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Nuclear Instruments and Methods in Physics Research A 600 (2009) 434–439

Contents lists available at ScienceDirect

Nuclear Instruments and Methods inPhysics Research A

0168-90

doi:10.1

�Corr

E-m

(A. Mich

journal homepage: www.elsevier.com/locate/nima

Characterisation of Gd2O2S:Pr phosphor screens for water windowX-ray detection

George Fern a,�, Terry Ireland a, Jack Silver a, Robert Withnall a, Alan Michette b,�, Chris McFaul b,Slawka Pfauntsch b

a Wolfson Centre for Materials Processing, Brunel University, Uxbridge, London, UB8 3PH, UKb King’s College London, Department of Physics, Strand, London, WC2R 2LS, UK

a r t i c l e i n f o

Article history:

Received 24 June 2008

Received in revised form

24 November 2008

Accepted 30 November 2008Available online 6 December 2008

Keywords:

Detector

X-ray luminescence

Laser generated plasma

Gd2O2S:Pr

Phosphor

02/$ - see front matter & 2008 Elsevier B.V. A

016/j.nima.2008.11.116

esponding author.

ail addresses: george.fern@brunel.ac.uk (G. Fe

ette).

a b s t r a c t

A soft X-ray scintillator phosphor (Gd2O2S:Pr) has been synthesised which is composed of discrete

crystals of 120�150 nm average size. This material has been deposited onto a transparent substrate so

that it can be used to detect X-rays generated by a laser generated plasma. A photodiode has then been

used to quantify the intensity of emission by the phosphor slide. This nano-crystalline Gd2O2S:Pr

phosphor shows greatly enhanced signal over a commercially available phosphor with larger particle

size.

& 2008 Elsevier B.V. All rights reserved.

1. Introduction

X-ray microscopes hold the potential to probe biologicalspecimens in their natural state and, due to the shorterwavelength, can provide much enhanced resolutions over opticalmicroscopy. For X-ray microscopes to become readily availableaway from synchrotron facilities it is necessary to generate thesoft X-rays via a more cost effective means. The 2.3–4.4 nmwavelength range of the X-rays used in these systems lies in theregion termed as the water window (photon energies of0.28–0.53 keV), where water is much less absorbing than carbon.These wavelengths can be generated using pulsed laser generatedplasmas (LGPs). The detectors used to measure X-ray intensity aretherefore operating in electromagnetically noisy environments.An answer to the detection problem that this presents is to use anX-ray phosphor coupled to a photodiode as the detector, whichcan be activated after a time delay so that it follows the trailingedge of the laser pulse and the noise that it introduces. This paperreports on the characteristics of a detector that uses a powderphosphor engineered for measuring these X-rays in conjunctionwith a photodiode. Such detectors will almost certainly also findapplications at other sources, e.g., synchrotrons where, in modern

ll rights reserved.

rn), alan.michette@kcl.ac.uk

sources, direct detection via photon counting can be problematicdue to the high brilliances.

Research into the characterisation and use of LGP X-ray sourcesis still a topic of interest [1–3]. Such sources use moderatelyintense pulsed laser beams, typically with pulse lengths in thepicosecond to nanosecond range. In addition to generating X-rayswhen focused to small spots on solid, liquid or gas targets, such abeam is a source of intense electromagnetic noise, essentiallytemporally coincident with the X-ray pulse. In applications suchas microscopy, where the use of focusing optics leads to very lowX-ray optical apertures and efficiencies [4], typically around 1 in1010 of the emitted X-rays reaches the detector. The electro-magnetic noise can then be the limiting factor on the systemperformance, especially as it is correlated with the X-ray signaland so cannot be reduced to a manageable level by integrating oraveraging signals. Applications of other short wavelength pulsedsources, some using more intense beams which can cause morenoise, such as XUV lasers [5] and high-harmonic generation [6],may well suffer from similar limitations. To overcome thisproblem one approach would be to use very careful shielding.However, this can be expensive and it is difficult to completelyshield out the noise to the levels required. Another possibility is touse two detectors, one of which is shielded from the X-rays, andsubtract the signals to remove the noise. The difficulty with thisapproach would be drift in the electronics of the two detectors,which could, for example, behave differently under temperaturevariation. An alternative is to avoid direct X-ray detection by

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Diode

Light Cap

Washer

TitaniumFoil

Phosphorcoated slide

Sleeve

Fig. 1. Schematic representation of detector arrangement.

