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Particle Identification using Digital Pulse Shape Analysisfor new generation NTD Silicon detector arrays

V. V. Parkar1,∗ J. A. Duenas1, D. Mengoni2, R. Berjillos1,A. M. Sanchez-Benıtez1, I. Martel1, M. Assie3, and D. Beaumel31Departamento de Fısica Aplicada, Universidad de Huelva, E-21071 Huelva, Spain

2Dipartimento di Fisica, Universita di Padova,via F. Marzolo, 8 - 35131 Padova, Italy and

3Institut de Physique Nucleaire, Universite Paris-Sud-11-CNRS/IN2P3, 91406 Orsay, France

Particle identification from silicon detectors with analog electronics using the energyloss, rise-time or time of flight information is known from few decades. However, there isa limitation in the pulse shape analysis (PSA) due to the non-homogeneity of the siliconwafer. The recent advances in semiconductor detector technology brought the use of NTD(Neutron Transmutation Doped) silicon for making detectors. The higher resistivityand more homogeneity from NTD technique has provided the opportunity for betterPSA. The high density, compact and efficient charged particle detector arrays namelyHYDE, GASPARD, FAZIA and TRACE being built in Europe will use NTD silicon formaking double sided silicon strip detectors (DSSSDs). At present, PAD detectors andfew prototype of DSSSDs which belongs to HYDE-GASPARD collaboration are beingused for doing some studies on Digital Pulse Shape Analysis (DPSA). The limits in the Zand A separation from DPSA as well as the lowest cutoff in energy are the main concernsof this study. The results from the recent experiments along with the future plans arediscussed here.

1. Introduction

The upcoming radioactive ion beam (RIB)facilities at Germany (FAIR, GSI) and France(SPIRAL2, GANIL) will provide new exoticnuclear species with higher intensities. Thenuclear reaction and spectroscopy studies withthese nuclei also demand for highly efficientdetector arrays with the modern technology.This has stimulated an interest in building thehigh density, compact and efficient chargedparticle detector arrays. The arrays namelyHYDE (Hybrid Detector array) [1], GAS-PARD (Gamma Spectroscopy and ParticleDetector) [2], FAZIA (Four π A and Z Identi-fication Array) [3] and TRACE (Tracking Ar-ray for Light Charged Particle Ejectiles) [4]being built in Europe will use NTD siliconstrip detectors along with the state-of-the-artelectronics to process the signal directly fromthe preamplifier. These arrays will consist ofNTD silicon strip detectors of various thick-

∗Electronic address: parkarvivek@gmail.com

nesses (20 µm - 2000 µm) [5]. The NTD sil-con is much more homogeneous than normalsilicon which is advantageous for better PSAand hence clean particle identification. Thebasic philosophy of all these detector arraysis to digitize the charge and current signalsfrom the preamplifier and store in the digi-tizer for further analysis. For better DPSA,it is well known that the detectors have to bemounted in low field injection mode [6, 7]. Thehigh bandwidth charge sensitive preamplifierPACI [8] is already fabricated and tested. Forthe digitizer, currently the commercial avail-able digitizers viz., CAEN [9] are being used.The recent experiments with these NTD-PADand DSSSD (prototype of HYDE-GASPARDarrays), supplied by Micron SemiconductorsLtd. [10], PACI preamplifier and different dig-itizers are presented here in the following sec-tions.

2. Experimental Details

This experiment was dedicated for findingthe lowest energy thresholds for light chargedparticles (Z=1,2) and also isotopic separation

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in DPSA [11]. The mono-energetic beams ofdeuterium at energies of 2, 2.5 and 10 MeVand proton at 2 MeV from TANDEM-ALTOaccelerator facility at IPN-Orsay, France werebombarded on a thin (∼ 100 µg/cm2) 197Autarget. In addition to it, we have also per-formed 7Li +12C reaction at 34 MeV. Theresulting charged particles from the reactionwere collected by a 500 µm NTD-Silicon (crys-tal orientation 〈111〉) detector placed at 55with respect to beam axis, and 8 out off re-action plane to avoid channeling effects. Thedetector (20×20 mm2) with a measured capac-itance of 106 pF and resistivity of 4200 Ω·cm,was operated at a bias of 300 V in a low-fieldinjection configuration. To avoid the influ-ence of the resistivity non-uniformity on thePSA [12, 13] the detector was collimated (∅ 3mm). It was connected to a PACI preampli-fier that provided charge Q(t) and current I(t)outputs with gain of 32 mV/MeV and 7000V/A respectively. The two signals were ac-quired using a four channel NIM-based card(N1728B from CAEN) with 14 bit, 100 MHzsampling rate, and a maximum input voltageof 1 V. It is important to mention that thepreamplifier was kept in the chamber just afew centimetres away from the detector andhad a water cooling system attached to ensureits stability along the experiment. Continuedclose monitoring of the detector leakage cur-rent showed values between 3 and 6 nA andthus there was no need for bias compensation.Moreover, the energy resolution in situ of thedetector plus electronic chain before and af-ter the experiment yields values of 18-22 keVusing a triple α-source. The typical experi-mental setup is shown in Fig. 1.

