- Email: [email protected]
Requirements for synthetic diamond devices for radiotherapy dosimetry applications
Diamond & Related Materials 13 (2004) 2046 – 2051 www.elsevier.com/locate/diamond
Requirements for synthetic diamond devices for radiotherapy dosimetry applications M.J. Guerrero*, D. Tromson, M. Rebisz, C. Mer, B. Bazin, P. Bergonzo LIST (CEA-Recherche Technologique) DETECS/SSTM/LTD-CEA/Saclay, F-91191 Gif-sur-Yvette, Cedex, France Available online 6 October 2004
Abstract
CVD diamond is a remarkable material for the fabrication of radiation detectors. Radiation hardness, chemical resistance and hightemperature operation capabilities of diamond motivate its use for fabrication of devices operating in hostile environments such as that encountered in nuclear industry and high energy physics. Its potentialities for such applications have been well documented and recent studies have led to the developments of a few applications that are addressing specific industrial needs. One particular interest of diamond stands in the fact that its atomic number is close to that of human tissues. This implies that the response of a diamond device to radiation is close to that received by the human body. Its thus enables the straightforward measurement of the dose for radiotherapy applications. However, this requires high reproducibility and linearity. It is widely observed that radiation exposure is modifying the initial performances of diamond detectors and priming devices is therefore required to obtain the required linearity. However, the nature of defects in the material strongly influences the type of priming required. This paper will address this problem from the study of trapping levels and their influence on the device response. We present here the current status of the development of polycrystalline diamond for this type of application, and propose new techniques of improving the material characteristics toward the optimisation of ionisation chamber performances as well as that of thermoluminescent dosimeters for the particular field of radiotherapy applications. D 2004 Elsevier B.V. All rights reserved. Keywords: Polycrystalline CVD diamond; Radiotherapy; Dosimetry; Ionisation chamber; Thermoluminescence
1. Introduction Radiation therapy, also referred to as radiotherapy, radiation oncology or therapeutic radiology are one of the three principal modalities used in treatment of malignant diseases (cancer). In this medical specialty that commonly relies on the use of ionising radiations, one requires new techniques providing radiation dosimetry measurement for several application domains, such as for the calibration of accelerator sources, the accurate verification of the dose received by the patient, or also the accurate mapping of the irradiated areas (including healthy parts), as well as * Corresponding author. Tel.: +33 1 69088745; fax: +33 1 69087679. E-mail addresses: [email protected] (M.J. Guerrero)8 [email protected] (P. Bergonzo). 0925-9635/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2004.07.026
irradiation caused by secondary emission due to the collimation apparatus. For such, radiation dosimeters must exhibit several desirable characteristics and of these are accuracy and precision, together with linearity to the dose and dose rate, and stable response in spite of the variation of the energy of incident photons, and all those with good spatial resolution. For the last few years, new techniques of treatment have also appeared in the field of radiotherapy (IMRT—Intensity Modulated Radiation Treatment, etc.). Such treatments require high accuracy and geometrical precision in the measurement of the delivered dose. In this respect, a recently awarded European Integrated Project (IP), called Methods and Advanced Equipment for Simulation and Therapy in Radiation Oncology (MAESTRO) is aiming at the development within the next few years of a range of new generation diagnostic tools providing opti-
M.J. Guerrero et al. / Diamond & Related Materials 13 (2004) 2046–2051
mised dose measurement for the patient. Part of the project is focussing on the dose monitor devices, and diamond stands as an excellent candidate due to its unique combination of properties. The interests in using diamond as a radiation dosimeter stems principally from its tissue equivalence. The atomic number of diamond (Z=6) matches that of biological tissues (the effective number of human tissue is 7.42 for muscle and 5.92 for fat), thus it comes that the energy deposited by any radiation per unit of mass in diamond equals virtually that for human tissues. Furthermore, the use of diamond enables to obviate correction factors to the detector signals for the dose conversion in tissues equivalent media. Moreover, new techniques such as IMRT show new dosimetric problems due to the characteristics of elementary beams used: namely that it leads to poor lateral electronic equilibrium [1] and to high dose gradient [2]. This dosimetric problem could be minimized using a small (for high gradient dose) tissue equivalent detector (for lack of electronic equilibrium). Diamond appears as the best solution to solve this dosimetric problem. Other attractive properties of diamond as a dosimeter are that it is chemically stable, non-toxic, mechanically robust and insensitive to radiation damage. Diamond detectors of small volumes can be fabricated and then are particularly suitable for accurate dose measurements in small radiation fields. In addition, diamond is an extremely versatile material and its dosimetric properties can be exploited in two manners: as a solid state ionisation chamber [3] as well as a thermoluminescent (TL) dosimeter [4]. So far, commercial diamond detectors have been fabricated for this application and rely on the use of high-quality natural gems. These devices equip today several hospital irradiation facilities. However, the main drawback of such dosimeters are the high cost and long delivery times, mostly resulting from the scarcity of suitable stones, or in other words of the lack of a technological technique enabling their easy fabrication. The use of natural stones implies a severe gem selection, and one of the consequences is also an observed poor reproducibility. This happens to be particularly detrimental for the use of diamond as TL dosimeters. However, the recent availability of synthetic samples, grown under controlled conditions using the chemical vapour deposition technique, has rendered possible the widespread application of diamond for this particular field of medical radiation dosimetry. In this paper, we will describe the difficulties encountered when polycrystalline synthetic diamond (from the chemical vapour deposition technique) is used for this application, and namely the influence of the defect levels regularly observed in this material. The consequences of those defect populations will be discussed for both types of device applications, namely ionisation chamber and TL dosimeters.
