Recent activities in neutron standardization at NMIJ/AIST

Transcription

1 Recent activities in neutron standardization at NMIJ/AIST Tetsuro Matsumoto, Hideki Harano, Akihiko Masuda Quantum Radiation Division, National Metrology Institute of Japan (NMIJ), National Institute of Advanced Industrial Science and Technology (AIST), Japan 1. Introduction This report described recent activities on neutron standardization at the National Metrology Institute of Japan (NMIJ) in the National Institute of Advanced Industrial Science and Technology (AIST). The neutron fluence standard with a heavy water moderated 252 Cf source was developed. We started calibration service of the neutron fluence standard with a heavy water moderated 252 Cf source in We developed a high energy neutron fluence rate standard at neutron peak energy of 45 MeV using TIARA (Takasaki Ion Accelerators for Advanced Radiation Application) of JAEA. We will start calibration service of the high energy neutron fluence rate in The 3rd CIPM peer review was carried out in February High energy neutron fluence standard We are developing high energy quasi mono-energetic neutron fields with neutron peak energy of 45 MeV at TIARA (Takasaki Ion Accelerators for Advanced Radiation Application) of JAEA (Japan Atomic Energy Agency) [1, 2]. Quasi mono-energetic neutrons are produced in the 7 Li(p,n) 7 Be reaction where a proton beam from an AVF cyclotron impinges on an enriched metal lithium-7 target. A thickness of the target is determined so that the proton energy loss in the target is 5 MeV. The neutrons reach an irradiation room through a collimator of a 10-cm diameter made of an iron and a concrete as shown in figure 1. The neutron field has a 15-m neutron flight path. A transmission plastic scintillator with a thickness of 0.5 mm placed on the neutron beam line is used as a neutron monitor. Neutron energy spectra were measured by the time-of-flight (TOF) method. In the TOF measurements, the beam burst from the cyclotron are chopped by an electrical chopper (S chopper) at a sub-multiple (1/5 or 1/6) of repetition rate of the cyclotron. However, it is difficult to measure the neutron energy spectra below approximately 10 MeV by limitations for pulse intervals (200 ns ~ 300 ns) of ion beams from the AVF cyclotron and the distance from the neutron production target. Recently, JAEA developed a new chopper system (P chopper) that was located in a beam injection line of the cyclotron [3]. It is possible to obtain ion pulses with intervals of micro second by using 1/8

2 the S- and P- chopper systems. Therefore, the neutron energy down to kev region is measured with the TOF method at TIARA. A liquid scintillator (BC501A, Diameter: 7.62 cm, Thickness: 7.62 cm) and a 6 Li-glass scintillator (GS20, Diameter: 5 cm, Thickness: 5 mm) were used in the TOF measurements. The response function of the liquid scintillator was determined by calculation with the SCINFUL-QMD code [4]. The detection efficiency of 6 Li-glass scintillator was obtained from calibration using mono-energetic neutrons at AIST. The liquid scintillator and the 6 Li glass scintillator were placed 6.5 m away the target. Figures 2(a) and 2(b) show the source neutron energy spectra obtained by the TOF method for the proton energy of 50 MeV. A slight thermal neutron component is added to the source neutron energy spectra shown in figure 2 in actual energy spectra at the detector position because of neutron scattered in the irradiation room. We also measured the neutron energy spectra in the thermal neutron energy region using a Bonner sphere spectrometer by means of an unfolding method. The neutron fluence was determined using a proton recoil telescope (PRT) that consists of a Si semiconductor detector (Depletion depth: 500 m, Sensitive area: 60 mm diameter) and the liquid scintillator that is used in the TOF measurements as shown in figure 3. An aluminum aperture with a 58-mm diameter and a 15-mm long is placed between the liquid scintillator and the Si detector. A high density polyethylene radiator (Diameter: 100 mm, Thickness: 2 mm) and a carbon radiator (Diameter: 100 mm, Thickness: 1 mm) were set in the center of the beam axis and the PRT was placed out of the beam axis at a recoil angle of 10 degrees. Measurements without the radiators were performed to obtain background. Figure 4 shows the two dimensional plot of pulse height measured with the liquid scintillator and the Si detector for the 45 MeV quasi mono-energetic neutrons. The detection efficiency of the PRT was determined using the MCNPX code [5]. The neutron energy spectra measured with the TOF method was used in the calculations. The time of flight (TOF) for the proton is further measured by the liquid detector in order to improve the reliability for results of peak fluence. We will start calibration service in Figure 1. Schematic view of a TIARA beam line. 2/8

3 (a) Neutron Fluence per neutron monitor (/cm 2 /MeV) Neutron Energy (MeV) (b) Figure 2. Source neutron energy spectrum measured with (a) the liquid scintillator and (b) the liquid scintillator and the 6 Li-glass scintillator. Figure 3. Schematic view of the PRT. 3/8

