JP2013019678A - High-speed xafs measurement device and operation method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a technique for performing in-situ observation of a change in the structure and electronic state of various materials having low concentration due to a chemical reaction with time resolution of 10 ms or less.SOLUTION: An XAFS measurement device includes: an incoming X-ray detector having an X-ray response time of 10 μs or less; a transmission X-ray detector having the X-ray response time of 10 μs or less; and a fluorescence X-ray detector having the X-ray response time of 10 μs or less.

Description

  The present invention relates to a high-speed XAFS measuring apparatus capable of continuously performing time-resolved measurement with a time resolution of millisecond on a low-concentration sample, and an operating method thereof.

  In order to understand the effects of chemical reactions of material substances such as catalysts at the atomic level, it is necessary to observe the structure and chemical state of the material substances in the chemical reaction process in situ. Many of these material substances form an amorphous structure without taking a crystalline state, and as the most effective technique for carrying out structural analysis and chemical state analysis in an in-situ environment such as a gas atmosphere / solution atmosphere, An XAFS measurement method can be mentioned. In the XAFS measurement method, a change in the X-ray absorption amount in the vicinity of the X-ray absorption edge energy of a measurement target atom is measured by measuring an X-ray dose transmitted through a sample (transmission XAFS method) or by measuring a fluorescent X-ray dose from a sample ( This is a method of measuring with high accuracy by the fluorescent XAFS method (for example, Non-Patent Document 1).

  To track transient chemical reactions such as catalyst activity, it is necessary to continuously perform XAFS measurements on a time scale from the millisecond to minute order. Conventional XAFS measurement with a time resolution on the order of milliseconds has been performed by an energy dispersive XAFS measurement method using a curved crystal X-ray spectrometer (for example, Non-Patent Document 2). This measurement method uses a position-sensitive X-ray detector to measure the X-ray intensity transmitted through the sample in a batch over the entire XAFS spectral region, so the measurement time resolution is on the order of milliseconds. Is possible. However, it can be applied only to a sample with a high concentration of element to be measured (for example, a high concentration sample of about 1% by weight or more), and a sample with a low concentration represented by many actual catalysts and thin film devices. It is not applicable to (for example, a low concentration sample of less than about 1% by weight).

  Under such circumstances, a quick XAFS measurement method has been developed. The quick XAFS measurement method is a technique in which a fluorescent XAFS method that can be applied to a low concentration sample is used in addition to a transmission XAFS method that can be applied only to a high concentration sample (for example, Non-Patent Document 3). In the quick XAFS measurement method, the X-ray spectrometer is measured at high speed (for example, the angle range of about 1 ° is angled at a speed of about 10 ms to about 1 minute), and the X-ray energy is measured. A system that measures the intensity of X-rays incident on the sample and the intensity of X-rays transmitted through the sample at high speed, high efficiency, and high accuracy (transmission quick XAFS measurement system) is used. From the viewpoint of efficiency, it is desirable that the intensity of the X-ray incident on the sample is designed to detect about 10 to 30%, and the intensity of the X-ray transmitted through the sample measures almost 100%. It is considered desirable. In addition, the quick XAFS measurement method uses a system that measures the intensity of fluorescent X-rays emitted from a sample at high speed, high efficiency, and high accuracy (fluorescence quick XAFS measurement system).

  So far, a quick XAFS measurement system having a time resolution of the order of 10 milliseconds is known (Non-Patent Document 4). In addition, there is a high-speed angular scanning X-ray spectrometer that can be used to perform quick XAFS measurement with a time resolution of 10 milliseconds or less (Non-Patent Document 5).

  As an X-ray detector used for conventional transmission quick XAFS measurement, there is a gas ionization X-ray detector (ion chamber detector) having a response time performance on the order of 10 microseconds. As an X-ray detector used for conventional fluorescence quick XAFS measurement, there is a photomultiplier tube or a silicon PIN photodiode detector having a response time performance of 1 microsecond or less (Non-Patent Documents 4 and 6). Here, the “response time” is determined by the time width of the electric signal when the X-ray enters the detector and is measured as an electric signal by the measurement circuit. When the response time is short, that is, the response speed is high, the time width of the measured electrical signal is narrowed, and the time resolution of the measurement is improved accordingly.

