- Email: [email protected]
Simulation study of accelerator based quasi-mono-energetic epithermal neutron beams for BNCT
Author’s Accepted Manuscript Simulation study of accelerator based quasi-monoenergetic Epithermal neutron beams for BNCT M. Adib, N. Habib, I.I. Bashter, M.S. El-Mesiry, M.S. Mansy www.elsevier.com/locate/apradiso
PII: DOI: Reference:
S0969-8043(15)30201-3 http://dx.doi.org/10.1016/j.apradiso.2015.10.003 ARI7150
To appear in: Applied Radiation and Isotopes Received date: 9 July 2015 Revised date: 17 September 2015 Accepted date: 4 October 2015 Cite this article as: M. Adib, N. Habib, I.I. Bashter, M.S. El-Mesiry and M.S. Mansy, Simulation study of accelerator based quasi-mono-energetic Epithermal neutron beams for BNCT, Applied Radiation and Isotopes, http://dx.doi.org/10.1016/j.apradiso.2015.10.003 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Simulation study of accelerator based quasi-mono-energetic epithermal neutron beams for BNCT M. Adib a, N. Habib a, I.I. Bashter b, M.S. El-Mesiry a and M. S. Mansy b, a
Reactor Physics Department, NRC, Atomic Energy Authority, Cairo, Egypt b Physics Department, Faculty of Science, Zagazig University, Egypt
Abstract Filtered neutron techniques were applied to produce quasi-mono-energetic neutron beams in the energy range of 1.5–7.5 keV at the accelerator port using the generated neutron spectrum from a Li (p, n) Be reaction. A simulation study was performed to characterize the filter components and transmitted beam lines. The feature of the filtered beams is detailed in terms of optimal thickness of the primary and additive components. A computer code named "QMNB-AS" was developed to carry out the required calculations. The filtered neutron beams had high purity and intensity with low contamination from the accompanying thermal, fast neutrons and γ-rays. Keywords: BNCT- Accelerator based neutron sources – Li (p, n) Be reaction- Epithermal beamsEpithermal neutron filters.
1. Introduction Boron Neutron Capture Therapy (BNCT) has for many decades been advocated as an innovative form of radio therapy that, in principle, has the potential to be the ideal form of treatment for many types of cancers. BNCT is complex yet at the same time relatively simple, in that it uses the high propensity of the non- radioactive nuclide boron-10 to capture thermal neutrons, which results into the prompt nuclear reaction 10B(n,α)7Li. The products of this reaction have high linear energy transfer characteristics (α particle, Eα≈ 150 keV µm-1, 7Li ion, ELi ≈175 keV µm-1) (Moss, 2014). The path lengths of these particles are in the range of 4-10 µm. Hence their energy deposition is limited to the diameter of a single cell. Therefore, it is possible to selectively irradiate cancer cells that have taken up a sufficient amount of 10B, while simultaneously sparing normal, healthy cells. However, this requires two conditions that are seen to underlie the difficulties perceived by others, namely: there must be a sufficient number of boron-10 atoms (thousands) deposited in to each cancer cell and there must be a sufficient number of thermal neutrons (tens of millions per second) targeted into each tumor cell (Moss, 2014). For BNCT, neutrons must be delivered with a high fluency rate, at the “right” energy, with as little as possible “contaminants” in the radiation beam and directed into the tumor bed. BNCT irradiations on patients performed at nuclear research reactors are reported by Moss et al., 1997, Nigg, 2006 and Harling & Riley, 2012. Many of these reactors are either no longer open, have ceased their BNCT activities or are under the threat of closure. Only a very small number are available today. Whilst these facilities remain essential for the further development of BNCT, the long-term outlook is not tenable. The only apparent hope for multi-institutional, largescale clinical trials is to use hospital-based accelerators, producing neutrons of the requisite characteristics, together with all the patient care possibilities of a modern hospital (Moss, 2014). It is well known that, the neutron spectrum from a nuclear reactor covers a wide energy range, from meV to several MeV and it is accompanied with γ-rays with average energy Eγ ≈ 2 MeV (Miyamaru, 2011). The disadvantages which may limit the use of research reactor sources for BNCT are: i. High gamma rays contamination from both the fission reaction at the reactor core and from the neutron capture through filtering materials used for selecting the energy band for the treatment process. ii. Low flux density of the filtered beams which means irradiation time will be prolonged.
