Doctoral Thesisµ-PIC gaseous area detector for small-angle X-ray scatteringexperiments

Kaori Hattori
Department of Physics, Graduate School of Science, Kyoto University
January, 2009

Abstract
A detector with such properties, a two-dimensional counting detector based on a micro-pixel gas
chamber (µ-PIC ), is described.
Small-angle X-ray scattering (SAXS) is useful for investigating structures on a scale of 1-100
nm. It has a wide range of applications in biology, material science, and the study of soft condensed matter. The SAXS measurements are becoming increasingly popular in this field. The
narrow dynamic range of the imaging detectors currently in use has limited the resolution of many
experiments. Now integrated-type area detectors, Imaging plates (IPs) and CCDs, are widely used.
Due to internal noise, the accuracy of these detectors are limited. Therefore, a new detector thatprovides high-resolution measurements is desired. In theory, photon-counting type detectors isfree from the internal noise and its accuracy is limited by only statistical error. Therefore, theywould attain higher resolution than the integrated-type detectors. Then, the photon-counting typedetectors would be able to measure low-intensity scattering and widen the application field.
The application of the µ-PIC detector to high-resolution SAXS, its performance, and refinementsof the detector system, are reported. We have developed the X-ray area detector based on the µPIC , which is a photon-counting type detector. The µ-PIC is a micro-pattern gaseous detector
with a pitch of 400 µm fabricated by printed circuit board technology. In the present work, a µ-PIC
with an active area of 100×100 mm was used. To reduce the number of channels, a strip readout isused and a position encoding system reconstructs the positions of incident X-rays. The µ-PIC has
only 256 × 2 channels readout though it has 256 × 256 pixels (= 6.6 × 10 4 ). This paper will focus
on refinements of the position encoding system because it plays a key role in the high-resolution
SAXS experiments.
With the refined encoding system, solution scattering experiments were carried out. The performance was investigated at the RIKEN structural biology beamline (BL45XU-SAXS station) at
SPring-8, Japan. We successfully showed that a solute scattering curve maintained linearity in arange of > 106 . This range is wider by an order of magnitude than that observed by the IPs and thewidest one achieved by X-ray area detectors. The wide dynamic range is essential to observe weak
scattering and enhance the resolution of measurements. In this way, we showed that the µ-PIC
detector has an ability to perform the high-resolution SAXS measurements. We also showed thatthe position resolution was enhanced using charges. The position resolution was refined and was
σ = 93.6 µm. Scattering curves observed by the new µ-PIC system and an imaging plate agreed
well. The pixel pitch of the IP was 100 µm.
The improved position encoding system was applied to a Compton camera that consists ofa Time Projection Chamber (TPC) read by the µ-PIC and scintillators surrounding the TPC.
Then, three-dimensional fine tracks of cosmic muons and Compton-recoil electrons generated by
662 keV gamma-ray were observed within the TPC. The new encoding system doubled the hitsof these tracks. Using charges generated by recoil electrons, the system is expected to provide astronger criterion used for determining Compton-scattered points than that currently used. Then,the angular resolution of the camera would be enhanced.
This µ-PIC area detector also functions as a thermal neutron imaging detector. The µ-PIC
detector successfully measured small-angle neutron scattering (SANS) and observed a Bragg peak.
Using a knife edge test, a fine position resolution of < 1 mm was obtained while conventional
detectors used for thermal neutron scattering have a position resolution of a few mm.

Contents
1 Introduction
1.1 Principles of X-ray scattering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.2 Small-angle X-ray scattering (SAXS) . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.2.1 Small-angle scattering curves . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.2.2 Requirements for X-ray detectors . . . . . . . . . . . . . . . . . . . . . . . . .
1.2.3 Solution scattering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.3 X-ray detectors for SAXS techniques . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.3.1 Imaging Plate (IP) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.3.2 Charge-coupled device (CCD)-based area detectors . . . . . . . . . . . . . . .
1.3.3 Silicon pixel detector (PILATUS) . . . . . . . . . . . . . . . . . . . . . . . . .
1.3.4 Refined ADC Per Input Detector system (RAPID) . . . . . . . . . . . . . . .
1.3.5 Micro Stip Gas Chamber (MSGC) . . . . . . . . . . . . . . . . . . . . . . . .
1.4 X-ray area detector based on µ-PIC . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.5 Time resolved X-ray detectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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2 X-ray area detector based on µ-PIC
2.1 The detector system based on µ-PIC . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2 Readout system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.1 Amplifier-shaper-discriminator (ASD) chips . . . . . . . . . . . . . . . . . . .
2.2.2 Time walk at comparator output . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.3 Encoding module . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.4 ASD reference threshold . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.5 X-ray imaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.6 Time Projection Chamber based on µ-PIC (micro-TPC) . . . . . . . . . . . .
2.3 Calibration of X-ray area detector . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.3.1 Camera length . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.3.2 Refinement of non-orthogonality . . . . . . . . . . . . . . . . . . . . . . . . .
2.4 Time-resolved measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.4.1 Time-resolved X-ray diffraction . . . . . . . . . . . . . . . . . . . . . . . . . .
2.4.2 Continuous rotation photograph (CRP) method . . . . . . . . . . . . . . . .

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3 Performance characteristics of the µ-PIC detector for high-count rate environments
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3.1 Experimental setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
3.2 Performance characteristic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
3.2.1 Linearity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
3.2.2 Distortion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
3.2.3 Dynamic range . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
3.2.4 Accidental coincidence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
3.3 X-ray detection efficiency with position encoding system . . . . . . . . . . . . . . . . 42
3.4 New position encoding method (mode-II) . . . . . . . . . . . . . . . . . . . . . . . . 44
1

3.4.1 Two-dimensional images obtained by mode-II . . . . . . . . . . . . . . . . . .
3.4.2 Aging behavior in high-counting rate environment . . . . . . . . . . . . . . .
3.4.3 External trigger . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.4.4 Solution scattering measurement . . . . . . . . . . . . . . . . . . . . . . . . .
3.4.5 Guinier plot . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Further improvements of position encoding method (mode-III) . . . . . . . . . . . .
3.5.1 Position calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.5.2 Position resolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.5.3 Refinements in scattering curves . . . . . . . . . . . . . . . . . . . . . . . . .
Future improvements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.6.1 Gain stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.6.2 Counting rates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.6.3 Detection efficiency for X-rays . . . . . . . . . . . . . . . . . . . . . . . . . .
3.6.4 µ-PIC with large active area . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.6.5 Compact system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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4 Application to Compton camera
4.1 MeV gamma-ray observation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2 Reconstruction of Compton events . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.3 Current Status of Compton camera based on the µ-PIC . . . . . . . . . . . . . . . .
4.4 New position encoding system for micro-TPC . . . . . . . . . . . . . . . . . . . . . .
4.4.1 TPC mode-II . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.4.2 TPC-modeIII . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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5 Application to small-angle neutron scattering
5.1 Small-angle neutron scattering (SANS) . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2 Neutron detector based on µ-PIC . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2.1 3 He(n, p) Reaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2.2 Advantage of neutron detector based on µ-PIC . . . . . . . . . . . . . . . . .
5.3 Overview on study of neutron time-resolved imaging . . . . . . . . . . . . . . . . . .
5.4 Thermal neutron imaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.4.1 Position resolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.4.2 Bragg curve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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6 Summary

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3.5

3.6

2

Chapter 1

Introduction
Nowadays, X-ray scattering is widely used to investigate a wide range of substances, such as
crystals, industrial materials, and biological macromolecules.

1.1

Principles of X-ray scattering

In this section, we will treat the principles of pure elastic scattering by a sample [1]. We consider
the scattering of a plane monochromatic wave of X-rays. Once electrons in the sample interact
with the X-rays, they generate secondary spherical waves. Figure 1.1 shows a schematic view ofthe scattering.
scattering wave vector k f
scattering vectorq = kf - k i2θr

incident wave vector k iscatterer

Figure 1.1: Schematic view of X-ray scattering.
The phase difference of the secondary wave is represented by r · q as a function of a position, r,
where q = kf −ki and ki and kf are the incident and scattering wave vectors with |k i | = |kf | = 2π/λ,
λ denoting the wavelength. The resultant scattering wave is a superposition of scattering waves
generated by the sample whose electron density is given by ρ(r). Then the scattering is representedbyZ
F (q) =
ρ(r)exp(−iq · r)dr,(1.1)
V

F (q) is called a structure factor. A scattering intensity generated by the sample per volume
unit I(q) is given by
F (q)F ∗ (q).
V
The scattering intensity may be rewritten in the formZγ(r)exp(−q · r)dr,
I(q) =
I(q) =

V

3

(1.2)

(1.3)

whereγ(r) =1
VZ

ρ(r 0 )ρ(r + r 0 )dr 0 .

(1.4)

V

Expression (1.3) represents a Fourier transform of γ(r) that is a self-correlation function of theelectron density ρ(r). In a case of an isotropic sample, equation (1.3) is given byZ ∞sin qrI(q) =
4πr 2 γ(r)
dr.
(1.5)
qr
0

1.2

Small-angle X-ray scattering (SAXS)Intensity

The “small-angle X-ray scattering” (SAXS) method is applied to disordering objects [2]. The
diffraction pattern is diminishing as objects are disordered. The scale of the structure in the lowordered samples is much larger than crystals. Thus, it is sufficient for low-ordered samples toobserve a central part of the diffraction pattern. This leads to the SAXS method, which is usefulto study low-ordered samples.
To obtain enough information on the inhomogeneities with a typical size D, the measurementsof the scattering curve up to approximately q = 1/D is required, where q = 4πsinθ/λ. The size of
1-100 nm corresponds to the angular region up to 2θ = 0.8 − 8 degrees at wavelength λ = 1.54 Å
(the X-ray Cu Kα line). SAXS is a promising tool for investigating such structural scales, especiallyfor determining the shape and size distribution of the particles, and the electron density fluctuationof the system. The fields investigated successfully by this method are biology and material science.
Recently, studies of soft condensed matter using this method has been popular. Thus, various
characteristics of disperse systems can be studied with small-angle scattering techniques.

shapeinterparticle
interference
surfacesize ofparticles

wide angle
X-ray scattering

scattering angle 2θ
or scattering vector q

Figure 1.2: Typical information on particle sample in small angular region

1.2.1

Small-angle scattering curves

We now consider the behavior of the intensity of scattering, I(q) as shown in Fig.1.2.
Guinier plot
In the vicinity of q = 0,4

sin qr
q2 r2 q4 r4=1−+− · · ·.
qr
3!
5!

(1.6)

Then, equation (1.5) is given by
1
lnI(q) = lnI(0) − Rg2 q 2 ,3
where Rg is the radius of gyration expressed by
R∞ 2r ρ(r)dr2
Rg = R0 ∞.
0 ρ(r)dr

(1.7)

(1.8)

Since the scattering intensity is proportional to q 2 in the vicinity of q = 0, Rg can be determinedfrom the lnI(q) vs q 2 plot (the Guinier plot). Guinier approximation neglects higher order of qR g .
Therefore, it is valid
q mainly in the interval qR g < 1. A sphere of radius D is determined by theequation of Rg =3
5 D.

Shape of particles
In the angular region of qRg >> 1, the scattering intensity depends on the shape of the particles.
To simplify calculations, we deal with scattering from spheres of radius D. We assume that theelectron density in the sphere is constant and the density of spheres is low. Then the intensity isgiven by
4π
I(q) =q

Z D

rγ(r) sin(qr)dr.

(1.9)

0

γ(D) is equal to 0 due to the low density and ρ(r + De) where e is a unit vector. The derivativeof γ(D) is also equal to 0 at r = D. Considering them, we perform integration by parts twice andobtain4πI(q) = 3qZ D[rγ(r)]00 sin(qr)dr.

(1.10)

0This may be rewritten in the form
4πI(q) = 4 (−2γ 0 (0) + cos(qD)Dγ 00 (D) −q

Z D[rγ(r)]000 cos(qr)dr).

(1.11)

0

The third term tends to 0 as q tends to infinity. In general, γ 00 is 0 and the second term canbe neglected. Therefore, only the first term contributes to the intensity in the angular region of
qRg >> 1. It can be concluded that the scattering intensity of spherical particles are proportional
to q −4 . The scattering intensities of rod-like particles and lamellar particles, are proportional toq −1 , q −2 , respectively, which are valid around q = 1/D.
The Porod law
Considering the behavior of I(q) as q tends to infinity, the asymptotic trend of I(q) is proportional
to q −4
lim I(q) ∝

q→∞

1 S
,
q4 V

(1.12)

where V and S are a volume and a surface area of scatterers, respectively. Thus a specificsurface area S/V can be obtained. This law known as the Porod law holds only in the case ofparticles for which the scattering density distribution is a continuous smooth function.
5

1.2.2Requirements for X-ray detectors

Thus the scattering intensity frequently changes by several orders of magnitude within a narrow
range of values of q. This suggests that a wide dynamic range of the intensity is essential for
detectors of the scattering radiation. This also implies necessity of high-angle resolution in the
study of disperse systems. Since the range of a camera length that is a distance between a sample
and the detector is constrained by instrumentation, the spatial resolution of the detectors should
be high (< 100 µm). The large active area is a supplementary advantage for the high-angle region.
It should be noted that stability operation for a long duration and under intense X-ray radiationis also crucial for detectors intended for synchrotron radiation.

1.2.3

Solution scattering

SAXS measurements is also a powerful tool to study solution scattering, which allows the determination of the structure of objects under investigation without crystallization. Since many biologicalsamples dissolve in solvent, measurements of solution scattering is an ideal method to investigate
their nature. Moreover, it takes a long duration, sometimes several years, to achieve crystal growth.
Thus the ability to make measurements without crystallization is very helpful.
For application to the solution scattering, there are more strict requirements for X-ray detectors
than those mentioned above. To obtain a solution scattering curve, one should measure the scattering pattern of solvent and solution. The scattering from the solution includes the backgroundand the scattering from solvent. Therefore, the scattering from the solute can be determined bysubtracting the solvent scattering curve from the solution curve. This is a general method forinvestigating SAXS.
The local counting rate with the solution scattering is sometimes about 100 times higher thanthat with the solvent only. Thus, it is crucial for the detectors to respond proportional to incoming
photons in a wide range. From now we refer the proportionality as linearity. In addition, thedifference of the scattering curve between the solution and the solvent is usually very small at wide
scattering angles. To obtain the solute curve with a wide dynamic range, correct measurements ofthis small difference is essential. For this reason, solution scattering measurements are one of themost difficult method for structure determination. This fact implies that once a detector attains
successful results in solution scattering measurements, it can be applied to almost all fields of X-ray
structure determination.

1.3

X-ray detectors for SAXS techniques

These days there are a great number of models and devices used for small-angle scattering studies.
In this section, features of several detectors are summarized. Basically, detectors of the scatteringradiation are classified into two categories: counting and integration-type detectors.
In the early 80s, position-sensitive gas proportional counters (PSPC) were used for SAXS experiments in most synchrotron radiation facilities and laboratories. The PSPC is a one-dimensional
photon-counting-type gaseous detector. Its counting rate was limited to 10 4 − 105 counts per second, which is limited by the speed of its electronics. Third-generation synchrotron radiation sources
have been available since the mid-90s, and the use of high-intensity and high-brilliance X-ray beams
has begun. Most counting-type detectors have no tolerance for operating under such conditions.
Gaseous detectors have also been employed as a two-dimensional X-ray position encoding instrument ([3] and the references therein). A classical example can be observed in X-ray protein
crystallography, wherein two-dimensional multi-wire proportional counters (MWPC) were being
used in many synchrotron radiation facilities in the 1980s [4]. However, limitations in counting
rates and position resolutions prevented their further usage in this field.
To satisfy requirements in third-generation synchrotron radiation sources, an imaging plate [5]and Charge-coupled device (CCD)-based area X-ray detectors [6] [7] have found wide use as an
6integration-type X-ray detector for SAXS experiments and macromolecular crystallography. They
are free from counting limitations due to electronics and are thus suitable for radiation with intense
X-rays. Overviews of the CCD-detectors are also given in [8].
However, an integration-type X-ray detector has an intrinsic limitation in the signal-to-noise
ratio of its data caused by internal noise such as readout noise and dark current in the CCD, except
for quantum fluctuation of the signal. Another disadvantage of the detectors is low time resolution
given in Tab. 1.1. Time-resolved measurements allow one to reveal dynamics of materials. Somekinds of experiments can be repeated and then its accuracy could be enhanced. The detail of thetime-resolved measurements will be described in the subsequent chapter.
From the experimantal requirements, photon-counting-type detectors such as gaseous detectors
and silicon pixel detectors have been developed. They have dramatically improved the signal-tonoise ratio and the time resolution.
A novel photon counting area detector, the micro strip gas chamber (MSGC) [9] [10], with afine position resolution of 100 µm was being studied. Recently, novel gaseous detectors such as
MICROMEGAS [11] and RAPID [12] [13] have been developed, and have helped meet the challengesof crystallography in high X-ray flux environments. Also, another type of photon counting area
detector, silicon pixel detectors have proved stable operation under radiation with intense X-rays.
Now we treat with an imaging plate and and CCD detectors which are integration-type detectors. We also deal with two counting-type gaseous detectors, the RAPID and the MSGC, and acounting-type silicon detector, the PILATUS [14]. The features of the detectors are summarizedin Table 1.1. In general, the maximum count of each pixel is limited by the size of the counters.
Therefore, a dynamic range often refers to the maximum count. In this paper, a dynamic range
represents a range where a linearity in the detector response is confirmed.

1.3.1

Imaging Plate (IP)The imaging plates are integration-type detectors. They are similar to photographic
film but they have replaced conventional
crystalline phosphorescent material
X-ray film in most laboratories because ofimaging
their advantages. They are at least 10
platetimes more sensitive than X-ray film. The
exposure to
dynamic range of the IPs is much wider
X-rays5−6(10 ) while the X-ray film has a narrow
dynamic range of 102 .
The IPs have a thin layer of an photo- laser scans the plate
PMT measures
stimulable phosphor, BaF(Br,I):E2+ , where stored energy isemitted lightreleased
X-rays are absorbed. The principle of theimaging plate is shown in Fig.1.3. X-rays
excite electrons in the phosphor to higher
erasure of residual
image by exposure
energy levels. The excited electrons emit
to white light
light in the visible wavelength region verysoon as normal fluorescent. However, thepart of the energy is stored in the phosFigure 1.3: Principle of the imaging plate.
phor by electrons trapped in color centers.
It is emitted only slowly for the duration of
more than several days. This stored energy
is released when light is illuminated. Mainly a red laser is employed for scanning the phosphor andthe stored energy is released as blue light. The red light is removed by a filter and the blue lightis observed with a photomultiplier (PMT). The number of the emitted photons is proportional tothat of absorbed X-ray photons. The spatial resolution mainly depends on the reading system.

7

Parameter

Type
AbsorberSpatialResolution
Max. counting
Rates (local)
Dynamic Range
Dynamic Range(for solute scattering)Time Resolution

Table 1.1: Summary of X-ray area detectors
Imaging
X-ray Image
fiber optic
Plate
Intensifier
taper / CCD/ CCD
integrated
integrated
integrated
phosphor
phosphor
phosphor50-200 µm∼100 µm∼100 µm

PILATUSphoton counting
silicon172 µm108 cps/mm2

106−7 cps/mm2

108 cps/mm2

5 × 106 cps/mm2

105−6
> 106

103−4
103−4

103−4
103−4

106
no data> a few 100 ms> 1 ms

>1 ms

10 ms

Parameter
Type
AbsorberSpatialResolution
Max. counting
Rates (local)
Dynamic Range
Dynamic Range(for solute scattering)Time Resolution

RAPID
photon countinggas
375 µm(FWHM)
106 cps/mm2

MSGC
photon counting
gas
100 µm107 cps/mm2µ-PIC
photon countinggas
220 µm(FWHM)
107 cps/mm2

infiniteno data

infinite106

infinite> 106

100 ns>20 ns>10 ns8

At present, the pixel size of 50 - 200 µm is available. After reading which takes a few minutes
the plates need to be erased by exposure to intense white light. Then, the plates can be reused.
Therefore, the imaging plates are favorable for measurements of static objects. However, it is notsuitable for continuous operations and time resolved experiments.

1.3.2

Charge-coupled device (CCD)-based area detectors

Another integrated-type detector found wide use is a CCD-based detector. The CCD detectors
consist of components such as phosphors, fiber optics, image intensifiers and a CCD. Figure 1.4
shows typical CCD detector systems. Incident X-rays are converted to visible light by phosphorsand then transmitted through fiber optics, occasionally amplified by the intensifiers. The positions
are recorded by the CCDs.
The CCDs are directly sensitive to X-rays and applied to astrophysics. However, the effective
area of available CCDs are too small. The CCDs are suffered from radiation damage, and have too
low X-ray stopping power. Therefore a phosphor screen is employed to absorb the X-rays in large
area and coupled to a CCD. To enhance a position resolution, the screen of long, thin, opticallyisolated columns of luminescent material has been fabricated as shown in Fig.1.5. The phosphor
columns are deposited on fiber optics which transform scintillation light to an adjacent component.
Since the area of the phosphor screen is larger than that of the CCD, demagnification of phosphor image is done by a fiber-optic taper or an image intensifier.
fiber-optic taper/CCD
A fused fiber-optic bundle consisting of a coherent bundle of glass-fiber light guides is often usedbecause of easy-handling. Photons from the phosphor screen are collected through the bundle andthus the image can be demagnified. A distortion and a non-uniformity are always found in the
demagnified image and lead to necessity for corrections.
Image intensifier/CCD
The system is schematically represented in Fig.1.4. In this detector, the incident X-rays are collected
on a fluorescent screen. X-ray photons are transformed into visible photons by CsI located on thescreen. Then the visible photons are transformed into electrons by photo cathodes. The resultant
electrons are accelerated by the electric field applied in an image intensifier and end with anotherfluorescent screen. The amount of photons emitted at the fluorescent screen is proportional to theenergy of the incident electron. Therefore the accelerated electrons produces more photons than
primary photons generated in the phosphor screen. The amplified photons are registered into the
CCD.
In a case of a large demagnification ratio, number of photons registered to the CCD is low,
and thus intensification is useful. Since the intensifiers are expensive and fragile, the use should be
avoided whenever possible. The introduction of low noise CCDs decreases necessity of the image
intensifiers.
The cooled CCDs with low noise and low-time resolution are used for measurements of static
objects. On the other hand, the CCDs of high noise and high frame rates are used for time resolved
measurements. Even in the case of the cooled CCDs, readout noise contributes to their narrow
dynamic range of 103−4 .
Some CCDs designed for the operation at high frame rates have two kinds of pixels: normal
pixels and shaded pixels. After one frame exposure, charge accumulated in the normal pixels istransformed to the shaded pixels. The charge in the shaded pixels is registered during the next
frame. In this method, all pixels should be read within a frame. Therefore, it restricts the time
resolution of the detector, which has reached 1 msec.

