This paper presents bent crystals obtained by low energy plasma enhanced chemical vapour deposition technique, which could be further improved for medical applications to focus gamma rays. Medical imaging plays an important role in the field of nuclear medicine and the use of bent crystals to focus energetic gamma rays used for imaging of radioactivity in the human body is our interest. An innovative technique to obtain curved crystals is proposed in this work. Surface profilometry measurements were done to find the curvature of curved crystals

Arivuoli, D.

Guidi, V.

Joice Sophia, P.

Neri, I.

English

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DOI 10.1393/ncc/i2011-10956-0Colloquia: Channeling 2010IL NUOVO CIMENTO Vol. 34 C, N. 4 Luglio-Agosto 2011Bent crystals obtained by low energy plasma enhanced chemicalvapour deposition for medical applicationsP. Joice Sophia(1)(2)(∗), I. Neri(1), V. Guidi(1) and D. Arivuoli(2)(1) Dipartimento di Fisica, Universita` di Ferrara - Ferrara, Italy(2) Crystal Growth Centre, Anna University Chennai, India(ricevuto il 22 Dicembre 2010; pubblicato online il 12 Settembre 2011)Summary. — This paper presents bent crystals obtained by low energy plasmaenhanced chemical vapour deposition technique, which could be further improvedfor medical applications to focus gamma rays. Medical imaging plays an importantrole in the field of nuclear medicine and the use of bent crystals to focus energeticgamma rays used for imaging of radioactivity in the human body is our interest.An innovative technique to obtain curved crystals is proposed in this work. Surfaceprofilometry measurements were done to find the curvature of curved crystals.PACS 87.57.U- – Nuclear medicine imaging.PACS 61.05.cp – X-ray diffraction.1. – IntroductionRadioactive materials used as a diagnostic tool can identify the status of a disease andminimize the need for surgery by reducing the risks from postoperative infection. Nuclearimaging is performed by injecting a radiopharmaceutical into the patient and measuringthe intensity distribution of gamma radiation emitted from the patient’s body [1].In order to have a good reflection efficiency and broad energy band-pass, special crys-tals like mosaic crystals and bent crystals with the controlled lattice deformation areneeded. Curved crystals are promising because their curved diffracting planes allow fo-cusing and concentrating gamma rays with higher diffraction efficiency in comparisonwith mosaic crystals leading to increasing energy bandwidth due to their continuum an-gular spread [2,3]. Bent crystals are nowadays largely used to excite coherent interactionsof high-energy particles in beam collimation and also in beam extraction [4, 5].The development of bent crystals in order to focus gamma rays will find significantapplication in the field of medical imaging. Short-lived radioisotopes are preferred foruse in these tracers to minimize the radiation dose to the patient. In most cases, these(∗) E-mail: joicesophia@gmail.comc© Societa` Italiana di Fisica 461462 P. JOICE SOPHIA, I. NERI, V. GUIDI and D. ARIVUOLIradioisotopes decay to stable elements within minutes, hours, or days, allowing patientsto be released from the hospital in a relatively short time. The radioisotope used in about80 percent of nuclear diagnostic procedures is Tc-99m. The penetrating properties of itsgamma rays and its short half-life (6-hours) help in reducing risk to the patient frommore prolonged radiation exposure. Short-lived radio-nuclides such as technetium-99m,gallium-67, and thallium-201 are often used to diagnose the functioning of the heart,brain, lung, kidney or liver.Cancerous cells have high growth rate and multiply very rapidly. The radioactiveisotope injected into the body of the patient normally migrates to high growth ratelocations and will incorporate in this new growth and if the injected substance is a short-lived radioactive isotope, the tumour location can be identified from the region of highradioactivity.2. – General backgroundThe diagnostics is based on Bragg diffraction of gamma rays from crystal planes.Diffracting crystals (Bent crystals) are used for focusing the gamma radiation and di-recting the radiation to a detector which could be used to analyze the data with respectto the location of the radiation source.2.1. Crystal diffraction. – Diffraction inside the crystal can either occur near thesurface, i.e. Bragg geometry or in the volume, where the beam propagates through theentire crystal, namely Laue geometry. In