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Recent Results from Lohengrin on Fission Yields and Related Decay Properties
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Nuclear Data Sheets 119 (2014) 320–323 www.elsevier.com/locate/nds
Recent Results from Lohengrin on Fission Yields and Related Decay Properties O. Serot,1, ∗ C. Amouroux,2 A. Bidaud,3 N. Capellan,3 S. Chabod,3 A. Ebran,4 H. Faust,5 G. Kessedjian,3 U. K¨ oester,5 A. Letourneau,2 O. Litaize,1 F. Martin,1, 3 T. Materna,2 1, 6 L. Mathieu, S. Panebianco,2 J.-M. Regis,7 M. Rudigier,7 C. Sage,3 and W. Urban5 1
CEA, DEN-Cadarache, F-13108 Saint-Paul-lez-Durance, France 2 CEA, DSM-Saclay, F-91191 Gif-sur-Yvette, France 3 LPSC, 53 rue des Martyrs, F-38026 Grenoble Cedex, France 4 CEA, DAM-Ile de France, F-91290 Arpajon, France 5 Institut Laue Langevin, 6 rue Jules Horowitz, F-38042, Grenoble, France 6 CENBG, Chemin du Solarium, F-33175 Gradignan, France 7 IKP, Universit¨ at zu K¨ oln, 50937 K¨ oln, Germany The Lohengrin mass spectrometer is one of the 40 instruments built around the reactor of the Institute Laue-Langevin (France) which delivers a very intense thermal neutron flux. Usually, Lohengrin was combined with a high-resolution ionization chamber in order to obtain good nuclear charge discrimination within a mass line, yielding an accurate isotopic yield determination. Unfortunately, this experimental procedure can only be applied for fission products with a nuclear charge less than about 42, i.e. in the light fission fragment region. Since 2008, a large collaboration has started with the aim of studying various fission aspects, mainly in the heavy fragment region. For that, a new experimental setup which allows isotopic identification by γ-ray spectrometry has been developed and validated. This technique was applied on the 239 Pu(nth ,f) reaction where about 65 fission product yields were measured with an uncertainty that has been reduced on average by a factor of 2 compared with what was that previously available in nuclear data libraries. The same γ-ray spectrometric technique is currently being applied to the study of the 233 U(nth ,f) reaction. Our aim is to deduce charge and mass distributions of the fission products and to complete the experimental data that exist mainly for light fission fragments. The measurement of 41 mass yields from the 241 Am(2nth ,f) reaction has been also performed. In addition to these activities on fission yield measurements, various new nanosecond isomers were discovered. Their presence can be revealed from a strong deformed ionic charge distribution compared to a ’normal’ Gaussian shape. Finally, a new neutron long-counter detector designed to have a detection efficiency independent of the detected neutron energy has been built. Combining this neutron device with a Germanium detector and a beta-ray detector array allowed us to measure the beta-delayed neutron emission probability Pn of some important fission products for reactor applications. I.
INTRODUCTION
The Lohengrin recoil mass spectrometer located at the Institut Laue-Langevin in Grenoble (France) became operational in 1975. Since then, most of the experiments performed on Lohengrin are devoted to the determination of the fission product observables, such as isobaric and isotopic yields, odd-even effects, and kinetic energy distributions, from thermal neutron-induced fission of various actinides. The other main activity is related to the investigation of the nuclear structure of short-lived fission products, i.e. neutron-rich nuclei far from stability. After nearly 40 years of existence, beam times on Lohengrin are
∗
Corresponding author: [email protected]
http://dx.doi.org/10.1016/j.nds.2014.08.088 0090-3752/© 2014 Elsevier Inc. All rights reserved.
still highly requested by experimentalists for mainly two reasons: • The target position of Lohengrin is located at about 50 cm from the reactor core where the thermal neutron flux reaches about 5.3×1014 n/s/cm2 , which is one of the highest thermal neutron fluxes in the world. Due to this extremely high neutron flux, yields down to 10−7 % can be measured. • The excellent mass and energy resolutions, which are estimated to be about 0.3 % (mass resolution) and 1 % (energy resolution) for a typical 70 x 10 mm target. Fission products entering the spectrometer are selected according to their mass (A), ionic charge (q) and kinetic energy (E) by the use of a magnetic field followed by
Recent Results from Lohengrin . . .
