Nuclear Instruments and Methods in Physics Research B 296 (2013) 78–81

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Nuclear Instruments and Methods in Physics Research B

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Energy resolution measurement and application of the F series ORTEC SSBdetector in TOF-ERDA spectrometry

Zdravko Siketic ⇑, Iva Bogdanovic RadovicRu -der Boškovic Institute, Bijenicka c. 54, 10000 Zagreb, Croatia

a r t i c l e i n f o a b s t r a c t

Article history:Received 9 October 2012Received in revised form 7 November 2012Available online 3 December 2012

Keywords:ORTEC SSB detector-F seriesHeavy ionsEnergy and mass resolutionTOF-ERDA

0168-583X/$ - see front matter � 2012 Elsevier B.V.http://dx.doi.org/10.1016/j.nimb.2012.11.006

⇑ Corresponding author. Tel.: +385 1 4561 012; faxE-mail address: zsiketic@irb.hr (Z. Siketic).

In the Elastic Recoil Detection Analysis (ERDA) and Heavy Ion Rutherford Backscattering Spectrometry(HIRBS), silicon surface barrier (SSB) detectors as well as other types of Si charged particle detectorsare conventionally used for measuring ion energy due to their simplicity and ease of use. Energy resolu-tion of such detectors is a limiting factor for both, mass and depth resolution in the experiment. In thepresent work we have studied performance of the commercially available F series ORTEC SSB detectordesigned in particular for heavy ion spectroscopy (BF-023-300-60). Detector energy and mass resolutionwere measured for a wide range of ion masses (7Li, 16O, 28Si, 35Cl and 81Br) and energies (2–20 MeV).Obtained results were compared with the already published data for a standard A series ORTEC SSBdetector (BA-017-100-100) and gas ionization chambers. Possible application of the F series SSB detectorin time-of-flight ERDA (TOF-ERDA) spectrometer was discussed.

� 2012 Elsevier B.V. All rights reserved.

1. Introduction

Ion beam analysis (IBA) techniques ERDA and HIRBS are power-ful methods for measuring elemental composition and depth pro-files of elements in the thin films [1–3]. Requirements on theanalysis of thin film composition are pushed nowadays towardsthe nanometer scale demanding more and more sophisticated en-ergy detectors with superior energy resolution to minimize theirinfluence on mass and depth resolution of the measurements.

In the ERDA and HIRBS measurements ions heavier than 7Li arenormally used. It is well known that energy resolution of the Sicharged particle detectors for those ions is significantly worse thanfor light ions, H and He.

There are three main contributions to the intrinsic energy reso-lution of the Si charged particle detector [4]: (a) entrance windoweffect (energy loss and straggling through the entrance window),(b) nuclear collision effect (non-ionizing energy loss) and (c) plas-ma effect (dense cloud of produced electron–hole pairs creates azero initial electric filed region and consequently recombinationof the charge carriers can occur). By reducing those contributions,in particular plasma effect, energy resolution of the detector can beimproved. Recently research on the gas ionization chambers (GIC)has shown that those detectors have better energy resolution thanSi charged particle detectors for heavier ions and therefore aremore suitable for TOF-ERDA and HIRBS [5–7]. The only problemin using GIC detector is a thin entrance window (�50 nm) and

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therefore special care must be taken during detector operation.However if gas ionization detector is not available, energy resolu-tion of the Si charged particle detector can be improved by increas-ing the value of the internal electric field in order to reduce plasmaeffect (delay time and plasma time decrease) [4].

In the present work commercially available F series ORTEC SSBdetector (BF-023-300-60) designed especially for heavy ion spec-troscopy was studied. Detector is characterized with a high valueof an electric filed under the entrance window (P15 kV/cm). Sofar, energy resolution of F series SSB detector was measured inthe low energy range (up to 3 MeV) for several different masses[8,9]. Since TOF-ERDA measurements are usually performed athigher energies, detector energy resolution was determined for awider range of ion masses and energies. Obtained results werecompared with the published data for GICs as well as with stan-dard A series Ortec SSB charge particle detector (BA-017-100-100).

As mentioned before, energy resolution affects the mass resolu-tion of the experiment, which is very important in TOF-ERDA.Therefore, in addition to the energy resolution, measurements ofthe TOF-ERDA spectrometer mass resolution using F series SSBdetector are presented.

2. Experimental setup

Energy resolution measurements were performed for differention species (7Li, 16O, 28Si, 35Cl and 81Br) in the 2–20 MeV energyrange using new (undamaged) detector. Ions were acceleratedusing 6 MV Tandem Van de Graaff accelerator, and scattered to-ward the TOF-ERDA telescope using thin gold target (30 nm of

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120ORTEC F series, present measurementsGas ionization chamber, Ref. [6]Gas ionization chamber, Ref. [7]ORTEC A series, Ref. [11]

E (k

eV)

E (keV)

16O

Fig. 2. Energy resolution of the F series ORTEC SSB detector as a function of energyfor 16O ions compared with the data existing in the literature.

