- Email: [email protected]
Measurements of the Mass and Isotopic Yields of the 233U(nth,f) Reaction by the Lohengrin Spectrometer
Available online at www.sciencedirect.com
Nuclear Data Sheets 119 (2014) 328–330 www.elsevier.com/locate/nds
Measurements of the Mass and Isotopic Yields of the Reaction by the Lohengrin Spectrometer
233
U(nth ,f )
F. Martin,1, 2 C. Sage,1, ∗ G. Kessedjian,1 O. S´erot,2 C. Amouroux,3 C.O. Bacri,4 A. Bidaud,1 A. Billebaud,1 N. Capellan,1 S. Chabod,1 X. Doligez,4 H. Faust,5 U. K¨ oster,5 A. Letourneau,3 T. Materna,3 L. Mathieu,6 O. M´eplan,1 and S. Panebianco3 1
LPSC, CNRS/IN2P3, UJF Grenoble 1, INPG, Grenoble, France 2 DEN/DER/SPRC/LEPh, CEA Cadarache, France 3 DSM/IRFU/SPhN, CEA Saclay, France 4 IPN, CNRS/IN2P3, Univ. Paris-Sud, France 5 Institut Laue-Langevin, Grenoble, France 6 CENBG, Universit´ e de Bordeaux 1, France
Analysis and results of the measurements of the independent isotopic and mass fission yields for the 233 U(nth ,f) reaction are presented, which were performed at the Lohengrin recoil mass spectrometer in ILL. The spectrometer separates the fission products according to their mass to ion charge ratio and kinetic energy to ion charge ratio. Mass yields were obtained by means of an ionisation chamber after separation by the spectrometer. Isotopic yields were determined by placing two high-purity Germanium clover detectors downstream the spectrometer. An innovative analysis method developed for the Lohengrin spectrometer is presented, from which the final results are independent from the previous evaluations, which is usually not the case.
I.
INTRODUCTION
Growing world energy consumption has renewed the interest in nuclear power. The rise of nuclear reactors implies the study of innovative systems that could rely not only on the commonly used fuel cycle of UraniumPlutonium (U-Pu), but also on Thorium-Uranium (ThU). The latter might indeed provide cleaner nuclear energy than the U-Pu fuel cycle. Studies of different aspects of innovative fuel cycles, such as calculations of residual heat or poison concentration in the fuel, require detailed knowledge of isotopic and mass fission yields. An experimental campaign has been initiated within a collaborative between the Institut Laue-Langevin (ILL) and French CEA and CNRS laboratories to extend and improve the existing fission products yields data for the key nuclei within the innovative Th-233 U cycle. Available experimental data for 233 U(nth ,f) exist mainly for light fission fragments. Several measurements have been performed to determine the mass yields of 233 U, although were limited to cumulated yields, i.e. measured within a few minutes or hours after fission occurred. Only two independent measurements for the light region exist: Wehring et al. [1] and Quade et al. [2]. As for
∗
Corresponding author: [email protected]
http://dx.doi.org/10.1016/j.nds.2014.08.090 0090-3752/© 2014 Elsevier Inc. All rights reserved.
the heavy region, the only independent measurements available ([3]-[4]) were estimated with a three-mass unit uncertainty, smoothing drastically any structure in the heavy-fragment peak. For these reasons, our recent independent yield measurements presented here were then focused on the heavy fission fragment parts.
II.
THE LOHENGRIN SPECTROMETER
The Lohengrin mass spectrometer is located in the ILL research reactor facility, which allows to study fragment distributions from thermal neutron induced fission with very good resolution. A fissile actinide target is placed close to the reactor core, which operates at a thermal neutron flux reaching 5×1014 neutrons×cm−2 ×s−1 . Fission fragments emerge from the target with an average ionic charge state of approximately 21. Those fragments that are emitted along the beam tube axis undergo horizontal deflection in a magnetic field, directly followed by vertical deflection in an electric field. These combined fields separate ions according to their A/q and E/q values (with A, q and E the mass, ion charge state and kinetic energy of the ions respectively). See Refs. [5, 6] for more information concerning the spectrometer performance.
