Residual stress instrument with double-crystal monochromator at research reactor IR-8

Accepted Manuscript Residual stress instrument with double-crystal monochromator at research reactor IR-8 V.T. Em, I.D. Karpov, V.A. Somenkov, V.P. Glazkov, A.M. Balagurov, V.V. Sumin, P. Mikula, J. Ŝaroun PII:

S0921-4526(18)30162-5

DOI:

10.1016/j.physb.2018.02.042

Reference:

PHYSB 310759

To appear in:

Physica B: Physics of Condensed Matter

Received Date: 10 August 2017 Revised Date:

22 February 2018

Accepted Date: 26 February 2018

Please cite this article as: V.T. Em, I.D. Karpov, V.A. Somenkov, V.P. Glazkov, A.M. Balagurov, V.V. Sumin, P. Mikula, J. Ŝaroun, Residual stress instrument with double-crystal monochromator at research reactor IR-8, Physica B: Physics of Condensed Matter (2018), doi: 10.1016/j.physb.2018.02.042. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Residual stress instrument with double-crystal monochromator at research reactor IR-8. V.T. Ema,*, I.D. Karpova, V.A. Somenkova, V.P. Glazkova, A.M. Balagurovb, V.V. Suminb,

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P. Mikulac, J. Ŝarounc

a

NRC “Kurchatov Institute”, 1, Akademika Kurchatova pl., Moscow, 123182, Russia

b

Joint Institute for Nuclear Research, Joliot-Curie, 6, Dubna, 141980, Russia

Nuclear Physics Institute, v.v.i., ASCR, 25068 Řež, Czech Republic

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c

Keywords: neutron diffraction, neutron diffractometer, double-crystal monochromator, residual

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stress, maximal path length.

Abstract

Residual stress instrument STRESS at 8MW research reactor IR-8 at National Research Center “Kurchatov Institute” is described. Using double-crystal monochromator PG002(flat)/ Si220(horizontal focusing perfect crystal), providing fixed neutron wavelength λ = 1.56Å,

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resulted in compact arrangement of the instrument, high intensity and low background. The experiments showed that in maximal available path length (76mm in ferritic steel) the difractometer is comparable with other modern stress-difractometers at more powerful reactors.

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1. Introduction

Neutron diffraction method is only method, which can measure all three components of stress tensor in the bulk of materials [1]. Therefore during last two decades the dedicated stress-

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diffractometers were created at neutron research centers for fundamental researches and industrial application.

Available path length (~60mm in steel) and spatial resolution (∼1mm3) of the method are limited and measurement time is long because even at the modern neutron sources the neutron flux is not sufficient [2–4] . Therefore optimization of stress-diffractometer for higher intensity and resolution is very important.

----------------------------------------*Corresponding author. E-mail address: [email protected] 1

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Neutron diffractometer STRESS is installed at research reactor IR-8 (maximal power 8MW) at NRC “Kurchatov institute”. The reactor has 12 radial type horizontal experimental channels (HEC) with diameter 100mm and length ~2.5m. The flux of thermal neutrons at the end face of the horizontal channels in beryllium reflector is about 1x1014cm-2s-1. The scattering angle 2θS ≈ 900 is usually used for stress measurements because such geometry provides good spatial resolution and minimal variation of a gauge volume when a

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sample is reoriented to measure different strain components [1]. For 2θS ≈ 900 the optimal relationship between intensity and resolution can be achieved at take-off angle of monochromator 2θМ ≈ 900 for mosaic crystals [5] and 2θМ ≈ 500 for bent perfect silicon crystals [6–9]. For these take-off angles and typical in a stress-difractometer monochromator to sample

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distance LMS ≈ 2m and sample to detector distance LSD ≈1m, the space, correspondingly with ∼ 2.5m and ∼1.7m dimension perpendicular to the channel axis is necessary. There was not such

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space at the horizontal experimental channel #3 (HEC-3) of reactor IR-8, assigned for the stress instrument. The HEC-3 is radial type channel and, in contrast with tangential type channels, is directed to the reactor core. Therefore, there is problem with high background, because the neutron beam along with thermal neutrons contains high flux of fast neutrons and gamma-rays. In this paper we report the technical characteristics and performance of compact and high luminosity stress-diffractometer STRESS, in which these problems were solved by using double-

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crystal monochromator and optimized diffractometer optics.

2. Neutron diffractometer STRESS

The schematic of diffractometer STRESS is shown in Fig. 1. Neurons from the reactor core

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pass through the first collimator and impinge on the first crystal- monochromator. Consequently

Fig.1. The schematic of difractometer STRESS 2

ACCEPTED MANUSCRIPT reflected from the first and the second crystal, the monochromatic neutron beam passes through the second collimator and impinges on a sample. Neutrons scattered from the sample are detected by a position sensitive detector (PSD).

