Experimental Equipments for Mossbauer spectroscopy at SIST

1. High temperature Mossbauer Spectroscopy
2. Conversion electron Mossbauer Spectroscopy
3. Mossbauer Spectroscopy under uniaxtial stress
4. High temperature Mossbauer Spectroscopy under gas atmosphere
5. "Iron microscope" (2-dim position sensitive Mossbauer Spectrometer)

1　High temperature Mossbauer Spectroscopy

　In order to study the diffusion and the segregation of Fe atoms in materials in the thermal equilibrium, we performed the absorber experiments using ⁵⁷Fe containing materials and a standard ⁵⁷Co-in-Rh source. The total resonance area depends not only the number of the probe nucleus in the sample, but also a Debye-Waller factor, which depends on the mean square displacement of the probe atoms, and becomes smaller with increasing temperature. It is therefore extremely difficult to measure a system with very small probe concentration at high temperature.

Figure 1 and 2 show a high temperature furnace, which enables us to measure a temperature range from 300 K to 1800 K under a vacuum of 10^-7Pa. As is seen in the cross section of the furnace (Fig.3 (b)), a strong ⁵⁷Co-in-Rh source (F) with 1.85〜3.7 GBq is mounted on the Mossbauer transducer (A), which produces Doppler shifts of γ-rays. The transducer in a lead box is placed on a water-cooled stainless-steel chamber (B). The γ-ray detector (G) is inserted in the chamber. The specimen is mounted in a Ta-holder (Fig.3 (c)). In order to measure spectra at high temperatures, where the line-width could become broader due to diffusion and simultaneously the Debye-Waller factor could be vanishing, a good S/N must be achieved without losing the γ-rays intensity. Thin aluminum foils are used for the γ-ray windows (a) (Fig.3 (b)). A compact electrical furnace (b) consists of Mo wire with a diameter of 0.5 mm and a pure alumina cylinder. This furnace built in a small tantalum-pot (c) is thermally well isolated by three Ta-heat shields (d). Along the g-ray path, three thin zirconium foils (e) of 2mm thickness are fixed on the Ta-heat shields above and below the furnace. The whole chamber is well cooled by water. Two pairs of Pt-PtRh thermocouple (f), which are inserted into the Ta holder, measure the specimen temperature. Standard temperature controllers control the temperatures of both the furnace and the cooling water separately, yielding to a temperature variation of specimen within 0.2 degrees. This system can be operated without degrading the furnace vacuum, so that a sequence of measurements can be continued for several months.

Fig.1　High temperature furnace for Mossbauer spectroscopy and inside the furnace。

Fig.2 Ta sample holder and surounding elecdtrical furnace fixed on an Al2O3 core.

Fig.3 cross section of the furnace and Ta sample holder

Fig.4 Mossbauer spectra of pure Fe at 20, 700, and 777^oC

２　Conversion electron Mossbauer Spectroscopy (under construction!)

　メスバウアスペクトルの測定は通常実験室では⁵⁷Feの親核である⁵⁷Co線源（半減期270日）を用い、⁵⁷Feを含む試料を吸収体として透過吸収スペクトルを線源のDoppler速度（エネルギーシフト）の関数として計測する。この時、14.4keVγ線は吸収体中の⁵⁷Feで無反跳共鳴吸収（メスバウア効果）され、⁵⁷Feは14.4keVの励起状態に遷移する。その後、100nsの寿命で基底状態に戻るがこのとき再び14.4keVγ線が無反跳共鳴放射されるか、または軌道電子にエネルギーを与えて電子が放出される（内部転換電子）。後者の過程の割合は⁵⁷Feの場合９０％以上で内部転換電子を計測すれば極めて高いS/Nでスペクトル測定が可能になる。しかも、透過法で得られる情報はγ線透過方向の試料厚さ全域であるのに対して、内部転換電子メスバウア分光法は試料表面近傍のみ、すなわち最大100nmまでの深さに分布する⁵⁷Feからの情報は選択的に獲得できる。以下に、本研究室で利用している2つの異なる内部転換電子計測法、（１）平行平板アバランチカウンター法、（２）マイクロチャンネルプレート法を紹介しよう。

