proposal PR12-11-105

Why PEPPo ?


Polarized Electron Source

200 kV Inverted Gun + SSL GaAs/GaAsP + RF-Fiber Laser 4 mA

Test Parameter Value

Laser Rep Rate 1500 MHz

Laser Pulselength 50 ps

Laser Wavelength 780 nm

Laser Spot Size 350 µm FWHM

High Polarization Photocathode

SSL GaAs/GaAsP

Gun Voltage 200kV DC

CW Beam Current

4 mA

Run Duration 1.4 hr

Extracted Charge 20 C

1/e Charge Lifetime

85 C

R. Suleiman et al., PAC’11, New York (NJ, USA), March 28 – April 1, 2011

QE(q) = QE0 * e–(q / 84.8)

• High QE ~ 1.5% ( ~6 mA/W/%).

• Current-limited by available laser power.

• Higher 200kV voltage => supersede 1mA demo (PAC’07).

• Pushes technology in support of Electron Ion Colliders > 50 mA, High-P e- Drivers.

Annihilation Detector



Signal integral

Co60

Signal integral

Na22

Two NaI detectors are used to measure the back-to-back photons emitted by the annihilation of positrons in an insertable target.

Radioactive source measurements with the PEPPo DAQ system have been worked out to determine optimum operating conditions and set a

coincidence trigger.

Fiber Array Detector



Detector (FIU) used for the TPE experiment

The beam profiler at T2 consists of a 2 layers fiber array detector read by a multi-anode PMT.

The fibers are currently changed to recover a linear detector

response.

90Sr response profile of the present fiber array showing radiation damages after the TPE run.

Polarimeter Calorimeter



The original E166 calorimeter has been modified for PMT readout for an optimum energy resolution.

New measurements of the energy resolution with the PEPPo DAQ are on-

going.

e+ Beam Concept

Geometric Emittance < 5 mm-mrad Absolute Energy Spread < 1 MeV

Beam Current > 50 nA Bunch Length < 2 ps Duty Factor = 100 % Frequency = 1497 MHz

Beyond PEPPo…


e+ Chicane

Accelerator magnetsMost magnet power supplies are reversible except the arc dipoles which requires a manual action.

Beam diagnosticsBeam position monitors and viewers will work as long as the e+ current is ≥ 50 nA.

Beam modesThe tune mode based on a pulsed beam about tens of µA and 250 µs long and used for beam steering will need to be redefined because of the small e+ current.

RF systemAll passes in the linac are adjusted in phase with each other via the adjustement of their pathlength with the arc dogleg sections. The diagnostic that measures the phase difference between passes require an upgrade to the system.

A. Freyberger, Proc. of the International Workshop on Positrons at Jefferson Lab, Newport News (VA, USA), March 25-27, 2009

Beyond PEPPo…


PAC38, Newport News, August 22-26, 2011

Documents

polarized beam

polarized positronsa

polarized positron source

jlab energies

gpds hq

experimental observables

na e beam

usanewport news