2047
2. Influence of defect levels in diamond Diamond has a wide band gap of 5.5 eV within which several defects caused by impurities, dislocations and crystal defects may co-exist. Such trapping levels can be empirically classified in two categories: (i) shallow levels, that are unstable at room or near-to-room temperatures and (ii) deep levels that remain stable at room temperature. This classification of trapping levels directly corresponds to the type of priming required: shallow levels or deep levels do not behave in the same way whether the device is used as an ionisation chamber or as a TL dosimeter. Priming defines the pre-irradiation that enables to fill up the shallow as well as deep levels in order to stabilise the photo-sensitivity of the device [5]. The populations of the trapping levels can be studied probing the signature of thermally stimulated currents (TSC) or/and of the luminescence (TL). Fig. 1 shows the TSC measured on one CVD sample (right axis) as well as the TL signature of the same CVD sample (left axis). The TSC characteristic shows a strong peak corresponding to a deep level emptied at 550 K, whether shoulders are observed around 350 K corresponding to shallow levels but with negligible contribution as compared with the deep level. However, when TL signals are probed, it comes that the optical contribution of the shallow levels is much more significant with respect to the deep levels. The effect is sometimes more visible on few natural samples and Fig. 1 shows the particular case of one sample exhibiting a luminescence contribution at 350 K thus from shallow levels that is more radiative than that from deep levels. The comparison between TSC and TL thus give insights into the radiative or non-radiative nature of the thermally stimulated processes (see also Ref. [6]). 2.1. Shallow levels They are unstable at room temperature and thus strongly influence the device response when used as ionisation
Fig. 1. TSC signature of a CVD diamond sample (right scale) and corresponding TL characteristic (left scale). TL spectra are also given for two distinctive natural IIa type samples.
2048
M.J. Guerrero et al. / Diamond & Related Materials 13 (2004) 2046–2051
chamber. During irradiation, shallow and deep levels will progressively be filled up to an equilibrium where trapping kinetics compensate thermal detrapping ones. As such, when irradiation is stopped, shallow levels are slowly emptied and when the next irradiation starts the signal will only progressively reach back its equilibrium. In addition, if the device temperature is modified, the trapping–detrapping equilibrium may significantly be altered and thus result in observed variations of the device photosensitivity. This phenomenon is a highly detrimental problem in radiotherapy when diamond is used as an ionisation chamber. In the second issue for diamond use in this scope, and namely as a TL dosimeter, the presence of shallow levels do not constitute such a problem during irradiation, since here the signal readout is obtained by thermal heating well after the sample irradiation. Therefore, the progressive linear heating will effectively start close to room temperature and therefore will result in the immediate detrapping of those shallow levels and instantly annihilate the detrimental effect they may have caused. The TL reading will therefore only probe the behaviour of deep levels. 2.2. Deep levels They are stable at room temperature and therefore when the irradiation stops the traps remain filled for long times unless the sample is heated to high temperatures (NN450 K) [7]. TL dosimeters are based on this principle and the more stable the trapped population the better. On the other hand, in a diamond ionisation chamber, when the deep levels are emptied, a significant part of the photo-generated carriers may progressively be used to fill those emptied levels, and therefore will not contribute to the device signal. After a prolonged irradiation, fewer traps are available, thus a smaller portion of carriers are lost, and thus the device appears to be more sensitive. This common effect of priming is necessary to improve the stability of the signal. However, here again if the device is heated up to elevated temperatures (N450 8C [7]); the sensitivity may falsely appear as collapsing whether it is just the effect of an unstable trapped population on a defect level.
very close to those measured on high-quality natural diamonds, but with much higher reproducibility since they can be synthesised [12]. We have made such experiments using thick polycrystalline CVD diamonds of varying active volumes (30, 150, 300 Am in thickness for devices referenced as Det1, Det2 and Det3, respectively) and grown using the microwave-assisted chemical vapour deposition technique. Electrical contacts were prepared on each surface of the diamond sample using gold metal evaporation. The devices were then assembled to PCB boards (back contacts) and wire bound (top contact) (Fig. 2). The experiments were performed using a Saturne 43, a medical accelerator for photons and electrons at Laboratoire National Henri Becquerel-Saclay (LNHB) using 6 MeV photons, at varying dose rates (3, 1.6 and 0.75 Gy/ min). The dose variation was achieved from the reduction of the current in the accelerator. The three ionisation chambers were biased at 50 V and therefore not to the same electric field due to their varying thickness. The photo-current measurements were performed with a Keithley 6517A in the common mode. In order to avoid air ionisation and particularly around the biased signal electrode, the devices were coated with insulative resin (wax). It comes clear that there is here an interest in using for this configuration a tissue equivalent resin. Additionally, the use of gold in the device may not be optimal for tissue equivalence and may have to be optimised for future developments (graphite, etc.). Fig. 3 shows the observed response of the devices, obtained after the devices were exposed to about 10 Gy of low-energy photons for the priming reasons discussed above. Attempts to vary the dose show a close to linear variation of the device response. However, too few points being used this surely requires further measurements and will not be discussed here. After irradiation is stopped and put back on, unexpected behaviours can be observed in Fig. 3: the photo-current measured on Det2 always requires a couple of minutes to reach equilibrium. This can be related to the decay rate of shallow levels that almost immediately get detrapped after irradiation and thus times is required each time for trapped carriers to reach equilibrium. A similar
3. Consequences for CVD diamond ionisation chambers The dosimetric properties of pure IIa type natural diamond solid state ionisation chambers is well documented and has demonstrated how diamond detectors are more appropriate than Si diode as well as small standard ionisation chamber [8–10]. Kozlov et al. [11] have empirically proposed ways to select gems suitable for radiation detection: they should exhibit high resistivities (N1012 V) and low impurity content. Since this earlier work, CVD diamond synthesis enabled a lot of progress, up to a point where some synthetic diamonds even though of polycrystalline nature exhibit dosimetric properties that are
Fig. 2. Schematic representation and picture of a diamond ionisation chamber.