O. Serot et al.
NUCLEAR DATA SHEETS
an electric field. Details on the Lohengrin are given in Refs. [1, 2]. At the exit slit of Lohengrin, fission products with the same A/q and E/q values can be detected using a high resolution ionization chamber with a split anode, which allows a good nuclear charge discrimination within a mass line [3]. Nevertheless, this technique works only for fission products with a nuclear charge below around 42. This explains why quite extensive data sets for kinetic energy, nuclear mass, and nuclear charge have been measured in the light mass region (see Ref. [5] and references therein), but almost nothing was measured in the heavy mass region. In this context, by taking advantage of the high performance of the Lohengrin facility, we started in 2008 a new collaboration with the aim of studying various properties of heavy fission products.
8
233
U(nth,f)
235
U(nth,f)
Yield (%)
6
4
2
0 120
124
128
132
136
140
144
148
152
156
Heavy Mass Number
FIG. 1. Independent fission yields measured with Lohengrin coupled to an ionisation chamber for 233 U(nth ,f) [8, 9] and 235 U(nth ,f) [5] reactions.
152
II.
MASS YIELDS AND KINETIC ENERGY
148
1.000
Heavy Mass Number
0.9000
The experimental setup used to investigate mass yields and fission product kinetic energy distributions is the coupling of the Lohengrin spectrometer with a high resolution ionization chamber (see details in Ref. [3]). This type of measurements requires a thin target (typically between 50 and 150 μg/cm2 ) which is put on a thick 9×2 cm2 Ti backing. A very thin Ni-foil (0.25 μm or 0.5 μm) covers the actinide deposit in order to reduce its sputtering by the fission fragments. In this way, it is possible to avoid a too rapid burn-up of the target [4]. Various fission reactions were investigated: 233 U(nth ,f), 235 U(nth ,f), 239 Pu(nth ,f), 241 Pu(nth ,f), 241 Am(2nth ,f). For each reaction, we have tried to cover in detail the heavy fission fragment mass range, from symmetric to very asymmetric regions, except for the 241 Am(2nth ,f) reaction, where the more produced masses in the light and heavy regions were investigated. For a consistency check also several well-known masses in the light mass peak were also measured. Details of the data analysis are given in Refs. [5, 8, 9]. An example of mass yields measured for 233 U [8, 9] and 235 U [5] is plotted in Fig. 1. For 233 U(nth ,f) a variance-covariance analysis was performed. Preliminary results of the correlation matrix obtained by accounting for target burnup, normalization and the use of two different targets are presented in Fig. 2. In the case of 241 Am(2nth ,f) (reaction induced after successive capture of 2 thermal neutrons), 41 mass yields were measured in the light and heavy mass regions. Preliminary results show that mass yields from the neutroninduced fission of 242g Am and from 242m Am(n,f) do not differ by more than 25% [13]. Moreover, we have observed for the light peak a good agreement with the ENDF/BVII.0 library, while for the heavy peak, our results are consistent with both ENDF/B-VII.0 and JEFF-3.1.1 libraries. In addition, for each mass investigated a post-neutron
144
0.8000 0.7000 0.6000
140
0.5000 0.4000 0.3000
136
0.2000 0.1000 0
132
-0.1000 -0.2000 -0.3000
128 124 120 120
124
128
132
136
140
144
148
152
Heavy Mass Number
FIG. 2. Partial experimental correlation matrix for the independent fission yields from 233 U(nth ,f) reaction.
kinetic energy distribution was measured. From this distribution, the average kinetic energy (KE) and the rms width (σKE ) can be extracted as illustrated in Fig. 3 for the 239 Pu(nth ,f) reaction. III.
