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RadInspector Radioisotope Identification and Measurement

RadInspector Radioisotope Identification and Measurement. Aman Kataria (EE), Johnny Klarenbeek (EE), Dean Sullivan(EE ), David Valentine(EE ). Design Goals and Motivation. Goal: Portable Small , light weight, and compact LCD touch display, rechargeable and power efficient Low Cost

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RadInspector Radioisotope Identification and Measurement

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  1. RadInspectorRadioisotope Identification and Measurement RadInspector AmanKataria(EE), Johnny Klarenbeek(EE), Dean Sullivan(EE), David Valentine(EE)

  2. Design Goals and Motivation Goal: • Portable • Small , light weight, and compact • LCD touch display, rechargeable and power efficient • Low Cost • Robust Motivation: • The involvement and integration of multiple electrical engineering disciplines • The challenge; foreseen difficulty of research, design, and implementation RadInspector

  3. Specifications • Operating conditions: • Low power consumption (< 2W) • Long battery life (2+ hrs) • Room temperature (25°C) • Energy resolution: • < 10% FWHM @ 0.611 MeV (137Cs) • Data collection: • Plot spectrum and perform real-time dosimetry • Adjustable energy range (400 keV 800 keV 1.2 MeV) • Low cost • < $500.00 RadInspector

  4. Project Overview RadInspector

  5. Detector Design Goals • High stopping ability • Increases with increasing Z • High probability of photoelectric interaction • Increasing probability with increasing Z ( • Charge generation should be linear • Determined experimentally and then calibrated • High Q.E. and responsivity 100% • Both improve with increasing density • Good resolution • Charge generation would resemble dirac-delta pulse RadInspector

  6. RadInspector

  7. Probability of Photoelectric Interaction Calculated Quantum Efficiency Generated Photocurrent Ideal Resolution RadInspector

  8. Cost: $90.00 • 100 mm2 active area and 5 mm thick • Trading resolution for efficiency • Issues • Charge trapping • Lack of material uniformity • Microphonic CadmiumZincTelluride 10 mm 10 mm 5 mm RadInspector

  9. Charge Trapping Effects attenuation • Efficiency • Corrected efficiency Efficiency Correction characteristic tailing Charge Collection Efficiency vs. Depth for CZT Images courtesy of Amptek RadInspector

  10. Charge Sensitive Preamplifier • Ideal response is a single sided exponential decay (RC) • Detector charge pulse charges capacitor according to • Pulse decays with time constant RadInspector

  11. Charge Sensitive Preamplifier • Test circuit with OPA827, Cf = 1pF, Rf = 1Gohm using BPW34 Si photodiode • Signal is noisy • Low energy rays close to noise floor • Detector is only efficient at low energies, need to maximize SNR • Which components to optimize? • Noise Analysis: • Detector contribution • Opamp current/voltage noise • Thermal noise RadInspector

  12. Charge Sensitive Preamplifier Noise Analysis Feedback Resistor (voltage) • Detector has shunt resistance and capacitance • Thermal noise and shot noise modelled • Opamp current noise and voltage noise modelled • Flatband and 1/f • Thermal noise (Rf typ. ~ 100M-1G) • Find transfer function for each source • Obtain output referred noise Detector (current) Opamp (current + voltage) RadInspector

  13. Current noise is a major source of noise • Lower voltage noise op amps typically have higher current noise • Hybrid implementation: buffer current noise with JFET, select low voltage noise opamp • Op amp current noise now negligible with respect to detector current noise • Negligible increase in voltage noise from JFET NXP BF862 N-channel JFET • Cost: $0.49 • Equivalent noise input voltage = 0.8 nV/√Hz • Operating temp. as low as - 65° C if TEC is needed 100M-1G 2.5 mm 1k 3.0 mm RadInspector

  14. Charge Sensitive Preamplifier Noise Analysis = * RadInspector

  15. Observations: • Current noise dominates • To reduce noise: • Lower to reduce gain peaking • Lower 1/f corner freq • Minimize bandwidth with • and GBWP • Lower will reduce current • noise but increase BW • Reduce detector contribution. Lower dark current (temp) RadInspector

  16. Noise Analysis Verification • OPA827, BPW34 photodiode, NXP BF862 JFET, Rf = 1Gohm, Cf = 1pF • Dark current = 5nA @ 10V reverse bias • Cin = Cd + Cgs = 15pF + 10pF = 25pF • Integrate under the area of each curve to find that component’s RMS contribution • Total RMS noise is root of sum of squares • Using these specs we get: 2.67mV (RMS) • Experimental measurement yields 1.78mV (RMS) RadInspector