M.J. Guerrero et al. / Diamond & Related Materials 13 (2004) 2046–2051
Fig. 3. Photo-current measurements as measured on three different CVD diamonds under 6 MeV photons beam on a medical accelerator.
2049
Fig. 5. Current measurements on the same samples as Fig. 3 under proton beam (energy is 100 MeV, fluency is 6.71011 protons/h).
behaviour is observed on Det3 but here the detrapping effect was only visible after long non-irradiation stays, as shown in Fig. 4 after one night. These behaviour are strongly detrimental as they may lead to nonlinear responses of the device: if several minutes are required before equilibrium is reached in the device, it comes that the device may not perfectly reflect all fast intensity changes in the photon fluency modulation. Similarly, the over-estimated values observed on Det3 after one night are inevitably going to cause signal nonlinearities. As long as the time constants and behaviour could be precisely predicted according to the device history, this problem may be controlled experimentally. However, we see that whether Det3 is left 5 min or one night unattended, the response will not be reproducible. This is the most problematic behaviour of diamond that requires correction before an industrial application using synthetic diamond is used; and is one of the driving aspects of diamond developments in the MAESTRO project. Other devices such as Det1 here seems not to exhibit this problem, however the sensitivities are far too low and therefore signal to noise ratios lower. In fact, the existing commercially available devices used today on such medical facilities and
based on natural diamond also require daily pre-irradiation (recommended dose is typically of 8 Gy). It shows that such problem may be industrially empirically solved using either an external source of irradiation or an optimised use of the device, but surely it does not consist the goal of the MAESTRO project, where new techniques are initiated to solve these problems. Nevertheless, the actual state of the art has ended up to the point where synthetic CVD diamond almost matches the performance of high quality natural stones, but the goal is of course to reach material grades for whom properties are beyond those of natural diamond. Further measurements were performed on the same three devices and in the same conditions under proton beams at the Orsay Proton Therapy Centre (CPO), and using 100 MeV protons at fluencies of 6.71011 protons/h (Fig 5). This preliminary experiment showed that the devices were exhibiting stable behaviours during irradiation. Unfortunately, no calibration data was available here, therefore the only comparison is to be made between the devices themselves. The goal of the MAESTRO project is also to
Fig. 4. Photo-current measurements on the same samples under photon beam after one night at room temperature.
Fig. 6. Comparison of the TL signatures of two CVD diamond with a LiF dosimeter.
2050
M.J. Guerrero et al. / Diamond & Related Materials 13 (2004) 2046–2051
Fig. 7. Relative linearity of the response with the dose on CVD diamond (see text).
support more tests under proton beams, an application where diamond advantages for miniature dosimeter fabrication is expected to have a strong impact on the improvement of the metrology of the dose given to the patient.
4. Consequences for CVD diamond TL dosimeters Thermoluminescence (TL) is the thermally stimulated emission of light from a semiconductor following the previous absorption of energy from radiation [13]. Part of the carriers created by the absorbed ionising radiation remains trapped in discrete energy levels within the forbidden energy gap due to impurities or lattice imperfections. Subsequent heating of the material induces the release of the trapped electrons/holes, which can recom-
Fig. 8. TL mapping of the dose measured on a
F
bine radiatively. The energy of emitted photons is then particular to the materials, or, better to the trapping level distribution, whereas the TL intensity is a function of the trapping cross section, temperature, trap density and absorbed energy of radiation. After passive irradiation of the TL device, the readout requires the use of a system that measures the overall number of photons released by the sample when the sample is heated in a dedicated black cell. Commercial devices are today equipping several irradiation facilities and it is essential here that diamond fits the existing readout systems in terms of photon energies and temperature ranges. One alternative material commonly used is lithium fluoride (LiF). One drawback however is that LiF is toxic, hygroscopic, and therefore not easy to use for passive dosimetry in clinic routine. Diamond superior properties here concern its non toxicity, and rather biocompatibility, and chemical inertness that make it compatible with body fluids as well as physiological liquids. Here, also the absence of electrical measurement requirement enables such devices to be used as small size inserts in the body during treatments, for post irradiation integrated dose verifications. Natural diamond crystals exhibit very unequal TL properties depending on their types (Ia, Ib, IIa, IIb) and namely the nitrogen contents and other sites caused by the presence of impurities or crystal defects [6]. Here, we have compared the TL signatures as probed on two CVD diamond TL dosimeters with the response of a typical lithium fluoride (LiF) dosimeter and using photons from a Cs source. Fig. 6 shows that the response of CVD diamond closely matches that of LiF in terms of photon intensities and wavelengths. Complementary measurement is shown in Fig. 7 and demonstrates the relatively poor linearity of the device response with respect to the dose. In
5 cm CVD diamond irradiated with a brachytherapy ruten eye-applicator (radium needle).
