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Development and performance of a 14-MeV neutron generator
Nuclear Inst. and Methods in Physics Research, A 959 (2020) 163495
Contents lists available at ScienceDirect
Nuclear Inst. and Methods in Physics Research, A journal homepage: www.elsevier.com/locate/nima
Development and performance of a 14-MeV neutron generator S.J. Vala a,b ,∗, M. Abhangi a,b , Ratnesh Kumar a , S. Tiwari a , R. Kumar a,b , H.L. Swami a , M. Bandyopadhyay a,b a b
Institute for Plasma Research, Bhat, Gandhinagar 382428, India Homi Bhabha National Institute (HBNI), Anushaktinagar, Mumbai 400094, India
ARTICLE
INFO
Keywords: D-T neutron source 2.45 GHz ECR ion source Tritium target Neutron diagnostics
ABSTRACT An accelerator-based 14-MeV neutron generator for fusion neutronics studies has been developed in Fusion Neutronics Laboratory (FNL) at Institute for Plasma Research (IPR). The generator is initially designed to produce a neutron yield of 1010 n/s and operated for 1.1×109 n/s. It will be further upgraded to the neutron yield of 1012 n/s. The neutron generator consists of various components such as 2.45 GHz ECR ion source, 300 kV linear accelerator, beam diagnostic system, TMP based vacuum system, solid tritium target, and control system. Various direct and indirect neutron detection techniques are deployed to evaluate the performance of the neutron generator in terms of neutron emission. These techniques include foil activation, alpha particle diagnostic, and He-3 proportional counter. The reaction rates for the foil activation are estimated using the Monte Carlo technique. Results of all independent diagnostics are obtained and compared. The paper describes the experimental setup and neutron diagnostics. It also describes the performance of the 14-MeV neutron generator highlighting its stability under continuous operation.
1. Introduction The intense 14 MeV neutron source is one of the prime requirements for the upcoming research plan for the International fusion program (DEMO, future nuclear fusion reactor). The development of a safe, cost-effective, and environmentally secured neutron source is one of the prime requirements for the fusion research community. The charged particle induced acceleration based neutron source is one of the most promising approaches [1–4], especially in terms of lowcost and low tritium consumption. The Fusion Neutronics Laboratory (FNL) at Institute for Plasma Research (IPR) has started the program to develop an accelerator based 14-MeV Neutron Generator (NG). It will support the Indian fusion program as well as the international fusion program [5,6]. The IPR has designed and developed the NG, which can produce neutrons yield of 1010 n/s [5]. It is currently producing 109 n/s, and it will be further upgraded to 1012 n/s in the next phase [7]. The Associated Alpha Particle Diagnostic (AAD) technique is used for the direct neutron yield measurement [8]. It measures the associated alpha particle coming out with neutron in the T(D, n)42 He reaction. It has one to one corresponds to the neutron emission. The Silicone Surface barrier Detector (SSD) is used to measure the alpha particle. The neutron yield is also measured by other indirect methods, like the He-3 detector and foil activation techniques [9]. The simulation model has been developed to estimate the reaction rates in foil. The neutron
source details have been generated using the NueSDesc code [10], using the deuterium beam parameter and tritium target geometry details. The NG will be used for benchmark experiments regarding Fusion Evaluated Nuclear Data (FENDL), neutron detector testing measurements, double differential cross-section measurements, blanket mockup experiments, and study on neutron as well as charged particle induced damage to the structural material [11–15]. The paper covers the details description of the neutron generator, its components, and comprehensive performance evaluation using various diagnostics.
2. Description of neutron generator The schematic diagram of the neutron generator components and its assembly is shown in Fig. 1. The neutron generator is mainly described in the following parts. (1) High voltage deck assembly (2) Acceleration system (3) Beam diagnostic system (4) Target assembly and vacuum system (5) Control system
∗ Corresponding author at: Institute for Plasma Research, Bhat, Gandhinagar 382428, India. E-mail address: [email protected] (S.J. Vala).
https://doi.org/10.1016/j.nima.2020.163495 Received 18 October 2019; Received in revised form 17 January 2020; Accepted 20 January 2020 Available online 23 January 2020 0168-9002/© 2020 Elsevier B.V. All rights reserved.
S.J. Vala, M. Abhangi, R. Kumar et al.
Nuclear Inst. and Methods in Physics Research, A 959 (2020) 163495
Fig. 1. Schematic of 14-MeV neutron generator.
2.1. High voltage deck assembly
2.4. Target assembly and vacuum system
The high voltage deck assembly housed an Electron Cyclotron Ion Source (ECRIS), Deuterium gas generator, and water cooling system. It is kept on 300 kV floating potential. The input electric power to the entire high voltage deck assembly is provided through a DC isolation transformer (350 kV, 15 kVA). The ECRIS consists of a water cooled plasma chamber, 2.45 GHz microwave system, ion extraction system, and Einzel lens. The 2.45 GHz microwave system contains a 2 kW magnetron, which is coupled to the plasma chamber followed by the circulator, dummy load, three-stub tuner, and high voltage DC break [16,17]. The two-electrode extraction system is installed for the extraction of the deuterium ions beam from the plasma chamber. The aperture diameter of the plasma electrode and the puller electrode are 8 mm and 10 mm, respectively. The gap between the two electrodes is 10 mm. The plasma electrode is supplied by a 50 kV High voltage power supply, and the puller electrode is kept at ground potential. The extracted deuterium ion beam is focused into the acceleration column via an Einzel lens.
The two numbers of Pfeiffer makes (Hipace-700) Turbo Molecular Pump (TMP) sets are integrated with the beamline of the NG to achieve the vacuum in order of 10−7 mbar. The pneumatically controlled gate valve is installed between the tritium target assembly and the beamline for isolation of the target chamber from the beamline. The 10 Ci TiT target is mounted in a water-cooled target holder and placed at the end of the target chamber. The heat removal of the target is done by a closed water circulation loop at a flow rate of 3 liter per minute and inlet temperature of 18 ◦ C. 2.5. Control system The control system of the NG is divided into two parts, the first part based on Programmable Logic Control (PLC) and the second part of the based on PXIe. The PLC controls the equipment kept on the high voltage deck assembly, and PXIe controls the equipment kept at the ground potential. Both systems are controlled and monitored by a centralized control system.
