Bremsstrahlung converter to plots

Abstract

We have developed a new type of target for linear particle accelerators, capable of converting electrons into X-rays with an efficiency comparable to that of conventional targets (such as bulk tungsten), and a greater resistance to high irradiation (reduced risk of target melting). Our target prototype consists of several layers of a substrate material (combining good thermal conductivity, low atomic number of its constituents, and high melting temperature), in which are implanted pads made of elements with high Bremsstrahlung conversion power (i.e., elements with high atomic number). We have presented the advantages and limitations of the different types of substrates and pads (ceramic or metallic), and modeled, via a Monte-Carlo approach, the photon spectra obtained with the prototypes with Cu, SiC and diamond substrates, and with W and TaC pads.

Description

Title of the invention: Bremsstrahlung converter with studs Summary of the invention

[0001] Design of a new type of target for converting electrons into X-rays for linear particle accelerators, based on the implantation of high-power Bremsstrahlung conversion pads within a substrate material. Technical problem

[0002] FLASH radiotherapy is a new generation oncological treatment which consists of delivering to the patient a high dose of irradiation (of the order of ten Gy) in a very short period of time (less than one second), as opposed to conventional radiotherapies based on the delivery of moderate doses of irradiation, over a longer period of time and over several sessions. This FLASH irradiation, whose antitumor power is equivalent to conventional irradiation, has a double advantage: on the one hand, greater comfort for the patient, and on the other hand, better preservation of healthy tissues irradiated during the treatment of the tumor. It has proven particularly effective in the treatment of cutaneous lymphoma (Refl).

[0003] Current FLASH radiotherapy treatments mainly involve irradiation with electrons, generated via a linear particle accelerator. This type of particle, easily screened by human tissues, has the disadvantage of only being able to treat superficial tumors. To be able to treat more internal tumors by FLASH radiotherapy, it is necessary to use X-rays, whose tissue penetration power is much greater than that of electrons. It is then necessary to adapt a conversion target at the output of the linear accelerator, which transforms the electrons from the accelerator into X-ray photons via the Bremsstrahlung phenomenon (continuous bremsstrahlung radiation). In FLASH radiotherapy based on X-rays (Ref2), one of the major difficulties is to design a Bremsstrahlung conversion target capable of withstanding high doses of radiation, in particular the melting induced by the rise in temperature.Current conversion targets are generally made of a few millimeters of a high Bremsstrahlung power metal (W (Ref3,Ref4), Ta (Ref5), Mo (Ref6)...), coupled with a high thermal conductivity material allowing heat to be dissipated (diamond (Ref4), Al (Ref5), Cu (Ref3)) which can be arranged in the form of a flange (Ref5) or a support surface (Ref3). Technical solution

[0004] To overcome the problem of the fusion of conversion targets during FLASH irradiation, we propose here a new prototype of a target for converting electrons into photons, consisting of a substrate material (acting as a thermal regulator) etched by pads of material with a high atomic number (acting as a Bremsstrahlung converter). The substrate material must combine, as far as possible, high thermal conductivity (to properly evacuate the heat from the pads resulting from the Bremsstrahlung), high melting temperature (to prevent its fusion), and low atomic number of its constituents (which implies a low Bremsstrahlung, therefore low heating during irradiation).

[0005] Compared to conventional conversion targets, made of a bulk material with a high atomic number (W, Ta, Mo), our prototype must have an equivalent power for converting electrons into X-rays, and be more resistant to a strong irradiation flux (less risk of fusion).

[0006] Our prototype conversion target, shown in [Fig.l], consists of a superposition of several layers of a substrate material, the thickness of each layer being a few hundred micrometers. Each of the layers of substrate material will have previously undergone a plot etching (Ref7): within each plot (a few hundred micrometers deep) will be injected a material consisting of elements with a high atomic number (and, consequently, with a high Bremsstrahlung power). The etched plots may have various geometric shapes depending on the manufacturing conditions (hemispherical, cylindrical, parallelepiped, etc.).