M.J. Guerrero et al. / Diamond & Related Materials 13 (2004) 2046–2051
fact, this surely demonstrates the requirement for developing new growth processes that aim at controlling the impurity nature as well as concentrations within the material. Since diamond is known to be atomically extremely dense so that very little species can diffuse through its lattice, it comes that the material will have to be doped from its growth and that new candidates have to be explored. From the theory of transition metals incorporation in diamond [14], Nickel and Cobalt have been identified as promising candidate and will be tested during the project. Such material may then be extremely attractive for TL dosimetry on large scale devices in order to probe the dose distribution profile of radiotherapy beams. In Fig. 8, a 5-cm diameter diamond wafer is used to probe the dose distribution profile when exposed to a Brachtherapy ruten eye applicator. The TL map is then probed using a CCD type imaging system and enable the geometrical measurement of the dose distribution profile.
5. Conclusion CVD diamond is studied as a ionisation chamber and TL dosimeter for radiotherapy application. The technology has demonstrated that current grade poly-crystalline material grown using the CVD technique is a good candidate for this application even though improvements can be expected from the growth aspects as well as from the optimisation of the measurement protocol. Diamond ionisation chambers remain stable and reproducible as long as the duration between irradiations is kept short. Diamond TL dosimeters are luminescent in the same spectral region as LiF but linearity to the dose response can be improved from addressing the impurity concentration in the material. It is expected that the MAESTRO project will enable us to address these aspects in order to aim at the reproducible production of devices compatible with irradiation facility precision requirements.
Acknowledgements The authors are acknowledging colleagues from the Henry Becquerel National Laboratory (CEA-Saclay) and namely Josianne Daures and Aime´ Ostrowsky for enabling the measurements on the Saturne 43 photon accelerators. Similarly, Re´gis Ferrand from the Orsay Proton Therapy
2051
Centre (CPO) is acknowledged together with Jean-Paul Kleider from LGEP for the experiments performed under proton beams. The authors would like to thank Pawel Olko and Barbara Marcevska for Fig. 8, a measurement that was conducted in the frame of the polonium project no. 03292PF. The MAESTRO project should shortly officially start with EC support under the reference IP CE503564. Jean Barthe and Jean-Philippe NicolaR are acknowledged for the managing of this application as well as for useful discussions.
References [1] A. Wu, R.D. Zwicker, A.M. Kalend, Z. Zheng, Med. Phys. 20 (3) (1993) 777 – 779. [2] C. Martens, C. De Wagter, W. De Neve, Phys. Med. Biol. 45 (2000) 2519 – 2530. [3] A. Fidanzio, L. Azario, P. Viola, P. Ascarelli, E. Cappelli, G. Conte, A. Piermattei, Nucl. Instrum. Methods Phys. Res., Sect. A, Accel. Spectrom. Detect. Assoc. Equip. 524 (2004) 115 – 123. [4] J. Krasa, L. Juha, V. Vorlicek, A. Cejnarova, Nucl. Instrum. Methods Phys. Res., Sect. A, Accel. Spectrom. Detect. Assoc. Equip. 524 (2004) 332 – 339. [5] D. Tromson, P. Bergonzo, A. Brambilla, C. Mer, F. Foulon, V.N. Amosov, J. Appl. Phys. 87 (7) (2000) 3360. [6] M. Rebisz, M.J. Guerrero, D. Tromson, M. Pomorski, P. Bergonzo, Diamond Relat. Mater. 13 (2004) 796 – 801. [7] D. Tromson, A. Brambilla, P. Bergonzo, A. Mas, C. Hordequin, C. Mer, F. Foulon, J. Appl. Phys. 90 (2001) 1608 – 1611. [8] S.N. Rustgi, Med. Phys. l22 (5) (1995) 567 – 570. [9] A. Fidanzio, L. Azario, R. Miceli, A. Russo, A. Piermattei, Med. Phys. 27 (11) (2000) 2589 – 2593. [10] C. De-Angelis, S. Onori, M. Pacilio, G.-A.-P. Cirrone, G. Cuttone, L. Raffaele, M. Bucciolini, S. Mazzocchi, Med. Phys. 29 (2) (2002) 248 – 254. [11] S.F. Kozlov, E.A. Konorov, Y.A. Kuznetsov, Y.A. Salikov, V.I. Redko, V.R. Grinberg, M.L. Meilman, IEEE Trans. Nucl. Sci. 24 (1) (1977) 235 – 237. [12] A.J. Whitehead, R. Airey, C.M. Buttar, J. Conway, G. Hill, S. Ramkumar, G.A. Scarsbrook, R.S. Sussmann, S. Walker, Nucl. Instrum. Methods Phys. Res., Sect. A, Accel. Spectrom. Detect. Assoc. Equip. 460 (2001) 20 – 26. [13] S.W.S Mc Keever, Thermoluminescence of Solids, Cambridge Solid State Science Series, Cambridge University Press, 1985. [14] K. Johnston, A. Mainwood, Diamond Relat. Mater. 11 (2002) 631 – 634; K. Johnston, A. Mainwood, Diamond Relat. Mater. 12 (3–7) (2003) 516 – 520; K. Johnston, A. Mainwood, A.T. Collins, G. Davies, D. Twitchen, M. Newton, J.M. Baker, Diamond Relat. Mat. 9 (3–6) (2000) 424 – 427.