Measurements of the Mass . . .
MASS YIELD MEASUREMENTS A.
8
Method of Analysis
The first step consists in getting the count rate of a fission product with a given mass independently from their energy or charge. As it would be too long to measure the kinetic energy distribution for each charge state, the following hypothesis is usually introduced: the two variables are independent [7]. The total count rate of a mass MA is then calculated from a single energy scan and a charge scan. The first one is obtained by measuring different energies at a fixed charge q, and the second one by measuring different ionic charges at a fixed energy E [7]. During the experiment, the amount of fissile material within the spectrometer’s acceptance decreases, principally by sputtering but also from the burning due to neutron induced reactions. Thus the monitoring of the 233 U target is obtained by periodically measuring the kinetic energy distribution of a given mass at a fixed ionic charge, so-called “burnup” measurements. Provided that all the heavy mass rates are measured, it is possible to auto-normalise the data by defining to 100% the sum of the whole heavy peak yields. As a consequence, these new measurements are independent from another experiment or assessment and may be compared with the existing data. This auto-normalization introduces correlations between all the measurements and increases the uncertainties on Y (Ai ) . Completion of the first step depends on the quality of the kinetic energy and charge distribution assessments. The hypothesis of independence has been tested on some nuclei and has been validated for the fission products without nanosecond isomers. For the other ones, the correction due to the correlation between kinetic energy and charge has been estimated by fitting the evolution of the mean kinetic energy with the ionic charge, this correlation depends strongly on the presence of nanosecond isomers and on the width of the ionic charge distribution. Such a correction has been estimated between 3% and 15% for the few involved masses, it is specific to and needs to be determined for each mass.
B.
JEFF-3.1.1 ENDF/B-VII.0 this work
9
Mass yield (% per fission)
III.
F. Martin et al.
NUCLEAR DATA SHEETS
7 6 5 4 3 2 1 0 120
125
130
135
140
Mass (A)
145
150
155
FIG. 1. 233 U(nth ,f) mass yield measurements compared with JEFF-3.1.1 and ENDF/B-VII.0 evaluations.
IV.
ISOTOPIC YIELD MEASUREMENTS A.
Method of Analysis
The 233 U isotopic fission yields were determined by means of gamma-ray spectroscopy, on the basis of the fission-product beta decay occurring after their flight in the spectrometer (about 1 μs). The fission products are implanted in a movable tape, the position of which is fixed for each particular gamma-ray measurement (about 30 min). Two HPGe clover detectors are placed on each side of the tape to measure the gamma decay. Only the fragments with given A/q and E/q ratios corresponding to the Lohengrin tuning can be gathered on the tape [7]. The targets used for the isotopic measurements were too thick, which would result in a significant deterioration in the energy resolution and would need larger corrections to the fits and normalization. Instead of a direct estimation of the isotopic yields, we chose to determine the probability distribution of each isotope P (Z|A) for a given mass A by self-normalization to minimize the systematic effects (HPGe efficiency, target burnup correction, partial ionic charge and kinetic energy distributions, etc.) and obtain independent data. The isotopic yield is determined by multiplying this value by the mass yield Y (A, Z) = Y (A) × P (Z|A). The latter can then be estimated from the gamma-ray spectroscopy measurement
Results and Discussion
The 233 U(nth ,f) mass yield measurements from the Lohengrin spectrometer are presented in Fig. 1 and are compared with the JEFF-3.1.1 [8] and ENDF/B-VII.0 [9] evaluations. Two structural features are seen in the JEFF and ENDF libraries at mass values equal to 136 and 142. Our results at mass 136 are in agreement with the ENDF library within ENDF’s large uncertainties. However, our results are in complete disagreement with the European library. Additionally, the second structure appears at mass 141 and not 142 according to our results.
Si =
0
tf
f γ Iγ λi Ni (t) dt,
(1)
where Si is the integral of the corresponding photopeak of a given gamma-ray, γ is the efficiency of the detection system at this gamma energy, Iγ is the relative intensity taken from Ref. [13], Ni (t) is the number of nuclei for isotope i, tf is the measurement time, and f is the summing correction factor ranging from around 1.1 to 0.8 and 329
Measurements of the Mass . . .
