JP4925133B2 - Terahertz electron beam spectroscopy method and apparatus - Google Patents

Description

  The present invention relates to the use of an accelerator using Compton scattering by an electron beam and a laser and coherent radiation from the electron beam, and more particularly to a terahertz electron beam spectroscopy measurement technique.

The terahertz wavelength region was located in the middle band between the wavelength of light waves and electromagnetic waves, and there were few light sources suitable for spectroscopic measurement, and this was a region where research and development was delayed.
Recently, however, this wavelength band has begun to attract attention in fields such as communication and imaging technology. Terahertz light emitted by coherent radiation emitted from high-energy electron bunches, oscillation by backward and backward wave tubes, and current switching of semiconductors, etc. The research has been remarkably advanced by using (see Patent Documents 1 and 2).

The terahertz wavelength band has an absorption spectrum characteristic of molecular bonding, and is expected to be applied in fields such as substance identification and dangerous substance detection.
However, in the wide wavelength region of the terahertz band, there are still few strong continuous spectrum light sources, and in order to perform spectroscopic measurement, the wavelength is changed little by little using a wavelength variable light source, or the wide wavelength region is covered, or the time domain The light source is designed to cover a wide wavelength region by changing the delay time of the probe light by spectroscopy.
JP 2007-057407 A JP 2007-327897 A

In the above prior art, it is difficult to obtain a highly sensitive array type detector in the terahertz band, and therefore, simultaneous spectrum measurement over a wide wavelength region is difficult. That is, since the response to different wavelengths will observe events at different times, in order to measure over a wide wavelength region, the measurement results of different wavelengths at different times must be spliced together.
In view of the above problems, an object of the present invention is to provide a terahertz electron beam spectroscopy measuring method and apparatus for performing spectral measurement of events at the same time when terahertz light is irradiated.

In the present invention, the spectrum of terahertz light is converted into a wavelength region that is easy to observe simultaneously by inverse Compton scattering with relativistic electron energy, enabling spectroscopic measurement of events at the same time irradiated with terahertz light. is there.
Specifically, coherent radiation emitted from a relativistic electron beam of an electron accelerator is utilized to generate terahertz band long-wavelength high-intensity light.
Coherent radiation is a phenomenon that becomes coherent when the wavelength of the radiation beam exceeds the bunch length of the electron bunch and becomes high brightness because the radiation intensity is proportional to the square of the number of electrons in the bunch.
It is known that there are several forms of coherent radiation, depending on the radiation used. Coherent synchrotron radiation (CSR) or coherent transition radiation, which is one of these forms, produces strong continuous spectrum terahertz light. Obtainable.

  After the terahertz light is propagated, it is condensed and applied to the material, the transmitted or reflected light is propagated, and the electron bunches after the next are collided in front. Thereby, inverse Compton scattering occurs, and high energy photons are generated. Terahertz light must be collected just before collision, but high-energy photons that are inversely Compton-scattered pass through the same path (it is also possible to pass electron beams). Make a hole. If the focusing lens can absorb high energy photons well, this also serves as an aperture. Since terahertz light is a continuous spectrum, photons generated by inverse Compton scattering have a continuous spectrum even on the electron beam trajectory axis. Since there is a difference in the scattered photon spectrum depending on the presence or absence of data, it becomes an absorption or scattering spectrum.

In particular, a terahertz electron beam spectrometer has at least a deflecting magnet for generating a CSR beam, and a first optical system that focuses the spread CSR beam and propagates it to the sample, and extracts the spectral characteristics of the sample. A second optical system that guides the CSR beam to collide with the electron beam trajectory, and a third optical system that causes the components having the same energy of the inverse Compton scattered photons generated by the front collision to enter the spectroscope. The
In addition, a perforated focusing lens and perforated reflected light are used in common with the first to third optical systems.

