GALTRACE is an array of segmented silicon detectors specifically built to work as an ancillary of the GALILEO γ-ray spectrometer at Legnaro National Laboratory of INFN. GALTRACE consists of four telescopic ΔE-E detectors which allow discriminating light charged particles also via pulse-shape analysis techniques. The good angular and energy resolutions, together with particle discrimination capabilities, make GALTRACE suitable for experiments where coincidences with specific emitted particles allow for the selection of reaction channels with very low cross section. The first in-beam experiment is reported here, aiming at identifying a narrow resonance, near-proton-threshold state in 11B, currently under discussion

Capra, S.

Dueñas Díaz, José Antonio

Galtarossa, F.

English

Arias Montano: Institutional Repository of the University of Huelva

DOI 10.1393/ncc/i2022-22098-3Communications: SIF Congress 2021IL NUOVO CIMENTO 45 C (2022) 98GALTRACE: A highly segmented silicon detector arrayfor charged particle spectroscopy and discriminationS. Capra(1)(2), S. Ziliani(1)(2), A. Goasduff(3)(4)(5), S. Leoni(1)(2),A. Pullia(1)(2), G. Benzoni(2), S. Bottoni(1)(2), F. Camera(1)(2),F. C. L. Crespi(1)(2), E. Gamba(1)(2), L. Iskra(2), B. Million(2),M. Polettini(1)(2), O. Wieland(2), B. Fornal(6), N. Cieplicka-Orynczak(6),M. Ciemala(6), J. Duenas(7), A. Gadea(8), D. Mengoni(3)(4),D. Brugnara(3)(4), L. Cortes(5), A. Gottardo(5), E. Gregor(5),G. Pasqualato(3)(4), F. Recchia(3)(4), J. J. Valiente Dobon(5),J. Pellumaj(5)(9), I. Zanon(5)(9), G. Zhang(3)(4)and F. Galtarossa(10)(1) University of Milan - Milano, Italy(2) INFN of Milan - Milano, Italy(3) University of Padua and INFN - Padova, Italy(4) INFN of Padua - Padova, Italy(5) INFN Legnaro National Laboratories - Legnaro, PD, Italy(6) Nuclear Physics Institute, Polish Academy of Sciences - Krakow, Poland(7) Universidad de Huelva - Huelva, Spain(8) IFIC, CSIC-Universitat de Valencia - Valencia, Spain(9) University of Ferrara - Ferrara, Italy(10) IPN-Orsay - Orsay, Francereceived 2 February 2022Summary. — GALTRACE is an array of segmented silicon detectors specificallybuilt to work as an ancillary of the GALILEO γ-ray spectrometer at Legnaro Na-tional Laboratory of INFN. GALTRACE consists of four telescopic ΔE-E detec-tors which allow discriminating light charged particles also via pulse-shape anal-ysis techniques. The good angular and energy resolutions, together with particlediscrimination capabilities, make GALTRACE suitable for experiments where coin-cidences with specific emitted particles allow for the selection of reaction channelswith very low cross section. The first in-beam experiment is reported here, aiming atidentifying a narrow resonance, near-proton-threshold state in 11B, currently underdiscussion.1. – IntroductionLight nuclei, such as carbon, oxygen, nitrogen, . . ., represent a fertile ground to searchfor new experimental evidence which can be exploited to benchmark modern theoreticalapproaches. In particular, such systems are an ideal laboratory to attempt a unified de-scription of the atomic nucleus as on open quantum system, where bound and unboundCreative Commons Attribution 4.0 License (https://creativecommons.org/licenses/by/4.0) 12 S. CAPRA et al.excitations are described on equal footing. While traditional Shell-Model approachesfail in describing weakly bound nuclei, recent theory models, like the Shell Models Em-bedded in the Continuum (SMEC) [1] provide, for example, first predictions for thestructure and decay properties of near-threshold resonance states. New challenging mea-surements are therefore needed, which can greatly benefit from the synergic interactionof high-resolution γ spectroscopy with high-performance particle spectroscopy and dis-crimination.The technological answer to such demand is the development of new-generation detec-tor arrays. High-Purity Germanium (HPGe) detectors constitute the gold standard in γspectroscopy due to their unpaired energy resolution. At the Legnaro National Laborato-ries (LNL) the resident HPGe array is GALILEO [2], comprising a total of 50 