Proceedings of IBIC2012, Tsukuba, Japan

MOPA48MEASUREMENT OF TEMPORAL RESOLUTION AND DETECTIONEFFICIENCY OF X-RAY STREAK CAMERA BY SINGLE PHOTONIMAGES
A. Mochihashi∗ , M. Masaki, S. Takano, K. Tamura, H. Ohkuma,
JASRI/SPring-8, 1-1-1 Kouto, Sayo, Hyogo, 679-5198 Japan

In the third generation and the next generation synchrotron radiation light sources, the electron beam bunch
length of ps∼sub-ps is expected to be achieved. An X-ray
streak camera (X-SC) can directly measure the temporal
width of X-ray synchrotron radiation pulse. The temporal
resolution of X-SC depends on the initial velocity distribution of the photoelectrons from a photocathode which converts the X-ray photons to the photoelectrons. To measure
the temporal resolution of the X-SC, we have observed ’single photon’ streak camera images and measured the temporal spread of the images. We have also tried to evaluate thedependence of the temporal resolution and the detection efficiency on the thickness of the photocathode. For this purpose, we have developed a multi-array type CsI photocathode with 3 different thickness of the photocathodes. The
experimental setups, and the results of the measurementsof the temporal spread and the detection efficiency of thesingle photon events are presented.

INTRODUCTION
An X-ray streak camera (X-SC) can measure directly Xray pulses whose lengthes are nano to pico seconds. In caseof a single shot observation, the temporal resolution of theX-SC depends on an initial velocity distribution of photoelectrons at an X-ray incident photocathode and a spacecharge effect of the photoelectrons on the photocathode. A
conversion process in which incident photons change to thephotoelectrons at the photocathode creates primary electrons whose energy is ∼ keV because of the incident photon energy. On the other hand, secondary electrons whichhave typically the energy of several ten eV and the energy
spread of several eV are also created. Because the secondary electrons contribute formation of the streak images,
the initial velocity distribution of the secondary electrons
affects the temporal resolution of the X-SC. The temporal spread due to the initial velocity distribution of the secondary electrons can be observed as a temporal spread of
the streak image when single photon hits the photocathode.
We have observed the temporal spread of the streak imagesof the single photon events by decreasing intensity of theincident photons up to single photon counting level. Bychanging the incident photon energy, we have observed thedependence of the temporal resolution of the X-SC on thephoton energy. Because the secondary electrons which are∗ mochi@spring8.or.jp

created by multiple scattering process of the photoelectrons
in the photocathode contribute the formation of the streak
image, it is supposed that the detection efficiency and thetemporal resolution depend on the thickness of the photocathode. To investigate this, we have developed a multiarray CsI photocathode which can set three different thickness of the photocathodes simultaneously and oserved thedependence of the temporal resolution and the detection efficiency on the thickness of the photocathode.

EXPERIMENTAL SETUP
Figure 1 shows the block diagram and the timing setupof the single photon experiment. In the experiment, wehave used a streak camera system (Hamamatsu, C568006) with a synchroscan unit(Hamamatsu, M5675) anddual sweeping unit (Hamamatsu, M5679). The RF signal (508.58MHz) of the SPring-8 storage ring is transferredfrom the RF station to the beamline station via optical cable, and the optical signal is converted to the electronic signal by O/E module. The timing signal of the synchroscan
unit is generated by the frequency divider which generates 7th subharmonics of the RF frequency (72.65MHz).
The timing signal is used for the input signal of the delay
unit (Hamamatsu, C6878) which can adjust timing delayof the synchroscan unit. The reference signal from the synchroscan unit is used for the reference input of the delayunit.
The timing signal of the dual sweeping unit is generated
by the frequency divider and the streak trigger unit (Hamamatsu, C4547). The streak trigger unit generates 9Hz repetition signal synchronized with the RF signal and the signal is used for the trigger of the digital delay pulse generator (Stanford Research Systems, DG535). The pulse generator generates 3 timing signals: (A)exposure trigger for
CCD camera, (B)gate signal for image intensifier (I.I.) and(C)trigger signal for the dual sweeping unit. Because theCCD camera has internal delay (11μs) for the external trigger, the CCD trigger signal precedes in 11μs for both theI.I. gate trigger and the sweeping trigger. The fluorescent
substance at the end of the streak tube is P43[1] for C568006; the 100%→10% decay time of the fluorescence is 1ms.
Because of the decay time, it is necessary for identification of each single photon event on the fluorescent screen toprovide timing gate shorter than 1ms for the photoelectronswhich hit the screen. We have used the dual sweeping unit
as a timing gate for the photoelectrons which hit the screen:
we have operated the dual sweeping unit whose operation