Table 1Commercial phosphor performance under X-ray excitation.

Type Gd2O2S:Pr

Mean grain size 3.5mm

Percentage absorption at 3.37 nm 499.99%

Conversion efficiency �15%

Peak emission wavelength 510 nm

Decay time �7ms

G. Fern et al. / Nuclear Instruments and Methods in Physics Research A 600 (2009) 434–439 435

incorporating a component with a delay so that the detector needonly become sensitive after the laser pulse and hence when thenoise has diminished. In addition, the remaining stochastic noisecan be minimised by integration of the detector output. One wayof achieving this is to use a phosphor coating which absorbsincoming X-rays and subsequently relaxes, emitting visible, ornear-visible electromagnetic radiation which can be detectedusing a photodiode. For a sufficiently long phosphor decay time agating circuit can be employed to trigger the detector a short timeafter the laser pulse, combined with a circuit to integrate thephosphor output over a defined time.

Earlier tests using a prototype detector have shown a dramaticreduction in the influence of noise [7]. A non-optimised detectorwas used in terms of Gd2O2S:Pr phosphor thickness and grain size,efficiency and emission wavelength matching with the peakresponse of the photodiode.

Many phosphors used for X-ray imaging are available, but lightextraction and matching the emission spectrum of the phosphorto the wavelength sensitivity of the photodiode is important.Gd2O2S:Pr was chosen for further study, as it has a reasonablylong decay time, especially when used without additionaldopants. This phosphor also gives 1.8 times greater light outputthan CaWO4 (the latter was the commonly used phosphor prior tothe introduction of Gd2O2S:Pr). It should be noted that the X-raysfor the application of interest in this work have a long wavelengthand, thus, will interact with the phosphor differently to when it isused in a common (for this phosphor) application such as X-rayComputed Tomography (XCT).

The scientific literature has mostly been aimed at the detectionof the higher energies associated with techniques such as XCT,positron emission tomography and gamma ray detectors, nuclearand high energy physics detectors [8]. Commercial phosphors arein general not suitable in this application as the particle sizes tendto be too large for efficient low energy X-ray excitation. Control ofthe emitted light is paramount in optimising the capabilities forX-ray detection and increasing the spatial resolution of X-raydetectors; the latter will be crucial for beam position monitoringat the sub-micrometre level in the X-ray microscope and otherapplications.

2. Experimental

In characterisation experiments using the prototype detector, aphosphor-coated glass cover slip was placed in front of a lightsensitive photodiode with a titanium foil to act as a water-windowfilter (Fig. 1). The foil transmitted water-window X-rays butabsorbed lower and higher energies. Two 1mm thick titaniumdiscs were used to prevent visible light/UV transmission throughpinholes; this gave a transmission of about 1% at the energy of367 eV dominant in the LGP spectrum of the Mylar targets used inthe detector characterisation.

A (simple) photodiode was used in photovoltaic mode and thesetup was placed inside the LGP target chamber, relatively close tothe plasma. When used in X-ray microscopy, the detector has tomeasure signals �3 orders of magnitude lower, requiring higherefficiency phosphors with peak spectral output matched to thephotodiode peak response, as well as more efficient photodiodes.

For use in the X-ray microscope the Ti foil will be replaced by areflective cap to reflect X-rays emitted in the direction away fromthe photodiode; a small hole will allow the focused X-ray beam topropagate through. A commercially available phosphor has beenused previously and has the properties given in Table 1; theconversion efficiency quoted here is for hard X-rays, for soft X-raysit is likely to be considerably less. The decay time of the phosphoris much longer than the duration of X-ray emission, which to a

first approximation follows the laser pulse. Thus, the detector wasused in conjunction with a custom-built gated integratingamplifier, which was turned on �100 ns following the trailingedges of the laser and associated noise pulses, entirely eliminatingthese sources of noise, while statistical fluctuations in the signalwere reduced via the integrator.

