Photon Factory Activity Report 2019 #37 (2020)

BL-11D/ 2017G638

Calibration of a CCD-Based Energy Monitor for High Power Soft X-Ray Lasers

Thanh-Hung DINH1,*, Masaharu NISHIKINO1, Masahiko ISHINO1,Takeshi HIGASHIGUCHI2, and Tadashi HATANO3
1
Kansai Photon Science Institute, National Institutes for Quantum and Radiological Science andTechnology (QST), 8-1-7 Umemidai, Kizugawa, Kyoto 619-0215, Japan,
2
Department of Electrical and Electronic Engineering, Faculty of Engineering and Center for Optical
Research and Education (CORE), Utsunomiya University, 7-1-2 Yoto, Utsunomiya, Tochigi 3218585, Japan3Institute of Multidisciplinary Research for Advanced Materials (IMRAM), Tohoku University, 2-1-1
Katahira, Aoba-ku, Sendai 980-8577, Japan
1 Introduction
The interaction of a solid material with focused, intense
pulses of x-ray radiations creates a strong electronic
excitation state within an ultra-short time and on ultrasmall spatial scales. This offers the possibility to control
the response of a material on a spatial scale of less than ananometer which is crucial for the next generation of nanodevices [1]. So far, generation of a short duration pulse ofcoherent x-ray radiation has been realized by techniques as
well-known as laser-driven coherent x-ray sources, andfree electron laser (FEL). Plasma-based x-ray laser (XRL),
beside high harmonic generations (HHGs), is one of laserdriven coherent x-ray sources having along been studied.
The population inversion of XRL is usually created by theelectron collisional excitation in hot and dense plasma
medium produced by intense optical laser pulses. The
advantages of XRLs are their compactness – table-topscales, and the narrow spectral line of the emitted radiation.
In National Institutes for Quantum and Radiological
Science and Technology (QST), we have a Ni-like Ag
plasma XRL operated at a wavelength of 13.9 nm with anarrow bandwidth of ∆𝜆𝜆/𝜆𝜆 ≤ 1 × 10−4 , a pulse durationof 7 ps and a pulse energy of sub-µJ [2]. The XRL has beenused to investigate damage thresholds of solid materials.
Recently, x-ray free-electron lasers (XFELs) – the fourth
generation light sources have been introduced in the largescale facilities of modern synchrotrons. In Japan, thebeamline-1 (BL1) at the Japanese x-ray free-electron laser
facility, SACLA, has started operation for users, providing
FEL radiations at soft x-ray regions (around 13 nm) with apulse duration of ~70 fs and a pulse energy of up to 100 µJ[3]. A critical issue of both XRLs and XFELs is their SASE
(self-amplified spontaneous emission) lasing mode whichusually leads to a large shot-by-shot fluctuation of the laser
output. Therefore, a monitor that indicates energy of eachlaser pulse, is essential for quantitative analysis.
For the illumination experiments using the 13.9-nm
XRL at QST and the soft x-ray free-electron laser (SXFEL)at SACLA, an energy monitor which consists of an x-ray
charge-coupled device (CCD) image sensor and x-rayfilters has been employed. Here we report the calibrationof the CCD-based energy monitor and its preliminary
performance in application study.

2 Configuration of the CCD-based energy monitor
The schematic view of the CCD-based energy monitor
is depicted in Fig. 1. A set of thin-film zirconium (Zr) andsilicon (Si) filters is motorized and be used as an attenuator
to regulate the pulse energy. So far, there are many
advanced energy detectors which can operate at very highrepetition rate (up to 100 kHz), e.g., commercial x-ray
photo-diodes, gas monitor detector [4]. Although, an x-ray
CCD image sensor can only be operated at a relatively low
repetition rate ranging from 1 Hz to few-tens Hz, its
advantage is both laser pulse energy and its spatial beamprofile can be monitored simultaneously. From the beamprofile data, one can retrieve the illuminated laser spot onthe target surface by a ray tracing calculation. Thesensitivity of x-ray CCD camera and the transmittance of
x-ray filters were calibrated by using synchrotron radiationfrom the BL-11D beamline of the PF.

Fig. 1: Schematic view of the CCD-based energy monitor
3 Measurement of the sensitivity of x-ray CCD cameras
The detailed procedure of the calibration has beenreported previously in Ref. 5. In briefly, the experimental
setup is shown in Fig. 2(a). To minimize the influence of
the impure light included in the beamline, an aluminum(Al) filter with a thickness of 200 nm was used. Acalibrated AXUV100G x-ray diode was used to determine
the number of photons incident onto the x-ray CCD camera.
The CCD chip was cooled at −50 ℃ with a pressure in thecalibration chamber of 5 × 10−5 Pa. Since the highvacuum degree was achieved, influence of the residual
contamination such as nitrogen, water vapor, and carbon
compounds, which deposit and condense onto the cooled
CCD surface, was negligibly small. This type ofcontamination usually can be removed by warming up thechip.
Figure 2(b) indicates an image recorded by the CCD
camera setting at an exposure time of 0.5 sec and a pixelPhoton Factory Activity Report 2019 #37 (2020)

readout rate of 3 MHz. The beamline shutter was opened
until the readout finished. The red- and yellow colored
circles in Fig. 2(b) indicated the smearing parts caused by
the opening process of the digital shutter and the readout
process of the CCD camera, respectively. As shown in Fig.
3, the count number of CCD camera linearly increases withraising the exposure times. The count number of thesmearing is equal to the count number at the exposure timeof 0 second; therefore one can derived it from the fitting
equation of the data as show in Fig. 2(c) . By considering
amount of the smearing, the effective exposure time wascalculated to be 1.54 sec with a discrepancy of less than3%.

