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PERFORMANCE EVALUATION OF GAGG+ AND TUNGSTEN CARBIDEBLADES IN AN X-RAY PINHOLE CAMERA
S. Burholt∗ 1 , L. Bobb , N. Vitoratou, Diamond Light Source, Didcot, UK
Abstract
At Diamond Light Source two X-ray pinhole cameras areused to measure the transverse profile of the 3 GeV electron
beam. The current pinhole assembly is formed using tungsten blades with chemically etched shims to produce a 25 µm
x 25 µm aperture and the imager incorporates a 0.2 mm
LuAG:Ce scintillator. Tungsten carbide is a machinablehigh-Z material which at millimetre thicknesses is opaqueto X-rays. With a slight change in pinhole design, similar tothat already in place at the ESRF, tungsten carbide blades
could offer a well-controlled aperture size for the pinhole
camera with simpler assembly. Further to this, improvements to the photon yield of scintillators mean that the new
scintillator GAGG+ has an almost two-fold increase in yieldcompared to the current LuAG:Ce scintillator. An evaluation of the tungsten carbide blades and GAGG+ scintillator
is presented.

INTRODUCTION
X-ray pinhole cameras are important diagnostic systems
used for beam emittance monitoring at synchrotron lightsources. Diamond currently uses two X-ray pinhole cameras
downstream of two different bending magnets to provide
non-invasive transverse beam profile measurements. These
two pinhole cameras are referred to as pinhole camera 1(P1), and pinhole camera 2 (P2). A schematic layout can beseen in Fig. 1. There is another pinhole camera (P3) set up
in a similar way to the other pinhole cameras where researchand development projects are carried out.

Figure 1: Diamond X-ray pinhole camera layout, including
source, pinhole aperture, scintillator, mirror, lens, and camera.
The synchrotron radiation with an energy range of 15 keV
to above 60 keV, with a peak around 26 keV, exits vacuum
to air through a 1 mm aluminum window. The X-rays thenpass through a tungsten pinhole assembly which is formed∗ sam.burholt@diamond.ac.uk

of 25 µm × 25 µm apertures [1]. This X-ray beam then interacts with a 0.2 mm thick Luttetium aluminium garnet dopedwith cerium (LuAG:Ce) scintillator screen and is imaged by
a CCD camera (Flea 2), allowing for beam profile measurement and fitting.
The 25 µm pinhole aperture is formed from stacks of
25 mm(h) x 1 mm(v) x 5 mm(d) tungsten blades separated
by chemically etched shims of the required thickness [2, 3].
There is one stack vertically and one horizontally so that an
aperture of 25 µm in both planes is presented to the X-ray
beam. This also forms a 10 mm long tunnel, fully attenuating the beam outside of the aperture. A low flow of nitrogengas is used to reduce metal oxide growth inside the pinholeassembly.
The spatial resolution of a pinhole camera system canbe described by the point spread function (PSF). The PSF
is assumed to be constant on relatively long timescales fora given system. For a pinhole camera imaging system theoverall PSF may be represented as
𝜎2𝑃𝑆𝐹 = 𝜎2𝑝𝑖𝑛ℎ𝑜𝑙𝑒 + 𝜎2𝑐𝑎𝑚𝑒𝑟𝑎(1)

𝜎2𝑝𝑖𝑛ℎ𝑜𝑙𝑒 = 𝜎2𝑑𝑖𝑓 𝑓 𝑟𝑎𝑐𝑡𝑖𝑜𝑛 + 𝜎2𝑎𝑝𝑒𝑟𝑡𝑢𝑟𝑒(2)

𝜎2𝑐𝑎𝑚𝑒𝑟𝑎 = 𝜎2𝑠𝑐𝑟𝑒𝑒𝑛 + 𝜎2𝑙𝑒𝑛𝑠 + 𝜎2𝐶𝐶𝐷

(3)