2.2. Acceleration system 3. Neutron generator performance evaluation The acceleration system consists of four numbers of National Electrostatic Corporation (NEC) make general-purpose acceleration columns, each having a voltage rating of 75 kV in air. It is used to accelerate the deuterium ion beam up to 300 keV. One end of this acceleration column is coupled to the Einzel lens on the high voltage deck, and another end connected to the vacuum system at ground potential. The Glassman makes (300 kV, 10 mA) high voltage power supply is used to supplied high voltage to the acceleration system.
To evaluate the performance of the neutron generator, the measurement of accurate neutron yield throughout the experiments is very important. Hence various neutron diagnostic systems have been installed, such as alpha particle, foil activation, and gas-filled detectors. The results of neutron diagnostic techniques and their associated uncertainty are presented in the following sections. 3.1. Associated alpha particle technique
2.3. Beam diagnostic system
An Associated Alpha particle Diagnostic (AAD) is used to measure the source strength of the neutron source in absolute terms [8,9]. An associated alpha particle of energy 3.5 MeV is emitted along with a neutron during a T(D, n)42 He reaction. This technique is used to measure the neutron yield by counting the associated 𝛼-particles with a semiconductor detector. From the alpha count rate, the absolute total neutron is measured using Eq. (1). In the center of the mass frame, the neutron and alpha emission is isotropic for the deuteron beam of energy (𝐸𝐷 ) < 400 keV, but in the laboratory frame, the neutron and alpha emission is not isotropic [8,9].
The beam diagnostic system includes NEC make Beam Profile Monitor (BPM) and Faraday cup (FC), for the measurement of beam size and beam current, respectively. The BPM consists of a rotating helical wire which rotates at 18 cycles per second. Its sweeps twice across the beam during each rotation, and provide a signal for X and Y profile. The beam-induced secondary electrons are collected by a cylindrical collector placed surrounding wire. It provides a signal proportional to the intercepted beam intensity at every instant. The FC has a tantalum cone, which measures the beam current. It has the suppressor to return secondary electrons for accurate beam current measurement.
𝑌𝑛 = 𝑌𝛼 = 2
4𝜋 𝐶 𝑅(𝐸𝐷 , 𝜃𝛼 ) 𝛥𝛺 𝛼
(1)
S.J. Vala, M. Abhangi, R. Kumar et al.
Nuclear Inst. and Methods in Physics Research, A 959 (2020) 163495
Fig. 2. Schematic of the experimental setup of AAD.
Fig. 3. Silicon Surface Barrier Detector (SBD) detection scheme.
Fig. 5. Alpha particle spectrum for d(T, n)4 He reaction with Al foil.
Table 1 Parameter of alpha monitor.
where, 𝛥𝛺 represents the solid angle subtended by the detector aperture on the target, 𝐶𝛼 represents the true alpha count detected by the detector, 𝑅(𝐸𝐷 , 𝜃𝛼 ) represents the anisotropy correction factor whose value depends on the incident energy 𝐸𝐷 of the beam, and the emitted angle of the alpha particle 𝜃𝛼 . The anisotropy correction factor 𝑅(𝐸𝐷 , 𝜃𝛼 ) can be calculated from Eq. (2) [18]. ( ) E dE ∫0 D d𝜎(E) ∕ dX (E)dE ( ) d𝜔′ Ti−T 𝑅 𝐸𝐷 , 𝜃𝛼 = (2) d𝜎(E) ED
(
′ )d𝜔 dE (E) dX Ti−T
Values
Detector active area (a) Solid angle (𝛥𝛺) Anisotropy correction factor
1.172 ± 0.042 mm2 (3.33158 ± 0.13151) × 10−06 sr 1.23
spectroscopy amplifier Model 2026, and PC based FASTCOMTEC MCA3. The FASTCOMTEC MCA-3 also accepts scaling input and used as Multichannel Scalar (MCS) mode. The detection scheme is shown in Fig. 3. An aluminum foil of 7 μm with an aperture diameter of 1.22165 ± 0.02180 mm is placed in front of the SSB detector. The purpose of thin aluminum foil is to eliminate most of the Rutherford scattered deuterons [19]. The diameter of the aperture is measured by an optical microscope for accurate measurement of a solid angle. The detector is kept at a distance of 593 ± 5 mm away from the tritium target to protect the detector from neutron damage. The parameter of the alpha monitor is summarized in Table 1. The energy calibration of the detector is carried out by using Pu-239 and Am-241 alpha sources. The charge particle spectrums of Am-241 are obtained with and without the aluminum foil. It gives the energy shifting of the peak due to the energy loss of particles in aluminum foil, as shown in Fig. 4. The measured alpha particle spectrum for d(T, n)4 He reaction with Al foil is shown in Fig. 5. The anisotropy correction factor is evaluated for the deuterium beam of energy (𝐸𝐷 )190 keV and for emission angle of the alpha particle (𝜃𝛼 )175◦ . The measured neutron yield is 1.20 × 108 n/s. The uncertainty in the neutron yield measurement is estimated by accounting uncertainty in anisotropy correction factor (±1.5%) and solid angle (±3.9%). An error in the anisotropy correction factor is due to the non-uniform tritium distribution in the TiT target, and this unknown error evaluated using references [18]. The errors in the anisotropy correction factor and the solid angle are independent, hence can be summed using a
Fig. 4. Alpha particle spectra of an Am source with and without Al foil.
∫0
Parameter
d𝜔′ (E, 𝜃𝛼 )dE d𝜔
where the term d𝜎∕d𝜔′ represents the D-T differential cross-section in the center of mass frame, dE∕dX represents the rate of the energy loss of deuterons in the Ti-T target, and d𝜔′ ∕d𝜔 represents the solid angle conversion factor from the center of the mass frame to lab frame. The ORTEC make Silicone Surface Barrier detector (SBD) is used for the measurement of the associated charged particle, which has a surface area of 50 mm2 and a depth of 100 μm (model no. BA-1550-100). It is installed in the target chamber and the schematic of the detection scheme shown in Fig. 2. The detected alpha is counted by FASTCOMTEC make preamplifier model CSP10, CANBERRA make 3
S.J. Vala, M. Abhangi, R. Kumar et al.
