- Email: [email protected]
ERDA at the 9 MV Tandem and at the 3 MV Tandetron of IFIN-HH
Nuclear Instruments and Methods in Physics Research B xxx (2017) xxx–xxx
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
ERDA at the 9 MV Tandem and at the 3 MV Tandetron of IFIN-HH H. Petrascu a,⇑, M. Petrascu a, D. Pantelica a, F. Negoita a, P. Ionescu a, M.D. Mihai a, T. Acsente b, M. Statescu a, A.C. Scafes a a b
Horia Hulubei National Institute for R&D in Physics and Nuclear Engineering, IFIN-HH, Magurele 077125, Romania National Institute for Laser, Plasma and Radiation Physics, INFLPR, Magurele 077125, Romania
a r t i c l e
i n f o
Article history: Received 3 August 2016 Received in revised form 8 December 2016 Accepted 7 February 2017 Available online xxxx Keywords: ERDA LI-ERDA RBS
a b s t r a c t Recoil spectrometry using heavy ions proposed in 1976 by L’Ecuyer has evolved into a universal IBA technique. Few years later an experimental setup for simultaneous light and medium heavy element detection including a compact DE(gas)–Er(solid) telescope, was developed at the Tandem accelerator of IFINHH. To increase the resolution, an integrated preamplifier was mounted close to the ionization chamber. The calibration procedure for the telescope and the software for the quantitative evaluation of the data are briefly presented. Recently, a 3 MV Tandetron accelerator has been installed and commissioned at the IFIN-HH. Among several ion-beam techniques for detection and depth profiling of hydrogen isotopes, Elastic Recoil Detection Analysis (ERDA) technique using a low energy 4He beam, proposed by Doyle and Peercy, is particularly advantageous. By measuring simultaneously both the H or D recoiling at a forward angle and backscattered 4He ions, a rather complete characterization of the sample can be achieved. Selected results from our investigations, obtained using these facilities, are presented. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction The Elastic Recoil Detection Analysis (ERDA) technique has been proposed in 1976 by L’Ecuyer et al. [1]. This method came out from heavy-ion nuclear physics following nuclear reaction studies in an inverse geometry, in which heavy projectiles are incident on lighter target nuclei. However, the use of an absorber in order to discriminate between scattered projectiles and recoiling atoms limits the method. This article presents the newest results of a work based on the experience accumulated in more than 30 years of research activity using ERDA technique in IFIN-HH. In 1984 an experimental setup for recoil spectroscopy, based on a compact DE(gas)–Er(solid) telescope, was developed at the Heavy Ion Department of IFIN-HH [2]. The telescope presented in Fig. 1, consists in both a DE pulseionization chamber and a residual energy Er silicon detector placed at the end of the ionization chamber. As can be seen from the figure the pulse ionization chamber contains three electrodes: the cathode 2, the anode 4, and the Frisch grid 3 whose role is to annihilate the dependence of the pulse height collected at the anode on the position between the grid and the cathode of the heavy ion path. The grid was made using 20 lm W-Au wire, having a transparency close to 95%. The anode and the cathode were made from 3 mm ⇑ Corresponding author. E-mail address: [email protected] (H. Petrascu).
thick Printed Circuit Boards (PCBs). The residual energy detector is a 100 mm2 active surface ion-implanted silicon detector having a depletion depth of 300 lm. As a filling gas, a mixture of 88% Ar + 12% CH4 is used, at pressures between 60 and 190 torr. By introducing a getter inside compartment 7, one could work with a high purity gas inside the ionization chamber. The telescope is placed inside the HV (High Vacuum) scattering chamber. The separation window between the gas volume of the telescope and the high vacuum zone of the scattering chamber is a 1.8 lm mylar foil, enforced with two 20 lm W wires. The operating voltages are +400 V for the anode and +150 V for the Frisch grid. The cathode was grounded. The resolution characteristics of this chamber were tested initially in a measurement performed at JINR, Dubna, on deep inelastic collisions in the system 40Ar(300 MeV) + 197Au [3] in which all elements in the range of Z = 2–22 could be distinguished, and afterwards in measurements on 19F(72 MeV) + 24Mg nuclear interaction [4], in which 13 elements with Z = 2–14 could be separated.
2. Experimental In order to improve the resolution of the ionization chamber, we tested the possibility to couple directly the preamplifier to the anode inside the scattering chamber. The preamplifier we used has 10 identical channels and is designed to work in HV. For each
http://dx.doi.org/10.1016/j.nimb.2017.02.015 0168-583X/Ó 2017 Elsevier B.V. All rights reserved.