  In addition, time-resolved XAFS measurement can be performed only in a synchrotron radiation experimental facility where high X-ray intensity can be obtained. In the conventional quick XAFS measurement method having a time resolution on the order of 10 milliseconds, the time when the synchrotron X-ray pulse is uniform. Measurement can be performed only under a synchrotron radiation operating condition having a distribution (for example, the synchrotron radiation facility SPring-8 occupies about 60 to 70% of the entire operation time) (Non-patent Document 7).

“Extended x-ray absorption fine structure (EXAFS) of dispersed metal catalysts”, G. H. Via, J. H. Sinfelt and F. W. Lytle, J. Chem. Phys., 71, 690 (1979). “A Fast X-Ray Absorption Spectrometer for Use with Synchrotron Radiation”, T. Matsushita, R. P. Phizackerley, Jpn. J. Appl. Phys., 20, 2223 (1981). “New method for time dependent x-ray absorption studies”, R. Frahm, Rev. Sci. Instrum., 60, 2515 (1989). “Quick XAFS System using Quasimonochromatic Undulator Radiation at SPring-8”, T. Uruga et al., AIP Conf. Proc., 882, 914 (2007). “Development of millisecond time-resolved quick XAFS measurement system”, Uruga et al., 24th Annual Meeting of the Japanese Society for Synchrotron Radiation Optics p160 (2011). Basics of synchrotron radiation (Hiroyuki Oyanagi, Maruzen, 1996), Chapter 6 Synchrotron radiation measurement technology 6.1.1 Introduction to X-ray detectors SPring-8 storage ring operation mode http://www.spring8.or.jp/en/users/operation_status/schedule/bunch_mode

  The response time of an ion chamber detector that has been used as a detector for detecting transmitted X-rays in the conventional quick XAFS measurement method is on the order of 10 microseconds. Therefore, in order to perform XAFS measurement having time resolution on the order of milliseconds, it is necessary to develop an X-ray detector capable of measuring the intensity of transmitted X-rays with a response time of 10 microseconds or less.

  The photomultiplier tube used as the fluorescent X-ray detector had a response time on the order of 10 microseconds when used in the current measurement mode. When the photomultiplier tube is operated in the photon counting mode, the response time can be 1 microsecond or less, but counting loss occurs when high-intensity incident X-rays are used. It was difficult to do. On the other hand, the silicon PIN photodiode detector has a thin depletion layer thickness of a silicon element that absorbs X-rays, and it has been difficult to measure high energy X-rays of 20 keV or more with high efficiency. Therefore, there is a need for a method for measuring high-intensity fluorescent X-rays with a small counting loss.

  Further, as an operating condition of the X-ray source used for XAFS, when the synchrotron radiation X-ray pulse has a non-uniform time distribution, the intensity of the X-ray incident on the sample varies with time in the order of microseconds. Therefore, conventionally, it has been difficult to use such operating conditions for quick XAFS measurement having a time resolution on the order of milliseconds. For example, the synchrotron radiation facility SPring-8 occupies about 30 to 40% of the synchrotron radiation operating conditions in which the time distribution of the X-ray pulses is non-uniform. It is very inefficient to be unavailable for measurement. Therefore, a technique capable of performing XAFS measurement under such operating conditions is demanded.