ISBN 978-3-95450-119-9
Longitudinal Diagnostics and Synchronization

1c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○Abstract

MOPA48

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PVc 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)Copyright ○Figure 1: Setup of the experiment.
range is 100μs. To detect the single-photon events clearly,
we have operated the I.I. as a gate trigger mode: the gate
trigger mode can decrease dark signal from the micro channel plate inside the I.I.. We have settled the I.I. gate widthof 1ms to collect the fluorescence from the screen sufficiently. The output window of the I.I. is also made of P43;
therefore, we have set the exposure time of CCD camera tobe 2ms.
The profile of the usual photocathode in X-SC is 200μmin vertical and 6mm in horizontal. In the experiment we puta 4 jaws X-ray slit made of Ta in front of X-SC. We shaped
the spatial profile of X-ray by the slit so that the X-ray beam
spot held the photocathode region. To measure the photon
flux into the X-SC, between the slit and the X-SC we put anion chamber which has Kapton sheeted input/output windows and is filled with pure nitrogen gas. We applied DChigh voltage (1kV) inside the chamber and monitored ion
current by an ultra high resistance meter (ADVANTEST,R8340).

OBSERVATION OF SINGLE PHOTONEVENTS
The experiments have been performed in the beam diagnostic beamline II (BL05SS)[2, 3, 4] in the SPring-8 storage ring. BL05SS has a multipole wiggler as a light source
and a double crystal monochromator. We have adjusted
the gap of the multipole wiggler and the bragg angle ofthe monochromator and injected the monochromatic X-ray
into the X-SC. To identify individual single photon events,
we have adjusted the beamline component to decrease sufficiently the photon flux into the X-SC. A typical streak
image of the single photon in dual sweeping and singleshot operation is shown in Fig.2. A dot cluster inside thecircle in the figure corresponds to the streak image due tothe single photon event. The enlargement and the temporalprofile of the dot cluster in the Fig. 2 is shown in Fig. 3. The

dot cluster appears frequently when we adjust the beamline
component to increase photon flux to the X-SC. We typically have got 10000 shots of the streak image in the same
experimental condition, and picked up the single photon
events from the series of the streak image data.
In the streak image, not only the real single photon
events but also dark events due to the dark current in themicro channel plate inside the I.I. are included. To separate
the real and dark events, we have analyzed the intensity ofthe dots by making an intensity histogram. Figure 4 shows
typical example of the intensity histogram when the photon energy is 10keV. Squares correspond to the histogram
when the beam is in timing (occupied RF bucket timing)and circles correspond to that when the beam is out of timing (empty RF bucket timing). As seen in the figure, we candistinguish the real photon events and the dark events byapplying an intensity threshold. In the analysis, we firstly
have picked up all of the dots including not only the photon events but also the dark events, and next, we have madethe intensity histogram of the picked dots such as Fig. 4, anddefine the intensity threshold not to pick up the dark events.
After defining the threshold, we have picked up again thedots whose intensity is over the threshold, and have analyzed statistically averaged temporal structure of the single
photon events.
Figure 5 shows the statistically averaged temporal profile of the single photon events for 10keV photons. In theanalysis, we firstly analyzed the first order moment of thetemporal profile for each dot cluster, and to get statisticallyaveraged profile we have superimposed them to make theweighted center of each dot the same timing. We have fitted
the temporal profile data with Lorenz distribution whose
amplitude is treated as a free parameter and analyzed theFWHM for each X-ray energy data.

