# Investigations of high mobility single crystal chemical vapor deposition diamond for radiotherapy photon beam monitoring

Dominique Tromson, Caroline Descamps, Nicolas Tranchant, Philippe Bergonzo, Milos Nesladek, Aurélie Isambert

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## To cite this version:

Dominique Tromson, Caroline Descamps, Nicolas Tranchant, Philippe Bergonzo, Milos Nesladek, et al. Investigations of high mobility single crystal chemical vapor deposition diamond for radiotherapy photon beam monitoring. Journal of Applied Physics, 2008, 103, 054512 (6 p.). 10.1063/1.2872193. cea-05466805v2

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# Investigations of high mobility single crystal chemical vapor deposition diamond for radiotherapy photon beam monitoring

D. Tromson; C. Descamps; N. Tranchant; P. Bergonzo; M. Nesladek; A. Isambert

The intrinsic properties of diamond make this material theoretically very suitable for applications in medical physics. Until now ionization chambers have been fabricated from natural stones and are commercialized by PTW, but their fairly high costs and long delivery times have often limited their use in hospital. The properties of commercialized intrinsic polycrystalline diamond were investigated in the past by many groups. The results were not completely satisfactory due to the nature of the polycrystalline material itself. In contrast, the recent progresses in the growth of high mobility single crystal synthetic diamonds prepared by chemical vapor deposition (CVD) technique offer new alternatives. In the framework of the MAESTRO project (Methods and Advanced Treatments and Simulations for Radio Oncology), the CEA-LIST is studying the potentialities of synthetic diamond for new techniques of irradiation such as intensity modulated radiation therapy. In this paper, we present the growth and characteristics of single crystal diamond prepared at CEA-LIST in the framework of the NoRHDia project (Novel Radiation Hard CVD Diamond Detector for Hadrons Physics), as well as the investigations of high mobility single crystal CVD diamond for radiotherapy photon beam monitoring: dosimetric analysis performed with the single crystal diamond detector in terms of stability and repeatability of the response signal, signal to noise ratio, response speed, linearity of the signal versus the absorbed dose, and dose rate. The measurements performed with photon beams using radiotherapy facilities demonstrate that single crystal CVD diamond is a good alternative for air ionization chambers for beam quality control.

Topics

Signal processing, Diamond, Polycrystalline material, Raman spectroscopy, Chemical vapor deposition, Semiconductor detectors, Ionization chamber, Hadrons, Medical physics, Radiation therapy

## I. INTRODUCTION

In the framework of the MAESTRO and NoRHDia projects, the CEA-LIST is studying the potentialities for using optimized synthetic diamond for new radiotherapy treatments such as intensity modulated radiation therapy (IMRT). In that particular case, the improvement expected is the irradiation of the tumour without deterioration of the surrounding tissue that implies change of the small-size radiation fields during the treatment. Detectors adapted to this technique have to follow all the requirements of the conventional radiotherapy treatments and some more specific conditions due to IMRT such as very little volumes of detection (ideal case is around $\square 1\text{mm}^3$) with high sensitivities (more than three decades for signal to noise ratio) and a very fast response time in order to follow the sharp transient of the irradiation fields («second»). Diamond exhibits several assets for its use as online detector for radiotherapy beam dosimetry.¹ Its intrinsic properties such as tissue equivalence (atomic number Z=6 close to that of human tissues) radiation and mechanical hardness as well as wide band gap (5.5 eV) make it of high interest for use as a small-size solid state detector.

Diamond ionization chambers fabricated from natural diamond gemstones are currently commercialized by PTW.²,³ However, the production of devices, that exhibit suitable dosimetric characteristics, requires a severe selection of high-quality natural gems. It implies that such detectors are highly expensive, long delivery times are common, and there is a lack in the reproducibility from one device to another. Several studies have already proven the compatibility of polycrystalline chemical vapor deposition (CVD) diamonds for dosimetry applications.⁴ However, even though their synthesis is more straightforward, polycrystalline material performances are highly altered by the presence of defects. The use of synthetic single crystal diamond, grown in a reproducible way, with optimized properties close to the selected natural ones is a good alternative for the fabrication of diamond detector.

