- Email: [email protected]
Neutron spectrum determination of d(20)+Be source reaction by the dosimetry foils method
Radiation Physics and Chemistry xxx (xxxx) xxx–xxx
Contents lists available at ScienceDirect
Radiation Physics and Chemistry journal homepage: www.elsevier.com/locate/radphyschem
Neutron spectrum determination of d(20)+Be source reaction by the dosimetry foils method ⁎
Milan Stefanika,b, , Pavel Bema, Mitja Majerlea, Jan Novaka, Eva Simeckovaa a b
Nuclear Physics Institute of The Czech Academy of Sciences, p.r.i, Rez 130, Rez 250 68, Czechia Czech Technical University in Prague, Faculty of Nuclear Sciences and Physical Engineering, Brehova 7, Prague 115 19, Czechia
A R T I C L E I N F O
A BS T RAC T
Keywords: Multi-foil activation technique Accelerator-based neutron source Neutron spectrometry Gamma-ray spectrometry Reaction rate Charged particle accelerator
The cyclotron-based fast neutron generator with the thick beryllium target operated at the NPI Rez Fast Neutron Facility is primarily designed for the fast neutron production in the p+Be source reaction at 35 MeV. Besides the proton beam, the isochronous cyclotron U-120M at the NPI provides the deuterons in the energy range of 10–20 MeV. The experiments for neutron field investigation from the deuteron bombardment of thick beryllium target at 20 MeV were performed just recently. For the neutron spectrum measurement of the d(20)+Be source reaction, the dosimetry foils activation method was utilized. Neutron spectrum reconstruction from resulting reaction rates was performed using the SAND-II unfolding code and neutron cross-sections from the EAF-2010 nuclear data library. Obtained high-flux white neutron field from the d(20)+Be source is useful for the intensive irradiation experiments and cross-section data validation.
1. Introduction
2. Materials and methods
The accelerator driven fast neutron sources with broad neutron spectra are mostly built on thick beryllium targets, because the deuteron induced (d+Be) and proton induced (p+Be) source reactions on beryllium target have the increasing value of cross-sections for neutron production in the energy range from several MeV to tens of MeV and thus the high value of the neutron spectral yield. The melting point of the beryllium has a value of 1 287 °C (Chu et al., 2016), so it is possible to use the charged particle beams of high intensity. One of the most important issues connected with beryllium as a target material is its high toxicity (Petzow et al., 2005). The NG-2 accelerator driven fast neutron generator with beryllium target station operated at the Nuclear Physics Institute (NPI) of The Czech Academy of Sciences (CAS) uses the proton beam delivered by the isochronous cyclotron U-120M for the high-energy neutron field production up to 34 MeV. This neutron field is used in the irradiation experiments carried out within the fusion related research applications (e.g. IFMIF research program – International Fusion Material Irradiation Facility (Mollendorf et al., 2002)). However, the neutron field produced in deuteron bombardment of the beryllium and suitable for the intensive irradiation experiments up to 20 MeV was recently studied in close vicinity from the source target, and obtained results are presented in this paper.
2.1. The d+Be neutron source reaction
⁎
The d+Be source interaction was intensively investigated by several scientists in the 70s and 80s of the last century. Neutron spectra were experimentally studied by Lone et al. (1977), (Meulders et al., 1975), Weaver et al. (1973); Graves et al. (1979); Brede et al. (1989); Saltmarsh et al. (1977); Meadows et al. (1993); Waterman et al. (1979), and Madey et al. (1977). It was found (especially from Lone's and Meulders’ measurements) that for deuteron beam energy Ed above 10 MeV, the mean energy of neutron spectrum En above 2 MeV in the forward direction can be described by the following empirical equation (Lone et al., 1977; Meulders et al., 1975; Cierjacks, 1983):
En = 0.4 × Ed − 0.3
Ed > 10MeV,
(1)
For 20 MeV deuteron beam, the fluence averaged neutron energy should be 7.7 MeV. The main neutron producing reactions when beryllium target is bombarded by deuteron beam are summarized in Table 1. The (d,n) deuteron stripping process on beryllium is the most important neutron producing reaction; it has a Q-value of 4.4 MeV (Q-value Calculator, 2016), and it is responsible for the high energy part of neutron energy spectrum (En > 0.8 × Ed ) (Allisy et al., 1989). For a thick target, the Gaussian-like broad maximum around fourty percent of the deuteron
Corresponding author at: Nuclear Physics Institute of The Czech Academy of Sciences, p.r.i, Rez 130, Rez 250 68, Czechia. E-mail address: milan.stefanik@fjfi.cvut.cz (M. Stefanik).
http://dx.doi.org/10.1016/j.radphyschem.2017.03.029 Received 30 September 2016; Received in revised form 12 March 2017; Accepted 14 March 2017 0969-806X/ © 2017 Elsevier Ltd. All rights reserved.
Please cite this article as: Stefanik, M., Radiation Physics and Chemistry (2017), http://dx.doi.org/10.1016/j.radphyschem.2017.03.029