Nuclear Inst. and Methods in Physics Research, A 959 (2020) 163495
quadratic law. The overall uncertainty in the yield measurement is 4.22%. The contributions from other competing nuclear reactions are also studied. It is observed that the accumulation of deuterium on the target does not affect neutron yield measurement. As the tritium target is almost 10 years old, the possibility of 3 He(d, p)4 He examined. However, the deuteron beam energy is 190 keV measured pulse height spectrum does not show any peak due to parasitic reaction on the target [20]. The reaction rate of this reaction becomes comparable with the D-T reaction rate for Ed > 250 keV. 3.2. Activation technique A 27 Al(n, 𝛼)24 Na reaction is chosen for the determination of neutron yield by activation technique because of its half-life (14.9 h) and threshold energy for this reaction (3.2 MeV). It gives sufficient counts in shorter irradiation time and appropriates for the detection of fast neutrons. It is a standard dosimetry reaction used for cross-section measurement of other reactions also [9]. A total five number of aluminum foils are placed on the outside of the stainless steel (SS) target holder. Each foil is having a diameter of 13 mm and a thickness of 0.5 mm. One foil is placed at 0◦ at a distance of 1.7 ± 0.1 cm, and the remaining four placed at the periphery of the target holder at 90◦ interval. The purpose of these four foils is to validate the source definition card generated by the NeuSDesc code [10]. The activity inside the foil after the irradiation is measured by the High Purity Germanium (HPGe) detector. The efficiency calibration of the HPGe detector is done using a point source. However, actual samples (foils) are volume sources. Monte Carlo simulation is also carried out for modeling of the HPGe detector. This model is validated for a point source of different energies and distance between source to a detector. This model is used to find efficiency for foil activation diagnostic. The recorded spectrum of gamma-rays emitted from the irradiated Al foil is shown in Fig. 6. Neutron yield can be determined from a mathematical Eq. (3). 𝑌𝑛 =
4 𝜋r 2 𝑃𝑘 𝜆𝐵 𝐴 ( ) ( ) 𝐾𝐺 𝐾𝐴 𝐾𝑅 𝐼𝛾 𝜀𝐴 𝜎𝐴 𝐿𝑚𝑎 1 − e−𝜆𝐵 𝑡𝑖𝑟𝑟 e−𝜆𝐵 𝑡𝑐 1 − e−𝜆𝐵 𝑡𝑚
Fig. 6. Gamma-ray spectrum of irradiated aluminum foil.
forward direction of the beam at 0◦ . The proportional counter has an inbuilt pulse processing unit and the bias supply. It measures the count rate as well as the dose rate. The detector is calibrated using an Am–Be standard neutron source. The flux at the detector location is estimated from the count rate, which gives neutron yield by Eq. (4). 𝑌𝑝𝑐 =
4 𝜋d2 𝜙 . 𝐾𝐺 𝐾𝐴 𝐾𝑅
(4)
KG , KA , and KR are geometrical correction factor, anisotropy correction factor, and attenuation correction, respectively. The d is a distance between the source and the detector. The measured neutron yield is 1.17 × 108 n/s. The uncertainty in the neutron yield measurement is due to the uncertainty of the He-3 detector position (±0.4%) and its counting statistics (±1.3%). The overall uncertainty in the neutron yield is ∼1.4%.
(3) 3.4. Monte Carlo modeling
The term Pk is the photopeak count, A is the atomic mass of the foil material, 𝜆B is the decay constant, r is source to foil distance, KG is the geometrical correction factor, KA is the anisotropy correction factor, KR is the attenuation correction factor, I𝛾 is the gamma-ray intensity, 𝜀A is the peak efficiency, 𝜎A is the spectrum average cross-section, L is Avogadro number, m is the foil mass, a is the isotopic abundance, tirr is irradiation time, tc is the cooling time, and tm is the measurement time. The measured activity is used to determine the neutron yield of the neutron generator by activation analysis. This foil activation technique is reported elsewhere [9,21]. Neutron yield measured by this technique is also corrected for the source anisotropy and the attenuation correction. Attenuation correction is due to the target holder geometry. The attenuation correction factor is calculated by considering the copper plate, cooling water, and SS material of the target holder. The uncertainty involved in this method is being evaluated separately and will be reported later. The measured neutron yield is 1.20 × 108 n/s. Uncertainty in neutron yield measurement by foil activation technique is due to uncertainty in peak area (±2%), HPGE detector efficiency calibration (±2%), cross-section (±2%), foil position (±3%), and foil mass (±1.15%). The overall uncertainty in neutron yield is ∼4.6%.
The reaction rate of foil is calculated using the Monte Carlo NParticle (MCNP) radiation transport code and IRDF-2002 cross-section library. The DT neutron source and foil are modeled as per the actual experimental arrangement [10]. The scattering effect is also considered for obtaining the spectrum at the sample location. The neutron yield is obtained from Eq. (5) for 27 Al (n, a)24 Na reaction [9]. 𝑌𝑚𝑐 = 𝐹𝑎𝑐𝑡 ⟨𝜎∅⟩𝑚𝑐
(5)
The term Fact and ⟨𝜙𝜎⟩mc represents the experimental reaction rate and the calculated reaction rate from the MNCP transport code, respectively. The calculated neutron yield is 1.27 × 108 n/s. The uncertainty in the reaction rate calculation and experiments are less than ±1.0% and ±4.95%, respectively. The overall uncertainty in the neutron yield is 4.70%. 4. Assessment of operational stability The stability of the NG is measured to ensure the performance for long operation. The NG is operated for 5.42 h at deuterium beam parameters of ∼200 μA and 190 keV while monitoring corresponding neutron yield. Alpha particle spectrum and time-varying neutron emission are recorded by Multichannel Analyzer (MCA) and multichannel Scaler (MCS), respectively. The time profile of neutron emission is measured using the AAD technique. A small fluctuation in the emission of neutron yield is observed due to the disruption in the ion source reflected in Fig. 7. The standard deviation in the neutron emission is below 1%, which is quite promising for long-duration experiments.
