MCP Testing Using Square Conductive Pads as Readout Anode Sagar Setru 1/19/2014

Motivation Purpose: measure MCP characteristics – Uniformity – Gain – Stability – MCP Charge depletion/rate dependence (10 3 Hz) – Position resolution (10 -6 m) – Dark current Needs to be in vacuum (10 -6 Torr) Complementary to APS/UChicago apparatus – Near ALD lab, fast, stand-alone, limited

Apparatus layout No cathode Incoming photon generates electron on MCP surface Resulting charge cloud lands on anode with padded conductors Apparatus layout based on SSL Cross Strip/Cross Delay Line 1 and APS test apparatus 2 MCP Signal ~2.5mm Padded Anode Analog to Digital Converter MCPs Bias Voltage Readout channels photon Laser 1 O.H.W. Siegmund, A. Tremsin, J.V. Vallerga and J. Hull. Microchannel Plate Imaging Photon Counters for Ultraviolet through NIR Detection with High Time Resolution. Proc SPIE May 12; 8033: Bernhard Adams, Matthieu Chollet, Andrey Elagin, Eric Oberla, Alexander Vostrikov, Matthew Wetstein, Razib Obaid and Preston Webster.. A test-facility for large-area microchannel plate detector assemblies using a pulsed sub-picosecond laser. Rev. Sci. Instrum. 84, (2013).

Readout overview Measure, digitize gain from each pad Find the center (X,Y) of charge cloud from charge distribution – Use modified center of mass algorithm (‘center of charge’) Readout pads each have their own channel Mounted on inside of CF flange Readout pads Conflat Flange Analog-to-Digital Converter X axis Y axis Readout pads Charge Cloud Computer Signals from pads

What is the trade-off between good spatial resolution and practicality?

6σ ≈ 3.5 mm Approximate signal from MCP as 2D Gaussian cloud of charge 1 Dimensions of cloud fixed – σ = mm – 6*σ ≈ 3.5 mm – 6σ gives approx. diameter of cloud A ≈ 10 6 electrons, RMS – Represents gain (x 0, y 0 ) is center 1 A. S. Tremsin, J. V. Vallerga, O. H. W. Siegmund, J. S. Hull, Proc. SPIE, vol. 5164, Centroiding algorithms and spatial resolution of photon counting detectors with cross strip anodes, UV/EUV and Visible Space Instrumentation for Astronomy II, San Diego (2003) Modelling MCP Output A = 10 6 e - x0x0 y0y0

Monte Carlo simulations to determine resolution vs. pad size 1.Place Gaussian charge cloud on array of square pads – Placed at random position – Noise per pad ≈ 500 electrons, RMS 2.Read out the pads 3.Calculate position of center of charge cloud 4.Compare calculated position of center with actual, Monte Carlo position of center 5.Repeat – Many times per pad size – Several pad sizes Signal 1 Signal 2 Signal 3 Signal 4

Example calculation of reading out pads in simulation Integrate 2D Gaussian function Bounds of integration are positions of pad border Example: a 4 mm by 4 mm pad: -4 4 (0,0) 4 X Y σ = mm A = 10 6 electrons, gain of signal from MCP Cloud charge is centered at (1,1) Charge cloud (1,1)

Center of charge algorithm

Initial results Why does resolution get worse as pad size decreases?!

Answer: noise from pads 1 A. S. Tremsin, J. V. Vallerga, O. H. W. Siegmund, J. S. Hull, Proc. SPIE, vol. 5164, Centroiding algorithms and spatial resolution of photon counting detectors with cross strip anodes, UV/EUV and Visible Space Instrumentation for Astronomy II, San Diego (2003)

Solution: center of charge algorithm with a threshold 1 A. S. Tremsin, J. V. Vallerga, O. H. W. Siegmund, J. S. Hull, Proc. SPIE, vol. 5164, Centroiding algorithms and spatial resolution of photon counting detectors with cross strip anodes, UV/EUV and Visible Space Instrumentation for Astronomy II, San Diego (2003) Threshold

Resolution now improves as pads become smaller! But why does resolution start going up for the smallest pads?

Answer: different pad sizes have different ideal threshold So must fix pad size and compare different thresholds Looked at 0.25, 0.5, 1, 1.25, 1.5 mm pads Thresholds used (as percent of total charge) – 0.001, 0.002, 0.005, 0.01, 0.02, 0.05, 0.1, 0.2, 0.5, 1, 2, 5, 10, 20, 50, 100

Example results: resolution vs. threshold (fixed pad size) 1.25 mm pads 0.5 mm pads

Another study: resolution vs. noise Noise increases position resolution at the submicron level Very low noise (< 100 e -, RMS) —nearing single micron/sub micron resolution

Conclusions Standard center of charge centroiding fails with small pads because of noise – Must use a threshold to remove far away signals Different pad sizes have different ideal thresholds Less noise is better for resolution, even with threshold centroiding

Conclusions

To do 1. Tank design – MCP holder, spacing 2. Anode – Mounting, design, electronics 3. Laser spot size 4. Laser scanner 5. Frequency of laser 6. Determine electronic noise

Appendix: Investigating the sudden jump in resolution vs. threshold Occurs at a different threshold for different padsizes 1.25 mm pads 0.5 mm pads

Appendix: Investigating the sudden jump in resolution vs. threshold Before jump/at low thresholds: – Identical to standard centroiding w/out threshold – Makes sense (letting nearly all charge through) 1.5 mm 0.5 mm

Appendix: Investigating the sudden jump in resolution vs. threshold Very high thresholds – Undefined resolution – Makes sense (eventually letting no data through) 1.5 mm 0.5 mm No resolution