2.1. The first collimator

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The first collimator is a beam limiter with step-wise decreasing diameter of the hole for a neutron beam. It consists of 4 steel sections that are installed in 4 sections of the beam shutter. Each collimator section has 280mm length and 98mm outer diameter. The total length of the second collimator is 1400mm. The diameter of hole in the sections decreases along the beam

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direction in the sequence ∅58- ∅52-∅46- ∅40mm so, that through the outlet hole ∅40mm the

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first monochromator can “see” the whole irradiating channel end face ∅100mm.

2.2. Monochromator

We showed [10] that in case of limited space for a stress-diffractometer the best choice is using double-crystal monochromator (double monochromator) with two different crystals, providing fixed wavelength. The main aim of the first monochromator (pre-monochromator) is to deflect the direct neutron beam. Therefore the first monochromater is pyrolytic graphite (PG)

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with reflection planes (002) that maximally reflect neutrons. PG pre-monochromator ( 150x50x3mm3 ) composed from 3 plates with dimensions 50x50x3mm3 and mosaicity η ≈10 is placed possible close (~150mm) to the outlet of the channel to increase intensity. At take-off angle 2θМ1 = 270 it provides neutrons with wavelength λ = 1.56 Å. The optimal for a stress-

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diffractometer a bent perfect crystal Si monochromator with reflection planes (220) in symmetric reflection geometry [8,9,11] is used as the second monochromator. A slab of silicon crystal

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(200x40x4mm3) with optimizer curvature is oriented for take-off angle 2θМ2 = 480 (λ = 1.56Å). In such geometry the angle φ between the monochromatic beam and the channel axis is comparatively small: φ =210. It makes possible compact arrangement of a stress-diffractometer (0.9m to one side and 0.7m to the other side from the channel axis) as shown in Fig.1. There is no Soller collimator before monochromator. Therefore, loss in intensity (~ 2 times), caused by pre-monochromator, is less than gain (~ 4 times), due to shorter channel end face to monochromator

distance (∼2.5m) than in conventional instruments with a single

monochromator (∼5m). In modern stress diffractometers [12–14] there is possibility of tuning wavelength to settle the optimal for stress measurements scattering angle 2θS ≈ 900 for different materials. In case of double monochromator with different crystals the neutron wavelength is fixed. Therefore we 3

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chose the wavelength λ = 1.56 Å, for which the scattering angle 2θS for main industrial metals lies near 900: Metal / lattice

Fe / bcc

Fe / fcc

Al /fcc

Cu / fcc

Ni / fcc

Cr/bcc

(reflecting plane)

(211)

(311)

(311)

(311)

(311)

(211)

2θS0

83.8

91.6

79.4

91.4

94.5

82.94

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The wavelength λ = 1.56 Å is chosen also because it lies near the local minimum of neutron total cross-section in ferritic steel, corresponding to the Bragg edge (321) at λ = 1.53А and therefore is advantages for stress measurements at depth in ferritic steel [15].

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2.3 The second collimator

The second collimator is a steel beam limiter 1200mm length and 120(h)x100(w)mm2 section

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with 40(h)x50(w)mm2 inlet window and 40(h)x20(w)mm2 outlet window. The vertical focusing is not used because it deteriorates definition of a gauge volume in vertical direction. For 40mm height silicon crystal the vertical divergence of the beam incident on the sample through 5mm height cadmium slit is comparatively small (±0.70). Therefore it is not necessary additional vertical collimation of the incident beam to reduce the penumbra region of a gauge volume [16].

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2.4. Sample table

The possibly minimal second monochromator to sample distance LM2S = 2100mm was used as it increases intensity [9]. The sample table consists (from bottom to top) of one circle goniometer (0 ÷ 3600, accuracy ± 20”) and X (±150mm),Y (±150mm), Z(±45mm) positioning translators

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(accuracy ±5µm) mounted on goniometer platform (Ø400mm). For alignment of the sample coordinate axes XS,YS, ZS along the positioning translators axes X,Y,Z, a sample goniometer is

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mounted on Z-platform. The platform of the sample goniometer (S-platform) can rotate around vertical Z’(0-3600) and horizontal X’(±140 ),Y’(±140) axes with accuracy ±20”. The maximal load on S-platform is 100kg.