Corresponding author: M. S. Mansy, Email: [email protected], [email protected] Address: Physics Department, Faculty of Science, Zagazig University, Egypt. Tel: +20 1142173078,Fax: +202 44620787
1
To overcome these problems accelerator based neutron sources have been developed. There are many different types of accelerators that have and are being considered for BNCT applications, ranging from low-energy electrostatic machines to higher energy cyclotrons and to still much higher energy Linacs or synchrotrons. They have the advantages to be sited in hospitals and to be able to follow a much easier licensing procedure than a dedicated nuclear reactor. They are also more compact and less expensive. Accelerators offer a number of advantages over reactor-based sources, such as: i. They can be easily turned off when the neutron field is no longer required ii. Installation and maintenance of accelerators are simpler. iii. The capital expense is much less. iv. Radiotherapy departments in hospitals have had experience with accelerators for years (Moss, 2014). Thermal neutrons are capable of being captured immediately by all elements in the body, and hence have only a limited depth to which they can penetrate before reacting. Therefore, epithermal neutron beams are accessible that permit to treat deep-seated tumours (such as glioblastoma multiform (GBM)). Epithermal neutrons in the energy range from 1 eV to 10 keV, cannot as such be captured efficiently by the atoms of the body (Bisceglie et al. 2000). Filtered neutron techniques are applied to produce quasi-mono-energetic neutron beams at keV energies at both research reactor and accelerator based neutron sources. These filters are based on interference between the resonance and potential scattering amplitudes give rise to a minimum (dip) in the total cross-section just preceding a resonance (peak). If this dip is pronounced and narrow, it serves as a window through which neutrons of the requisite energy can be transmitted while neutrons of other energies in the spectrum are either scattered from or captured in the filter employed (Kondaiah et al. 1973). Morcos and Naguib (2014) have reported the composition and characteristics of accelerator based filtered epithermal beams in the energy range (1.5- 10) keV. However in their work, they did not present the basis for selecting the optimal amounts of primary and secondary filter elements. Moreover, Morcos and Naguib (2014) have performed their simulation study using an accelerator spectrum transmitted through beam shaping assembly (BSA) which works as a filter with wide neutron range (1-106) eV (Minsky et al. 2011). Also the intensity of transmitted neutron beam lines reported by Morcos and Naguib (2014) were low, which requires a long time of irradiation for cancer treatment process. In the present work, the suggested filters in the energy range from (1 – 7.5) keV are installed directly at the beam port of accelerator. A computer code have been developed to perform the required calculations. Moreover, the optimal amounts of the primary and secondary filter elements to obtain higher intensity and purity of the monoenergetic main peak are provided at the maximum rate of change of its purity.
2. Methodology A computer code called "QMNB-AS" (Quasi-Mono-energetic Neutron Beams from Accelerator Source) was developed in the MATLAB programming language to calculate the neutron spectrum transmitted through the filter materials. The “QMNB-AS” code is an adapted version of the computer code “QMNB” written by Mansy et al., (2015). The filter components and their amounts were optimized to obtain the highest possible intensity without disturbing the main energy line and the lowest-energy parasitic lines in the filtered neutron spectrum. The total neutron cross-section data for filter materials were obtained from JENDL-4.0, BROND-2.2, CENDL-3.1 and ENDF/B-7.1 libraries using JANIS software. The transmission T ( E ) , of neutrons at energy E through the filter is
T ( E ) Ti ( E )
(1)
i
Here,
Ti ( E ) is the neutron transmission of the i th filter component, which is Ti ( E) exp (ti NA t ( E) / Ai )
(2)
and the transmitted spectrum is
T ( E) T ( E) ( E) 2
(3)
Here,
NA
ti
is the thickness of component filter
i
in g/cm2,
t ( E ) is the total neutron cross-section in cm2/atom,
is Avogadro’s number, Ai is the atomic weight in g, and ( E ) is the incident neutron spectrum. The purity of the filter P is 1MeV Ef (4) P T ( E )dE ( E ) dE 100 T E 0.1eV i Here,
Ei and E f
are the energy boundaries of the main peak.
Because the cross section data calculated by JANIS software package were not defined at equal energy intervals, a linear interpolation rule was applied for all cross-section data to equalize the energy steps. Also, to calculate the area under curves, we use the trapezoidal rule: b
i n
f ( x)dx i 1
a
f ( xi 1 ) f ( xi ) xi1 xi 2
(5)
a , xn b and n is the total number of tabulated points. The optimal thickness t of the primary filter element was determined by identifying the maximum rate of Here, xo
change of the purity (t ) . (t ) is given by the following: (t ) [ P(t t ) P(t )] / t
(6)
Here, ( t ) is the difference between each successive thickness value.
3. Results and discussions As reported by Kiyanagi et al., (2012), the p-Li reaction produces larger number of neutrons than the p-Be reaction at low proton energy region around 2-3 MeV, and the neutron energy is much lower than that produced by the p-Be reaction at higher proton energy. Therefore, in the present work our filter calculations were based on the use of neutron spectrum generated at proton energy 2.9 MeV for the p-Li reaction with a beam current of 50 mA. The neutron spectrum reported by Kiyanagi et al., (2012) is displayed in Fig. (1). Also a schematic geometry for BNCT facility (accelerator/filter/patient) setup is illustrated in Fig. 2.