  In order to achieve the above object, an XAFS measurement apparatus of the present invention includes an incident X-ray detector having an X-ray response time of 10 microseconds or less, a transmission X-ray detector having an X-ray response time of 10 microseconds or less, and an X-ray A fluorescent X-ray detector having a response time of 10 microseconds or less;

  As the incident X-ray detector and the transmission X-ray detector of the present invention, any transmission X-ray detector having an X-ray response time of 10 microseconds or less can be used, but an ion chamber detector is preferable. In this case, in order to reduce the response time to 10 microseconds or less, the interval between the two electrodes constituting the ion chamber detector is set to 1 to 3 mm, and the length of each electrode in the X-ray optical axis direction is set to 200 mm. It is necessary to be set to ~ 400 mm. If the distance between the electrodes is narrower than 1 mm, it is difficult to pass X-rays between the electrodes, which is not preferable. If the electrode length is shorter than 200 mm, the moving speed to the electrode is high, but it is not preferable because a light element inert gas (for example, helium or nitrogen) having a low X-ray absorption rate cannot be used. By setting the electrode interval and the electrode length within this range, an X-ray response time of 10 microseconds or less can be obtained.

  As the fluorescent X-ray detector of the present invention, any fluorescent X-ray detector having an X-ray response time of 10 microseconds or less can be used, but low to high energy fluorescent X-rays (for example, 4 to 40 keV). ), Preferably, a pulse distributor that replicates a voltage pulse composed of a plurality of X-ray photons into a plurality of voltage pulses, and a pulse counter that measures a voltage pulse corresponding to the number of X-ray photons. Yes. By adopting such a configuration, the number of photons counted by counting a voltage pulse consisting of a plurality of X-ray photons as one photon is extremely reduced, and a highly efficient X-ray response time of 10 microseconds or less is achieved. Can do.

The XAFS measurement apparatus of the present invention may further include an apparatus connected to the X-ray source for synchronizing the time variation of the incident X-ray intensity and the timing of the X-ray measurement.
When the XAFS measurement apparatus of the present invention is operated, the synchronization device is used to measure a synchronization signal corresponding to the intensity of X-rays emitted from the X-ray source, and for X-ray measurement based on the synchronization signal. Generates a trigger signal, inputs the trigger signal to a transmission X-ray detector and a fluorescent X-ray detector, and synchronizes the time variation of the incident X-ray intensity with the timing of the X-ray measurement, thereby varying the time of the incident intensity. Even in the case where X-rays having the above are used as incident X-rays, XAFS measurement can be continuously performed at time intervals of 10 microseconds or less.

  By utilizing the XAFS measuring apparatus and its operating method of the present invention, the structure and electronic state of various kinds of catalytic materials (fuel cell electrode catalyst, storage battery electrode catalyst, photocatalyst, chemical synthesis catalyst, etc.) and material materials having a low concentration can be obtained. It is possible to observe in situ the state of change caused by a chemical reaction with a time resolution of 10 milliseconds or less.

It is a schematic diagram which shows an example of the structure of the ion chamber detector which is a kind of the conventional transmission X-ray detector. It is a schematic diagram of the long electrode length narrow gap ion chamber detector and the precision position adjustment stage which are one embodiment of the transmission X-ray detector of the present invention. It is a conceptual diagram which shows the principle of count-down generation | occurrence | production with respect to the pulse voltage output which adjoined temporally. It is a schematic diagram of the system which comprises the fluorescent X-ray detector of this invention. It is a conceptual diagram which shows the principle which reduces the countdown with respect to a pulse voltage output with the system which comprises the fluorescent X-ray detector of this invention. It is the conceptual diagram which showed an example of the distribution state of the electron group accumulate | stored in the synchrotron radiation storage ring in synchrotron radiation nonuniform distribution operation mode. It is the conceptual diagram which showed an example of the time fluctuation | variation of the X-ray intensity in synchrotron radiation nonuniform distribution operation mode. It is a conceptual diagram which shows the principle which synchronizes a synchrotron radiation train and the timing of X-ray measurement using the synchronizing apparatus of this invention.

  The XAFS measurement apparatus of the present invention can detect the intensity of X-rays incident on a sample with a response time of 10 microseconds or less, and can measure the intensity of X-rays transmitted through the sample with a response time of 10 microseconds or less. And a detector capable of measuring the intensity of fluorescent X-rays generated from the sample with a response time of 10 microseconds or less.

