Exploring the spatial resolution of positionsensitive microchannel plate detectors Blake Wiggins, Davinder Siwal, Romualdo T. desouza Cold neutrons A. S. Tremsin et al, Nucl. Instr. Meth. A 652, 400 (2011). Good spatial information is essential for quality imaging. Whether detecting photons, ions, or neutrons inevitably one is concerned with the detection of electrons. Goal: Development of a detector with (a) single-electron sensitivity, (b) submillimeter spatial resolution, (c) sub-nanosecond time resolution, and (d) the capability of resolving two spatially separated, simultaneous electrons. Supported by the NNSA under award no. DE-NA0002012 E. H. Gschweng et al, Cancer Res. 74, 5173 (2014).
Introduction to the Induced Signal Approach A single electron is amplified to a cloud of 10 7-10 8 electrons, which is sensed by a wire plane (2 orthogonal planes can provide 2D). 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. Resolution = 466 μm FWHM R. T. desouza et al, Rev. Sci. Instrum. 83, 053305 (2012). Blake Wiggins Indiana University 2
Improvement to the Induced Signal Approach Digitized Sense Wires x 2 FFT (150 MHz cutoff frequency) Linear Interpolation (0.5ns step -> 0.05 ns step) Take Derivative 1) FFT (500 MHz cutoff frequency) 2) Extract Max With digital signal processing, resolution = 115 μm FWHM 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. Blake Wiggins Indiana University 3
An Approach to Characterize the Induced Signals Idea: Use independent measure of position to characterize the induced signals. Replaced sense wires and metal anode with a resistive anode (Quantar Technology). Q 1 Y=1 Q 0 Charge-Division Method: X=0 X=1 Q 2 Y=0 Q 3 Total charge of the event is measured from the MCP. Position is derived from the four corners of the resistive anode using conventional electronics: charge-sensitive amplifiers, shaping amplifiers, and a peak-sensing ADC. Using only charge division, resolution = 157 μm FWHM Can the joint use of charge division and pulse shape information improve the spatial resolution? B. B. Wiggins et al, Rev. Sci. Instrum. 86, 083303 (2015). Blake Wiggins Indiana University 4
Spatial Resolution of the Resistive Anode (RA) A clear correlation is evident between the signal risetime and Y position. Use of the signal risetime in addition to the charge-division method results in a significantly improved resolution. Using pulse shape analysis, resolution = 64 μm (FWHM) However, insertion of a sense wire plane prior to the RA resulted in large coupling of RA to the sense wire plane. Thus, it is not feasible to use the RA to characterize the induced signals. D. Siwal et al, Nucl. Instr. Meth. A 804, 144 (2015). Blake Wiggins Indiana University 5
Spatial Resolution of the Resistive Anode (RA) A clear correlation is evident between the signal risetime and Y position. Use of the signal risetime in addition to the charge-division method results in a significantly improved resolution. Using pulse shape analysis, resolution = 64 μm (FWHM) However, insertion of a sense wire plane prior to the RA resulted in large coupling of RA to the sense wire plane. Thus, it is not feasible to use the RA to characterize the induced signals. D. Siwal et al, Nucl. Instr. Meth. A 804, 144 (2015). Blake Wiggins Indiana University 5
Multi-Strip Anode Anode strips are 250 μm wide with 75 μm interstrip isolation. Anode area is approximately 3 cm x 3 cm. Even and odd strips are independently coupled to the taps of a delay line. Signals from the delay line are processed in a similar fashion to the induced signal approach. Z-Stack MCP Resolution = 94 μm FWHM Sense Wire Plane (1 mm pitch) Multi-Strip Anode Can correlating the induced signal shape with the position measured by the multi-strip anode be used to improve the spatial resolution of the induced signal approach? Blake Wiggins Indiana University 6
Conclusions Position-Sensitive MCP Detector Spatial Resolution FWHM (μm) Resistive Anode 157 Resistive Anode- Risetime Analysis 64 Multi-Strip Anode (delay line) 94 First Generation Induced Signal 466 Induced Signal with DSP 115 1) 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. 2) Using a multi-strip anode with delay line readout, we achieved a sub 100 μm resolution typically obtained only with high density, complex charge sensitive readout. 3) 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. Blake Wiggins Indiana University 7
Outlook: Neutron Imaging 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. Low Energy Neutron Source (LENS) Characteristics of LENS: 13 MeV proton linac driver 9 Be(p,n) reaction to produce neutrons Thermalization (polyethylene, solid CH 4 at 6.5K) 100 n/(ms.cm 2 ) neutron flux 10 B + n 11 B* 6% 94% 7 Li* 478 kev ϒ 7 Li Blake Wiggins Indiana University 8
Acknowledgements Indiana University Nuclear Chemistry: D. Siwal, S. Hudan, R. T. desouza, Z. desouza, J. Huston, T. K. Steinbach, V. Singh, J. Vadas Indiana University Chemistry Department: Mechanical Instrument Services and Electronic Instrument Services National Nuclear Security Association: Award No. DE-NA0002012 Blake Wiggins Indiana University 9