2.5. Detector

Helium gas two dimensional position-sensitive detector (PSD) with delay line read-out has 150(w)x250(h)mm2 active window and spatial resolution 2mm in horizontal and 3mm in vertical direction. Efficiency of detector is 60% for neutrons with wavelength λ = 1.6 Å. The width of one channel in horizontal direction corresponds to 0.02630 in scattering angle 2θS. For sample to detector distance LSD = 1050mm, typical width of cadmium slit before detector 2mm and horizontal spatial resolution of PSD 2mm, the equivalent distance collimation before PSD is 4

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α3 = 0.120 , which is quite enough for high resolution diffractometer. For PSD height of 250mm the uncertainty in the direction of measured strain in vertical plane is ±70. PSD is surrounded by bulky shielding from boron polyethylene (~150mm thickness) and mounted on the cart, which can rotate around the diffractometer axis in the interval 200÷ 1100 with accuracy ±20”.

2.6.Cadmium slits

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Cadmium slits on incident (IS) and diffracted (DS) beams define the gauge volume. The slits are mounted at the end faces of collimators that can move along incident and diffracted neutron beams so, that cadmium slit to the diffractometer center distance can be set in the interval (0÷ 300mm) for IS and (0÷350mm) for DS. In order to decrease background the moving collimator

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on diffracted beam before PSD is surrounded by boron polyethylene (~50mm thickness).

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3. Experiment 3.1. The second monochromator curvature.

The curvature of the Si220 monochromator in the horizontal plane was optimized

by

measuring the (112) diffraction peak from a ferritic steel pin (∅2mm, h=40mm), which was placed at the diffractometer center and acted as a gauge volume (~120 mm3). Experiments showed that optimal relation between integral intensity and resolution (FWHM =19.4’, ∆d/d ≈

3.2. Stability.

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0.003) was achieved for bending radius R ≈ 8.5m.

Since stress measurements takes long time the stability of the detector performance is

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important. The position of the diffraction peak may change with time because of instability of electronics. In order to estimate such instability, the same diffraction peak (112) of the ferritic

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steel pin (∅2 mm, h=40mm) was measured for 70h with 1 h time slices. Statistical error (from fitting) of the peak position for a 1 h exposure was ±0.00060 . Smooth variation of peak position with time was observed. The maximum difference in peak position during 70h measurement (±0.0030) was much larger than the statistical error and corresponded to the strain uncertainty ±25µε. However, such stability is quite acceptable, because usually in stress measurements the strains are measured with uncertainty ±100µε. 3.3. Maximal available path length. In order to estimate the instrument ability for stress measurements at depth the dependence of strain error on neutron beam path length in ferritic steel was studied. Through depth measurements (reactor power 6MW) were carried out in “reflection geometry” like in measurement of normal strain component (Fig.2). 5

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Fig. 2. The reflection geometry for the measurement of the normal (N) component. Gauge volume is defined by slits in the incident (IS) and diffracted (DS) beams. The total neutron beam path length (lt ) is the sum of the incident (li ) and diffracted (ld ) beam path lengths: lt = li +ld .

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Incident slit 2(w)x20(h)mm2 and diffracted slit 2(w)x60(h)mm2 defined the gauge volume GV≈ 80mm3. The sample was ferritic steel disk with diameter 120mm and thickness 40mm

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annealed at 8500С to randomize texture. For example, the diffraction peak (112) measured for

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path length 75mm is shown in Fig. 3.

Fig. 3. Diffraction peak (112) of ferritic steel measured at path length 75mm (depth 25mm, 2θS = 83.80, λ = 1.56Å, 1channal = 0.02630), 1h measurement with gauge volume ≈80mm3.

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Fig. 4 The dependence of the strain error on the neutron penetration path length in ferritic steel. The depth scan was carried out in reflection geometry (1h measurement , 80mm3 gauge volume) by measuring

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the 112 reflection. Strain error 100µε corresponds to the path length 76mm.

The dependence of strain error on neutron beam path length is shown in Fig.4. One can see that strain error 100µε corresponds to the maximal path length lm = 76mm. It means that strain components in ferritic steel plate with thickness of 50mm can be measured with uncertainty ±100 µε. The result well coincides with the result of the same experiment at the residual stress

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instrument (RSI) at 30MW reactor HANARO (lm = 77mm) [15]. 4. Conclusion

Compact and high luminosity stress-difractometer can be created using double-crystal monochromator PG002/Si220. Experiments showed that for stress measurements the

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diffractometer STRESS with such monochromator at reactor IR-8 is comparable with modern

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stress-diffractometers at more powerful reactors.

Acknowledgements

This work was partially supported by RSF grant 16-12-10065.

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