- Email: [email protected]
CR-39 detector based thermal neutron flux measurements, in the photo neutron project
Available online at www.sciencedirect.com
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 266 (2008) 3656–3660 www.elsevier.com/locate/nimb
CR-39 detector based thermal neutron flux measurements, in the photo neutron project A. Mameli a, F. Greco a, A. Fidanzio a, V. Fusco b, S. Cilla c, G. D’Onofrio c, L. Grimaldi c, B.G. Augelli c, G. Giannini d, R. Bevilacqua d, P. Totaro d, L. Tommasino e, L. Azario f, A. Piermattei f,* a U.O. di Fisica Sanitaria Policlinico A. Gemelli, Universita` Cattolica S. Cuore, Roma, Italy U.O. di Radioterapia, Centro di Riferimento Oncologico della Basilicata, CROB Rionero Pz, Italy c U.O. di Fisica Sanitaria, Centro di Ricerca e Formazione ad Alta Tecnologia nelle Scienze Biomediche dell’Universita` Cattolica S. Cuore, Campobasso, Italy d Dipartimento di Fisica-Universita` di Trieste e INFN Sez Trieste, Padriciano, Trieste, Italy e Consultant, Via Cassia 1727, 00123 Roma, Italy f Istituto di Fisica, Universita` Cattolica del S. Cuore, Roma, Italy b
Received 7 February 2008; received in revised form 12 May 2008 Available online 10 June 2008
Abstract PhoNeS (photo neutron source) is a project aimed at the production and moderation of neutrons by exploiting high energy linear accelerators, currently used in radiotherapy. A feasibility study has been carried out with the scope in mind to use the high energy photon beams from these accelerators for the production of neutrons suitable for boron neutron capture therapy (BNCT). Within these investigations, it was necessary to carry out preliminary measurements of the thermal neutron component of neutron spectra, produced by the photo-conversion of X-ray radiotherapy beams supplied by three LinAcs: 15 MV, 18 MV and 23 MV. To this end, a simple passive thermal neutron detector has been used which consists of a CR-39 track detector facing a new type of boron-loaded radiator. Once calibrated, this passive detector has been used for the measurement of both the thermal neutron component and the cadmium ratio of different neutron spectra. In addition, bubble detectors with a response highly sensitive to thermal neutrons have also been used. Both thermal neutron detectors are simple to use, very compact and totally insensitive to low-ionizing radiation such as electrons and X-rays. The resultant thermal neutron flux was above 106 n/cm2s and the cadmium ratio was no greater than 15 for the first attempt of photoconversion of X-ray radiotherapy beams. Ó 2008 Elsevier B.V. All rights reserved. PACS: 87.56.Bd; 25.20. x; 29.25.Dz Keywords: Neutron; CR-39; BNCT
1. Introduction Boron neutron capture therapy (BNCT) is a radiotherapy approach based on the capture of thermal neutrons by the isotope 10B followed by emission of an a particle and a nucleus of 7Li. If boron is present inside cancer cells, by inducing the 10B(n,a)7Li reaction, highly-ionizing short*
Corresponding author. E-mail address: [email protected] (A. Piermattei).
0168-583X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2008.05.122
range (5 lm) a and 7Li particles deliver most of their energy inside the cancer cells [1]. At present, there are a limited number of BNCT facilities around the world, which are located at nuclear research reactors which use epithermal neutron fluxes of more than 108 n/cm2s. Since most reactors are located far away from hospitals the international BNCT community is engaged in developing alternative neutron sources such as compact proton accelerators [2] or D-D and D-T fusion sources [3] suitable to be installed in radiotherapy departments for in-hospital BNCT treatments.
A. Mameli et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 3656–3660
The photo neutron source (PhoNeS) project, funded by Italian Nuclear Physics National Institute (INFN), is a feasibility study to investigate whether, new neutron sources can be developed through the exploitation of linear highenergy-electron accelerators (15–25 MeV) already operating in hospital radiotherapy units. In this case, the neutron source consists of a neutron photo-converter made of a high Z core for neutron production, surrounded by light nuclei material for moderating fast neutrons down to thermal or epithermal energies. This facility can be easily installed and located in front of any accelerator head. In order to carry out a feasibility study, a first prototype of this facility has been set up, for the first time with a 18 MV X-ray beam [4,5]. Within these investigations, it was necessary to carry out preliminary measurements of the thermal component of different neutron spectra, produced by two other linear accelerators (LinAcs) supplying 15 MV and 23 MV X-rays. To this end, a simple passive thermal neutron detector was developed based on a CR39 damage-track-detector [6] facing an advanced-type of boron-loaded radiator. Once calibrated, this passive detector was used for the measurement of the thermal neutron component and the cadmium ratio of different neutron spectra. In addition, bubble detectors with a response highly sensitive to thermal neutrons have been also used. Both types of detectors are simple, compact and totally insensitive to the low-ionizing radiations, such as electrons and X-rays produced by the radiotherapy LinAcs.