Fig.5 Mossbauer transmission measurement

2.1　Pararell plate avalanche counter （PPAC)(under construction!)

　試料中の⁵⁷Feが無反跳共鳴吸収の後に放出する内部転換電子を平行平板電極でガス増幅を利用して検出する。このために試料とグラファイト薄膜で形成する平行平板電極をアクリル容器内（図５）に固定し、計数ガス間に約800Vの電圧を印加する。内部転換電子に対する検出効率はほぼ100％で高いS/Nでのスペクトル測定が可能になる。ただ、試料は高い電場中に置かれることになるので、Si中の⁵⁷Fe格子間原子のように電荷を有する場合には電場の効果がスペクトル計測結果に影響を及ぼす可能性がある。検出器の構造は極めて簡単であるので、PPACは自作可能である。ただ、図５のような封入方の検出器の場合には時間と共に計数ガスが電荷を蓄積し、検出効率が低下する傾向にある。一定のS/Nの計数が必要な場合にはガスフロータイプの検出器とすることも可能である。

　このPPACはデンマークのaahus大学のProf. G. Weyerが多くの仕事を残しており、鉄シリサイドの問題などこれまでに多くの報告がある。この検出器はHahn-Meitner Instituteや理研の線源が試料となる加速器実験（インビーム・メスバウア実験）では、検出器内部に⁵⁷Feを富化したステンレススチール薄を吸収体として、高いバックグラウンド下の”メスバウア効果”のみに敏感な検出器として利用されている。

Fig.6　PPAC and prarell plate electrode (graphite and sample)

Fig.7　cross section of PPAC

　図７はSi表面に⁵⁷Feを左図が100nm、右図が10nm真空蒸着し、異なる熱処理を施した後、試料をPPACに組み込み室温で測定した内部転換電子メスバウア・スペクトルである。両方の試料ではSiと⁵⁷Feの界面反応の結果様々なFe-Siが形成されるが、100nm蒸着試料はこの界面が試料表面から100nmの深さにあり、界面Fe-Si成分はほとんど見えず、αFe成分のみが873Kまでの熱処理で観測される。さらに、1173Kの熱処理では全ての⁵⁷Feが常磁性Fe-Siに変化したことを示している。一方、10nm蒸着試料はαFe成分に加えて界面成分を含む全ての⁵⁷Feが観測されている。内部転換電子メスバウア分光の検出深さが実際に100nm以下であることを実験的に検証した例となっている。

Fig.8 Conversion electron Mossbauer spectra of　100nm-, 10nm-⁵⁷Fe deposited Si after different heat treatments.

2.2　Conversion electron spectrometer using microchannel Plate（MCP)

Fig.9　MCP Mossbauer spectrometr and MCP( right inset)

Fig.10　Mossbauer spectrum of α-⁵⁷Fe measured by MCP spectrometer.

　　A new type of conversion electron Mossbauer spectrometer (CEMS) is developed using a micro
channel plate (MCP). A Mossbauer spectrum of α-Fe is successfully obtained using the new system, but there remain still several problems to be improved. This system will be incorporated into a 2-dim position sensitive Mossbauer spectrometer.

３　Mossbauer Spectroscopy under uniaxtial stress

3.1 Introduction

External stress may induce a wide variety of phenomena in materials such as elastic and plastic deformation, fracture, structural transformation, segregation and diffusion, which have a strong influence on the mechanical and electrical properties of the materials. A stress-strain curve may usually be measured to study the mechanical properties, and subsequently, optical microscopy and/or electron microscopy are often used to investigate morphological changes by the external tensile stress. In order to clarify the microscopic mechanisms of the stress induced phenomena, however, an atomistic experimental tool is highly desirable, which performs an in-situ observation of structural changes under the external stress. Accordingly, we have developed, for the first time, a new experimental set-up for the in-situ Mossbauer studies of stress-induced atomistic effects in materials. The system consists of a conventional Mossbauer spectrometer and an Instron-type tensile test-machine specially designed for a thin foil sample. This set-up can be used not only for a laboratory experiment, but also for an in-beam experiment at a synchrotron facility or a heavy-ion accelerator facility as well.

