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X-Ray CCD Detectors

Course Notes

September 2002


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1. Basics

We are all familiar with the electromagnetic spectrum, illustrated in Figure 1, however the vast majority ofapplications, which we encounter, utilize the relatively small visible region of the spectrum with photonenergies of a few electron volts (eV). This section is intended to introduce you to the x-ray region of thespectrum which is currently being widely exploited by science and technology sectors and returningsignificant opportunities for growth.

Figure 1: The different regions of the electromagnetic spectrum with wavelengths indicated along the topand photon energies along the bottom.

The x-ray region has higher photon energies than the Ultraviolet region but lower than those associated

with gamma rays. There are no hard and fast boundaries between the different regions, however for thepurpose of this introduction we will consider the x-ray region to contain photons with energies between0.1-100keV (kilo-electron volts) and corresponding wavelengths between approximately 100-0.1(Angstroms, 10 1nm). As indicated in Figure 1 the x-ray region is normally sub-divided into tworegions: soft x-rays at lower photon energies and hard x-rays at the higher end. Again these regionsare not separated by a well defined boundary, however for the purpose of this introduction we will definesoft x-rays to have photon energies between 0.1-10keV and hard x-rays energies between 10-100keV.


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0

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1

0.1 1 10 100

X-Ray Photon Energy (keV)

FractionalTransmission

1cm of air

100cm of air

Figure 2: Transmission of x-rays through air at atmospheric pressure.

One important point to note regarding the x-ray region of the spectrum is that a large section of it isstrongly attenuated by air, as illustrated in Figure 2. This means that many x-ray experiments (particularlythose involving soft x-rays) must utilize vacuum technology to enable the x-rays to propagate from source

to detector.

2. X-Ray Detection Using CCDSUntil recently photographic film has been the most commonly used detection media during x-ray basedexperiments, offering large area coverage, good spatial resolution and relatively low material costs.However the many advantages of using cooled CCD detectors have resulted in their adoption as thedetector of choice for such experiments. The most obvious advantage is the convenience of detection the image can be displayed and recorded digitally within a matter of seconds, whereas using film,

significant time must be spent developing and digitising the exposed film.

There are two main methods for detecting x-ray photons using CCDs, commonly known as direct andindirect detection, the basic features of each method are outlined in the following sections.

2.1 Direct Detection

In this method of detection the x-ray photon is absorbed within the silicon of the CCD, resulting in theproduction of multiple electron-hole pairs. If this absorption occurs within the depletion region of the CCD,

as illustrated in Figure 3, the electrons and holes are separated by the internal electric field, with the holesrapidly undergoing recombination whilst the electrons are trapped in the pixel until being read-out [Note:photoelectrons generated outside the depletion region will not contribute to the signal]. The number of


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photoelectrons, NE, generated within the CCD can be related to energy of the absorbed photon, E (in eV),by the simple relation:

3.65ENE =

[Note this is different to the case for absorption of visible / UV wavelengths which produce only onephotoelectron per detected (i.e. absorbed) photon.]

Figure 3: Schematic illustration of the direct detection of an X-ray photon.

The probability of a photon being absorbed within the depletion region is displayed in the QuantumEfficiency graph, Figure 4. The low QE of front illuminated devices for soft x-ray photons with energiesbelow ~0.6keV is due to the electrode structure on the surface of the device. This structure stronglyattenuates photons below this energy thereby preventing them from reaching the depletion region wherethey could be detected. This limitation may be overcome by using a back illuminated (or back thinned)

device which offers superior QE across the entire energy range of interest, as can be seen from theFigure 4. The discontinuities in the QE traces are the result of intrinsic properties of Silicon, namely theabsorption edges [L-edge at 100.6eV and K-edge at 1.8keV]. As the x-ray photon energy increases uptowards 10keV the probability of the photons being stopped in the depletion layer decreases, i.e. thephotons pass through the depletion region without creating a signal resulting in a decrease in QE. Theuse of a thicker depletion region (i.e. a deep depletion device) increases the probability of a photon beingstopped in this region and therefore adds a shoulder to the QE curve in the higher energy photon range.