2. Material and methods 2.1. The neutron photo-converter
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In the PhoNeS project, the bremsstrahlung X-ray beams from conventional LinAcs are collimated toward a passive photo-converter: a shield of high Z material (such as lead) producing an isotropic neutron flux u (neutron/cm2s) [8]. The mean energy is in the range 700 keV–1 MeV [7]. The neutrons are then slowed down by elastic scattering on light nuclei by introducing suitable moderating structures, to obtain thermal and epithermal neutrons, which have energies E < 0.5 eV and 0.5 < E < 10 keV, respectively. Different types of materials can be selected to optimize the neutron fluence for the BNCT, such as lead and tungsten for the photo-production, graphite, heavy water, plexiglass and polyethylene for the moderation, lead and bismuth for the X-ray shielding. A geometrical configuration of the various components has been designed to shape the thermal beam from the photo-converter, in order to maximize the neutron component for the BNCT [9,10]. A small modular type of photo-converter which is easy to position and remove from in front of the accelerator head has been set up. Fig. 1 shows the first photo-converter used. The main components are: a lead target (30 30 15 cm3), graphite blocks external moderator (60 cm width, 75 cm height, 30 cm depth), moderators in polyethylene (30 30 3 cm3) and heavy water (D2O 99% purity, 8 kg) in a suitably shaped plexiglass box to obtain an irradiation cavity (20 20 10 cm3). The total weight of the photo-converter is about 300 kg [5]. The photo-converter was placed almost in contact with the accelerator head and used to obtain a neutron flux by a 18 MV X-ray beam (30 30 cm2 in size at 100 cm from the source) supplied by the General Electric linear accelerator Saturne 43, located at the Gemelli Radiotherapy Department of the Roma Catholic University (UCSC). Neutron flux measurements were carried out using:
High-energy (15–25 MeV) electron linear accelerators, widely used for electron and X-ray radiotherapy beams, are sources of undesired neutrons produced in the accelerator head by giant dipole resonance (GDR) reactions [7].
– A 15 MV beam from the Elekta Precise linear accelerator located at the Radiotherapy Department (UCSC Campobasso).
x-RAY BEAM
10 cm
LEAD TARGET GRAPHITE
GRAPHITE POLYETHYLENE HEAVY WATER IRRADIATION CAVITY
Fig. 1. The photo-converter prototype, (a) placed against the accelerator head of the 18 MV accelerator (UCSC Roma) and (b) scheme of the photoconverter (top view).
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– A 23 MV beam from the Varian 2100/C linear accelerator located at the Radiotherapy Department of the Reference Oncology Centre (CROB) at Rionero (Potenza). For these two beams, the LinAc collimator jaws were totally closed obtaining a lead thickness of 16 cm (similar to that used by the photo-converter reported in Fig. 1) for the neutron photo-conversion. In front to the accelerator head, a slab of polyethylene 30 30 cm2 in size, 1 cm thick and a graphite box filled with heavy water total thickness of 10 cm, were used to moderate the neutron fluence. 2.2. The CR-39 thermal neutron detector The detection of thermal neutrons through the registration of damage tracks of neutron-induced alpha particles or 7Li in 10B has been already reported [6]. This neutron detection requires just two elements: a track detector and a radiator loaded with 10B. A suitable track detector for the registration of alpha particles is the carbonate plastic first developed by Pittsburgh Plate Glass, commercially known as CR-39 (Columbia Resin, 1939) [6]. One attractive characteristic of these passive track detectors is that they are highly sensitive to alpha particles. Boron-loaded organic scintillators are commercially available which can be successfully used as radiators for thermal neutron detection [11]. These scintillators (from the Bicron Corporation) are available with concentrations of natural boron which may vary from 0.02% to 10% by weight. Unfortunately all these scintillators have a thickness greater than 1 mm, which may cause a relatively large self-absorption of thermal neutrons. In the present work, novel custom-made scintillators with a thickness of about 25 lm, having natural boron-concentration of 10%, are used. The 25 lm thickness is sufficiently large for the range of the alpha particles involved and sufficiently thin to ensure little or no perturbation of the thermal neutron fluence / (neutron/cm2) [8]. The response of track detectors is linear within a range of 102 tracks/cm2 and about 104 tracks/cm2. The thermal neutron detector, developed ad-hoc for the present investigation, consists of a 25 lm-thick radiator sandwiched between two different track detectors (13 37 1 mm3), which are both used for track counting. CR-39 detectors register both alpha particles and proton recoils induced by fast neutrons with energies greater than a few hundred keV. The etching conditions (2.5 h at 80 °C with 30% KOH in water) have been optimised in order to facilitate the etching of the alpha particle tracks from the (n,a) reactions, while the etching of tracks due the fast-neutron-induced proton recoils is less effective. The track counting under the microscope has been carried at a magnification of 100, where at least ten fields have been manually counted forming a total scanned area of about 16 mm2.