with
and

where the subscripts denote the sources of the PSF contributions [1]. It is well known that there are many contributions
to the point spread function. A significant contribution arises
from the pinhole aperture [1].
The pinhole aperture material must be opaque to X-rays.
A tungsten blade and shim assembly is currently used due tothe opacity of tungsten being a high-z material. Shims are
required because tungsten is not readily machinable. Therefore the pinhole apertures must be formed using chemicallyetched shims between the tungsten blades. Due to this theabsolute size of the pinhole aperture differs from the shimthickness and is thus unknown. A few factors can cause
this discrepancy including; the deformation from cutting or
handling the chemically etched shims, misalignment duringassembly, and the pressure of the blade-shim-blade assembly. It is also not trivial to measure the pinhole aperturesize due to the 5 mm depth of the tungsten blades and threedimensional structure of the pinhole assembly.
In order to minimise the PSF contribution from the pinhole aperture it is necessary to be able to control the pinhole
aperture size and ensure it is matched to the spectrum of
the incident synchrotron radiation. Previously machinedmolybdenum blades were investigated as an alternative tothe tungsten/shim design, however the 5 mm molybdenum

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blade aperture scans showed a non-linear response indicating insufficient opacity to the X-ray beam [2]. LIGA (Xray lithography) to produce well-defined apertures in a thin
250 µm gold screen has also been studied, however due tothe thickness constraints of the LIGA process, images may
require background subtraction. LIGA apertures or slits may
be more suitable for monochromatic Fresnel diffractometry
and/or interferometry [3].
Tungsten carbide is machinable and provides similar opacity to X-rays as tungsten. At the ESRF tungsten carbide
blades using a C-D design have been tested [4]. In this paper,
similarly machined tungsten carbide blades are evaluated forthe pinhole cameras at Diamond.
From Eq. (3) the overall PSF may be reduced by improving the spatial resolution of the scintillator. Gadolinium aluminium gallium garnet doped with cerium (GAGG+) is a relatively new commercially available scintillator and has beenstudied for diagnostic applications at other facilities [5–7].
This material has an almost two-fold increase in photon yield(@23 °C) of 40 - 60 x 103 Ph/MeV vs. 25 x 103 Ph/MeV for
LuAG:Ce which allows for better imaging of the X-ray beam
at low current, and enables shorter exposure times thereby
reducing the contribution of jitter within images [8]. In thispaper the performance of GAGG+ is also evaluated for the
X-ray pinhole cameras at Diamond.

TUNGSTEN CARBIDE BLADES
Tungsten carbide blades have been manufactured in the CD design used at the ESRF [4] with the aperture being formed
from an extrusion (D) and void (C) within the stacked blades,
rather than with the traditional shim design, or void/flatassembly. The reason for the C-D design is for ease of
manufacture: overall it is better to grind larger steps into thetungsten carbide and combining the C-D blades to form thedesired aperture size, than trying to grind the 25 µm aperture
exactly.
Two tungsten carbide stacks of matching pairs were manufactured with an aperture size of 25 µm. These two pairs
were set up within the normal blade stack as shown with a
traditional tungsten aperture using 25 µm chemically etchedshims. This formed an overall pinhole stack with a 25 µm
tungsten-shim-tungsten aperture and a C-D tungsten carbide
aperture which allowed for comparative measurements asshown in Fig. 2. Both blades types were manufactured tohave overall dimensions of 5 mm in depth, 1 mm in height,
and 20 mm in length.
The tungsten carbide blades were manufactured by Midland Carbide [9] and had aperture specification tolerancesof ±1 µm. The blades were manufactured using electrodischarge machining (EDM) to form the overall shape andsize then ground and polished to the desired thickness. The
stock material was tungsten carbide grade K10, which is theultra-fine particle size with a material composition of 5.5 %cobalt (Co) and 94.5% tungsten carbide (WC), hardness(HRA) of 92.8, and density of 14.95 g/cm3 [10].

Figure 2: Pinhole blade stack design (left) constructed oftungsten blades (light grey), 25 µm chemically etched shims(blue), and the tungsten carbide C-D design with 25 µm
machine gap (dark grey). Stack will be put under pressure
when in use to hold all of these as flush as possible. Gaps
are not to scale, but to show the C-D design. Total stackformation (right) showing how the apertures are formed fromthe orthogonal stacks A and B.
Using a back-lit microscope a rough measurement of thepinhole aperture size can be made. With the use of image
processing software ImageJ [11] both set of tungsten carbide
apertures were measured to be ≈35 µm in size. This is 10 µm
larger than the specification.

