核データニュース，No.131 (2022)

新博士誕生

My Doctoral Research and Life Experience in Japan
Kim Tuyet, Tran
The High Energy Accelerator Research Organization (KEK)
tuyet@post.kek.jp――――――――――――――――――――――――――――――――――――――

1. Accelerators: Risks and Usage
Accelerators are state-of-art machines leading the charged particles to relativistic speeds.
Technology used in development of these accelerators is continuously upgraded, and they
have been proved to provide benefits in fundamental research and applications for humanlife. On the other hand, beams in these accelerators can hit and irradiate the surrounding
material via nuclear reactions. Products from irradiation processes are harmful for humantissues, so radiation shielding, and radiation control must always be considered to designand operate accelerators according to the knowledge of radioactivities induced by nuclear
reactions and their secondary particle products.
For induced radioactivities, the construction material of the accelerators is irradiated via
nuclear reactions and transform nuclides of the material to radioactive isotopes. For
secondary particle production, various types of particles can be produced at differentenergies depending on the primary beam and the target material. Among secondary particles,
neutrons are difficult to be shielded and produce extra exposure by activating other materials.
To describe the phenomena properly in radiation control, we need enough basic data, hence,
many researches have been carried out including experiments and theory calculations.

2. Photoneutron production in accelerators
The electron accelerators with energies higher than tens of MeV can produce high energy
photons known as bremsstrahlung radiations due to electron beam dump. Throughphotonuclear reactions, those photons induce production of (photo)neutrons. In order toestimate the induced activity of these neutrons and perform shielding design, energy and- 36 -angular distributions of neutrons are important. Monte Carlo simulation, evaluated nuclear
data, and nuclear reaction models can be used to estimate the activity and shielding
calculation. However, there is lack of experimental data on photoneutron production tovalidate these calculation tools.
In 1950s, there were many experiments set up to measure the cross section, energy, andangular distributions of photoneutrons [1-10]. Those experiments used bremsstrahlung andquasi-monoenergetic photon sources, which came with many disadvantages such as a hugebackground, continuous spectrum, and low intensity. In recent years, with the laser Compton
scattering technology, one can study photoneutron production using a monoenergetic andlinearly polarized photon beam. Recently, a research group at High Energy Accelerator
Research Organization (KEK) has conducted a series of measurement of photoneutron
productions using the laser Compton scattering photon source at NewSUBARU, Japan [10].
According to their study, polarization of photons causes anisotropic photoneutron emission,
which is not accounted in current nuclear models when producing photoneutrons.

3. Experiment at NewSUBARU
The information on the energy and angular distributions of (γ, xn) reactions are extremely
essential in shielding calculation and accelerator design. Additionally, the experimental data
are an important resource to check the accuracy of Monte Carlo simulation tools, evaluatednuclear data, and nuclear reaction models. In this experiment, I measured the double
differential cross section (DDX) of (γ, xn) reactions on different targets using linearlypolarized monoenergetic photon beam at 14, 17, and 20 MeV [13]. Here, I only report theresults at 17 MeV. For results of 14 and 20 MeV, one can read the reference [13]. The photon
is generated at the BL-01, NewSUBARU, using the laser Comtpon scattering (LCS)
technology.
Figure 1 shows the overview of the BL-01 at NewSUBARU. The electrons with 982.4
MeV energy are circulated in a storage ring, which is operated in the single bunch mode. ANdYVO4 laser system generates photons with wavelength of 1.064 mm at 20 W. The laser
photons are guided to the storage ring, collide with the electron bunches, and arebackscattered as high-energy photons. These photons are collimated to Hutch 2, where I set
up the target and neutron detectors to detect photoneutrons.
Figure 2 is the experimental setup in Hutch 2. On the upstream, a plastic scintillator isplaced to monitor the photon beam. Most of the LCS photons penentrate through the plastic
scintillator, are incident on the target, and induce photonuclear reactions. Neutrons producedat the target are detected by 6 neutron detectors surrounding the target. The neutron- 37 -detectors are cylindrical with both diameter and length of 12.7 cm, filled with organic liquid
scintillator NE213 (Nuclear Enterprises, Ltd., UK, or equivalent). They are placed
horizontally at 30°, 60°, 90°, 120°, 150°, and vertically at 90°.

