Haruo Ohkuma

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.

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.

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.

Characterizations and Applications of the Insertion Device of the SPring‐8 Diagnostics Beamline II

An insertion device (ID05) of the SPring‐8 diagnostics beamline II (BL05SS) was characterized from the aspects of both the magnetic field performance and the spectral performance as a high K wiggler, which was confirmed to have the field performance with the rms phase error less than 2 degree. Meanwhile, the spectral performance of ID05 as an undulator with small K was also investigated by the energy spectrum measurements. The rms phase error less than 2degree leads us to apply the wiggler radiation on the higher harmonics to the beam diagnostics. We successfully demonstrated an application to the energy‐spread diagnostics of electron beam using the 19th harmonics.

HARPO, prototype of a gamma-ray polarimeter: Results of a polarised photon beam test between 1.7 and 74 MeV

Access to the photon polarisation in the 1-100 MeV energy range is a challenge for the next generation of space telescopes. The current telescopes in space are almost blind in this energy range, mainly due to the degradation of the angular resolution of e+e- pair and due to elastic scattering in the matter. Pair-conversion detector technologies as gaseous detectors are a promising alternative to the technologies based on tungsten-converter/thin-sensitive-layer stacks such as COS-B/EGRET/Fermi-LAT, firstly to improve the single-photon angular resolution and secondly for the polarisation information. The use of a time projection chamber (TPC) as a target and a tracking detector will improve by up to one order of magnitude the single-photon angular resolution (0.5° @ 100 MeV) with respect to the Fermi-LAT (5° @ 100 MeV), and by up to a factor of three with respect to what can be expected for silicon detectors (1.0-1.5° @ 100 MeV). With such a good angular resolution, a TPC can close the sensitivity gap at the level of 10^-6 MeV/cm².s) between 3 and 300 MeV despite having e lower sensitive mass. Furthermore, this good single-track angular resolution allows us to measure the linear polarisation fraction. The HARPO (Hermetic ARgon POlarimeter) detector prototype that we built is a high pressure (0.5-4 bar) low pile-up and low-diffusion gas detector. We will present the results of its high-statistics characterisation in the 1.7-74 MeV fully-polarised and non-polarised gamma-ray beam provided by the BL01 line at NewSUBARU. The excellent value of the polarisation asymmetry dilution factor that we measured opens the possibility of having a polarimeter in space working in the MeV-GeV energy range. In conclusion, we will present the design of a balloon-flight prototype ST3G (Self-Triggered Time projection chamber as a Gamma-ray Telescope) which is being developed. We will discuss its expected performance.

Evidence for the {theta}{sup +} in the {gamma}d{yields}K{sup +}K{sup -}pn reaction by detecting K{sup +}K{sup -} pairs

The {gamma}d{yields}K{sup +}K{sup -}pn reaction has been studied to search for the evidence of the {theta}{sup +} by detecting K{sup +}K{sup -} pairs at forward angles. The Fermi-motion-corrected nK{sup +} invariant mass distribution shows a narrow peak at 1.524{+-}0.002+0.003 GeV/c{sup 2}. The statistical significance of the peak calculated from a shape analysis is 5.1 {sigma}, and the differential cross section for the {gamma}n{yields}K{sup -}{theta}{sup +} reaction is estimated to be 12{+-}2 nb/sr in the photon energy range from 2.0 to 2.4 GeV in the LEPS angular range by assuming the isotropic production of the {theta}{sup +} in the {gamma}n center-of-mass system. The obtained results support the existence of the {theta}{sup +}.The γδ → K + K - pn reaction has been studied to search for the evidence of the Θ + by detecting K + K - pairs at forward angles. The Fermi-motion-corrected nK + invariant mass distribution shows a narrow peak at 1.524 ± 0.002 + 0.003 GeV/c 2 . The statistical significance of the peak calculated from a shape analysis is 5.1 σ, and the differential cross section for the γn → K - Θ + reaction is estimated to be 12 ± 2 nb/sr in the photon energy range from 2.0 to 2.4 GeV in the LEPS angular range by assuming the isotropic production of the Θ + in the yn center-of-mass system. The obtained results support the existence of the Θ + .