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A magnetically pulsed neutron time-of-flight spectrometer for inelastic scattering
NUCLEAR INSTRUMENTS AND METHODS 116 (1974) 205-216; (D NORTH-HOLLAND PUiSLISHING CO . A MAGNETICALLY PULSED NEUTRON TIME-OF-FLIGHT SPECTROMETER FOR INELASTIC SCATTERING* H. A. MOOKt, F. W. SNODGRASS+ and D. D. BATES+
Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830, U.S.A . Received 26 November 1973 A
time-of-flight spectrometer utilizing a magnetically pulsed beam has been placed in operation at the High Flux Isotope Reactor at the Oak Ridge National Laboratory . The pulsing is accomplished by rapidly changing the magnetic moment direction in a ferrite crystal that serves to both monochromate and pulse the beam . Good neutron reflectivity is obtained from the ferrite crystal and pulse risetimes of the order of 1 lus are available. The spectrometer is interfaced to a Digital Equipment Corpora-
tion PDP-15-30 computer which drives pulsed motors to set up the desired scattering geometry and collects and analyzes the time-of-flight data . The magnetically pulsed beam is ideal for use with the cross-correlation technique and spectra are: presented which show the accuracy of the pulsing technique and the power of the cross correlation methods in obtaining neutron scattering data.
1 . Introduction
technique has the disadvantage that both the momentum and energy transfer are varied from point to point in a time-of-flight spectrum from a single detector while a triple-axis spectrometer varies one of the quantities while the other is held fixed . This problem can be circumvented to some extent by using many detectors for a time-of-flight scan and picking the points from detector to detector in which either the momentum or energy is a constant. However, for many experiments no difficulty is encountered from both the energy and momentum transfer varying in the time-of-flight scan. Most present day pulsed beam spectrometers for inelastic scattering involve at least two high-speed' , phased rotors to obtain a neutron pulse with a small' ` velocity spread. Both multiple-rotor choppers and rotating crystal spectrometers that are phased with'` rotating collimators are successfully used. However, high-speed rotating equipment is expensive to construct, quite often needs a fair amount of maintenance, and requires safety precautions in case the rotor should shatter . Also, conventional rotor systems are not very flexible because it is impossible to vary the pulse length, the wavelength and the repetition rate of the neutron burst independently without changing rotors . Furthermore it is difficult to construct a mechanical correlation chopper that can run at a sufficiently high speed to produce a reasonably wide neutron beam and a short time pulse for good resolution . It is also very important in using the correlation technique that the chopper have a high degree of time stability. This is relatively easy for an electronically pulsed device but considerably more difficult for a mechanical chopper. Pulsed beams have been developed that make use of a polarized beam and a spin flip chopper") . These
Neutron inelastic scattering investigations can provid-- valuable information concerning the dynamic properties of materials. Especially with the advent of high-flux reactors a large number of measurements have been made on the phonon and magnetic excitations in a wide class of materials . Most neutron inelastic scattering experiments are performed with either a triple-axis crystal spectrometer or a pulsed time-of-flight spectrometer. These techniques are complementary and each offers specific advantages for certain types of investigations. The pulsed-beam spectrometer usually has the advantage of a higher rate of data accumulation because neutrons of all velocities in each burst are counted and thus neutrons of all energy transfers are measured simultaneously . In addition, multiple neutron detectors can be easily used since the detectors remain stationary in the course of a measurement . This means that neutron scattering distributions can be measured simultaneously at many different momentum transfers . Since the sample also remains stationary in the course of the measurement, the time-of-flight method is preferable in experiments where the sample must be connah: ed in large equipment such as a high-pressure apparatus or a large magnet. Furthermore the possibility of using the correlation method of obtaining time-of-flight data can in many cases result in a large gain in the signal-to-noise ratio over other types of spectrometers . The time-of-flight " Research sponsored by the U. S. Atomic Energy Commission under contract with the Union Carbide Corporation. t Solid State Division. + Instrumentation and Controls Division. APRIL 1974
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H. A. MOOK et al .
&vices areelectronic in nature and thus circumvent the
difficulties ofmechanical devices.However, theneutron intensity from such devicesis low since only relatively weak polarized beams are available at present. In addition two scatterings from polarizing crystals are necessary to produce a beam in which the intensity is varied as a function of time and the resulting neutron beam may be twoorders of magnitudelowerin intensity than beams from other types of spectrometers . In view of these considerations a method of magnetically pulsing a neutron beam by means of a ferrite crystal has been developed'-6). The neutron beam is pulsed by diffracting it from a magnetic crystal in which the moment direction can be switched rapidly. The same crystal serves to both pulse and monochromate the neutron beam . The use of a magnetic crystal as a neutron shutterwas first suggested by Shull et al.') who showed that the intensity of a magnetic reflection from a crystal is dependent on the direction of the atomic magnetic moments within the crystal relative to the scattering vector. When the atomic moments are aligned along the scattering vector the magnetic intensity is zero End when the moments are aligned perpendicular to the scattering vector the magnetic intensity becomes a maximum. Therefore the magnetically reflected neutron intensity can be varied from zero to the maximum value if the magnetic moment orientation in a crystal can be changed by an external magnetic field . A neutron pulse can then be produced by applying an external pulsed magnetic field to the crystal and the neutron pulse length and frequency can be changed by simply changing the length and frequency or the current pulse that generates the magnetic field . 2. Thepulsing crystal In orderto produce the magnetically pulsed beam a crystal with certain characteristics is necessary. The crystal must be an insulator so that the magnetic moment direction can be switched rapidly without the crystal getting hot and the magnetic anisotropy must be small so that the switching can be done quickly with reasonably sized fields. The spin system must be collinear and a ringle magnetic domain must be produced so that tie spins in theentire crystal can be alignedtogether in thesame`direction. Thecrystal must also have good neutron scattering'prope>tics ; thus ;low neutron absorption and a large magnetic women; are desirable. Furthermore the nuclear scattering must be able to be eliminated so that it does not provide a constant background that would always be present
even when the magnetic component of the scattering is made zero. Certain of the ferrites that crystallize in the spinet structure possess these characteristics. There are two sites that the metal ions occupy in the spinel structure which are called thetetrahedral andoctahedral sites or more simply theA andBsites. The structurefactors for the (111) and (331) reflections ofthespinel structureare Fut = 8i«t -4V2f«t+d, F331
= 8f-1- 4N/2f-+ S .
where fA and fs are the scattering amplitudes of the atomson the A and Bsites and d and ~ aretheoxygen contributions to the reflection. By placing atoms of the right scattering amplitude on the two sets of sites the nuclear part of the structure factor may be made to vanish. Unfortunately, for the (111) reflection it is difficult to produce amaterial with the rightatoms on the right sites especially since the oxygen contribution is positive. For the (331) reflection it is easy to find a material for which cancellation takes place since the oxygen contribution is negative . The nuclear intensity in the (111) reflection can be made quite small for the compound 7Lio,Fe2, s 04 . Unfortunately, the cancellation is not exact and the thicker the crystal used the more extinction is present and the ratio between the nuclear and magnetic components of the reflection becomes smaller. For the (331) reflection any composition of Ni .,Fe 3 _x04, where x can be anything from 0 to 1 results in good cancellation of the nuclear scattering. The magnetic moments on the A and B sites are oppositely directed so that the magnetic scattering is in phase for the reflections and the magnetic intensity is at a maximum. The(331) reflection is about2.5 times smaller than the (111) reflection fora reasonably sized crystal because of the magnetic form factor. For the (111) reflection the ratio of magnetic to nuclear scattering is between five and ten to one for a reasonably -sized crystal of 7Lio, s Fe2,s04. For the (331) reflection of Fe 2,4NiO.6 04 the nuclear contribution to the reflection is 1 part in 20. The cancellation for the (I11) reflection can be made exact if one uses 62 Ni, which has a large negative scattering amplitude, in the compound 62Ni e ,4 Fe2 .604 . Such a. crystal is in the process of being grown and we feel that it should be the optimum crystal for producing a magnetically pulsed neutron beam. The ferrites mentioned above have low magnetic anisotropy and can be pulsed rapidly in small:fields. Crystals of these compositions have been grown` at
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NEUTRON TIME-OF-FLIGHT SPECTROMETER
ORNL by several techniques ; however, the most successful technique appears to be the Bridgman method . Crystals 11" in diameter by 4" long can be grown fairly routinely by using platinum crucibles and slow growth rates . Large crystals can be grown by the flux technique ; however, these crystals always contain flux inclusions and they are very difficult to saturate magnetically. . Good reflectivity can be obtained with the ferrite crystals . Fig. 1 shows a rocking curve for the (111) reflection of 7Lio.5Fez.504 . The crystal was a flat plate 0.090" thick and about 1" square. Thecrystal was measured in symmetric transmission through the flat plate using a wavelength of 1.44 A . The dip in the incident beam is shown also as the neutron detector was placed after thecrystal in theincident beam andthe crystal rocked through the reflection . The crystal reflects about 1/3 of the beam in the center of the reflection . This is notas high areflectivity as is obtained in some of the crystals such as Be used on the tripleaxis spectrometers, especially since these crystals may be used in reflection instead of symmetrictransmission through a thin plate. However, this is generally more
than compensated for when one considers that two crystal reflections are needed for a triple-axis spectrometer. The thin plate is used for the magnetic crystal so that the magnetization direction can be kept in the plate and demagnetization effects are kept small. If large magnetic fields can be used, the crystal thickness could be increased, although it would probably be difficult to magnetize a crystal along the scattering vector when it is used in reflection. In any case the reflectivity is sufficiently high to produce the intense neutron beams needed for inelastic scattering experiments. Since the beam can be pulsed magnetically, pulses can be of anylength and can be made to occur at any time making thedevice ideal for usewith thecorrelation technique. The correlation technique has been discussed by several authors"') and we will not consider it in detail in this article. In the correlation technique, instead of using a single neutron burst and counting all the neutrons scattered from the sample for each burst separately, a pseudo-random code of neutron bursts is produced . A portion of such a code is shown in fig . 2 and is denoted by Sit)- The code is made up of basic increments of width At and is Ndt long, where N is typically 500. The result of an experiment to measure the response F(t) of a sample is then given by N
; Fi, Si, Zj(t) = Y_ i
(2)
where Ft is the value of F(t) for the ith time channel. If the code S(t) is chosen correctlythen thefunction Ft that one wishes to measure is given by inverting eq . (2) giving N
Fi = E Si Zi+;.