The present invention provides a wavelength region that is easily observed simultaneously by inverse Compton scattering with relativistic electron energy for terahertz light over a wide wavelength region having transmission or reflection spectral information of the sample by transmitting or reflecting the sample. By performing energy conversion, simultaneous spectrum measurement over a wide wavelength region in the terahertz band can be performed.
The generation of continuous spectrum terahertz light in a wide wavelength band utilizes coherent synchrotron radiation (CSR) and coherent transition radiation emitted from a multi-bunch electron beam, so that powerful terahertz light generation and inverse Compton scattering can be performed with the same accelerator. There is an advantage that can be done.

Embodiments of the present invention will be described in detail with reference to the drawings. These examples do not limit the invention.
(Example)
A small S-band linac with a photocathode RF electron gun installed at the National Institute of Advanced Industrial Science and Technology is used as the accelerator.
For the electron beam characteristics and laser characteristics, numerical values that are considered to be available with existing technology are used.
The terahertz light source uses a CSR that is emitted when an electron beam passes through a 90-degree deflecting magnet.
Table 1 shows the main parameters used in the following calculations.

FIGS. 1 to 6 are main part configuration diagrams of the terahertz wave electron beam spectrometer of the present invention, and are schematic diagrams showing temporal changes in the optical system and electron beam trajectory including the CSR beam and transmitted / scattered light from the sample. It has become.
The terahertz electron beam spectrometer includes a first optical system that has at least a deflecting magnet for generating a CSR beam, focuses the spread CSR beam, and propagates it to the sample, and a CSR that extracts the spectral characteristics of the sample. The second optical system guides the beam so as to collide with the electron beam trajectory, and further includes the third optical system that causes the components having the same energy of the inverse Compton scattered photons generated by the front collision to enter the spectroscope.

  The first optical system includes a movable stage 10 including a CSR beam generation point 4, a perforated focusing lens 5, a perforated reflecting mirror 6, a focusing lens 7, a sample 8, and a reflecting mirror 9. Since the CSR beam is a coherent component of synchrotron radiation, when the relativistic electron beam is accelerated by an electromagnetic field, it is emitted in a wide range around the tangential direction of the electron trajectory. Only the light is condensed by the focusing lens 5, propagated with a wavefront close to a parallel beam, changed in direction by the perforated reflecting mirror 6, condensed by the focusing lens 7, and transmitted (or reflected) by the sample 8.

  In the second optical system, the CSR transmitted through the sample 8 is reversed in direction while the wavefront is kept in the original state by the reflecting mirror 9, and is again transmitted through the sample 8 to converge the focusing lens 7, the perforated reflecting mirror 6, and the focusing lens. The light is condensed at the collision point 11 through the path 5. When the light is reflected by the sample 8, the light is condensed at the collision point 11 through the path of the converging lens 7, the perforated reflecting mirror 6, and the converging lens 5. The movable stage 10 is used to adjust the optical path length.

  The third optical system includes a perforated focusing lens 5, a perforated reflecting mirror 6, an absorption aperture 12, a lens 13, and a spectroscope 14 arranged in a straight line from the collision point 11. When the CSR beam including the spectral characteristics of the sample 8 and the electron beam collide head-on at the collision point 11, inverse Compton scattered light is generated within an extremely narrow solid angle, and the focusing lens 5, the perforated reflector 6, the absorption aperture Only the component that has passed through the 12 apertures reaches the lens 13. The scattered light is converged by the lens 13 and enters the spectroscope 14. The spectroscope 14 has a high-sensitivity multichannel photon detector 18 for measuring the spectral characteristics of the sample 8, and a spectrum reflecting the spectral characteristics of the sample 8 is obtained.

In this terahertz wave electron beam spectrometer 1, there is a short straight line portion 2 delimited by a small angle steering 17, and therefore a CSR beam that interacts with the straight line portion 2 among multi-bunch electron beams that arrive at regular intervals. There is only one electron bunch, and only the inverse Compton scattered photons with uniform energy are incident on the spectroscope 14 installed in the extending direction of the straight line portion 2.
The vacuum chamber 15 is configured to house an optical system including the linear portion 2 and the deflecting portion 3 and mainly the movable stage 10, and is appropriately divided as necessary.

