Taking pictures at near the one electron limitnuchem.iucf.indiana.edu/Presentations/RdS_FSU_June_2015.pdfRomualdo T. deSouza, Indiana University Taking pictures at near the one electron

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


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


• 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.


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.


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.


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).


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

3210

20

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QQX

3210

10

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QQY


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.


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!


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.


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..


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.


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

Documents

Taking pictures at near the one electron limitnuchem.iucf.indiana.edu/Presentations/RdS_FSU_June_2015.pdfRomualdo T. deSouza, Indiana University Taking pictures at near the one electron