Please cite this article in press as: H. Petrascu et al., ERDA at the 9 MV Tandem and at the 3 MV Tandetron of IFIN-HH, Nucl. Instr. Meth. B (2017), http://dx. doi.org/10.1016/j.nimb.2017.02.015
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H. Petrascu et al. / Nuclear Instruments and Methods in Physics Research B xxx (2017) xxx–xxx
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1 - Window 2 - Cathode 3 - Frish Grid 4 - Anode 5 - Teflon Insulator 6 - Si Detector 7 - Geer Space 8 - Anode HV 9 - Grid HV 10 - Si Det HV 11 - Vacuum and Gas Fill-in Pipe 12 - Metallic Mesh 13 - Support
Fig. 1. The DE–Er telescope.
channel, the main component is one CS AMP-3 integrated circuit, realized in Surface Mount Device (SMD) technology. We have chosen this technology in order to achieve both high amplification factor (>5000) and very low noise, with a good stability of these parameters in vacuum. The measurements with the preamplifier directly coupled to the anode of the ionization chamber were done first with an a source and then by using a heavy ion beam in ERDA geometry. The measurements with the a source have shown a 25% improvement in the resolution of the ionization chamber, comparing with the best resolution obtained using an external preamplifier. This was confirmed by the identification map shown in Fig. 2 obtained with a 63Cu beam on a glass sample. To get the simultaneous detection of very light elements (H and He) and of the heavier ones (C, O, Mg, Al), the two outputs from the preamplifier were fed into two main amplifiers, operated with high and low gain, respectively.
For the measurements, the telescope was placed at 30° with respect to the beam direction. The samples were placed at 15° with respect to the beam. The solid angle of the telescope was around 1 msr. The electronic setup was fairly conventional. The energy loss signal and the residual energy signal were fed to two analog-todigital converters that were parts of a multiparameter acquisition system based on a workstation, where the data was stored. The data acquisition software made possible on-line studies for both 1-dimensional and 2-dimensional data. The scattering chamber is made of aluminum having 781 mm in diameter and 227 mm in height. In the last couple of years, we upgraded the scattering chamber by adding both a digital movement controller and a gas handling system for the telescopes. Also, the getter was removed from the detectors’ volume. The digital movement controller has five stepper motors, four of them for detectors angular positioning and the fifth for target rotation. The system’s accuracy is better than 0.1°. A view of the scattering chamber’s content is shown in Photo 1. In this photography one can see two gas DE–Er telescopes mounted on two arms together with another two free arms. Longitudinal movement of all detectors can be also done but
Fig. 2. DE–Er spectrum of a glass sample.
Photo 1. Scattering chamber’s internal components.
Please cite this article in press as: H. Petrascu et al., ERDA at the 9 MV Tandem and at the 3 MV Tandetron of IFIN-HH, Nucl. Instr. Meth. B (2017), http://dx. doi.org/10.1016/j.nimb.2017.02.015
H. Petrascu et al. / Nuclear Instruments and Methods in Physics Research B xxx (2017) xxx–xxx
manually only. The gas pipes are blue and they are connected to the gas handling controller, outside the scattering chamber. The vacuum resistant 10 channel charge sensitive preamplifier can be seen in the picture, too. The gas controller has two working channels and is able to handle gas telescopes over a pressure range from few to 1000 torr. In fact, the gas handling system can even mix two pure gases in any concentration for one or more detectors, if required. Our configuration is shown Fig. 3. The system is based on a multi-channel flow ratio/pressure controller 647C, MKS Instruments type. The baratron is a regular one for 1000 torr full scale and the MFCs can manage up to 1000 sccm (standard cubic centimeter per minute) of Nitrogen. The system, as presented, can stabilize only the pressure. The flow rate of the gas is chosen roughly using the R6 needle valve. An upgrade of this system is already in progress, having as purpose the stabilization of the gas flow too, by adding to the actual system a 250E controller together with a V248 valve, both from MKS Instruments. The pressure’s value we work with was between 60 torr and 190 torr, P-10 gas. During the experiment the stabilization of the pressure was better than 1%. The gas flow rate was chosen 250 sccm. It is well known that ionization chambers request higher rates for the gas flow than other similar gas detectors, like proportional counters, to maintain their parameters constant. That’s why we preferred to use a rather higher than the usual 100 sccm gas flow rate for our small volume telescopes. One of the outstanding advantages of ERDA is the ability to provide accurate quantitative determination of elemental composition (at.%) of elements in the surface region of solid sample and depth profiling of thin films without need of calibration by external standards. A prerequisite for the precise quantitative evaluation of the ERDA data is the correct energy calibration of the data acquisition system. The data acquisition system is used to register all multiparametric events during the experiment; data are registered in list mode on disk for further processing. The total 2-parameter
3