For example, the deflection unit 3 has a length of 600 mm in the length direction of the straight portion 2, a radius ρ of 500 mm from the center of the arc serving as an electron trajectory, and a radius R from the center of the arc of the arc formed by the inner tube wall is 420 mm. Has been. The vertical height in the vacuum is 46 mm.
In particular, the terahertz electron beam spectrometer 1 includes at least a deflecting magnet 16 for generating a CSR beam, a first optical system that focuses a spread CSR beam and propagates it to a sample, and spectral characteristics of the sample. A second optical system that guides the extracted CSR beam to collide with the electron beam trajectory, and a third optical system that causes the components having the same energy of the inverse Compton scattered photons generated by the front collision to enter the spectrometer. is necessary.

1 to 6 show the timing of the electron bunch, CSR beam, and inverse Compton scattered light. Description will be made in accordance with the timing order shown in FIGS.
After the generation point 4, the CSR beam (C1) and the electron beam (E1) emitted in the vicinity of the generation point 4 are separated and move along a straight line and a circular orbit, respectively (FIG. 1).
The CSR beam (C1) is converted into a plane wave by the perforated focusing lens 5 (FIG. 2), deflected by the perforated reflecting mirror 6 (FIG. 3), and irradiated onto the sample 8.
The CSR beam (C1) having the spectral information of the sample 8 is returned to the original optical system by the reflecting mirror 9 in the opposite direction. At this time, the next electronic bunch (E2) approaches toward the collision point 11 (FIG. 4).

The CSR beam (C1) again travels in the opposite direction through the perforated reflecting mirror 6 and perforated focusing lens 5 (FIG. 5), and collides frontally with E2 at a collision point 11 set slightly upstream from the generation point 4. . At this time, scattered light S1 is generated by inverse Compton scattering.
The scattered light S1 passes through the holes of the perforated focusing lens 5 and the perforated reflecting mirror 6 (FIG. 6), further adjusts the energy resolution by the absorption aperture 12, and enters the spectroscope 14 via the lens 13. At this time, C2, which is the next CSR beam, moved behind the scattered light S1 in the same direction, but was deflected by the perforated reflecting mirror 6 to prepare for the next collision with the next electron bunch (E3). Become.

The detailed calculation until an inverse Compton scattering spectrum is obtained is shown below.
The electron beam passes through the achromatic arc and is bunch compressed to about 0.5 ps.
By using a quadrupole electromagnet (not shown) for converging the electron beam, the beam size is narrowed down to an arbitrary value, for example, 1 mm, at the collision point 11 of inverse Compton scattering.
A small-angle deflection steering coil 17 is installed downstream of the quadrupole electromagnet, and the straight part 2 is formed, for example, about 20 cm up to the 90-degree deflecting magnet 16 to be added to the reverse Compton scattering of the frontal collision. Limit bunch to one.
That is, in order to limit the electron bunch in the straight portion 2 to only one collision point 11, the trajectory is bent by the steering coil 17 for minute angle deflection so that no other electron bunch is generated on the electron beam locus.

By doing so, the wavelength of the terahertz light can correspond to the photon energy caused by inverse Compton scattering with a narrow resolution. In order to simplify the model, the leakage magnetic field of the 90-degree deflecting magnet is ignored, and the collision point 11 is set immediately before the 90-degree deflecting magnet 16 (for example, 10 mm upstream), so that the beam size is not expanded so much. Can perform TRON radiation.
The coherent synchrotron radiation emitted while the electron bunch moved from 0 degree to 0.12 rad is collected by the perforated focusing lens 5 installed, for example, about 230 mm downstream from the 0 degree position. The height of the lens 5 is equal to the height in the vacuum chamber 15 and is, for example, 46 mm. (In this case, the vacuum chamber is uniformly vacated by 46 mm in the height (vertical) direction. The lens is placed so as to be sandwiched there, so the height of the lens is 46 mm).

