Nuclear Science and Engineering ISSN: 0029-5639 (Print) 1943-748X (Online) Journal homepage: http://www.tandfonline.com/loi/unse20 Measurement of the Photonuclear (γ,n) Reaction Cross Section for Photons 129 I Using Bremsstrahlung Abul Kalam Md. Lutfor Rahman, Shigeyuki Kuwabara, Kunio Kato, Hidehiko Arima, Nobuhiro Shigyo, Kenji Ishibashi, Jun-ichi Hori, Ken Nakajima, Tetsuo Goto & Mikio Uematsu To cite this article: Abul Kalam Md. Lutfor Rahman, Shigeyuki Kuwabara, Kunio Kato, Hidehiko Arima, Nobuhiro Shigyo, Kenji Ishibashi, Jun-ichi Hori, Ken Nakajima, Tetsuo Goto & Mikio 129 Uematsu (2008) Measurement of the Photonuclear (γ,n) Reaction Cross Section for I Using Bremsstrahlung Photons, Nuclear Science and Engineering, 160:3, 363-369, DOI: 10.13182/ NSE160-363 To link to this article: http://dx.doi.org/10.13182/NSE160-363 Published online: 10 Apr 2017. Submit your article to this journal View related articles Citing articles: 2 View citing articles Full Terms & Conditions of access and use can be found at http://www.tandfonline.com/action/journalInformation?journalCode=unse20 Download by: [University of Missouri-Columbia] Date: 27 October 2017, At: 04:27 NUCLEAR SCIENCE AND ENGINEERING: 160, 363–369 ~2008! Measurement of the Photonuclear (g, n) Reaction Cross Section for 129 I Using Bremsstrahlung Photons Downloaded by [University of Missouri-Columbia] at 04:27 27 October 2017 Abul Kalam Md. Lutfor Rahman,* Shigeyuki Kuwabara, Kunio Kato, Hidehiko Arima, Nobuhiro Shigyo, and Kenji Ishibashi Kyushu University, Department of Applied Quantum Physics and Nuclear Engineering 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan Jun-ichi Hori and Ken Nakajima Kyoto University Research Reactor Institute, Osaka 590-0494, Japan and Tetsuo Goto and Mikio Uematsu Toshiba Corporation, Yokohama 235-8523, Japan Received October 28, 2007 Accepted March 15, 2008 Abstract – Nuclear waste contains a significant amount of long-lived non-gamma-emitting nuclei such as 129 I and 14 C. A method of nondestructive detection for monitoring long-lived waste products is proposed as an application of the (g, n) reaction. This method is useful for surveying long-lived “difficultto-measure” nuclides, e.g., 129 I. Iodine-128 produced from the reaction of 129 I(g, n) 128 I emits gamma rays that can easily be measured by a gamma-ray counter. We measured the inclusive photonuclear 129 I(g, n) 128 I reaction cross section induced by bremsstrahlung photons. The photons were produced at a Ta target bombarded by 30-MeV electrons from a linear accelerator. The intensity of the slow neutrons was considered in the reactions of 127 I(n, g) 128 I and 129 I(n, g) 130 I. The activity of 128 I was measured by a high-purity germanium spectrometer. The gamma-ray flux and the neutron flux were calculated using the EGS and MCNP codes, respectively. The average activation cross section of the 129 I(g, n) 128 I reaction had a 12% deviation from the evaluated International Atomic Energy Agency photonuclear data. I. INTRODUCTION nor actinides such as neptunium, americium, and curium; and long-lived fission products like technetium, iodine ~ 129 I!, and cesium. The solution proposed for disposal of this waste is to isolate it from the biosphere for periods of hundreds of thousands of years in deep underground geological repositories. Prior to disposal of longhalf-life radionuclides, it is necessary to know their amounts in nuclear waste. Measurement based on neutroninduced reaction is sometimes not usable because it tends to produce rather long half-life nuclides; for example, the neutron capture reaction of 129 I produces 130 I, which has a half-life of 12.36 h. In contrast, using the 129 I ~g, n! reaction produces 128 I with