Use of large volume LaBr3:Ce detectors for low-energy Coulomb excitation measurements

N. Marchini1,2, M. Rocchini2, A. Nannini2, G. Benzoni3, F. Camera3,4, D.T. Doherty5, M. Zielińska6, K. Hadyńska-Klȩk5,S.D. Bakes5,7,8, D. Bazzacco8, A. Goasduff8,9, A. Gozzelino7, A. Illana7, D. Mengoni8,9,F. Recchia8,9, M. Siciliano7,

D. Testov8,9, J.J. Valiente Dobón7, S. Ceruti10

1 Università degli Studi di Firenze, Firenze, Italy. 2 INFN, Sezione di Firenze, Firenze, Italy. 3 INFN, Sezione di Milano, Milano, Italy.4 Università degli Studi di Milano, Milano, Italy. 5 University of Surrey, Guildford, UK. 6 CEA Saclay, IRFU/SPhN, France.

7 INFN, Laboratori Nazionali di Legnaro, Legnaro (Padova), Italy. 8 INFN, Sezione di Padova, Padova, Italy.9 Università degli Studi di Padova, Padova, Italy. 10 KU Leuven, Instituut voor Kern-en Stralingsfysica, Leuven, Belgium.

INTRODUCTION

Low-energy Coulomb excitation is a well-establishedexperimental method used to study the electromagneticproperties of low-lying nuclear states, such as transitionprobabilities and quadrupole moments. The basic assump-tion of this method is that the excitation of the nuclearstates is caused solely by the electromagnetic field actingbetween the colliding partners, while the contribution ofshort-range nuclear forces can be neglected, allowing thenuclear structure to be studied in a model-independent way.

The most viable experimental technique for Coulombexcitation studies with heavy ions is high-resolutioncoincident γ-ray and particle spectroscopy, which requiresthe use of an array of γ-ray detectors coupled with a chargedparticle detector. A newly implemented set-up of thiskind has been successfully commissioned in 2016 at LNL,coupling the GALILEO γ-ray array [1] with the SPIDERheavy-ion detector [2]. At present, GALILEO is composedof 25 Compton suppressed HPGe detectors arranged in4 rings at 152◦, 129◦, 119◦ and 90◦ with respect to thebeam direction. SPIDER is composed of 7 segmented Sidetectors (each divided in 8 independent strips) assembled ina cone-like shaped configuration. The detector is positionedat backward angles in the GALILEO scattering chamber toavoid excessive radiation damage effects, covering a polarangular range from 123 to 163 degrees in the laboratoryframe.

When the number of the states populated in Coulombexcitation is limited, as in the case of even-even nuclei inproximity of the shell-closures, the resolution of the γ-raydetectors might not be of primary importance, while it isimportant to maximize the detection efficiency. To this aim,we have studied the capabilities of large volume 3”×3”LaBr3:Ce detectors [3] for Coulomb excitation studies.These detectors represent a powerful tool when investigatingγ-ray transitions with energy around 1 − 3 MeV: theytypically have one of the best energy resolution amongscintillators and their high density entails to high γ-rayefficiency.

Fig. 1. The GALILEO-SPIDER Coulomb excitation setup withthe addition of six LaBr3:Ce detectors, positioned in the forwarddirection.

SOURCE MEASUREMENTS AND SIMULATIONS

Fig. 1 shows 6 large-volume 3”×3” LaBr3:Ce detectorscoupled to the GALILEO-SPIDER setup. The distance ofthe detectors can be adjusted from ∼ 12 cm to ∼ 20 cm withrespect to the target position. In the following, we comparethe characteristics of one of the GALILEO HPGe detectors,placed at 22.5 cm with respect to the target position, withone LaBr3:Ce detector positioned at the same distance.

The LaBr3:Ce signals have been acquired using theGALILEO DAQ, as described in [4]. Two differentprocedures are available in the GALILEO DAQ to processthe signals: in the first approach the Long-Trace Energy(LTE) is computed from the input pulse using a MovingWindow Deconvolution (MWD) algorithm [5], tuned witha set of programmable parameters, while in the secondthe Short-Trace Energy (STE) is obtained analyzing thedigitized pulse in a time window around the triggeringsample and performing a simple pulse-height estimate.While the LTE estimate provides a better energy resolutionfor both GALILEO and SPIDER, we found that the bestenergy resolution for the LaBr3:Ce detectors is obtainedusing the STE approach. This is due to the fact that theprogrammable parameters of the MWD algorithm cannot be