ISBN 978-3-95450-119-92

Longitudinal Diagnostics and Synchronization

Proceedings of IBIC2012, Tsukuba, Japan

MOPA48410Dark Timing
Beam Timing

3Frequency

10210110010850.05

900.04

95100
−0.5
00.5
1Intensity (arb. unit)

95100
5055
Time (µs)

60Figure 3: The enlargement and the temporal profile of thedot cluster in Fig. 2.

EXPERIMENTS WITH MULTI-ARRAY
PHOTOCATHODE
In the X-SC, secondary electrons created by multiple
scattering in the photocathode contribute the formation ofthe streak image, therefore, it is supposed that the detection efficiency and the temporal resolution depend on thethickness of the photocathode. To investigate this, we havedeveloped a multi-array CsI photocathode which can set
three different thickness of photocathodes simultaneously.
Figure 6 shows a picture of the multi-array photocathodedisk. At the center of the cathode disk, there are 3 different
thickness of the CsI membranes. Figure 7 shows a focus
image when we set the multi-array CsI photocathode disk.
The size of each photocathode is 1.3mm in horizontal and200μm in vertical. According to Henke et al.[5], the escapelength of the CsI is 250nm; so we have prepared 50, 300
and 1000nm thickness of the photocathode membranes.
To remove/mount the photocathode disk in the X-SC, wehave also improved a photocathode adapter in the X-SC.
Due to the adapter, we can easily remove/mount the photocathode disk.
In the experiment, we have adjusted the X-ray slit so200300
Intensity (arb. unit)

400

0.03
0.02
0.01
0−15

−10−50Time (ps)51015Figure 5: Statistically averaged temporal profile of the single photon(10keV) events. Circles correpond to the experimental result and a curve corresponds to a fitting result.
that the X-ray beam spot held a proper photocathode andobserved the single photon spread in various photon energies. Figure 8 summarizes the temporal spread of thesingle photon events for 5.3∼30keV region for 3 different
photocathode thicknesses. As seen in the figure, the single
photon spread distributes around 4∼5 ps (FWHM) and theenergy dependence of the spread cannot be seen clearly inthis experiment. The dependence of the photon spread onthe photocathode thickness cannot be seen clearly, either.
Figure 9 summarizes yields of the single photon eventsin different thicknesses of the photocathode. As seen in
the figure, the yield has clear dependence on the photon
energy: in the energy region of 5.3 to 5.8keV, the yields
increase rapidly and beyond the peak the yields decrease
slowly. Such energy dependence is caused by the absorption edge energy of the photocathode material; Cs and I.
Figure 10 shows the numerical calculation[6] of dependence of the X-ray transmission on the photon energy forISBN 978-3-95450-119-9
Longitudinal Diagnostics and Synchronization

3c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○85

Intensity (arb. unit)

80

90

100

Figure 4: An intensity histogram of dots in the streak image
in the single-photon experiment. Squares correspond to thedata when the synchroscan timing is adjusted to filled RF
bucket (beam timing). Circles corrsponds to the data in thesame setup but in empty RF bucket timing (dark timing).

80

Time (ps)Time (ps)Figure 2: A streak image of the single photon event.

