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A data acquisition system for a high intensity powder diffractometer at HANARO
ARTICLE IN PRESS Nuclear Instruments and Methods in Physics Research A 620 (2010) 445–449
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
A data acquisition system for a high intensity powder diffractometer at HANARO Jong-Kyu Cheon a,n, Myung-Kook Moon b, Chang-Hee Lee b, V. T Em b, Seong-Su Lee b, Uk-Won Nam c, Jung-Hwan Yi d, Je-Geun Park e, Hong-Ju Kim f a
Department of Radiation, Sorabol College, 165 Chunghyodong, Gyeongju, Gyeongsangbuk-do 780-711, South Korea Neutron Science Division, Korea Atomic Energy Research Institute, 150 Duckjindong, Yuseong-gu, Daejeon 305-353, South Korea c Space Science Division, Korea Astronomy and Space Science Institute, 61-1 Whaamdong, Yuseong-gu, Daejeon 305-348, South Korea d Department of Physics, Sungkyunkwan University, 300 Cheoncheondong, Jangan-gu, Suwon, Gyeonggi-do 440-746, South Korea e Department of Physics and Astronomy, Seoul National University, 599 Gwanak-ro, Gwanak-gu, Seoul 151-747, South Korea f School of Physics and Energy Sciences, Kyungpook National University,1370 Sankyukdong, Bukgu, Daegu 702-701, South Korea b
a r t i c l e in fo
abstract
Article history: Received 25 January 2010 Received in revised form 23 February 2010 Accepted 30 March 2010 Available online 10 April 2010
A data acquisition (DAQ) and control system for the high intensity powder diffractometer (HIPD) at High-flux Advanced Neutron Application ReactOr (HANARO) is developed in this paper. The HIPD provides the advantage of wide-angle coverage of neutron diffraction operations, which can reduce the time that is required for experiments. The HIPD DAQ and control system was commissioned into operation and the diffraction data of Al2O3 was recorded successfully. The details of the detector, the data acquisition process and the instrument controls system used in the operation of the HIPD are presented. & 2010 Elsevier B.V. All rights reserved.
Keywords: Instrumentation Powder diffraction Neutron diffraction
1. Introduction A neutron powder diffractometer is used to study microscopic atomic structures by measuring the angular distribution of neutrons diffracted from the sample. In powder diffraction, angular scanning of a larger area is an important factor, as this reduces the experiment time and hence the beam time. To record diffraction peaks, a traditional powder diffractometer using a tube detector with a small angular coverage area and step scanning is necessary. These types of experiments run for few days for a single sample. However, using a curved PSD with a wide angular scanning area, it is possible to measure the diffraction peaks much faster compared to the tube detector. Moreover, the reactor time can be used judicially. The diffractometer D20 at ILL and Wombat at ANSTO have detector systems that are capable of wide-angle coverage. The diffractometer D20 uses a 1-D large curved microstrip gas chamber system while Wombat is equipped with a 2-D position sensitive detector; however, these diffractometers are costly, and the development of a large-area PSD is a high-budget project that requires a multitude of tasks [1–3]. The most feasible option used at HIPD, HANARO, involves the use of eight modules for 1-D PSDs with a two-step scanning process.
n
Corresponding author. Tel.: + 82 11 9598 6942; fax: + 82 53 939 1259. E-mail address: [email protected] (J.-K. Cheon).
0168-9002/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2010.03.166
A high-resolution powder diffractometer is also installed at HANARO, but this instrument is heavily used for services. For certain diffraction experiments, high intensity is preferred over a high resolution. Thus, the installation of HIPD was essential for specific applications, allowing HRPD to be devoted to more precise experiments. A HIPD instrument capable of wide-angle coverage was developed for high-intensity diffraction applications. This diffractometer consists of eight multiwire-based 1-D PSDs. Each detector covering a scattering angle of 81 is placed at 161 intervals. To record a full diffraction pattern, two-step scanning is required. The conceptual design and diffraction experiments were recently completed by the authors [4,5]. It is similar to D4c at ILL [6]. The raise in the intensity and faster data collection enabled the study of microsamples in a volume range of 1–20 mm2. This represents a very important factor in the study of rarely available samples and/or those that deteriorate rapidly. This paper discusses the construction and performance of the HIPD. Complete indigenous development of the instrument was carried out using a custom-built detector, a custom-built step scan control device and a data acquisition system.
2. Development of the HIPD The HIPD is installed at beam port number ST-3 of HANARO, a 30 MW research reactor operated by KAERI. Fig. 1 shows the HIPD instrument. The neutron wavelength used is 1.478 A˚ with an
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J.-K. Cheon et al. / Nuclear Instruments and Methods in Physics Research A 620 (2010) 445–449
filled with 3He gas at 450 kPa and CF4 gas at 100 kPa as a ˚ quenching gas, which give a detection efficiency of 66% at 1.478 A. Pulses from the detector are processed by a current-sensitive preamplifier and then fed to a constant fraction discriminator (CFD) through a fixed additional delay at a scanning height of 7.61. The PSD assembly covers a total scanning angle of 1281.