a half-life of 25 min. Therefore, it is considered easy to measure the amount of 129 I The production of electricity by nuclear power plants is very effective; however, handling spent nuclear fuel is still a very great challenge. Part of the nuclear waste includes radioactive isotopes with half-lives on the order of millions of years, and therefore, this material is an unwanted legacy for future generations. Figure 1 is a schematic diagram of the radioactive waste source. Various radioactive products are formed during the operation of nuclear reactors. Some are long-lived and highly radiotoxic. The main products are plutonium; mi*E-mail: [email protected] 363 Downloaded by [University of Missouri-Columbia] at 04:27 27 October 2017 364 RAHMAN et al. Fig. 1. Schematic diagram of the source of radioactive waste. by gamma-induced reaction. In this context, photoninduced reaction has the potential to measure long-lived radionuclides nondestructively in nuclear waste. Some efforts were recently directed 1,2 toward photonuclear reaction as laser transmutation of 129 I. The technology of laser transmutation may be usable as a “nondestructive detection” method in the future. The present work proposes to measure the photonuclear 129 I~g, n! 128 I reaction cross section using bremsstrahlung photons produced by an electron linac. When a fast electron interacts with matter, part of its energy is converted into electromagnetic radiation in the form of bremsstrahlung. The fraction of the energy converted into bremsstrahlung increases with electron energy and is the largest for absorbing materials of high atomic number. The process is important in a conventional X-ray tube. In a typical bremsstrahlung spectrum, low-energy photons are predominant, and the average photon energy is a small fraction of the incident electron energy. Radioactive waste contains long-lived non-gammaemitting nuclides. They are deemed “difficult-to-measure” radioisotopes. For safe disposal, it is necessary to know their radioactivities. The measurement of such isotopes is usually determined using an indirect method such as the scaling factor method.3 This method works by exploiting a radiochemical process that uses mixed bed and cation bed resins. The resins remove ionic fission products such as anionic iodine with a cation. Although its effective removal efficiency is high, this method is costly and time-consuming. If a long-lived beta radionuclide such as 129 I ~halflife ⫽ 1.57 ⫻ 10 7 yr! can be changed into the short-lived gamma emitter 128 I ~half-life ⫽ 25 min!, its radioactivity can easily be measured by counting gamma rays. We have devised a nondestructive detection technique for this purpose. The nondestructive detection method measures gamma rays that are emitted from the beta decay of ~g, n! products in a photon-irradiated waste drum ~asphalt solidification drum! without any destruction. In the destructive process, radionuclides are separated from the waste and then analyzed by the radiochemical technique.4 The concept of the nondestructive detection method is illustrated in Fig. 2. The method consists of an electron accelerator, a heavy metal target ~Ta, W, etc.!, a radioactive waste drum ~asphalt drum!, and a Ge gammaray detector. High-energy electrons strike the heavymetal target and generate high-energy bremsstrahlung photons. The photons directly react with the long-lived radionuclide, and the result is conversion of long-lived beta-radio nuclei into short-lived gamma emitters. The radioactivity of the residual nuclide is measured using a Ge detector. Since the photonuclear cross section is high at the giant resonance region, the electron energy is chosen so that the bremsstrahlung photon spectrum covers