Radiation Physics and Chemistry xxx (xxxx) xxx–xxx
M. Stefanik et al.
Table 1 Neutron producing reactions on beryllium target bombarded by deuteron beam. (Cierjacks, 1983, Q-value Calculator, 2016). Reaction
Q (MeV)
Ethresh (MeV)
9
Be(d,n)10B Be(d,pn)9Be 9 Be(d,p2n)8Be 9 Be(d,p)25He *
+4.36 −2.22 −3.89 −5.27
0.00 2.71 4.76 6.44
9
−0.83 −4.07
1.02 4.99
9
Be(d,5He)6Li Be(d,2n)9B
9
energy is formed by the multi-body break-up interactions of 9 Be(d,pn)9Be, 9Be(d,p2n)8Be, 9Be(d,5He)6Li, and 9Be(d,p)25He * with subsequent 5He break-up to neutron and α-particle (Saltmarsh et al., 1977; Cierjacks, 1983). The inelastic deuteron scattering on beryllium and 9Be(d,2 n)9B and 9Be(d,pn)9Be reactions create the low energy fraction of spectrum below 2 MeV as well as they form the peak at a energy of 800 keV (Allisy et al., 1989); this underlying continuum starts at very low neutron energies and falls off almost exponentially with the energy (Cierjacks, 1983). The d+Be interaction provides the best neutron source presently available for radiotherapy when accelerators with energies of at least 15 MeV are used (Wyckoff et al., 1976). So, the typical utilization of the fast neutron spectra from beryllium targets is for neutron radiotherapy, study of radiation damage of materials, and radiation hardness of electronics against the fast neutron fields. The spectra are also used for integral benchmarks and validation of nuclear data and as the reference spectra in nuclear data libraries. Typically, from beryllium target the broad (or white) neutron spectrum is obtained and oriented in forward direction as the deuteron energy increases. The experimentally found shapes of neutron spectra from the d+Be source reaction are depicted in Fig. 1 and 2. In Fig. 1, the spectra measured by M.A. Lone are displayed, the time-of-flight (TOF) method was utilized. In Fig. 2, the neutron spectral flux reported by J.P. Meulders for three energies is depicted; his measurement was performed by the NE-111 scintillation probe 3.
Fig. 2. The d+Be neutron energy spectra measured by Meulders using NE-111 scintillation probe for deuteron beam energy of 16 MeV, 33 MeV, and 50 MeV (Meulders et al., 1975).
2.2. Beryllium target station NG-2
Fig. 3. Beryllium target station of the NG-2 neutron generator at the NPI with aluminium holder of activation foils.
Since 2012, the beryllium target station of the NG-2 neutron generator at the NPI in Rez near Prague has been standardly operated together with the source reaction of the p+Be and for a proton beam energy up to 35 MeV. The beryllium target has a thickness of 8 mm and diameter of 5 cm, and during the operation, it is cooled by ethanol to the temperature of 5 °C. The beam power is about 420 W. The proton
beam current on the target and both carbon collimators, temperature, pressure, and flow rate of cooling ethanol are online monitored and registered during the experiments. In the standard operation, the beryllium target station produces the white spectrum up to 34 MeV with neutron spectral yield up to 1011 cm−2s−1 in the close distance from the source target. The obtained neutron spectrum of the p(35)+Be source reaction was reported Stefanik et al. (2014b), and the spectral characteristics were analyzed Stefanik et al. (2014a) in detail. However, new experiment with deuteron beam we performed recently at the NPI. For this white neutron source based on the d+Be source reaction, the maximum deuteron energy (20 MeV) provided by the U-120M cyclotron was utilized. 2.3. Multi-foil activation technique For the neutron field spectrometry in the irradiation system where the source-to-sample distances are short and the dimensions of production target and foils are comparable, the multi-foil activation technique is the most appropriate method. This method uses a set of dosimetry foils of various materials for measurement of responses to the neutron spectrum, i.e. the reaction rates. Afterwards, the neutron spectrum is reconstructed by the unfolding code (e.g. SAND-II) using the reaction rates determined by means of the γ-ray spectrometry, activation cross-sections, and initial guess neutron spectrum. At the
Fig. 1. The d+Be neutron energy spectra measured by Lone using TOF technique for deuteron beam energy of 14.8 MeV, 18 MeV, and 23 MeV. (Lone et al., 1977).
2
Radiation Physics and Chemistry xxx (xxxx) xxx–xxx
M. Stefanik et al.
NG-2 neutron generator, the multi-foil activation technique was successfully used for the neutron spectrum reconstruction of heavywater target station with the p(37)+D2O source reaction (Stefanik et al., 2012, 2014) and verified also on the neutron spectrum reconstruction of the p(35)+Be source reaction from the beryllium target station (Stefanik et al., 2014b).
Table 2 Neutron activation reactions observed in irradiation experiment and successfully used for neutron field reconstruction (Chu et al., 2016, Q-value Calculator, 2016).
Reaction 27 Al(n,α)24Na
2.4. The d(20)+Be neutron spectrum determination The cyclotron based neutron generators at NPI were developed for the purposes of the IFMIF and ADTT (Accelerator Driven Transmutation Technologies) research programs. Both these programs require the good knowledge of neutron field in the close vicinity of the source target where the studied samples are irradiated. For the neutron spectrum determination of the d+Be source reaction at the sample position, the multi-foil activation method was employed. The set of eight activation detectors for irradiation experiment consisted of Al, Au, In, Ti, Fe, Y, Lu, and Co. The activation foils were in the form of spectroscopic thin dosimetry foils with a diameter of 15 mm and delivered from GoodFellow Company. Activation detectors were sensitive to various parts of the neutron energy spectrum according to the activation cross-sections. The irradiation experiment lasted for 14 h; the energy and intensity of deuteron beam was 20 MeV and 7.25 μA. The