9

A

C

B
D

Figure 1.4: Schematics of CCD detectors [8]. (A) Combination of phosphor screen, an image
intensifier, and CCD; (B) Combination of phosphor screen, an image intensifier, and a lens coupled
to a CCD; (C) Combination of phosphor or scintillating fiber-optics screen, a fiber-optic taper,
an image intensifier, a second taper and the CCD; (D) Combination of phosphor or scintillatingfiber-optics screen, a fiber-optic taper and the CCD.

Figure 1.5: Schematic of CsI(Na) phosphor columns grown on an fiber-optic plate [8].

10

1.3.3

Silicon pixel detector (PILATUS)

The PILATUS detector (pixel apparatus for the SLS) is another type of photon counting detectors.
It is a silicon pixel array detector based on the CMOS hybrid pixel technology. Figure 1.6 shows aschematic of the silicon pixel array detector. Figure 1.7 (a) displays a side view of the PILATUS
detector. In the semiconductor sensor, incident X-rays liberate electrons and the electrons generatemany electron-hole pairs along their track. The charge is transferred to the ASIC CMOS chips via asmall indium bump bond shown in Fig. 1.7(c). Each pixel comprises a preamplifier, a comparator
and a counter. In the sensor, X-rays are transformed to charge enforced by the preamplifiers.
The comparator generates a digital signal if the charge exceeds a threshold. The detector equips
counters where number of digital signals per pixel is stored. The maximum count of this detector is
106 , which is determined by the 20-bit counters. The maximum counting rate is 1.5 MHz/pixel/s.
As shown in Fig. 1.7 (b), the counter occupies most of the pixel area.
This readout method is free from readout noise and dark current. The readout time is 3 ms
and the frame rate ranges from 10 Hz to 100 Hz. During the readout, the detector is insensitive toincident X-rays. It is a disadvantage to time resolved measurements and continuous measurements.
Main advantage of the silicon detectors is a high quantum efficiency. The efficiency attains 50 %even at 16 keV while that of gaseous detectors falls rapidly in such high energy region. Anotheradvantage is a good spatial resolution determined by the pixel size of 0.172 mm.
At present, three kinds of PILATUS detector systems have been developed, the PILATUS 100K,
1M, and 6M. The PILATUS 100K detector is a single module consisting of 487 x 195 pixels. The
active area is over 84×34mm 2 . The PILATUS 2M detector consists of 3×7 modules with 1475×1467
pixels. The total active area is 254×252mm 2 . The PILATUS 6M detector consists of 5×12 modules
with 2463 × 2527 pixels. The total active area is 424 × 435mm 2 . The detector has attained many
advantages in comparison with the integrated-type X-ray area detectors. However, the cost of the
PILATUS is unrealistic for uses at a laboratory. Thus a photon counting detector with reasonable
cost is eagerly awaited. This requirement leads to a detector with a simpler structure and readout.

semiconductor sensor
bump bonding
CMOS chips
Figure 1.6: Schematic of a silicon pixel array detector.

1.3.4

Refined ADC Per Input Detector system (RAPID)

The RAPID detector is one of photon counting-type detectors. It consits of a fine-pitch MWPCand a multi-channel readout system. The MWPC has 256 × 256 anode wires with a pitch of 750µm. An anode to cathode gap is 500 µm as illustrated in Fig.1.8. The RAPID used the narrowanode to cathode gap to overcome the space charge effects in wire chambers which limits the local
counting rates. This enhances the local counting rates in the RAPID detector to approximately 10 6
photons/mm2 /s. The local rate of the delay line readout MWPCs were limited to 3 × 10 3 counts
s−1 mm −2 and the global rate to 106 counts s−1 [16].
When an incident X-ray liberates an electron trapped by a gas atom, the liberated electron
causes the formation of clouds of ions and electrons in the gas. The electron clouds drift toward theanode wires along an electric field applied across the gas volume. Then amplification is applied byaccelerating the electron clouds in an electric field around the anode wires. The resultant electrons11(a) Side view of the PILATUS detector.

(b) Magnification of a single pixel. A bump
pad is next to the analog part. A digitalcounter occupies most of the pixel area.

(c) Each silicon pixel sensor is electrically connected to a readout chip via a small indium
bump bond.

Figure 1.7: The PILATUS detector [15].

12are collected by the anode wires. Ions formed through the amplification are collected by the cathodeplaced below the anode wires. Since there is no material absorbing ions between the anode wires
and the window, ion feedback could be observed. Therefore, signals from the anode wires andcathode wires should coincide. The signals in coincidence allows interpolation of two-dimensional
position of the incident X-ray.
Each wire of the detector is instrumented with a preamplifier and an Analogue to Digital
Converter (ADC). The ADCs collect the signals from the most closet four channels surrounding
an incident X-ray. Then calculation of the position of the event is performed independently in X
and Y and the two co-ordinates combined to reconstruct the position of the X-ray. The number ofpixels is 1024 × 1024 in typical use. The resulting position, the charge/energy value, and time arestored in a histogramming memory. The spatial resolution of 384 µm is better than the pixel pitch
because of position interpolation. The readout is a complex and huge system. The fast ADC andthe position interpolation system are need to be operated with cooling water.
The photon-counting detectors including the RAPID is free from noise accompanied by readout.
This outstanding feature of the photon-counting detectors provides repeated measurements. Then,
low-intensity signals can be detected as the result of the sum of data. The disadvantage of the
RAPID is a low tolerance for high-counting rate environments. The RAPID can detect all X-rays
at 20 MHz when the distribution of the incident X-rays are approximately uniform. However,
localized incident X-rays (1 MHz/mm 2 ) causes the counting loss even if the global counting rates
are less than 20 MHz. The RAPID has not been easy to handle and thus further improvements arenecessary. For common use, performance characteristics of detectors must be the same. However,wire chambers with the same design often show different performance. The RAPID using wires has
the same feature that prevents the RAPID from wide spread.

Figure 1.8: Schematic of the 2D micro-gap detector

1.3.5

Micro Stip Gas Chamber (MSGC)

The MSGC shown in Fig.1.9 is designed from the basis of the MWPC. The structure of the MSGC
is similar to the RAPID while the MSGC has thin conductor strips printed on a substrate. The
potential of the anode is positive relative to the two adjacent cathode strips. The pitches of anodes
and cathodes are 200 µm, respectively. The narrowly spaced strips prevent the space charge effects
seen in wire chambers. To yield two-dimensional images backstrips with a 200 µm pitch are arrangedon the opposite side of the 20-µm thick substrate. Each strip is instrumented with an amplifier and13

a discriminator. The discriminated signals are used for calculation of the position of an incident
X-ray.
The MSGC features a fine spatial resolution
in comparison with the RAPID. The spatial resolution was 50 µm using ADCs, similar to the
RAPID. Without the ADCs, it was 100 µm. A
solute scattering curve determined by subtracting the solution curve from the solution curve
attained a good linearity in the wide range of
106 [17]. Thus, the MSGC has proved the advantages of photon-counting detectors. However, operation at a sufficient gas gain of > 10 3
causes discharges between the anodes and thecathodes. It is impossible to apply a high voltage to the MSGC once it was damaged by serious
discharges. To solve this problem, a preamplifier
such as a capillary plate [18] and a Gas Electron Multiplier (GEM) [19] has been installed
above the MSGC. However, the instability problem has not solved yet. Therefore the need ofa new gaseous photon counting detector with a Figure 1.9: Schematic of the MSGC. Above thegood position resolution, a wide dynamic range MSGC a preamlifier such as a capillary plate is
installed due to supply a gas gain.
and a stable operation has been arisen.

1.4 X-ray area detector basedon µ-PIC
This study is aiming at providing powerful methods for structural determination, especially in
SAXS field. SAXS measurements can be conducted using low-intensity X-ray source such as X-ray
tubes but the use of third-generation synchrotron radiation sources is popular. Since the thirdgeneration synchrotron radiation sources produce intense X-rays, measurement time can be shorten
and dynamics of samples can be observed. However, radiation damage is a serious problem. To
avoid radiation damage, there is a necessity of structural analysis of biological macromolecules andmaterials for industrial use conducted in a few minutes.
High-precision measurements are desired to analyze structure of materials containing light elements. Intensity of scattered X-rays by samples is roughly proportional to the electron density
and thus atomic number Z. Thus, the contribution of the light elements to the scattering is small.
Another reason for the request of the high-precision measurements is the feature of SAXS curves
decreasing exponentially as q increases. Thus, precise observations of very low-intensity scattering
patterns are crucial for the high-precision measurements. A detector that ensures linearity in a wide
range can provide the high-precision measurements. At present, the IPs have observed scattering
curves with the widest dynamic range of about 10 6 . However, a solute scattering curve determinedby subtracting a solvent curve from a solution curve maintain linearity in a range of about 10 5 . It
implies that accuracy of X-ray counts recorded by the IPs is 5 orders of magnitude. Our goal is
to obtain solvent scattering maintaining linearity in a range of 10 6−7 and thus accuracy of X-ray
counts must be 6-7 orders of magnitude. The photon-counting type detectors are suitable for thehigh-precision experiments because fluctuation in X-ray counts is limited to statistical error.
The MSGC proved the advantages of the photon-counting detectors but it have not exhibitedenough performances to withstand high-counting rates environments. It is not in practical use atpresent. As refinement of the MSGC, we have been developing an X-ray area detector based on

14

a micro-pixel gas chamber (µ-PIC ) that is a micro-pattern gaseous detector [20]. The µ-PIC has
totally different pixel structure from the MSGC and is robust for discharges. The performance ofthe µ-PIC will be described in the next chapter.
The X-ray area detector based on the µ-PIC is photon-counting type. Theoretically a dynamic
range of the µ-PIC is infinite. Only cosmic muons and electronic noise contribute the background.
The electronic noise can be easily eliminated because this system samples signals exceeding athreshold. The counting rates of the cosmic muons is limited to a few cps. Therefore, the contribution of the background is quite small and statistical uncertainty is dominant. This featureallows to measure low-intensity diffraction spots and scattering pattern precisely and will lead tohigh-precision measurements.
The µ-PIC has strip readout and charges are converted to digital signals. In consequence, thesystem is simple and then is able to deal with high-counting rates X-rays. Because of the simplicity,
the cost of the µ-PIC detector system is much cheaper than the PILATUS that is another photoncounting detector.
To apply an X-ray detector to SAXS measurements, it needs to meet severe requirements
mentioned above. Once a detector successfully proves its usefulness in this field, it can be applied
to another field such as measurements using X-ray tubes whose intensity is much weaker. In theory,the background of photon-counting type detectors is quite low and do not depend on exposure
time. Thus, they are suitable for a long-term exposure. They can be applied straightforward to
the laboratory measurements once they show fine performance in high-counting rate environments.

1.5Time resolved X-ray detectors

Recently time resolved measurements of SAXS has been attractive and expected to reveal a wide
range of new phenomena. In spite of lots of efforts for a long duration, a time resolved X-ray
detector applied to a wide range of fields has not been invented yet. At present, one should choosea detector suitable for each experiment and adjustments of the detector to the experiment are
sometimes necessary.
Time resolved experiments are conducted using fast shutters or time resolved X-rays detectors. The fast shutters can open in a short period and control their timings. The time-resolved
scattering pattern is observed with successive periods of measurements. Therefore the number ofphotons scattered in each experiment is quite small and an integrated-type detector with a highdetection efficiency is employed for recording all incident photons. This method requires repetitionof measurements and thus the range of the application is quite narrow. Especially, it limits useof biological material because of radiation damage. To use fresh object in each experiment one
should prepare large amount of the object under investigation. To solve these problems X-ray
detectors with a fine time resolution and capability of performing continuous measurements havebeen desired.
The photon courting-type detector is favorable for the time resolved experiments because it caneasily associate the arrival time with the position of the incident X-ray. The MSGC and RAPID
have already succeeded in it. In addition, they have no dead time due to readout. Thus they
allow one to investigate the change of the scattering pattern continuously in a single measurement.
However, the instability of these detectors avoids a wide range of uses. The µ-PIC developed at ourlaboratory encodes successively the position and the time of the incident X-rays. Thus, it is possible
to arrange time-resolved measurements without dead time. Details of time-resolved measurements
performed by the µ-PIC will be dealt with in the subsequent chapter.

15

Chapter 2

X-ray area detector based on µ-PIC
In this chapter, the characteristics of the µ-PIC , such as the structure of the detector and theprinciple of X-ray detection will be described. I will also discuss how the µ-PIC functions as an
X-ray area detector. Currently, the X-ray area detector based on the µ-PIC is being developed withthe aim of accurate observation of SAXS at third generation synchrotron facilities. The µ-PIC had
been used mainly in low-counting rate environments before the application to the X-ray detector
was started. Therefore, refinements to the system are required for operation in the high-counting
rate environment of the third generation synchrotron facilities. These refinements will be consideredin subsequent chapters.

2.1

The detector system based on µ-PIC

A µ-PIC is a micro-pattern gaseous detector fabricated by printed circuit board technology. The
printed circuit board technology easily introduces
an extension of the size of the detector. Figure
2.1 represents its schematic structure and Fig.
3.5 mm
2.2 shows the pixel structure of the µ-PIC . An
0.7 kV/cm
anode electrode structure surrounded by cathode rings prevents discharges in the µ-PIC [21][22]. This structure makes the µ-PIC more ro3.5 mm
bust against discharges than the MSGC.
3 kV/cm
Anode and cathode strips on the µ-PIC are
formed orthogonally on a polyimide substratewith a pitch of 400 µm. We employed a strip
readout to reduce the number of channels. As
shown in Fig. 2.2, anodes are located in the center of each pixel and connected by a strip un400 µm
der a 100-µm thick substrate fabricated by anelectric plating process. Figure 2.3 shows electric field around a pixel of the µ-PIC , calculated Figure 2.1: The schematic of the X-ray area deby Maxwell3D. A positive potential is applied to tector based on the µ-PIC . Above the µ-PIC
the anodes, and the cathodes are grounded, and the GEM is installed and operated at a low gas
then a strong electric field develops around the gain.
anodes. Electrons produced by charged particles
drift along the electric field applied across the gas
volume and reach at the anodes located in the center of the µ-PIC pixels. The electrons drift in thestrong electric field around the anodes and produce avalanches. Thus, the electrons are amplified
and detectable by electronics connected to the µ-PIC .
16

In the present work, a µ-PIC with an active area of 100×100 mm was used (manufactured by
Dai Nippon Printing Co. Ltd.). Owing to the strip structure, the µ-PIC has 256 × 2 channels
readout while it has 256 × 256 pixels (= 6.6 × 10 4 ). The system where each strip is instrumented
with an amplifier and a discriminator is simple and thus provides fast readout. Figures 2.4(a) and(b) show an overview and internal picture of the µ-PIC detector system. The electrode geometry
was optimized by a three-dimensional simulation [23] [24]. The gas gain of the µ-PIC as a functionof the voltage applied to the anodes is shown in Fig. 2.6. The maximum gain of 2 × 10 4 wasachieved. Stable operation was achieved for more than 1000 h at a gas gain of 6000, with an ArC2 H6 (90:10) gas mixture. The uniformity is an important issue for X-ray area detectors. The gain
map is shown in Fig. 2.7 where the gas gain uniformity was ∼ 5% (RMS) [25]. A two-dimensional
position resolution of 120 µm rms, discussed in a subsequent section, was obtained for continuum
X-rays around 10 keV with a Xe-C2 H6 (70:30) gas mixture. A larger µ-PIC with an active area of
300 mm×300 mm has also provided a stable operation for over one month.

Figure 2.2: Schematic cross-section of the pixel structure of the µ-PIC .

ICUECVJQFGCPQFG RQN[KOKFG

Figure 2.3: Electric field around a pixel of the µ-PIC . It was calculated by Maxwell3D. Electrons
produced by charged particles drift along the electric field applied across the gas volume and reachat the anodes located in the center of the µ-PIC pixels. The electron drift in the strong electricfield around the anodes and produce avalanches.
The principle of X-ray detection is almost the same as that of RAPID mentioned in the previous
chapter. An X-ray photon is stopped in a gas (cf. Xe 70% C 2 H6 30%, Xe/Ar/CO2 ), followed byinduction of an electron, which creates electron clouds in the gas. The electron clouds then drift
toward the GEM installed above the µ-PIC. The GEM consists of a polyimide insulator and copper
electrodes. The copper electrodes are formed on both sides of the polyimide insulator, with holesof 50 µm diameter and a pitch of 140 µm. When a voltage is applied between these electrodes,
17

(a)

(b)

Figure 2.4: (a)Overview and (b)internal picture of the µ-PIC detector system.

O-rings

poly-imide window

drift planevacuum
pomp

GEM
µ-PIC

pre
-amplifiers

sealed vessel

printed circuitboard

effective gas gain

Figure 2.5: To protect the polyimide window, it is covered when the air is removed.

Ar 80% + C2H6 20%
Ar 90% + C2H6 10%

3500
3000

104

2500
2000
1500
1000
500

103
400

420

440

460

480

500

520

540

560

580 600 620
anode HV [V]Figure 2.6: Gas gain of the µ-PIC as a functionof the voltage applied to the anodes.

0
10
An 8
od
e[ 6
cm
]

4

2

0 0

2

4

10
8
[cm]
6
e
d
o
Cath

Figure 2.7: Gain map of the µ-PIC .

18

electrons are multiplied while passing through the holes. The electrons from the GEM drift toward
the anodes on the µ-PIC and create avalanches around the anodes. The GEM, introducing a gainof 2 - 3, is temporarily installed due to lack of a gas gain of the µ-PIC. After enhancing the gas
gain of the µ-PIC , the GEM will be removed. The GEM is positioned 3.5 mm from the µ-PIC
and also 3.5 mm from the entrance window. We employed a GEM manufactured by Scienergy Co.
Ltd. Japan. The electric field applied across the induction region was 3 kV/cm, and that acrossthe drift region was 0.7 kV/cm, as shown in Fig. 2.1. The µ-PIC was operated at 2 × 10 3 , theGEM at 3, and the total gain was about 5 × 10 3
The detector is contained in a sealed aluminium vessel with a polyimide entrance window of
0.1-mm thickness. The use of the polyimide limits 8-keV X-ray absorption at the entrance window
to 10 %. The detector is filled with a Xe-C 2 H6 (70:30) gas mixture at a pressure of 1 atm. The
vessel is designed to withstand a gas pressure of 1 atm in order to create a vacuum inside thevessel before filling it with gas. This operation shorten a time to renew gas inside the vessel. Since
the creation of a vacuum reduces outgas after filling a sealed vessel with gas, this operation isnecessary for a long-duration operation. We used Araldite to adhere the polyimide window to thevessel. Thus the entrance window may come loose when one side of the window remains at 1 atm
and the other side is exposed to a vacuum. When the air is removed, the window is covered byan aluminium cap shown in Fig.2.5 and thus both sides of the window are kept under a vacuum.
After filling the vessel with gas the cap is removed. The detector works stably for more than one
month without renewing gas supply. The gas gains of the µ-PIC and the GEM decline by 10-20%
after one month operation, but remain well above a threshold.

2.2

Readout system

Signal extension board
To connect all electrodes of the µ-PIC to the amplifiers outside the gas vessel, we developed thesignal-extension printed circuit board shown in Fig. 2.8. It connects the µ-PIC inside the vesselto IC-amplifiers placed outside. In general, signals generated inside a sealed vessel are sent toelectronics located outside through field-through connectors arranged at a wall of the vessel. The
connectors using a large space are not suitable for 512-ch readout of the µ-PIC . On contrary,
the extension board with only two connectors saves a space and provides a compact system. The
extension board was held securely between the upper gas vessel and the bottom plate with O-rings,
as shown in Fig. 2.5. On the board, resistors of 1 MΩ that function as dumping ones, jumpers
and capacitors of 100 pF are arranged. Figure 2.9 shows a circuit diagram of the signal extension
board. Each strip is connected to the high voltage supply via a resistor of 1 MΩ and a jumper. Each
strip is also instrumented to readout via a capacitor of 100 pF and thus the high voltage applied
to the µ-PIC is cut by the capacitor. The jumpers would be removed when one needs to isolate
electronically a strip involving a discharging electrode. After the bad strip is isolated the µ-PIC
can be operated without alteration of the detector properties. On the other hand, the resistors of
1 M Ω are arranged to reduce damage caused by discharges. The effects of a few missing strips on
profiles of SAXS is negligible. On average, a few strips per detector are removed during one year
operation. In the case of GEMs, once they incur serious damage caused by discharges, they need
to be replaced with new ones. Therefore the µ-PIC is easy to handle and has attained a tolerance
for long duration operation.

2.2.1

Amplifier-shaper-discriminator (ASD) chips

The data-acquisition system consists of amplifier-shaper-discriminator (ASD) chips [27], a position
encoding module [28], and a memory module on a VME bus. Each ASD chip contains four channelsof amplifiers and discriminators. A block diagram of the data acquisition system is shown in
Fig.2.10. The output charges of the 256+256 channels are pre-amplified, shaped, and discriminated19

holes for screws
fixing O-rings

connector
to µ-PIC
holes for keeping
both sides of theboard under vacuum

onboard resistors
1 MΩ and 0Ω

onboard capacitors
100 pF

connector to
ASDs and HV supply

Figure 2.8: The signal-extension printed circuit board has connectors to µ-PIC , onboard resistorsof 1 MΩ and jumpers, capacitors of 100 pF, and a connector to ASDs and high voltage supply. The
jumpers are arranged for isolation of a strip involving a discharging electrode.

100 pF

1 MΩ

ASD ch1

HV
Jumper cut

electrode ch1

100 pF

1 MΩ

ASD ch2Jumper cut

electrode ch2

100 pF

1 MΩ

ASD ch256
Jumper cut

electrode ch256

Figure 2.9: The circuit diagram of the signal extension board connecting the µ-PIC and the ASDs.
Each strip is connected to high voltage supply via a resistor of 1 MΩ and a jumper.

20

in parallel by the ASD chips with a decay time constant of 80 ns. ASD chips with a decay time
constant of 16 ns are also available. However, the decay time of 16 ns is short to integrate all electron
clouds drifting toward the µ-PIC and thus their gain is sometimes insufficient to meet needs of some
applications such as X-ray detectors and Time Projection Chambers (TPCs) observing Minimum
Ionization Particles (MIPs). Therefore we used the ASD with decay time constant of 80 ns. A
common reference threshold voltage was supplied to each of the ASD chips.
The time constant of signals integrated by the ASD chips was also determined by the integration
time of charges generated in the µ-PIC . The oscillograms in Fig.2.11 show the resultant signal at
the analog and digital outputs of the ASD with decay time constant of 80 ns against impulse inputs
ranging from 0.1 pC to 0.4 pC. Figure 2.12 demonstrates the signal shape at the analog outputsof the ASDs with decay time constant of 80 ns and 16 ns. For plots of Fig. 2.12, the ASDs wereconnected to the µ-PIC irradiated with 5.9 keV X-rays. It should be noted that the time constantof the ASD analog output slowed down when it is connected to the µ-PIC . This fact implies thatthe time constant of signals generated by the µ-PIC is obviously slow. The charges generated by
the µ-PIC are converted to the digital signals by comparators contained in the ASD chips in order
to simplify the subsequent readout system. The timing chart with the analog output and the digital
output of a channel contained in an ASD chip is shown in Fig. 2.13.

