- Email: [email protected]
System on chip (SoC) microcontrollers (μC) as digitisers for ion beam analysis (IBA) instruments
Nuclear Instruments and Methods in Physics Research B xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
System on chip (SoC) microcontrollers (lC) as digitisers for ion beam analysis (IBA) instruments q Harry J. Whitlow University of Applied Sciences (HES-SO), Haute Ecole Arc Ingénierie, Eplatures-Gris 17, CH-2300 La Chaux-de-Fonds, Switzerland
a r t i c l e
i n f o
Article history: Received 30 June 2015 Received in revised form 18 May 2016 Accepted 31 May 2016 Available online xxxx Keywords: Microcontroller Ion beam analysis Multi-channel analyser Event mode data collection
a b s t r a c t Data digitisation of the analogue signals from detectors to digital data is an essential process in ion beam analysis (IBA). The low-cost, easy availability and development environments that have a low learning threshold makes system-on-chip (SoC) microcontrollers (lC) attractive for this task. These lC combine, on one die, analogue and digital inputs and outputs with serial USB interfaces, which opens up simple implementation of tailor-made interfaces for specific IBA measurement systems. We have investigated the design and performance limitations based on development of three different digitisation interfaces for IBA. These were a two-channel nuclear instrumentation module (NIM) ADC event mode interface (EMI) for a high-resolution magnetic RBS spectrometer, a simple headless-multi-channel analyser (MCA) and a combined dual channel headless MCA and EMI. It is shown that SoC lC based interfaces for digitisation of analogue spectroscopy pulses in IBA systems can be implemented for material costs less than 100 €. The performance of the SoC devices for many IBA applications is close to what can be achieved with state-of-the-art instruments. The simple pulse spectroscopy interface circuit and software are included in the auxiliary archive. Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction Many ion beam analysis (IBA) measurements employ similar electronics readout chains. The most common is conventional analogue pulse electronics [1,2] with a pulse-height digitising analogue to digital converter (ADC) feeding a computer for data presentation and storage. Often digitisers are implemented in the standard Nuclear Instrumentation Modules (NIM) [3] which is expensive, heavy (17 kg per crate) and power consuming (5– 20 W per slot) although more compact externally powered units are becoming available. Direct digitisation of the pre-amplifier signal by a digital signal processor (DSP) with using digital filtering is emerging as an alternative, particularly for high counting rates. The cost per channel for these approaches is approximately the same. The DSP approach has several drawbacks. (i) Maintaining synchronicity between DSP modules for read-out of many channels in event mode can be difficult. (ii) Important details about how the pulses are processed, which may influence the results and the device interfacing, may be proprietary information that is not available to the user, or cannot be re-configured for customisation
q This work was originally submitted at the 22nd International Conference on Ion Beam Analysis, June 14 to 19, 2015, Opatija, Croatia. E-mail address: [email protected]
because they are implemented in firmware. (iii) Non-standard proprietary interfaces can also lead to obsolescence problems with change of the host computer or operating system. More advanced programmable pulse digitiser and DSP systems may be based on field programmable gate array (FPGA) systems, e.g. Ref. [4]. Making full-use of the potential of these flexible and higher performance advanced systems requires extensive programming skills. This is a drawback because it gives a high learning threshold for these flexible instruments. System on chip (SoC) microcontrollers (lC) with a USB interfaces have become readily available at low cost [5–7]. These SoC have an on-chip processor, flash and dynamic memory as well as peripheral digital input/output (i/o) and serial/USB interfaces, analogue to digital converters (ADC) and digital to analogue converters (DAC) [5,6]. SoCs allow a dramatic reduction in component numbers and internal wiring complexity compared with conventional ADCs/digitisers. This is significant because it reduces cost, simplifies construction and fault-finding. Furthermore, less internal wiring improves reliability. The use of open source SoC lC hardware and software and a standard USB serial class interfacing facilitates modification, reconfiguration and repair by the user and reduces obsolescence problems. The low, learning thresholds for programming, cost and power (0.1 W, supplied via the USB interface) makes SoC lC interesting for users to tailor-make instruments
http://dx.doi.org/10.1016/j.nimb.2016.05.033 0168-583X/Ó 2016 Elsevier B.V. All rights reserved.
Please cite this article in press as: H.J. Whitlow, System on chip (SoC) microcontrollers (lC) as digitisers for ion beam analysis (IBA) instruments, Nucl. Instr. Meth. B (2016), http://dx.doi.org/10.1016/j.nimb.2016.05.033
2
H.J. Whitlow / Nuclear Instruments and Methods in Physics Research B xxx (2016) xxx–xxx
with built-in pulse spectroscopy capabilities, [8] (e.g. for automatic channelling measurements). This is a more flexible approach and reduces reliance on shared central laboratory data acquisition systems. Here, the performance and implementation of several low-cost pulse-height digitising instruments for IBA are presented. (i) A two-channel event mode interface (EMI) for a magnetic Rutherford backscattering spectrometry (RBS) spectrometer that reads-out legacy NIM ADCs. (ii) A headless multi-channel analyser (MCA) for Si detectors. (iii) A two channel EMI and MCA for Time of Flight-Energy Elastic Recoil Detection (ToF-E ERDA). The materials costs for each of these instruments was less than 100 €. The difference between a MCA and a EMI interface is that in the former, the contents of the address in the buffer corresponding to the ADC conversion values are histogramed to create a 1D pulse height spectrum, while in the EMI case; the sets of ADC conversion values themselves are stored as tuple records in the buffer which are subsequently sorted by the host computer to generate 1D and multidimensional histograms. In Time of Flight – Energy Elastic Recoil Detection Analysis (ToF-E ERDA) an EMI with pair of ADCs is used to measure the 2D energy detector and time spectra. The Arduino open source platform [5] was chosen because of, low cost, low learning threshold and a real-time environment without illdefined operating system latencies. Software and an accompanying technical note describing a practical generic interface circuit [9] are deposited in the auxiliary archive. 