The CEA-LIST is one of the laboratories in the world which is able to grow synthetic single crystal diamond using the CVD technique. In the present work, single crystal diamonds were elaborated and characterized at CEA-LIST and the dosimetric performances of one of the device are presented. The aim of the present study was to perform a complete analysis of the dosimetric performances including the stability and repeatability of the signal, signal to noise ratio, response speed, linearity with the absorbed dose, and dose rate dependence. The purpose here was to demonstrate the feasibility of using synthetic single crystal diamonds for radiotherapy applications.

## II. MATERIALS AND METHODS

The single crystal diamond device was elaborated from homoepitaxy on a high pressure and high temperature Ib diamond, with (100) crystalline orientation. The synthesis was performed using an ASTEX 5400 reactor from H2-CH4 precursor mixtures, in a microwave plasma at 500W power and a pressure of following a high pressure process described in previous studies.⁵,⁶ After synthesis, the substrate used for growth was fully removed and the layer was mechanically polished on both sides.⁷

This highly transparent diamond exhibited dimensions equal to 2.72*3.06*0.68 mm²□. Optical and electronic properties of the diamond were then characterized using on the one hand Raman spectroscopy, and on the other hand using thermally stimulated current (TSC) and time of flight (TOF) measurements, in order to check, respectively, the structural quality of the sample and the detector quality via the presence of electronic defects in the material and the estimation of the carriers mobilities. The Raman spectrum was obtained using a Jobin-Yvon confocal micro-Raman HR800 with two excitation laser lines at 632.82 and 514.5 nm □. The TSC response after 50kV□, 1 mA x-ray irradiation was recorded using a measurement setup built in the laboratory. The TSC setup measurement enables to perform successive irradiation at controlled temperature without exposing the sample to light between each successive measurement.⁸ The TOF setup used in our study is based on a coherent second harmonic generation of an optical parametric oscillator laser, tuned to 218 nm excitation wavelength and pumped with a tripled neodymium doped yttrium aluminum garnet. A beam splitter placed across the laser beam allows the use of a Si detector to trig the signal. The 218 nm laser light penetration was calculated to be lower than 20 µm in diamond, based on the optical absorption data,⁹,¹⁰ therefore carriers are generated in the close vicinity of the top electrode (with respect to the 680 µm thickness of the freestanding sample presented). During all our experiments, the intensity of the laser beam was kept low (□ 125 mJ without filtration), thus the photogenerated charge was significantly smaller (□□1.3*10⁻⁸ C, calculated from the area of the maximum charge induced) than the CU product, where C is the sample capacity and U the maximum applied
voltage, CU=4*10⁻¹³*10⁻⁷*700*10⁻⁴=3*10⁻⁷ (i.e., we were not in space charge limited current regime). The photocurrent signal was then collected using a 1 GHz bandwidth Tectronics oscilloscope linked to a computer. The photocurrent traces were recorded for both carrier polarities and applied electric fields in the range of 0-8.5 kV/cm.

