Investigation of the surrogate-reaction method via the simultaneous measurement of gamma-emission and fission probabilities in 3 He Pu reactions

Transcription

1 Investigation of the surrogate-reaction method via the simultaneous measurement of gamma-emission and fission probabilities in 3 He Pu reactions M.Aiche, S.zajkowski, D. Denis-Petit, B. Jurado, L. Mathieu, R. Pérez Sánchez, I. Tsekhanovich entre d Etudes Nucléaires de Bordeaux-Gradignan, France P. Marini, V. Méot, O. Roig EA DAM, Bruyères le hâtel, France L. Audouin, M. Lebois, D. Ramos, L. Tassan-Got, J. N. Wilson Institut de Physique Nucléaire d Orsay, France O. Bouland, O. Sérot EA, adarache, France G. Boutoux ELIA, Bordeaux, France A. Plompen, S. Oberstedt E-JR, Geel, Belgium G. Kessedjian LPS, Grenoble, France K. Nishio Japan Atomic Energy Agency, Tokai, Japan A. Goergen, M. Guttormsen, A.-. Larsen, S. Siem University of Oslo, Norway A. Moro University of Sevilla, Spain A. Andreyev University of York, UK Abstract: The surrogate-reaction method is an indirect method for determining neutron-induced cross sections of short-lived nuclei. It is based on the measurement of decay probabilities induced by alternative or surrogate reactions (transfer or inelastic scattering) and on the assumption that the decay probabilities of the compound nuclei formed in these reactions are equal to the neutron-induced probabilities. Most models predict a significant dependence of the fission and gamma-emission probabilities on the angular momentum J and parity π of the decaying nucleus. Since neutron-induced and surrogate reactions populate different initial J π distributions, large differences between the decay probabilities induced by neutron absorption and by surrogate reactions are expected. However, previous work shows that the fission probabilities induced by surrogate reactions are in good agreement with the neutron-induced ones, whereas there are very large discrepancies between the gamma-emission probabilities. Additional and dedicated measurements are needed to fully understand this finding. The aim of the present experiment is to simultaneously measure fission and gammaemission probabilities induced by several surrogate reactions involving a 3 He beam and a 240 Pu target. These reactions will lead to the formation of the 239,240 Pu and 240,241 Am compound nuclei, which are important for reactor applications. The measured data will be useful for understanding the surrogatereaction method and for constraining calculations based on the statistical model, which will lead to a significant improvement of the quality of evaluations. The measurement needs a 27MeV (20-30 nae) 3 He beam and a total number of 31 UT. This measurement is the subject of the PhD thesis of R. Pérez Sánchez. This proposal will be also submitted to the PA of the European project HANDA in February

2 1. Introduction: The surrogate-reaction method Neutron-induced reaction cross sections of short-lived nuclei are important in several domains such as fundamental nuclear physics, nuclear astrophysics and applications in nuclear technology. These cross sections are key input information for modeling stellar element nucleosynthesis via the s and r-processes. They also play an essential role in the design of advanced nuclear reactors for the transmutation of nuclear waste, or reactors based on innovative fuel cycles, like the Th/U cycle. However, very often these cross sections are extremely difficult (or even impossible) to measure, due to the high radioactivity of the targets involved. Figure 1: Schematic representation of the surrogate-reaction method. The surrogate reaction depicted here is a transfer reaction X(y,w)A*. Three of the possible exit channels (fission, gamma emission and neutron emission) are represented. The surrogate-reaction method was first developed at the Los Alamos National Laboratory by ramer and Britt [1]. This indirect technique aims to determine neutron-induced cross sections of reactions involving short-lived nuclei that proceed through the formation of a compound nucleus. In this method, the same compound nucleus as in the neutron-induced reaction of interest is produced via an alternative, or surrogate, reaction (e.g. a transfer or an inelastic scattering reaction). The principle of the surrogate-reaction method is schematically represented in Fig. 1. The left part of figure 1 illustrates a neutron-induced reaction on target A-1, which leads to the formation of nucleus A* at an excitation energy E*. The nucleus A* can decay via different exit channels: fission, gamma-decay, neutron emission, etc. On the right part of figure 1, the same nucleus A* is produced via a surrogate reaction. In figure 1, the surrogate reaction is a transfer reaction between a projectile y (a light nucleus) and a target X, leading to the heavy recoil nucleus A* and an ejectile w. The charge and mass identification of the ejectile w allows one to deduce the charge and mass of the decaying nucleus A*, and the measurement of the ejectile kinetic energy and emission angle provides its excitation energy E*. In most applications of the surrogate method, the surrogate reaction is used to measure the decay probability P χ and the desired neutron-induced reaction cross section is simulated by applying the equation: 2

