https://aapm.onlinelibrary.wiley.com/action/showCampaignLink?uri=uri%3A69545a18-c783-44d0-be73-eebc22001723&url=https%3A%2F%2Fbit.ly%2F3Hi8qNW&pubDoi=10.1002/mp.16333&viewOrigin=offlinePdf


Received: 6 October 2022 Revised: 27 January 2023 Accepted: 20 February 2023

DOI: 10.1002/mp.16333

TECHNICAL NOTE

Technical note: Measurement of the bunch structure of a
clinical proton beam using a SiPM coupled to a plastic
scintillator with an optical fiber

Miguel García Díez1,2 Andrea Espinosa Rodriguez1,2

Victor Sánchez Tembleque1,2 Daniel Sánchez Parcerisa1,2

Victor Valladolid Onecha1,2 Juan A. Vera Sanchez3 Alejandro Mazal3

Luis Mario Fraile1,2 Jose Manuel Udias1,2

1Grupo de Física Nuclear, EMFTEL &
IPARCOS, Universidad Complutense de
Madrid, CEI Moncloa, Madrid, Spain

2Instituto de Investigación del Hospital Clínico
San Carlos (IdISSC), Ciudad Universitaria,
Madrid, Spain

3Centro de Protonterapia Quironsalud,
Madrid, Spain

Correspondence
Andrea Espinosa Rodriguez, Grupo de Física
Nuclear, EMFTEL & IPARCOS, Universidad
Complutense de Madrid, CEI Moncloa,
Madrid, Spain.
Email: anespi04@ucm.es

Funding information
Comunidad de Madrid, Grant/Award Number:
B2017/BMD-3888; Spanish Government and
EU Regional Funds, Grant/Award Numbers:
RTI2018-098868-B-I00, RTC-2015-3772-1;
Spanish Ministerio de Educación, Cultura y
Deporte, Grant/Award Number: FPU18/02551

Abstract
Background: Recent proposals of high dose rate plans in protontherapy as
well as very short proton bunches may pose problems to current beam monitor
systems. There is an increasing demand for real-time proton beam monitoring
with high temporal resolution, extended dynamic range and radiation hardness.
Plastic scintillators coupled to optical fiber sensors have great potential in this
context to become a practical solution towards clinical implementation.
Purpose: In this work,we evaluate the capabilities of a very compact fast plastic
scintillator with an optical fiber readout by a SiPM and electronics sensor which
has been used to provide information on the time structure at the nanosecond
level of a clinical proton beam.
Materials and methods: A 3 × 3 × 3 mm3 plastic scintillator (EJ-232Q Eljen
Technology) coupled to a 3 × 3 mm2 SiPM (MicroFJ-SMA-30035, Onsemi) has
been characterized with a 70 MeV clinical proton beam accelerated in a Pro-
teus One synchrocyclotron. The signal was read out by a high sampling rate
oscilloscope (5 GS/s). By exposing the sensor directly to the proton beam, the
time beam profile of individual spots was recorded.
Results: Measurements of detector signal have been obtained with a time sam-
pling period of 0.8 ns.Proton bunch period (16 ns), spot (10 µs) and interspot (1
ms) time structures could be observed in the time profile of the detector signal
amplitude. From this, the RF frequency of the accelerator has been extracted,
which is found to be 64 MHz.
Conclusions: The proposed system was able to measure the fine time structure
of a clinical proton accelerator online and with ns time resolution.

KEYWORDS
fine time structure measurement, FLASH-RT, plastic scintillator fiber optic detector, RF frequency,
time resolution

1 INTRODUCTION

Radiation therapy (RT) has undergone significant tech-
nological advances in the last few decades, with proton
therapy leading the way for increased dose conformal-

This is an open access article under the terms of the Creative Commons Attribution License,which permits use,distribution and reproduction in any medium,provided
the original work is properly cited.
© 2023 The Authors. Medical Physics published by Wiley Periodicals LLC on behalf of American Association of Physicists in Medicine.

ity. New temporal patterns of dose modulation, such as
ultra-high dose rate (UHDR) treatments, are emerging
as a revolutionary tool to further increase the thera-
peutic ratio, sparing healthy tissues while maintaining a
similar efficacy for tumor control. This has been termed

