Instrumentation for Neutron Diffraction
Instrumentation Associates was formed in the early 1980s to supply instrument control electronics and computer interfaces for neutron and X-Ray diffraction instrumentation. Since 2000, the company has concentrated on the design and development of components for neutron diffraction. The suite of instrument components offered grew out of our experiences in the development of instrumentation at a number of neutron diffraction laboratories in the US and elsewhere. Instrumentation Associates provides design, fabrication and software for instrument subsystems or complete instruments: monochromators, position sensitive detector arrays, electronics for position decoding with linear PSDs, support electronics for linear PSD systems, detector shield assemblies, diffractometer mechanics assemblies, rotating oscillating collimators, monochromator shield assemblies, and in-pile primary collimators and filters. Instrumentation Associates also supplies custom software for the control, maintenance and operation of linear position sensitive detector neutron powder diffractometers.
Neutron Diffractometer Systems
Instrumentation Associates can supply complete linear PSD neutron diffractometer systems configured for your installation site. We can supply all elements of the instrument or we can provide major subsystems to mate with your existing shield and site configuration. Components are custom designed, built and assembled for each installation so that it is not possible to quote prices here. Please contact Instrumentation Associates so that we can configure a quotation for individual components or for a complete system tailored to your needs and instrument geometry.
Two recent examples of such turnkey systems are the SAND diffractometer, recently installed at the 3 MW TRIGA reactor outside of Dhaka, Bangladesh and the HIPD diffactometer installed at the new 60 MW CARR reactor near Beijing.
CARR-HIPD. The High Intensity Powder Diffractometer (HIPD) is one of the initial suite of instruments to be installed at the 60 MW CARR reactor. HIPD is located on HT3 at CARR, a port that is shared with the High Resolution Powder Diffractometer. IA was responsible for the design, fabrication, supply and installation of all of the instrument components, including the monochromator and monochromator goniometer. The instrument was designed to mate with the existing primary shield.
CARR-HIPDThe photo above shows the HIPD in-place. A full description of the instrument can be viewed here.
SAND The Savar Neutron Diffractometer (SAND) was built and installed over the period January 2008 - June 2010. IA was responsible for the design and fabrication of all elements of the instrument, including the primary shield. A drawing of the instrument layout is shown below.
elements of the SAND design were dictated by the conditions and
facilities available at the BAEC Atomic Energy Research Establishment
laboratory, the characteristics of the reactor and beamport.
The SAND diffractometer.Back to Top
Neutron Diffractometer Components
Double Bent Perfect Single Crystal Silicon Monochromator
The doubly bent perfect single crystal focusing silicon monochromator (Popovici monochromator) is composed of 9 slabs of perfect single crystal silicon cut from a single wafer with nominal dimensions of 0.57 x 0.021 x 7.5 in3. The blades are mechanically bent horizontally and supported so that they form a polygonal approximation to the surface of a sphere in the vertical direction. Bending is provided by a mechanical screw with the horizontal and vertical curvatures optimized for the diffractometer geometry (Source-Monochromator and Monochromator-Sample distances). The figure below shows a Popovici monochromator.
The neutron optical theory underlying the monochromator was developed by Popovici and his collaborators and perfected by Popovici at the University of Missouri (MURR) in the 1990s. About 15 of these monochromators are in service at neutron beam laboratories in various locations.
Double bent perfect single crystal focusing silicon monochromator.
The normal to the surface of the monochromator blades is cut at an offset angle from the natural (111) direction; the optimum offset being determined by the instrument geometry. The (1-10) direction is held vertical, which allows a number of planes to be accessed. For a 90o takeoff angle, the reflections and wavelengths accessible for the monochromator are: (113) 2.316 A, (115) 1.478 A, (335) 1.171 A, (117) 1.075 A, (331) 1.762 A, (551) 1.075 A. The first 4 reflections are accessible by simply rotating the monochromator since these reflections are in the (110) zone. The remaining two reflections can be obtained by inverting the monochromator (exchanging top and bottom). The most intense reflection is the (115) although the other reflections are also usable for diffraction experiments.
