SMILE-2+ Balloon Experiment Overview

• SMILE-2+ is an innovative balloon experiment that utilizes the Electron-Tracking Compton Camera (ETCC) to fully reconstruct Compton events for high-sensitivity MeV gamma-ray imaging.
• The experiment achieved nearly 100% detection efficiency and an order-of-magnitude improvement in signal-to-noise ratio near 400 keV through precise tracking and advanced background rejection techniques.
• SMILE-2+ successfully detected gamma-ray emissions from key cosmic sources, such as the Galactic center and Crab Nebula, setting a new benchmark for future astrophysical observations.

The SMILE-2+ balloon experiment is a milestone in observational MeV gamma-ray astrophysics, enabling high-sensitivity, wide-field imaging spectroscopy of cosmic sources with unprecedented background rejection. Central to SMILE-2+ is the Electron-Tracking Compton Camera (ETCC), which for the first time allows full reconstruction of the Compton scattering process for each incident gamma ray. By tracking both the recoil electron and scattered photon, the ETCC provides point-like imaging—transcending the ring ambiguities of prior Compton telescopes—and delivers precise spectral measurements across a large swath of the sky. The project establishes a new observational paradigm, demonstrating robust detection of diffuse gamma-ray backgrounds, Galactic center emission, and discrete astrophysical sources, and achieves this with an efficacy that legitimatizes the ETCC concept for deployment in future balloon and satellite missions.

1. Experimental Objectives and Scientific Context

The SMILE-2+ experiment was designed to achieve two primary goals: (1) perform wide-field, high-sensitivity imaging-spectroscopic observations of MeV gamma rays, and (2) enable robust discrimination of astrophysical signals from dominant backgrounds in the MeV regime. SMILE-2+ addresses longstanding problems in MeV gamma-ray astronomy, notably the large statistical and systematic uncertainties that have limited previous missions (such as COMPTEL and INTEGRAL) in mapping Galactic diffuse and cosmic background MeV gamma-ray (CBMG) fluxes, as well as the 511 keV annihilation line from the Galactic center. By reliably measuring all Compton event parameters, including recoil electron track, energy, and time, SMILE-2+ provides new indirect constraints on light dark matter models and cosmic nucleosynthesis and delivers observational data with low systematics in one-day exposures (Tanimori, 2020, Takada et al., 2021).

2. Instrumentation: Electron Tracking Compton Camera (ETCC)

The core technological advance underpinning SMILE-2+ is the ETCC, coupling a gaseous time projection chamber (TPC) with surrounding pixelated scintillator arrays (PSAs). The TPC, with a 30 cm cube volume filled with an Ar-based gas mixture at 2 atm, records the complete three-dimensional track of recoil electrons created by Compton scattering. Readout is implemented via a micro pixel chamber (μ-PIC) and gas electron multiplier, achieving a readout pitch of 800 μm and energy resolution of ~45.9% for low-energy gamma rays (~43 keV), and with the support of 100 MHz-clocked FPGAs handling the fine-scale time structure (Mizumura et al., 2013, Mizumoto et al., 2015, Takada et al., 2021).

Each event’s scattered gamma ray is absorbed and positionally resolved in the PSAs, constructed from GSO:Ce crystals (6 mm × 6 mm × 13 mm pixels, with ~11% FWHM energy resolution at 662 keV), providing coincident energy and position measurement of the scattered photon (Mizumura et al., 2013). This arrangement enables full reconstruction of the incident gamma ray's direction, energy, and arrival time by combining TPC and PSA data to form bijective mapping on the celestial sphere.

3. Data Acquisition System and Real-Time Processing

The increased detector volume and segmentation (30 cm³ TPC, 108 PSAs) required a scalable, high-throughput data acquisition (DAQ) system. The DAQ harnesses parallel data flow, ASIC-based front-end electronics (FE2009bal: 16 channels/chip, 0.6 V/pC preamplifier gain), and per-channel time-over-threshold (TOT) encoding, enabling a detection efficiency for recoil electron tracks improved from ~10% in SMILE-I to nearly 100% in SMILE-2+ (Mizumoto et al., 2015).

Data rates of up to 1 kHz (tested in laboratory) and sustained rates of 100–200 Hz in flight are handled with multievent buffering, FPGA logic, and optimized trigger control units (TCUs) which guarantee low dead time (>70% live-time at 100 Hz). The system triggers on valid TPC-PSA coincidences (with an 8 μs programmable delay for TPC drift) and efficiently filters spurious, out-of-time, or otherwise invalid events (Mizumoto et al., 2015).