Med Phys. 2023;1–7. wileyonlinelibrary.com/journal/mp 1

https://orcid.org/0000-0002-3258-6064
https://orcid.org/0000-0001-9804-2900
https://orcid.org/0000-0002-4153-4055
https://orcid.org/0000-0002-2245-9539
https://orcid.org/0000-0002-3594-9457
https://orcid.org/0000-0003-1391-8446
https://orcid.org/0000-0002-6281-3635
https://orcid.org/0000-0003-3714-764X
mailto:anespi04@ucm.es
http://creativecommons.org/licenses/by/4.0/
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2 BUNCH STRUCTURE WITH NOVEL DETECTOR

the FLASH effect.1 The differential biological effect of
extreme average dose rates (100 Gy/s) compared to
conventional ones (<0.1 Gy/s) aroused a huge interest
to monitor the irradiation structure up to the nanosecond
level.2

Furthermore, it has been shown that the onset of the
FLASH effect is deeply influenced by the beam pulse
structure (instantaneous dose-rate, dose-per-pulse or
the number of pulses) as it modifies the cell’s expo-
sure to free radicals.2–5 Therefore, fast and accurate
monitoring of the time structure of the beam is vital for
FLASH treatments, which challenges existing solutions.
Bunch monitors are also crucial for some range veri-
fication techniques in particle therapy that rely on the
prompt-gamma ray production within the patient.6

Conventional ionization chambers, which are typically
used in clinical practice, may exhibit ion recombi-
nation effects7 under FLASH irradiation conditions,
which can reach dose rates as high as 109 Gy/s2.
This compromises their use for online monitoring of
UHDR treatments,8 and correction factors need to be
considered.9 To deal with UHDR, the ultra-thin paral-
lel plate ionization chamber (UTIC) has been proposed
as an alternative solution.8 A first prototype has been
tested, which does not show ion recombination effects8

at dose rates up to 2.5⋅106 Gy/s8. However, the intrinsic
difficulty of fabricating this system and its low avail-
ability challenge its use. Recombination effects are less
significant with proton beams. In this case, acceptable
results have been obtained with commercially avail-
able models.10,11 Recently, other IC-based dosimetry
systems, including strip ionization chambers and multi-
gap ionization chambers, have been also successfully
proven to perform online FLASH beam monitoring.12,13

Since recombination effects depend on the temporal
structure of the proton beam,further studies are needed
to assess the performance of these systems under
different pulse structures.

Other detectors used in particle therapy and UHDR
dosimetry include radiochromic films14 or alanine and
thermoluminescence detectors.15,16 Although all of
them are dose-rate independent, do not provide online
information of the treatment. As an alternative, diamond
detectors have been proposed to perform dosimetry in
this area.17 Despite their favorable features, the detec-
tor response at these dose per pulse conditions is still
under investigation.18,19

Scintillators display unique features for UHDR mea-
surements, including excellent temporal resolution
(<<ns), dose and dose-rate linearity as well as high
radiation hardness.20,21 Moreover, they can be imple-
mented in a great variety of geometries and in several
dimensions.22,23 Some previous work has been done
with singular sensors capable of ultra-fast beam diag-
nosis in real time. However, most of them rely on an
indirect measurement of the beam, which is based
on the scattered photons,20 prompt gamma rays6 or
neutrons.24

Due to their composition, organic plastic scintilla-
tors are by nature water-equivalent,energy-independent
(down to of kV)25 and can be placed along the proton
beam path. Traditionally, plastic scintillators are cou-
pled to PMTs for beam monitoring measurements.26,27

However, SiPMs are more compact and have an easily
tunable dynamic range, while displaying similar capabil-
ities. Both the plastic scintillator and the SiPM can be
coupled by means of optical fibers. This allows us to
perform measurements in the control room, outside the
treatment bunker, thereby preventing radiation damage
to electronics. In addition, for clinical proton beams, the
contribution of Cherenkov radiation to the measured sig-
nal can be neglected and the signal will be solely due to
scintillation light.28,29

The use of scintillating fibers for online dosimetry has
been previously reported in different RT modalities.30–32

They exhibit several advantages over common com-
mercial devices, such as small size, ruggedness, ease
of use or high dynamic range. Not only do they pro-
duce scintillation light, but also they guide the produced
optical photons with negligible self -absorption. The reg-
istered signal is directly proportional to the energy lost
in the fiber, thus providing a direct measurement of
the beam, without the need of relying on secondary
radiation detection.