Requirements for synthetic diamond devices for radiotherapy dosimetry applications M.J. Guerrero*, D. Tromson, M. Rebisz, C. Mer, B. Bazin, P. Bergonzo LIST (CEA-Recherche Technologique) DETECS/SSTM/LTD-CEA/Saclay, F-91191 Gif-sur-Yvette, Cedex, France Available online 6 October 2004
Abstract
CVD diamond is a remarkable material for the fabrication of radiation detectors. Radiation hardness, chemical resistance and hightemperature operation capabilities of diamond motivate its use for fabrication of devices operating in hostile environments such as that encountered in nuclear industry and high energy physics. Its potentialities for such applications have been well documented and recent studies have led to the developments of a few applications that are addressing specific industrial needs. One particular interest of diamond stands in the fact that its atomic number is close to that of human tissues. This implies that the response of a diamond device to radiation is close to that received by the human body. Its thus enables the straightforward measurement of the dose for radiotherapy applications. However, this requires high reproducibility and linearity. It is widely observed that radiation exposure is modifying the initial performances of diamond detectors and priming devices is therefore required to obtain the required linearity. However, the nature of defects in the material strongly influences the type of priming required. This paper will address this problem from the study of trapping levels and their influence on the device response. We present here the current status of the development of polycrystalline diamond for this type of application, and propose new techniques of improving the material characteristics toward the optimisation of ionisation chamber performances as well as that of thermoluminescent dosimeters for the particular field of radiotherapy applications. D 2004 Elsevier B.V. All rights reserved. Keywords: Polycrystalline CVD diamond; Radiotherapy; Dosimetry; Ionisation chamber; Thermoluminescence
1. Introduction Radiation therapy, also referred to as radiotherapy, radiation oncology or therapeutic radiology are one of the three principal modalities used in treatment of malignant diseases (cancer). In this medical specialty that commonly relies on the use of ionising radiations, one requires new techniques providing radiation dosimetry measurement for several application domains, such as for the calibration of accelerator sources, the accurate verification of the dose received by the patient, or also the accurate mapping of the irradiated areas (including healthy parts), as well as * Corresponding author. Tel.: +33 1 69088745; fax: +33 1 69087679. E-mail addresses: [email protected] (M.J. Guerrero)8 [email protected] (P. Bergonzo). 0925-9635/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2004.07.026
irradiation caused by secondary emission due to the collimation apparatus. For such, radiation dosimeters must exhibit several desirable characteristics and of these are accuracy and precision, together with linearity to the dose and dose rate, and stable response in spite of the variation of the energy of incident photons, and all those with good spatial resolution. For the last few years, new techniques of treatment have also appeared in the field of radiotherapy (IMRT—Intensity Modulated Radiation Treatment, etc.). Such treatments require high accuracy and geometrical precision in the measurement of the delivered dose. In this respect, a recently awarded European Integrated Project (IP), called Methods and Advanced Equipment for Simulation and Therapy in Radiation Oncology (MAESTRO) is aiming at the development within the next few years of a range of new generation diagnostic tools providing opti-
M.J. Guerrero et al. / Diamond & Related Materials 13 (2004) 2046–2051
mised dose measurement for the patient. Part of the project is focussing on the dose monitor devices, and diamond stands as an excellent candidate due to its unique combination of properties. The interests in using diamond as a radiation dosimeter stems principally from its tissue equivalence. The atomic number of diamond (Z=6) matches that of biological tissues (the effective number of human tissue is 7.42 for muscle and 5.92 for fat), thus it comes that the energy deposited by any radiation per unit of mass in diamond equals virtually that for human tissues. Furthermore, the use of diamond enables to obviate correction factors to the detector signals for the dose conversion in tissues equivalent media. Moreover, new techniques such as IMRT show new dosimetric problems due to the characteristics of elementary beams used: namely that it leads to poor lateral electronic equilibrium [1] and to high dose gradient [2]. This dosimetric problem could be minimized using a small (for high gradient dose) tissue equivalent detector (for lack of electronic equilibrium). Diamond appears as the best solution to solve this dosimetric problem. Other attractive properties of diamond as a dosimeter are that it is chemically stable, non-toxic, mechanically robust and insensitive to radiation damage. Diamond detectors of small volumes can be fabricated and then are particularly suitable for accurate dose measurements in small radiation fields. In addition, diamond is an extremely versatile material and its dosimetric properties can be exploited in two manners: as a solid state ionisation chamber [3] as well as a thermoluminescent (TL) dosimeter [4]. So far, commercial diamond detectors have been fabricated for this application and rely on the use of high-quality natural gems. These devices equip today several hospital irradiation facilities. However, the main drawback of such dosimeters are the high cost and long delivery times, mostly resulting from the scarcity of suitable stones, or in other words of the lack of a technological technique enabling their easy fabrication. The use of natural stones implies a severe gem selection, and one of the consequences is also an observed poor reproducibility. This happens to be particularly detrimental for the use of diamond as TL dosimeters. However, the recent availability of synthetic samples, grown under controlled conditions using the chemical vapour deposition technique, has rendered possible the widespread application of diamond for this particular field of medical radiation dosimetry. In this paper, we will describe the difficulties encountered when polycrystalline synthetic diamond (from the chemical vapour deposition technique) is used for this application, and namely the influence of the defect levels regularly observed in this material. The consequences of those defect populations will be discussed for both types of device applications, namely ionisation chamber and TL dosimeters.