DE–Er spectra are reconstructed after the experiment. A monoparametric total energy spectrum for each element is obtained by projections of the region delimited by a polygonal line corresponding to this element. However, to reconstruct the total energy spectrum we have to know the energy calibration for both the ionization chamber and the Si detector. Concerning the energy calibration of the Si detector, it is well known that such detectors exhibit a Pulse Height Defect (PHD) when used with heavy ions [5]. Semi empirical formulae for PHD calculations have been established by systematic measurements. The PHD depends on mass, charge and energy of the ion as well as on the resistivity and electric field strength characterizing the detector. In order to obtain reliable energy calibration for the Si detector, the pulse height versus energy response was measured for a variety of ions from 1H to 63 Cu, at energies ranging from 2 to 30 MeV. The ions were obtained by elastic scattering of the corresponding beams on a thin Au target. The energies for the elastically scattered particles were calculated from the known beam energy and the detector angle with respect to the beam, taking into account the energy loss of the incident and scattered ions in the gold foil. The foil thickness was measured in a separate experiment using Rutherford Backscattering Spectrometry (RBS) technique. The thickness of the Au foil used for calibrations was d = 700 Å. No change in the response of the silicon detector has been observed. Spectra measured without gas in the ionization chamber and without mylar window were used to investigate the pulse height versus energy relationship for different ions. For the same ions elastically scattered from the Au foil we measured two parameter DE–Er spectra at different gas pressures and ion energies. Using calculated energies for the elastically scattered ions, corrected for energy loss in the gold foil and in the mylar window, we get the total energy of the ions when entering the ionization chamber. The residual energy Er can be determined using the calibration curves measured for different ions. The difference between the total energy and residual energy represents the energy loss in gas. So, an energy calibration of the ionization chamber can be accomplished. The procedure followed to extract the information about the composition of a sample form experimental data taken in ERDA measurements consists in three steps: – getting the calibration coefficients from calibration runs; – build the total experimental energy spectra (histograms) for each sample, and for each element present in the sample; – adjust the parameters describing the sample composition so that the theoretical spectra reproduce the experimental ones. The first and second steps require handling of a biparametric list of events. The Physics Analysis Workstation (PAW) code [6], developed at CERN, has been chosen for this purpose. The numerical algorithm used to generate the simulated spectra consists of a loop over many thin sublayers of constant stoichiometry, each giving in the final spectrum a contribution approximated by the convolution of a rectangle and a Gaussian. Stopping powers are computed with the Stopping and Range of Ions in Matter (SRIM) code [7]; the energy straggling and energy response of the detector are taken as gaussian. The interactive modification of the input parameters is made by another routine. The link between these programs is done inside a PAW macro-procedure named SURFAN (SURFace ANalysis) [8].
3. Results and discussion
Fig. 3. Gas handling arrangement: MFC = mass flow controller; T = telescopes; M = manometers; B = baratron; R = valves.
Crystalline oxide ceramics as aluminate spinel (MgAl2O4) were identified as promising matrices for actinide transmutation. The behavior of He produced by the disintegration of actinides is a very
Please cite this article in press as: H. Petrascu et al., ERDA at the 9 MV Tandem and at the 3 MV Tandetron of IFIN-HH, Nucl. Instr. Meth. B (2017), http://dx. doi.org/10.1016/j.nimb.2017.02.015
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H. Petrascu et al. / Nuclear Instruments and Methods in Physics Research B xxx (2017) xxx–xxx
Fig. 4. DE–Er spectra of MgAl2O4 sample (normal and high amplification).
important issue. Fig. 4 shows the DE–Er spectra corresponding to normal and high amplification of a MgAl2O4 single crystal sample implanted with 30 keV He ions at a fluence of 2 1016 at./cm2. The spectra were taken with an 80 MeV 63Cu beam. The identification matrix corresponding to normal amplification shows O, Mg and Al, the components of the spinel. Because the surface of the sample was covered with a thin carbon layer in order to avoid charging effects, the carbon signal from the surface is also present. The H recoils from the sample surface and He recoils are seen in the identification matrix corresponding to high amplification of the DE signal. The total energy spectra are presented in Fig. 5. The results of a calculation using SURFAN code are presented with points. The evaluated detection limits are around 2-3 1014 at./cm2. The calculation reproduced quite well the experimental spectra in the case of C, O, H and He. The small differences for magnesium and aluminum are most probably due to imperfect knowledge of the stopping power. We also have to mention that our simulation program is using the Bragg rule of stopping power additivity in compounds and small differences might also occur between this model and their real values. Among several ion-beam techniques for detection and depth profiling of hydrogen isotopes, ERDA technique using the Light incident Ion (LI-ERDA) 4He beam, proposed by Doyle and Peercy [9], is particularly advantageous because all hydrogen isotopes can be profiled simultaneously with a sensitivity as high as 0.1 at.%, the measurements can be performed using a relatively low energy accelerator and the samples undergoes less damage compared to the use of high-Z analysis or Heavy incident Ion ERDA (HI-ERDA). The large recoil cross sections led to a rapid development of the technique. Recently, a 3 MV Tandetron accelerator, shown in Fig. 6 has been installed and commissioned at the IFIN-HH. The first beam line at 30° is dedicated to Ion Beam Analysis (IBA) experiments: ERDA, RBS, Particle Induced X-ray and c-ray Emission (PIXE and PIGE), Nuclear Reaction Analysis (NRA) and micro-beam experiments (l-PIXE). Charged particles are detected by two AMETEK type BU-012-050-100 solid state detectors, which are mounted at a fixed (165°) and movable (10°–150°) positions with a solid angular acceptance of 1.641 msr and 5.57 msr, respectively. The IBA scattering chamber is equipped with a total of six steppers: four on the sample manipulator (phi rotation, theta