Figure 2.10: A block diagram of the data acquisition system.

2.2.2

Time walk at comparator output

Since digital signals are generated by the comparators at the instant, when the analog signals crossthe threshold, the ASDs exhibit time walk for variations in the amplitudes of the analog signals.
Figure. 2.14 shows the propagation delay at the comparator output as a function of input charge.
The ASD with a decay time constant of 16 ns was used. Overall time walk is less than 4 ns when
the input charges are in the range of 0.05 pC to 2 pC.

21

Figure 2.11: Resultant signal at the analog and
LVDS digital outputs of an ASD against impulse
inputs ranging from 0.1 pC to 0.4 pC. Decay time
constant of the ASD is 80 ns. The grid cell is of
40-ns × 50-mV size[27].

Figure 2.12: Signal waveform at the analog outputs of the ASDs with decay time constant of 80
ns (lower) and 16 ns (upper). The ASDs amplified signals from the µ-PIC irradiated with 5.9
keV X-rays.

Analog output

Threshold

Digital output

Figure 2.13: Timing chart with the analog output and the digital output of a channel contained Figure 2.14: Propagation delay at comparator
in an ASD chip. The rising edge is sampled by output as a function of input charge. An ASD
the position encoding module.
with a decay time constant of 16 ns was used.
Overall time walk is less than 4 ns when input
charges are in the range of 0.05 pC to 2 pC.

22

2.2.3

Encoding module

All discriminated digital signals are sent to the position encoding module consisting of Field Programmable Gate Arrays (FPGA) with an internal clock of 100 MHz, allowing the recording of theanode and cathode coincident position= (X, Y ) and the time (T ) to the memory module. Figure
2.15 shows a schematic of the encoding system. The position encoding is performed in the following
steps:
1. Edge detection The FPGAs sample digital signals from the ASDs at their rising edge shownin Fig. 2.13. Because only the rising edges are monitored, each channel is insensitive to any
subsequent signals while the signal from the µ-PIC exceeds the threshold.
2. Synchronization The timing of the signal is synchronized with the internal clock of the
FPGAs.
3. Coincidence When the synchronized signals originating from the anodes and the cathodes
coincide in a clock interval (10 ns), the two dimensional position time-stamped with a resolution of 10 ns, is calculated. We used the coincident signals to improve signal-to-noise ratio.
The time resolution is determined by the internal clock of the FPGAs (10 ns).
4. P osition encoding If two or more channels generate signals within a clock of 10 ns, theaverage of the channels for the maximum and minimum coordinates is calculated. If thedifference between the maximum and minimum coordinates is more than eight, the eventsare discarded. The range of electrons liberated by 10-keV X-rays in a gas is much shorter
than 3.2 mm (corresponding to 8 channels ×0.4 mm). Therefore most events having a wider
width than 8 channels are considered to involve noise.
5. Data Readout The resultant data are buffered in a FIFO on the encoding module unless the
FIFO overflows with data. Then the data are output to the memory board at a rate of 50
MHz and stored. The memory board can hold 8.7 Mevents. Taking into account the transfer
rate to the memory board, the FIFO overflows at counting rates of more than 50 MHz andas a consequence, a part of the data are lost.
Thus, the position encoding is completely executed by digital processing. This method dramatically increased the limit on counting rates to >1 Mcps, while conventional methods based on
analogue pulse processing, such as delay lines and charge division, have achieved a counting rateof several hundreds of kcps. Moreover, the simple digital readout contributes much to the stabilityof the detector system and the convenience for users.

2.2.4

ASD reference thresholdIn this experiment, the ASD reference threshold voltage was set to 25 mV, which corresponds toabout 7 × 104 electrons (= 0.01 pC). This value is determined by both the gain of the ASD chips(0.3 pC/V) and the buffer amplification for the ASD comparator (×7). This number of electrons
corresponds to the energy deposition of about 0.17 keV X-ray photons in Xe gas, when the gas
gain is 104 .
An X-ray photon of 13.8 keV generates about 600 primary electrons in Xe gas, which distributeover approximately 4 strips in the µ-PIC . These electrons are amplified to 6 × 10 6 by the µ-PIC
and the GEM. Then, the number of amplified electrons deposited is 1.5 × 10 6 per strip on average,
which corresponds to 3keV. Considering a Gaussian-like distribution for an electron cloud, a 5 keVenergy deposit is expected on at least one strip. Despite of the large uncertainty of gains and nonuniformity (a few 10% at maximum), the threshold allows sufficient margin for counting loss. The
background counts reached a few cps due to cosmic-ray muons and electrical noise. This counting
rate is much lower than the counting rates of X-rays. Thus, the background counts were negligible
in the experiments performed at SPring-8.
23

256 ch LVDS output from cathodes

256 ch LVDS output from anodes

FPGAs work with an internal clock of 100 MHz
FPGA encoding anode strips
FPGA calculating the anode
Rising edges

and cathode coincident positionsaresampledWhen signals from anodes
and cathodes coincide within(X1, T)

one clock interval (10 ns)(X2, T)......

If (|XN - X1| < 9 && |YN-Y1| < 9),(XN, T)((X1+XN)/2, (Y1+YN)/2, T)

X1 < X2 < ... < XN

is sent to a memory board.

FPGA encoding cathode strips

33 bits per event

Rising edges

1bit: write flagare32sbit: (X1+XN)/2, (Y1+YN)/2, T)

sampled(Y1, T)(Y2, T)

FIFO......
(YN, T)

50 MHzY1 < Y2 < ... < YN
memory board
with 64 MB of SRAM
(8.7 M events can be stored)
Figure 2.15: A block diagram of the position encoding system used for the X-ray area detector.

242.2.5

X-ray imaging

Figure 2.16: X-ray image of a test chart irradiated with an X-ray generator [26]. The slits ofvarious width are scribed in the 0.05 mm thicklead layer on the plastic plate. The slits of 0.5
mm width are separated clearly.

Figure 2.17: Projection along the edge
√ of the test
chart fittedbya+aerf((x−a)/(2σ)), where
1
2
3
Rx
√
erf(x) = 0 exp(−t2 )dt/ π and a1 , a2 , a3 , σ aredetermined by fitting.

Using this position encoding system, two-dimensional X-ray imaging was performed. The output of the encoding module was integrated without corrections and two-dimensional images wereobtained. Figure 2.16 is the image of a test chart irradiated with an X-ray generator [26]. The
acceleration voltage of the X-ray tube was set to 12 kV with a 1 mm thick aluminum filtering to
cut low energy X-rays. To enhance the position resolution, the vessel was filled with a Xe basedgas mixture (Xe 70% C2 H6 30%). Xenon has a large stopping power, and thus a practical range forelectrons liberated by 12 keV X-ray is 1 mm in xenon gas whereas the range in argon gas is 2 mm.
For this reason, a xenon gas was used. The detection depth was set to 2 mm to reduce diffusion
effects. The position resolution was evaluated from this image.
If the fluctuation of encoded positions is characterized by a Gaussian distribution with deviation σ, the projection of an edge will be described by the error function.√ Figure 2.17 shows aprojection
along the edge of the test chart fitted by a 1 + a2 erf((x − a3 )/( 2σ)), where erf(x) =Rx√
20 exp(−t )dt/ π. The parameter σ, corresponding to the position resolution of the µ-PIC, wasdetermined to be 120 µm, . The √position resolution of a pixel gaseous detector with
√ a pitch of dis theoretically limited by σ = d/ 12. The theoretical limit of the µ-PIC is 400/ 12 = 115 µm,
which is close to the measured position resolution. Thus the position resolution is reasonable andof good quality.

2.2.6

Time Projection Chamber based on µ-PIC (micro-TPC)

The µ-PIC also delivers another application, a Time Projection Chamber (micro-TPC). The twodimensional data combined with timing information allows the reconstruction of the three-dimensional
tracks of charged particles. Figure 2.18 shows a schematic of the micro-TPC. The charged particles
causes the formation of clouds of ions and electrons in gas. The electron clouds drift toward theµ-PIC along the electric field applied across the gas volume. Because of the constant drift velocityof electrons within the gas of the detector, electron clouds generated at different height above theµ-PIC will reach the surface of the µ-PIC at different times. This time lag is proportional to theheight of the electron cloud, and thus their relative position can be reconstructed using the timing

25

information. If the encoding module is triggered by a simultaneous event, the absolute height canbe obtained.
Figure 2.19 shows a block diagram of the position encoding system used for the micro-TPC.
The system is almost the same as that mentioned in the previous section, but the format of
the output data is different. In the case of the
micro-TPC, two-dimensional positions are notaveraged. Instead, the maximum and minimum
coordinates are recorded and thus one can derive
the sizes of the electron clouds. The output dataalso contain additional information to identify
events belonging to the same trigger (event ID,
see Fig.2.19). In this method, events with a big
difference between the maximum and minimum
coordinates must be removed because they occaFigure 2.18: A schematic of a micro-TPC. Besionally involve noise. Thus, deriving the maxicause of the delay in arrival times of electron
mum and minimum coordinates is useful to imclouds at the surface of the µ-PIC , the ASDs
prove signal-to-noise ratio. To measure absolute
amplify the signals from the µ-PIC at different
positions, the position encoding system is trigtimes.
gered by a signal produced in coincidence withthe charged particles. Then, the absolute positions can be calculated using the drift velocity in the gas and the time difference between thetrigger and the arrival times of the electron clouds.
Figure 2.20 shows a typical cosmic muon track. The micro-TPC with the detection depth of
15 cm was filled with a 1atm Ar-C2 H6 (90:10) gas mixture. The detector was operated at a highgas gain of 3 × 104 to ensure that the small charge deposition of MIPs, 4 electrons per strip (witha pitch of 400 µm), exceeds the detection threshold. Two plastic scintillators were located aboveand below the micro-TPC. A coincidence of these scintillators was used for the muon trigger andthe absolute height of the track was measured. The position resolution of muon tracks was about
400 µm.

2.3

Calibration of X-ray area detector

In this section, fundamental issues in calibration of X-ray area detectors, calculations of a camera
length and a refinement of non-orthogonality to the beam, will be described.

2.3.1

Camera length

To obtain a scattering angle, the camera length defined as the distance between the sample and thedetector plane must be measured. In general a well-known powder having strong Debye-Scherrer
rings is favorable for the camera length calibration. Since the diffraction angles of the rings are
known, the camera length can be calculated. In addition, the shape of the rings provides informationof the non-orthogonality of the detector. We used silver behenet and CeO 2 powder for calibration
at the SPring-8 and the KEK-PF beam line, respectively.
We used the two-dimensional data analysis program FIT2D developed by Hammerley [29] forthe correction. The FIT2D provides 2-theta scans of the rings. It also allows a beam center andthe non-orthogonality of the detector to be refined.

26

256 ch LVDS output from anodes

FPGAs work with an internal clock of 100 MHz

trigger

FPGA encoding anode strips

clock is set to 0

Rising edges
are

FPGA calculating the anodesampledand cathode coincident positions
When signals from anodes(X1, T)and cathodes coincide within(X2, T)

one clock interval(10 ns)......
(XN, T)

If (|XN - X1| < 9 && |YN-Y1| < 9)

X1 < X2 < ... < XN

(X1, XN, Y1,YN, T)

256 ch LVDS output from cathodes

is sent to the memory board.
FPGA encoding cathode strips

33 bits×2 per event

Rising edes

1 bit: write flag (for memory board)are32 bits: (X1, XN, event_ID)

sampled1 bit: write flag
32 bits: (Y1, YN, T)(Y1, T)
50 MHz(Y2, T)......
(YN, T)

memory board

Y1 < Y2 < ... < YN

with 64 MB of SRAM
(4.4 M events can be stored)

Figure 2.19: A block diagram of the position encoding system for use with the micro-TPC.

27

Z [cm]16
14

Z [cm]

16
14

12

12
10

10

8

8

6
4

6

2
0
Y
10

4

[c

2
0
0

2

4

6

8

6

10
Y [cm]

4

2

0 0

2

4

6

10
8 X [cm]

10

Y [cm]

X [cm]

10

8

8

6

6

4

4

2

2

0
0

m8
]

2

4

6

8

0
0

10
X [cm]

2

4

6

Figure 2.20: Typical cosmic muon track.

28

8

10

12

14
16
Z [cm]

2.3.2

Refinement of non-orthogonality

Occasionally, the detector is irradiated with the beam at a slant to reduce the parallax effect ina wide-angle region. In general, the detector should be aligned perpendicular to the beam but it
is difficult to eliminate a small deviation from the correct position. Therefore, there is a need tocorrect for this non-orthogonality.
We now consider the projection of the rings to the detector plane. A spatial orientation of theframe of the detector is expressed by Euler angles. The detector system is denoted by the letters(X, Y , Z) and a reference system is denoted by the letters (x, y, z), shown in Fig. 2.21. The
direction of the z-axis is defined to be parallel to the beam. The Euler angles φ 0 , θ0 , ψ0 are defined
as below:• φ0 represents a rotation angle around the axis z of the original reference frame.
• θ0 represents a rotation angle around an intermediate X axis, which corresponds to an angle
between the z-axis and the Z-axis.
• ψ0 represents a rotation angle around the axis Z of the final frame identical to the detector
frame.
The position of a point on the detector is expressed by (X, Y , 0) in the detector coordinate
system. A coordinate transformation to the reference system is done by multiplication of thematrices representing the three simple rotations. The resultant coordinate is expressed as in theequation    


 
x
0
cosψ0 sinψ0 0
cosθ0 0 −sinθ0
cosφ0 sinφ0 0X
y  = 0 + −sinψ0 cosψ0 0  0
1
0  −sinφ0 cosφ0 0  Y  , (2.1)
z
l
0
0
1
sinφ0 0 cosφ0001Z
where l is a distance between the sample and the detector. A Debye-Scherrer ring with ascattering or diffraction angle of 2θ is included in a region expressed as in the equation
x2 + y 2 − z 2 tan2 2θ = 0.

(2.2)

X 2 (cos2 θ0 − sin2 θ0 tan2 θ) + Y 2 + 2lXsinθ0 tan2 θ − l2 tan2 θ = 0.

(2.3)

On substituting (x, y, z) by equation (2.1), the projection of equation (2.2) to the detector
plane is given by

Since the geometry has symmetry with respect to rotations about the beam (the z-axis), φ 0and ψ0 vanish in equation (2.3). The projection is a hyperbola when cos 2 θ − sinθ0 tan2 θ < 0. It isan ellipse when cos2 θ − sinθ0 tan2 θ > 0. The plots of the lefthand side of Fig. 2.22 show examplesof the projections for various values of θ. They demonstrate simulations of a diffraction pattern
for a CeO2 powder irradiated with 0.7744 Å X-rays. The camera length was set to 125 mm. The
points follow a Gaussian distribution with width σ = 0.08 mm. The plots of the lefthand side ofFig. 2.22 were transformed to a plane vertical to the beam. The figures on the righthand side show
the transformed plots.

2.4

Time-resolved measurements

In this section, two examples of time-resolved measurements using the X-ray area detector basedon the µ-PIC and the encoding system are described: time-resolved X-ray diffraction and theContinuous Rotation Photograph (CRP) method. The detector system was able to perform timeresolved measurements without significant dead time. Advantages of continuous time-resolved
measurements are also discussed in following sections.
29

X
x
O

Z

l

θ0 z

sampleyY

Figure 2.21: Definition of the detector coordinate system (X, Y , Z) and the reference system (x,y, z). The direction of the z-axis is defined to be parallel to the beam.

2.4.1

Time-resolved X-ray diffraction

Figure 2.24 and 2.25 show a pyromellitic acid hydrate and anhydrate powder diffraction patternobtained with the X-ray area detector based on the µ-PIC . The hydrate is a compound formed by
the addition of water to the pyromellitic acid and the anhydrate is one that loses the water. Since
they are powder diffractions, the intensity of diffracted photons depends on only 2θ. The objects
were irradiated with 0.7 Å X-rays at KEK-PF. The distance between the detection area and thesample was 137 mm. The experimental setup is shown in Fig. 2.23. The diffraction patterns of thehydrate and anhydrate are clearly different and it is easy to distinguish these objects. The detector
was placed perpendicular to the beam and thus the peaks in high-angle region are wide. To reduce
this parallax effect, the detection depth should be reduced and the detector should be placed on atilt to the beam. In the case of SAXS measurements, 2θ is typically less than a few degrees andthe parallax is negligible.
The hydrate loses the water and becomes the anhydrate when heat is applied (140 ◦ C). The
removal of the water from the hydrate is called dehydration. The anhydration diffraction patternshown in Fig. 2.24(b) and 2.25 was observed after the dehydration reaction had finished.
To prove the efficiency of continuous time-resolved measurements and measure the time resolution, the dehydration reaction was observed in real time. The reaction occurs so quickly that thedehydration process itself can not be observed by the µ-PIC detector system. On the other hand,
the weight fraction of the hydrate changes much more slowly, within several seconds. Thus theobserved diffraction pattern is determined by the weight fraction of the hydrate. Figure2.25 shows
the time-resolved profile as a function of time. In Fig. 2.25 and Fig. 2.27, one can see that thereaction occurred in about 7 sec and the diffraction pattern clearly changed.
Assuming the background and mass absorption coefficient are constant, the observed intensityI(2θ, t) is expressed as I = xId (2θ) + (1 − x)Ih (2θ), where Id (2θ) (Ih (2θ)) is the intensity of theanhydrate (the hydrate) including the constant background. The mass absorption coefficient of
the object will change as unbound water is evaporated. However, the contribution of evaporated
water to the X-ray absorption is small and thus the mass absorption coefficients for the hydrate
and the anhydrate are sufficiently close. The diffraction pattern at each frame is fitted by I =xId (2θ) + (1 − x)Ih (2θ) and x is obtained from the fit.
The time-resolved x as a function of time is shown in Fig. 2.28. The weight diffraction of
hydrate starts from 1 and ends with approximately 0, which means the dehydration has finished.
The x values are plotted every 0.65 sec using 4.3 × 10 4 events. In the case of this system, analysis
of diffraction patterns generally requires more than 10 4 events. Therefore the time resolution isdetermined by not only the counting-rate limit of the encoding system but also the intensity of Xrays. We used the low-intensity X-ray beam at KEK-PF and observed the slow diffraction change.
30

10

10

8

25

6
4

20

2

25

6
4

20

2
15

0
-2

10

-4
-6

5

-8
-10
-10

30

8

0

15

-2

10

-4
-6

5

-8
-8

-6

-4

-2

0

2

4

6

8

10 0

-10
-10

-8

-6

-4

-2

0

2

4

6

8

10 0

(a) θ = 10◦

10

10

8

25

6

25

6
20

4
2

15

0
-2

10

-4
-6

5

-8
-10
-10

8

20

4
2

15

0
-2

10

-4
-6

5

-8
-8

-6

-4

-2

0

2

4

6

8

10 0

-10
-10

-8

-6

-4

-2

0

2

4

6

8

10 0

(b) θ = 20◦

10

10
25

8
6

20

4
2

15

0
-2

10

-4
-6

5

-8
-10
-10

30

8

25

6
4

20

2
0

15

-2
10

-4
-6

5

-8
-8

-6

-4

-2

0

2

4

6

8

10 0

-10
-10

-8

-6

-4

-2

0

2

4

6

8

10 0

(c) θ = 30◦

10

24

10

8

22

8

20

6

18

4

16

2

14

2

12

0

-2

10
8
6

-6

4

-8
-10
-10

2
-8

-6

-4

-2

0

2

4

6

8

10 0

30

4

0

-4

35

6

25
20
15

-2
-4

10

-6

5

-8
-10
-10

-8

-6

-4

-2

0

2

4

6

8

10 0

(d) θ = 40◦

Figure 2.22: Projection of diffraction cones to a slanted detector plane with φ = 0 ◦ and ψ = 2◦
(left). The plots of the lefthand side are converted to ones in a plane vertical to the beam (right).

31

The counting rates of 70 kcps at this experiment has enough margin for encoding system. If samples
are irradiated with the higher intensity third-generation synchrotron radiation sources, higher time
resolution would be expected. Considering the encoding system (100 MHz), our goal is to observe
X-rays at a counting rate of 10 MHz. At this counting rate, one can obtain 10 4 events every 1 msec.
Therefore, adjustments of the detector system to high counting rate environments is important interms of fine-time-resolved experiments.
For most of the X-ray area detectors currently available, dead time caused by readout is unavoidable and diffraction patterns are recorded in limited intervals. Even the PILATUS, anotherphoton counting detector, is not free from the dead time, and repeated measurements using fast
shutters are required. Instead of continuous measurements one should prepare a large amount ofsamples and use repeated measurements to collect the data of diffraction pattern in each period.
Thus it takes a long time to measure all periods of the diffraction. A solution to this problem is
to combine three-fast CCDs, successively irradiate X-rays and read charges generated by X-rays[30]. This method provides continuous measurements at a frame rate of 3.4 msec. However, fast
CCDs have large readout noise that narrows the dynamic range. Furthermore, the CCD system
is so complex that they cannot be triggered by other instruments such as a beam monitor. Thesefeatures also prevent the CCD system from achieving wide utility.
Another solution for avoiding dead time is counting photons one-by-one and recording continuously. One-by-one photon counting is easy for gas detectors because there is no need to integrate
charges in the detection area and one can obtain signals in real time. We successfully developed
the system that count photons one-by-one and encodes their positions in real time. The X-ray area
detector based on the µ-PIC will provide a powerful tool in time-resolved X-ray measurements if
the maximum global counting rate is enhanced to 10 7 photons/sec.
In this paper, we will focus on measurements of Small Angle X-ray Scattering, which also
requires adjustments of the detector system to high counting rates environments. Therefore, detector systems succeeding in measurements of SAXS also lead to fine-time resolution for time-resolved
measurements.
&GVGEVQTEO$GCOUVQR:TC[DGCO
5CORNG

Figure 2.23: Schematic top view of instrumentation generally employed in this paper.

