High energy gamma production: analysis of LAL 4-mirror cavity data

Transcription

1 High energy gamma production: analysis of LAL 4-mirror cavity data Iryna Chaikovska LAL, Orsay POSIPOL 211, August, 28 1

2 Experiment layout Electron energy Electron charge Revolution period Electron bunch length 1.28 GeV ~1.6 nc 463 ns 25 ps Bunch spacing 5.6 ns Simulated gammas spectrum LASER photon energy LASER frequency Power stored in cavity Crossing angle LASER pulse length 1.2 ev MHz ~ 2 W 8 deg. 2 ps The collimators only accept photons with an energy above ~15 MeV 2

3 Detector to measure gamma rays Two pieces of BaF2 1mm x 7mm x 7 mm each 2 mm x 7 mm x 7mm All surfaces are polished Wrapping: absorbing tape and 3M scotch super88 for light shielding UV - filter (2-24 nm) to rid off the slow component of scintillation PMT: Hamamatsu Photonics R3377 (rise time.7 ns) Developed by Hiroshima university Material: BaF2 Geant4 simulation 3

4 Location where the optical photons are produced inside scintillator The high energy Compton gammas interact along the crystal while some of them pass the calorimeter without interacting..98 of impinging gammas interact in the 2 cm BaF2 crystal. Most of the optical photons are produced at the beginning of the scintillator. 4

5 Contribution of the reflections to the signal collected red: opt photons reaching PMT directly black: ALL opt photons reaching PMT Photons generated at the beginning of the crystal have smaller probability to reach the PMT. 11% of the photons reaching the PMT undergo at least one reflection in the crystal. 5

6 Summary of data taking During the data taking we usually save the waveforms of the Compton signal, the transmitted power from the FP cavity, the 357 MHz clock and the BPMs data, injection trigger. Data acquisition was performed using LeCroy WS454 scope (1Gs/s, 5MHz bandwidth). All information such as the type of the data, names of the files, the date and time are sent to a DataBase. MatLab/Octave framework is used to process the data. After processing, the results are saved in the DataBase for further analysis. Data was acquired parasitically during ATF runs (1 train/1 bunch, 2 trains/1 bunch, 3 trains/1 bunch, 1 train/ 6 bunches) 6

7 Data analysis approach The 357 MHz clock is used to define the beginning of the periods in time during which the collisions occurred. So the length of the period is naturally 924 ns (two full turns of ATF DR). All periods belonging to one data file (usually.2 ms) are stacked to find the timing of the peak and number of the peaks (multi-train operation). Gating of signal is applied for each period. The length of the gate was chosen to be 12 ns. 7

8 Data analysis approach The background level and rms are calculated within each period to be subtracted after from the peak height. The peak height and peak integral are calculated. 8

9 Data analysis. Laser power. Peak maximum [V] peak max laser power x Laser power Photodiode signal is noisy (limited dynamic range). 1 2 Time [s] x 1 4 Smoothing with time constant comparable to the FP cavity time constant (tens of μs) needed. Strong correlations between laser power and peak height are observed. 9

10 Data analysis. Quality of the data. To increase the purity of the sample analyzed fiducial cuts are applied: Correct phase of peak max wrt ATF clock (6 ns after beginning of the gate). Shape of the electronic pulse (relation between the peak max and peak integral). Correct shape of the signal. By putting constraints on them, we reject noise and improve the quality of the data. Entries Position of the peak max Integral of the peak [V s] x Peak maximum [V] Peak maximum [V] 1 x B A Samples C 1

11 462 ns & random peaks background In some files there are peaks spaced by 462 ns (DR revolution period) This is not possible because a bunch can not interact with laser on two consecutive turns. Such background is rejected. In some files there are peaks spaced randomly. The source of such a background is unclear. To ensure high purity data, such background is rejected. 11

12 Calibration layout PMT 1 PMT 3 PMT 2 Calibration is performed using cosmic rays. Two plastic scintillators are used to trigger the data acquisition. FAST LAL 12

13 Calibration using cosmic rays BaF2 calorimeter calibration (preliminary) - integrated value snr snr 1 8 Entries 5 Mean 5.861e-12 RMS 2.992e-12! / ndf / 18 Width 1.253e-13 ± 5.928e-14 MP 5.154e-12 ± 1.733e-13 Area 4.735e-1 ± 2.168e-11 GSigma 1.659e-12 ± 9.661e-14 2 BaF2 calorimeter calibration (preliminary) - peak value 5 4 Entries 5 Mean.125 RMS.5848! / ndf / 24 Width 2.38e-5 ± 2.58e-4 MP.1146 ± Area.5124 ± GSigma.4382 ± snr " The most probable value for the integrated signal is 5.2 mv.ns and most probable value for the peak is 1.1 mv Assuming minimum ionization, the energy deposition in BaF2 is MeV/cm. The values above correspond to approximately 45 MeV of energy deposited. However, this needs to take into account the geometry and the acceptance of the system. 13

