Takuya Kotaka

Performance measurement of HARPO: A time projection chamber as a gamma-ray telescope and polarimeter

Abstract We analyse the performance of a gas time projection chamber (TPC) as a high-performance gamma-ray telescope and polarimeter in the e + e − pair-creation regime. We use data collected at a gamma-ray beam of known polarisation. The TPC provides two orthogonal projections (x, z) and (y, z) of the tracks induced by each conversion in the gas volume. We use a simple vertex finder in which vertices and pseudo-tracks exiting from them are identified. We study the various contributions to the single-photon angular resolution using Monte Carlo simulations, compare them with the experimental data and find that they are in excellent agreement. The distribution of the azimuthal angle of pair conversions shows a bias due to the non-cylindrical-symmetric structure of the detector. This bias would average out for a long duration exposure on a space mission, but for this pencil-beam characterisation we have ensured its accurate simulation by a double systematics-control scheme, data taking with the detector rotated at several angles with respect to the beam polarisation direction and systematics control with a non-polarised beam. We measure, for the first time, the polarisation asymmetry of a linearly polarised gamma-ray beam in the low energy pair-creation regime. This sub-GeV energy range is critical for cosmic sources as their spectra are power laws which fall quickly as a function of increasing energy. This work could pave the way to extending polarised gamma-ray astronomy beyond the MeV energy regime.

HARPO: beam characterization of a TPC for gamma-ray polarimetry and high angular-resolution astronomy in the MeV-GeV range

A time projection chamber (TPC) can be used to measure the polarization of gamma rays with excellent angular precision and sensitivity in the MeV-GeV energy range through the conversion of photons to e+e- pairs. The Hermetic ARgon POlarimeter (HARPO) prototype was built to demonstrate this concept. It was recently tested in the polarized photon beam at the NewSUBARU facility in Japan. We present this data-taking run, which demonstrated the excellent performance of the HARPO TPC.

Damage Analysis of Plasma-Etched n-GaN Crystal Surface by Nitrogen K Near-Edge X-ray Absorption Fine Structure Spectroscopy

The surface of an n-GaN crystal etched with an Ar, Kr, or Xe plasma was analyzed by nitrogen K near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. The NEXAFS spectroscopy was carried out with the total electron yield (TEY) method in a sample current mode and the total fluorescence yield (TFY) method by measuring the amount of fluorescence using a photodiode. The shapes of the spectra of Ar plasma-etched samples obtained by the TEY method became smooth (blunt) with increasing Ar pressure from 10 to 200 mTorr. However, those obtained by the TFY method did not change with pressure. These results indicate that the etching damage was restricted in the shallow region of less than a few nm from the surface. A change in the NEXAFS spectral shape of Kr or Xe plasma-etched samples was not observed even when measured by the TEY method. This result indicates that the surface damage in Kr or Xe plasma-etched samples was less pronounced than that in Ar plasma-etched samples.

First measurement of the polarisation asymmetry of a gamma-ray beam between 1.7 to 74 MeV with the HARPO TPC

Current γ-ray telescopes suffer from a gap in sensitivity in the energy range between 100 keV and 100 MeV, and no polarisation measurement has ever been done on cosmic sources above 1 MeV. Past and present e+e- pair telescopes are limited at lower energies by the multiple scattering of electrons in passive tungsten converter plates. This results in low angular resolution, and, consequently, a drop in sensitivity to point sources below 1 GeV. The polarisation information, which is carried by the azimuthal angle of the conversion plane, is lost for the same reasons. HARPO is an R&D program to characterise the operation of a gaseous detector (a Time Projection Chamber or TPC) as a high angular-resolution and sensitivity telescope and polarimeter for γ-rays from cosmic sources. It represents a first step towards a future space instrument in the MeV-GeV range. We built and characterised a 30cm cubic demonstrator [SPIE 91441M], and put it in a polarised γ-ray beam at the NewSUBARU accelerator in Japan. Data were taken at photon energies from 1.74MeV to 74MeV and with different polarisation configurations. We describe the experimental setup in beam. We then describe the software we developed to reconstruct the photon conversion events, with special focus on low energies. We also describe the thorough simulation of the detector used to compare results. Finally we will present the performance of the detector as extracted from this analysis and preliminary measurements of the polarisation asymmetry. This beam-test qualification of a gas TPC prototype in a γ-ray beam could open the way to high-performance -ray astronomy and polarimetry in the MeV-GeV energy range in the near future.

Investigation of analyzing depth of N-K absorption spectra measured using TEY and TFY methods

In order to investigate the analyzing depth of N-K absorption spectra measured using the TEY and TFY methods, the spectra of Si3N4 crystals covered with Cr thin films were measured. The thicknesses of Cr films were 1 – 100 nm. The N-K absorption intensity of Si3N4 measured using the TEY method was observed at Cr thickness of less than 3 nm, while that measured with the TFY method was observed for more than 100 nm. The decrease in the TFY peak intensity was explained by the transmittance of Cr film. The results were applied to estimate the damage depth of plasma-etched GaN crystal. The deformation of the crystal structure in n-GaN etched by Ar plasma was restricted in a shallow region of less than 3 nm from the surface.