3 σ 1 ( E ) = σ ( E ) P ( E*) (1) A A A χ n N n χ where the index χ represents the decay channel (e.g. fission or gamma-ray emission) and A σ N ( En) is the cross section for the formation of a compound-nucleus A* after the absorption of a neutron of energy E n by nucleus A-1. The compound-nucleus formation cross section A σ N ( En) can be calculated with phenomenological optical-model calculations with an accuracy of about 10% for nuclei not too far from the stability valley [2]. The excitation A 1 energy E* and the neutron energy E n are related via the equation E* = Sn + En, where S n A is the neutron separation energy of nucleus A. The usefulness of the surrogate-reaction method is that, in some cases, one can find a surrogate reaction where the target X is stable or less radioactive than the target A-1. For the surrogate-reaction method to be valid, several conditions have to be fulfilled [2]. First, both the neutron-induced and the surrogate reactions must lead to the formation of a compound nucleus. In that case the decay of the compound nucleus A* is independent of the entrance channel, and the reaction cross section can be factorized into the product of the compound-nucleus formation cross section and the decay probability into a channel χ, as in eq. (1). The second condition is that the decay probability measured in the surrogate reaction has to be equal to the decay probability in the neutron-induced reaction. However, models predict a strong dependence of the decay probabilities on the spin J and parity π of the decaying nuclei, in particular at low excitation energies. Since the J π distributions populated via the surrogate reactions can be very different from the ones populated via neutron absorption, there may be significant discrepancies between the probabilities induced by both types of reactions. For most surrogate reactions it is not yet possible to predict the populated J π distribution [2] and the validity of the surrogate method has to be verified a posteriori, by comparing the experimental results obtained with the method with well known neutroninduced data. Surrogate-reaction studies performed in the last decade have shown that fission cross sections obtained via the surrogate-reaction method are generally in good agreement with the corresponding neutron induced data, see e.g. [3]. However, discrepancies as large as a factor 10 have been observed when comparing radiative-capture cross sections of rare-earth nuclei obtained in surrogate and neutron-induced reactions [4, 5]. These significant differences have been attributed to the higher angular momenta populated in the surrogate reaction. At excitation energies close to S n, neutron emission is very sensitive to the angular momentum of the decaying nucleus A*, as only the ground state and the first excited states of the residue nucleus A-1 can be populated. When the angular momentum of A* is considerably higher than the angular momentum of the first states of A-1, neutron emission is hindered and the nucleus A* predominantly decays by gamma emission, which is the only open decay channel [5]. Similarly to the situation at energies close to the ground state, the energy region close to the fission barrier is also characterized by a low density of states and a significant dependence of 3

4 the fission probability on the angular momentum is expected by theory [2]. Therefore, it is surprising that the spin/parity mismatch between the surrogate and neutron-induced reactions has no major impact on the measured fission probabilities, not even at energies around the fission threshold. To shed light into this puzzling observation, it is first of all necessary to demonstrate the much weaker sensibility of the fission probability to angular momentum by simultaneously measuring fission and gamma-decay probabilities for the same nucleus at the same excitation energy. This was never been done before and is the aim of an experimental campaign we are performing. Note that the measurement of the gamma-decay probability at excitation energies where the fission channel is open is challenging because of the background of gamma rays emitted by the fission fragments. 2. Previous measurements The Tandem of the ALTO facility is particularly well suited for surrogate-reaction studies because of the high quality of the beams. Indeed, the very good energy and position resolution of the beams, as well as the excellent beam energy definition allow us to reach an adequate excitation-energy resolution to explore regions where the decay probabilities change very rapidly with excitation energy, such as the fission or the neutron-emission thresholds. In April 2015 we performed an experiment at the Tandem of Orsay to simultaneously measure the fission and gamma-decay probabilities of several surrogate reactions induced by a 3 He beam on a 238 U target with a new setup developed by the ENBG. For some of the reactions investigated there exists good quality neutron-induced data to which we can compare our results. The analysis of the data is almost completed. The results of this work have been presented in numerous international conferences (e.g. [6]) and the corresponding article is currently under preparation [7]. Some of our results are displayed in the figures below. Figure 3: Preliminary results for fission (blue) and gamma-decay (red) probabilities of the 238 U( 3 He,d) 239 Np reaction as a function of the excitation energy of 239 Np*. The sum of the fission and gamma-decay probabilities is represented by black circles. The vertical dotted line indicates the neutron separation energy of 239 Np. Figure 3 shows preliminary results for the gamma-emission and fission probabilities of the 238 U( 3 He,d) 239 Np reaction as a function of excitation energy. The corresponding neutroninduced reaction is n Np. 238 Np has a half-life of only 2.1 days and there are no neutron- 4