1.1. Pulse spectroscopy requirements for IBA The specific characteristics of IBA pulse spectroscopy measurements are: (i) Series of samples are often measured with automated settings. (ii) Usually only a single, or a few detectors in coincidence are used. E.g. ToF-E ERDA, (iii) The pulse spectroscopy system and other components used are connected to a PC which performs display and data storage functions. (iv) The relative energy resolution (DEfwhm =E) does not exceed 0.05–0.1% for standard Si X-ray and charged-particle detectors. For standard ToF-E ERDA with 0.05–0.5 MeV/u recoil energies and 250 ps FWHM time resolution, the flight times are 50–160 ns over 0.5 m. This implies digitisation with more than 1024 or 2048 channels resolution is not meaningful. (v) Dead-time associated spectrum distortion is detrimental for the quantitativeness and sensitivity of IBA measurements. (vi) The accuracy may be degraded by differential and integral non linearities in pulse height conversion. Special consideration needs to be given to dead-times which restrict system throughput. Dead-time can be characterised as
paralysed, sP , and non-paralysed, sNP , dead-time [1]. sP gives a loss of registered pulses pP ¼ sP f , where f is the mean pulse rate. A trivial correction of pP can be applied for quantitative IBA applications. sNP is more problematic because if two pulses occur within sNP , they are treated as a single composite pulse giving pulse pile-up, which in IBA limits the instrument detection level (IDL) for PIXE and RBS analysis of trace elements. The pile-up probability is, pPU ¼ 1 ef sNP [1]. For a fast ADC with negligible conversion time, the sNP and sP are governed by the duration of the pulse at the ADC input. For a semi-Gaussian pulses with 3 ls pulse peaking time at f ¼ 1000 pulses s1 ; pP = 0.83% and pPU = 0.3%. (Fast ComTec 7072 dual fast ADC [15], Table 1.) These losses increase with increasing f, with pP and pPU reaching 4.2% and 1.5%, respectively at f ¼ 5000 pulses s1 . 1.2. Basic lC interface considerations In both MCA and EMI interfaces, two processes must take place concurrently. Firstly, the ADC(s) must be read out at random times and the ADC data stored in a buffer. Secondly, the EMI and MCA buffer(s) must be read out to the host. Service functions, (startstop ADCs, zero buffers, set threshold and delays etc.) must also be performed. This situation lends itself to a program structure based on a two concurrent autonomous threads. One thread reads out the ADC and stores data in the buffer under interrupt control and the other sequentially performs service functions such as serial readout under host control. This two-thread approach was adopted because it is easy to implement and can make a more efficient use of processor time than other possible structures, such as a hierarchical state machine. Where both threads share a single processor, the maximum throughput is set by the requirement that the time taken to process a single event must be less than 1=f . The interrupt driven ADC service thread performs coincidence checks, reads the ADC(s) and writes the data to the output buffer (s), increments counters and finally resets the ADC interface [9]. The readout thread is simply a polling loop that interrogates the incoming serial port for commands such as binary read out of buffer contents, start/stop/zero etc. If any commands are found the appropriate thread is branched to. 2. Two-NIM ADC event mode interface (EMI) A system for event mode readout of a pair of 13-bit legacy NIM ADCs [10] was developed. These are used to digitise the position P ¼ L=ðL þ RÞ and total charge C ¼ ðL þ RÞ analogue signals from a charge division multi-channel plate focal plane detector on a mag-
Table 1 Digitiser performance comparison. ADC type and interface NIM MCA [10] NIM EMI [10] Simple 10-bit MCA 12-bit dual MCA 12-bit EMI 7070 ADC [14] 7072 ADC [15] DSA-1000 DSP [16] 1 2 3 4 5 6 7 8 9
Input chan. 1 2 1 1 2 1 1 1
sP /ls 1,2
19.1 29.41,2 125 8.36 9.62 17.97 8.36 3.78
sNP /ls Para. loss (1000 s1)
Pile-up (1000 s1)
USB rate2/events s1
Rel. speed3
8.9 8.9 20 5.86 7.82 3.06 3.06 0.04
0.89% 0.89% 2.0% 0.58% 0.58% 0.30% 0.30% 0.005%9
5400 2700 5500 11000 5400
1.00 0.7 0.15 1.3 1.3
1.9% 2.9% 12.5% 0.83% 0.96% 1.8% 0.83% 0.75%
r/chan d BSL4 % full span
<1.25 1.5 1.5
not spec. not spec. 0.1–0.25 0.2–0.65 0.2–0.65
<1.79
No buffering, 2048 channels, 50% max span. At max USB Baud rate. Relative throughput to NIM MCA with Canberra 8713 ADC [10]. p.–p. deviation from Best Straight Line (BSL). Over 15–90% of max span. Limited by input pulse peaking time and duration. For 12-bit operation. Trapezoidal shaping 2.6 ls rise/fall time, 0.6 ls flat-top. These are the closest preset values that correspond to a Semi-gassian pulse with 3 For 4096 channels.
ls peaking time.
Please cite this article in press as: H.J. Whitlow, System on chip (SoC) microcontrollers (lC) as digitisers for ion beam analysis (IBA) instruments, Nucl. Instr. Meth. B (2016), http://dx.doi.org/10.1016/j.nimb.2016.05.033
H.J. Whitlow / Nuclear Instruments and Methods in Physics Research B xxx (2016) xxx–xxx
3
requires that the ADC conversion must take place after the peak of the input pulse. This achieved by an adjustable digital delay that is set via the USB serial interface. The on-chip 10-bit ADC [11,12] was used to digitise the analogue pulse into 1024 channels. The on-chip 2.56 V band-gap reference voltage defined the input span of the ADC with a temperature drift of about 25 ppm/°C. An attenuator and buffer is used to adapt the NIM preferred practice 0 ± 10 V analogue signal to the ADC span. The analogue signal processing circuit and software is described in detail in the associated technical note [9]. 4. ToF-E ERDA EMI/MCA interface
Fig. 1. Schematic of event mode interface (EMI).