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0

10

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30

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50

60

70

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0,01 0,1 1 10 100

Photon Energy (keV)

PhotonsDetected(%)

BN

FI

FI DD

Figure 4: QE curves for Back Illuminated (BN), Front Illuminated (FI) and Front Illuminated DeepDepletion (FI DD) Devices.

Direct detection offers good spatial resolution (comparable to pixel dimensions) and good QE over quite awide range of photon energies. In comparison to typical x-ray film, direct detection also possessesseveral performance advantages including:

Higher Dynamic Range

The dynamic range (the ratio of the largest detectable signal to the smallest) of the detector, in the x-rayregion, is dependant upon the energy of the incident photons. The number of counts, NC, generated bythe detection of a photon of energy, E(eV), is given by

3.65g

E

NC =

where g is the gain of the system in photoelectrons per count. Using this equation the dynamic rangesfor several photon energies (incident on a 16-bit device with a gain of 7) were calculated, as shown inTable 1.


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X-Ray Photon Energy(keV)

Wavelength () Counts per DetectedPhoton

Dynamic Range

0.1 124 3.9 16 804

0.5 24.8 19.6 3 3431 12.4 39 1 6805 2.48 196 334

10 1.24 391 167

Table 1: Counts per detected photon and dynamic range for several x-ray photon energies. (Assuming a16-bit device and gain = 7 photoelectrons per count)

Typical x-ray films have a dynamic range of 100 for photon energies around 1keV falling to 50between 3-10keV.

I. Increased SensitivityTypical x-ray film has a low flux detection limit of approximately 0.1 photons per m2. Wecan see, from Table 1, that for a CCD with read noise ~1 count and negligible dark currentnoise a single detected photon should register a signal above the noise level. Therefore thelow flux detection limit for this detector is 1 photon per pixel which corresponds to a flux of~1.510-3 photons perm2, i.e. a sensitivity increase of approximately 67 when compared tofilm.

II. LinearityCCDs exhibit good linearity to x-ray fluxes, whereas film responds logarithmically and also

has a variable gamma.III. Improved Signal to Noise

The noise on a signal detected by a CCD has three main components: a) read noise, b) darkcurrent and c) shot noise. The contributions from a) and b) can be considered negligible for acooled, scientific grade sensor, thus leaving only the shot noise. This will be the same forboth film and CCD detection, however additional noise contributions from film fog andmicrodensitometer scanning result in CCDs having superior signal to noise figures.

IV. Energy ResolutionAt low flux levels, i.e. where no more than one photon is likely to be incident on any given

pixel during the exposure time, it is possible to determine the energies of the incident x-rayphotons from the number of counts generated in the detector. A histogram of the incidentphoton energies may be constructed to provide information on the source of the x-rayphotons. This technique of determining the energy spectrum of the detected signal is knownas Energy Dispersive Spectroscopy (EDS).

However direct detection does also have its own advantages / limitations:

I. Poor QE for photon energies above 20keVAs mentioned previously, photons with energies above ~20keV are not stopped within thedepletion layer and therefore are not detected. As a result direct detection is not a valid

option for applications involving hard x-rays above ~20keV.


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II. Relatively Small Image AreaThe majority of CCD sensors have an active area (25 25)mm2, which is small comparedto most sheets of photographic film.