5 induced fission or capture cross sections data for this nucleus at fast neutron energies. However, the data of 239 Np* are of particular interest since they can be used to test the validity of the method we have used to simultaneously infer the fission and gamma-emission probabilities. Indeed, the fission barrier of 239 Np is lower than its neutron separation energy S n. Therefore, for this nucleus, the two main decay modes below S n are fission and gamma emission, and the sum of the corresponding decay probabilities must be equal to 1. As can be seen in figure 1, the gamma-emission probability is first 100% and decreases when fission sets in. Both probabilities decrease at S n because of the competition with the neutron-emission channel. We can see that below S n the sum of the two decay probabilities is consistent with 1 within the error bars. This validates the experimental measurement and the analysis procedure. Figure 4 shows preliminary results for the gamma-emission and fission probabilities of the 238 U( 3 He, 4 He) 237 U reaction compared to different evaluations of the decay probabilities for the n U reaction. The gamma-decay probability obtained with the surrogate-reaction method is several times higher than the neutron-induced one, whereas the fission probability is in much better agreement with the neutron-induced data. Note that this is also the case in the excitation-energy region where fission and gamma emission are in competition (between 5.3 and 6.3 MeV), which demonstrates that the fission probability is much less sensitive to the differences in the entrance channel than the gamma-emission probability. Figure 4: Preliminary gamma-decay (left) and fission (right) probabilities of the 238 U( 3 He, 4 He) 237 U reaction as a function of the excitation energy of 237 U compared to the corresponding neutron-induced decay probabilities according to different evaluations. The vertical dotted line in the left panel represents the neutron separation energy S n of 237 U. We are currently working with Olivier Bouland from the EA-adarache to investigate whether we can explain the results obtained in the 238 U( 3 He, 4 He) reaction within the frame of the statistical model. As shown in [8], the model of O. Bouland well reproduces the neutroninduced cross sections 236 U(n,f) and 236 U(n,γ), which were used to tune some of the few model parameters. Figure 5 shows that this model reproduces also our decay probabilities fairly well when the J π probability distribution populated in the n U reaction is replaced by J π distribution populated in the 238 U( 3 He, 4 He) reaction calculated by I. Thompson and J. 5

6 Escher [9]. Note that the two J π distributions are very different and that the model parameters were not tuned to reproduce our data. Since the decay probabilities, in particular the gammaemission probability, are sensitive to the populated J π distributions, the good reproduction of our data is a remarkable result that gives confidence in both the Hauser-Feshbach calculation and the predicted J π distribution. Therefore, these model calculations will help us to gain insight into the origin of the observed weaker sensibility of the fission probability to the angular momentum. Moreover, the good agreement shown in figure 5 demonstrates that it is possible to have a common theoretical frame for describing neutron-induced and surrogate reactions. This is highly interesting because it means that the decay probabilities measured for surrogate reactions can be used to tune the parameters of the Hauser-Feshbach calculation, and the tuned Hauser-Feshbach calculation can then be employed to provide reliable predictions for neutron-induced cross sections of short-lived nuclei that cannot be directly measured. Figure 5: Preliminary results for gamma-decay and fission probabilities of the 238 U( 3 He, 4 He) 237 U reaction as a function of excitation energy. The data are compared to a preliminary Hauser-Feshbach calculation by O. Bouland. To fully understand why we do not observe the predicted sensibility of the fission probability to the angular momentum it is necessary extend the range of fissioning nuclei and surrogate reactions investigated. In particular, the study of even-even fissioning nuclei is highly interesting since the density of states near the fission barrier is low and the fission probability should be more sensitive to angular momentum than in the case of the odd-a or odd-odd nuclei we studied so far. 3. Reactions to be investigated in this experiment 3.1. The 240 Pu( 3 He, 3 He ) reaction One of the main objectives of the proposed experiment is to simultaneously measure the gamma-emission and fission probabilities for the inelastic scattering reaction 240 Pu( 3 He, 3 He ) 240 Pu. The interest of this reaction is manifold. First, the excited 240 Pu nucleus is an even-even nucleus. In addition, there are good-quality neutron-induced cross-section data to which we can compare our results since the associated neutron-induced reaction, n + 6