3.3. He-3 proportional counter The LB 6411 Berthold make He-3 proportional counter is used to monitor the source neutron strength. A detector is enclosed in a spherical polyethene moderator having an outer diameter of 25 cm. It is placed at a distance of 56.5 cm away from the target holder in the 4
S.J. Vala, M. Abhangi, R. Kumar et al.
Nuclear Inst. and Methods in Physics Research, A 959 (2020) 163495
with respect to intellectual property. In so doing we confirm that we have followed the regulations of our institutions concerning intellectual property. CRediT authorship contribution statement S.J. Vala: Conceptualization, Data curation, Formal Analysis, Methodology, Project administration, Writing - original draft, Writing - review & editing, Supervision. M. Abhangi: Data curation, Formal Analysis, Methodology, Writing - original draft, Writing - review & editing. Ratnesh Kumar: Conceptualization, Data curation, Methodology, Writing - original draft. S. Tiwari: Data curation, Formal Analysis, Writing - original draft. R. Kumar: Conceptualization, Formal Analysis, Writing - review & editing, Supervision. H.L. Swami: Data curation, Writing - review & editing. M. Bandyopadhyay: Conceptualization, Writing - review & editing. References Fig. 7. Time-varying neutron yield measurement.
[1] A. Ibarra, et al., The IFMIF-DONES project: preliminary engineering design, Nucl. Fusion 58 (2018) 105002. [2] J. Knaster, et al., Overview of the IFMIF/EVEDA project, Nucl. Fusion 57 (2017) 102016. [3] A. Pietropaolo, et al., The Frascati Neutron Generator: A multipurpose facility for physics and engineering, J. Phys. Conf. Ser. 1021 (2018) 012004. [4] E. Surrey, et al., Application of accelerator based neutron sources in fusion materials research, in: 2013 IEEE 25th Symposium on Fusion Engineering, SOFE, San Francisco, CA, 2013, pp. 1–6. [5] P.K. Sarkar, S. Basu, M. Nandy, Accelerator and Radiation Physics, Narosa Publishing House, New Delhi, India, 2013, p. 170. [6] H. Swami, et al., Occupational radiation exposure control analyses of 14 MeV neutron generator facility: A neutronic assessment for the biological and local shield design, Nucl. Eng. Technol. (2020) http://dx.doi.org/10.1016/j.net.2020. 01.006. [7] Sudhirsinh Vala, et al., Rotating tritium target for intense 14-MeV neutron source, Fusion Eng. Des. 123 (2017) 77–81. [8] T.R. Fewell, An evaluation of the alpha counting technique for determining 14 MeV neutron yields, Nucl. Instrum. Methods 61 (1968) 61–71. [9] M. Angelone, et al., Absolute experimental and numerical calibration of the 14 MeV neutron source at the Frascati neutron generator, Rev. Sci. Instrum. 67 (1996) 2189. [10] E. Birgersson, G. Lövestam, NeuSDesc — neutron source description, EUR 23794 EN-2009. [11] H.L. Swami, et al., A neutronic experiment to support the design of an Indian TBM shield module for ITER, Plasma Sci. Technol. 21 (2019) 065601, 6. [12] P.N. Maya, et al., Evaluation of tungsten as divertor plasma-facing material: results from ion irradiation experiments and computer simulations, Nucl. Fusion 59 (2019) 076034. [13] Mayank Rajput, et al., Primary knock on atom spectra, gas production and displacement cross section for tungsten and chromium irradiated with neutrons at energies up to 14.1 MeV, Fusion Eng. Des. 130 (2018) 114–121. [14] M. Rajput, et al., Calculated differential and double differential cross section of DT neutron induced reactions on natural chromium (Cr), Indian J. Phys. 92 (1) (2018) 91–96. [15] Shrich Jakhar, et al., Tritium breeding mock-up experiments containing lithium titanate ceramic pebbles and lead irradiated with DT neutrons, Fusion Eng. Des. 95 (2015) 50–58. [16] A.T.T. Mostako, et al., Effect of microwave power on electron temperature and electron density in deuterium plasma generated by electron cyclotron resonance, IEEE Trans. Plasma Sci. 44 (2016) 7–14. [17] S. Vala, et al., Development of a test bench of 2.45 GHz ECR ion source for RFQ accelerator, J. Instrum. 14 (2019). [18] J. Benveniste, et al., The problem of measuring the absolute yield of 14-MeV Neutrons by means of an alpha counter, Nucl. Instrum. Methods 7 (1960) 306. [19] H. Maekawa, et al., Report JAERI-M 83-219. [20] D.W. Mingay, et al., Neutron induced reactions in silicon semiconductor detector, Nucl. Instrum. Methods 94 (1971) 497–507. [21] Shrich Jakhar, et al., Neutron flux spectra investigations in breeding blanket assembly containing lithium titanate and Lead irradiated with DT neutrons, Fusion Eng. Des. 100 (2015) 619–628.
