Fast Digitizing and Digital Signal Processing of Detector ... · Fast Digitizing and Digital Signal Processing of Detector Signals. Diploma Thesis of Roland Hannaske, ... Fast Digitizing

Institute of Radiation Physics Roland Hannaske www.fzd.de Member of the Leibniz Association

Nuclear Safety Research

Fast Digitizing and Digital Signal Processing of Detector SignalsDiploma Thesis of Roland Hannaske, TU Dresden

Content: 1. Fast-Digitizer Data Acquisition2. Pulse-Height Analysis of HPGe Preamplifier Signals3. Time Resolution of Fast Scintillation Detectors

Seminar IKTP, TU Dresden, 20.11.2008

Institute of Radiation Physics Roland Hannaske www.fzd.de 20.11.2008


2

What means digitizing?

Fast Digitizing and Digital Signal Processing of Detector Signals 1. Fast-Digitizer Data Acquisition

digitizing = converting an analog voltage signal to a discrete signal(both in amplitude and time)



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+ resolution better energy and time resolution compared to analog systems

+ versatility digitized, recorded signals can be analyzed several times with different methods, application of digital methods

Fast-Digitizer Data Acquisition System (FDDAS)

Fast-Digitizer Data Acquisition


PC/serverDSP

(online/offline)

recorder(embedded PC)

fast digitizer(flash ADC)

- trigger preselection of signals (eventually by external trigger)

- hardware requirements transfer at very high data rates, storage and analysis of huge amounts of data

storage area network (SAN)

detector



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HPGe

high-purity Germanium detector (preamplifier signal)• rise time ≈

200 ns, decay time constant ≈

60 µs →

fsampling = 100 MHz (Δt = 10 ns)• 10 bit, 4 channels →

data rate = 760 MB/s

What means fast ?


audio CD• Nyquist–Shannon sampling theorem: fsampling > 2 · fmax

• fmax ≈

20 kHz →

fsampling = 44.1 kHz• 16 bit, 2 channels (stereo) →

data rate = 170 kB/s

BaF2

fast scintillation detector (photomultiplier signal)• rise time ≈

5 ns, decay time ≈

5 ns →

fsampling = 2 GHz (Δt = 0.5 ns)• 10 bit, 4 channels →

data rate = 15 GB/s



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nELBE FDDAS


Acqiris DC282Acqiris DC282 digitizing card- 4 channels, 10 bit resolution- 2 GS/s sampling rate, 2 GHz bandwidth- 256k points acquisition memory- dead time ≈

300 ns (trigger rearming)- time stamps, simultaneous multi-buffer data acquisition and readout

VMETRO C5000

VMETRO C5000 recording card- IBM 440GX PowerPC, OS: VxWorks, recording software- cPCI backplane (64 Bit / 66 MHz), max. data transfer rate ≈

400 MB/s (sustained)

Storage Area Network (SAN)- JBOD (Just A Bunch of Disks) = 12 x 300 GB, no redundancy- dual 2 GBit/s fibre channel connections

Analysis server- self-developed analysis program (C, ROOT)



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[figure from Knoll]

HPGe detector + charge-sensitive preamplifier = ballistic deficit

Fast Digitizing and Digital Signal Processing of Detector Signals 2. Pulse-Height Analysis of HPGe Preamplifier Signals

• incident γ-rays →

electron-hole pairs (ε

= 2.96 eV)• charge collection →

detector current• resistive feedback charge-sensitive preamplifier→ U(t) independent of Cd

→ discharging with time constant τ = Rf · Cf

→ reduced pulse height (ballistic deficit)→ rise time fluctuations lead to amplitude fluctuations

detector current(idealized)

detector charge

preamp. voltageballi

stic

def

icit



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Moving Window Deconvolution (MWD)


• restoring original charge signal from preamplifier signal• parameters → window length m, → decay time constant τ, → sampling interval Δt

Georgiev IEEE-TNS 40-4 (1993) 770Stein NIM B113 (1996) 145

m

detector charge

preamp. voltage

MWD signal

digitized preamp. signal

belonging MWD-signal



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Shaping algorithms


a) differentiation (CR filter)b) integration (RC filter)c) moving averaged) quasi-Gaussian (CR-(RC)n)e) triangular / trapezoidalf) cusp-like / truncated cusp-like

a), b), c) d) n=1, n=4

e), f): Jordanov NIM A345 (1994) 337

e), f)

input signal



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pulse height spectrum (DSP)