(Incident X-ray detector and transmitted X-ray detector)
FIG. 1 is a schematic diagram of a parallel plate ion chamber detector that has been widely used as an incident X-ray detector and a transmission X-ray detector. The parallel plate ion chamber detector is a case 1 in which an inert gas (helium, nitrogen, argon, or a mixed gas thereof) is filled between two electrodes (a high-voltage electrode 2 and an electron collecting electrode 3). Yes (Figure 1). A high voltage is loaded between the two electrodes 2 and 3, and a strong electric field is formed. X-rays enter from a window 4 installed on the side of the ion chamber, ionize the inert gas, and generate gas ions and electrons. According to the electric field loaded between the electrodes 2 and 3, gas ions move to the high voltage electrode 2 side, and electrons move to the electron collecting electrode 3 side, so that a minute current flows between the two electrodes. It is measured using a high-speed minute current measuring device 6 (for example, a high-speed current amplifier and a high-speed digitizer), and the X-ray intensity is measured. X-rays that have not been absorbed by the inert gas are emitted from the window 5 installed on the other side surface. The response time of the ion chamber is determined by the time for each ionized ion and electron to move to the electrode when using a microcurrent measuring device with a sufficiently high response speed (for example, having a response time of about 1 microsecond). Is done.

  In one embodiment of the invention, as described in FIG. 2, the two electrodes 2, 3 comprising an ion chamber detector are used to detect transmitted X-rays with a response time of 10 microseconds or less. Was set to 3 mm (narrow gap structure), which is 1/3 or less of the conventional interval (mostly 10 mm or more), and the electric field strength between the electrodes was increased. Furthermore, if the electrode interval is shortened to 3 mm, the moving distance of ions and electrons ionized by the incidence of X-rays to the electrode is reduced to 1/3 or less. In addition, in this embodiment, the length of the electrode in the X-ray optical axis direction is 280 mm. By setting the electrode length to 200 mm or more, it is possible to use a light element inert gas having a low X-ray absorption rate, such as helium and nitrogen, although the moving speed to the electrode is high. As a result, the time required for ionized ions and electrons to reach the electrode can be shortened to about 1/5 times that of the prior art. This makes it possible to measure the X-ray incident on the sample and the X-ray intensity transmitted from the sample with a response time on the order of microseconds. The long electrode long narrow gap ion chamber detector of the present invention allows the X-ray to pass with high accuracy near the center between the long and narrow electrodes, and therefore the automatic height adjustment stage 10 and the automatic tilt angle adjustment stage. 9 can be used for remote position adjustment with high accuracy. First, the height of the ion chamber is adjusted so that the X-ray passes through the center between the electrodes while measuring the intensity of the X-ray passing through the long electrode length narrow gap ion chamber detector using an automatic height adjustment stage. I do. Next, using the automatic tilt angle adjustment stage, the direction of the X-ray passing through the electrode is adjusted in parallel while measuring the intensity of the X-ray passing through the long electrode length narrow gap ion chamber detector. By the above operation, the X-rays pass near the center of the electrode over the entire length of the long electrode length narrow gap ion chamber detector.

(X-ray fluorescence detector)
In one embodiment of the present invention, the X-ray fluorescence detector further comprises a measurement system. As shown in FIG. 4, the measurement system connected to the fluorescent X-ray detector includes a high-speed preamplifier 13, a high-speed pulse distributor 14, a high-speed threshold discriminator 15, and a high-speed pulse counter 16. These high-speed preamplifier 13, high-speed pulse distributor 14, high-speed threshold discriminator 15, and high-speed pulse counter 16 all desirably have a response speed of at least 500 MHz to 1 GHz. In particular, it is desirable that the high-speed preamplifier has an amplification factor of 5 times or more, and the high-speed threshold discriminator has a minimum separation voltage of 10 mV or less.

With such a measurement system, it is possible to measure the intensity of fluorescent X-rays emitted from a sample with high response efficiency in the order of microseconds and with low counting loss.
As the fluorescent X-ray detector of the present invention, for example, an X-ray detector equipped with a high-speed scintillator 11 (for example, plastic scintillator) that converts X-rays into visible light at high speed on the front surface of a photomultiplier tube 12 of a high-speed response system. A fluorescent X-ray detector (Non-patent Document 5) using a plurality of elements and arranging a plurality of the elements to increase the light receiving area is used. The high-speed scintillator 11 desirably has a fluorescence lifetime of at least 1 nanosecond to 3 nanoseconds or less. For example, BC-420 manufactured by Saint-Gobain Co., Ltd. can be used.