A rapid characterisation of a neutron facility for BNCT can be made through the evaluation of the thermal neutron component. When neutrons interact with cadmium, the thermal neutrons are very effectively captured by cadmium because of its large neutron cross-section due to a very high resonance at 0.2 eV. For this reason, thermal neutrons are very often referred to as sub-cadmium neutrons. A simple way to measure thermal neutrons is through the difference of the responses of two alpha-track detectors exposed without and with the cadmium cover respectively. A parameter which characterises the thermal neutron component within a complex neutron spectrum is the cadmium ratio, which is given by the ratio of the responses of two thermal neutron detectors exposed without and with the cadmium cover, respectively. In the present investigations, cadmium foils 1 mm thick were used to make a small box for the CR-39 detectors. The calibration has been made at the TRIGA MARK II nuclear reactor from Pavia by exposing the CR-39 based detectors to a thermal neutron fluence of (1.70 ± 0.17) 107 n/cm2 (±1SD) characterised by a cadmium ratio of about 50 ± 5 (±1SD). The thermal neutron fluence from the TRIGA reactor and the relative cadmium ratio has been measured by the Applied Nuclear Energy Laboratory of the Pavia University. For this calibration, the neutron fluences and the cadmium ratio have been measured through the activation of gold foils with an uncertainty of 10% (±1SD). The calibration response for the thermal neutron detector based on 10% boron-loaded scintillators resulted in approximately (7.8 ± 1.4) 10 4 (±1SD) tracks/neutron. The cadmium ratio obtained with these detectors resulted in approximately 53 ± 13 (±1SD), which, within one standard deviation, is equal to that measured with gold foils. Thermal neutron detectors based on CR-39 with and without the cadmium cover have been exposed to different neutron fields produced by the X-ray beams supplied by the three different LinAcs reported in Section 2.1. Four detectors (two CR-39 track detectors with the 25 lm thick radiator) were positioned in the irradiation cavity of the photo-converter (Fig. 1) or in front of the polyethylene slabs used for the 15 MV and 23 MV X-ray beams. Five sets of four detectors were irradiated for 1 min using the three X-ray beams. The applied dose rate of photons were about 400 cGy/min at 100 cm from the source. 2.3. Bubble dosimeters Bubble detectors provide instant visible detection and measurement of neutron fluence. Inside the detector, tiny droplets of a superheated liquid (a cylinder 5 cm length and 1 cm in diameter) are dispersed throughout a clear polymer. When a neutron strikes a droplet, the droplet immediately vaporizes, forming a visible gas bubble trapped in the gel. The number of droplets provides a direct measurement of the neutron dose and/or fluence. They are compact, lightweight and robust [12]. In this work, bubble
A. Mameli et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 3656–3660
detectors (BD), sensitive to thermal neutrons, developed by Bubble Technology Industries BTI, were used. Even though the response of these detectors has not been studied and calibrated by us, they have been used in the 18 MV X-ray irradiation experiment for a preliminary comparison with thoroughly investigated CR-39 detectors. According to the manufacturer, these bubble detectors are sensitive to neutrons with energies between 0.025 and 0.4 eV. Exposures of these detectors have been planned in order to obtain approximately a total of 100 bubbles, which are easy to count and provides counting statistics of 10%. However, the total uncertainty of the fluence measurement was of about ±20% (±1SD) as reported by the calibration supplied by the manufacturer [12]. Four different bubble detectors have been used together with the same number of track-etch detectors, respectively. All these devices have been placed in the same positions of the CR-39 detectors and irradiated for 1.5 s, using the 18 MV X-ray beam (UCSC Roma) that delivered a photon dose rate of approximately 400 cGy/min at 100 cm from the source. 3. Results Table 1 shows the thermal neutron flux obtained by the CR-39 and the bubble detectors by the photo-conversion of a 18 MV X-ray beam (at the UCSC of Roma). The number of tracks without and with cadmium were used to determine a cadmium ratio of 10 ± 3. The two neutron fluxes obtained by the two kinds of detectors are the same at least within two standard deviations of the measurements. The neutron flux measurements carried out with 15 MV and 23 MV X-ray beams using the CR-39 detectors are shown in Table 2. The cadmium ratios determined for these two LinAcs ranged between 10 ± 3 for the 15 MV and 15 ± 3 for the 23 MV X-ray beam.
Table 1 Results of the measurements at the UCSC (Roma) obtained by a 18 MV X-ray beam, using CR-39 and BT dosimeters CR-39 Tracks/cm2 (without cadmium) CR-39 Tracks/cm2 (with cadmium) CR-39 Fluence / (n/cm2) CR-39 Flux u (n/cm2s) BD Flux u (n/cm2s)
(3.4 ± 1.5) 104 (3.3 ± 0.5) 103 (3.8 ± 0.8) 107 (0.7 ± 0.2) 106 (1.5 ± 0.5) 106
Table 2 Thermal neutron flux measurements by the photo-conversion of 15 MV and 23 MV X-ray beams Flux u CR-39 (n/cm2s) 15 MV 23 MV CR-39 detectors were used.