stacks of the dosimetry foils were located on the aluminium holder in close target-to-sample-distance (see Fig. 4), in particular at a position of P0 and position P14. Position P0 corresponded to the beryllium-to-sample distance of 14 mm; position P14 was related to the sample distance of 15.4 cm from the back of beryllium. After irradiation, the induced radioactivity of activation foils was repeatedly measured by two semiconductor HPGe detectors. The reaction products observed in measured gamma-ray spectra were identified on the basis of energies, intensities, and half-life periods. From measured induced activity, the reaction rates per one target nuclei were determined in the same way as described earlier Stefanik et al., (2012, 2014a); subsequently, the neutron spectra for both positions were reconstructed. Overall, 22 activation and thresholds reactions were observed in all dosimetry foils. The experimental setup was also simulated in the MCNPX transport code (Waters et al., 2007) in order to obtain some preliminary information on the neutron spectrum in the activation experiment. In the MCNPX model, the target station with aluminium holder and samples, and cyclotron room with the approximation of the accelerator body were included. For simulation, the ENDF/B-VII.1 neutron crosssection library (Chadwick et al., 2011) was utilized; however, the
Ethresh
T1/2
Eγ
Iγ
14.96 h 83.79 d
(keV) 1 368.63 1 120.55
(%) 100.00 99.99
nat
Ti(n,x)46Sc
(MeV) 3.25 1.57
nat
Ti(n,x)48Sc
3.28
43.67 h
1 312.10
100.00
nat
Fe(n,x)54Mn
3.17
312.30 d
834.85
99.98
nat
Fe(n,x)56Mn
2.87
2.58 h
846.77
98.90
nat
Fe(n,x)51Cr 59 Co(n,γ)60Co 59 Co(n,2 n)58Co 59 Co(n,3 n)57Co 59 Co(n,p)59Fe 59 Co(n,α)56Mn 89 Y(n,2 n)88Y
0.00
27.70 d
320.08
10.00
115
In(n,n′)115m In
0.00 10.51 17.33 1.49 0.00 11.05 0.34
5.27 y 70.86 d 271.79 d 44.50 d 2.58 h 106.65 d 4.49 h
1 332.50 810.78 122.06 1 099.25 846.77 1 836.06 336.24
99.99 99.00 85.60 56.50 98.90 99.20 45.83
Lu(n,γ)177Lu
11.00
0.00
6.73 d
208.37
nat
14.19
1.37 y
78.63
11.87
197
0.00 8.19 8.19
2.70 d 6.18 d 6.18 d
411.80 355.68 355.68
96.00 87.00 87.00
14.79
186.09 d
98.85
10.90
176
Lu(n,x)173Lu 197 Au(n,γ)198Au 197 Au(n,2 n)196Au Au(n,2 n)196m Au Au(n,3 n)195Au
197
deuteron library was not available, and thus the d+Be interaction was calculated by the MCNPX built-in analytical model for this preliminary information. For neutron spectrum reconstruction from measured reaction rates, the modified version of SAND-II unfolding code (SAND-II-SNL, 1996) was utilized. Altogether 19 reactions (see Table 2) out of 22 and corresponding cross-sections from the EAF2010 data library (Forrest et al., 2010) were successfully used in neutron spectra unfolding procedure for both irradiation positions. As the initial guess neutron spectra for unfolding code, the MCNPX predicted spectra were utilized.
2.5. Results The SAND-II adjusted neutron spectra for the NG-2 neutron generator with the d(20)+Be source reaction and the MCNPX calculations for positions P0 and P14 are depicted in Fig. 5. As it can be seen from Fig. 5, the MCNPX calculation is underestimated by a factor 2–3 for energy range below 16 MeV with respect to the SAND-II spectrum reconstructed from experimentally measured reaction rates. Above 20 MeV energy range, the MCNPX prediction (based on built-in physics model) for low-intensity neutron spectrum tail is overestimated
Fig. 5. Neutron field of NG-2 generator with d(20)+Be source reaction at two irradiation positions measured by multi-foil activation technique at NPI for 20 MeV deuteron beam.
Fig. 4. Aluminium holder with activation foils stacks at beryllium target station head of NG-2 neutron generator at NPI.
3
Radiation Physics and Chemistry xxx (xxxx) xxx–xxx
M. Stefanik et al.
Table 3 The C /E ratios for unfolded neutron spectra. Reaction Al(n,α)24Na nat
Ti(n,x)46Sc
Position P0 1.03 1.02
nat
Ti(n,x)48Sc
0.97
0.99
nat
Fe(n,x)54Mn
1.07
1.07
nat
Fe(n,x)56Mn
1.04
1.03
nat
Fe(n,x)51Cr 59 Co(n,γ)60Co 59 Co(n,2n)58Co 59 Co(n,3n)57Co 59 Co(n,p)59Fe 59 Co(n,α)56Mn 89 Y(n,2n)88Y
0.97
0.99
115
1.02 1.02 1.01 0.94 0.98 1.02 0.94
1.00 0.87 0.96 0.92 0.99 0.96 1.02
27
In(n,n′)115m In
Position P14 1.03 1.02
0.99
0.98
nat
1.02
–
197
1.00 0.96 1.07
1.01 0.96 1.04
–
1.16
Lu(n,γ)176m Lu
nat
Lu(n,x)173Lu 197 Au(n,γ)198Au 197 Au(n,2n)196Au Au(n,2n)196m Au Au(n,3n)195Au
197
Fig. 7. The d+Be neutron spectra in lin-lin scale measured by M.A. Lone by means of TOF technique (data adopted from Lone et al. (1977)).
in comparison with the SAND-II reconstruction. This is given by the fact the MCNPX code does not take into account the forward orientation of the neutron spectrum from the d+Be source reaction. The corresponding C / E ratios (calculated over experimental reaction rates ratios) for all reactions successfully used in reconstruction procedure for both positions are summarized in Table 3 and they assess the uncertainties of unfolded spectra. These C / E ratios are very close to unity, and they confirm the correctness of the SAND-II reconstructed spectra. The shape comparison of neutron spectrum determined by the multi-foil activation technique at the NPI for the NG-2 generator with results from another author for similar incident deuteron energies is performed in Fig. 6 and 7. The newly measured neutron spectrum of the NG-2 generator is depicted in Fig. 6 with linear scale of neutron spectral flux. In Fig. 7, the neutron spectra adopted from Lone et al. (1977) are displayed also with linear scale of neutron spectral flux. There is a very good agreement in the shape of the compared spectra. Moreover, the mean energy of the determined neutron spectrum is really 7.7 MeV which confirms the fluence averaged energy according to the empirical equation (1). Besides the standard irradiation positions P0 and P14, the aluminium foils were also irradiated at the positions P2 and P6 (berylliumto-sample distance of 34 mm and 74 mm) to precisely study the
Fig. 8. The distance.