Gd2O2S:Pr phosphor was prepared by an analogous method tothe previously published procedure for the synthesis of (Y,Tb)2O2S[9]. The reactions proceed according to the following chemicalequations:

H2NCOHN2 ! NHþ4 þ OCN�

OCN� þ 2Hþ þ 2H2O! H2CO3 þ NHþ4

½ðY;PrÞOHðH2OÞn�2þ þH2CO3

! ðY;PrÞOHCO3 � H2Oþ ðn� 1ÞH2O

2ðY;PrÞOHCO3 �H2Oþ 3Na2CO3 þ 4Sþ O2

! ðY;PrÞ2O2Sþ 4CO2 þ SO2 þ 3Na2Oþ 2H2OÞ þH2S

Optimisation of the Pr activator concentration (from 1.0 to10.0 mol%) by cathodoluminescent (CL) measurements indicatedthat a 1.0 mol% Pr concentration gave the highest luminancevalues, for example, 1100 cd/m2 with 5000 V, 1.46mA e-beamemission current and 30000 cd/m2 with 5000 V and 8.6mA e-beam emission current. The body colour of the synthesisedGd2O2S:Pr phosphor powder was slightly off-white. X-ray powderdiffraction (XRPD) patterns obtained from the Gd2O2S:Pr phos-phor were shown to have good agreement with the publishedGd2O2S structure [10,11]. The gadolinium atom is bonded to fouroxygen and three sulphur atoms and the crystal parameters are,a=3.855 A, c=6.670 A with a Rwp of 3.39. Under 254 nm radiation astrong green luminescence was observed and a weak greenluminescence when excited with 366 nm radiation.

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Table 2X-ray luminescence intensity ratio of commercial large particle phosphor to small

particle phosphor screens measured with a photodiode with optimum response at

550 nm.

Average phosphor deposit thickness (mm) Ratio ref/RAL Sample/ref ratio

Commercial non-optimised screen 1.736

3.3 3.401 1.96

20 0.436 0.25

Ref refers to the signal from the photodiode in arbitrary units. RAL refers to an

X-ray diode characterized at the Rutherford Appleton Laboratory [12].

G. Fern et al. / Nuclear Instruments and Methods in Physics Research A 600 (2009) 434–439436

For electrophoretic deposition (EPD) of the phosphor ontoscreen substrates, the samples (�0.5–1.0 g) were suspended in anelectrolyte solution of magnesium nitrate in isopropanol. Toimprove dispersion the suspensions were agitated in an ultrasonicbath for a minimum of ten minutes. An electric field of 350 V wasapplied for between 10–480 s with the screen substrate beingmade of 80O/m2 indium tin oxide (ITO) coated poly(ethyleneterephthalate) (PET). The ITO coated PET was placed into thephosphor suspension and coated for various lengths of time(hereon termed the ‘phosphor slide’). After coating, the samplewas removed and dried in an oven at 80 1C.

The LGP was formed by focusing 70 mJ, 800 ps laser pulses at532 nm onto a Mylar (C10H8O4)n tape. Measurements were takenat focus, with a 10 Torr N2 atmosphere to provide spectral filteringso that only emission from carbon ions, specifically at 3.37 nm,reached the detector. Readings were compared to a referenceX-ray diode and relative efficiencies found (shown in Table 2,brightest and least bright small particle screens only shown). Thephosphor slide was cut into small sections and placed in thearrangement shown schematically in Fig. 1 for testing using theLGP source. The X-ray luminescence emission was detected with aphotodiode with maximum sensitivity at 550 nm. The screenswith various deposition thicknesses were compared to those ofthe phosphor screen prepared using large particle commerciallyavailable Gd2O2S:Pr phosphor. A JEOL 2000FX transmissionelectron microscope (TEM) fitted with a Gatan Orious SC1000 11MegaPixel CCD and operated at 200 kV was used to image theprimary particles. Crystal/particle sizes were determined bymeasuring a total of 45 particles from two images. Fieldemission scanning electron microscopy (FESEM) was carried outusing a Zeiss Supra 35VP. Particle size analysis was carriedout using a Horiba LS920 particle size analyser with an ultra-sonic disperser in Dispex solution. Photoluminescence (PL)measurements were carried out using a Bentham Instrumentsdual monochromator system. It has not yet been possible todevelop a method to separate these particles in an aqueoussuspension to allow measurement of the particle size distribution.Due to aggregation such methods indicate a mean particle size of3.2mm which is much greater than revealed by electronmicroscopy. XRPD measurements were made using a Bruker D8powder diffractometer. The phase information including crystalsize was fitted using Bruker AXS TOPAS version 3.