7 239 Pu, is highly important in reactor physics. Moreover, as shown in table 1, 240 Pu is a fissile nucleus whose fission barrier is lower than its neutron separation energy. Therefore, we will have access to the region where only fission and gamma-emission compete. In the same way as for the 239 Np case discussed in section 2, this will represent a stringent test of the measurement procedure since the sum of the measured probabilities should be 1 below S n. To cover excitation energies near the fission and the neutron-emission threshold we need an incident 3 He beam energy of 27 MeV. Surrogate reaction Neutron-induced reaction S n of decaying nucleus Fission barrier B f of decaying nucleus 240 Pu( 3 He, 3 He ) 240 Pu n+ 239 Pu (T 1/2 = years) 6.53 MeV 5.70 MeV 240 Pu( 3 He, 4 He) 239 Pu n+ 238 Pu (T 1/2 = years) 5.65 MeV 6.08 MeV 240 Pu( 3 He,t) 240 Am n+ 239 Am (T 1/2 = 11.9 hours) 5.95 MeV 6.50 MeV 240 Pu( 3 He,d) 241 Am n+ 240 Am (T 1/2 = 50.8 hours) 6.64 MeV 6.05 MeV Table 1: Surrogate reactions that will be investigated in this experiment and the associated neutron-induced reactions. The second column gives also the half-life of the target of the neutron-induced reaction. The last column gives the higher of the inner and outer fission barriers of the decaying nuclei according to the experimental systematic of [10]. The fission threshold of 240 Pu is not accessible in neutron-induced reactions but it has been carefully studied via the 239 Pu(d,p) reaction by Back et al. [11] and via the 238 Pu(t,p) reaction by ramer and Britt [12] with an excitation-energy resolution similar to ours. The comparison of our results with this previous data will allow us to further investigate the sensitivity of the fission probability to different reaction entrance channels. An additional clear advantage of this reaction is that we have the theoretical support needed for the interpretation of the results. Indeed, inelastic scattering reactions are easier to model than the majority of transfer reactions [2]. The populated J π distribution will be calculated with a model based on the Distorted Wave Born Approximation (DWBA) in collaboration with Antonio Moro from the University of Sevilla. The decay of 240 Pu will be described with the model by O. Bouland, which was recently tuned to compute the neutron-induced cross sections of a long chain of Pu isotopes [13]. In fact, the comparison of the model calculations with our data, in particular the comparison with the 240 Pu gamma-emission probability, which we will measure for the first time, will provide a stringent test of the calculations The 240 Pu( 3 He, 4 He), 240 Pu( 3 He,t) and 240 Pu( 3 He,d) reactions A significant advantage of the surrogate method is that several nuclei can be studied in one measurement, i.e. with a single projectile-target combination. The ensemble of reactions that are simultaneously populated in the interaction of a 3 He beam at 27 MeV with a 240 Pu target are listed in table 1. As can be seen, for the transfer reactions the half life of the targets needed in the corresponding neutron-induced reaction is much shorter than the half life of 240 Pu of T 1/2 = 6571 years. The neutron-induced cross sections of 238 Pu are important for reactor applications because 238 Pu can be produced via 239 Pu(n,2n) reactions, specially in fast reactors that use a fuel based 7