III. Exposure to high X-ray doses will damage the CCDDirect exposure of the CCD to x-rays causes damage to the sensor, this normally occurs intwo stages: a) The dark current increases with accumulated x-ray dosage (this effect will bemost pronounced in NIMO devices, e.g. deep depletion sensors). b) A voltage shift isinduced in the sensor which will compromise its performance. This shift can be corrected, upto a point, beyond which the performance will deteriorate irreversibly AIMO devices willoperate as NIMO with the associated increase in dark current.The number of photons which must be detected before noticeable damage occurs is energydependent but in the x-ray region is of the order of one million therefore the lifetime of thedevice could be as low as 3000 fully saturated images! However, this is the worst-case

scenario for front illuminated NIMO devices and it should be pointed out that the assumptionsused in the derivation of these figures are likely to overestimate the rate at which damageoccurs. We believe that Marconi sensors are superior to those of other manufacturers, suchas Thompson or SITe, and are the most radiation resistant currently available. Additionally BIdevices are much more resistant to damage than FI devices and therefore we wouldrecommend the use of BI type detectors where funding permits.

Bearing these points in mind, it is clear that for many applications direct detection may not be a viableoption.

2.2 Indirect Detection

This method of detection employs a material to convert the x-ray photons into visible photons, which arethen detected by the CCD in the usual manner, as illustrated in Figure 5. These converter materials areknown as phosphors or scintillators. [The two words are often (incorrectly) used interchangeably strictly speaking scintillators are crystalline materials whereas phosphors are granular in nature]. We willlimit ourselves to the example of a phosphor called Gadolinium Oxysulphide (Gd2O2S:Tb) often referredto a GADOX, but also known as P43, which has become the standard for CCD-based x-ray imaging.This phosphor absorbs x-ray photons and emits visible photons predominately at 545nm (2.28eV), withapproximately a 15% conversion efficiency, i.e. 15% of the absorbed x-ray photon energy is converted

into visible photons. Therefore the number of visible photons, N, emitted per absorbed x-ray photon ofenergy E (eV) can be calculated:

2.28

0.15EN

=

Thus absorption of a 10keV photon will generate approximately 658 visible photons, however not all ofthese will reach the detector as they are emitted into 4, i.e. all directions.


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Figure 5: Schematic illustration showing indirect detection of x-ray photons.

The absorption emission process is noisy both in terms of intensity and spatial distribution as scatteringcan occur within the phosphor itself. The characteristics of a phosphor layer, e.g. spatial resolution, lightoutput and quantum efficiency, are dependent upon parameters such as the phosphor thickness, meanparticle size and of course the phosphor material itself.

The performance of a system using indirect detection will depend upon the method used to couple thelight from the phosphor onto the CCD. The simplest technique is to directly coat the CCD with thephosphor material, however as the phosphor will not stop every incident x-ray photon some will passthrough it and be detected directly as outlined in the previous section. This technique will exhibitincreased noise due to the direct detection of some x-ray events and will not protect the sensor fromdamage due to the accumulation of a large x-ray dose. Also phosphor coatings can become damagedduring usage and it is difficult to get a sensor re-coated.

The most common method of coupling the phosphor output to the CCD is by means of a fibre optic taper,as illustrated in Figure 6. The phosphor is deposited directly on to one end of the taper and the othercoupled to the CCD sensor, should the coating become damaged it is relatively easy to get the fibre optic

polished and re-coated.


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X-Ray photon

Phosphor coatingon fibre optic

Fibre Optic

CCD

Visible photonsemitted by phosphor

Only a fraction of the photonsemitted by the phosphor will

propagate down the fibre opticand be detected by the CCD

Figure 6: Illustration of a phosphor coating coupled to CCD via a fibre optic.

The fibre optic may be tapered to provide a larger effective image area or it may simply be a 1:1 taper,both illustrated in Figure 7. The magnification of the taper (simply the ratio of the large diameter to thesmall diameter) enables the effective image area to be much larger than the active area of the CCD,however this is comes at the expense of the throughput of the taper.

CCD

CCD

1:1 Fibre Optic TaperFibre Optic Taper coupling

large area phosphor to

CCD dimensions Input area, equal

to dimensions ofCCD active areaInput area, larger than

dimensions of CCD active area(depends on magnification of taper)

Figure 7: Image area is determined by magnification of the taper.