3. Digital Pulse Shape Analysis

The off-line analysis of the data was per-formed on the two output signals of thepreamplifier, which were digitized at a fre-quency of 100 MHz (see Fig. 2-top). We la-beled them as I(t) for the current signal, andQ(t) for the charge. The digital signal process-ing flow diagram is shown in Fig. 2-bottom.Both signals were baselined by simple averagealgorithm that took the samples just before

DAQ100 MS/s

14 bit

100 MS/s

14 bit

Reaction

products

3 mm

collimator Reverse

mount

NTD silicon

PACICharge

Current

Digitizers

FIG. 1: (Colour online) Typical setup during theexperiment.

the rising/falling of the signals, yielding Ib(t)and Qb(t). Then we separated pulses fromnon-pulses by setting a threshold discrimina-tor, obtaining Ith(t) and Qth(t), the currentand the charge signals, which underwent dif-ferent treatment. The energy information wasobtained by Qth(t) using a trapezoidal filter(for recursive shaping algorithm see for exam-ple [14]) with a risetime of 0.5 µs and a flattop of 1 µs. Its output Qtf (t) passed througha “find-maximum” algorithm to yield the non-calibrated Energy value. We curve fittedIth(t) to find its maximum (Imax), by applyinga cubic interpolation algorithm (third degreepolynomial equation). The number of interpo-lated points between two adjacent ADC sam-ples (10 ns spaced) was 50, yielding a new in-terpolated sampling rate of 200 ps. We foundthat the Energy - Imax correlation improves(i.e. better particle identification) as the num-ber of interpolated points increases, reachinga non-improvement situation for more than 50points.

4. Results and discussion

Figure 3 shows the correlation betweenEnergy and Imax for proton and deuteriumat 2 MeV. Good separation is achieved bytaking the maximum of the current signalafter fitting Icb(t) (see Fig. 3-top). However,if one takes the maximum directly from thedigitized signal without any fitting algorithmIth(t) (just the ADC values), then the identi-fication resolution is lost (see Fig. 3-middle).The projection histogram of the Imax for boththe cases can be seen in Fig. 3-bottom, wherethe separation between the proton (right

Proceedings of the DAE Symp. on Nucl. Phys. 57 (2012) 113

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time (ns)200 300 400 500 600 700 800

Am

plit

ud

e (a

.u.)

0

200

400

600

800

1000I(t)

Q(t)

Preamplifier current I(t) and charge Q(t) outputs

Deuterium at 2 MeV

FIG. 2: (Colour online) Top, preamplifier outputsfor a given event after digitization (100 MHz).The digital values are marked with asterisks, whilethe solid lines correspond to the interpolated val-ues. Bottom, digital signal processing flow dia-gram. The correlation of Energy vs Imax is usedfor particle identification.

side) and deuterium (left side) contributionsare well established in the fitting case. It isapparent that the sampling rate of our digi-tizer (10 ns) cannot reproduce the real shapeof the analog current signals, particularly itspointed part (see the lower peak of the currentsignal in Fig. 2-top). Therefore, the use of afitting algorithm is mandatory. Nevertheless,interpolation can however not modify theshape of the measured signal: indeed it wasshown by Hamrita et al. [8] that the currentsignal for low energy protons and deuterons(3 MeV) display a peculiar shape where one

(a.u.)maxI950 1000 1050 1100

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rgy

(MeV

)

1.96

1.98

2

2.02

2.04

from interpolation)maxMono-energetic beams at 2 MeV (I

d

p

(a.u.)maxI900 950 1000 1050 1100

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rgy

(MeV

)

