Photon Factory Activity Report 2016 #34 (2017)

BL-14A/2015P018

Incident position dependence of Si-APD temporal response for a single X-ray photon
at 13.6 keV
Takahiko Masuda1,* Takahiro Hiraki1, Sho Okubo2, Ryota Ozaki2, Noboru Sasao1, Akihiro Yoshimi1
and Koji Yoshimura1
1
Research Institute for Interdisciplinary Science, Okayama University, Okayama, 700-8530, Japan
2
Graduate School of Natural Science and Technology, Okayama University, Okayama, 700-8530,
Japan
Detail of temporal response of a silicon avalanche photodiode for a single X-ray photon is investigated. We measured
the temporal response of a Si-APD by changing the incident position of X-ray photons with 50 µm steps. Obvious
difference is observed between the response at the outer region and that at the inner region: the response at the outerregion has a tail structure.
1 Introduction
Silicon avalanche photodiode (Si-APD) is widely used
for detection of a single X-ray photon in experiments
using synchrotron radiation X-ray beams. [1] One ofadvantages of Si-APD is fast temporal response. We
measured the incident position dependence of temporal
response of Si-APD to investigate possibility to increase
the fastness of temporal response by limiting the incident
position of X-ray photons.
2 Experiment
The tested Si-APD is S12053-05 (Hamamatsu Photonics). The nominal sensitive area is 0.5 mm in diameter.
The thickcness of the depletion layer is estimated to be
10 µm. The Si-APD is adopted because of the fast time
response due to the small sensitive area and the thin
depletion layer.
The beam line used in the measurement is BL-14A inKEK-PF. Figure 1 shows the experimental setup. Thebeam current in the storage ring is 450 mA. The energy of
X-ray beam is tuned to 13.6 keV by using monochromators. The Si-APD is located directly head-on the beam
and is able to be moved both horizontal and vertical byauto-stages. A plate with a pin-hole whose diameter isapproximately 5 µm is placed in front of the Si-APD.
The Si-APD is housed in a metal box which is anelectromagnetic shield. The box is made of aluminumwalls and a brass front collimator. The collimator has ahole the diameter of which is 3 mm. The Si-APD is
located just behind the collimator hole so as to receive theX-ray photons directly. The bias voltage of 154 V isapplied. A preamplifier that amplifies a Si-APD output
signal is also embedded in the box. The preamplifier
output signal is sent to a lab-build constant-fraction
discriminator which converts the analog pulses to fastNIM logic pulses. Finally the logic pulses are transmitted
to a time-to-digital converter (MCS6, FAST ComTec)and digitized timing data is stored in a personal computer.

X-rayslit

IC att. filterpin holeslit

Si-APD ina Metal box

Fig. 1: Photo of the experimental setup around the SiAPD. IC is an ion chamber used to monitor the beam
intensity. The pin hole is mounted on a three-axis auto
stage. The Si-APD is housed in a metal box and is notshown in the figure. The two slits and att. filter are not
relevant to this study.
3 Results and Discussion
During the measurement, the pin-hole is shifted with
50 µm step horizontally or vertically. Data take time foreach position is 600 s. Data rate is 1.4×103 counts persecond. Figure 2 shows the temporal response at eight
positions in the Si-APD. While the nominal radius of theSi-APD is 0.25 mm, the single hit rate is uniform up to0.3 mm along the radial axis from the center. The overall
time resolution of 120 ps (full width at half maximum) isalmost uniform independently of the incident positions.
On the other hand, the temporal response within 0.25 mm
is almost same but that at 0.3 mm has bigger tailcomponent. The other three axes have the similar
structure.
Figure 3 shows calculated time spectra based on thespectra shown in Fig. 2 by assuming X-ray photons areirradiated uniformly in the sensitive area. The difference
between two spectra is whether outer region is included
or not. Here the boundary between the outer and inner
regions is defined as 0.25 mm from the center. It suggeststhat a guard ring that covers the edge region of thePhoton Factory Activity Report 2016 #34 (2017)#events / 25 ps

sensitive area can suppress the tail component and obtaina more gaussian like profile down to 10-6. This method isused for visible light region. [2] It is not easy to make aguard ring against X-ray photons in comparison to visible
photons because of the small crosssection; however, if theguard ring can be prepared, the time spectrum changes
from the red spectrum in Fig. 3 to the black spectrum inthe same figure. It would be useful if one needs tosuppress the tail component.
The statistics in this study is limited only 105. An
investigation with higher statistics is motivated for afuture work.
1010101050.5 mm

4320.05 mm

10100.5

11.52time [ns]#events / 25 ps

Fig. 2: Time response of the Si-APD for each position.
The up-right circle represents the sensitive area of the SiAPD and the eight small colored circles indicate
irradiated positions. The colors of histograms correspondto those of small circles.

101010
10
10

654

3210
100.5

11.5

2time [ns]Fig. 3: Weighted time response of the Si-APD. The black
histogram shows the time response with only innersensitive region the radius of which is 0.25 mm. The
red histogram shows that with the whole sensitivearea.

Acknowledgement
This work is supported by Japan Society for thePromotion of Science KAKENHI grant number
15H03661.

References[1] S. Kishimoto Nucl. Instrum. Methods A 309, 603(1991).
[2] F. Acerbi, A. Ferri, A. Gola, N. Zorzi, C. Piemonte,
Nucl. Instrum. Methods A 787, 34 (2015).
* masuda@okayama-u.ac.jp