When the monochromator is optimized for the instrument geometry, it accepts a broad beam from the neutron source plane and focuses this beam on to the specimen position. A strong Fankuchen compression of the beam in the horizontal direction adds to the effect of (conventional) vertical focusing and to the diffraction focusing to produce line profiles at the detector plane that are optimized for the use of a linear position sensitive detector.Back to Top
Instrumentation Associates can supply the primary (monochromator) shield custom designed for your instrument installation, reactor and beamport geometry. Calculation via MCNP is used to qualify the initial design which is supplemented by our experience in developing effective radiation shields. Primary shielding can be developed for a single takeoff angle installation, for multiple takeoff angle instruments or for multiple instruments installed at the same beam port. Since a Popovici monochromator is essentially transparent to neutrons (removing only a small slice of the white beam spectrum) it is easy to install secondary instruments downstream. The adjacent figure illustrates the design of the SAND primary shield, built from 7 steel jacketed, heavy concrete filled blocks. The capacity of the building crane at the reactor laboratory limited the weight of the individual shield pieces to 5 tons. The shield was designed for 2 takeoff angles: 97o and 83o and makes provision for the use of the white beam downstream of the monochromator for a second instrument installation.Back to Top
The Popovici monochromator is optimized for an open primary beam (no a1 collimator) while obtaining high resolution diffraction linewidths at the focusing condition. This enormously simplifies the design and fabrication cost of the collimator. The collimator for SAND is of welded aluminum construction and can be equipped with a sapphire filter to suppress fast neutrons. Primary collimators can be designed to be flooded with deionized water, effectively making a very cost-effective shutter. With the collimator flooded, radiation levels in the monochromator cavity are much reduced, making maintenance activities easier. During measurements, the collimator can be back-filled with He gas so that the air scattering and attenuation of the beam is reduced. The neutron loss in air (typically 6%/m) is then eliminated.Back to Top
Instrumentation Associates can supply the monochromator rotation, single or double translation and tilt stages to mount the monochromator appropriately at the intersection of the white beam and the monochromatic beam line. The monochromator is fastened to the goniometer with custom designed jigs so that the tilt and translation stages are appropriately oriented for optimum decoupled tilt and rotation monochromator adjustments. The adjacent figure shows the moochromator tilt and translation stages mounted below the monochromator mushroom shield of the SAND diffractometer.
We recommend mounting the monochromator rotation stage outside of the monochromator cavity in the primary shield so that it can be equipped with an incremental optical encoder – encoders usually have a short lifetime in the radiation field of the monochromator cavity.Back to Top
Linear Position Sensitive Detector Array
Arrays of linear position sensitive neutron proportional counters are a practical and economical alternative to monolithic large area neutron detectors. Small numbers of linear PSDs were used at Brookhaven National Laboratory and at the KFA-Julich in the 1970s for SANS instruments but it was at the University of Missouri where the linear PSD array was adopted and developed for neutron powder diffraction applications.
Instrumentation Associates can fabricate neutron detector arrays to your specifications. We have generally produced detector arrays with 3, 5, 7, 11 or 15 detector elements, each 1” (2.54 cm ) dia and 12” (30.5 cm) or 24” (61 cm) long but other element diameters and lengths can be provided.
The detector arrays are complete with integral preamplifiers and high-voltage decoupling capacitors mounted on printed circuit “mother boards” at each end of the array. The preamplifier output signals connect via MMCX connectors to the external position decoding electronics. The figure above shows the 15-element detector array that is installed at the SAND diffractometer in Dhaka, Bangladesh.
The position decoding resolution of the detector elements is nominally 3 mm along the detector axis although the shorter (12”) detector elements can be expected to provide substantially better performance while detectors longer than 24” will have poorer performance.
The detector array requires +/- 6 V for preamplifier power and a separate high voltage power supply for detector bias.Back to Top
Power Module: PWR
Instrumentation Associates provides a 1-wide NIM module that supplies preamplifier power to the detector array. The PWR module supplies clean +/- 6 V from 2 DB-9 connectors mounted at the back of the module. One PWR module will supply power for up to 15 detector elements (30 preamplifiers).
The PWR module also contains a detector preamplifier that can be used to exercise PEMs (see below). Two BNC connectors at the back of the module and two BNC connectors on the front of the module are connected to the internal preamplifier. A single input BNC connector on the front panel is used for test (pulser) input signals.Back to Top
Position Encoding Module (PEM)
The Instrumentation Associates Position Encoding Module (PEM) was developed specifically to determine event positions in linear position sensitive proportional counters. It is a 1-wide NIM device that accepts the signals from the detector preamplifiers, digitizes them and calculates the event positions from the ratio of the signal amplitudes. It maintains pulse height and event position histograms in its internal memory and delivers this information to the instrument host computer via USB bus on command. One PEM is used to service the signals from a single detector element. A PEM is shown in the adjacent figure.