detectors:20 single Compton-suppressed crystals and 10 triple clusters with Compton shields. Theneed for a particle detector characterized by high granularity, high resolution, high an-gular coverage and good particle-discrimination capability led to the development ofGALTRACE [3], an array of segmented silicon detectors in ΔE-E telescopic configura-tion. Designed to fit inside the GALILEO reaction chamber, this array features a totalof 256 acquisition channels, distributed among four pairs of crystals. In order to copewith such a high-density application, a compact, integrated front-end electronics hasbeen developed. This is based on a custom ASIC pre-amplifier realized in 350 nm AMSCMOS technology. This pre-amplifier [4] ensures high energy resolution (up to 1.2 keVmeasured on a dedicated test-bench [5]), low-power consumption (≤15mW/channel) andwide bandwidth (between 15 and 45MHz, adjustable with digital slow-control [6]). Thesepre-amplifiers are designed to fully exploit the potential of the technology, minimizingthe 1/f noise at the input and optimizing the input capacitance. The only discretecomponent required by these pre-amplifiers is the 1GΩ feedback resistor. This choice isaimed at minimizing the parallel noise [7] and avoiding non-white noise components [8]produced by integrated feedback devices.The pre-amplifiers are equipped with an evolution of the fast-reset [9] circuit technol-ogy. The reset current generator draws charge directly from the input node of the firstpre-amp stage, extending the dynamic range of the pre-amplifier above its natural satu-ration limit imposed by the power supply voltage. In case of saturation, this techniquealso reduces the dead time roughly by a factor 1000 (from milliseconds to microseconds).The channels that perform the readout of the detectors’ non-segmented back electrodesimplement an innovative Time-To-Amplitude (TAC) circuit [10] that rejects the base-linestray contribution [11,12] from reset duration measurements.The first ΔE layer, segmented in 60 pads, is 200μm thick and the second layer,divided in 4 pads, is 1 or 1.5mm thick. The 4-by-4mm2 detector segmentation ensuresan angular resolution around 4◦ in φ, the azimuthal angle with respect to the verticalaxis, and between 3 and 4◦ in θ, the polar angle with respect to the beam direction.2. – Physics caseA perfect physics case to evaluate the performance of the GALTRACE silicon detectorarray, to be used as an ancillary of the GALILEO gamma spectrometer, is represented bythe decay of the unbound 11B nucleus. In recent experiments performed at ISOLDE andTRIUMF [13,14], a very rare β−-delayed proton emission was measured from 11Be: 11Bewas found to decay to 11B, that can in turn decay into 10Be after the emission of the newlycreated proton. Although the energy window available for this decay is only 280 keV, theprocess was observed with a branch (upper limit) of 10−6, namely two orders of magnitudeGALTRACE: A HIGHLY SEGMENTED SILICON DETECTOR ARRAY 3higher than predicted by theory. These results are compatible with the existence in11B of a resonant state, just above the proton-separation energy, unobserved thus far.An alternative approach to prove the existence of such resonance is the measurementof its possible gamma decay, which is predicted by the SMEC model with a branch of∼3·10−3 [15], namely within the experimental sensitivity of the GALILEO+GALTRACEarray.3. – Experimental setup and performanceA week-long experiment was performed at LNL in February 2021, employing the fusionreaction 6Li + 6Li to populate 11B after one-proton emission from the 12C compoundnucleus. The experimental setup consists of the GALILEO array, coupled with threeGALTRACE detectors placed inside the GALILEO reaction chamber. A 11μm-thickaluminum absorber was put in front of each telescope to protect it from the scattered 6Liions. The counting rate for each telescope detector was between 4 and 6 kHz. The signalswere acquired with the same 100MHz, 14-bit digitizers of the GALILEO array. Theenergy was calculated with on-line Moving-Window Deconvolution (MWD) algorithmsperformed