8 on 239 Pu. However, the measurement of these cross sections is very complicated. The very high alpha activity of the sample makes extremely difficult the discrimination between signals generated by alpha pile-up and by fission fragments. Besides, the alpha radioactivity strongly damages the fission detector. This makes the measurement of the radiative capture cross section of this nucleus at neutron energies where the fission channel is open (above ~ 200 kev) very difficult because the gamma-rays arising from the fission fragments have to be tagged with a fission detector for subtraction. As a matter of fact, there are no capture crosssection measurements for this nucleus above 200 kev. The neutron-induced fission crosssection data for 238 Pu agree fairly well at low neutron energies. However, there are strong discrepancies above 5 MeV. Our data can help to solve the discrepancies observed at higher energies between neutron-induced fission data. In addition, similarly to the inelastic scattering reaction described above, for the 240 Pu( 3 He, 4 He) we have the necessary theoretical support to provide more reliable predictions for the radiative capture cross section of 238 Pu above 200 kev. Our fission probabilities of 240,241 Am can be used to extract the fission barriers of 240,241 Am (see e.g. [10]) and significantly improve the predictions for the neutron-induced second- and third-chance fission cross section of 241 Am. These n th -chance fission cross sections are important for simulation studies of the incineration of 241 Am, where significant amounts of this isotope are placed in a fast reactor. Note that 241 Am is present in large amounts in the waste generated by current nuclear reactors. It is one of the few isotopes that can be fully separated and extracted from spent fuel rods, and for which transmutation could be seriously considered in a relatively near future. No neutron-induced cross-sections have been measured for 239,240 Am because of their very short half-lifes, see Table 1. There exist a measurement of the fission probability of 240 Am formed via the 239 Pu( 3 He, d) reaction [14] that can be compared with our result for the 240 Pu( 3 He,t) reaction to investigate the influence of the entrance channel. 4. Experimental setup In a surrogate-reaction experiment, the decay probability for a given de-excitation channel χ, A,exp P χ, is given by the following expression: P ( E ) = A,exp * χ * χ ( E ) * * e χ N ( E ) ( E ) singles N (2) where N singles is the total number of detected ejectiles, N χ is the number of detected ejectiles in coincidence with the observable that is used to identify the decay mode χ, e.g. detection of a fission fragment or a gamma ray, and ε χ is the efficiency for detecting the decay products. To determine the gamma-emission probability we have to remove the background originating from prompt gamma rays emitted by the fission fragments. As we show in [15], this is done by correcting the measured ejectile-gamma coincidence spectrum, tot N γ as follows: 8

9 N,, ( *) tot γ f E γ ε f ( E*) N ( E*) = N ( E*) (3) γ where N γ, f is the number of gamma cascades detected in coincidence with an ejectile and a fission fragment, and ε f is the fission detection efficiency. The key for a good subtraction of the fission-fragment gamma background is to have a setup with a large fission efficiency whose value is known with good precision (only 3-4% absolute uncertainty), see our article [15] for more details. The experimental setup for the proposed measurements is schematically represented in figure 6. It consists of a vacuum chamber housing the target, two particle telescopes centered at 140 degrees with respect to the beam axis and the fission detector. The chamber is surrounded by two types of gamma detectors: four 6 D 6 liquid scintillators and six high-purity germanium detectors. Figure 6: Schematic view of the experimental setup for the proposed experiment. Fission Si-Telescope Gamma-cascade Detection efficiency (65±4)% of 4π 9% (6 ± 0.6)% ( 6 D 6 detectors) Table 2: Efficiencies of the detectors used in the present experimental setup. The E-E Si telescopes are used to identify and measure the kinetic energies and angles of the ejectiles. Each telescope is composed of a 100 μm position-sensitive Si detector ( E) and a 5 mm thick SiLi detector (E). A 30 µm thick aluminum foil will be placed in front of the telescopes to stop fission fragments and also the alpha particles originating from the activity of the 240 Pu target. The 6 D 6 scintillators are very well adapted for determining the gammadecay probability, as shown in [5]. One important advantage of the 6 D 6 detectors is that they allow one to distinguish gamma-rays from neutrons via pulse-shape discrimination. The fission detector consists of 16 solar cells surrounding the beam in a very compact cylindrical geometry. Each side of the detector, 40 mm long, is composed of two cells of variable length (1/4, 1/2 or 3/4 of the detector length), in order to obtain a segmentation of the polar angle. The forward cells are centered at angles ranging from 15 to 63 degrees and the backward cells 9