2047
2. Influence of defect levels in diamond Diamond has a wide band gap of 5.5 eV within which several defects caused by impurities, dislocations and crystal defects may co-exist. Such trapping levels can be empirically classified in two categories: (i) shallow levels, that are unstable at room or near-to-room temperatures and (ii) deep levels that remain stable at room temperature. This classification of trapping levels directly corresponds to the type of priming required: shallow levels or deep levels do not behave in the same way whether the device is used as an ionisation chamber or as a TL dosimeter. Priming defines the pre-irradiation that enables to fill up the shallow as well as deep levels in order to stabilise the photo-sensitivity of the device [5]. The populations of the trapping levels can be studied probing the signature of thermally stimulated currents (TSC) or/and of the luminescence (TL). Fig. 1 shows the TSC measured on one CVD sample (right axis) as well as the TL signature of the same CVD sample (left axis). The TSC characteristic shows a strong peak corresponding to a deep level emptied at 550 K, whether shoulders are observed around 350 K corresponding to shallow levels but with negligible contribution as compared with the deep level. However, when TL signals are probed, it comes that the optical contribution of the shallow levels is much more significant with respect to the deep levels. The effect is sometimes more visible on few natural samples and Fig. 1 shows the particular case of one sample exhibiting a luminescence contribution at 350 K thus from shallow levels that is more radiative than that from deep levels. The comparison between TSC and TL thus give insights into the radiative or non-radiative nature of the thermally stimulated processes (see also Ref. [6]). 2.1. Shallow levels They are unstable at room temperature and thus strongly influence the device response when used as ionisation
Fig. 1. TSC signature of a CVD diamond sample (right scale) and corresponding TL characteristic (left scale). TL spectra are also given for two distinctive natural IIa type samples.
2048
M.J. Guerrero et al. / Diamond & Related Materials 13 (2004) 2046–2051
chamber. During irradiation, shallow and deep levels will progressively be filled up to an equilibrium where trapping kinetics compensate thermal detrapping ones. As such, when irradiation is stopped, shallow levels are slowly emptied and when the next irradiation starts the signal will only progressively reach back its equilibrium. In addition, if the device temperature is modified, the trapping–detrapping equilibrium may significantly be altered and thus result in observed variations of the device photosensitivity. This phenomenon is a highly detrimental problem in radiotherapy when diamond is used as an ionisation chamber. In the second issue for diamond use in this scope, and namely as a TL dosimeter, the presence of shallow levels do not constitute such a problem during irradiation, since here the signal readout is obtained by thermal heating well after the sample irradiation. Therefore, the progressive linear heating will effectively start close to room temperature and therefore will result in the immediate detrapping of those shallow levels and instantly annihilate the detrimental effect they may have caused. The TL reading will therefore only probe the behaviour of deep levels. 2.2. Deep levels They are stable at room temperature and therefore when the irradiation stops the traps remain filled for long times unless the sample is heated to high temperatures (NN450 K) [7]. TL dosimeters are based on this principle and the more stable the trapped population the better. On the other hand, in a diamond ionisation chamber, when the deep levels are emptied, a significant part of the photo-generated carriers may progressively be used to fill those emptied levels, and therefore will not contribute to the device signal. After a prolonged irradiation, fewer traps are available, thus a smaller portion of carriers are lost, and thus the device appears to be more sensitive. This common effect of priming is necessary to improve the stability of the signal. However, here again if the device is heated up to elevated temperatures (N450 8C [7]); the sensitivity may falsely appear as collapsing whether it is just the effect of an unstable trapped population on a defect level.
very close to those measured on high-quality natural diamonds, but with much higher reproducibility since they can be synthesised [12]. We have made such experiments using thick polycrystalline CVD diamonds of varying active volumes (30, 150, 300 Am in thickness for devices referenced as Det1, Det2 and Det3, respectively) and grown using the microwave-assisted chemical vapour deposition technique. Electrical contacts were prepared on each surface of the diamond sample using gold metal evaporation. The devices were then assembled to PCB boards (back contacts) and wire bound (top contact) (Fig. 2). The experiments were performed using a Saturne 43, a medical accelerator for photons and electrons at Laboratoire National Henri Becquerel-Saclay (LNHB) using 6 MeV photons, at varying dose rates (3, 1.6 and 0.75 Gy/ min). The dose variation was achieved from the reduction of the current in the accelerator. The three ionisation chambers were biased at 50 V and therefore not to the same electric field due to their varying thickness. The photo-current measurements were performed with a Keithley 6517A in the common mode. In order to avoid air ionisation and particularly around the biased signal electrode, the devices were coated with insulative resin (wax). It comes clear that there is here an interest in using for this configuration a tissue equivalent resin. Additionally, the use of gold in the device may not be optimal for tissue equivalence and may have to be optimised for future developments (graphite, etc.). Fig. 3 shows the observed response of the devices, obtained after the devices were exposed to about 10 Gy of low-energy photons for the priming reasons discussed above. Attempts to vary the dose show a close to linear variation of the device response. However, too few points being used this surely requires further measurements and will not be discussed here. After irradiation is stopped and put back on, unexpected behaviours can be observed in Fig. 3: the photo-current measured on Det2 always requires a couple of minutes to reach equilibrium. This can be related to the decay rate of shallow levels that almost immediately get detrapped after irradiation and thus times is required each time for trapped carriers to reach equilibrium. A similar
3. Consequences for CVD diamond ionisation chambers The dosimetric properties of pure IIa type natural diamond solid state ionisation chambers is well documented and has demonstrated how diamond detectors are more appropriate than Si diode as well as small standard ionisation chamber [8–10]. Kozlov et al. [11] have empirically proposed ways to select gems suitable for radiation detection: they should exhibit high resistivities (N1012 V) and low impurity content. Since this earlier work, CVD diamond synthesis enabled a lot of progress, up to a point where some synthetic diamonds even though of polycrystalline nature exhibit dosimetric properties that are
Fig. 2. Schematic representation and picture of a diamond ionisation chamber.