3. Results and discussion

A prototype detector with a thick, large grain, commercialphosphor screen had previously been employed in the KCL LGPsource. This was functional but the results manifested the need forimprovement. For 367 eV X-rays the 1/e attenuation length ofGd2O2S:Pr, with a density of 7340 kg m�3, is �0.14mm usingtabulated absorption coefficients [13]. Even if the phosphor isdeposited at non-bulk density, the attenuation length will still be

a small fraction of a micrometre. A smaller mean grain size of0.5mm or less, with (say) 95% of grains smaller than 1mm, wouldgive essentially the same X-ray absorption but with better lighttransmission. An even smaller grain size would allow moreuniform coatings by using several layers. Early prototype coatingswere made with no control of thickness, uniformity or reprodu-cibility between different coatings. This has therefore led us tosynthesise a scintillator phosphor with an average particle size of120 nm width by 150 nm breadth with a standard deviation of 47and 55 nm, maximum values of 250 and 297 nm and minimumvalues of 63 and 78 nm respectively (i.e. More than 20 timessmaller than the commercially available material). Moreover,XRPD data indicates a crystal size of 116 nm for this materialwhich is in good agreement with the values measured by TEMobservation.

Control of the crystal size has resulted from earlier investiga-tions into the synthesis of Y2O3:Eu monosized submicronspherical phosphor particles that have diameters ranging from100 to 500 nm for field emission device (FED) display applications.These particles resulted from annealing of spherical europium-doped hydroxycarbonate particles at approximately 1000 1C.Adjusting the experimental conditions during the precipitationof these particles resulted in a range of particle sizes. From thesestudies it was reasoned that it may be possible to use theseprecursor particles to produce submicron sized rare earth-dopedoxysulfide particles for FED applications. In the case of intention-ally tailoring the crystal size of the Gd2O2S:Pr particles, europium-doped hydroxycarbonate with a diameter of approximately400 nm was chosen. Sulfurisation of these particles was achievedby mixing with sulfur followed by annealing at the desiredtemperature yielding Gd2O2S:Pr with a rounded morphology anda relatively small particle size distribution.

Optimisation of the developed phosphor has been carried outusing CL light intensity measurements over a range of excitationenergies and e-beam emission currents. Comparison between thedeveloped and commercial phosphors at low excitation energies(o5000 V) shows the luminous efficacy of the developedGd2O2S:Pr (1.0 and 2.0 mol%) phosphor to be superior tocommercial samples. It must be stated that the commercialsamples are supplied as X-ray phosphors that will have beenoptimised with regards to Pr activator concentration for thisapplication. Therefore a precise quantitative comparison is notpossible. From our experiments on the optimisation of submicronphosphor particles for low-voltage applications it was found thatas the particle size decreases a lower activator concentration wasrequired. This is due in some part to the higher effective packingfraction of the smaller size and rounded morphology of thedeveloped phosphor forming the screen.

TEM has shown that this developed material is composed ofdiscrete particles that have a tendency to agglomerate but they donot appear to be sintered together (Fig. 2). Particle size wasinvestigated using FESEM after ultrasonic dispersion in alcohol(since TEM studies use small sample populations). The FESEM ofthe discrete powder shows that this material is indeed composedof particles substantially smaller than 1mm with no large discreteparticles but does provide evidence that these sub-micrometreparticles agglomerate on drying (Fig. 3).