the Laue mode, the incident beam of radiationenters from one surface of the diffracting bent crystal and interacts with a particulararray of parallel atomic layers. After that, the radiation splits into transmitted anddiffracted beam produced by a coherent superposition of scattering by the atoms in theparallel atomic layers. In the Bragg mode, the diffracted beam (which consists of almostall the incident energy) is produced by the multiple scattering with the atoms in theparticular array of parallel atomic layers and the diffracted beam exits from the samesurface where the radiation entered. For instance, bent crystals in Bragg geometry arecurrently used as Goebel mirrors to transform a divergent X-ray beam into an intenseparallel beam.It is well known demonstrated that gamma rays interact coherently in a crystal latticeif and only if the angles of the incident photons are very small and satisfy the Braggrelation and the crystal diffraction of gamma rays is given bysin θ =(nΛ2dhkl),(1)where Λ is the wavelength of the gamma ray in A˚, n is the order of diffraction, dhkl isthe d spacing of the planes hkl and θ is the crystal diffraction angle.Λ =(hcE),(2)where E is the gamma-ray energy (in keV), h is Planck’s constant and c is the speed oflight.The approximate focal length (in metres) of a Laue-based lens is given asF = 1.65 · 10−2 · E.(3)BENT CRYSTALS OBTAINED BY LOW ENERGY PLASMA ETC. 463Smither [6] calculated the distance from the source to lens (LS) and the distance fromthe lens to detector (LD) and also the values of LS and LD for different gamma-raysources were tabulated using the formula1F=1LS+1LD,(4)In his experiment, for a 100 keV gamma ray diffracted by the (111) planes of germa-nium, θ is found to be 1.0895 degrees or 0.01902 rad and hence the surface of a Braggdiffraction crystal has to be 52.6 times longer than the height of a Laue crystal used indiffracting the same beam. As reported by Smither, for energetic gamma rays the diffrac-tion angle (Bragg angle) is quite small and Laue geometry makes much more efficientuse of the crystalline material.Bending of the crystal eliminates the spreading of the diffracted beam in the planeperpendicular to the diffraction beam. Roa [1] used copper crystals due to their highreflectivity for gamma-ray energies of 100–200 keV. Halloin [7] explained that the use ofperfect monocrystals for gamma-ray lens is not applicable due to the narrow diffractingangular range and also the thickness of a perfect crystal is greater than the extinctionlength. This latter leads to the formation of the constructive and destructive interferenceslimiting the diffracting power of crystal.2.2. Laue lens for medical application. – A Laue lens is generally made of crystalswhich are arranged in concentric rings and oriented in such a way to focus X-raysor gamma rays through Bragg diffraction in Laue geometry. Laue lens is an emerg-ing technology to realize the space-borne telescopes which are more sensitive within100 keV–1.5MeV and the concentration of gamma rays has been studied [8]. Develop-ment of Laue lens in focusing gamma rays in medical application finds a new way inthe field of nuclear imaging. Smither [9] in his new experimental approach used crystalswith curved diffracting planes including Si crystals and Si-rich SiGe gradient crystals todiffract high energy X-ray from advanced photon source and gamma rays from radioac-tive source and it was found that with the help of this approach one can increase thediffraction efficiency and bandwidth by a factor of 5 compared to mosaic crystals.2.3. Experimental layout for medical imaging . – Diagnostic techniques in nuclearmedicine use radioactive tracers which emit gamma rays from the inside of the body.These tracers are generally short-lived isotopes linked to chemical compounds which per-mit specific physiological processes to be scrutinized. They can be imparted to a patientby injection, inhalation or orally. The first type is where single photons are detected bya gamma camera which can view organs from many different angles. The camera buildsup an image from the points from which radiation is emitted; this image is enhanced bya computer and displayed on a monitor for indications of abnormal conditions.Bent crystals are expected to have both high diffraction efficiency and relatively broadenergy bandwidth and would be capable of detecting the radiopharmaceuticals that emit50–200 keV gamma rays which are generally used in diagnostic molecular imaging andin nuclear medicine. This system will focus the incoming gamma rays into a small focalpoint on a detector’s sensitive area.Figure 1 shows the experimental arrangement of the treatment method. A commonmethod to locate cancerous tissue in the human body is to inject a biological compoundthat is readily incorporated in cancer cells and can carry a radioactive nucleus with464 P. JOICE SOPHIA, I. NERI, V. GUIDI and D. ARIVUOLIFig. 