Fig. 1

Overview of laser Compton scattering set up at BL-01 NewSUBARU facility

NaI(Tl)NE213

Fig. 2

Experimental setup to detect photoneutrons- 38 -

On the downstream, a NaI(Tl) detector is placed and functioned as a beam absorber as well
as beam intensity monitor. I respectively investigated the DDX of photoneutrons produced
from different targets, including natPb, Au, natSn, natCu, natFe, and natTi. The target shape iscylindrical, diameter of 1 cm and thickness of 1-4 cm.
The data taking time is about 1-2 hours for every target, depending on the statistics. Theg-rays are the background in this measurement, and we need to carry out pulse shapediscrimination (PSD) measurement to distinguish neutrons and gammas. The energy of eachneutron is measured using the time-of-flight (TOF) method. A Versa Module Europa (VME)
data acquisition (DAQ) system is set up for collecting events from NE213 and plastic
scintillator detectors. It includes charge-to-digital converter (QDC) and time-to-digitalconverter (TDC) modules for PSD and TOF purposes. The collected data are analyzed
event-by-event using C++ and ROOT.
To distinguish between neutron and g-ray, for every signal, I calculated the ratio of the tailcharge to the total charge, which are measured by the QDC modules. Figure 3 shows thecharge ratio as a function of time-of-flight measured by the TDC. The gamma background
events are clearly discriminated from the neutrons. By selecting proper ranges of charge
ratio and TDC, neutrons can be selected.

Fig. 3

Neutron-gamma seperation- 39 -

In Figure 4, the TOF distributions of neutrons, -rays and both events are plotted in red,black, and blue, respectively. The resolution of the TOF system can be determined by thefull-width at half-maximum of the prompt-gamma peak, which is 0.95 ns. The small peak at
6450 TDC channel in the -ray TOF distribution indicates photons back-scattered on theNaI(Tl) detector. With the TOF and the flight path distance from the center of target to eachneutron detector, it is possible to calculate the energy of neutrons observed in NE213detectors. In this experiment, the traveling time difference between photon and neutron (tn-) is measured. Because neutrons arrive the NE213 detectors after photons, the TOF ofneutron is calculated as a sum of TOF of photon and the traveling time difference: TOFn =TOF + tn-Fig. 4

Time-of-flight spectra of gamma (black), neutron (red) and sum of them (blue)

4. Results and discussion
For every target, the neutron TOF distributions obtained in every NE213 detector are
normalized with solid angle, number of photons, the attenuation factors of photon andneutron in the target. Figure 5 shows the experimental results of the DDXs of the (γ, xn)
reactions of 17 MeV linearly polarized photons on six targets, and the DDXs are measured
at six different laboratory angles. The data points of DDXs are all above 2 MeV, which isabout the energy threshold. The energy resolution of neutron detectors from 2 MeV to 8- 40 -

MeV is ranging from 0.1 MeV to 0.6 MeV, which is equivalent to 5% to 7.5%.

Fig. 5Experimental results of DDXs of (γ, xn) reactions at six detector positions using 17

MeV linearly-polarlized photons on different targets- 41 -

From the distributions of the DDX, there are two components: the low-energy component
occupying from 2 MeV to around 4 MeV with the shape like the tail of Maxwellian
distribution; and the high-energy component occurs in the energy range above 4 MeV. The
high-energy component is also known as a direct and pre-equilibrium component [10]. Theenergy distribution obtained from Au(γ, xn) is consistent with the one reported in previous
study [10]. The yield of high-energy component decreases with the smaller target mass
number. For Pb, Au, Sn, Cu, and Fe targets, the maximum and minimum DDX values can befound at horizontal 90o and vertical 90o. However, for the Ti target, the minimum andmaximum values are observed at horizontal 90o and vertical 90o. For more details on thestudy, one can check references [13] and [14].

