Supported by the NNSA under award

Supported by the NNSA under award

no. DE-NA0002012

Modeling microchannel

plate

detectors for improved performance

Whether detecting photons, ions, or neutrons inevitably one is concerned with the detection of electrons.

Good spatial information is essential for quality imaging.

Goal: Development of a detector with (a) single-electron sensitivity, (b) sub-millimeter spatial resolution, (c) sub-nanosecond time resolution, and (d) the capability of resolving two spatially separated, simultaneous electrons.

A. S. Tremsin et al, Nucl. Instr. Meth. A 652, 400 (2011).

Romualdo T. deSouza

Neutron imaging of gunpowder grains in a bullet casing

Positron emission tomography of red drum

W. B. Feller, “MCP Detector Development.” Neutron and Photon Detection Workshop, Gaithersburg, MD (2012).

Z. S. Browning et al., J. Fish Dis.

36

, 911-919 (2013).

R.T. deSouza Indiana University

Microchannel plate detectors (MCP)

Microchannel plate consists

of

a

Pb

-glass material with millions of channels fused together

Typically ~0.5 mm thick with a L/D of 40:1 or 60:1

Gain of 1 x 10

3

Can stack 2 or three plates sequentially

Sub-nanosecond

time resolution

R.T. deSouza Indiana University

Position sensitive MCP

Resistive anode

Helical delay lineCross-strip anode

Notice that determining the position accurately is related to

charge centroiding.How about if we have two “hits” within the active area?

Cross-strip anode

Introduce a position sensitive element following MCP

Readout charge on strips with high quality, high density ASIC circuitry

Simulating the growth of the electron cloud

R.T. deSouza Indiana University

Black = 1

st PlateRed = 2nd Plate

Set up 2nd plate pore map10 μm diameter pores12 μm center-to-center pore spacingMove single pore from 1st plate over 2nd plateKeep track of number of overlapping pores and overlapping area for each position

Plate 1

Plate 2

e

-

Simulating the growth of the electron cloud

R.T. deSouza Indiana University

# Pores

123Emerging Cloud radius (μm)51111.93

For a 2 plate stack of MCPs (Chevron arrangement) one observes a maximum of three pores activated in the second plate.

The maximum radius of the electron cloud from this geometric analysis is ~12µm.

Simulating the growth of the electron cloud

R.T. deSouza Indiana University

Increasing the number of MCP plates to three extends the pore distribution to the most probable number of pores being five with a maximum of seven pores.

The size of the electron cloud increases to a maximum diameter of approximately

40

m.

B.B. Wiggins et al. Rev. Sci.

Instrum

.

86

, 083303 (2015)

Simulations of the electron cloud propagation

R.T. deSouza Indiana University

Simulations of the propagation of the electron cloud

R.T. deSouza Indiana University

The velocity reaches an asymptotic value within ~200

ps

while the radius grows approximately linearly.

Simulations of the propagation of the electron cloud

R.T. deSouza Indiana University

Simulations of the propagation of the electron cloud

R.T. deSouza Indiana University

The highest current slice determines the maximum radius.

Simulations of the propagation of the electron cloud

R.T. deSouza Indiana University

Operate at

t

he lowest total amplification (charge) necessary to achieve the desired S/B

Minimizing the distance between the MCP and the anode is extremely important

A simple

approach to a PS MCP

Total charge of the event is measured from the MCP.Position can be derived from the four corners of the resistive anode using conventional electronics: charge-sensitive amplifiers, shaping amplifiers, and a peak-sensing ADC.

Q

1

Q

0

Q

2

Q

3

Y=0

X

=0

X=1

Y=1

R.T. deSouza Indiana University

Charge-Division Method:

Position Spectrum for the Resistive Anode (RA)

The active area of the MCP is evident.

All slits in the mask are visible (100 μm wide with a 4.2mm pitch).There is a non-linear distortion at the edges of the RA.

Start by using conventional electronics (charge sensing amplifiers, shaping amplifiers, peak sensing ADCs).

Using only charge

division, resolution

=

157 μm FWHM

B. B. Wiggins et al, Rev. Sci. Instrum. 86, 083303 (2015).

R.T. deSouza Indiana University

Improving spatial Resolution of the RA

detector

A clear correlation is evident between the signal risetime and Yposition.

R.T. deSouza Indiana University

D

.