240

Z. Siketic, I. Bogdanovic Radovic / Nuclear Instruments and Methods in Physics Research B 296 (2013) 78–81 79

Au on the thick Si substrate). F series ORTEC SSB detector wasmounted at the end of the TOF-ERDA spectrometer. More detailsabout TOF-ERDA spectrometer can be found in Refs. [2,3]. Inci-dence angle of the ions was 20� toward the sample surface, andscattering angle was 37.5� toward the beam direction. Countingrate during the experiment was kept in all cases below 1000 Hz.Total fluence received by the detector during all performed mea-surement was �4 � 106 particles/cm2, two orders of magnitudelower than the threshold fluence after which detector resolutionstarts to degrade (�108 particles/cm2).

Electronic noise was monitored using test pulse from the ORTEC419 Precision Pulse Generator connected to the test input of theCANBERRA 2003BT charge sensitive preamplifier. Pulse from thecharge sensitive preamplifier was shaped and amplified usingCANBERRA 2022 amplifier with a 0.5 ls timing constant.

Energy spectra of the ions scattered from the gold target weresimulated using simulation code SIMNRA [10]. Energy stragglingin the timing foils of the TOF-ERDA spectrometer was included inthe simulation. Energy resolution of the F series SSB detector wastreated as a free parameter, fitted to reproduce well energy spectraedge width for ions scattered from Au. For the each measurement,electronic noise was subtracted. It is worth to mention that due tothe pulse height defect, electronic noise does not have a constantvalue and is mass dependent.

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220ORTEC F series, present measurements

Gas ionization chamber, Ref. [6] Gas ionization chamber, Ref. [7]

E (k

eV)

E (keV)

28 Si

Fig. 3. Energy resolution of the F series ORTEC SSB detector as a function of energyfor 28Si compared with the data existing in the literature.

3. Results

3.1. Energy resolution of the F series SSB detector

Energy resolution of the F series ORTEC SSB detector for 7Li and16O as a function of the ion energy is shown on Figs. 1 and 2. As canbe seen from Fig. 1, energy resolution of the F series SSB detectorfor 7Li is worse than energy resolution of the GIC and A seriesSSB detector [11]. Data provided by the producer (ORTEC) givenoise width (FWHM) of 18 keV, and alpha resolution 26 keV(FWHM, including noise) for the F series detector used for thismeasurements. Measured noise level (using precision pulser), inthe case of 7Li ions, was 18.6 keV (FWHM), which is in good agree-ment with a value given by producer. Thus, it can be concludedthat F series ORTEC SSB detector is, concerning the energy resolu-tion for the light ions, worse than GIC and A series SSB detector[11]. In case of 16O (Fig. 2), energy resolution for ORTEC F series

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50ORTEC F series, present measurementsGas ionization chamber, Ref. [7]ORTEC A series, Ref. [11]

E (k

eV)

E (keV)

7Li

Fig. 1. Energy resolution of the F series ORTEC SSB detector as a function of 7Lienergy compared with the data existing in the literature.

detector is slightly better than one set of the GIC results publishedin Ref. [6] but is worse than another GIC data set [7]. It is interest-ing that A series SSB detector has almost the same energy resolu-tion as F series for 16O ions in the studied energy range.

Figs. 3 and 4 show energy resolution of the F series ORTEC SSBdetector for 28Si and 35Cl ions as a function of the ion energy. FromFig. 3 it is clear that in case of 28Si energy resolution of gas ioniza-tion chamber is almost two times better than energy resolution ofF series SSB detector. For 35Cl no data about the energy resolutionof GIC detector can be found in the literature and therefore F seriesdetector was compared only with A series detector [11]. FromFig. 4 can be seen that energy resolution for both ORTEC detectorsis almost the same.

Fig. 5 shows energy resolution of the F series ORTEC SSB detec-tor as a function of the ion energy for 81Br ions. Results for ORTEC Aseries SSB detector are also shown, however for energies lowerthan energies measured in our case.

Experimental data for ORTEC F series SSB detector for all mea-sured ions and energies are shown at Fig. 6. It can be seen that en-ergy resolution strongly depends on ion mass and it is worse forheavier masses and higher energies.

4000 6000 8000 10000 12000 14000 16000180

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340 ORTEC F series, present measurements ORTEC A series, Ref. [11]

E (k

eV)

E (keV)

35Cl

Fig. 4. Energy resolution of the F series ORTEC SSB detector as a function of energyfor 35Cl compared with the data existing in the literature.

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450

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800ORTEC F series, present measurements ORTEC A series, Ref. [11]

E (k

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E (keV)

81Br

Fig. 5. Energy resolution of the F series ORTEC SSB detector as a function of 81Br ionenergy compared with the data existing in the literature.

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100

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300

400

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8007Li16O28Si35Cl

81Br

E (k

eV)

E (keV)

Fig. 6. Energy resolution of the F series ORTEC SSB detector as a function of energyfor 7Li, 16O, 28Si, 35Cl and 81Br.