4 Figure 4. Two dimensional plot of pulse height measured with the liquid scintillator and the Si detector. 3. High energy neutron experiments at RCNP We performed high energy neutron experiments at RCNP (Research Center of Nuclear Physics) of the Osaka University in cooperative research with the Japan Atomic Energy Agency, the High Energy Accelerator Research Organization, etc. from 2009 to Quasi-monoenergetic neutrons with an energy region from 140 to 400 MeV are produced using the 7 Li(p,n) reaction and a proton beam from a ring cyclotron. We tried to calibrate response functions of Bonner spheres with a 3 He spherical proportional counter for energies of 100 MeV, 140 MeV, 200 MeV, 250 MeV, 300 MeV and 390 MeV [6, 7]. Measurements at two angles of 0 and 30 degrees, or 0 and 25 degrees were performed to obtain mono-energetic response by subtracting the tail contribution [8]. Reference energy spectra were measured with an NE213 scintillation detector (25.4 cm diameter and 25.4 cm thickness) by means of the TOF method [7]. We try to measure the peak neutron fluence with the proton recoil telescope. 4. Neutron standard field using a heavy water moderated 252 Cf source A neutron standard field with a heavy-water moderated 252 Cf source was developed. It can produce a neutron energy spectrum consisting of a remaining spontaneous fission spectrum and a slowing-down continuum to the low-energy region. Figure 5 shows the calculated heavy-water moderated neutron spectrum compared with the bare 252 Cf source spectrum. The heavy-water moderated 252 Cf source consists of a standard 252 Cf source with an aluminum sustaining rod and enclosed stainless steel heavy-water tank covered with a cadmium cover, as shown in Figure 6. The 4/8

5 252 Cf source is encapsulated in a type-x1 stainless steel capsule with a nominal activity of 200 MBq and its neutron emission rate was calibrated by means of a relative measurement for a standard 241 Am-Be source in a graphite pile. The tank is 30 cm in diameter and weighs approximately 20 kg when it is filled with heavy water. The isotropic enrichment of the heavy water is more than 99.9 %. The shadow-cone method is used to subtract background due to neutrons scattered in an irradiation room. Three types of shadow cones of high-density polyethylene were designed and fabricated for use with representative detectors with consideration for their size, weight and cost. Their shielding abilities were evaluated by calculations with the MCNPX code. A fluence measurement was performed at the distance of 150 cm from the center of the source assembly using a cm diameter Bonner sphere detector calibrated with a 252 Cf neutron source. The measured value agrees with the value predicted from the MCNPX calculation within an uncertainty. Spectral evaluation using a Bonner sphere spectrometer and the unfolding method with the MAXED code [9] was also performed. The unfolding result well reproduced the calculated spectrum. Figure 5. Heavy-water moderated 252 Cf neutron spectrum compared with bare 252 Cf. 5/8

6 Figure 6. Heavy water moderated 252 Cf neutron source assembly 5. Neutron emission rate We are developing a multi-layer type manganese bath with a manganese alloy for determination of the neutron emission rate. The multi-layer type manganese bath is composed of high density polyethylene pates with a 8-mm thickness and manganese alloy plates with a 12-mm thickness as shown in Figure 7. The manganese alloy (D2052, STAR SILENT, DAIDO STEEL Co.) contains Mn (70.3 wt%), Cu (22.4 wt%), Ni (5.1 wt%) and Fe (2.0 wt%). The manganese alloy has chemical stability. It is furthermore possible to cut a manganese alloy plate with machine work as well as a stainless steel. The size of the manganese bath is 30 cm (W) x 30 cm (D) x 30 cm (H). In measurements of neutron emission rate, a neutron source is set in the center of the manganese bath at first. After neutron moderation with the polyethylene, neutrons emitted from the neutron source are absorbed by manganese in the manganese alloy plates with the 55 Mn(n, ) 56 Mn reaction. A NaI(Tl) detector with a 5.08 cm diameter and 5.08 cm length instead of the neutron source is placed in the center of manganese bath to measure gamma rays with energy of 847 kev from the activated manganese, 56 Mn, produced in the side and bottom parts of manganese bath as shown in Figure 8. Radioactivity of each manganese alloy plate in the top part of the manganese bath is individually measured with a high purity (HP) Ge detector with relative efficiency of 60 %. Figures 9(a) and 9(b) show the pulse height spectra of the NaI(Tl) detector and HPGe detector, respectively. The behavior of neutrons emitted from the neutron source in the manganese bath is also evaluated by the MCNP Monte-Carlo simulation. The 61.4 % neutrons are absorbed by the 55 Mn(n, ) Mn reaction. The Fe(n,p) Mn reaction also produce Mn. However, the 55 Mn(n, ) 56 Mn reaction rate in the manganese bath is 0.61 per emitted neutron, while the 56 Fe(n,p) 56 Mn reaction rate is 6.1 x 10-6 per emitted neutron. Therefore, effects due to the 6/8