108
Li(p,n)Be 2.9 MeV
Neutron Flux (n/cm2.s)
107
106
105
104
103 10-1
100
101
102
103
104
105
106
Neutron energy (eV) [
Fig. 1. Generated neutron spectrum from a 2.9 MeV proton with a lithium target.
3
Collimator Reflector Accelerated Proton Beam
Filtered Epithermal Neutron Beam
Generated Neutrons
Patient
Accelerator
Li- Target Filter Primary Element
Filter Secondary Element
Irradiation Port
Fig. 2. Schematic geometry for the BNCT facility setup. The physical parameters of the elements used as filter for calculations are listed in Table 1. Table 1. Physical parameters of the elements used as filter components. Atomic weight (a.m.u) 44.95 50.99 54.00 58.93 60.00 63.55 64.00 80.00
Element 45-Sc Nat-V 54-Fe 59-Co 60-Ni Nat-Cu 64-Zn 80-Se
Density (g/cm3) 2.98 6.11 7.87 8.90 8.91 8.92 7.14 4.82
Abundance (%) 100.00 100.00 5.84 100.00 26.22 100.0 48.83 49.61
The filtered beam intensity of the main peak (1.5 keV) of 80Se and its purity at different thicknesses for a normalized accelerator neutron spectrum were calculated. The results of the calculation are displayed in Fig. 3. 1.35x10-4
50 Main Peak Purity (%) Main Peak Intensity
1.20x10-4
45 40 35
9.00x10-5
30 7.50x10-5
25
6.00x10-5
20
4.50x10-5
15 10
3.00x10-5
5 1.50x10-5
0
30
60
90
120
Thickness (g/cm2)
4
150
180
0 210
Purity (%)
Main Peak Intensity (a.u.)
1.05x10-4
Fig. 3. Intensity of the main 80Se peak and its purity at different thicknesses. Fig. 3 demonstrates that with increasing thickness of 80Se, the purity of the transmitted main peak is increased and reached only about 50% while its intensity is decreased. Such behavior is due to the multiple resonance dips in the 80 Se cross-section. So secondary elements having a resonance in their cross-sections above energy of 2.0 keV were added to improve the purity of the main peak at low accompanying background from other energy lines. It is observed from Fig. 3 that the rate of change of the intensity of the main peak is decreasing with increasing the thickness. While the rate of change of its purity is increasing up to thickness of 120 g/cm2 and then decreasing with increasing the thickness. Consequently the optimal thickness of the primary filter component is at the maximum rate of its purity. The rate of change of purity (t ) versus thickness (t) is calculated for different primary filter component. The result of the calculation is displayed in Fig. 4. Because the filters at 2.0 and 7.5 keV exhibited the same dependence of the intensity and purity on thickness as at 1.5 keV did, the same procedure was used to obtain the optimum thickness of the filters primary elements for the energy range of (1.5 – 7.5) keV. The obtained results are also displayed in Fig. 4. 5.0 4.5
Change of purity / thickness
4.0 3.5 3.0 2.5 2.0 1.5
Nat- Cu 80-Se 64-Zn
1.0 0.5 0.0 0
30
60
90
120
150
180
210
240
270
300
330
Thickness (g/cm2)
Fig. 4. Rate of change of purity versus thicknesses for the primary filter components. From Fig. 4 the optimal thickness t of the primary filter element is determined at the peak of the rate of change of the purity per thickness (t ) . While the optimal thickness of secondary elements was selected to reach the maximum purity value of the main peak. The filter components in the energy range from (1.5 – 7.5) keV suggested in the present work along with those reported by Morcos and Naguib (2014) are listed in Table 2. Table 2. The thicknesses of filter components in g/cm2 Filter components (g/cm2) (1.5) keV 45 Sc
Filter 80
Present work (F1) Morcos and Naguib (2014) 64
Present work (F2) Morcos and Naguib (2014)
Zn 95.0 356.65
Se
nat-V
120.0
90.0
---
335.3
---
10.4
45
Sc 43.0 2.989
(2.0) keV 54 Fe 5.0 ---
60
59
Ni 130.0 62.314
Co 5.0 ---
(7.5) keV Present work (F3)
nat-Cu
5
45
Sc
230.0
90.0
The distributions of the transmitted filtered neutron beam intensities were calculated using the components listed in Table 2 where the first element is the primary one. The results of calculation are displayed in Fig.5. For comparison the distribution of the filtrated neutron beam intensities reported by Morcos and Naguib (2014) are shown in Fig.5. 5x107
Neutron flux density (n.cm-2s-1)
Filter (1.5 keV)
4x107
Present Work (F1) Morcos and Naguib (2014)
3x107
2x107
1x107
0 102
103
104
Neutron Energy (eV)
Neutron flux density (n.cm-2s-1)
5x107
4x107
Filter (2.0 keV) Present Work (F2) Morcos and Naguib (2014)
3x107
2x107
1x107
0 102
103
104
Neutron Energy (eV) 1.8x107
Neutron Flux density (n.cm-2s-1)
1.6x107 1.4x107 1.2x107 1.0x107
Filter (7.5 keV) Present Work (F3)
8.0x106 6.0x106 4.0x106 2.0x106 0.0 5x103
6x103
7x103
8x103
9x103
104
Neutron Energy (eV)
Fig. 5. The distribution of the transmitted neutrons through different filters.
