A Pure Photon Source for use with Solid Polarized Targets Progress Report UVa Option

Transcription

1 A Pure Photon Source for use with Solid Polarized Targets Progress Report UVa Option Donal Day, Dustin Keller, Darshana Perera, Jixie Zhang and friends NPS Collaboration Meeting January 19, 2017 Jefferson Lab 1

2 Outline Background Real Photons on a Polarized Target - the Physics Dynamically Polarized Targets and their behavior with electrons Bit of History Concept is simple Not so easy to achieve in practice Benefits and FOM multiples with a pure photon source Details Work planned 2

3 Wide Angle Compton Scattering One of the most fundamental processes yet it is still not well understood at medium energy For wide angle kinematics (s,-t,-u >> M 2 ) there is consequential untapped information on nucleon structure WACS provides complimentary information to elastic FF at high Q 2 and DVCS, TCS, DDVCS, DVMP Common thread: large energy scale leading to factorization of scattering amplitude into a hard perturbative kernel and a factor expressing soft nonperturbative WF Polarized observables can provide access to information not otherwise available 3

4 WACS Polarization Observables K LL (θcm=120) - HB, CQM,SCET, Miller - YES, pqcd - NO K LL (θcm=70) - CQM,SCET,HB,pQCD - NO Status Relation between K LL and A LL pqcd: K LL = A LL but < 0 HB: K LL = A LL SCET: K LL = A LL CQM: K LL!= A LL at large angles K LS small and > 0 HB: K LS = -A LS pqcd: K LS = A LS = 0 CQM: K LS = A LS = 0 HB, pqcd, SCWT, CQM all have predictions for s-dependence and θ-dependence What if: K LL = A LL ; HB/SCET on track; we provide constraints on GPDs, and data need to refine theory K LL and A LL about equal Kroll: learn about helicity flip Kiev (SCET): learn about power corrections K LL!= A LL SCET gets a reset, HB (Kroll) can be interpreted in terms of helicity flip This is what we will learn in PR

5 Bit of history 1. Initial State Helicity Correlation in Wide Angle Compton Scattering, PR , deferred with regret by PAC 27 - mixed electron/photon beam. 2. E approved for 14 days by PAC 28 (A - ) - mixed electron/photon beam. 3. Pure photon source (radiator, dipole, dump) proposed at Jan Hall C meeting. 4. E was planned to run with SANE (2009) but did not - lack of beam time. 5. E (2014) approved at PAC 42 for 15 days (B) - essentially a resubmission of E05-101, with a mixed electron/photon beam 6. 2kW pure photon beam (single dipole and dump upstream of target) presented at at NPS meeting on 10/9/ Polarization Observables in Wide-Angle Compton Scattering at Photon Energies up to 8 GeV, PR , deferred by PAC 43, 10 kw Compact Photon Source introduced. 8. Longitudinal and Transverse Target Correlation Asymmetries in Wide Angle Compton Scattering, PR , deferred by PAC kW pure photon beam with downstream beam dump presented. 9. Both PAC 43 and PAC 44 strongly encouraged a creation of a single collaboration to present optimized photon source and experiment. 5

6 At Hall C Workshop January 7, 2006 Radiator 1 μa on radiator instead of 100 na Magnet(s) Polarized target Beam dump on floor Existing beam dump Also at joint 2010 Hall A/Hall C workshop on High Intensity Polarized Targets at 12 GeV 6

7 WACS with Polarized Target Solid polarized proton target, NH 3 4 He evaporation refrigerator 5 T polarizing field Dynamic Nuclear Polarization Mixed photon/electron beam Polarization (%) Radiation Damage Target Proton High Momentum Spectrometer (10 15 e - /cm 2 ) 5x10 15 e - /cm 2 = 9 hours at 100 na Significant Overhead Radiator 2005 (2014): 88/506 = 17% (16%) Beam Photon Electron BigCal Calorimeter Beam Dump Change targets (top/bottom) every 8 hours Polarize Anneal, Cooldown, TE, Polarize [2 times/day - 2 hours] Replace target material at least once/week: Warm up, pull stick, replace material, take TE s top and bottom, polarize!< 8 hours 7

8 Benefits of a real photon beam Heat load on target reduced Higher maximum target polarization with beam: 90% ==> 95% Production of depolarizing free radicals much reduced Higher average polarization FOM = (90/70) 2 = 1.65 Fewer target changes Significant reduction in target overhead, Higher electron intensity on radiator - more photons - factor of 18 Reduced running time Push to higher energies where σ are << Overall improvement in FOM by a factor of 30 Reduced electron background, no ep and no epγ Transverse running trivial - no chicane required New Physics TCS Photo-disintegration of tensor polarized deuteron 8

9 Pure Photon Source Separated function dipole and dump October 2014 Compact Photon Source Combined Function dipole/dump A beam dump at the target only if there are absolutely no other possible choices! B ~ 2.5T e - PR , June 2015 Target hangs on a platform above the pivot post 9

10 Experiment Setup: PR NPS 1) Use Pure Photon Beam Dipole Target 2) 10% radiator 3) 3 ua beam current Dipole Dipole Dipole Hall C Dump 4) FZ dipole(s) chicane HMS 5) Electron beam goes under the target chamber, then either drift to a local dump in the hall or transported to the standard Hall C dump. Dipole Target Dipole Dipole Dipole Hall C Dump Pivot Post Local Dump 10

