High Energy Photons at HI S

Transcription

1 High Energy Photons at HIS Rob Pywell High Intensity Gamma Source Duke University Thanks to Dr. Ying Wu, Duke University, for supplying some of the information in this talk.

2 Precision Photo-Reaction Measurements Goals: Test our understanding of the nucleonnucleon interaction. Test fundamental symmetries. Look for physics beyond the standard model. Requires: Precision absolute cross-section measurements

3 Current Experiments GDH: Direct measurement of the Gerasimov-Drell-Hearn (GDH) sum rule for the Deuteron. Based on very general principles: causality, unitarity, gauge and Lorentz invariance. de P A 2 t Imminent start of data taking ( E ) ( ) 2 E St E 0 M t Precision measurements of Deuteron Photodisintegration. Test basic theory of nucleon-nucleon interactions in the simplest nucleus. Precision measurements of 4 He photodisintegration. Resolve long-standing disagreements between earlier measurements. Test of nucleon-nucleon interaction models. Bethe-Heitler pair production asymmetry in γz e + e Z Fundamental test of QED prediction of energy asymmetry of electron positron pair. 2

4 High Intensity Gamma Source: HIS At the, Duke University Free Electron Laser Laboratory (DFELL) HIS has several advantages for these measurements. Monoenergetic photons (low E) High intensity (> 10 7 s 1 on target) Linear and circular polarization available Pulsed (micropulses every ~180 ns)

5 DFELL 160 MeV Linac pre-injector 160 MeV 1.2 GeV Booster injector 240 MeV 1.2 GeV Storage ring Revolution Freq MHz Circumference m Bunch length ps Mirror Mirror Typical Modes FEL: single-bunch, up to 95 ma HIGS: two-bunch, ma

6 Wiggler Switchyard Four helical wigglers: OK-5 Circularly polarized photons Two linear wigglers: OK-4 Linearly polarized photons Interchangeable

7 HIGS Operating Mode High-flux, quasi- CW operation Micro-pulses with sub-ns durations at 5.58MHz. (180 ns apart) Typical energy spread (FWHM): <5% Gamma-ray fluxes: ~10 9 s 1 Energy spread and flux depend on collimation of gamma-ray beam Smaller collimator better energy spread. Linear (<25 MeV) and circularly (<100 MeV) polarized photon beams

8 HIGS Operation HIGS operation movie

9 Circular Polarization HIGS Operation 60 m downstream E depends on f Collimation can reduce E C. Sun and Y. K. Wu, Phys. Rev. ST Accel. Beams 14, (2011)

10 Linear Polarization HIGS Operation 60 m downstream E depends on f Collimation can reduce E C. Sun and Y. K. Wu, Phys. Rev. ST Accel. Beams 14, (2011)

11 Energy Ranges Available. FEL: nm Gamma-ray: MeV

12 Energy Spread Example

13 Photon Beam Stability Pointing stability: 2.5 rad (peak-to-peak, 36 hr)

14 Photon Flux HIGS User Flux Capabilities with OK-5 FEL With New Mirrors

15 Neutrons Detected by Blowfish Example: 6 Li at E = 13 MeV Linear polarized photons

16 Photon Counting Precision cross section measurements require accurate knowledge of the number of photons hitting a target. This has notoriously been a significant source of error in photonuclear measurements. Photon tagging would in principle solve this problem.

17 Photon Counting However subtle effects involving random coincidences require careful analysis and simulation to correctly determine the incident photon flux. e.g. MAX IV: Nuclear Instruments and Methods in Physics Research A729(2013)

18 Photon Flux Monitor HIGS beam is not continuous. Pulsed at 5.58 MHz (180 ns between bunches) A direct counting photon detector with an efficiency known to better than 2% has been designed and commissioned. Low efficiency 1 2 % Very stable efficiency Insensitive to small changes in gain Wide energy range MeV Wide photon flux range Now in regular use at HIGS

19 Photon Flux Monitor 5 thin (~1 mm) scintillator paddles Detects recoil electrons and positrons from Compton scattering and pair production from a thin Al radiator. Described well with a GEANT4 simulation. Gains can be monitored by sampling paddle spectra. Photon Beam Scintillators Radiator Recoil e + or e Discriminators Monitor Output Veto

20 Photon Flux Monitor Data compared to GEANT4 simulation Coincidence of paddles 0, 1 and 2. Used for determining gain and threshold of paddle 1. Coincidence of paddles 2, 3 and 4 in anticoincidence with paddle 1. Black Measured Red Simulation Threshold

21 Photon Flux Monitor We do not rely on the simulation to predict efficiency. Inter-calibrated with a large NaI detector. Regularly during a measurement. N f Calib N Monitor Can determine f Calib to better than 2%. (Pywell et at. NIM A 606 (2009) 517)

22 Photon Flux Monitor There are only a few photons in each bunch (bunch rate 5.58 MHz) At high photon rates there is chance that more than one photon can trigger the Flux monitor but only one can be counted per bunch. A simple correction can be made using Poisson statistics and using the measured rates in veto paddle. Operation of the flux monitor has now been verified in several experiments.

23 HIGS2 Proposal Increased photon flux for precision experiments at low energies. A Prospectus Document for NSAC (Aug. 2012) HIGS2: The Next Generation Compton -ray Source, M. W. Ahmed, A. E. Champagne, C. R. Howell, W. M. Snow, R. P. Springer, Y. Wu

24 HIGS2 Version 2012 The photon beam: high power CW laser beam (10 to 20 kw average power) built-up inside a high-finesse Fabry-Perot resonator driven by an external infrared laser. 32 bunches (instead of 2) in ring MHz photon bunch rate.

25 HIGS2

26 HIGS2 Projection ELI-NP ELI-NP = Extreme Light Infrastructure Nuclear Physics (Romania)