6
The intensity of neutron flux density transmitted through different filters was calculated. The results are presented in Table 3. Because the absorbed neutrons in the filter materials generates γ-quanta, the amounts of γ-rays generated from 100 (n, γ) capture were calculated using “PGGA” database obtained from IAEA website (Database of “PGAA”). The results of calculation are also listed in Table 3. Table 3. Features of filtered neutron beams
Filter
Present work (F1) Morcos and Naguib (2014) Present work (F2) Morcos and Naguib (2014) Present work (F3)
Energy (keV)
1.5 2.0 7.5
Purity (%)
FWHM (keV)
92.3 99.6 90.7 94.5 91.3
0.20 0.14 0.50 0.23 0.09
Features Flux density ×109 (n.cm-2.s-1)
11.4 3.50 15.0 12.5 1.70
(Nγ/100) capture
274 85 97 84 340
Table 3 indicates that the features of the suggested filters (F1, F2 and F3) in the present work are favorable in application for both BNCT and nuclear measurements because they have high resolution at high flux density. Moreover, the new developed filter (F3) at energy (7.5) keV in the present work is more preferable in BNCT for deep-seated tumours (Glioblastoma) because it has high energy and resolution. As can observed from Table 3, the flux density of the filters (F1 and F2) is higher than the other that reported by Morcos and Naguib (2014) at almost the same purity, resolution and low contamination of generated γ-rays. Such features support their use in skin cancers (melanoma) at shorter time of irradiation.
4. Conclusion The developed computer code was found to be sufficient for calculating the optimal component amounts of a quasi-mono-energetic neutron beam filters in the energy range from (1 to 7.5) keV. The method used to select the optimum thickness as the maximum of the rate of change in purity was proved to be useful and cost-effective for filter compositions. The developed filters at energy (1.5, 2.0 and 7.5) keV in the present work allows a wide range application in BNCT. Because they have high intensity and purity at low contamination of generated γ-rays.
References Bisceglie, E. Colangelo, P., Colonna, N., Santorelli, P., and Variale, V. 2000, “On the optimal energy of epithermal neutron beams for bnct,” Physics in medicine and biology, vol. 45, no. 1, p. 49, doi:10.1088/0031-9155/45/1/304 Database of “PGAA” retrieved from Harling, O.K., Riley, K.J., 2012. Fission reactor-based irradiation facilities for neutron capture therapy. In: Sauerwein,W.A.G., Wittig, A., Moss, R., Nakagawa, H. (Eds.), Neutron Capture Therapy: Principles and Applications, 2012. Springer-Verlag, Heidelberg. Kiyanagi,Y., et al. A Project of Boron Neutron Capture Therapy System based on a Proton Linac Neutron Source, 2012. Physics Procedia, 26, 223 – 230, doi: 10.1016/j.phpro.2012.03.029 Kondaiah E., Anand R.P. and Bhattacharya D., 1973. 25 keV neutron beam facility at the reactor “Apsara”. Nuclear Instruments and Methods. 111, 337-343, DOI: 10.1016/0029-554X(73)90081-5.
7
Mansy, M.S., Bashter, I.I., El-Mesiry, M.S. , Habib, N., and M. Adib, 2015 Filtered epithermal quasimonoenergetic neutron beams at research reactor facilities, Applied Radiation and Isotopes, Volume 97, 7883, doi: 10.1016/j.apradiso.2014.12.014. Minsky, D., Kernier, A., and Valda, A., 2011. Ab-bnct beam shaping assembly based on 7Li (p, n)7Be reaction optimization, Applied radiation and isotopes, Volume 69, 1668-1671, doi:10.1016/j.apradiso.2011.02.047 Miyamaru, H. and Murata, I., 2011. Neutron and Gamma-ray Dose Evaluation on Accelerator Neutron Source using p-Li Reaction for BNCT. Progress in Nuclear Science and Technology. 1, 533-536. Morcos, H. N. Naguib, K. 2014. Production of Optimal Epithermal Neutron Beams for BNCT, SOP Transactions on Applied Physics, Volume 1, Number 2, pp.7-13. Moss, R.L., Critical review, with an optimistic outlook, on Boron Neutron Capture Therapy (BNCT). Appl. Radiat. Isotopes (2014), http://dx.doi.org/10.1016/j.apradiso.2013.11.109i Moss, R.L.,Aizawa,O.,Beynon,D.,Brugger,R.,Constantine,G.,Harling,O.,Liu,H.B., Watkins,P.,1997.The requirements and development of neutron beams for neutron capture therapy of brain cancer. J. Neuro oncol.33, 27–40. Nigg, D.W., 2006. Neutron sources and applications in radiotherapy – a brief history and current trends. In: Nakagawa, Y., Kobayashi, T., Takamatsu, Fukuda H. (Eds.), Advances in Neutron Capture Therapy2006 – Proceedings of the 12th International Congress on Neutron Capture Therapy, October 9–13. Japan.