The PEM is supplied with the program PEMTest, written in C++, that allows the user to exercise one PEM at a time, acquire data, display the amplitude, sum and position histograms on a graph and save the histograms to disk. A second program, PEMDataViewer, written in Java, is also supplied. It permits the user to open the data files produced by PEMTest and plot the individual histograms. The source code for both programs is provided.
The program examples will allow a user to write his own programs to control the operation of many PEMs working together and to implement the calibration and conversion operations that are needed to convert the raw position spectra provided by the PEMs.
The PEMs provide the event positions as “position spectra” and these must be related to the instrument geometry and detector calibration in order to convert them to angular histograms and to combine the data from the different PEMs and detector elements. Instrumentation Associates has developed an extensive instrument control software system, the Neutron Diffractometer Control System (NDCS, described below) that accomplishes these tasks and much more. Users are strongly advised to purchase this software when buying a position sensitive detector array.Back to Top
The large size and high efficiency of the linear PSD detector array makes an effective shield a necessity. Instrumentation Associates can supply the detector shield for your instrument. The adjacent figure shows the detector shield for the 15-element detector at SAND. It is composed of high density polyethylene, borated polyethylene and is lined with cadmium metal.
The detector array can be moved within the shield assembly to two different distances from specimen position: typically 1.6 m is used as the sample-detector separation for the highest resolution with 24” (61 cm) long detector elements. At this distance, the detector spans 20o two-theta. The detector can also be placed at 1.05 m from the specimen, spanning 30o two-theta at somewhat lower resolution. Switching from one position to another is a simple task requiring only that the back of the detector array be opened and the detector moved to the new location. The detector array is mounted on rails inside the shield assembly and has two preset locations set by mechanical stops.
At the top of the shield is a slot for the insertion of a precision comb mask for detector calibration. The comb mask is supplied along with the detector array. The back of the shield assembly is a hinged door to facilitate detector servicing. The detector can generally be serviced without the need to remove the array from the shield.Back to Top
The Arc-Shield, shown in the SAND system drawing and CARR-HIPD photo, is a secondary shield placed in front of the detector array to block neutrons that would otherwise enter the large entrance aperture of the detector shield. The Arc-Shield, supplied with the detector shield, is custom designed for the instrument mechanics and shield configuration.
The mechanical elements of the diffractometer design must be of high quality and precision in order to obtain the best diffraction data. Instrumentation Associates believes that the most economical, high-precision configuration of the diffractometer is obtained using air-pad/air-bearing support for the diffractometer shield combined with robust rotation tables to provide the angular motion of the assembly. The general elements of the instrument configuration are shown in SAND system drawing. The detector shield (typically ~ 1000 Lbs) is supported on 3 air-bearings on top of a thick aluminum plate. The aluminum instrument support base is jack-screw leveled and covered in turn by a tough plastic protective cover. The plastic protects the aluminum surface (which is soft) from damage. The plastic cover will last many years and can be very economically replaced if necessary. A further advantage of this configuration is that installation of the instrument does not limit access to cable trenches or other services that are present in the floor below. The instrument can, when necessary, be disassembled to permit access.
The motion of the detector assembly about the specimen position is provided by a rotation stage, mechanically attached to the aluminum instrument support slab. When building compressed air is supplied to the air-pads (P > 90 psi), motion of the detector assembly on its circular track about the specimen is virtually frictionless and easily driven.
The diffractometer mechanics for an instrument must be designed for your particular installation and site constraints. Please contact Instrumentation Associates so that we can work with you to prepare a quotation for your instrument.Back to Top
Rotating Oscillating Collimator
In order to perform experiments on specimens in special environment chambers (cryorefrigerators, cryostats, furnaces, …) as well as to reduce background from the environment, it is necessary to suppress the scattering from vacuum and heat shield walls that are external to the specimen itself. The rotating oscillating collimator (ROC) is used to accomplish this task.
The adjacent figure shows the ROC for a 15-element detector array. In normal operation, the ROC is placed directly in front of the opening of the detector shield. Additional neutron shields (not shown) are placed at the front and back of the ROC.