Based on the experience from the simple MCA, a two channel MCA/EMI interface for readout of a Time of Flight-Energy elastic recoil detection analysis (ToF-E ERDA) system was constructed. To reduce the dead-time losses an Arduino Due was used. This digitiser interface is based on a 85 MHz ARM RISC processor with configurable 1 ls conversion time 12-bit ADC [13], where the default ADC settings were modified to give a 2.9 ls conversion time. This corresponds to a 24 ps channel width with a 100 ns ToF span, which is adequate for most ToF-E ERDA applications. This lC was chosen over alternative faster Linux based lCs which showed long and ill-defined operating system latencies. This interface can be operated as two independent MCAs or a two channel EMI. Two identical peak detector interface circuits [9] with the addition of a 2.50 V ADC voltage reference circuit with 10 ppm/°C temperature drift, were used in the interface. 5. Host computer and software
Fig. 2. Schematic of multichannel (MCA) interface configuration.
netic spectrometer of Kimura type. Here, L and R refer to the charge signals collected from the left and right ends of the resistive anode, respectively. The MCA/EMI, which is based on an Arduino Mega 2560 [11] is illustrated schematically in Fig. 1. This lC was chosen because it has short interrupt latency, and had sufficient i/o lines (24 needed) to connect directly to the ADC bus, without the need for additional logic. The 8 kbyte SRAM also provides adequate buffer space. The EMI program follows the two thread approach discussed above. The 2-byte conversion values were read out sequentially by fast port-wise readout and stored in a circular buffer that holds 512 events, (Fig. 1). Housekeeping tasks such as starting and stopping the acquisition by enabling/disabling the ADC interrupts, initiating binary readout and zeroing of the circular buffer, are selected depending on the value of a single ASCII character received by the USB serial interface. 3. Simple MCA interface The simple MCA interface schematic configuration and program structure is shown in Fig. 3. This also used a Arduino Mega 2560 board [11] because a 1024 channel 4-byte wide data buffer could be contained in the built-in SRAM [12]. A fast precision peak detector is used to capture the maximum peak amplitude [9]. This
The function of the host computer is to read-out the EMI or MCA interface via the USB serial interface and display the data as well as performing storage and some data-processing and -transformation functions. The use of a USB serial port interface with a simple protocol implies the host computer software can be implemented with different platform and program environments (e.g. (LabVIEW, Matlab, Scilab and Octave). The data is transferred on-demand from the EMI and MCA buffers in blocks at modest rates for modern PCs, implying a single program thread can be used. This makes it straightforward to integrate with other devices such as motorised sample stages and goniometers to facilitate automatic IBA measurements. In our work we have chosen to use the proprietary LabVIEW environment in order to easily adapt and develop software for different applications. Recognising that only a few states are important for IBA analysis (start, stop, zero, store, set-parameter, end of session) a statemachine controlled by polling for commands from the graphical user interface (GUI), or a sequential list, was implemented for both the EMI and MCA. Fig. 2 shows a screen-shot of the GUI for the MCA interface. 6. Results and discussion Table 1 summarises the performance results for the different interfaces and compares them with a 100 MHz Wilkinson nuclear ADC (Fast ComTec 7070 [14]), a fast nuclear ADC (Fast ComTec 7072 [15]) and a DSP (Canberra DSA-1000 [16]). To allow direct comparison of the digitisers a semi-Gaussian shaped pulse with 3ls peaking time (1.7 ls shaping time, 8.3ls return to baseline time) was taken as the input pulse. In the case of the DSP [16] where the shaping is trapezoidal, the rise and fall times were 2.6ls with a 0.6ls flat top. These values were chosen to give approximately the same detector noise filtering characteristics as for the semi-Gaussian pulse.
Please cite this article in press as: H.J. Whitlow, System on chip (SoC) microcontrollers (lC) as digitisers for ion beam analysis (IBA) instruments, Nucl. Instr. Meth. B (2016), http://dx.doi.org/10.1016/j.nimb.2016.05.033
4
H.J. Whitlow / Nuclear Instruments and Methods in Physics Research B xxx (2016) xxx–xxx
Fig. 3. Graphic user interface (GUI) for the simple MCA.
The throughput performance is defined by dead times sP and sNP that govern the paralysed count loss and pile-up fraction. The fastest ADC is the 7072 which had a 600 ns conversion and readout time that is considerably shorter than the return-to-baseline time for the input signal. This defines the minimum sP ¼ 8:3 ls and hence the smallest paralysed pulse loss at 1000 counts s1. The Wilkinson ADCs (NIM ADC/EMI and 7070) have a long sP that depends on pulse height and hence the paralysed dead time is ill-defined. This gives paralysed dead time losses at 1000 pulses/s of 2–3%. The worst case is the Simple 10-bit ADC which is limited by the 120 ls conversion time of the Arduino Mega ADC to 12.5%. In this case, to bring the paralysed dead time loss down to the level of the Wilikinson ADCs at 1000 s1the count rate must be reduced to 250 pulses s1. Alternatively, a correction to the spectrum could be applied by various software and hardware approaches [17]. Using a faster ADC such as was done in the 12-bit dual ADC/EMI (Arduino Due, 2:9ls conversion time) brings the paralysed dead time performance close to that of the 7072. The non-paralysed dead time sNP is more problematic, because two pulses are recorded as a single composite pulse, that gives a background contribution to the spectrum. This time is set by the rise time of the semi-Gaussian peak and some latency. For the SoC lC this is set by the interrupt initiation time. Since no readout latencies were taken into consideration for the 7070 and 7072 ADCs, these values represent lower limits for sP . Hence for the 12-bit dual MCA/EMI the sP values are closely comparable. Presumably, the longer sP for the NIM ADC MCA/EMI and Simple 10-bit MCA is due to the longer interrupt latency for the Arduino Mega.