rotation, tilt, lift) and a movable RBS-detector with six positions foil-changer. These step motors have some kind of reset function, where a pre-defined reference position is approached. The target holder is stainless steel disc of 44 mm diameter connected to an electric charge integrator. In order to prevent secondary electrons emission a 300 V positive bias is applied directly to the sample holder. A very important aspect in the engineering design activities of the fusion reactors is represented by the selection and study of the materials suitable for development of such plants. A continuous research effort is sustained for the fusion reactors design and material selection, state of the art solutions being adopted; still, some of the implemented technical solutions present drawbacks which have to be solved. Fig. 7 shows ERDA and RBS spectra corresponding of a CHD/Si sample deposited by magnetron sputtering. The IBA measurements were performed using a 2.5 MeV 4 He++ particle beam. The thin film samples to be investigated were mounted in a scattering chamber where a vacuum of 10 5 Pa was achieved. The energy of recoiled and backscattered particles was measured by the two ORTEC silicon detectors that were mentioned previously. A 3.2 mm diameter collimator was placed in front of the mobile detector, resulting in a solid angle of 1.000 msr. The experimental setup allows simultaneous investigations by ERDA and RBS of the samples. To filter out the scattered helium ions, mylar foils (11 lm thick) were placed in front of the mobile detector. All spectra were registered using conventional electronics consisting of a charge sensitive preamplifier, a linear amplifier and an analog to digital converter. Counting rates were always kept small enough in order to have a negligible dead time during the measurements. For ERDA–RBS measurements the samples were placed on the sample holder that had an incidence angle of 75° with respect to the surface normal. The recoiling hydrogen isotopes were observed at a laboratory angle of 30° corresponding to an exit angle of 75° with respect to the surface normal. The backscattered helium ions were detected at 165° with respect to the beam direction in the first detector. Quantitative determination of C, H, and D concentration and thickness was performed using the SIMNRA [10] code. Accurate simulation requires an accurate knowledge of the H and D differential cross section. To analyze our data, we used the cross sections reported by V. Quillet et al. [11]. The results are presented with a continuous line.
Please cite this article in press as: H. Petrascu et al., ERDA at the 9 MV Tandem and at the 3 MV Tandetron of IFIN-HH, Nucl. Instr. Meth. B (2017), http://dx. doi.org/10.1016/j.nimb.2017.02.015
5
H. Petrascu et al. / Nuclear Instruments and Methods in Physics Research B xxx (2017) xxx–xxx
Ge
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ine
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ed
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IBA et. 2
IBA s
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er
3 MV Tandetron
Fig. 5. Total energy spectra and simulation of the MgAl2O4 sample.
300 magnet
900 magnet
Fig. 6. The 3 MV HVE TandetronTM accelerator sketch.
Please cite this article in press as: H. Petrascu et al., ERDA at the 9 MV Tandem and at the 3 MV Tandetron of IFIN-HH, Nucl. Instr. Meth. B (2017), http://dx. doi.org/10.1016/j.nimb.2017.02.015
6
H. Petrascu et al. / Nuclear Instruments and Methods in Physics Research B xxx (2017) xxx–xxx
Fig. 7. ERDA and RBS spectra corresponding of a CHD/Si sample deposited by magnetron sputtering.
4. Conclusions A compact DE(gas)–Er(solid) telescope with an integrated preamplifier mounted close to the ionization chamber is well suited for ERDA with heavy ions. For a quantitative data analysis, a reliable calibration of both ionization chamber and Si detector is necessary. In particular, the pulse height defect in the Si detector must be taken into account. The measured energy spectra can be converted into depth profiles using the code SURFAN. Analysis of the hydrogen and deuterium with foil-ERDA in thin layers was performed with 4He from 3 MV Tandetron accelerator. The foil-ERDA has the advantage that the detector and the electronics are not overloaded. Thus, relatively high beam currents can be used resulting in good statistics in shorter measuring time. By measuring simultaneously both the H and D recoiling at a forward angle and backscattered 4He ions, a rather complete characterization of the sample can be achieved. Acknowledgements
