SciPost Phys. Proc. 12, 041 (2023)

ISAI: Investigating Solar Axion by Iron-57
Tomonori Ikeda1⋆ , Toshihiro Fujii2 , Takeshi Go Tsuru1 , Yuki Amano1 , Kazuho Kayama1 ,Masamune Matsuda1 , Hiromu Iwasaki1 , Mizuki Uenomachi1 , Kentaro Miuchi3 ,Yoshiyuki Onuki4 , Yoshizumi Inoue4 and Akimichi Taketa5
1 Graduate School of Science, Kyoto University, Kitashirakawa Oiwake-cho,
Sakyo-ku, Kyoto-shi, Kyoto, 606-8502, Japan
2 Graduate School of Science, Osaka Metropolitan University, Sugimoto-cho,Sumiyoshi-ku, Osaka-shi, Osaka, 558-8585, Japan
3 Department of Physics, Graduate School of Science, Kobe University,
Rokkodai-cho, Nada-ku, Kobe-shi, Hyogo, 657-8501, Japan
4 ICEPP, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
5 ERI, The University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-0032, Japan⋆ ikeda.tomonori.62h@st.kyoto-u.ac.jp

14th International Conference on Identification of Dark Matter
Vienna, Austria, 18-22 July 2022
doi:10.21468/SciPostPhysProc.12

Abstract
The existence of the axion is a unique solution for the strong CP problem, and theaxion is one of the most promising candidates of the dark matter. Investigating Solar
Axion by Iron-57 (ISAI) is being prepared as a complemented table-top experiment toconfirm the solar axion scenario. Probing an X-ray emission from the nuclear transitions
associated with the axion-nucleon coupling is a leading approach. ISAI searches forthe monochromatic 14.4 keV X-ray from the first excited state of 57 Fe using a state-ofthe-art pixelized silicon detector, dubbed XRPIX, under an extremely low-backgroundenvironment. We highlight scientific objectives, experimental design and the latest statusof ISAI.
Copyright T. Ikeda et al.
This work is licensed under the Creative Commons
Attribution 4.0 International License.
Published by the SciPost Foundation.

1

Received 02-10-2022
Accepted 24-04-2023
Check forupdatesPublished 04-07-2023
doi:10.21468/SciPostPhysProc.12.041

Introduction

Quantum chromodynamics requires extremely fine-tuning to explain the observed electric
dipole moment of the neutron [1, 2]. This problem is generally known as the strong CP problem. Peccei and Quinn proposed a global U(1) symmetry whose breaking gives a rise to a
new pseudoscalar boson called axion [3–5]. Moreover, the axion could be a part of the dark
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SciPost Phys. Proc. 12, 041 (2023)

matter [6]. Therefore, the axion search is one of the most important topics in the fields ofastrophysics and particle physics. The axion mass (ma ) and the coupling strength are proportional to the inverse of the breaking scale ( f a ). The axion mass can be written with thefollowing relation:pz 1.3 × 107
ma =
eV ,
(1)
1 + z f a /GeV
where z = mu /md is the mass ratio of the up and down quark. Since the higher breaking scalethan the electroweak scale makes the weakly interacting, axions become invisible. One of theinvisible axion models is the Kim-Shifman-Vainstein-Zakharov (KSVZ) axion, or called as the
hadronic axion, which does not have tree-level couplings to leptons and ordinary quarks. The
brightest source of such axion is the Sun, and the solar axion flux on Earth is calculated in
Ref [7]. Some experiments have been performed as the axion helioscope and searched forthe axion-photon and axion-electron coupling using the Primakoff effect or Compton interaction [8, 9]. The other novel method to independently verify the axion-nucleon couplingof the hadronic axion was proposed by Moriyama [10]. 57 Fe nuclei are thermally excited in
the Sun and can decay by emitting monochromatic axions corresponding to 14.4 keV. Emitted monochromatic axions can resonantly excite 57 Fe nuclei thanks to the doppler broadeningof the axion energy due to the thermal motion of the 57 Fe nuclei in the Sun. The expected
excitation rate per unit mass of 57 Fe is written as,2