4. Observation Strategy, Imaging Methodology, and Results

A single-day balloon flight over Australia (April 2018, altitudes ~37–39 km) enabled SMILE-2+ to observe MeV gamma rays from key sky regions: Galactic center, Crab nebula, southern sky CBMG field (~3/5 of all sky). The ETCC reconstructs Compton events using both geometrical and kinematic relations:

• Geometric angle: $$\cos \alpha_{\mathrm{geo}} = \mathbf{g} \cdot \mathbf{e}$$
• Kinematic angle: $$\cos \alpha_{\mathrm{kin}} = 1 - (m_e c^2/E_\gamma) \sqrt{K_e/(K_e + 2 m_e c^2)}$$

where $\mathbf{g}$ and $\mathbf{e}$ are unit vectors for scattered gamma and recoil electron directions, $E_\gamma$ is measured gamma energy, $K_e$ is electron kinetic energy, and $m_e$ the electron mass.

Selection on $|\Delta\cos\alpha| < 0.05$ (with $$\Delta\cos\alpha = \cos\alpha_{\mathrm{geo}} - \cos\alpha_{\mathrm{kin}}$$) yields nearly ideal background rejection: direct Compton events cluster at $\Delta\cos\alpha\approx 0$, while backgrounds (multi-site, non-electron, or accidentals) populate a broader distribution (Ikeda et al., 2023, Matsuoka et al., 2014). This selection delivers an S/N improvement of about an order of magnitude near 400 keV (Ikeda et al., 2023).

Observationally, SMILE-2+ detected:

• A $\sim$30% enhancement in gamma-ray flux during Galactic center transits, matching the CBMG/GDMG ratio (Tanimori, 2020).
• Clear identification of 511 keV line emission ($\gtrsim$5–10σ near the Galactic center) with very low systematic error (residual backgrounds of only a few 10% of total CBMG events) (Tanimori, 2020).
• Crab Nebula detection at 4.0σ significance in the 0.15–2.1 MeV energy range in only 5.1 h of live time (Takada et al., 2021).
• Light curves and spectral maps consistent with cosmic gamma-ray source expectations and pre-launch backgrounds, providing robust validation of both detection sensitivity and modeling (Takada et al., 2021).

5. Background Characterization and Rejection Techniques

Comprehensive Monte Carlo modeling and empirical data identified three dominant background classes:

Energy Regime	Dominant Background Sources	ETCC Mitigation Techniques
<400 keV	Atmospheric gamma rays, cosmic-ray/secondary particles, accidentals	Compton-kinematics test, dE/dx particle discrimination, event topology selection
>400 keV	Residual unidentified (non-Compton) component	Stringent kinematic selection, further material optimization
All	Accidental coincidences (TPC/PSA), multi-site/scatter	TCU gating, event time matching, anti-coincidence candidate development

Distinguishing direct Compton events (full arc reconstruction, spatial/temporal correlation) from backgrounds (double Compton, scatter in surrounding material, accidental TPC-PSA coincidences) was critical (Ikeda et al., 2023). The Compton-kinematics selection ($|\Delta\cos\alpha| < 0.05$), dE/dx-based electron/proton discrimination in the TPC, and optimized fiducial volume constraints proved central. Atmospheric gamma-ray and secondary cosmic-ray backgrounds dominate below 400 keV; above 400 keV unresolved backgrounds—potentially including untagged scatter or internal contamination—were identified, suggesting directions for improved instrument design.

6. Scientific Impact and Prospects for Next-Generation MeV Gamma-Ray Telescopes

SMILE-2+ validates the ETCC as a high-resolution gamma-ray imager capable of bijective imaging spectroscopy: reconstructed event-by-event PSFs allow conventional ON–OFF background subtraction, spectral extraction, and imaging—approaches not previously available in the MeV band (Takada et al., 2021). This methodology enables the detection of point and diffuse sources (including the 511 keV line and continuum excess), mapping of the CBMG, and constraints on MeV emission processes tied to nucleosynthesis, compact-object physics, and dark matter (Tanimori, 2020).

The demonstration of robust background rejection with minimal systematic bias over short exposures paves the way for enhanced ETCC instruments (e.g., SMILE-3), which will target increased effective area (~10 cm²), improved angular resolution (few degrees), broadened FOV, and integration with advanced event reconstruction (including machine learning) (Takada et al., 2021). The performance seen in flight—quantitatively matching ground calibration and simulation—shows that next-generation balloon or satellite experiments can surpass the legacy sensitivity of COMPTEL, yielding high-statistics, low-background MeV gamma-ray spectra and images crucial for multi-messenger astrophysics.

7. Conclusion

The SMILE-2+ balloon experiment represents a technological and scientific advance for MeV gamma-ray astronomy, demonstrating the ETCC’s capability for full Compton event reconstruction, high-precision source localization, and effective background suppression. Its successful detection of cosmic gamma-ray sources (Galactic center, Crab nebula) and extraction of diffuse sky emission, achieved within severe background environments and with low systematic uncertainty, establishes a blueprint for future wide-field, high-resolution MeV gamma-ray observatories (Tanimori, 2020, Takada et al., 2021, Ikeda et al., 2023). The proven reliability, scalability, and diagnostic acuity of the ETCC concept mark the SMILE-2+ experiment as a key reference in the evolution of observational gamma-ray astrophysics in the MeV domain.