Ⅱ-9. Instrumentation

RIKEN Accel. Prog. Rep. 56 (2023)
RIKEN
Accel. Prog. Rep. 56 (2023)

Highly-eﬃcient γ-ray linear polarization measurementby multi-layer CdTe Compton Camera
S. Go,∗1 Y. Tsuzuki,∗2,∗3,∗1 Y. Ichikawa,4,∗1 T. Ikeda,∗1 N. Imai,∗5 K. Imamura,∗1 M. Niikura,∗1D. Nishimura,∗6,∗1 R. Mizuno,∗2,∗1 S. Takeda,∗3,∗1 H. Ueno,∗1 S. Watanabe,∗7,∗3,∗1 T. Y. Saito,∗5S. Shimoura,∗5,∗1 S. Sugawara,∗6,∗1 A. Takamine,∗1 H. Yoneda,∗1 and T. Takahashi∗2,∗3,∗1

F = ϵQ2 ,(1)where ϵ is the eﬃciency of the detector to measure
Compton-scattering events and Q is the polarization
sensitivity of the polarimeter. The development of theγ-ray polarimeters which has a suﬃcient ﬁgure of meritis important for the linear polarization measurements
on rare isotopes.
Therefore, we implement a multi-layer semiconductor Compton Camera,2,3) which has been developed
originally for the X- and gamma-ray observatory inspace, for the linear polarization measurement. Thedetector consists of 20-layer pixelated cadmium telluride. The thickness of each layer is 0.75 mm andthe eﬀective area of 51.2 × 51.2 mm2 is pixelated into
16 × 16. The detector measured the intensity distribution of Compton-scattering events of γ ray as a function of azimuth angle.
To demonstrate the capability of measuring the linear polarization for sub-MeV γ-ray, the experiment
was conducted at the RIKEN Pelletron facility in May
2022. The performance as a polarimeter was investigated with polarized γ-ray.4) The iron target foil of
10 µm was irradiated with the proton beam energy
around 3.1 MeV. The ﬁrst excited 2+ state of 56 Fe
was populated by the proton inelastic reactions. Theazimuth angle distribution of 847-keV γ ray were measured by the detector placed at 90◦ degrees to thebeam direction. The segmented Ge detector (CNSGRAPE5) ) with a lead collimator was also placed at90◦ to the beam direction to measure the axial asymmetry of scattered radiation as described in Ref. 6).
The intensity distribution of Compton scattering
∗1
∗2
∗3
∗4
∗5
∗6
∗7

RIKEN Nishina Center
Department of Physics, University of Tokyo
Kavli Institute for the Physics and Mathematics of the Universe, Institutes for Advanced Study
Department of Physics, Kyushu University
Center for Nuclear Study, University of Tokyo
Department of Physics, Tokyo City University
Institute of Space and Astronautical Science, JapanAerospace Exploration Agency- 106 -events as a function of azimuth angle obtained by theCompton Camera is shown in Fig. 1. The curve wasobtained by calculating the ratio of the experimental
and simulated azimuth angle intensity for polarized
and unpolarized photons, respectively. The known
electric character of the γ-ray was successfully conﬁrmed. Further analysis to quantify the polarization
sensitivity and ﬁgure of merit of the polarimeter is inprogress.

1.5
1.4
1.3
1.2

Ratio

Linear-polarization measurements of photons are
typically applied to determine the electromagnetic
character of gamma-ray transitions. However, themeasurement has not been applied in the wide range of
nuclei because it needs to measured the azimuth andpolar scattering angle of Compton Scattering events.
As for the performance for the polarimeter, a ﬁgure of
merit was proposed as:1)

1.1
1
0.9
0.8
0.7
0.6
0.5

3

2

10

1

2

3

Azimuth angle (rad)Fig. 1. Intensity distribution of Compton scattering eventsof the 847-keV γ-ray as a function of azimuth angle.

References
1) B. A. Logan et al., Nucl. Instrum. Methods Phys. Res.
108, 603 (1973).
2) T. Takahashi, Nucl. Phys. B (Proc. Suppl.) 134, 191(2004).
3) S. Watanabe et al., Nucl. Instrum. Methods Phys. Res.
A 505, 118 (2003).
4) J. Simpson et al., Nucl. Instrum. Methods Phys. Res.
204, 463 (1983).
5) S. Shimoura et al., Nucl. Instrum. Methods Phys. Res.
A 525, 188 (2004).
6) P. A. Butler et al., Nucl. Instrum. Methods Phys. Res.
108, 497 (1973).