Polarized positrons with the E-166 Experiment

Size: px

Start display at page:

Download "Polarized positrons with the E-166 Experiment"

Dominick Patrick
5 years ago
Views:

1 Polarized positrons with the E-166 Experiment Ralph Dollan Humboldt University, Berlin On behalf of the E-166 collaboration

2 Outline The goal of E-166 The helical undulator Photon transmission polarimetry The E-166 setup Data taking First results on photon and positron asymmetries 15/11/05 R. Dollan 2

3 E-166 Demonstration experiment to proof the possibility, to produce polarized positrons using a helical undulator Collaboration of >50 people from 17 Institutions from 3 continents In the final focus test beam (FFTB) at SLAC with ~50 GeV (unpolarized) electrons 1 m long helical undulator produces circular polarized photons Conversion of photons to positrons in thin W-target Measurement of polarization of photons and positrons by Photon transmission method 15/11/05 R. Dollan 3

4 E-166 Demonstration experiment to proof the possibility, to produce polarized positrons using a helical undulator Collaboration of >50 people from 3 continents In the final focus test beam (FFTB) at SLAC with ~50 GeV (unpolarized) electrons 1 m long helical undulator produces circular polarized photons Conversion of photons to positrons in thin W-target Measurement of polarization of photons and positrons by Photon transmission method ~50 GeV e - Undulator D1 1 m Energy spectrum e - Target e - Dump D2 e - D3 e - Dump γ e + Gamma polarimetry Positron polarimetry 15/11/05 R. Dollan 4

x e- beam y Helical winding: z component of the induced magnetic field

5 Helical winding where I 1 and I 2 are in opposite directions. I 1 I 1 = - I 2 The helical undulator Undulator photons I 2 I 1 z I 2 x e- beam y Helical winding: z component of the induced magnetic field cancels remaining magnetic field describes a helical profile 15/11/05 R. Dollan 5

6 Undulator parameters r u wound left handed λ u Parameter Period λ u On axis field Value 2.4mm 0.76 T K factor 0.17 E 0 =ωh (Energy cut-off 1 st harmonic) Feeding current Rate Heating/pulse r u Undulator aperture 9.6 MeV (50GeV e- beam) ~ 2 ka up to 30 Hz ~3 degc 0.88 mm K - factor (Undulator strength) K E 0 = eh λ u 93.4 H [ T ] λ [ m ] 2 u 2πmc The average photon polarization depends on the angular photon selection (K factor) and also on the quality of the photon collimation (before the conversion target). First harmonic Energy cut-off 2 hc λ γ 2 u Photon intensity - inverse proportional to the undulator aperture. 15/11/05 R. Dollan 6

7 Photon Energy and Polarization 1 st Harmonic 2 nd Harmonic 1 st Harmonic 2 nd Harmonic Undulator Photon energy spectrum Undulator Photon degree of polarization 15/11/05 R. Dollan 7

8 The Positron production target 0.5 X0 W (Tungsten) -> E166 X0 W (Tungsten) = 3.5 mm Polarized γ beam From the Helical Undulator e+ e+ e- e- e- e+ e- e+ Polarization transfer in e+ e- pair creation Energy spectrum Positron Polarization profile created by the undulator photons (creation point) e+ Energy distribution (in and out the 0.5 X0 W target) 15/11/05 R. Dollan 8

9 Production efficiency Escape length d 10 e+ Absorbed: W e+ Absorbed: Ti N e +/N γ [%] 1 e+ produced: W e+ produced: Ti 0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0 1,1 Target Thickness [X0] Positron production efficiency (positron yield) N (e+) / N (gamma) e+, z distribution (in the W target) For different target thickness 15/11/05 R. Dollan 9

10 Expected Polarization Expected positron polarization vs. positron energy 15/11/05 R. Dollan 10

Photon transmission polarimetry Transmission: T ( L) = e nl ( σ phot+ σ pair+ σcomp0 ) ± nlpγ Pe σ pol e Y Analyzing magnet γ B Analyzing magnet Y γ B Sig(+) counter Sig(-) counter

11 Photon transmission polarimetry Transmission: T ( L) = e nl ( σ phot+ σ pair+ σcomp0 ) ± nlpγ Pe σ pol e Y Analyzing magnet γ B Analyzing magnet Y γ B Sig(+) counter Sig(-) counter Asymmetry: δ ( L) T T + T + T = + nlp e P σ By knowing Pe => Pγ can be calculated: P γ δ = nlσ Asym pol = P e δ = A P γ e γ Pol E166 measures Sig( ) Sig( + ) Sig( ) + Sig( + ) 15/11/05 R. Dollan 11

12 Positron Analyzing Power Positron Energy E e + (MeV) Positron Polarisation P e+ (%) Positron Asymmetry δ (%) Analyzing Power A e+ (%) Expected asymmetries and analyzing power versus positron energy G3 simulation based on the experimental setup of the proposal V. Gharibyan Most challenging task for E166 was to measure asymmetries 1% in the CsI - Calorimeter 15/11/05 R. Dollan 12

