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Portable multi-leaf Faraday cup for quality assurance in radiation therapy with charged particles


Based on a multi‑leaf Faraday cup – developed specifically for PTB’s research electron accelerator – which is used to determine the energy of high‑energy electron radiation, a new, enhanced model has now been completed, calibrated and successfully tested. Compared to its predecessor, the new device has been optimized for the electron energies relevant to radiation therapy. In addition, it is portable. It has also been successfully tested for use in ion beams for radiotherapy.

For quality assurance in radiation therapy with high‑energy electrons, protons or carbon ions, the beam energy has to be routinely validated. In clinical practice, the dose is therefore recorded in a water phantom as a function of the penetration depth, since this depth depends on the energy. These measurements are very time‑consuming and require expensive equipment.

However, the range of high‑energy charged particles such as electrons or ions can also be measured by means of a multi‑leaf Faraday cup [1, 2]. Here, the initial distribution of the charge deposited by the beam in a solid is determined as a function of the penetration depth. The corresponding energy can be determined from such charge distributions.

An MLFC detector was developed specifically for PTB’s research electron accelerator to be able to determine the energy in real time while the settings are being adjusted and while an electron beam of a defined energy and fluence is being prepared or optimized [3 - 5]. Based on this work, an enhanced MLFC was completed, calibrated and tested last year. Compared to its predecessor, the new device has been optimized for the electron energies relevant to radiation therapy. In addition, this new device is portable. Figure 1 shows the new MLFC detector in front of the electron applicator of a medical linear accelerator.

Multi-leaf Faraday cup detector

Figure 1: New, portable MLFC detector in front of a medical linear accelerator.

This new detector design also allows the energy of proton and carbon ion beams to be determined precisely and in real time in the energy range relevant to ion beam therapy. PTB’s new detector is equipped with stand‑alone read‑out electronic device with a display. This device can also be remote‑controlled via LAN (Figure 2).

read-out device

Figure 2: Stand‑alone read‑out device of the new portable MLFC.

The MLFC detector mainly consists of 128 galvanically insulated aluminum plates that are stacked vertically to the angle of incidence of the charged particles. The aluminum plates act like capacitors and store the charge that has reached the respective plate until their sequential readout is performed by the read‑out electronic device to determine the charge quantity. The beam energy is determined from the distribution of the charge on the plates (that is, in the 128 corresponding read‑out channels). The thickness of the plates was dimensioned in such a way that the charge distributions of electron beams with energies relevant to clinical applications (that is, between 3 MeV and 25 MeV) can be determined with good resolution.

The new, portable MLFC was calibrated with monoenergetic electron beams at the output of a 180° magnetic spectrometer at PTB’s research electron accelerator. For this purpose, charge distributions were recorded as a function of the known beam energies. Figure 3 shows the charge distributions (normalized to the maximum) as a function of the plate number, where #1 is the first plate and #128 the last.

diagram charge distributions electron beam energies

Figure 3: Charge distributions for different electron beam energies: charge normalized to the maximum as a function of the plate number.

In cooperation with MedAustron, the center for ion therapy and research in Wiener Neustadt (Austria), the new, portable MLFC system was also tested for use in clinical ion beams. Figure 4 shows measured charge distributions for different proton energies within the range relevant to proton therapy. Contrary to electron beams, ions that have the same primary energy all stop at the same material depth, so that the charge collected by the MLFC comes from few plates only. Since the range of ions at energies relevant to proton therapy is larger than the entire aluminium plate stack of the MLFC, a 3 cm thick aluminium plate was placed in front of the detector as a range modulator. For energies larger than 160 MeV and 220 MeV, 9 cm and 15 cm thick aluminium plates were used, respectively.

diagram charge distributions proton energies

Figure 4: Charge distributions at different proton energies when using a 3 cm thick aluminum plate in front of the MLFC detector as a range modulator.

The innovations described above allow the energy and the charge of each of the synchrotron spills (5-second ion pulse) to be recorded in real time. In principle, it should be possible to calibrate the position of the charge maximum in the MLFC against a measurement of the position of the dose maximum (Bragg peak) in the water phantom. With the portable MLFC, it would be possible to perform measurements to validate the beam energy for quality assurance very fast and without much effort. In cooperation with PTB, MedAustron is planning to install a permanent MLFC detector to replace the existing beam dump to be able to perform “parasite” measurements to monitor the energy stability while a patient is being treated.


[1]        B. Gottschalk et al., “Nuclear interactions of 160 MeV protons stopping in copper: A test of Monte Carlo nuclear models”, Med. Phys. 26 (1999) 2597-2601. https://doi.org/10.1118/1.598799

[2]        K. Nesteruk et al., “Measurement of the Beam Energy Distribution of a Medical Cyclotron with a Multi‑Leaf Faraday Cup”, Instruments 2019, vol. 3, p. 4. doi: 0.3390/instruments3010004

[3]        C. Makowski und A. Schüller, “Development and calibration of a Multi-Leaf Faraday Cup for the determination of the electron beam energy of PTB’s research electron accelerator in real time”, Scientific News 2019

[4]        C. Makowski and A. Schüller, “Development and Calibration of a Multi‑Leaf Faraday Cup for the determination of the Beam Energy of a 50 MeV Electron LINAC in real‑time” in Proceedings of IBIC2019, Malmö, Sweden, MOPP004; https://doi.org/10.18429/JACoW-IBIC2019-MOPP004

[5]        Zum Patent angemeldet: PCT/EP2019/065254

Contact persons:

Opens local program for sending emailC. Makowski, Department 6.2, Working Group 6.21

Opens local program for sending emailA. Schüller, Department 6.2, Working Group 6.21