Highlights
Epithermal neutron beams used in BNCT.
Quasi-monoenergetic neutron beams in energy range from (1.5-13) keV.
Interference between the resonance and the potential scattering amplitudes.
8
PII: DOI: Reference:
S0969-8043(15)30201-3 http://dx.doi.org/10.1016/j.apradiso.2015.10.003 ARI7150
To appear in: Applied Radiation and Isotopes Received date: 9 July 2015 Revised date: 17 September 2015 Accepted date: 4 October 2015 Cite this article as: M. Adib, N. Habib, I.I. Bashter, M.S. El-Mesiry and M.S. Mansy, Simulation study of accelerator based quasi-mono-energetic Epithermal neutron beams for BNCT, Applied Radiation and Isotopes, http://dx.doi.org/10.1016/j.apradiso.2015.10.003 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Simulation study of accelerator based quasi-mono-energetic epithermal neutron beams for BNCT M. Adib a, N. Habib a, I.I. Bashter b, M.S. El-Mesiry a and M. S. Mansy b, a
Reactor Physics Department, NRC, Atomic Energy Authority, Cairo, Egypt b Physics Department, Faculty of Science, Zagazig University, Egypt
Abstract Filtered neutron techniques were applied to produce quasi-mono-energetic neutron beams in the energy range of 1.5–7.5 keV at the accelerator port using the generated neutron spectrum from a Li (p, n) Be reaction. A simulation study was performed to characterize the filter components and transmitted beam lines. The feature of the filtered beams is detailed in terms of optimal thickness of the primary and additive components. A computer code named "QMNB-AS" was developed to carry out the required calculations. The filtered neutron beams had high purity and intensity with low contamination from the accompanying thermal, fast neutrons and γ-rays. Keywords: BNCT- Accelerator based neutron sources – Li (p, n) Be reaction- Epithermal beamsEpithermal neutron filters.
1. Introduction Boron Neutron Capture Therapy (BNCT) has for many decades been advocated as an innovative form of radio therapy that, in principle, has the potential to be the ideal form of treatment for many types of cancers. BNCT is complex yet at the same time relatively simple, in that it uses the high propensity of the non- radioactive nuclide boron-10 to capture thermal neutrons, which results into the prompt nuclear reaction 10B(n,α)7Li. The products of this reaction have high linear energy transfer characteristics (α particle, Eα≈ 150 keV µm-1, 7Li ion, ELi ≈175 keV µm-1) (Moss, 2014). The path lengths of these particles are in the range of 4-10 µm. Hence their energy deposition is limited to the diameter of a single cell. Therefore, it is possible to selectively irradiate cancer cells that have taken up a sufficient amount of 10B, while simultaneously sparing normal, healthy cells. However, this requires two conditions that are seen to underlie the difficulties perceived by others, namely: there must be a sufficient number of boron-10 atoms (thousands) deposited in to each cancer cell and there must be a sufficient number of thermal neutrons (tens of millions per second) targeted into each tumor cell (Moss, 2014). For BNCT, neutrons must be delivered with a high fluency rate, at the “right” energy, with as little as possible “contaminants” in the radiation beam and directed into the tumor bed. BNCT irradiations on patients performed at nuclear research reactors are reported by Moss et al., 1997, Nigg, 2006 and Harling & Riley, 2012. Many of these reactors are either no longer open, have ceased their BNCT activities or are under the threat of closure. Only a very small number are available today. Whilst these facilities remain essential for the further development of BNCT, the long-term outlook is not tenable. The only apparent hope for multi-institutional, largescale clinical trials is to use hospital-based accelerators, producing neutrons of the requisite characteristics, together with all the patient care possibilities of a modern hospital (Moss, 2014). It is well known that, the neutron spectrum from a nuclear reactor covers a wide energy range, from meV to several MeV and it is accompanied with γ-rays with average energy Eγ ≈ 2 MeV (Miyamaru, 2011). The disadvantages which may limit the use of research reactor sources for BNCT are: i. High gamma rays contamination from both the fission reaction at the reactor core and from the neutron capture through filtering materials used for selecting the energy band for the treatment process. ii. Low flux density of the filtered beams which means irradiation time will be prolonged.