An important consideration is the data transfer speed of the USB serial interface. For semi-Gaussian shaping as described above typical maximum count rates are 1000 pulses s1 to restrict pile-up in systems where piledup pulses are not rejected. The speed of the USB serial interface depends on the SoC lC and host PC. Even for the slowest speed of 115 kBaud, the sustainable data rate is 5400 pulses s1or 2700 double ADC conversion values s1. This is several times faster than the typical maximum count rates limited by the input pulse duration. The USB protocol allows up to 127 peripherals to be connected to a single USB cable connection to the host. This and the moderate serial data rates opens up the possibility of using constellations of headless MCA and EMI interfaces to simply read out 10–100 channels from e.g. a Si strip detector. The use of standard hubs and standard SoC lC can give considerable cost reductions while the concurrency from the many digitisers allows a high data rate to be achieved. In MCA mode the effective USB serial link data rates can be further increased by irreversible data compression where only the change in channel contents between readout cycles is transmitted. This is not possible in EMI mode as the ADC conversion values must be transmitted as tuples. Measuring the response of the simple MCA with a Ortec 142IH preamplifier, Tennelec main amplifier and BNC-4 pulser revealed the pulse width expressed as r (Table 1), the standard deviation of the variance corresponds to a Gaussian fwhm contribution of < 0:3% fwhm/full-scale [9]. We are unable to ascertain if the resolution is significantly contributed to by the MCA, or the remainder of the electronics chain, so this should be regarded as an upper limit. This level corresponds to a small broadening ( 1 keV for a
Please cite this article in press as: H.J. Whitlow, System on chip (SoC) microcontrollers (lC) as digitisers for ion beam analysis (IBA) instruments, Nucl. Instr. Meth. B (2016), http://dx.doi.org/10.1016/j.nimb.2016.05.033
H.J. Whitlow / Nuclear Instruments and Methods in Physics Research B xxx (2016) xxx–xxx
12 keV fwhm resolution detector) in 2 MeV He RBS measurements which is adequate for many applications. The non-linearity measured as the p.-p. departure from the best fit straight line (d BFL) is less than 0.2% of full scale. For the dual MCA/EMI where an Arduino Due the pipeline ADCs [13] configured to give a 2.9 ls conversion time Table 1 and Ref. [9] show that r is 1.5 channels which corresponds to a FWHM of 0.09 % of full scale, which following the argument above is more than adequate for IBA applications. The non-linearity is somewhat worse than for the simple MCA (Table 1, [9]). This might originate from the pipeline ADC [18]. The smooth departures [9]) from the Best Fit Line (BFL), will mainly effect identification of edge and peak energies in IBA, may be corrected using a polynomial calibration function. In Table 1 a comparison is also made with a DSP (Canberra DSA1000 [16]). DSPs digitise the pre-amplifier output signal directly and can handle very fast count rates using software pile-up rejection. The DSP paralysed loss associated with this is similar to the fast 7072 ADC and the 12-bit MCA/EMI at 1000 pulses s1 (Table 1). This loss increases to 15% at f = 20 000 pulses s1. The pileup rate is very low for the DSP because the first and piled up pulse(s) are removed from the data stream. Similar pile-up rejection can be implemented with the conventional ADC and SoC lC based digitisers [19] but requires more external components. 7. Conclusions Three USB readout interfaces suitable for IBA measurements have been developed using low-cost USB connected SoC lCs. Interface circuits and software are presented in the auxiliary archive. The speed was found to be limited by the ADC dead-time losses and not by the speed of the USB serial interface. Using a lC interface with a 2.9 ls ADC conversion time a deadtime performance that was close to a high performance NIM ADC with sub-ls conversion time could be achieved. Acknowledgements Edouard Guibert, Patrick Jeanneret and Emilien Kobi are thanked for contributions to the experimental work. Jean-Michel Kissling, Mario Delia and Jean-Paul Sandoz are thanked for help. This work was funded in-part by the HES-SO RadioBeam project No 38396.
5
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.nimb.2016.05. 033. References [1] W.R. Leo, Techniques for Nuclear and Particle Physics Experiments: A How-to Approach, Springer, Heidelberg, 1994. [2] H.J. Whitlow, Electronics for application of ion beams in nanoscience, in: H.J. Whitlow, Y. Zhang, R. Hellborg (Eds.), Ion Beams in Nanoscience and Technology, Springer, Heidelberg, 2009, p. 431. [3] US NIM Committee, Standard NIM instrumentation system (US Department of Energy, May 1990, DOE/ER-0457T). [4] H. Kivistö, M. Rossi, P. Rahkila, P. Jones, R. Norarat, N. Puttaraksa, T. Sajavaara, M. Laitinen, V. Hänninen, K. Ranttila, P. Heikkinen, L. Gilbert, V. Marjomäki, H.J. Whitlow, Advanced time-stamped total data acquisition control front-end for MeV ion beam microscopy and proton beam writing, Microelectron. Eng. 102 (2013) 9, http://dx.doi.org/10.1016/j.mee.2012.02.011. [5] Arduino home page, http://arduino.cc. [6] Beagleboard home page, http://beagleboard.org. [7] Raspberry Pi home page, http://www.raspberrypi.org/. [8] J. Cardoso, V. Amorim, R. Bastos, R. Madeira, J.B. Simões, C.M.B.A. Correia, A Very Low-Cost Portable Multichannel Analyzer, in: IEEE Nuclear Science Symposium 2000, Lyon, France, 2000. [9] H.J. Whitlow, Technical note: A pulse spectroscopy interface for system-onchip (SoC) microcontrollers (lC), Document included in the suplementary data. [10] Model Canberra, 8713 ADC Users Manual, (9231459A 4/99, Canberra Industries CT, USA, 1999. [11] Arduino Mega 2560, http://arduino.cc/en/Main/ArduinoBoardMega2560. [12] Atmel, 8-bit Atmel Microcontroller with 64K/128K/256K Bytes In-System Programmable Flash (2549P-AVR-10/2012), Atmel Corporation 2325 Orchard Parkway, San Jose, CA 95131, USA. [13] Atmel SAM1995)3X/SAM3A SMART ARM-based MCU Datasheet, Atmel11057C-ATARM-SAM3X-SAM3A-Datasheet.pdf 23-Mar-15. [14] Analog-to-Digital Converter Model 7070 Fast ComTec, http:// www.fastcomtec.com/products/product-lines/nim-modules/7070.html. [15] Dual ADC Model 7072, ADC/ SVA; Fast Comtec Gmbh, http:// www.fastcomtec.com/ftp/manuals/7072doc.zip. [16] Canberra; Manual Utilisateur, DSA-1000; March 2007. [17] R.M. Lindstrom, R.J. Flemming, Dead time, pileup and accurate gamma-ray spectrometry, Radioact. Radiochem. 6 (1995) 20. [18] A. Larsson, S. Sonkusale, A background calibration scheme for pipelined ADCs including non-linear operational amplifier gain and reference error correction, in: SOC Conference, 2004. Proceedings. IEEE International, 2004, pp. 34–37, http://dx.doi.org/10.1109/SOCC.2004.1362343. [19] H.J. Whitlow, Low Energy Ion Implantation of Silicon, University of Sussex, 1980. D.Phil thesis, p. 86–87, Fig. 4.19.