The dosimetric study was performed at the Institut Gustave Roussy (IGR, Villejuif, France). Single crystal diamond detector (SCDD) was fabricated by evaporation of 50nm thick gold electrical contacts on both faces of the sample and mounting on a subminiature version A (SMA) connector. To avoid air ionization around the electrodes, the diamond, holder, and electrodes were encapsulated with a polymer resin. Even though this resin has a density close to one, the device is not completely optimized as a tissue equivalent device due to the metallic foil (□ 200 µthick, Cu) below the sample to make the background contact. The detector signal current was recorded using a Keithley 6517A. The medical linear accelerator used for this study was the Clinac (Varian). The photon energies used were tuned with a linac accelerator from 6 to 20MV□. In the rest of the paper, we will misuse the term "MV" instead of "MeV" since in medical physics, the beam photons energy are usually describe by the applied bias set. We will speak about 6 and 20MV photon beams. The variation of the dose rates from 1 to 6 Gy / min is obtained by varying the pulse repetition frequency (i.e., the number of monitor units per minute). The detector parameters were evaluated with the device placed between polymethylmethacrylate sheets to be in dosimetric conditions. We investigated the stability and repeatability of the signal, the signal to noise ratio, the rapidity of the detector, the linearity of the signal versus the delivered dose, and finally the dose rate dependence for two different energies of 6 and 20 MV □. We compared the results obtained with the recommendation of the code of practice of IAEA (International Atomic Energy Agency) as well as by internal recommendation within the MAESTRO integrated project, which are slightly different due to the fact that the MAESTRO project deals with new techniques with very drastic criteria of use for the device. We always compare our results with the most severe criteria between the two recommendations. For all tests, the SCDD was biased at a 4V fixed voltage throughout the whole device volume corresponding to a best compromise between the signal to noise ratio and the detector response. The source-detector distance was fixed at 100 cm□, and a 10*10 cm² flat field was used in order to cover the whole area of the detector.

### III. RESULTS

#### A. Sample characterization

As depicted in Fig. 1, the Raman spectrum of the single crystal diamond under study exhibits a sharp absorption band at 1332 cm⁻¹ corresponding to the diamond first-order phonon mode revealing the good quality of the film elaborated. There are no additional features due to amorphous carbon or graphitic phases in the diamond. There are no indication of nitrogen related signal from photoluminescence measurements using the laser at 514nm□.

The TSC response of the single crystal diamond under □50kV, 1 mA x rays is presented in Fig. 2. Sample current versus temperature was recorded from 300 to 550K with a linear heating rate β=0.5K/s □. In polycrystalline and some natural samples several structures could be observed on TSC curve: peak near room temperature due to the presence of shallow trapping levels and a main peak at higher temperature near 500-550 K □. It is known that the shallow trapping levels could be responsible for the instabilities observed in diamond detector due to the thermal release of the trapped carriers at room

temperature.¹¹ In the present case, there is no peak near room temperature, and the x-ray response of the SCDD shows a steady signal (Fig. 2). The TSC curve shows a main peak at a maximum temperature Tm=511 K that is usually observed in polycrystalline and natural diamond even in IIa type. The determination of the energy level was obtained from the Hoogenstraaten method with various heating rates.¹² The slope of the Ln (Tm²/β) with respect to 1/Tm gives the energy level of the trap. The values of the maximum temperature peaks and the corresponding heating rates used are summarized in Table I. We find here □1.9±. This method is usually used for the first-order kinetics where we supposed that there is no retrapping on the level, but is still a good approximation for the next orders.

<figure><img src="image_3.png" /><figcaption>**Figure 2**
(a) TSC obtained after 90 kV x-ray irradiation; (b) x-ray SCDD response.</figcaption></figure>

<table><thead><tr><td>Heating rate (K/s)</td><td>Maximum temperature of TSC peak</td></tr></thead><tbody><tr><td>0.782</td><td>505.0</td></tr><tr><td>0.485</td><td>500.3</td></tr><tr><td>0.194</td><td>487.8</td></tr><tr><td>0.09</td><td>483.8</td></tr></tbody></table>

The drift mobility was estimated from TOF measurements and studied as a function of the applied electrical field across the SCDD. The zero electric field mobility and drift saturation velocity were calculated using an asymptotic behavior of the µ(E) (Ref. 13) that has been obtained by asymptotic fitting of the calculated drift mobilities at high

<figure><img src="image_4.png" /><figcaption>**Figure 1:** Raman spectrum of single crystal CVD diamond.</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>**Figure 3:** Hole drift mobilities obtained using UV laser excitation TOF.</figcaption></figure>
electric fields. The transit time needed for the induced charge to go through the sample thickness was calculated from the intercept of pretransit and post-transit slopes. The precision of the transit time evaluation at 300K was estimated to be better than 10%. The fitting of the zero electrical field mobility data yielded mobility values of 2700±50 cm² V⁻¹s⁻¹ for electrons and 2426±50 cm² V⁻¹s⁻¹ for holes.¹⁴ These mobility results emphasize the high quality of the synthesized diamond material. The zero field electrical charge mobilities for holes are shown in Fig. 3.