Corresponding author: M. S. Mansy, Email: [email protected], [email protected] Address: Physics Department, Faculty of Science, Zagazig University, Egypt. Tel: +20 1142173078,Fax: +202 44620787
1
To overcome these problems accelerator based neutron sources have been developed. There are many different types of accelerators that have and are being considered for BNCT applications, ranging from low-energy electrostatic machines to higher energy cyclotrons and to still much higher energy Linacs or synchrotrons. They have the advantages to be sited in hospitals and to be able to follow a much easier licensing procedure than a dedicated nuclear reactor. They are also more compact and less expensive. Accelerators offer a number of advantages over reactor-based sources, such as: i. They can be easily turned off when the neutron field is no longer required ii. Installation and maintenance of accelerators are simpler. iii. The capital expense is much less. iv. Radiotherapy departments in hospitals have had experience with accelerators for years (Moss, 2014). Thermal neutrons are capable of being captured immediately by all elements in the body, and hence have only a limited depth to which they can penetrate before reacting. Therefore, epithermal neutron beams are accessible that permit to treat deep-seated tumours (such as glioblastoma multiform (GBM)). Epithermal neutrons in the energy range from 1 eV to 10 keV, cannot as such be captured efficiently by the atoms of the body (Bisceglie et al. 2000). Filtered neutron techniques are applied to produce quasi-mono-energetic neutron beams at keV energies at both research reactor and accelerator based neutron sources. These filters are based on interference between the resonance and potential scattering amplitudes give rise to a minimum (dip) in the total cross-section just preceding a resonance (peak). If this dip is pronounced and narrow, it serves as a window through which neutrons of the requisite energy can be transmitted while neutrons of other energies in the spectrum are either scattered from or captured in the filter employed (Kondaiah et al. 1973). Morcos and Naguib (2014) have reported the composition and characteristics of accelerator based filtered epithermal beams in the energy range (1.5- 10) keV. However in their work, they did not present the basis for selecting the optimal amounts of primary and secondary filter elements. Moreover, Morcos and Naguib (2014) have performed their simulation study using an accelerator spectrum transmitted through beam shaping assembly (BSA) which works as a filter with wide neutron range (1-106) eV (Minsky et al. 2011). Also the intensity of transmitted neutron beam lines reported by Morcos and Naguib (2014) were low, which requires a long time of irradiation for cancer treatment process. In the present work, the suggested filters in the energy range from (1 – 7.5) keV are installed directly at the beam port of accelerator. A computer code have been developed to perform the required calculations. Moreover, the optimal amounts of the primary and secondary filter elements to obtain higher intensity and purity of the monoenergetic main peak are provided at the maximum rate of change of its purity.
2. Methodology A computer code called "QMNB-AS" (Quasi-Mono-energetic Neutron Beams from Accelerator Source) was developed in the MATLAB programming language to calculate the neutron spectrum transmitted through the filter materials. The “QMNB-AS” code is an adapted version of the computer code “QMNB” written by Mansy et al., (2015). The filter components and their amounts were optimized to obtain the highest possible intensity without disturbing the main energy line and the lowest-energy parasitic lines in the filtered neutron spectrum. The total neutron cross-section data for filter materials were obtained from JENDL-4.0, BROND-2.2, CENDL-3.1 and ENDF/B-7.1 libraries using JANIS software. The transmission T ( E ) , of neutrons at energy E through the filter is
T ( E ) Ti ( E )
(1)
i
Here,
Ti ( E ) is the neutron transmission of the i th filter component, which is Ti ( E) exp (ti NA t ( E) / Ai )
(2)
and the transmitted spectrum is
T ( E) T ( E) ( E) 2
(3)
Here,
NA
ti
is the thickness of component filter
i
in g/cm2,
t ( E ) is the total neutron cross-section in cm2/atom,
is Avogadro’s number, Ai is the atomic weight in g, and ( E ) is the incident neutron spectrum. The purity of the filter P is 1MeV Ef (4) P T ( E )dE ( E ) dE 100 T E 0.1eV i Here,
Ei and E f
are the energy boundaries of the main peak.
Because the cross section data calculated by JANIS software package were not defined at equal energy intervals, a linear interpolation rule was applied for all cross-section data to equalize the energy steps. Also, to calculate the area under curves, we use the trapezoidal rule: b
i n
f ( x)dx i 1
a
f ( xi 1 ) f ( xi ) xi1 xi 2
(5)
a , xn b and n is the total number of tabulated points. The optimal thickness t of the primary filter element was determined by identifying the maximum rate of Here, xo
change of the purity (t ) . (t ) is given by the following: (t ) [ P(t t ) P(t )] / t
(6)
Here, ( t ) is the difference between each successive thickness value.
3. Results and discussions As reported by Kiyanagi et al., (2012), the p-Li reaction produces larger number of neutrons than the p-Be reaction at low proton energy region around 2-3 MeV, and the neutron energy is much lower than that produced by the p-Be reaction at higher proton energy. Therefore, in the present work our filter calculations were based on the use of neutron spectrum generated at proton energy 2.9 MeV for the p-Li reaction with a beam current of 50 mA. The neutron spectrum reported by Kiyanagi et al., (2012) is displayed in Fig. (1). Also a schematic geometry for BNCT facility (accelerator/filter/patient) setup is illustrated in Fig. 2.