2.4.2

Continuous rotation photograph (CRP) method

Although the photon-counting area detectors such as PILATUS and the µ-PIC have been developed
recently , diffraction patterns from a single crystal were recorded by integrated-type area detectors.
The crystal is oscillating during the measurements to observe the diffraction patterns at various
angles. To observe a wide range of 2θ one should conduct many experiments with changing slightly
the rotation angle of the crystal.
A photon-counting type area detector, MSGC showed the new approach [31] [32]. The continuous rotation photograph (CRP) method was employed allowing all diffraction spots from the
32

8

100

6

80

100

8

80

6

60

60

4

120

counts120

10

Y-coordinate [cm]countsY-coordinate [cm]

10

4

40

40
2

2

20

0
0

2

4

6

8
10
X-coordinate [cm]

20

0
0

0

2

4

6

8
10
X-coordinate [cm]0(b) Anhydrate(a) Hydrate

Figure 2.24: Raw images of a hydrate and an anhydrate. The diffraction from the anhyadrate wasmeasured after the dehydration reaction had finished.

100

time [sec]Intensity [arbitrary unit]

R-T
dehydrate
hydrate

1

50

80

40
60

10-1

30
40

20

20

0

10-2

10

5

10

15

20

0
0

25
30
2 theta [degree]

50

100

150

200
250
radius [pixel]

Figure 2.25: Profiles of a pyromellitic acid hy- Figure 2.26: Time resolved diffraction pattern as
drate and anhydrate. The diffraction from the a function of time. The dehydration reaction ofanhydrate was measured after the dehydration a pyromellitic acid hydrate occured within 7 sec.
reaction had finished.

33

Weight fraction of hydrate

Intensity [arbitrary unit]

0 sec
1.95 sec
3.90 sec
6.50 sec

10

1

0.9

0.8
0.7

8

0.6

6

0.5
0.4

4

0.3
0.2

2

0.1
0
0

5

10

15

20

25

0
0

30
2 theta

1

2

3

4

5

6
7
Time [sec]Figure 2.27: Time resolved diffraction pattern as Figure 2.28: Weight fraction of the hydrate asa function of time. 0 sec is defined as the time a function of time. 0 sec is defined as the time
when the dehydration reaction started.
when the dehydration reaction started. The reaction is almost finished at 7 sec.

crystal to be observed in a short period of a few seconds. In this method, the crystal rotatesaround an axis using a goniometer. The CRP method requires a time-resolved area detector which
can associate the arrival time of a photon to the instantaneous rotation angle of the crystal. The
detector then provides a movie of the diffraction spots as the crystal rotates 360 ◦ or 180◦ . The
angular resolution of the diffraction spots is expected to be less than 0.1 ◦ . Using intense X-rays, themeasurement would be completed after a few rotations of the crystal. A measurement during many
rotations would enhance the accuracy of the data. The CRP method introduces a new parameter,
the rotation angle and hence it allows to remove most of the background X-rays.
The MSGC proved that the CRP method was useful and drastically shortened the measurement
time to a few seconds, which was 100 to 1000 times faster than CCDs and four-axis diffractometercurrently in use in this field. However, the MSGC, suffering from serious discharges and criticaldamages caused by discharges, was operated at a low gas gain of 1000 and maintained stable
operation for only a short period. The signal-to-noise ratio was deteriorated by the low gas gain andlack of data. Thus 9% internal agreement factor (R int ) that MSGC achieved could not be improved.
Due to the instability, the MSGC did not meet the requirements of X-ray crystallography [21]. R int
represents the degree of an agreement between experimental values and calculated values. R int is
0 % when these values completely agree and high-precision measurements reduces R int .
Alternatively, we examined the CRP method using the µ-PIC , which is more robust against
discharges and has attained stable operation for more than one month. Figure 2.29 shows integrated
diffraction spots form a rotating Ylid (C 11 H10 O2 S) crystal. Figure 2.30 shows the diffraction spots
as a function of the rotation angle. In this experiment, the µ-PIC was used without a GEM andworked stably at a gas gain of 3000. A 1atm Ar-C 2 H6 gas was filled in the vessel. We used the
Ar-based gas for maintaining a stable operation though the gas has less stopping power than the
Xe-based gas generally used. Although the raw data shown in Fig. 2.30(a) contains a large amountof X-ray scattered by the air, the clusters of diffracted photons are clearly recognized in the threedimensional space. Thus, it is possible to eliminate the background X-rays and Fig. 2.30(b) shows
the diffraction spots after the background reduction. Using these diffraction spots an internal
agreement factor, Rint of 3.7 % was obtained for an exposure time of 3700 sec. The Ylid crystal
was irradiated with an X-ray generator, and thus the burst rate of the strongest diffraction spots
34

was at most 3.6 × 105 cps, which was well within the capabilities of the current encoding system.
Irradiation with intense X-rays generated by third-synchrotron radiation sources would provide an
Rint of less than 1%.
It can be concluded that adjustments of the detector system to high counting rate environments
is useful not only for SAXS experiments but also measurements of time-resolved powder diffractions
and the CRP method with high accuracy.

Figure 2.29: Raw image of diffraction from an organic crystal Ylid integrated over 3700 sec.

(a)

(b)

Figure 2.30: X-ray diffraction from Ylid plotted in three-dimensional space. The diffraction pattern
forms many three-dimensional clusters. (a)Raw data. (b)Data after background reduction.

35

Chapter 3

Performance characteristics of theµ-PIC detector for high-count rateenvironments
The development of the µ-PIC was originally motivated by an application as a Compton camera
on board a satellite. The Compton camera consists of the micro-TPC surrounded by scintillatorsand the encoding system. An incident gamma-ray is scattered within the micro-TPC and thescattered gamma-ray is absorbed by the scintillator. Using the kinetics of the recoil electron andthe scattered gamma-ray, the incident gamma-ray can be reconstructed. The camera aims at
observations of cosmic gamma-rays for which counting rates are low. The encoding system for the
X-ray area detector based on the µ-PIC described in the previous section shares the same idea to
encode positions of signals with that of the Compton camera. This system is referred to as theclassical system.
To adjust the encoding system to high-counting rate environments, some improvements of thesystem are necessary. In this section, first we will demonstrate the performance characteristics
using the classical encoding system, point out its problems, and then show how the encoding
system should be improved. New logic for encoding positions is also demonstrated in this section.
We will also show the performance characteristics of the µ-PIC for high-counting rate environments
investigated mainly in the RIKEN Structural Biology beamline I BL45XU at SPring-8, Japan. This
beamline was designed for SAXS measurements.

3.1

Experimental setup

At the BL45XU, the photon flux can reach 10 12 photons per second. A pair of four-blade slits was
set in front of the sample to define the beam size. The beam intensity was adjusted using absorbers
installed in front of the first slit. The beam size in front of the sample was a rectangle of 1.0
mm (horizontal)×0.8 mm (vertical). A vacuum path shown in Fig. 3.1 was inserted between thesample and the detector to prevent from scattered X-rays by the sample interacting with the airand producing the background. At the end of the beam path, a polyimide window is arranged tokeep inside the path under a vacuum and reduce an absorption loss. A beam-stopper is arranged on
the window. The µ-PIC was set perpendicular to the X-ray beam and placed next to the polyimide
window.

36

sampleX-ray area detector
based on µ-PIC

beam

entrance
window

Figure 3.1: Overview of experimental setup in the RIKEN Structural Biology beamline I BL45XU
at SPring-8, Japan.

3.2

Performance characteristic

3.2.1

Linearity

Linearity in local counting rates
Linearity of the µ-PIC in local counting rates was investigated [21]. The X-ray area detector
without the GEM was locally irradiated with X-rays generated by an X-ray generator with a targetof molybdenum (Kα : 17.4 keV). The gas gain was 2000. Linearity in an anode current was plotted
as a function of counting rates in Fig. 3.2(a). A good proportionality to the anode current wasmaintained at a counting rate of up to 10 Mcps/mm 2 .
Linearity in global data acquisition rates
The measurement described above was conducted to investigate the performance of the µ-PIC
itself. The global data acquisition rate (DAR) of the encoding system must also be examined forthe actual applications. This was studied at BL45XU at SPring-8 using SAXS from a piece of glassy
carbon, whose scattering pattern is close to uniform. The camera length was set to 2200 mm. The
wavelength of the X-ray beam was adjusted to 0.9 Å (13.8 keV). A 1 atm Xe-C2 H6 (70:30) gas
mixture was used as an absorber of X-rays. In this section, all experiments were conducted withthis gas filling. The DAR was measured for a counting rate ranging from 20 cps to 5 Mcps. The
anode current of the µ-PIC depends linearly on the beam intensity at the detector, as discussed
above. Therefore, we compared the DAR and the leakage current of the µ-PIC to evaluate thelinearity in the DAR.
The DAR and the anode current demonstrated a good linear correlation from 20 cps to 5 Mcps,
as shown in Fig. 3.2 (b). The current limit of the high voltage supply module, which supplies
current up to 10 mA, limits the DAR. The anode current limit was roughly equivalent to a DAR
of 5 Mcps.

3.2.2

Distortion

In principle, micro-pixel gas detectors with strip-readout are free from an image distortion. It is
one of advantages of these detectors. On the other hand, CCDs suffered from a strong distortionneed corrections. In general the image of CCD-II includes an image distortion and a non-uniform37

(a) Anode current as a function of the intensity of X-rays [21](b) Global data acquisition rates as a functionof an anode current

Figure 3.2: Linearity in (a) local counting rates and (b) global data acquisition rates

response, due to the aberration of the electron lens in the Imaging Intensifier and the non-uniformityof the phosphor thickness.
Through measurements using the µ-PIC , a negligibly small distortion was detected in imagesof a grid mask. On the grid mask, holes were perpendicularly arranged at a constant interval. The
entrance window of the µ-PIC was masked and irradiated with scattering from a piece of a glassy
carbon. The wavelength of the X-rays was 0.9 Å (13.8 keV) and the camera length was 2200 mm.
Figure 3.3(a) and (b) show images of the grid mask observed by the µ-PIC . The global counting
rates of (a) and (b) are 8 kcps and 200 kcps, respectively. For reference, an image of the mask shownin Fig. 3.3(c) was observed by a CCD in the same condition. One can see a strong distortion in Fig.
3.3(c). However, the differences in the distance of the holes, as observed by the µ-PIC were within
1% of the entire active area. The deviation from the perpendicular was < 1 degree. Moreover,the change in the image was negligible when the beam intensity was varied. The peaks observedat the holes on the mask moved 2-8 µm when the beam intensity was varied from 8 to 200 kcps.
The change of 2-8 µm is negligible considering the pixel pitch of 400 µm. We concluded from theseresults that the strip readout and the coincidence of the strips caused no distortion. Even in thecase of micro pattern gas detectors, an analogue position encoding method would distort images
under moderate count rate environments due to their readout circuits. In the present evaluation,
all image profiles were performed without correction for image distortion.

3.2.3

Dynamic range

We examined the dynamic range of a solution scattering curve decreasing exponentially as q increases. Figure 3.4 shows the scattering curve. The target is a polystyrene latex whose diameteris 120 nm. The wavelength of the X-rays was 0.9 Å (13.8 keV) and the camera length was 2200
mm. The scattering curve of the latex, approximated by a solid sphere, is expected to follow thepower law of q −4 . A wide dynamic range of over 105 is obtained and the intensity clearly follows
the power law of q −4 .

38(a) µ-PIC (8 kcps)(b) µ-PIC (200 kcps)(c) CCD

Figure 3.3: (a)(b)Image of a grid mask. The entrance window of the µ-PIC was masked andirradiated with scattering from a piece of a glassy carbon. (c)Image of the grid mask recorded by
a CCD.

39

Figure 3.4: Solution scattering curve from polystyrene latex. The diameter is 120 nm. A scattering
curve of the latex approximated by a solid sphere follows the power law of q −4 . The dynamic rangeof over 105 is obtained and the intensity clearly follows the power law of q −4 . The classical encoding
system was employed.

3.2.4

Accidental coincidence

The strip readout of 256 ch + 256 ch simplifies the system. However, the encoding system needs
to overcome difficulties in encoding the position in high-rate environments. The encoding systemmonitors the whole detector and finds coincident strips. Therefore, in high-rate environments where
several X-rays are likely to arrive closely enough in time to pile up, the system often determines
incorrect positions and thus produces a ghost image. Diffraction pattern images of an Au nanoparticle solution at a DAR of 120 kHz and 2 MHz are shown in Fig. 3.5. One can see the clear
ghost image that consists of two rectangles crossing the beam center in Fig. 3.5 (a). The ghost
image is not seen in Fig. 3.5 (b) that was obtained in the low counting rate environment. The
incorrect calculations in high-counting rate environments occur as follows.
It is assumed that two X-rays arrive closely enough in time to pile up. Figure 3.6 demonstrates
such events. One of the X-rays hits the µ-PIC at (X1, Y 1) and the other at (X2, Y 2). If X1 and Y 1
coincide with the clock interval of T 1, X2 and Y 2 coincides with T 2, and T 1 6= T 2, two events are
successfully recorded. When X1, X2 are synchronized with the same clock T and |X1 − X2| > 8,both events are discarded. If signals generated by the anodes and cathodes completely coincide,
this encoding system works well and always records the correct coordinates. In this chapter, we
refer to an analog signal generated by the ASD as just a ‘signal’ and a digital signal as a ‘hit’.
Since an electron liberated by an X-ray crosses a few µ-PIC pixel, a few signal and then a few hits
are generated by the ASDs.
However, due to the time walk of the ASDs mentioned in the previous chapter, anode strips
and cathodes strips occasionally fail to coincide. In the previous chapter, using signals obtainedby a pulse generator, we proved that overall time walk was less than 2 ns when input charges
are in the range of 0.05 pC to 2 pC. However, due to the slow rise of signals, that is tens of
nanoseconds, generated by the µ-PIC , the time walk would increase when the ASDs are connectedto the detector. In addition, the low threshold of 0.01 pC makes the situation worse.
Figure 3.7 shows oscillograms of discriminated signals and analog output of the ASDs. Discrim-

40

6

10

8

5

10

4

6

103

10

3

4

4
102

2

0
0

10

countscounts104

8

Y-coordinate [cm]Y-coordinate [cm]

10

2

4

6

10

2
0

8
10
X-coordinate [cm](a) At a DAR of 2 MHz

0

2

4

2

10
8
6
X-coordinate [cm](b) At a DAR of 120 kHz

Figure 3.5: Diffraction pattern images of an Au nano particle solution at a DAR of 120 kHz and 2
MHz. Ghost induced by accidental coincidence can be clearly seen in (a). The classical encoding
system was used.

inated signals from anode strips were ORed together. Discriminated signals from cathode strips
were also ORed together. The 32-channel summed analog output in the Fig. 3.7 (b) shows thatthe signals sent from the anodes have the different waveform from those from the cathodes. The
difference in the waveform is due to the ASDs, whose performance is different for positive andnegative inputs. Given the difference in rising time constant of the anodes and cathodes, 20 to
30 % hits fail to coincide. It should be noted that a capacitor of 1µF connects the analog output
and a ground in order to reduce noise. The sloping rise and fall at the analog output is causedby the capacitor. Since an X-ray photon usually deposits its energy on only a few strips, anodes
and cathodes produce few signals. Due to the timewalk of the ASDs, not all discriminated signalsof the anodes and cathodes are in coincidence. In the worst case such as Fig. 3.7(c), all hits fail
to coincide and thus the incoming X-ray is not detected though the pulse height is well above thethreshold.
In high-counting rate environments, strips failing to coincide properly are likely to coincidewith other strips generated by other X-rays. For instance, X1 and Y 2 coincide in accident and anincorrect coordinate of (X1, Y 2) is recorded. Many such incorrect results form the ghost and distortthe image. In SAXS, most X-rays concentrate on the area close to the beam center. Therefore,
accidental coincidences involving an X-ray hitting close to the beam center are most likely to beobserved. When both of the two X-rays are observed near the beam center within an intervalof tens of nanoseconds, the resultant positions are likely to be wrong and would cause an image
distortion there, but do not contribute to the ghost image. When two X-rays hit the µ-PIC within
an interval of tens of nanoseconds and one is around the beam center and the other is apart fromthe center, they are likely to contribute to the ghost image. In this case, the X = X1 or Y = Y 2
cross the region close to the beam center and the other coordinate (Y = Y 2 or X = X1) ranges in
the whole detector. Therefore, the ghost image forms a cross.

41

counts

Y-coordinate [cm]

Y1

10

10 4

8

6
10 3
4

Y2

10 2
2

0
0

2

4

6

X1

8
10
X-coordinate [cm]

X2

Figure 3.6: Diffraction pattern image of an Au nano particle solution at a DAR of 2 MHz. The
classical encoding system was used. Two X-rays hit at (X1, Y 1) and (X2, Y 2) close enough intime to pile up. Due to the timewalk of the ASDs, X1 and Y 1 occasionally fail to synchronize withthe same clock T of the encoding module. Alternatively, X1 and Y 2 coincide in accident and anincorrect position of (X1, Y 2) is recorded.

3.3

X-ray detection efficiency with position encoding system

In the previous section, we showed that the encoding system sometimes fails to detect X-rays though
the pulse heights are well above the threshold. Then, X-ray detection efficiency was examined andthe result is shown in Fig. 3.8. The vessel containing the detector was filled with a Xe-C 2 H6
(70:30) gas mixture and was irradiated with 109 Cd (22.2 keV, 24.9 keV). In this experiment, theASD reference threshold voltage was set to ±23 mV, 64-cathode strips were ORed, and the analog
and digital output was monitored.
The numbers of hits (digital signals) generated by the cathodes were shown as a function ofpulse height of the 64-channel ORed analog output in Fig. 3.8. X-rays generated by 109 Cd emit
copper fluorescent of 8 keV when they interact with the copper electrodes of the µ-PIC . Thus awide range of X-rays can be obtained. The ratio of the hits having coincident hits generated by
the anodes (256 channels) was also determined. Hits sent from the anodes were measured in thesame way.
From these measurements, it is concluded that most hits are lost because of the failure ofcoincidence. An X-ray generates a few hits and thus the ratio of the lost X-rays is about 50 %.
In high-rate environments, the ratio of the lost hits is expected to be almost the same because
the detector system maintains linearity up to 5 MHz. The count loss is slightly decreased by theaccidental coincidence as counting rates increase. However, it mainly depends on the time walkcharacteristics of the ASDs. To avoid the count loss, all hits should be taken into account even if
they do not synchronize to any other hits. We will discuss such a method in subsequent sections.
It should be noted that the number of digital generated by the cathodes is larger than thatgenerated by the anodes because the threshold supplying the cathodes is higher than that supplying
the anodes. Since the minimum threshold level is dictated by the noise level that strongly depends
on the environment, the threshold level needs to be well-adjusted every time the environment
42

anodes(256 ch ORed)cathodes(256 ch ORed)
anodes and cathodes
ANDedanalog output of cathodes(32 ch summed)(a)anodes(256 ch ORed)cathodes(256 ch ORed)analog output of cathodes(32 ch summed)

analog output of anodes(32 ch summed)(b)

anodes(256 ch ORed)cathodes(256 ch ORed)
anodes and cathodes
ANDedanalog output of cathodes(32 ch summed)(c)

Figure 3.7: Discriminated signals and analog output of ASDs. Discriminated signals from anodestrips were ORed together. Discriminated signals from cathode strips were also ORed together.
Not all discriminated signals of anodes and cathodes are in coincidence. Oscillograms also show
32-strips summed analog output.
43

Number of hits

hit_c
hit_a
hit_ac654
32
1-80 -60 -40 -20

020 40 60 80
Pulse height [mV]Figure 3.8: X-ray detection efficiency. Numbers of hits (digital signals) generated by the anodes
and cathodes per photon, hita , hitc are plotted as a function of pulse height at analog output.
Ratio of hits (digital signals) having coincident ones, hit ac is also shown.

changes.

3.4

New position encoding method (mode-II)

In this section, improvements in the position encoding method will be discussed. From now, we
will refer to the classical encoding method as mode-I and the new method mentioned in this section
as mode-II. Figure 3.9 shows a block diagram of the mode-II position encoding system. Mode-II
allows one to record all hits. On the other hand, mode-I only records hits from the anodes and thecathodes when they coincide in a clock interval (10 ns). One can easily load new files to the FPGAs
which are components of the position encoding system, and then the new logic of the system canbe used. According to Fig. 3.8, the data acquisition rate doubles using the mode-II.
The number of generated hits strongly depends on the energy of the X-rays, the gas gain, andgains of the ASDs. In the case of an X-ray photon of 8 - 13.8 keV available at SPring-8, it generates
0 to 2 pairs of the hits which coincide within in a clock interval. We integrated the resultant hits
and obtained two-dimensional images shown in the previous sections.
The uniformity of the µ-PIC is quite good at about 5 % RMS. However, the low thresholdlevel of 25 mV (corresponding to 3.6 mV at the comparator input) supplied to the ASDs is in aregion where input charges are very sensitive to propagation delay at the comparator output. The
propagation delay is the main reason for the counting loss. Thus, the small gain variation leads tonon-uniformity of the signal detection efficiency.
Figure. 3.10 shows an approximately uniform illumination, the scattering pattern image ofglassy carbon, whose scattering curve declines gentle. The encoding system used was mode-I. The
non-uniformity can clearly be seen in Fig. 3.10. The many vertical and horizontal lines are due
entirely to the gain variation of the ASDs.

44

256 ch LVDS output from anodes

FPGAs work with an internal clock of 100 MHz
FPGA encoding anode strips
FPGA calculating the anode
Rising edges

and cathode coincident positionsaresampledif (|XN - X1| < 9 && |YN-Y1| < 9),((X1+XN)/2, (Y1+YN)/2, T).

(X1, T)else if(|XN - X1| < 9 && cathodes no hit),(X2, T)((X1+XN)/2, 2047, T).

......

else if(anodes no hit && |YN-Y1| < 9),(XN, T)(2047, (Y1+YN)/2, T).

X1 < X2 < ... < XN

The output is sent to a memory board

256 ch LVDS output from cathodes

33 bits per event
FPGA encoding cathode strips
Rising edges

1 bit: write flagare32 bits: (X, Y, T)

sampled(Y1, T)(Y2, T)

FIFO......
(YN, T)

50 MHzY1 < Y2 < ... < YN
memory board
with 64 MB of SRAM
(8.7 M events can be stored)
Figure 3.9: A block diagram of the mode-II position encoding system

45

10

Y [cm]

8

6

4

2

0

0

6

4

2

8

10

X [cm]

0.1

1

10

100
Intensity

1000

10000

Figure 3.10: Raw scattering pattern image of glassy carbon. The sample was irradiated with 0.9
Å (13.8) X-rays at BL45-XU SPring-8. The mode-I encoding system was employed.