(5.7 ± 1.1) 105 (1.7 ± 0.3) 106
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4. Discussion and conclusion The CR-39 detectors appear to be of interest for thermal neutron measurements for BNCT facilities. In particular, because of their compactness, they could be useful in the study of the dose distribution within a phantom. CR-39based neutron detectors were used in this study for the quantitative assessments of the thermal neutron component, 106 n/cm2s, produced by a first photo-converter prototype with a 18 MV X-ray beam used in a Radiotherapy Department. Moreover, for the same beam, investigation was carried out with the thermal neutron sensitive bubble detectors, supplying a useful confirmation of the neutron flux. Even if the measurements were carried out with different photo-converters, the measurements show a small increase of the thermal neutron flux with the X-ray beam quality from 15 MV to 23 MV, while the cadmium ratio ranged between 10 and 15. Even with the different geometries and materials used for the photo-conversion and the moderation, the detectors used in this work do not supply any information about the fractions of epithermal and fast neutrons produced. In the PhoNeS project it is the intention to investigate how the fast neutron and the epithermal neutron components can be moderated toward the thermal neutron energy range. However, from the measurements of both the thermal flux and the cadmium ratios of these neutron fields it is possible to conclude that the use of this first prototype is not yet adequate for a complete single fraction BNCT treatment. Nevertheless the PhoNeS project can start its initial studies into photo-converter prototypes, using different geometries and combinations of materials selected by Monte Carlo simulations. Once an optimised neutron spectrum is found (with a high value of cadmium ratio) the neutron flux can be further increased, which would confirm that a dedicated LinAc beam, with a high electron current, could be useful for BNCT. Moreover, the results of this feasibility study show that it is possible to use a thermal neutron facility in a hospital centre, for radiobiological research, where lower neutron fluxes are sufficient. The neutron source can be obtained using a modular photo-converter easy to be positioned and removed from the radiotherapy accelerator head. Acknowledgements This work was financially supported by the INFN (Istituto Nazionale di Fisica Nucleare) and by a PRIN project (Progetti di ricerca di Rilevante Interesse Nazionale) of the MIUR (Ministero dell’Istruzione, Universita` e della Ricerca). This work was also supported by the M.I.U.R. Project no 4210011 ‘‘Sviluppo di nuovi approcci terapeutici al problema clinico della resistenza alla chemioterapia antitumorale”. The authors would like to acknowledge Dr. S. Altieri (Triga reactor in Pavia) for his help during the CR-39 track detector calibration.
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References [1] IAEA TECDOC-1223 Current status of neutron capture therapy, May 2001. [2] K.N. Leung et al., BNCT dose distribution in liver with epithermal D-D and D-T fusion – based neutron beams, Appl. Radiat. Isotope 61 (2004) 853. [3] T. Kobayashi et al., Depth-dose characteristics in a water phantom for BNCT using neutrons from 7Li(p,n)7Be reactions at nearthreshold & at 2.5 MeV and KUR-HWNIF, in: Proceedings of the 10th International Symposium on Neutron Capture Therapy for cancer, Monduzzi Editore, Bologna, 2002, p. 353. [4] O. Borla, Fotoproduzione di neutroni da acceleratori lineari per trattamenti di BNCT: studio di fattibilita`, Thesis, University of Torino, Italy, 2004. [5] G. Giannini et al., Photoneutron source for in-hospital BNCT treatment: feasibility study, in: Proceedings of the 12th International Congress on Neutron Capture Therapy Takamatsu, Kagawa, Japan, 2006.
[6] R.V. Griffith, L. Tommasino, in: K.R. Kase, B.E. Bjarnard, F.H. Attix (Eds.), From the Dosimetry of Ionizing Radiation, Vol. 3, Academic Press, Inc., 1990, p. 323. [7] S. Followil et al., Neutron source strength measurements for Varian, Siemens, Elekta, and general electric linear accelerators, J. Appl. Clin. Med. Phys. (2003) 189. [8] ICRU Report 60: Fundamental Quantities and Units for Ionizing Radiation, 1998. [9] C. Ongaro et al., Analysis of photoneutron spectra produced in medical accelerators, Phys. Med. Biol. 45 (2000) 55. [10] K. Vittor, Studio di un convertitore per radioterapia fotoneutronica mediante e-LinAcs, Radiazioni, ricerca ed applicazioni (SIRR) IX, 2006. [11] G. Zapparoli, L. Tommasino, S. Djeffal, A. Maiorana, Additional results with electro-chemically etched CR-39 neutron dosimeters, Nucl. Tracks Rad. Meas. 12 (1986) 675. [12] Bubble Technology Industries Instruction manual for the bubble detector spectrometer (BDS), ChalkRiver, Ontario, Canada, 2003.