108
Li(p,n)Be 2.9 MeV
Neutron Flux (n/cm2.s)
107
106
105
104
103 10-1
100
101
102
103
104
105
106
Neutron energy (eV) [
Fig. 1. Generated neutron spectrum from a 2.9 MeV proton with a lithium target.
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Collimator Reflector Accelerated Proton Beam
Filtered Epithermal Neutron Beam
Generated Neutrons
Patient
Accelerator
Li- Target Filter Primary Element
Filter Secondary Element
Irradiation Port
Fig. 2. Schematic geometry for the BNCT facility setup. The physical parameters of the elements used as filter for calculations are listed in Table 1. Table 1. Physical parameters of the elements used as filter components. Atomic weight (a.m.u) 44.95 50.99 54.00 58.93 60.00 63.55 64.00 80.00
Element 45-Sc Nat-V 54-Fe 59-Co 60-Ni Nat-Cu 64-Zn 80-Se
Density (g/cm3) 2.98 6.11 7.87 8.90 8.91 8.92 7.14 4.82
Abundance (%) 100.00 100.00 5.84 100.00 26.22 100.0 48.83 49.61
The filtered beam intensity of the main peak (1.5 keV) of 80Se and its purity at different thicknesses for a normalized accelerator neutron spectrum were calculated. The results of the calculation are displayed in Fig. 3. 1.35x10-4
50 Main Peak Purity (%) Main Peak Intensity
1.20x10-4
45 40 35
9.00x10-5
30 7.50x10-5
25
6.00x10-5
20
4.50x10-5
15 10
3.00x10-5
5 1.50x10-5
0
30
60
90
120
Thickness (g/cm2)
4
150
180
0 210
Purity (%)
Main Peak Intensity (a.u.)
1.05x10-4
Fig. 3. Intensity of the main 80Se peak and its purity at different thicknesses. Fig. 3 demonstrates that with increasing thickness of 80Se, the purity of the transmitted main peak is increased and reached only about 50% while its intensity is decreased. Such behavior is due to the multiple resonance dips in the 80 Se cross-section. So secondary elements having a resonance in their cross-sections above energy of 2.0 keV were added to improve the purity of the main peak at low accompanying background from other energy lines. It is observed from Fig. 3 that the rate of change of the intensity of the main peak is decreasing with increasing the thickness. While the rate of change of its purity is increasing up to thickness of 120 g/cm2 and then decreasing with increasing the thickness. Consequently the optimal thickness of the primary filter component is at the maximum rate of its purity. The rate of change of purity (t ) versus thickness (t) is calculated for different primary filter component. The result of the calculation is displayed in Fig. 4. Because the filters at 2.0 and 7.5 keV exhibited the same dependence of the intensity and purity on thickness as at 1.5 keV did, the same procedure was used to obtain the optimum thickness of the filters primary elements for the energy range of (1.5 – 7.5) keV. The obtained results are also displayed in Fig. 4. 5.0 4.5
Change of purity / thickness
4.0 3.5 3.0 2.5 2.0 1.5
Nat- Cu 80-Se 64-Zn
1.0 0.5 0.0 0
30
60
90
120
150
180
210
240
270
300
330
Thickness (g/cm2)
Fig. 4. Rate of change of purity versus thicknesses for the primary filter components. From Fig. 4 the optimal thickness t of the primary filter element is determined at the peak of the rate of change of the purity per thickness (t ) . While the optimal thickness of secondary elements was selected to reach the maximum purity value of the main peak. The filter components in the energy range from (1.5 – 7.5) keV suggested in the present work along with those reported by Morcos and Naguib (2014) are listed in Table 2. Table 2. The thicknesses of filter components in g/cm2 Filter components (g/cm2) (1.5) keV 45 Sc
Filter 80
Present work (F1) Morcos and Naguib (2014) 64
Present work (F2) Morcos and Naguib (2014)
Zn 95.0 356.65
Se
nat-V
120.0
90.0
---
335.3
---
10.4
45
Sc 43.0 2.989
(2.0) keV 54 Fe 5.0 ---
60
59
Ni 130.0 62.314
Co 5.0 ---
(7.5) keV Present work (F3)
nat-Cu
5
45
Sc
230.0
90.0
The distributions of the transmitted filtered neutron beam intensities were calculated using the components listed in Table 2 where the first element is the primary one. The results of calculation are displayed in Fig.5. For comparison the distribution of the filtrated neutron beam intensities reported by Morcos and Naguib (2014) are shown in Fig.5. 5x107
Neutron flux density (n.cm-2s-1)
Filter (1.5 keV)
4x107
Present Work (F1) Morcos and Naguib (2014)
3x107
2x107
1x107
0 102
103
104
Neutron Energy (eV)
Neutron flux density (n.cm-2s-1)
5x107
4x107
Filter (2.0 keV) Present Work (F2) Morcos and Naguib (2014)
3x107
2x107
1x107
0 102
103
104
Neutron Energy (eV) 1.8x107
Neutron Flux density (n.cm-2s-1)
1.6x107 1.4x107 1.2x107 1.0x107
Filter (7.5 keV) Present Work (F3)
8.0x106 6.0x106 4.0x106 2.0x106 0.0 5x103
6x103
7x103
8x103
9x103
104
Neutron Energy (eV)
Fig. 5. The distribution of the transmitted neutrons through different filters.