3.4.1

Two-dimensional images obtained by mode-II

In mode-I, hits generated by an X-ray are recorded and integrated to obtain a two-dimensional
image. The recorded hits are the projection of the electron track liberated by the X-ray photon
to the µ-PIC detector plane. Thus, the two-dimensional image obtained is the integration of the√electron tracks. Considering the gas diffusion (the diffusion coefficient of 210 µm cm) and therange of the electrons, this method deteriorates the position resolution. Moreover, the number ofhits generated by an X-ray photon depends on the gain of the gas detector and the ASDs. Thus
the integration of the electron tracks emphasizes non-uniformity of the system.
To improve these problems, in addition to mode-II, the calculation of the positions of the X-rays
was improved. An X-ray generates several hits. Instead of integrating all the hits, the averaged
position of the hits is calculated. Since hits generated by an X-ray form a cluster, the averaged
time of the anode and the cathode clusters is refered. When the time difference is less than 100
ns, they make an anode-cathode pair. This method improves the position resolution because thediffusion that follows a Gaussian distribution is compensated. The averaged positions are filled ina two-dimensional histogram and the image can be obtained. This method is independent of thegain variations while an X-ray generates at least two hits, one by the anodes and the other by thecathodes.
To eliminate incorrect position calculations, we used X-rays which had no possibility of pile-up.
When the arrival time difference between two X-rays is less than 200 ns, they are likely to pileup, considering the time constant of the ASDs. Therefore, these X-rays are removed in the off-line
analysis. We refer to this cut as a pile-up cut.
For the evaluations of the mode-II position encoding systems, described in this section, we useda solution scattering patterns from SiO 2 nano particles and from Apo-Ferritin that is a protein.
The diameter of the SiO2 particles is 100 nm. These samples were irradiated with 1.5 Å (8 keV)
X-rays at BL45-XU SPring-8. The intensity of the X-ray beam in front of the samples was 1.5×10 10that is almost the same as the intensity of the beam used for the IPs. The camera length was set
to 2167 mm. Figure 3.11 shows solution scattering patterns from SiO 2 nano particles. Since a dataacquisition rate was high (6.3 MHz), a ghost image can be clearly seen in Fig. 3.11(a). The ghost

46

10

8

8

6

6

Y [cm]

Y [cm]

10

4

4

2

2

0

0

6

8

1000 10000
Intensity

1.E5

4

2

0

10

0

6

4

2

0.1

1

10

100

8

10

X [cm]

X [cm]

0.1

1.E6

(a)

1

10

100

1000 10000 1.E5
Intensity

1.E6

(b)

Figure 3.11: Solution scattering patterns from SiO 2 nano particles. The diameter is 100 nm. The
data was collected by the mode-II encoding system. The sample was irradiated with 1.5 Å (8 keV)
X-rays at BL45-XU SPring-8. (a)We caluculated the average the positions of the hits generated by
the X-rays but did not applied pile-up cut. A ghost image crossing a beam center can be clearly
seen. (b)The ghost image vanished when applying a pile-up cut.

image vanished after applying the pile-up cut.
Figure 3.12(a) and (b), produced using the same data show a solution scattering pattern from
Apo-Ferritin. In Fig. 3.12(a), all the events with the anode and cathode hits in the same clock (=10 ns) are filled in a histogram. In Fig. 3.12(b), the averaged positions of the hits are calculated andthe pile-up cut was applied. In Fig. 3.12(a) there are many vertical and horizontal lines while Fig.
3.12(b) is quite smooth. Two clear valleys are seen between the intense rings in Fig. 3.12(b). Thus,
the combination of the mode-II encoding system and the calculations of the averaged positions of
the hits drastically improve the detection uniformity and enhance the quality of X-ray images.

3.4.2

Aging behavior in high-counting rate environment

X-ray area detectors for use at third-generation synchrotron radiation sources must be resistant todamage caused by intense X-rays. The detectors are also required to confirm linearity and stability
during a long duration radiation.
Environments with high levels of X-ray radiation create special design challenges of the µ-PIC. In the case of the µ-PIC , after radiation with intense X-rays, the gas gain in the area aroundthe beam center drops permanently. An area irradiated with a strong scattering peak shows thesame feature. In Fig. 3.12(b), a small ring ((X, Y ) = (3.5, 7.5)) which corresponds to an area
damaged by intense X-rays distributing around the beam center can be seen. Figure 3.12(b) also
shows the drop in gain caused by two strong scattering peaks which had been observed before thismeasurement.
In principle, the small drop in the gain does not affect the detection efficiency when all signals
generated by X-rays are counted by the detector system. For the data presented in Fig. 3.12, the

47

10

8

8

6

6

Y [cm]

Y [cm]

10

4

4

2

2

0

0

6

4

2

8

0

10

X [cm]

0.1

1

10

100
Intensity

0

6

4

2

8

10

10000

1.E5

X [cm]

1000

0.1

10000

(a)

1

10

1000
100
Intensity(b)

Figure 3.12: Solution scattering pattern image of Apo-Ferritin. The sample was irradiated with
1.5 Å (8 keV) X-rays at BL45-XU SPring-8. Plots (a) and (b) use the same data set. (a)Events
having the anodes and the cathodes hit in the same clock (= 10 ns) are integrated. (b)All events
are analyzed on the basis of mode-II.

common reference threshold voltage supplied to each of the ASD chips is still high and the detector
failed to detect a part of X-rays hitting the damaged area, which do not exceed the threshold. Thiscan be avoided by applying the lower threshold. Therefore, it is important to improve signal-tonoise ratio.
The easiest way to obtain the fine signal-to-noise ratio is to raise the gas gain by installing
another GEM above the current one. However, when the amount of electrons amplified by theµ-PIC is large, they would damage the surface of the µ-PIC . Thus, it is important to reduce thenoise level. The extension board connecting the µ-PIC with the ASDs mainly contributes to thenoise problem because its connection to the ground level is weak and the length is too long. We
are developing a new extension board and will place readout circuits closer to the µ-PIC .
Another solution to this problem is to remove the ethane gas and use another kind of a quencher
gas such as CO2 . Intense X-rays allow hydrocarbon families to form larger complexes (polymerization), which pile on the electrodes of the µ-PIC . The polymers could change the electric field aroundelectrodes and reduce the gas gain. Instead of the hydrocarbon families, inorganic compounds like
CO2 or CF4 , which has more radiation-hardness , should be added.

3.4.3

External trigger

Since the intensity of the X-ray beam occasionally fluctuates, it must be monitored during a measurement by a reliable detector such as an ion chamber. To conduct a precise measurement of
the intensity of scattered X-rays, the X-ray area detector should synchronize with the monitoring
detector, using, for example, a common gate signal. Then, we improved the encoding system byallowing it buffer data in the FIFO only while receiving the gate signal.

48

3.4.4

Solution scattering measurement

With the gating and the introduction of the mode-II position encoding system, solution scattering
patterns were measured at SPring-8. A solute scattering curve is determined by subtracting thesolvent curve from the solution curve. Since their exposure times are sometimes different, thescattering curves are corrected by the integrated intensities of incoming photons. Another correction
to derive the correct solute scattering curve is for the pile-up cut.
In principle, the ratio of discarded events depends on the data acquisition rate because a probability of pile-up depends on it. The data acquisition rate is different between the solution andthe solvent scattering measurements. Thus, a correction for the ratio of the discarded events issignificant.
Then, the scattering curve without applying the pile-up cut was divided by the one with thecut. The resultant curve shown in Fig. 3.13 is almost flat except in the region shaded by the beam
stopper. The correction coefficient for the pile-up cut C p is the constant values of these curves.
Taking the corrections into account, the solution scattering curve must be multiplied by (C p )solvent /(Cp )solution .
The subtraction of scattering curves is sensitive to the correction coefficient and thus careful measurements and calculation are required. A 1%-deviation of C p disturbs a wide angle region where
the difference between the solution and solvent is quite small.
The resultant scattering curve is shown in Fig. 3.14 (c). It almost agrees with the scattering
curve measured by an IP as reference while (a) apparently deviates from the reference. Plots (a),(b) and (c) used the same data set. In the case of (a), hits from the anodes and the cathodes
when they coincide in a clock interval (10 ns) are used (equivalent to the mode-I). The peaks andthe valleys shown in Fig. 3.14(a) deviate from those obtained using the IP. The mode-I encoding
system caused the non-uniformity of the X-ray detection efficiency and thus the profile of thesolution scattering is slightly changed. The solute scattering curve determined by subtracting thesolvent curve from the solution curve is very sensitive to this change. Fig . 3.14 (b) shows the plot
without the pile-up cut. The plot almost agrees with the reference but the deviation from it islarger in the wide-angle region in comparison with Fig. 3.14 (c). Table 3.1 summarizes the resultsof the analysis of solution scattering from an Apo-Ferritin.
Figure 3.15 shows the intensity difference between the µ-PIC and the IP (R-Axis). The scattering patterns from Apo-Ferritin shown in Fig. 3.14(c) were used for the evaluation. The degreeof difference, 100(Iµ-PIC )(q) − IIP (q))/IIP (q) [%], is shown on the y-axis. Since the scattering patterns of the µ-PIC and the IP are consistent in the small-angle region, the intensity of the IP isnormalized to that of the µ-PIC using an averaged intensity in a range of 0.1 ≤ q ≤ 0.15 [nm −1 ].
The reason why the difference in the wide-angle region is especially large is the non-uniformityof the detection efficiency for X-rays. The drift plane located above the µ-PIC would be slantedslightly. The slanted plane leads to the non-uniformity of the thickness of the gas layer and thusof the detection efficiency.
Figure 3.16 shows solution scattering curves from SiO 2 nano particles. Plots (a), (b) and (c)used the same data set. In the case of (a), hits from the anodes and the cathodes when they coincidein a clock interval (10 ns) in the same clock (= 10 ns) were used (equivalent to the mode-I). Recall
that the scattering curve of the particles approximated by a solid sphere follows the power law.
The curve shown in Fig. 3.16 (a) approximately follows the power law of q −4 but the exponent
changes at q ∼ 0.6 [nm−1 ]. In the wide-angle region, the count loss due to the accidental coinciden
is significant because the difference between the solution and the solvent curve is very small. The
counting rate of the solution scattering is higher than that of the solvent scattering and thus thecount loss is larger. Therefore, the solution curve shown in Fig. 3.16 (a) would have more counts.
The plot shown in in Fig. 3.16 (b) also has the count loss for the same reason and thus the pile-up
cut is necessary to obtain the correct curves. The dynamic range of solute scatterings shown in Fig.
3.16 (c) was widened by introduction of mode-II and the intensity clearly follows the power lawof q −3.7363 . The scattering curve deviates from the power law of q −4 because the particles are not

49

Table 3.1: Results of an analysis of solution scattering from Apo-Ferritin and water are summarized.
‘Events (mode-I)’ represents the number of hits from the anodes and the cathodes when they
coincide in a clock interval (10 ns). A few hits are generated by an X-ray. ‘After analysis’ represents
the number of the X-rays hitting the detector. These two numbers do not need to be the same but
agree by accident in this case.
Apo-Ferritin
Exposure time473.63737 sec
Events (total)
724,820,905
Data acquisition rate
1.530328 MHz
Hits (mode-I)
137,616,751(18.9863%)
After analysis
137,588,418 photons (18.9824%)
Counting rate of X-rays
290.493 kHz

Water
Exposure time
Events (total)
Data acquisition rate
Hits (mode-I)
After analysis
Counting rate of X-rays

405.06771 sec
290,734,627
717.743 kHz
55,642,762
55,630,853 photons
137.337 kHz(19.1387%)(19.1346%)

perfect spheres. The difference between the solute scattering and the power law q −3.7363 is shownat the bottom. It shows that the detector follows the power law in a range of over 10 6 and thus
maintains linearity there.
Obviously, the X-ray area detector based on the µ-PIC has attained the widest linearity insolute scattering. This range exceeds that provided by the MSGC. Well-tuned IPs have a dynamic
range of about 106 but the solute scattering curves determined by subtracting the solvent curvefrom the solution curve only maintain linearity in a range of about 10 5 . This implies that theaccuracy of the X-ray intensity measured by the IPs is limited to 10 −5 by the internal backgroundsuch as readout noise. On the other hand, the solute scattering curve measured by the µ-PIC has a
10 times wider dynamic range. Thus the detector system proves that the accuracy of X-ray counts
is better than 10−6 . This high accuracy was attained because the detector is free from the internal
background and the statistical uncertainty is dominant. The µ-PIC proved that it has the key
features of the photon-counting detectors. The high accuracy of the µ-PIC allows to measure weakdiffraction spots and scattering pattern precisely and will lead to high-precision measurements.
The PILATUS is also a photon-counting detector and has the same feature as the µ-PIC .
However, the dynamic range of the 20-bit counter that each pixel comprises is 10 6 while that of theµ-PIC is infinite. Theoretically the 20-bit counter limits that of solute scattering curves obtainedby a single frame to 106 . To use a larger-bit counter, the pixel structure needs renewal and it
requires a long-term development. Thus, the measurements must be repeated to achieve a wider
dynamic range.

3.4.5

Guinier plot

According to equation (1.7), the gyration R g can be obtained from the Guiner plot. A part of thedata in Fig. 3.14 (b) is plotted in Fig. 3.17. The interval is limited to the region where qR g <∼ 1
where the Guinier approximation is valid. Since the IP has a finer position resolution, it has more50

no cut
with cut
ratio

104

105

Intensity

Intensity

105

104

103

103

102

102

10

10

1

1

10-10

0.5

1

no cutwith cut
ratio

10-10

1.5
2
2.5
Q (Inverse Nanometer)

(a)

0.5

1

1.5
2
2.5
Q (Inverse Nanometer)(b)

Figure 3.13: Scattering curve without applying the pile-up cut is divided by one with the cut.
(a) shows scattering curves from an Apo-Ferritin solution and (b) shows those from a water. The
samples were irradiated with 1.5 Å (8 keV) X-rays at BL45-XU SPring-8. The whole detection
area of the µ-PIC was used to obtain the scattering curves.

data in the very small angle region of q < 0.05nm −1 and thus the small error of the gyration. The
gyration obtained by the µ-PIC was 5.357 ± 0.006 nm while the gyration obtained by the IP was
5.320 ± 0.031 nm. Considering the errors, the gyrations derived by the µ-PIC and the IP seemconsistent.

3.5

Further improvements of position encoding method (modeIII)

The new position encoding method, mode-II, improved the performance of the detector system interms of linearity, uniformity, accidental coincidence, and counting loss. However, there is a roomfor further improvements because a new parameter, a pulse width of a hit, is available. The hits
having maximum and minimum coordinates sampled in a clock interval (10 ns) are considered andthe average of them is recorded by the mode-I and mode-II encoding system. Thus, the hits placed
between the maximum and minimum coordinates are neglected. However, it is necessary to use all
hits to calculate the averaged position. Thus, mode-III that is an improved version of mode-II and
can measure pulse widths was developed.
Figure 3.18 shows a block diagram of the mode-III position encoding system. The mode-III
system records both leading edges and falling edges of the hits (the digital signals) of the ASDs andthus the widths of the hits can be measured. If two or more ASD channel generate hits within aclock of 10 ns, positions of all the channels are recorded. The pulse heights of each channel can be
deduced from the widths. Therefore, this method also allows one to find a centroid of the charge
distribution and then improve the position resolution.
Figure 3.19 shows hits and their widths generated by 1.5 Å (8 keV) X-rays produced at SPring8. Since the charges concentrate in the center of the electron cloud, strips located in the center

51

Intensity

Intensity

105

104

104

103

103

102

102

10

10

1
10-10

1
0.5

1

10-10

1.5
2
2.5
Q (Inverse Nanometer)

1

1.5
2
2.5
Q (Inverse Nanometer)(b) Mode-II without the pile-up cut.

Intensity(a) Mode-I

0.5

105

104
103
102
10
1

10-10

0.5

1

1.5
2
2.5
Q (Inverse Nanometer)(c) Mode-II

Figure 3.14: Solution scattering patterns from Apo-Ferritin. The samples were irradiated with 1.5
Å (8 keV) X-rays at BL45-XU SPring-8. The red, black, blue, pink lines refer to a scattering curveof the solution, a solvent (water), their subtraction, and a scattering curve measured by IP (R-Axis)
for reference, respectively. Plots (a), (b) and (c) used the same data set obtained by the mode-II
encoding system. In the case of (a), hits from the anodes and the cathodes when they coincide ina clock interval (10 ns) are used (equivalent to the mode-I). (b)An X-ray generates a few hits andtheir averaged position is used. The pilse-up cut was not applied. (c)An X-ray generates a few hits
and their averaged position is used. The pile-up cut was applied.

52q [nm -1 ]Figure 3.15: Intensity difference between the µ-PIC and the IP (R-Axis). The scattering patterns from Apo-Ferritin shown in Fig. 3.14(c) were used for the evaluation. 100(I µ-PIC )(q) −IIP (q))/IIP (q) [%] is shown on the y-axis. The intensity of the IP is normalized to that of theµ-PIC using an averaged intensity in a range of 0.1 ≤ q ≤ 0.15[nm −1 ].
have long pulse widths and cross the threshold first. Hits generated by strips surrounding thecenter usually have short pulse widths and follow the signals generated at the center. This featuresuggests that the entire electron cloud arrives at the µ-PIC at almost the same time. The observed
propagation delay occurs due to the time walk of the ASDs. It should be noted that electrical noise
has short length of less than a few clocks while most of the hits have lengths of more than 5 clocks.
Considering the features of the signals generated by X-rays and the electrical noise, it is easy toseparate noise and signal.
Since the mode-III system has been constructed without ADCs, it is simple and easy to handle.
The encoding system was upgraded by loading new files to the FPGAs. On contrary, RAPID uses
the fast ADCs with necessity of cooling water and the detector system is complex.
The mode-III system could reduce the possibility of making noise and signal pairs. A counting
rate of noise-noise pairs can be examined by recording the output of the µ-PIC in the absence ofradiation. In this way, the rate for noise-noise pairs was seen to be several Hz and thus is negligible.
In high-counting rates environments, the possibility of making noise-signal pairs is small but theircontribution to non-linearity is considerable. Our next goal is to achieve a wider dynamic range of
106−7 and thus making noise-signal pairs should be avoided. By considering the widths of hits andthe number of hits, noise will be eliminated and the dynamic range will be enhanced.

3.5.1

Position calculation

Now we assume that the shape of the analog output is a triangle whose length of the base, W 0 , isindependent of its height. while its height h, is proportional to the input charge. The definition of
the parameters is shown in Fig. 3.20. The pulse width of the digital output, W , is then expressedbyh − Vthhwhere Vth is the threshold voltage. A simple calculation gives
W = W053(3.1)

Intensity

Intensity

106

106

105

105

q -4

4

10

104

103

103

102

102

10

10

1

1

10-1

10-1

10-1

1
Q (Inverse Nanometer)

Intensity(a) Mode-I

10-1

1
Q (Inverse Nanometer)(b) Mode-II without the pile-up cut.

6

10

105
104
3

10

102
10
1

Difference

10-1

10-1

1
Q (Inverse Nanometer)

15
10
5
0
-5
-10
-15
-20
-25
10-1

1
Q (Inverse Nanometer)(c) Mode-II

Figure 3.16: Solution scattering curve from SiO 2 nano particles. The diameter of the particles is
100 nm. The red, black, and blue lines represent the scattering curve of the solution, the solvent(water) and their subtraction, respectively. The samples were irradiated with 1.5 Å (8 keV) X-rays
at BL45-XU SPring-8. A scattering curve of the latex approximated by a solid sphere follows thepower law of q −4 . Plots (a), (b) and (c) used the same data set. In the case of (a), hits from theanodes and the cathodes when they coincide in a clock interval (10 ns) are used (equivalent to themode-I). (b)An X-ray generates a few hits and their averaged position is used. The pile-up cut
was not applied. (c)An X-ray generates a few hits and their averaged position is used. The pile-up
cut was applied. The difference between the solute scattering and the power law is shown at thebottom.
54

log I(q)

10.65
10.6
10.55
10.5
10.45
10.4
10.35
10.3
10.25
10.2

0

0.01

0.02

0.03

0.04
q2 [nm -2 ]

0.03

0.04
q 2 [nm -2 ](a) µ-PIC

log I(q)

10.65
10.6
10.55
10.5
10.45
10.4
10.35
10.3
10.25
10.2
10.15

0

0.01

0.02

(b) Imaging plate

Figure 3.17: Guinier plots for an Apo-Ferritin.

55

256 ch LVDS output from anodes

FPGAs work with an internal clock of 100 MHz
FPGA encoding anode strips
FPGA calculating the anode
all edges

and cathode coincident positions

are(edge_flag1, X1, T)

encoded(edge_flag2, X2, T)......

positve edge : edge_flag = 0(edge_flagN, XN, T)

negative edge : edge_flag = 1(edge_flag1, Y1, T)(edge_flag1, X1, T)(edge_flag2, Y2, T)(edge_flag2, X2, T)......

......

(edge_flagN, YN, T)(edge_flagN, XN, T)

is sent to a memory board

256 ch LVDS output from cathodes

33 bit per event
FPGA encoding cathode strips
1bit: write flag

all edgesare1bit: write flag

32bits: (edge_flag, X, T) 32bits: (edge_flag, Y, T)

encodedpositive edge : edge_flag = 0
negative edge : edge_flag = 1(edge_flag1, Y1, T)

FIFO

FIFO

(edge_flag2, Y2, T)......

50 MHz50 MHz(edge_flagN, YN, T)
memory board
with 64 MB of SRAM
(8.7 M events can be stored)
Figure 3.18: A block diagram of the mode-III position encoding system

56

Z

Z

30

30

25

25

20

20

15

15

10

10

5

5

0

0

-5

-5

-10

-10
40

178 180 182 184 186 188 190 192 194 196
X

42

44

46

48

50

52

54

56

58

60
Y

(a)

Z

30

Z

30

25

25

20

20

15

15

10

10

5

5

0

0

-5

-5

-10
176 178 180 182 184 186 188 190 192 194 196
X

-10

34

36

38

40

42

44

46

48

50

52
Y

(b)

Z

30

Z

30

25

25

20

20

15

15

10

10

5

5

0

0

-5

-5

-10

-10
24

170 172 174 176 178 180 182 184 186 188
X

26

28

30

32

34

36

38

40

42

44
Y

(c)

Figure 3.19: Typical X-ray events monitored by the mode-III encoding system. The detector was
irradiated with X-rays of 1.5 Å (8 keV) at SPring-8. X refers to the strip number of anodes: Y
to the strip number of cathodes; Z to the clock. Z = 0 is defined as the time when the first hit
is detected. Red-filled triangles display the timing of leading edges and falling edges while lines
denote clocks when the signals exceed the threshold.
57hThresholdW
W0

Figure 3.20: Schematic of the analog output of the ASD and definition of the parameters.
width Anode3
×10

width Cathode
3
×10

1200

1000

1000
800
800
600

600

400

400

200
0

200
0

5

10

15

20

0

25
30
width [clock]

0

5

10

15

20

25
30
width [clock]Figure 3.21: Distribution of the pulse width. The detector was irradiated with an X-ray generator.
The acceleration voltage of the X-ray tube was set to 45 kV.

h=

1
V
W th
1− W
0

(3.2)

Thus, the relative pulse height h/V th is determined by the width W and the length of the base
W0 . The width W tends to W0 as h tends to infinity. Therefore, W0 corresponds to the maximum
width under a condition where pile-up is does not occur. Using the expression given in Eq. (3.2),
the centroid of the charge distribution is calculated by the width of the hits. The calculation of
the position of the X-ray is performed independently in X and Y.
W0 was determined by the maximum width. Under high-counting rate environments, pile-up is
likely to occur and the width occasionally becomes longer than that of a single pulse. Therefore,
W0 was invesitaged under a low-counting rate environment where the counting rate per pixel isless than 10 Hz. Figure 3.21 shows the resultant distribution of the pulse height. The detector
was irradiated with an X-ray generator without a monochrometer. The acceleration voltage of the
X-ray tube was set to 45 kV. The X-ray tube mainly emits copper fluorescent of 8 keV but alsoemits X-rays with higher energy up to 45 keV. In this experiment, the ASD reference thresholdvoltage was set to -45 mV (anodes) and 55 mV (cathodes). This assymetric threshold level makes
the width distributions shown in Fig. 3.21 different. All the experiments using the mode-III systemwere applied to this threshold voltage. The maximum width for the anodes and the cathodes areabout 300 ns and 280 ns, respectively. The position resolution weakly depends on W 0 and theerror of about 50 ns is allowed. Therefore, we set W 0 to 300 ns for the anodes and 280 ns for thecathodes.

