Kimihiro Norizawa

Radially polarized terahertz waves from a photoconductive antenna with microstructures

The generation of radially polarized terahertz (THz) waves from a photoconductive antenna (PCA) with radial microstructures was investigated. Microstructured electrodes with 10 μm lines and spaces were fabricated on a semi-insulating InP wafer using photolithography. The PCA with three layers was driven by an amplified femtosecond laser and DC power supply. Radially polarized THz waves from the PCA were measured by a Michelson interferometer with a 4.2 K silicon bolometer. The polarization components of THz waves were analyzed by a wire grid polarizer.

Multimode terahertz-wave generation using coherent Cherenkov radiation

Multimode terahertz(THz)-wave generation using coherent Cherenkov radiation (CCR) was investigated. The frequency spectra of CCR, which utilized a metal-wrapped hollow dielectric tube of 7 mm outer radius and a picosecond electron bunch of 27 MeV beam energy, were measured by a Michelson interferometer with a 4.2 K silicon bolometer. In this study, discrete spectral components at frequencies of 0.09, 0.14, and 0.36 THz were observed experimentally and explained as transverse magnetic (TM) modes of TM03, TM04, and TM09, respectively, according to a theoretical calculation for the tube.

Radiation effects in dry ice: models for a peak on the Arrhenius curve

Dry ice (solid CO2) occurs in the polar caps of Mars, on the surface of Triton, and in places in the outer planets of our solar system. Radicals in gamma-irradiated solid CO2 have been studied by ESR for future applications of ESR dating on outer planets. The annealing curves for CO3- radical (ESR signal at g = 2.0126) can be described neither by the first-order nor the second-order decay kinetics. The peak observed in the Arrhenius plot can result from two parallel first-order kinetic processes. Radicals that provide overlapping signals are CO3- (g1 = 2.0057, g2 = 2.0126, g3 = 2.0161; activation energy E = 0.10 eV; frequency factor v0 = 4 x 10(1) s(-1)) and HO2 (g1 = 2.0040, g2 = 2.0055, g3 = 2.0360), which have E = 0.28 eV and v0 = 7 x 10(5) s(-1)). Hence, HO2 is more thermally stable, and use of HO2 is promising for ESR dating.

Development of double-decker pulse radiolysis

Double-decker pulse radiolysis (DDPR), which utilizes double-decker electron beams, was investigated to develop a new pulse radiolysis with a high time resolution. The double-decker electron beams were generated by injecting two UV pulses into a photocathode radio-frequency gun. In the pulse radiolysis, one electron beam was used as a pump beam, and the other was converted to a probe pulse. Finally, as its first application, the DDPR was successfully used for observing solvated electrons in water, with a 10%-90% rise time of 8.6 ps.

Collective Energy Loss of Attosecond Electron Bunches

The formalism of the stopping power for cluster beams was adapted to the stopping power for short electron bunches using the wake field of a medium characterized by plasma frequency. It was shown that, if the bunch length is in the 100 as range, the energy loss of the bunch is proportional to the square of the number of electrons in the bunch. If the number of electrons is large, the collective loss is able to excite a high-energy density state in the target. The target medium and beam parameters were examined to demonstrate the collective effect, and an accelerator system consisting of an accumulation ring and an inverse free-electron laser (IFEL) was considered to produce attosecond bunches.

Optically Stimulated Luminescence Study on γ-Irradiated Ice Frozen from H2O and D2O

Optically stimulated luminescence (OSL) and thermoluminescence (TL) for γ-irradiated ice samples have been investigated as future dating techniques for icy bodies in the solar system. The OSL around 400 nm lasted more than 600 s for γ-irradiated H2O ice and D2O ice under 623-nm-light stimulation at 90 K; the latter was used to study the migration of hydrogen atoms. A defect containing trapped electrons is the most suitable explanation of the OSL emissions. The intensity of the TL peak at 120 K increased linearly with γ-dosage increasing up to 15 kGy for both D2O ice and H2O ice. Intensities of both OSL and TL for D2O ice were larger than those for H2O ice. The TL peak related to H2O was observed but its thermal characteristics did not agree with those of OH and HO2 radicals measured by ESR. The OSL method should be employed in future surveys in the solar system.

THz time-domain spectroscopy and vibration analysis of DNA-related base molecules

The vibration modes of hydrogen bonds and intermolecular interaction appear in the THz region. Particularly, biomolecules show a unique fingerprint spectrum. We have measured the THz absorption spectra of simple base molecules like adenine (A), thymine (T), guanine (G), cytosine (C), deoxyadenosine monophosphate (dAMP), deoxyguanosine monophosphate (dGMP) and deoxyadenosine triphosphate (dATP). Each spectrum had a unique structure. Adenine and dAMP had little absorption. Cytosine and dCMP had larger absorption than the others. The fingerprint spectra and their identification with molecular orbital theory are presented.