27
Al(n,α)24Na reaction rates ratios in dependence on beryllium-to-sample
evolution of neutron field in dependence on the target-to-sample distance. In Fig. 8, the experimentally determined reaction rates for the 27Al(n,α)24Na reaction in all distances are normalized to the reaction rate at the position P14. The observed deflection of the rate values from the 1/ R 2 law is apparent due to the non-point-like geometry of the irradiation system, where the dimensions of target and activation foils as well as their distance are comparable. 3. Conclusions At the NPI, new neutron field based on the d(20)+Be source reaction was developed employing the 20 MeV deuteron beam extracted from the U-120 M isochronous cyclotron and beryllium target station of the NG-2 generator. The mean energy of obtained spectra is about 7.7 MeV, and it corresponds well to the empirical equation (1) earlier derived from measurements of the other scientists (Lone et al., 1977; Meulders et al., 1975; Weaver et al., 1973; Graves et al., 1979; Brede et al., 1989; Saltmarsh et al., 1977; Meadows et al., 1993; Waterman et al., 1979; Madey et al., 1977; Cierjacks, 1983). The shape of neutron spectra is in very good agreement with results reported from other authors, in particular Brede et al. (1989), Meulders et al. (1975), and Lone et al. (1977). The fast neutron spectral flux amounts 5.5 × 1010 cm−2s−1 for position P0 and 1.7 × 109 cm−2s−1 for position P14 with an uncertainity of 8% and for proton beam current of 7.25 μA. The newly obtained neutron spectra from the d(20)+Be source reaction represent a useful tool for integral benchmark experiments,
Fig. 6. Neutron spectrum of NG-2 generator at NPI with source reaction of d(20)+Be in lin-lin scale for position P14 for 20 MeV deuteron beam.
4
Radiation Physics and Chemistry xxx (xxxx) xxx–xxx
M. Stefanik et al.
344. Madey, R., et al., 1977. Neutron spectra at 0 degrees from 83.7 MeV deuterons and 100.2 MeV protons on beryllium. Med. Phys. 4, 322. Meadows, J.W., et al., 1993. The thick-target 9Be(d,n) neutron spectra for deuteron energies between 2.6 and 7.0 MeV. Nucl. Inst. Meth. A 324, 239–246. Meulders, J.P., et al., 1975. Fast neutron yields and spectra from targets of varying atomic number bombarded with deuterons from 16 to 50 MeV. Phys. Med. Biol. 20, 235–243. Mollendorf, U., et al., 2002. A nuclear simulation experiment for the International Fusion Materials Irradiation Facility (IFMIF), Karlsruhe. Petzow, G., et al., 2005. Beryllium and Beryllium Compounds, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. Q-value Calculator, 2016. National Nuclear Data Center BNL. Saltmarsh, M.J., et al., 1977. Characteristics of an intense neutron source based on the d.Be reaction. Nucl. Inst. Meth. 145, 81–90. SAND-II-SNL, 1996. PSR-345, UKAEA FUS. Stefanik, M., et al., 2012. The p-D2O generator neutron spectrum determination by multi-foil activation method. Trans. ANS 106, 894. Stefanik, M., et al., 2014a. High-flux white neutron source based on p(35)-Be reactions for activation experiments at NPI. Radiat. Phys. Chem. 104, 302–305. Stefanik, M., et al., 2014b. Neutron spectrum determination of the p(35 MeV)-Be source reaction by the dosimetry foils method. Nucl. Data Sheets 119, 422–424. Stefanik, M., et al., 2014. Accelerator driven p(37)-D2O fast neutron source at NPI Rez. In: Proceedings of the the 15th International Scientific Conference on Electric Power Engineering, EPE-2014. Waterman, F.M., et al., 1979. Neutron spectra from 35 and 46 MeV protons, 16 and 28 MeV deuterons, and 44 MeV 3He ions on thick beryllium. Med. Phys. 6, 432–435. Waters, L.S., et al., 2007. MCNPX User’s Manual V. 2.1.5. Weaver, K.A., et al., 1973. Neutron spectra from deuteron bombardment of D, Li, Be, and C. Nucl. Sci. Eng. 52, 35–45. Wyckoff, H.O., et al., 1976. ICRU Report 26 - Neutron Dosimetry for Biology and Medicine. In: International Commision on Radiation Units and Measurements, Maryland.