5. Conclusion A 14 MeV neutron generator has been designed and developed for neutron yield of 1010 n/s having beam energy up to 300 keV using T(D, n)42 He reaction. The beam parameters are monitored through a beam diagnostic system for the reliable and stable operation of the neutron generator. The 10 Ci of solid tritium target is fitted in a watercooled target holder to produce neutrons. The performance of the NG has been evaluated for continuous operation. The neutron yield has been measured and estimated using various techniques such as Associated Alpha Particle, Foil Activation, He-3 Proportional Counter, and Monte Carlo Calculation. The uncertainties and correction factors associated with all measurement techniques have been assessed and incorporated into the final results. The neutron yield with an uncertainty measured by associated alpha particle techniques, foil activation technique, and He-3 proportional counter is 1.20 × 108 n/s ± 4.5%, 1.19 × 108 n/s ± 4.6%, and 1.17 ×108 n/s ± 1.38%, respectively. The associated alpha particle technique is the most accurate and sensitive among all three. The neutron yield with an uncertainty predicted by the Monte Carlo radiation transport code is 1.27 × 108 n/s ± 4.70%. The neutron yield evaluated by all techniques lies within ±10%. The neutron generator has been tested for continuous operation of 5.42 h with an average yield of 1.1 × 109 n/s for conducting fusion related neutronic mock-up experiments. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Funding No funding was received for this work. Intellectual property We confirm that we have given due consideration to the protection of intellectual property associated with this work and that there are no impediments to publication, including the timing of publication,
5
Contents lists available at ScienceDirect
Nuclear Inst. and Methods in Physics Research, A journal homepage: www.elsevier.com/locate/nima
Development and performance of a 14-MeV neutron generator S.J. Vala a,b ,∗, M. Abhangi a,b , Ratnesh Kumar a , S. Tiwari a , R. Kumar a,b , H.L. Swami a , M. Bandyopadhyay a,b a b
Institute for Plasma Research, Bhat, Gandhinagar 382428, India Homi Bhabha National Institute (HBNI), Anushaktinagar, Mumbai 400094, India
ARTICLE
INFO
Keywords: D-T neutron source 2.45 GHz ECR ion source Tritium target Neutron diagnostics
ABSTRACT An accelerator-based 14-MeV neutron generator for fusion neutronics studies has been developed in Fusion Neutronics Laboratory (FNL) at Institute for Plasma Research (IPR). The generator is initially designed to produce a neutron yield of 1010 n/s and operated for 1.1×109 n/s. It will be further upgraded to the neutron yield of 1012 n/s. The neutron generator consists of various components such as 2.45 GHz ECR ion source, 300 kV linear accelerator, beam diagnostic system, TMP based vacuum system, solid tritium target, and control system. Various direct and indirect neutron detection techniques are deployed to evaluate the performance of the neutron generator in terms of neutron emission. These techniques include foil activation, alpha particle diagnostic, and He-3 proportional counter. The reaction rates for the foil activation are estimated using the Monte Carlo technique. Results of all independent diagnostics are obtained and compared. The paper describes the experimental setup and neutron diagnostics. It also describes the performance of the 14-MeV neutron generator highlighting its stability under continuous operation.
1. Introduction The intense 14 MeV neutron source is one of the prime requirements for the upcoming research plan for the International fusion program (DEMO, future nuclear fusion reactor). The development of a safe, cost-effective, and environmentally secured neutron source is one of the prime requirements for the fusion research community. The charged particle induced acceleration based neutron source is one of the most promising approaches [1–4], especially in terms of lowcost and low tritium consumption. The Fusion Neutronics Laboratory (FNL) at Institute for Plasma Research (IPR) has started the program to develop an accelerator based 14-MeV Neutron Generator (NG). It will support the Indian fusion program as well as the international fusion program [5,6]. The IPR has designed and developed the NG, which can produce neutrons yield of 1010 n/s [5]. It is currently producing 109 n/s, and it will be further upgraded to 1012 n/s in the next phase [7]. The Associated Alpha Particle Diagnostic (AAD) technique is used for the direct neutron yield measurement [8]. It measures the associated alpha particle coming out with neutron in the T(D, n)42 He reaction. It has one to one corresponds to the neutron emission. The Silicone Surface barrier Detector (SSD) is used to measure the alpha particle. The neutron yield is also measured by other indirect methods, like the He-3 detector and foil activation techniques [9]. The simulation model has been developed to estimate the reaction rates in foil. The neutron
source details have been generated using the NueSDesc code [10], using the deuterium beam parameter and tritium target geometry details. The NG will be used for benchmark experiments regarding Fusion Evaluated Nuclear Data (FENDL), neutron detector testing measurements, double differential cross-section measurements, blanket mockup experiments, and study on neutron as well as charged particle induced damage to the structural material [11–15]. The paper covers the details description of the neutron generator, its components, and comprehensive performance evaluation using various diagnostics.
2. Description of neutron generator The schematic diagram of the neutron generator components and its assembly is shown in Fig. 1. The neutron generator is mainly described in the following parts. (1) High voltage deck assembly (2) Acceleration system (3) Beam diagnostic system (4) Target assembly and vacuum system (5) Control system
∗ Corresponding author at: Institute for Plasma Research, Bhat, Gandhinagar 382428, India. E-mail address: [email protected] (S.J. Vala).
https://doi.org/10.1016/j.nima.2020.163495 Received 18 October 2019; Received in revised form 17 January 2020; Accepted 20 January 2020 Available online 23 January 2020 0168-9002/© 2020 Elsevier B.V. All rights reserved.
S.J. Vala, M. Abhangi, R. Kumar et al.
Nuclear Inst. and Methods in Physics Research, A 959 (2020) 163495
Fig. 1. Schematic of 14-MeV neutron generator.
2.1. High voltage deck assembly
2.4. Target assembly and vacuum system
The high voltage deck assembly housed an Electron Cyclotron Ion Source (ECRIS), Deuterium gas generator, and water cooling system. It is kept on 300 kV floating potential. The input electric power to the entire high voltage deck assembly is provided through a DC isolation transformer (350 kV, 15 kVA). The ECRIS consists of a water cooled plasma chamber, 2.45 GHz microwave system, ion extraction system, and Einzel lens. The 2.45 GHz microwave system contains a 2 kW magnetron, which is coupled to the plasma chamber followed by the circulator, dummy load, three-stub tuner, and high voltage DC break [16,17]. The two-electrode extraction system is installed for the extraction of the deuterium ions beam from the plasma chamber. The aperture diameter of the plasma electrode and the puller electrode are 8 mm and 10 mm, respectively. The gap between the two electrodes is 10 mm. The plasma electrode is supplied by a 50 kV High voltage power supply, and the puller electrode is kept at ground potential. The extracted deuterium ion beam is focused into the acceleration column via an Einzel lens.
The two numbers of Pfeiffer makes (Hipace-700) Turbo Molecular Pump (TMP) sets are integrated with the beamline of the NG to achieve the vacuum in order of 10−7 mbar. The pneumatically controlled gate valve is installed between the tritium target assembly and the beamline for isolation of the target chamber from the beamline. The 10 Ci TiT target is mounted in a water-cooled target holder and placed at the end of the target chamber. The heat removal of the target is done by a closed water circulation loop at a flow rate of 3 liter per minute and inlet temperature of 18 ◦ C. 2.5. Control system The control system of the NG is divided into two parts, the first part based on Programmable Logic Control (PLC) and the second part of the based on PXIe. The PLC controls the equipment kept on the high voltage deck assembly, and PXIe controls the equipment kept at the ground potential. Both systems are controlled and monitored by a centralized control system.
