The NUMEN Experiment aims to get information on the Nuclear Matrix Elements of the Neutrinoless Double Beta Decay, by measuring heavyion induced Double Charge Exchange (DCE) reactions cross sections. A good energy resolution is needed to clearly distinguish energy states of DCE products. To measure the energy of reaction products with the required resolution, the target must be thin and uniform to minimise dispersion and straggling effects on the ejectile energy. Few hundreds of nanometers of the target isotope are deposited on a Highly Oriented Pyrolytic Graphite substrate a few micrometers thick. The results of the characterisation of the first target prototypes of tin and tellurium are presented. The Scanning Electron Microscopy was used to qualitatively analyse the samples surface. A setup to study Alpha Particle Transmission has been assembled to measure thickness and uniformity of the targets; the thickness results have been verified by the Rutherford Backscattering measurements. To evaluate the effects of the thickness on the resolution of the DCE products energy, a Monte Carlo code has been implemented, using the measured thickness and uniformity as input data for the simulation

Calvo D.

Capirossi V.

Delaunay F.

Fisichella M.

Iazzi F.

Pinna F.

Rigato V.

English

PORTO@iris (Publications Open Repository TOrino - Politecnico di Torino)

18 October 2022POLITECNICO DI TORINORepository ISTITUZIONALEThickness and uniformity characterization of thin targets for intense ion beam experiments / Capirossi, V.; Delaunay, F.;Iazzi, F.; Pinna, F.; Calvo, D.; Fisichella, M.; Rigato, V.. - In: ACTA PHYSICA POLONICA B. - ISSN 0587-4254. -ELETTRONICO. - 51:3(2020), pp. 661-666. [10.5506/APhysPolB.51.661]OriginalThickness and uniformity characterization of thin targets for intense ion beam experimentsPublisher:PublishedDOI:10.5506/APhysPolB.51.661Terms of use:openAccessPublisher copyright(Article begins on next page)This article is made available under terms and conditions as specified in the  corresponding bibliographic description inthe repositoryAvailability:This version is available at: 11583/2843484 since: 2020-08-31T12:32:29ZJagellonian UniversityVol. 51 (2020) Acta Physica Polonica B No 3THICKNESS AND UNIFORMITYCHARACTERIZATION OF THIN TARGETSFOR INTENSE ION BEAM EXPERIMENTS∗V. Capirossia,b, F. Delaunaya,b,c, F. Iazzia,b, F. Pinnaa,bD. Calvob, M. Fisichellabfor the NUMEN CollaborationaDISAT — Politecnico di Torino, Torino, ItalybINFN — Sezione di Torino, Torino, ItalycLPC Caen, Normandie Université, ENSICAEN, UNICAEN, CNRS/IN2P3Caen, FranceV. RigatoINFN — Laboratori Nazionali di Legnaro, Legnaro, Italy(Received January 8, 2020)The NUMEN Experiment aims to get information on the Nuclear Ma-trix Elements of the Neutrinoless Double Beta Decay, by measuring heavy-ion induced Double Charge Exchange (DCE) reactions cross sections. Agood energy resolution is needed to clearly distinguish energy states of DCEproducts. To measure the energy of reaction products with the requiredresolution, the target must be thin and uniform to minimise dispersion andstraggling effects on the ejectile energy. Few hundreds of nanometers ofthe target isotope are deposited on a Highly Oriented Pyrolytic Graphitesubstrate a few micrometers thick. The results of the characterisation ofthe first target prototypes of tin and tellurium are presented. The ScanningElectron Microscopy was used to qualitatively analyse the samples surface.A setup to study Alpha Particle Transmission has been assembled to mea-sure thickness and uniformity of the targets; the thickness results have beenverified by the Rutherford Backscattering measurements. To evaluate theeffects of the thickness on the resolution of the DCE products energy, aMonte Carlo code has been implemented, using the measured thicknessand uniformity as input data for the simulation.DOI:10.5506/APhysPolB.51.661∗ Presented at the XXXVI Mazurian Lakes Conference on Physics, Piaski, Poland,September 1–7, 2019.(661)662 V. Capirossi et al.1. IntroductionOne of the most investigated topics in the modern nuclear physics is re-lated to the neutrino nature. In particular, many experiments are directlysearching for the Neutrinoless Double Beta Decay (0νββ) to find out if neu-trino is a Majorana or a Dirac particle. The 0νββ half-life is related to theNuclear Matrix Element (NME) that up to now is calculated with a con-siderable uncertainty by different nuclear models. The NUMEN project fitsin this context, measuring the cross sections of the heavy-ion induced Dou-ble Charge Exchange (DCE) reactions, whose NME shares many similaritieswith that of 0νββ [1]. NUMEN is a fixed target experiment based at INFN-LNS in Catania. Since DCE reactions are rare (cross sections range from fewnb/sr to some µb/sr), very intense ion beams are required to collect a sta-tistically significant set of data, in a reasonable time. For this reason, beamintensity will reach 