Supported by the NNSA under award no. DE-NA0002012

Measuring position in 2-dimensions using induced signals in a microchannel plate detector

Blake Wiggins, Romualdo deSouza

§  Good spatial information is essential for quality imaging.§  Whether detecting photons, ions, or neutrons inevitably one is concerned with detecting electrons.§  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.

Courtesy of Paul Scherrer Institut

Blake Wiggins Indiana University 2

Microchannel Plate (MCP) Motivation

MCP Motivation:①  Sensitivity to a broad range of particles

including charged particles and neutrons②  High gain ③  Fast temporal response④  Compact size⑤  Stable operation even in high magnetic fields

A.S. Tremsin et al. NIM A539, 1 (2005). P. Schagen, Advances in image pick-up and display, 1, 1 (1974).

J. M. Walters, M.S. Thesis, Oregon State University, Corvallis, OR, (2014).

§  An MCP is composed of millions of leaded glass tubes (2-10 μm in diameter) where each channel acts as an independent secondary electron emitter.

§  An applied voltage across the plate causes the initial electron to be cascaded with an amplification of ~103-104.

§  Individual plates can be stacked to achieve a gain of 106–108.

188 CHAPTER 10 MCP-PMT

10.1 Structure

10.1.1 Structure of MCPs

Figure 10-1 (a) illustrates the schematic structure of an MCP. The MCP consists of a two-dimensional arrayof a great number of glass capillaries (channels) bundled in parallel and formed into the shape of a thin disk.Each channel has an internal diameter ranging from 6 to 20 microns with the inner wall processed to have theproper electrical resistance and secondary emissive properties. Accordingly, each channel acts as an indepen-dent electron multiplier. The cross section of a channel and its principle of multiplication are illustrated inFigure 10-1 (b). When a primary electron impinges on the inner wall of a channel, secondary electrons areemitted. Being accelerated by the electric field created by the voltage VD applied across both ends of the MCP,these secondary electrons bombard the channel wall again to produce additional secondary electrons. Thisprocess is repeated many times along the channel and as a result, a large number of electrons are released fromthe output end.

CHANNEL

CHANNEL WALL

STRIP CURRENT

INPUTELECTRON

OUTPUTELECTRONS

VD

OUTPUT ELECTRODE

INPUT ELECTRODE

THBV3_1001EA

(a) Schematic structure of an MCP (b) Principle of multiplication

Figure 10-1: Schematic structure of an MCP and its principle of multiplication

MCPs are quite different in structure and operation from conventional discrete dynodes and therefore offerthe following outstanding features:

1) High gain despite compact size

2) Fast time response

3) Two-dimensional detection with high spatial resolution

4) Stable operation even in high magnetic fields

5) Sensitive to charged particles, ultraviolet radiation, X rays, gamma rays, and neutrons

6) Low power consumption

There are various types of detectors that utilize the advantages offered by MCPs, for example image inten-sifiers for low-light-level imaging, fast time response photomultiplier tubes that incorporate an MCP (MCP-PMTs), position-sensitive multianode photomultiplier tubes, streak tubes for ultra-fast photometry, and pho-ton counting imaging tubes for ultra-low light level imaging.

© 2007 HAMAMATSU PHOTONICS K. K.

Hamamatsu Photonics K. K., Photomultiplier Tubes Basics and Applications 3rd ed (2007).

TP209/MAR08

4.1. DETECTION EFFICIENCY

The detection efficiency of channel multipliers to various kinds of primary radiation is summarized in table 1 from Schagen17) and includes data from single channel multipliers and MCPs. Measurements made with the former are easier to interpret since single channels are usually operated in space charge saturation, allowing the use of pulse counting techniques. More recently, a review of single channel and MCP detection efficiencies has been published by Macau et al.35). 4.1.1. Charged particles The secondary electron emission coefficient, δ, for lead glasses typically used in channel plates reaches a maximum of about 2 at an incident primary electron energy of 300 eV36). At low energies, where the incident electrons do not have a range sufficient for multi-channel excitation, the detection efficiency should approach the open area ratio of the MCP which is typically 50%. However, electrons striking the interstitial electrode material produce secondaries which can excite neighboring channels. Galanti et al.37) have observed the channel plate efficiency to increase from 50% at 50 eV to 70% at 1 keV. These authors also see a variation of efficiency with the electron angle of incidence, and have reported a maximum at 20° for 1 keV incident electrons. Clearly, if the electron trajectories are almost parallel to the channel axes, there is a high probability for deep penetration into the channels before a primary interaction; this results in low gain output pulses near 0º. TABLE I: Detection efficiency of channel multipliersa.

Type of radiation Detection efficiency (%)

Electrons 0.2 - 2 keV 50-85 2 - 50 keV 10-60

Positive ions 0.5 - 2 keV 5-85 (H+, He+, A+) 2 - 50 keV 60-85

50 -200 keV 4-60 U.V. radiation 300 - 1100 Å 5-15

1100-1500 Å 1-5 Soft X-rays 2 -.50 Å 5-15 Diagnostic X-rays 0.12 - 0.2 Å ~1

a From Schagen17).

Most positive ion detection efficiency measurements reported in the literature have been with single channels, and quantitative results exist only for energies less than 30 keV. However, measured efficiencies are of the same order of magnitude as those for electrons. Tatry et al.38) have found the detection efficiency for 40 keV protons and alpha particles to be about 80%.