acknowledged for the financial support through the project: Core Program No. PN09450103, Partnerships in Priority Areas Program No. 143/2012. References [1] J. L’Ecuyer, C. Brassard, C. Cardinal, J. Chabbal, L. Deschênes, J.P. Labrie, B. Terreault, J.G. Martel, R. St-Jacques, J. Appl. Phys. 47 (1976) 381. [2] M. Petrascu, I. Berceanu, I. Brancus, A. Buta, M. Duma, C. Grama, I. Lazar, I. Mihai, M. Petrovici, V. Simion, Nucl. Instr. Meth. B 4 (1984) 396. [3] I. Berceanu et al., Rev. Roum. Phys. 32 (1987) 733–742. [4] I. Berceanu et al., Rev. Roum. Phys. 31 (1986) 29–50. [5] J.J. Grob, Theses, Strasbourg, 1971. [6] CERN, PAW Manual Version 1.14, Application Software Group, Computing and Network Division, CERN, Geneva, Switzerland, 1992. [7] J.F. Ziegler, Handbook of Stopping Cross-Section for Energetic Ions in All Elements, Vol. 5, Pergamon, 1980. [8] D. Pantelica, M. Petrascu, F. Negoita, N. Scintee, H. Petrascu, A. Isbasescu, Report WP8, IDRANAP 10-01, 2001, p. 508. [9] B.L. Doyle, P.S. Peercy, Appl. Phys. Lett. 34 (1979) 811. [10] SIMNRA home page,
The National Authority for Scientific Research from The Ministry of Education, Research and Youth of Romania is gratefully
Please cite this article in press as: H. Petrascu et al., ERDA at the 9 MV Tandem and at the 3 MV Tandetron of IFIN-HH, Nucl. Instr. Meth. B (2017), http://dx. doi.org/10.1016/j.nimb.2017.02.015
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
ERDA at the 9 MV Tandem and at the 3 MV Tandetron of IFIN-HH H. Petrascu a,⇑, M. Petrascu a, D. Pantelica a, F. Negoita a, P. Ionescu a, M.D. Mihai a, T. Acsente b, M. Statescu a, A.C. Scafes a a b
Horia Hulubei National Institute for R&D in Physics and Nuclear Engineering, IFIN-HH, Magurele 077125, Romania National Institute for Laser, Plasma and Radiation Physics, INFLPR, Magurele 077125, Romania
a r t i c l e
i n f o
Article history: Received 3 August 2016 Received in revised form 8 December 2016 Accepted 7 February 2017 Available online xxxx Keywords: ERDA LI-ERDA RBS
a b s t r a c t Recoil spectrometry using heavy ions proposed in 1976 by L’Ecuyer has evolved into a universal IBA technique. Few years later an experimental setup for simultaneous light and medium heavy element detection including a compact DE(gas)–Er(solid) telescope, was developed at the Tandem accelerator of IFINHH. To increase the resolution, an integrated preamplifier was mounted close to the ionization chamber. The calibration procedure for the telescope and the software for the quantitative evaluation of the data are briefly presented. Recently, a 3 MV Tandetron accelerator has been installed and commissioned at the IFIN-HH. Among several ion-beam techniques for detection and depth profiling of hydrogen isotopes, Elastic Recoil Detection Analysis (ERDA) technique using a low energy 4He beam, proposed by Doyle and Peercy, is particularly advantageous. By measuring simultaneously both the H or D recoiling at a forward angle and backscattered 4He ions, a rather complete characterization of the sample can be achieved. Selected results from our investigations, obtained using these facilities, are presented. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction The Elastic Recoil Detection Analysis (ERDA) technique has been proposed in 1976 by L’Ecuyer et al. [1]. This method came out from heavy-ion nuclear physics following nuclear reaction studies in an inverse geometry, in which heavy projectiles are incident on lighter target nuclei. However, the use of an absorber in order to discriminate between scattered projectiles and recoiling atoms limits the method. This article presents the newest results of a work based on the experience accumulated in more than 30 years of research activity using ERDA technique in IFIN-HH. In 1984 an experimental setup for recoil spectroscopy, based on a compact DE(gas)–Er(solid) telescope, was developed at the Heavy Ion Department of IFIN-HH [2]. The telescope presented in Fig. 1, consists in both a DE pulseionization chamber and a residual energy Er silicon detector placed at the end of the ionization chamber. As can be seen from the figure the pulse ionization chamber contains three electrodes: the cathode 2, the anode 4, and the Frisch grid 3 whose role is to annihilate the dependence of the pulse height collected at the anode on the position between the grid and the cathode of the heavy ion path. The grid was made using 20 lm W-Au wire, having a transparency close to 95%. The anode and the cathode were made from 3 mm ⇑ Corresponding author. E-mail address: [email protected] (H. Petrascu).
thick Printed Circuit Boards (PCBs). The residual energy detector is a 100 mm2 active surface ion-implanted silicon detector having a depletion depth of 300 lm. As a filling gas, a mixture of 88% Ar + 12% CH4 is used, at pressures between 60 and 190 torr. By introducing a getter inside compartment 7, one could work with a high purity gas inside the ionization chamber. The telescope is placed inside the HV (High Vacuum) scattering chamber. The separation window between the gas volume of the telescope and the high vacuum zone of the scattering chamber is a 1.8 lm mylar foil, enforced with two 20 lm W wires. The operating voltages are +400 V for the anode and +150 V for the Frisch grid. The cathode was grounded. The resolution characteristics of this chamber were tested initially in a measurement performed at JINR, Dubna, on deep inelastic collisions in the system 40Ar(300 MeV) + 197Au [3] in which all elements in the range of Z = 2–22 could be distinguished, and afterwards in measurements on 19F(72 MeV) + 24Mg nuclear interaction [4], in which 13 elements with Z = 2–14 could be separated.
2. Experimental In order to improve the resolution of the ionization chamber, we tested the possibility to couple directly the preamplifier to the anode inside the scattering chamber. The preamplifier we used has 10 identical channels and is designed to work in HV. For each
http://dx.doi.org/10.1016/j.nimb.2017.02.015 0168-583X/Ó 2017 Elsevier B.V. All rights reserved.