Siwal

et al

,

Nucl

.

Instr.

Meth

.

A

804

, 144

(2015)

.

Pulse shape analysis of the Resistive Anode (RA)

A clear correlation is evident between the signal

risetime and Yposition.Peaks correspond to slits in the maskUse of the signal risetime in addition to the charge-division method results in a significantly improved resolution.

R.T. deSouza Indiana University

Using pulse shape analysis, resolution = 64 μm (FWHM)

D. Siwal et al, Nucl. Instr. Meth. A 804, 144 (2015).

Multi-Strip Anode

R.T. deSouza Indiana University

Anode strips are 250

μm wide with 75 μm inter-strip isolation. Signal propagates from strip to the delay line where it splits. Time difference of signal arrival at either end of delay line is related to signal position.Even and odd strips are independently coupled to the taps of a delay line. Anode area is approximately 3 cm x 3 cm.

Resolution = 94 μm FWHM

Note: This approach does not rely on charge

centroiding

and is capable of distinguishing multiple particles simultaneously incident that are spatially separated.

Application: Position sensitive E x B detector

Crossed electric and magnetic field transports electrons from secondary emission foil to the

microchannel

plate (MCP)

20 neodymium permanent magnets produce magnetic field (~85 gauss)6 grid plates produce electric field (~101,000 V/m)C foil frame biased to -1000 VMCP with 18 mm diameterTime resolution (MCP-MCP) ~ 350 ps

Bowman et al., Nucl. Inst. and Meth. 148, 503 (1978) Steinbach et al., Nucl. Inst. and Meth. A 743, 5 (2014)

By making the MCP position sensitive one can image the beam in the horizontal dimension.

R.T. deSouza Indiana University

R.T. deSouza Indiana University

Introduction to the Induced

Signal Approach

A single electron is amplified to a cloud of 107-108 electrons, which is sensed by a wire plane (2 orthogonal planes can provide 2D).As the charge cloud approaches the wires electrons in the wires are repelled resulting in a negative signal. As the electron cloud recedes from the wires a positive signal results.Wires in the sense wire plane have a 1 mm pitch and are connected to taps on a delay line.Position is related to the time difference between the signals arriving at the ends of the delay line.

R. T.

deSouza et al, Rev. Sci. Instrum. 83, 053305 (2012).

The inherent bipolar shape of this induced signal approach is a potential advantage.

R.T. deSouza Indiana University

Introduction to the Induced

Signal Approach

R. T. deSouza et al, Rev. Sci. Instrum. 83, 053305 (2012).

Printed circuit board measures approximately 5 in. x 5 in. with 5cm x 5cm square cut out

Across opening 25

μ

m Au-W wires are strung 1 mm apart

Delay of ~1 ns/tap

By measuring the time difference between arrival of signal at two ends of the delay line (XL and XR) or (YU and YD), the position can be determined.

Notice that this encodes position into time rather than relying on charge

centroiding

hence multiple particles are distinguishable.

Spatial Resolutio

n of the Induced Signal Approach

Digitized signals with a 2 GS/s waveform digitizer (CAEN V1729A).The induced signals have the expected bipolar shape, where the zero-crossing point corresponds to the passage of the charge cloud past the sense wire plane. Each induced signal is amplified by a low-noise amplifier with a gain of 30.

R. T. deSouza, Z. Q. Gosser, and S. Hudan, Rev. Sci. Instrum.

83, 053305 (2012).

R.T. deSouza Indiana University

Mask with 50

μ

m wide slits

Spatial Resolutio

n of the Induced Signal Approach

First attempt: Resolution = 466 μm FWHM

R. T. deSouza, Z. Q. Gosser, and S. Hudan, Rev. Sci. Instrum. 83, 053305 (2012).

Q

2

100

5

0

Q

2

-10000

-5000

0

Yield (counts)

Position (μm)

50 μm wide slits with 900 μm pitch

Individual slits in a mask visible as peaks

R.T. deSouza Indiana University

Mask with 50

μ

m wide slits

Expected anti-correlation between time-of-arrival of signals at ends of the delay line is observed

Non-

linearities

observed at ends of detector

Improvement to the Induced Signal Approach

Improved resolution indicates that better digitization would improve the resolution

To date only the zero-crossing point in the induced signal has been utilized. However, the entire pulse shape contains information. To use this information, we need to understand the dependence of the detailed shape of the induced signal on position.