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 266 (2008) 3656–3660 www.elsevier.com/locate/nimb
CR-39 detector based thermal neutron flux measurements, in the photo neutron project A. Mameli a, F. Greco a, A. Fidanzio a, V. Fusco b, S. Cilla c, G. D’Onofrio c, L. Grimaldi c, B.G. Augelli c, G. Giannini d, R. Bevilacqua d, P. Totaro d, L. Tommasino e, L. Azario f, A. Piermattei f,* a U.O. di Fisica Sanitaria Policlinico A. Gemelli, Universita` Cattolica S. Cuore, Roma, Italy U.O. di Radioterapia, Centro di Riferimento Oncologico della Basilicata, CROB Rionero Pz, Italy c U.O. di Fisica Sanitaria, Centro di Ricerca e Formazione ad Alta Tecnologia nelle Scienze Biomediche dell’Universita` Cattolica S. Cuore, Campobasso, Italy d Dipartimento di Fisica-Universita` di Trieste e INFN Sez Trieste, Padriciano, Trieste, Italy e Consultant, Via Cassia 1727, 00123 Roma, Italy f Istituto di Fisica, Universita` Cattolica del S. Cuore, Roma, Italy b
Received 7 February 2008; received in revised form 12 May 2008 Available online 10 June 2008
Abstract PhoNeS (photo neutron source) is a project aimed at the production and moderation of neutrons by exploiting high energy linear accelerators, currently used in radiotherapy. A feasibility study has been carried out with the scope in mind to use the high energy photon beams from these accelerators for the production of neutrons suitable for boron neutron capture therapy (BNCT). Within these investigations, it was necessary to carry out preliminary measurements of the thermal neutron component of neutron spectra, produced by the photo-conversion of X-ray radiotherapy beams supplied by three LinAcs: 15 MV, 18 MV and 23 MV. To this end, a simple passive thermal neutron detector has been used which consists of a CR-39 track detector facing a new type of boron-loaded radiator. Once calibrated, this passive detector has been used for the measurement of both the thermal neutron component and the cadmium ratio of different neutron spectra. In addition, bubble detectors with a response highly sensitive to thermal neutrons have also been used. Both thermal neutron detectors are simple to use, very compact and totally insensitive to low-ionizing radiation such as electrons and X-rays. The resultant thermal neutron flux was above 106 n/cm2s and the cadmium ratio was no greater than 15 for the first attempt of photoconversion of X-ray radiotherapy beams. Ó 2008 Elsevier B.V. All rights reserved. PACS: 87.56.Bd; 25.20. x; 29.25.Dz Keywords: Neutron; CR-39; BNCT
1. Introduction Boron neutron capture therapy (BNCT) is a radiotherapy approach based on the capture of thermal neutrons by the isotope 10B followed by emission of an a particle and a nucleus of 7Li. If boron is present inside cancer cells, by inducing the 10B(n,a)7Li reaction, highly-ionizing short*
Corresponding author. E-mail address: [email protected] (A. Piermattei).
0168-583X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2008.05.122
range (5 lm) a and 7Li particles deliver most of their energy inside the cancer cells [1]. At present, there are a limited number of BNCT facilities around the world, which are located at nuclear research reactors which use epithermal neutron fluxes of more than 108 n/cm2s. Since most reactors are located far away from hospitals the international BNCT community is engaged in developing alternative neutron sources such as compact proton accelerators [2] or D-D and D-T fusion sources [3] suitable to be installed in radiotherapy departments for in-hospital BNCT treatments.
A. Mameli et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 3656–3660
The photo neutron source (PhoNeS) project, funded by Italian Nuclear Physics National Institute (INFN), is a feasibility study to investigate whether, new neutron sources can be developed through the exploitation of linear highenergy-electron accelerators (15–25 MeV) already operating in hospital radiotherapy units. In this case, the neutron source consists of a neutron photo-converter made of a high Z core for neutron production, surrounded by light nuclei material for moderating fast neutrons down to thermal or epithermal energies. This facility can be easily installed and located in front of any accelerator head. In order to carry out a feasibility study, a first prototype of this facility has been set up, for the first time with a 18 MV X-ray beam [4,5]. Within these investigations, it was necessary to carry out preliminary measurements of the thermal component of different neutron spectra, produced by two other linear accelerators (LinAcs) supplying 15 MV and 23 MV X-rays. To this end, a simple passive thermal neutron detector was developed based on a CR39 damage-track-detector [6] facing an advanced-type of boron-loaded radiator. Once calibrated, this passive detector was used for the measurement of the thermal neutron component and the cadmium ratio of different neutron spectra. In addition, bubble detectors with a response highly sensitive to thermal neutrons have been also used. Both types of detectors are simple, compact and totally insensitive to the low-ionizing radiations, such as electrons and X-rays produced by the radiotherapy LinAcs.
