Medipix Project: Characterization and Edge Analysis

Matthijs DamenStudent ID: 5887453

University of Amsterdamand

Nikhef, National Institute for Subatomic Physics

July 5, 2011

Abstract

This thesis is a description of a Bachelor project at the Detector R&D group of Nikhef(National Institute for Subatomic Physics). The main research topic within this group isMedipix based detector systems. This report will give a short overview of the Medipix andTimepix detector systems and then focus on the presentation of the results of experimentsdone with these detector systems. There are roughly two parts in this project. The firstpart consists of the characterization of the Medipix Detector using a dedicated laser setup.This involves investigating the response of the detector while changing several laser and chipparameters. The second part is edge analysis of the silicon sensor on top of the Timepixchip using the same setup. The results are used to validate simulations of the sensors edgedone by [5].

The setup appears to be well suited to be used for characterization. Especially the factthat the laser can be focused on a single pixel is very useful. Also, most of the results appearto be in accordance with the simulations.

12 EC Bachelor ThesisFaculty: Physics and AstrophysicsFrom 01-05-2011 to 1-07-2011Supervisor: Jan VisserSecond Assessor: Auke Pieter Colijn

Contents

1 Introduction 1

2 The Medipix and Timepix Detectors 12.1 Concept . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.2 Generations of the Medipix chip . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.3 Analog side of the pixel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32.4 Modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2.4.1 Photon Counting Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42.4.2 Time Over Threshold Mode(ToT) . . . . . . . . . . . . . . . . . . . . . . 42.4.3 Time of Arrival Mode (ToA) . . . . . . . . . . . . . . . . . . . . . . . . . 4

3 Characterization of a Medipix Detector 53.1 The setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53.2 Results of the characterization experiments . . . . . . . . . . . . . . . . . . . . . 6

3.2.1 Laser frequency and pile-up . . . . . . . . . . . . . . . . . . . . . . . . . . 63.2.2 Pulse width of the laser . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73.2.3 THL scans and pulse width . . . . . . . . . . . . . . . . . . . . . . . . . . 73.2.4 Laser intensity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83.2.5 Intensity and pulse width . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

4 Sensor Edge Analysis 114.1 The setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114.2 Results of experiments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

4.2.1 Stepping to the edge of the sensor . . . . . . . . . . . . . . . . . . . . . . 114.2.2 Hecht relation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134.2.3 Systematic even-odd pixel response . . . . . . . . . . . . . . . . . . . . . . 154.2.4 Pixel size as a function of bias voltage . . . . . . . . . . . . . . . . . . . . 16

5 Conclusion and discussion 185.1 Characterization using laser setup . . . . . . . . . . . . . . . . . . . . . . . . . . 185.2 Sensor Edge Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

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1 Introduction

This Bachelor thesis gives an overview of two months at the Detector R&D group of Nikhef(National Institute for Subatomic Physics). The design and development of the Medipix chipis a collaboration of many institutes including Nikhef, hosted by CERN [1]. The Medipix chipand its spin off, the Timepix chip, are both hybrid pixel detectors capable of detecting photonsand charged particles. A new feature is the use of one or more thresholds to discriminate certainphoton or particle energies.

This project consists of roughly two parts. The first part is the characterization of a Medipixdetector system using a dedicated laser setup. The response of one or more pixels is monitoredwhile changing several parameters of both the laser and the chip, such as frequency, pulse widthand threshold value (THL).

The second part of the project is an analysis of the edge of the sensor material. One Medipixor Timepix chip measures 1.5 by 1.5 cm. If one would like to cover a larger area, the chips needto be tiled. To do this in an useful way, the edges of the chip need to respond in more or lessthe same way as the rest of the sensor, or a correction for the difference in response needs tobe applied. By doing several measurements on the sensors edge, these possible differences areanalyzed and compared to the predicted differences.

Before the experiments are explained and the results are shown, a short introduction on theMedipix and Timepix chips is given.