(3) i This involves N multiplications for each channel one wishes to measure, making an on-line computer highly desirable. With the correlation technique the beam maybe incident upon the sample as much as 50 of the time giving a much improved signal-to-noise ratio over a conventional neutron chopper which may have a duty cycle of about 0.01. Also, frame overlap may be eliminated since the :;ode can always be made as long as is necessary to avoid frame overlap without ot Fig. 1. Rocking curve of ferrite crystal used in time-of-flight spectometer. The upper curve shows the neutrons removed from the incident beam as the crystal is rotated through the reflection.
Lfl
i
Nat
Fig. 2. Part of a typical pseudo-random code for use with the cross-correlat.antechnique.
H. A. MOOK et al.
any loss in intensity. Depending on the shape of F(t) variouslength and duty cycle codesgive thebest relative error inF(t) andthusit is desirableto have aselection of codes available for various experiments. The relative merits of then correlation technique and methods for choosing' the best code' for a given experiment are discussed in the references quoted . 3. The maget lntlser Since the ferrite moment direction can be rotated very rapidly, it is desirable to be able to supply short current pulses to the pulsing coil surrounding the crystal so that short neutron bursts can be produced for good time-of-flight resolution. Fields of the order of 1000 Oe are needed to pulsethe ferrite crystal . Since short pulses are desired the coil impedance must be kept low, limiting thenumber ofturnson thepulsingcoil and making high pulsecurrents necessary. Themagnetic pulser used is illustrated in fig . 3. The input signal code to the pulser is sent from a computer on line to the experiment. In order to achieve the high currents necessary for the magnet coil at reasonable voltages, the magnet coil was designed to be an integral part of a pulse-forming
network system such that the risetime of the puls . would be, determined by the resonant frequency of an individual mesh of the network and'the pulse width is determined by the number of meshesin the network"). The magnet pulser utilizes the energy stored in the pulse formingnetwork (PFN)to generate thebasicAT pulse to the crystal coil . A total of 16 networks is used in the pulser and these are triggered sequentially by a ring counter to produce the desired pulse width. For example, if a pulse of 3A T is required, the outputs of the first three networks are combined sequentially in themagnet coil to produce a rectangular pulse of 3d T. A stable clock in the computer provides the input to the ring counter to insure proper overlap of thePFN's. Once the capacitors in a PFN are discharged, they rechargethrough resonant charging from thedo supply, and have at least 16AT to recharge before that particular PFN is fired again. Thecharacteristic impedance of the PFN is approximately 2.5 0 which allows 100 A current pulses to be generated at relatively low voltages and thus allows silicon control rectifiers to be used as the electronic switch during the discharge cycle. RCA developmental type TA7395 SCR's provide the fast turn-on and turn-
Magnet Pulser. Fig. 3. Schematic diagram ofcurrent pulser used to switch the moment direction of the ferrite crystal.
NEUTRON TIME-OF-FLIGHT SPECTROMETER
off of the current pulses needed during the discharge the rotatable monochromator shield to allow the use cycle. The SCR's also isolate the magnet coil during of large take-off angles wit~tout shielding problems. the resonant charging of the PFN's. A small saturable The wedges in the directi, in of the main beam lift reactor (L,) in each discharge path delays the turn-on above it to provide fora 2" square beam while adjacent of the SCR'sto allowthe gate charge to spread across wedges remain in place to provide shielding. The the base region and therefore eliminate di/dt failure shield is quite massive measuring 92" in diameter and weighing 84 000 lbs, but this shielding is necessary in the SCR's. Two types of coil arrangements have been success- becauseof the very intense radiation that emerges from fully used. The first of these uses a standard electro- the HFIR. In practice the variable take-off angle magnet to hold the moment direction parallel to which allows variation of the incoming energy, is of the scattering vector and a coil wound at right angles extremeimportance. Theresolution ofthe spectrometer to this direction to pulse the crystal when a neutron is avery sensitive function oftheenergy transfer so that burst is desired. The pulsing coil consists of about to measure an excitation with a given energy with good 15 turns of # 16 aluminum wire anodized on the resolution the incident energy must be chosen accordsurface for insulation . The pulsing current needed is ingly . The spectrometer is generally used so that the about 50 A and the coil is cooled by blowing a small neutron loses energy in the scattering process and thus amount of air on it. The total power dissipated in the slows down,making thetime-of-flight resolution letter. whole pulsing system including the coil, for a 25% Thus the incoming neutron energy must be set higher than the excitation one wishes to measure but not so duty cycle, is 280 W. The other type of arrangement consists of placing high that the energy transfer is small compared to the the crystal in a ferrite transformer core to serve as a incident energy, giving poor resolution. The flight path is 1.5 m long and is shielded by 2" return magnetic path so that demagnetization effects are eliminated . This requires along crystal in order to of masonite and 2" of 13 C mixed with epoxy. Thetotal keep the core out of the beam but the arrangement weight of the flight path assembly is 3650lbs and it is requires less pulsing current and shorter pulses can be carried over the floor on three air bearings. The floor produced for a given power input. 5its bursts can be is covered with 1/8" steel sheet welded together and it produced with 30 A pulses and total power consump- does no . have to be particularly flat for the airbearings tion is about 180 W at a25 % duty cycle . The crystal is to work well . It is important that the floor have no placed in the ferrite core so that the scattering vector grooves in it so that air can escape from the air is along the crystalslab and a coil is kept energized to bearings . The shield lifts about 1/4" offthe floorwhen cancel the magnetic scattering intensity . When a the air is supplied to the bearings and the whole neutron pulse is desired a current pulse is sent through system works well and is trouble-free. the same coil in the opposite direction to the steady do current so that the magnetic moments are rotated to a direction more perpendicular to the scattering vector. A crystal slab about4" long and 1" high with a thickness of around 1/8" is used in this configuration . Both techniques are completely satisfactory although 'he last is to be preferred if a crystal of sufficient size is available. 4. Spectrometer layout A schematic drawing of the spéctrometer, which is installed at the HB-4 beam port at the High Flux Isotope Reactor, is shown in fig. 4. The monochromator-pulser crystal is mounted in a shield of the same type as thoseemployed on the triple-axis spectrometers") in operation at the HFIR. The sample and flight path assembly areattached to the monochromator shield so that scattering angles from the monochromator can be varied continuously between zero and 90°. A system of movable wedges is employed in
Fig. 4. Layout of the time-of-flight spectrometer installed at the HFIR.