2.2. Acceleration system 3. Neutron generator performance evaluation The acceleration system consists of four numbers of National Electrostatic Corporation (NEC) make general-purpose acceleration columns, each having a voltage rating of 75 kV in air. It is used to accelerate the deuterium ion beam up to 300 keV. One end of this acceleration column is coupled to the Einzel lens on the high voltage deck, and another end connected to the vacuum system at ground potential. The Glassman makes (300 kV, 10 mA) high voltage power supply is used to supplied high voltage to the acceleration system.
To evaluate the performance of the neutron generator, the measurement of accurate neutron yield throughout the experiments is very important. Hence various neutron diagnostic systems have been installed, such as alpha particle, foil activation, and gas-filled detectors. The results of neutron diagnostic techniques and their associated uncertainty are presented in the following sections. 3.1. Associated alpha particle technique
2.3. Beam diagnostic system
An Associated Alpha particle Diagnostic (AAD) is used to measure the source strength of the neutron source in absolute terms [8,9]. An associated alpha particle of energy 3.5 MeV is emitted along with a neutron during a T(D, n)42 He reaction. This technique is used to measure the neutron yield by counting the associated 𝛼-particles with a semiconductor detector. From the alpha count rate, the absolute total neutron is measured using Eq. (1). In the center of the mass frame, the neutron and alpha emission is isotropic for the deuteron beam of energy (𝐸𝐷 ) < 400 keV, but in the laboratory frame, the neutron and alpha emission is not isotropic [8,9].
The beam diagnostic system includes NEC make Beam Profile Monitor (BPM) and Faraday cup (FC), for the measurement of beam size and beam current, respectively. The BPM consists of a rotating helical wire which rotates at 18 cycles per second. Its sweeps twice across the beam during each rotation, and provide a signal for X and Y profile. The beam-induced secondary electrons are collected by a cylindrical collector placed surrounding wire. It provides a signal proportional to the intercepted beam intensity at every instant. The FC has a tantalum cone, which measures the beam current. It has the suppressor to return secondary electrons for accurate beam current measurement.
𝑌𝑛 = 𝑌𝛼 = 2
4𝜋 𝐶 𝑅(𝐸𝐷 , 𝜃𝛼 ) 𝛥𝛺 𝛼
(1)
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Nuclear Inst. and Methods in Physics Research, A 959 (2020) 163495
Fig. 2. Schematic of the experimental setup of AAD.
Fig. 3. Silicon Surface Barrier Detector (SBD) detection scheme.
Fig. 5. Alpha particle spectrum for d(T, n)4 He reaction with Al foil.
Table 1 Parameter of alpha monitor.
where, 𝛥𝛺 represents the solid angle subtended by the detector aperture on the target, 𝐶𝛼 represents the true alpha count detected by the detector, 𝑅(𝐸𝐷 , 𝜃𝛼 ) represents the anisotropy correction factor whose value depends on the incident energy 𝐸𝐷 of the beam, and the emitted angle of the alpha particle 𝜃𝛼 . The anisotropy correction factor 𝑅(𝐸𝐷 , 𝜃𝛼 ) can be calculated from Eq. (2) [18]. ( ) E dE ∫0 D d𝜎(E) ∕ dX (E)dE ( ) d𝜔′ Ti−T 𝑅 𝐸𝐷 , 𝜃𝛼 = (2) d𝜎(E) ED
(
′ )d𝜔 dE (E) dX Ti−T
Values
Detector active area (a) Solid angle (𝛥𝛺) Anisotropy correction factor
1.172 ± 0.042 mm2 (3.33158 ± 0.13151) × 10−06 sr 1.23
spectroscopy amplifier Model 2026, and PC based FASTCOMTEC MCA3. The FASTCOMTEC MCA-3 also accepts scaling input and used as Multichannel Scalar (MCS) mode. The detection scheme is shown in Fig. 3. An aluminum foil of 7 μm with an aperture diameter of 1.22165 ± 0.02180 mm is placed in front of the SSB detector. The purpose of thin aluminum foil is to eliminate most of the Rutherford scattered deuterons [19]. The diameter of the aperture is measured by an optical microscope for accurate measurement of a solid angle. The detector is kept at a distance of 593 ± 5 mm away from the tritium target to protect the detector from neutron damage. The parameter of the alpha monitor is summarized in Table 1. The energy calibration of the detector is carried out by using Pu-239 and Am-241 alpha sources. The charge particle spectrums of Am-241 are obtained with and without the aluminum foil. It gives the energy shifting of the peak due to the energy loss of particles in aluminum foil, as shown in Fig. 4. The measured alpha particle spectrum for d(T, n)4 He reaction with Al foil is shown in Fig. 5. The anisotropy correction factor is evaluated for the deuterium beam of energy (𝐸𝐷 )190 keV and for emission angle of the alpha particle (𝜃𝛼 )175◦ . The measured neutron yield is 1.20 × 108 n/s. The uncertainty in the neutron yield measurement is estimated by accounting uncertainty in anisotropy correction factor (±1.5%) and solid angle (±3.9%). An error in the anisotropy correction factor is due to the non-uniform tritium distribution in the TiT target, and this unknown error evaluated using references [18]. The errors in the anisotropy correction factor and the solid angle are independent, hence can be summed using a
Fig. 4. Alpha particle spectra of an Am source with and without Al foil.
