VCI - V. Conti February,19-24 2007 1

A TIME OF FLIGHT DETECTOR FOR THERMAL NEUTRONS FROM

RADIOTHERAPY LINACS

V. Conti, G.Bartesaghi, D.Bolognini, M.Prest, S.Scazzi, P.Cappelletti, M.Frigerio, S.Gelosa, A.Monti, A.Ostinelli,

G.Giannini, E.Vallazza, A.Mozzanica

VCI - V. Conti February,19-24 2007 2

OUTLINE

• The Boron Neutron Capture Therapy and its future in a hospital environment

• A novel TOF detector : description and comparison with standard neutron detectors

• Spectra and flux measurements with a Clinac 2100

• Simulation developments and comparison with data

• Conclusion

VCI - V. Conti February,19-24 2007 3

RADIOTHERAPY

The radiotherapy goal local control of the primary tumour

Ideal situation : a large amount of energy deposited in the tumour volume and none in the surrounding healthy tissue

Radiotherapy beams

Electrons for superficial or semideep cancers

Photons are useful to treat deep-seated tumours that is tumours located several

cm below the skin surface

DEPTH IN WATER (cm)

VCI - V. Conti February,19-24 2007 4

Although the percentage of survival is increased in the years, some tumours have a very low five-year survival probability

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For localized tumours Hadron therapy

BRAGG PEAK

Hadron radiation :

Negative pions, protons and light ions (e.g. helium, carbon, neon, silicon and argon nuclei)

The dose increases with penetration depth from a low dose at the entrance to a sharp maximum at the end of the particle range

Depth in water (cm)

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PROTONX-RAYS

HADRON THERAPY ADVANTAGE

but this treatment cannot be used for some kinds of

tumour like

Extended tumours (liver, stomach or lung) Radio-resistant ones (melanoma)

Tumours surrounded by vital organs (brain)

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The BNCT : Boron Neutron Capture therapy

The BCNT is an oncology radiotherapy treatment that exploits the capture of thermal neutrons by 10B and the following emission of an α particle and a nucleus of 7Li

Four years after the discovery of neutrons in 1932 by Chadwick, Locker introduced the concept of BNCT

VCI - V. Conti February,19-24 2007 8

specific drugs (BSH or BPA) are being developed for this purpose

boron is delivered mainly in the tumoral cells

Irradiating tumor cells with a thermal neutron beam thus producing heavy charged particles releasing their whole energy in the cell

High damage only in the target cells

healthy celltumor cell

10B

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The BNCT requires:

• a high thermal neutron flux (> 5x108 n cm-2 s-1)

• with a low energy neutron spectrum (neutron energy < 10 keV)

Up to now such a beam is produced only in nuclear reactors

a reactor would be too expensive and too dangerous to be put inside a hospital

to develop a neutron source that can be used inside a hospital for BNCT treatments

PHONES PROJECT

AN IDEA

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PhoNeS (PHOton NEutron Source)

The aim of the PhoNeS project is to provide a competitive thermal neutron source using clinac radiotherapy Linacs, maximizing with a dedicated photo-neutron converter the neutrons produced by Giant Dipole Resonance (GDR) by a high energy photon beam (> 8 MeV)

photo-neutron converter (PhoNeS)

Accelerator head (Clinac 1800, 2100)

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• high energy photons hit a high Z target

• fast neutrons are produced by GDR

• neutrons are slowed down by a moderating material

• neutron capture inside the device should be kept low

• with as low as possible gamma field

To obtain a Photon Neutron Source useful for medical purposes the following requirements have to be fulfilled:

• a thermal neutron flux greater than 108 n cm-2 s-1

• a fast neutron and gamma dose per thermal neutron lower than 2x10-12 Sv cm2 n-1

Giant Dipole Resonance

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Different prototypes and simulation have been developed to optimize the following parameters:

Photoconverter lead thicknessModerator shape

and thickness

Box of moderator material (D2O)

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MCNP-4B GN code

Simulation WITH and WITHOUT PhoNeS prototype

without PhoNeS with PhoNeS

34 % slow 76 % slow

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In this framework a development of neutron detection methods to evaluate neutron and gamma doses is of primary importance

COMMERCIAL NEUTRON DETECTORS:

TLD• passive detector, it integrates the flux in a certain energy range over the whole measurement duration

• not real time, it needs a long calibration procedure, an annhealing cycle before the use and after the readout

