Detector evaluation for micro-tomography experiments at BL20B2
Kentaro UESUGI and Masato HOSHINO
1.

Introduction and explain about BL20B2
A) Optics hutch
Monochromator and stabilized water cooling systemB) Experimental hutch
Micro-imaging setup including tomography

2.

Monochromator alignment checkA) fixed-exit, energy calibrations

3.

i.

changing the X-ray energyii.

adjusting the monoiii.

measure the beam positioniv.

measure the flux densityv.

measure the stability by intensity and beam position

Micro-imaging setupA) high-precision stagesi.

check the movements of translation stageB) image detectori.

measure MTF

ii.

measure conversion gain

4.

Basic introduction for synchrotron radiation micro-tomography

5.

Micro-tomography measurement at hutch 1A) CT scan (samples: woods, plastics, etc.)
i.

ORCA Flash4.0 (f=50mm)+BM2(f=35mm)+P43(10um-thick)ii.

ORCA II ER (f=50mm)+BM2(f=35mm)+P43(10um-thick)B) Tomographic reconstruction using GPGPU on PC

Measurement of CONVERSION GAIN for X-ray imaging detector
Measurement of the conversion gain helps us to understand sensitivity of X-ray
detectors. In this practice, an X-ray detector which is composed of a beam monitor and acharged coupled device (CCD) camera is used. The incident X-ray beam onto the beammonitor is converted into visible light by a phosphor screen. The visible light image isfocused on the CCD camera via relay lenses. The measurement procedure of theconversion gain is as follows.

I. Detection of incident X-ray beam using X-ray detector
You can use image acquisition software “HiPic” to view and analyze X-ray images.
A) Adjust exposure time of the CCD camera so that adequate signals are obtained.
Note that long exposure may induce saturation of signals.
B) Adjust aperture size of a slit (TCslit1) using GUI on the X-terminal. In this case,
X-ray beam size should be smaller than effective field of view of the X-ray detector.
C) Measure the effective pixel size (μm/pixel) of the X-ray detector used.

II. Measurement of X-ray beam sizeA) Select “Rectangle ROI” from a tool bar on HiPic.
B) Indicate the shape of the X-ray beam by ROI. The ROI size should be same as theX-ray beam size.
C) Read out the ROI size in unit of pixel from “ROI interface” window. Then, actual
X-ray beam size can be calculated from the ROI size and the effective pixel size.

III. Measurement of X-ray photon flux using ionization chamber.
A) Measure intensity of the incident X-ray beam (I0). You can read out the digitalizedvalue from ORTEC Counter.
B) In the same manner, measure the dark (background) signal (Idark) without X-ray
beam.
C) Calculate real intensity by subtracting Idark from I0.
D) Convert the digitalized value into voltage. Here, you can use a conversion factor of aV/F converter; 1MHz/10Volts.
E) Convert the voltage into ionization current generated in the ionization chamber.
Here, you can use a gain factor of a current amplifier; 10xVolts/1Ampere. Actual
gain factor is displayed in the current amplifier installed on beamline.
F) Calculate X-ray photon flux using a following equation.

n( photons / sec) Wi
Ee 1  exp l 

W: W-value of gas in the ionization chamber. In this case, it is air. Wair=33.97eV
E: Incident X-ray energy (eV)e: Electrical charge; 1.602×10-19(C)

Table. 1 Linear absorption coefficient

μ: Linear absorption coefficient (cm-1) of air. See Table.1.
l: Length of electrode in the ionization chamber (cm).
i: Ionization current (A).

If you have derived the X-ray photon flux, let’s calculate the X-ray photon flux per unit
area (photons/sec/mm2) using the X-ray beam size.

IV. Measurement of total amount of signals produced by X-ray detectorA) Acquire an incident X-ray beam image. Remember that signals on the CCD camera
should not be saturated. In the case of HiPic, a saturated pixel is represented as redcolor. Note the exposure time in order to estimate the total amount of signals per
1sec in the following calculation.
B) Acquire a dark image with the same exposure time.
C) Measure the total amount of signals (ADCtotal) produced by the X-ray detector.
Indicate the shape of the X-ray beam using a “rectangle ROI” tool. Here, the ROI
size should be larger than the size of X-ray beam to capture all of signals. You canknow the total amount of signals using an analytical tool on HiPic. Find “Analysis”
located at a menu bar on HiPic and select “Histogram display”. Then, you willobtain the total amount of signals (Total count) by pushing “Get histogram” button.
D) In the same manner, measure the total amount of dark signals (ADCdark) at thesame ROI position.
E) Calculate the real signal by subtracting ADCdark from ADCtotal.
F) Convert the real signal calculated above into the total amount of signals per 1sec(ADC/sec) by considering the exposure time.

V. Calculation of conversion gain for X-ray detector
A) The conversion gain (ADC/photon), which is the amount of signals generated by asingle X-ray photon, will be calculated by dividing the total amount of signals per
1sec (ADC/sec) by the incident X-ray photon flux (photons/sec).