on dedicated FPGAs, obtaining an average resolution of 60 keV on the frontpixels. The pulse-shape was analyzed partially on-line and partially off-line by applyingdigital algorithms.The GALTRACE array was used to select the protons emitted from the compoundnucleus 12C, feeding excited states in 11B, in coincidence with gamma rays detectedby the GALILEO spectrometer. This particle-gamma coincidence greatly enhanced theoverall sensitivity of the apparatus for the desired reactions, significantly suppressing thehuge background from uncorrelated gamma events. In panel (a) of fig. 1 the PSA matrixis shown for those particles that are stopped in the first telescope layer [16]. The chosenfigure of merit is the maximum signal derivative (Imax). Event clusters for protons,Fig. 1. – (a), (b): particle-identification matrices obtained respectively with the PSA algorithmand ΔE-E technique. (c), (d): projections of γ-γ matrices in coincidence with protons.4 S. CAPRA et al.deuterons and α particles are clearly visible and partially separable. Panel (b) showninstead the ΔE-E matrix built on the collection of particles that cross the first layer andare stopped in the second one.The combination of the information obtainable with these two techniques allows forefficient identification of the particles detected by the GALTRACE array. The selectionof the gamma events in coincidence with the desired particle ensures a great backgroundrejection and isolation of peaks otherwise absolutely not visible. In panels (c) and (d)of fig. 1 gamma spectra in coincidence with protons stopped in the ΔE layer (blue line)and stopped in the E layer (red line) are shown. Most of the visible peaks are identifiedwith the corresponding excited nucleus. The spectrum is divided in two halves: under2.5MeV (panel (c)) and above 2.5MeV (panel (d)).4. – Conclusion and perspectivesThe experiment discussed in this contribution represents the first in-beam validationof the GALTRACE array’s functionality. Future developments will be focused on thecoupling of GALTRACE with the AGATA array. Further improvements may include thepossibility to sample the pre-amplifier signals directly inside the reaction chamber withanalog memories [17]. The data analysis is ongoing and will contribute to shed light onthe existence of the near-threshold resonance in 11B which is currently under debate.REFERENCES[1] Okolowicz J., Ploszajczak M. and Rotter I., Phys. Rep., 374 (2003) 271.[2] Goasduff A. et al., Nucl. Instrum. Methods Phys. Res., Sect. A, 1015 (2021) 165753.[3] Capra S. et al., Nucl. Instrum. Methods Phys. Res., Sect. A, 935 (2019) 178.[4] Capra S. et al., in IEEE 2014 NSS/MIC - Proceedings (IEEE) 2016, p. 7431043.[5] Capra S. and Pullia A., IEEE Trans. Nucl. Sci., 67 (2020) 1877.[6] Capra S. et al., in IEEE 2015 NSS/MIC - Proceedings (IEEE) 2016, p. 7581814.[7] Capra S. and Pullia A., in IEEE 2014 NSS/MIC - Proceedings (IEEE) 2016, p. 7431042.[8] Capra S., IEEE Trans. Nucl. Sci., 67 (2020) 722.[9] Pullia A., Zocca F. and Capra S., Rev. Sci. Instrum., 89 (2018) 026107.[10] Pullia A. and Capra S., in IEEE 2015 NSS/MIC - Proceedings (IEEE) 2016, p. 7581811.[11] Pullia A. and Capra S., J. Instrum., 13 (2018) C12004.[12] Capra S., Secci G. and Pullia A., An Innovative Analog Circuit toRetrieve Energy Information from Signals of Deeply Saturated Pre-AmplifiersConnected to Semiconductor Detectors, IEEE Transactions on Nuclear Science,https://doi.org/10.1109/TNS.2022.3178760.[13] Riisager K. et al., Eur. Phys. J. A, 56 (2020) 100.[14] Ayyad Y. et al., Phys. Rev. Lett., 123 (2019) 082501.[15] Okolowicz J. et al., Phys. Rev. Lett., 124 (2020) 042502.[16] Cieplicka-Oryńczak N. et al., Eur. Phys. J. A, 54 (2018) 209.[17] Aliaga R. J. et al., Nucl. Instrum. Methods Phys. Res., Sect. A, 800 (2015) 34.

GALTRACE: A highly segmented silicon detector array for charged particle spectroscopy and discrimination

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GALTRACE: A highly segmented silicon detector array for charged particle spectroscopy and discrimination

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