F. Martin et al.
NUCLEAR DATA SHEETS
calculated as follows using the TrueCoinc code [10] f= (1 − Iγ tot γ ).
provided P (Z|A, Ei ) = P (Z|A, Ei ) for all Ei and Ei , and where N (Z|A, E) is the measurement of isotope A ZX at kinetic energy E using gamma-ray spectroscopy, R(Ei ) is the energy resolution of the Lohengrin spectrometer, and P (E|A) is the energy probability per mass A obtained with the ionisation chamber.
(2)
j=i
P (Z|A) values were estimated via several characteristic gamma lines in the decay of a given isotope, the P (Z|A) adopted was the averaged value of the individual ones. The detector efficiency was obtained via the comparison
B.
Isotopic studies on the 131-145 mass range have been completed and the analysis is under way. Preliminary results are presented in Fig. 2 for mass 137 isotopic production, normalised to the JEFF-3.1.1 evaluation or with our new method giving totally independent data, and compared to previous measurements from Piksaykin et al. [12]. 137 Te contribution remains to be evaluated, whereas 137 Cs is not reachable via our measurement due to its 30 years half-life. The mass 139 isotopic yield measurement was taken as the reference one for the cross normalisation since more than 99% of the isotopic distribution has been measured in this experiment. The agreement with the JEFF-3.1.1 evaluation and a previous measurement is quite good, yet a thorough analysis of the several corrections to apply is still under way.
FIG. 2. Preliminary results for the isotopic production of mass 137 (independent yields).
of calibrated point source measurements with a thorough modelling of the geometry using the MCNP5 code [11]. Relative agreement between the measurements and the simulation induces an uncertainty of approximately 3% in the efficiency. For the mass chain with one or few isotopes with a long lifetime or no gamma rays (pure β decay to the ground state), the isotopic probability P (Z|A) couldn’t be normalised with the method described above. In these cases we used a different method that allows cross normalisation on a complete isotopic measurement Aref based on the following relationship Y (Zi |Ai ) = Y (Zref |Aref ) P (Eref |Aref ) R(Eref ) N (Zi |Ai , Ei ) , P (Ei |Ai ) R(Ei ) N (Zref |Aref , Eref )
Results and Discussion
V.
CONCLUSIONS
A new campaign of 233 U(nth ,f) fission yield measurements has begun using the Lohengrin spectrometer. The goal of these new measurements is to assess precisely the fission yields in the heavy mass region. Fission fragment mass yields have been determined by coupling the Lohengrin spectrometer to an ionization chamber, while the isotopic yields were derived from gamma-ray spectroscopy. Isotopic yields were obtained from the results of the mass yield measurements, and the self-normalization process generated independent measurements of the existing data.
(3)
[1] B.W. Wehring et al., Trans. American Nucl. Soc. 11 (1980). [2] U. Quade et al., Nucl. Phys. A 487, 1 (1988). [3] M. Asghar et al., Nucl. Phys. A 368, 328 (1981). [4] H. Baba et al., J. Nucl. Sci. Technol. 34 (1997). [5] O. S´erot et al., Nucl. Data Sheets 119, 320 (2014). [6] H. Faust et al., presented in ND2013. [7] A. Bail et al., Phys. Rev. C 84, 034605 (2011). [8] The JEFF-3.1.1 nuclear data library. OECD, NEA, JEFF Report 21, ISBN 92-64-02314-3 (2006).
[9] M.B. Chadwick et al., Nucl. Data Sheets 107, 2931 (2006). [10] Truecoinc, a software utility for calculation of the true coincidence correction, IAEA Tecdoc 1275, 37 (2002). [11] X-5 Monte Carlo Team, LANL Report LA-UR-03-1987 (2003). [12] V.M. Piksaykin et al., Joint Inst. for Nucl. Res., Dubna Reports 2004-169, 342 (2004). [13] ENSDF, NNDC, http://www.nndc.bnl.gov.