Please cite this article in press as: H.J. Whitlow, System on chip (SoC) microcontrollers (lC) as digitisers for ion beam analysis (IBA) instruments, Nucl. Instr. Meth. B (2016), http://dx.doi.org/10.1016/j.nimb.2016.05.033
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
System on chip (SoC) microcontrollers (lC) as digitisers for ion beam analysis (IBA) instruments q Harry J. Whitlow University of Applied Sciences (HES-SO), Haute Ecole Arc Ingénierie, Eplatures-Gris 17, CH-2300 La Chaux-de-Fonds, Switzerland
a r t i c l e
i n f o
Article history: Received 30 June 2015 Received in revised form 18 May 2016 Accepted 31 May 2016 Available online xxxx Keywords: Microcontroller Ion beam analysis Multi-channel analyser Event mode data collection
a b s t r a c t Data digitisation of the analogue signals from detectors to digital data is an essential process in ion beam analysis (IBA). The low-cost, easy availability and development environments that have a low learning threshold makes system-on-chip (SoC) microcontrollers (lC) attractive for this task. These lC combine, on one die, analogue and digital inputs and outputs with serial USB interfaces, which opens up simple implementation of tailor-made interfaces for specific IBA measurement systems. We have investigated the design and performance limitations based on development of three different digitisation interfaces for IBA. These were a two-channel nuclear instrumentation module (NIM) ADC event mode interface (EMI) for a high-resolution magnetic RBS spectrometer, a simple headless-multi-channel analyser (MCA) and a combined dual channel headless MCA and EMI. It is shown that SoC lC based interfaces for digitisation of analogue spectroscopy pulses in IBA systems can be implemented for material costs less than 100 €. The performance of the SoC devices for many IBA applications is close to what can be achieved with state-of-the-art instruments. The simple pulse spectroscopy interface circuit and software are included in the auxiliary archive. Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction Many ion beam analysis (IBA) measurements employ similar electronics readout chains. The most common is conventional analogue pulse electronics [1,2] with a pulse-height digitising analogue to digital converter (ADC) feeding a computer for data presentation and storage. Often digitisers are implemented in the standard Nuclear Instrumentation Modules (NIM) [3] which is expensive, heavy (17 kg per crate) and power consuming (5– 20 W per slot) although more compact externally powered units are becoming available. Direct digitisation of the pre-amplifier signal by a digital signal processor (DSP) with using digital filtering is emerging as an alternative, particularly for high counting rates. The cost per channel for these approaches is approximately the same. The DSP approach has several drawbacks. (i) Maintaining synchronicity between DSP modules for read-out of many channels in event mode can be difficult. (ii) Important details about how the pulses are processed, which may influence the results and the device interfacing, may be proprietary information that is not available to the user, or cannot be re-configured for customisation
q This work was originally submitted at the 22nd International Conference on Ion Beam Analysis, June 14 to 19, 2015, Opatija, Croatia. E-mail address: [email protected]
because they are implemented in firmware. (iii) Non-standard proprietary interfaces can also lead to obsolescence problems with change of the host computer or operating system. More advanced programmable pulse digitiser and DSP systems may be based on field programmable gate array (FPGA) systems, e.g. Ref. [4]. Making full-use of the potential of these flexible and higher performance advanced systems requires extensive programming skills. This is a drawback because it gives a high learning threshold for these flexible instruments. System on chip (SoC) microcontrollers (lC) with a USB interfaces have become readily available at low cost [5–7]. These SoC have an on-chip processor, flash and dynamic memory as well as peripheral digital input/output (i/o) and serial/USB interfaces, analogue to digital converters (ADC) and digital to analogue converters (DAC) [5,6]. SoCs allow a dramatic reduction in component numbers and internal wiring complexity compared with conventional ADCs/digitisers. This is significant because it reduces cost, simplifies construction and fault-finding. Furthermore, less internal wiring improves reliability. The use of open source SoC lC hardware and software and a standard USB serial class interfacing facilitates modification, reconfiguration and repair by the user and reduces obsolescence problems. The low, learning thresholds for programming, cost and power (0.1 W, supplied via the USB interface) makes SoC lC interesting for users to tailor-make instruments
http://dx.doi.org/10.1016/j.nimb.2016.05.033 0168-583X/Ó 2016 Elsevier B.V. All rights reserved.
Please cite this article in press as: H.J. Whitlow, System on chip (SoC) microcontrollers (lC) as digitisers for ion beam analysis (IBA) instruments, Nucl. Instr. Meth. B (2016), http://dx.doi.org/10.1016/j.nimb.2016.05.033
2
H.J. Whitlow / Nuclear Instruments and Methods in Physics Research B xxx (2016) xxx–xxx
with built-in pulse spectroscopy capabilities, [8] (e.g. for automatic channelling measurements). This is a more flexible approach and reduces reliance on shared central laboratory data acquisition systems. Here, the performance and implementation of several low-cost pulse-height digitising instruments for IBA are presented. (i) A two-channel event mode interface (EMI) for a magnetic Rutherford backscattering spectrometry (RBS) spectrometer that reads-out legacy NIM ADCs. (ii) A headless multi-channel analyser (MCA) for Si detectors. (iii) A two channel EMI and MCA for Time of Flight-Energy Elastic Recoil Detection (ToF-E ERDA). The materials costs for each of these instruments was less than 100 €. The difference between a MCA and a EMI interface is that in the former, the contents of the address in the buffer corresponding to the ADC conversion values are histogramed to create a 1D pulse height spectrum, while in the EMI case; the sets of ADC conversion values themselves are stored as tuple records in the buffer which are subsequently sorted by the host computer to generate 1D and multidimensional histograms. In Time of Flight – Energy Elastic Recoil Detection Analysis (ToF-E ERDA) an EMI with pair of ADCs is used to measure the 2D energy detector and time spectra. The Arduino open source platform [5] was chosen because of, low cost, low learning threshold and a real-time environment without illdefined operating system latencies. Software and an accompanying technical note describing a practical generic interface circuit [9] are deposited in the auxiliary archive. 