2 The Medipix and Timepix Detectors

In this section a short overview of the Medipix detector is given. The type of detector will bediscussed and the different generations of the Medipix/Timepix detectors are summarized alongwith their detection modes. The detailed functioning and structure of the pixels and the chip asa whole goes beyond the scope of this report. For a thorough explanation refer to [2].

2.1 Concept

A Medipix based detector is a so called hybrid pixel detector, see Figure 1. This means that thesensor material and the read-out chip are separately produced and connected with solder bumpsat each pixel. The sensor can be a cube of gas or be made out of semiconductor material suchas silicon or gallium-arsenide. For the latter, the material is segmented in the same pattern asthe read-out. The advantage of hybrid detectors is that the sensor and electronics on the pixelcan be optimized separately. Also, it makes it easy to use different kinds of sensor chips withthe same read out, for instance semiconductor sensors or gas sensors.

The Medipix project was started to develop a theoretically noise-free hybrid pixel-detector.The purpose of the detector would be particle tracking in high-energy experiments. The sensoris capable of detecting and counting photons as well as charged particles. The signal to noiseratio (SNR)of this chip is greatly increased because thresholds can be set in each pixel separately.With these thresholds, a energy window can be created which gives the opportunity to collectinformation about the energy of particles or photons or count particles within a specific energyrange.

2.2 Generations of the Medipix chip

The first version of the Medipix detector, called Medipix1 or Photon Counting Chip (PCC), wasfinished in 1997. It was a sensor with 64 x 64 pixels and a pixel pitch of 170 µm, see Figure

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Figure 1: The concept of hybrid detectors with the sensor and read-out connected through bumpbonds [2].

2. Although the detector was a succes, it had some limitations. The main limitations are listedbelow.

• The spatial resolution was limited due to the pixel size.

• Only positive input charges (holes) could be measured.

• The chips could not be tiled due to the large dead area around it.

These limitations lead to the development of the Medipix2.

Figure 2: A picture of the electronics chip (no sensor chip on top) of a Medipix1 detector [2].

With the arrival of the Medipix2 chip, most of the limitations were removed. The pixel pitchdecreased to 55 µm (256 x 256 pixels), the dead area was reduced (although tiling was still aproblem, more about this later in the report) and both positive and negative charges could bemeasured.

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After the success of Medipix1 and Medipix2 there was a need to improve the detailedcharge/energy measurements and to obtain information about the arrival time of particles andphotons. A modification of the Medipix2 chip was made, called Timepix. This detector wascapable of measuring Time of Arrival (ToA), Time over Threshold (ToT) and/or event countingindependently in each pixel. These counting modes are explained in detail in section 2.4.

The newest generation of the Medipix chip is the Medipix3. The most significant improvementcompared to previous versions is the capability of dealing with charge sharing (overflow of chargeto neighboring pixels). It was also made possible to read and write at the same time on a singlepixel, because each pixel has two counters.

The experiments presented here are all done with a Medipix2 chip or the Timepix chip.

2.3 Analog side of the pixel

To understand the various counting modes of the Medipix/Timepix chips, some understandingof the pixel electronics is needed. Figure 3 shows the analog part of the pixel electronics.

The charge generated in the sensor material is collected at the anode, which is designated”input” in the figure. The charge then goes to the Charge Sensitive Amplifier (CSA) thatintegrates and shapes the signal. As the name suggests, the output of the CSA is charge sensitive.This means that the amplitude of the output signal is proportional to the collected charge.Because the charge in turn is proportional to the deposited energy, the amplitude of the CSAoutput is related to the energy.

The CSA output is compared with two thresholds; Discriminators (or Disc). The lowerthreshold is called THL, the higher THH. If the signal falls within the (energy-)window createdby these two thresholds, the discriminators will give a high output signal. Depending on thecounting mode, the counter on the pixel will be incremented.

Figure 3: A schematic of the analog part of the pixel electronics [2].

2.4 Modes

In this section a brief overview of the different counting modes is given.