330
Nuclear Data Sheets 119 (2014) 328–330 www.elsevier.com/locate/nds
Measurements of the Mass and Isotopic Yields of the Reaction by the Lohengrin Spectrometer
233
U(nth ,f )
F. Martin,1, 2 C. Sage,1, ∗ G. Kessedjian,1 O. S´erot,2 C. Amouroux,3 C.O. Bacri,4 A. Bidaud,1 A. Billebaud,1 N. Capellan,1 S. Chabod,1 X. Doligez,4 H. Faust,5 U. K¨ oster,5 A. Letourneau,3 T. Materna,3 L. Mathieu,6 O. M´eplan,1 and S. Panebianco3 1
LPSC, CNRS/IN2P3, UJF Grenoble 1, INPG, Grenoble, France 2 DEN/DER/SPRC/LEPh, CEA Cadarache, France 3 DSM/IRFU/SPhN, CEA Saclay, France 4 IPN, CNRS/IN2P3, Univ. Paris-Sud, France 5 Institut Laue-Langevin, Grenoble, France 6 CENBG, Universit´ e de Bordeaux 1, France
Analysis and results of the measurements of the independent isotopic and mass fission yields for the 233 U(nth ,f) reaction are presented, which were performed at the Lohengrin recoil mass spectrometer in ILL. The spectrometer separates the fission products according to their mass to ion charge ratio and kinetic energy to ion charge ratio. Mass yields were obtained by means of an ionisation chamber after separation by the spectrometer. Isotopic yields were determined by placing two high-purity Germanium clover detectors downstream the spectrometer. An innovative analysis method developed for the Lohengrin spectrometer is presented, from which the final results are independent from the previous evaluations, which is usually not the case.
I.
INTRODUCTION
Growing world energy consumption has renewed the interest in nuclear power. The rise of nuclear reactors implies the study of innovative systems that could rely not only on the commonly used fuel cycle of UraniumPlutonium (U-Pu), but also on Thorium-Uranium (ThU). The latter might indeed provide cleaner nuclear energy than the U-Pu fuel cycle. Studies of different aspects of innovative fuel cycles, such as calculations of residual heat or poison concentration in the fuel, require detailed knowledge of isotopic and mass fission yields. An experimental campaign has been initiated within a collaborative between the Institut Laue-Langevin (ILL) and French CEA and CNRS laboratories to extend and improve the existing fission products yields data for the key nuclei within the innovative Th-233 U cycle. Available experimental data for 233 U(nth ,f) exist mainly for light fission fragments. Several measurements have been performed to determine the mass yields of 233 U, although were limited to cumulated yields, i.e. measured within a few minutes or hours after fission occurred. Only two independent measurements for the light region exist: Wehring et al. [1] and Quade et al. [2]. As for
∗
Corresponding author: [email protected]
http://dx.doi.org/10.1016/j.nds.2014.08.090 0090-3752/© 2014 Elsevier Inc. All rights reserved.
the heavy region, the only independent measurements available ([3]-[4]) were estimated with a three-mass unit uncertainty, smoothing drastically any structure in the heavy-fragment peak. For these reasons, our recent independent yield measurements presented here were then focused on the heavy fission fragment parts.
II.
THE LOHENGRIN SPECTROMETER
The Lohengrin mass spectrometer is located in the ILL research reactor facility, which allows to study fragment distributions from thermal neutron induced fission with very good resolution. A fissile actinide target is placed close to the reactor core, which operates at a thermal neutron flux reaching 5×1014 neutrons×cm−2 ×s−1 . Fission fragments emerge from the target with an average ionic charge state of approximately 21. Those fragments that are emitted along the beam tube axis undergo horizontal deflection in a magnetic field, directly followed by vertical deflection in an electric field. These combined fields separate ions according to their A/q and E/q values (with A, q and E the mass, ion charge state and kinetic energy of the ions respectively). See Refs. [5, 6] for more information concerning the spectrometer performance.
Measurements of the Mass . . .