Detector Bank
2.2. Pulse processing and data acquisition (DAQ) system Sample
Beam shutter
Fig. 1. High-intensity powder diffractometer installed at beam ST-3, at Hanaro, with a detector bank consisting of eight 1-D PSDs and shielding.
Table 1 HIPD instrument and detector specifications. Beam port
ST-3
Monochromator crystal Flux at sample (n/cm2/s) ˚ Wavelength (A)
Si bent perfect crystal 106 1.478
Angular area covered (deg.) Detector Type Position encoding Sample to Detector (mm) Angular Resolution (deg.) Position Resolution (mm) Effective area (mm2) Operating voltage (V) Detector Gas Detection gap (mm) Efficiency at 1.478 A˚ (%)
128 1-D neutron position sensitive detector Delay line 1050 0.109 2 140 180 3200 3 He + CF4 22.8 66
˚ Flux at the Si effective mosaic spread of 1.6 10 3 A. monochromator crystal is 109 n/cm2/s and the sample position is 106 n/cm2/s. The sample-to-detector distance was selected as 1 050 mm. The detectors were developed with a scanning height of 14 cm, and the values for the spatial resolution in the angular direction were 2 mm and 0.1091. Data from the detectors is processed using a delay line readout method with digital 12 bit data that is processed in a data acquisition system. The step scanning operations of the detectors is controlled by a high-power and high-precision motor controller and driver. The specifications of the HIPD instrument and detectors are shown in Table 1.
2.1. Detector All of the detectors used at HIPD were developed by the authors. These detectors are multiwire-based proportional counters and use a delay line readout method for position encoding. The sensing length of the detector is optimized to minimize parallax error and the height is optimized for intensity such that no broadening is experienced at a very low angle. Each detector consists of a wire frame and a strip readout frame. An anode frame, with an array of 10 mm wires at a pitch of 5 mm, is placed between the cathode readout frame and the window. The cathode is made up of a printed circuit board with 2.5 mm spacing strips. Each strip is terminated with a lumped delay. The distance between the anode and the cathode frame is 3.2 mm and total detector delay time is 230 ns. The effective area of the detector is 140 180 mm2 and the detection gap is 22.8 mm. The detector is
Fig. 2 shows a schematic of the control and data acquisition system of the HIPD. The signals of eight detectors through a preamplifier are converted into logic pulses at the CFD and are sent to DAQ USB modules. A DAQ USB module is the device that can read and record the information from the delay line of the detector. This module was developed by the authors in collaboration with the Korea Astronomy and Space Science Institute. They are NIM cassette-type modules and communicate with a PC through a USB 2.0 port. At the HIPD, eight DAQ USB modules are run simultaneously. Other devices that compose the HIPD DAQ system include a motor controller to move the detector bank and a temperature controller that monitors and controls the sample environment. All of the devices are controlled by SPEC based on LINUX. A SPEC macro was developed to conduct the basic neutron diffraction experiments. 2.2.1. DAQ USB module The DAQ USB module consists of two basic parts. The first is a time-to-digital converter (TDC) that processes the signal of the delay line detector and the second is a data acquisition module that interfaces to the PC via a USB connection. Two signals from the readout cathode X1, X2 and the anode are amplified with the current-sensitive preamplifier, and this signal is converted into a constant-amplitude fast NIM signal at the CFD. This signal carries data in the form of the arrival time. The converted signal enters the DAQ USB module. The DAQ USB module consists of the F1 TDC, FPGA, DRAM and DSP components. The F1 TDC measures the time difference between X1 and X2. The programmed FPGA logic rejects pileups, grabs data and controls the preset time or the counter. The DSP controls the F1-based TDC and FPGA. It plots a 1-D or 2-D histogram using the position data from the FPGA. It communicates with the PC via USB [7,8]. This module supports the synchronized mode, which can be used for the present multiPSD system. The synchronized mode has two types of modules: the master module and the slave module. When a user starts the master module, it sends a gate signal to the slave modules. When the gate signal is active, slaves are operational. The master has all of the preset information but the slaves keep only the histogram data. 2.2.2. Motor control unit A complete detector bank consisting of eight PSD chambers and three preamplifiers mounted on each PSD is quite heavy and thus it is moved by an air pad cushion. Currently, the PSD remains on air during the measurement. We will remove the air during the detector operation later to improve position stability. A highpower motor driver and a micro step controller are required for precise and controlled movement of the detector bank. A stepper motor control unit that uses an attached controller and a driver was developed for precise motion control. The control unit communicates with the PC through RS-232. It can control the stepping motor by micro-steps of 50,000 pulses/revolution with a maximum current of 4 A/phase. Commands to the motor controller are also accessed through a data acquisition system through the PC. The data acquisition is automated so that it can continue to monitor all of the controls, such as the readout
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447
Step Motor Controller
Detector Bank Gonio -meter
Motor Temperature Controller
1DPSD PA Delay
CFD
GPIB 1 Trig.OUT
2
3
4
5
6
7
RS-232
8
DAQ USB module(ATDC) USB
Linux server (spec)
Trig.IN
Fig. 2. Block diagram of the HIPD DAQ system: a Linux server installed with SPEC can control the HIPD DAQ system using USB, GPIB and RS-232. Synchronized DAQ USB modules can record the data of the eight 1-D PSDs simultaneously.
system, the motor control system and the temperature parameters. This allows the HIPD instrument to continue its operation unattended and facilitates judicial use of reactor time.