the giant resonance region. Table I lists typical longlived nuclides. Among these, 129 I comprises a significant amount of nuclear waste. It can easily be measured because the half-life of the residual nuclide 128 I is 25 min. As far as we know, there are no experimental photonuclear cross-section data available for 129 I. The photoabsorption cross sections were thus evaluated from giant dipole resonance ~GDR! and quasi-deuteron model calculations.5 Figure 3 shows the evaluated photoabsorption cross section of 129 I. In addition, the production cross sections were calculated with the GNASH code.6 The aim of this study is to measure the average inclusive 129 I~g, n! 128 I reaction cross section using continuous-energy bremstrahlung photons emittted from a Ta target bombarded by a 30-MeV electron linac. The EGS5 ~Ref. 7! and MCNP ~Ref. 8! Monte Carlo codes were used in the data analysis. Fig. 2. Concept of nondestructive detection method. NUCLEAR SCIENCE AND ENGINEERING VOL. 160 NOV. 2008 PHOTONUCLEAR CROSS SECTION FOR 129 I 365 TABLE I Example of Photonuclear Reactions Applicable to Nondestructive Detection Nuclide 14 C 36 Cl 59 Ni 79 Se 99 Tc 129 Downloaded by [University of Missouri-Columbia] at 04:27 27 October 2017 93 I Zr Half-Life ~yr! 5.73 ⫻ 10 3 3.01 ⫻ 10 5 7.50 ⫻ 10 5 6.50 ⫻ 10 4 2.13 ⫻ 10 5 1.57 ⫻ 10 7 1.53 ⫻ 10 6 Giant Estimated Cross Resonance Peak Section s Residual Reaction ~MeV! ~mb! Nuclide Half-life ~g, p! ~g, 2n! ~g, p! ~g, p! ~g, 3n! ~g, n! ~g, p! Fig. 3. Evaluated cross section of 25 25 20 20 22 15 22 129 I~g, 10 8 30 25 20 300 20 13 B 34m Cl 58 Co 78As 96m Tc 128 92 I Y Decay Mode Major Gamma Energy ~keV! 17.4 ms b⫺ 32.3 min b⫹ 70.9 days Electron capture 1.5 h b⫺ 51.5 min Electron capture 25 min b⫺ 3.54 h b⫺ 3684 2128 811 614, 1309 778 443 934, 1405 abs!. Fig. 4. Irradiation room of the electron linac facility at Kyoto University Research Reactor Institute. II. EXPERIMENTAL II.A. 129 I(g, n) Reaction Measurement TABLE II For the analysis of the photonuclear ~g, n! reaction cross section of radioactive 129 I ~activity of 3 MBq!, an irradiation experiment was carried out at the electron linear accelerator facility of Kyoto University Research Reactor Institute. The experimental layout is shown in Fig. 4. The irradiation conditions are summarized in Table II. Bremsstrahlung photons were generated at a thick tantalum target ~50-mm diameter and 61-mm thickness! near the extraction port of the accelerator. The target was actually 29 mm thick. Inside the target capsule there were some channels through which cooling water flowed. A shielded source of radioactive iodine was used as a sample, and it was covered with a titanium metal sheet. The iodine sample ~27.8-mm diameter and 2.8-mm thickness! contained radioactive 129 I at 68% proportion and stable 127 I at 32% proportion. The sample was irradiated together with 197Au foils placed in the front and the back of the iodine sample to measure the photon and NUCLEAR SCIENCE AND ENGINEERING VOL. 160 NOV. 2008 Irradiation Condition in the Experiment Electron Energy Irradiation Time Beam Current Pulse Width Frequency 30 MeV 30 min 27.8 mA 100 ns 100 Hz neutron fluxes at the sample position. The gold foil at the back of the iodine was covered by a cadmium sheet to estimate the low-energy neutron flux. Gold foils of different sizes were used to ensure uniform distribution of the flux in the iodine sample. The geometry of the sample and the gold foils is presented in Fig. 5. The sizes and weights of the foils are listed in Table III. Gamma rays from residual nuclides of irradiated iodine and foils were 366 RAHMAN et al. TABLE IV Standard Calibration Sources Used in the Experiment Calibration Source 109 Cd Co 137 Cs 60 Co 60 