Experience with the Oak Ridge automatic three-circle neutron diffractometer

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Submitted on 1 Jan 1964

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Experience with the Oak Ridge automatic three-circle neutron diffractometer

W.R. Busing, H.G. Smith, S.W. Peterson, H.A. Levy

To cite this version:

W.R. Busing, H.G. Smith, S.W. Peterson, H.A. Levy. Experience with the Oak Ridge au- tomatic three-circle neutron diffractometer. Journal de Physique, 1964, 25 (5), pp.495-496.

�10.1051/jphys:01964002505049500�. �jpa-00205815�

495.

EXPERIENCE WITH THE OAK RIDGE AUTOMATIC THREE-CIRCLE NEUTRON DIFFRACTOMETER

By ^{W. R.} BUSING, ^{H. G.} SMITH, ^{S. W. PETERSON} (1) ^{and H. A.} LEVY,

Chemistry Division, ^Oak Ridge ^National Laboratory (2), ^Oak Ridge, Tennessee, ^{U. S. A.}

Resume. - Le diffractomètre automatique à neutrons ^« trois cercles ^» d’Oak Ridge ^{est un dis-} positif programmeur ^{à bande} perforée (papier) ^avec déroulement de la bande de papier. ^{Il a été}

mis en service en juin 1961 et il est capable ^{de mesurer les} intensités de 120 réflexions d’un mono-

cristal par jour. L’appareil ^et les programmes ^de calcul qui ^{lui sont} adjoints ^{seront brièvement}

décrits. Les méthodes d’orientation du cristal seront discutées et quelques difficultés rencontrées seront mentionnées.

Abstract.

The Oak Ridge automatic three-circle neutron diffractometer is a punched paper

tape controlled device with paper tape output. It has been in operation since June 1961 and is

capable ^of measuring the intensities of 120 single-crystal reflections per day. ^The instrument and

computer programs associated with ^it will be described briefly. Methods of crystal orientation will be discussed, and some of the difficulties encountered will be mentioned.

LE JOURNAL DE PHYSIQUE ^TOME 25, ^MAI 1964,

The Oak Ridge automatic three-circle ^neutron diffractometer is a punched paper tape ^controlled

device with paper tape output which has been in

operation since June 1961. During ^{this time it}

has been used ^to measure of the order of 16,000

reflections from about 18 samples. Some of the structures which have been determined are repor- ted in other papers ^at this meeting.

The basis of the instrument is a General Electric

Spectrogoniometer mounted with its axis hori- zontal so that it can be used with a vertical mono-

chromatic neutron beam. This device maintains the theta-two theta relationship by ^{means of a} system ^{of levers. Mounted on the} sample ^table

is the orienter which positions ^the crystal by ^rota- ting ^{it about two axes} designated ^{chi and} phi.

Associated with each degree ^{of freedom is a motor}

which drives the equipment ^{and a} digitizer ^which

senses the position ^{to 0. 01} degrees.

A system of electronic circuits causes the three

angles ^{to be set} sequentially according ^{to infor-}

mation supplied through the seven-hole paper tape

reader. Each angle ^{is read to an} instruction regis- ter, ^{and the motor} operates ^{until the} digitizer- position register agrees with this instruction. A system ^{of fast and slow drives is} provided ^{so that}

the final position ^is always approached ^{from the}

same direction and the setting ^is correct to 0.01

degrees. ^{When the three} angles ^{have been set the}

instrument proceeds ^to step ^{scan in a theta-two}

theta mode punching ^the neutron counts at each step ^{on a} paper tape output.

Data collection is controlled through ^a computer (1) ^{Present address :} Department ^of Chemistry, ^Was- hington ^State University, Pullmann, Washington.

(’) Operated for the U. S. Atomic Energy ^Commision by

Union Carbide Corporation.

program which prepares the paper control tape

and a printed listing ^{of the} angles ^{to be set. The} output tape punched by ^the automatic instrument

serves as input ^{to a} computer program for proces-

sing the results. An auxiliary tape ^{is used to}

transmit the indices of the reflections from the control program ^to the processor ^so that the data

can be identified. Output ^{for each reflection}

includes the indices, ^the background count, ^the

net count with its standard _error, and the value of two theta for the peak ^{center. This} output ^is printed ^{and also written on} magnetic tape ^{for use} by ^later computer programs.

Primary input ^to the control _program includes the cell parameters, ^the wavelength, orientation

information, ^{and a} description ^{of the} reflections to be measured. Orientation information is given ⁱⁿ

the form of the chi and phi ^{values for two reflec-}

tions. The reflections to be measured can be listed or a range of Bragg angles ^{can be} specified.

Provision is also made for repeating ^{the measu-}

rement of standard reflections periodically. ^Be-

cause the two-theta drive is much slower than the

others, ^the reflections are usually arranged ^{in the}

order of increasing Bragg angle.

With ^a three-circle instrument the sample ^orien-

tation can be established either mechanically by adjusting ^the goniometer ^{arcs or} computationally by accurately determining ^the existing orientation.

The form of the orientation input ^{to the} program

permits ^{the use of either of these} equivalent

methods. In practice mechanical orientation is used initially ^{with the Oak} Ridge instrument. A convenient procedure ^{is to} bring ^the plane ^normal

of one reflection into coincidence with the phi ^axis by setting ^{chi at 90} degrees ^and scanning ^the

reflection at phi values of 0, 90, 180, ^{and 270}

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:01964002505049500

496

degrees. From the two-theta values of these four measurements the arc corrections can be calcu- lated. This scheme is independent ^{of the cell} pararneters ^or wavelength ^and requires ^{no masks}

or apertures ^{on the counter.}

The control input parameters ^{can then be} adjus-

ted by the method of least _squares using ^{as obser-}

vations the two-theta peak positions ^{of several}

reflections. Included as input ^{with each such}

reflection are the chi and phi settings ^{at which}

these observations were made. The parameters

which may be adjusted include the cell constants, tlle wavelength ^{if not} all three cell edges ^are varied, and three of the four angles defining ^the

orientation. Two instrumental parameters, ^the

theta zero correction and the correction for an error in the perpendicularity ^{of the chi and} phi

axes, may also be determined.

The only apparent disadvantage ^of this least squares approach ^{to the} orientation problem ^is

that it-requires reasonably rapid ^{access to a com-} puter. It should yield ^{an accurate} crystal ^orien-

tation together ^{with a set of} reasonably good ^cell

parameters, although the latter hypothesis ^will require ^some testing. The method can give ^an

indication of mechanical troubles with the orienter

or with the sample mounting. It appears ^to be an

appropriate technique ^{for use} by diffraetometers with direct access to a computer.

Discussion

Dr MUELLER. - Employez-vous normalement le _programme de calcul de l’orientation d’une f a~on

routinière pour le contr6le de ^tous les cristaux dont les données doivent être recueillies ?

Dr BUSING.

Nous orientons normalement le cristal aussi bien _que possible ^{d’une façon méca-} nique. Nous observons ensuite les positions optima-

lisées des 20 ou 30 premieres réflexions. Nous les utilisons _pour un raffinement _par moindres carrés des parametres ^{d’entr6e. Si ce} raffinement montre des changements significatifs, ^nous utilisons alors les nouveaux parametres pour recalculer les posi-

tionnements.