Multichannel Analyzers Based on Digital Signal Processing

Valentin T. Jordanov

YANTEL, LLCDurham, NH 03824, USA

INTRODUCTION• Early 1990s first practical DSP

based MCA proposed

• 1992-1994, Real Time, Time-Invariant DSP algorithms developed

• Communication/Internet tech revolution leads to High-Resolution, Fast ADC development in late 1990s

• FPGA technology progress also follows the communication bum

• 2000+ Radiation Instrumentation Industry switches almost entirely to DSP technology

DET

PA

DIF

AMP

PKD

ADC

MCA

BASIC ANALOG SPECTROMETER

DET

PA

DIF

AMP

PKD

ADC

MCA

BASIC DIGITAL SPECTROMETER

FAST DISCRIMINATOR

FAST DISCRIMINATOR

PILE-UP REJECTOR

PILE-UP REJECTOR

RISE-TIME DISCRIMINATOR

RISE-TIME DISCRIMINATOR

Pulse Shaping Goals

• Noise Suppression (Resolution)

– White Noise: parallel and series;

– 1/f Noise: parallel and series;

• Throughput Optimization (Dead Time)

– Finite Pulse Duration;

• Ballistic Deficit (Resolution, Peak Distortion)

– Flat Top;

Optimum Shaping: White Noise

– Parallel

– Series

V. Radeka, IEEE Trans. Nucl. Sci. NS-15 (1968) p 455

Optimum Shaping: 1/f Noise

E. Gatti and M. Sampietro, Nucl. Instr. and Meth., A287, (1990) p513.

E. Gatti et al., Nucl. Instr. and Meth., A394, (1997) p268.

Digital Pulse Shape Synthesis

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Digital Pole-Zero Cancellation

X

ACC

M

+

+

R

C

v(n)

p(n)

q(n)

s(n)Σ

τ 1

X - multiplierACC - accumulatorΣ - adder

M ≈ τ1 /Tclk - 0.5

Dig

itize

r

τ 2= RC

τ 2

Pile-up EffectsPeak Detection

Real Time Operation

PA

DIF

SHP

PKD

ADC

MCA

DAC

DET

DIGITAL PULSE-SHAPE DISCRIMINATION

Pulse Processing for Pulse-Shape Discrimination

• Convolution of the Integrated Pulse with Impulse Response for Optimum Pulse-Shape Discrimination

• Amplitude Normalization

• Threshold Discrimination

Ratio Spectra at Different Energies

100 200 300 400 500 600Channel

100 200 300 400 500 600Channel

20-30 keV ee

M=0.78

30-60 keV ee

M=1.14

60-100 keV ee

M=1.59

100-150 keV ee

M=2.16

M=3.39

300-400 keV ee

400-500 keV ee

M=3.46

200-300 keV ee

M=2.95

M=2.55

150-200 keV ee

n γ

Neutron-Gamma Separation

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DSP

Digital MCA in the Near Future

Miniaturization

Spectrometer on a Programmable Chip

DIGITALPULSE

SHAPER

DIGITALGAIN

CONTROL

DIGITALFAST

SHAPER

MAINSHAPER

LLD

FASTDISCRIMINATOR

PILE-UPREJECTER

DUAL PORTMEMORY

CONTROLLER

REAL and LIVETIMERS

DIGITALBLR

RISE TIMEDISCRIMINATOR

AUTOMATICTHRESHOLDSCONTROLLER

DIGITALPEAK

DETECTOR

DEAD TIMEESTIMATOR

EXTERNAL ANDINTERNAL GATE

CONTROLLER

HOUSE KEEPINGAND INTERFACE

CIRCUITS

DATAACQUISITION

DIGITALOSCILLOSCOPE

COUNTERS

• High Density FPGA

• Low Power

• Flexible Design

• In Circuit Reprogram

• One Hardware Platform –Multiple Device Functions

Small Size and Low Power Benefits

• Integration of the Detector and the MCA in one package.

• Elimination of signal and power cables.

• Low Power and the Integration reduce the external noise pickup and ground loop effects.

• MCA is completely tuned to the attached Detector which eliminates most of the traditional adjustments –Pole/Zero, Gain, Polarity, HV polarity etc.

• MCA is always calibrated – user chooses energy range. Efficiency, calibration and other data storage.

• Extra power and space allows the integration of analytical software.

EXAMPLE: Niton XRF Analyzers

• Integrated Detector, MCA and X-Ray Source in One Package

• Ultra small, Ultra light

• Battery Operated

• Low Power

• Integrated Analytical Software

Field Use of Portable Digital MCA

CONCLUSION

Digital Pulse Processing Benefits• Improved Spectroscopy Performance

• High Count Rate, High Resolution

• Temperature Stability

• Improved Pulse Shape Analysis

• Size and Weight of Instruments

• Battery Operation

• Overall Flexibility, Hardware and Software Integration