integral validation of cross-sections for fusion related program IFMIF, testing the radiation hardness of electronics against the fast neutron fields, and for application of neutron activation analysis. Acknowledgements The irradiation experiments in the neutron field of accelerator driven fast neutron source NG-2 carried out at the CANAM infrastructure of the NPI CAS Rez are supported through the MSMT project no. LM2011019. References Allisy, A., et al., 1989. ICRU Report 45 - Clinical Neutron Dosimetry Part I: Determination of Absorbed Dose in a Patient Treated by External Beams of Fast Neutrons. In: International Commision on Radiation Units and Measurements, Maryland. Brede, H.J., et al., 1989. Neutron yields from thick Be targets bombarded with deuterons or protons. Nucl. Inst. Meth. A 274, 332–344. Chadwick, M., et al., 2011. ENDF/B-VII.1: NuclearDataforScienceandTechnology: Cross Sections, Covariances, Fission Product Yields and Decay Data, Nuc. Data Sheets 112, 2887. Chu, S.Y.F., et al., 2016. Lund/LBNL Nuclear Data Search Version 2.0. Cierjacks, S., 1983. Neutron Sources For Basic Physics and Applications. Pergamon Press, Oxford. Forrest, R.A., et al., 2010. EAF-2010 48, UKAEA FUS. Graves, R.G., et al., 1979. Neutron energy spectra of d(49)-Be and p(41)-Be neutron radiotherapy sources. Med. Phys. 6, 123–128. Lone, M.A., et al., 1977. Thick target neutron yields and spectral distributions from the 7 Li(d,n), 7Li(p,n) and 9Be(d,n), 9Be(p,n) reactions, Nucl. Inst. Meth. 143, pp. 331–
5
Contents lists available at ScienceDirect
Radiation Physics and Chemistry journal homepage: www.elsevier.com/locate/radphyschem
Neutron spectrum determination of d(20)+Be source reaction by the dosimetry foils method ⁎
Milan Stefanika,b, , Pavel Bema, Mitja Majerlea, Jan Novaka, Eva Simeckovaa a b
Nuclear Physics Institute of The Czech Academy of Sciences, p.r.i, Rez 130, Rez 250 68, Czechia Czech Technical University in Prague, Faculty of Nuclear Sciences and Physical Engineering, Brehova 7, Prague 115 19, Czechia
A R T I C L E I N F O
A BS T RAC T
Keywords: Multi-foil activation technique Accelerator-based neutron source Neutron spectrometry Gamma-ray spectrometry Reaction rate Charged particle accelerator
The cyclotron-based fast neutron generator with the thick beryllium target operated at the NPI Rez Fast Neutron Facility is primarily designed for the fast neutron production in the p+Be source reaction at 35 MeV. Besides the proton beam, the isochronous cyclotron U-120M at the NPI provides the deuterons in the energy range of 10–20 MeV. The experiments for neutron field investigation from the deuteron bombardment of thick beryllium target at 20 MeV were performed just recently. For the neutron spectrum measurement of the d(20)+Be source reaction, the dosimetry foils activation method was utilized. Neutron spectrum reconstruction from resulting reaction rates was performed using the SAND-II unfolding code and neutron cross-sections from the EAF-2010 nuclear data library. Obtained high-flux white neutron field from the d(20)+Be source is useful for the intensive irradiation experiments and cross-section data validation.
1. Introduction
2. Materials and methods
The accelerator driven fast neutron sources with broad neutron spectra are mostly built on thick beryllium targets, because the deuteron induced (d+Be) and proton induced (p+Be) source reactions on beryllium target have the increasing value of cross-sections for neutron production in the energy range from several MeV to tens of MeV and thus the high value of the neutron spectral yield. The melting point of the beryllium has a value of 1 287 °C (Chu et al., 2016), so it is possible to use the charged particle beams of high intensity. One of the most important issues connected with beryllium as a target material is its high toxicity (Petzow et al., 2005). The NG-2 accelerator driven fast neutron generator with beryllium target station operated at the Nuclear Physics Institute (NPI) of The Czech Academy of Sciences (CAS) uses the proton beam delivered by the isochronous cyclotron U-120M for the high-energy neutron field production up to 34 MeV. This neutron field is used in the irradiation experiments carried out within the fusion related research applications (e.g. IFMIF research program – International Fusion Material Irradiation Facility (Mollendorf et al., 2002)). However, the neutron field produced in deuteron bombardment of the beryllium and suitable for the intensive irradiation experiments up to 20 MeV was recently studied in close vicinity from the source target, and obtained results are presented in this paper.
2.1. The d+Be neutron source reaction
⁎
The d+Be source interaction was intensively investigated by several scientists in the 70s and 80s of the last century. Neutron spectra were experimentally studied by Lone et al. (1977), (Meulders et al., 1975), Weaver et al. (1973); Graves et al. (1979); Brede et al. (1989); Saltmarsh et al. (1977); Meadows et al. (1993); Waterman et al. (1979), and Madey et al. (1977). It was found (especially from Lone's and Meulders’ measurements) that for deuteron beam energy Ed above 10 MeV, the mean energy of neutron spectrum En above 2 MeV in the forward direction can be described by the following empirical equation (Lone et al., 1977; Meulders et al., 1975; Cierjacks, 1983):
En = 0.4 × Ed − 0.3
Ed > 10MeV,
(1)
For 20 MeV deuteron beam, the fluence averaged neutron energy should be 7.7 MeV. The main neutron producing reactions when beryllium target is bombarded by deuteron beam are summarized in Table 1. The (d,n) deuteron stripping process on beryllium is the most important neutron producing reaction; it has a Q-value of 4.4 MeV (Q-value Calculator, 2016), and it is responsible for the high energy part of neutron energy spectrum (En > 0.8 × Ed ) (Allisy et al., 1989). For a thick target, the Gaussian-like broad maximum around fourty percent of the deuteron
Corresponding author at: Nuclear Physics Institute of The Czech Academy of Sciences, p.r.i, Rez 130, Rez 250 68, Czechia. E-mail address: milan.stefanik@fjfi.cvut.cz (M. Stefanik).
http://dx.doi.org/10.1016/j.radphyschem.2017.03.029 Received 30 September 2016; Received in revised form 12 March 2017; Accepted 14 March 2017 0969-806X/ © 2017 Elsevier Ltd. All rights reserved.
Please cite this article as: Stefanik, M., Radiation Physics and Chemistry (2017), http://dx.doi.org/10.1016/j.radphyschem.2017.03.029