2.2.3. Temperature controller To study the magnetic structure of a sample, the installation of a low-temperature sample environment is in progress. A spec macro for temperature control was formulated in the setup of the next set of experiments. Communication with the temperature controller is managed through a GPIB. The creation of the macro and a test of its properties were completed using a closed cycle refrigerator (CCR).
2.2.4. 1U rack server computer A 1U rack mount-type server PC was used to construct the stable and compact DAQ system. The space around HIPD is limited and the voluminous electronics demands a compact computer using space efficiently. A thin computer increases the efficiency of the space. For remote control from outside the HANARO reactor hall, a computer can access the system via FTP and through a VNC client program. The HIPD computer requires three types of interfaces: USB for the USB DAQ modules, GPIB for the temperature controller and RS-232 for the motor controller. However, the 1U rack server has only 1 PCI slot; thus, a RS-232 to USB and a GPIB to USB converter are used. The complete electronics system and the computer were installed in a 19 in rack of a detector bank and extension cables for the keyboard, mouse and PC monitor were used to control the instruments. The operating system of the PC is Ubuntu Linux, which enables the securing of an appropriate library for the HIPD programs.
2.2.5. SPEC macro based on LINUX SPEC is data acquisition software that is based on macros. It is used especially at neutron and X-ray facilities [9–12]. SPEC has a built-in macro for general purposes but the present system requires a more specific macro for the aforementioned instruments. The DAQ USB module and the motor controller are registered for SPEC and a specific macro was created for the HIPD instrument. Fig. 3 shows the flow of the SPEC macro for the HIPD data acquisition process. The macro functions start the instrument, the monitoring diffraction spectrum, the data acquisition and the data storage processes. The macro requires the three angle parameters of the start, target and step angle parameters. When the macro starts, it examines the angle parameters. The user can fill in the experimental information, which the macro can save as a file header. DAQ USB modules are initialized and a preset time or preset count of the monitor detector is set up. The measurement loop consists of four parts: the first process moves the detector bank by means of the step angle, the second process starts the DAQ USB modules, the third process shows the full spectrum of the eight detectors for every update instance on the screen and the fourth process saves the data of the eight DAQ USB modules to a disk when the measurement is completed. Up to the target angle, the macro repeats the measurement loop. The data file, which uses the ASCII format, has two parts: the header and the data. The file header includes the user information, the time and the measurement condition. The data section has the detector entry number, the present angle and the spectrum data from 10 columns. To trace the data easily, it was determined that the file name would be in the form of digits that were automatically numbered (i.e., HIPD000001.dat). The digits refer to the absolute number of measurements, and the digits increase with every instance of data acquisition.
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J.-K. Cheon et al. / Nuclear Instruments and Methods in Physics Research A 620 (2010) 445–449
Start : Start/Target/Step angle Up to target angle Declare Variables
Move Detector bank
Input Parameter bad
StartmasterDAQmodule
Check parameters
Until DAQ stop
good
Preview spectra
Initialize DAQ modules
Wait for update time
Receive & Save data
End Fig. 3. Spec macro algorithm: the HIPD spec macro repeats acquiring the data from the eight 1-D PSD and from the scanning of the detector bank.
Record Al2O3 data for channel calibration 0-8° at a step angle 1°
3000 Uniform irradiation by plexiglass
Calculate channel-degree conversion coefficient
Record Plexiglas data for 2 hour
Intensity (counts/3hrs)
for 20 min.
Record Al2O3 0-8° at a step angle 8° for 2 hour
1° = 95.3055 channels 1 channel = 0.0105°
2500 2000
Differential nonlinearity = 3.5%
1500 1000
Effective area >850 channels 500
Find center channel of each detector
0 0
100 200 300 400 500 600 700 800 900 1000 1100 Channel Number
Convert into 2 theta
Fig. 5. Uniformity of the responses of a 1-D PSD to Plexiglas spectra: the effective angular coverage of the PSD is 8.91.
Cutting, normalizing, binning
8000
Create Output File
7000 Fig. 4. Process of system calibration and normalization of the data.
2.2.6. HIPD system calibration The data obtained from all of the PSDs is in a raw state. It is calibrated and converted to a scattering angle of 2y. Fig. 4 shows a flow chart of the calibration of the raw data and the channel for the 2y conversion. A correction for the parallex error from the flat detectors was not made because the optimum geometry was found to minimize parallex broadening below 10 at previous work [4]. To calibrate the channel, an Al2O3 sample was recorded for a start angle of 01, an end angle of 81 and a step angle of 11. Using variation of the peak position for rotation of the detector bank, the channel-angle conversion coefficients of all detectors were derived. To compensate for offsets among the detectors, the
Intensity
6000 5000 4000 3000 2000 1000 0 0
500
1000
1500
2000
2500
3000
Channel Fig. 6. Raw data of the Al2O3 taken by the 5th and 6th PSDs (761r 2y r1021).
