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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
Nuclear Safety Research
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)
Institute of Radiation Physics Roland Hannaske www.fzd.de 20.11.2008
Nuclear Safety Research
3
+ 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
Fast Digitizing and Digital Signal Processing of Detector Signals 1. 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
Institute of Radiation Physics Roland Hannaske www.fzd.de 20.11.2008
Nuclear Safety Research
4
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 ?
Fast Digitizing and Digital Signal Processing of Detector Signals 1. Fast-Digitizer Data Acquisition
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
Institute of Radiation Physics Roland Hannaske www.fzd.de 20.11.2008
Nuclear Safety Research
5
nELBE FDDAS
Fast Digitizing and Digital Signal Processing of Detector Signals 1. Fast-Digitizer Data Acquisition
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)
Institute of Radiation Physics Roland Hannaske www.fzd.de 20.11.2008
Nuclear Safety Research
6
[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
Institute of Radiation Physics Roland Hannaske www.fzd.de 20.11.2008
Nuclear Safety Research
7
Moving Window Deconvolution (MWD)
Fast Digitizing and Digital Signal Processing of Detector Signals 2. Pulse-Height Analysis of HPGe Preamplifier Signals
• 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
Institute of Radiation Physics Roland Hannaske www.fzd.de 20.11.2008
Nuclear Safety Research
8
Shaping algorithms
Fast Digitizing and Digital Signal Processing of Detector Signals 2. Pulse-Height Analysis of HPGe Preamplifier Signals
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
Institute of Radiation Physics Roland Hannaske www.fzd.de 20.11.2008
Nuclear Safety Research
9
pulse height spectrum (DSP)
Experimental setup
Fast Digitizing and Digital Signal Processing of Detector Signals 2. Pulse-Height Analysis of HPGe Preamplifier Signals
• 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
Institute of Radiation Physics Roland Hannaske www.fzd.de 20.11.2008
Nuclear Safety Research
10
Energy resolution (FWHM)
Fast Digitizing and Digital Signal Processing of Detector Signals 2. Pulse-Height Analysis of HPGe Preamplifier Signals
@ 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)
Institute of Radiation Physics Roland Hannaske www.fzd.de 20.11.2008
Nuclear Safety Research
11
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
Institute of Radiation Physics Roland Hannaske www.fzd.de 20.11.2008
Nuclear Safety Research
12
Timing methods
Fast Digitizing and Digital Signal Processing of Detector Signals 3. Time Resolution of Fast Scintillation Detectors
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)
Institute of Radiation Physics Roland Hannaske www.fzd.de 20.11.2008
Nuclear Safety Research
13
Neutron time-of-flight (tof)
Fast Digitizing and Digital Signal Processing of Detector Signals 3. Time Resolution of Fast Scintillation Detectors
• 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)
Institute of Radiation Physics Roland Hannaske www.fzd.de 20.11.2008
Nuclear Safety Research
14
Comparison of timing methods
Fast Digitizing and Digital Signal Processing of Detector Signals 3. Time Resolution of Fast Scintillation Detectors
• LET, ELET → not competitive• ZCT, ZCCFT → improved time resolution with BaF2 detectors• CFT, FCFT → best time resolution with both detector types
Institute of Radiation Physics Roland Hannaske www.fzd.de 20.11.2008
Nuclear Safety Research
15
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
Institute of Radiation Physics Roland Hannaske www.fzd.de 20.11.2008
Nuclear Safety Research
16
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