  A voltage pulse output when an X-ray photon enters the X-ray fluorescence measuring instrument is first amplified five times or more using the high-speed preamplifier 13. Thereafter, in a threshold discrimination circuit (discriminator) built in the high-speed threshold discriminator 15, by comparing the voltage pulse height with a set threshold voltage, each pulse is generated by one X-ray photon. It discriminates from noise and sends only the signal to the high-speed pulse counter 16 to count the number of pulses.

  Here, under conditions where high-intensity X-rays are incident on the fluorescent X-ray measuring instrument, the time interval at which the voltage pulse is generated may be closer in time than the average width of the voltage pulse. In this case, a plurality of (N) voltage pulses are overlapped to generate a voltage pulse having a height approximately N times that of one X-ray photon. As shown in FIG. 3, if this voltage pulse is counted as one X-ray photon, a counting loss (counting down) occurs. In such a case, in the measurement system of the present invention, the high-speed pulse distributor 14 divides a voltage pulse having a height N times into N equivalent voltage pulses. When dividing a voltage pulse composed of a plurality of photons, it is possible to perform counting with very few countdowns by setting the threshold voltage of the threshold classification circuit in accordance with the number of photons contained in the voltage pulse. For example, as shown in FIG. 5, the threshold voltage may be set to a voltage slightly higher than the voltage pulse for incident photons (N−1).

(Continuous measurement by synchronization with X-ray source)
In one embodiment of the present invention, the XAFS measurement apparatus further includes an apparatus for synchronizing the time variation of the incident X-ray intensity and the timing of the X-ray measurement with the incident X-ray source. The apparatus includes a synchronization signal generator, a timing adjustment circuit, and a measurement circuit.

  With such an apparatus, the quick XAFS measurement apparatus can be operated even when the operation mode of the radiation beam that is the incident X-ray source is the non-uniform distribution operation mode. As schematically shown in FIGS. 6 and 7, when the operation mode of the X-ray source is the non-uniform distribution operation mode, electrons that circulate around the radiation storage ring of the X-ray source (for example, in the case of SPring-8, electrons Takes about 4.8 microseconds to make one round), so that X-ray photons are concentrated in a time-space region called a train (for example, in the case of SPring-8, 70-80% of the total X-ray photons) Degree), the distribution of X-ray photons in other regions is dilute. Therefore, the X-ray intensity incident on the sample varies with time in the order of microseconds. Here, in the quick XAFS measurement, the X-ray intensity is continuously measured at a time interval obtained by dividing the measurement time of one XAFS spectrum into about 1/100 to 1/1000. Therefore, in the case of quick XAFS measurement with a time resolution on the order of 10 milliseconds, the measurement is performed at time intervals on the order of 10 microseconds. It does not appear and does not matter. On the other hand, in the measurement with a time resolution of 10 milliseconds or less, the measurement is performed at a time interval of 10 microseconds or less. Therefore, the XAFS spectrum is affected by the time variation of the X-ray intensity, and the spectrum measurement is performed. I can't. For example, in the case of SPring-8, the non-uniform distribution operation mode occupies about 30 to 40% of the total operation time. Therefore, if XAFS cannot be used in the non-uniform distribution operation mode, the period during which measurement is impossible increases. Is inefficient.