ISOTOPIC YIELDS
As already stated, the ionization chamber can be used to determine isotopic yields only for fission products with nuclear charges below 42. In order to determine heavy isotopic yields, a new setup was developed, which is based on gamma spectrometry for isotopic identification. Indeed, as the beta-decays of fission products are often followed by gamma de-excitation, and because these decays occur after their flight path in the spectrometer (the travel time in the separator being about 2 μs), gammarays can be used to determine isotopic yields. For this purpose the fission products are implanted in a moving tape which is coupled to a vacuum chamber at the focus position of the spectrometer. Two germanium clover detectors are used to measure the gamma decay with high 321
Recent Results from Lohengrin . . .
O. Serot et al.
NUCLEAR DATA SHEETS 8
88
80
6
Fission Yields (%)
76
KE (MeV)
Mass Yield Z=51 (Sb) Z=52 (Te) Z=53 (I) Z=54 (Xe) Z=55 (Cs) Z=56 (Ba) Z=57 (La) Z=58 (Ce)
7
84
72 68 64 60 56
5 4 3 2 1
8.5 8.0
0 126
VKE (MeV)
7.5
129
132
135
138
141
144
147
150
Heavy Mass Number 7.0
FIG. 4. Independent isotopic fission yields for 239 Pu(nth ,f) measured with Lohengrin coupled to the new gamma spectrometry setup [7].
6.5 6.0 5.5 5.0 124
128
132
136
140
144
148
152
Heavy Mass Number
products a strong asymmetric ionic charge distribution was observed. This phenomenon is interpreted as due to nanosecond isomers that decay by a highly converted internal transition. Owing to the short half-life of these isomeric states (of the order of some nanoseconds), conversion and Auger electrons, which are emitted between the target and the first dipole of the spectrometer, increase the ionic charge of the fission product. This new ionic charge state is maintained during the flight through Lohengrin and therefore can be detected. An example of such an isomeric state is given in Fig. 5, where the ionic charge distributions were measured by γ-spectrometry for both 140 Xe and 140 Cs. A Gaussian shape was found for 140 Xe, indicating no nanosecond isomer, while for the 140 Cs nucleus, a deformed distribution was observed, showing the presence of a nanosecond isomer.
FIG. 3. Average kinetic energy (top) and rms width (bottom) obtained from the measured kinetic energy distributions as a function of the fission product mass (239 Pu(nth ,f) case [5]).
efficiency. The tape system allows the removal of longlived activity. Note that the isotopic yield can be determined only if the branching ratios and decay constants of the isotopes are known. The analysis of the data is based on integration of the Bateman equations. Details of the analysis can be found in Ref. [7]. Some isotopic fission product yields from 239 Pu(nth ,f) were measured in the light mass region and compared with the results obtained by Schmitt [12] with an ionisation chamber. Excellent agreement was found which is very important to validate both the good functioning of our experimental setup and the procedure used for the data analysis. Data of our measured isotopic fission product yields in the heavy mass region are shown in Fig. 4, for Z=51 up to Z=58 [7]. For the large majority of fission products, a very good agreement between the Lohengrin data and the European library JEFF-3.1.1 values is observed, but with a significant reduction of the uncertainties (except for nuclei where gamma branching ratios are poorly known). The average yield uncertainty reaches 11.9% for our measurements and 23.3% for JEFF-3.1.1 respectively, which corresponds to a reduction of nearly a factor 2. Preliminary results of isotopic yields measured for the 233 U(nth ,f)-reaction are given in [10].
IV.
50 140 55
45
Cs
Intensity (arb. unit)
40 35 30 25
140 54
Xe
20 15 10 5 16
18
20
22
24
26
28
30
32
Ionic charge
FIG. 5. Ionic charge distributions measured for Cs by γ-spectrometry [7].
140
NANOSECOND ISOMERS
140
Xe and
By using this method, new nanosecond isomers were identified and reported by Materna et al. [14]. In addition, the technique was improved to provide an estimate of the lifetime of these isomeric states, see Ref. [6] for details.
In principle, the ionic charge distribution should be roughly Gaussian with an average q-value of about 21-22 in the heavy mass region. Nevertheless, for some fission 322
Recent Results from Lohengrin . . .