Therefore, it is possible to capture a CSR beam emitted at a solid angle of 0.12 rad (horizontal) × 0.2 rad (vertical). The CSR beam is converted into a plane wave by the perforated focusing lens 5 that is a perforated aspherical convex lens, reflected by the perforated reflecting mirror 6, and reflected by the focusing lens 7 of a short focus lens or a parabolic mirror (not shown). It is focused on a spot size as small as the wavelength. A sample 8 to be spectroscopically measured is disposed at a condensing position (in this example, an intermediate position between the focusing lens 7 and the reflecting mirror 9), and the scattered light is returned to the same optical system again, or a concave mirror is provided on the opposite side of the sample 8. A reflecting mirror 9 is arranged and reflected so as not to change the wavefront, and is again transmitted through the sample 8 and returned to the same optical system. The CSR beam that has returned to the original optical system has the transmission or reflection spectrum information of the sample 8 and is condensed around 0 degrees. By making the round-trip optical path length shorter by, for example, 10 mm than the electron train bunch interval of 3.791 m in the pulse train, inverse Compton scattering can be generated in the straight line portion 2. The experimental layout is shown in FIGS. Moreover, the calculated value of the CSR spectrum per pulse irradiated to a lens is shown in FIG.
FIG. 7 is a characteristic diagram of CSR spectral characteristics obtained by the terahertz wave electron beam spectroscopy of the present invention. The characteristics of FIG. 7 are shown in Table 2 below. Here, the unit of CSR output represents the energy emitted in a wavelength width of 1% per pulse and per 100 mrad in the horizontal direction, as in the vertical axis of FIG. 7, in joule units.

Scattered photons are generated in the visible to vacuum ultraviolet region when the next electron bunch and the CSR beam cause inverse Compton scattering with respect to the electron bunch that generated the CSR beam. The photons scattered by the inverse Compton scattering are given energy from the electrons, so that the energy becomes high and has a concentric energy distribution from the scattering axis.
In general, since the electron energy is high, most photons are scattered within a very narrow solid angle. In the case of the example, since the electron energy is relatively low at 20 MeV, inverse Compton scattered photons having an energy width of about 3% enter the range of 4.4 mrad from the optical axis.

Taking into account the accuracy of the holes in the lens and mirror, a square hole of 20 mrad (H: horizontal direction) × 20 mrad (V: vertical direction) is used as a lens or mirror such as the perforated focusing lens 5 or perforated reflector 6. Open to allow transmission of inverse Compton scattered photons.
The final absorption aperture 12 that determines the energy resolution is installed in front of the spectrometer 14. The absorption aperture 12 is a variable aperture type capable of absorbing ultraviolet to visible light, and plays a role of absorbing stray light in addition to the energy cut described above.
The inverse Compton scattered photons are split by the spectroscope 14 in the vacuum ultraviolet to visible range. FIG. 8 shows the calculated values of the inverse Compton scattered photon spectrum obtained by using only one lens 19 having a transmittance of 90% and spectrally splitting with an energy width of 3% in the absence of the sample 8.
FIG. 8 is a characteristic diagram of inverse Compton scattering spectrum characteristics in the absence of a sample in the terahertz wave electron beam spectroscopy of the present invention. The characteristics of FIG. 8 are shown in Table 3 below. Here, the number of photons is based on the number of photons emitted at a wavelength width of 3% per second, as in the vertical axis of FIG.

An array-type high-sensitivity photodetector capable of single photon detection at a wavelength of 120 nm or more is commercially available, and calculation shows that spectrum measurement is possible in a wide wavelength range of 120 nm to 480 nm.
Recently, an energy recovery type linac has been developed, and the repetition frequency can be increased to several tens of MHz while maintaining a short pulse property. Although the amount of charge per bunch is as small as about 0.1 nC (nanocoulomb), if the same optical arrangement is used, it is possible to obtain a photon number more than 100 times that of the above-described embodiment, and in a short time terahertz Wave spectroscopy measurement is possible. Therefore, if the sample is moved two-dimensionally, the same time spectroscopic imaging measurement data can be acquired.