Radiation Physics and Chemistry xxx (xxxx) xxx–xxx
M. Stefanik et al.
Table 1 Neutron producing reactions on beryllium target bombarded by deuteron beam. (Cierjacks, 1983, Q-value Calculator, 2016). Reaction
Q (MeV)
Ethresh (MeV)
9
Be(d,n)10B Be(d,pn)9Be 9 Be(d,p2n)8Be 9 Be(d,p)25He *
+4.36 −2.22 −3.89 −5.27
0.00 2.71 4.76 6.44
9
−0.83 −4.07
1.02 4.99
9
Be(d,5He)6Li Be(d,2n)9B
9
energy is formed by the multi-body break-up interactions of 9 Be(d,pn)9Be, 9Be(d,p2n)8Be, 9Be(d,5He)6Li, and 9Be(d,p)25He * with subsequent 5He break-up to neutron and α-particle (Saltmarsh et al., 1977; Cierjacks, 1983). The inelastic deuteron scattering on beryllium and 9Be(d,2 n)9B and 9Be(d,pn)9Be reactions create the low energy fraction of spectrum below 2 MeV as well as they form the peak at a energy of 800 keV (Allisy et al., 1989); this underlying continuum starts at very low neutron energies and falls off almost exponentially with the energy (Cierjacks, 1983). The d+Be interaction provides the best neutron source presently available for radiotherapy when accelerators with energies of at least 15 MeV are used (Wyckoff et al., 1976). So, the typical utilization of the fast neutron spectra from beryllium targets is for neutron radiotherapy, study of radiation damage of materials, and radiation hardness of electronics against the fast neutron fields. The spectra are also used for integral benchmarks and validation of nuclear data and as the reference spectra in nuclear data libraries. Typically, from beryllium target the broad (or white) neutron spectrum is obtained and oriented in forward direction as the deuteron energy increases. The experimentally found shapes of neutron spectra from the d+Be source reaction are depicted in Fig. 1 and 2. In Fig. 1, the spectra measured by M.A. Lone are displayed, the time-of-flight (TOF) method was utilized. In Fig. 2, the neutron spectral flux reported by J.P. Meulders for three energies is depicted; his measurement was performed by the NE-111 scintillation probe 3.
Fig. 2. The d+Be neutron energy spectra measured by Meulders using NE-111 scintillation probe for deuteron beam energy of 16 MeV, 33 MeV, and 50 MeV (Meulders et al., 1975).
2.2. Beryllium target station NG-2
Fig. 3. Beryllium target station of the NG-2 neutron generator at the NPI with aluminium holder of activation foils.
Since 2012, the beryllium target station of the NG-2 neutron generator at the NPI in Rez near Prague has been standardly operated together with the source reaction of the p+Be and for a proton beam energy up to 35 MeV. The beryllium target has a thickness of 8 mm and diameter of 5 cm, and during the operation, it is cooled by ethanol to the temperature of 5 °C. The beam power is about 420 W. The proton
beam current on the target and both carbon collimators, temperature, pressure, and flow rate of cooling ethanol are online monitored and registered during the experiments. In the standard operation, the beryllium target station produces the white spectrum up to 34 MeV with neutron spectral yield up to 1011 cm−2s−1 in the close distance from the source target. The obtained neutron spectrum of the p(35)+Be source reaction was reported Stefanik et al. (2014b), and the spectral characteristics were analyzed Stefanik et al. (2014a) in detail. However, new experiment with deuteron beam we performed recently at the NPI. For this white neutron source based on the d+Be source reaction, the maximum deuteron energy (20 MeV) provided by the U-120M cyclotron was utilized. 2.3. Multi-foil activation technique For the neutron field spectrometry in the irradiation system where the source-to-sample distances are short and the dimensions of production target and foils are comparable, the multi-foil activation technique is the most appropriate method. This method uses a set of dosimetry foils of various materials for measurement of responses to the neutron spectrum, i.e. the reaction rates. Afterwards, the neutron spectrum is reconstructed by the unfolding code (e.g. SAND-II) using the reaction rates determined by means of the γ-ray spectrometry, activation cross-sections, and initial guess neutron spectrum. At the
Fig. 1. The d+Be neutron energy spectra measured by Lone using TOF technique for deuteron beam energy of 14.8 MeV, 18 MeV, and 23 MeV. (Lone et al., 1977).
2
Radiation Physics and Chemistry xxx (xxxx) xxx–xxx
M. Stefanik et al.
NG-2 neutron generator, the multi-foil activation technique was successfully used for the neutron spectrum reconstruction of heavywater target station with the p(37)+D2O source reaction (Stefanik et al., 2012, 2014) and verified also on the neutron spectrum reconstruction of the p(35)+Be source reaction from the beryllium target station (Stefanik et al., 2014b).
Table 2 Neutron activation reactions observed in irradiation experiment and successfully used for neutron field reconstruction (Chu et al., 2016, Q-value Calculator, 2016).
Reaction 27 Al(n,α)24Na
2.4. The d(20)+Be neutron spectrum determination The cyclotron based neutron generators at NPI were developed for the purposes of the IFMIF and ADTT (Accelerator Driven Transmutation Technologies) research programs. Both these programs require the good knowledge of neutron field in the close vicinity of the source target where the studied samples are irradiated. For the neutron spectrum determination of the d+Be source reaction at the sample position, the multi-foil activation method was employed. The set of eight activation detectors for irradiation experiment consisted of Al, Au, In, Ti, Fe, Y, Lu, and Co. The activation foils were in the form of spectroscopic thin dosimetry foils with a diameter of 15 mm and delivered from GoodFellow Company. Activation detectors were sensitive to various parts of the neutron energy spectrum according to the activation cross-sections. The irradiation experiment lasted for 14 h; the energy and intensity of deuteron beam was 20 MeV and 7.25 μA. The