Bull. Soc. Photogr. Imag. Japan. (2020) Vol. 30 No. 2: 22–25

LetterObservation of Double-strangeness Nuclei using Nuclear-emulsion Technology
Aung Nay Lin Nyaw1 *, Hiroyuki EKAWA2, Manami FUJITA3, Shuhei HAYAKAWA3, Ayumi KASAGI1, 2, PhyoMyat Lin1, Kazuma NAKAZAWA1, 4, Junya YOSHIDA2, 5, Masahiro YOSHIMOTO4

Abstract: e J-PARC E07 experiment aimed to systematically detect double-strangeness nuclei with emulsion technologies. Thirty-three events wereTh
detected within 2 years by following of Ξ - tracks. One event of D001 among them was examined to identify the production and decay processes by energy and momentum conservation obtained from the track lengths and emission angles of daughter particles. The event was determined to be Λ1 Λ0 Be (8Be and two Λ particles) as the most likely case assuming Ξ - capture in the atomic 3D state of 14N. The binding energyof the two Λ hyperons and the core nucleus of this double-Λ hypernucleus was 15.22 ± 2.78 MeV, and the Λ-Λ interaction strength was 1.80
± 2.78 MeV.

Key words:

Double hypernuclei, Double strangeness, Binding energy, J-PARC E07, Nuclear emulsion

Double hypernuclei (DH) with two units of strangeness (S) -suchas double-Λ hypernuclei (DLH) or Ξ hypernuclei-have been studied experimentally with Λ and Ξ hyperons having one and two unitsof S, respectively for more than 50 years 1 -9) . In the case of DLH, wecan analyze the binding energy (BΛΛ) between two Λ hyperons andthe core nucleus, as well as the interaction strength (ΔBΛΛ) of the twoΛ hyperons with each other. The BΛΛ and ΔBΛΛ gives us an information about Λ-Λ and Ξ-N (nucleon) interactions, and ΛΛ - ΞN mixing effect. This information is important for understanding not only
specific issues in nuclear physics but also the formation of neutronstars. To measure BΛΛ and ΔBΛΛ is equal to measure the mass of DH,
whose mass energy can be obtained from the track ranges of daughter nuclei by the decay of DH. Nuclear emulsion is the best way toanalyse DH, as it can detect very rare, low-energy, or short-lived
particles with sub-μm spatial resolution. This article introduces nuclear-emulsion technology and the nuclide identification method of
DH events analysed by using an optical microscope with an example
of D001 event.
In the previous experiment of KEK E373 (E373), we detected 9
DH 10) . Among these, a double-Λ hypernuclear event named
NAGARA was the first case uniquely identified as ΛΛ6He 3) 5) . From
the NAGARA event, ΔBΛΛ was measured to be 0.67 ± 0.17 MeV,
which means Λ-Λ interaction is weakly attractive. In order to know
the Λ-Λ interaction depending on the differences of core nuclei, several DLH events were necessary to be identified uniquely. The mostrecent experiment of J-PARC E07 (E07), which aimed at detecting
about 100 DH events using approximately 10 times as much data as

E373. One event in the 14 DLH events, named MINO 8) , was likely interpreted as 1Λ1ΛBe with ΔBΛΛ of 1.87 ± 0.37 MeV, which is notconsistent with that of the NAGARA event. The other DLH events
are now under analysis.
The experimental process at both experiments includes 1) emulsion sheet making, 2) beam exposure, 3) photographic development
and 4) detection and analyses of DH, and that of E07 is as follows.
We produced dedicated photographic-emulsion sheets from nuclear
emulsion gel for the use of this E07 experiment. We prepared 2.1
tonnes of the emulsion gel (Fuji: NU-3GIFL), which was threetimes as much as the amount used in E373. The gel of 2,930 g ismelted in a hot water bath of 40 °C with plasticizer of 164 mL (181g). The melted emulsion was poured into an area of 71 × 70 cm2
framed by acrylic square bars on a polystyrene (PS) film support of
40 μm-thick, which was spread on an acrylic plate on a level block
made of stone. Since the fringe of the emulsion gel undergoes heavy
distortion in the drying process, dummy gel made of gelatin andplasticizer was applied around the layer of emulsion gel in a 76 ×
76 cm2 region after the emulsion gel was set with cool air (20 °C)
from an air conditioner. The gel layers on the acrylic plate were installed in the drying cabinet, and dried at 30 °C and 75% relative
humidity (R.H.) for 2 days, then we got 450 μm-thick emulsion
sheets (thick sheet). On the third day after coating, we coated thesurface of emulsion layer with 0.3% gelatin solution to prevent the
precipitation of silver on a photographic development process. On
the fourth day, we repeated the above processes on the backside of
the sheet and finally dried at 25 °C and 60% R.H. for one day. The