Co 57 Gamma Energy ~keV! 88.03 122.1 661.6 1173 1333 Downloaded by [University of Missouri-Columbia] at 04:27 27 October 2017 Fig. 5. Position of iodine and gold foils in the experiment ~inset below: All samples are packed together in a holder!. measured by a high-purity germanium detector for 30 min. The cooling time for the iodine sample was 24 min. The gold foils were placed 4 mm and the iodine sample 170 mm from the end surface of the detector. II.B. Efficiency Calibration of the Ge Detector Detection efficiency is essential to measure the absolute emission rates of gamma rays. The efficiency of the Ge detector was measured using standard sources listed in Table IV. The detector efficiency was also calibrated by the EGS5 code to determine the shape of the efficiency curve. Figure 6 plots the measured and calculated efficiencies at different energies. The calculated values agree well with the measured ones. The absolute value of the efficiency was taken from the following fitting function of the standard source value: Efficiency ⫽ K0 ⫹ K1 ⫻ [email protected]⫺~ln~ x0K2 !0K3 !# , ~1! where K0 , K1 , K2 , K3 are fitting parameters and x is energy in kilo-electron-volts. Fig. 6. Efficiency at different energy ~17-cm distance from detector surface to sample!. III. RESULTS AND DISCUSSION III.A. Results and Discussion on Photon and Neutron Fluxes Photon fluxes were derived from data of gold foils irradiated together with the iodine sample. The measured fluxes were then checked by the EGS5 code taking combinatorial geometry. The photon flux at the iodine TABLE III Sizes and Weights of Different Gold Foils Sample Position Size Weight ~mg! Small Au Small Au Big Au Small Au covered by Cd Front of I Back of I Back of I Back of I 6-mm diameter ⫻ 15 mm 6-mm diameter ⫻ 15 mm 20-mm diameter ⫻ 10 mm 6-mm diameter ⫻ 30 mm 8.32 8.33 76.67 17.81 NUCLEAR SCIENCE AND ENGINEERING VOL. 160 NOV. 2008 Downloaded by [University of Missouri-Columbia] at 04:27 27 October 2017 PHOTONUCLEAR CROSS SECTION FOR sample’s position was determined by averaging the fluxes on the gold foils at the front and the back of the iodine. Since 32% of the radioactive iodine sample was 127 I, the contribution of the 127 I~n, g! 128 I reaction to the yield of total 128 I was considered to determine only the 129 I~g, n! 128 I reaction cross section. To obtain the ~n, g! reaction contribution, the fast neutron flux was calculated using the MCNP code. In addition, low-energy neutron fluxes in the thermal and epithermal regions were derived from experimental data on gold foils with and without cadmium covers. The thermal neutron flux was assumed to have a Maxwellian distribution with amplitude and temperature parameters. The two parameters were determined from the gold activation results with and without cadmium sheets. The neutron attenuation through the cadmium sheet was taken into account. The temperature of the thermal neutrons was 2.69 ⫻ 10⫺2 keV. The contribution of ~n, g! was mainly from the thermal neutron flux. 129 Activity ⫽ A 0 ⫽ I 367 l ⫻ net gamma-ray counting area « ⫻ exp~⫺l ⫻ t1 ! ⫺ exp~⫺l ⫻ t2 ! ~2! where l ⫽ decay constant « ⫽ counting efficiency of the Ge detector t1 , t2 ⫽ start and finish times of the gamma counting. Activation rates were derived as follows: Activation rate ⫽ sfN ⫽ A0 ~1 ⫺ exp~⫺l ⫻ t !! , where f ⫽ flux Since the photonuclear reaction cross section of gold is known, gold foil was irradiated together with radioactive 129 I to check the evaluated cross section of the 129 I~g, abs! reaction. In the gold foil irradiation, occurring reactions are 197Au~g, n! 196Au and 197Au~n, g! 198Au. At high energy, the cross section of the Au~g, n! reaction 5 is dominant over the Au~n, g! reaction.9 Figure 7 