EVALUATION OF DATA FROM 1D-PSD USED IN TOF METHOD

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EVALUATION OF DATA FROM 1D-PSD USED IN TOF METHOD

N. Niimura

To cite this version:

N. Niimura. EVALUATION OF DATA FROM 1D-PSD USED IN TOF METHOD. Journal de Physique Colloques, 1986, 47 (C5), pp.C5-129-C5-136. �10.1051/jphyscol:1986517�. �jpa-00225834�

EVALUATION O F DATA FROM I D - P S D USED I N TOF METHOD

N. NIIMURA

Laboratory of Nuclear Science, Faculty of Science, Tohoku University, Mikamine 1-2, 982 Sendai, Japan

Resume - Un detecteur B localisation spatiale B 2 dimensions a BtQ construit specialement pour des mesures neutroniques de temps de vol. Ce detecteur fonctionne avec un scintillateur en verre au 6 ~ i et un syst&me de codage par fibres optiques. Un programme d'integration des intensites mesurees est decrit.

Abstract - ^A specially designed PSD using 6 ~ i glass scintillators and the fibre optic encoding method has been constructed and is used to record two- dimensional data by the TOF method. A computer program which evaluates inte- grated intensities is described.

I - INTRODUCTION

Position sensitive detectors (PSD) have been widely used for neutron scattering exp- eriments. When a two-dimensional PSD is used on a time-of-flight (TOF) single-cry- stal diffractometer, a large volume of reciprocal space can be observed simul- taneously. When a one-dimensional PSD is used, a large area of reciprocal space can be accessed, because white neutrons of continuous wavelengths are used in TOF diff- ractometry /I/. This distinctive feature makes such an instrument promising for the measurement of intensity distributions in reciprocal space and for the collection of many Bragg reflections from a single crystal to determine the crystal structure. The most troublesome problems of the PSD-TOF system are that (i) it records a huge amount of data, all of which should be accepted and displayed effectively and quickly, and

(ii) the accumulated data have to be corrected for the incident neutron intensity spectrum i(A) to get a meaningful result. We have built a new I-D PSD and written data evaluation programs to overcome these problems. The detector and software are described in this paper.

We have constructed a new PSD using 6 ~ i glass scintillators and the fibre optic encoding method / 2 / based on our experience of the construction of the prototype PSD 131. The principle of the fibre optic encoding is illustrated in Fig. 1 , where ten scintillator elements are given as an example. Each detector element is connected to two fibre optic channels and the 10 elements are covered by 5 photomultipliers (PMs). Suppose that the 5th scintillator is irradiated by a neutron and a scintill- ation event occurs. Photons created by the event are divided into two channels and are transferred to the PMs B and C. The correspondence between the position of a detector element and the combinations of two PMs are tabulated in the read-only- memory (ROM) in the decoder circuits. By surveying the table, the position of an irradiated scintillator is assigned definitely. In this method it is only important whether there are output signals from PBfs or not, that is, the information is digitized. This means that it takes only several hundred nanoseconds to handle the signal

.

The layout of the PSD is shown in Fig. 2. One detector element consists of 3 cylin- drical scintillators (3mm in diameter and 30 mm in height). Flexible plastic optical

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986517

C5-130 JOURNAL DE PHYSIQUE

fibres (3 mm in diameter) are connected to both ends of the scintillator.

The PSD is divided into two sections, that is, a right and left section. Each section has 64 detector elements (192 scintillators), and they are covered by 12 PMs.

A total of 384 scintillator elements sit side by side in a semicircle 45 cm in radius. As described in 111, data in each section are further divided into 4 blocks.

The PSD combined with a TOF four-circle single-crystal diffractometer (FOX) is installed at the pulsed neutron source in the National Laboratory for High Energy Physics (KEK). The performance of the PSD has been examined by using the pulsed neutron source.