TUNGSTEN CARBIDE APERTURE
INVESTIGATIONSAperture Scans
The pinhole assembly is mounted to translation and rotation stages which allow for remote alignment with the beam.
This is required because misalignment can cause reductionin the aperture size for the passing synchrotron radiation.
Using the rotation stages the angle of the pinhole aperture
relative to the incident beam is varied. During this aperturescan the 2D Gaussian fitted beam sizes (𝜎𝑥 , 𝜎𝑦 ), peak intensity and fit error are recorded via the normalisation ofthese parameters to their maximum and minimum values asa function of angle.
Higher photon flux through the aperture occurs when theaperture is parallel with the X-ray beam. Thus the measured
peak intensity reduces from a maximum value as the angleof the aperture becomes misaligned to the beam.
The horizontal rotation scans of the tungsten and tungsten
carbide apertures can be seen in Fig. 3. The horizontal rotation of the blades show the expected increase and decreasein the peak intensity around the aligned position, this canalso be seen by the consistent horizontal and vertical beam
size measurements throughout the scans.
The vertical aperture scans show a similar response forthe peak intensity, as seen in Fig. 4, but the tungsten carbide
vertical beam size increases as the system becomes aligned
to the X-ray beam, whilst the tungsten blades do not.
This response could be attributed to the larger aperturesize of the tungsten carbide assembly when compared to
the 25 µm tungsten aperture, as it was found to be 35 µm
instead of 25 µm via microscope images. To further show
this effect a 50 µm tungsten aperture was tested similarly and
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12th Int. Beam Instrum. Conf.
ISBN: 9 7 8 - 3 - 9 5 4 5 0 - 2 3 6 - 3

IBIC2023, Saskatoon, Canada
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750

1.4
700

650

1
600

0.8
x
y

0.6

/min (tungsten)
Y (Pixel)

normalised value

1.2/min (tungsten)peak/max (tungsten)normalised fit error/min (tungsten)/min (tungsten carbide)
x

0.4

y

/min (tungsten carbide)

550

500

normalised fit error/min (tungsten carbide)peak/max (tungsten carbide)

0.2
-0.15

-0.1

-0.05

0

0.05

0.1

0.15

450

rotation (deg)Figure 3: Horizontal rotation scans on pinhole camera 3 ofthe tungsten aperture (solid lines) and the tungsten carbide
aperture (dashed lines).

400

350
50

100

150

200

250

300

350

200

250

300

350

X (Pixel)

1.6

1.4

750

1.2
700

1
650

0.8
x
y

0.6

/min (tungsten)
600/min (tungsten)peak/max (tungsten)normalised fit error/min (tungsten)/min (tungsten carbide)
x

0.4

y

Y (Pixel)

normalised value

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1.6/min (tungsten carbide)peak/max (tungsten carbide)
normalised fit error/min (tungsten carbide)

0.2
-0.15

-0.1

-0.05

0

0.05

550

500

0.1

0.15

rotation (deg)Figure 4: Vertical rotation scans on pinhole camera 3 ofthe tungsten aperture (solid lines) and the tungsten carbide
aperture (dashed lines).
found to have the same response in the fitted vertical beam
size (𝜎𝑦 ) and none in the horizontal beam size (𝜎𝑥 ). Thisshows that the measured beam size of an aperture varies
depending on the alignment and size of the aperture dueto the PSF contribution. The optimal aperture size for theincident X-ray beam at Diamond is 15 µm [3].

GAGG+ EXPERIMENTS

450

400

350
50

100

150
X (Pixel)

Figure 5: Tungsten blade knife-edge images at the same gainand exposure settings for LuAG:Ce (Top) and GAGG+ (Bottom). ROIs used for knife-edge MTF measurement (dashedorange) and ROIs used for photon yield measurement (solidblue).

Modulation Transfer Function Experiments
For these experiments knife-edge images were acquired
using a tungsten blade. This blade was placed in front of thescintillator, at a slight angle, to block a portion of the X-ray
fan. Knife-edge images are shown in Fig. 5 from which
the Edge Spread Function (ESF) is derived. Differentiating

the ESF provides the Line Spread Function (LSF) which isanalogous to a one-dimensional PSF. Finally the LSF undergoes Fourier Transform to derive the Modulation Transfer
Function (MTF) [12–14].