Recently, some experimental work has been done
in the field of online FLASH-RT dosimetry with
electrons33,34 and protons.35–37 Rahman et al.38 used
a 2D dosimetry system composed of a CMOS cam-
era and a scintillation matrix to solve pencil beam
scanning (PBS) protons with a temporal resolution of
10 ms. Another study by Kanouta et al.36 improved
this time resolution. Spot and transition durations in a
PBS proton FLASH treatment were measured using
four fiber-coupled inorganic scintillators. More recently,
they have shown the feasibility of this system to
perform time-resolved in-vivo dosimetry.37 However, in
both studies, the authors were limited by the sampling
rate of the readout method, 50 kHz, corresponding
to 20 µs. Casolaro et al.35 have also reported on
a small detector system with similar configuration as
the one proposed in this work. In that case, the
system was tested with proton FLASH beams and
using several fast photosensors, including SiPMs and
PMTs, but the minimum time resolution achieved was
100 ns.

In our work, we have evaluated the timing capabilities
of a novel plastic scintillator fiber optic detector coupled
to a SiPM and read out by an ultrafast data acquisition
system (DAQ). The main design requirements for the
clinical application of this prototype are direct exposure
to the proton beam, compact size, ease of use, radia-
tion hardness, while keeping superior (better than a few
ns) time resolution and a dynamic range spanning from
normal dose rate to UHDR conditions. The experiment
consisted in the irradiation of the small plastic scintilla-
tor with a 70 MeV proton beam and subsequent analysis

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BUNCH STRUCTURE WITH NOVEL DETECTOR 3

F IGURE 1 (a) Detector system used in the measurements: the plastic scintillator is coupled to a 3 × 3 mm2 SiPM via an optical fiber. (b)
Position of the detector and beam direction during the irradiations.

of the individual pulses with subnanosecond sampling.
This allowed us to determine the RF frequency and the
particle bunch structure of the clinical accelerator.

2 MATERIALS AND METHODS

2.1 Clinical facility

The experiment was carried out at the Quironsalud
protontherapy center (Madrid, Spain). The facility is
equipped with an IBA Proteus-ONE system,39 consist-
ing of a compact synchrocyclotron (S2C2) with a RF
frequency in the 60–90 MHz range.40 The single-room
proton therapy system delivers protons with energies up
to 230 MeV with a pulse repetition rate and width of
1000 µs and 10 µs, respectively.41

2.2 The detector system

The measurements were performed using an in-house
design, depicted in Figure 1a. A 3 × 3 mm2 SiPM
(MicroFJ-SMA-30035, Onsemi, formerly SensL)42 was
coupled to a 3 × 3 × 3 mm3 fast plastic scintillator
(Eljen Technology EJ-232Q) quenched with 0.5% of
benzophenone.43 The quencher renders the scintillator
must faster, with rise and decay times of 110 ps and
700 ps, respectively,44 at the expense of a reduced light
yield. Both elements were coupled using an assembly
of ten optical fibers with a diameter of 0.8 mm each
and a length of 63 cm. To improve the optical coupling
and fix the entire detector assembly, a layer of Norland
Optical Adhesive 61 (NOA-61) was applied between the
optical fibers and the SiPM and the plastic. Finally, a
standard black heat-shrinkable tubing was used to cover
the optical fiber and to protect the SiPM from external
light.

The SiPMs were biased with a programmable voltage
supply (Tenma 72–2550) at 28 V (22◦C) which war-
rants that the detector was not saturated during the
irradiations. The SiPM was mounted in a PCB board

TABLE 1 Time resolution (FWHM) for different SiPM42,47

+optical coupler+plastic scintillator measured from coincidences for
511 keV gamma photons from a 22Na source, using the methodology
described in a previous work.48

SiPM Fiber
Length
(cm)

Time
resolution
(ps)

S13360-3075CS Plastic coupled directly 58 (10)

MicroFJ-SMA-
30035

Plastic coupled directly 128 (4)

S13360-3075CS Set of fibers 60 638 (5)

MicroFJ-SMA-
30035

Set of fibers 60 715 (5)

S13360-3075CS Solid 10 176 (4)

S13360-3075CS Solid 60 329 (5)

