1. Tests of AWAKE spectrometer screen and camera at PHIN Introduction Layout Procedure Setup, results (runs 1 – 5) Conclusions L. Deacon, S. Mazzoni, B. Biskup et. al. AWAKE tech. board meeting, Wednesday 19 August 2015

2. Introduction The AWAKE electron spectrometer will use a scintillator screen to detect the positions of the accelerated electrons after the dipole field. The screen will be imaged by a intensified CCD camera placed 17 metres away for radiation protection. We used a 5.5 MeV test beam at PHIN [1] at the CLIC Test Facility in order to test the screen output and camera sensitivity.

3. 3 Screen holder 8” square protected Al mirror 91.4% unpol. Reflectivity @ 550 nm Flatness 5 @ 632.8 nm Layout (1)

4. 4 Layout (2) Intensified camera 17 metres from edge of support table

5. 5 Layout (3) 300 mm f/4 NIKKOR camera lens Magnification: 0.0171 Field of view: 1480+/- 10 mm (0.79 mm/pix)

6. Scintillator screens Screen samples supplied by Applied Scintillation Technologies (now Scintacor), UK Phosphor = P43 (GOS:Tb, Gadox, Lanex) All screens are 0.2mm plastic backing coated with phosphor layers of varying thicknesses and phosphor grain sizes We measured the thicknesses. Screen 1: “Medex Portal” – phosphor thickness = 0.85 +/- 0.03 mm, particle size = 25 micron Screen 2 “Medium” – 0.55 +/- 0.01, particle size = 6 micron Screen 3 “HB” - 0.546 +/- 0.006, particle size = ? – need to ask supplier Screen 4 “HE” – 0.510 +/- 0.004, particle size = 15 micron

7. Procedure The screens were installed at either 45 degrees or 90 degrees to the beam line. The bunch charge was varied by either attenuating the laser or changing the length of the bunch change (range from 50 ns to ~ ms) The signal was recorded with the camera, using the appropriate gain setting to get a good peak signal (~10000 counts per pixel) if possible Noise ~ 520 counts per pixel

8. Setup – run1 Used screen 1 (thickest screen) at 90 degrees to beam. Copper photocathode (lower charge range). Camera gate width: 1 ms (phosphor decay width ~ 1ms) Camera gate delay: 0

9. First test 100 images taken Charge = 290 +/- 90 pC RMS width ~ 10mm Charge dens ~ 0.9 pC/mm2 Gain ~200 (1775 V) Peak S/N ~ 40

10. First test – very rough comparison with AWAKE spectrometer beam AWAKE spectrometer possible beam size ~ 100mmX2mm= 2000 mm2 0.06nC per bunch So AWAKE spectrometer charge density ~ 0.03 pC/mm2 = 30 times less However, the gain of the camera can be increased by factor 75.

11. Further tests 11 further tests performed, with increasing charge Here: charge = 12.1 +/- 0.5 pC Sigma ~ 12mm Gain ~200 (1775 V)

12. Data analysis procedure For each charge setting: –100 images were taken. –The number of counts was summed over each screen image, and the mean and standard deviation were found. –The mean background was subtracted. –Camera gain normalization was applied. –The signal and error were plotted vs. the charge measurement and error.

13. Data analysis procedure The following function was fit to the data set by minimizing chi2: Where Sn is the normalised signal (y-axis), Q is the total charge hitting the screen, k is a constant and B is a constant. The equation becomes non linear with increasing charge (screen saturation). [2]

14. Run 1 – summary of results m=1.6 +/- 0.1 B= 0.009+/-0.006 k= 0.3+/-0.3 Χ 2 =1.7 NDF=8 Χ 2 /NDF=0.21

15. Setup – run 2 Same as run 1, except the screen was rotated to 45 degrees

16. Run 2 – summary of results m=2.0 +/- 0.2 B=0.026+/- 0.009 k=-0.3 +/- 0.3 Χ 2 =0.95 NDF=4 Χ 2 /NDF=0.24

17. Comparison: run 2 and run 1 With the screen at 45 degrees (run 2), the output is increased by a factor of 1.3 +/- 0.1 The non-linearity is also increased

18. Setup – run 3 In run 3, screen 1 was used again at 90 degrees. The bunch charge was increased further, using a different type of photocathode. We began saturating the camera. In subsequent tests, we reduced the gate width in order to be able to saturate the screen, not the camera

19. Setup – run 4 In run 4, screen 3 was used (“HB”), at 90 degrees to the beam. The camera gate width was set to 5 us Camera gate delay: 1us

20. Run 4 – summary of results M=(2.92 +/- 0.04)X10 -2 B=(8.7+/-0.3) X10 -3 k=(4.4+/-0.1) X10 -3 Χ 2 =120 NDF=12 Χ 2 /NDF=10.0

21. Setup – run 5 Screen was changed to screen 2, “medium”.

22. Run 5 – summary of results M=(2.72 +/- 0.02)X10 -2 B=(8.8+/-0.2) X10 -3 k=(4.9+/-0.1) X10 -3 Χ 2 =96 NDF=9 Χ 2 /NDF=11

23. Run 5 – summary of results M=(2.72 +/- 0.02)X10 -2 B=(8.8+/-0.2) X10 -3 k=(4.9+/-0.1) X10 -3 Χ 2 =96 NDF=9 Χ 2 /NDF=11

24. Run 5 – summary of results M=(2.72 +/- 0.02)X10 -2 B=(8.8+/-0.2) X10 -3 k=(4.9+/-0.1) X10 -3 Χ 2 =96 NDF=9 Χ 2 /NDF=11

25. Analysis still to be done We took data of the camera gate width vs. signal in order to determine the decay curve – to be plotted. Normalise run 4-5 WRT gate width for direct comparison with run 1-2 Calculate absolute photon output from screen.

26. Conclusions More analysis to be done. Screen 3 maybe slightly brighter than screen 2. For comparison with screen 1, need to analyse gate width vs output data because different gate widths were used. Preliminary results suggest good signal for AWAKE spectrometer. Good linearity at AWAKE charge densities. Output increased by factor for electrons incident at 45 degrees. Thanks to the PHIN team for their support and use of the the beam.

27. References [1]THE PHIN PHOTOINJECTOR FOR THE CTF3 DRIVE BEAM, R. Losito et. al., EPAC06 Edinburgh [2]THE PHIN PHOTOINJECTOR FOR THE CTF3 DRIVE BEAM, R. Losito et. al., EPAC06 Edinburgh A. Buck et al, Absolute charge calibration of scintillating screens for relativistic electron detection, Review of Scientific Instruments 81, 033301 (2010); doi: 10.1063/1.3310275