stacks of the dosimetry foils were located on the aluminium holder in close target-to-sample-distance (see Fig. 4), in particular at a position of P0 and position P14. Position P0 corresponded to the beryllium-to-sample distance of 14 mm; position P14 was related to the sample distance of 15.4 cm from the back of beryllium. After irradiation, the induced radioactivity of activation foils was repeatedly measured by two semiconductor HPGe detectors. The reaction products observed in measured gamma-ray spectra were identified on the basis of energies, intensities, and half-life periods. From measured induced activity, the reaction rates per one target nuclei were determined in the same way as described earlier Stefanik et al., (2012, 2014a); subsequently, the neutron spectra for both positions were reconstructed. Overall, 22 activation and thresholds reactions were observed in all dosimetry foils. The experimental setup was also simulated in the MCNPX transport code (Waters et al., 2007) in order to obtain some preliminary information on the neutron spectrum in the activation experiment. In the MCNPX model, the target station with aluminium holder and samples, and cyclotron room with the approximation of the accelerator body were included. For simulation, the ENDF/B-VII.1 neutron crosssection library (Chadwick et al., 2011) was utilized; however, the
Ethresh
T1/2
Eγ
Iγ
14.96 h 83.79 d
(keV) 1 368.63 1 120.55
(%) 100.00 99.99
nat
Ti(n,x)46Sc
(MeV) 3.25 1.57
nat
Ti(n,x)48Sc
3.28
43.67 h
1 312.10
100.00
nat
Fe(n,x)54Mn
3.17
312.30 d
834.85
99.98
nat
Fe(n,x)56Mn
2.87
2.58 h
846.77
98.90
nat
Fe(n,x)51Cr 59 Co(n,γ)60Co 59 Co(n,2 n)58Co 59 Co(n,3 n)57Co 59 Co(n,p)59Fe 59 Co(n,α)56Mn 89 Y(n,2 n)88Y
0.00
27.70 d
320.08
10.00
115
In(n,n′)115m In
0.00 10.51 17.33 1.49 0.00 11.05 0.34
5.27 y 70.86 d 271.79 d 44.50 d 2.58 h 106.65 d 4.49 h
1 332.50 810.78 122.06 1 099.25 846.77 1 836.06 336.24
99.99 99.00 85.60 56.50 98.90 99.20 45.83
Lu(n,γ)177Lu
11.00
0.00
6.73 d
208.37
nat
14.19
1.37 y
78.63
11.87
197
0.00 8.19 8.19
2.70 d 6.18 d 6.18 d
411.80 355.68 355.68
96.00 87.00 87.00
14.79
186.09 d
98.85
10.90
176
Lu(n,x)173Lu 197 Au(n,γ)198Au 197 Au(n,2 n)196Au Au(n,2 n)196m Au Au(n,3 n)195Au
197
deuteron library was not available, and thus the d+Be interaction was calculated by the MCNPX built-in analytical model for this preliminary information. For neutron spectrum reconstruction from measured reaction rates, the modified version of SAND-II unfolding code (SAND-II-SNL, 1996) was utilized. Altogether 19 reactions (see Table 2) out of 22 and corresponding cross-sections from the EAF2010 data library (Forrest et al., 2010) were successfully used in neutron spectra unfolding procedure for both irradiation positions. As the initial guess neutron spectra for unfolding code, the MCNPX predicted spectra were utilized.
2.5. Results The SAND-II adjusted neutron spectra for the NG-2 neutron generator with the d(20)+Be source reaction and the MCNPX calculations for positions P0 and P14 are depicted in Fig. 5. As it can be seen from Fig. 5, the MCNPX calculation is underestimated by a factor 2–3 for energy range below 16 MeV with respect to the SAND-II spectrum reconstructed from experimentally measured reaction rates. Above 20 MeV energy range, the MCNPX prediction (based on built-in physics model) for low-intensity neutron spectrum tail is overestimated
Fig. 5. Neutron field of NG-2 generator with d(20)+Be source reaction at two irradiation positions measured by multi-foil activation technique at NPI for 20 MeV deuteron beam.
Fig. 4. Aluminium holder with activation foils stacks at beryllium target station head of NG-2 neutron generator at NPI.
3
Radiation Physics and Chemistry xxx (xxxx) xxx–xxx
M. Stefanik et al.
Table 3 The C /E ratios for unfolded neutron spectra. Reaction Al(n,α)24Na nat
Ti(n,x)46Sc
Position P0 1.03 1.02
nat
Ti(n,x)48Sc
0.97
0.99
nat
Fe(n,x)54Mn
1.07
1.07
nat
Fe(n,x)56Mn
1.04
1.03
nat
Fe(n,x)51Cr 59 Co(n,γ)60Co 59 Co(n,2n)58Co 59 Co(n,3n)57Co 59 Co(n,p)59Fe 59 Co(n,α)56Mn 89 Y(n,2n)88Y
0.97
0.99
115
1.02 1.02 1.01 0.94 0.98 1.02 0.94
1.00 0.87 0.96 0.92 0.99 0.96 1.02
27
In(n,n′)115m In
Position P14 1.03 1.02
0.99
0.98
nat
1.02
–
197
1.00 0.96 1.07
1.01 0.96 1.04
–
1.16
Lu(n,γ)176m Lu
nat
Lu(n,x)173Lu 197 Au(n,γ)198Au 197 Au(n,2n)196Au Au(n,2n)196m Au Au(n,3n)195Au
197
Fig. 7. The d+Be neutron spectra in lin-lin scale measured by M.A. Lone by means of TOF technique (data adopted from Lone et al. (1977)).
in comparison with the SAND-II reconstruction. This is given by the fact the MCNPX code does not take into account the forward orientation of the neutron spectrum from the d+Be source reaction. The corresponding C / E ratios (calculated over experimental reaction rates ratios) for all reactions successfully used in reconstruction procedure for both positions are summarized in Table 3 and they assess the uncertainties of unfolded spectra. These C / E ratios are very close to unity, and they confirm the correctness of the SAND-II reconstructed spectra. The shape comparison of neutron spectrum determined by the multi-foil activation technique at the NPI for the NG-2 generator with results from another author for similar incident deuteron energies is performed in Fig. 6 and 7. The newly measured neutron spectrum of the NG-2 generator is depicted in Fig. 6 with linear scale of neutron spectral flux. In Fig. 7, the neutron spectra adopted from Lone et al. (1977) are displayed also with linear scale of neutron spectral flux. There is a very good agreement in the shape of the compared spectra. Moreover, the mean energy of the determined neutron spectrum is really 7.7 MeV which confirms the fluence averaged energy according to the empirical equation (1). Besides the standard irradiation positions P0 and P14, the aluminium foils were also irradiated at the positions P2 and P6 (berylliumto-sample distance of 34 mm and 74 mm) to precisely study the
Fig. 8. The distance.
