Development and Characterization of the Next-GenerationFront-End Electronics for the TRACE Detector Array

S. Capra1,2, A. Pullia1,2, R. J. Aliaga3, D. Mengoni4,5, P. R. John4,5, A. Gadea3, V. Herrero6

1 Physics Department, University of Milan, Milan, Italy. 2 INFN of Milan, Milan, Italy.3 Instituto de Fisica Corpuscolar, Paterna, Spain. 4 INFN, Sezione di Padova, Padova, Italy.

5 Dipartimento di Fisica dell’Universita di Padova, Padova, Italy. 6 Universitat Politecnica de Valencia, Valencia, Spain.

INTRODUCTION

The new facilities for the production of radioactive ionbeams, such as SPES (LNL), SPIRAL2 (GANIL), FAIR(GSI), will enable the study of nuclear structures far fromthe valley of stability as well as nuclear astrophysics. Inthese contexts, experiments where a very high counting rateand high energy range is expected are foreseen. Such ex-periments will benefit greatly from the high granularity andenergy resolution of modern particle detectors but will alsorequire particular specifications for the front-end electron-ics. Due to the increasing number of channels, for ex-ample, some logistical problems may rise. If the numberof channels is low (in the order of 100) each channel canby directly connected with a cable to the back-end. If thenumber of channels increases over 1k, smarter solutions be-come mandatory. In the case of the TRACE[1] detector ar-ray, where the expected number of channels is above severalthousands, all the front-end electronics must be placed in-side the reaction chamber and the number of connections tothe back-end should be minimized.

The solution we propose is to use integrated multichannelpreamplifiers together with analog memory ASICs[2]. Thelatter stores a time window of 1 µs for every channel thatfires. The data is then packed with relevant information suchas number of channel and timestamp and finally sent to theback-end which digitizes the signals and stores the informa-tion. Pulse-shape analysis techniques will be implementedin order to discriminate different particles impinging on thedetectors.

THE INTEGRATED MULTICHANNEL FAST-RESETPREAMPLIFIER

We designed and tested[3] a five-channel integratedcharge-sensitive preamplifier that can be seen in figure 1.Four channels are specifically designed for hole signals com-ing from the front pads of the detector. The last channel isdesigned for electron signals coming from the back. Thisone is separately powered and can be shut down if not used.The chip is realized in AMS C35 technology and works witha dual ±2.5 V power rail. Since the electronics must workin vacuum it is of paramount importance to keep the powerconsumption as low as possible. There is generally a trade-off between power consumption of a device and its perfor-

Fig. 1. This photograph of the preamplifier chip was taken with amicroscope attached to a PC through USB connection. The totalarea of the chip is 5 mm2.

mance, both in terms of bandwidth and noise. Even if in ourchip every channel dissipates only ≈ 12 mW, we managed toachieve an energy resolution in the order of 1 keV and a rise-time of only 10ns, which is good for the pulse-shape analy-sis we want to perform[4]. The front channels are equippedwith a smart device which reduces dead-time in case of sat-uration. Thanks to a clever design of the output stage, thepreamplifiers have an intrinsic dynamic range of ≈ 40 MeV.The back channel implements the Fast-Reset device whichnot only reduces dead-time but also boosts the natural dy-namic range by more than order of magnitude thanks to aTime-Over-Threshold technique. In this way an extendedenergy range, up to a factor 20 respect to the natural one, isachieved. A I2C engine provides additional flexibility to thecircuit, enabling an on-line adjustment of some critical pa-rameters, such as gain and bandwidth. In this way the samepreamplifier can be connected to different detector types ob-taining in each case the best risetime possible and avoidingunwanted ringing or instability.

EXPERIMENTAL RESULTS

In order to test the preamplifier we realized at first a smallPCB and characterized the chip using a pulser. We also con-nected the preamplifier to an open-source microcontrollerprogrammed as an I2C master. The ASIC is fully I2C com-pliant and the programmability feature works as expected.

Fig. 2. TRACE32ch v1.1 board connected to a TRACE silicon PADdetector.

Table 1. Absolute and relative FWHM values for the seven peaksin the spectrum of figure 3 .

E [keV] FWHM [keV] FWHM %

5143 39.45 0.775155 23.81 0.465388 135.36 2.515443 26.28 0.485486 21.56 0.395763 24.80 0.435805 22.02 0.38

After these preliminary results we designed and realized adedicated PCB (figure 2) with eight ASIC sockets for a totalof 32 channels. This board is realized with a Rogers 4003Claminate, which has a very low dispersion coefficient and isfully compatible with FR4 fabrication processes. The boardis compliant with the connector layout of the TRACE siliconpad detector prototypes. In order to evaluate the spectro-scopic performance of the system we put the board and thedetector into a vacuum chamber and acquired the spectrumof a triple Am-Cm-Pu alpha source. The signals were digi-tized with four N1728A CAEN modules. These are FPGA-powered 100 MHz 14-bit resolution digitizer cards with fourdifferential inputs each. These modules record simultane-ously the waveforms and the energies of the events calcu-lated on-line with adjustable digital trapezoidal filters. Thespectrum acquired from one of the front pads is reported infigure 3. The energy resolution for different peaks is re-ported in table 1 .

CONCLUSIONS

Even if the first results are very promising we are improv-ing the experimental setup in order to achieve better resolu-tion. In the meanwhile a new version of the ASIC pream-plifier is being designed. It will include more channels andimplement the time-over-threshold algorithm with an analog

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Energy [ MeV ]Fig. 3. Experimental spectrum of a triple 241Am-244Cm-239Pu al-pha source. The signals come from a front pad of a TRACE silicondetector prototype.

circuitry. In this way the fast-reset device used to boost thedynamic range of the back channel will no longer requireany digital post-processing: a time-to-amplitude converterwill produce analog signals with amplitude proportional tothe energy of the corresponding event.

At the moment discrete components are used as feed-back resistances because large integrated polysilicon resis-tors have noise performance that do not comply with ourspectroscopic requirements. This fact is due to their largecapacitive coupling to bulk: this phenomenon is being stud-ied both theoretically and experimentally[5]. Some solutionsare being investigated in order to integrate feedback resis-tors or equivalent active circuits inside our preamplifier chipwithout affecting the overall noise performance.

[1] https://web.infn.it/spes/index.php/nuclear-physics/spes-instrumentation/150 trace .

[2] Aliaga, R. et al. Conceptual design of the TRACE detectorreadout using a compact, dead time-less analog memory ASIC.Nuclear Instruments and Methods in Physics Research Sec-tion A: Accelerators, Spectrometers, Detectors and AssociatedEquipment 800, 34 – 39 (2015).

[3] Capra, S. et al. Design of an integrated low-noise, low-powercharge sensitive preamplifier for gamma and particle spec-troscopy with solid state detectors. To be published in 2014IEEE Nuclear Science Symposium and Medical Imaging Con-ference Record, NSS/MIC 2014, Seattle, WA .

[4] Mengoni, D. et al. Digital pulse-shape analysis with a TRACEearly silicon prototype. Nuclear Instruments and Methods inPhysics Research Section A: Accelerators, Spectrometers, De-tectors and Associated Equipment 764, 241 – 246 (2014).

[5] Capra, S. & Pullia, A. Study of the effects of parasitic ca-pacitance on large integrated feedback resistors for charge-sensitive preamplifiers. To be published in 2014 IEEE NuclearScience Symposium and Medical Imaging Conference Record,NSS/MIC 2014, Seattle, WA .