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2.4.1 Photon Counting Mode

When a photon or particle hits the sensor material, it deposits energy and this results in thecreation of electron-hole pairs. The amount of charges created is directly proportional to thedeposited energy of the particle or photon. In counting mode, the charges created by an incomingphoton or particle are collected and compared to a certain threshold (the THL and THH values).If the total amount of charge exceeds this threshold, a counter is incremented. This methodthus gives information about the number of photons that hit the sensor during the shutter oracquisition time (hence photon counting mode). The threshold gives the opportunity to eliminatenoise, and therefore this increases the SNR. It also makes it possible to discern lower from higherphoton energies. Both the Medipix and the Timepix detector are capable of operating in thismode.

2.4.2 Time Over Threshold Mode(ToT)

In the right part of Figure 4, a simulated output of a pixel in ToT mode is shown. Here theShutter shows the time interval for which the pixel is active. The CSA amplifies the collectedcharge and the signal, which goes to the discriminator (Disc). If the output of the CSA is overthreshold, the discriminators output is high. Then a clock starts giving pulses at a certain rate(40 MHz) to the pixel counter, which gets incremented at that rate. When the CSA output goesunder threshold again, the clock stops. In this way, the number of pulses is a measurement of thetime the signal was over threshold. Because the amplitude of the CSA signal is energy dependent,so is the number of counts. This means that the number of counts is a direct measurement ofthe deposited energy of the incoming photon or particle. Only the Timepix chip is able to countin this mode.

Figure 4: Output of a pixel in time of arrival mode (left) and ToT mode (right) [2]. The top partshows the shutter or acuisition time interval. The second part shows the discriminator output.The third part shows the CSA output. Here, the straight horizontal line indicates the thresholdlevel. The fourth part shows the clock to counter signal.

2.4.3 Time of Arrival Mode (ToA)

In time of arrival mode, the counter starts to get incremented from the moment the outputof the discriminator goes high. The pulses are again given by the same clock as in ToT mode.Independently of what happens during the rest of the shutter time, the counter gets incremented.The number of counts is therefore a measurement of the time the pixel was first over threshold

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and thus of the time the charge arrived at the pixel. This mode is for instance used with a gasbased detector system (instead of a semiconductor sensor) to capture particle tracks. Only theTimepix chip is able to count in this mode.

3 Characterization of a Medipix Detector

The first part of the project was to characterize the Medipix chip using a laser setup. This meansseveral parameters of both the chip and the laser are changed and the behavior of a single or agroup of pixels is monitored. If the pixels do not behave as expected, the result is analyzed anda possible explanation is given. If possible the hypothesis is tested with another experiment.

3.1 The setup

For all of these measurements a single Medipix chip is used in photon counting mode. A ”Fitpix”USB interface is used to read out the chip. The program Pixelman [8] can preview the chip’sresponse and is used to store the counts of the pixels in a file. In most of the experiments thelaser was focused and the settings were applied such that only a single pixel was responding.

(a) The laser and chip within theenclosure

(b) Laser and stage/holder (c) The Medipixchip with Fitpix

Figure 5: On the left an overview of the setup within the enclosure to shield it from light. Inthe middle the optical fiber (yellow) which is held in place by the metal cylinder and the stagewhich holds, in this case, the double Timepix chip. On the right a photo of the Medipix chipwith the Fitpix USB interface.

Figure 5 shows some pictures of the laser setup. Because the chip is used to detect photons,it needs to be shielded from unwanted light. For this reason the whole setup is inside a metalenclosure. The wires that connect the laser with the control devices go through a hole in theenclosure. The hole is filled with foam to prevent light entering.

The two lasers used, one with a wavelength of 660 nm, the other of 980 nm, are inside theenclosure. The input for these laser comes from a pulse generator, which is outside. The pulsesare monitored using an oscilloscope. A yellow optical fiber with a focuser at the end transports

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the laser pulses to the detector. This fiber can be seen in the left and middle picture of Figure 5.The focuser is held in place by the metal cylinder. The holder is attached to a height adjustablestage which can be moved with micrometer accuracy in the z direction. The detector is placedunderneath the laser holder on a precision table which can be moved with the same accuracy inthe x and y direction. The movement of both the laser and the detector are controlled througha Labview program.