Note: Contribution from the PMT+LaBR detector has been subtracted from the
total coincidence resolving time.

providing separate fast and slow signals, also from
Onsemi (MicroFJ-SMA-30035-GEVB).45 Both output
signals from the SiPMs processing board were acquired
by an 8-bit digital scope with a bandwidth of 350 MHz
(Picoscope 6403D) and a sampling rate of up to 5 Gs/s.
The time resolution of this assembly was tested with
the fast timing setup in our laboratory46 using 511-keV
photons from a Na-22 source in gamma-gamma coin-
cidence, considering Compton and photopeak regions.
It was found to be less than 715 ps, limited mainly
by the small scintillation light produced by the individ-
ual gamma photon in the plastic scintillator (Table 1).
These values are in good agreement with coincidence
resolving times of SiPMs coupled to plastic scintillators
reported in other works,44,46 and we conclude that they
are very well suited for the proposed measurements.

2.3 Experimental methods

Throughout each irradiation, the detector was placed on
the surface of the treatment couch perpendicular to the
beam direction, as shown in Figure 1b. The irradiations

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4 BUNCH STRUCTURE WITH NOVEL DETECTOR

F IGURE 2 (a) Signal intensity as a function of time for a sequence of 10 spots delivered at 1 kHz. (b) Individual measurement of a single
pulse. (c) 200 ns zoom of the fast output in the planar region of one pulse, indicated in Figure 2b. The signal shows a clear repetition pattern
with spikes every 16 ns. The FWHM of the proton bunch is of the order of 3.5 ns. (d) FFT frequency spectra of the accelerator RF signal, which
reveals an operating frequency of 64 MHz.

were made at the isocenter plane of a 70-MeV proton
beam (8 mm2 sigma spot size) with the beam current
set to 8.98 nA.

Time measurements of the beam were performed
with two different recording lengths. First, the beam sig-
nal was acquired for seven seconds with a sampling rate
of 4.76 MS/s (210 ns in between samples), which was
enough to resolve the 10 µs macropulses delivered at 1
kHz repetition rate.To resolve the time structure of these
10 µs proton pulses, the sampling rate was increased to
1.25 GS/s (0.8 ns). This high sampling rate allowed us
to overcome the limitations reported in previous studies,
consequently increasing the time resolution.36,37

The frequency spectrum of each pulse was obtained
using the Fast Fourier Transform (FFT) spectrum
analyzer tool available in MATLAB (R2021b). The anal-
ysis included a total of 2048 samples, with the sampling
period set to the sampling interval of 0.8 ns.

3 RESULTS AND DISCUSSION

Figure 2a illustrates the characteristic pulse structure of
the Proteus One accelerator in a 10-ms region taken
from the first measurement, acquired during seven sec-
onds. The 1 ms period has been extracted from it and
corresponds to the expected 1 kHz pulse repetition rate
of the accelerator.40 This serves as a verification of the

measurement procedures and time calibrations of the
setup. This time structure was repeated during the full
irradiation.

Figure 2b shows an individual acquisition of one
micropulse. Each pulse has a characteristic width of 10
µs and ripples are observed due to the accelerator RF
system.Figure 2c presents a 200 ns zoom over the inter-
mediate region indicated in Figure 2b, corresponding
to the fast signal. A clear repetition pattern with visi-
ble spikes can be observed. Examination of this region
yields a micro-bunch period of 16 ns and a bunch width
of about 3.5 ns FWHM.

The FFT frequency spectra resulting from the analy-
sis of 2048 consecutive samples (∼1.6 µs) in this region
is shown in Figure 2d. The analysis reveals a main peak
component at 64 MHz followed by other minor peaks
with decreasing intensities resulting from the contribu-
tion of higher harmonics. FFT analysis of the set of 32
individual pulse measurements demonstrates the same
characteristic frequency. This value is in the nominal RF
range of the S2C2/Proteus ONE accelerator, which is
between 60 and 90 MHz.40

Proton beam diagnosis systems based on scintilla-
tion dosimeters have been evaluated in silico22 and
experimentally49 in several dimensions, also reporting
nanosecond timing resolutions.New detector prototypes
with a similar configuration have also been developed in
the field of FLASH-RT with proton beams, and tests are

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BUNCH STRUCTURE WITH NOVEL DETECTOR 5

ongoing in several clinical facilities. Previous systems
have achieved sub-millisecond36,37 or sub-microsend35

temporal resolutions. In the current work, we have
improved this value by including a high bandwidth and
sampling rate digitizer and the signal was readout with
a sub-nanosecond (0.8 ns) frame rate.