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449
28000 hipd al2O3 300k.PRF:
24000
Yobs Ycalc Yobs-Ycalc Bragg_position
Intensity (arb. units)
20000 16000 12000 8000 4000 0 -4000 -8000 26
36
46
56
66
76
86
96
106
116
126
2 (°) Fig. 7. Entire diffraction pattern of an Al2O3 rod from the HIPD overlapped in two steps.
center channel was found by referring to the Al2O3 pattern of the HRPD. As the neutron wavelength of the HRPD is different from that of the HIPD, a HRPD pattern was simulated for the neutron wavelength of the HIPD using Fullproof [13]. Normalization was accomplished by Plexiglas data, and the channels were converted into 2y using the pre-obtained channel-angle conversion coefficients. A cutting process was used to merge 16 patterns according to the detector offset values. A binning process was then used to adjust the step size. The resulting file is thus created with continuous data from 11 to 1281 of 2y versus the intensity.
3. Results The uniformity of response from a 1-D PSD is recorded using a Plexiglas sample. Fig. 5 shows the detector uniformity over the sensitive length. The effective coverage per detector is 8.91, and the differential nonlinearity is 3.5%. As the minimum angular coverage to create a full pattern is 81, this coverage is sufficient for analyzing the diffraction pattern. Similar data is recorded for all eight detectors and the responses were found to be satisfactory. The spatial resolution of a 1-D PSD is measured using a cadmium collimator plate with five slits. The slits have a width of 1 mm at a spacing of 10 mm. They are exposed to the Plexiglas spectra. The resolution obtained is 2 mm FWHM at 3 200 V. As the sample-detector distance is 1 050 mm, the angular resolution of 2y is 0.1091 FWHM. Variation of the spatial resolution as a function of the applied voltage is recorded, and better resolution is obtained at a higher anode bias. The best resolution obtained is 2 mm FWHM. Similar tests were performed on all eight PSDs, and all of the results were found to be in agreement with these results. The response of the entire system was studied using the Al2O3 test sample, which was recorded for 2 h. The raw data of the Al2O3 was recorded and a channel number for the raw data was then reconstructed into a 2-y angle by an external converter program. The raw data of the fifth and sixth PSDs before calibration are presented in Fig. 6. The interfaces of the two PSDs that overlap are indicated by ellipses. The entire diffraction pattern of the Al2O3 after the calibration step is shown in Fig. 7. The differences of calculated and observed intensities in Fig. 7 are due to texture of the alumina rod and imperfection in the efficiency calibration.
4. Conclusions A HIPD DAQ system using eight 1-D PSDs has been constructed. The proper counting of diffraction patterns of the instrument was verified using a test sample. Various samples will be measured in an effort to define the capabilities of the diffractometer. As most of the parts of HIPD were developed in-house, the diffractometer is economically constructed. The response speed of the delay line detector and the TDC in this system is faster compared to a charge division readout. The utilization of SPEC as the DAQ software simplifies the adaptation of various auxiliary devices such as temperature controllers and motor controllers, making the proposed system capable of diverse experiments through modification of the macro. The HIPD system will be modified by adding an algorithm for more convenient access. The custom-built instrument is easy to be maintained and to be modified for new scientific fields. The instrument will eventually be handed over to the users. References [1] Thomas C Hansen, Paul F Henry, Henry E Fischer, Jacques Torregrossa, Pierre Convert, Meas. Sci. Technol. 19 (2008) 034001. [2] J. Fried, J.A. Harder, G.J. Mahler, D.S. Makowiechi, J.A. Mead, V. Radeka, N.A. Schaknowski, G.C. Smith, B. Yu, Nucl. Instr. and Meth. Phys. Res. A 478 (2002) 415. [3] Andrew J Studer, Mark E. Hagen, Terrence J. Noakes, Physica B 385–386 (2006) 1013. [4] Chang-Hee Lee, Myung-Kook Moon, V.T. Em, Young-Nam Choi, Hwa-Suk Oh, Uk-Won Nam, Nucl. Instr. and Meth. Phys. Res. A 508 (2003) 353. [5] Chang-Hee Lee, Vyacheslav Em, Myung-Kook Moon, Kyun-Pyo Hong, Jong-Kyu Cheon, Young-Hyun Choi, Uk-Won Nam, Kyung-Nam Kong, Physica B 385–386 (2006) 1016. [6] H.E. Fischer, G.J. Cuello, P. Palleau, D. Feltin, A.C. Barnes, Y.S. Badyal, J.M. Simonson, Appl. Phys. A 74 (2002) S160 (Suppl.). [7] U.W. Nam, S.G. Lee, J.G. Bak, M.K. Moon, J.K. Cheon, C.H. Lee, Rev. Sci. Instrum. 78 (2007) 103504. [8] /http://www.aisolutions.co.kr/product_1.htmlS. [9] /http://www.certif.com/S. [10] T.-S. Gau, Y.-C. Jean, K.-Y. Liu, C.-H. Chung, C.-K. Chen, S.-C. Lai, C.-H. Shu, Y.-S. Huang, C.-H. Chao, Y.-R. Lee, C.T. Chen, S.-L. Chang, Nucl. Instr. and Meth. Phys. Res. A 466 (2001) 569. [11] D.A. Walko, Nucl. Instr. and Meth. Phys. Res. A 582 (2007) 196. [12] R. Baudoing-Savois, M. De Santis, M.C. Saint-Lager, P. Dolle, O. Geaymond, P. Taunier, P. Jeantet, J.P. Roux, G. Renaud, A. Barbier, O. Robach, O. Ulrich, A. Mougin, G. Be´rard, Nucl. Instr. and Meth. Phys. Res. B 149 (1999) 213. [13] /http://www.ill.eu/sites/fullprof/S.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