50 µA with energy from 15 to 60 MeV/u. The NUMENtarget isotopes are also candidates for 0νββ. Some of the planned reac-tions that will be studied are: 116Cd(20Ne, 20O)116Sn, 130Te(20Ne, 20O)130Xe,76Ge(20Ne, 20O)76Se, 116Sn(18O, 18Ne)116Cd, 76Se(18O, 18Ne)76Ge. The in-tense beams will release a large amount of energy inside the target, thus anefficient cooling system is necessary. Therefore, target isotopes will be de-posited on a thin substrate of Highly Oriented Pyrolytic Graphite (HOPG),which will quickly transfer the heat outside the target thanks to its highin-plane thermal conductivity [2].2. Target requirementsDCE are detected by measuring the energy of the ejectile and a good en-ergy resolution is mandatory to clearly identify the reaction. There are somefixed contributions to the NUMEN energy resolution, due to the cyclotronand the spectrometer, evaluated as 1/1000 of the energy each. The targetfeatures must be also carefully evaluated in order to estimate the energydispersion and energy straggling effects.The energy straggling is due to statistical fluctuations of the number ofcollisions between the ion and target electrons. Straggling occurs both inthe target and in the HOPG substrate and is proportional to the square rootof the crossed thickness. Instead, the energy dispersion is related only tothe target, since it is due to the uncertainty on the depth where the DCEreaction takes place. This effect is proportional to the target thickness.For these reasons, the target has to be thin (300–500 nm, depending onthe isotope) and as uniform as possible, and the substrate thickness mustbe limited (around a few micrometers).Thickness and Uniformity Characterization of Thin Targets for Intense . . . 6633. Target productionFirst prototypes of tellurium and tin targets were produced by the Elec-tron-Beam Physical Vapour Deposition. Deposition parameters (substratetemperature, chromium buffer to facilitate the adherence) have been ad-justed in order to increase the uniformity of the target films [4]. First explo-rative prototypes were produced using a 10 µm thick backing. Once the bestdeposition parameters have been established, new samples were depositedon a 5 µm thick HOPG substrate. To minimise straggling and dispersioneffects, the tellurium and tin targets must be around 250 µg/cm2 thick.The Field Emission Scanning Electron Microscopy (FESEM) was usedto quickly and qualitatively examine the target surface. Figure 1 shows topview images of tellurium (B1) and tin (B6) depositions on 5 µm HOPG. Inthe case of tellurium, the substrate was kept at room temperature and nochromium buffer was used. The deposition is quite uniform and flat, withsome small structures. For the tin deposition, the HOPG was heated toaround 400 K, using a chromium buffer to obtain a uniform cover of thetin layer. A homogeneous deposition covers all the area, with some flat andirregular structures over an underlying layer made of smaller grains. Thesame study has been performed also on targets deposited on 10 µm HOPG:in general, there are no significant differences between the depositions onsubstrates of different thickness.(a) (b)Fig. 1. FESEM images of tellurium (a) and tin (b) depositions on 5 µm HOPG.4. Target thickness and uniformity evaluationLow-energy ion beams can be used to evaluate the thickness of a samplemeasuring the beam energy loss. In order to evaluate both thickness anduniformity of targets and substrates, a set-up to perform Alpha-ParticlesTransmission (APT) measurements has been assembled at Politecnico diTorino. The α particles are provided by a 241Am source and collimatedby a cylindrical collimator with diameter ' 3 mm (close to the size of the664 V. Capirossi et al.NUMEN beam). The energy of α particles after passage through the sam-ple under study was measured by an ion-implanted silicon detector, placeddownstream from the target. A chain of electronics (pre-amplifier, ampli-fier, ADC) and MAESTRO software [3] were used for data readout. Theenergy calibration of the set-up was performed before every data taking, byusing an additional source of 148Gd, to correct the electronics drift due totemperature variations.The obtained energy spectra were analysed in the ROOT framework,performing a fit of the peaks with Crystal Ball functions. The Crystal Ballpeak shape was preferred over a Gaussian shape because of the low-energytail of the APT energy spectra. Figure 2 (a) shows an example of APTspectra of the analysed tin targets, acquired before the deposition (onlyHOPG) and after the deposition (target + HOPG). The HOPG thicknessfor samples A19 and B6 was equal to 10 µm and 5 µm, respectively. The peakof the energy spectrum represents the most probable thickness crossed by thebeam, while the standard deviation σ is related to the thickness uniformityand to the energy straggling. The thickness of the target is given by theshift between the two peaks before and after the