The majority of light coupled into a high magnification taper will be lost and never reach the CCD, thefraction reaching the detector can be approximated by 1/m2, where m is the magnification of the taper,e.g. a 2:1 taper (m=2) will only allow ~_ of the light to reach the CCD compared to a 1:1 taper.

Indirect detection methods also offer the user some additional advantages over direct detection,including:

I. Higher Dynamic RangeThe dynamic range is increased, particularly at higher energies, due to the productions of fewerphotoelectrons in the device per detected x-ray photon.


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II. Wide Photon Energy CoverageThe phosphor coating can be tailored to suit virtually any application and can enable detection of

x-ray photons with energies from ~5keV up to (and beyond) the hard x-ray region.

III. Protection of CCDThis is by far the most important advantage of indirect detection over direct detection. The useof a phosphor coated fibre optic prevents any x-ray photons reaching the CCD and thereforecompletely protects the sensor from x-ray damage, thereby prolonging the lifetime of the device.

However there are also some disadvantages incurred when using indirect detection methods, such as:

I. Loss of Spatial ResolutionThe spatial resolution of a phosphor is typically quoted as between 10-12 line pairs per mm,

therefore a single detected photon could produce a spot of light with a diameter of approximately100m in the phosphor. However, if an application requires only a thin phosphor layer thisresolution can improve to ~30m.

II. Loss of Energy ResolutionAs we previously stated, the absorption-emission process within the phosphor is noisy both interms of spatial distribution and intensity, therefore there is a greater uncertainty in the value ofphoton energy required to generate a given number of counts in the CCD.

III. Reduced SensitivityThis arises as a combination of both fewer emitted photons per detected x-ray and lower spatialresolution: the smaller number of visible photons produced by the x-ray event may be spreadover a greater area (i.e. more pixels) of the detector, resulting in the signal level decreasing intothe noise level.

2.3 X-Ray Filters and Light Barriers

These are usually required to screen the detector from ambient (predominantly visible) light. The twomost commonly used materials are Beryllium and Aluminium.


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0%

20%

40%

60%

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X-Ray Photon Energy (keV)

PercentageTransmissio

n 200nm Al

0.25mm Be

0.5mm Be

1mm Be

Figure 8 : X-Ray transmission of Aluminium and Beryllium Filters of different thickness.

Beryllium is particularly transparent in the x-ray region and still has relatively high x-ray transmission at athickness of 1mm, see Figure 8. Aluminium barriers are generally much thinner, usually a few hundrednanometers (transmission also shown in Figure 8) and must be supported on some form of backing.Aluminium coatings are often applied to the surface of phosphor coatings as they not only prevent visiblelight leaking through the coating to the detector but they also reflect more of the photons emitted by thephosphor towards the sensor. This increases the number of counts recorded in the device per detectedphoton, but at the expense of spatial resolution.

3. ANDOR X-Ray ProductsAs mentioned previously, some x-ray energies cannot pass through air, therefore many experimentsinvolve vacuum technology. In response to this requirement ANDOR TECHNOLOGY offers a range of x-ray detection products specifically designed for in-vacuum experiments. These products are designatedby either a DO or DX prefix.

3.1 DO Systems

These open-front systems are designed so that they can be coupled to a vacuum chamber via thefaceplate. There is no window isolating the sensor from the outside world and the internal design is suchthat the CCD enclosure will be vacuum tight once a seal between the faceplate and vacuum chamber hasbeen made, as illustrated in Figure 9.


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Figure 10: Picture of a DO system with a ConFlat flange for mounting directly onto a vacuum chamber.

DO systems can, in principle, be used for either direct or indirect detection (depending on the sensor

format) and incorporate any individual chip.

