CVD diamond for thermoluminescence dosimetry: optimisation of the readout process and application

Diamond and Related Materials 13 (2004) 796 801 CVD diamond for thermoluminescence dosimetry: optimisation of the readout process and application a,b, a a a,b c d M. Rebisz *, M.J. Guerrero, D. Tromson, M. Pomorski, B. Marczewska, M. Nesladek, P. Bergonzo a a LIST (CEA Recherche Technologique) DIMRIySIARyLTD CEAySaclay, 91191 Gif-sur-Yvette, France b Faculty of Physics and Nuclear Techniques, University of Science and Technology, Krakow, Poland c Institute of Nuclear Physics (INP), Krakow, Poland d Linburgs Universitair Centrum, Diepenbeek, Belgium Received 9 September 2003; received in revised form 14 November 2003; accepted 20 November 2003 Abstract Diamond is known to be an extremely attractive material for dosimetry applications due to its intrinsic properties: tissue equivalence, physically robust, chemically inert and safe for in vivo use. Polycrystalline diamond substrates exhibit defective levels which trapped population is of major importance for the fabrication of radiation detectors based on diamond ionisation chambers or thermoluminescent dosimeter. The thermoluminescence (TL) is based on the measurement of the population that has been trapped during irradiation. Several problems however have been reported, namely fading of the TL signal when the device is exposed to light, poor linearity with respect to the absorbed dose and often poor reproducibility of the measurements. To address this we have set up a technique that enables irradiation and sample probing in the same apparatus. Irradiation was performed using an X-ray source with 50 kv accelerating voltage. Simultaneous measurements of TL and thermally stimulated currents (TSC) were performed. This way the fine control of the parameters (including temporal aspects) known to affect the reproducibility of the measurement could be varied, and a procedure for improving the linearity factor (TL signal with respect to the dose) of the diamond TL detectors was optimised. The technique was implemented on a set of CVD films to compare in a reproducible way the material properties according to their growth characteristics. 2003 Elsevier B.V. All rights reserved. Keywords: Polycrystalline CVD diamond; TL and TSC detector; Bleaching; Linearity 1. Introduction Diamond is known to be an extremely attractive material for dosimetry applications. Its near tissue equivalence (atomic number of 6 as compared with that of effective atomic number of human soft tissue: 7.4) avoids the need for energy dependence corrections as usually required in dosimetry. Further, diamond is physically robust, chemically inert and thus safe for in vivo use. It can be sterilized by thermal heating and is reusable. Recent studies on CVD diamond samples have demonstrated the possibility to rely on CVD diamond passive detectors based on thermoluminescence (TL) to probe their total irradiation history w1x. The dose depend- *Corresponding author. Tel.: q33-1-69084053; fax: q33-1- 69087679. E-mail address: pbergonzo@cea.fr (M. Rebisz). ence of CVD diamond can be compared with that of commercial LiF dosimeters w2 4x. In the form of what is usually performed on LiF detectors an optimised annealing procedure is required in order to achieve stable TL signals and reproducibility. The possible use of diamonds as TL dosimeters depends on the presence of defects in the materials. Diamond has a wide band gap of 5.5 ev within which deep levels could exist due to impurities or defects in the material. Principle of thermoluminescent dosimeter is based on the capability to trap carriers created by irradiation at room temperature and the low probability to release them. A thermal energy supply leads to the release of trapped carriers into the conduction or valence band. The free carriers can recombine with a radiative emission and the emitted photons give a TL signal. When an electrical field is applied, the carriers occurring 0925-9635/04/$ - see front matter 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2003.11.061

M. Rebisz et al. / Diamond and Related Materials 13 (2004) 796 801 797 Table 2 Sensitivity values of the main glow peaks of the diamond detectors 3 after X-ray radiation (dose is 0.7 Gy as normalised to 1 cm of diamond) BSL E07 SA1 MTS-N TL 49.08 Vycm 3 22.12 Vycm 3 4.12 Vycm 3 19.25 Vycm 3 TSC 210 maycm 3 546 naycm 3 360 paycm 3 Fig. 1. Typical TL glow curves for CVD diamond detectors (broken lines) and for LiF:Mg,Ti MTS-N detector (solid line) after X-ray irradiation up to a dose 0.7 Gy. The curve for the BSL detector has been scaled down by a factor of 0.5. in the bands are collected and give a thermally stimulated current (TSC). The temperature at which a carrier can be released from a trap depends on the depth of the level of the defect whose population has been modified by irradiation. One inherent problem