[0007] This conversion target will be positioned at the output of the electron source (see [Fig.2]). The X-ray beam obtained will then be collimated, then quantified via an X-ray detector (for example, a SiC diode (Ref8)). This detector will then serve as a pilot to control the electron source, and allocate the desired X-ray dose for the patient.

[0008] Design of prototypes: choice of constituent materials

[0009] First of all, it is necessary to choose constituent materials for the pads, whose role will be to carry out the Bremsstrahlung conversion (electrons-photons), and the substrate, whose role will be to evacuate the heat from the pads. • For the pads: a material with a high Bremsstrahlung conversion power. Typically, a metal with a high atomic number (W, Ta) or a ceramic based on one of these metals (for example, TaC) will be chosen. Generally, ceramic compounds have a lower Bremsstrahlung conversion power than their metallic counterparts, but they have the advantage of having very low thermal expansion coefficients (which prevents possible damage to the substrate by stress during the temperature rise). • For the substrate: a material combining, if possible, low atomic number of its constituents (which implies low Bremsstrahlung conversion power, therefore little heating during irradiation), high thermal conductivity (which promotes the evacuation of heat from the pads resulting from Bremsstrahlung), and high melting temperature (which prevents melting of the substrate). Typically, one can choose either an ultra-refractory ceramic (SiC, diamond), or a metal or alloy with high thermal conductivity (for example, Cu).

[0010] If necessary, a cooler can be adapted (see for example (Ref5)) to better evacuate the heat transported by the substrate (itself coming from the pads during Bremsstrahlung).

[0011] The advantages and disadvantages of the different substrate / pad pairs are presented in Table 1. The “ceramic pad on metal substrate” case is deliberately omitted, because it combines two major disadvantages: on the one hand the lower Bremsstrahlung conversion power of the ceramic pads, and on the other hand the risk of cracking of the latter during thermal expansion of the metal substrate. [Tables 1] Substrate Plot Advantages Disadvantages Ceramic Metallic Excellent resistance of the substrate to high radiation fluxes High thermal conductivity of the substrate allows heat to be easily evacuated from the pads Risk of cracking of the substrate during thermal expansion of the pads Metallic Metallic No risk of cracking of the substrate (“soft” metal) during thermal expansion of the pads Risk of melting of the pad, and of the surrounding substrate zone (substrate with moderate thermal conductivity and melting temperature) Ceramic Ceramic Excellent resistance of the substrate to high radiation fluxes High thermal conductivity of the substrate allows heat to be easily evacuated from the pads No risk of cracking of the substrate (low thermal expansion of the pads) Conversion power Br emsstrahlung of a ceramic pad generally higher s weaker than that of a metal stud Advantages and disadvantages of the different substrate / pad pairs to be used for the design of the Bremsstrahlung converter

[0012] For good robustness of the converter under high irradiation, the following combinations can be favored: • Metal pads on metal substrate: the flexibility of the metal substrate will prevent cracking induced by thermal expansion stresses of the pads during temperature rise. • Ceramic pads on ceramic substrate: when the temperature rises, the ceramic pads will only expand very slightly, the resulting stresses on the (fragile) ceramic matrix will then be negligible, which will prevent the latter from cracking.