Received 1st December, 2020; Accepted 20th December, 2020; Published 21st December, 2020.
1 Graduate School of Engineering, Gifu University, Gifu 501-1193, Japan
2 High Energy Nuclear Physics Laboratory, Cluster for Pioneering Research, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
3 Advanced Science Research Center, Japan Atomic Energy Agency, Tokai 319-1195, Japan
4 Faculty of Education, Gifu University, Gifu 501-1193, Japan
5 Department of Physics, Tohoku University, Sendai 980-8578, Japan* mgmgnyaw@gmail.com

Aung Nay Lin Nyaw, Hiroyuki Ekawa, Manami Fujita, Shuhei Hayakawa, Ayumi Kasagi, Phyo Myat Lin, Kazuma Nakazawa, Junya Yoshida, Masahiro Yoshimoto

EmulsionStack

1.8 GeV/c
K−

K+

A
ΛΛ Z

Ξ−

A′ ′
ΛZ

X ray

Fig 1. Experimental setup of the E07 experiment.
Table 1 Experimental condition in J-PARC E07

Beam line

K1.8

K - beam quality [%]K - beam momentum

80-85

Expected numbers of Ξ stopΞ tracking detector--

1.80 GeV/c
~104

SSD

KURUMA Magnet

Diamond
TargetGe (Hyperball-X)

SSD

SSD

large dried sheet with a thickness of nearly 1 mm was cut into 35.0
× 34.5 cm2. One manufacturing cycle produced 144 emulsion sheetsof this size. A thin sheet of a PS film of 180 μm-thick with 100 μmthick emulsion layers on either side was produced in the same way as
described above. Both sheets have a thickness accuracy of 5%. The
numbers of prepared thick and thin sheets were 1,298 and 236, respectively.
To avoid the recording of tracks in the sheet from cosmic radiation, all the sheets were kept in an underground laboratory of Kamioka Observatory (University of Tokyo) before the beam exposure.
Because our beam times were postponed to 3 years (20% emulsion)and 4 years (80% emulsion) after the production of sheets, the sheets
were stored in a box of 40 × 40 × 150 cm3 dimensions at 17 ± 1 °C
shielded by lead blocks of 10 cm thick to prevent Compton electrons
scattered by γ-rays in the Kamioka mine. By keeping the sheets in
Kamioka for 3 years, the record of cosmic ray and Compton electron
were respectively suppressed to 6.5% and 20% from the ones kept ina refrigerator of Gifu University for 2 years. Moreover, we conducted
the forced fading 11) just before beam exposure to erase the recorded
latent images by keeping all the sheets for 100 h at 25 °C and 90%
R.H.
Figure 1 presents the setup for the beam exposure of E07. We irradiated the diamond target with 1.13×1011 K - particles of momentum 1.8 GeV/c to produce Ξ - particles for DH measurement. In theK- reaction with a proton (p), we tagged K+ as a signal for Ξ - production with a magnet spectrometer (KURAMA), in which the reaction of K -+ p → K++ Ξ - proceeded. During tagging, we recorded theΞ - signal in the silicon-strip detectors (SSD) to reconstruct the Ξ tracks in three dimensions. The stacked emulsion sheets were composed of 11 thick sheets sandwiched between 2 thin sheets, andequipped in the midst of 2 sets of SSD. All emulsion stacks were
irradiated with the beam at J-PARC for 2 months. Table 1 summarises the outlines of E07.