In the case of a 2-position detector shield, two sets of ROC shields are supplied. When the detector array is moved from one sample-detector distance to another, the appropriate ROC shield must be put in place.
The blades of the ROC are made of stainless steel and are coated with neutron absorbing paint. They are oriented radially and effectively prevent neutrons scattering from objects greater than 2 cm distant the specimen position from reaching the detector. In order to prevent the appearance of shadows from the blades on the detector plane, the entire array of collimator blades is rotated back and forth over a range of several degrees. In this manner the absorption of the blades is averaged out. The rotation/oscillation of the ROC is powered by a small stepper motor. With the ROC, scattering from heat shields and vacuum walls greater than ~2 cm from the diffractometer axis becomes essentially invisible.Back to Top
Stepper Motor Controllers
Instrumentation Associates can supply the stepper motor controllers required for your installation. The figure below shows the front panel of a 19” rack panel mount 3-motor driver assembly designed for the control of the HIPD diffractometer. These motor drivers can be configured for a wide variety of 2-phase bipolar stepper motors with and without incremental optical encoders. The motor drivers also have inputs for hard limit switches and can be configured for software limit switches. These systems have a rich inventory of commands, parameters and capabilities that can be user configured. The triple motor driver assembly communicates with the host computer via serial port and comes with software for experimenting with the motor configuration and operation. Finally, they are compatible with the Instrumentation Associates NDCS instrument control software and their configuration and operation are thereby brought under unified computer control.Back to Top
IA Counter/Timer Shutter Control Module
Neutron diffraction experiments are normally conducted using a neutron beam monitor to control the duration of the experiment. The IA Counter/Timer – Shutter Control (C/T-S) is designed to provide control of the duration of neutron diffraction experiments either by the preset number of events recorded in a neutron monitor or by a preset time. The Counter/Timer-Shutter control module is a 1-wide NIM module with three BNC counter inputs and one BNC control signal output. The C/T-S communicates with the host computer via USB bus.
Under control of the host computer, any of the 3 input counters or the timer may be used as the “preset device” that will control the experiment duration. When the preset device is loaded with the preset count (or time) from the host, and commanded to start, all counter channels and the timer count. When the preset device reaches its preset count or time, it signals the host that it is done and all the counters and timer halt. The C/T-S can be programmed to output a logic control signal automatically at start and/or stop or it can output the signal on command from the host computer.
The polarity and amplitude of the control signal can be configured from the host computer as can the thresholds for the front and back panel module counter inputs. The front of the C/T-S is shown in the adjacent figure.Back to Top
Neutron Diffractometer Control System – NDCS
A linear position sensitive detector array presents a series of challenges that must be overcome in order to obtain the best system performance. The PEM ADC (analog-to-digital converter) gains must be matched to the detector pulse height spectrum and balanced one side to the other. The system electronic gains must be carefully monitored in order to ensure that system drift does not compromise performance. PEM data is NOT in angle or true detector position but in relative position channels. In order to obtain scattering angle or position histograms, the detectors must be calibrated and their data appropriately processed. In addition to controlling the characteristics of the detector array, the experimenter is also faced with maintaining control over the instrument mechanical motions and combining this information with the detector array data to produce diffraction spectra over the complete instrument angular range. Finally, the instrument can produce an avalanche of data in comparison to conventional, single or multi-detector detector instruments. This data must be managed. The techniques that worked well when one spectra was accumulated in 3 to 5 days of instrument time will not work for an instrument than can produce 5 (or 50) spectra a day.
The Neutron Diffractometer Control System (NDCS) is a completely integrated instrument control program that was written (in Java) to manage the operation, electronic adjustment, maintenance, scheduling, data analysis and data visualization for neutron diffractometers employing arrays of linear position sensitive proportional counters.
The Neutron Diffractometer Control System includes:
The complete description and illustration of the capabilities of the NDCS is too large to be presented here but can be supplied on request.Back to Top
The Popovici double focusing monochromator combined with the linear position sensitive detector array presents an instrument that is optimized for small diameter specimens. This is a distinct advantage when the user wishes to study new materials where there may only be a small amount of specimen material available. To match the instrument optimization, Instrumentation Associates offers two thin wall vanadium specimen holders: 0.125” dia (3.17 mm) and 0.25” dia (6.35 mm).The small diameter sample holder is shown below.