0 500 1000 1500 2000 2500 30000.100

5.10–41.10–3

1.5.10–32.10–3

2.5.10–33.10–3

3.5.10–34.10–3

4.5.10–35.10–3

Energy [keV]

ε γHPGe exp

HPGe fit

LaBr3:Ce exp

LaBr3:Ce fit

Fig. 2. Experimental absolute photo-peak efficiency for aGALILEO HPGe detector (red) and a LaBr3:Ce detector (blue),both positioned at a distance of 22 cm from the source (located atthe target position). The experimental values have been fitted withan empirical function (continuous line).

optimized for the very fast LaBr3:Ce signals.Without sources in the scattering chamber we observed

the typical γ lines coming from the LaBr3:Ce natural activity(due to the presence of the unstable 138La isotope) andthe γ lines due to the presence of the 227Ac contaminantelement [6]. These γ-rays do not disturb Coulomb excitationmeasurements since γ-particle coincidences select the eventsof interest.

In typical Coulomb excitation experiments, the range ofγ-ray energies of interest is between ∼ 100 keV and ∼ 2−3 MeV, thus we concentrate our analysis in this energy range.In Fig. 2, the experimental absolute photo-peak efficiency ofthe GALILEO HPGe and the LaBr3:Ce detectors is reportedas a function of the γ-ray energy. Only those γ-lines ofthe sources 60Co, 152Eu, 133Ba, 88Y, which were clearlydistinguished in the LaBr3:Ce detector, have been included.The values have been deduced considering the sourcesactivity, the relative γ-ray intensities, the measurementduration and the acquisition dead time. As it is visible,a factor of two and even more is always gained using aLaBr3:Ce with respect to a GALILEO HPGe detector.

In Fig. 3 the FWHM for several γ-ray lines measuredwith the LaBr3:Ce is shown. The experimental points havebeen fitted with a quadratic function (continuous line), theresulted curve has been used as an input parameter fora Monte Carlo simulation aimed to investigate the finalFWHM that is possible to obtain after the Doppler correctionprocedure. A safe Coulomb excitation experiment of a60Co beam impinging on a lead target (1 mg/cm2 thick) hasbeen considered. The real geometry of SPIDER (7 sectors,cone-like configuration) and a LaBr3:Ce detector at 22.5 cmfrom the target position have been included. An isotropicγ-ray and particle emission distributions have been assumedand the energy loss effect in the target has been included.The results are shown in Fig. 3, dashed line. The simulatedcurve provides an estimation of the final FWHM which can

0 500 1000 1500 2000 2500 300010

20

30

40

50

6070

Energy [keV]

FWH

M [k

eV]

Intrinsic exp

Intrinsic fit

DC simulation

Fig. 3. Experimental FWHM for a LaBr3:Ce as a functionof the γ-ray energy (blue dots) has been fitted with a secondorder polynomial (continuous line). The FWHM obtained afterDoppler-correction in a simulated experiment is also shown(dashed line).

be obtained in a Coulomb excitation measurement usingSPIDER and LaBr3:Ce detectors.

CONCLUSIONS AND FURTHER PERSPECTIVES

The plots reported in Fig. 2 and Fig. 3 show the efficiencyand resolution of 3”×3” LaBr3:Ce detector mounted in theGALILEO-SPIDER setup. In Fig. 2 also a comparison withthe efficiency of a GALILEO HPGe detector is provided.The results hereby reported can be used to plan futureCoulomb excitation experiments using also the LaBr3:Cedetectors. A first experiments with GALILEO-SPIDER andthe LaBr3:Ce detectors has been performed in March 2018:in this experiment, a 94Zr beam of 370 MeV energy and1 pnA intensity was accelerated by the TANDEM-ALPIaccelerator, impinging on a 1 mg/cm2 thick 208Pb target.Aim of the experiment is to study how collectivity evolvesin Zr isotopes and the coexistence observed between varioussingle-particle configurations. On the basis of the performedsimulation, we expect that many of the transitions of interestfor this experiment will be resolved using the LaBr3:Cedetectors.

[1] J.J. Valiente-Dobon et al., LNL-INFN Annual Report 2014, 95(2015).

[2] M. Rocchini et al., Physica Scripta 92, 074001 (2017).[3] A. Giaz et al., Nuclear Instruments and Methods in Physics

Research A 729, 910 (2013).[4] G. Benzoni et al., LNL-INFN Annual Report 2015, 84 (2016).[5] D. Barrientos et al., arXiv:1406.3925v1.[6] R. Nicolini et al., Nuclear Instruments and Methods in Physics

Research A 582, 554 (2007).