2. Material and methods 2.1. The neutron photo-converter
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In the PhoNeS project, the bremsstrahlung X-ray beams from conventional LinAcs are collimated toward a passive photo-converter: a shield of high Z material (such as lead) producing an isotropic neutron flux u (neutron/cm2s) [8]. The mean energy is in the range 700 keV–1 MeV [7]. The neutrons are then slowed down by elastic scattering on light nuclei by introducing suitable moderating structures, to obtain thermal and epithermal neutrons, which have energies E < 0.5 eV and 0.5 < E < 10 keV, respectively. Different types of materials can be selected to optimize the neutron fluence for the BNCT, such as lead and tungsten for the photo-production, graphite, heavy water, plexiglass and polyethylene for the moderation, lead and bismuth for the X-ray shielding. A geometrical configuration of the various components has been designed to shape the thermal beam from the photo-converter, in order to maximize the neutron component for the BNCT [9,10]. A small modular type of photo-converter which is easy to position and remove from in front of the accelerator head has been set up. Fig. 1 shows the first photo-converter used. The main components are: a lead target (30 30 15 cm3), graphite blocks external moderator (60 cm width, 75 cm height, 30 cm depth), moderators in polyethylene (30 30 3 cm3) and heavy water (D2O 99% purity, 8 kg) in a suitably shaped plexiglass box to obtain an irradiation cavity (20 20 10 cm3). The total weight of the photo-converter is about 300 kg [5]. The photo-converter was placed almost in contact with the accelerator head and used to obtain a neutron flux by a 18 MV X-ray beam (30 30 cm2 in size at 100 cm from the source) supplied by the General Electric linear accelerator Saturne 43, located at the Gemelli Radiotherapy Department of the Roma Catholic University (UCSC). Neutron flux measurements were carried out using:
High-energy (15–25 MeV) electron linear accelerators, widely used for electron and X-ray radiotherapy beams, are sources of undesired neutrons produced in the accelerator head by giant dipole resonance (GDR) reactions [7].
– A 15 MV beam from the Elekta Precise linear accelerator located at the Radiotherapy Department (UCSC Campobasso).
x-RAY BEAM
10 cm
LEAD TARGET GRAPHITE
GRAPHITE POLYETHYLENE HEAVY WATER IRRADIATION CAVITY
Fig. 1. The photo-converter prototype, (a) placed against the accelerator head of the 18 MV accelerator (UCSC Roma) and (b) scheme of the photoconverter (top view).
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– A 23 MV beam from the Varian 2100/C linear accelerator located at the Radiotherapy Department of the Reference Oncology Centre (CROB) at Rionero (Potenza). For these two beams, the LinAc collimator jaws were totally closed obtaining a lead thickness of 16 cm (similar to that used by the photo-converter reported in Fig. 1) for the neutron photo-conversion. In front to the accelerator head, a slab of polyethylene 30 30 cm2 in size, 1 cm thick and a graphite box filled with heavy water total thickness of 10 cm, were used to moderate the neutron fluence. 2.2. The CR-39 thermal neutron detector The detection of thermal neutrons through the registration of damage tracks of neutron-induced alpha particles or 7Li in 10B has been already reported [6]. This neutron detection requires just two elements: a track detector and a radiator loaded with 10B. A suitable track detector for the registration of alpha particles is the carbonate plastic first developed by Pittsburgh Plate Glass, commercially known as CR-39 (Columbia Resin, 1939) [6]. One attractive characteristic of these passive track detectors is that they are highly sensitive to alpha particles. Boron-loaded organic scintillators are commercially available which can be successfully used as radiators for thermal neutron detection [11]. These scintillators (from the Bicron Corporation) are available with concentrations of natural boron which may vary from 0.02% to 10% by weight. Unfortunately all these scintillators have a thickness greater than 1 mm, which may cause a relatively large self-absorption of thermal neutrons. In the present work, novel custom-made scintillators with a thickness of about 25 lm, having natural boron-concentration of 10%, are used. The 25 lm thickness is sufficiently large for the range of the alpha particles involved and sufficiently thin to ensure little or no perturbation of the thermal neutron fluence / (neutron/cm2) [8]. The response of track detectors is linear within a range of 102 tracks/cm2 and about 104 tracks/cm2. The thermal neutron detector, developed ad-hoc for the present investigation, consists of a 25 lm-thick radiator sandwiched between two different track detectors (13 37 1 mm3), which are both used for track counting. CR-39 detectors register both alpha particles and proton recoils induced by fast neutrons with energies greater than a few hundred keV. The etching conditions (2.5 h at 80 °C with 30% KOH in water) have been optimised in order to facilitate the etching of the alpha particle tracks from the (n,a) reactions, while the etching of tracks due the fast-neutron-induced proton recoils is less effective. The track counting under the microscope has been carried at a magnification of 100, where at least ten fields have been manually counted forming a total scanned area of about 16 mm2.