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'He detectors 3/4" in diluneter with 20 atm fill to the system. The major portion of the electronics is pressure areused as input to 15 separate data channels . constructed in a CAMACt3 )Icrate. CAMAC is a Allthe detectors are set on atrack and canbe position- complement of NIMt4); NIM is used foranalog signal ed at anyscattering anglefrom 5°-140°. Three separate processing and CAMAC for digital data handling. banks of detectors can be positioned by means of Wire-wrap integrated circuit boards are used in all the pulsed motors that are interfaced to the computer. CAMAC modules. Other electronics, including the The sample angle is also setable with a pulsed motorso input-output signal buffering transmitter-receivers, that a desired scattering geome~ry may be set similar motor drivers and feedback degree marker amplifiers, to a triple-axis spectrometer . Thus, while constant- and solenoid driversarebuiltin standard logicmountmomentum-transfer scans may not be made, scans ing panels with plug-in logic cards. Motor circuits (fig. 5) provide the computer with the maybe made along adesiredsymmetry direction in the sample. This is very useful in measuring phonon or capability of driving up to eight stepping motors to spin wave excitations, where one is usually most position thedetectors, sample, main shield, etc. Degree interested in theexcitations along symmetry directions. photo-marker signals from each motor are amplified The motors are also used in step scans for sample in the degree marker amplifier circuits and cabled to alignment and to measure the mosaic structure of the interrupt register . These pulses, occurring once each motor shaft revolution, allow the computer to sample crystals. verify proper motor operation . The sample and flight 5. Spectrometer control system path assembly shield are moved together with the A diagram of the control electronics is shown in monochromator shield by a single pulsed motor. movement by the fig. 5. The computer is a PDP-15-30 with 24 k core Activation of the air bearing during memory, disc, magnetic-tape units, foreground and computer is achieved through a solenoid driver . The CAMAC crate has a built-in dataway for background typewriters, display, andhigh-speed punch and reader. A Houston Complot plotter is also on line coveying digital data, control signals, and power. There
------_--DEC TAPE DRIVERS
COMPUTER
MOTOR CLOCK GGENERATO
J
INTERRUPT REGISTER /1 A16 STAGES
TRANSMITTERS AND RECEIVERS
CONTROLLER
PROGRAMME PULSE GENERATOR
SOLENOID DRIVERS
TO AIR BEARINGS
MOTOR DRIVERS
TO MOTORS
I -
I I ( I
DEGREE MARKERS
FROM MOTORS
MAGNET PULSER
To MAGNET COIL
I MONITOR PRESCALER
PLOTTER
1
I
i TOF SCALERS _
DISC
I
I
CAMAC_RAT CE I L __- _-- - -_ - -J
FROM BEAM INTENSITY MONITOR FROM TOF DETECTORS
Fig. 5. Diagram of the controlelectronics used to operate thetime-of--flight spectrometer.
NEUTRON TIME-OF-FLIGHT SPECTROMETER are25 86-pin edge-connector socketsforuse by plug-in modules. One station, thecontrolstation, hasindividual lines to each of the other 24 stations . Each of the 24 stations connectto 24 input and 24 output data lines, and several control lines . The controller module is a multiple-width module using the control station and oneof the other24 stations.It serves as the supervisory module between the various station modules and the computer . It provides routing and synchronization for program-controlled input-output (IOT) transfers and commands, autonomous (Data Channel) data transfer, and automatic priority interrupts (API). Due to the complexity of this system, and for economic and other reasons, the CAMAC specifications were not strictly adhered to . However, the sacrifice was in flexibility, not reliability . When a module requires some action by the computer it sends a "look-at-me" (LAM) signal to the controller . The controller initiates the proper communication with the computer according to themodule making therequest. A given module is selected by the controller by placing a signal level on themodules' Nselect line. In addition
to this signal, three sub-address lines are coded by the controller to specify just what action is to take place within the module. Also, two strobe pulses, S1 andS2, may be generated by the controller. By decoding combinations of these signals, the station modules perform the required response. The programmed pulsegenerator (PPG) generates a recycling pseudo-random train of pulses which drittes the neutron pulser . The binary sequence of the pulse train resides in the computer memory. There may be up to 4096 bits in the sequence. A stable oscillator operating at four times the frequency of the sequence provides the time base, or basic period, of each bit (AT) . AT may be as short as 1.4 ,us. Prior to run time a status register (fig. 6) is loaded by the computer . Twelve of theeighteen bits in the status register control the scaling modulus of the AT counter. Also, prior to run time, the A register is loaded with the first 18 bits of the sequence . All other bits of the sequence will be requested from the computer by the PPG as required and held in the B register until needed by the shift register.
Fig. 6. Diagram of the programmed pulse generator. Two types of PPG output wave forms are shown on the lower part of the diagram.
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H. A. MOOK et al.
When the run command is given, the contents of the A register are loaded into the shift register anda LAM signal is sent to the controller module to request the second 18 bits of the sequence from the computer memory via a high-priority multi-cycle data channel. As the sequence is shifted out, both the dT counter and modulo-18 counter are incremented, once for each shift . These counters determine when the shift register is to be loaded with more information andwhether it is to come from the A or B register. In general, when the modulo-18 counter overflows, the shift register is loaded from the B register and when the dT
counter overflows, the shift register is loaded from the A register to repeat the sequence from the beginning. Each time the shift register is loaded from the B register, a LAM is sent to the controller to request more binary information for the B register. By maintaining the first 18 bits of the sequence in thePPG A register, the system is guaranteed sufficient time (18AT) to always have the B register loaded when its information is needed. Two types of PPG output waveforms (fig. 6) are generated : returnto-zero (RZ) and nonretum-to-zero (NRZ). Two error signals may be produced by the PPG at
(DATAWAY LINES)
Fig. 7. Layout of the time-of--flight sealer module.
NEUTRON TIME-OF-FLIGHT SPECTROMETER
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front-panel BNC connectors. These signals may be process all the incoming data . At a 40 us per detector connected to an interrupt register module, to be scan rate, the maximumtotal countrate is 25 000events described later. A "sequence" error occurs if the last persecond. Aswitch allows theoperator to select fewer bitin the sequence output does not compare with one than 16 (8, 4, or 2) detectors to allow a proportionately bit in the status register. The "lag" error occurs if higher rate perdetector selected, butthe maximumtotal data have not been loaded in the B register by the time rate remains 25 kHz. they are required by the shift register . Data from the TOF staler module are transferred Another importantfunction of the PPG is to control into the computer memory through the controller via and synchronize the TOF staler module to the PPG high-priority data channel and API circuits as described output sequence. The TOF control signals are: a run- above. The data are stored in oneof two bluffer regions enable signal level, async signal which resets the staler in memory. While one region is being filled, the to zero at the beginning of each sequence, and a clock computer sorts data from the other buffer and updates signal which advances the staler . The staler may be histograms residing elsewhere in memory. At the end advanced one, two, or four times per dT to give of the run the sorted data are dumped on magnetic corresponding time-channel-per-AT ratios. This ratio tape or disc. Later, thedata are read back into memory is specified by two bits in the PPG status register. This where an algorithm performs the cross correlation to ratio multiplied by the number of bits in the PPG extract the scattering information . sequence cannot be greater than 4096, since the TOF In the present operating system core memory size staler has 12 bits. Also,onestatus register bitcauses the restricts the number of detectors - number of time staler to be enabled even though the PPG is not in the channels per detector combination. New programs will operate mode. This will be described as aspecial mode be developed whereby the histograms can be stored on in the following section. the disc and swapped in and out of memory for The time-of-flight (TOF) staler module (fig . 7) updating. consists of four basic sections: a staler, detector input In one mode of operation the TOF staler module is circuits, a buffer memory, and output circuits . The operated in a manner different than in the normal staler provides up to 4096 time channels and, as was time-of-flight experiment. This is a scanning operation described previously, is controlled by the run, sync, and to count neutron events versus scattering angles . The clock signals from the programmed pulse generator . module is enabled by one of the PPG status register When a pulse arrives from a detector, the time bits and the staler remains in the zero (reset) stab . (within the sequence) of arrival is remembered by ThePPG is not operating and no clock pulses are sent loading the content of the staler into a 12-bit semi- to the staler. The internal operation of theTOF module conductor memory register and, at the same time, a is the same as described above, except, sincethe staler flag (flip-flop) is set. Each detector has its dedicated is not advanced, the twelve bits of time information memory and flag. The sync circuits shown in fig. 7 will all contain zeros. Thus, effectively, nnly the four delay the memory loading process if a detector pulse bits which identify the detectors are transferred to the arrives when the staler is in transition. Once a flag is computer where a profile of rate versus angle can be set, its corresponding input circuit is blocked until the performed. The detectors are moved to desired angles information stored in the buffer register is transferred by program-controlled stepping motors. A prescaler module serves to scale down the number to the computer and the flag reset. A scanner examines the flags at a rate of one every of incident neutrons as detected by the monitor 40 us, and if the scanned flag is set, a LAM signal is detector. Output pulses from the prescaler cause sent to the controller requesting a data channeltransfer interrupts (via the interrupt register module) to the to the computer. During the transfer cycle a transfer computer at a rate proportional to the neutron beam command is sent to the TOF mc~_'ule to gate the data intensity, and enable the computer to determine how outand to clear the flag. Twelve bits of data are gated long to take data . The prescaler is basically a 13-bit through the output multiplexer in accordance with the counter with a front panel switch for setting the scale scannerposition . The four ID bits (detectorindentifica- factor . Output pulses are produced at scaling factors of tion) correspond directly with the scanner position. 