Observation of Double-strangeness Nuclei using Nuclear-emulsion Technology

23

Next, we conducted the photographic-development process tothe irradiated sheets to amplify latent images along the path ofcharged particles to silver grains visible with a microscope. We used
an amidol 12) developer. For the thin sheets, the temperature was
kept on 10 °C in the process from presoak through fixing. For the
thick sheets, we maintained all solutions in the process at 5 °C except the hot developer on 20 °C applied just after a cold (5 °C) developer. After washing of processed sheets, we soaked them into thesolution with 10% glycerin to soften the dried sheets. It was taken
for 8 months to develop all sheets.
After the photographic development, we searched for the Ξ tracks at the top emulsion sheet in the module and followed thosetracks to downstream direction sheet by sheet using the automatedmicroscope scanning system 13). When we arrived at the stopping
point of the Ξ -, we carefully observed the topology of the tracks with
an optical microscope. A topology of three vertices of the tracks is agood evidence for the formation and decay of DH.
By following of Ξ - tracks for 2 years, 33 DH events were detected,
and some of these events, such as the MINO 8) and IBUKI 9) events,
have been analysed and reported. Here, the newly detected D001
event is introduced as a new analysis of DLH. Our analysis consistsof measuring the mass of the DH. To achieve this, the production
and decay processes of DH were reconstructed by energy and momentum conservation obtained from the ranges and emission anglesof daughter particles.
In DLH, the values of BΛΛ and ΔBΛΛ are obtained from the mass
as follows:(1)BΛΛ(ΛΛAZ)＝M(A-2Z) + 2M(Λ) - M(ΛΛAZ),
A-1
A
A
ΔBΛΛ(ΛΛ Z)＝BΛΛ(ΛΛ Z) - BΛ( ΛZ),(2)where Z, A, and M are the charge, atomic mass number, and mass,
respectively. BΛ is the Λ-binding energy of a single hypernucleus of A -1ΛZ.
The track ranges were measured via image processing on a seriesof several hundred micrographs taken with an objective lens 100
times in a moving lens barrel with changing a focus depth of 0.1 μm
interval. The pixel size of the image corresponds to 0.143 μm on the
projected image. The kinetic energy of a charged particle stopping in
the emulsion layer can be obtained by summing the segmented
ranges using the following range energy relation14):m
R＝ ─2 ∙λ(β ) + m Z2/3 ∙ CZ(β /Z),(3)zWhere R and m are the range and the mass ratio of particle forming the track to proton mass. λ(β) is the range of a proton at a velocity of βc, and CZ is an empirical function that compensates an extension of the range of positive-charged particles caused by absorptionsof electrons.
Because the thickness of the sheet was reduced via the photographic development process, the range-energy relation must be calibrated to obtain the original track ranges before shrinking. For thisprocess, nearly 140 tracks of α-particle in the emulsion sheet, inwhich the D001 event was detected, were used to obtain the shrinkage factor (SF ). The α-particles from the decay of 212Po, which originated from natural isotopes of the Thorium series in the emulsion
included as impurities, have a monochromatic energy of 8.784 MeV.
The range of charged particles also depends upon the emulsion den-24

Bull. Soc. Photogr. Imag. Japan. Vol. 30 No. 2 (2020)

Fig 2. (a) Error of the α-range distribution as a function of SF. (b) Distribution of α range for the assigned
SF of 1.881 ± 0.012.

sity. Since our emulsion has a different density from that of the standard emulsion (Ilford G5), the following equation (4) was applied tocorrect the range 6) .

