FlashDC, A fluorescence-based beam monitor for Ultra-High dose rates

Vincenzo Patera

Università di Roma “Sapienza”

&

INFN

Legnaro 12 May 2025

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FLASH therapy & beam monitoring

The FLASH effect is emerging as a new paradigm, with an increasing number of experiments underway, new machines being commissioned, and new theories developing.

Beam monitoring is a cornerstone of this research and must provide reliable assessment of beam parameters, which are sometimes extreme.

The FLASH radiotherapy environment poses significant challenges to beam monitoring detectors.. The activity in the field, both in the upgrade current technology or in the development of new detectors has been impressive in the last years.

FLASH beam monitoring

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• At FLASH intensities, instantaneous dose-rate and dose per pulse can be orders of magnitude higher than in conventional radiotherapy, leading to issues of saturation and/or non-linear response of standard dosimeters.

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The ideal device should feature:�• Dose rate linearity (up to 10⁵ Gy/s)�• Real-time, in-beam monitoring�• No energy dependence�• No material obstructing the beam�• Spatial resolution on the millimeter scale

Ashraf MR et al, “Dosimetry for FLASH Radiotherapy”doi: 10.3389/fphy.2020.00328

Air fluorescence and .. EAS detector

• Air fluorescence detector are largely used in very high energy EAS obstervation
• The air is excited by the EAS and the fluorescence is collected by a large mirror and then focused on a light detector composed by PMTs
• This mechanism, which uses AIR as active medium, can in principle be applied also to FLASH beam monitoring…

The need of very low budget, low signal, high linearity made us look in other fields of physics (the best idea is always by someone else.. )

Fluorescence & FLASH beam monitor

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What about the air (nitrogen) fluorescence as radiation detection mechanism? Conceptually, a fluo-based BM is an empty box filled of air, with walls of thin black mylar with a light read-out at the end (maybe PMT?)

The beam excites the air that produces light collected by the readout (PMT)

Appealing features:

• No energy threshold
• Photons emitted isotropically
• Signal emitted within ~ns
• Simple and cheap to produce
• Minimal impact on the beam
• Range limitation by UV filter

However:

• Not yet well measured
• …. Does it work???

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Fluorescence and FLASH BM

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• In air, fluorescence occurs mainly on the nitrogen molecule.
• Nitrogen fluorescence has a lifetime of around 10⁻⁸ s, with emission spectrum that nicely fits in a standard photo-sensor response window (es: PMT)

The wavelength spectrum is known, and fits nicely with the response of standard photosensors.

Fluo for FLASH monitoring.. a journey!

2022

2023

2024

2025

First assesment @FLASH beam

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Testing has been done on the (easily) available sources of beams with UHDR intensities: low energy (6-12 MeV) electrons usually used for intra-operative applications or dedicated to FLASH study

LIAC-HWL

Energy: 12 MeV

Dpp: 0.3 Gy

ElectronFlash

Energy: 7-9 MeV

Dpp: up to 20 Gy

We started with very basic exp setup….

First: proof of principle..

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First prototype: a volume of 7x7x90 cm³ of air, enclosed by a thin layer of Teflon sheet, with a PVC supporting structure and two PMTs on the opposite squared faces.

• LIAC HWL: linac for intra-operative electron RT modified to reach 10¹⁰ electrons/pulse.
• Electron energy: 6MeV.
• Pulse duration: 2 µs.
• Dose per pulse ~ 0.3 Gy.
• Large area beam (5-10 cm diameter )
• FLUKA MC simulation of accelerator delivery and the detector

Inside

Reflecting

Inside

Absorbing

Looking for the fluorescence signal

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• The results confirmed the expected signal sensitivity to the detector position with respect to the beam.

Centered

Moved 30 cm along length

Off beam

PMT collected signals

MC simulations (FLUKA 2021) used to evaluate signal and background and ratio of optical photons on the PMTs when the beam moves along the bar.

Next step: intensity sensitivity

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• ElectronFlash: up to 10¹² electrons/pulse.
• Electron energy at the linac exit: 7MeV.
• Dose(-rate) per pulse: up to 20 Gy (5*10⁶ Gy/s).
• Field diameter: 5-6 cm at BEW

• The linearity of the fluorescence VS beam intensity has been tested @ ElectronFlash by S.I.T., an electron LINAC developed specifically to perform FLASH studies

PDDs of electrons delivered were measured using flash Diamond detector and compared to FLUKA MC simulation).

Moving around the detector

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The active media of the prototypes was an air volume, with dimensions : 2x2x60 cm³

HV: 550 V

HV: 850 V

Two PMTs on both ends equipped with UV filters

studied both position and charge sensitivity.

Moving along Y

Near-far inside the beam

Moving along X

(inside-outside the beam)

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Data vs MC simulation

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• The comparison is quite fair, but some beam parameters are not correctly modeled in the simulation, like beam secondaries and uncertainties in the energy and angular divergence
• The background modeling is the culprit of the MC simulation in the low signal region.
• The presence of this background not under control triggered an optimization of the geometry.

MC simulation has been developed to model the detector response.