with CVD diamond is that commonly observed trapping populations have been probed in the range 0.75 1.2 ev. These energy values correspond to detrapping temperatures close to 250 8C. However, since these energies remain below that of visible light, the latter can provide sufficient energy in the trapped defect populations to create their progressive alteration. This effect is called bleaching and results in the progressive loss of the TL signal. This will be illustrated in detail herein, where we report on the comparative TL and TSC properties of a few polycrystalline CVD diamond samples as well as of a monocrystalline natural IIa diamond. The study is oriented toward the medical physics application of this type of devices implying in our experiment irradiation at room temperature. In Fig. 1 the glow curves probed on two CVD diamond samples are plotted with that of a LiF:Mg,Ti (MTS-N) detector. The dose here consists of 0.7 Gy emitted by a Kevex X-ray tube with acceleration voltage of 50 kv. A tungsten target is used and resulting photons have an energy range from 10 to 50 kev with a maximum at approximately 35 kev. The diamond samples studied consist of (i) one sample (named BSL) fabricated in a conventional ASTEX reactor at 5 KW MW power at IMO-Hasselt, (ii) a sample (named E07) fabricated at much lower plasma densities at CEA- Saclay. Table 1 gives the parameters used for the microwave CVD samples BSL and E07. The sensitivity per unit volume of CVD diamonds is in the same range of that probed from the MTS-N detector (Fig. 1 and Table 2). The curve for BSL TL detector has been scaled down by a factor of 0.5. The CVD diamond glow characteristics are very similar with respect to the others, and a main peak at approximately 225 8C is observed after X-ray irradiation. The energy relative to this peak can be estimated using several methods, such as (i) the initial rise method, (ii) Chen s method w5x and (iii) numerical fitting, etc. The resulting energies of the trapping population associated with this 225 8C peak can be estimated at approximately 0.75 1.2 ev depending on the authors w6 9x. It seems therefore that the trapped population as induced by the pre-irradiation of the device is likely to decay when the sample is exposed to light. Further, it appears that other trap release peaks can be observed (Fig. 1) and result in the progressive rise of the sample glow at temperatures as low as 100 8C. It is likely that the environment to which the sample is exposed between irradiation and TL characterisation may affect the glow curves, and particularly if the sample is exposed to light or to varying temperatures. A special apparatus was optimised for this study as described in the following. 2. Experimental set-up The system comprises a heating stage, enabling linear temperature ramps of the sample from room temperature up to 550 8C. The heating stage was driven by a Eurotherm controller, using analog and amplified control of the heating power up to 40 W. All measurements were made in vacuum, in order to avoid problems Table 1 Parameters used for the microwave CVD samples BSL and E07 Methane (%) Microwave power (W) Pressure (mbar) Substrate temperature (8C) E07 0.5 1500 70 750 BSL 2.7 6000 8000 160 170 820 840

798 M. Rebisz et al. / Diamond and Related Materials 13 (2004) 796 801 3. Comparison between TL and TSC Fig. 2. Schematic description of the apparatus used and what enables in same set-up to irradiate the samples and receive simultaneously TL and TSC characteristics. associated with the progressive rise in temperature of the apparatus. This way only the sample stage temperature was varying, and care was required in order to ensure the best thermal contact between the sample and the stage. Using test samples, or also performing the measurement on a non-irradiated sample, it was made possible to probe the variation in temperature during the ramp-up and down of the system. Typical variations were remaining below 3 8C in all configurations. The system enables the possibility to irradiate the samples via a view port in the apparatus and using either X-ray or UV illumination (Fig. 2). As described earlier, X-rays were delivered typically at 30 kev with a40gyymin at the tube exit. However, a 3 mm thick entrance quartz window was used that resulted in the strong attenuation of the X-ray flux down to approximately 1.4 Gyyh. This way the quartz window could also be used in order to measure the same characteristics but using the full UV spectrum emitted by a deuterium light source. The TSC and TL characteristics were both recorded in this apparatus simultaneously. Sample was biased in a sandwich configuration and the TSC current was measured using a 6517A Keithley, whereas a Hamamatsu photomultiplier tube was used to measure the TL signal. In order to reduce the glow signal as emitted by the heating stage at high temperature (from 150 8C), a BG 18 