A data acquisition system for a high intensity powder diffractometer at HANARO Jong-Kyu Cheon a,n, Myung-Kook Moon b, Chang-Hee Lee b, V. T Em b, Seong-Su Lee b, Uk-Won Nam c, Jung-Hwan Yi d, Je-Geun Park e, Hong-Ju Kim f a
Department of Radiation, Sorabol College, 165 Chunghyodong, Gyeongju, Gyeongsangbuk-do 780-711, South Korea Neutron Science Division, Korea Atomic Energy Research Institute, 150 Duckjindong, Yuseong-gu, Daejeon 305-353, South Korea c Space Science Division, Korea Astronomy and Space Science Institute, 61-1 Whaamdong, Yuseong-gu, Daejeon 305-348, South Korea d Department of Physics, Sungkyunkwan University, 300 Cheoncheondong, Jangan-gu, Suwon, Gyeonggi-do 440-746, South Korea e Department of Physics and Astronomy, Seoul National University, 599 Gwanak-ro, Gwanak-gu, Seoul 151-747, South Korea f School of Physics and Energy Sciences, Kyungpook National University,1370 Sankyukdong, Bukgu, Daegu 702-701, South Korea b
a r t i c l e in fo
abstract
Article history: Received 25 January 2010 Received in revised form 23 February 2010 Accepted 30 March 2010 Available online 10 April 2010
A data acquisition (DAQ) and control system for the high intensity powder diffractometer (HIPD) at High-flux Advanced Neutron Application ReactOr (HANARO) is developed in this paper. The HIPD provides the advantage of wide-angle coverage of neutron diffraction operations, which can reduce the time that is required for experiments. The HIPD DAQ and control system was commissioned into operation and the diffraction data of Al2O3 was recorded successfully. The details of the detector, the data acquisition process and the instrument controls system used in the operation of the HIPD are presented. & 2010 Elsevier B.V. All rights reserved.
Keywords: Instrumentation Powder diffraction Neutron diffraction
1. Introduction A neutron powder diffractometer is used to study microscopic atomic structures by measuring the angular distribution of neutrons diffracted from the sample. In powder diffraction, angular scanning of a larger area is an important factor, as this reduces the experiment time and hence the beam time. To record diffraction peaks, a traditional powder diffractometer using a tube detector with a small angular coverage area and step scanning is necessary. These types of experiments run for few days for a single sample. However, using a curved PSD with a wide angular scanning area, it is possible to measure the diffraction peaks much faster compared to the tube detector. Moreover, the reactor time can be used judicially. The diffractometer D20 at ILL and Wombat at ANSTO have detector systems that are capable of wide-angle coverage. The diffractometer D20 uses a 1-D large curved microstrip gas chamber system while Wombat is equipped with a 2-D position sensitive detector; however, these diffractometers are costly, and the development of a large-area PSD is a high-budget project that requires a multitude of tasks [1–3]. The most feasible option used at HIPD, HANARO, involves the use of eight modules for 1-D PSDs with a two-step scanning process.
n
Corresponding author. Tel.: + 82 11 9598 6942; fax: + 82 53 939 1259. E-mail address: [email protected] (J.-K. Cheon).
0168-9002/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2010.03.166
A high-resolution powder diffractometer is also installed at HANARO, but this instrument is heavily used for services. For certain diffraction experiments, high intensity is preferred over a high resolution. Thus, the installation of HIPD was essential for specific applications, allowing HRPD to be devoted to more precise experiments. A HIPD instrument capable of wide-angle coverage was developed for high-intensity diffraction applications. This diffractometer consists of eight multiwire-based 1-D PSDs. Each detector covering a scattering angle of 81 is placed at 161 intervals. To record a full diffraction pattern, two-step scanning is required. The conceptual design and diffraction experiments were recently completed by the authors [4,5]. It is similar to D4c at ILL [6]. The raise in the intensity and faster data collection enabled the study of microsamples in a volume range of 1–20 mm2. This represents a very important factor in the study of rarely available samples and/or those that deteriorate rapidly. This paper discusses the construction and performance of the HIPD. Complete indigenous development of the instrument was carried out using a custom-built detector, a custom-built step scan control device and a data acquisition system.