shows an example of a gamma-ray spectrum of the irradiated gold. The peaks are at 355.73 keV ~Ig ⫽ 87%! for 196Au decaying through the 197Au~g, n! 196Au reaction and at 411.80 keV ~Ig ⫽ 95.6%! for 198Au decaying through 197Au~n, g! 198Au. Activities of the decaying isotopes were calculated at the end of irradiation by Eq. ~2!: N ⫽ number of nuclides t ⫽ irradiation time. The activation rates measured for 197Au~g, n! and 197Au~n, g! at different positions are summarized in Table V. It is obvious from Table V that the ~g, n! reaction is dominant over the ~n, g! reaction. The photon flux on the gold foil was calculated with the EGS5 code taking 8 million particles per batch ~Fig. 8!. The average statistical error in the calculation was 4%. The peak energy at the giant dipole resonance for the 197 Au~g, n! 196Au reaction is 13.5 MeV. The calculated average flux deviated by 20% from the experimental one and can be expressed by Eq. ~4!: Measured activation Calculated activation~sevl fcal N ! ⫽ 20% , TABLE V Measured Activation Rate of Different Gold Foils Fig. 7. Gamma-ray spectrum of irradiated gold. VOL. 160 ~3! s ⫽ cross section III.B. Results and Discussion on the Gold Foil Experiment NUCLEAR SCIENCE AND ENGINEERING , NOV. 2008 196Au 198Au Gold Foil ~Bq! ~Bq! Small Au ~front of I! Small Au ~back of I! Big Au ~back of I! Au with Cd cover 2.39 ⫻ 10 5 2.32 ⫻ 10 5 2.00 ⫻ 10 6 2.17 ⫻ 10 5 1.39 ⫻ 10 4 1.19 ⫻ 10 4 1.34 ⫻ 10 5 5.28 ⫻ 10 3 ~4! Downloaded by [University of Missouri-Columbia] at 04:27 27 October 2017 368 RAHMAN et al. Fig. 8. Photon flux from 30-MeV incident electron by EGS5 code. Fig. 9. Gamma-ray spectrum of irradiated iodine. TABLE VI where sevl ⫽ evaluated cross section of the 197Au~g, n! 196Au reaction fcal ⫽ flux calculated by the EGS5 code. The calculated flux was normalized by Eq. ~5! to reproduce the experimental gold foil data at the iodine sample position: Activation Yields Obtained from Measurement of Iodine Yield of 128 I Bq 5.19 ⫻ 10 6 ~62.6 ⫻ 10 5 ! 5.10 ⫻ 10 6 ~62.5 ⫻ 10 5 ! 9.69 ⫻ 10 4 ~64.9 ⫻ 10 3 ! Total 129 I~g, n! 128 I 127 I~n, g! 128 I Normalized flux, fnor ⫽ Normalizing factor ⫻ fcal , ~5! where Normalizing factor is the ratio of the measured and calculated activation rates. III.C. Results and Discussion on the Iodine Sample Experiment Figure 9 shows the irradiation spectrum of the iodine sample. The spectrum peaks at 442.9 keV ~Ig ⫽ 12.62%! and 526 keV ~Ig ⫽ 1.2%! originate from 128 I decaying through the 129 I~g, n! 128 I reaction, and those at 536 keV ~Ig ⫽ 99%!, 668.5 keV ~Ig ⫽ 96%!, and 739.5 keV ~Ig ⫽ 82%! are from 130 I decaying through 129 I~n, g! 130 I. As the iodine sample contained 32% 127 I, 128 I was also produced through the reaction of 127 I~n, g! 128 I. The contribution of the 127 I~n, g! reaction to the production of 128 I was estimated on the basis of the neutron flux by using the following relation: Activation rate of 127 I(n,g) 128 I ⫽ @~Normalizing factor ⫻ activation at the fast neutron energy region! ⫹ activation at the thermal neutron energy# , where Normalizing factor is reproduced from the ratio of measured and calculated gold activation in the fast neutron energy region. The total and individual activation yields of 128 I obtained from the irradiation of the iodine sample are listed in Table VI. The contribution of the ~n, g! reaction was 1.9% of the total amount of 128 I production, due to the flow of cooling water inside the tantalum target capsule through some narrow channel that reduced the thermal neutron production. The average cross section of the 129 I~g, n! 128 I reaction was 0.099 6 0.005 b, as determined by Eq. ~6!: Activation rate N ⫻ fnor ⫽s , ~6! where fnor ⫽ normalized flux s ⫽ cross section of the 129 I~g, n! 