Please cite this article in press as: H. Petrascu et al., ERDA at the 9 MV Tandem and at the 3 MV Tandetron of IFIN-HH, Nucl. Instr. Meth. B (2017), http://dx. doi.org/10.1016/j.nimb.2017.02.015
2
H. Petrascu et al. / Nuclear Instruments and Methods in Physics Research B xxx (2017) xxx–xxx
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1 - Window 2 - Cathode 3 - Frish Grid 4 - Anode 5 - Teflon Insulator 6 - Si Detector 7 - Geer Space 8 - Anode HV 9 - Grid HV 10 - Si Det HV 11 - Vacuum and Gas Fill-in Pipe 12 - Metallic Mesh 13 - Support
Fig. 1. The DE–Er telescope.
channel, the main component is one CS AMP-3 integrated circuit, realized in Surface Mount Device (SMD) technology. We have chosen this technology in order to achieve both high amplification factor (>5000) and very low noise, with a good stability of these parameters in vacuum. The measurements with the preamplifier directly coupled to the anode of the ionization chamber were done first with an a source and then by using a heavy ion beam in ERDA geometry. The measurements with the a source have shown a 25% improvement in the resolution of the ionization chamber, comparing with the best resolution obtained using an external preamplifier. This was confirmed by the identification map shown in Fig. 2 obtained with a 63Cu beam on a glass sample. To get the simultaneous detection of very light elements (H and He) and of the heavier ones (C, O, Mg, Al), the two outputs from the preamplifier were fed into two main amplifiers, operated with high and low gain, respectively.
For the measurements, the telescope was placed at 30° with respect to the beam direction. The samples were placed at 15° with respect to the beam. The solid angle of the telescope was around 1 msr. The electronic setup was fairly conventional. The energy loss signal and the residual energy signal were fed to two analog-todigital converters that were parts of a multiparameter acquisition system based on a workstation, where the data was stored. The data acquisition software made possible on-line studies for both 1-dimensional and 2-dimensional data. The scattering chamber is made of aluminum having 781 mm in diameter and 227 mm in height. In the last couple of years, we upgraded the scattering chamber by adding both a digital movement controller and a gas handling system for the telescopes. Also, the getter was removed from the detectors’ volume. The digital movement controller has five stepper motors, four of them for detectors angular positioning and the fifth for target rotation. The system’s accuracy is better than 0.1°. A view of the scattering chamber’s content is shown in Photo 1. In this photography one can see two gas DE–Er telescopes mounted on two arms together with another two free arms. Longitudinal movement of all detectors can be also done but
Fig. 2. DE–Er spectrum of a glass sample.
Photo 1. Scattering chamber’s internal components.
Please cite this article in press as: H. Petrascu et al., ERDA at the 9 MV Tandem and at the 3 MV Tandetron of IFIN-HH, Nucl. Instr. Meth. B (2017), http://dx. doi.org/10.1016/j.nimb.2017.02.015
H. Petrascu et al. / Nuclear Instruments and Methods in Physics Research B xxx (2017) xxx–xxx
manually only. The gas pipes are blue and they are connected to the gas handling controller, outside the scattering chamber. The vacuum resistant 10 channel charge sensitive preamplifier can be seen in the picture, too. The gas controller has two working channels and is able to handle gas telescopes over a pressure range from few to 1000 torr. In fact, the gas handling system can even mix two pure gases in any concentration for one or more detectors, if required. Our configuration is shown Fig. 3. The system is based on a multi-channel flow ratio/pressure controller 647C, MKS Instruments type. The baratron is a regular one for 1000 torr full scale and the MFCs can manage up to 1000 sccm (standard cubic centimeter per minute) of Nitrogen. The system, as presented, can stabilize only the pressure. The flow rate of the gas is chosen roughly using the R6 needle valve. An upgrade of this system is already in progress, having as purpose the stabilization of the gas flow too, by adding to the actual system a 250E controller together with a V248 valve, both from MKS Instruments. The pressure’s value we work with was between 60 torr and 190 torr, P-10 gas. During the experiment the stabilization of the pressure was better than 1%. The gas flow rate was chosen 250 sccm. It is well known that ionization chambers request higher rates for the gas flow than other similar gas detectors, like proportional counters, to maintain their parameters constant. That’s why we preferred to use a rather higher than the usual 100 sccm gas flow rate for our small volume telescopes. One of the outstanding advantages of ERDA is the ability to provide accurate quantitative determination of elemental composition (at.%) of elements in the surface region of solid sample and depth profiling of thin films without need of calibration by external standards. A prerequisite for the precise quantitative evaluation of the ERDA data is the correct energy calibration of the data acquisition system. The data acquisition system is used to register all multiparametric events during the experiment; data are registered in list mode on disk for further processing. The total 2-parameter
3
DE–Er spectra are reconstructed after the experiment. A monoparametric total energy spectrum for each element is obtained by projections of the region delimited by a polygonal line corresponding to this element. However, to reconstruct the total energy spectrum we have to know the energy calibration for both the ionization chamber and the Si detector. Concerning the energy calibration of the Si detector, it is well known that such detectors exhibit a Pulse Height Defect (PHD) when used with heavy ions [5]. Semi empirical formulae for PHD calculations have been established by systematic measurements. The PHD depends on mass, charge and energy of the ion as well as on the resistivity and electric field strength characterizing the detector. In order to obtain reliable energy calibration for the Si detector, the pulse height versus energy response was measured for a variety of ions from 1H to 63 Cu, at energies ranging from 2 to 30 MeV. The ions were obtained by elastic scattering of the corresponding beams on a thin Au target. The energies for the elastically scattered particles were calculated from the known beam energy and the detector angle with respect to the beam, taking into account the energy loss of the incident and scattered ions in the gold foil. The foil thickness was measured in a separate experiment using Rutherford Backscattering Spectrometry (RBS) technique. The thickness of the Au foil used for calibrations was d = 700 Å. No change in the response of the silicon detector has been observed. Spectra measured without gas in the ionization chamber and without mylar window were used to investigate the pulse height versus energy relationship for different ions. For the same ions elastically scattered from the Au foil we measured two parameter DE–Er spectra at different gas pressures and ion energies. Using calculated energies for the elastically scattered ions, corrected for energy loss in the gold foil and in the mylar