7 56 Fe(n,p) 56 Mn reaction are negligible. The percentage of leakage neutrons from the manganese bath is 27.9 %. The leakage neutrons will decrease and the absorbed neutrons with the 55 Mn(n, ) 56 Mn reaction will increase, if the size of the manganese bath becomes large. Other neutrons are absorbed by the H(n, ) and the Cu(n, ) reactions. Moreover, we plan to establish a manganese bath with manganese sulfite solution again for the neutron emission rate standard by We will start to study on the manganese bath with manganese sulfite solution from Figure 7. A cross sectional view and a picture of a new type of manganese bath. Figure 8. Configuration for measurements of activated manganese with a NaI(Tl) detector. (a) (b) Figure 9. Pulse height spectra of (a) the NaI(Tl) detector and (b) the HPGe detector. 7/8

8 References [1] H. Harano et al., Radiat Meas. 45 (2010) [2] Y. Shikaze et al., Nucl. Instrum. Meth. Phys. Res. A 615 (2010) 211. [3] S. Kurashima et al., Proc. CYCLOTRONS 2010, MOPCP090 (2010). [4] D. Satoh et al., JAEA-Data/Code (2006). [5] D. B. Pelowitz, MCNPX use s manual Ver 2.5.0, Los Alamos National Laboratory, LA-CP (2005). [6] Y. Iwamoto et al., Prog. Nucl. Sci. Technol. 4 ( [7] A. Masuda et al., IEEE Trans. Nucl. Sci. 59 (1) (2012) 161. [8] R. Nolte et al., Nucl. Inst. Meth. A476 (2002) 369. [9] M. Reginatto and P. Goldhagen, Health Phys., 77 (1999) /8

9 Recent publications from the neutron group of NMIJ/AIST From 2013 to T. Matsumoto, H. Harano, A. Masuda, J. Nishiyama, H. Matsue, A. Uritani, Two-dimensional differential calibration method for a neutron dosimeter using a thermal neutron beam, Radiat. Prot. Dosim. 155 (4), (2013). 2. V. Mares, C. Pioch, W. Ruhm, H. Iwase, Y. Iwamoto, M. Hagiwara, D. Satoh, H. Yashima, T. Itoga, T. Sato, Y. Nakane, H. Nakashima, Y. Sakamoto, T. Matsumoto, A. Masuda, H. Harano, J. Nishiyama, C. Theis, E. Feldbaumer, L. Jaegerhofer, A. Tamii, K. Hatanaka and T. Nakamura, Neutron dosimetry in quasi-monoenergetic neutron fields of 244 MeV and 387 MeV, IEEE Trans. Nucl. Sci. 60 (1), pp (2013). 3. A. Masuda, H. Harano, T. Matsumoto, K. Kudo, J. Nishiyama, Development of a neutron standard field using a heavy-water moderated 252Cf source at NMIJ-AIST, Prog. Nucl. Sci. Technol. 4, (2014). 4. T. Matsumoto, A. Masuda, J. Nishiyama, H. Harano, H. Iwase, Y. Iwamoto, M. Hagiwara, D. Satoh, H. Yashima, Y. Nakane, H. Nakashima, Y. Sakamoto, C. Pioch, V. Mares, A. Tamii, K. Hatanaka, T. Nakamura, Measurement of neutron energy spectra behind shields for quasi-monoenergetic neutrons generated 246-MeV and 389-MeV protons using a Bonner sphere spectrometer, Prog. Nucl. Sci. Technol. 4, (2014). 5. Y. Iwamoto, M. Hagiwara, H. Iwase, H. Yashima, D. Satoh, T. Matsumoto, A. Masuda, C. Pioch, V. Mares, C. Theis, C, Urscheler, J, Lukas, T. Shima, A. Tamii, K. Hatanaka, T. Nakamura, Characterization of quasi-monoenergetic neutron source using 137, 200, 246 and 389 MeV 7Li(p,n) reaction, Prog. Nucl. Sci. Technol., 4, (2014). 6. M. Hagiwara, H. Iwase, Y. Iwamoto, D. Satoh, T. Matsumoto, A. Masuda, H. Yashima, Y. Nakane, H. Nakashima, Y. Sakamoto, A. Tamii, K. Hatanaka, Shielding benchmark experiment using hundreds of MeV quasi-monoenergetic source by a large organic scintillator, Prog. Nucl. Sci. Technol., 4, (2014). 7. Y. Nakane, M. Hagiwara, Y. Iwamoto, H. Iwase, D. Satoh, T. Sato, H. Yashima, T. Matsumoto, A. Masuda, T. Nunomiya, Y. Sakamoto, H. Nakashima, A. Tamii, K. Hatanaka, T. Nakamura, Response measurement of various neutron dose equivalent monitors in MeV neutron fields, Prog. Nucl. Sci. Technol., 4, (2014).

10 8. V Gressier, A C Bonaldi, M S Dewey, D M Gilliam, H Harano, A Masuda, T Matsumoto, N Moiseev, J S Nico, R Nolte, S Oberstedt, N J Roberts, S Rottger and D J Thomas, International key comparison of neutron fluence measurements in monoenergetic neutron fields: CCRI(III)-K11, Metrologia 51, (2014).