Fig. 3: The sensitivity curves of x-ray CCD cameras.
4

Characterization of a zirconium filter
We have also measured the transmittance of a Zr filter
with a nominal thickness of 200 nm by the x-ray
photodiode. The measured result, denoted by the redcircles in Fig. 4, is well-fitted with the solid- blue curve
calculated from the data base [6] with the thickness of Zr
layer of 184 nm and ZrO2 layer of 56 nm. This result
implies that the surface of the Zr filter had been oxidized.
Our detailed investigation shows that the oxidation of Zr
filters would start during their production process. The
maximum thickness of ZrO2 layer was observed to be of~70 nm for Zr filters which have been used in for morethan three years. It should be noted that our x-ray filters
have been safekeeping in a desiccator after usage.

Fig. 2: Calibration of an x-ray CCD camera. (a) isexperimental setup. (b) is a typical image recorded by theCCD camera. (c) indicates the count number of the CCD
cameras as function of the exposure time for thewavelength of 13.9 nm.
Figure 3 indicates the measured sensitivity of the CCD
cameras at the soft x-ray region. Each CCD camera usually
has a different sensitivity curve. An obvious reason is thatthe sensitivity depends on performance of the assembledcomponents. The CCD chip would also be degraded upon
exposure of intense x-ray radiation. Therefore, a regularlycalibration is strongly recommended for CCD cameras
used in x-ray region.

Fig. 4: Transmittance of a Zr filter with a nominal
thickness of 200 nm. The red circles is the measured value.
The green-dashed-line and purple dotted-line are thetransmittance of Zr filters calculated from the data base forthe thickness of 200 nm and 350 nm, respectively. The
solid-blue-line is the calculated curve with the thicknessesof Zr and ZrO2 layers of 184 nm and 56 nm, respectively.

Photon Factory Activity Report 2019 #37 (2020)5Operation of the energy monitor in application studies
The calibrated CCD-based energy monitor has been
installed into the XRL and the SACLA SXFEL beam lines
for the damage studies of several solid materials [1,7].
Figure 5(a) depicts the schematic diagram of the irradiation
experiment performed at the SACLA SXFEL. Single x-ray
pulse from the SXFEL is focused onto the surface of solidmaterials by using a Kirkpatrick-Baez (K-B) mirrors. The
target surface is normal with respect to the incident laser
beam. The diameters of the laser spot size at full width at
half maximum, which were measured by knife-edge
scanning, were ~5.5 μm and ~4.5 μm for horizontally andvertically, respectively. An attenuator consist of calibrated
thin-film filters was motorized to regulate the pulse energy.
These filters were also helpful to block the undesired highharmonic orders of the FEL radiation. During theexperiment, the pulse energy was monitored by a gas
detector monitor on a shot-to-shot basis with a relative
uncertainty of 10%. The throughput energy was evaluatedby the calibrated CCD monitor. Figure 5(b) indicates thelinearity of the CCD-based energy monitor versus the gas
detector monitor.

CCD-based energy monitor using synchrotron radiations
from the soft x-ray beam line BL-11D of the PF has beenreported. Such energy monitor is essential instrument for
the quantitative experiment of light-matter-interaction
using the XRL and the SXEFL.
Acknowledgement
A part of this study was supported by support from JSPS
KAKENHI Grant Number JP19K15402.
References
1.
Dinh, T.-H. et al. Controlled strong excitationof silicon as a step towards processing
materials at sub-nanometer precision.
Commun. Phys. 2, 150 (1–9) (2019).
2.
Nishikino, M. et al. Demonstration of a soft-xray laser at 13.9 nm with full spatial
coherence. Phys. Rev. A 68, 061802 (2003).
3.
Owada, S. et al. A soft X-ray free-electron
laser beamline at SACLA: The light source,photon beamline and experimental station. J.
Synchrotron Radiat. 25, 282–288 (2018).
4.
Tiedtke, K. et al. Gas detectors for x-ray
lasers. J. Appl. Phys. 103, 094511 (2008).
5.
Dinh, T.-H. et al. Evaluation of a flat-fieldgrazing incidence spectrometer for highly
charged ion plasma emission in soft x-ray
spectral region from 1 to 10 nm. Rev. Sci.
Instrum. 87, 123106 (2016).
6.
Powell, C. J. & Jablonski, A. Evaluation of
Calculated and Measured Electron Inelastic
Mean Free Paths Near Solid Surfaces. J.
Phys. Chem. Ref. Data 28, 19 (1999).
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Mikami, K. et al. Laser-induced damage
thresholds and mechanism of silica glass
induced by ultra-short soft x-ray laser pulse
irradiation. Opt. Lett. 45, 2435 (2020).
* dinh.thanhhung@qst.go.jp

Fig. 5: Operation of the CCD-based energy monitor in an
application study at the SACLA SXFEL. (a) Schematic
view of the damage test experiment. (b) indicates thelinearity of the CCD-based energy monitor versus the gas
detector monitor which has been installed at the SACLASXFEL.
In summary, the calibration of each optical element isvery importance for quantitative studies, which use theradiation in soft x-ray region. Here the calibration of the