O. Serot et al.
NUCLEAR DATA SHEETS
An initial experiment has been carried out at the Lohengrin facility using the new LOENIE detector and the ’gamma-neutron spectrometry’ method, where a Gedetector was inserted in the central hole of the longcounter close to the vacuum chamber. The delayed neutron emission probability can then be deduced from the following equation Pn =
(1)
where Nγ is the number of detected γ-rays which follow the β-decay of the precursor. Nγ must be corrected by the gamma efficiency detection (γ ) and the branching ratio of the γ-ray (BR). Nn is the number of detected delayed neutrons which is corrected by the neutron detection efficiency n . As reported in Ref. [11], the measured 99 Y Pn-value, (1.77 ± 0.19) %, is in excellent agreement with data from literature but with a smaller uncertainty. The 136 Te Pn-value, (1.34 ± 0.13) %, is less precise but confirms that the zero value adopted in JEFF-3.1.1 is wrong. In addition, other experiments are foreseen with LOENIE setup, but coupled with a beta detector, applying the beta-neutron spectrometry method.
FIG. 6. Experimental setup used for the Pn measurements [11].
V.
Nn BRγ × , Nγ n
DELAYED NEUTRON EMISSION PROBABILITY
A new neutron detector called LOENIE (LOng-counter with ENergy Independent Efficiency) has been built. Our aim is to measure the delayed neutron emission probability (Pn) for various important delayed neutron precursors. The detector has an octagonal shape and contains 18 proportional counters 3 He in two concentric rings (inner and outer) as shown in Fig. 6. Since we want to thermalize the delayed neutrons before their detection, the 3 He tubes are embedded in a moderating polyethylene matrix. Details of this neutron detector can be found in Ref. [11]. The experimental setup includes a vacuum chamber, where the selected fission products are stopped. This vacuum chamber is placed in the middle of the long-counter. Again, a tape system enables to remove fission products far from the detection system in order to reduce long-lived background.
VI.
CONCLUSIONS
By combining the Lohengrin mass spectrometer and a new experimental setup based on γ-spectrometry, we were able to investigate various fission product properties in the heavy mass region. Results obtained for several fission reactions are very encouraging and uncertainties have been decreased compared with other experiments and evaluated data. The beam from Lohengrin is a combination of various fission products with the same A/q and E/q values. Coupling of Lohengrin with a Gas Filled Magnet is under study [15]. in order to purify the extracted beam and to get a pure isobaric fission product beam.
[1] P. Armbruster et al., Nucl. Instrum. Methods 139, 213 (1976). [2] G. Fioni et al., Nucl. Instrum. Methods A 335, 175 (1993). [3] E. Moll et al., Nucl. Instrum. Methods 139, 213 (1976). [4] U. K¨ oster et al., Nucl. Instrum. Methods A 613, 363 (2010). [5] A. Bail, Ph.D. Thesis, University of Bordeaux (2009). [6] T. Materna et al., Proc. 5th Int. Conf. on Fission and Properties of Neutron Rich Nuclei, Dec. 2012, Sanibel Island (USA), World Scientific. [7] A. Bail et al., Phys. Rev. C 84, 034605 (2011).
[8] F. Martin et al., Proc. 2nd Int. Conf. Advancements Nuclear Instrumentation, Measurement Methods and their Applications, June 2011, Ghent (Belgium). [9] C. Sage et al., Proc. 7th Int. Conf. Dynamical Aspects Nuclear Fission, Oct. 2011, Smolenice (Slovak Republic). [10] F. Martin et al., Nucl. Data Sheets 119, 328 (2014). [11] L. Mathieu et al., JINST 7, P08029 (2012). [12] C. Schmitt et al., Nucl. Phys. A 430, 21 (1984). [13] C. Amouroux et al., EPJ Web of Conferences, 42 (2013), in press. [14] T. Materna et al., AIP Conf. Proc. 1175, 367 (2009). [15] G. Kessedjian et al., EPJ Web of Conferences 42, (2013), in press.