In the present paper, we report on the new experimental set-up and the first experimental results on a stress-induced martensite transformation in SUS304 stainless steel and in a Fe68.3Ni31.7 alloy.

3.2 A set-up for Mossbauer spectroscopy under uniaxial tensile stress

Figure 11(a) shows a new apparatus combined with an Instron-type tensile-test-machine (Shimadzu Co Ltd.). The main part of the set-up consists of two pairs of specimen chuck (a) built in a stainless-steel cylinder (b), a Mossbauer transducer (c), and a gas proportional counter (d). The g-ray source of 57Co-in-Rh (e) is mounted on the transducer (c). The g-rays will be transmitted through specimen foil (f) to the gas proportional counter (d). The whole g-ray path is well collimated by two lead-plates (g) with a rectangular hole of 4 mm 5 6 mm.

Specimen foil (f) is fixed between two pairs of chuck (a). The upper part of the chuck is directly connected to a load cell (h) mounted on the top of the Instron, which measures the force applied to the foil. While the lower part of the chuck, which is connected to a crossbar (i) of the Instron through the stainless steel cylinder, will be shifted to the lower direction with increasing tensile strain. The maximum force measurable with this tensile test-machine will be 980 N, the value of which is mainly limited from the strength of the stainless steel cylinder. The diameter of the cylinder (b) is 60 mm, and the whole cylinder can be inserted into a top-loading-type cryostat for the measurements at low temperatures.

In order to apply a homogeneous uniaxial tensile stress within the measuring area on the specimen foil, we cut the foil in a special form as is shown in Fig.2 (a). The measured area is marked with black. Figure 12 (b) and (c) shows the measured sample before and after the tensile-test, respectively. After rupture each marker square in 2.0x2.0mm drawn on a SUS304 foil was deformed into rectangular in 3.0x1.3mm. This indicates that the external stress applied on the foil can be thought to be homogeneous and uniaxial within the measuring area of the sample illuminated by the g-rays.

Fig. 11 (a) Mossbauer set-up under uniaxial tensile stress, (b) details around the specimen.

FIg.12　picture fo Mossbauer spectrometer

Fig.13 （a）SUS304 sample, （ｂ）stress-strain curve of SUS304, （ｃ）Mossbauer spectra of stress induced Martensite measure by this equipment.

In order to measure Mossbauer spectra as a function of the external stress by keeping the stress at different values during a uniaxial tensile test, some difficult conditions must be overcome. (1) A thin foil with a thickness of 30 to 50 mm can be spanned between the chucks without breaking during the tensile test. (2) A stress-strain curve measured with the present system must coincide with that obtained from a conventional tensile-test machine, where a much thicker sample of some mm is usually used. To test the new set-up for the in-situ measurement under a uniaxial tensile stress, we first measured a SUS304 stainless steel foil (Cr 18%, Ni 8%, Mn < 2%, Si < 1%, Fe balance) with 30 mm in thickness. The foil was cut into the special shape as shown in Fig.2 (a) and a stress-strain curve for both elastic and plastic region of SUS304 was successfully measured as in Fig.3. after rupture, respectively. Above 600 N/mm2 the magnetic bcc phase ferrite appears in addition to the nonmagnetic fcc austenite phase. It was reported using Conversion Electron Mossbauer spectroscopy that after cold working, the transformation from the austenite to the martensite phase was observed only near the specimen surface at room temperature [1]. In the present experiment, on the other hand, the fraction of the martensite phase increases monotonically with increasing stress (Fig.5), and finally, after rupture, even reached to nearly 50%.