1. – Pictorial representation of the clinical treatment method.it, which is also incorporated in the cancerous cells. We are planning to carry out anexperiment with these bent crystals so that they can be used in medical imaging.3. – Fabrication of bent crystals by LEPECVDBent crystals can be obtained in different methods like by mechanically bending aperfect single crystal [6], by using mixed crystals with a composition gradient [10], byapplying a thermal gradient to the planes of a perfect crystal [6] and by indentationmethod [2, 11]. In this present work bent crystals are obtained by low energy plasmaenhanced chemical vapour deposition (LEPECVD) technique. The significant featuresof this technique are wide range of epitaxial growth rate at low substrate temperatureand high reproducibility [12].With the use of LEPECVD, a crystalline germanium film is grown on the surfaceof the silicon substrate (100) which is tilted 6◦ towards 〈111〉 and/or vice versa. Thethickness of the silicon substrate is 400μm. The sample details are given in table I. Theimportance of this method is to produce intrinsically curved crystals without the use ofany mechanical means. The sample gets curved due to the lattice mismatch between Siand Ge and the different thermal expansion coefficients [13]. Thanks to this technique,curved crystals can be obtained by depositing tensile or compressive films over bulkysubstrates. This technique is chosen for its reproducibility, its capability to bend thecrystal to the desired curvature, bending is strong due to high stress imparted whichcan be matched with stability without delamination and also the interjunction stress isdistributed over a range of distances.Table I. – Details of the samples with the different thickness of Ge.Sample no. Film thickness (nm)1 5052 7623 11224 1800BENT CRYSTALS OBTAINED BY LOW ENERGY PLASMA ETC. 465Fig. 2. – Optical surface profilometry of sample 4. (a) gives the false-colour representation ofthe deformation. The deformation pattern cross-section along the x (b) and y (c) directions isseen with the two curvatures. The isotropic bending has been achieved.4. – Preliminary results and discussion-surface profilometry measurementsWyko surface profiler systems are non-contact optical profilers to measure a widerange of surface heights. The basic principle used here is that the light reflected from areference mirror combines with the light reflected from a sample to produce interferencefringes, where the best-contrast fringe occurs at best focus. We used the vertical scanninginterferometry mode in which the white-light source is filtered with a neutral densityfilter, which preserves the short coherence length of the white light, and the systemmeasures the degree of fringe modulation, or coherence, instead of the phase of theinterference fringes. The radius of curvature is measured with the help of surface profilersWyko NT1100.As an example, the optical surface profilometry of sample 4 is shown in fig. 2. Thecurvature has a significant effect on the diffraction efficiency of the crystal. The originof the curvature can be interpreted in terms of the thickness of deposition and also dueto lattice mismatch. The bent crystal samples of different thickness of germanium overthe silicon substrate were grown and characterized. As the thickness of deposition of Geis increased, the radius of curvature is decreased. Table II shows the variation of thecurvature radius along the y direction with respect to the different thickness of Ge. TheTable II. – Variation of the curvature radius with respect to the different thickness.Sample no. Radius of curvature (m)1 24.682 10.833 9.214 4.08466 P. JOICE SOPHIA, I. NERI, V. GUIDI and D. ARIVUOLIsignificance of this study is that it is easy to tailor the curvature to a wanted curvatureafter deposition of a suitable thickness and highly bent crystals can be produced by sucha technique.All the samples have been produced and optically characterized at Sensor and Semi-conductor Laboratory (Ferrara, Italy).5. – Simulation to optimize crystal diffraction propertiesPerformance of Laue medical lens relies on