SMILE: MeV gamma-ray imaging-spectroscopy with well-defined point spread function using gaseous electron-tracking Compton camera

2. Electron-Tracking Compton Camera (ETCC)

T. Takemura, T. Tanimori, A. Takada, H. Kubo, T. Mizumoto, Y. Mizumura, T. Sawano1, K. Nakamura, Y. Matsuoka, S. Komura, T. Kishimoto, M. Oda, S. Miyamoto, Y. Nakamasu, K. Yoshikawa, K. Miuchi2, S. Kurosawa3

(Kyoto University, 1Kanazawa University, 2Kobe University, 3Tohoku University)

SMILE-chan

1. MeV gamma-ray astronomy

3. Balloon/satellite observation by ETCC

R. Diehl+, 2014, Science, 345, 1162

Universe in MeV gamma-ray

Nucleosynthesis

SNR (Radio-isotopes)

Galactic Plane (26Al, 60Fe, and annihilation )

Particle Acceleration

Jets in AGN, GRB (Synchrotron radiation and Inverse Compton)

SNR (π0-decay and Inverse Compton)

Early universe

GRB of first stars

Problems of current telescopesConventional Compton imaging and coded aperture imaging cannot obtain two directional angles of incident gamma-ray event by event. Therefore current telescopes cannot reject completely huge background created by interaction of cosmic-ray and a satellite body.

Additionally, the energy spectra of current telescopes are interfered by background gammas from sources in the outside of the FoV, because they have no well-defined PSF.

For MeV gamma-ray telescopes, the capability of rejection of background is important.

Prototype ETCC for Sub-MeV gamma ray Imaging Loaded-on-balloon Experiment (SMILE)

We plan SMILE project to certificate the performance of the ETCC as a telescope before the observation loading on a satellite. The target of SMILE is the observation of bright sources (Crab or annihilation line from Galactic center), and we have developed the prototype ETCC for establishment of a MeV gamma-ray telescope with well-defined PSF.

The ETCC is composed of a gaseous micro-time projection chamber (μ-TPC), which measures track and energy of electrons, and pixel scintillator arrays (PSAs), which detect absorption position and energy of scattered gamma-ray.

The information of these, especially electron track, provides us resolution of the arrival direction of gamma-ray as a point on the sky event by event.

ETCC = μ-TPC + PSAs

Gas Ar:C2H6 (90:10), 1 atm

Size 30x30x30 cm3

Energy resolution (FWHM) 22% (@ 20 keV)

Scintillator GSO:Ce

Pixel size 6x6x13 mm3

Energy resolution (FWHM) 10% (@ 662 keV)

Well-defined Point Spread Function (PSF)

The ETCC has two angular resolutions: Angular Resolution Measure (ARM) and Scatter Plane Deviation (SPD). A PSF depends on both resolutions and right figure shows PSFs of an ETCC and a conventional Compton camera at each ARM and SPD. A conventional Compton camera has only ARM, therefore the Half Power Radius (HPR) is limited at 40 °by overlaying of Compton circles (@662 keV, electron energy:3-200 keV). In contrast, ETCC has the sharp PSF and the HPR of our current ETCC represents 15°(@662 keV) at the ARM of 5°and the SPD of 100°. Because a PSF depends strongly on a SPD, improvement of SPD is necessary to improve a HPR. Because a SPD is limited by the effect of multiple scattering, we use gas, which has few effect of multiple scattering, as the electron tracker. Therefore we established the MeV gamma-ray telescope with well-defined PSF.

To SMILE-II+We plan to observe electron-positron annihilation line in the galactic center by one day balloon experiment. However this experiment require a HPR of 10°and an effective area of 1 cm2 (@ 511 keV), and prototype ETCC is not enough. Therefore we are developing the prototype ETCC to the SMILE-II+ ETCC to satisfy the requirements. PSAs of current ETCC are placed outside of the gas vessel of TPC. In contrast, PSAs of SMILE-II+ are placed inside of it. Due to this changes, SMILE-II+ ETCC measure high energy electrons, and high energy electron is impervious to multiple scattering, therefore we expect the improvement of HPR of SMILE-II+ ETCC.

Expected effective area and sensitivitiesWe estimate sensitivities of future ETCCs using simulated effective areas, background flux and HPRs. HPRs were assumed with a half ARM resolution. (the expected SPD resolution is 5-10° )

⇒ Significance detection of Crab by SMILE-II+ ETCC I is more than 6σ in live time 3 hours @1.2 MeV.

⇒The sensitivity of SMILE-satellite reach to 100 times higher that of COMTEL.

SMILE-III

Effective areaSMILE-II+ ETCC

26Al is a probe of thermal diffusion of the material in galaxy, however current observations by COMPTEL and INTEGRAL/SPI is not enough clear.

We estimate the observation of 26Al by SMILE-satellite, using extragalactic diffuse and galactic diffuse as background and assuming sky map observed by DIRBE/COBE as the distribution of 26Al. Right figure shows the expected observation of SMILE-satellite using each template. The simulated sky maps in 1.8 MeV have the different features with the different tracers.We can obtain a detail distribution of 26Al, which enable us to discuss the distribution of nucleosynthesis source in galaxy.

Effective area: ~200 cm2

PSF: ~2.3°Energy resolution:2.4% (@1.8 MeV FWHM)Energy cut: ±3σ(@1.8 MeV)

Gas vesselDetector sensitivities

Prototype ETCC

SMILE-II+

SMILE-III

SMILE-satellite

COMPTEL

EGRET

Fermi

SPI

IBIS

OSSE

Tlive = 106 secΔE = E3σ detection

SMILE-satellite

5 years

Etc. gamma-ray pulsar, solar flare, and π0-decay in accretion disk of black hole

Expected observation by future ETCCs

Laboratory tests of prototype ETCC

Significant maps of three 137Cs, which is obtained by conventional Compton camera method and by electron-tracking Compton camera method, using the prototype ETCC.

The simulated TOF distribution of COMPTEL

COMPTEL reject background using the measurement of time of flight between a Compton target and an absorber. However, the background rejection is not completely because it cannot reject all instrument background.

A image for whole energy band and energy spectra from one 137Cs with each angular radii. By applying a smaller radius cut, the contamination of air-scattered photons are suppressed dramatically.

Conventional Compton method

ETCC has a high sensitivity and a capability of rejection of background gammas from the outside of the FoV.

Schematic view and picture of prototype ETCC

We need a new telescope with well-defined point spread function (PSF) of ~1 °

G.Weidenspointner, et.al. (A&A, 2001)

Prototype ETCC

By these balloon experiments and observation of satellite using ETCC, we open the window of MeV gamma-ray astronomy

Detected 56Ni line by INTEGRAL from SN2014J

Ar 2atm

μ-TPC

gamma-ray

PSAs

μ-TPC

for 662 keV

Electron 3-200 keV

μ-TPC: gaseous electron tracker as Compton-scattering target

PSAs: gamma-ray absorber

Prototype ETCC

Half Power Radius (HPR) 15°(@ 662 keV)

ARM, SPD 5°, 100°(@ 662 keV)

Energy resolution (FWHM) 10% (@ 662 keV)

Effective area 0.15 cm2 (@ 662 keV)

FoV ~ 3 str

SMILE-II+

T. Tanimori, et.al. (ApJ, 2015)

T. Tanimori, et.al. (ApJ, 2015)

Electron-tracking Compton camera method