M.J. Guerrero et al. / Diamond & Related Materials 13 (2004) 2046–2051
Fig. 3. Photo-current measurements as measured on three different CVD diamonds under 6 MeV photons beam on a medical accelerator.
2049
Fig. 5. Current measurements on the same samples as Fig. 3 under proton beam (energy is 100 MeV, fluency is 6.71011 protons/h).
behaviour is observed on Det3 but here the detrapping effect was only visible after long non-irradiation stays, as shown in Fig. 4 after one night. These behaviour are strongly detrimental as they may lead to nonlinear responses of the device: if several minutes are required before equilibrium is reached in the device, it comes that the device may not perfectly reflect all fast intensity changes in the photon fluency modulation. Similarly, the over-estimated values observed on Det3 after one night are inevitably going to cause signal nonlinearities. As long as the time constants and behaviour could be precisely predicted according to the device history, this problem may be controlled experimentally. However, we see that whether Det3 is left 5 min or one night unattended, the response will not be reproducible. This is the most problematic behaviour of diamond that requires correction before an industrial application using synthetic diamond is used; and is one of the driving aspects of diamond developments in the MAESTRO project. Other devices such as Det1 here seems not to exhibit this problem, however the sensitivities are far too low and therefore signal to noise ratios lower. In fact, the existing commercially available devices used today on such medical facilities and
based on natural diamond also require daily pre-irradiation (recommended dose is typically of 8 Gy). It shows that such problem may be industrially empirically solved using either an external source of irradiation or an optimised use of the device, but surely it does not consist the goal of the MAESTRO project, where new techniques are initiated to solve these problems. Nevertheless, the actual state of the art has ended up to the point where synthetic CVD diamond almost matches the performance of high quality natural stones, but the goal is of course to reach material grades for whom properties are beyond those of natural diamond. Further measurements were performed on the same three devices and in the same conditions under proton beams at the Orsay Proton Therapy Centre (CPO), and using 100 MeV protons at fluencies of 6.71011 protons/h (Fig 5). This preliminary experiment showed that the devices were exhibiting stable behaviours during irradiation. Unfortunately, no calibration data was available here, therefore the only comparison is to be made between the devices themselves. The goal of the MAESTRO project is also to
Fig. 4. Photo-current measurements on the same samples under photon beam after one night at room temperature.
Fig. 6. Comparison of the TL signatures of two CVD diamond with a LiF dosimeter.
2050
M.J. Guerrero et al. / Diamond & Related Materials 13 (2004) 2046–2051
Fig. 7. Relative linearity of the response with the dose on CVD diamond (see text).
support more tests under proton beams, an application where diamond advantages for miniature dosimeter fabrication is expected to have a strong impact on the improvement of the metrology of the dose given to the patient.
4. Consequences for CVD diamond TL dosimeters Thermoluminescence (TL) is the thermally stimulated emission of light from a semiconductor following the previous absorption of energy from radiation [13]. Part of the carriers created by the absorbed ionising radiation remains trapped in discrete energy levels within the forbidden energy gap due to impurities or lattice imperfections. Subsequent heating of the material induces the release of the trapped electrons/holes, which can recom-
Fig. 8. TL mapping of the dose measured on a
F
bine radiatively. The energy of emitted photons is then particular to the materials, or, better to the trapping level distribution, whereas the TL intensity is a function of the trapping cross section, temperature, trap density and absorbed energy of radiation. After passive irradiation of the TL device, the readout requires the use of a system that measures the overall number of photons released by the sample when the sample is heated in a dedicated black cell. Commercial devices are today equipping several irradiation facilities and it is essential here that diamond fits the existing readout systems in terms of photon energies and temperature ranges. One alternative material commonly used is lithium fluoride (LiF). One drawback however is that LiF is toxic, hygroscopic, and therefore not easy to use for passive dosimetry in clinic routine. Diamond superior properties here concern its non toxicity, and rather biocompatibility, and chemical inertness that make it compatible with body fluids as well as physiological liquids. Here, also the absence of electrical measurement requirement enables such devices to be used as small size inserts in the body during treatments, for post irradiation integrated dose verifications. Natural diamond crystals exhibit very unequal TL properties depending on their types (Ia, Ib, IIa, IIb) and namely the nitrogen contents and other sites caused by the presence of impurities or crystal defects [6]. Here, we have compared the TL signatures as probed on two CVD diamond TL dosimeters with the response of a typical lithium fluoride (LiF) dosimeter and using photons from a Cs source. Fig. 6 shows that the response of CVD diamond closely matches that of LiF in terms of photon intensities and wavelengths. Complementary measurement is shown in Fig. 7 and demonstrates the relatively poor linearity of the device response with respect to the dose. In
5 cm CVD diamond irradiated with a brachytherapy ruten eye-applicator (radium needle).