1.1. Pulse spectroscopy requirements for IBA The specific characteristics of IBA pulse spectroscopy measurements are: (i) Series of samples are often measured with automated settings. (ii) Usually only a single, or a few detectors in coincidence are used. E.g. ToF-E ERDA, (iii) The pulse spectroscopy system and other components used are connected to a PC which performs display and data storage functions. (iv) The relative energy resolution (DEfwhm =E) does not exceed 0.05–0.1% for standard Si X-ray and charged-particle detectors. For standard ToF-E ERDA with 0.05–0.5 MeV/u recoil energies and 250 ps FWHM time resolution, the flight times are 50–160 ns over 0.5 m. This implies digitisation with more than 1024 or 2048 channels resolution is not meaningful. (v) Dead-time associated spectrum distortion is detrimental for the quantitativeness and sensitivity of IBA measurements. (vi) The accuracy may be degraded by differential and integral non linearities in pulse height conversion. Special consideration needs to be given to dead-times which restrict system throughput. Dead-time can be characterised as
paralysed, sP , and non-paralysed, sNP , dead-time [1]. sP gives a loss of registered pulses pP ¼ sP f , where f is the mean pulse rate. A trivial correction of pP can be applied for quantitative IBA applications. sNP is more problematic because if two pulses occur within sNP , they are treated as a single composite pulse giving pulse pile-up, which in IBA limits the instrument detection level (IDL) for PIXE and RBS analysis of trace elements. The pile-up probability is, pPU ¼ 1 ef sNP [1]. For a fast ADC with negligible conversion time, the sNP and sP are governed by the duration of the pulse at the ADC input. For a semi-Gaussian pulses with 3 ls pulse peaking time at f ¼ 1000 pulses s1 ; pP = 0.83% and pPU = 0.3%. (Fast ComTec 7072 dual fast ADC [15], Table 1.) These losses increase with increasing f, with pP and pPU reaching 4.2% and 1.5%, respectively at f ¼ 5000 pulses s1 . 1.2. Basic lC interface considerations In both MCA and EMI interfaces, two processes must take place concurrently. Firstly, the ADC(s) must be read out at random times and the ADC data stored in a buffer. Secondly, the EMI and MCA buffer(s) must be read out to the host. Service functions, (startstop ADCs, zero buffers, set threshold and delays etc.) must also be performed. This situation lends itself to a program structure based on a two concurrent autonomous threads. One thread reads out the ADC and stores data in the buffer under interrupt control and the other sequentially performs service functions such as serial readout under host control. This two-thread approach was adopted because it is easy to implement and can make a more efficient use of processor time than other possible structures, such as a hierarchical state machine. Where both threads share a single processor, the maximum throughput is set by the requirement that the time taken to process a single event must be less than 1=f . The interrupt driven ADC service thread performs coincidence checks, reads the ADC(s) and writes the data to the output buffer (s), increments counters and finally resets the ADC interface [9]. The readout thread is simply a polling loop that interrogates the incoming serial port for commands such as binary read out of buffer contents, start/stop/zero etc. If any commands are found the appropriate thread is branched to. 2. Two-NIM ADC event mode interface (EMI) A system for event mode readout of a pair of 13-bit legacy NIM ADCs [10] was developed. These are used to digitise the position P ¼ L=ðL þ RÞ and total charge C ¼ ðL þ RÞ analogue signals from a charge division multi-channel plate focal plane detector on a mag-
Table 1 Digitiser performance comparison. ADC type and interface NIM MCA [10] NIM EMI [10] Simple 10-bit MCA 12-bit dual MCA 12-bit EMI 7070 ADC [14] 7072 ADC [15] DSA-1000 DSP [16] 1 2 3 4 5 6 7 8 9
Input chan. 1 2 1 1 2 1 1 1
sP /ls 1,2
19.1 29.41,2 125 8.36 9.62 17.97 8.36 3.78
sNP /ls Para. loss (1000 s1)
Pile-up (1000 s1)
USB rate2/events s1
Rel. speed3
8.9 8.9 20 5.86 7.82 3.06 3.06 0.04
0.89% 0.89% 2.0% 0.58% 0.58% 0.30% 0.30% 0.005%9
5400 2700 5500 11000 5400
1.00 0.7 0.15 1.3 1.3
1.9% 2.9% 12.5% 0.83% 0.96% 1.8% 0.83% 0.75%
r/chan d BSL4 % full span
<1.25 1.5 1.5
not spec. not spec. 0.1–0.25 0.2–0.65 0.2–0.65
<1.79
No buffering, 2048 channels, 50% max span. At max USB Baud rate. Relative throughput to NIM MCA with Canberra 8713 ADC [10]. p.–p. deviation from Best Straight Line (BSL). Over 15–90% of max span. Limited by input pulse peaking time and duration. For 12-bit operation. Trapezoidal shaping 2.6 ls rise/fall time, 0.6 ls flat-top. These are the closest preset values that correspond to a Semi-gassian pulse with 3 For 4096 channels.
ls peaking time.
Please cite this article in press as: H.J. Whitlow, System on chip (SoC) microcontrollers (lC) as digitisers for ion beam analysis (IBA) instruments, Nucl. Instr. Meth. B (2016), http://dx.doi.org/10.1016/j.nimb.2016.05.033
H.J. Whitlow / Nuclear Instruments and Methods in Physics Research B xxx (2016) xxx–xxx
3
requires that the ADC conversion must take place after the peak of the input pulse. This achieved by an adjustable digital delay that is set via the USB serial interface. The on-chip 10-bit ADC [11,12] was used to digitise the analogue pulse into 1024 channels. The on-chip 2.56 V band-gap reference voltage defined the input span of the ADC with a temperature drift of about 25 ppm/°C. An attenuator and buffer is used to adapt the NIM preferred practice 0 ± 10 V analogue signal to the ADC span. The analogue signal processing circuit and software is described in detail in the associated technical note [9]. 4. ToF-E ERDA EMI/MCA interface
Fig. 1. Schematic of event mode interface (EMI).