FLUKA 2021

Beam window border

Beam window center

Towards a true FLASH beam monitor

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• Next step is to prove that the linearity of signal vs beam current is really due to fluorescence => subtract background.
• The detector design has been directly tailored to the Beam Exit Window dimensions. is

readout

Towards a true FLASH beam monitor

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• Next step is to prove that the linearity of signal vs beam current is really due to fluorescence => subtract background.
• The detector design has been directly tailored to the Beam Exit Window dimensions. is

PMT inside

active volume mounted on the beam exit window.

N.B.: no material on the beam line! Just air…

• The active volume is the air immediately after the BEW, enclosed in a cylindrical case. A sliding leaf on the external face can be closed and opened for background measurement.

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PMT is wrapped in a plastic shield with thickness of 2 cm, at 1.2 m from the beam exit window

UHDR beam monitoring

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FLUORESCENCE+BKG

BKG

FLUORESCENCE+BKG

BKG

FLUORESCENCE+BKG

BKG

[Gy/pp]

[Gy/pp]

S/N can be largely optimized → dedicated readout system in development!!!

UHDR beam monitoring

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FLUORESCENCE+BKG

BKG

FLUORESCENCE+BKG

BKG

FLUORESCENCE+BKG

BKG

S/N can be largely optimized → dedicated readout system in development!!!

[Gy/pp]

[Gy/pp]

FLASH beam monitoring

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FLUORESCENCE-bck

Beam current 1.5 Gy/pp

Beam current 0.9 Gy/pp

Beam current 2.3 Gy/pp

• Fluorescence signal linearity as a function of the beam current is confirmed!
• The statistics is very low… only 30 pulses have been acquired (for each current).

• 3% syst. err on current
• 3% syst.err + stat.err on charge

[Gy/pp]

Beam monitoring @SPES intensity…?

Fluorescence technique could be safely applied to the low end of the intensity range of SPES… application to high end is uncertain

• FlashDC showed no saturation at the maximum intensity/energy available at electron FLASH machine→ higher intensity can be explored
• FlashDC prototype seems to suffer from low signals with respect to bck→ could gain S/N at higher intensities
• Proton beam with reduced transverse size opens to transverse position accuracy measurement using dedicated readout
• However, several unknown need to be explored:
• Fluorescence saturation?
• Background effect?
• ….?

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🤷‍♂️❓🤷‍♂️❓🤷‍♂️❓

HIC SUNT LEONES…

BTW, good ideas have a lot of fathers…

Of course we discovered the the effect was already observed in 2013 not only in EAS physics, but also in radiotherapy!!!

Seeing the invisible: Direct visualization of therapeutic radiation beams using air scintillation

Benjamin Fahimian et al Medical Physics, Volume: 41, Issue: 1, 2013, DOI: (10.1118/1.4851595)

And now, back from FLASH to EAS…

• SPES can have a charged particle density that can mimic the highest EAS energy (10²¹ Ev)
• A tailored FLASH-DC detector can measure the Fluorescence Yeld induced by very high Energy EAS

Courtesy of Marvin Gottowik

Fluorescence Yeld experiments

AIRFLY is the current standard for most major UHE EAS observatories

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Today, the overall FY uncertainty propagates to about 7–8% energy uncertainty

Improving it to 3–5% total would require new experiment carefully simulating atmospheric conditions all along the shower.

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No hadron beam!!

EAS structure

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The EAS shower is not made only by electron but has a hadron core contributing to the fluorescence

Exp@SPES could Improve FY meas?

A FD measurement useful for High Energy EAS the measurement could:

• Extend pressure and humidity range:�Test very low pressure and very dry/wet air to simulate full EAS altitudes realistically (down to 100 hPa).
• Measure heavier gas admixtures:�Air is not 100% N2 and O2 — minor gases (Ar, CO₂) could slightly affect fluorescence.
• Wavelength-resolved FY at all conditions:�Some detectors are sensitive across 300–400 nm. Better mapping of spectrum vs conditions would reduce systematic error.
• Direct ion beam excitation:�Instead of secondary electrons, use light ion beams (p, He) to study more realistic energy deposition profiles of the inner hadronic core.
• Nonlinear effects at high ionization densities:�Study whether high particle densities (core of showers) suppress FY

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Peculiar

@SPES??

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Summary and conclusions

• Air fluorescence can be used as detection principle for FLASH beam monitoring

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• Careful geometry and readout optimization needed to deal with background of FLASH beams

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• The functionality of such a technique could be explored at SPES with UHDR proton beams

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• A FlashDC could be used @SPES to refine the measurement of the air fluorescence for UHE EAS

Thanks!!!

Position sensitivity test

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AGGIUNGENDO QUI:

• sole
• cuore
• amore
• ..
• cacca
• pipì

For this test we positioned the BM near the LINAC nozzle so to have a beam size of the order of ˜cm. Then we moved the beam position along the detector

The PMT charge has a nice sensitivity with respect to the beam position…

PMT

PMT

Left PMT

Beam

Preliminary tests

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Centered

Off beam

PMT collected signals

Moved 20 cm along length

• 1st question: can we observe in-beam/off-beam difference?
• Experimentally, we move the device 10 cm away from the beam axis.
• A FLUKA* MC simulation estimates a reduction of optical photons of a factor 10³.

• 2nd question: what if one of the PMTs is farther from the aperture?
• With a 20 cm shift, FLUKA* estimates a reduction of optical photons of a factor 2 with respect to the configuration with the beam shooting at the center of the device.

Schulz