27
Al(n,α)24Na reaction rates ratios in dependence on beryllium-to-sample
evolution of neutron field in dependence on the target-to-sample distance. In Fig. 8, the experimentally determined reaction rates for the 27Al(n,α)24Na reaction in all distances are normalized to the reaction rate at the position P14. The observed deflection of the rate values from the 1/ R 2 law is apparent due to the non-point-like geometry of the irradiation system, where the dimensions of target and activation foils as well as their distance are comparable. 3. Conclusions At the NPI, new neutron field based on the d(20)+Be source reaction was developed employing the 20 MeV deuteron beam extracted from the U-120 M isochronous cyclotron and beryllium target station of the NG-2 generator. The mean energy of obtained spectra is about 7.7 MeV, and it corresponds well to the empirical equation (1) earlier derived from measurements of the other scientists (Lone et al., 1977; Meulders et al., 1975; Weaver et al., 1973; Graves et al., 1979; Brede et al., 1989; Saltmarsh et al., 1977; Meadows et al., 1993; Waterman et al., 1979; Madey et al., 1977; Cierjacks, 1983). The shape of neutron spectra is in very good agreement with results reported from other authors, in particular Brede et al. (1989), Meulders et al. (1975), and Lone et al. (1977). The fast neutron spectral flux amounts 5.5 × 1010 cm−2s−1 for position P0 and 1.7 × 109 cm−2s−1 for position P14 with an uncertainity of 8% and for proton beam current of 7.25 μA. The newly obtained neutron spectra from the d(20)+Be source reaction represent a useful tool for integral benchmark experiments,
Fig. 6. Neutron spectrum of NG-2 generator at NPI with source reaction of d(20)+Be in lin-lin scale for position P14 for 20 MeV deuteron beam.
4
Radiation Physics and Chemistry xxx (xxxx) xxx–xxx
M. Stefanik et al.
344. Madey, R., et al., 1977. Neutron spectra at 0 degrees from 83.7 MeV deuterons and 100.2 MeV protons on beryllium. Med. Phys. 4, 322. Meadows, J.W., et al., 1993. The thick-target 9Be(d,n) neutron spectra for deuteron energies between 2.6 and 7.0 MeV. Nucl. Inst. Meth. A 324, 239–246. Meulders, J.P., et al., 1975. Fast neutron yields and spectra from targets of varying atomic number bombarded with deuterons from 16 to 50 MeV. Phys. Med. Biol. 20, 235–243. Mollendorf, U., et al., 2002. A nuclear simulation experiment for the International Fusion Materials Irradiation Facility (IFMIF), Karlsruhe. Petzow, G., et al., 2005. Beryllium and Beryllium Compounds, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. Q-value Calculator, 2016. National Nuclear Data Center BNL. Saltmarsh, M.J., et al., 1977. Characteristics of an intense neutron source based on the d.Be reaction. Nucl. Inst. Meth. 145, 81–90. SAND-II-SNL, 1996. PSR-345, UKAEA FUS. Stefanik, M., et al., 2012. The p-D2O generator neutron spectrum determination by multi-foil activation method. Trans. ANS 106, 894. Stefanik, M., et al., 2014a. High-flux white neutron source based on p(35)-Be reactions for activation experiments at NPI. Radiat. Phys. Chem. 104, 302–305. Stefanik, M., et al., 2014b. Neutron spectrum determination of the p(35 MeV)-Be source reaction by the dosimetry foils method. Nucl. Data Sheets 119, 422–424. Stefanik, M., et al., 2014. Accelerator driven p(37)-D2O fast neutron source at NPI Rez. In: Proceedings of the the 15th International Scientific Conference on Electric Power Engineering, EPE-2014. Waterman, F.M., et al., 1979. Neutron spectra from 35 and 46 MeV protons, 16 and 28 MeV deuterons, and 44 MeV 3He ions on thick beryllium. Med. Phys. 6, 432–435. Waters, L.S., et al., 2007. MCNPX User’s Manual V. 2.1.5. Weaver, K.A., et al., 1973. Neutron spectra from deuteron bombardment of D, Li, Be, and C. Nucl. Sci. Eng. 52, 35–45. Wyckoff, H.O., et al., 1976. ICRU Report 26 - Neutron Dosimetry for Biology and Medicine. In: International Commision on Radiation Units and Measurements, Maryland.
integral validation of cross-sections for fusion related program IFMIF, testing the radiation hardness of electronics against the fast neutron fields, and for application of neutron activation analysis. Acknowledgements The irradiation experiments in the neutron field of accelerator driven fast neutron source NG-2 carried out at the CANAM infrastructure of the NPI CAS Rez are supported through the MSMT project no. LM2011019. References Allisy, A., et al., 1989. ICRU Report 45 - Clinical Neutron Dosimetry Part I: Determination of Absorbed Dose in a Patient Treated by External Beams of Fast Neutrons. In: International Commision on Radiation Units and Measurements, Maryland. Brede, H.J., et al., 1989. Neutron yields from thick Be targets bombarded with deuterons or protons. Nucl. Inst. Meth. A 274, 332–344. Chadwick, M., et al., 2011. ENDF/B-VII.1: NuclearDataforScienceandTechnology: Cross Sections, Covariances, Fission Product Yields and Decay Data, Nuc. Data Sheets 112, 2887. Chu, S.Y.F., et al., 2016. Lund/LBNL Nuclear Data Search Version 2.0. Cierjacks, S., 1983. Neutron Sources For Basic Physics and Applications. Pergamon Press, Oxford. Forrest, R.A., et al., 2010. EAF-2010 48, UKAEA FUS. Graves, R.G., et al., 1979. Neutron energy spectra of d(49)-Be and p(41)-Be neutron radiotherapy sources. Med. Phys. 6, 123–128. Lone, M.A., et al., 1977. Thick target neutron yields and spectral distributions from the 7 Li(d,n), 7Li(p,n) and 9Be(d,n), 9Be(p,n) reactions, Nucl. Inst. Meth. 143, pp. 331–
5