A rapid characterisation of a neutron facility for BNCT can be made through the evaluation of the thermal neutron component. When neutrons interact with cadmium, the thermal neutrons are very effectively captured by cadmium because of its large neutron cross-section due to a very high resonance at 0.2 eV. For this reason, thermal neutrons are very often referred to as sub-cadmium neutrons. A simple way to measure thermal neutrons is through the difference of the responses of two alpha-track detectors exposed without and with the cadmium cover respectively. A parameter which characterises the thermal neutron component within a complex neutron spectrum is the cadmium ratio, which is given by the ratio of the responses of two thermal neutron detectors exposed without and with the cadmium cover, respectively. In the present investigations, cadmium foils 1 mm thick were used to make a small box for the CR-39 detectors. The calibration has been made at the TRIGA MARK II nuclear reactor from Pavia by exposing the CR-39 based detectors to a thermal neutron fluence of (1.70 ± 0.17) 107 n/cm2 (±1SD) characterised by a cadmium ratio of about 50 ± 5 (±1SD). The thermal neutron fluence from the TRIGA reactor and the relative cadmium ratio has been measured by the Applied Nuclear Energy Laboratory of the Pavia University. For this calibration, the neutron fluences and the cadmium ratio have been measured through the activation of gold foils with an uncertainty of 10% (±1SD). The calibration response for the thermal neutron detector based on 10% boron-loaded scintillators resulted in approximately (7.8 ± 1.4) 10 4 (±1SD) tracks/neutron. The cadmium ratio obtained with these detectors resulted in approximately 53 ± 13 (±1SD), which, within one standard deviation, is equal to that measured with gold foils. Thermal neutron detectors based on CR-39 with and without the cadmium cover have been exposed to different neutron fields produced by the X-ray beams supplied by the three different LinAcs reported in Section 2.1. Four detectors (two CR-39 track detectors with the 25 lm thick radiator) were positioned in the irradiation cavity of the photo-converter (Fig. 1) or in front of the polyethylene slabs used for the 15 MV and 23 MV X-ray beams. Five sets of four detectors were irradiated for 1 min using the three X-ray beams. The applied dose rate of photons were about 400 cGy/min at 100 cm from the source. 2.3. Bubble dosimeters Bubble detectors provide instant visible detection and measurement of neutron fluence. Inside the detector, tiny droplets of a superheated liquid (a cylinder 5 cm length and 1 cm in diameter) are dispersed throughout a clear polymer. When a neutron strikes a droplet, the droplet immediately vaporizes, forming a visible gas bubble trapped in the gel. The number of droplets provides a direct measurement of the neutron dose and/or fluence. They are compact, lightweight and robust [12]. In this work, bubble
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detectors (BD), sensitive to thermal neutrons, developed by Bubble Technology Industries BTI, were used. Even though the response of these detectors has not been studied and calibrated by us, they have been used in the 18 MV X-ray irradiation experiment for a preliminary comparison with thoroughly investigated CR-39 detectors. According to the manufacturer, these bubble detectors are sensitive to neutrons with energies between 0.025 and 0.4 eV. Exposures of these detectors have been planned in order to obtain approximately a total of 100 bubbles, which are easy to count and provides counting statistics of 10%. However, the total uncertainty of the fluence measurement was of about ±20% (±1SD) as reported by the calibration supplied by the manufacturer [12]. Four different bubble detectors have been used together with the same number of track-etch detectors, respectively. All these devices have been placed in the same positions of the CR-39 detectors and irradiated for 1.5 s, using the 18 MV X-ray beam (UCSC Roma) that delivered a photon dose rate of approximately 400 cGy/min at 100 cm from the source. 3. Results Table 1 shows the thermal neutron flux obtained by the CR-39 and the bubble detectors by the photo-conversion of a 18 MV X-ray beam (at the UCSC of Roma). The number of tracks without and with cadmium were used to determine a cadmium ratio of 10 ± 3. The two neutron fluxes obtained by the two kinds of detectors are the same at least within two standard deviations of the measurements. The neutron flux measurements carried out with 15 MV and 23 MV X-ray beams using the CR-39 detectors are shown in Table 2. The cadmium ratios determined for these two LinAcs ranged between 10 ± 3 for the 15 MV and 15 ± 3 for the 23 MV X-ray beam.
Table 1 Results of the measurements at the UCSC (Roma) obtained by a 18 MV X-ray beam, using CR-39 and BT dosimeters CR-39 Tracks/cm2 (without cadmium) CR-39 Tracks/cm2 (with cadmium) CR-39 Fluence / (n/cm2) CR-39 Flux u (n/cm2s) BD Flux u (n/cm2s)
(3.4 ± 1.5) 104 (3.3 ± 0.5) 103 (3.8 ± 0.8) 107 (0.7 ± 0.2) 106 (1.5 ± 0.5) 106
Table 2 Thermal neutron flux measurements by the photo-conversion of 15 MV and 23 MV X-ray beams Flux u CR-39 (n/cm2s) 15 MV 23 MV CR-39 detectors were used.