323
Nuclear Data Sheets 119 (2014) 320–323 www.elsevier.com/locate/nds
Recent Results from Lohengrin on Fission Yields and Related Decay Properties O. Serot,1, ∗ C. Amouroux,2 A. Bidaud,3 N. Capellan,3 S. Chabod,3 A. Ebran,4 H. Faust,5 G. Kessedjian,3 U. K¨ oester,5 A. Letourneau,2 O. Litaize,1 F. Martin,1, 3 T. Materna,2 1, 6 L. Mathieu, S. Panebianco,2 J.-M. Regis,7 M. Rudigier,7 C. Sage,3 and W. Urban5 1
CEA, DEN-Cadarache, F-13108 Saint-Paul-lez-Durance, France 2 CEA, DSM-Saclay, F-91191 Gif-sur-Yvette, France 3 LPSC, 53 rue des Martyrs, F-38026 Grenoble Cedex, France 4 CEA, DAM-Ile de France, F-91290 Arpajon, France 5 Institut Laue Langevin, 6 rue Jules Horowitz, F-38042, Grenoble, France 6 CENBG, Chemin du Solarium, F-33175 Gradignan, France 7 IKP, Universit¨ at zu K¨ oln, 50937 K¨ oln, Germany The Lohengrin mass spectrometer is one of the 40 instruments built around the reactor of the Institute Laue-Langevin (France) which delivers a very intense thermal neutron flux. Usually, Lohengrin was combined with a high-resolution ionization chamber in order to obtain good nuclear charge discrimination within a mass line, yielding an accurate isotopic yield determination. Unfortunately, this experimental procedure can only be applied for fission products with a nuclear charge less than about 42, i.e. in the light fission fragment region. Since 2008, a large collaboration has started with the aim of studying various fission aspects, mainly in the heavy fragment region. For that, a new experimental setup which allows isotopic identification by γ-ray spectrometry has been developed and validated. This technique was applied on the 239 Pu(nth ,f) reaction where about 65 fission product yields were measured with an uncertainty that has been reduced on average by a factor of 2 compared with what was that previously available in nuclear data libraries. The same γ-ray spectrometric technique is currently being applied to the study of the 233 U(nth ,f) reaction. Our aim is to deduce charge and mass distributions of the fission products and to complete the experimental data that exist mainly for light fission fragments. The measurement of 41 mass yields from the 241 Am(2nth ,f) reaction has been also performed. In addition to these activities on fission yield measurements, various new nanosecond isomers were discovered. Their presence can be revealed from a strong deformed ionic charge distribution compared to a ’normal’ Gaussian shape. Finally, a new neutron long-counter detector designed to have a detection efficiency independent of the detected neutron energy has been built. Combining this neutron device with a Germanium detector and a beta-ray detector array allowed us to measure the beta-delayed neutron emission probability Pn of some important fission products for reactor applications. I.
INTRODUCTION
The Lohengrin recoil mass spectrometer located at the Institut Laue-Langevin in Grenoble (France) became operational in 1975. Since then, most of the experiments performed on Lohengrin are devoted to the determination of the fission product observables, such as isobaric and isotopic yields, odd-even effects, and kinetic energy distributions, from thermal neutron-induced fission of various actinides. The other main activity is related to the investigation of the nuclear structure of short-lived fission products, i.e. neutron-rich nuclei far from stability. After nearly 40 years of existence, beam times on Lohengrin are
∗
Corresponding author: [email protected]
http://dx.doi.org/10.1016/j.nds.2014.08.088 0090-3752/© 2014 Elsevier Inc. All rights reserved.
still highly requested by experimentalists for mainly two reasons: • The target position of Lohengrin is located at about 50 cm from the reactor core where the thermal neutron flux reaches about 5.3×1014 n/s/cm2 , which is one of the highest thermal neutron fluxes in the world. Due to this extremely high neutron flux, yields down to 10−7 % can be measured. • The excellent mass and energy resolutions, which are estimated to be about 0.3 % (mass resolution) and 1 % (energy resolution) for a typical 70 x 10 mm target. Fission products entering the spectrometer are selected according to their mass (A), ionic charge (q) and kinetic energy (E) by the use of a magnetic field followed by
Recent Results from Lohengrin . . .