４．High temperature Mossbauer Spectroscopy under gas atmosphere

　　　　　　　　　　　　　　　　　　　　　　　(under construction!)

　高温メスバウア分光でγ線の透過吸収スペクトルを計測する場合、通常は試料を真空チェンバー内の小型電気炉内に固定し、10^-6Pa程度の真空中で保持しながら測定する。ところで、試料をガス雰囲気ちゅうで保持しながら高温でスペクトル測定を行いたい場合がある。この場合には、高温測定炉にガスを単純に導入するだけでは断熱真空を破ってしまうので測定できない。つまり、試料室にのみガスを流し、その周りは断熱真空を保持して高温を実現する必要がある。そこで、本研究室では図１３のようなガス雰囲気透過メスバウア分光用の試料ホルダーを作製し、これを真空容器に固定し、赤外線過熱ヒータで試料室上部を直接加熱する装置を開発した。図13中の細いステンレスパイプガス導入ラインで、銅のホルダーにつながれている。γ線導入窓としてはグラファイト板をメタルOリングで固定し、断熱真空と分断している。

Fig.14　Sample holder for spectrometer under gas atomosphere

　このシステムの典型的な例が、図１４に示す鉄ゼオライト触媒をNOｘ還元中にスペクトル測定を行った研究例である。ここでは、高温でガスと反応中のFeの化学変化を追跡しながら触媒反応の素過程を解明ている。(a)はFe/MFI触媒の大気中、室温のメスバウア・スペクトルで2種類のFe³⁺（I),(II)のダブレットが観測される。(b)はHeガス中で573Kで測定したもので、外側のFe³⁺(II)のダブレットが減少して斜線で示すFe²⁺のダブレットが出現する。この試料をNO1000ppm+C2H41000ppm, O2 5%+He balanceのガス雰囲気、523Kで測定したのが（ｃ）である。Fe²⁺のダブレットの相対強度が（ｂ）と比較して少し増加しており、NOｘ還元反応中ではFe²⁺が触媒反応の活性種となっていることを直接示している。

Fig.15　Mossbauer spectra of Fe/MFI catalysts by ion exchange method：(a)in air at RT, (b)　in He at 523 K, (c) in NO1000ppm+C2H41000ppm, O2 5%+He balance at 523K.

５． “Mossbauer-Spectroscopic Iron Microscope”

We report on a “Mossbauer-spectroscopic iron microscope”, i.e. a two-dimensional position sensitive Mossbauer spectrometer for ⁵⁷Fe. 14.4keV γ-rays emitted from a standard ⁵⁷Co-in-Rh source are focused on a sample by Multi-Capillary-X-ray lens (MCX), which has been developed for X-ray microanalysis[1,2]. We use a new lens especially designed for 14.4keV γ-rays, which provides the spot size of 265μm in diameter on a sample. The sample is mounted on a X-Y goniometer, which is placed in a vacuum chamber. The conversion electrons are measured using a micro channel plate (MCP) as a function of the position of γ-ray spot, producing an image of ⁵⁷Fe concentration profile near the surface. The position is varied with a step size between 100 nm and 100 mm, yielding images with different resolutions (Fig.16). In addition, the image depends on the nuclear resonance condition, which is determined by the Doppler-velocity of the source.

In the present study, a 100 nm-⁵⁷Fe deposited Si wafer is used as a sample. Figure 17 presents the image of the sample observed near aluminum mask. The source is vibrated with a maximum velocity of 7 mm/s. About 1/3 region in the lower part of the picture corresponds to aluminum mask, where no ⁵⁷Fe exists. On the other hand, about 2/3 region of the upper part shows white contrasts, spots and lines, corresponding to higher counts of electrons.

Fig.16　principle for “Mossbauer-spectroscopic iron microscope”　

Fig. 17 Mapping image of conversion electrons

６．The students and technitians.