3.5.2

Position resolution

We achieved improvements in the position resolution using the mode-III system. The position
resolution was evaluated from an image of a grid mask where holes with a radius of 250 µm were58

Y [cm]drilled. The entrance window of the µ-PIC was masked and irradiated with scattering from a pieceof a glassy carbon. The scatterer was placed 1666 mm in front of the detector plane and wasirradiated with X-rays of 1.5 Å (8 keV) at SPring-8. The centroid of the charge distribution was
calculated using pulse widths and the resultant image is displayed in Fig. 3.22.
The counts distribution around a hole as afunction of the distance from the hole center r
10
is shown in Fig. 3.23. The holes close to thebeam center (< 32 mm) were summed up. If the9
fluctuation of the calculated positions is charac8
terized by a Gaussian distribution with deviation
7
σ, the counts distribution is represented by the6
curves shown in Fig. 3.24. The polar cooridanate
5system (r, θ) is used to denote the distribution.
4
Figure 3.24(a) shows counts per pixel; (b) shows3
the integral of (a) on the interval of 0 ≤ θ ≤ 2π.
2
According to Fig. 3.24(b), the tails of the curves
1strongly depend on σ. The area around the cen0
1
2
8
10
0
3
6
4
9
7
5
ter is less important to determine σ because theX [cm]
curves are almost propotional to r there. The
distribution shown in Fig .3.23(a) and (b) is fit1000
100
10
0.01
0.1
1
Intensity
ted by the curve shown in Fig. 3.24(b). The
resultant position resolution is σ = 93.6 µm. For
reference, the same image was recorded by the Figure 3.22: Image of a grid mask. The entrance
mode-II system and the position resolution was window of the µ-PIC was masked and irradiateddetermined to σ = 157.9 µm that is larger than with scattering from a piece of a glassy carbon.
The bin size is set to 200µm × 200µm.
σ = 120 µm described in section 2.2.5.
The counts distribution shown in Fig. 2.17 is
the projection along the edge at a 45 ◦ angle fromthe µ-PIC strips. The position resolution of the µ-PIC is minimum at a 45 ◦ angle and maximum
at a right angle. In the case of the holes, the position resolution of their edges is the average over
all the angles. If the position resolution at a right angle is expressed by σ, the averaged value isgiven by
R 4π
0cosθdθ
14π

4σ = √ σ = 0.900σ.
2π

(3.3)

This equation implies
that the position resolution of σ = 120 µm at a 45 ◦ angle corresponds to√
σ = 120 × 0.900 × 2 = 153 µm on the edges of the holes. This value is similar to the resolutionof mode-II (157.9µm).
We have assumed that the shape of the analog output is a triangle and the width is expressedby eq. 3.2. This is a rough approximation of the X-ray position but improves the resolution.
However, the actual shape, shown in Fig. 2.11, is not a triangle. Therefore, the correlation of thepulse widths with the pulse heights determined by real measurements will enhance the position
resolution.

3.5.3

Refinements in scattering curves

Chicken collagenWe also examined the mode-III system using diffraction from chicken collagen and solution scattering. Figure 3.25 shows a diffraction pattern of chicken collagen. The scatterer was placed 1666
mm in front of the detector plane and was was irradiated with X-rays of 1.5 Å (8 keV) at SPring-8.

59

×103

CountsCounts

×103
250

350
300

200

250

150

200
150

100

100

50
0
0

50

1

2

3

4

5
6
r [X 200 µm]

0
0 0.2 0.4 0.6 0.87(a) mode-III

1 1.2 1.4 1.6 1.8 2 2.2

r [X 400 µm](b) mode-II2arbitrary unit

arbitrary unit

Figure 3.23: Counts distribution as a function of the distance from the center of the holes. Holes
close to the beam center (< 32 mm) were summed up.

σ = 0 µm

1.8
1.6

5

σ = 0 µm

4

1.4
1.2

3

1
σ = 50 µm
0.82

σ = 150 µm

0.6
0.4
0.2
0
0

σ = 100 µm

1

σ = 150 µm

σ = 50 µm
00100 200 300 400 500 600 700 800 900 1000r [µm](a) Counts per pixel.

σ = 100 µm

100 200 300 400 500 600 700 800 900 1000r [µm](b) Integral of (a) on the interval of 0 ≤θ ≤ 2π.

Figure 3.24: Counts distribution around a hole with a diameter of 500 µm.

60The resultant images are displayed in Fig. 3.25 (a) and (b). The bin size is set to 200µm × 200µm.
The first peak about 1.5 mm apart from the shade of the beam stopper is clearly separated. For
reference, the averaged positions of hits recorded by the mode-II system were integrated and theresultant images are shown in Fig. 3.25 (c), (d) and (e). Due to the position resolution, there aremany empty bins in Fig. 3.25(c) where the bin size is set to 200µm × 200µm.
The angular region designated by the diagonal lines in Fig. 3.25 (a) is used to determine the
distributions shown in Fig. 3.26. The black lines referring to the diffraction curve determined by
Fig. 3.25(a) have higher diffraction peaks and show a improved resolution. The oscillating curves
in Fig. 3.26 (c) and (d) are due to many empty bins in Fig. 3.25(c). We adjusted the bin size ofthe scattering curve drawn by the red line in Fig. 3.26 (c) to smooth, and examined the position
resolution of the first and the second peak of the sample shown in Table 3.2. Then it was comparedwith the resolution of the peaks obtained by the mode-III system. It can be concluded that theposition resolution of the peaks was improved well by position interpolation. The diffraction curve
determined by Fig. 3.25(a) was also compared with that observed by the IP for reference shown in
Fig. 3.25(e) and these two curves agree well. The pixel pitch of the IP is 100 µm.
Table 3.2: The position resolution (FWHM) of the first and the second peaks of chicken collagen.
mode-II
mode-III−1
1st peak 0.0148 nm
0.00807 nm−1
−1
2nd peak 0.0111 nm
0.00161 nm−1

Solution scattering
We measured solution scattering from SiO 2 nano particles using the mode-III system. The diameterof the particles is 110 nm. The instrumentation is same as that used for the observation of thediffraction from the collagen metioned before. The resultant scattering curve is shown in Fig. 3.27.
Using the same data set, the scattering curves with and without interpolation are compared. The
curve with interpolation has steeper valleys and agrees better with that observed by the IP with apitch of 100 µm. The first peak that corresponds to the edge of the shadow of the beam stopper
is lower in comparison with that measured by the IP. It might be due to the aging mentioned in
Section 3.4.2. The scattered X-rays by the sample concentrated on the edge of the shadow and thegain there dropped.
Table 3.3 summarizes the results of the analysis of solution scattering from the SiO 2 nanoparticles. Since the mode-III system records both leading edges and falling edges of the hits of theASDs, the number of events generated by an X-ray doubles in comparison with mode-II.

3.6

Future improvements

3.6.1

Gain stability

The ideal detection efficiency of photo-electrons generated by X-rays in the detection volume is
100 %. To achieve it, the pulse height of signals generated by the X-rays must be well above thethreshold applied to the ASDs. The detection efficiency is not effected by the gain variation of thedetector while the pulse heights of the signals are well above the threshold. In principle, the gain of
the µ-PIC (and many micro-pixel gaseous detectors) vary within a time-frame of several hours after
a high voltage is applied. The duration of this gain variation is determined by the time constantof the polarization of the polyimid placed between the anodes and the cathodes. Moreover, thegain could be changed by intense X-rays, as mentioned in 3.4.2. Therefore, to keep the detection
efficiency of the liberated electrons at 100 % is an important issue. To eliminate the fluctuation
61

10

8.0

9

7.8

8

7.6
7.4

6

Y [cm]

Y [cm]

7
5
4
3

6.8

2

6.6

1

6.4

0

0

0.1

1

2

3

4

10

1

5

6

X [cm]

100

Intensity

7

8

6.2

10

9

8.0

8.0

7.8

7.8

7.6

7.6

1000

10000

100

10

(b) Integration of interpolated
positions from pulse widths.
The bin size is set to 200µm ×200µm.

108

7.0

Y [cm]

Y [cm]

7.2

7.2
7.0

6.8

6.8

6.6

6.6

6.4

6.4

2

6.2
0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4

X [cm]

X [cm]

1

10

100

Intensity

1000

10000

(c) Averaged positions of hits
were integrated. The bin size
is set to 200µm × 200µm, thesame as (a) and (b).

6

4

0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4

0.1

10000

1000

Intensity

7.4

7.4

Y [cm]

0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4

X [cm](a) Integration of interpolated
positions from pulse widths.
The bin size is set to 200µm ×200µm.

6.2

7.2

7.0

100

1000

Intensity

10000(d) Averaged positions of hits
were integrated. The bin size
is set to 400µm × 400µm

0

0

2

4

0.1

1

10

6

8

10

100

1000

10000

X [cm]

Intensity(e) Averaged positions of hits
were integrated. The bin size
is set to 400µm × 400µm

Figure 3.25: Diffraction pattern of chicken collagen. The detector was irradiated with X-rays of 1.5
Å (8 keV) at SPring-8. Counts per bin is displayed in the two-dimensional histograms. (a) (b) The
centroid of the charge distribution was calculated using pulse widths. The first peak about 1.5 mm
distanced from the shade of the beam stopper is clearly separated. (c) (d) (e) Averaged positionsof hits recorded by the mode-II system were integrated. Due to the pixel size of the µ-PIC , there
are many empty bins in (c).

62Intensity [arbitrary unit]

Intensity [arbitrary unit]

103

103

102

102

10
10
0

0.2

0.4

0.6

0.8

1 1.2 1.4 1.6 1.8
Q (Inverse Nanometer)

0Intensity [arbitrary unit](b) The Red line refers to thediffraction curve of Fig. 3.25 (e).

Intensity [arbitrary unit](a) The Red line refers to thediffraction curve of Fig. 3.25 (e).

103

103

102

102

10

0

0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45
Q (Inverse Nanometer)

10

0.2

0.4

0.6

0.8

1 1.2 1.4 1.6 1.8
Q (Inverse Nanometer)

0

0.2

0.3

0.4
0.5
0.6
Q (Inverse Nanometer)(d) The Red line refers to thediffraction curve of Fig. 3.25 (c).

Intensity [arbitrary unit](c) The Red line refers to thediffraction curve of Fig. 3.25 (c).

0.1

104

103

102

10

1
0

0.2

0.4

0.6

0.8

1
1.2 1.4 1.6
Q (Inverse Nanometer)(e) The Red line refers to thediffraction curve observed by the IP
for reference.

Figure 3.26: Comparison of the diffraction curve determined by mode-III with Fig. 3.25 (c) and(e). The angular region used to determine the distributions is designated by the diagonal lines
shown in Fig. 3.25 (a). The black lines refer to the diffraction curve of Fig. 3.25.
63

Table 3.3: Results of an analysis of solution scattering from the SiO 2 nano particles and water are
summarized. The number of events generated by an X-ray doubles because the mode-III system
records both leading edges and falling edges of the hits of the ASDs. ‘After analysis’ represents thenumber of the X-rays hitting the detector.
SiO2 nano particles
Exposure time
Events (total)
Data acquisition rate
After analysis
Counting rate of X-rays

800.949 sec
598,680,543
747.464 kHz
46,797,583 photons
58.43 kHz

Water
Exposure time
Events (total)
Data acquisition rate
After analysis
Counting rate of X-rays

3960.94 sec
376,858,883
95.1438 kHz
28,874,951 photons
7.289 kHz

105

104

104

103

q

-4

q -4

103

102

102

10

10

1

1

-1

10

10-2

(7.662%)

Intensity

Intensity

105(7.817%)

10-1
10-1

10-2

1
Q (Inverse Nanometer)(a) Integration of interpolated positions from pulse widths.

10-1

1
Q (Inverse Nanometer)(b) Averaged positions of hits were integrated.

Figure 3.27: Solution scattering patterns from SiO 2 nano particles. The diameter is 110 nm. (a) and(b) uses the same data collected by the mode-III encoding system. The sample was irradiated with
1.5 Å (8 keV) X-rays at BL45-XU SPring-8. The red, black, pink, blue lines refer to a scattering
curve of the solution, a solvent (water), their subtraction, and a scattering curve measured by IP(R-Axis) for reference, respectively.

64

of the detection efficiency, we need to improve the signal-to-noise ratio and ensure that all signals
generated by the X-rays exceed the threshold. The solution is described in Section 3.4.2.

3.6.2

Counting rates

To avoid radiation damage of samples and obtain fine scattering patterns, exposure of intense Xrays for a short duration is required. Thus, the X-ray area detector must work properly under
the high-counting rate environments. Now we confirmed that the mode-I position encoding system
functions at a DAR of 5 MHz and the mode-II system at a DAR of 7 MHz. Our goal is to operate theµ-PIC under the high-counting rates where IPs are used. We successfully measured the solution
scattering in such an environment and achieved the dynamic range of > 10 6 . The DAR of themode-III system is estimated at > 10 MHz there. However,the mode-III system is more complex
and records more information. Thus, the data flow is bottlenecked in an FPGA controlling theoutput to the memory module at a DAR of > 2 MHz. Therefore, the position encoding system isin need of a renewal.
The current encoding system uses three FPGAs; one for the anodes, another for the cathodes,
and the other functions as a controller. The controller receives encoded positions from the other two
FPGAs and sends the data to the memory module. A propagation delay caused by communications
between the controller and the other two FPGAs is the main reason of the counting rate limit. theoutput of encoded positions should be sent directly to the memory module.

3.6.3

Detection efficiency for X-rays

X-rays scattered by a sample are absorbed by a 4-mm Xe-C 2 H6 (70:30) gas layer kept at 1 atm. The
detection efficiencies for 8 keV and 13.8 keV X-rays, which are commonly used at BL45-XU SPring8, are 24% and 8%, respectively. As shown in Table 3.4, the absorption by the gas layer locatedbetween the entrance window and the drift plane is a major loss of incident X-rays. Therefore, thedrift plane must be glued to the internal side of the polyimide entrance window. Since incident
X-rays need to penetrate an entrance window, the window must be kept thin, and does not have atolerance for a pressure of > 1atm. For this reason, increasing the gas pressure is not an easy way
to improve the detection efficiency. However, since the detector is aiming at small-angle scattering
measurements, it is unnecessary to consider parallax. Thus one can safely double the thickness ofthe gas layer and the detection efficiency will also be doubled.
Table 3.4: Absorption rates. The gas layer represents that located between the entrance window
and the drift plane and its thickness is 2 mm. X-rays of 8 keV and 13.8 keV are commonly used at
BL45-XU, SPring-8.
EnergyEntrance window and drift plane gas layer detection region
8 keV (1.5 Å)
∼ 10%
17%
32%
13.8 keV (0.9 Å)
∼ 2%
4.4%
8.5%
The diffusion is an important issues which must be taken into account when one extends the
thickness of the gas layer. The diffusion of the electrons as they drift toward the µ-PIC is pro√portional to z, where z is the drift length. The diffusion coefficient of a Xe-C 2 H6 (70:30) gas is√
210 µm cm at a drift field strength of 0.7 kV/cm. The root mean square of the electron cloud
distribution after 4-mm drift is 132 µm and after 8-mm drift is 187 µm. Even in the case of the
8-mm gas layer, the diffusion is considerably smaller than the pitch of the µ-PIC . In addition, we
derive the centroid of the electron cloud distribution using the pulse widths and thus, it is easy
to estimate the diffused distribution of the electron cloud. It can be concluded that the detector
with the 8-mm gas layer will double the detection efficiency for X-rays and will maintain almost
the same position resolution as the current detector.
65

3.6.4µ-PIC with large active area

One solution for improvements of the angular resolution of scattering curves is to obtain the centroidof the electron distribution, and another is to extend the active area of the detector, which allows
extension of a camera length. In this case, the position resolution remains the same but the angularresolution, which is a function of the camera length, improves.
We have successfully fabricated a µ-PIC with an active area of 300 mm×300 mm, shown in Fig.
3.28(a), and demonstrated its stability for long-duration operation. We used the same readouts asthat of the µ-PIC with a detection area of 100 mm×100 mm. X-ray imaging using a µ-PIC with alarge active area is now under investigation. We successfully tested the detector system based on
the large µ-PIC work at SPring-8. Figure 3.28(b) shows the setup. The detector was irradiatedwith 0.9 Å (13.8 keV)scattering from a silver behenet powder and yielded the two-dimensional
image shown in Fig. 3.28 (c). One can see two diffraction rings in this image.
For this measurement, the detector is contained in a sealed aluminium vessel with an aluminum
entrance window of 1-mm thickness. We used an Ar-C 2 H6 (90:10) gas mixture, which can stabilize
the µ-PIC better than a Xe-based gas mixture. Since the experiment was conducted without a
GEM, the signal-to-noise ratio was not good. We are planning to install a large GEM with anactive area of 230 mm× 280 mm, the largest currently available; replace the aluminum window
with polyimide; and use a Xe-based gas mixture. As a consequence, the detection efficiency andthe signal-to-noise ratio should improve.

3.6.5

Compact system

For delivering compact readouts with low power, a CMOS-ASIC chip with amplifiers and discriminators is being developed at KEK, Japan. The number of channels is doubled to 8 channels per
chip. In a few years, these CMOS-ASIC chips will allow all the readout electronics to be locatedon the rear of the gas vessel, and make it possible to develop an imaging detector similar in size toa PC-notebook at a reasonable cost.

66

ASDs

400 µm

30 cm

30 cm
cm

beam

µ-PIC

(a) µ-PIC with an active area of 300
mm×300 mm.

(b) X-ray area detector based on the µ-PIC with an active areaof 300 mm×300 mm at beamline BL45XU.

24

10

4

10

3

10

2

20

Y [cm]

16
12
8

10

4
0

0

4

8

12

16

20

X [cm](c) The first image observed by the largeµ-PIC . Scattering pattern from a silver behenet powder irradiated with 0.9 Å
(13.8 keV) X-rays at SPring-8.

Figure 3.28: µ-PIC with a large active area.

67Chapter 4

Application to Compton camera
As mentioned in chapter 2, the developmentof the µ-PIC has been originally motivated by
an application to a Compton camera on board
a satellite. Figure 4.1 shows a schematic of thecamera. A Time Projection Chamber based on aµ-PIC (micro-TPC) is a component of the Compton camera and a gamma-ray Compton-scattered
there is observed. The scattered gamma-ray isabsorbed by pixellated scintillators surrounding
the micro-TPC while the recoil electron is absorbed by the micro- TPC. The micro-TPC andscintillators acquire a complete set of kinematicand geometry information and then the incident
gamma-ray can be fully reconstructed. In thischapter, we report on improvements of the dataacquisition (DAQ) for the micro-TPC. The concept of mode-II was used for the improvement on
DAQ for the micro-TPC.

4.1 MeV gamma-ray observation~1MeV γ~10MeV γ
Drift plane

e-

e-

µ-PIC
Scintillator

e+

PMTs

Figure 4.1: Schematic of a Compton camera
based on the µ-PIC . The Compton camera consists of the micro-TPC surrounded by scintillators and the encoding system.

Many attractive phenomena - supernova remnants, spin-down pulsers, black holes, the galactic center, and active galactic nuclei - radiate
MeV gamma-rays. Production of gamma-rays in
the universe involves several mechanisms such as synchrotron radiation, Bremsstrahlung, inverse
Compton scattering, nuclear transition and decay and annihilation. Thus, MeV gamma-ray observations bring many useful information about the universe.
The dominant interaction in the energy band from a sub MeV up to several tens is Compton
scattering. Observing Compton scattering is the most challenging matter. Tracking recoil electrons
is difficult because of the high probability of multiple scattering in material. The multiple scattering
deteriorates an anglular resolution of incident gamma-rays. In addition, gamma-rays Comptonscattered by a satellite induce internal background. It is also difficult to separate signals frombackground because of the insufficient anglular resolution.
A few observations for MeV gamma-rays on board satellites were carried out in the past. It
should be noted that the number of observations in this energy band is quite small in comparison

68with other energy bands. The most successful observation in this energy band is COMPTEL [33] onboard the Compton Gamma Ray Observatory (CGRO) satellite. In spite of the successful all-sky
survey conducted by COMPTEL, the number of celestial objects found in the MeV gamma-ray
band was smaller by an order of magnitude than that found in the GeV band. Thus, a Compton
telescope with an improved detection sensitivity is awaited.
COMPTEL consists of two planes of position sensitive detectors: modules filled with organic
liquid scintillator and NaI(Tl) crystal arrays. The liquid scintillator has a low atomic number toprevent multiple-Compton scattering. Gamma-rays Compton-scattered in the liquid scintillator
are absorbed by the inorganic crystals. The NaI(Tl) crystals which have a high atomic number
and a high density were chosen to enhance the photon absorption properties. The two detectors
measure energy and hit positions of a recoil electron and a scattered gamma-ray. A momentum of
the scattered gamma-ray can be reconstructed by the hit positions. A measure of a time difference
between the two detectors (time-of-flight) is used for background reduction. Since the direction of
a Compton recoil electron was not measured with COMPTEL, the direction of the incident photon
could only be reconstructed as a cone demonstrated in Fig. 4.2. Accordingly, it was difficult toreject the background, thereby limiting the sensitivity. The time-of-flight value was useful for the
background reduction but insufficient.
Measuring the direction of the recoil electron reduces the Compton cone to a small segment of
the cone, and leads to the stronger background rejection [34] [35]. Figure 4.3 shows a principle ofthe electron tracking camera. To measure both the three-dimensional track and the energy of therecoil electron, the micro-TPC based on the µ-PIC was introduced.
E0
E0

φ
Ke
E1

α

E2

Eγ

Figure 4.2: Schematic view of Compton scatter- Figure 4.3: Schematic view of an electron tracking occurred in COMPTEL.
ing camera.

