Yoshitaka Mizumura

JAXA operates scientific balloon campaigns, aiming at obtaining scientific results through safe and reliable balloon flights. The development of the prototype of the flight prediction and control system began more than 20 years ago. It has become a mature system through many years of operation and functional enhancement and modification. The main functions of the system are implemented by a database system, which has been used for at least 82 heavy balloon experiments and 102 light balloon experiments since 2007. The applications used in client computers include more than 180 graphical user interface panels. The system is designed to incorporate redundancy for availability during balloon flight operations. Although various constraints face balloon flights, such as scientific requirements, flight safety, and severe high-altitude wind conditions, the flight prediction and control system enable us to construct a detailed flight plan and to control the flight based on predictions. In addition to the report of the system, flight prediction is explained with an example of boomerang flight control planning.

MeV gamma-rays provide a unique window for the direct measurement of line emissions from radioisotopes, but observations have made little significant progress since COMPTEL on board the Compton Gamma-ray Observatory (CGRO). To observe celestial objects in this band, we are developing an electron-tracking Compton camera (ETCC) that realizes both bijective imaging spectroscopy and efficient background reduction gleaned from the recoil-electron track information. The energy spectrum of the observation target can then be obtained by a simple ON–OFF method using a correctly defined point-spread function on the celestial sphere. The performance of celestial object observations was validated on the second balloon SMILE-2+ , on which an ETCC with a gaseous electron tracker was installed that had a volume of 30 × 30 × 30 cm³. Gamma-rays from the Crab Nebula were detected with a significance of 4.0σ in the energy range 0.15–2.1 MeV with a live time of 5.1 hr, as expected before launch. Additionally, the light curve clarified an enhancement of gamma-ray events generated in the Galactic center region, indicating that a significant proportion of the final remaining events are cosmic gamma-rays. Independently, the observed intensity and time variation were consistent with the prelaunch estimates except in the Galactic center region. The estimates were based on the total background of extragalactic diffuse, atmospheric, and instrumental gamma-rays after accounting for the variations in the atmospheric depth and rigidity during the level flight. The Crab results and light curve strongly support our understanding of both the detection sensitivity and the background in real observations. This work promises significant advances in MeV gamma-ray astronomy.

For future large-scale structures with high accuracy, we have researched and developed an alignment monitor system. In particular, we have focused on the measurement of the relative positions of both ends of a one-dimensionally long structure such as the support structure of an X-ray telescope. The alignment monitor system consists of a laser source, a beam splitter, a retroreflector, and a PSD (Position Sensitive Device). The laser source and retroreflector are attached to the reference and target for which relative displacement is to be measured. The developed alignment monitor system was used for measurements in ground tests of a large astronomical observation satellite, and its usefulness was confirmed. To apply this alignment system to astronomical observations in space and the stratosphere, it is necessary to verify the compatibility with each environment. Therefore, the DemonstRation Experiment of Alignment Monitor (DREAM) were conducted on July 9, 2021 to evaluate the environmental compatibility of the alignment monitor system in the stratosphere for the future astronomical observation system of balloon experiments. The maximum altitude was 29 km, and the flight was for 2 hours 54 minutes. Due to the upper limit of the size of the gondola for balloon experiment, the laser source and the retroreflector were installed at a distance of 1m. Through the balloon experiment, it was confirmed that this alignment monitor system functioned normally in the stratosphere. In this experiment, artificial periodic thermal deformation of the structure was adopted to give a predetermined displacement to the measurement target (retroreflector) in the stratosphere. The difference between the displacement estimated from the measured temperatures and the displacement measured by this alignment monitor system was 0.4 μm RMS or less.

<title>Abstract</title> The Electron-Tracking Compton Camera (ETCC), which is a complete Compton camera that tracks Compton scattering electrons with a gas micro time projection chamber, is expected to open up MeV gamma-ray astronomy. The technical challenge for achieving several degrees of the point-spread function is precise determination of the electron recoil direction and the scattering position from track images. We attempted to reconstruct these parameters using convolutional neural networks. Two network models were designed to predict the recoil direction and the scattering position. These models marked 41$^\circ$ of angular resolution and 2.1 mm of position resolution for 75 keV electron simulation data in argon-based gas at 2 atm pressure. In addition, the point-spread function of the ETCC was improved to 15$^\circ$ from 22$^\circ$ for experimental data from a 662 keV gamma-ray source. The performance greatly surpassed that using traditional analysis.