deposition. The targetnon-uniformity can be estimated from its contribution σtarget non−uniformityto the total standard deviation σtot asσtarget non−uniformity =√σ2tot − σ2HOPG − σ2target straggling − σ2detector+noise ,where σtot is the standard deviation of the distribution after the depo-sition, approximately given by the Crystal Ball fit, σHOPG is the samebefore the deposition (it contains HOPG non-uniformity and straggling),σtarget straggling is the standard deviation due to straggling inside the tar-get, and σdetector+noise takes into account the detector resolution and theelectronics noise.To verify the accuracy of the APT thickness results, the samples werealso systematically studied with the Rutherford Backscattering (RBS) at theAN2000 accelerator at INFN-LNL. The RBS measurements were performedwith an α beam of 2 MeV (1 mm in diameter), the scattered beam detectedat 150◦. The average energy of the low-energy edge of the spectrum isused to evaluate the average sample thickness. Figure 2 (b) shows, as anexample, the RBS spectra of the tin targets: the two deposition thicknessesare equal (the same average energy), even though the respective substrateshave different thicknesses. These RBS spectra correspond to the targetonly, without a HOPG substrate contribution, because of the low energyof the α beam. RBS is not suited to obtain quantitative information onthe thickness uniformity, even though the slope of the left edge of a RBSspectrum is somehow related to it.Thickness and Uniformity Characterization of Thin Targets for Intense . . . 665(a) (b)Fig. 2. (a) APT energy spectra of B6 and A19 tin samples before and after thetarget deposition. (b) RBS energy spectra of B6 and A19 tin targets.Table I shows the APT and RBS results of thickness and the APT resultsof uniformity for tellurium and tin targets. The agreement on the thicknessresults between the two analysis techniques is generally good, below 3% forall but B6, whose measurement was affected by a mistake in the energycalibration due to a drift of electronics. These results confirm that the APTset-up is a reliable tool to obtain not only the sample thickness but also aquantitative measure of the thickness uniformity.TABLE IThickness (APT and RBS) and uniformity (APT) results obtained for A7 and B1targets (tellurium on 10 and 5 µm thick HOPG, respectively) and for A19 andB6 targets (tin on 10 and 5 µm thick HOPG, respectively). ∆ is evaluated as|RBS−APT|(APT+RBS)/2 and the non-uniformity asσtarget non−uniformity (APT)thickness (APT) .Target Thickness Thickness ∆ Non-uniformity(APT) (RBS) (APT)A7 435 µg/cm2 440 µg/cm2 1.1% 17%B1 470 µg/cm2 480 µg/cm2 2.1% 17%A19 175 µg/cm2 180 µg/cm2 2.8% 29%B6 235 µg/cm2 195 µg/cm2 19% 28%A Monte Carlo code has been written to simulate the energy distributionof DCE ejectiles in order to evaluate the target effects on the energy res-olution. Experimental values of target thickness and uniformity, measuredby ATP, are used as input parameters. Contributions from the cyclotronand spectrometer are taken into account as well. The same cross sections666 V. Capirossi et al.for the ground state, first and second excited levels of the residual nucleuswere assumed. Figure 3 shows the energy distribution of the DCE ejectilein the case of a simulated reaction 130Te(20Ne,20O)130Xe at 15 MeV/u withthe target system B1 (Fig. 3 (a)) and the reaction 116Sn(18O,18Ne)116Cd at15 MeV/u with the target system B6 (Fig. 3 (b)). With the current tin andtellurium target non-uniformity, it is possible to clearly distinguish the sec-ond excited state. The separation between the ground state and the firstexcited level is much less evident and requires further analysis.(a) (b)Fig. 3. Energy distribution of DCE ejectiles with the target systems B1 and B6.5. ConclusionsPrototypes of tellurium and tin target for the NUMEN Experiment wereproduced and studied. Their characterization was performed with FESEM,APT and RBS. The APT technique is a useful tool to study both thicknessand uniformity of thin samples, as verified by the results on thickness ob-tained in the RBS measurements. Thickness and uniformity of the presentedtellurium and tin target prototypes are good enough to allow the identifica-tion of a DCE reaction, clearly distinguishing between the ground state ofthe target nucleus and excited levels higher than the first one.REFERENCES[1] F. Cappuzzello et al., Eur. Phys. J. A 54, 72 (2018).[2] F. Pinna et al., Nuovo Cim. C 42, 67 (2019) and references therein.[3] https://www.ortec-online.com/products/application-software/maestro-mca[4] V. Capirossi et al., Nucl. Instrum. Methods Phys. Res. A 954, 161122 (2020).

Thickness and uniformity characterization of thin targets for intense ion beam experiments

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Thickness and uniformity characterization of thin targets for intense ion beam experiments

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