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Romualdo T. deSouza, Indiana University
Taking pictures at near the one electron limit
“A picture is worth a thousand words”
Personnel : Mr. Blake Wiggins (third year graduate student), Dr. Davinder Siwal (postdoc),
Romualdo deSouza (PI)
Whether detecting photons, electrons, or ions ultimately one is inevitably dealing
with electrons.
Goal: Develop a detector with single electron sensitivity that has sub-millimeter
position resolution, sub-nanosecond time resolution and the capability of
resolving two spatially separated, simultaneous electrons.
Neutron radiography Individual gunpowder grains
inside a bullet
A.S. Tremsin et al., Nucl. Instr.
Meth A652 (2011) 400
Positron Emission
Tomography (PET)
image of a mouse
Good position and time
resolution are necessary
for good imaging!
Achieving good resolution
at the lowest signal to
background levels possible
is often very important
Romualdo T. deSouza, Indiana University
Concept behind our approach
• Amplification of a single electron to a pulse
of 107 or 108 electrons is sensed by two wire
planes oriented in orthogonal directions.
• The sense wire plane consists of wires
spaced by 1 mm which are connected to taps
in a delay line.
• Position is related to the time difference of
the signal arriving at the two ends of the
delay line.
Sense wire plane of first prototype
detector with 50 mm x 50 mm
active area
Generation of a single electron
Romualdo T. deSouza, Indiana University
• Digitize signals with 2GS/s waveform digitizer
(Caen V1729)
• Induced signals exhibit the expected bipolar shape
• Zero crossing time corresponds to the passage of
the charge cloud past the sense wire plane
• The entire signal shape provides information about
the particle’s position
• Cross-talk from the anode is evident at the 2-3%
level
Position resolution : 465 μm
In the first year an oil-free vacuum test station
and wire winder were designed and built.
Romualdo T. deSouza, Indiana University
Signal to background (S/B) We measure the microchannel plate Z-stack to have a background rate of 5
cts/cm2/s in agreement with manufacturer’s datasheet ~60 cts/s for 40 mm
detector.
Typical measurements of position resolution involve use of large incident
signals e.g. 1,000 incident photons [Floryan Rev. Sci. Instr. 60 (1989) 339.] to
achieve a resolution of 50-70 m.
Detection at low S/B is important because:
• Enables imaging of faint images (e.g. astronomy)
• Reduces radiation exposure (e.g. medical imaging)
• Improves sample processing time (e.g. neutron radiography)
What position resolution is achievable at low S/B?
After considering other approaches we decided that the approach of generating an
electron via bombardment of a foil by an particle is a good approach since the
distribution of ejected electrons is peaked at a single electron.
To improve the efficiency of our experimental setup we replaced the
relatively small silicon detector with a scintillator/PMT. A five-fold
improvement in coincidence rate was achieved.
Romualdo T. deSouza, Indiana University
How does the induced signal depend on the
trajectory of the charge cloud between wires?
Selecting the electron trajectory in this manner will
allow us to characterize the dependence of the
induced signal on the position of the electron cloud
resulting in a database of induced signal shapes.
Characterizing the position dependence of the induced signal
As the resistive anode is key to the further development of the induced signal
approach, we have focused on characterizing its performance.
By replacing the metal anode with a resistive
anode which independently measures position,
we anticipate being able to select on different
electron cloud positions with 100-200 μm
position resolution.
Delivery of a resistive anode (Quantar Technology) and its integration into the existing
experimental setup (mechanics, electronics, and DAQ) was slower than anticipated.
Romualdo T. deSouza, Indiana University
Need to vary MCP to resistive anode distance required redesign of experimental setup
New setup facilitates ease of handling detector
Dependence of resolution on distance between MCP and resistive anode
Timing signal is taken from MCP not anode
Total charge of event is measured from MCP
Position is derived from four corners of resistive anode using charge sensitive amplifiers, shaping amplifiers, and peak sensing ADC (Caen V785).
Romualdo T. deSouza, Indiana University
Masked spectrum of RA reveals several features:
Distribution of pulse heights from MCP is
broad
Two component nature (one electron vs two
electron?)
• All ten slits visible (100 µm, ~4mm pitch)
• Non-linear position response at edges of RA
• Slight rotation of mask relative to RA
• “hot” spot on MCP associated with a visible
crack
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Romualdo T. deSouza, Indiana University
Open question : Is this the best RA resolution that can be achieved for a single electron?
After selection on:• MCP pulse height• Rotation of two dimensional spectrum• Slits well described by gaussians
With decreasing MCP to RA distance the
resolution of all slits improves.
Central slits (5and 6) have the best resolution
(due to angle of incidence)
Resolution of 270 µm (FWHM) achieved for
essentially single electron using coincidence
At 4.5 mm (distance necessary for sense
wires) resolution not degraded significantly.
Romualdo T. deSouza, Indiana University
Comparison of total charge produced by
MCP with that measured by RA shows
that incomplete charge collection
occurs. Selecting on the uppermost
band (complete charge collection)
restricts the slit image to approximately
20mm diameter (center)
Pulses associated with the edge of the RA have long risetimes incompatible with the 1us shaping time!
Romualdo T. deSouza, Indiana University
Taking into account the finite slit width of 100 m this corresponds to an intrinsic
resolution of 157 m!
Resolution improves with larger amplification
There is an optimal accelerating voltage for the electron cloud
Under optimal conditions a resolution of 170 m is achieved
Resolution gets worse if the bias is lowered
(less amplification by MCP)
At low bias middle slits show best resolution
(somewhat insensitive to bias) but edge slits
which degrade more exhibit a large
variation in resolution.
Romualdo T. deSouza, Indiana University
Implementation of digital signal processing (DSP)
Can pulse shape (e.g. risetime) be used
to improve the position resolution?
To test this question each corner of the resistive anode was coupled to a high
quality charge sensitive amplifier (CSA) and digitized by an oscilloscope,
triggered by the scintillator/PMT (detection of an particle)
TKS 3054C scope 10 GS/s
CSA
PC
Signals from the corners of the RA
exhibit dramatically different shapes
no doubt associated with the position
of the electron cloud on the RA..
Romualdo T. deSouza, Indiana University
Results from DSP (preliminary)
Acquisition and analysis of 9100 events (limited statistics)
Resolution using DSP ~ 170 m. Trapezoidal filter gives same result within ~10 m
First step: Reproduce resolution achieved with analog electronics
With limited statistics only broad gate on MCP
charge is possible
• Used Gaussian filter and peak sensing
algorithm to produce 2D position spectrum
• Distortion in two dimensional image
indicates improper gain matching of four
corners.
Romualdo T. deSouza, Indiana University
Accomplishments
Outlook
• In the past year : improved efficiency of
experimental setup five-fold.
• Implementation and characterization of resistive
anode for few electron incident on MCP
(FWHM:170 µm); Rev. Sci. Instr. Manuscript
submitted
• Begun using DSP techniques to improve the
resolution (achieved same as analog electronics)
• Designed and fabricated the mechanical assembly
for testing the second generation detector with
differential signal readout.
Use resistive anode to characterize position dependence of induced signals
Implement use of second generation detector with differential readout/analysis
Test whether an MCP with 5 m channels improves the position resolution
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