MASS YIELD MEASUREMENTS A.
8
Method of Analysis
The first step consists in getting the count rate of a fission product with a given mass independently from their energy or charge. As it would be too long to measure the kinetic energy distribution for each charge state, the following hypothesis is usually introduced: the two variables are independent [7]. The total count rate of a mass MA is then calculated from a single energy scan and a charge scan. The first one is obtained by measuring different energies at a fixed charge q, and the second one by measuring different ionic charges at a fixed energy E [7]. During the experiment, the amount of fissile material within the spectrometer’s acceptance decreases, principally by sputtering but also from the burning due to neutron induced reactions. Thus the monitoring of the 233 U target is obtained by periodically measuring the kinetic energy distribution of a given mass at a fixed ionic charge, so-called “burnup” measurements. Provided that all the heavy mass rates are measured, it is possible to auto-normalise the data by defining to 100% the sum of the whole heavy peak yields. As a consequence, these new measurements are independent from another experiment or assessment and may be compared with the existing data. This auto-normalization introduces correlations between all the measurements and increases the uncertainties on Y (Ai ) . Completion of the first step depends on the quality of the kinetic energy and charge distribution assessments. The hypothesis of independence has been tested on some nuclei and has been validated for the fission products without nanosecond isomers. For the other ones, the correction due to the correlation between kinetic energy and charge has been estimated by fitting the evolution of the mean kinetic energy with the ionic charge, this correlation depends strongly on the presence of nanosecond isomers and on the width of the ionic charge distribution. Such a correction has been estimated between 3% and 15% for the few involved masses, it is specific to and needs to be determined for each mass.
B.
JEFF-3.1.1 ENDF/B-VII.0 this work
9
Mass yield (% per fission)
III.
F. Martin et al.
NUCLEAR DATA SHEETS
7 6 5 4 3 2 1 0 120
125
130
135
140
Mass (A)
145
150
155
FIG. 1. 233 U(nth ,f) mass yield measurements compared with JEFF-3.1.1 and ENDF/B-VII.0 evaluations.
IV.
ISOTOPIC YIELD MEASUREMENTS A.
Method of Analysis
The 233 U isotopic fission yields were determined by means of gamma-ray spectroscopy, on the basis of the fission-product beta decay occurring after their flight in the spectrometer (about 1 μs). The fission products are implanted in a movable tape, the position of which is fixed for each particular gamma-ray measurement (about 30 min). Two HPGe clover detectors are placed on each side of the tape to measure the gamma decay. Only the fragments with given A/q and E/q ratios corresponding to the Lohengrin tuning can be gathered on the tape [7]. The targets used for the isotopic measurements were too thick, which would result in a significant deterioration in the energy resolution and would need larger corrections to the fits and normalization. Instead of a direct estimation of the isotopic yields, we chose to determine the probability distribution of each isotope P (Z|A) for a given mass A by self-normalization to minimize the systematic effects (HPGe efficiency, target burnup correction, partial ionic charge and kinetic energy distributions, etc.) and obtain independent data. The isotopic yield is determined by multiplying this value by the mass yield Y (A, Z) = Y (A) × P (Z|A). The latter can then be estimated from the gamma-ray spectroscopy measurement
Results and Discussion
The 233 U(nth ,f) mass yield measurements from the Lohengrin spectrometer are presented in Fig. 1 and are compared with the JEFF-3.1.1 [8] and ENDF/B-VII.0 [9] evaluations. Two structural features are seen in the JEFF and ENDF libraries at mass values equal to 136 and 142. Our results at mass 136 are in agreement with the ENDF library within ENDF’s large uncertainties. However, our results are in complete disagreement with the European library. Additionally, the second structure appears at mass 141 and not 142 according to our results.
Si =
0
tf
f γ Iγ λi Ni (t) dt,
(1)
where Si is the integral of the corresponding photopeak of a given gamma-ray, γ is the efficiency of the detection system at this gamma energy, Iγ is the relative intensity taken from Ref. [13], Ni (t) is the number of nuclei for isotope i, tf is the measurement time, and f is the summing correction factor ranging from around 1.1 to 0.8 and 329
Measurements of the Mass . . .