4.2

Reconstruction of Compton events

The electron tracking camera provides complete reconstruction of incident gamma-rays using kinetic
energy and a momentum vector of a recoil electron, and energy and a momentum vector of ascattered gamma-ray. Figure 4.4 shows a schematic of a Compton event. Here, we define somevariables.
The momentum vector of the incident gamma-ray, ~s rcs , which normalized to |~srcs | = 1 and its
energy E = 0 are given by69E0 = E γ + K e


sin φ
sin φ
~srcs = cos φ −
~e
~g +
tan α
sin α

(4.1)
(4.2)

where Eγ , Ke , ~g , ~e, φ, α represent the energy of the scattered gamma-ray, the kinetic energyof the recoil electron, a scattering angle of the gamma-ray, and a recoil angle of the electron,
respectively. The scattering angle φ is given byme c2 KeEγ + K e Eγ
The recoil angle α is derived by ~g and ~e. This value α geo is given bycos φ = 1 −cos αgeo = ~g · ~e

(4.3)

(4.4)

The recoil angle is also derived by the kinetic energy of the recoil electron and the gamma-ray.
This value αkin is given by
r

Ke
me c2
(4.5)
cos αkin = 1 −EγKe + 2me c2
αgeo and αkin are derived by the independent variables and thus they are also independent. In
the case of the background events, α geo and αkin have different values. Selection of events whose
αgeo is the same as αkin brings the strong background rejection. Now we refer to the selection as
an alpha cut.
Electron tracking cameras can reconstruct gamma-rays event by event. Thus two angularresolutions for an incident gamma-ray can be defined when a position of a gamma-ray source is
known.
• ARM (Angular Resolution Measure) : resolution of the scattering angle of the gamma-ray, φ.
ARM = φgeo − φkin
• SPD (Scatter Plane Deviation) : resolution of a plane including ~g and ~e
 
 
  
~g ×~s~g ×~srcs

 

×
|~g ×~srcs ||~g ×~s|~g × ~s~g × ~srcs





 

arccos·SPD = ~g ·~g ×~srcs|~g × ~s||~g × ~srcs |× ~g×~s|~g ×~srcs |

(4.6)

(4.7)

|~g ×~s|where ~s and ~srcs represent a actual direction of the incident gamma-ray and its reconstructed
direction.

4.3

Current Status of Compton camera based on the µ-PIC

In this section, the current status of the Compton camera using the position encoding system shownin Fig. 2.19 will be described. Figure 4.6 shows the Compton camera currently used. A TPC was
mounted on a structure that can rotate, and was filled with an Ar-C 2 H6 gas mixture at a pressureof 1 atm. A detection volume of the TPC was 10 cm×10 cm×9.5 cm where a drift field of 0.26
kV/cm was applied. An induction field applied between the GEM and the µ-PIC was 1.06 kV/cm.
The gas gains of a µ-PIC and a GEM were 3 × 10 3 and 10, respectively, and thus the total gas gainis 3 × 104 .
Since most gamma-rays are scattered forward, scintillators used as absorbers are located behind
the TPC. We arrayed 8×8 GSO:Ce scintillators with a pixel size of 6 mm× 6 mm× 13 mm read bya multi-anode Flat Panel PMT H8500. These components are combined and form 3 × 3 arrays.
70

~s, E0φψ~e, Ke~g , EγαδFigure 4.4: Definition of variables

Figure 4.5: Definition of ARM and SPD

Figure 4.7(a) shows reconstructed
images of gamma-rays from a point Amplifier Shaper Discriminator (ASD)
Scintillators
137 Cs source. The spectrum of incident
gamma-rays is shown in Fig. 4.7(b).
The 137 Cs source has a strong peak at
662 keV. Energy of an incident gammaray should be the same as sum of that ofa recoil electron and a scattered gammaray when their energy is completely abTime Projection
sorbed by the camera. The reason why
Chamber(TPC)
one can see another peak in lower energy region is that the recoil electron
gamma-rays
could escape from the TPC or the scattered gamma-ray could escape from thePowerscintillators. It is impossible to resupply
construct these events and thus theymust be discarded. To yield the twodimensional image, the blue shaded partaround 662-keV peak was used. ARM
and SPD were 6.5◦ , 132.8◦ (FWHM), respectively.
Position
Figure 4.8 shows an image of
encodergamma-rays from three gamma-ray
sources, 137 Cs (662 keV), 22 Na (511 Figure 4.6: Compton camera system. The time projeckeV), 133 Ba (356 keV). One can see tion chamber is mounted on a structure that can rotate.
three peaks from the sources in Fig. 4.8(b). The images of the sources are successfully separated concerning the energy of the incident gamma-rays. Figure 4.8 (c)(d)(e) show
events included in the yellow, green, and blue region in Fig. 4.8(b), respectively.
To check the analysis program used for our detector, GEANT4 simulations were performed[25]. Events generated by GEANT4 provide correct energy, positions, and momentum vectors andthen values derived by the analysis program were compared with the real values. The position
resolution of the Compton-scattered points is shown in Fig. 4.9 and the the angular difference
between analyzed momentum vectors and real ones of recoil electrons in Fig. 4.10.

71

Number of events

Y [cm]

10

800

8
6

700

4

600

2

500

0

400

2

300

4

200

6

100

8
1010

8

6

4

2

0

2

4

6

0

8 10
X [cm]

600

800

1000
1200
Energy [keV](b) Reconstructed spectrum of the gammarays. Blue shaded part was used to yield theimage.

800

1400

700

1200

600

1000

500

800

400

600

300

400

200

200
0
80

400

Number of events

Number of events(a) Reconstructed image of gamma-rays froma 137 Cs source 22.5 cm apart from the TPC.
The center of the µ-PIC is defined as (0 cm, 0
cm).

200

60

40

20

0

20

40

100

60
80
ARM [°](c) ARM 6.5◦ (FWHM)

150

100

50

0

50

100

(d) SPD 132.8◦ (FWHM)

Figure 4.7: Irradiation with a 137 Cs point source.

72

150
SPD [ °]

Y [cm]Number of events

15

500

10
5

400

0

300

5

200

10

100

15

15

10

5

0

5

10

0

15
X [cm]

200

400

600

800

1000
1200
Energy [keV]

15

15

Y [cm](b) Spectrum of the gamma-rays.
Each shaded part was used to yield(c), (d) and (e).

Y [cm](a) Reconstructed image of gammarays from three gamma-ray sources.
The center of the µ-PIC is definedas (0 cm, 0 cm).

10

10

5

5

0

0

5

5

10

10

15

15

10

5

0

5

10

15

15
X [cm](c) Events included in the yellow
region corresponding to the 137 Cs
source.

15

10

5

0

5

10

15
X [cm](d) Events included in the green
region corresponding to the 22 Na
source.

Y [cm]

15
10
5
0
5

10
15

15

10

5

0

5

10

15
X [cm](e) Events included in the blue region corresponding to the 133 Ba
source.

Figure 4.8: Irradiation with 137 Cs, 22 Na, 133 Ba point sources. The three sources were successfully
separated using the energy information.
73

100

100

80

80

60

60

40

40

20

20

00

1

2

3

4

5

00

6
7
8
9
10
residual of initial point [mm]Figure 4.9: Position resolution of Comptonscattered points simulated by GEANT4 [25].
The TPC-modeI position encoding system wasused.

after the α cut

20

40

60

80

100

120

140

160
180
∆ [degree]

Figure 4.10: Angular difference between analyzed momentum vectors and real ones of recoil
electrons [25]. Compton events were generated
by GEANT4 and analyzed and then the analyzed
momentum vectors and real ones were compared.

4.4

New position encoding system for micro-TPC

4.4.1

TPC mode-II

Our goal is to achieve a sensitivity of 10 times as high as that of COMPTEL. The goal necessitatesa micro-TPC with a large volume (> 50 × 50 × 50 cm 3 ) and a µ-PIC with a large detection area.
Considering the low consumption power provided by satellites, the readout electronics of the µ-PIC
must be simplified. To reduce the number of channels, anodes in the same column are connectedby a strip and so are cathodes in the same row. Hits from the anodes and the cathodes when
they coincide in a clock interval (10 ns) are recorded and the three-dimensional tracks of the recoil
electrons can be observed. We used the hits in coincidence in order to improve the signal-to-noise
ratio. A block diagram of the position encoding system for use of the micro-TPC is demonstrated
in Fig. 2.19.
The strip readout delivers the simple system whereas it is difficult to obtain the correct positions.
Deposited charges by the recoil electrons over the anode and cathode strips depend on how many
anode and cathode strips the electrons cross and the asymmetric deposition leads to the propagation
delay.
In comparison with ARM, SPD is much worth due to the insufficient accuracy of the momentum
vectors of the recoil electrons, as shown in Fig. 4.10. Since the electrons are multi-scattered in agas, it is essential to measure their tracks around Compton-scattered points. However, the recoil
electrons deposit less energies around the Compton-scattered points than around the endpointsof the tracks. As mentioned in Chapter 3, the propagation delay at the digital outputs of theASDs increases as dE/dx is low. In the case of the encoding system shown in Fig. 2.19, hits
from the anodes and the cathodes are recorded when they coincide in a clock interval (10 ns), anddelayed signals are discarded. Therefore, the number of hits around the Compton-scattered pointis less than an end point of the electron. The resolution of the momentum vectors of the recoil
electrons deteriorates due to the loss of the hits. Figure 4.9 shows that the position resolution of theCompton points is much worse than the pitch of the µ-PIC due to the loss of the hits. Therefore,
some improvements similar to mode-II are necessary.
We developed a new encoding system for use of the micro-TPC. The concept of the new systemis the same as the mode-II encoding system: all hits are recorded. Figure 4.11 shows a block
diagram of the new system. Now we refer to the new position encoding system as TPC mode-II74

and the old one as TPC mode-I.
The performance of the TPC mode-II system was investigated with cosmic muons. Two plastic
scintillators were located above and below the TPC. A coincidence of these scintillators was used
for the muon trigger. Figure 4.12 shows typical muon tracks. X refers to the strip numbers of theanodes: Y to the strip numbers of the cathodes: Z to the clock. Z = 0 is defined as the time
when the first hit is detected. For conversion to length, X and Y should be multiplied by 0.04(= pixel pitch) and Z by 0.042 determined by a drift velocity. To compare TPC mode-I with II,
Fig. 4.12(b)(d) show hits from the anodes and cathodes only when they coincide in a clock interval(equivalent to TPC mode-I). The X-Z and Y-Z planes in Fig. 4.12(a)(c) show all the hits. The
propagation delay is within two clocks in most cases. Thus, in the X-Y plane and three-dimensional
space of Fig. 4.12(a)(c), anode and cathode hits when they coincide in two clocks interval are paired
together and plotted. Figure 4.12 shows that the propagation delay is likely to occur and TPC
mode-II drastically improved the quality of the muon tracks.
Figure 4.13 shows tracks of recoil electrons when the detector was irradiated with 137 Cs. The
Compton events are demonstrated in the same manner as in Fig. 4.12. Figure 4.13 shows that
the TPC mode-II system increase the number of hits per recoil electron. Electrons in a gas are
multi-scattered and thus precise tracks are necessary to determine Compton-scattered points andmomentum vectors there.
The distribution of the number of hits per muon track is shown in Fig. 4.14. The ASDs
connected with the cathode strips generate more hits than those for the anode ones because thelower threshold is applied to the cathode readout. The absolute values of the threshold for thecathode and anode readout are usually different because of the noise level and the offset of thecomparators in the ASDs. Figure 4.14(c) shows that many hits are discarded by TPC mode-I.
Figure 4.15 demonstrates the distribution of the number of hits per recoil electron track. The
trend of the distribution agrees with that of the muon tracks in most part. One can see peaks at
0 hit in Fig. 4.14 and 4.15. The 0-hit events are generated when only hits from the anodes arerecorded, and vice verasa. Thus, these events are considered as the electronic noise accidentallycoincided with the triggers. These results imply that TPC mode-II would improve the position
resolution of the Compton-scattered points and SPD.

4.4.2

TPC-modeIII

It is essential to determine which edge of a track is a Compton-scattered point. To determine whichedge of the track is the Compton-scattered point, a track is divided by two. The ratio of the track
length to the distance between edges of each fragment is calculated [25]. A possibility of multiple
scatterings increases as an electron deposits its energy. Thus, the ratio of the fragment including
the Compton-scattered point must be smaller. This criterion was applied to the data describedin Section 4.3. According to GEANT4 simulation, the criterion gives 60% recoil electrons correct
directions. Since the ratio of 60% is slightly better than a random selection of the Compton points,
this result indicates another criteria is strongly required. We will suggest a new criteria using thewidths of the hits.
To measure the widths, TPC-modeIII, similar to mode-III for use of the X-ray area detector,
was suggested. With minor change in the position encoding module, shown in Fig. 4.16, the widths
are available. From the widths, one can deduce approximate pulse heights and then dE/dx.
Since the TPC-modeIII encoding system is being designed now, it is unavailable. Instead of the
new system, we studied tracks of recoil electrons monitored by the mode-III system for use of the
X-ray area detector. Figure 4.17 shows typical tracks of recoil electrons. X refers to a strip numberof anodes: Y to a strip number of cathodes: Z to a clock. Z = 0 is defined as a time when analog
signal from cathodes or anodes generated by a liberated electron cross the threshold for the firsttime. A TPC with a detection volume of 10 cm×10 cm×0.4 cm filled with Xe-C 2 H6 (70:30) gas
mixture at a pressure of 1 atm. Since the detection volume is small, most of the electrons would75

trigger

256 ch LVDS output from anodes

FPGAs work with an internal clock of 100 MHz

clock is set to 0

FPGA encoding anode strips
FPGA calculating the anode

Rising edgesareand cathode coincident position

encodedif (|XN - X1| < 9 && |YN-Y1| < 9),(X1, XN, Y1,YN, T).
else if(|XN - X1| < 9 && cathode no hit),(X1, T)(X1, XN, 1023, 1023, T).

(X2, T)else if(anode no hit && |YN-Y1| < 9),......

(1023, 1023, Y1, YN , T).

(XN, T)

The output is sent to a memory board

X1 < X2 < ... < XN

256 ch LVDS output from cathodes

33×2 bits per event
FPGA encoding cathodes strips

1 bit: write flag (for memory board)
32 bits: (X1, XN, event_ID)

Rising edges

1bit: write flagare32 bits: (Y1, YN, T)

encoded(Y1, T)

50 MHz(Y2, T)......

memory board(YN, T)with 64 MB of SRAM

Y1 < Y2 < ... < YN

(4.4 M events can be stored)

Figure 4.11: A block diagram of a new position encoding system for use of the micro-TPC. The
idea of the new system is similar to mode-II.

76

(a) All events are used.

(b) Hits from the anodes and cathodes only when they coincide in a clock interval are used.

(c) All events are used.

(d) Hits from the anodes and cathodes only when they coincide in a clock interval are used.

Figure 4.12: Tracks of cosmic muons. (a)(c)All hits are plotted in the X-Z and Y -Z plane. In the
X-Y plane, hits from the anodes and cathodes when they coincide in two clocks interval are paired
together and plotted. (b)(d)Hits from the anodes and cathodes only when they coincide in a clockinterval are used (equivalent to TPC mode-I).
77

(a) All events are used.

(b) Hits from the anodes and cathodes only when they coincide in a clock interval are used.

(c) All events are used.

(d) Hits from the anodes and cathodes only when they coincide in a clock interval are used.

Figure 4.13: Tracks of recoil electrons. The detector was irradiated with 137 Cs. (a)(c)All hits areplotted in the X-Z and Y -Z plane. In the X-Y plane, hits from the anodes and cathodes when
they coincide in two clocks interval are paired together and plotted. (b)(d)Hits from the anodes
and cathodes only when they coincide in a clock interval are used (equivalent to TPC mode-I).
78

(a)

(b)

(c)

Figure 4.14: Number of hits per cosmic muon track. (a)Hits from the cathodes. (b)Hits from theanodes. (c)Hits from the anodes and the cathodes which coincided in one clocks interval.

(a)

(b)

(c)

Figure 4.15: Number of hits per recoil electron track. The detector was irradiated with 137 Cs.
(a)Hits from the cathodes. (b)Hits from the anodes. (c)Hits from the anodes and the cathodes
which coincided in one clock interval.

79escaped from the detector. The total gas gain was about 5 × 10 3 (the µ-PIC at 2 × 103 , the GEMat 3). This encoding system cannot be triggered and thus an absolute height of the tracks are
unknown. The tracks shown in Fig. 4.17 have a feature in at least one of X-Z and Y-Z plane: thewidths on the right-hand or left-hand side are larger. When the electron crosses more anode strips
than cathode ones, the feature is likely to be found in the X-Z plane (X refers to the strip numbersof the anodes) and vice versa. Then, one can determine which edge is the Compton-scatteredpoint considering the widths in the plane and then in three-dimensional space. In this way, theTPC-modeIII system will enhance the ratio of events with the correct Compton points.

80triggerclock is set to 0
256 ch LVDS output from anodes

FPGAs work with an internal clock of 100 MHz
FPGA encoding anode strips

FPGA calculating the anodeand cathode coincident positions

all edges

X : event_flag = 0

are

Y : event_flag = 1

encoded(0 ( = event_flag), edge_flag1, X1, T)(0, edge_flag2, X2, T)

positive edge : edge_flag = 0......

negative edge : edge_flag = 1(0, edge_flagN, XN, T)(edge_flag1, X1, T)(1 ( = event_flag), edge_flag1, Y1, T)(edge_flag2, X2, T)(1, edge_flag2, Y2, T)......

......

(edge_flagN, XN, T)(1, edge_flagN, YN, T)

256 ch LVDS output from cathodes

is sent to a memory board
FPGA encoding cathode strips

33 bit per event

all edges

1bit: write flagare

32bit: (event_flag, edge_flag, X or Y, T)

encodedlevel of LVDS output:
positive edge : edge_flag = 0
FIFO

negative edge : edge_flag = 1(edge_flag1, Y1, T)
50 MHz(edge_flag2, Y2, T)......
(edge_flagN, YN, T)

memory board
with 64 MB of SRAM
(8.7 M events can be stored)

Figure 4.16: A block diagram of a new position encoding system for use of the micro-TPC, TPCmodeIII. The idea of the new system is similar to the mode-III encoding system for use of the X-ray
area detector.

81

Z

Z
30

30

25

25

20

20

15

15

10

10

5

5

0

0

-5

-5

-10

-10

114 116 118 120 122 124 126 128 130 132
X

206 208 210 212 214 216 218 220 222 224
Y

Z

Z

(a)

30

30

25

25

20

20

15

15

10

10

5

5

0

0

-5

-5

-10
62

64

66

68

70

72

74

76

78

80

-10

82
X

114 116 118 120 122 124 126 128 130 132
Y

Z

Z

(b)

35

35

30

30

25

25

20

20

15

15

10

10

5

5

0

0

-5

-5

-10
132 134 136 138 140 142 144 146 148 150 152X-10
162 164 166 168 170 172 174 176 178 180 182
Y(c)Figure 4.17: Signals generated by recoil electrons. The detector was irradiated with gamma rays of
137 Cs point source (662 keV). Digital output of the ASDs are monitored by the mode-III encoding
system. Red-filled triangles displays timings of leading edges and falling edges while lines denotes
clocks when the signals exceed the threshold.
82

Chapter 5

Application to small-angle neutron
scattering
In this paper, we mainly discuss the gas detectors for small-angle X-ray scattering. An additionof gas 3 He enables for gaseous detectors to detect neutrons. Using a 3 He-mixed gas, the µ-PIC
detector system functions as a two-dimensional neutron imaging detector. In this chapter, we
will briefly summarize motivations for developing the two-dimensional thermal neutron imaging
detector and then refer to the first observation of thermal-neutron scattering using the µ-PIC at
JRR-3, Japan.

5.1

Small-angle neutron scattering (SANS)

Small-angle neutron scattering (SANS) probes structure in samples on almost the same scale as
SAXS, 1 nm to 1 µm scale. Since X-rays interact with electron clouds, their scattering cross section
is proportional to an atomic number Z. On the other hand, the neutrons interact with nuclei andthus their scattering cross section is totally different from X-rays. SANS probes the distributionsof nucleus in samples while SAXS probes the distributions of electron clouds there. Therefore, they
are complementary to each other and provide useful information about structure determination.
Table 5.1 shows the scattering lengths and cross sections for various elements. It should be
noted that most of the neutron scattering cross sections shown in Tab. 5.1 have the same scalethough their atomic numbers vary in the wide range. Since neutrons are uncharged, they interact
much more strongly with the atomic nuclei rather than the electrons. Thus, neutron scattering issuitable to observe the positions of light atoms with few electrons. In the case of SAXS, it is hard
to separate scattering from heavy elements and hydrogen because the X-rays interact weakly withlight atoms.
The key feature of the neutron scattering is that the cross section of hydrogen is distinct fromthat of deuterium. Because the chemical properties of hydrogen and deuterium are almost the same,
deuterium can be easily exchanged with hydrogen and the resultant sample maintains the original
chemical property. Their differential scatter is applied to the technique of contrast variation that isuseful for structure determination of materials dissolved in water. Exchanging a part of H 2 O with
D2 O in a solvent at certain ratios of H2 O to D2 O, the scatter length density from a component of
the solute can be the same as that of the solvent. After subtracting the solution scattering fromthe solvent scattering, components of which scattering length density differ from that of the solvent
are observed. In addition, neutrons have magnetic moments and thus they can probe magnetic
structure in samples.
As mentioned in section 1.2, the size of 1-100 nm corresponds to the angular region up to2θ = 0.8 − 8 degrees at wavelength λ = 1.54 Å. Therefore, a sample is irradiated with thermal
neutrons with wavelength of Å scale to observe SANS.

