1. 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

2. 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

3. 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)

4. 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

5. VCI - V. Conti February,19-24 2007 5 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)

6. VCI - V. Conti February,19-24 2007 6 PROTON X-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)

7. VCI - V. Conti February,19-24 2007 7 The BNCT : Boron Neutron Capture therapy The BCNT is an oncology radiotherapy treatment that exploits the capture of thermal neutrons by 10 B and the following emission of an α particle and a nucleus of 7 Li Four years after the discovery of neutrons in 1932 by Chadwick, Locker introduced the concept of BNCT

8. 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 10 B

9. VCI - V. Conti February,19-24 2007 9 The BNCT requires: a high thermal neutron flux (> 5x10 8 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

10. VCI - V. Conti February,19-24 2007 10 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 )

11. VCI - V. Conti February,19-24 2007 11 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 10 8 n cm -2 s -1 a fast neutron and gamma dose per thermal neutron lower than 2x10 -12 Sv cm 2 n -1 Giant Dipole Resonance

12. VCI - V. Conti February,19-24 2007 12 Different prototypes and simulation have been developed to optimize the following parameters: Photoconverter lead thickness Moderator shape and thickness Box of moderator material (D 2 O)

13. VCI - V. Conti February,19-24 2007 13 MCNP-4B GN code Simulation WITH and WITHOUT PhoNeS prototype without PhoNeS with PhoNeS 34 % slow 76 % slow

14. VCI - V. Conti February,19-24 2007 14 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

15. VCI - V. Conti February,19-24 2007 15 BF 3 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

16. VCI - V. Conti February,19-24 2007 16 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

17. VCI - V. Conti February,19-24 2007 17 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)

18. VCI - V. Conti February,19-24 2007 18 THE FIRST DETECTOR A polystyrene plastic scintillator 2x2x1 cm 3 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

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

20. VCI - V. Conti February,19-24 2007 20 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

21. VCI - V. Conti February,19-24 2007 21 WE HAVE TWO GOALS: TO MEASURE The neutron number The neutron energy spectrum

22. VCI - V. Conti February,19-24 2007 22 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)

23. VCI - V. Conti February,19-24 2007 23 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?

24. VCI - V. Conti February,19-24 2007 24 Scan of PMMA Accelerator 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

25. VCI - V. Conti February,19-24 2007 25 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 ….

26. VCI - V. Conti February,19-24 2007 26 THE SECOND PROTOTYPE using a boron loaded plastic scintillator to evaluate an α and 7 Li contributions BC-454(Bicron) a boron loaded plastic scintillator diameter = 2 cm thickness=0.5cm P30CW5 photomultiplier by Electron Tubes The assembled detector

27. VCI - V. Conti February,19-24 2007 27 Comparison of the scintillator data with the activation method and the simulation data DETECTOR DOSIMETER It counts the photons produced by the neutrons in the scintillator plus in all surrounding materials

28. VCI - V. Conti February,19-24 2007 28 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

29. VCI - V. Conti February,19-24 2007 29 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

30. 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