The detector also needs a bias voltage. This is supplied using an adjustable power supply,which is also outside the enclosure.

3.2 Results of the characterization experiments

3.2.1 Laser frequency and pile-up

Figure 6: A graph of the counts of a single pixel on the y-axis versus the frequency of the laserpulses on the x-axis. The first part of the graph is fitted (red line) with a linear relation. The insetat the top right shows the values of the parameters of the fit, which has the form y = p1x+ p0

Figure 6 shows the counts of a single pixel as a function of the frequency of the laser pulses.Because the Medipix chip is in counting mode, the pixel will add a count when it goes overthreshold. An individual photon of 660 nm is not enough to give a count, but a single pulse ofthe laser, consisting of multiple photons, is. This means the chip counts the laser pulses. Theacquisition time of the experiment was 0.001 seconds. This means that the shutter of the pixelis opened for 0.001 seconds and this explains that for instance with a frequency of 200 kHz, thepixel gives 200 counts.

The result is fitted with a linear relation (red line) up to 800 kHz. At frequencies above 800kHz the relation breaks down. This is due to the ”pile-up” effect.

Pile-up happens when one injects too much charge in one pixel or the counts come too fastafter each other. This can happen when raising the pulse width, intensity or frequency of thelaser too much. The amplitude of the CSA output is proportional to the injected charge. Whenthe output of the CSA has gone over thresholds, the signal needs some time to decay, to go underthreshold again, before another pulse can be measured and counted. The slope of this decay is

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more or less constant (independent of the amplitude of the signal), which means that the morecharge is injected, the longer the CSA needs to discharge.

Now if there is too much charge injected, the amplitude of the CSA output goes far overthreshold. The signal starts to decay, but before it can go under threshold, another pulse/signalis coming. This new pulse raises the output of the CSA and the pulse is not counted. Also,when the pulses are too close together in time, the same effect happens. Because the trigger fora count is the moment the signal goes over threshold, instead of counting the number of photons,or in this case pulses, the pixel will count 0.

3.2.2 Pulse width of the laser

Figure 7: A graph of the counts of a single pixel on the y-axis versus the pulse width of the laserpulses on the x-axis. This type of measurement is also used to create the graphs of Figure 8.

Besides the frequency of the laser, the pulse width can also be tuned. The pulse width is theduration (in seconds) of the pulse given to the laser. Figure 7 shows the counts of a single pixel asa function of the pulse width of the laser pulses. At low pulse widths, the charge deposited is notenough to get any counts. When there is enough charge, the counts rise fast to the maximum.The pulse at which this occurs will be called the rise pulse width from now on. At high pulsewidths, the pixel goes in pile-up, and does not count anything anymore. This is called the fallpulse width. The difference between rise pulse width and fall pulse width is called pulse widthdurations. The lower threshold (THL) of the pixel determines the pulse width at which the pixelstarts and stops counting. Rising the THL will make the graph move to the right, while loweringit will make it move to the left.

3.2.3 THL scans and pulse width

At different pulse widths of the laser pulses, a scan is done of the THL (lower threshold) value.When the THL value is increased, at a certain point the pixel stops responding, because thesignal does not go over threshold anymore (do not mistake this for pile-up). One of those scans

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(a) Example scan with fit (b) Final result

Figure 8: THL scan at different pulse widths. The graph on the left shows the counts of a pixelon the y-axis versus the THL value. The graph on the right shows the THL50 value on the y-axisversus the pulse width on the x-axis.

is shown in Figure 8a. This curve is fitted with a s-curve (red line) for every value of the pulsewidth:

s(x) =p0

1 + p1 exp [−p2(x− p3)]+ p4 (1)

The parameters of the fit are used to compute the THL value at which the pixel stops countingand the errors on this value. The mid point of the curve is taken as this THL value (which willbe called THL50), while the error is given by the 5% and 95% point of the s-curve. The values ofTHL50 for different pusle widths are all plotted in a final graph, which is figure 8b. This resultis fitted with a linear fit.