128 I reaction. The average was made taking the continuous-energy bremsstrahlung photons, and the energy range was 9.0 to 30.0 MeV. On the other hand, the International Atomic Energy Agency ~IAEA! evaluated photonuclear data library gives an average of 0.113 b ~Ref. 5!. The crosssection ratio ~measured0evaluated! is 88%. The evaluated value deviates from the experimental data by 12%. NUCLEAR SCIENCE AND ENGINEERING VOL. 160 NOV. 2008 PHOTONUCLEAR CROSS SECTION FOR Because of lack of a photonuclear cross section on the photoabsorption cross section was calculated from the GDR and quasi-deuteron model adopting the GDR parameters of 127 I ~Ref. 5!. The present 129 I~g, n! value is based on the experiment and is supposed to be more reliable than the IAEA calculation. For this reason, the experimental ~g, n! reaction cross-section value is considered to be useful for the assessment of the longlived radioactive 129 I nuclide. 129 I 369 REFERENCES 129 I, Downloaded by [University of Missouri-Columbia] at 04:27 27 October 2017 2. J. MAGILL, H. SCHWOERER, F. EWALD, J. GALY, R. SCHENKEL, and R. SAUERBREY, “Laser Transmutation of Iodine-129,” Appl. Phys. B Laser Optics, 77, 387 ~2003!. 3. K. H. HWANG and K. J. LEE, “Modeling the Activity of 129 I and 137 Cs in the Primary Coolant and CVCS Resin of an Operating PWR,” J. Nucl. Mater., 350, 2, 153 ~2006!. IV. CONCLUSION Long-lived waste management is one of the major problems in the nuclear world. In this study, we measured the 129 I~g, n! 128 I reaction cross section using continuous-energy bremsstrahlung photons. An important aspect of this measurement was measuring the influence of the ~n, g! reaction caused by low-energy neutrons in the experimental room. The influence of the 127 I~n, g! 128 I reaction on the 128 I yield was obtained with the help of the well-known 197Au~n, g! 198Au reaction measurement. The contribution of the ~n, g! reaction was 1.9% to the 128 I production in this experiment. The average cross-section value obtained from the experiment is 0.099 6 0.005 b. The IAEA evaluated data deviate by 12% from the measured data. The use of the measured average cross section of the 129 I~g, n! 128 I reaction may give reliability in the assessment of long-lived betaactive radionuclides. The authors express their gratitude to the staffs of the Kyoto University Research Reactor Institute for giving us the opportunity and support to perform the experiment. VOL. 160 4. P. ORMAI, A. FRITZ, J. SOLYMOSI, I. GRESITS, E. HERTELENDI, Z. SZOCS, N. VAJDA, ZS. MOLNAR, and P. ZAGYVAI, “Inventory Determination of Low and Intermediate Level Radioactive Waste of Paks Nuclear Power Plant Origin,” J. Radioanal. Nucl. Chem., 211, 2, 443 ~1996!. 5. “IAEA Photo Nuclear Data Library,” IAEA Coordinated Research Project, International Atomic Energy Agency ~1996–1999!. 6. P. G. YOUNG, E. D. ARTHUR, and M. B. CHADWICK, “Comprehensive Nuclear Model Calculations: Theory and Use of the GNASH Code,” Proc. Workshop Nuclear Reaction Data and Nuclear Reactors: Physics, Design and Safety, Trieste, Italy, April 15–May 17, 1996, p. 227, A. GANDINI and G. REFFO, Eds., World Scientific Publishing, Ltd. ~1998!. 7. H. HIRAYAMA et al., “The EGS5 Code System,” SLACR-730~2005! and KEK Report 2005-8, Stanford Linear Accelerator Center and High Energy Accelerator Research Organization ~2005!. 8. L. J. COX et al., “MCNP Version 5,” LA-UR-02-1527, Los Alamos National Laboratory ~2002!. ACKNOWLEDGMENT NUCLEAR SCIENCE AND ENGINEERING 1. K. W. D. LADINGHUM et al., “Laser-Driven PhotoTransmutation of 129 I—A Long-Lived Nuclear Waste Product,” J. Phys. D Appl. Phys., 36, L79 ~2003!. NOV. 2008 9. P. G. YOUNG, “Evaluated Nuclear Data File ~ENDF!0VII,” LA-10069-PR, DIST-DEC 06 ~2006!.