window, we get the total energy of the ions when entering the ionization chamber. The residual energy Er can be determined using the calibration curves measured for different ions. The difference between the total energy and residual energy represents the energy loss in gas. So, an energy calibration of the ionization chamber can be accomplished. The procedure followed to extract the information about the composition of a sample form experimental data taken in ERDA measurements consists in three steps: – getting the calibration coefficients from calibration runs; – build the total experimental energy spectra (histograms) for each sample, and for each element present in the sample; – adjust the parameters describing the sample composition so that the theoretical spectra reproduce the experimental ones. The first and second steps require handling of a biparametric list of events. The Physics Analysis Workstation (PAW) code [6], developed at CERN, has been chosen for this purpose. The numerical algorithm used to generate the simulated spectra consists of a loop over many thin sublayers of constant stoichiometry, each giving in the final spectrum a contribution approximated by the convolution of a rectangle and a Gaussian. Stopping powers are computed with the Stopping and Range of Ions in Matter (SRIM) code [7]; the energy straggling and energy response of the detector are taken as gaussian. The interactive modification of the input parameters is made by another routine. The link between these programs is done inside a PAW macro-procedure named SURFAN (SURFace ANalysis) [8].
3. Results and discussion
Fig. 3. Gas handling arrangement: MFC = mass flow controller; T = telescopes; M = manometers; B = baratron; R = valves.
Crystalline oxide ceramics as aluminate spinel (MgAl2O4) were identified as promising matrices for actinide transmutation. The behavior of He produced by the disintegration of actinides is a very
Please cite this article in press as: H. Petrascu et al., ERDA at the 9 MV Tandem and at the 3 MV Tandetron of IFIN-HH, Nucl. Instr. Meth. B (2017), http://dx. doi.org/10.1016/j.nimb.2017.02.015
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Fig. 4. DE–Er spectra of MgAl2O4 sample (normal and high amplification).
important issue. Fig. 4 shows the DE–Er spectra corresponding to normal and high amplification of a MgAl2O4 single crystal sample implanted with 30 keV He ions at a fluence of 2 1016 at./cm2. The spectra were taken with an 80 MeV 63Cu beam. The identification matrix corresponding to normal amplification shows O, Mg and Al, the components of the spinel. Because the surface of the sample was covered with a thin carbon layer in order to avoid charging effects, the carbon signal from the surface is also present. The H recoils from the sample surface and He recoils are seen in the identification matrix corresponding to high amplification of the DE signal. The total energy spectra are presented in Fig. 5. The results of a calculation using SURFAN code are presented with points. The evaluated detection limits are around 2-3 1014 at./cm2. The calculation reproduced quite well the experimental spectra in the case of C, O, H and He. The small differences for magnesium and aluminum are most probably due to imperfect knowledge of the stopping power. We also have to mention that our simulation program is using the Bragg rule of stopping power additivity in compounds and small differences might also occur between this model and their real values. Among several ion-beam techniques for detection and depth profiling of hydrogen isotopes, ERDA technique using the Light incident Ion (LI-ERDA) 4He beam, proposed by Doyle and Peercy [9], is particularly advantageous because all hydrogen isotopes can be profiled simultaneously with a sensitivity as high as 0.1 at.%, the measurements can be performed using a relatively low energy accelerator and the samples undergoes less damage compared to the use of high-Z analysis or Heavy incident Ion ERDA (HI-ERDA). The large recoil cross sections led to a rapid development of the technique. Recently, a 3 MV Tandetron accelerator, shown in Fig. 6 has been installed and commissioned at the IFIN-HH. The first beam line at 30° is dedicated to Ion Beam Analysis (IBA) experiments: ERDA, RBS, Particle Induced X-ray and c-ray Emission (PIXE and PIGE), Nuclear Reaction Analysis (NRA) and micro-beam experiments (l-PIXE). Charged particles are detected by two AMETEK type BU-012-050-100 solid state detectors, which are mounted at a fixed (165°) and movable (10°–150°) positions with a solid angular acceptance of 1.641 msr and 5.57 msr, respectively. The IBA scattering chamber is equipped with a total of six steppers: four on the sample manipulator (phi rotation, theta
rotation, tilt, lift) and a movable RBS-detector with six positions foil-changer. These step motors have some kind of reset function, where a pre-defined reference position is approached. The target holder is stainless steel disc of 44 mm diameter connected to an electric charge integrator. In order to prevent secondary electrons emission a 300 V positive bias is applied directly to the sample holder. A very important aspect in the engineering design activities of the fusion reactors is represented by the selection and study of the materials suitable for development of such plants. A continuous research effort is sustained for the fusion reactors design and material selection, state of the art solutions being adopted; still, some of the implemented technical solutions present drawbacks which have to be solved. Fig. 7 shows ERDA and RBS spectra corresponding of a CHD/Si sample deposited by magnetron sputtering. The IBA measurements were performed using a 2.5 MeV 4 He++ particle beam. The thin film samples to be investigated were mounted in a scattering chamber where a vacuum of 10 5 Pa was achieved. The energy of recoiled and backscattered particles was measured by the two ORTEC silicon detectors that were mentioned previously. A 3.2 mm diameter collimator was placed in front of the mobile detector, resulting in a solid angle of 1.000 msr. The experimental setup allows simultaneous investigations by ERDA and RBS of the samples. To filter out the scattered helium ions, mylar foils (11 lm thick) were placed in front of the mobile detector. All spectra were registered using conventional electronics consisting of a charge sensitive preamplifier, a linear amplifier and an analog to digital converter. Counting rates were always kept small enough in order to have a negligible dead time during the measurements. For ERDA–RBS measurements the samples were placed on the sample holder that had an incidence angle of 75° with respect to the surface normal. The recoiling hydrogen isotopes were observed at a laboratory angle of 30° corresponding to an exit angle of 75° with respect to the surface normal. The backscattered helium ions were detected at 165° with respect to the beam direction in the first detector. Quantitative determination of C, H, and D concentration and thickness was performed using the SIMNRA [10] code. Accurate simulation requires an accurate knowledge of the H and D differential cross section. To analyze our data, we used the cross sections reported by V. Quillet et al. [11]. The results are presented with a continuous line.