The phosphor used in this work has been optimised for CL at5 kV by adjustment of the praseodymium concentration in thephosphor. This method for the production of a bright phosphorwas carried out because an X-ray source was not available formeasuring the luminous efficiency of the phosphor for eachphosphor synthesis. It was decided that this was reasonable sinceX-ray phosphors and cathode ray phosphors both excite via thephosphor lattice rather than the activator ion directly, which canoccur when using UV or visible light excitation [14].

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PL and CL emission spectra, normalised at 512 nm for the smallparticle phosphor, are shown in Fig. 4. The emission bands alter inintensity ratio with screen thickness and this effect is shown inFig. 4 for just the 3.3mm thick phosphor slides for clarity. The PLand CL spectra (Fig. 4) gave evidence that the most efficientphosphor slide (prepared at 3.3mm average deposit thickness)shows a strong preference for the 512 nm peak over the 671 nmpeak, which is most likely to be due to small excitation bandsbeing present in the material between 450–520 nm. Indeed all thelonger wavelength emission bands from 564 to 800 nm (the longwavelength limit of these measurements) are suppressed in theemission spectra of this phosphor when it is coated onto thephosphor slides.

The EPD technique has been shown as an effective method ofdepositing thin layers onto electrically conductive substrates[15–17]. This is a fast method for the preparation of ceramic thickfilms and has the benefits of being relatively simple, low cost andworking at room temperature. Using EPD to coat the phosphoronto the substrate gives high uniformity and packing density asshown in Fig. 5a (which shows slides of the new phosphor). Thephosphor coated slide that showed the optimum X-rayluminescence in these experiments has an average thickness of3.3mm and manifests a milky appearance (Fig. 5a and b). Thesmall particle phosphor slides comprise of �2mm sizedagglomerations of particles inter-dispersed with much finermaterial that was too small for the FESEM to resolve. The3.3mm thick slide has a very smooth appearance and theFESEMs show that the deposit is uniform; without pinholesthat, if present, could allow the photodiode to become damagedby incident X-rays. Fig. 5b shows the agglomeration of the small

Fig. 2. TEM of small particle in-house synthesised phosphor powder.

Fig. 3. FESEM of (a) Commercial 3.5mm phosphor and (b) Small p

particle phosphor on the 3.3mm average thickness phosphor slidewhich manifested the brightest X-ray luminance. It is evident thatthin layers can be achieved by control of the coating time whensmall particles are used. Fig. 5c shows the same material with anaverage deposit thickness of 20mm. These thicker and rougherdeposits will reabsorb more of the emitted light which willprevent it from reaching the photodiode (due to the thickness ofthe coating on the substrate). Thicker slides (up to 20mm) showedno visible light transparency over these wavelengths and werehighly susceptible to charging in the FESEM (which reduced theimage quality of these micrographs). Fig. 5d shows the EPDcoating possible with commercially available phosphor with largeparticle size. This material is much more challenging to depositwhen using EPD since the particles settle before they can bedeposited onto the substrate; a settling procedure must thereforebe used when making phosphor slides of this material. This isdifferent to the small particle phosphor synthesised for this workwhich stays in suspension for around 30 min before coating timesare affected and which allows a much thinner and morecontrolled phosphor coating to be achieved.

Screen noise properties of powder phosphor screens used invarious X-ray imaging detector applications have been studied interms of a noise transfer function (NTF), where the inherentphysical properties of the materials have a significant effect on theimage detector transfer characteristics [18]. In both types of

article phosphor showing agglomerations of small particles.

Fig. 4. PL (excited at 254 nm) spectra of the new small particle phosphor and the

brightest screen prepared with this phosphor and the CL spectrum of this slide.

Spectra normalised at 512 nm to show the reduction in emission intensity at longer

wavelengths on the phosphor coated slides.

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Fig. 5. FESEM images of small particle phosphor on ITO PET: (a) and (b) 3.3mm average phosphor thickness, (c) 20.0mm average phosphor thickness and (d) 11.5mm thick

deposit of the commercial large particle phosphor showing the poorly coated layer.