4.1.2. U.V. and soft X-rays Single channel multipliers and channel plates have surface

work functions which allow photoelectron production at incident wavelengths shorter than 2000 A. Measurements by Paresce39) on single channels suggest an almost exponential decrease in efficiency from 2% at 1200 Å to 10-9 at 2600 Å. Other measurements suggest an efficiency near 10% in the 12-70 A region40). More recently, Bjorkholm et al.41) have re ported peak MCP efficiencies ranging from 27% at 0.86 keV (~14 Å) to 5% at 3 keV (~4.1 Å). These authors used a Chevron configuration with bias angles of (0/8°) and α =80. They found that the quantum efficiency increases as the angle of incidence decreases until a critical angle is reached after which there is a rapid fall off of efficiency. The peak occurs in the 1°-6° range, and the small angle dip is due to reflection down the channel. They explained the energy variation of quantum efficiency by the variation of the X-ray absorption coefficient, and of the photo and Auger electron ranges; their model assumed a 100 A layer of SiO2 over the lead glass matrix, which is in agreement with the MCP surface analysis studies of Siddiqui42).

Quantum efficiency enhancement can be obtained by vacuum deposition of various high yield photo-cathode materials on the input face of an MCP. Henry et al.43) report an increase of 65% in the quantum efficiency at 1.48 keV using a MgF2 coated plate. The efficiency of electron multipliers with Au, LiF, MgF2, BeO, SrF2, KC1 and CsI photocathodes in the 23.6-113 A wavelength range has been measured by Lukirskii et al.44); CsI is often used at longer wave lengths.

Finally, it should be noted that the use of α = 80 Chevrons for photon detection ensures space charge saturation at small angles of incidence; grazing incidence reflections down the channels for α= 40 plates degrade the resulting pulse height distribution so that a discriminator level becomes difficult to set. The use of an interplate bias voltage27) improves the performance with α = 40 plates, however. 4.1.3. Hard X-rays

As the photon energies increase, so does range within the MCP matrix glass. The primary interaction then occurs throughout the MCP rather than at a front surface. The thicker the lead glass matrix, the higher the photon absorption coefficient; channel to channel spacing must be small enough, however, to allow photoelectrons produced in the matrix glass to excite a channel wall.

Standard Galileo MCPs are made from Corning 816145) glass and have an elemental composition given in table 2. It is of interest to note that the photoelectron absorption coefficient of this material for 662 keV X-rays is 30% larger than that of sodium iodide. The glass density is nominally 4.0 g/crn3.

Thermal Neutrons 0 - 25 meV 14-25

Blake Wiggins Indiana University 3

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).

§  Wires in a 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.

Resolution = 466 μm FWHM

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

Improving the Spatial Resolution

§  Minimized the distance between the MCP and the sense wire plane.§  Optimized the electric field of the system.§  Optimized the grounding of the signals.

Blake Wiggins Indiana University 4

FFT(150 MHz cutoff frequency)

Digitized Sense Wire

Determine Zero-Crossing

With optimizations, resolution = 240 μm FWHM

Resolution = 466 μm FWHM

Improving the Spatial Resolution

Blake Wiggins Indiana University 5

FFT (150 MHz cutoff

frequency)

Digitized Sense Wire

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

With digital signal processing and use of a 10 GS/s Digitizer (Tektronix DPO5204B), resolution = 95 μm FWHM

Slow Neutron Radiography

Blake Wiggins Indiana University 6

Characteristics of LENS: §  13 MeV proton linac driver§  9Be(p,n) reaction to produce neutrons §  Thermalization (polyethylene, solid CH4 at 6.5K) §  100 n/(ms.cm2) neutron flux

Low Energy Neutron Source (LENS)

Slow neutron radiography was performed at the LENS facility at Indiana University.

XY Anode

Cd Mask*

* 2mm Wide Slits, Horiz. Oriented, 5mm Pitch (Thermal capture σ for 113Cd = 19,820 b)

10B + n(25 meV) 7Li + 4He σ = 3840 b

Slow Neutron Radiography: Online Analysis

Blake Wiggins Indiana University 7

XY_r869-r870_pyEntries 78376Mean 0.1476− Std Dev 42.71

Y Position (a.u.)100− 50− 0 50 100

0

100

200

300

400

500

600

XY_r869-r870_pyEntries 78376Mean 0.1476− Std Dev 42.71

In the first 2D neutron image (using sense wires):§  Can clearly distinguish the MCP-B (25mm diameter), and the MCP-Z (40mm diameter), and the 3 slits in the mask.§  Measured expected slit width(2mm) and pitch(5mm) with resolution = 861 μm FWHM. §  There is a non-uniform intensity fluctuation in X (offline anlalysis is underway).§  Our Peak/Bkdg is only ~2.§  S/B = 0.7 and S+B =170 cps, and are currently rate limited by our DAQ.

LENS in low power mode (intensity/10)

Summary and Outlook

Blake Wiggins Indiana University 8

Summary:I.  Demonstrated proof-of-principle of the induced signal method.

II.  Using only the zero-crossing point of the induced signal, resolution = 95 μm FWHM.III.  Acquired first 2D image for slow neutrons, resolution = 861 μm FWHM.

Outlook:I.  Improve Signal Processing

i.  Utilize the entire induced signal shape.ii.  Implement a differential readout method.

II.  Neutron Radiography – Improvement to S/Ni.  Assemble a faster waveform digitizer DAQ.ii.  Incorporate a TDC for a coincidence (MCP & proton beam) measurement.

Indiana University: D.V. Baxter, Z. deSouza, S. Hudan, J. Huston, V. Singh, J. Vadas

LENS Staff

Indiana University Chemistry Department: Mechanical Instrument Services and Electronic Instrument Services

National Nuclear Security Association:Award No. DE-NA0002012

Acknowledgements

Blake Wiggins Indiana University 9