filter was used. Using this apparatus, irradiation and sample characterisation could be performed in the same experimental set-up, but also the control of all external parameters and transitory effects that could affect the measurements become possible. Therefore, the sample was maintained in the dark, in vacuum, at a stable temperature during a controlled time between irradiation and measurement. Using the samples presented in Table 1 and on a commercially available natural IIa type diamond, semitransparent gold contacts were evaporated in order to enable biasing of the devices. As described earlier, even though gold is not the best theoretical candidate for ohmic contact fabrication, it suffices well for the present case where currents are probed, and the possible rectifying behaviour such devices may exhibit would vanish at the relatively high electric fields used here (0.7 Vy mm). Goldydiamond contacts have also proven to be extremely stable and particularly in the wide temperature ranges explored. The samples were irradiated in vacuum at room temperature either using the D2 UV source (200 400 nm, 50 W, and corresponding typically to 15 2 7=10 phycm photons) or X-rays (W target, 50 kv, 1mA) at a dose rate of 23 mgyymin (1.4 Gyyh). The read-out procedure was started exactly 2 min after the end of the excitation. The samples were heated from RT to 350 8C at a rate of 0.5 8Cys. After each measurement, the sample was annealed for 30 min at 350 8C w1x. Table 2 shows the sensitivity of the main glow peak for all investigated samples after irradiation to a dose of 3 0.7 Gy normalised to 1 cm. Fig. 3 shows the resulting curves as observed on the three samples studied. The results seem to correlate correctly to each other, demonstrating that TL and TSC, involving similar detrapping phenomena, occur at similar temperatures. This indirectly shows that since TSC enables the observation of non-radiative defects, whether TL does not, the observed similar characteristics give insights on the nature of the defects probed. This result, even though not in full agreement with Briand et al. w6x tends to demonstrate that TSC can in fact be a good candidate in diamond for dosimetry since it may enable the sample to be fully encapsulated in the dark and therefore not exposed to light to prevent bleaching. This remarkable advantage over the drawback that it requires contact electrodes makes TSC a good candidate over TL for medical dosimetry with diamond. 3.1. Bleaching As discussed earlier the strong limitation of diamond use in TL dosimetry is the possible release of trapped carrier between irradiation and measurement due to light illumination w10 12x. Following what was concluded before on the effect of TSC, it was necessary to probe if other techniques could be used to prevent bleaching. This study was conducted on sample BSL, exhibiting the most absolute TL signals. After X-ray exposure at RT (0.7 Gy), the sample was illuminated for 3 min using a solar spectrum OL 100A AM1 lamp in the range 300 750 nm. As expected, the observed TL signal

M. Rebisz et al. / Diamond and Related Materials 13 (2004) 796 801 799 Fig. 3. TL and TSC glow curves for two CVD diamond detectors after X-ray irradiation (broken lines) and natural diamond detector after UV illumination 20 min with deuterium lamp (straight line), for one IIa type diamond. Heating rate is 0.5 8Cys. vanished after such illumination (Fig. 4). The experiment then consisted of filtering the visible light using a set band-pass filters centered at 403, 483, 578 and 657 nm with 10-nm width band. All trends appear clearly similar, with slightly more expression of bleaching when photons of higher energies are used. Red light (657 nm) as used here was not sufficient in order to prevent for photo-induced detrapping in diamond. It is to be noted that the same trends are observed both on TL as well as on the TSC characteristics. Since TL emission characteristics are located at approximately 500 nm (green light), it appears that it is possible to protect the diamond from light using a filter that would also enable the glow measurement. 3.2. Linearity One of the important features of the detectors used for dosimetry is the response linearity, i.e. the range where the measured signal remains proportional to the applied dose. Here, since the trapping characteristics are caused by the inherent presence of defect levels in the diamond band gap, it is clear that the operator for growing high intrinsic quality diamond does not have a fine control of the defect concentration in the material. Therefore, one could expect that all the traps available get shortly filled, and therefore that the absolute resulting TL or TSC signals will tend to reach a limit. Its variation Fig. 4. The behaviour of TL and TSC peaks as a function of the wavelength of the bleaching light in the case of a priming dose of 0.7 Gy for the BSL sample.