∫0
Parameter
d𝜔′ (E, 𝜃𝛼 )dE d𝜔
where the term d𝜎∕d𝜔′ represents the D-T differential cross-section in the center of mass frame, dE∕dX represents the rate of the energy loss of deuterons in the Ti-T target, and d𝜔′ ∕d𝜔 represents the solid angle conversion factor from the center of the mass frame to lab frame. The ORTEC make Silicone Surface Barrier detector (SBD) is used for the measurement of the associated charged particle, which has a surface area of 50 mm2 and a depth of 100 μm (model no. BA-1550-100). It is installed in the target chamber and the schematic of the detection scheme shown in Fig. 2. The detected alpha is counted by FASTCOMTEC make preamplifier model CSP10, CANBERRA make 3
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Nuclear Inst. and Methods in Physics Research, A 959 (2020) 163495
quadratic law. The overall uncertainty in the yield measurement is 4.22%. The contributions from other competing nuclear reactions are also studied. It is observed that the accumulation of deuterium on the target does not affect neutron yield measurement. As the tritium target is almost 10 years old, the possibility of 3 He(d, p)4 He examined. However, the deuteron beam energy is 190 keV measured pulse height spectrum does not show any peak due to parasitic reaction on the target [20]. The reaction rate of this reaction becomes comparable with the D-T reaction rate for Ed > 250 keV. 3.2. Activation technique A 27 Al(n, 𝛼)24 Na reaction is chosen for the determination of neutron yield by activation technique because of its half-life (14.9 h) and threshold energy for this reaction (3.2 MeV). It gives sufficient counts in shorter irradiation time and appropriates for the detection of fast neutrons. It is a standard dosimetry reaction used for cross-section measurement of other reactions also [9]. A total five number of aluminum foils are placed on the outside of the stainless steel (SS) target holder. Each foil is having a diameter of 13 mm and a thickness of 0.5 mm. One foil is placed at 0◦ at a distance of 1.7 ± 0.1 cm, and the remaining four placed at the periphery of the target holder at 90◦ interval. The purpose of these four foils is to validate the source definition card generated by the NeuSDesc code [10]. The activity inside the foil after the irradiation is measured by the High Purity Germanium (HPGe) detector. The efficiency calibration of the HPGe detector is done using a point source. However, actual samples (foils) are volume sources. Monte Carlo simulation is also carried out for modeling of the HPGe detector. This model is validated for a point source of different energies and distance between source to a detector. This model is used to find efficiency for foil activation diagnostic. The recorded spectrum of gamma-rays emitted from the irradiated Al foil is shown in Fig. 6. Neutron yield can be determined from a mathematical Eq. (3). 𝑌𝑛 =
4 𝜋r 2 𝑃𝑘 𝜆𝐵 𝐴 ( ) ( ) 𝐾𝐺 𝐾𝐴 𝐾𝑅 𝐼𝛾 𝜀𝐴 𝜎𝐴 𝐿𝑚𝑎 1 − e−𝜆𝐵 𝑡𝑖𝑟𝑟 e−𝜆𝐵 𝑡𝑐 1 − e−𝜆𝐵 𝑡𝑚
Fig. 6. Gamma-ray spectrum of irradiated aluminum foil.
forward direction of the beam at 0◦ . The proportional counter has an inbuilt pulse processing unit and the bias supply. It measures the count rate as well as the dose rate. The detector is calibrated using an Am–Be standard neutron source. The flux at the detector location is estimated from the count rate, which gives neutron yield by Eq. (4). 𝑌𝑝𝑐 =
4 𝜋d2 𝜙 . 𝐾𝐺 𝐾𝐴 𝐾𝑅
(4)
KG , KA , and KR are geometrical correction factor, anisotropy correction factor, and attenuation correction, respectively. The d is a distance between the source and the detector. The measured neutron yield is 1.17 × 108 n/s. The uncertainty in the neutron yield measurement is due to the uncertainty of the He-3 detector position (±0.4%) and its counting statistics (±1.3%). The overall uncertainty in the neutron yield is ∼1.4%.
(3) 3.4. Monte Carlo modeling
The term Pk is the photopeak count, A is the atomic mass of the foil material, 𝜆B is the decay constant, r is source to foil distance, KG is the geometrical correction factor, KA is the anisotropy correction factor, KR is the attenuation correction factor, I𝛾 is the gamma-ray intensity, 𝜀A is the peak efficiency, 𝜎A is the spectrum average cross-section, L is Avogadro number, m is the foil mass, a is the isotopic abundance, tirr is irradiation time, tc is the cooling time, and tm is the measurement time. The measured activity is used to determine the neutron yield of the neutron generator by activation analysis. This foil activation technique is reported elsewhere [9,21]. Neutron yield measured by this technique is also corrected for the source anisotropy and the attenuation correction. Attenuation correction is due to the target holder geometry. The attenuation correction factor is calculated by considering the copper plate, cooling water, and SS material of the target holder. The uncertainty involved in this method is being evaluated separately and will be reported later. The measured neutron yield is 1.20 × 108 n/s. Uncertainty in neutron yield measurement by foil activation technique is due to uncertainty in peak area (±2%), HPGE detector efficiency calibration (±2%), cross-section (±2%), foil position (±3%), and foil mass (±1.15%). The overall uncertainty in neutron yield is ∼4.6%.
The reaction rate of foil is calculated using the Monte Carlo NParticle (MCNP) radiation transport code and IRDF-2002 cross-section library. The DT neutron source and foil are modeled as per the actual experimental arrangement [10]. The scattering effect is also considered for obtaining the spectrum at the sample location. The neutron yield is obtained from Eq. (5) for 27 Al (n, a)24 Na reaction [9]. 𝑌𝑚𝑐 = 𝐹𝑎𝑐𝑡 ⟨𝜎∅⟩𝑚𝑐
(5)
The term Fact and ⟨𝜙𝜎⟩mc represents the experimental reaction rate and the calculated reaction rate from the MNCP transport code, respectively. The calculated neutron yield is 1.27 × 108 n/s. The uncertainty in the reaction rate calculation and experiments are less than ±1.0% and ±4.95%, respectively. The overall uncertainty in the neutron yield is 4.70%. 4. Assessment of operational stability The stability of the NG is measured to ensure the performance for long operation. The NG is operated for 5.42 h at deuterium beam parameters of ∼200 μA and 190 keV while monitoring corresponding neutron yield. Alpha particle spectrum and time-varying neutron emission are recorded by Multichannel Analyzer (MCA) and multichannel Scaler (MCS), respectively. The time profile of neutron emission is measured using the AAD technique. A small fluctuation in the emission of neutron yield is observed due to the disruption in the ion source reflected in Fig. 7. The standard deviation in the neutron emission is below 1%, which is quite promising for long-duration experiments.