The sample holder vanadium wall is 0.005” thick and the the cylinder is of welded construction. The small specimen holder typically will contain less than 1 g of specimen material. The sample holders are not vacuum tight. At the bottom is a 0.250” dia stud for mounting the sample holder in the beam. The stud-flange is screwed on with small socket head cap screws. At the other end of the specimen is another screw that can be removed to facilitate cleaning tightly packed old specimen materials out of the specimen can. The sample holders are supplied with filling tools that make it easy to load them with powdered specimens.
Nominally, the 3 mm dia. sample cans are used with the detector at 1.6 m from the specimen position for the best resolution. Higher speed data acquisition at lower resolution can be obtained by placing the detector at 1.05 m from the specimen and employing the 6 mm dia. specimens.
Data acquisition at the 1.05 m detector distance is nominally 6 x faster than that at the 1.6 m detector distance using the 3 mm specimen holder.Back to Top
Cryorefrigerator/Low-Temperature Sample Environment
Instrumentation Associates can supply a cryorefrigerator/low-temperature sample environment for neutron diffraction consisting of the cryo-cooler compressor, cold-head, neutron scattering tailpiece and temperature controller. Cryo-cooler compressor/cold head combinations can be supplied for 10 K, 5.5 K or 4 K systems.
The neutron scattering tailpiece has been designed for experiments using the Instrumentation Associates thin-wall vanadium sample holders. These are placed in an indium-wire sealed aluminum can that is mounted to the system cold-finger as shown in the adjacent figure with the with the radiation and vacuum walls removed. The samples are loaded into the exchange gas can in a helium gas atmosphere which ensures that there will be good thermal contact between the specimen and the cold-finger.
In operation, the cold-head/neutron scattering tailpiece is bolted in place on the diffractometer sample table as shown in the figure on the right. Use of the cryorefrigerator system will require a clean high-vacuum pumping system to evacuate the cold-head and neutron scattering tailpiece. Crossed X-Y slides mounted to the diffractometer sample table to center the specimen on the diffractometer axis are also required.
The Cryorefrigerator/Low-Temperature sample environment is completely integrated with the NDCS, allowing the user to specify and schedule a series of experiments at different temperatures, adjusting the temperature controller to the optimum control parameter settings for each experiment. NDCS provides a real-time plot of the system temperature and produces a time-temperature record coordinated with the diffraction data record.Back to Top
Neutron Diffraction Furnace
The left hand figure shows a top view the neutron diffraction furnace built for the SAND diffractometer and the right hand figure is a cutaway of the central portion. The central portion consists of a tube (stainless steel for lower temperature operation, alumina for higher temperatures) into which the sample is lowered (with appropriate heat shields on the sample down-rod). The SAND design required that the central tube be filled with a static gas (typically Ar) but a minor modification to the design will allow for continuous controllable gas flow, with the gas introduced below the sample and vented at the top. Two heaters surround the central tube, one above and one below the sample location. These are potted super Kantal (or similar) elements that can be operated above 1000 C. A type K thermocouple is mounted on the surface of each heater closest to the sample, while two additional thermocouples are mounted to the top and bottom of the sample can.
heat shield immediately surrounds the heaters and one or more
additional heat shields (depending on the operating demands) are at
larger diameters inside the vacuum space. The shell of the furnace
is of aluminum. In general, samples are changed without breaking the
vacuum so the lifetime of the elements in the vacuum space should be
quite long. Total attenuation of the neutron beam by the vacuum
jacket, heat shields and central tube is of order 15%. The furnace controller is arranged to individually drive each heater, based on the reading from its respective thermocouple or from the sample thermocouple in the same position i.e. upper or lower. In this way the temperature gradient across the sample can be minimized (by the development of a calibration table and careful adjustment of the set points). The furnace controller (shown on right) is fully integrated into the control system, but can also be operated in stand-alone mode to produce the calibration data.
The complete neutron diffractometer includes a variety of additional components that should be included to obtain the functionality of the instrument. None of the elements listed below are “optional” in the sense that they can be omitted from the portfolio of accessories for a functioning instrument.
The monochromatic beam shutter, monochromatic beam transport tubing, exit slit holder and interchangeable exit slit for the SAND diffractometer are shown above. The monochromatic beam transport tubing can be fabricated in several sections to provide flexibility.Back to Top