Ⅱ-4. Hadron Physics

RIKEN Accel. Prog. Rep. 52 (2019)

Isotope identification in nuclear emulsion plate for double-hypernulcear
study†
S. Kinbara,∗1 H. Ekawa,∗2 T. Fujita,∗3 S. Hayakawa,∗4 S. H. Hwang,∗5 Y. Ichikawa,∗3 K. Imamura,∗3 R. Murai,∗1M. K. Soe,∗1 M. Sweet,∗1 A. Takamine,∗3 A. M. M. Theint,∗1 H. Ueno,∗3 J. Yoshida,∗6 M. Yoshimoto,∗1 andK. Nakazawa∗1
Double hypernulcei such as double-Λ and Ξ hypernuclei provide information about Λ-Λ and Ξ-N interactions, which are important to understand the inner
structure of a neutron star. We have only two reliable
information from the NAGARA1,2) and KISO3,4) eventsin the E373 experiment. Other events are not uniquely
identiﬁed for the production and decay processes owing
to the remaining possible interpretations. The charge
identiﬁcation method is a key technique to understandthe multi-strangeness system.
To develop the method, we exposed eight nuclides (1 H,2H, 3 H, 3 He, 4 He, 7 Li, 9 Be, and 11 B) to a nuclear emulsion at RIPS. To study the halo eﬀect in the ﬁnite focal
depth of objective lens, the exposed angle θ perpendicular to the emulsion surface was set to be θ ≈ 25◦ , 50◦ ,and 75◦ .
To recognize the charge of the particle, we measured
the track width and estimated the track volume thatreﬂect the energy-losses. Raw images were taken by microscope with a 100× objective lens and an 8 bits CCD
camera. A focused image, as shown in Fig. 1(a), consists of the most focused layers of raw images. Figure 1(b) is illustrated according to the following equation, Bout = 255 × (Bin − Bmin )/(Bmax − Bmin ), where
Bin , Bmax , and Bmin are the brightness of each pixel,maximum, and minimum brightness in Fig. 1(a), respectively. Bin was enhanced to Bout . Figure 1(c) was illustrated by applying an algorithm called “diﬀerence ofGaussian” to Fig. 1(b). Then, a uniform backgroundimage was obtained by subtracting Fig. 1(b) from 1(c),as shown in Fig. 1(d). We measured the brightness
perpendicular to the track in the Fig. 1(d) and de-Fig. 1. Image processing method to obtain track width.
†∗1
∗2
∗3
∗4∗5
∗6Condensed from the article in Prog. Theor. Exp. Phys. 2019,
011H01 (2019).
Physics Department, Gifu University
Department of Physics, Kyoto University
RIKEN Nishina Center
Department of Physics, Osaka University
Korea Research Institute of Standards and Science
Japan Atomic Energy Agency

Fig. 2. Vrα of each nucleus to α particle as a function oflog(1/sinθ).

ﬁned the track width as a distance between two inﬂection points, which were obtained by applying a ﬁtting
function, f = a × tanh(gauss(x, µ, σ)), to the data inFig. 1(e).
The track width depends on the photographic development. As a calibration source, we used α particles
which have monochromatic kinetic energy emitted fromnatural isotope 212 Po in the emulsion. We obtained thecalibration function of v α (d) with 68 α-particles, wherev α is the average track volume of an α-particle at depth
d from the emulsion surface.
The widths were measured for every 1 µm cell along
the track. Because the depths of the measured cellschanged along the track, a volume ratio Vrα to normalize
the α-particle for each nucleus was ∑obtained ∑for measured
9090αV/volume Vi in ith cell as Vrα =ii=1i=1 v (di ),where di is the depth of the ith cell. We put 200 tracks
together at four areas for each nucleus in the exposed
emulsion and ﬁtted them according to log(1/sinθ), wherewe set log(1/sinθ) to be ξ, as angle dependence of thevolume ratio, Vrα (ξ), to the volume of the α particle, as
shown in Fig. 2.
To conﬁrm the utility of the above method, it wasapplied to one track of a Ξ hypernucleus candidate detected in the E373 experiment. The nucleus of this trackis known as 3 H or 6 He by kinematical analysis. TheirVrα can be estimated by the data points of 1 H and 4 He.
Thus, we concluded that the nucleus would be a 6 Henucleus with a likelihood ratio of 0.9.
References

1) H. Takahashi et al., Phys. Rev. Lett. 87, 212502 (2001).
2) J. K. Ahn et al., Phys. Rev. C 88, 014003 (2013).
3) K. Nakazawa et al., Prog. Theor. Exp. Phys. 2015,033D02 (2015).
4) E. Hiyama, K. Nakazawa, Annu. Rev. Nucl. Part. Sci. 68,
131 (2018).

- 92 -