3.3. He-3 proportional counter The LB 6411 Berthold make He-3 proportional counter is used to monitor the source neutron strength. A detector is enclosed in a spherical polyethene moderator having an outer diameter of 25 cm. It is placed at a distance of 56.5 cm away from the target holder in the 4
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with respect to intellectual property. In so doing we confirm that we have followed the regulations of our institutions concerning intellectual property. CRediT authorship contribution statement S.J. Vala: Conceptualization, Data curation, Formal Analysis, Methodology, Project administration, Writing - original draft, Writing - review & editing, Supervision. M. Abhangi: Data curation, Formal Analysis, Methodology, Writing - original draft, Writing - review & editing. Ratnesh Kumar: Conceptualization, Data curation, Methodology, Writing - original draft. S. Tiwari: Data curation, Formal Analysis, Writing - original draft. R. Kumar: Conceptualization, Formal Analysis, Writing - review & editing, Supervision. H.L. Swami: Data curation, Writing - review & editing. M. Bandyopadhyay: Conceptualization, Writing - review & editing. References Fig. 7. Time-varying neutron yield measurement.
[1] A. Ibarra, et al., The IFMIF-DONES project: preliminary engineering design, Nucl. Fusion 58 (2018) 105002. [2] J. Knaster, et al., Overview of the IFMIF/EVEDA project, Nucl. Fusion 57 (2017) 102016. [3] A. Pietropaolo, et al., The Frascati Neutron Generator: A multipurpose facility for physics and engineering, J. Phys. Conf. Ser. 1021 (2018) 012004. [4] E. Surrey, et al., Application of accelerator based neutron sources in fusion materials research, in: 2013 IEEE 25th Symposium on Fusion Engineering, SOFE, San Francisco, CA, 2013, pp. 1–6. [5] P.K. Sarkar, S. Basu, M. Nandy, Accelerator and Radiation Physics, Narosa Publishing House, New Delhi, India, 2013, p. 170. [6] H. Swami, et al., Occupational radiation exposure control analyses of 14 MeV neutron generator facility: A neutronic assessment for the biological and local shield design, Nucl. Eng. Technol. (2020) http://dx.doi.org/10.1016/j.net.2020. 01.006. [7] Sudhirsinh Vala, et al., Rotating tritium target for intense 14-MeV neutron source, Fusion Eng. Des. 123 (2017) 77–81. [8] T.R. Fewell, An evaluation of the alpha counting technique for determining 14 MeV neutron yields, Nucl. Instrum. Methods 61 (1968) 61–71. [9] M. Angelone, et al., Absolute experimental and numerical calibration of the 14 MeV neutron source at the Frascati neutron generator, Rev. Sci. Instrum. 67 (1996) 2189. [10] E. Birgersson, G. Lövestam, NeuSDesc — neutron source description, EUR 23794 EN-2009. [11] H.L. Swami, et al., A neutronic experiment to support the design of an Indian TBM shield module for ITER, Plasma Sci. Technol. 21 (2019) 065601, 6. [12] P.N. Maya, et al., Evaluation of tungsten as divertor plasma-facing material: results from ion irradiation experiments and computer simulations, Nucl. Fusion 59 (2019) 076034. [13] Mayank Rajput, et al., Primary knock on atom spectra, gas production and displacement cross section for tungsten and chromium irradiated with neutrons at energies up to 14.1 MeV, Fusion Eng. Des. 130 (2018) 114–121. [14] M. Rajput, et al., Calculated differential and double differential cross section of DT neutron induced reactions on natural chromium (Cr), Indian J. Phys. 92 (1) (2018) 91–96. [15] Shrich Jakhar, et al., Tritium breeding mock-up experiments containing lithium titanate ceramic pebbles and lead irradiated with DT neutrons, Fusion Eng. Des. 95 (2015) 50–58. [16] A.T.T. Mostako, et al., Effect of microwave power on electron temperature and electron density in deuterium plasma generated by electron cyclotron resonance, IEEE Trans. Plasma Sci. 44 (2016) 7–14. [17] S. Vala, et al., Development of a test bench of 2.45 GHz ECR ion source for RFQ accelerator, J. Instrum. 14 (2019). [18] J. Benveniste, et al., The problem of measuring the absolute yield of 14-MeV Neutrons by means of an alpha counter, Nucl. Instrum. Methods 7 (1960) 306. [19] H. Maekawa, et al., Report JAERI-M 83-219. [20] D.W. Mingay, et al., Neutron induced reactions in silicon semiconductor detector, Nucl. Instrum. Methods 94 (1971) 497–507. [21] Shrich Jakhar, et al., Neutron flux spectra investigations in breeding blanket assembly containing lithium titanate and Lead irradiated with DT neutrons, Fusion Eng. Des. 100 (2015) 619–628.
5. Conclusion A 14 MeV neutron generator has been designed and developed for neutron yield of 1010 n/s having beam energy up to 300 keV using T(D, n)42 He reaction. The beam parameters are monitored through a beam diagnostic system for the reliable and stable operation of the neutron generator. The 10 Ci of solid tritium target is fitted in a watercooled target holder to produce neutrons. The performance of the NG has been evaluated for continuous operation. The neutron yield has been measured and estimated using various techniques such as Associated Alpha Particle, Foil Activation, He-3 Proportional Counter, and Monte Carlo Calculation. The uncertainties and correction factors associated with all measurement techniques have been assessed and incorporated into the final results. The neutron yield with an uncertainty measured by associated alpha particle techniques, foil activation technique, and He-3 proportional counter is 1.20 × 108 n/s ± 4.5%, 1.19 × 108 n/s ± 4.6%, and 1.17 ×108 n/s ± 1.38%, respectively. The associated alpha particle technique is the most accurate and sensitive among all three. The neutron yield with an uncertainty predicted by the Monte Carlo radiation transport code is 1.27 × 108 n/s ± 4.70%. The neutron yield evaluated by all techniques lies within ±10%. The neutron generator has been tested for continuous operation of 5.42 h with an average yield of 1.1 × 109 n/s for conducting fusion related neutronic mock-up experiments. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Funding No funding was received for this work. Intellectual property We confirm that we have given due consideration to the protection of intellectual property associated with this work and that there are no impediments to publication, including the timing of publication,
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