[0013] As part of our modeling, we tested different types of materials, the thermodynamic characteristics of which are explained in Table 2 and in [Fig.3]: • Three types of substrate: - Metallic Cu, the metal with the highest thermal conductivity in the fourth period of the periodic table, - SiC and diamond ceramics, distinguished by their high thermal conductivity (for diamond in particular, see Table 2) and their excellent resistance to irradiation (commonly used for the design of radiation detectors (Ref9,ReflO)). • Two types of plots: - Metallic W: the metal with the highest melting temperature, and one of the highest Bremsstrahlung conversion efficiencies, - TaC ceramic: one of the ceramics with the highest melting temperature, and a significant Bremsstrahlung conversion power due to its Ta content (metal with high Bremsstrahlung conversion power). [Tables 2] Substrates SiC Pads Diamond Cu W TaC Degradation temperature (°C) 2700 (Refll) (decomposition) 700 (ignition) 1084.6 (R efl5) (melting) 3422 (Ref 20) (melting) 3768(Ref 22) (melting) Density at ambient T ( g ■ cm3) 3.21 (Refl2) 3.50-3.53 (Refl8) 8.96 (Ref 20) 19.25 (Re f20) 14.3-14.65 (Ref20) Thermal conductivity at ambient temperature (W ffC Æ1) 280 (n-type) 347 (v-doped) (Refl3) 2400-2500 (Refl9) 386 (Refl 6) 176 (Ref2 D 26 (Ref23) Specific heat capacity at ambient temperature (J kg1- 551.7 (n-type) 734.1 (v-doped) (Refl3) 510 (Refl4 ) 380 (Refl 7) 133 (Ref2 D 256 (Ref2 3) Properties of the test materials constituting the converter

[0014] Design of prototypes: simulation of the obtained X-ray spectra

[0015] To illustrate the operation of our prototype, we superimposed 4 layers of support material (SiC, diamond, or Cu), with individual thickness 350^w / and square section 2mm x 2mm, etched by cylindrical plots (in W or TaC) with height 300^ÎW and diameter 3QQ^m, the minimum gap between the lateral surfaces of two neighboring cylinders being fixed at 100^OT.

[0016] We modeled the electron irradiation of such a structure, represented in [Fig.4], by a Monte-Carlo approach via the MCNPX code (Ref29).

[0017] For this purpose, we used a source of electrons with energy of 10 MeV, delivered via a monodirectional disk with a radius of 0.2 cm, placed 5 cm upstream of the converter. The maximum number of source particle histories was set at 40,000. For each simulation, we recorded the energy spectrum associated with the average surface flux tally of photons crossing the output surface of the last plate constituting the detector.

[0018] The obtained Bremsstrahlung spectra are plotted in [Fig.5], in comparison with a reference spectrum corresponding to that of a 2mm thick tungsten plate.

[0019] Among our different prototypes, the best Bremsstrahlung conversion efficiency is obtained with the Cu substrate with W pads (quantity of photons produced slightly higher than that relative to the 2mm plate of pure W). This high production of photons is explained by the combination of the very high conversion power of the W pads, coupled with the minor but non-negligible Bremsstrahlung conversion carried out by the Cu substrate. This excellent electron / photon conversion capacity is nuanced by the potential risk of melting of the Cu substrate (heating due to the intrinsic Bremsstrahlung of the substrate, and “low” melting temperature of 1084°C), which can nevertheless be corrected by an adequate cooling system.

[0020] In second place in the Bremsstrahlung conversion efficiencies are the prototypes based on ceramic substrate with W-plot implantation: the conversion is equivalent for the two substrates used (SiC and diamond), and it corresponds to approximately 85% of the efficiency obtained in the case of a 2mm thick pure W plate. This type of prototype has the advantage of coupling the high conversion efficiency of the W pads with the excellent heat dissipation capacity from the pads via the ceramic substrates (i.e. high thermal conductivity of the substrates, particularly for diamond: see Table 2). The limit remains the risk of cracking of the ceramic substrate during thermal expansion of the metal pads.

[0021] In third place in Bremsstrahlung conversion efficiencies are the prototypes based on ceramic substrates with implantation of ceramic pads: the conversion efficiency represents approximately two thirds of that obtained with a 2mm W plate. This lower conversion efficiency is nevertheless compensated by the excellent thermodynamic properties: very little thermal expansion of the ceramic pads (which greatly minimizes the risks of cracking of the ceramic substrate), high melting temperatures of the pads and substrates (very little risk of melting), and high thermal conductivity of the substrates (excellent capacity of the substrate to evacuate heat from the pads). Description of the figures

[0022] [Fig-1] Diagram of the conversion target prototype.