3.2 DX Systems

In some instances it may not be practical to simply bolt the detector to a port on the vacuum chamber, forexample it may be necessary to have the detector closer to the sample under investigation or orienteddifferently to the positions available on the vacuum chamber. In such cases it is clear that the customer isbeing constrained by the shape and dimensions of the vacuum chamber used. In order to remove thisconstraint we offer in vacuum systems, which as the name suggests, have heads which are completelyvacuum compatible, i.e. the entire CCD, including casing and electronics, can be mounted inside the

vacuum chamber and then pumped down. Once again, as the customer provides the vacuum, suitableprecautions must be taken in order to prevent contamination / condensation damage to the CCD. Thesame standard faceplate, which acts as a filter holder, is used on both DX and DO systems.


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Figure 11: Picture of a ANDOR DX system(Copper block protruding through sides)

Figure 12: Feedthru and detector cable

DX systems, shown in Fig. 11, have a few noticeable differences when compared to other Andor CCDs:

I. There is no heat sink or fan since, by definition, inside a vacuum there is no air to be blown past theheat sink! Also water cooling is not offered as standard on DX systems, because typically most vacuumusers are reluctant to have water circulating within their vacuum chamber. Therefore the head must bepassively cooled by means of a copper block, which may be coupled to other objects within the vacuumchamber to aid this process. A PS150 should not be used with this system.

II. The cable from the head goes to a vacuum compatible feed-thru connector which is inserted through aflange in the vacuum chamber. This vacuum tight electrical feed through enables the head inside thevacuum chamber to be connected to the controller card in the customers PC. Therefore each DXsystem comes with two cables a 1.5m cable which connects the head to the feed-thru and a standard3m cable which connects the feed-thru to the PCI card. The addition of the Feed-thru part numberorders both connector and feed through connector assembly.

DX systems may also be used for either direct or indirect detection (depending on the sensor format) andincorporate any individual chip.

3.3 DY Systems

For some applications, which dont involve vacuum technology, a stand alone DY head may be used.This type of head incorporates both a Beryllium window, Figure 12(a) and a phosphor coated, fibre opticsensor, Figure 12(b). Due to the mechanical properties of Beryllium it is not technically possible to build aDY system containing a large area sensor nor is it possible to vacuum seal a DY system, therefore theseheads are backfilled with Argon. The systems should not be used with a PS150 and achieve a minimumtemperature of approximately 35C. A schematic illustration of the main components of a DY system isshown in Figure 13.


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Figure 13: (a) DY faceplate with Be window (b) Phosphor coated, fibre optic sensor used in DY systems

X-RAYP

HOTONS

Faceplate

BerylliumWindow

Backfilled

CCD enclosure

CCD

Fibre Optic

on CCD

Phosphor coatingon fibre optic

Visible photonsare reflected by

Beryllium window

Cooler

Figure 14: Illustration of the main features of DY systems.

The phosphor coating on the fibre optic sensor determines both the spatial resolution and the energyrange over which the device is useable. At present these DY systems are offered for a limited range ofsensors only.

3.4 Phosphor Coated, Taper Devices

For large area coverage it is necessary to use a CCD in conjunction with a phosphor coated taper. Aschematic illustration of the proposed design for such a system is shown in Figure 14. The taper housingcouples onto the front of a fibre optic camera and is designed to be removable so that the system can beused as a fibre optic camera or the taper may be polished and recoated as required.

Beryllium

window in

face late

Phosphor coating on fibre optic sensor

(a) (b)


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FIBRE OPTIC CAMERA

PHOSPHOR COATINGON TAPER

SPRING TO PREVENT

DECOUPLING OFTAPER AND CAMERA

FIBRE OPTIC TAPER

TAPER HOUSING

COUPLING GELBETWEEN TAPER ANDFIBRE OPTIC CAMERA

FIBRE OPTICPROTRUDING

FROM CAMERA

Figure 15: Illustration of a CCD system with a large area phosphor coated fibre optic taper.

The spring loading within the taper housing is designed to prevent decoupling of the taper from the fibreoptic camera. The sensitivity of the system will depend on a number of factors including the magnificationof the taper, the coupling between the different components and the properties of the phosphor itself.

Figure 16: Image of a CCD system with a large area phosphor coated fibre optic taper.


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