Please cite this article in press as: H. Petrascu et al., ERDA at the 9 MV Tandem and at the 3 MV Tandetron of IFIN-HH, Nucl. Instr. Meth. B (2017), http://dx. doi.org/10.1016/j.nimb.2017.02.015
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H. Petrascu et al. / Nuclear Instruments and Methods in Physics Research B xxx (2017) xxx–xxx
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Fig. 5. Total energy spectra and simulation of the MgAl2O4 sample.
300 magnet
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Fig. 6. The 3 MV HVE TandetronTM accelerator sketch.
Please cite this article in press as: H. Petrascu et al., ERDA at the 9 MV Tandem and at the 3 MV Tandetron of IFIN-HH, Nucl. Instr. Meth. B (2017), http://dx. doi.org/10.1016/j.nimb.2017.02.015
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H. Petrascu et al. / Nuclear Instruments and Methods in Physics Research B xxx (2017) xxx–xxx
Fig. 7. ERDA and RBS spectra corresponding of a CHD/Si sample deposited by magnetron sputtering.
4. Conclusions A compact DE(gas)–Er(solid) telescope with an integrated preamplifier mounted close to the ionization chamber is well suited for ERDA with heavy ions. For a quantitative data analysis, a reliable calibration of both ionization chamber and Si detector is necessary. In particular, the pulse height defect in the Si detector must be taken into account. The measured energy spectra can be converted into depth profiles using the code SURFAN. Analysis of the hydrogen and deuterium with foil-ERDA in thin layers was performed with 4He from 3 MV Tandetron accelerator. The foil-ERDA has the advantage that the detector and the electronics are not overloaded. Thus, relatively high beam currents can be used resulting in good statistics in shorter measuring time. By measuring simultaneously both the H and D recoiling at a forward angle and backscattered 4He ions, a rather complete characterization of the sample can be achieved. Acknowledgements
acknowledged for the financial support through the project: Core Program No. PN09450103, Partnerships in Priority Areas Program No. 143/2012. References [1] J. L’Ecuyer, C. Brassard, C. Cardinal, J. Chabbal, L. Deschênes, J.P. Labrie, B. Terreault, J.G. Martel, R. St-Jacques, J. Appl. Phys. 47 (1976) 381. [2] M. Petrascu, I. Berceanu, I. Brancus, A. Buta, M. Duma, C. Grama, I. Lazar, I. Mihai, M. Petrovici, V. Simion, Nucl. Instr. Meth. B 4 (1984) 396. [3] I. Berceanu et al., Rev. Roum. Phys. 32 (1987) 733–742. [4] I. Berceanu et al., Rev. Roum. Phys. 31 (1986) 29–50. [5] J.J. Grob, Theses, Strasbourg, 1971. [6] CERN, PAW Manual Version 1.14, Application Software Group, Computing and Network Division, CERN, Geneva, Switzerland, 1992. [7] J.F. Ziegler, Handbook of Stopping Cross-Section for Energetic Ions in All Elements, Vol. 5, Pergamon, 1980. [8] D. Pantelica, M. Petrascu, F. Negoita, N. Scintee, H. Petrascu, A. Isbasescu, Report WP8, IDRANAP 10-01, 2001, p. 508. [9] B.L. Doyle, P.S. Peercy, Appl. Phys. Lett. 34 (1979) 811. [10] SIMNRA home page,
The National Authority for Scientific Research from The Ministry of Education, Research and Youth of Romania is gratefully
Please cite this article in press as: H. Petrascu et al., ERDA at the 9 MV Tandem and at the 3 MV Tandetron of IFIN-HH, Nucl. Instr. Meth. B (2017), http://dx. doi.org/10.1016/j.nimb.2017.02.015