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The intensity of neutron flux density transmitted through different filters was calculated. The results are presented in Table 3. Because the absorbed neutrons in the filter materials generates γ-quanta, the amounts of γ-rays generated from 100 (n, γ) capture were calculated using “PGGA” database obtained from IAEA website (Database of “PGAA”). The results of calculation are also listed in Table 3. Table 3. Features of filtered neutron beams
Filter
Present work (F1) Morcos and Naguib (2014) Present work (F2) Morcos and Naguib (2014) Present work (F3)
Energy (keV)
1.5 2.0 7.5
Purity (%)
FWHM (keV)
92.3 99.6 90.7 94.5 91.3
0.20 0.14 0.50 0.23 0.09
Features Flux density ×109 (n.cm-2.s-1)
11.4 3.50 15.0 12.5 1.70
(Nγ/100) capture
274 85 97 84 340
Table 3 indicates that the features of the suggested filters (F1, F2 and F3) in the present work are favorable in application for both BNCT and nuclear measurements because they have high resolution at high flux density. Moreover, the new developed filter (F3) at energy (7.5) keV in the present work is more preferable in BNCT for deep-seated tumours (Glioblastoma) because it has high energy and resolution. As can observed from Table 3, the flux density of the filters (F1 and F2) is higher than the other that reported by Morcos and Naguib (2014) at almost the same purity, resolution and low contamination of generated γ-rays. Such features support their use in skin cancers (melanoma) at shorter time of irradiation.
4. Conclusion The developed computer code was found to be sufficient for calculating the optimal component amounts of a quasi-mono-energetic neutron beam filters in the energy range from (1 to 7.5) keV. The method used to select the optimum thickness as the maximum of the rate of change in purity was proved to be useful and cost-effective for filter compositions. The developed filters at energy (1.5, 2.0 and 7.5) keV in the present work allows a wide range application in BNCT. Because they have high intensity and purity at low contamination of generated γ-rays.
References Bisceglie, E. Colangelo, P., Colonna, N., Santorelli, P., and Variale, V. 2000, “On the optimal energy of epithermal neutron beams for bnct,” Physics in medicine and biology, vol. 45, no. 1, p. 49, doi:10.1088/0031-9155/45/1/304 Database of “PGAA” retrieved from
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Mansy, M.S., Bashter, I.I., El-Mesiry, M.S. , Habib, N., and M. Adib, 2015 Filtered epithermal quasimonoenergetic neutron beams at research reactor facilities, Applied Radiation and Isotopes, Volume 97, 7883, doi: 10.1016/j.apradiso.2014.12.014. Minsky, D., Kernier, A., and Valda, A., 2011. Ab-bnct beam shaping assembly based on 7Li (p, n)7Be reaction optimization, Applied radiation and isotopes, Volume 69, 1668-1671, doi:10.1016/j.apradiso.2011.02.047 Miyamaru, H. and Murata, I., 2011. Neutron and Gamma-ray Dose Evaluation on Accelerator Neutron Source using p-Li Reaction for BNCT. Progress in Nuclear Science and Technology. 1, 533-536. Morcos, H. N. Naguib, K. 2014. Production of Optimal Epithermal Neutron Beams for BNCT, SOP Transactions on Applied Physics, Volume 1, Number 2, pp.7-13. Moss, R.L., Critical review, with an optimistic outlook, on Boron Neutron Capture Therapy (BNCT). Appl. Radiat. Isotopes (2014), http://dx.doi.org/10.1016/j.apradiso.2013.11.109i Moss, R.L.,Aizawa,O.,Beynon,D.,Brugger,R.,Constantine,G.,Harling,O.,Liu,H.B., Watkins,P.,1997.The requirements and development of neutron beams for neutron capture therapy of brain cancer. J. Neuro oncol.33, 27–40. Nigg, D.W., 2006. Neutron sources and applications in radiotherapy – a brief history and current trends. In: Nakagawa, Y., Kobayashi, T., Takamatsu, Fukuda H. (Eds.), Advances in Neutron Capture Therapy2006 – Proceedings of the 12th International Congress on Neutron Capture Therapy, October 9–13. Japan.
Highlights
Epithermal neutron beams used in BNCT.
Quasi-monoenergetic neutron beams in energy range from (1.5-13) keV.
Interference between the resonance and the potential scattering amplitudes.
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