M.J. Guerrero et al. / Diamond & Related Materials 13 (2004) 2046–2051
fact, this surely demonstrates the requirement for developing new growth processes that aim at controlling the impurity nature as well as concentrations within the material. Since diamond is known to be atomically extremely dense so that very little species can diffuse through its lattice, it comes that the material will have to be doped from its growth and that new candidates have to be explored. From the theory of transition metals incorporation in diamond [14], Nickel and Cobalt have been identified as promising candidate and will be tested during the project. Such material may then be extremely attractive for TL dosimetry on large scale devices in order to probe the dose distribution profile of radiotherapy beams. In Fig. 8, a 5-cm diameter diamond wafer is used to probe the dose distribution profile when exposed to a Brachtherapy ruten eye applicator. The TL map is then probed using a CCD type imaging system and enable the geometrical measurement of the dose distribution profile.
5. Conclusion CVD diamond is studied as a ionisation chamber and TL dosimeter for radiotherapy application. The technology has demonstrated that current grade poly-crystalline material grown using the CVD technique is a good candidate for this application even though improvements can be expected from the growth aspects as well as from the optimisation of the measurement protocol. Diamond ionisation chambers remain stable and reproducible as long as the duration between irradiations is kept short. Diamond TL dosimeters are luminescent in the same spectral region as LiF but linearity to the dose response can be improved from addressing the impurity concentration in the material. It is expected that the MAESTRO project will enable us to address these aspects in order to aim at the reproducible production of devices compatible with irradiation facility precision requirements.
Acknowledgements The authors are acknowledging colleagues from the Henry Becquerel National Laboratory (CEA-Saclay) and namely Josianne Daures and Aime´ Ostrowsky for enabling the measurements on the Saturne 43 photon accelerators. Similarly, Re´gis Ferrand from the Orsay Proton Therapy
2051
Centre (CPO) is acknowledged together with Jean-Paul Kleider from LGEP for the experiments performed under proton beams. The authors would like to thank Pawel Olko and Barbara Marcevska for Fig. 8, a measurement that was conducted in the frame of the polonium project no. 03292PF. The MAESTRO project should shortly officially start with EC support under the reference IP CE503564. Jean Barthe and Jean-Philippe NicolaR are acknowledged for the managing of this application as well as for useful discussions.
References [1] A. Wu, R.D. Zwicker, A.M. Kalend, Z. Zheng, Med. Phys. 20 (3) (1993) 777 – 779. [2] C. Martens, C. De Wagter, W. De Neve, Phys. Med. Biol. 45 (2000) 2519 – 2530. [3] A. Fidanzio, L. Azario, P. Viola, P. Ascarelli, E. Cappelli, G. Conte, A. Piermattei, Nucl. Instrum. Methods Phys. Res., Sect. A, Accel. Spectrom. Detect. Assoc. Equip. 524 (2004) 115 – 123. [4] J. Krasa, L. Juha, V. Vorlicek, A. Cejnarova, Nucl. Instrum. Methods Phys. Res., Sect. A, Accel. Spectrom. Detect. Assoc. Equip. 524 (2004) 332 – 339. [5] D. Tromson, P. Bergonzo, A. Brambilla, C. Mer, F. Foulon, V.N. Amosov, J. Appl. Phys. 87 (7) (2000) 3360. [6] M. Rebisz, M.J. Guerrero, D. Tromson, M. Pomorski, P. Bergonzo, Diamond Relat. Mater. 13 (2004) 796 – 801. [7] D. Tromson, A. Brambilla, P. Bergonzo, A. Mas, C. Hordequin, C. Mer, F. Foulon, J. Appl. Phys. 90 (2001) 1608 – 1611. [8] S.N. Rustgi, Med. Phys. l22 (5) (1995) 567 – 570. [9] A. Fidanzio, L. Azario, R. Miceli, A. Russo, A. Piermattei, Med. Phys. 27 (11) (2000) 2589 – 2593. [10] C. De-Angelis, S. Onori, M. Pacilio, G.-A.-P. Cirrone, G. Cuttone, L. Raffaele, M. Bucciolini, S. Mazzocchi, Med. Phys. 29 (2) (2002) 248 – 254. [11] S.F. Kozlov, E.A. Konorov, Y.A. Kuznetsov, Y.A. Salikov, V.I. Redko, V.R. Grinberg, M.L. Meilman, IEEE Trans. Nucl. Sci. 24 (1) (1977) 235 – 237. [12] A.J. Whitehead, R. Airey, C.M. Buttar, J. Conway, G. Hill, S. Ramkumar, G.A. Scarsbrook, R.S. Sussmann, S. Walker, Nucl. Instrum. Methods Phys. Res., Sect. A, Accel. Spectrom. Detect. Assoc. Equip. 460 (2001) 20 – 26. [13] S.W.S Mc Keever, Thermoluminescence of Solids, Cambridge Solid State Science Series, Cambridge University Press, 1985. [14] K. Johnston, A. Mainwood, Diamond Relat. Mater. 11 (2002) 631 – 634; K. Johnston, A. Mainwood, Diamond Relat. Mater. 12 (3–7) (2003) 516 – 520; K. Johnston, A. Mainwood, A.T. Collins, G. Davies, D. Twitchen, M. Newton, J.M. Baker, Diamond Relat. Mat. 9 (3–6) (2000) 424 – 427.