crystals as its most important elements.Hence, in order to maximize diffraction properties of the lens, crystal parameters have tobe optimized. For this aim, a simulation code has been developed. This has been workedout through Python high-level programming language, and it is specifically designed forbent crystals. Indeed, the software describes diffraction in crystals with curved diffractingplanes and generates the physical quantities which are typically used to characterizethe diffraction properties of a crystal. These are diffraction efficiency and reflectivityas defined in [8]. The former is the ratio of the diffracted beam intensity over thetransmitted one when no diffraction condition occurs and the latter is the ratio of thediffracted beam intensity to the incident beam intensity. Reflectivity can be determinedby simply multiplying efficiency to the attenuation factor due to linear absorption in thesample. Moreover, for a given atomic number and reflection plane, the software codecomputes the thickness maximizing reflectivity and efficiency as a function of photonenergy and angular spread, this latter being the bending angle of the crystal. Crystalthickness is a crucial parameter for optimization of a Laue lens. In Laue geometry, thisis considered as the part of the crystal which is totally traversed by the radiation, givingthe contribution to photon diffraction. In medical imaging applications of Laue lens,the thickness of crystal element gives the spatial resolution of the whole lens. Thus,this feature has to be taken into account when maximizing reflectivity and diffractionefficiency of the crystals composing the lens.As a result of simulations, 5mm thickness of sample 1 would diffract 140.5 keV ofTc-99m featuring 83% diffraction efficiency (which turns into a reflectivity of 42%), over42 arcsec angular spread. Crystalline planes of Ge have been considered for diffraction.Hence, provided that the crystalline planes of the sample are curved, high-efficiencydiffraction of gamma rays would be attained, with higher spatial resolution than forcurrently operating devices.6. – ConclusionsThe curved crystals obtained by LEPECVD could be used to focus and concentrategamma rays coming from the body of the patient towards the detector. This work relatesto a method of medical imaging and a Laue lens made of bent crystals would act as adevice to efficiently focus gamma radiation. The basic application of crystal diffractionin focusing the energetic gamma rays has been explained. Thanks to these crystals, highspatial resolution could be attained, even preventing the patient from heavy doses ofradioactivity, can enhance the overall performance of focusing the gamma rays and canalso eliminate the filters provided by gamma camera.BENT CRYSTALS OBTAINED BY LOW ENERGY PLASMA ETC. 467∗ ∗ ∗We are very grateful to S. Baricordi, R. Camattari, M. Gambaccini,L. Pozzetti, A. Tartari and D. Vincenzi for their valuable support and technicalassistance.REFERENCES[1] Roa D., Smither R., Zhang X., Nie K., Shieh Y., Ramsinghani N., Milne N.,Kuo J., Redpath J., Al-Ghazi M. and Caligiuri P., Exp. Astron., 20 (2005) 229,10.1007/s10686-005-9017-y; URL http://dx.doi.org/10.1007/s10686-005-9017-y.[2] Barrie`re N., Guidi V., Bellucci V., Camattari R., Buslaps T., Rousselle J.,Roudil G., Arnaud F.-X., Bastie P. and Natalucci L., J. Appl. Crystallogr., 43(2010) 1519; URL http://dx.doi.org/10.1107/S0021889810038343.[3] Frontera F., Loffredo G., Pisa A., Virgilli E., Carassiti V., Evangelisti F.,Landi L., Squerzanti S., Auricchio N., Caroli E., Landini G., Silvestri S. andStephen J., Mem. Soc. Astron. Ital., 79 (2008) 807.[4] Maisheev V., in Proceedings of 51st Workshop of the INFN ELOISATRON ProjectCharged and Neutral Particles Channeling Phenomena - Channeling 2008, edited byDabagov S. and Palumbo L. (World Scientific Publishing Company) 2008, p. 95.[5] Afonin A., Baranov V., Britvich G., Chepegin V., Chesnokov Y., Kotov V.,Maisheev V., Terekhov V. and Yazynin I., in Proceedings of 51st Workshop of theINFN ELOISATRON Project Charged and Neutral Particles Channeling Phenomena -Channeling 2008, edited by Dabagov S. and Palumbo L. 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Bent crystals obtained by low energy plasma enhanced chemical vapour deposition for medical applications

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Bent crystals obtained by low energy plasma enhanced chemical vapour deposition for medical applications

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