80 Z. Siketic, I. Bogdanovic Radovic / Nuclear Instruments and Methods in Physics Research B 296 (2013) 78–81

From all results can be concluded that in general GICs have sig-nificantly better energy resolution than ORTEC F series detector.Also, energy resolution of ORTEC F series is comparable, in the caseof 7Li even worse, to that of the standard ORTEC A series SSB detec-tor, making questionable manufacturers intention to classify it asspecially designed for heavy ion detection.

3.2. Mass resolution of the TOF-ERDA spectrometer using ORTEC Fseries SSB detector

Besides depth (energy) resolution, for TOF-ERDA experimentsmass resolution is also very important. In the TOF-ERDA experi-ment, recoiled ion energy is measured in coincidence with its timeof flight between two timing stations separated by 0.5 m. Due tothe better energy resolution time axis is usually used instead ofthe energy axis to extract information about elemental depth pro-files [2,3]. Information from the energy axis (SSB detector) is usedonly for the mass separation.

Using known relation between kinetic energy and velocity, rel-ative mass resolution is given by: Dm=m ¼ ððDE=EÞ2 þ ð2Dt=tÞ2Þ1=2.Relative time resolution Dt/t is much better than relative energyresolution in the energy range used for TOF-ERDA [2]. Therefore,energy resolution of the SSB detector is crucial for the mass resolu-tion of the system.

To determine mass resolution of the applied F series SSB detec-tor, energy of the scattered ions from the thin Au target was mea-sured in coincidence with the TOF (F series SSB detector was usedfor energy detection). Widths of the time and energy edges for ionsscattered from Au were fitted using error function in order to ob-tain Dt and DE. Mass resolution was calculated according to aboverelation.

Energy dependence of the mass resolution for 28Si, 35Cl and 81Brions is shown at Fig. 7. Mass resolution dependence for 7Li and 16Oions is not displayed while it is significantly below 1 amu for thestudied energy range. From the figure can be seen that mass reso-lution depends strongly on the mass and energy of the detectedion. For 81Br mass resolution is around 5 amu while for 28Si isaround 1 amu.

For an additional test we measured TOF-ERDA spectra of twosamples using 25 MeV 127I ions. Figs. 8 and 9 show mass spectraof the MgxByOz and AlxSiyOz samples calculated from the TOF-Ecoincidence map using multivariate analysis [12]. Higher part ofthe TOF-E energy spectra was used for the mass projection. To ex-tract mass resolution, multi peak Gauss fit was performed.

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0.51.0

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6.028Si35Cl81Br

m (a

mu)

E (keV)

Fig. 7. Energy dependence of the mass resolution for 28Si, 35Cl and 81Br ions.

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nts

m (amu)

Experimental Gauss fit

m=0.64

10B

11B 12C

16O

24Mg

25Mg26Mg

Fig. 8. Mass spectra of the MgxByOz sample.

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29Si

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Experimental Gauss fit 16O 27Al

28Si

Fig. 9. Mass spectra of the AlxSiyOz sample.

Z. Siketic, I. Bogdanovic Radovic / Nuclear Instruments and Methods in Physics Research B 296 (2013) 78–81 81

It can be seen that magnesium isotopes (Fig. 8.) are nicely sep-arated, and mass resolution is around 0.64 amu. In Fig. 9 it isshown that 27Al and 28Si are resolved with mass resolution of0.87 amu. Also, silicon isotopes are clearly separated.

Generally it is difficult to define fixed value for mass resolutionof the TOF-ERDA spectrometer due to the fact that it depends on

the energy as well as on the mass of the detected ion. From themeasurements we can conclude that mass resolution of our TOFERDA spectrometer, using F series ORTEC SSB detector, is 1 amufor mass 35 for energies above 10 MeV (Fig. 7.). As typical ion usedin TOF-ERDA is 25–40 MeV 127I, recoil energies of 35Cl fit into thatenergy range. Also, with this type of the detector, 27Al and 28Si canbe well separated with mass resolution better than 1 amu (Fig. 9).

4. Conclusions

To test ability of the F series ORTEC SSB detector, which is spec-ified as detector for heavy ion detection, we have performed en-ergy resolution measurement using different heavier ions (7Li,16O, 28Si, 35Cl and 81Br) in the 2–20 MeV energy range. From thecomparison of our measurements with the already published datafrom the literature for GIC, it can be concluded that energy resolu-tion of GIC is superior comparing to energy resolution of F seriesSSB detector. Another important advantage of GICs is that theyare not sensitive to the radiation damage. We have found thatSSB detector starts to degrade for about 108 fission fragments/cm2. Two advantages of SSB detectors are simplicity of handlingand operation. On the other hand, achievable mass resolution ofTOF-ERDA systems using F series SSB detector is 1 amu for mass35 amu, which is satisfactory. Using GIC mass resolution is better,close to 1 amu for mass 40 amu [6]. It is important to point out thatenergy resolution of the F series SSB detector for heavier ions isvery similar to the energy resolution of the A series SSB detector.There is actually no improvement in the energy resolution forthe heavy ion detection in the specially designed F series ORTECSSB detector.

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