64 to 8192, in eight binary steps. Atlhough normally The scanner used in this TOF scheme satisfies two under computer control, front panel switches permit major requirements : (1) that there be no favoritism in manual control. the recognition of events from any detector and (2) The motor clock generator module is simply a 300that the computer have a guaranteed minimumtime to pulse-per-second clock under program control. It
H. A. MOOK et al. provides a time base for running the stepping motors. When enabled, it interrupts the computer 300 times per second via the interrupt register module. The interrupt register module provides automatic priority interruptforup to 16 devices. Device functions connected to this register typically include the motor photodegree markers, the motor clock generator (300 pps) output, the monitor prescaler output and error signals from the PPG. One stage of the register is shown in fig. 8. When any stage flip-flop is set, the LAM signal causes the controller API circuits to interrupt the computer. The computer determines which stage(s) caused the interrupt after (nondestructively) transferring the register contents to its accumulator. Clearing of therecognized stages is accomplished by the computer by placing data "ones" on the lines correspondingto the stages to be reset andexecuting the conditional clear JOT command. For test purposes, all stages can be setto the "one" condition by the set-allbits IOT instructions . 6. Spectrometer operation For many types of experiments the spectrometer will be operated in the following manner. A guess will be made of the energy of the excitation one wishes to measure. The incoming energy Eo will then be chosen so that the excitation can be measured with the desired
resolution . Aprogram is then enteredinto thecomputer from a disk that calculates the detector and sample angle needed to measure a given excitation along a particular, symmetry direction and with a given momentum transfer. Sometimes such a condition is not possible and a scan must be chosen not along the desired symmetry direction but along a direction that will hopefully intersect the symmetry direction at the desired energy and momentum transfer. Once a scan is chosen the computer sets the desired angles for the monochromator, sample, and detectors by means ofthe pulsed motors. A guess must then be made about the shape of the spectrum to be measured so that an optimum pseudorandom code can be chosen . The guess need not be particularly accurate, only if the spectrum will be a broad distribution covering a lotof energies or a more sharp distribution such as a phonon or a spin wave . A code is then chosen and sent from the disk and additional information such as number of detectors, channel width, channels per dT, and counting time is given as inputdata. The experiment is runand thedata are stored on the disk or tape for cross correlation . After cross correlation various data handling and analysis programs are run to display the data, convert from time of flight to energy, average channels, add to otherdata or subtract backgrounds, etc. Once the data
Interrupt Register-One Stage of 16 .
Fig. 8. Diagram of one stage of the interrupt module.
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NEUTRON TIME-OF-FLIGHT SPECTROMETER 10,000 N C oO U C Gl C
8000 6000 4000 2000 TIME-OF-FLIGHT 8 .81csec PER CHANNEL
Fig. 9 . Time-of-flight spectrum obtained from a monitor detector placed in the direct beam from the pulsing crystal for a 25% duty cycle pseudo-random code.
are in the form desired, they can be plotted on the Complot plotter and printed on the teletype. The data are left on dectape so that they can be examined at any later time. If complicated data analysis is desired for a large quantity of data the dectape can be taken to the computer center and read into the large IBM computers by means of a PDP-10 computer that serves as a data link . However, the PDP-15 computer is sufficiently powerful that most analyses can be performed on line immediately. If several scans are to be performed these will be done in turn forthe desired counting time and thedata read outon tape. Thus aseries of runs may be perfoemed overnight and all the data examined in themorning . The computer is capable of data collection andanalysis simultaneously although the time-sharing programs for this are still being written, and at the moment data collection must be interrupted to do cross correlations and analysis. 7. Spectrometer performance In order to determine the performance of a correlation time-of-flight spectrometer the beam directly incident upon the sample must be examined carefully . Fig. 9 shows part of the time-of-flight spectrum from a monitor counter placed in the direct beam at the sample position. The monitor counter had to be quite inefficient since thebeam intensity is around 10'n/cm' s in the bursts for incident energies of the order of 40 meV. The spectrum shown is for a 25 % code with two time-of-flight channels per At i :_crement. Fig. 10 shows the cross-correlated result obtained from this incident spectrum. It is important that no peaks appear in this spectrum other than the desired peak at the neutronflight time from the monochromator-to-sample position. Thebackground should be flat within counting errors in this spectrum for the spectrometer to be operating correctly. It is of great importance that each pulse in the correlation code occurs at thecorrect time
and be of the correct height. This is easy to do electronically and the spectrum in fig. 10 is flat except for the elastic peak, and no spurious peaks are observed . Once one is satisfied the incident beam_ is satisfactory the spectrometer may be used to perform neutron scattering experiments. The spectrometer hasbeen used foralarge number of different scattering experiments . Ameasurement of the crystal field excitations in TmAs is shown in fig. 11 as an example of thetype of data that can be obtained with the spectrometer . Tm has a neutron absorption cross section of about200b at the incident neutron energies used in the experiment; however, the spectrum shown in fig. 10 was obtained in about 15 h. The various crystal field transitions are identified in the figure for the ground state multiplet . The data were obtained with an incident neutronenergy of 13.5 meVanda 25 % duty cycle correlation code . Data were taken simultaneously in several other detectors at different angles so that the momentum transfer dependence of thedata could be examined. t>
24
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TIME-OF-FLIGHT
Fig. 10. Cross-correlated spectrum obtained from the data in fig . 9. Theinsert shows some of theflat part of thespectrum on an expanded scale. The scatter in the points is no larger than that expected from statistical counting errors demonstrating the accuracy of thepulsing sysb ::n .
21 6
H. A. MOOK et al .
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Fig. 11 . Spectrum obtained with the time-of-flight spectrometer for TmAs. Crystal field transitions can easily be observed despite the large cross section for neutron absorption of Tm. The statistical errors are only slightly larger than the data points themselves despite a counting time of only 15 h. 8 . Conclusion A very flexible time-of-flight spectrometer has been constructed that makes use of a magnetic crystal to pulse the neutron beam . The spectrometer is ideally suited for the correlation technique since the electronic pulsing permits high accuracy for the pulsing times. The spectrometer has variable incident energy and variable
scattering angles, so desired scattering geometries and energy resolutions can be easily obtained. Computer control of the spectrometer contributes to the great flexibility ofthe instrument and permits rapid handling of the large amount of data that the spectrometer is capable of obtaining . References 1 ) L.Pit, N.Kro6,J .Gordon, P.Pellionis.,F.SZUvikandI .Vizi, Neutroninelasticscattering, vol. 2(IAEA,Vienna, 196ß)p.407. 2) O. Steinsvoll and A . Virgo, Neutron inelastic scattering, vol. 2 (IAEA, Vienna, 1968) p. 395 . 3) H . Rauch, J. Harms and H. Moldaschl, Neutron inelastic scattering, vol. 2 (IAEA, Vienna, 1968) p. 387; Nucl. Instr. and Meth . 58 (1968) 261 . 4) H. Kendrick, J . S . King, S. A. Werner and A . Arrott, Nucl. Instr. and Meth . 79 (1970) 82. s) H. A. Mook and M. K . Wilkinson, J. Appl. Phys. 39 (1968) 447. 9) H. A . Mook and M. K. Wilkinson, Instrumentation for neutron inelastic scattering research (IAEA, Vienna, 1970) p . 173 . 7) C. G . Shull, E . O. Wollan and W. A. Strauser, Phys. Rev. 81 (1951) 483. 8) F. Hossfeld and R . Amadori, Institut für FestkSrperforschung, Jülich, Report 684 (1970). 9) G. Wilhelmi and F. Gompf, Nucl. Instr. and Meth. 81 (1970) 36. 10) R. Von Jan and R. Scherm, Nucl . Instr. and Meth. 80 (1970) 69. 11) D . D. Bates, IEEE Trans . Nucl. Sci. NS-15 (1968) 349. 12) M. K. Wilkinson, H. G. Smith, W . C. Koehler, R . M. Nicklow and R. M. Moon, Neutron inelastic scattering, vol. 2 (IAEA, Vienna, 1968) p. 253 . 13) ESONE Committee Report EUR 4100e (March 1969) . 14) L. Costrell, Standard nuclear instrument modules, TID-20893, Rev . 3 (December 1969).
Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830, U.S.A . Received 26 November 1973 A
time-of-flight spectrometer utilizing a magnetically pulsed beam has been placed in operation at the High Flux Isotope Reactor at the Oak Ridge National Laboratory . The pulsing is accomplished by rapidly changing the magnetic moment direction in a ferrite crystal that serves to both monochromate and pulse the beam . Good neutron reflectivity is obtained from the ferrite crystal and pulse risetimes of the order of 1 lus are available. The spectrometer is interfaced to a Digital Equipment Corpora-
tion PDP-15-30 computer which drives pulsed motors to set up the desired scattering geometry and collects and analyzes the time-of-flight data . The magnetically pulsed beam is ideal for use with the cross-correlation technique and spectra are: presented which show the accuracy of the pulsing technique and the power of the cross correlation methods in obtaining neutron scattering data.
1 . Introduction
technique has the disadvantage that both the momentum and energy transfer are varied from point to point in a time-of-flight spectrum from a single detector while a triple-axis spectrometer varies one of the quantities while the other is held fixed . This problem can be circumvented to some extent by using many detectors for a time-of-flight scan and picking the points from detector to detector in which either the momentum or energy is a constant. However, for many experiments no difficulty is encountered from both the energy and momentum transfer varying in the time-of-flight scan. Most present day pulsed beam spectrometers for inelastic scattering involve at least two high-speed' , phased rotors to obtain a neutron pulse with a small' ` velocity spread. Both multiple-rotor choppers and rotating crystal spectrometers that are phased with'` rotating collimators are successfully used. However, high-speed rotating equipment is expensive to construct, quite often needs a fair amount of maintenance, and requires safety precautions in case the rotor should shatter . Also, conventional rotor systems are not very flexible because it is impossible to vary the pulse length, the wavelength and the repetition rate of the neutron burst independently without changing rotors . Furthermore it is difficult to construct a mechanical correlation chopper that can run at a sufficiently high speed to produce a reasonably wide neutron beam and a short time pulse for good resolution . It is also very important in using the correlation technique that the chopper have a high degree of time stability. This is relatively easy for an electronically pulsed device but considerably more difficult for a mechanical chopper. Pulsed beams have been developed that make use of a polarized beam and a spin flip chopper") . These
Neutron inelastic scattering investigations can provid-- valuable information concerning the dynamic properties of materials. Especially with the advent of high-flux reactors a large number of measurements have been made on the phonon and magnetic excitations in a wide class of materials . Most neutron inelastic scattering experiments are performed with either a triple-axis crystal spectrometer or a pulsed time-of-flight spectrometer. These techniques are complementary and each offers specific advantages for certain types of investigations. The pulsed-beam spectrometer usually has the advantage of a higher rate of data accumulation because neutrons of all velocities in each burst are counted and thus neutrons of all energy transfers are measured simultaneously . In addition, multiple neutron detectors can be easily used since the detectors remain stationary in the course of a measurement . This means that neutron scattering distributions can be measured simultaneously at many different momentum transfers . Since the sample also remains stationary in the course of the measurement, the time-of-flight method is preferable in experiments where the sample must be connah: ed in large equipment such as a high-pressure apparatus or a large magnet. Furthermore the possibility of using the correlation method of obtaining time-of-flight data can in many cases result in a large gain in the signal-to-noise ratio over other types of spectrometers . The time-of-flight " Research sponsored by the U. S. Atomic Energy Commission under contract with the Union Carbide Corporation. t Solid State Division. + Instrumentation and Controls Division. APRIL 1974
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&vices areelectronic in nature and thus circumvent the
difficulties ofmechanical devices.However, theneutron intensity from such devicesis low since only relatively weak polarized beams are available at present. In addition two scatterings from polarizing crystals are necessary to produce a beam in which the intensity is varied as a function of time and the resulting neutron beam may be twoorders of magnitudelowerin intensity than beams from other types of spectrometers . In view of these considerations a method of magnetically pulsing a neutron beam by means of a ferrite crystal has been developed'-6). The neutron beam is pulsed by diffracting it from a magnetic crystal in which the moment direction can be switched rapidly. The same crystal serves to both pulse and monochromate the neutron beam . The use of a magnetic crystal as a neutron shutterwas first suggested by Shull et al.') who showed that the intensity of a magnetic reflection from a crystal is dependent on the direction of the atomic magnetic moments within the crystal relative to the scattering vector. When the atomic moments are aligned along the scattering vector the magnetic intensity is zero End when the moments are aligned perpendicular to the scattering vector the magnetic intensity becomes a maximum. Therefore the magnetically reflected neutron intensity can be varied from zero to the maximum value if the magnetic moment orientation in a crystal can be changed by an external magnetic field . A neutron pulse can then be produced by applying an external pulsed magnetic field to the crystal and the neutron pulse length and frequency can be changed by simply changing the length and frequency or the current pulse that generates the magnetic field . 2. Thepulsing crystal In orderto produce the magnetically pulsed beam a crystal with certain characteristics is necessary. The crystal must be an insulator so that the magnetic moment direction can be switched rapidly without the crystal getting hot and the magnetic anisotropy must be small so that the switching can be done quickly with reasonably sized fields. The spin system must be collinear and a ringle magnetic domain must be produced so that tie spins in theentire crystal can be alignedtogether in thesame`direction. Thecrystal must also have good neutron scattering'prope>tics ; thus ;low neutron absorption and a large magnetic women; are desirable. Furthermore the nuclear scattering must be able to be eliminated so that it does not provide a constant background that would always be present
even when the magnetic component of the scattering is made zero. Certain of the ferrites that crystallize in the spinet structure possess these characteristics. There are two sites that the metal ions occupy in the spinel structure which are called thetetrahedral andoctahedral sites or more simply theA andBsites. The structurefactors for the (111) and (331) reflections ofthespinel structureare Fut = 8i«t -4V2f«t+d, F331
= 8f-1- 4N/2f-+ S .