10 from 110 to 150 degrees. This segmentation will allow us to measure the fission-fragment angular anisotropy, which can have a significant impact on the fission-detection efficiency. The geometrical efficiency of the fission detector will be measured with a 252 f source of known activity. The effective fission efficiency, which accounts for the fission-fragment angular anisotropy and kinematic effects, will be obtained with a Monte-arlo simulation, previously validated with the data obtained with the 252 f source. To determine the gammadecay probability we need to know the gamma-cascade detection efficiency of the 6 D 6 detectors. The latter can be obtained with the extrapolated efficiency method that we describe in [16, 17]. The efficiencies of the different detectors used in our setup are listed in table 2. The kinetic energy of the fission fragments can be measured with solar cells with a resolution of few percent. The geometry of our fission detector allows us to detect both fission fragments in coincidence for some fission events. With this information it is possible to determine the fission-fragment mass distributions using the iterative double kinetic-energy technique [18]. This would allow us to extract an additional very valuable observable from our measurements, the fission-fragment mass distributions. In particular, the evolution of the fission-fragment yields with excitation energy is highly interesting as the data are scarce. However, one of the main difficulties associated to this technique is to evaluate the mass resolution. With the present set-up it is possible to obtain this information by using the Ge detectors to identify a gamma transition from a fission fragment and representing the pulseheight spectrum of the solar cells in coincidence with that particular transition. In this way, we obtain the response of the cells for a particular fission-fragment mass. The energy calibration of the solar cells can be performed with the 252 f source [19]. The 240 Pu target will be produced end of February 2017 by the radiochemistry group of the IPN of Orsay. It will have a thickness of 200 μg/cm 2 and an isotopic purity of 99.89%, corresponding to an activity of ~ 475 kbq. The sample will be supported by a natural carbon foil of 100 μg/cm 2. As shown in figure 6, the vacuum chamber is equipped with an airlock in order isolate the radioactive samples from the environment during the transportation from the glove box, where they will be mounted, to the experimental setup. We will use two airlocks, one will contain the 252 f source and the other will contain a target ladder with the 240 Pu, a target backing (natural carbon), and a 208 Pb target. A measurement with the target backing is necessary to subtract from the singles spectrum of the 240 Pu( 3 He,d) reaction the background coming from reactions of the 3 He projectile with 12. The population of the first excited states of 207 Pb and 209 Bi formed in the 208 Pb( 3 He, 4 He) and 208 Pb( 3 He,d) reactions, respectively, will serve us to accurately calibrate the telescopes in energy, see [20] for more details. 5. Requested beam time and planning of the experiment In this measurement, the beam intensity has to be restricted to about na to limit the damage of the detectors caused by the irradiation with the elastic scattered 3 He beam. The quantity that is subject to the largest uncertainty in our measurement is the number of gamma cascades detected in coincidence with the ejectiles N γ, see eq. (3). As we discuss in [15], the relative uncertainty of the number of gamma cascades measured in coincidence is given by: 10

11 , tot γ N N γ γ, f ε 2 f = + δ + γ γ ε f γ ε f N N N N (4) where we have neglected parameter correlations, δ is: PM γ f f δ = (5) γ PM γ γ where P f is the fission probability, P γ is the gamma-emission probability, and M γ f and M γ are γ the gamma-ray multiplicities for fission and gamma-emission, respectively. Typically, M γ f is two times larger than M γ γ. The two last terms in eq. (4) come from the subtraction of the gamma cascades arising from the fission fragments. Eq. (4) shows that the relative uncertainty of N γ will significantly increase when fission sets in and competes with gamma-emission. The planning of the experiment and the requested units of time (UT) for each step are presented in table 3. From our previous measurement, 3 He+ 238 U, performed at the tandem of the ALTO facility with the same set-up as in the present proposed experiment, we estimate a total cross section (integrated over all the telescope angles and over the full excitation-energy range) for the 240 Pu( 3 He, 3 He ) and 240 Pu( 3 He, 4 He) reactions of about 0.5 mb. Note that from all the reactions listed in Table 1, we expect that these will be the transfer channels with the lowest cross section. With 22 UT of 3 He on 240 Pu we estimate that the relative uncertainty of P γ will vary from 10% when P γ =1 and P f =0 to nearly 30% when P f =2P γ. The number of UTs needed for the 208 Pb (200 µg/cm 2 ) and natural-carbon (100 µg/cm 2 ) targets has been estimated from the measured counting rates of our last experiment, taking into account that in the present measurement the targets are two times thicker. 6 UTs are needed to populate the first excited states of 207 Pb and 208 Bi with sufficient statistics to ensure a very accurate energy calibration of the telescopes. The latter is particularly important for the measurement of the decay probabilities at the fission and neutron-emission thresholds. Beam Intensity Target Tasks Number of UT 20 nae 208 Pb alibration of telescopes. 6 3 He, 27 MeV 30 nae - Measurement of background due to the 2 20 nae backing target backing. 240 Pu Data taking. 22 alibration of Ge and 6 D 6 detectors and measurement of the geometrical 1 efficiency of the fission detector with the 252 f source. Total 31 Table 3: Steps of the proposed experiment and corresponding UT. 11

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