0MOPA48

Proceedings of IBIC2012, Tsukuba, Japan
5.550nm
300nm
1000nm

FWHM (ps)54.5

43.500Vertical (CH)

2003001000nm

300nm

0.04

50nm

500
0100

200
300
400
Horizontal (CH)

500

600

SUMMARY
In this article, We have tried to measure temporal resolution of the X-ray streak camera by observing temporal
spread of the single photon events. Because the single photon event has the shortest duration in principle, it is possible

350.030.025
0.020.015
0.010.005
510152025Photon Energy (keV)3035

Figure 9: Yield of the single photon events for 3 different
thickness of photocathode.
1

Figure 7: A focus image of the multi-array CsI photocathode.
CsI membrane whose thickness is 300nm. As seen in thefigure, the transmission has a valley around 5keV: Cs and I
have L-edge absorption energy in the region 4.5∼6keV[7].
In the absorption energy region, the quantum efficiency islarger; that introduces the increase in the yields in the energy region.
As seen in the Fig. 9, the yield becomes better when thethickness of the photocathode is larger in this experiment.
As seen in Fig. 8, the temporal resolution have little dependence on the thickness; therefore, 1000nm thickness isbetter in this experimental condition. It is supposed that
there is an optimum thickness to realize better yield andtemporal resolution.

3050nm300nm
1000nm0.035

004000.98Transmission

c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○10010152025
Photon Energy (keV)

Figure 8: Temporal spread of the single photon events for
3 different thickness of photocathode.

Yield (Count Rate/Photon Flux)

Figure 6: A picture of the multi-array photocathode disk.
At the center of the cathode disk, there are 3 different thickness of CsI photocathode membrane: from left to right, 50,
300 and 1000nm.

50.96
0.94
0.92
0.9
0510
1520
Photon Energy (keV)

2530

Figure 10: The dependence of the transmission on the photon energy for CsI 300nm membrane[6].
to measure precisely the temporal resolution of the streakcamera. Because the measurement has been performed
with the single shot observation mode, we can measure thetemporal resolution without timing jitter. The single photon
also has the smallest intensity, so the space charge effect on
the photocathode can be minimized.

ISBN 978-3-95450-119-9
4Longitudinal Diagnostics and Synchronization

Proceedings of IBIC2012, Tsukuba, Japan

This work was partly supported by MEXT Grant-in-Aidfor Young Scientists (B) 21740215.

REFERENCES[1] Hamamatsu
Photonics
Web
http://jp.hamamatsu.com/en/index.html

Page,[2] M. Masaki et al., ”Characterizations and applications of theinsertion device of SPring-8 diagnostic beamline II”, AIP
Conference Proceedings 1234 (2010) 560-563.
[3] S. Takano et al., ”Overview of the SPring-8 diagnostic beamlines”, AIP Conference Proceedings 1234 (2010) 399-402.
[4] S. Takano et al., ”Status and Activities of the SPring-8 Diagnostics Beamlines”, MOPB52, these proceedings.
[5] B. L. Henke, J. P. Knauer and K. Premaratne, ”The characterization of x-ray photocathodes in the 0.1 - 10-keV photon energy region”, Journal of Applied Physics, 52(3) March (1981)
1509-1520.
[6] The Center of X-ray Optics, http://www.cxro.lbl.gov/[7] A. Thompson et al., ”X-ray Data Booklet”, Lawrence Barkeley National Laboratory, University of California, Barkeley,
California 94720

ISBN 978-3-95450-119-9
Longitudinal Diagnostics and Synchronization

5c 2012 by the respective authors — cc Creative Commons Attribution 3.0 (CC BY 3.0)
Copyright ○We have measured the dependence of the temporal resolution on the photon energy and the thickness of the photocathode. To investigate this, we have developed the multiarray photocathode disk which can mount different 3 thickness of the photocathode membranes simultaneously. In
our experiment, we have concluded that the temporal resolution of our streak camera system is 4∼5ps (FWHM). The
resolution doesn’t have clear dependence on the photon energy and the thickness of the photocathode. The yield, on
the other hand, has clear energy dependence because of theabsorption edge energy of the photocathode materials. In
this experiment, the photocathode of 1000nm thickness has
better yield than other thinner ones. It is supposed that there
is an optimum thickness to realize better yield and temporal
resolution. Investigation for better condition of the photocathode is a future plan.

MOPA48