where fA and fs are the scattering amplitudes of the atomson the A and Bsites and d and ~ aretheoxygen contributions to the reflection. By placing atoms of the right scattering amplitude on the two sets of sites the nuclear part of the structure factor may be made to vanish. Unfortunately, for the (111) reflection it is difficult to produce amaterial with the rightatoms on the right sites especially since the oxygen contribution is positive. For the (331) reflection it is easy to find a material for which cancellation takes place since the oxygen contribution is negative . The nuclear intensity in the (111) reflection can be made quite small for the compound 7Lio,Fe2, s 04 . Unfortunately, the cancellation is not exact and the thicker the crystal used the more extinction is present and the ratio between the nuclear and magnetic components of the reflection becomes smaller. For the (331) reflection any composition of Ni .,Fe 3 _x04, where x can be anything from 0 to 1 results in good cancellation of the nuclear scattering. The magnetic moments on the A and B sites are oppositely directed so that the magnetic scattering is in phase for the reflections and the magnetic intensity is at a maximum. The(331) reflection is about2.5 times smaller than the (111) reflection fora reasonably sized crystal because of the magnetic form factor. For the (111) reflection the ratio of magnetic to nuclear scattering is between five and ten to one for a reasonably -sized crystal of 7Lio, s Fe2,s04. For the (331) reflection of Fe 2,4NiO.6 04 the nuclear contribution to the reflection is 1 part in 20. The cancellation for the (I11) reflection can be made exact if one uses 62 Ni, which has a large negative scattering amplitude, in the compound 62Ni e ,4 Fe2 .604 . Such a. crystal is in the process of being grown and we feel that it should be the optimum crystal for producing a magnetically pulsed neutron beam. The ferrites mentioned above have low magnetic anisotropy and can be pulsed rapidly in small:fields. Crystals of these compositions have been grown` at
207
NEUTRON TIME-OF-FLIGHT SPECTROMETER
ORNL by several techniques ; however, the most successful technique appears to be the Bridgman method . Crystals 11" in diameter by 4" long can be grown fairly routinely by using platinum crucibles and slow growth rates . Large crystals can be grown by the flux technique ; however, these crystals always contain flux inclusions and they are very difficult to saturate magnetically. . Good reflectivity can be obtained with the ferrite crystals . Fig. 1 shows a rocking curve for the (111) reflection of 7Lio.5Fez.504 . The crystal was a flat plate 0.090" thick and about 1" square. Thecrystal was measured in symmetric transmission through the flat plate using a wavelength of 1.44 A . The dip in the incident beam is shown also as the neutron detector was placed after thecrystal in theincident beam andthe crystal rocked through the reflection . The crystal reflects about 1/3 of the beam in the center of the reflection . This is notas high areflectivity as is obtained in some of the crystals such as Be used on the tripleaxis spectrometers, especially since these crystals may be used in reflection instead of symmetrictransmission through a thin plate. However, this is generally more
than compensated for when one considers that two crystal reflections are needed for a triple-axis spectrometer. The thin plate is used for the magnetic crystal so that the magnetization direction can be kept in the plate and demagnetization effects are kept small. If large magnetic fields can be used, the crystal thickness could be increased, although it would probably be difficult to magnetize a crystal along the scattering vector when it is used in reflection. In any case the reflectivity is sufficiently high to produce the intense neutron beams needed for inelastic scattering experiments. Since the beam can be pulsed magnetically, pulses can be of anylength and can be made to occur at any time making thedevice ideal for usewith thecorrelation technique. The correlation technique has been discussed by several authors"') and we will not consider it in detail in this article. In the correlation technique, instead of using a single neutron burst and counting all the neutrons scattered from the sample for each burst separately, a pseudo-random code of neutron bursts is produced . A portion of such a code is shown in fig . 2 and is denoted by Sit)- The code is made up of basic increments of width At and is Ndt long, where N is typically 500. The result of an experiment to measure the response F(t) of a sample is then given by N
; Fi, Si, Zj(t) = Y_ i
(2)
where Ft is the value of F(t) for the ith time channel. If the code S(t) is chosen correctlythen thefunction Ft that one wishes to measure is given by inverting eq . (2) giving N
Fi = E Si Zi+;.
(3) i This involves N multiplications for each channel one wishes to measure, making an on-line computer highly desirable. With the correlation technique the beam maybe incident upon the sample as much as 50 of the time giving a much improved signal-to-noise ratio over a conventional neutron chopper which may have a duty cycle of about 0.01. Also, frame overlap may be eliminated since the :;ode can always be made as long as is necessary to avoid frame overlap without ot Fig. 1. Rocking curve of ferrite crystal used in time-of-flight spectometer. The upper curve shows the neutrons removed from the incident beam as the crystal is rotated through the reflection.
Lfl
i
Nat
Fig. 2. Part of a typical pseudo-random code for use with the cross-correlat.antechnique.
H. A. MOOK et al.
any loss in intensity. Depending on the shape of F(t) variouslength and duty cycle codesgive thebest relative error inF(t) andthusit is desirableto have aselection of codes available for various experiments. The relative merits of then correlation technique and methods for choosing' the best code' for a given experiment are discussed in the references quoted . 3. The maget lntlser Since the ferrite moment direction can be rotated very rapidly, it is desirable to be able to supply short current pulses to the pulsing coil surrounding the crystal so that short neutron bursts can be produced for good time-of-flight resolution. Fields of the order of 1000 Oe are needed to pulsethe ferrite crystal . Since short pulses are desired the coil impedance must be kept low, limiting thenumber ofturnson thepulsingcoil and making high pulsecurrents necessary. Themagnetic pulser used is illustrated in fig . 3. The input signal code to the pulser is sent from a computer on line to the experiment. In order to achieve the high currents necessary for the magnet coil at reasonable voltages, the magnet coil was designed to be an integral part of a pulse-forming
network system such that the risetime of the puls . would be, determined by the resonant frequency of an individual mesh of the network and'the pulse width is determined by the number of meshesin the network"). The magnet pulser utilizes the energy stored in the pulse formingnetwork (PFN)to generate thebasicAT pulse to the crystal coil . A total of 16 networks is used in the pulser and these are triggered sequentially by a ring counter to produce the desired pulse width. For example, if a pulse of 3A T is required, the outputs of the first three networks are combined sequentially in themagnet coil to produce a rectangular pulse of 3d T. A stable clock in the computer provides the input to the ring counter to insure proper overlap of thePFN's. Once the capacitors in a PFN are discharged, they rechargethrough resonant charging from thedo supply, and have at least 16AT to recharge before that particular PFN is fired again. Thecharacteristic impedance of the PFN is approximately 2.5 0 which allows 100 A current pulses to be generated at relatively low voltages and thus allows silicon control rectifiers to be used as the electronic switch during the discharge cycle. RCA developmental type TA7395 SCR's provide the fast turn-on and turn-
Magnet Pulser. Fig. 3. Schematic diagram ofcurrent pulser used to switch the moment direction of the ferrite crystal.
NEUTRON TIME-OF-FLIGHT SPECTROMETER
off of the current pulses needed during the discharge the rotatable monochromator shield to allow the use cycle. The SCR's also isolate the magnet coil during of large take-off angles wit~tout shielding problems. the resonant charging of the PFN's. A small saturable The wedges in the directi, in of the main beam lift reactor (L,) in each discharge path delays the turn-on above it to provide fora 2" square beam while adjacent of the SCR'sto allowthe gate charge to spread across wedges remain in place to provide shielding. The the base region and therefore eliminate di/dt failure shield is quite massive measuring 92" in diameter and weighing 84 000 lbs, but this shielding is necessary in the SCR's. Two types of coil arrangements have been success- becauseof the very intense radiation that emerges from fully used. The first of these uses a standard electro- the HFIR. In practice the variable take-off angle magnet to hold the moment direction parallel to which allows variation of the incoming energy, is of the scattering vector and a coil wound at right angles extremeimportance. Theresolution ofthe spectrometer to this direction to pulse the crystal when a neutron is avery sensitive function oftheenergy transfer so that burst is desired. The pulsing coil consists of about to measure an excitation with a given energy with good 15 turns of # 16 aluminum wire anodized on the resolution the incident energy must be chosen accordsurface for insulation . The pulsing current needed is ingly . The spectrometer is generally used so that the about 50 A and the coil is cooled by blowing a small neutron loses energy in the scattering process and thus amount of air on it. The total power dissipated in the slows down,making thetime-of-flight resolution letter. whole pulsing system including the coil, for a 25% Thus the incoming neutron energy must be set higher than the excitation one wishes to measure but not so duty cycle, is 280 W. The other type of arrangement consists of placing high that the energy transfer is small compared to the the crystal in a ferrite transformer core to serve as a incident energy, giving poor resolution. The flight path is 1.5 m long and is shielded by 2" return magnetic path so that demagnetization effects are eliminated . This requires along crystal in order to of masonite and 2" of 13 C mixed with epoxy. Thetotal keep the core out of the beam but the arrangement weight of the flight path assembly is 3650lbs and it is requires less pulsing current and shorter pulses can be carried over the floor on three air bearings. The floor produced for a given power input. 5its bursts can be is covered with 1/8" steel sheet welded together and it produced with 30 A pulses and total power consump- does no . have to be particularly flat for the airbearings tion is about 180 W at a25 % duty cycle . The crystal is to work well . It is important that the floor have no placed in the ferrite core so that the scattering vector grooves in it so that air can escape from the air is along the crystalslab and a coil is kept energized to bearings . The shield lifts about 1/4" offthe floorwhen cancel the magnetic scattering intensity . When a the air is supplied to the bearings and the whole neutron pulse is desired a current pulse is sent through system works well and is trouble-free. the same coil in the opposite direction to the steady do current so that the magnetic moments are rotated to a direction more perpendicular to the scattering vector. A crystal slab about4" long and 1" high with a thickness of around 1/8" is used in this configuration . Both techniques are completely satisfactory although 'he last is to be preferred if a crystal of sufficient size is available. 4. Spectrometer layout A schematic drawing of the spéctrometer, which is installed at the HB-4 beam port at the High Flux Isotope Reactor, is shown in fig. 4. The monochromator-pulser crystal is mounted in a shield of the same type as thoseemployed on the triple-axis spectrometers") in operation at the HFIR. The sample and flight path assembly areattached to the monochromator shield so that scattering angles from the monochromator can be varied continuously between zero and 90°. A system of movable wedges is employed in
Fig. 4. Layout of the time-of-flight spectrometer installed at the HFIR.
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H. A. MOOK et al.