83

Table 5.1: Neutron scattering lengths and cross sections [36]. b c , bi , σc , σi and σa represent a
coherent scattering length, an incoherent scattering length, a coherent cross section, an incoherent
cross section, and an absorption cross section, respectively.
Element bc [10−13 cm] bi [10−13 cm] σc [10−24 cm2 ] σi [10−24 cm2 ] σa [10−24 cm2 ]1H-3.73925.278
1.757
80.30
0.333
2 H(D)
6.671
4.04
5.592
2.05
0.000
12 C
6.646
0
5.550
0.001
0.003
14 N
9.36
2.0
11.01
0.50
1.90
16 O
5.803
0
4.232
0.000
0.000
19 F
5.654
0
4.232
0.001
0.000
23 Na
3.63
3.59
1.66
1.62
0.530

Now neutrons generated by nuclear fission reactors are mainly used for SANS experiments.
Due to absence of fine converging neutron guides, the reactor-based sources produce much lowerintensity flux in comparison with synchrotron sources. Thus, samples irradiating with the reactorbased sources must be large. This is a serious disadvantage to some kinds of samples such as
biological materials.
Another neutron sources available are high-energy particle accelerators developed recently. A
proton accelerator, J-PARC in Japan, has neutron beamlines where the world highest intensity
pulsed neutron will be produced. The estimate of time-integrated intensity and peak intensitywith 1 meV neutron is roughly 1014 n/cm2 /s/sr/eV and 1016 n/cm2 /s/pulse/sr/eV at 1 MW. A
repetition rate is 25 Hz [37]. To conduct high-intensity SANS, an area detector working in highcounting rate environments is desired. The pulsed neutron source produces neutrons with a certain
range of energy. The energy of the neutrons is measured by Time-of-flight (TOF) on a scale of µs.
Therefore, a counting-type area detector with the time resolution of several µs allows one to obtainwavelength dependence of scattering patterns.
Other requests to the detector is low sensitivity to gamma-rays which could be background,and a wide dynamic-range. IPs and MWPCs widely used for SANS have a narrow dynamic rangeof up to 104 . A detector consisting of scintillators located in the middle of the detection areasurrounded by MWPCs has the widest dynamic range of 10 6 . Similar to SAXS measurements, wider
dynamic range is required for high-precision experiments. Counting-type detectors are superior to
integrated-type ones in terms of a dynamic range because counting-type detectors are free fromnoises proportional to an integration time. Since the high-energy particle accelerators produces agamma-ray background, an ability to discriminate against gamma-rays is important. Our goal isto achieve a dynamic range of more than 10 7 and thus sensitivity to gamma-rays must be less than
10−6 sensitivity to neutrons.

5.2

Neutron detector based on µ-PIC

5.2.1

3

He(n, p) Reaction

The 3 He is used as a detection medium for thermal and thermal neutrons. A 3 He(n, p) reaction
converts a neutron to detectable particles. The reaction may be written
31
3
1
2 He +0 n →1 H +1 p

Q-value = 0.764 MeV
The Q-value of 764 keV is converted to kinematic energies of reaction products oppositely
directed.
84

Ep = 0.573 MeV
E3 He = 0.191 MeV
The neutron cross section with a wavelength of 1.8 Å is 5333 barn, and its value falls off witha 1/λ dependence. Figure 5.1 shows an energy deposit along the proton and triton tracks in 1 atm
Ar-C2 H6 -3 He (>99 : 10 : <1) mixture gas. The values were calculated by GEANT4 (ver 5.0 patch01) [38]. The conversions of 25 meV-thermal neutrons were assumed. The proton have a longer
track length so that one can separate tracks of two reaction-products and derive the conversion
point. In the case of a gas with a bigger ratio of 3 He, the track length is longer because dE/dx is
proportional to atomic number Z.

Figure 5.1: Energy deposit along the proton and triton tracks in 1 atm Ar-C 2 H6 -3 He (>99 : 10 :<1) mixutre gas. The values were calculated by GEANT4 (ver 5.0 patch-01) [38].

5.2.2

Advantage of neutron detector based on µ-PIC

The area detector based on the µ-PIC has been developed for purpose of practical use at thirdgeneration synchrotron radiation sources. The detector system suitable for high-counting rateenvironments has being developed and is expected to be applied to high-counting rate SANS.
Considering the requirements for pulsed neutron sources, the µ-PIC detector is suitable becauseof the fine time resolution of 10 ns for incoming particles. Moreover, the µ-PIC is free from deadtime if one prepares a large memory to store data from the position encoder. Data obtained byintegrated-type area detectors cannot be combined with TOF information. The µ-PIC detector
has shown the wide-dynamic range of the solute scattering using the third-synchrotron radiation
source.
The energy of the gamma-rays, which are found with neutrons, is transferred to recoil electrons
through Compton-scattering. dE/dx of the recoil electrons is small and thus their track lengths
are long. On contrary, dE/dx of protons and tritium is quite large and their track lengths areshort. Since one can measure three-dimensional tracks of charged particles using the position
encoding module, it is easy to separate signals generated by the neutrons and background. Using
the mode-III encoding system described in section 3.5, one can also measure pulse widths andobtain an approximate value of dE/dx. It could be a strong background rejection unique to a
detector system based on a TPC.
A position resolution of less than 1 mm is desired for neutron detectors while currently available
neutron detectors have a position resolution of up to a few mm. Now we are considering two ways85

to enhance the position resolution of the neutron detector based on the µ-PIC . One way is toraise a gas pressure to about 10 atm and then the short track lengths of the reaction products leadto a better position resolution. Other way is to obtain fine tracks of the reaction products at amoderate-gas pressure using the mode-III system. Since a detection efficiency of at least 20 - 30 %is required, there is a need of a thick gas layer in this case. The detector is aiming at small-angle
scattering measurements. Thus it is unnecessary to take into account parallax even if the gas layer
is thick and heights of the conversion points are unknown. With the approximate value of dE/dx,
one could determine directions of the reaction products, which are oppositely directed. Since theirkinematic energies are known, the conversion point can be calculated.

5.3

Overview on study of neutron time-resolved imaging

We studied a TPC based on the µ-PIC for a time-resolved neutron imaging detector [39]. The TPC
was irradiated with neutrons produced by the fission decay of 252 Cf (about 2 MBq). Since gammarays emitted from the decay were detected by Yttrium Aluminum Perovskite (YAP) scintillator forthe neutron trigger, one can measure the absolute height of the conversion points. In addition, every
32-ch analog outputs from the ASDs were summed and sampled by a 100 MHz flash ADC (FADC)
in spite of measuring the widths of the digital output. Referring to the waveform of the analog
output, one can observe the Bragg curves and deduce the directions of the reaction products. Thismethod solves the parallax problem. However, the neutron beam does not have the simultaneous
signals and thus this method is unavailable.
Now we will briefly summarize the performances of the time-resolved neutron imaging detector.
In this experiment, the vessel containing the TPC with a detection volume of 10 cm × 10 cm × 10
cm was filled with an Ar-CF4 -3 He (78:20:2) mixture gas at 1.1 atm. We used a 252 Cf source mainly
emitting fast neutrons. Polyethylene blocks were arranged next to the source and thermalizes thefast neutrons. We chose a CF4 having a small cross section of fast neutrons in elastic scattering.
The TPC mode-I position encoding system shown in Fig. 2.19 was applied to this experiment.
Figure 5.2 shows tracks of charged particles generated by 3 He(n, p) reactions. The double-peaked
Bragg curve can be clearly seen in every event. Through this measurement, we proved that theµ-PIC has an ability to detect tracks of the reaction products and determine the conversion points.
Instead of the TPC mode-I encoding system, we will use the mode-III encoding system, which
provides the pulse widths. The mode-III system provides approximate pulse heights without the
FADC and continuous measurements. We are planning to conduct neutron scattering experimentwith the mode-III system.

5.4

Thermal neutron imaging

To prove an ability to thermal neutron imaging, we conducted SANS experiments at NOP beamline,
JRR-3, Japan. Since a detector for neutron imaging was under construction at that time, we usedthe detector for X-ray imaging. Thus, the gas layer of 4 mm is so thin that most reaction products
penetrate the detection volume. Since the vessel containing the detector was designed to withstanda gas pressure of about 1 atm, we filled a 3 He-Ar-C2 H6 (11.1 : 80.0 : 8.9) mixed gas with a pressureof 1.08 atm. The low gas gain of < 103 was applied in order to discriminate against the gammarays. We removed the GEM because the µ-PIC maintains stable amplifications at a gas gain of
103 . To enhance the intensity, we removed a slit and a monochrometer in front of a sample. In
consequence, the beam had a broad wavelength distribution. The thermal-neutron beam had an
average wavelength of 7.6 Å. The counting rate was 5.9 × 103 cps and the beam size was about
φ25 mm at the detection area without a collimator. The mode-II position encoding system was
employed because the mode-III system had not been suggested at that time. The possibility of theconversions in the detection volume was 2.96 × 10 −2 at a wavelength of 7.6 Å. An actual counting
86

(a) Three-dimensional tracks of tritonsand protons and their waveforms.

(b) Waveform of protons and tritons.

Figure 5.2: Tracks of charged particles generated by 3 He(n, p) reactions.

rate was found to be smaller by two orders of magnitude than this value. Because most of thereaction products penetrate the gas layer, they deposit a small fraction of its initial energies within
the gas. Therefore, reaction products which happen to deposit most of their energy in the gas
volume might be detected.
Figure 5.3(b)(c) display images of a cadmium test chart irradiated with a large absorption crosssection. The exposure time was 7.4 × 10 4 seconds. To enhance a signal-to-noise ratio, events withmore than 5 hits (32 bits × 5) were used. The image yielded by an integration of the reaction
products tracks shown in Fig. 5.3(b) broadens and the structure of the test chart is smeared dueto the bad position resolution. The track length of the reaction products including penetrating
ones ranges from approximately 2 mm to 7 mm. Some of the products penetrate the thin detection
region and lead to the wide range of the track length. Each bright line in Fig. 5.3(b) corresponds toa noisy ASD channel. The low intensity and detection efficiency emphasis the contribution of thenoise. The image yielded by an integration of the averaged positions of the reaction products tracks
is shown in Fig. 5.3(c). The noisy ASD channels were removed and then the averaged positionsof the tracks were calculated. Since the tracks of two reaction products are on the same line,
the difference between the conversion point and the averaged position must be smaller than thatbetween the conversion point and hits close to the end of the tracks. Thus, the position resolution
was drastically improved.

5.4.1

Position resolution

We conducted the knife edge test to evaluate the position resolution of the image yield by the
integration of the averaged positions. Figure 5.4 shows an image of a cadmium slit with a 8-mm
hole. The exposure time was 4.6 × 104 seconds. Since a uniform neutron beam with a large-crosssection was unavailable, we used a region where the beam intensity is approximately uniform,shown in the sector in Fig. √5.4(a). Figure 5.4(b) shows a projection along the edge of the hole
fitted by a1 +a2 erf((x−a3 )/( 2σ)). The parameter σ was 0.956 mm, corresponding to the position
resolution. Irradiating with a uniform beam, the position resolution would be better than this value.
The parameter a3 was 3.992 mm, which agrees with the radius of the hole. Neutrons scattered by
the vessel walls and the µ-PIC are supposed to produce the long tail in Fig. 5.4(b). Therefore, thebackground estimation and reduction are important to enhance a wide-dynamic range. A thicker
87

7.2
6.4

Y [cm]

5.6
4.8
4.0
3.2
1.6

0.8

0

(a) Circled region was irradiatedwith a neutron beam.

20

2.4

40

60

3.2
4.0
X [cm]
80

4.8

5.6

100 120 140 160 180
Intensity(b) Integration of reaction products tracks

7.2
6.4

Y [cm]

5.6
4.8
4.0
3.2
0.8

1.6

0

10

2.4

20

3.2
4.0
X [cm]

4.8

5.6

30
Intensity4050(c) Integration of the averaged positions of reaction
products tracks

Figure 5.3: Images of a cadmium test chart irradiated with thermal neutrons (7.6 Å).

88

gas layer avoids penetration of the reaction products and the mode-III encoding system would
provide more accurate conversion points. With these improvements the position resolution could
be enhanced.
6.8
6.4

50

6.0
5.6

40

5.2

35

4.8

30

4.4
count

Y [cm]"1_2D_ave.chi"a*(1+erf((b-x)/(sqrt(2)*c)))

45

4.0

25

3.6

20

3.2

15

2.4

0

10

3.2

20

4.0
X [cm]
30

40

4.8

10

5.6

5

50

60

Intensity(a) Image of a cadmium slit; a 8-mm
hole is arranged.

70

0

0

2

4

6

8

10

12

R [mm](b) A region defined by a sector shown in (a) is integrated.

Figure 5.4: Images of a cadmium test chart irradiated with thermal neutrons (7.6 Å). The edge ofthe hole is at R = 4 [mm].

5.4.2

Bragg curve

We observed a diffraction pattern from a 4-mm thick F 12 H20 crystal, shown in Fig. 5.5. The
distance between the sample and the drift plane 4.5 mm in front of the µ-PIC was 2.3 × 10 2 mm.
The exposure time was 1.8 × 105 seconds. A black line refers to a diffraction curve observed by
the µ-PIC ; a red line to one observed by a neutron scintillator (ZnS/ 6 LiF) as a reference. The
positions of the first Bragg peak observed by the µ-PIC and the scintillator agree. According to
Fig. 5.5, counts per pixel are less than 1 at the Bragg peak due to the low-detection efficiency.
It leads to the bad signal-to-noise ratio and thus the Bragg peak observed by the µ-PIC was lessclear. A detector with a thicker-gas layer could improve the signal-to-noise ratio and provide aclear Bragg peak.

89

Intensity

2_2D_track_q.chi
IvsQ_F12H20_2.txt

102

10

1

10-1

10-2

10-1

1
q [1/nm]

Figure 5.5: Diffraction curve of F12 H20 . A black line refers to a diffraction curve observed by theµ-PIC ; a red line to one observed by a neutron scintillator (ZnS/ 6 LiF) as a reference. The intensityof the reference is an arbitrary unit. The curve measured by the µ-PIC shows counts per pixel. A
Bragg peak of q =1.02 nm−1 can be recognized.

90

Chapter 6

Summary
We have successfully shown the advantages of photon-counting type detectors to SAXS measurements, especially the wide dynamic range, using the gaseous area detector based on the µ-PIC.
Since the µ-PIC has the strip structure, the positions of incident X-rays are reconstructed by
the position encoding module. The mode-I position encoding system is the oldest one: it records
hits from the anodes and cathodes only when they coincide in a clock interval. Using the mode-I
position encoding system, the µ-PIC detector had showed the fine performances for time-resolved
experiments and the CRP method. However, the performance of the detector, especially of theposition encoding system, was insufficient to achieve high-precision SAXS measurements in termsof linearity, uniformity, accidental coincidence, and counting loss. The solution scattering curvesof Apo-Ferritin measured by the mode-I system and the IP were compared. Then, the positionsof the valleys and the peaks did not agree. Therefore, we developed the new position encoding
systems, mode-II and mode-III. The performance of the improved system was studied at the RIKEN
structural biology beamline (BL45XU-SAXS station) at SPring-8, Japan.
The mode-II system recording all hits from anodes and cathodes solved the counting-loss problem due to the propagation delay at the digital outputs of the ASDs. Instead of integrating all
hits, we used the averaged positions of them to yield two-dimensional images. Then, the effect of
the non-uniformity of the gas gain and the ASD gain were eliminated and the images became quite
uniform. When the arrival time difference between two X-rays is less than 200 ns, they are likely to
pile up and thus removed in the off-line analysis. This cut, called a pile-up cut, was used to remove
the ghost images produced by hits from the anodes and the cathodes when they accidentally coincide. The resultant scattering curve of Apo-Ferritin was consistent with that measured by the IP.
The important value of gyration obtained by the µ-PIC was 5.320 ± 0.031 nm while that obtainedby the IP was 5.357 ± 0.006. Considering the errors, these two values seem consistent. This result
proves that the curves in the very small-angle region measured by them agree with each other.
The dynamic range of the µ-PIC detector was investigated using the solution scattering from
SiO2 nano particles. In theory, the scattering curve of the particles approximated by a solid spherefollows the power law of q −4 . The intensity clearly followed the power law, and the dynamic rangeof the solute scatterings was widened to > 10 6 . This is the widest linearity in solution scatteringobserved by X-ray area detectors. This wide dynamic range was attained because the detector isfree from the internal background and the statistical uncertainty is dominant. In this way, theµ-PIC detector proved that it has the key features of the photon-counting detectors. The detector
will provide high-precision measurements and more accurate structure determination. Using thedetector, structure of materials containing light elements would be revealed by SAXS measurements.
Many experiments carried out on high-flux and high-brilliance third generation synchrotron X-ray
sources have been limited by the performance of imaging detectors. A new photon-counting imaging
device such as the µ-PIC will enhance some of those experiments.
To perform more accurate position calculations, the mode-III position system was designed.
91

The system can measure pulse widths of the hits and allows one to find a centroid of the charge
distribution. The position resolution was improved using mode-III. The position resolution ofσ = 93.6 µm was determined by holes with 500 µm diameter. We also used a diffraction patternof chicken collagen to evaluate mode-III and succeeded in separating the first peak. The position
resolution of the peak recorded by the mode-III was compared with that by the mode-II system.
Then, it became two times better when the mode-III system was used. The diffraction curve of
the collagen was compared with that observed by an imaging plate for reference, and these two
curves agreed well. The pixel pitch of the IP was 100 µm. The solution scattering from SiO 2 nanoparticles was also observed by these detectors. The resultant scattering curves agreed well.
There is a need for improvements in terms of the gain stability, the aging behavior in highcounting rate environment and the DAR. In principle, gaseous detectors including the µ-PIC detector have the gain variation, which is especially large within a time-frame of several hours after
a high voltage is applied. At the current gain flactuation of the µ-PIC detector and the common
threshold applied to the ASDs, the detection efficiency of charges generated by the X-rays is not
100% and thus is effected by the gain fluctuation. The electronic noise reduction is necessary tolower the threshold level and then the the detection efficiency for the charges will be 100%. Therefore, we are developing a new extension board and will place readout circuits closer to the µ-PIC. The noise reduction will also compensate the small drop in the gain due to the aging behavior.
Another solution to the aging is to remove the ethane gas and use another kind of a quencher gas
such as CO2 .
Our goal is to operate the µ-PIC under the high-counting rates where the IPs are used. Using
mode-II, we successfully measured the solution scattering in such an environment and achieved the
dynamic range of > 106 . However, mode-III is more complex and has achieved the DAR of only 2MHz. For use in the high-counting rate environment, the renewal of the system is required.
These mode-II and -III position encoding systems were applied to the Compton camera consisting of a TPC with readout of the u-PIC and a scintillation camera. Using mode-I, the camera
had successfully produced two-dimensional images of gamma-ray sources. However, this system
also had the counting loss problem similar to the X-ray area detector, because hits from the anodes
and the cathodes were recorded when they coincide in a clock interval. Thus, we developed TPCmodeII based on mode-II that allows one to record all hits. As consequent, the number of hits was
doubled and the fine three-dimensional tracks of muons and recoil electrons were obtained. Now
the angular resolution of SPD is much worse than ARM because of the multiple-scattering of therecoil electrons. Thus, the more precise tracks would be able to determine the initial momentum
vectors of the recoil electrons more precisely.
Furthermore, the widths of the hits were measured by the TPC-modeIII encoding system. The
key concept of TPC-modeIII is the same as that of mode-III. Then, we showed that Compton points
would be determined by the widths and thereby dE/dx. Another main reason for deteriorating
SPD is the incorrect Compton points determined by only the positions of the hits. The GEANT4
simulation shown that the criterion gives 60% recoil electrons correct directions. It is slightly better
than a random selection. Therefore, TPC-modeIII will improve the situation drastically.
Using mode-II, we also showed that the area detector can detect thermal neutron scattering.
It was the first observation of the thermal neutrons using the µ-PIC detector and was conducted
SANS experiments at NOP beamline, JRR-3, Japan. We obtained the two-dimensional imageof the test chart and position resolution of 1 mm using the knife edge test. Currently available
neutron imaging detectors have a position resolution of up to a few mm. Thus the µ-PIC detector
would provide high-resolution measurements. The first Bragg curve of the F 12 H20 crystal wasobserved at the correct position. In this way, we proved that the µ-PIC detector functioned as a
two-dimensional neutron imaging detector. The pulsed neutron sources produced by high-energy
particle accelerators need detectors with the time resolution of several µs to measure the energy ofthe neutrons. The µ-PIC detector with time resolution of 10 ns is promising for the application.
We are developing a detector with a thicker gas layer of 5 cm and will operate it at a higher pressure
92

of about 2 atm. Then, the detection efficiency of the thermal neutrons would be 20-30 % that is
sufficient for the practical use. The detector with the thicker gas layer will provide the whole tracksof the neutrons. Moreover, the track length will be short at the high pressure. The thick gas layer
and the short tracks are expected to improve the position resolution to a few 100 µm.

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Acknowledgements
The work was conducted with lots of support given by a large number of people. I am particularly
indebted to my supervisor Prof. Dr. T. Tanimori for his guidance, support and valuable discussion.
Special thanks to my former colleagues, Dr. K. Tsuchiya and Y. Okada(Tsuchiya) who used to do
this work and helped me when I started this work.
I am deeply grateful to the collaborators of this work: Dr. H. Kubo, Dr. K. Miuchi, Dr. S.
Kabuki, Dr. J. D. Parker, H. Nishimura, K. Ueno, S. Kurosawa, C. Ida, and M. Takahashi. I
also would like to thank other members at my laboratory for their funny talks and creative jokes.
Particular thanks to Dr. Parker and C. Ida who helped SAXS measurements at SPring-8. Dr.
Parker proofread a part of this paper and gave useful advice. After I graduate, he will do this workin place of me. I am looking forward to see how he improves the detector in the near future. I am
grateful to Dr. Kubo who helped improvements on the position encoding module and gave lots ofadvice on not only this work but lots of other things. VHDL programs for the encoding module
were written by ARKUS.Ltd, in Japan.
I thank SPring-8 staff for the operation of SPring-8 and the support for this work. I am
particularly indebted to Dr. K. Ito (RIKEN Harima Institute/SPring-8) for lots of support duringexperiments at SPring-8, valuable advice, and significant discussion. I am also grateful to Dr.
M. Takata (RIKEN Harima Institute/SPring-8, JASRI, The University of Tokyo) for giving us
valuable advice. I also thank Prof. Hideki Seto (Kyoto University) for his support of SAXS using
an X-ray generator. I am very grateful to Prof. Yoshiyuki Amemiya (The University of Tokyo)
for significant advice. Our appreciation is extended to RIGAKU co. Ltd for supporting the X-ray
generator.
I thank JRR-3 staff, particularly T. Oku for the operation of SPring-8 and the support for thiswork. I am grateful to Prof. H. Shimizu for valuable advice on SANS experiments.
This work was supported by KEK Renkei, a Grant-in-Aid for the 21st Century COE ”Centre for Diversity and Universality in Physics”, and a Grant-in-Aid for the Global COE Program”The Next Generation of Physics, Spun from Universality and Emergence”; Grant-in-Aids in Scientific Research of the Ministry of Education, Culture, Sports, Science and Technology (MEXT).
The author is supported by Japan Society for the Promotion of Science Research Fellowship for
Young Scinentists. The synchrotron radiation experiments were performed at BL45XU in SPring-8
with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposals No.
2007A1630, 2007B1407, and 2008A1532).

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