2. Development of the HIPD The HIPD is installed at beam port number ST-3 of HANARO, a 30 MW research reactor operated by KAERI. Fig. 1 shows the HIPD instrument. The neutron wavelength used is 1.478 A˚ with an
ARTICLE IN PRESS 446
J.-K. Cheon et al. / Nuclear Instruments and Methods in Physics Research A 620 (2010) 445–449
filled with 3He gas at 450 kPa and CF4 gas at 100 kPa as a ˚ quenching gas, which give a detection efficiency of 66% at 1.478 A. Pulses from the detector are processed by a current-sensitive preamplifier and then fed to a constant fraction discriminator (CFD) through a fixed additional delay at a scanning height of 7.61. The PSD assembly covers a total scanning angle of 1281.
Detector Bank
2.2. Pulse processing and data acquisition (DAQ) system Sample
Beam shutter
Fig. 1. High-intensity powder diffractometer installed at beam ST-3, at Hanaro, with a detector bank consisting of eight 1-D PSDs and shielding.
Table 1 HIPD instrument and detector specifications. Beam port
ST-3
Monochromator crystal Flux at sample (n/cm2/s) ˚ Wavelength (A)
Si bent perfect crystal 106 1.478
Angular area covered (deg.) Detector Type Position encoding Sample to Detector (mm) Angular Resolution (deg.) Position Resolution (mm) Effective area (mm2) Operating voltage (V) Detector Gas Detection gap (mm) Efficiency at 1.478 A˚ (%)
128 1-D neutron position sensitive detector Delay line 1050 0.109 2 140 180 3200 3 He + CF4 22.8 66
˚ Flux at the Si effective mosaic spread of 1.6 10 3 A. monochromator crystal is 109 n/cm2/s and the sample position is 106 n/cm2/s. The sample-to-detector distance was selected as 1 050 mm. The detectors were developed with a scanning height of 14 cm, and the values for the spatial resolution in the angular direction were 2 mm and 0.1091. Data from the detectors is processed using a delay line readout method with digital 12 bit data that is processed in a data acquisition system. The step scanning operations of the detectors is controlled by a high-power and high-precision motor controller and driver. The specifications of the HIPD instrument and detectors are shown in Table 1.
2.1. Detector All of the detectors used at HIPD were developed by the authors. These detectors are multiwire-based proportional counters and use a delay line readout method for position encoding. The sensing length of the detector is optimized to minimize parallax error and the height is optimized for intensity such that no broadening is experienced at a very low angle. Each detector consists of a wire frame and a strip readout frame. An anode frame, with an array of 10 mm wires at a pitch of 5 mm, is placed between the cathode readout frame and the window. The cathode is made up of a printed circuit board with 2.5 mm spacing strips. Each strip is terminated with a lumped delay. The distance between the anode and the cathode frame is 3.2 mm and total detector delay time is 230 ns. The effective area of the detector is 140 180 mm2 and the detection gap is 22.8 mm. The detector is
Fig. 2 shows a schematic of the control and data acquisition system of the HIPD. The signals of eight detectors through a preamplifier are converted into logic pulses at the CFD and are sent to DAQ USB modules. A DAQ USB module is the device that can read and record the information from the delay line of the detector. This module was developed by the authors in collaboration with the Korea Astronomy and Space Science Institute. They are NIM cassette-type modules and communicate with a PC through a USB 2.0 port. At the HIPD, eight DAQ USB modules are run simultaneously. Other devices that compose the HIPD DAQ system include a motor controller to move the detector bank and a temperature controller that monitors and controls the sample environment. All of the devices are controlled by SPEC based on LINUX. A SPEC macro was developed to conduct the basic neutron diffraction experiments. 2.2.1. DAQ USB module The DAQ USB module consists of two basic parts. The first is a time-to-digital converter (TDC) that processes the signal of the delay line detector and the second is a data acquisition module that interfaces to the PC via a USB connection. Two signals from the readout cathode X1, X2 and the anode are amplified with the current-sensitive preamplifier, and this signal is converted into a constant-amplitude fast NIM signal at the CFD. This signal carries data in the form of the arrival time. The converted signal enters the DAQ USB module. The DAQ USB module consists of the F1 TDC, FPGA, DRAM and DSP components. The F1 TDC measures the time difference between X1 and X2. The programmed FPGA logic rejects pileups, grabs data and controls the preset time or the counter. The DSP controls the F1-based TDC and FPGA. It plots a 1-D or 2-D histogram using the position data from the FPGA. It communicates with the PC via USB [7,8]. This module supports the synchronized mode, which can be used for the present multiPSD system. The synchronized mode has two types of modules: the master module and the slave module. When a user starts the master module, it sends a gate signal to the slave modules. When the gate signal is active, slaves are operational. The master has all of the preset information but the slaves keep only the histogram data. 2.2.2. Motor control unit A complete detector bank consisting of eight PSD chambers and three preamplifiers mounted on each PSD is quite heavy and thus it is moved by an air pad cushion. Currently, the PSD remains on air during the measurement. We will remove the air during the detector operation later to improve position stability. A highpower motor driver and a micro step controller are required for precise and controlled movement of the detector bank. A stepper motor control unit that uses an attached controller and a driver was developed for precise motion control. The control unit communicates with the PC through RS-232. It can control the stepping motor by micro-steps of 50,000 pulses/revolution with a maximum current of 4 A/phase. Commands to the motor controller are also accessed through a data acquisition system through the PC. The data acquisition is automated so that it can continue to monitor all of the controls, such as the readout
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Step Motor Controller
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Fig. 2. Block diagram of the HIPD DAQ system: a Linux server installed with SPEC can control the HIPD DAQ system using USB, GPIB and RS-232. Synchronized DAQ USB modules can record the data of the eight 1-D PSDs simultaneously.
system, the motor control system and the temperature parameters. This allows the HIPD instrument to continue its operation unattended and facilitates judicial use of reactor time.