'He detectors 3/4" in diluneter with 20 atm fill to the system. The major portion of the electronics is pressure areused as input to 15 separate data channels . constructed in a CAMACt3 )Icrate. CAMAC is a Allthe detectors are set on atrack and canbe position- complement of NIMt4); NIM is used foranalog signal ed at anyscattering anglefrom 5°-140°. Three separate processing and CAMAC for digital data handling. banks of detectors can be positioned by means of Wire-wrap integrated circuit boards are used in all the pulsed motors that are interfaced to the computer. CAMAC modules. Other electronics, including the The sample angle is also setable with a pulsed motorso input-output signal buffering transmitter-receivers, that a desired scattering geome~ry may be set similar motor drivers and feedback degree marker amplifiers, to a triple-axis spectrometer . Thus, while constant- and solenoid driversarebuiltin standard logicmountmomentum-transfer scans may not be made, scans ing panels with plug-in logic cards. Motor circuits (fig. 5) provide the computer with the maybe made along adesiredsymmetry direction in the sample. This is very useful in measuring phonon or capability of driving up to eight stepping motors to spin wave excitations, where one is usually most position thedetectors, sample, main shield, etc. Degree interested in theexcitations along symmetry directions. photo-marker signals from each motor are amplified The motors are also used in step scans for sample in the degree marker amplifier circuits and cabled to alignment and to measure the mosaic structure of the interrupt register . These pulses, occurring once each motor shaft revolution, allow the computer to sample crystals. verify proper motor operation . The sample and flight 5. Spectrometer control system path assembly shield are moved together with the A diagram of the control electronics is shown in monochromator shield by a single pulsed motor. movement by the fig. 5. The computer is a PDP-15-30 with 24 k core Activation of the air bearing during memory, disc, magnetic-tape units, foreground and computer is achieved through a solenoid driver . The CAMAC crate has a built-in dataway for background typewriters, display, andhigh-speed punch and reader. A Houston Complot plotter is also on line coveying digital data, control signals, and power. There
------_--DEC TAPE DRIVERS
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INTERRUPT REGISTER /1 A16 STAGES
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Fig. 5. Diagram of the controlelectronics used to operate thetime-of--flight spectrometer.
NEUTRON TIME-OF-FLIGHT SPECTROMETER are25 86-pin edge-connector socketsforuse by plug-in modules. One station, thecontrolstation, hasindividual lines to each of the other 24 stations . Each of the 24 stations connectto 24 input and 24 output data lines, and several control lines . The controller module is a multiple-width module using the control station and oneof the other24 stations.It serves as the supervisory module between the various station modules and the computer . It provides routing and synchronization for program-controlled input-output (IOT) transfers and commands, autonomous (Data Channel) data transfer, and automatic priority interrupts (API). Due to the complexity of this system, and for economic and other reasons, the CAMAC specifications were not strictly adhered to . However, the sacrifice was in flexibility, not reliability . When a module requires some action by the computer it sends a "look-at-me" (LAM) signal to the controller . The controller initiates the proper communication with the computer according to themodule making therequest. A given module is selected by the controller by placing a signal level on themodules' Nselect line. In addition
to this signal, three sub-address lines are coded by the controller to specify just what action is to take place within the module. Also, two strobe pulses, S1 andS2, may be generated by the controller. By decoding combinations of these signals, the station modules perform the required response. The programmed pulsegenerator (PPG) generates a recycling pseudo-random train of pulses which drittes the neutron pulser . The binary sequence of the pulse train resides in the computer memory. There may be up to 4096 bits in the sequence. A stable oscillator operating at four times the frequency of the sequence provides the time base, or basic period, of each bit (AT) . AT may be as short as 1.4 ,us. Prior to run time a status register (fig. 6) is loaded by the computer . Twelve of theeighteen bits in the status register control the scaling modulus of the AT counter. Also, prior to run time, the A register is loaded with the first 18 bits of the sequence . All other bits of the sequence will be requested from the computer by the PPG as required and held in the B register until needed by the shift register.
Fig. 6. Diagram of the programmed pulse generator. Two types of PPG output wave forms are shown on the lower part of the diagram.
212
H. A. MOOK et al.
When the run command is given, the contents of the A register are loaded into the shift register anda LAM signal is sent to the controller module to request the second 18 bits of the sequence from the computer memory via a high-priority multi-cycle data channel. As the sequence is shifted out, both the dT counter and modulo-18 counter are incremented, once for each shift . These counters determine when the shift register is to be loaded with more information andwhether it is to come from the A or B register. In general, when the modulo-18 counter overflows, the shift register is loaded from the B register and when the dT
counter overflows, the shift register is loaded from the A register to repeat the sequence from the beginning. Each time the shift register is loaded from the B register, a LAM is sent to the controller to request more binary information for the B register. By maintaining the first 18 bits of the sequence in thePPG A register, the system is guaranteed sufficient time (18AT) to always have the B register loaded when its information is needed. Two types of PPG output waveforms (fig. 6) are generated : returnto-zero (RZ) and nonretum-to-zero (NRZ). Two error signals may be produced by the PPG at
(DATAWAY LINES)
Fig. 7. Layout of the time-of--flight sealer module.
NEUTRON TIME-OF-FLIGHT SPECTROMETER
21 3
front-panel BNC connectors. These signals may be process all the incoming data . At a 40 us per detector connected to an interrupt register module, to be scan rate, the maximumtotal countrate is 25 000events described later. A "sequence" error occurs if the last persecond. Aswitch allows theoperator to select fewer bitin the sequence output does not compare with one than 16 (8, 4, or 2) detectors to allow a proportionately bit in the status register. The "lag" error occurs if higher rate perdetector selected, butthe maximumtotal data have not been loaded in the B register by the time rate remains 25 kHz. they are required by the shift register . Data from the TOF staler module are transferred Another importantfunction of the PPG is to control into the computer memory through the controller via and synchronize the TOF staler module to the PPG high-priority data channel and API circuits as described output sequence. The TOF control signals are: a run- above. The data are stored in oneof two bluffer regions enable signal level, async signal which resets the staler in memory. While one region is being filled, the to zero at the beginning of each sequence, and a clock computer sorts data from the other buffer and updates signal which advances the staler . The staler may be histograms residing elsewhere in memory. At the end advanced one, two, or four times per dT to give of the run the sorted data are dumped on magnetic corresponding time-channel-per-AT ratios. This ratio tape or disc. Later, thedata are read back into memory is specified by two bits in the PPG status register. This where an algorithm performs the cross correlation to ratio multiplied by the number of bits in the PPG extract the scattering information . sequence cannot be greater than 4096, since the TOF In the present operating system core memory size staler has 12 bits. Also,onestatus register bitcauses the restricts the number of detectors - number of time staler to be enabled even though the PPG is not in the channels per detector combination. New programs will operate mode. This will be described as aspecial mode be developed whereby the histograms can be stored on in the following section. the disc and swapped in and out of memory for The time-of-flight (TOF) staler module (fig . 7) updating. consists of four basic sections: a staler, detector input In one mode of operation the TOF staler module is circuits, a buffer memory, and output circuits . The operated in a manner different than in the normal staler provides up to 4096 time channels and, as was time-of-flight experiment. This is a scanning operation described previously, is controlled by the run, sync, and to count neutron events versus scattering angles . The clock signals from the programmed pulse generator . module is enabled by one of the PPG status register When a pulse arrives from a detector, the time bits and the staler remains in the zero (reset) stab . (within the sequence) of arrival is remembered by ThePPG is not operating and no clock pulses are sent loading the content of the staler into a 12-bit semi- to the staler. The internal operation of theTOF module conductor memory register and, at the same time, a is the same as described above, except, sincethe staler flag (flip-flop) is set. Each detector has its dedicated is not advanced, the twelve bits of time information memory and flag. The sync circuits shown in fig. 7 will all contain zeros. Thus, effectively, nnly the four delay the memory loading process if a detector pulse bits which identify the detectors are transferred to the arrives when the staler is in transition. Once a flag is computer where a profile of rate versus angle can be set, its corresponding input circuit is blocked until the performed. The detectors are moved to desired angles information stored in the buffer register is transferred by program-controlled stepping motors. A prescaler module serves to scale down the number to the computer and the flag reset. A scanner examines the flags at a rate of one every of incident neutrons as detected by the monitor 40 us, and if the scanned flag is set, a LAM signal is detector. Output pulses from the prescaler cause sent to the controller requesting a data channeltransfer interrupts (via the interrupt register module) to the to the computer. During the transfer cycle a transfer computer at a rate proportional to the neutron beam command is sent to the TOF mc~_'ule to gate the data intensity, and enable the computer to determine how outand to clear the flag. Twelve bits of data are gated long to take data . The prescaler is basically a 13-bit through the output multiplexer in accordance with the counter with a front panel switch for setting the scale scannerposition . The four ID bits (detectorindentifica- factor . Output pulses are produced at scaling factors of tion) correspond directly with the scanner position. 64 to 8192, in eight binary steps. Atlhough normally The scanner used in this TOF scheme satisfies two under computer control, front panel switches permit major requirements : (1) that there be no favoritism in manual control. the recognition of events from any detector and (2) The motor clock generator module is simply a 300that the computer have a guaranteed minimumtime to pulse-per-second clock under program control. It