11 Setup Floor in vertical: mm Dump: Z = mm Radiator: Z= mm Dipole Center : Z = mm Post and target Z = 0 mm Magnet container length : 2320 mm FZ Magnet : 2m (Bdl~4)

12 Setup Dipole (tilted at 5 o, ends 4.3m upstream of target. Large space available to shield dipole, radiator, collimator, beam pipe - no possibility of charged particles to be transported by HMS No disruptive magnetic forces on target. Opportunity for radiation exposure minimized. Distance and shielding minimizes singles background in NPS. Hot spot at collimator/absorber at end of dipole. Experience from PREX instructive Hermetic local dump can be made with as many meters of material as necessary - no space restrictions. Sliding door when access to hall necessary. ALARA in command

13 Electron energy spread out of radiator is broad Electrons < ~6000 MeV will intercept a spacer bar in the dipole magnet. Low energy electrons produced by pair production kw/ua

14 XY distribution at dipole exit - no collimator/absorber If this beard of electrons were allowed to drift to the dump it would create a 10 m long hot spot. Plan is to absorb them at the end of the dipole using a combined photon collimator and electron absorber.

15 Photon Collimator/ Electron Absorber In progress Zoom to photon collimator W L =5cm Radius 2 = 1 mm Radius 1 = 5 mm WNiCu Alloy L = 10 cm 60 cm Height 2= 1.5 cm Height 1 = 2 cm 15 cm 50 cm in width

16 Just before collimator; collimator in place The square shape at the beginning is due to the shape of our slit (rectangular hole in collimator), backscattered 16

17 Just after collimator The square shape at the beginning is due to the shape of our slit (rectangular hole in collimator), forward going. 17

18 XY distribution at local dump: no collimator/absorber Vertical spread about 100 cm, rms 3 cm x(mm) y(mm) 18

19 XY distribution at local dump: with collimator Spread reduced to about 10 cm, rms 1.5 cm 19

20 Fluka Work Cylinder core: R=5cm, L=20cm (HD17) Dump box: 30cm x 30cm x 40cm (Lead) The core is aligned to the back face of the dump, therefore there is 20 cm entrance space inside lead box Concrete shielding box: 2m x 2m x 2m, with entrance tunnel - 80 cm Pivot cm Activated Dose Rate After 1-hour Decay Pivot 1400 cm cm, upstream 75cm, upstream 100cm, upstream 50cm, sideways 75cm, sideways 100cm, sideways 4 weeks of 3uA 8.8 GeV 4 weeks of 3uA 8.8 GeV Equivalent Dose Rate (mrem/h) no shielding upstream of entrance 4 weeks of 3uA 8.8 GeV Distance To Core (cm) 20

21 Fluka Work Cylinder core: R=5cm, L=20cm (HD17) Dump box: 30cm x 30cm x 40cm (Lead) The core is aligned to the back face of the dump, therefore there is 20 cm entrance space inside lead box Concrete shielding box: 2m x 2m x 2m, with entrance tunnel - 80 cm Pivot cm 4 weeks of 3uA 8.8 GeV Pivot 1400 cm Activated Dose Rate After 1-hour Decay cm, upstream 100cm with 50cm door, upstream 100 Equivalent Dose Rate (mrem/h) weeks of 3uA 8.8 GeV cm concrete door in front of tunnel Distance To Core (cm) 21

22 Pure Photon Source Performance Beam Energy (GeV) Beam Current (ua) Radiator Distance Flux Lost (%) gamma/s % E % E % E % E % E % E % E % E < Eγ/Beam < 0.95, requiring the spot size on target within a 2mm radius circle. Past use of PT: 100nA, 0.36 W are deposited in target: 10 times more than from the photon flux generated by 1 μa on a 10% radiator! If cooling power was only issue we could put 8-10 μa on radiator to illuminate the PT: target would operate normally. 22

23 Power Deposition Ebeam (MeV) Radiator length(%) Power deposit 1uA beam (Watts) Electron experiments use 100nA and 0.36 W are deposited in target: an order of magnitude more than from the photon flux generated by 1 μa on a 10% radiator. If cooling power was only issue we could put 8-10 μa on radiator to illuminate the PT: target would operate normally. 23

24 Work in progress Fluka & Geant4/G4Beamline Shielding around absorber/collimator guided by PREX/CREX Incorporate dipole into Fluka, dump into Geant4 Alternative shielding configurations for the dump Brindza shielding; borated and plasticized concrete 1 m concrete plus plastic around local dump Modify dump core following JLAB tuneup dump (120kW) design Hope to have both Geant4 and Fluka models at some point Other tasks as exposed by discovery 24

25 Work in progress Shorter, higher field dipole super ferric? superconducting? allows dipole to move closer to pivot Fewer photons lost in collimation Optimize location of 2T dipole - could be moved closer Optimize tradeoff between radiator thickness, photon yield and radiation at collimator/absorber and local dump Other tasks as exposed by discovery 25

26 Photon Flux/μA 26

27 Spot size on target: +/- 2 mm to insure NPS resolution is fully exploited. Configuration Beam(GeV) Current(μA) Distance(cm) Photon/s PR E+11 27

28 Raster Over Face of Target Incident electron not subject to large scale raster Full and uniform irradiation of target still needed Rotation already implemented at UVa though for different reasons. Photon spot fixed in space, target cell moves up and down with rotation 28