2.2.3. Temperature controller To study the magnetic structure of a sample, the installation of a low-temperature sample environment is in progress. A spec macro for temperature control was formulated in the setup of the next set of experiments. Communication with the temperature controller is managed through a GPIB. The creation of the macro and a test of its properties were completed using a closed cycle refrigerator (CCR).
2.2.4. 1U rack server computer A 1U rack mount-type server PC was used to construct the stable and compact DAQ system. The space around HIPD is limited and the voluminous electronics demands a compact computer using space efficiently. A thin computer increases the efficiency of the space. For remote control from outside the HANARO reactor hall, a computer can access the system via FTP and through a VNC client program. The HIPD computer requires three types of interfaces: USB for the USB DAQ modules, GPIB for the temperature controller and RS-232 for the motor controller. However, the 1U rack server has only 1 PCI slot; thus, a RS-232 to USB and a GPIB to USB converter are used. The complete electronics system and the computer were installed in a 19 in rack of a detector bank and extension cables for the keyboard, mouse and PC monitor were used to control the instruments. The operating system of the PC is Ubuntu Linux, which enables the securing of an appropriate library for the HIPD programs.
2.2.5. SPEC macro based on LINUX SPEC is data acquisition software that is based on macros. It is used especially at neutron and X-ray facilities [9–12]. SPEC has a built-in macro for general purposes but the present system requires a more specific macro for the aforementioned instruments. The DAQ USB module and the motor controller are registered for SPEC and a specific macro was created for the HIPD instrument. Fig. 3 shows the flow of the SPEC macro for the HIPD data acquisition process. The macro functions start the instrument, the monitoring diffraction spectrum, the data acquisition and the data storage processes. The macro requires the three angle parameters of the start, target and step angle parameters. When the macro starts, it examines the angle parameters. The user can fill in the experimental information, which the macro can save as a file header. DAQ USB modules are initialized and a preset time or preset count of the monitor detector is set up. The measurement loop consists of four parts: the first process moves the detector bank by means of the step angle, the second process starts the DAQ USB modules, the third process shows the full spectrum of the eight detectors for every update instance on the screen and the fourth process saves the data of the eight DAQ USB modules to a disk when the measurement is completed. Up to the target angle, the macro repeats the measurement loop. The data file, which uses the ASCII format, has two parts: the header and the data. The file header includes the user information, the time and the measurement condition. The data section has the detector entry number, the present angle and the spectrum data from 10 columns. To trace the data easily, it was determined that the file name would be in the form of digits that were automatically numbered (i.e., HIPD000001.dat). The digits refer to the absolute number of measurements, and the digits increase with every instance of data acquisition.
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Start : Start/Target/Step angle Up to target angle Declare Variables
Move Detector bank
Input Parameter bad
StartmasterDAQmodule
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good
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End Fig. 3. Spec macro algorithm: the HIPD spec macro repeats acquiring the data from the eight 1-D PSD and from the scanning of the detector bank.
Record Al2O3 data for channel calibration 0-8° at a step angle 1°
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1° = 95.3055 channels 1 channel = 0.0105°
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Fig. 5. Uniformity of the responses of a 1-D PSD to Plexiglas spectra: the effective angular coverage of the PSD is 8.91.
Cutting, normalizing, binning
8000
Create Output File
7000 Fig. 4. Process of system calibration and normalization of the data.
2.2.6. HIPD system calibration The data obtained from all of the PSDs is in a raw state. It is calibrated and converted to a scattering angle of 2y. Fig. 4 shows a flow chart of the calibration of the raw data and the channel for the 2y conversion. A correction for the parallex error from the flat detectors was not made because the optimum geometry was found to minimize parallex broadening below 10 at previous work [4]. To calibrate the channel, an Al2O3 sample was recorded for a start angle of 01, an end angle of 81 and a step angle of 11. Using variation of the peak position for rotation of the detector bank, the channel-angle conversion coefficients of all detectors were derived. To compensate for offsets among the detectors, the
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Channel Fig. 6. Raw data of the Al2O3 taken by the 5th and 6th PSDs (761r 2y r1021).
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2 (°) Fig. 7. Entire diffraction pattern of an Al2O3 rod from the HIPD overlapped in two steps.
center channel was found by referring to the Al2O3 pattern of the HRPD. As the neutron wavelength of the HRPD is different from that of the HIPD, a HRPD pattern was simulated for the neutron wavelength of the HIPD using Fullproof [13]. Normalization was accomplished by Plexiglas data, and the channels were converted into 2y using the pre-obtained channel-angle conversion coefficients. A cutting process was used to merge 16 patterns according to the detector offset values. A binning process was then used to adjust the step size. The resulting file is thus created with continuous data from 11 to 1281 of 2y versus the intensity.