Generation and Diagnosis of Ultrashort Electron Bunches from a Photocathode RF Gun Linac

Ultrashort electron bunches are essential for timeresolved measurement methods such as pulse radiolysis from the viewpoint of time resolutions. On the other hand, generation of electro-magnetic wave in the THz range using short electron bunches has been investigated. Frequency spectra of coherent transition radiation (CTR) emitted by an electron bunch depend on bunch form factor (BFF), which is expressed by Fourier coefficients of longitudinal distribution in the electron bunch. In this study, the bunch length measurement was demonstrated by analyzing THz-waves generated by CTR. Femtosecond electron bunches were generated by a laser photocathode RF gun linac and magnetic bunch compressor. THz-waves generated by CTR, which was emitted on an interface of an aluminum mirror along the beam trajectory, were transported to a Michelson interferometer. The bunch length was measured by analyzing interferogram, which was an infrared detector output as a function of a moving mirror position. Finally, the bunch length was measured according to fitting curves for the interferogram near the centerburst. Minimum bunch length of 1.3 fs was obtained at a bunch charge of ~1 pC. INTRODUCTION Ultrashort electron bunches whose durations are picoseconds and femtoseconds have been applied to the accelerator physics applications including free electron lasers and laser Compton X-rays. Such electron bunches are also the key elements in time resolved measurements such as pulse radiolysis [1]. Pulse radiolysis is a powerful and useful tool for investigating ultrafast phenomena induced by the electron bunches. Recently, 100-fs electron bunches were generated using a laser photocathode RF gun linac and a time resolution of ~240 fs was achieved in pulse radiolysis [2]. The time resolution of pulse radiolysis strongly depends on the bunch length of the electron bunches. Therefore, the ultrashort electron bunches are demanded to improve the time resolution of pulse radiolysis and observe the initial process of the radiation chemistry. On the other hand, it is also important to develop a measurement system to diagnose the bunch length of such ultrashort electron bunches. Femtosecond streak camera is generally used as a measurement technique of the bunch length, however, its time resolution is limited to ~100 fs in rms. Therefore, alternative methods to obtain information of the longitudinal profile of the ultrashort electron bunches have been investigated. One of the promising techniques is a method to observe CTR using an interferometer [3]. CTR is a phenomenon that an electron bunch emits intense radiation at a wavelength longer than the bunch length when crossing a boundary between different media. The CTR spectrum depends on the bunch form factor, which is a square modulus of Fourier transform (FT) of the longitudinal bunch distribution. In other words, information of the bunch length can be obtained from the observation of CTR. In the present study, to improve the time resolution of the pulse radiolysis technique, generation and bunch length measurement of the femtosecond electron bunches were investigated by monitoring CTR using a Michelson interferometer. The ultrashort electron bunches were generated by a photocathode-based linac with a magnetic bunch compressor. Bunch length measurement is also important for characterizing electron bunches. Enabling analysis of broadband electromagnetic (EM) waves up to 50 THz, the technique using a Michelson interferometer with two detectors could be a useful and effective method for characterizing a wide range of bunch lengths. EXPERIMENTAL ARRANGEMENT Generation of Femtosecond Electron Bunches using a Photocathode-based Linac In this study, ultrashort electron bunches were generated using an S-band (2.856 GHz) photocathode-based RF gun linac and an arc-type magnetic bunch compressor [4,5,6]. The linac is composed of a 1.6-cell S-band (2.856 GHz) RF electron gun with a copper cathode, a 2-m-long S-band acceleration tube, and a magnetic bunch compressor. Irradiating the cathode by the third harmonic of Ti:Sapphire femtosecond laser, femtosecond electron bunches were generated at the RF gun. The incident angle of the laser on the cathode was ~2° with respect to the electron beam axis. The beam energy was 4 MeV at the exit of the gun and 32 MeV at the exit of the accelerating tube and the charge of the electron bunches were estimated using an integrating current transformer. The bunch charges were suppressed to the pico coulomb order to reduce the bunch-length growth due to the space charge effect. The femtosecond electron bunch was compressed by rotating the phase-space distribution in the magnetic bunch compressor. The compressor was composed of a pair of 45° bending magnets, two pairs of quadrupole magnets, ___________________________________________ #nozawa81@sanken.osaka-u.ac.jp 5th International Particle Accelerator Conference IPAC2014, Dresden, Germany JACoW Publishing ISBN: 978-3-95450-132-8 doi:10.18429/JACoW-IPAC2014-THPME132 06 Instrumentation, Controls, Feedback & Operational Aspects T03 Beam Diagnostics and Instrumentation THPME132 3553 Co nt en tf ro m th is w or k m ay be us ed un de rt he te rm so ft he CC BY 3. 0 lic en ce (© 20 14 ). A ny di str ib ut io n of th is w or k m us tm ai nt ai n at tri bu tio n to th e au th or (s ), tit le of th e w or k, pu bl ish er ,a nd D O I.

Pulse Radiolysis Using Terahertz Probe Pulses

Pulse radiolysis, which utilizes a pump electron beam and a probe pulse, is a powerful tool that can be used for the time-resolved observation of ultrafast radiationinduced phenomena. Recently, double-decker pulse radiolysis using visible probe pulses were demonstrated based on a photocathode RF gun driven by two UV pulses, which enabled synchronized pump electron beam and visible probe pulses. In this study, pulse radiolysis using terahertz (THz) probe pulses which were realized by the “double-decker” electron beams and dynamics of transient quasi-free electrons in semiconductors are presented.

THz Time-Domain Spectroscopy of Thin-Film DNA Oligomer Having Mismatch

THz spectroscopy and imaging is useful for hybridization assessment compared with conventional methods using fluorescence indicators, because THz method can give the hybridization information without any indicators. We have successfully obtained the THz spectrum using 11mers having G-G mismatch and found a very clear peak structure in the spectrum. We also report the THz spectra of 11mers with naphthyridine dimmer binding to G-G mismatch.