The points seem to agree with the fit quite well, although some more points (at higher pulsewidths) would have been useful to be more certain about this. As the injected charge should golinear with the pulse width, a linear relation is indeed expected.

3.2.4 Laser intensity

The specifications of both lasers show how the output power behaves as a function of a DC inputvoltage. Because the laser is not used in continuous operation, but pulsed, the relation mightbe slightly different. Figure 9 shows the output of the laser, measured using a PIN diode, as afunction of the input voltage. Here, the input voltage is the height of the pulses. Between 500mV and 2000 mV the relation seems exponential. This fact is used in subsequent experiments.

3.2.5 Intensity and pulse width

The figures 10, 11 and 12 all belong to the same measurement. At a certain laser intensity (givenin mV), a scan is done of the pulse width (as in figure 7). This relation is fitted with the s-curveof eq. 1 and the rise pulse width, fall pulse width and width of the function is determined. Thedifferent values of the rise pulse widths obtained, are plotted against the intensity (this is relatedto the input voltage), as is done with the fall pulse widths and the width of the step function.This leads to the figures shown.

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Figure 9: Laser intensity, on a logarithmic y-axis, as a function of the input voltage, on thex-axis. The output is measured using a PIN diode.

(a) At rise (b) At fall

Figure 10: The graph on the left shows the rise pulse width (y-axis) for different laser intensities(x-axis). On the right the fall pulse width. Both are for the 660 nm laser

According to Figure 9 the output of the laser goes exponentially with the input voltagebetween 500 mV and 2000 mV. Also, the charge deposited is expected to be linear with the pulsewidth. This means that the graphs of the rise and fall pulse widths and the widths of the stepfunction should decay exponentially with the input voltage. This indeed seems to be the case.

Note that there are only 4 points in Figure 11a. This is because the pixel was already countingthe maximum counts at the minimum pulse width, which is 5 ns. Therefore, no rise pulse widthcould be determined. This also means that we can only plot the width of the block for these 4points.

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(a) At rise (b) At fall

Figure 11: The same two graphs as in Figure 10, but now for the 980 nm laser.

(a) 660 nm laser (b) 980 nm laser

Figure 12: The width (rise pulse width - fall pulse width, called pulse width duration on they-axis) of the response curve for different laser intensities. The results for both lasers are shown.

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4 Sensor Edge Analysis

4.1 The setup

For the second part of this project, the same laser setup was used. Only some little modificationshave been made. For the experiments done on the edge, two Timepix assemblies with edgelesssensors positioned next to each other were used instead of a single Medipix. The sensor materialis provided by VTT [6]. These two chips use the Relaxd-board [7] to communicate with thecomputer. The chip and board are mounted on a holder, which is bolted on a stage. Theconstruction is shown in Figure 13. A Newport x-y-stage was used with the actuators removed,because the setup was already on a movable table. An advantage of this stage is that the chip ismore secure and movement is more precise. For the measurements done, the Timepix is used inTime over Threshold mode.

The rest of the setup is exactly the same as before. Note that some of the experiments aredone with the 980 nm laser instead of the 660 nm.

(a) Newport stage with Relaxd and chip (b) Double Timepix

Figure 13: Stage and chip setup used in the edge analysis experiments.

4.2 Results of experiments

The difference between the edge pixels and pixels in the middle of the chip is mostly due toelectric field anomalies near the edge of the sensor material. Simulations of this field have beendone by [5]. These simulations show the shape of the electric field near the edge for different biasvoltages, see Figure 14. The purpose of the experiments is to check whether the real situation isin accordance with the simulations.

4.2.1 Stepping to the edge of the sensor

To probe the ”shape” of the edge pixels as well as ”normal” pixels, the laser is focused in themiddle of a pixel. Then the detector is moved a (few) micron(s), while the response of thesurrounding pixels is monitored. In this way a graph can be made of the behavior of the pixels;when they turn on and off.