3. Results The uniformity of response from a 1-D PSD is recorded using a Plexiglas sample. Fig. 5 shows the detector uniformity over the sensitive length. The effective coverage per detector is 8.91, and the differential nonlinearity is 3.5%. As the minimum angular coverage to create a full pattern is 81, this coverage is sufficient for analyzing the diffraction pattern. Similar data is recorded for all eight detectors and the responses were found to be satisfactory. The spatial resolution of a 1-D PSD is measured using a cadmium collimator plate with five slits. The slits have a width of 1 mm at a spacing of 10 mm. They are exposed to the Plexiglas spectra. The resolution obtained is 2 mm FWHM at 3 200 V. As the sample-detector distance is 1 050 mm, the angular resolution of 2y is 0.1091 FWHM. Variation of the spatial resolution as a function of the applied voltage is recorded, and better resolution is obtained at a higher anode bias. The best resolution obtained is 2 mm FWHM. Similar tests were performed on all eight PSDs, and all of the results were found to be in agreement with these results. The response of the entire system was studied using the Al2O3 test sample, which was recorded for 2 h. The raw data of the Al2O3 was recorded and a channel number for the raw data was then reconstructed into a 2-y angle by an external converter program. The raw data of the fifth and sixth PSDs before calibration are presented in Fig. 6. The interfaces of the two PSDs that overlap are indicated by ellipses. The entire diffraction pattern of the Al2O3 after the calibration step is shown in Fig. 7. The differences of calculated and observed intensities in Fig. 7 are due to texture of the alumina rod and imperfection in the efficiency calibration.
4. Conclusions A HIPD DAQ system using eight 1-D PSDs has been constructed. The proper counting of diffraction patterns of the instrument was verified using a test sample. Various samples will be measured in an effort to define the capabilities of the diffractometer. As most of the parts of HIPD were developed in-house, the diffractometer is economically constructed. The response speed of the delay line detector and the TDC in this system is faster compared to a charge division readout. The utilization of SPEC as the DAQ software simplifies the adaptation of various auxiliary devices such as temperature controllers and motor controllers, making the proposed system capable of diverse experiments through modification of the macro. The HIPD system will be modified by adding an algorithm for more convenient access. The custom-built instrument is easy to be maintained and to be modified for new scientific fields. The instrument will eventually be handed over to the users. References [1] Thomas C Hansen, Paul F Henry, Henry E Fischer, Jacques Torregrossa, Pierre Convert, Meas. Sci. Technol. 19 (2008) 034001. [2] J. Fried, J.A. Harder, G.J. Mahler, D.S. Makowiechi, J.A. Mead, V. Radeka, N.A. Schaknowski, G.C. Smith, B. Yu, Nucl. Instr. and Meth. Phys. Res. A 478 (2002) 415. [3] Andrew J Studer, Mark E. Hagen, Terrence J. Noakes, Physica B 385–386 (2006) 1013. [4] Chang-Hee Lee, Myung-Kook Moon, V.T. Em, Young-Nam Choi, Hwa-Suk Oh, Uk-Won Nam, Nucl. Instr. and Meth. Phys. Res. A 508 (2003) 353. [5] Chang-Hee Lee, Vyacheslav Em, Myung-Kook Moon, Kyun-Pyo Hong, Jong-Kyu Cheon, Young-Hyun Choi, Uk-Won Nam, Kyung-Nam Kong, Physica B 385–386 (2006) 1016. [6] H.E. Fischer, G.J. Cuello, P. Palleau, D. Feltin, A.C. Barnes, Y.S. Badyal, J.M. Simonson, Appl. Phys. A 74 (2002) S160 (Suppl.). [7] U.W. Nam, S.G. Lee, J.G. Bak, M.K. Moon, J.K. Cheon, C.H. Lee, Rev. Sci. Instrum. 78 (2007) 103504. [8] /http://www.aisolutions.co.kr/product_1.htmlS. [9] /http://www.certif.com/S. [10] T.-S. Gau, Y.-C. Jean, K.-Y. Liu, C.-H. Chung, C.-K. Chen, S.-C. Lai, C.-H. Shu, Y.-S. Huang, C.-H. Chao, Y.-R. Lee, C.T. Chen, S.-L. Chang, Nucl. Instr. and Meth. Phys. Res. A 466 (2001) 569. [11] D.A. Walko, Nucl. Instr. and Meth. Phys. Res. A 582 (2007) 196. [12] R. Baudoing-Savois, M. De Santis, M.C. Saint-Lager, P. Dolle, O. Geaymond, P. Taunier, P. Jeantet, J.P. Roux, G. Renaud, A. Barbier, O. Robach, O. Ulrich, A. Mougin, G. Be´rard, Nucl. Instr. and Meth. Phys. Res. B 149 (1999) 213. [13] /http://www.ill.eu/sites/fullprof/S.