Experimental setup


• ORTEC GMX-100 (coaxial, n-type) • 22Na source (ε: 9.5%, β+: 90.5%)• analog signal processing with spectroscopy amplifier (ORTEC 671) and 14-bit peak-sensing ADC (SILENA 7423)

• digital signal processing (MWD + shaper) of signals acquired with nELBE FDDAS

22Na



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Energy resolution (FWHM)


@ 511 / 1275 keVanalog signal processing →

3.46 / 2.88 keVdigital signal processing (MWD + shaping):a) CR-(RC)12 →

7.55 / 10.52 keVb) trapezoidal →

4.69 / 5.39 keVc) truncated cusp-like →

4.75 / 5.37 keV

b) c)

a)



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nELBE: a neutron time-of-flight

Fast Digitizing and Digital Signal Processing of Detector Signals 3. Time Resolution of Fast Scintillation Detectors

• neutron cross section measurements (relevant for the transmutation of minor actinides in nuclear waste and for application to fission and fusion reactors)→

transmission, inelastic scattering• neutrons from (γ,n) reactions of bremsstrahlung in liquid-lead radiator

experiment at ELBE*

Electron Linac for beams with high Brilliance and low Emittance

*

Beyer NIM A575 (2007) 449• detectors: BaF2 , plastic scintillator (EJ-200)

• analog signal processing:- constant fraction discrim.- TDC (CAEN V1190A) with 100 ps per channel

• digital signal processing: - nELBE FDDAS



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Timing methods


a) leading edge timing (LET) →

amplitude walkb) extrapolated leading edge timing (ELET)c) zero-crossing shaper timing (ZCT)

→

CR-RC-CR filterd) zero-crossing constant-fraction timing (ZCCFT)

→

sn = a · vn - vn-m

e) constant fraction timing (CFT) →

pulse height analysis event by eventf) fitting constant fraction timing (FCFT)

→

as CFT, using fitting methods

c) d)

BaF2 signal

f)



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Neutron time-of-flight (tof)


• tof = detector time signal – reference time signal of accelerator• photon-flash peak followed by neutron distribution (≈

10 keV – 2 MeV)• time resolution: FWHM of photon-flash peak

photon-flash peak (ASP, BaF2 #9) tof-spectrum (DSP, plastic scintillator)



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Comparison of timing methods


• LET, ELET → not competitive• ZCT, ZCCFT → improved time resolution with BaF2 detectors• CFT, FCFT → best time resolution with both detector types



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Summary

Fast Digitizing and Digital Signal Processing of Detector Signals

The fast-digitizer data acquisition system (10 bit, 2 GS/s) of the nELBE experiment was tested successfully with different detector types and a self- developed analysis program.

Using digital timing methods (constant-fraction timing) the time resolution of BaF2 and plastic scintillation detectors improved compared to a state-of-the-art analog signal-processing system.

In pulse-height analysis of HPGe preamplifier signals, the ballistic deficit was corrected with the moving window deconvolution algorithm. The energy resolution of analog signal processing using a 14-bit ADC was not reached.

Publication: R. Hannaske, Wissenschaftlich-Technische Berichte, Forschungszentrum Dresden-Rossendorf, FZD-510, 2009



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Acknowledgement

Prof. R. Sauerbreysupervising professor and first reviewer

Prof. T. Cowansecond reviewer

Dr. A. Wagnersupervisor

Dr. A. Junghansproject leader nELBE

R. Beyer, Dr. P. Crespo, A. Dammrau, M. Fauth, Prof. E. Grosse, Dr. A. Matic, Dr. R. Schwengner

Nuclear Physics Division, Institute of Radiation Physics

Fast Digitizing and Digital Signal Processing of Detector Signals

www.efnudat.eu

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Fast Digitizing and Digital Signal Processing of Detector ... · Fast Digitizing and Digital Signal Processing of Detector Signals. Diploma Thesis of Roland Hannaske, ... Fast Digitizing