5. Japan: a wonderland for living and research
According to the report in 2017, Japan had 1711 accelerators, 75.6% of them were linearaccelerators, and most of linear accelerators were used in hospitals [11]. In my home country
Vietnam, the number of cancers is increasing. Therefore, the number of linear accelerators
used for radiation therapy is also increasing, such as 3 times increment from 2006 to 2013[12]. In future, our government plans to invest more accelerators for radiation therapy. The
information indicates the need of understanding nuclear models to reproduce photoneutron.
To be prepared for the future of radiation therapy in my country, I want to study thephenomenon of photoneutron production, and experimental research is a crucial step for thispurpose.
Before my Doctor course, I had two chances coming to Japan to attend short-term schools
in Osaka University in 2016 (SAKURA program) and KEK in 2017 (ATHENA program).
Since that time, Japan, in my eyes, has been a beautiful country, a perfect mixture of modern
technology and amazing nature. On top of that, the two schools were fantastic, where I canfind a lot of interests in scintillation detectors, data acquisition system, and pulse shapeanalysis. After the ATHENA program, Professor Sanami agreed to be my supervisor for myDoctor course.
In 2018, I left my home institution in Vietnam to come to KEK as a Doctoral student at
Sokendai University. Under the supervision of Professor Toshiya Sanami and Professor
Hirohito Yamazaki, I joined in the research project studying the photoneutron production
with monoenergetic and linearly polarized photon source set up at BL-01, NewSUBARU,Hyogo, Japan. I inherited the knowledge and the techniques from the group, and expanded
the investigation from medium to heavy target mass.
To me, coming to Japan is not only a great opportunity for doing research but also a good- 42 -decision to enjoy the environment, beautiful nature, and unique culture. When I came toJapan, staff at Sokendai and KEK helped me to rent my first apartment in Tsukuba city, andto prepare documents to be a resident in Japan. For a person who lives far from homecountry and family, it means a lot to me. KEK lent me a bicycle, helping me enjoy a dailyride from my apartment to the KEK’s Tsukuba campus. Before coming to KEK, I had
practiced vegetarian for 3 years. After a hard-working day in the laboratory, I really enjoy
cooking some delicious dishes using a large variety of fresh vegetables of Ibaraki prefecture.
Indian and Chinese restaurants with vegetarian options are also perfect choices to hang out
with friends. It must be a pity if we do not spend time for exploring the nature in Japan. I
took many opportunities to visit different places in Japan, including Amanohashidate, Kyoto,Osaka, Nara, Toyama, and Hyogo after a business trip, a conference, or a trip to Spring-8
where I carried out my experiment.
For this new year, I visited pagodas in Nara and had amazing time there with allsightseeing and the vegan food (Figure 6). The New Year ceremony in the pagodas in Japanis similar to the ceremony in Viet Nam.

Fig. 6

New Year celebration for a vegetarian

6. Summary
I have experienced a great lifetime in Tsukuba city, Ibaraki prefecture, Japan, and studied
an exciting experiment together with great physicists at the High Energy Accelerator
Research Organization. For me, it was a life-changing experience, and Japan is obviously agreat place for any young scientists starting their academic study and learning from the bestpeople. It is also an opportunity to share your research with other researchers in Japan andall over the world. Thanks to the Atomic Energy Society of Japan (AESJ), I had a chance toshare my study with other scientists.

- 43 -References[1]G.S. Mutchler, "The angular distributions and energy spectra of photoneutrons fromheavy elements", Doctor thesis, Massachusetts Institute of Technology, Cambridge(1966).

[2]

G.A. Price, et al., "Energy Spectra and Angular Distributions of Photoneutrons from
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[3]

F. Tagliabue, et al., "Angular distributions of fast photoneutrons", Nuclear Phys., vol.
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R.G. Baker, et al., "Angular distribution of fast photoneutrons", Can. J. Phys., vol. 39,
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[6]

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[7]

A.V. Varlamov, "Atlas of Giant Dipole Resonances: Parameters and Graphs ofPhotonuclear Reaction Cross Sections, Work Performed under the CoordinatedResearch Project on Compilation and Evaluation of Photonuclear Data forApplications", IAEA Tech Rep. INDC(NDS)-394. IAEA; (1999).

[8]

A. Agodi, "On γ-polarization effects in photonuclear reactions", Nuovo Cimento vol.
5, 21–29 (1957).

[9]K. Horikawa, et al., "Neutron angular distribution in (γ, n) reactions with linearlypolarized γ-ray beam generated by laser Compton scattering", Phys. Lett. B, vol. 737,pp. 109–113 (2014).

[10] Y. Kirihara, et al., "Neutron emission spectrum from gold excited with 16.6 MeV
linearly polarized monoenergetic photons", J. Nucl. Sci. Technol., vol. 57, pp. 444–456 (2020).
[11] Japan Radioisotope Association, "Statistics on the use of radiation in Japan 2017"(2017) https://www.jrias.or.jp/e/pdf/the_use_of_radiation_2017.pdf.
[12] Tung Pham, et al., "Cancers in Vietnam—Burden and Control Efforts: A Narrative
Scoping Review", Cancer Control, vol. 26, pp. 1-4 (2019).
[13] Tran Kim Tuyet, "Study on neutron anisotropic emission from photonuclear reaction
in giant dipole resonance energy region", Doctor thesis, The Graduate University for
Advanced Studies, SOKENDAI, School of High Energy Accelerator Science,
Department of Accelerator Science (2021).
[14] Tran Kim Tuyet et al., "Energy and angular distribution of photo-neutrons for 16.6

- 44 -MeV polarized photon on medium–heavy targets", Nucl. Ins. Meth. A, vol. 989, pp.
164965 (2021).

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