Tests of spectrometer screens

•Introduction•Layout•Procedure•Results•Conclusions

L. Deacon, B. Biskup, S. Mazzoni, M.Wing et. al.

AWAKE collaboration meeting, Wednesday 30 September 2015

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.

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Screen holder

8” square protected Al mirror91.4% unpol. Reflectivity @ 550 nmFlatness 5 @ 632.8 nm

Layout (1)

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

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Layout (3)

• 300 mm f/4 NIKKOR camera lens• Magnification: 0.0171• Field of view: 1480+/- 10 mm (0.79

mm/pix)

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

different 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• Screen 4 “HE” – 0.510 +/- 0.004, particle size = 15 micron

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

Example image – first test – screen 1 at 90 degrees

• 100 images taken• Charge = 290 +/-

90 pC• RMS width ~

10mm• Charge dens ~ 0.9

pC/mm2 • Gain ~200 (1775

V)

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.

Data analysis procedure

• The Birks’ saturation formula 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]

Results – screen 3

• Similar results available for screen 1, screen 1 at 45 degrees, and screen3.

• Screen 1 ~ 2 X brighter but will have worse resolution

• All are linear at expected AWAKE charge densities without quad focusing

Results – low charge test, screen 4 (thinnest screen)

• 1) Measured quantities– Charge measured: 1.8 +/- 0.06 nC– (Measured camera output – background) : (4.70 +/- 0.07)X10^7 counts– Lens acceptance - fraction of total screen output entering lens: 2.2 X 10^-6 (assuming screen is lambertian

emitter, lens diamter =50mm, camera is 17 metres away).– Dark noise: standard deviation of 6.91 +/- 0.02 counts per pixel– Gate factor: 42.35 +/- 0.07

• 2) Results obtained from camera/mirror/lens specificatiions– Pixel area: 0.59 +/- 0.06 mm^2 (from pixel size and measured screen size)– Mirror reflactance 91.4%. Lens transmission 90+/-10% (guess).– Camera ADC counts per incident photon: 14 +/- 3

• 2) Derived results:– Normalised camera output: camera output X gate factor: (1.99 +/- 0.03)X10^9 counts– Minimum detectable charge density (assume visible when signal = 2Xnoise): (2.1 +/- 0.2)X 10^-8 nC/mm^2– Photons emitted per electron: 7000 +/- 2000

Comparison with expected AWAKE charge density

• Assume: – AWAKE bunch charge with 100% capture efficiency: 0.2 nC– 1 microsecond gate length (integrate

• Beam uniformly distributed on screen with width 50 cm and height 1 cm (large energy spread) – charge density = 4 X 10^-5 nC /mm^2 = 2000 times the minimum detectable charge density.

• To improve the signal:– The measured the Medex screen screen has double the output of the “medium” screen.– Using e.g. diameter 14 cm camera lens instead of 5cm, would gain ~ 8 times acceptance.– Using the above, minimum visible charge density is (1.3+/-0.1)X10^-9 nC/mm2, and AWAKE charge density is

30000 times this, assuming 100% capture efficiency– The charge density could be improved using upstream quadrupoles to focus the beam.

Absolute calibration: comparison with simulation

• BDSIM (geant) simulation with:– 850 mum thick scintillator layer (measured)– Particle size = 25 microns– 50/50 mix of phospor/substrate in scintillator layer– 200 micron plastic backing– 5cm air gap from window to screen– 200 micron Aluminium window

• Simulation results: photons per incoming electron vs input beam energy (statistical errors):– 5.0MeV: 6160 +/- 80– 5.5MeV: 5669 +/- 4– 6.0MeV: 5500 +/- 30

• PHIN test results: 7000 +/- 2000 photons/electron

Future tests: resolution of optical system

• A lens has been chosen: AF-S NIKKOR 400mm f/2.8E FL ED VR

• We will test the optical system (including mirrors) using this lens – resolution, distortion etc, in November

Conclusions• Tests of the AWAKE spectrometer detection system have been carried out using a 5.5 MeV electron beam at

PHIN• The minimum visible charge density of the system is (1.3 +/- 0.9)X10^-9 nC/mm2• Charge density at AWAKE expected to be at least 30000 times greater (assuming 100% capture efficiency)• Charge density could be further increased using quadrupole focusing• The absolute number of photons generated by the screen agrees with simulation• A suitable lens has been identified and optical (resolution etc.) tests will be carried out in November.

Thanks to the PHIN team for their support and use of the the beam.

References

[1]THE PHIN PHOTOINJECTOR FOR THE CTF3 DRIVE BEAM, R. Losito et. al., EPAC06 Edinburgh [2]

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