O. Serot et al.
NUCLEAR DATA SHEETS
an electric field. Details on the Lohengrin are given in Refs. [1, 2]. At the exit slit of Lohengrin, fission products with the same A/q and E/q values can be detected using a high resolution ionization chamber with a split anode, which allows a good nuclear charge discrimination within a mass line [3]. Nevertheless, this technique works only for fission products with a nuclear charge below around 42. This explains why quite extensive data sets for kinetic energy, nuclear mass, and nuclear charge have been measured in the light mass region (see Ref. [5] and references therein), but almost nothing was measured in the heavy mass region. In this context, by taking advantage of the high performance of the Lohengrin facility, we started in 2008 a new collaboration with the aim of studying various properties of heavy fission products.
8
233
U(nth,f)
235
U(nth,f)
Yield (%)
6
4
2
0 120
124
128
132
136
140
144
148
152
156
Heavy Mass Number
FIG. 1. Independent fission yields measured with Lohengrin coupled to an ionisation chamber for 233 U(nth ,f) [8, 9] and 235 U(nth ,f) [5] reactions.
152
II.
MASS YIELDS AND KINETIC ENERGY
148
1.000
Heavy Mass Number
0.9000
The experimental setup used to investigate mass yields and fission product kinetic energy distributions is the coupling of the Lohengrin spectrometer with a high resolution ionization chamber (see details in Ref. [3]). This type of measurements requires a thin target (typically between 50 and 150 μg/cm2 ) which is put on a thick 9×2 cm2 Ti backing. A very thin Ni-foil (0.25 μm or 0.5 μm) covers the actinide deposit in order to reduce its sputtering by the fission fragments. In this way, it is possible to avoid a too rapid burn-up of the target [4]. Various fission reactions were investigated: 233 U(nth ,f), 235 U(nth ,f), 239 Pu(nth ,f), 241 Pu(nth ,f), 241 Am(2nth ,f). For each reaction, we have tried to cover in detail the heavy fission fragment mass range, from symmetric to very asymmetric regions, except for the 241 Am(2nth ,f) reaction, where the more produced masses in the light and heavy regions were investigated. For a consistency check also several well-known masses in the light mass peak were also measured. Details of the data analysis are given in Refs. [5, 8, 9]. An example of mass yields measured for 233 U [8, 9] and 235 U [5] is plotted in Fig. 1. For 233 U(nth ,f) a variance-covariance analysis was performed. Preliminary results of the correlation matrix obtained by accounting for target burnup, normalization and the use of two different targets are presented in Fig. 2. In the case of 241 Am(2nth ,f) (reaction induced after successive capture of 2 thermal neutrons), 41 mass yields were measured in the light and heavy mass regions. Preliminary results show that mass yields from the neutroninduced fission of 242g Am and from 242m Am(n,f) do not differ by more than 25% [13]. Moreover, we have observed for the light peak a good agreement with the ENDF/BVII.0 library, while for the heavy peak, our results are consistent with both ENDF/B-VII.0 and JEFF-3.1.1 libraries. In addition, for each mass investigated a post-neutron
144
0.8000 0.7000 0.6000
140
0.5000 0.4000 0.3000
136
0.2000 0.1000 0
132
-0.1000 -0.2000 -0.3000
128 124 120 120
124
128
132
136
140
144
148
152
Heavy Mass Number
FIG. 2. Partial experimental correlation matrix for the independent fission yields from 233 U(nth ,f) reaction.
kinetic energy distribution was measured. From this distribution, the average kinetic energy (KE) and the rms width (σKE ) can be extracted as illustrated in Fig. 3 for the 239 Pu(nth ,f) reaction. III.