1.96

1.98

2

2.02

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from ADC)max

Mono-energetic beams at 2 MeV (I

d

p

(a.u.)maxI950 1000 1050 1100

Co

un

ts

0

50

100

150

200

250

300d

p

projection using interpolated and ADC valuesmaxI

Interpolation

ADC values

FIG. 3: (Colour online) Energy vs Imax correla-tion for proton and deuterium at 2 MeV. Top, thevalues of Imax taken from the fitting of signals.Middle, the values of Imax taken from ADC sam-ples. Bottom, Imax projection of the above twocases.

can recognize the parts due to the electronsand holes migrations. The signal-to-noiseratio SNR (been defined as the maximum ofImax divided by the noise amplitude) for the2 MeV runs yielded gaussian distributions(not shown here) center at 12.26 and 12.45with standard deviations of 1.59 and 1.56 forproton and deuterium respectively.

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Figure 4 (top and midle) shows the Energyvs Imax correlation for the reaction productsobtained from a 34 MeV 7Li beam bom-barding a thin 12C target (information aboutthis reaction can be found at [15, 16]). Foridentification purposes the data from themono-energetic runs have been superimposed.The dynamic range, in our case dictated bythe maximum voltage (1 Vpp) at the ADCinput went from 1 to 18 MeV, which meanswe cannot see the elastically scattered 7Liparticles. The energy calibration was doneusing the deuterium mono-energetic beamsplus a triple alpha source. The deuterium line(labelled as d in the figures) was identifiedusing the mono-energetic beams, and thehelium one (labelled as α) using the datafrom the calibration alpha source. The datafrom the proton beam at 2 MeV also helpus to identify the proton line (labelled as p),see Fig. 4-midle. Good particle separation isobserved down to 3 MeV as shown in Fig. 4-bottom, where the projection of the Imax forenergies between 2950 and 3050 keV revealsthe proton, deuterium, tritium and alpha con-tributions. This energy threshold correspondsto a range in silicon of about 92.05, 60.97,49.51, and 12.04 µm respectively. Similarthreshold was found by Schmid et al. [17]for proton-alpha separation using an elec-tronic chain based on a time filter amplifier(TFA), leading edge discriminators (LED),and a time-to-digital converter (TDC).As the energy gets lower the alpha-tritiumseparation is lost, and then at even lower ener-gies, particle identification cannot be resolved.

Previous publications from FAZIA collab-oration [18–20] have also shown the Energyvs Imax correlation under different experimen-tal conditions, such as achieving Z (charge)identification down to α for particles fullystopped in a first layer of silicon. How-ever, FAZIA thresholds for PSA correspondto particle identification obtained from differ-ent kind of reactions using relatively low gainpreamplifier (due to their large dynamic range2-5 GeV involved), and therefore in FAZIA

(a.u.)maxI1000 2000 3000 4000 5000

Ene

rgy

(MeV

)

2

4

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10 α

d

mono-energetic deuterium beams

C at 34 MeV12Li + 7

(a.u.)maxI1300 1400 1500 1600

Co

un

ts

02468

1012141618 α

t

d

p

50 keV± projection for an energy window 3 MeV maxI

FIG. 4: (Colour online) Top and middle, Energyvs Imax correlation for the reaction 7Li + 12Cat 34 MeV. The distributions from the mono-energetic beams have been added to the reactiondata; the solid lines are used to guide an eye. Bot-tom, Imax projection for an energy window of 100keV centered at 3 MeV.

data Z-separation is not obtained for the lowenergy values studied in the present paper.A factor that may contribute to the qualityof the A/Z (Mass no./Atomic No.) separa-tion obtained here is the absence of heavierions. Other correlations such as Energy vs

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FIG. 5: (Colour online) Three prototype NTD Sil-icon strip detectors of thicknesses 100, 500 and1500 µm, from left to right respectively. At thetop one can see a dummy frame with the kaptonsand Molex connetors.

Risetime of both charge signal and currentsignal were studied with our data and they didnot show proton-deuterium separation, sincethe Risetime values (i.e. about 23 ns and 11ns for charge and current signal respectively)are at the very limit of the preamplifier ca-pability. From this observation, it would beadvisable to lower the bias applied to the de-tector in order to reduce the drift velocity ofthe charges. This finding is consistent with theFAZIA collaboration which recommend theuse of Imax rather than Risetime for Z < 10.

5. DPSA with NTD silicon stripdetectors

We have also received recently the proto-type of NTD silicon strip detectors of variousthicknesses (100, 500 and 1500 µm). The pic-ture of these detectors along with the connec-tors can be seen in Fig. 5.