Damage Analysis of n-GaN Crystal Etched with He and N2 Plasmas

To understand the details of etching-induced damage on a GaN surface, n-GaN crystals were plasma-etched with He and N2 gases. The etched surfaces were analyzed by X-ray photoelectron spectroscopy (XPS) and soft X-ray absorption spectroscopy (XAS) methods. The composition of the surface etched with He plasma changed significantly to being Ga-rich with the N/Ga ratio nearly equaling 0.4–0.5. The ratio of the surface etched with N2 plasma was about 0.6. The shape of the near-edge X-ray absorption fine structure (NEXAFS) of the N-K edge deformed with increasing gas pressure and processing time. The deformation can be explained by the increase in the band widths of a number of peaks in the NEXAFS spectra owing to the increase in the degree of structural disorder in the crystal. The increase in band width for the surface etched with N2 plasma was larger than that for the surface etched with He plasma. The above results can be explained with the model of the elastic energy transfer ratio of He+ and N2+ ions incident on the solid surface.

Characteristics of TiO2 Thin Film Surfaces Treated by Helium and Air Dielectric Barrier Discharge Plasmas

The characteristics of TiO2 thin film surfaces treated with He and air dielectric barrier discharge (DBD) plasmas at different gas pressures are investigated. There is a difference between the two DBD plasma characteristics: for He-DBD, which is an atmospheric pressure glow discharge (APGD), the breakdown voltage and discharge current hardly change with increasing gas pressure, whereas for air-DBD, which is basically a filamentary discharge, they increase with increasing gas pressure. There is also a difference between the characteristics of TiO2 surfaces treated with the two DBDs. The surface roughness for He-DBD is lower than the roughness of the as-grown surface, whereas that for air-DBD is higher. The surface hydrophilicity for He-DBD is more enhanced than the hydrophilicity of the as-grown surface regardless of UV irradiation. The hydrophilicity for air-DBD is dependent on UV irradiation. It is more enhanced with UV irradiation; it is not improved adequately without UV irradiation.

Electronics for HARPO: Design, development and validation of electronics for a high performance polarised-Gamma-ray detector

We designed and built an experimental apparatus based on a time projection chamber, a novel scheme for high performance γ-ray astronomy and polarimetry in the γ → e+e- regime. This presentation focuses on the electronics aspect of the detector and, in particular, on the versatile dedicated trigger system that we have developed which allowed us to take data on beam with a high γ-conversion signal efficiency and a high rejection factor for single tracks and upstream conversion background events. Our scheme allows for the selective collection of γ conversions in a high-background-rate environment, such as that which is present in orbit, with a fine 3D imaging of the events and very low (in particular electronics) background, at a mild cost in terms of the number of electronics channels and therefore of electrical power consumption.

Etching damage and its recovery by soft X-ray irradiation observed in soft X-ray absorption spectra of TiO2 thin film

Damage characteristics of TiO2 thin films etched by N2 plasma were analyzed using soft X-ray absorption spectroscopy. Changes in the spectra at the Ti-L2,3 near-edge X-ray absorption fine structure (NEXAFS) resulting from etching damage were observed more in the bulk region rather than in the sample surface. The damaged spectra were recovered to the spectra of the as-grown sample by soft X-ray irradiation for 10 min. Moreover, the once-recovered spectrum of the irradiated sample was returned to the disordered shape by storage in dark place for 10 days. These results could have been caused by electron-related structural relaxation in appearance.

X-ray Absorption Studies on the Growth Process of Radio-Frequency-Magnetron-Sputtered Boron Nitride Films: Effects of Bias Voltage and Substrate Temperature

We have investigated the effects of bias voltage and substrate temperature on the growth process of RF-magnetron-sputtered BN films. When the negative bias voltage was higher than 80 V, films including the sp3-BN phase grew even if the substrate temperature was RT. When the substrate temperature and the negative bias voltage were 600 °C and >100 V, respectively, films containing more than 90% sp3-BN phase were grown. The growth process of films deposited at 150 and 600 °C under a constant bias voltage of -100 V was studied by Near edge X-ray absorption fine structure (NEXAFS) measurements. These films consisted of two layers: an sp2-BN phase underlayer and an sp3-BN phase top layer. The sp2-BN underlayer had a preferential orientation of the c-axis parallel to the film surface. The thickness of the sp2-BN underlayer decreased with increasing substrate temperature. High substrate temperatures assisted the nucleation and growth of the sp3-BN phase.