Although we report on a proof-of -concept experiment,
the system can be readily implemented for online clini-
cal monitoring, especially for small-field and multipoint
dosimetry, which are typically used in FLASH-RT. To
ensure the clinical observation of the FLASH effect, its
reproducibility and translation, beam parameters must
be thoroughly measured in vivo. A recent work per-
formed by Ahsraf et al.50 using a point scintillator
detector has demonstrated this possibility in a murine
model,by measuring the number of pulses and the dose
per pulse. Moreover, if the exit dose is measured, the
procedure becomes minimally invasive.

Thanks to the mechanical flexibility of the com-
ponents employed in the current study, 2D and 3D
monitoring of the beam can be also readily implemented
to achieve spatial average dose-rate. This involves the
use of several probes36 covering the radiation field or
the use of a scintillator matrix.38 Spatial distribution
measurements are especially relevant for larger fields,
such as those achieved with spot scanning techniques,
where contributions from remote spots with lower dose
rates might affect the FLASH response.

4 CONCLUSION

In this work, we have shown the feasibility of a plas-
tic scintillator detector coupled to an optical fiber to
build a nanosecond time-resolved proton monitor. The
RF structure of a clinical accelerator has been mea-
sured in an individual proton pulse-by-pulse basis, with
a system time resolution of 0.8 ns, leveling other proton
bunch monitors based on plastic scintillators and bet-
ter than similar available detection systems proposed
in FLASH-RT. The signal has been FFT analyzed, and
the resulting frequency is in the range of the nominal
values of the accelerator. The system has an adaptable
dynamic range,with gain easily controlled in a very large
range, to avoid saturation even with very high dose rates.
Thus, it is very well suited to perform FLASH-RT online
monitoring with superior time resolution.

ACKNOWLEDGMENTS
This work was funded by Comunidad de Madrid
under project B2017/BMD-3888 PRONTO-CM “Pro-
tontherapy and nuclear techniques for oncology”
and by the Spanish Government and EU Regional
Funds (RTI2018-098868-B-I00, RTC-2015-3772-1). A.
Espinosa Rodriguez has been funded by a FPU predoc-
toral fellowship of the Spanish Ministerio de Educación,
Cultura y Deporte (FPU18/02551). This is a contribution

for the Moncloa Campus of International Excellence,
“Grupo de Física Nuclear-UCM”, Ref. 910059. Part of
the calculations of this work were performed in the
“Clúster de Cálculo para Técnicas Físicas”, funded in
part by Universidad Complutense de Madrid and in part
by EU Regional Funds.

CONFL ICT OF INTEREST STATEMENT
The authors have no conflicts of interest to disclose.

ORCI D
Miguel García Díez
https://orcid.org/0000-0002-3258-6064
Andrea Espinosa Rodriguez
https://orcid.org/0000-0001-9804-2900
Victor Sánchez Tembleque
https://orcid.org/0000-0002-4153-4055
Daniel Sánchez Parcerisa
https://orcid.org/0000-0002-2245-9539
Juan A. Vera Sanchez
https://orcid.org/0000-0002-3594-9457
Alejandro Mazal
https://orcid.org/0000-0003-1391-8446
Luis Mario Fraile
https://orcid.org/0000-0002-6281-3635
Jose Manuel Udias
https://orcid.org/0000-0003-3714-764X

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How to cite this article: García Díez M,
Espinosa Rodriguez A, Sánchez Tembleque V,
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structure of a clinical proton beam using a SiPM
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	Technical note: Measurement of the bunch structure of a clinical proton beam using a SiPM coupled to a plastic scintillator with an optical fiber
	Abstract
	1 | INTRODUCTION
	2 | MATERIALS AND METHODS
	2.1 | Clinical facility
	2.2 | The detector system
	2.3 | Experimental methods

	3 | RESULTS AND DISCUSSION
	4 | CONCLUSION
	ACKNOWLEDGMENTS
	CONFLICT OF INTEREST STATEMENT
	ORCID
	REFERENCES