H. A. MOOK et al. provides a time base for running the stepping motors. When enabled, it interrupts the computer 300 times per second via the interrupt register module. The interrupt register module provides automatic priority interruptforup to 16 devices. Device functions connected to this register typically include the motor photodegree markers, the motor clock generator (300 pps) output, the monitor prescaler output and error signals from the PPG. One stage of the register is shown in fig. 8. When any stage flip-flop is set, the LAM signal causes the controller API circuits to interrupt the computer. The computer determines which stage(s) caused the interrupt after (nondestructively) transferring the register contents to its accumulator. Clearing of therecognized stages is accomplished by the computer by placing data "ones" on the lines correspondingto the stages to be reset andexecuting the conditional clear JOT command. For test purposes, all stages can be setto the "one" condition by the set-allbits IOT instructions . 6. Spectrometer operation For many types of experiments the spectrometer will be operated in the following manner. A guess will be made of the energy of the excitation one wishes to measure. The incoming energy Eo will then be chosen so that the excitation can be measured with the desired
resolution . Aprogram is then enteredinto thecomputer from a disk that calculates the detector and sample angle needed to measure a given excitation along a particular, symmetry direction and with a given momentum transfer. Sometimes such a condition is not possible and a scan must be chosen not along the desired symmetry direction but along a direction that will hopefully intersect the symmetry direction at the desired energy and momentum transfer. Once a scan is chosen the computer sets the desired angles for the monochromator, sample, and detectors by means ofthe pulsed motors. A guess must then be made about the shape of the spectrum to be measured so that an optimum pseudorandom code can be chosen . The guess need not be particularly accurate, only if the spectrum will be a broad distribution covering a lotof energies or a more sharp distribution such as a phonon or a spin wave . A code is then chosen and sent from the disk and additional information such as number of detectors, channel width, channels per dT, and counting time is given as inputdata. The experiment is runand thedata are stored on the disk or tape for cross correlation . After cross correlation various data handling and analysis programs are run to display the data, convert from time of flight to energy, average channels, add to otherdata or subtract backgrounds, etc. Once the data
Interrupt Register-One Stage of 16 .
Fig. 8. Diagram of one stage of the interrupt module.
21 5
NEUTRON TIME-OF-FLIGHT SPECTROMETER 10,000 N C oO U C Gl C
8000 6000 4000 2000 TIME-OF-FLIGHT 8 .81csec PER CHANNEL
Fig. 9 . Time-of-flight spectrum obtained from a monitor detector placed in the direct beam from the pulsing crystal for a 25% duty cycle pseudo-random code.
are in the form desired, they can be plotted on the Complot plotter and printed on the teletype. The data are left on dectape so that they can be examined at any later time. If complicated data analysis is desired for a large quantity of data the dectape can be taken to the computer center and read into the large IBM computers by means of a PDP-10 computer that serves as a data link . However, the PDP-15 computer is sufficiently powerful that most analyses can be performed on line immediately. If several scans are to be performed these will be done in turn forthe desired counting time and thedata read outon tape. Thus aseries of runs may be perfoemed overnight and all the data examined in themorning . The computer is capable of data collection andanalysis simultaneously although the time-sharing programs for this are still being written, and at the moment data collection must be interrupted to do cross correlations and analysis. 7. Spectrometer performance In order to determine the performance of a correlation time-of-flight spectrometer the beam directly incident upon the sample must be examined carefully . Fig. 9 shows part of the time-of-flight spectrum from a monitor counter placed in the direct beam at the sample position. The monitor counter had to be quite inefficient since thebeam intensity is around 10'n/cm' s in the bursts for incident energies of the order of 40 meV. The spectrum shown is for a 25 % code with two time-of-flight channels per At i :_crement. Fig. 10 shows the cross-correlated result obtained from this incident spectrum. It is important that no peaks appear in this spectrum other than the desired peak at the neutronflight time from the monochromator-to-sample position. Thebackground should be flat within counting errors in this spectrum for the spectrometer to be operating correctly. It is of great importance that each pulse in the correlation code occurs at thecorrect time
and be of the correct height. This is easy to do electronically and the spectrum in fig. 10 is flat except for the elastic peak, and no spurious peaks are observed . Once one is satisfied the incident beam_ is satisfactory the spectrometer may be used to perform neutron scattering experiments. The spectrometer hasbeen used foralarge number of different scattering experiments . Ameasurement of the crystal field excitations in TmAs is shown in fig. 11 as an example of thetype of data that can be obtained with the spectrometer . Tm has a neutron absorption cross section of about200b at the incident neutron energies used in the experiment; however, the spectrum shown in fig. 10 was obtained in about 15 h. The various crystal field transitions are identified in the figure for the ground state multiplet . The data were obtained with an incident neutronenergy of 13.5 meVanda 25 % duty cycle correlation code . Data were taken simultaneously in several other detectors at different angles so that the momentum transfer dependence of thedata could be examined. t>
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Fig. 10. Cross-correlated spectrum obtained from the data in fig . 9. Theinsert shows some of theflat part of thespectrum on an expanded scale. The scatter in the points is no larger than that expected from statistical counting errors demonstrating the accuracy of thepulsing sysb ::n .
21 6
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Fig. 11 . Spectrum obtained with the time-of-flight spectrometer for TmAs. Crystal field transitions can easily be observed despite the large cross section for neutron absorption of Tm. The statistical errors are only slightly larger than the data points themselves despite a counting time of only 15 h. 8 . Conclusion A very flexible time-of-flight spectrometer has been constructed that makes use of a magnetic crystal to pulse the neutron beam . The spectrometer is ideally suited for the correlation technique since the electronic pulsing permits high accuracy for the pulsing times. The spectrometer has variable incident energy and variable
scattering angles, so desired scattering geometries and energy resolutions can be easily obtained. Computer control of the spectrometer contributes to the great flexibility ofthe instrument and permits rapid handling of the large amount of data that the spectrometer is capable of obtaining . References 1 ) L.Pit, N.Kro6,J .Gordon, P.Pellionis.,F.SZUvikandI .Vizi, Neutroninelasticscattering, vol. 2(IAEA,Vienna, 196ß)p.407. 2) O. Steinsvoll and A . Virgo, Neutron inelastic scattering, vol. 2 (IAEA, Vienna, 1968) p. 395 . 3) H . Rauch, J. Harms and H. Moldaschl, Neutron inelastic scattering, vol. 2 (IAEA, Vienna, 1968) p. 387; Nucl. Instr. and Meth . 58 (1968) 261 . 4) H. Kendrick, J . S . King, S. A. Werner and A . Arrott, Nucl. Instr. and Meth . 79 (1970) 82. s) H. A. Mook and M. K . Wilkinson, J. Appl. Phys. 39 (1968) 447. 9) H. A . Mook and M. K. Wilkinson, Instrumentation for neutron inelastic scattering research (IAEA, Vienna, 1970) p . 173 . 7) C. G . Shull, E . O. Wollan and W. A. Strauser, Phys. Rev. 81 (1951) 483. 8) F. Hossfeld and R . Amadori, Institut für FestkSrperforschung, Jülich, Report 684 (1970). 9) G. Wilhelmi and F. Gompf, Nucl. Instr. and Meth. 81 (1970) 36. 10) R. Von Jan and R. Scherm, Nucl . Instr. and Meth. 80 (1970) 69. 11) D . D. Bates, IEEE Trans . Nucl. Sci. NS-15 (1968) 349. 12) M. K. Wilkinson, H. G. Smith, W . C. Koehler, R . M. Nicklow and R. M. Moon, Neutron inelastic scattering, vol. 2 (IAEA, Vienna, 1968) p. 253 . 13) ESONE Committee Report EUR 4100e (March 1969) . 14) L. Costrell, Standard nuclear instrument modules, TID-20893, Rev . 3 (December 1969).