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Modulation transfer functions are normally used to investigate the camera lens and camera sensor spatial resolution [13]. The images from the pinhole cameras weretaken and processed using QuickMTF which can readily
produce the ESF, LSF, and MTF analyses from the regionof interest [14].
In order to compare the two scintillators, all aspects of
the imaging system (mirror, lens, and camera) remain unchanged. The magnification from the scintillator to the camera sensor is 1.04. The MTF results shown in Table 1 andFig. 6 have been converted from the image plane (camera
sensor) to the object plane (scintillator screen) using themagnification. The MTF10 and MTF50 values in Table 1
have been converted from cycles per pixel to line pairs per
mm (lp/mm) using the pixel size of the camera.

The spectral distribution of the emitted synchrotron radiation
was consistent for all studies. The intensity per pixel withinthese ROIs can be seen in Table 2. This shows an increasein the photon yield of 78 % between the current scintillator
LuAG:Ce and the new material GAGG+. These results arealso in good agreement with the photon yield specifications
for the scintillators which predicted a two-fold increase of
GAGG+ compared to LuAG:Ce [8].
Table 2: Intensity Results of a ROI of the Knife-edge Imagesof LuAG:Ce and GAGG+
Total Intensity (𝑎.𝑢)Pixels in ROI
Intensity per pixel (𝑎.𝑢)LuAG:Ce

GAGG+

6.04 x 106

1.07 x 107
68769
157

68769
88

100
GAGG+
LuAG:Ce90

CONCLUSIONS

80
70

MTF (%)

60
50
40
30
20
10
0

0

0.2
0.4
0.6
0.8
Spatial Frequency (cycles per pixel)

1

Figure 6: MTF Graph for LuAG:Ce and GAGG+.
Table 1: MTF Results of the Knife-edge Scans of Fig. 6
Value
MTF50 (lp/mm)
MTF10 (lp/mm)
10 - 90 % rise (pixels)

LuAG:Ce

GAGG+

27.8
70.7
4.5

32.0
78.6
3.56

The 10 - 90 % pixel value decreases by one pixel from
LuAG:Ce to GAGG+ demonstrating that the edge is sharper.
GAGG+ has a slightly increased spatial resolution than
LuAG:Ce when referring to the MTF10 and MTF50 values
in Table 1, however this increase can be seen to be marginal
when compared in Fig. 6. Overall Fig. 6 shows that LuAG:Ceand GAGG+ have similar spatial resolutions.

Photon Yield
A comparison of the photon yield from LuAG:Ce and
GAGG+ was undertaken using the ROIs indicated in Fig. 5.

This paper investigates whether the use of the new GAGG+
scintillator and tungsten carbide blades could improve thespatial resolution of the X-ray pinhole camera at DiamondLight Source. The microscope images and aperture scans
have shown that although specified to 25 µm the new tungsten carbide C-D designed blades have a larger apertureof approximately 35 µm. Aperture scans have shown tungsten carbide is sufficiently opaque to the incident X-ray synchrotron radiation. Although the tungsten carbide apertureshows an increase in vertical beam size during the verticalrotation scan, this can be attributed to its larger aperture sizeof 35 µm which is less optimal for the given spectrum ofsynchrotron radiation.
For camera images acquired using the same gain and exposure settings the spatial resolution of GAGG+ and LuAG:Ce
has been shown to be almost identical. The expected twofold increase in photon yield has been largely validated withan 78 % increase in the average intensity per pixel of GAGG+
over LuAG:Ce. This shows that GAGG+ would be a goodsubstitute for LuAG:Ce as it has a higher photon count without any loss in spatial resolution.

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IBIC2023, Saskatoon, Canada
JACoW Publishing
ISSN: 2 6 7 3 - 5 3 5 0
d o i : 1 0 . 1 8 4 2 9 / J A C o W - I B I C 2 0 2 3 - W E P 0 2 0Content from this work may be used under the terms of the CC-BY-4.0 licence (© 2023). Any distribution of this work must maintain attribution to the author(s), title of the work, publisher, and DOI

12th Int. Beam Instrum. Conf.
ISBN: 9 7 8 - 3 - 9 5 4 5 0 - 2 3 6 - 3

IBIC2023, Saskatoon, Canada
JACoW Publishing
ISSN: 2 6 7 3 - 5 3 5 0
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