14 Calibration factors Factor 1 A = all opt photons in BaF2 B = opt photons within PMT acceptance PMT 1 PMT 3 PMT 2 F1 = A/B = 39 Factor 2 C = all opt photons in BaF2 within calibration acceptance D = opt photons within calibration acceptance & PMT acceptance Egamma [MeV] = Vgamma[mV] Ecosm[MeV] / Vcosmic[mV])(F1/F2) F2 = C/D = 47 According to calibration, 1 mv peak value on the scope equals to 34 MeV of energy deposited and 1mV.ns integrated value corresponds to 7 MeV. Thus, 24 MeV gammas deposit 3.4 mv.ns with a peak of.7 mv. 14

15 7 Peak Height HQ Gamma spectrum 5 Peak Height normalized HQ 1.23mV dn di dn di Peak height [V] x 1 3 Peak Integral HQ Peak height [a.u.] Peak Integral normalized HQ V s dn di dn di Peak integral * 1e1 [V s] Peak integral normalized * 1e8 [a.u.]

16 Gamma spectrum. Laser power binning. 1.1 dn dpl mV dn di % of max LP, 1.38 mv 25% of max LP, 1.47 mv 31% of max LP, 1.81 mv 42% of max LP, 2.12 mv 49% of max LP, 2.67 mv 54% of max LP, 2.86 mv 59% of max LP, 3.19 mv 62% of max LP, 3.2 mv 1% of max LP, 3.21 mv Laser power normalized Peak height [mv] Since the power stored in the cavity is oscillating over a wide range, we have to study the gamma production within small laser power bins. The data have been split into nine bins of apprx 1 events each. With increasing laser power the mean of energy deposition distribution goes linearly towards higher values. 16

17 Data analysis. Highest integrated flux. One train. Voltage [V] Integrated flux over.2 ms Voltage stacking [V] Samples Time [s] x 1 4 Φint.2ms = V 3362 MeV Assuming Egamma = 24 MeV, 1265 gammas have been produced over.2 ms gammas * 1/.2 ms 6.3e6 gammas/s Taking into account Duty cycle of ATF 48ms/641ms (D =.75) we can extrapolate: gammas * 1/.2 ms * e6 gammas/s Peak maximum [V] Samples

18 Voltage [V] Data analysis. Highest integrated flux. Two trains Integrated flux over.2 ms Time [s] x 1 4 Φint.2ms = -.91 V 394 MeV Assuming Egamma = 24 MeV, 1289 gammas have been produced over.2 ms gammas * 1/.2 ms 6.4e6 gammas/s Taking into account Duty cycle of ATF 48ms/641ms (D =.75) we can extrapolate: 1289 gammas * 1/.2 ms * e6 gammas/s 18 Voltage stacking [V] Peak maximum [V] Samples x Samples

19 Data analysis. Highest integrated flux. Three trains. Voltage [V] Integrated flux over.2 ms Time [s] x 1 4 Φint.2ms = -1.8 V MeV Assuming Egamma = 24 MeV, 1428 gammas have been produced over.2 ms gammas * 1/.2 ms 7.1e6 gammas/s Taking into account Duty cycle of ATF 48ms/641ms (D =.75) we can extrapolate: 1428 gammas * 1/.2 ms * e6 gammas/s 19 Voltage stacking [V] Peak maximum [V] Samples x Iryna Chaikovska 1 LAL Orsay 15 Samples

20 Best integrated flux Data analysis. Best results. Electron pulse structure Integrated flux over.2 ms Integrated flux over 1 s (extrapolated) 1 train 1265 γ 6.3E+6 γ 2 trains 1289 γ 6.4E+6 γ 3 trains 1428 γ 7.1E+6 γ Best instantaneous flux Peak height = -.34 V (1156 MeV, 48 gammas) In average, approximately 4 gammas are produced per bunch crossing. As the repetition frequency of the collisions is about 1 MHz the flux of gamma rays achieved so far is ~4e6 gammas per second. 2

21 Longitudinal dynamics simulations. Energy spread [%] with Compton, 1 MW with Compton, 1 kw w/o Compton Bunch length [ps] with Compton, 1 MW with Compton, 1 kw w/o Compton Number of turns x Number of turns x 1 5 Property Initial Equilibr w/o Compton Equilibr w Compton (1 kw / 1 MW) ATF data Energy spread.34%.55%.6% /.1%.56% Bunch length 25 ps 18.4 ps 22 ps / 34 ps 18.7 ps Simulation show that for laser power below 1 kw, the effects of Compton scattering at the ATF DR are rather small which makes them difficult to observe. For high enough laser power bunch lengthening, increasing of the energy spread and electrons loss rate is significantly increased 21

22 Summary The Mightylaser project has demonstrated the production of polarized gamma rays using a non planar 4-mirror Fabry-Perot cavity. First gamma productions have been recorded and analyzed. In average, approximately 4 gammas are produced per bunch crossing. Best results: integrated flux of 1428 γ over.2 ms in 3-trains regime and instantaneous flux of 48 γ per shot were measured. These values are limited by the laser power achieved. Work in progress to improve laser stability and increase the cavity finesse. Simulation of the longitudinal dynamics of electron beam at the ATF DR in presence of Compton scattering shows that the effect of Compton scattering is quite small at ~1 kw of laser power and for high enough laser power (~ 1 MW) it becomes more important. 22