13 Measure the asymmetries Positron Polarimetry is similar to the photon Polarimetry. In a reconversion target the positrons are reconverted via Bremsstahlung and anihilation into photons. Reconversion target Analyzing magnet counter e+ γ B Asym = Sig( ) Sig( ) + Sig( + ) Sig( + ) The asymmetry is measured by flipping the magnet polarity. 15/11/05 R. Dollan 13

14 E-166 in the FFTB running parameters: beam energy: 46.6 GeV rep. Rate: 10 Hz N e- /pulse: ~ /11/05 R. Dollan 14

15 TOP VIEW Gamma Analyzing magnet E166 setup in the FFTB e+ Analyzing magnet helical undulator Gamma Table Positron Table collimators photons diag SIDE VIEW Positrons diag photons collimation ~30 m Polarized photons production Gamma Table Positron Table Undulator e- beam Dump magnets 15/11/05 R. Dollan 15

16 E166 setup in the FFTB Undulator table Bending magnets Positron table Gamma table 15/11/05 R. Dollan 16

Laihem Vacuum chamber e+ e+ e+ ReConversion target Analyzing

17 The spectrometer Polarized Photons Undulator Conversion target Polarized Photons SiW Calorimeter Analyzing magnet K. Laihem Vacuum chamber e+ e+ e+ ReConversion target Analyzing magnet Solenoid e+ e+ CsI Conversion target R. Poeschl 15/11/05 R. Dollan 17

18 Pulse Generator The Undulator setup Cooling system Undulator 15/11/05 R. Dollan 18

19 Setup Bending Magnets Solenoid Analyzing Magnet Helical Undulator 15/11/05 R. Dollan 19

20 Setup 15/11/05 R. Dollan 20

21 The CsI-Calorimeter Calorimeter 3x3 CsI crystals in a brass housing 15/11/05 R. Dollan 21

22 The CsI-Calorimeter Calorimeter Photo diodes every crystal is read out by 2 Si-PM s we are reading analog signals 15/11/05 R. Dollan 22

23 Data taking Original plan: two running periods in October 2004 and January 2005 Accident at SLAC -> delay June 2005: first run of E-166 September 2005: second run 15/11/05 R. Dollan 23

24 Data taking Original plan: two running periods in October 2004 and January 2005 Accident at SLAC -> delay June 2005: first run of E-166 September 2005: second run Data taking scheme: Beam energy 46.6 GeV 10 Hz beam Undulator at 10 Hz Every 2 nd pulse undulator off time -> undulator on -event followed by undulator off -event 15/11/05 R. Dollan 24

25 Data taking Signal : Undulator on/off No beam 15/11/05 R. Dollan 25

26 Collected positron data Spectrometer set for No. of beam pulses collected 5.6 MeV 2.0 * MeV 3.1 * MeV 1.2 * MeV 1.2 * MeV 1.2 * MeV 1.0 *10 6 Combined June- and September run 15/11/05 R. Dollan 26

27 How we obtain the asymmetries substract backgroundfrom signalevents average over certain bg-range test statistical methods with toy-monte carlo calculate the asymmetry between the two magnetization states 15/11/05 R. Dollan 27

28 The signals The signals after substracting the background for different methods A. Schälicke 15/11/05 R. Dollan 28

29 Photon Asymmetries preliminary Photon asymmetries from June data measured with 2 Detectors: W. Bugg Photon Calorimeter : 3.52 % ± 0.15 % Aerogel Counter : 3.50 % ± 0.40 % (stat. errors only) 15/11/05 R. Dollan 29

30 Photon Asymmetries preliminary Photon asymmetries from June data measured with 2 Detectors: W. Bugg Photon Calorimeter : 3.52 % ± 0.15 % Aerogel Counter : 3.50 % ± 0.40 % (stat. errors only) Expected photon asymmetries for 5 MeV eff. threshold: Beam Energy [GeV] Aerogel AG2 W-Si Cal. GCAL *) (G3 Simulation) *) energy weighted with calorimeter response function V. Gharibyan 15/11/05 R. Dollan 30

31 Positron Asymmetries preliminary Spectrometer Current [A] Positron Energy [MeV] Measured Asymmetry δ (%) Asymmetry error (stat. only) A. Schälicke (stat. errors only) 15/11/05 R. Dollan 31

32 Positron Asymmetries preliminary (stat. errors only) 15/11/05 R. Dollan 32

33 Summary E-166 was running and produced data with good quality The helical undulator was working We did a first analysis of the data and the asymmetries are in the expected range It still takes some time to come up with a number for the photon and positron polarization More simulation work has to be done The data analysis is ongoing 15/11/05 R. Dollan 33

A Test Experiment for a Polarized Positron Source - E-166 at SLAC

A Test Experiment for a Polarized Positron Source - E-166 at SLAC Ralph Dollan Humboldt University, Berlin Outline Why polarized beams at the ILC The goal of E-166 The helical undulator Positron production