• also sensitive to photons

• high dynamic range

BUBBLE DOSIMETERS• integrated measurement over time and energy

• not real time; they can be used only with low doses; the counting of the bubbles can introduce large systematic errors

• poor spatial resolution

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BF3 COUNTERS

• real time readout

• not tissue equivalent

• geometry constraints

MATERIAL ACTIVATION

• integrated measurement over time and energy

• a Ge or NaI detector nearby is needed

• Usually one sample per time. Almost real time

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BEAM TIME STRUCTURE

Electrons (as well as secondary photons) are emitted in a 3-5 µs wide bunch at a 150-300 Hz repetition rate (dose rate dependent)

It means up to tens ms “empty” time between two following bunches

In this time interval there are no more electrons or photons in the environment, only thermal neutrons survive

a 0.025 eV neutron has a speed of 2.2 mm/µs

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We are developing a real time neutron detector made of a scintillator coupled to a dedicated readout electronics

Plastic scintillators are low cost and flexible detectors, historically used for fast neutrons but can be employed also for slow ones in this particular configuration of the beam

For slow neutrons we can take advantage of the neutron capture by the hydrogen nuclei detecting the 2.2 MeV γ emitted in the reaction

n + H D + γ (2.2 MeV)

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THE FIRST DETECTOR

A polystyrene plastic scintillator 2x2x1 cm3

Two P30CW5 photomultipliers by Electron Tubes with integrated high voltage power supply facing the two opposite sides of the scintillator to avoid dark counts and improve reliability

The detector assembly

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Signals on the oscilloscope

Our detection system should be able to return the number of captured neutrons and their arrival time

The read out electronics

The signals from the scintillator are digitized by a NIM discriminator, threshold 30 mV

The coincidence between the two PMs is shifted to LVDS and then sent to sampler

sampling the discriminated signal with a 12.5MHz clock

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This is an output example:

each “1” corresponds to an impinging neutron

For each experimental condition several bunches have been collected, obtaining a time arrival profile

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WE HAVE TWO GOALS:

TO MEASURE

The neutron number The neutron energy spectrum

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The comparison between the Al sample activation and the real time detector measurements in the scan along a diagonal from the

accelerator head

Measurement setup in S.Anna Hospital (Como, Italy)

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This detector can be used to measure the energy spectrum

The TOF spectrum has been fitted with a double exponential and the result is :

Two different slopes:

• 135 µs

• 1.567 ms

These constants represent the contribution of a slower and a faster part of the neutron spectrum.

is it true?

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Scan of PMMAAccelerator head rotated of 90° detector

PMMA layer

• the slow constant from the accelerator head material (lead)

• the fast constant from a plastic material in the room

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The fast component is affected by the PMMA, since

The slow component doesn’t feel the effect of PMMA

Ratio fast/slow

More PMMA=more thermalization

From the TOF of each point ….

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THE SECOND PROTOTYPE using a boron loaded plastic scintillator

to evaluate an α and 7Li contributions

BC-454(Bicron) a boron loaded plastic scintillator

diameter = 2 cm thickness=0.5cm

P30CW5 photomultiplier by Electron Tubes

The assembled detector

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Comparison of the scintillator data with the activation method and the simulation data

DETECTOR

DOSIMETERIt counts the photons

produced by the neutrons in the scintillator plus in all surrounding materials

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the neutron energy spectrum is extracted from the simulation TOF

The TOF simulation comparison with data

This is a very important result

for a BNCT treatment you need the information on the spectrum

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CONCLUSION• BNCT in hospitals? … a dream becoming reality …

• A real time detector made of standard plastic scintillator that is both a relative DOSIMETER and a SPECTROMETER

• Can be used as a dosimeter at reactors?

• Flux measurement = real time equivalent of attenuation

Boron doped scintillating fibers or very thin scintillators

that will allow to disentangle the contribution due to the photons produced by neutron capture in the surrounding materials.

OUTLOOKS

•Absolute dose computation both with Linac and reactors

• Fiber system development

• Measurements with Phones with different Linacs

VCI - V. Conti February,19-24 2007 30

A TIME OF FLIGHT DETECTOR FOR THERMAL NEUTRONS FROM

RADIOTHERAPY LINACS

V. Conti, G.Bartesaghi, D.Bolognini, M.Prest, S.Scazzi, P.Cappelletti, M.Frigerio, S.Gelosa, A.Monti, A.Ostinelli,

G.Giannini, E.Vallazza, A.Mozzanica