Based on the experience from the simple MCA, a two channel MCA/EMI interface for readout of a Time of Flight-Energy elastic recoil detection analysis (ToF-E ERDA) system was constructed. To reduce the dead-time losses an Arduino Due was used. This digitiser interface is based on a 85 MHz ARM RISC processor with configurable 1 ls conversion time 12-bit ADC [13], where the default ADC settings were modified to give a 2.9 ls conversion time. This corresponds to a 24 ps channel width with a 100 ns ToF span, which is adequate for most ToF-E ERDA applications. This lC was chosen over alternative faster Linux based lCs which showed long and ill-defined operating system latencies. This interface can be operated as two independent MCAs or a two channel EMI. Two identical peak detector interface circuits [9] with the addition of a 2.50 V ADC voltage reference circuit with 10 ppm/°C temperature drift, were used in the interface. 5. Host computer and software
Fig. 2. Schematic of multichannel (MCA) interface configuration.
netic spectrometer of Kimura type. Here, L and R refer to the charge signals collected from the left and right ends of the resistive anode, respectively. The MCA/EMI, which is based on an Arduino Mega 2560 [11] is illustrated schematically in Fig. 1. This lC was chosen because it has short interrupt latency, and had sufficient i/o lines (24 needed) to connect directly to the ADC bus, without the need for additional logic. The 8 kbyte SRAM also provides adequate buffer space. The EMI program follows the two thread approach discussed above. The 2-byte conversion values were read out sequentially by fast port-wise readout and stored in a circular buffer that holds 512 events, (Fig. 1). Housekeeping tasks such as starting and stopping the acquisition by enabling/disabling the ADC interrupts, initiating binary readout and zeroing of the circular buffer, are selected depending on the value of a single ASCII character received by the USB serial interface. 3. Simple MCA interface The simple MCA interface schematic configuration and program structure is shown in Fig. 3. This also used a Arduino Mega 2560 board [11] because a 1024 channel 4-byte wide data buffer could be contained in the built-in SRAM [12]. A fast precision peak detector is used to capture the maximum peak amplitude [9]. This
The function of the host computer is to read-out the EMI or MCA interface via the USB serial interface and display the data as well as performing storage and some data-processing and -transformation functions. The use of a USB serial port interface with a simple protocol implies the host computer software can be implemented with different platform and program environments (e.g. (LabVIEW, Matlab, Scilab and Octave). The data is transferred on-demand from the EMI and MCA buffers in blocks at modest rates for modern PCs, implying a single program thread can be used. This makes it straightforward to integrate with other devices such as motorised sample stages and goniometers to facilitate automatic IBA measurements. In our work we have chosen to use the proprietary LabVIEW environment in order to easily adapt and develop software for different applications. Recognising that only a few states are important for IBA analysis (start, stop, zero, store, set-parameter, end of session) a statemachine controlled by polling for commands from the graphical user interface (GUI), or a sequential list, was implemented for both the EMI and MCA. Fig. 2 shows a screen-shot of the GUI for the MCA interface. 6. Results and discussion Table 1 summarises the performance results for the different interfaces and compares them with a 100 MHz Wilkinson nuclear ADC (Fast ComTec 7070 [14]), a fast nuclear ADC (Fast ComTec 7072 [15]) and a DSP (Canberra DSA-1000 [16]). To allow direct comparison of the digitisers a semi-Gaussian shaped pulse with 3ls peaking time (1.7 ls shaping time, 8.3ls return to baseline time) was taken as the input pulse. In the case of the DSP [16] where the shaping is trapezoidal, the rise and fall times were 2.6ls with a 0.6ls flat top. These values were chosen to give approximately the same detector noise filtering characteristics as for the semi-Gaussian pulse.
Please cite this article in press as: H.J. Whitlow, System on chip (SoC) microcontrollers (lC) as digitisers for ion beam analysis (IBA) instruments, Nucl. Instr. Meth. B (2016), http://dx.doi.org/10.1016/j.nimb.2016.05.033
4
H.J. Whitlow / Nuclear Instruments and Methods in Physics Research B xxx (2016) xxx–xxx
Fig. 3. Graphic user interface (GUI) for the simple MCA.
The throughput performance is defined by dead times sP and sNP that govern the paralysed count loss and pile-up fraction. The fastest ADC is the 7072 which had a 600 ns conversion and readout time that is considerably shorter than the return-to-baseline time for the input signal. This defines the minimum sP ¼ 8:3 ls and hence the smallest paralysed pulse loss at 1000 counts s1. The Wilkinson ADCs (NIM ADC/EMI and 7070) have a long sP that depends on pulse height and hence the paralysed dead time is ill-defined. This gives paralysed dead time losses at 1000 pulses/s of 2–3%. The worst case is the Simple 10-bit ADC which is limited by the 120 ls conversion time of the Arduino Mega ADC to 12.5%. In this case, to bring the paralysed dead time loss down to the level of the Wilikinson ADCs at 1000 s1the count rate must be reduced to 250 pulses s1. Alternatively, a correction to the spectrum could be applied by various software and hardware approaches [17]. Using a faster ADC such as was done in the 12-bit dual ADC/EMI (Arduino Due, 2:9ls conversion time) brings the paralysed dead time performance close to that of the 7072. The non-paralysed dead time sNP is more problematic, because two pulses are recorded as a single composite pulse, that gives a background contribution to the spectrum. This time is set by the rise time of the semi-Gaussian peak and some latency. For the SoC lC this is set by the interrupt initiation time. Since no readout latencies were taken into consideration for the 7070 and 7072 ADCs, these values represent lower limits for sP . Hence for the 12-bit dual MCA/EMI the sP values are closely comparable. Presumably, the longer sP for the NIM ADC MCA/EMI and Simple 10-bit MCA is due to the longer interrupt latency for the Arduino Mega.