R = 3.0 × 10

106 GeVfa

4

C 4 day−1 kg−1 ,(2)where C depends on the nuclear structure. The most stringent upper limit on the mass of theaxion was reported in 2007 to be ma < 216 eV using the Si PIN photodiodes [11]. In thispaper, we propose to search for the monochromatic 14.4 keV axion with a pixelized silicondetector called XRPIX. Furthermore, we describe the detector design, its performance, and thepredicted sensitivity to the mass of the axion.

2

Detector design and instruments

2.1

Investigating Solar Axion by Iron-57 (ISAI)

The detector design of the Investigating Solar Axion by Iron-57 (ISAI) is shown in Fig. 1. ISAI
consists of two detector modules. In one module, a 95.85% enriched 57 Fe foil, whose size is
3.2 cm × 3.2 cm × 40 µm, is set and used as the axion absorber. In the other module, a natural
Fe foil is set instead and used as a reference for the background study. The escape probability of 14.4 keV X-rays from such foil is 24.8%. Foils are covered by two XRPIX detectors,
whose detection area and the depletion thickness are 21.9 mm × 13.8 mm and 300 µm, respectively. The distance between the XRPIXs and the foil is 3 mm and its detection efficiency of the
14.4 keV X-rays is 14.5%. Oxygen-free copper plates with a thickness of 5 mm are arranged
around the XRPIXs as passive shieldings to protect from X- and beta-rays emitted from the leadshieldings outside. The lead shieldings, whose thickness is 50 mm are specified to reduce thepenetration of environment gamma rays. Anti-counters made by plastic scintillators are used
to measure the trajectory of cosmic-ray particles to reduce these backgrounds. These instruments are located inside a thermostatic chamber to keep the XRPIXs at a low temperature.

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SciPost Phys. Proc. 12, 041 (2023)

Plastic scintillator

Plastic scintillator

Thermostatic chamber

Thermostatic chamber

Lead shield

Oxygen free copper

Lead shield

XRPIX7

Foil holder (PEEK)

3 mm

Enriched 57Fe or natural Fe foil

Figure 1: Schematic drawing of the detector design of ISAI. The left picture shows theoverall detector, which consists of two sets of main detector modules surrounded byplastic scintillators as the anti-counter and a thermostatic chamber. The right picture
shows the details around the enriched 57 Fe or natural Fe foil.

2.2

XRPIX

XRPIX [12–18], which is one of the monolithic active pixel detectors based on the silicon-oninsulator (SOI) pixel technology, has been developed for a future X-ray astronomy mission,so-called FORCE [19], since 2011. We are planning to employ series number seven (XRPIX7)
which has the largest detection area. The XRPIX7 has 608 × 384 pixels and the pixel size
is 36 µm × 36 µm. Therefore, the XRPIX7 covers a detection area of 21.9 mm × 13.8 mm.
The picture of XRPIX7 is shown in Fig. 2(a). Each pixel is equipped with a CMOS circuit forthe signal processing, including comparators. The comparator gives the hit trigger, timing,and position. Furthermore, the XRPIX7 outputs the pulse heights around the hit pixels as theevent-driven readout mode. Therefore, the XRPIX7 offers a time resolution better than 10 µs,∼ 103−5 faster than scientific CCDs, and enables the use of an anti-coincidence technique toreduce backgrounds. Fig. 2(b) shows the observed spectrum of an 241 Am source with theevent-driven readout mode. The obtained energy resolution was 478 eV (FWHM) at 13.9 keVon the whole detection area. An X-ray image taken with a metal mask is shown in Fig. 2(c).
The conventional circuit board made of G10 glass epoxy included many backgroundsources of 238 U, 232 Th, and 40 K [20]. Hence, we developed a rigid flexible print circuit board
as a low-background readout system (Fig. 2(a)). The XRPIX7 bare chip is mounted on theflexible print circuit part without G10 glass epoxy and the rigid print circuit is part from thesensor. Only ceramic capacitors are implemented near the XRPIX7. Compared to the conventional circuit board, the background rate in the energy region of 14.4 keV is a factor of about
10−3 .

