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Development of an ultra-thin monitor for charged particle beams – PEPITES

PEPITES: a beam monitor thinner than a hair for proton therapy and the fight against cancer

High-performance, robust and reliable monitors ensure the quality of radiotherapy by proton beam used in cancer treatment. During the irradiation of the patient, the monitors are continuously crossed by the beam and must be as thin as possible so as not to disturb it; they must also have excellent radiation resistance to last over the long term. PEPITES offers an innovative approach in this area.

An ultrathin detector not to disturb the beams and tolerant to radiation

Proton therapy is a cancer-fighting technique using proton beams, which allows better preservation of healthy tissue than conventional radiotherapy while remaining effective in destroying tumours. However, during the treatment, the correct delivery of the dose requires continuous and precise measurement of the intensity, position and shape of the beam used during irradiation.<br />By crossing the thicknesses of material of a monitor, the beam undergoes a lateral spread which must remain less than one millimeter at the patient level to be acceptable. For monitors located a few meters upstream from the latter, this constraint limits the material budget to a thickness equivalent to less than 15 micrometers of water (the “water equivalent” thickness is a reference unit in radiotherapy). Moreover, a continuous control of the beam parameters requires the permanent presence of the monitor in the line, implying a good resistance to radiations.<br />The thinner a detector, the less it will disturb the beam, thus ensuring the quality of the treatments. Radioresistant, it will be able to remain operational for longer without having to be replaced, thus avoiding a stoppage of treatments in progress and making it possible to increase their number in a proton therapy center.<br />The existing monitors are ionization chambers. They consist of a volume of gas held between two membranes that must be as thin as possible, while still being able to sustain the gas pressure in the vacuum of the beam transport line. These systems are well known but suffer from limitations: on the thinness (the pressure of the gas imposes sufficiently resistant membranes), on the dynamic range (with problems of saturation in the gas), and on the resistance to radiation (these weaken the membranes over time), requiring preventive replacement.<br />The PEPITES project aims to produce and characterize a brand new operational prototype of an ultra-thin profiler, resistant to radiation, capable of operating continuously on charged particle accelerators used in the medical field. It aims to install this prototype at the ARRONAX cyclotron (Saint-Herblain), a facility that delivers proton, deuteron and alpha beams for the generation of radioelements in medicine and medical research.

Designing an ultra-thin detector means generating a measurement signal with very little material. The phenomenon of secondary electron emission allows this. When a charged particle passes through a medium, it extracts electrons from it by ionization. Those very close to the surface – less than ten nanometers from it – can leave the medium. If the latter is conductive, the departure of these electrons generates a current: this is the PEPITES measurement signal. The emission must take place in a vacuum, otherwise the electrons return to the emitting medium, canceling the signal. This monitor operating mode is suitable for an installation in the vacuum of the beam transport line. In addition, this characteristic has an advantage for resistance to radiation: since no element of the system undergoes mechanical stress, the alterations by radiation are of much less consequence than for ionization chambers and the lifetime of the system is increased. This phenomenon of secondary electron emission is well known. Very linear, it will allow the detector to operate over a large dynamic range.
Building a system using only the thicknesses of material necessary for this phenomenon requires producing objects as thin as a few tens of nanometers and the field of thin layers offers such possibilities. We then have the appropriate tools to build PEPITES.
The lateral profile of the beam is reconstructed using segmented electrodes: gold strips 50 nm thick are deposited by vacuum evaporation on an insulating substrate 1.5 microns thick. As it passes through the gold, the beam ejects secondary electrons, and the current thus formed in each band makes it possible to measure its lateral profile. In order to guarantee that the electrons do not return to their place of emission, a solid electrode, also thin, is placed opposite the strips and collects the electrons thanks to the application of an electric field. The substrate, which we want to be as thin and resistant as possible, is not an ordinary plastic, but a film produced for the manufacture of solar sails. An electronic chip, developed especially for the project, makes it possible to read these signals, the intensity of which can be as low as a few femtoamperes, i.e. millionths of a billionth of an ampere ! Particular care has therefore been taken to transport this signal from the gold strips in contact with the beam to the readout electronics. Finally, a computer based processing of the currents read by the chip provides the properties of the beam.
For the realization of this prototype, we have selected materials known for their intrinsic tolerance to radiation. This was tested on the sub-parts of the system undergoing the passage of the beam, using beams of electrons, protons at different energies, and gammas.

A complete prototype has been built and put into service at the ARRONAX cyclotron. PEPITES, with a final «water-equivalent« thickness of less than 10 micrometers (only one-fifth the diameter of a hair), successfully measured the beam parameters over a large dynamic range, validating the monitor and paving the way for multiple variations. Radiation tolerance studies have shown that operation for several years at ARRONAX or in proton therapy centers is possible.

The project is now entering a long-term behavior study phase, thanks to routine use at ARRONAX.
PEPITES is already of interest to the CNAO treatment center in Italy, in Pavia, and an agreement with the CNRS is being written. If PEPITES meets the needs of this center, it will be used clinically there.
Since the beginning of the project, in 2017, we witness the emergence of the new “FLASH” modality, which consists of depositing the therapeutic dose in a single, very brief and very intense irradiation. This technique, which significantly better preserves healthy tissue while maintaining tumor control, imposes new challenges on monitors: they must be able to measure short (a few ms or even less) and very intense beams (at least thousands times more than those currently used). PEPITES, with its large dynamic range and its extreme thinness which limits overheating problems, has the assets to meet these challenges and to become a major player in new FLASH therapies.