G. Fern et al. / Nuclear Instruments and Methods in Physics Research A 600 (2009) 434–439438

radiographic photodetector systems, traditional screen film ordigital, such as phosphors coupled with CCD arrays, total imagenoise has a detrimental effect on the quality of the resultingimage. The total image noise is composed of screen noise from thephosphor which is the main component and to a lesser degree byphotodetector noise. Screen noise has contributions from bothquantum and structure noise. Quantum noise has been shown tobe a consequence of the statistical nature of the spatialfluctuations of the X-ray quanta that have been absorbed anddominates in a low exposure environment. Significant structurenoise is a consequence of an inhomogeneous phosphor coatingcreating fluctuations of the absorbed X-ray quanta in highexposure conditions. The thickness and uniformity of thephosphor screen has been found to be critical for a low level ofstructure noise in high exposure conditions. A thicker phosphorlayer and a high degree of close packing of the phosphor particlesdecreases structure noise. Thus the phosphor particle morphologyand size will influence the effective filling fraction, particles thatare similar in size and morphology will result in a high effectivefilling fraction. There is also a reduction in structure noise with adecrease in particle size, due to a higher degree of particle packingand lower scattering of radiation [18]. It can therefore be putforward from the above considerations that these smallerGd2O2S:Pr particles will allow for a closer packed screen and alsothinner phosphor layer to yield a higher quality image under thehigh exposure conditions that were used in this work. FromFESEM studies shown in Fig. 5 the low magnification study of thesmaller Gd2O2S:Pr particles shows a much smoother surface andhigher effective packing fraction than that of the much largercommercial particles.

The PL and CL (see Fig. 4) emission spectra show evidence forslight Gd2O3:Pr contamination in the small particle phosphor. Thelevel of this contamination appears to remain constant in thephosphor slides and the uncoated phosphor powder. It is expectedthat these two materials would deposit at different rates usingEPD and therefore this small amount of contamination must be onthe surface of the Gd2O2S:Pr, having been introduced by sulphur

deficiency during the calcination process. If necessary it should bepossible to reduce the presence of this oxide contamination byimproving the calcination procedure.

Further future improvements in screens using Gd2O2S:Pr cannow be considered. Gd2O2S:Pr is generally employed in thedetection of higher energy X-rays and is not optimised for softX-rays; higher conversion efficiency could be obtained with peakemission wavelength being better optimised to the photodiodepeak efficiency or the integrated emission spectrum optimised tothe photodiode efficiency profile. Addition of dispersing agents tothe suspension might be able to reduce the tendency of theparticles to aggregate which would allow the use of even thinnerphosphor slides which would yield higher detection efficiency. Aphosphor with shorter decay time would give a higher photodiodecurrent, due to the shorter collection time, and a better signal tonoise ratio, the noise arising from a combination of the detectedphotocurrent and the dark current. This is feasible with thecurrently employed phosphor and will be investigated in furtherstudies by co-doping with cerium [19] and halogen doping [20].However the decay time should not be too short otherwise asignificant fraction of the light emission would take place beforethe amplifier is gated. Additionally, a shorter collection timewould result in more amplifier induced noise. These considera-tions suggest that a phosphor decay time in the range �10–100mswill be optimum.

4. Conclusions

Sub-micrometer sized phosphor powder shows higher detec-tion efficiency of soft X-rays when compared to commerciallyavailable phosphor with a mean particle size of 3.5mm. EPD hasbeen used to prepare phosphor coated slides of various thick-nesses. The 3.3mm thick phosphor coated slide shows the bestX-ray detection when compared to the commercial large particlephosphor slide prepared via a settling method. The spatialresolution achievable with a phosphor-based detector is directly

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related to the phosphor grain size, and so the smaller grain sizes ofthe custom produced phosphors will be superior to commercialphosphors in this respect.

Acknowledgements

The authors express their gratitude to J. Whitmarsh of AgmetESL Europe for thickness measurements. G. Fern thanks BrunelUniversity for a BRIEF award number 811. The King’s group iscurrently funded as part of the UK Smart X-ray Optics consortiumthrough a UKRC Basic Technology grant D04880X. Their work isalso supported by the European Science Foundation COST MP0601Action ‘‘Short Wavelength Laboratory Sources’’.

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