The positional resolution of the PSD depends on the size of the scintillator element and the gap between the arranged elements. This is determined as 9.7 mm by rotating a single crystal step-by-step and recording the peak position as a function of the rotating angle of the crystal. Furthermore, note that the resolution does not depend on the position because the total shape of the PSD is semicircular and the scattered neutrons from the specimen never strike the detector elements obliquely.

The linearity of the PSD is examined by using the Bragg reflection of a single crystal. The result shows that the linearity is excellent and that the encoding is satisfactory.

The uniformity of the PSD depends on the similarity of each scintillator element and its optical system. The adjustment of the high voltage bias to the PMs is espec- ially important. The uniformity of the PSD was examined by using the incoherent- scatterer vanadium as a sample. The results after several trials of high voltage adjustment are shown in Fig. 3(a). This then corresponds to the efficiency of the detector, and by the use of this efficiency the uniformity can be corrected as shown in Fig. 3 ( b ) . The values of the efficiency are stored in the interface memory, so the corrected data can always be displayed if desired.

The neutron detection efficiency of the scintillator of the PSD has been compared with that of a 3He detector by using a single crystal with cubic symmetry. When the

[loo] direction coincides with the incident beam direction, symmetry equivalents of the 110 Bragg reflection are perpendicular to the incident beam. Therefore, the 3He detector was placed opposite the PSD, so that equivalent 110 Bragg reflections were observed simultaneously with both the 3 ~ e detector and the PSD. The result is that the neutron detection efficiency of the PSD is 70-80% of the 3He detector.

I11 - DATA ACQUISITION AND REAL TIME DISPLAY

The block diagram of the data acquisition system for the right half of the detector is shown in Fig. 4 . The system for the other half is identical. 64 scintillators are connected to 12 PMs and the signals of the PM are fed to the amplifier and then to the discriminator. The discriminator level can be varied from 0 to -1 V. The detector position where a neutron strikes is determined by surveying the ROM table, and the time of the event is determined with a time analyzer. The 128 detectors are divided into 8 blocks,as shown in Fig. 2,in order to homogenize Q resolution (momen- tum transfer resolution) in Q-space, because Q depends on the scattering angle and the wavelength (Q = 4asinB/X). Each position block has an independent time analyzer (TA), the time channel width and the delay time of which can be selected independ- ently. The time channel widths are variable in each quarter of the 512 time

channels. This also helps to homogenize Q resolution. The specifications of the TA are summarized in Table 1. Since each detector element has 512 time channels, 128 times 512 words of memory are necessary for one measurement. A bulk memory (1 Mega- word) is provided in the system, and the final data are stored in the bulk memory.

However, the 64 K of data for one measurement cannot all be displayed in real time, so another 8192 words of memory are provided in the system for quick, real time display of the data. There are two modes of display, a position mode and a TOF mode In the position mode, integration of the data over all wavelengths is carried out in each detector element, so we can identify the detector elements where Bragp

time channels being summed, as shown in Fig. 5(b). The spectra of Fig. 5(a) or Fig. 5(b) are displayed in real time during the measurement.

IV

-

As shown in Fig. 2, the smallest scatterins angle is 10.54" and the largest one is 168.24" and the available wavelengths arc etween 0.5 A ^{and 6} 1. The accessible Q- space under these conditions is shown in L L ~ . 6, where the delay time and the time channel width of all the TAs are 0.5 msec and 10 psec, respectively. When the time channel width is changed, the region of accessible Q-space is modified accordingly.

V - PEAK SEARCH FROM SINGLE CRYSTAL DIFFRACTION DATA

A computer program which automatically locates peaks, determines integrated inten- sities and centroids,and calculates structure factors by considering several corrections has been developed. The flow chart is as follows:

1. Q-conversion: Each detector element and time channel pair corresponds to one point in Q-space. I(p,t) is converted to I(Qx,Qy), where p and t represent a detector element and a time channel, respectively.

2. To find the largest (remaining) peak: By surveying 128 times 512 data, the largest, the second largest, the third largest, etc.,.. peaks are found.