2.3

Plastic scintillator as anti-counter

An anti-counter system comprised of plastic scintillators is installed on the top and bottom of
the main detector as shown in Fig. 1. The cross-section of each scintillator is not a square,
but an isosceles triangle with a width of 5 cm and a height of 1 cm to enhance the position
accuracy by a signal ratio of two adjacent scintillators. There are 11 scintillators with a lengthof 30 cm in each layer. Two sets of two layers of X and Y directions are capable of measuring
the trajectory of an individual cosmic ray particle as a position-sensitive anti-counter. The

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SciPost Phys. Proc. 12, 041 (2023)

(a)

(b)

(c)

13.9 keVXRPIX721.9 mm

17.8 keV

13.8 mmFigure 2: (a) Photograph of XRPIX7. (b) Spectrum of 241 Am with event-driven readout mode. (c) X-ray image using the metal mask.
signals of scintillators are measured by SiPM (S13360-1375PE, Hamamatsu), and all of theirwaveforms are digitized by a dual 32ch 14-bit Flash ADC (ADS52J90, Texas Instrument).

3

Sensitivity

Using Eqs. (1) and (2), the axion mass is computed via the following equation:
ma = 5.55 ×R
1 day−1 kg−1

1/4

eV ,

(3)

where R is the 57 Fe de-excitation rate and rewritten as R = Nγ /(M ηε). We assumed the mass
ratio z and the nuclear structure C as 0.56 and 0.27, respectively. Here, Nγ is the count rate,
M is the 57 Fe mass, η is the emission probability of 14.4 keV X-rays, and ε is the detection
efficiency. The sensitivity is calculated by replacing Nγ with the background rate NBG . The
black solid line in Fig. 3 shows the expected sensitivity where we assumed that M = 127 mg,
η = 0.105, and ε = 0.145. Furthermore, NBG = 0.004 counts/day was estimated by theinternal background simulation associated with the energy resolution of 250 eV (FWHM) at
14.4 keV. A 95% confidence level upper limit on the axion mass of 145 eV corresponding to
Derbin’s result [21] is reachable in about four days. The dashed line indicates the expected
sensitivity of the future work with ten times the target mass.

4

Future prospects

We plan to start the commissioning run without the anti-counter and the 57 Fe foil at the endof 2022. Then, the primary performance of XRPIX7 and the ambient gamma-ray backgroundare verified over three months. Anti-counters are calibrated using muons of cosmic rays inparallel. In 2023, a scientific run will be carried out with the complete system of ISAI for one
year. While the current energy resolution of XRPIX7 reaches less than 250 eV, we will be able
to record the best upper limit in about one week. Moreover, we are developing the new XRPIX
of series number ten to overcome the energy resolution. The XRPIX7 will be replaced withXRPIX10 as soon as development is complete.
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Upper limit of axion mass (eV)SciPost Phys. Proc. 12, 041 (2023)

Namba (2007)
Derbin (2011)
102
ISAI

EISAI
1

10
102
Observation period (days)

103

Figure 3: Expected sensitivities. The solid and dashed lines show this work andfuture work of 10 times 57 Fe, respectively. The dotted and dash-dotted lines indicate
the constraints from Namba [11] and Derbin [21], respectively.

Acknowledgements
The authors thank S. Moriyama and T. Namba for productive and valuable discussions.
Funding information This work was partly supported by Japan Society for the promotion of
Science (JSPS), KAKENHI Grant-in-Aids for Scientific Research, Grant Number JP 21K18151,
22H04572, 21H05461, 21H04493 and JSPS Research Fellow, Grant Number JP 22J00064.

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