First such a graph is made for two pixels in the middle of the chip, as a reference. This isshown in Figure 15. The laser is focused on a certain pixel and then the table with the chip ismoved toward the neighboring pixel (in the x direction). Halfway the experiment, pixels belowthe pixel of interest (so in the y direction) started to respond, possibly due to the fact that the

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Figure 14: Simulations of the shape of the electric field at the edge of the sensor for differentbias voltages. Note that the sensor appears upside down. The y-axis indicates the thickness ofthe sensor (150 µm) with the pixel electronics at 0 µm and the top of the sensor at 150 µm. Thelines without arrows connect points of equal potential (field lines). The lines with arrows areperpendicular to the field lines and indicate the border between the last three pixels.

chip was not perfectly aligned with the table. As this was not intended, compensation was triedby summing the response of the pixels in the same column and use this as the correct value forthe pixel of interest. This procedure probably slightly increased the number of counts, whichcould explain the small rise in maximum pixel counts in Figure 15.

The response of the pixels below the pixel of interest could be due to a slight angle in theplacement of the chip. While the table is moved only in the x direction, the laser will move inthe x and y direction compared to the pixels. Because the stage cannot be easily rotated, it wasnot possible to correct for the angle.

From this experiment, the size of the laser spot on the sensor can also be determined. Thepixel pitch is 55 µm, but the second pixel, indicated by black squares, responded for about 97µm. This means the spot size is about 42 µm.

The same experiment is done for the edge pixels. The laser is focused at the fifth pixel fromthe edge. The chip is then moved in small steps until no pixel responded anymore. Note that

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Figure 15: A plot of the ToT counts versus the distance from the center of a pixel in the middleof the sensor. The 0 on the x-axis indicates the starting point. The difference in maximumcounts is probably an artifact from charge sharing.

the zero position indicates the assumed edge of the sensor. The result acquired with the 660 nmlaser is shown in Figure 16. Figure 17 shows the same, but now for the 980 nm laser.

As one can see the shape of the fifth, fourth and third pixel are quite normal for both lasers;the shape is the same as for the middle pixels. The shape of the second pixel is distorted andalso looks different for both lasers, as is the case with the first pixel. A comparison with thesimulation of the electric field indeed shows that the second pixel could be larger than normal.This effect is most present at the top of the sensor (bottom of Figure 14) and decreases whengoing deeper into the sensor.

The second pixel seems largest in the results of the 660 nm. This is because the photons ofthis wavelength are absorbed in the first few microns of the sensor material. Photons with awavelength of 980 nm are absorbed much deeper into the sensor, around 100 microns. The effectat 100 microns is much less prominent and therefore the second pixel looks more normal in themeasurements done with the 980 nm laser.

4.2.2 Hecht relation

The result of an experiment where the response of some pixels is monitored while the bias voltageover the sensor material changes is shown in Figure 18. The data points are fitted with a so-calledHecht relation:

Q(U) = Q0µτ

L2(U − U0)

[1 − exp

(− dL

µτ(U − U0)

)](2)

Here, Q is the charge collected by the pixel, Q0 is the total charge generated in the sensormaterial, µ is the mobility of the collected charge (electrons or holes), τ is the mean lifetime ofthe collected charge and U is the voltage over the sensor. L is the distance at which the charge

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Figure 16: The same type of plot as Figure 15, only this time the starting position is at pixel5 from the edge of the sensor (coordinates (5,160)). The 0 position indicates the assumed edgeof the sensor material. The ToT counts of all the pixels from the edge to pixel 5 are monitoreduntil all the pixels count nothing (which means the laser is not pointed at the chip anymore).

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Figure 17: Exactly the same measurement as is shown in Figure 16 but this one was done withthe 980 nm laser. The first 3 pixels look the same, but the second pixel from the edge is clearlya bit smaller. The response of the edge pixel is better than with the 660 nm laser.

is generated and d the thickness of the sensor. This relation states how efficient the pixel iscollecting charge, as a function of the bias voltage (and the sensor material properties like µ andτ). The product of µ and τ gives information about the recollection probability of the generated

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charge. A high value means that the probability is low, and the charge will likely be collected.A low value means the opposite.