#3
A
#1

rd-1
(ds-d) λ s
λs
─ ＝ ── +r ─── ∙ ─ (4)
rds-1
rds-1
λ
λw
Here, λs and λw are the ranges of proton tracks in the Ilford G5
emulsion and water, respectively 15) , and d and ds are the densities ofour emulsion and that of Ilford G5 (3.815 g/cm3), respectively. The
factor r (＝ΔV/ΔW ［cm3/g］) is the ratio of the increase of emulsion volume to the mass of water absorbed in the dried emulsion
sheet. The SF value was used as the variable in the range calculation
to obtain the standard deviation error (Stdev.) of the range distribution. The SF was assigned so as to give the smallest Stdev. as shownin Fig 2 (a). The mean range (R) in the sheet before development
was recalculated with the assigned SF by the distribution as presented in Fig 2 (b). The SF and R value of this region were 1.881 ± 0.012
and 49.46 ± 0.12 μm, respectively. Finally, the emulsion density wasdetermined to be 3.635 ± 0.014 g/cm3 with the use of the equation (4).
We analyzed the D001 event, and Figure 3 shows the results as asuperimposed image and a schematic drawing. Two charged particles (tracks #1 and #3) were emitted from the Ξ - particle at the
stopping point A, and track #1 decayed into two charged particles(tracks #2 and #4) at the point B. The endpoint C of track #2 showed
the tracks of two charged particles (tracks #5 and #6). Table 2 lists
the ranges and angles of the tracks.
The kinematic analysis described above was applied to D001. We#2

1.1 ± 0.2

33.7 ± 15.5

#45728.6 ± 5.9

108.9 ± 2.4#61762.6 ± 2.6#1

4.1 ± 0.3

#3

Fig 3. Superimposed image (left) and schematic drawing of the D001 event(right).

checked the kinematic consistency in all possible combinations of
the process for the production and decay of DH. In this study, themedium-heavy nuclei of 12C, 14N or 16O in the emulsion were examined for at-rest capture of a Ξ - particle 4) 10) . The possibility of a πparticle for tracks #4 and #6 was rejected because they did not show
the topology of π- stopping, such as the emission of auger electrons
and/or nuclear fragments. According to the topologies of vertices A,
B and C, at least one neutral particle will be emitted.
Table 3 lists the possible production modes. The nomination criterion was that the value of (ΔBΛΛ - B Ξ-) was between -5 and 5 MeV
within three standard deviations (3σ). In the case of atomic 3D capture of Ξ - particles by 12C, 14N or 16O, the B Ξ- values are 0.1 0.2 MeV, which is negligibly small with respect to 5 MeV. In theprevious E373, ΔBΛΛ has a maximal value of 2 MeV. Thus, we set the

Ξ + C
-

12

Ξ- + 14N

82.7 ± 2.8

153.3 ± 20.2

140.3 ± 2.1

1.8 ± 0.5

No. Ξ- captured

ϕ [°]53.6 ± 3.6

5.5 ± 0.4

#5

2.

#6

θ [°]

R [μm]

188.4 ± 2.7

Comment

Double-Λ hypernucleus
Single-Λ hypernucleus

2.2 ± 2.0

50.6 ± 6.7

187.4 ± 9.1

55.1 ± 1.9

91.5 ± 1.6

Table 3 Possible production modes at point A.
#1

→

8
ΛΛ

→

10
ΛΛ

Li

Be

#3

4
4

He
He

−

5 µm

Table 2 Ranges and angles of the tracks in D001 event.

Track

1.

#4

#5
B
#2 C

n
n

BΛΛ-B Ξ-[MeV]

ΔBΛΛ- BΞ-[MeV]

15.05 ± 2.78

1.63 ± 2.78

17.50 ± 1.46

6.34 ± 1.46

Aung Nay Lin Nyaw, Hiroyuki Ekawa, Manami Fujita, Shuhei Hayakawa, Ayumi Kasagi, Phyo Myat Lin, Kazuma Nakazawa, Junya Yoshida, Masahiro Yoshimoto

Table 4 Possible decay modes at point B.

No. DLH (#1)
1.

→

4
Λ

Li

→

4
Λ

Li

→

5
Λ

Be

→

6
Λ

Be

→

6
Λ

Be

→

7
Λ

Be

→

7
Λ

8
ΛΛ

3.

8
ΛΛ

4.

10
ΛΛ

5.

10
ΛΛ

6.

10
ΛΛ

7.

BΛΛ [MeV]

Li

8
ΛΛ

2.