ISOTOPIC YIELDS
As already stated, the ionization chamber can be used to determine isotopic yields only for fission products with nuclear charges below 42. In order to determine heavy isotopic yields, a new setup was developed, which is based on gamma spectrometry for isotopic identification. Indeed, as the beta-decays of fission products are often followed by gamma de-excitation, and because these decays occur after their flight path in the spectrometer (the travel time in the separator being about 2 μs), gammarays can be used to determine isotopic yields. For this purpose the fission products are implanted in a moving tape which is coupled to a vacuum chamber at the focus position of the spectrometer. Two germanium clover detectors are used to measure the gamma decay with high 321
Recent Results from Lohengrin . . .
O. Serot et al.
NUCLEAR DATA SHEETS 8
88
80
6
Fission Yields (%)
76
KE (MeV)
Mass Yield Z=51 (Sb) Z=52 (Te) Z=53 (I) Z=54 (Xe) Z=55 (Cs) Z=56 (Ba) Z=57 (La) Z=58 (Ce)
7
84
72 68 64 60 56
5 4 3 2 1
8.5 8.0
0 126
VKE (MeV)
7.5
129
132
135
138
141
144
147
150
Heavy Mass Number 7.0
FIG. 4. Independent isotopic fission yields for 239 Pu(nth ,f) measured with Lohengrin coupled to the new gamma spectrometry setup [7].
6.5 6.0 5.5 5.0 124
128
132
136
140
144
148
152
Heavy Mass Number
products a strong asymmetric ionic charge distribution was observed. This phenomenon is interpreted as due to nanosecond isomers that decay by a highly converted internal transition. Owing to the short half-life of these isomeric states (of the order of some nanoseconds), conversion and Auger electrons, which are emitted between the target and the first dipole of the spectrometer, increase the ionic charge of the fission product. This new ionic charge state is maintained during the flight through Lohengrin and therefore can be detected. An example of such an isomeric state is given in Fig. 5, where the ionic charge distributions were measured by γ-spectrometry for both 140 Xe and 140 Cs. A Gaussian shape was found for 140 Xe, indicating no nanosecond isomer, while for the 140 Cs nucleus, a deformed distribution was observed, showing the presence of a nanosecond isomer.
FIG. 3. Average kinetic energy (top) and rms width (bottom) obtained from the measured kinetic energy distributions as a function of the fission product mass (239 Pu(nth ,f) case [5]).
efficiency. The tape system allows the removal of longlived activity. Note that the isotopic yield can be determined only if the branching ratios and decay constants of the isotopes are known. The analysis of the data is based on integration of the Bateman equations. Details of the analysis can be found in Ref. [7]. Some isotopic fission product yields from 239 Pu(nth ,f) were measured in the light mass region and compared with the results obtained by Schmitt [12] with an ionisation chamber. Excellent agreement was found which is very important to validate both the good functioning of our experimental setup and the procedure used for the data analysis. Data of our measured isotopic fission product yields in the heavy mass region are shown in Fig. 4, for Z=51 up to Z=58 [7]. For the large majority of fission products, a very good agreement between the Lohengrin data and the European library JEFF-3.1.1 values is observed, but with a significant reduction of the uncertainties (except for nuclei where gamma branching ratios are poorly known). The average yield uncertainty reaches 11.9% for our measurements and 23.3% for JEFF-3.1.1 respectively, which corresponds to a reduction of nearly a factor 2. Preliminary results of isotopic yields measured for the 233 U(nth ,f)-reaction are given in [10].
IV.
50 140 55
45
Cs
Intensity (arb. unit)
40 35 30 25
140 54
Xe
20 15 10 5 16
18
20
22
24
26
28
30
32
Ionic charge
FIG. 5. Ionic charge distributions measured for Cs by γ-spectrometry [7].
140
NANOSECOND ISOMERS
140
Xe and
By using this method, new nanosecond isomers were identified and reported by Materna et al. [14]. In addition, the technique was improved to provide an estimate of the lifetime of these isomeric states, see Ref. [6] for details.
In principle, the ionic charge distribution should be roughly Gaussian with an average q-value of about 21-22 in the heavy mass region. Nevertheless, for some fission 322
Recent Results from Lohengrin . . .
