Development of a high-speed single photon
pixellated detector for visible wavelengths
Aaron Mac Raighne1, Antonio Teixeira2, Jason McPhate3, John Vallerga3, Pierre Jarron2, Colin Brownlee4, Val O’Shea1
1. Department of Physics and Astronomy, University of Glasgow, Scotland.
2. CERN, Genève, Switzerland.
3. University of California, Berkeley, U.S.A.
4. The Marine Biological Association of the United Kingdom, Plymouth, England
Introduction
Many biological imaging applications require high frame rates at low light intensities. Currently the EMCCD is the camera
of choice but limits the frame rates to ~500fps. We present the design of a camera capable of true single photon counting
across an array of 256x256 pixels at frame rates, over 3000fps, higher than ever achieved for similar arrays.
Optical Imaging with Medipix
Photocathode LNS20
• A photon incident on the photocathode produces a photoelectron.
Vacuum tube
• The electron is accelerated over a high voltage and is incident on the Si.
Photon
• A resulting charge cloud is created in the Si detector.
• The charge cloud is collected and read-out by the Medipix.
Ceramic Header
e-
High
Voltage
Interface PCB
5-10kV
Si Detector
Medipix
• Values above threshold increment the internal counter.
• The number of counts per pixel per acquisition is read out by Medipix,
through the ceramic header to the interface PCB.
Imaging Performance
Within the region 1-103
photons lies techniques
such
as
single
cell
fluorescence,
and
the
imaging of Ca2+ signalling.
These techniques and many
others are critical to the
imaging and understanding
of cellular and neural networks. They are limited by
the acquisition speeds of current low intensity
detector technology.
(b)
(a)
(c)
(d)
Maximum spatial frequencies transferred are
limited by the Medipix. Minimising the distance
of the photocathode to the Medipix chip and
maximising the accelerating voltage decreases the
effect of the point spread function of the
photocathode. In the design chosen the separation
is set at 2.27mm for a maximum voltage of 10kV.
Figures: (a) Shows the results of simulations demonstrating the effect of
the distance between the photocathode and the Medipix chip. By
multiplication of the calculated MTF of the Medipix with the results of
figure (a) the MTF for the system is found and displayed in figure (b).
Figures (c) and (d) show the MTFs calculated for both the photocathode
and the full detector system at a separation of 2.27mm for increasing
values of the accelerating voltage
Medipix
Ceramic Header
Ceramic header provides.
• A pad for attachment of the Medipix
detector assembly.
• Wirebonding pads for the chip which
connect to wire traces.
• Wire traces which lead to drilled feedthrough vias in the ceramic.
• Three offset dielectric layers to protect the
vacuum against micro-cracks in the metal
layers.
• A back surface containing a metal layer for
the attachment of surface mount components
and connectors.
•A surface on which to braze the flange onto
which the vacuum tube can be attached.
Ceramic disc
Drilled vias
filled with Au
Dielectric layer
Au layer
Metal layer for surface mount devices and
connectors
The Medipix chip contains 256x256 pixels each with a
square pixel size of side length 55 µm. The input
accepts positive or negative charge. The preamplifier
has a gain ~13mV/1 ke-, a peaking time of 150ns and a
return to of baseline<1µs, giving a count rate of
1MHz. An energy window is set by upper and lower
thresholds. A 13-bit counter per pixel is electronically
shuttered when the chip is reading out. Using a clock
of 100MHz and the parallel read-out the entire chip
can be read-out in 266µs which makes possible the
high frame rates
Conclusions
We present the design of a detector capable of imaging low intensity light at unprecedented speeds. These high-speeds are achieved
with SNR performance comparable to that of the highest performance EMCCD, in fact a higher quantum efficiency photocathode
would allow our detector to out-perform the EMCCD on SNR. MTF curves show that we can expect 30% contrast at spatial
frequencies up to ~15 line pair/mm.