10
ΛΛ

#2

25

He
He
He
Li

#4

ΔBΛΛ [MeV]<62.21 ± 2.53

<51.05 ± 2.53

p

3n<106.68 ± 0.90 <95.52 ± 0.90p2n<123.14 ± 1.36 <111.98 ± 1.36

d
p

Li

d

Li

d

Li

BΛΛ [MeV]

p

2n
3n
2n
2n
n

<104.29 ± 1.27 <90.64 ± 1.27
<62.17 ± 3.42

<48.52 ± 3.42

31.98 ± 6.65

18.33 ± 6.65

<106.91 ± 1.93 <93.26 ± 1.93

MINO
D001 ( ΛΛ8Li )DEMACHIYANAGI
10D001 ( ΛΛBe )10
NAGARA

5

6

7

the error is as large as one order of magnitude for the data from the
NAGARA and MINO event due to the neutron emission, the Λ-Λinteraction strength, ∆BΛΛ, agrees with these two events.
In the E07 experiment, 33 double-hypernuclear events were detected by following the Ξ - tracks for 2 years, meanwhile a new scanning method (called the overall-scanning method 18)) will be applied
to search for DH, and it is expected to detect nearly 1,000 DH
events within a few years.

We would like to thank the members of the J-PARC accelerator
and the Hadron facility for their support of the E07 experiment. We
are also very grateful to the staff of the Kamioka Observatory, ICRR,
for maintaining our emulsion sheets for over three years. This work
was supported by JSPS KAKENHI grants No. 23224006,
JP16H02180 and JP20H00155, and MEXT KAKENHI 15001001(Priority Area), 24105002, JP18H05403 and JP19H05417 (Innovative Area 2404).

E176

20

5

25

Acknowledgement● Past event
× D001

15

Observation of Double-strangeness Nuclei using Nuclear-emulsion Technology

8

9

10

11

12

13

14

References

A (ΛΛZ)Fig 4. The BΛΛ graph of observed DLH. The dots (●) and crosses (×) are
obtained by the previous events and this event, respectively. With the
exception of NAGARA, the past events show the most likely cases.

upper limit of the criterion to 5 MeV, and do not exclude repulsive
cases as a lower limit of -5 MeV. By this criterion, multiple-neutron
emissions were rejected.
At the vertex B in Fig 3, the decay modes with one-neutron emission are nominated, if the value of ΔBΛΛ are to be between -5 and5 MeV within 3σ. For the modes of multiple-neutron emission, we
don't set the upper limit (5 MeV) of ΔBΛΛ, because we can only
obtain the minimum momentum of neutrons due to the impossibility of specifying their emission direction. There are 42 possible casesof ΛΛLi, ΛΛBe, ΛΛB, and ΛΛC decays at the vertex B. Based on the
production modes in Table 3, the possible decay modes are restrictedto ΛΛ8Li and Λ1Λ0Be, and listed in Table 4.
Figure 4 presents a comparison of the BΛΛ to the mass number A(ΛΛZ) for the candidates of D001 according to the possible production process in Table 3 and the ones taken from the past analyses 4) 5) 8) .
Because the plotting point for the case No.1 in Table 3 is far fromthe line of BΛΛ to A (ΛΛZ), Λ1Λ0Be as the case No.2 seems most likely
to the nuclide in this D001 event.
Therefore, D001 can be interpreted as follows:(5)Ξ - + 14N → Λ1Λ0Be + 4He + n,(6) Λ6Li or Λ7Li (cases No.4 - 7 in Table 4).
6
7
where, ΛLi or ΛLi, in the equation (6) was understood as a decay
with multiple-neutron emissions.
Taking into account the BΞ - value of 0.17 MeV for the atomic 3D
orbit of the 14N + Ξ - system 16) , as well as the BΛ of 6.71 ± 0.04 MeV
of Λ9Be 17) at Λ1Λ0Be production, we find that the values of BΛΛ and ∆BΛΛare 15.22 ± 2.78 MeV and 1.80 ± 2.78 MeV, respectively. Although

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