O. Serot et al.
NUCLEAR DATA SHEETS
An initial experiment has been carried out at the Lohengrin facility using the new LOENIE detector and the ’gamma-neutron spectrometry’ method, where a Gedetector was inserted in the central hole of the longcounter close to the vacuum chamber. The delayed neutron emission probability can then be deduced from the following equation Pn =
(1)
where Nγ is the number of detected γ-rays which follow the β-decay of the precursor. Nγ must be corrected by the gamma efficiency detection (γ ) and the branching ratio of the γ-ray (BR). Nn is the number of detected delayed neutrons which is corrected by the neutron detection efficiency n . As reported in Ref. [11], the measured 99 Y Pn-value, (1.77 ± 0.19) %, is in excellent agreement with data from literature but with a smaller uncertainty. The 136 Te Pn-value, (1.34 ± 0.13) %, is less precise but confirms that the zero value adopted in JEFF-3.1.1 is wrong. In addition, other experiments are foreseen with LOENIE setup, but coupled with a beta detector, applying the beta-neutron spectrometry method.
FIG. 6. Experimental setup used for the Pn measurements [11].
V.
Nn BRγ × , Nγ n
DELAYED NEUTRON EMISSION PROBABILITY
A new neutron detector called LOENIE (LOng-counter with ENergy Independent Efficiency) has been built. Our aim is to measure the delayed neutron emission probability (Pn) for various important delayed neutron precursors. The detector has an octagonal shape and contains 18 proportional counters 3 He in two concentric rings (inner and outer) as shown in Fig. 6. Since we want to thermalize the delayed neutrons before their detection, the 3 He tubes are embedded in a moderating polyethylene matrix. Details of this neutron detector can be found in Ref. [11]. The experimental setup includes a vacuum chamber, where the selected fission products are stopped. This vacuum chamber is placed in the middle of the long-counter. Again, a tape system enables to remove fission products far from the detection system in order to reduce long-lived background.
VI.
CONCLUSIONS
By combining the Lohengrin mass spectrometer and a new experimental setup based on γ-spectrometry, we were able to investigate various fission product properties in the heavy mass region. Results obtained for several fission reactions are very encouraging and uncertainties have been decreased compared with other experiments and evaluated data. The beam from Lohengrin is a combination of various fission products with the same A/q and E/q values. Coupling of Lohengrin with a Gas Filled Magnet is under study [15]. in order to purify the extracted beam and to get a pure isobaric fission product beam.
[1] P. Armbruster et al., Nucl. Instrum. Methods 139, 213 (1976). [2] G. Fioni et al., Nucl. Instrum. Methods A 335, 175 (1993). [3] E. Moll et al., Nucl. Instrum. Methods 139, 213 (1976). [4] U. K¨ oster et al., Nucl. Instrum. Methods A 613, 363 (2010). [5] A. Bail, Ph.D. Thesis, University of Bordeaux (2009). [6] T. Materna et al., Proc. 5th Int. Conf. on Fission and Properties of Neutron Rich Nuclei, Dec. 2012, Sanibel Island (USA), World Scientific. [7] A. Bail et al., Phys. Rev. C 84, 034605 (2011).
[8] F. Martin et al., Proc. 2nd Int. Conf. Advancements Nuclear Instrumentation, Measurement Methods and their Applications, June 2011, Ghent (Belgium). [9] C. Sage et al., Proc. 7th Int. Conf. Dynamical Aspects Nuclear Fission, Oct. 2011, Smolenice (Slovak Republic). [10] F. Martin et al., Nucl. Data Sheets 119, 328 (2014). [11] L. Mathieu et al., JINST 7, P08029 (2012). [12] C. Schmitt et al., Nucl. Phys. A 430, 21 (1984). [13] C. Amouroux et al., EPJ Web of Conferences, 42 (2013), in press. [14] T. Materna et al., AIP Conf. Proc. 1175, 367 (2009). [15] G. Kessedjian et al., EPJ Web of Conferences 42, (2013), in press.
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