3. To read the local data of a Bragg peak and its vicinity: Since the CPU-accessible memory size is limited, all of the 128 times 512 data are stored in a magnetic disc file. The local data of the Bragg peak and its vicinity are transferred to the CPU work area to carry out the subsequent calculation quickly.

4. To calculate centroids.

5. To calculate backgrounds: If the profile of the Bragg reflection in the Q-space is described by a two-dimensional Gaussian, the radius of the Bragg reflection can be obtained and then the background range should be defined clearly.

However, since the profile has a long tail in the region of the lower Q of the peak, it is not so easy to define the background level. Fortunately, we can find the mean background in the lateral region of the peak; this is one of the distinctive features.

6. To calculate the structure factor: By considering the detector efficiency, the wavelength dependence of incident neutrons and the Lorentz factor, the final structure factor is calculated.

U I - APPLICATION TO STRUCTURE ANALYSIS OF SPINEL

The key question in the application of the PSD system to structure analysis is whether the integrated intensity can be measured accurately. We have attempted to answer this que .on by determining the crystal structure parameters of a spinel

(MgA1204). The lape of the specimen is a sphere 5 mm in diameter. The measure- ments were carried out with 7 crystal orientations. One example of the raw data is given in Fig. 7. The integrated intensity of the Bragg reflections was obtained by using the peak search program mentioned in the previous section. In total about 150 Bragg reflections were collected and among them there are 98 independent reflections.

In the TOF technique, the wavelength dependence of the secondary extinction is essential /4/. It is calculated from the formula of Becker & Coppens 151. The crystal structure is shown in Fig. 8, and the refined atomic parameters are listed in Table 2.

VII - CONCLUSION

This PSD has just been constructed, so we have had little experience with it as pet.

Nevertheless, we can conclude:

1. Since a very wide range of Q-space can be accessed simultaneously, this PSD can

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be used in complex structure analysis, for example, of a protein.

2. Slnce Q-space is surveyed continuously, it allows measurement of the diffuse scattering intensity distribution in Q-space.

VIII - ACKNOWLEDGEMENT

This study was carried out in collaboration with members of the PSD group and the 4-circle diffractometer group. I would especially acknowledge Dr. Kawada, the leader of the latter group and Dr. Isobe for the discussion of the structure analysis of the spinel.

1

I ) ) ) )

DECODER ( ROM )

Fig. 1 - The principle of the fibre optic encoding method.

Idetector dem

scintillator opkal

f i bre

Fig. 2 - The layout of the PSD.

Fig. 3 - (a) Uncorrected uniformity of the PSD. ( b ) Corrected uniformity of the P

control

Time Analyzer Time Analder

I- t--- ^-1

lFz!bP

select

r r y

-

- - - - -

SD.

Fig. 4

-

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Fig. 5 - (a) The position mode of the (b) The TOF mode of the real time

real time display. display.

Fig. 6 - The accessible Q-space of the PSD when the delay time and the time channel width of a l l the TAs are 0 . 5 msec and 10 usec, respectively.

+- ^{a * +}

Fig. 8 - The cryshal structure of a spinel (MgA1204).

C5-136 JOURNAL DE PHYSIQUE

Teeble 1 The specifications of the TA

';'able 2 The refined ato~fic 2arameters

-

Delay time Channel width

Total number of channels

Ng X(=~=Z) 0 U 0.0050(7) Al x(=y=z) 5/8

U 0.0035(7) 0 x(=y=z) 0.3876(2) U 0.0051 (7) Rfactor 0.086

0-7.5msec

1-32 p sec (variable in every qcarter )

522

/I/ NIDRRA, N.

,

/2/ ~IDSON,P.L.

,

/3/ NID1W,M., YWiA,K., KUBOTA,T., MATSwOTO:A., & lIOSHINO,S., Nucl. Instrm. &

i4ethods m(1983 ) 203.

/4/ iNIDiURL,N., TONIYOSIiI,S., TAKAHASHI,J., & HARADA,J.,J.Appl. Cr7st.H (1975) 560

/ 5 / BECKG?,P.J. & COPPETS,P., Ucta Cryst. @ (1974) 129,148