  Therefore, in the present invention, the synchronization signal generator, the timing adjustment circuit, and the measurement circuit are provided to synchronize the timing of the X-ray intensity measurement with the synchrotron radiation train in time, and even in the non-uniform distribution operation mode. It was possible to continuously measure at time intervals of 10 microseconds or less. Specifically, as shown in FIG. 8, a synchronization signal corresponding to the train supplied from the synchrotron radiation storage ring 21 which is an X-ray source is generated in the synchronization signal generator 22 and introduced into the timing adjustment circuit 23. Then, after adjusting the time delay appropriately by the timing adjustment circuit 23, it is input as a gate trigger signal to the measurement circuit 25 of the X-ray detector 24, whereby X-ray intensity measurement synchronized with the train timing becomes possible. Thereby, the time fluctuation which arises in incident X-ray intensity | strength can be prevented, and as a result, XAFS measurement can be implemented with the time resolution of 10 milliseconds or less with respect to all the radiation light operation modes.

  In the beam line BL40XU of SPring-8, which is a large synchrotron radiation facility, it is transmitted through a platinum foil using an ion chamber having an electrode length of 280 mm and an electrode interval of 3 mm as an incident X-ray detector and a transmission X-ray detector. Method XAFS measurements were performed. Pure nitrogen gas was used for the incident X-ray detector, and a mixed gas composed of 85% nitrogen and 15% argon gas was used for the transmission X-ray detector. The voltage between the electrodes was set to 1 kV. A small Si (111) channel crystal was used as the spectroscope, and angular scrutiny was performed at 250 Hz (± 0.2 °) using a galvano scanner. As a result, the measurement time of the transmission XANES spectrum at the platinum L3 absorption edge was 2 milliseconds.

DESCRIPTION OF SYMBOLS 1 Ion chamber housing | casing 2 Electron collection electrode 3 High voltage electrode 4 X-ray entrance window 5 X-ray exit window 6 High-speed microcurrent measuring instrument 7 High-voltage power supply 8 Electric field correction electrode 9 Automatic inclination angle adjustment stage 10 Automatic height adjustment stage 11 High-speed scintillator 12 high-speed photomultiplier tube 13 high-speed preamplifier 14 high-speed pulse distributor 15 high-speed threshold discriminator 16 high-speed pulse counter 21 synchrotron radiation ring 22 synchronization signal generator 23 timing adjustment circuit 24 X-ray detector 25 measurement circuit

Claims (6)

An incident X-ray detector for detecting X-rays incident on the sample;
A transmitted X-ray detector for detecting X-rays transmitted through the sample, and a fluorescent X-ray detector for detecting fluorescent X-rays emitted from the sample;
The incident X-ray detector and the transmitted X-ray detector are ion chamber detectors, and the length of the two electrodes constituting the ion chamber detector in the X-ray optical axis direction is 200 to 400 mm. A high-speed XAFS measurement apparatus in which the distance between the electrodes is 1 to 3 mm. X-ray fluorescence detector
A pulse distributor that replicates voltage pulses corresponding to the number of X-ray photons from a plurality of X-ray photon voltage pulses, and a pulse counter that measures voltage pulses corresponding to the number of X-ray photons,
The high-speed XAFS measurement apparatus according to claim 1, comprising: X-ray fluorescence detector
The high-speed XAFS measurement device according to claim 2, further comprising: a preamplifier that amplifies an X-ray voltage pulse; and a threshold discriminator that discriminates the voltage pulse from a signal and noise generated by an X-ray photon.   The high-speed XAFS measurement apparatus according to any one of claims 1 to 3, further comprising an apparatus connected to the X-ray source for synchronizing the time variation of the incident X-ray intensity and the timing of the X-ray measurement.   The high-speed XAFS measurement apparatus according to claim 4, wherein the apparatus for synchronizing the temporal variation of the incident X-ray intensity and the timing of the X-ray measurement includes a synchronization signal generator, a timing adjustment circuit, and a measurement circuit.   A trigger signal for X-ray measurement is generated based on a synchronization signal corresponding to the intensity of X-rays emitted from the X-ray source, and the trigger signal is input to a transmission X-ray detector and a fluorescent X-ray detector. By synchronizing the time variation of the incident X-ray intensity and the timing of the X-ray measurement, it is possible to perform continuous measurement at time intervals on the order of microseconds even when the time variation of the incident X-ray intensity exists. Item 6. A method for operating the XAFS measurement device according to Item 5.