F. Martin et al.
NUCLEAR DATA SHEETS
calculated as follows using the TrueCoinc code [10] f= (1 − Iγ tot γ ).
provided P (Z|A, Ei ) = P (Z|A, Ei ) for all Ei and Ei , and where N (Z|A, E) is the measurement of isotope A ZX at kinetic energy E using gamma-ray spectroscopy, R(Ei ) is the energy resolution of the Lohengrin spectrometer, and P (E|A) is the energy probability per mass A obtained with the ionisation chamber.
(2)
j=i
P (Z|A) values were estimated via several characteristic gamma lines in the decay of a given isotope, the P (Z|A) adopted was the averaged value of the individual ones. The detector efficiency was obtained via the comparison
B.
Isotopic studies on the 131-145 mass range have been completed and the analysis is under way. Preliminary results are presented in Fig. 2 for mass 137 isotopic production, normalised to the JEFF-3.1.1 evaluation or with our new method giving totally independent data, and compared to previous measurements from Piksaykin et al. [12]. 137 Te contribution remains to be evaluated, whereas 137 Cs is not reachable via our measurement due to its 30 years half-life. The mass 139 isotopic yield measurement was taken as the reference one for the cross normalisation since more than 99% of the isotopic distribution has been measured in this experiment. The agreement with the JEFF-3.1.1 evaluation and a previous measurement is quite good, yet a thorough analysis of the several corrections to apply is still under way.
FIG. 2. Preliminary results for the isotopic production of mass 137 (independent yields).
of calibrated point source measurements with a thorough modelling of the geometry using the MCNP5 code [11]. Relative agreement between the measurements and the simulation induces an uncertainty of approximately 3% in the efficiency. For the mass chain with one or few isotopes with a long lifetime or no gamma rays (pure β decay to the ground state), the isotopic probability P (Z|A) couldn’t be normalised with the method described above. In these cases we used a different method that allows cross normalisation on a complete isotopic measurement Aref based on the following relationship Y (Zi |Ai ) = Y (Zref |Aref ) P (Eref |Aref ) R(Eref ) N (Zi |Ai , Ei ) , P (Ei |Ai ) R(Ei ) N (Zref |Aref , Eref )
Results and Discussion
V.
CONCLUSIONS
A new campaign of 233 U(nth ,f) fission yield measurements has begun using the Lohengrin spectrometer. The goal of these new measurements is to assess precisely the fission yields in the heavy mass region. Fission fragment mass yields have been determined by coupling the Lohengrin spectrometer to an ionization chamber, while the isotopic yields were derived from gamma-ray spectroscopy. Isotopic yields were obtained from the results of the mass yield measurements, and the self-normalization process generated independent measurements of the existing data.
(3)
[1] B.W. Wehring et al., Trans. American Nucl. Soc. 11 (1980). [2] U. Quade et al., Nucl. Phys. A 487, 1 (1988). [3] M. Asghar et al., Nucl. Phys. A 368, 328 (1981). [4] H. Baba et al., J. Nucl. Sci. Technol. 34 (1997). [5] O. S´erot et al., Nucl. Data Sheets 119, 320 (2014). [6] H. Faust et al., presented in ND2013. [7] A. Bail et al., Phys. Rev. C 84, 034605 (2011). [8] The JEFF-3.1.1 nuclear data library. OECD, NEA, JEFF Report 21, ISBN 92-64-02314-3 (2006).
[9] M.B. Chadwick et al., Nucl. Data Sheets 107, 2931 (2006). [10] Truecoinc, a software utility for calculation of the true coincidence correction, IAEA Tecdoc 1275, 37 (2002). [11] X-5 Monte Carlo Team, LANL Report LA-UR-03-1987 (2003). [12] V.M. Piksaykin et al., Joint Inst. for Nucl. Res., Dubna Reports 2004-169, 342 (2004). [13] ENSDF, NNDC, http://www.nndc.bnl.gov.
330