It is a principal part block diagram of the terahertz wave electron beam spectrometer of this invention, and is the schematic diagram which showed the time change of the optical system and electron beam orbit including the CSR beam and the transmitted and scattered light from a sample. It is a principal part block diagram of the terahertz wave electron beam spectrometer of this invention, and is the schematic diagram which showed the time change of the optical system and electron beam orbit including the CSR beam and the transmitted and scattered light from a sample. The next time change state of FIG. 1 is shown. It is a principal part block diagram of the terahertz wave electron beam spectrometer of this invention, and is the schematic diagram which showed the time change of the optical system and electron beam orbit including the CSR beam and the transmitted and scattered light from a sample. The next time change state of FIG. 2 is shown. It is a principal part block diagram of the terahertz wave electron beam spectrometer of this invention, and is the schematic diagram which showed the time change of the optical system and electron beam orbit including the CSR beam and the transmitted and scattered light from a sample. The next time change state of FIG. 3 is shown. It is a principal part block diagram of the terahertz wave electron beam spectrometer of this invention, and is the schematic diagram which showed the time change of the optical system and electron beam orbit including the CSR beam and the transmitted and scattered light from a sample. The next time change state of FIG. 4 is shown. It is a principal part block diagram of the terahertz wave electron beam spectrometer of this invention, and is the schematic diagram which showed the time change of the optical system and electron beam orbit including the CSR beam and the transmitted and scattered light from a sample. The next time change state of FIG. 5 is shown. It is a characteristic view of the CSR spectrum characteristic obtained by the terahertz wave electron beam spectroscopy of the present invention. It is a characteristic view of the inverse Compton scattering spectrum characteristic in the state which does not have a sample in the terahertz wave electron beam spectroscopy of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Terahertz wave electron beam spectrometer 2 Straight line part 3 Deflection part 4 Generation point 5 Perforated focusing lens 6 Perforated reflecting mirror 7 Focusing lens 8 Sample 9 Reflecting mirror 10 Movable stage
DESCRIPTION OF SYMBOLS 11 Collision point 12 Absorption aperture 13 Lens 14 Spectrometer 15 Vacuum chamber 16 90 degree deflection magnet 17 Small angle steering coil 18 High sensitivity multichannel photon detector

Claims (7)

  The long-wavelength coherent radiation emitted by the multi-bunch relativistic electron beam is collected and irradiated onto the sample, and the transmitted light transmitted through the sample or the reflected light reflected by the sample is collected again, and then Measure the spectrum of scattered light generated by inverse Compton scattering in a state controlled to collide with an electronic bunch in front, and measure the absorption or reflection spectrum of the wavelength near the terahertz band based on the spectrum of the light. A terahertz electron beam spectroscopy measurement method.   The generation of the inverse Compton scattering is controlled so that the light generated by the inverse Compton scattering has a spectrum from an ultraviolet light region to a visible light region where spectrum is easy to measure, and simultaneous spectroscopy is performed. The terahertz wave electron beam spectroscopy measurement method according to 1.   3. The terahertz electron beam according to claim 1, wherein the reflected light or the transmitted light is collected by a perforated mirror to separate the long-wave coherent radiation and scattered light generated by inverse Compton scattering. Spectroscopic measurement method.   The electron beam trajectory is deflected by a small-angle deflecting magnet so as to maintain the spectral purity of the scattered light generated by inverse Compton scattering, and the trajectory upstream from the deflecting magnet and the electron beam trajectory at the collision point downstream from the deflecting magnet are shifted. The terahertz wave electron beam spectroscopy measurement method according to claim 1, wherein the terahertz wave electron beam spectroscopy measurement method is performed.   5. The terahertz electron beam spectroscopic measurement method according to claim 1, wherein the scattered light generated by the inverse Compton scattering is spectroscopically measured through an absorption type aperture.   A first optical system having at least a deflecting magnet for generating a coherent synchrotron radiation beam, converging the spread coherent synchrotron radiation beam to propagate to the sample, and a coherent synchrotron extracting the spectral characteristics of the sample A second optical system that guides the radiation beam to collide with the electron beam trajectory; and a third optical system that injects into the spectroscope components having the same energy of inverse Compton scattered photons generated by the frontal collision. Terahertz electron beam spectrometer.   7. A terahertz wave electron beam spectrometer according to claim 6, wherein a perforated focusing lens and perforated reflected light are used in common for the first, second and third optical systems.

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