  17. Opamp Noise Calculations • Comments: • Current noise dominant, so at room temp the difference between opamps is minor. • Can cool down the detector / feedback resistor to lower current and thermal noise. Picking the right opamp for this application is important! RadInspector

  18. Analog Pulse Shaping • Charge is integrated into a pulse with exponential decay (RC) • Initial “step” contains all the information we need, but preamp signal is noisy. • Charge amplifier pulse magnitude: ~1 – 10mV. Pulses close to noise floor. • Reduce signal bandwidth to limit noise and improve SNR RadInspector

  19. Noisy Charge Amplifier Data CR-3RC Filtered RadInspector

  20. Analog Pulse Shaper Implementation • Flexible 4 stage implementation using quad opamp • PCB design includes extra pads for each stage • Reconfigurable: integrator, inverting amplifier • Configure for analog pulse shaping or signal conditioning (DSP) RadInspector

  21. Analog Issue: Baseline Shift RadInspector Dependent on differentiator , pulse repetition period Amplitude measured is lower than what it should be Exaggerated by higher order low pass filter Analog baseline restoration relatively complex

  22. Digital Pulse Shaping • Replace analog pulse shaper with signal conditioning (antialiasing + gain) • Implement DPP using IIR or FIR filters, custom filters (trapezoidal, MWD) • Extremely flexible. Accuracy only limited by ADC and floating point precision RadInspector

  23. Digital Pulse Processing FIR filter: 12thorder low pass, RadInspector

  24. Digital Pulse Processing MWD signal integration: Separate pulses in “pile-up” region -> improve count rate and smooth peaks RadInspector

  25. Digital Peripherals RadInspector

  26. Microcontroller The ARM Cortex M4 based STMicro STM32F303VCT6 microcontroller High speed (72MHz), large flash (256K), and large RAM (48K) Integrated hardware floating point unit (FPU) High speed ADC peripheral Up to 5Msps 100 GPIO pins Cheap development board available STM Discovery F3 @ $9 RadInspector

  27. LCD SainSmart 3.2" TFT LCD Display+TouchPanel+PCB adapter SD Slot Standard widely used LCD controller (SSD1289) Standard interface controller (SPI interface) Inexpensive Size Parallel interface 20 GPIO pins 16 for data and 4 for control RadInspector

  28. Microcontroller LCD DATA LCD CONTROL USB DEBUGGING INTERFACE SPI LCD DATA STM32F303VCT6 microcontroller schematic RadInspector

  29. MCU Program Flow RadInspector

  30. Signal Acquisition RadInspector

  31. Power Management Overview RadInspector

  32. Battery Technologies • Lithium Ion Battery • Popular for portable electronic design • Little to no memory effect • Fast charging and high energy density per cell • 2- Cell Series Configuration • Utilize the higher voltage for linear step-down regulation RadInspector

  33. Charging Methodology RadInspector

  34. Power Regulation RadInspector

  35. High Voltage Bias Achieve high voltage to bias the semiconductor detector Larger electric field will sweep more charges to the preamplifier Utilization of the LT1930 boost converter IC with external voltage multiplier configuration 90 volts output at just a few µAmps RadInspector

  36. Next: Plot Spectrum • Perform system calibration: • Energy: ADC amplitude (channel) -> energy (keV) • Absorption efficiency of CZT • Correct for CZT charge trapping defects • Plotting procedure: • Bin samples according to amplitude. • Plot histogram of energy vs counts Americium-241 Energy Peaks RadInspector

  37. Experimental Energy Calibration Ba-133 Na-22 Cs-137 Ba-133 Na-22 Cs-137 88 keV Bin 851 Bin 2716 511 keV Bin 3463 662 keV channels energy E = m(ch)+b RadInspector

  38. Spectral fitting: Cd-109 88 keV peak clearly identified FWHM centroid RadInspector

  39. Cs-137 662keV peak clearly identified Compton back-scatter Compton edge centroid FWHM RadInspector

  40. RadInspector Schematic RadInspector

  41. Main PCB Stats: 1420 Traces 298 Vias / through holes 189 Components 546 SMD PADS Most resistors / caps are 0603 RadInspector

  42. Budget & Finance Total: $469.25 RadInspector

  43. Work Distribution RadInspector

  44. Issues • Semiconductor detector • Funding RadInspector

  45. QUESTIONS? RadInspector

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