## B. Evaluation of the dosimetric parameters

SCDD was irradiated successively five times under 6 MV photons, at a fixed dose of 2 Gy and using a dose rate of 2Gy/min ☐. The charge deposited was evaluated via the SCDD current, the mean value and the standard deviation were also calculated. We calculated the stability, repeatability, and the signal to noise ratio with respect to the definition of the IAEA code of practice.¹⁵

<figure><img src="image_6.png" /><figcaption>Figure 4:(a) Signal shape of the SCDD under 6MV photons with a dose rate of 2Gy / min (b) response speed.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>Figure 5:Signal linearity vs absorbed dose varying from 0.1 to 4 Gy.</figcaption></figure>

Figure 4(a) shows the detector response under 6 MV x-ray photons, with a dose rate of 2Gy/min ☐. Stability of the SCDD was estimated for each irradiation from the percentage ratio of the standard deviation to the mean value of the SCDD current. The SCDD shows a stability around 0.6% that is worse than the recommendations (0.5%). This could be partly due to the device that is not completely optimized for these measurements especially with regards to the noise. The repeatability is estimated by the percentage ratio of the standard deviation of the SCDD signal integral for five irradiations to the mean value of the SCDD signal integral. We find repeatability close to 0.5% that is the limit recommended by the IAEA 374 code of practice.

The signal to noise ratio was estimated from the mean values of the SCDD calculated with and without beam. We found a background signal B below 0.1 pA and a mean value of the SCDD current at 2.30 nA ☐. That implies a signal to noise ratio superior to 2300. This value is higher than the 1000 usually required in medical physics and recommended by IAEA. This result is of great importance with regards to application as IMRT that requires device with little detection volume and very high sensitivity. The response speed of the SCDD is estimated by the time required from the end of irradiation to reach the mean value of the background signal. As observed in Fig. 4(b), the signal stabilization is quasi-immediate. The response speed of the SCDD was evaluated at 0.78 s☐, which is very

close to the value obtained with air ionization chamber or PTW diamond detector. It is to be noted that these values could be modified by the electronic device setup and therefore are not only caused by the material response. The detector exhibits a fast response for doses greater than 1Gy☐, but it must be said that these results could be seen as insufficient for the device used in the case of IMRT dosimetry, i.e., where the detectors used should be able to follow extremely sharp transients of the IMRT

fields. For this application, other tests would have to be performed, including, for example, the dynamic response study from the evaluation of the signal linearity with the dose at very low monitor units.

In order to evaluate the dose dependence of the SCDD, the signal was recorded under 6 MV x-ray photons by varying the delivered dose from 0.1 to 4 Gy at a fixed dose rate of 2Gy/min ☐. The results obtained are shown in Fig. 5 and fitted using a linear regression fit. A good linearity of the signal with the absorbed dose was probed, with a corresponding regression coefficient of 0.9999. Moreover, the sensitivity of the detector was assessed to be around 69 nC/Gy (25 nC/Gy mm³)☐. These results are well correlated with those previously observed for natural diamond PTW chamber by Angelis et al.¹⁶ who calculated sensitivities of 85 and 88 nC/Gy for two PTW chambers as well as by Fidanzio et al.¹⁷ who probed sensitivities between 50 and 135 nC/Gy mm³☐.
<figure><img src="image_8.png" /><figcaption>Figure 6: (a) Ratio of the calculated dose to the reference dose for various dose rates; (b) dose rate behavior of the SCDD fitted with Fowler equation.</figcaption></figure>