Five articles following conferences (multi-partner), peer-reviewed:

o First results of PEPITES a new transparent profiler based on secondary electron emission for charged particle beams
- C. Thiebaux et al.
- Proceedings of IBIC2022, Kraków, Poland - Pre-Press Status 15-September 2022
- ibic2022.vrws.de/papers/mop21.pdf
- Poster
- IBIC 2022, International Beam Instrumentation Conference, 11-15 September 2022 (Kraków, Poland)

o Development of a transparent profiler based on secondary electrons emission for charged particle beams.
- C. Thiebaux et al.
- Proceedings of 22nd International Conference on Cyclotrons and their Applications, Cyclotrons 2019 (Cape Town, South Africa)
- jacow.org/cyclotrons2019/papers/thb04.pdf
- Invited talk
- 22nd International Conference on Cyclotrons and their Applications, Cyclotrons 2019 (Cape Town, South Africa)

o A new transparent beam profiler based on secondary electrons emission for hadrontherapy charged particles beams.
- C. Thiebaux et al.
- Physica Medica, Elsevier, 2019, 68, pp.42.
- doi.org/10.1016/j.ejmp.2019.09.149
- Poster
- 58èmes Journées Scientifiques de la Société Française de Physique Médicale 2019 (Angers, France)

o Exploring radiation hardness of PEPITES, a new transparent charged particle beam profiler.
- S. Elidrissi-Moubtassim et al.
- Nucl. Inst. Meth. (2020), 466, pp.8-11.
- dx.doi.org/10.1016/j.nimb.2020.01.003
- Poster
- 13th European Conference on Accelerators in Applied Research and Technology 2019 (Split, Croatia)

o Development of an ultra-thin beam profiler for charged particle beams.
- B. Boyer et al.
- Nucl. Inst. Meth. (2019), 936, pp.29-30.
- doi.org/10.1016/j.nima.2018.09.134
- Poster
- 14th Pisa Meeting on Advanced Detectors 2018 (Isola d’Elba, Italy)

Participation without articles in conferences (multi-partner):

o Observation multi-technique de l’endommagement de polyimides pour un nouveau profileur faisceau ultra-mince
- 8ème Rencontre «Ion Beam Applications Francophone« - IBAF 2021, Société Française du Vide 2021 (distanciel, France)
- Poster
- G. Pipelier on behalf of PEPITES consortium

o A new transparent beam profiler based on secondary electrons emission for hadrontherapy charged particles beams.
- 58th Annual Conference of the Particle Therapy Co-Operative Group 2019 (Manchester, England)
- Poster
- C. Thiebaux on behalf of PEPITES consortium

o Développement d’un profileur transparent à électrons secondaires pour faisceaux de particules chargées.
- Journées SFP Accélérateurs 2019 (Roscoff, France)
- Poster
- M. Verderi on behalf of PEPITES consortium

Participation manifestation grand public (monopartenaire) :

o Jeudis de la recherche de l’X
- M. Verderi
o Fête de la science Ecole polytechnique


o A patent on the technique was filed in 2017 and accepted in January 2020.

The project aims at realizing a fully working ultra-thin monitor prototype able to permanently operate on mid-energy (O(100 MeV)) charged particle accelerators. The targeted beam intensity ranges from a fraction of pA to about 10 nA. The active part of the prototype is built using thin film techniques and a low noise electronic provides the readout. The system itself is very simple to operate, has a small footprint and is expected to be more tolerant to cumulated doses than existing competing devices. The thinness of the materials employed makes it also less prone to heating from beam interaction. The development was initially motivated by the proton therapy needs, but the large flexibility of the techniques employed opens a range of applications that goes well beyond the medical needs. If particularly adapted in the energy and intensity ranges mentioned, the approach is not limited to these, being applicable well above the higher energy and intensity bounds indicated.
The Arronax center has expressed its interest for a 10 µm water-equivalent system to be operated for proton, deuteron and alpha beams in energy ranges 17-70 MeV, 8-17 MeV/u and 17 MeV/u for protons, deuterons and alphas, respectively, in the foreseen intensity range. Achieving a working prototype responding to these constraints would demonstrate the viability of the approach.
The project will proceed in four main stages. The signal generation will be studied with a simple set-up in the targeted energy ranges and in the medical one, up to 230 MeV, as existing measurements are quite scarce. The radiations hardness of the active part -its individual components and their assembly- will be studied to optimize the material choice and construction techniques and to additionally anticipate the operation time that a future system could tolerate. This will be conducted under an uniform protocol to allow direct comparison of merits. A dedicated low-noise electronic will be designed and realized. It will provide the readout for operations over about 5 orders of beam current magnitude. The final system performances will be studied in real operation conditions with proton, deuteron and alpha beams.
The project aims at proposing a patent on the technology developed.

Project coordination

Marc Verderi (Laboratoire Leprince Ringuet)

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.


Arronax GIP Arronax
IRFU Institut de Recherche sur les lois Fondamentales de l'Univers
LLR Laboratoire Leprince Ringuet

Help of the ANR 355,796 euros
Beginning and duration of the scientific project: September 2017 - 36 Months

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