(5.7 ± 1.1) 105 (1.7 ± 0.3) 106
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4. Discussion and conclusion The CR-39 detectors appear to be of interest for thermal neutron measurements for BNCT facilities. In particular, because of their compactness, they could be useful in the study of the dose distribution within a phantom. CR-39based neutron detectors were used in this study for the quantitative assessments of the thermal neutron component, 106 n/cm2s, produced by a first photo-converter prototype with a 18 MV X-ray beam used in a Radiotherapy Department. Moreover, for the same beam, investigation was carried out with the thermal neutron sensitive bubble detectors, supplying a useful confirmation of the neutron flux. Even if the measurements were carried out with different photo-converters, the measurements show a small increase of the thermal neutron flux with the X-ray beam quality from 15 MV to 23 MV, while the cadmium ratio ranged between 10 and 15. Even with the different geometries and materials used for the photo-conversion and the moderation, the detectors used in this work do not supply any information about the fractions of epithermal and fast neutrons produced. In the PhoNeS project it is the intention to investigate how the fast neutron and the epithermal neutron components can be moderated toward the thermal neutron energy range. However, from the measurements of both the thermal flux and the cadmium ratios of these neutron fields it is possible to conclude that the use of this first prototype is not yet adequate for a complete single fraction BNCT treatment. Nevertheless the PhoNeS project can start its initial studies into photo-converter prototypes, using different geometries and combinations of materials selected by Monte Carlo simulations. Once an optimised neutron spectrum is found (with a high value of cadmium ratio) the neutron flux can be further increased, which would confirm that a dedicated LinAc beam, with a high electron current, could be useful for BNCT. Moreover, the results of this feasibility study show that it is possible to use a thermal neutron facility in a hospital centre, for radiobiological research, where lower neutron fluxes are sufficient. The neutron source can be obtained using a modular photo-converter easy to be positioned and removed from the radiotherapy accelerator head. Acknowledgements This work was financially supported by the INFN (Istituto Nazionale di Fisica Nucleare) and by a PRIN project (Progetti di ricerca di Rilevante Interesse Nazionale) of the MIUR (Ministero dell’Istruzione, Universita` e della Ricerca). This work was also supported by the M.I.U.R. Project no 4210011 ‘‘Sviluppo di nuovi approcci terapeutici al problema clinico della resistenza alla chemioterapia antitumorale”. The authors would like to acknowledge Dr. S. Altieri (Triga reactor in Pavia) for his help during the CR-39 track detector calibration.
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References [1] IAEA TECDOC-1223 Current status of neutron capture therapy, May 2001. [2] K.N. Leung et al., BNCT dose distribution in liver with epithermal D-D and D-T fusion – based neutron beams, Appl. Radiat. Isotope 61 (2004) 853. [3] T. Kobayashi et al., Depth-dose characteristics in a water phantom for BNCT using neutrons from 7Li(p,n)7Be reactions at nearthreshold & at 2.5 MeV and KUR-HWNIF, in: Proceedings of the 10th International Symposium on Neutron Capture Therapy for cancer, Monduzzi Editore, Bologna, 2002, p. 353. [4] O. Borla, Fotoproduzione di neutroni da acceleratori lineari per trattamenti di BNCT: studio di fattibilita`, Thesis, University of Torino, Italy, 2004. [5] G. Giannini et al., Photoneutron source for in-hospital BNCT treatment: feasibility study, in: Proceedings of the 12th International Congress on Neutron Capture Therapy Takamatsu, Kagawa, Japan, 2006.
[6] R.V. Griffith, L. Tommasino, in: K.R. Kase, B.E. Bjarnard, F.H. Attix (Eds.), From the Dosimetry of Ionizing Radiation, Vol. 3, Academic Press, Inc., 1990, p. 323. [7] S. Followil et al., Neutron source strength measurements for Varian, Siemens, Elekta, and general electric linear accelerators, J. Appl. Clin. Med. Phys. (2003) 189. [8] ICRU Report 60: Fundamental Quantities and Units for Ionizing Radiation, 1998. [9] C. Ongaro et al., Analysis of photoneutron spectra produced in medical accelerators, Phys. Med. Biol. 45 (2000) 55. [10] K. Vittor, Studio di un convertitore per radioterapia fotoneutronica mediante e-LinAcs, Radiazioni, ricerca ed applicazioni (SIRR) IX, 2006. [11] G. Zapparoli, L. Tommasino, S. Djeffal, A. Maiorana, Additional results with electro-chemically etched CR-39 neutron dosimeters, Nucl. Tracks Rad. Meas. 12 (1986) 675. [12] Bubble Technology Industries Instruction manual for the bubble detector spectrometer (BDS), ChalkRiver, Ontario, Canada, 2003.