We have taken the data recently with theseNTD silicon strip detectors. The same reac-tion of 7Li+12C have been used in this experi-ment at IPN -Orsay. The typical experimental

setup is shown in Fig. 6. We have used twotelescopes in this experiment having the fol-lowing configurations. The first layer (∆E1)is the 16×16 normal DSSSD (thickness = 40µm) from where the n-side strips have beenelectrically shorted together to be grounded,also the p-side strips have been electricallyshorted to read only one signal. The secondlayer (∆E2) of the telescope, for which theDPSA was applied (for particles stopping fullyin this layer) was the NTD-DSSSD (64×64)(thicknesses = 100, 500 µm). In this case,we collected the data only from the four cen-tral pins of p and n-side. The n-side (back-side mounting) was kept facing the reactionproducts for better PSA. The third layer (E3)was the 500 µm Si-PAD detector for stoppingthe reaction products leaving the second layer(∆E2). The first and third layer signals wereprocessed via the preamplifier-amplifier chainand sent it to peak sensing ADC. The 4×4strips of the middle layer meant for DPSAstudies were given to individual PACI pream-plifiers (Fig. 6-top). The charge and currentoutputs from the PACI were given to MAT-ACQ digitizer having 2.5 Gs/s sampling rate.A stainless steel collimator of size 3 mm×3mm was kept in front of the telescope as-sembly. The dedicated data acquisition sys-tem (Narval) [21] from GANIL was used. Inthe present three element telescope assem-bly, particle identification can be performedby using energy loss information from dif-ferent combinations (∆E1:∆E2, ∆E2:E3 and∆E1+∆E2:E3). For the particles stopped inthe second layer, additional information of∆E2:Imax and ∆E2:∆E2Risetime was also pos-sible. The typical plots of charge and currentoutputs from p and n side from one of thestrips are shown in Fig. 7. A good quality ofsignal can be seen with very low noise (noiselevel < 5 mV). The data analysis is in progressand will be reported shortly.

6. Summary

Thanks to both a high quality NTD-Si de-tector and a low noise electronic chain, iso-topic separation for Z=1 at 3 MeV havebeen accomplished in a 7Li+12C reaction

Proceedings of the DAE Symp. on Nucl. Phys. 57 (2012) 116

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∆E1∆E2

E3

PACIs

Beam

FIG. 6: (Colour online) Top, The schematic ofthe telescope consisting of ∆E1: Si1, ∆E2: NTDSilicon, E: si-PAD (see text for details). Bottom,the picture of experimental setup. The coolingsystem was also connected to all the PACIs for itsbetter performance.

making use of DPSA. This energy thresholdwent down to 2 MeV for proton-deuteriumidentification separation using mono-energeticbeams. It has also been observed that thecurrent signal of the charge sensitive pream-plifier is needed for low energy particle identi-fication, in particular the peak value ratherthan its Risetime, which is limited by thebandwidth of the preamplifier. A long sam-pling rate (in our case 10 ns, 100 MHz ADC)implies the use of interpolated algorithm toobtain a good fit of the current signal. Oth-erwise, faster ADC would be recommendedwhen dealing with light particles at low en-ergies. A trade-off between low threshold par-ticle identification and energy dynamic rangemust be considered.

n-side currentn-side charge

p-side current

p-side charge

FIG. 7: (Colour online) The typical charge andcurrent signals from n and p-side strips are shown.

The NTD silicon strip prototype detectorsof HYDE-GASPARD collaboration are nowavailable with us. We have already carriedout one experiment (reading only four centralstrips) to see the capabilities of DPSA on thesedetectors. In future, we will be investigatingthe DPSA method for all the X and Y stripsof the detectors and decide the limits of iden-tification of isotopes and lowest energy thresh-olds.

Acknowledgments

This work has been partially supported bythe Spanish Ministry of Science and Inno-vation (MICINN) under projects, FPA2010-22131-C02-01 (FINURA) and INGENIO-2010(CPAN), and by the Italian Ministry of Edu-cation, University and Research (MIUR) un-der the project FIRB08. The research lead-ing to these results has received funding fromthe European Union Seventh Framework Pro-gramme FP7/2007-2013 under Grant Agree-ment n. 262010 - ENSAR. The authors wouldlike to extend appreciation to the technicalstaff of Tandem ALTO from Orsay for theirassistance during the experiment.

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