R.T. deSouza Indiana University

FFT

(150 MHz

c

utoff

frequency)

x

2

Digitized Sense Wires

Linear Interpolation

(0.5ns step

->

0.05 ns step)

1) FFT

(500 MHz

cutoff frequency)2) Extract Max

Take Derivative

With

digital signal processing

, resolution = 115

μm

FWHM

Determine resolution

 240 µm

Outlook: A Differential Readout

Implement a differential system by using independent delay boards one for odd numbered wires and the other for even numbered wires

The relative charge on adjacent wires is essentially linearly related to the position of a charge column between the wires.

Simulations with Maxwell 2D

R.T. deSouza Indiana University

Summary: Comparing different position sensing techniques

R.T. deSouza Indiana University

Position-Sensitive

MCP DetectorSpatial Resolution FWHM (μm)Resistive Anode157Resistive Anode- Risetime Analysis64Multi-Strip Anode (delay line)94First Generation Induced Signal466Induced Signal with DSP115

Using pulse

-shape (

risetime

)

analysis for the resistive anode, we have achieved more than a

factor of two better than

what has been previously obtained as well as less distortion.

Using a multi-strip anode with simple delay

line

readout, we achieved a sub 100

μm

resolution typically obtained only with high density, complex charge sensitive readout. Further improvements envisioned.

For

the induced signal

detector,

we have

achieved

a resolution of 115

μm

using only the

zero-crossing

point of the signal and are now poised to characterize and make use of the entire signal shape.

Outlook: Neutron Imaging

R.T. deSouza Indiana University

Characteristics

of LENS: 13 MeV proton linac driver9Be(p,n) reaction to produce neutrons Thermalization (polyethylene, solid CH4 at 6.5K) 100 n/(ms.cm2) neutron flux

Low Energy Neutron Source (LENS)

The detector will be used for neutron imaging using both

slow and fast neutrons. The measurements will be carried out at

the LENS

facility at Indiana University.

10

B + n

11

B*

7

Li*

7

Li

478

keV

ϒ

94%

6%

Indiana University

Nuclear Chemistry: B.B. Wiggins, D. Siwal, S. Hudan, Z. deSouza, J. Huston, T. K. Steinbach, V. Singh, J. Vadas Indiana University Chemistry Department: Mechanical Instrument Services and Electronic Instrument ServicesNational Nuclear Security Association:Award No. DE-NA0002012

Acknowledgements

R.T. deSouza Indiana University

Position sensitive E x B detector

SIMION calculations using a constant magnetic field of 85 gauss in the E x B detector

Can making the MCP in the E x B position sensitive aid us in defining the beam spot on the target?

Position sensitive E x B detector

To make the simulations more realistic we measured the magnetic field

The field is relatively constant for a given slice in Y with the strongest field closest to the iron plates

In the active region the field is between 85 and 95 gauss.

Position sensitive E x B detector

U

sing the measured magnetic field in the E x B detector the SIMION

calculations

predict that one can still measure the position on the target foil in the horizontal direction.

Experimental arrangement for measuring position resolution

(inside a vacuum chamber, P ~ 5 x 10

-7

torr

)

Testing the detector

Position-Sensitive

MCP Detector

Advantages

Disadvantages

Multi-Anode

Fast

signal

Can distinguish multi-hit

events

Spatial resolution

~ 25-50 µm

Cost of readout electronics

Cross-talk between anodes

Resistive Anode

Relatively slow signal

Simplicity of readout

Low

power consumption

Spatial resolution ~ 100 µm

Limited to

count rates <100kHz

Cannot distinguish multi-hit

events

Helical Delay Line Anode

Fast signal

Low power and electronic cost

Can

d

istinguish

multi-hit

s at 10 MHz

Spatial

resolution ~ 60 µm

Attenuation and dispersion of

signal in

delay line

Difficult fabrication process

Fragility of a single wound wire

Cross-Strip

Anode

Fast signal

Spatial resolution 10 – 20 µm

Chevron MCP (less charge spreading)

Can distinguish multi-hit

events

Cost of readout electronics

High power consumption

Induced Signal

Fast signal

Simplicity of readout

Can distinguish multi-hit

events

Low power consumption

Attenuation and dispersion of

signal in

delay line