[0023] [Fig.2] Location of the conversion target within the device radiotherapy.

[0024] [Fig.3] Thermal expansion of the test materials constituting the converter (experimental data from (Ref24-Ref28)).

[0025] [Fig.4] Diagram of the structure modeled with the MCNPX code.

[0026] [Fig.5] X-ray photon spectra obtained. References

[0027] (Refl) J. Bourhis et al., Radiotherapy and Oncology 139 (2019), 18.

[0028] (Ref2) P. Montay-Gruel et al., Medical Physics 49 (2022), 2055.

[0029] (Ref3) J. Wang et al., Medical Physics 44 (2017), 12.

[0030] (Ref4) D. Badali et al., Physics in Medicine and Biology 64 (2019), 245007.

[0031] (Ref5) N. Esplen et al., Physics in Medicine and Biology 67 (2022), 105003.

[0032] (Ref6) A. Tsechanski, US Patent No. USOO972.1691B2.

[0033] (Ref7) Ascent+ platform, cf. website “ascent.network”.

[0034] (Ref8) G. Bertuccio et al., Nuclear Instruments and Methods in Physics Research Section A 652 (2011), 19.

[0035] (Ref9) M. De Napoli, Frontiers in Physics 10 (2022), 898833.

[0036] (ReflO) M. Angelone et al., Journal of Nuclear Engineering 2 (2021), 422.

[0037] (Refl 1) Website “alfa.com”, numéro 014165 du catalogue.

[0038] (Refl2) T. Kimoto and J. A. Cooper, Fundamentals of Silicon Carbide Technology: Growth, Characterization, Devices, and Applications, First Edition (2014), John Wiley & Sons Singapore Pte Ltd. Appendix C.

[0039] (Refl3) R. Wei et al., Journal of Applied Physics 113 (2013), 053503.

[0040] (Refl4) C. Moelle et al., Diamond and Related Materials 7 (1998), 499-503.

[0041] (Refl5) Procès-verbaux du comité international des poids et mesures, 78e session (1989).

[0042] (Reflô) J. Carvill, Mechanical Engineer’s Data Handbook (1994), Elsevier Ltd.

[0043] (Refl7) NIST Chemistry WebBook, SRD 69.

[0044] (Refl8) Website “mindat.org”.

[0045] (Refl9) J. E. Graebner, ‘’Thermal conductivity of diamond”, The Kluwer international sériés in engineering and computer science.

[0046] (Ref20) D. R. Lide, CRC Handbook of Chemistry and Physics, 90ème édition (2009), CRC Press.

[0047] (Ref21) M. Zhao et al., Journal of Nuclear Materials 470 (2016), 236.

[0048] (Ref22) O. Cedillos-Barraza et al., Scientific Reports 6 (2016), 37962.

[0049] (Ref23) B. Zhang et al., Journal of Materiomics 7 (2021), 672.

[0050] (Ref24) L. S. Dubrovinsky et al., Physics and Chemistry of Minerais 24 (1997), 547.

[0051] (Ref25) K. Wang et al., High Température and Materials Science 35 (1996), 2.

[0052] (Ref26) B.-S. Li, Master Thesis, South Carolina University (2013).

[0053] (Ref27) R. O. Elliott et al., The Journal of Physical Chemistry 62 (1958), 630.

[0054] (Ref28) R. R. Reeber et al., Journal of Electronic Materials 25 (1996), 63.

[0055] (Ref29) L. S. Waters et al., AIP Conférence Proceedings 896 (2007), 81.

Claims

Claims

1. Composite material target for linear particle accelerators, capable of converting a flow of electrons from the particle accelerator into a flow of X-rays, via the Bremsstrahlung effect. The composite material consists of a substrate material having undergone plot etching, and a material having a high Bremsstrahlung conversion power implanted within the cavities which result from the plot etching.

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