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Pixel coordinate(1, 160)(2, 160)(103, 160)(104, 160)

Figure 18: The ToT counts of 4 pixels versus the bias voltage. One edge pixel, a pixel in thesecond column and 2 pixels in the middle of the sensor have been monitored. Obviously the edgepixel does not behave ”normally”. The second pixel is more like the middle pixels. The linesthrough the plots are fits using the Hecht relation of eq. 2

Figure 18 shows the Hecht relation for 2 pixels in the middle (103 and 104) and the first andsecond pixel from the edge. The results for the second pixel are more or less the same as forthe middle pixels. This means the charge collection properties of the second pixel are about thesame as for a regular pixel. The first pixel, however, collects far less charge. From the fit followsthat the product of µ and τ is about 10 times lower than for the other pixels.

That the collection capability of the edge pixels is so low could be due to impurities at theedge, but also due to damage (from cutting the sensor material) or the electric field anomaliesnear the edge. However, it is likely that an error was made in the analysis of the data. TheHecht relation is about charge collection efficiency of a pixel, under the condition that this pixelis able to collect all the charge created, were the efficiency 100%. In these measurements, thediffusion at lower bias voltages is large enough to let neighboring pixels respond. This meansthat the charge created is not totally collected by a single pixel, but the theory behind the Hechtrelation does not take this diffusion into account. A solution could be to sum the responses ofthe surrounding pixels for each bias voltage. Due to time limitations of this project this has notyet been done.

4.2.3 Systematic even-odd pixel response

Some of the results of the experiments done, show a hint of systematic error in the even-columnpixels versus the odd-column pixels. The maximum response of the even numbered columnsseemed systematically lower than of the odd numbered columns (see for instance Figure 17). Tocheck whether this is indeed the case, the maximum response of 20 pixels in a row was measured.The result is shown in Figure 19

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Pixel number98 100 102 104 106 108 110 112 114 116 118

Mea

n T

oT-v

alue

8000

8200

8400

8600

8800

9000

Figure 19: A test of systematic even-odd pixel column response. On the y-axis the number ofToT counts is shown, on the x-axis the pixel number. The variations seem random (althoughquite large), so systematic (mis)behavior does not seem to be the case.

In this graph the difference in maximum response does not look systematic. Although thedifferences in the responses are quite high, up to 800 counts, systematic even-odd pixel responsedifference does not seem to be the case.

4.2.4 Pixel size as a function of bias voltage

Another way of measuring the pixel size, this time also as a function of the bias voltage, is torecord the position at which the edge pixel starts to respond and the position at which it stopscounting again. This is basically the same as Figures 16 and 17, but faster, less detailed and fordifferent bias voltages.

The results of the turn off and turn on point for an edge pixel are shown in Figure 20. Themeasurement is performed for bias voltages of 1, 2, 3, 4, 5, 10, 20, 40 and 60 volts and for bothlasers. The 0 position indicates the edge of the sensor material. Because the laser spot has acertain extend (about 40 µm, so not a point), the pixel can still respond although the center ofthe laser spot is not on the sensor material anymore.

The size of the edge pixels seems to increase with larger bias voltages. This is partly inaccordance with the simulations of Figure 14. The border between pixel 1 and 2 indeed movesto the edge of the sensor when the bias voltage decreases. This makes pixel 2 larger and pixel 1smaller. But according to the simulations, at 20 V the edge pixel should be totally ”closed” bythe electric field. Unless one shoots into the side of the sensor material, the edge pixel shouldnot respond below 20 V. This is not what the results show, which means the shape of the electricfield could be different than the simulations show for lower bias voltages. In section 5 a differentexperiment is proposed to probe the shape of the pixels in a better way.