800 M. Rebisz et al. / Diamond and Related Materials 13 (2004) 796 801 with respect to the dose will hence progressively saturate. Few works report this problem of linearity of synthetic and CVD diamond detectors. In general, results show that CVD diamond detectors studied have a very short linearity range, e.g. Hopwood and Jones reported a linear behaviour up to 0.8 Gy w13x whether Borchi et al. came to the same conclusions in the range from 0.01 Gy up to 1 Gy w3x. Other authors have observed significantly wider linearity ranges for CVD diamond. For example, Benabdesselam et al. w14x investigated detectors with a wide linearity response up to 16 Gy for gamma photons, up to 30 Gy for X-rays and up to 20 Gy for beta radiation dose. Furetta et al. reported approximately three groups of CVD diamonds grown using different conditions, one of them exhibiting a linear response up to 20 Gy w15x. After this threshold the response always becomes sub-linear and saturation effect always appears well below 2000 Gy. Also, Hopwood and Jones reported approximately one group of CVD diamonds used for low doses measurement that exhibit smooth reproducible response at high doses up to at least 70 Gy w13x. Nam et al. found that three orders of magnitude of sensitivity were gained by decreasing the concentration of nitrogen from 100 ppm to between 1 and 10 ppm, linearity up to 10 Gy w16x. The inclusion of boron in the diamond matrix had the marked effect of improving the response linearity. Keddy et al., in particular reported that the incorporation of the 17 3 2=10 ycm of boron impurities in synthetic crystals leads to an increase of both the sensitivity and the linearity of the TL response by comparison with undoped crystals w16,17x. Also, synthetic crystals with incorporation of nickel present a significant TL response vs. the absorbed dose up to 700 Gy w7x. In our case, the TL response vs. dose has been carried out at RT up to 7.5 Gy by X-ray irradiation. We measured the maximum value of the glow peak for each dose value for samples BSL and E07. The results are reported in Fig. 5. Here on this particular BSL sample saturation is observed as early as 7 Gy. Detector E07 has a more linear response up to 6 Gy. Hence it appears crucial that the trend relies on the ability to reproducibility control the absolute amount of deep level defective centres in the material that will act as traps for the generated carriers. Such control could, to our knowledge, only be made from fine impurity concentration tuning in the material. One attractive way could be that of using transition elements like nickel or cobalt, that could be included in the material during CVD growth. 4. Conclusion TL and TSC measurements have been carried out in this apparatus simultaneously and compared on a set of diamond samples. The analyses have been performed Fig. 5. TL signals plotted as a function of the X-ray irradiation dose for both CVD samples. using either X-ray or UV illumination. For the two techniques the glow characteristics are very similar and a main peak at approximately 225 8C is observed. The TL technique is extremely attractive for diamond for several reasons, one being that the sensitivity of CVD diamonds is at the same range as commercially available LiF detector but since these energies remain below that of visible light, the latter can provide sufficient energy in the trapped defect populations to create their progressive alteration, namely via bleaching. Control of a deep trap population in the material would prevent visible light from bleaching only if the defect energy is above 2 ev. The TSC technique has been demonstrated as another way to exploit CVD diamond properties, since they enable the possibility to let the sample in the dark between irradiation and measurement. Our work now will be focused on CVD materials that are intentionally doped with impurities (e.g. transition metals like Ni or Co) for more reproducible control of the trapping concentrations. References w1x B. Marczewska, P. Bilski, M. Nesladek, M. Rebisz, M.P.R. Waligorski, Phys. Stat. Sol. A 193 (3) (2002) 470 475.

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