For an ideal detector the variation of the signal with the dose rate follows a linear behavior with a coefficient equal to 1. The calculation of the dose received by the device has to be constant if the dose delivered by the beam is kept constant during the measurement of the dose rate variation. In our measurements the dose rate behavior was estimated for 6 MV photons and a fixed dose at 4Gy with dose rate varying from 1 to 4 Gy/min. The dose rate behavior was estimated by the deviation of the calculated value for integrated dose at each dose rate, with respect to the reference dose that is presently 4 Gy at 2 Gy/min. Results in Fig. 6(a) show variation around 3% for each dose rate that indicates a slight variation of the measurement with the dose rate. This could be corrected by applying a correction factor after calibration of the device. Another usual way to investigate the dose rate behavior for PTW diamond detectors is to study the variation of the detector current using the Fowler relationship¹⁸ summarized in Fig. 6(b). The parameter $\Delta$ describes the deviation from linearity and is expected to be as close as possible to 1 to have a linear behavior with respect to the dose rate as for air ionization chamber. Figure 6(b) obtained with the same measurements than Fig. 6(a) shows directly the value of the SCDD current with varying the dose rate used for irradiations and the corresponding fit following the Fowler equation. The results of the fitting give a $\Delta$ value at 0.956±0.004, that is in good agreement with those obtained for PTW chambers with $\Delta$ values estimated around 0.98.¹⁶

<figure><img src="image_9.png" /><figcaption>Figure 7: 20 MVx-ray irradiation: (a) signal linearity vs absorbed dose from 0.1 to 4 Gy (b) dose rate behavior of the SCDD fitted with Fowler equation.</figcaption></figure>

Finally, we perform again the study of dose and dose rate behavior by varying the energy of the beam up to 20 MV. The determination of the energy dependence is very critical because the whole device, in other words both the diamond material and the experimental holder set-up, is involved. This may explain the high dispersions observed by several teams.¹⁹⁻²¹ The results are presented in Figs. 7(a) and 7(b) and have to be compared with Figs. 5 and 6(b). A very good linearity of the signal with the dose absorbed was observed and the linear fitting parameter was evaluated at $R=0.99989$. The sensitivity of SCDD to 20 MV photons beams was evaluated to be 71nC/Gy. With varying the dose rate we found a $\Delta$ value at 0.995±0.07. All those measurements will have to be repeated with an improved setup in order to minimize any possible influence of the holder. Despite this, the slight energy dependence observed is a well known problem for ionization chambers and may easily be solved from the preliminary determination of a calibration factor for each beam energy.

## IV. CONCLUSION

SCDD was elaborated and tested under radiotherapy photon beams. A single crystal diamond sample, grown at CEA-LIST, was characterized using Raman, TSC, and laser TOF measurements in order to check the crystalline and electronic quality of the sample. The Raman spectrum did not show any significant structure, and TSC and TOF results are similar to those obtained with natural IIa-type diamond. One main defect at 1.9 eV is observed from the TSC curve and the TOF techniques used yielded the zero field mobilities of 2700 cm²V⁻¹s⁻¹ for electrons and 2400 cm²V⁻¹s⁻¹ for holes, highlighting the high grade of the SC CVD diamond samples grown. This high mobility SCDD diamond grown at CEA-LIST was evaluated under photon beams. The results obtained appear to be in good agreement with the recommendations as mentioned by the MAESTRO IP project and consequently with International IAEA recommendations. These first tests assessing the signal linearity with the absorbed dose and the dose rate dependence lead to promising future developments for particular application as IMRT. They are in well agreement with the results obtained for commercial natural diamond PTW detector already used in several hospitals. This study demonstrates the feasibility of synthetic single crystal CVD diamond for radiotherapy field.

## ACKNOWLEDGMENTS

The authors acknowledge the European Commission for funding of part of this research from the MAESTRO project (IP CE503564) (C.D.) as well as from the I3HP JRA11 NORHDIA (Project No. RII3-CT-2004-506078) (N.T.). We also acknowledge the GeMAC laboratory (CNRS, Meudon, France) for their kind collaboration for Raman spectroscopy, the IMO University (Hasselt, Belgium) for their kind collaboration with UV laser TOF measurements, and A. Bridier from IGR for his useful contribution in the dosimetric parameter study.
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