An important consideration is the data transfer speed of the USB serial interface. For semi-Gaussian shaping as described above typical maximum count rates are 1000 pulses s1 to restrict pile-up in systems where piledup pulses are not rejected. The speed of the USB serial interface depends on the SoC lC and host PC. Even for the slowest speed of 115 kBaud, the sustainable data rate is 5400 pulses s1or 2700 double ADC conversion values s1. This is several times faster than the typical maximum count rates limited by the input pulse duration. The USB protocol allows up to 127 peripherals to be connected to a single USB cable connection to the host. This and the moderate serial data rates opens up the possibility of using constellations of headless MCA and EMI interfaces to simply read out 10–100 channels from e.g. a Si strip detector. The use of standard hubs and standard SoC lC can give considerable cost reductions while the concurrency from the many digitisers allows a high data rate to be achieved. In MCA mode the effective USB serial link data rates can be further increased by irreversible data compression where only the change in channel contents between readout cycles is transmitted. This is not possible in EMI mode as the ADC conversion values must be transmitted as tuples. Measuring the response of the simple MCA with a Ortec 142IH preamplifier, Tennelec main amplifier and BNC-4 pulser revealed the pulse width expressed as r (Table 1), the standard deviation of the variance corresponds to a Gaussian fwhm contribution of < 0:3% fwhm/full-scale [9]. We are unable to ascertain if the resolution is significantly contributed to by the MCA, or the remainder of the electronics chain, so this should be regarded as an upper limit. This level corresponds to a small broadening ( 1 keV for a
Please cite this article in press as: H.J. Whitlow, System on chip (SoC) microcontrollers (lC) as digitisers for ion beam analysis (IBA) instruments, Nucl. Instr. Meth. B (2016), http://dx.doi.org/10.1016/j.nimb.2016.05.033
H.J. Whitlow / Nuclear Instruments and Methods in Physics Research B xxx (2016) xxx–xxx
12 keV fwhm resolution detector) in 2 MeV He RBS measurements which is adequate for many applications. The non-linearity measured as the p.-p. departure from the best fit straight line (d BFL) is less than 0.2% of full scale. For the dual MCA/EMI where an Arduino Due the pipeline ADCs [13] configured to give a 2.9 ls conversion time Table 1 and Ref. [9] show that r is 1.5 channels which corresponds to a FWHM of 0.09 % of full scale, which following the argument above is more than adequate for IBA applications. The non-linearity is somewhat worse than for the simple MCA (Table 1, [9]). This might originate from the pipeline ADC [18]. The smooth departures [9]) from the Best Fit Line (BFL), will mainly effect identification of edge and peak energies in IBA, may be corrected using a polynomial calibration function. In Table 1 a comparison is also made with a DSP (Canberra DSA1000 [16]). DSPs digitise the pre-amplifier output signal directly and can handle very fast count rates using software pile-up rejection. The DSP paralysed loss associated with this is similar to the fast 7072 ADC and the 12-bit MCA/EMI at 1000 pulses s1 (Table 1). This loss increases to 15% at f = 20 000 pulses s1. The pileup rate is very low for the DSP because the first and piled up pulse(s) are removed from the data stream. Similar pile-up rejection can be implemented with the conventional ADC and SoC lC based digitisers [19] but requires more external components. 7. Conclusions Three USB readout interfaces suitable for IBA measurements have been developed using low-cost USB connected SoC lCs. Interface circuits and software are presented in the auxiliary archive. The speed was found to be limited by the ADC dead-time losses and not by the speed of the USB serial interface. Using a lC interface with a 2.9 ls ADC conversion time a deadtime performance that was close to a high performance NIM ADC with sub-ls conversion time could be achieved. Acknowledgements Edouard Guibert, Patrick Jeanneret and Emilien Kobi are thanked for contributions to the experimental work. Jean-Michel Kissling, Mario Delia and Jean-Paul Sandoz are thanked for help. This work was funded in-part by the HES-SO RadioBeam project No 38396.
5
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.nimb.2016.05. 033. References [1] W.R. Leo, Techniques for Nuclear and Particle Physics Experiments: A How-to Approach, Springer, Heidelberg, 1994. [2] H.J. Whitlow, Electronics for application of ion beams in nanoscience, in: H.J. Whitlow, Y. Zhang, R. Hellborg (Eds.), Ion Beams in Nanoscience and Technology, Springer, Heidelberg, 2009, p. 431. [3] US NIM Committee, Standard NIM instrumentation system (US Department of Energy, May 1990, DOE/ER-0457T). [4] H. Kivistö, M. Rossi, P. Rahkila, P. Jones, R. Norarat, N. Puttaraksa, T. Sajavaara, M. Laitinen, V. Hänninen, K. Ranttila, P. Heikkinen, L. Gilbert, V. Marjomäki, H.J. Whitlow, Advanced time-stamped total data acquisition control front-end for MeV ion beam microscopy and proton beam writing, Microelectron. Eng. 102 (2013) 9, http://dx.doi.org/10.1016/j.mee.2012.02.011. [5] Arduino home page, http://arduino.cc. [6] Beagleboard home page, http://beagleboard.org. [7] Raspberry Pi home page, http://www.raspberrypi.org/. [8] J. Cardoso, V. Amorim, R. Bastos, R. Madeira, J.B. Simões, C.M.B.A. Correia, A Very Low-Cost Portable Multichannel Analyzer, in: IEEE Nuclear Science Symposium 2000, Lyon, France, 2000. [9] H.J. Whitlow, Technical note: A pulse spectroscopy interface for system-onchip (SoC) microcontrollers (lC), Document included in the suplementary data. [10] Model Canberra, 8713 ADC Users Manual, (9231459A 4/99, Canberra Industries CT, USA, 1999. [11] Arduino Mega 2560, http://arduino.cc/en/Main/ArduinoBoardMega2560. [12] Atmel, 8-bit Atmel Microcontroller with 64K/128K/256K Bytes In-System Programmable Flash (2549P-AVR-10/2012), Atmel Corporation 2325 Orchard Parkway, San Jose, CA 95131, USA. [13] Atmel SAM1995)3X/SAM3A SMART ARM-based MCU Datasheet, Atmel11057C-ATARM-SAM3X-SAM3A-Datasheet.pdf 23-Mar-15. [14] Analog-to-Digital Converter Model 7070 Fast ComTec, http:// www.fastcomtec.com/products/product-lines/nim-modules/7070.html. [15] Dual ADC Model 7072, ADC/ SVA; Fast Comtec Gmbh, http:// www.fastcomtec.com/ftp/manuals/7072doc.zip. [16] Canberra; Manual Utilisateur, DSA-1000; March 2007. [17] R.M. Lindstrom, R.J. Flemming, Dead time, pileup and accurate gamma-ray spectrometry, Radioact. Radiochem. 6 (1995) 20. [18] A. Larsson, S. Sonkusale, A background calibration scheme for pipelined ADCs including non-linear operational amplifier gain and reference error correction, in: SOC Conference, 2004. Proceedings. IEEE International, 2004, pp. 34–37, http://dx.doi.org/10.1109/SOCC.2004.1362343. [19] H.J. Whitlow, Low Energy Ion Implantation of Silicon, University of Sussex, 1980. D.Phil thesis, p. 86–87, Fig. 4.19.
Please cite this article in press as: H.J. Whitlow, System on chip (SoC) microcontrollers (lC) as digitisers for ion beam analysis (IBA) instruments, Nucl. Instr. Meth. B (2016), http://dx.doi.org/10.1016/j.nimb.2016.05.033