Figure 21 shows the difference between the turn on and turn off point of Figure 20. Thisshould give the width (or size) of the pixel. The second line, which is labeled ”corrected”, showsthe same values, but now corrected for the assumed laser spot size. The laser spot size is derived

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Bias voltage [V]0 10 20 30 40 50 60

m]

µD

ista

nce

from

edg

e [

-20

0

20

40

60

80

100

120

660 nmright edgeleft edge

660 nmright edgeleft edge

980 nmright edgeleft edge

980 nmright edgeleft edge

Figure 20: The turn off and turn on point of an edge pixel (”pixel 1”) for different bias voltagevalues for both the 660 nm and the 980 nm laser. The micrometer scale on the y-axis designatesthe distance of the center of the laser spot from the edge of the sensor, where a negative valuemeans not on the sensor anymore.

Bias voltage [V]0 10 20 30 40 50 60

m]

µW

idth

[

0

20

40

60

80

100

120

660 nm

effective width

corrected

660 nm

effective width

corrected

980 nm

effective width

corrected

980 nm

effective width

corrected

Figure 21: The difference between the turn on and turn off point of Figure 20. This gives theeffective width of the edge pixel. The corrected value mean that the width of the laser pulse iscompensated for.

from Figure 15. For the 660 nm laser, the spot size is about 42 µm, for the 980 nm laser thisis about 35 µm. This corrected value should show the true size of the pixel for different biasvoltages.

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5 Conclusion and discussion

As this project has two quite distinct parts, both will have a separate conclusion and discussion.

5.1 Characterization using laser setup

The laser setup appeared to be very well suited for characterizing the Medipix chip. Especiallythe ability of focusing the laser spot on a single pixel makes the experiments very clear and theresult easy to interpret. All the attention can go to the response of this single pixel and datareduction while analyzing the results is quite easy.

An improvement of the setup could be a stage that can be used to rotate and tilt the chip. Inthis way the laser can also be used to shoot in from an angle, or even the side, and positioningof the chip is more accurate. Correcting for a tilt or angle in the placement of the detectoris easily done. Also, a type of laser which gives a smaller laser spot in the focus would makemeasurements of sub pixel features possible. Both these options will make future experimentsmore easy.

5.2 Sensor Edge Analysis

The edge analysis experiments show that the electric field at the edge is indeed distorted. Theresults of Figure 16 and 17 show that at bias voltages of at least 20V the effective width ofthe second pixel somewhere deep in the sensor is indeed smaller than at the top. This is inaccordance with Figure 14c and 14d

In contrast, the effective width of the first pixel at low bias voltages does not behave like thesimulations shown in Figure 14b and 14a. In those Figures, the last pixel is completely ”closed”by the electric field. The results show something different, see Figure 20. Although the pixelgets smaller for lower bias voltages, it will always show some counts. This means it cannot be”closed”.

An experiment where one shoots into the side of the laser could give more detailed informationabout the border between the first and second pixel. This can even be done with only a singlewavelength, which is absorbed around the assumed depth of the border. For this to be possible,the setup needs to be upgraded with the stage mentioned above.

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References

[1] Medipix website, available at: http://medipix.web.cern.ch/medipix/

[2] Xavier Llopart Cudie, Design and Characterization of 64K Pixels Chips Working in SinglePhoton Processing Mode. Mid Sweden University Doctoral Thesis 27, Department of Infor-mation Technology and Media, Sundsvall, 2007.

[3] Lukas Tlustos, Performance and Limitations of High Granularity Single Photon ProcessingX-ray Imaging Detectors. Doctoral Thesis, University of Technology, Vienne, Austria, 2005.

[4] Jan Visser et al, Particle Detection. [PowerPoint Slides] Master course UvA, VU and UU,Amsterdam, the Netherlands, 2010.

[5] Marten Bosma et al, Edgeless planar semiconductor sensors for a Medipix3-based radiographydetector. Proceedings of the 13th International Workshop on Radiation Imaging Detectors,Nikhef, Amsterdam, to be published.

[6] VTT, Technical Research Centre of Finland. Available at: http://www.vtt.fi/?lang=en

[7] Jan Visser et al, A Gigabit per second read-out system for Medipix Quads. Nuclear Instrumentsand Methods in Physics Research A 633, S22S25, 2011.

[8] T. Holy et al, Data acquisition and processing software package for Medipix2. Nuclear Instru-ments and Methods in Physics Research A 563, 254, 2006.

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