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Accueil du site > ANGLAIS > Research > ACEN: nuclear data for nuclear energy, new fuel cycle and nuclear waste transmutation > Activities > direct neutron measurements > cross section measurement relative to (n,p) scattering > 243Am neutron-induced fission cross section in the fast neutron energy range


243Am neutron-induced fission cross section in the fast neutron energy range

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A rather large amount of 243Am is present in the waste generated by current nuclear reactors. Americium isotopes can be fully separated and extracted from spent fuel rods and probably are the only nuclei for which fast neutron incineration could be seriously considered in the near future. A reliable incineration reactor can only be designed if the neutron-induced fission cross section of 243Am in a fast neutron spectrum is known precisely (with better than 5 % accuracy). However, in the 1 to 6 MeV neutron-energy range, the existing data showed systematic and significant discrepancies. These data can be categorized into two groups as follows: the first by Knitter et al.a, Fursov et al.b, and Seeger et al.c and the second by Behrens et al.d Goverdovsky et al.e, and Laptev et al.f. The second group finded systematically higher fission cross sections than the first group. Most of the data were obtained in reference to the fission cross section of 235U; only Fursov et al. have used 239Pu(n,f) as a reference. The most recent data from Laptev et al. was above the current evaluation values by more than 15 %. As pointed out by Talou et al.g, the discrepancy between the two groups’ results seems to be related to a normalization problem.

In order to solve this controversy, we have measured the 243Am fission cross section in reference to the neutron-proton (n,p) elastic scattering cross section, which is known with a precision better than 0.5 %, over a wide neutron-energy range of 1 meV to 20 MeV h. This is the first time such measurements have been performed. The high precision allows us to qualify these measurements as “quasi-absolute.” In addition, we have conducted measurements in reference to the 238U and 235U fission cross sections, which are known with an accuracy of 1 to 3 %, in the fast neutron energy range of 0.1 to 10 MeV. These data allowed us to compare the normalization procedures using three different standard reactions.

The measurements were performed at the 7MV Van-de-Graaff of the IRMM (Geel, Belgium) and at the 3.54 MV accelerator facility AIFIRA at the CENBG. Fast neutrons from 1 to 4 MeV were produced with the T(p,n3He reaction using a TiT solid target. Neutrons with energies from 4 to 8 MeV were produced with the D(d,n)3He reaction using a gaseous deuterium target. In these two neutron-energy ranges, we obtained a mono-energetic neutron beam. The experimental set-up is similar to the one used for the 233U(n,f) experiment and is illustrated in the following Figure.

Experimental setup for determining the fission cross section of 243Am in reference to the neutron-proton elastic scattering cross section: (1) Neutron source [D(d,n) or T(p,n) reactions]. (2) Two back-to-back 243Am targets*. (3) Fission fragment detectors. (4) Polypropylene foil (in blue) with the two Ta screen positions: (a) background measurement; (b) neutron flux measurement. (5) ΔE- E Si telescope. (6) External 3He monitor.
*Note that for some experiments of this programme, we used one 243Am target associated to a 238U or a 235U

Two targets of 243Am were placed back to back in a vacuum chamber at 39 mm from the neutron source and at 0° with respect to the incident neutron direction. The targets samples (546 μg/cm2 and 564 μg/cm2 thicknesses, 99.96 % isotopic purity) were prepared at the LBNL by electroplating techniques.

The neutron flux measurements were done with a proton-recoil telescope. It consists of a polypropylene (PP) foil [(C3H6)n] and a silicon ΔE- E telescope consisting of an energy-loss detector of 55 μm positioned in front of a residual-energy detector of 700 μm. Several thicknesses (10 to 50 μm) of the PP foil were used to keep the energy loss of the recoiling protons below 15 % for all incident neutron energies.

The recoiling proton spectrum was measured at each energy with two separate measurements, namely, a standard measurement followed by a background measurement. For the standard measurement, the telescope was in front of the polypropylene (PP) foil. For the background measurement, the recoiling protons were stopped in a tantalum screen placed between the PP foil and the telescope. The recoiling proton events are graphically selected on a ΔE- E plot.

In addition to the above-mentioned background, charged particles originating from the direct interaction of neutrons with the Silicon were also detected by the Si-telescope. For high neutron energies (> 4,5 MeV) the background is mainly due to the (n,p) and ( n,α) reactions within the telescope. The number of protons as a function of the total kinetic energy before and after background subtraction is shown.

The spectrum that results from the background subtraction presents only one peak corresponding to the protons produced by the interaction of the quasimonoenergetic incident neutrons with the PP foil. A 3He neutron monitor placed at 0 degrees with respect to the incident-neutron beam at 4.21 m from the neutron source was used to normalize the standard and background measurements.

In principle, the neutron flux on the PP foil is obtained by integrating the spectrum of recoil protons combined with the well-known n-p elastic cross section and the telescope efficiency. By computing the ratio of the solid angles subtended by the 243Am targets and the PP foil we infer the neutron flux on the 243Am targets. However, the neutron spectrum at the PP foil is not monoenergetic and one has to consider an average n-p cross section. Moreover, it is not obvious to determine precisely the telescope efficiency in an analytical way. For this reason, Monte Carlo simulations of neutrons and protons passing through the experimental setup have been done. They have allowed us to determine the neutron energy spectrum hitting the 243Am targets or the PP foil. The simulations took into account the resolution of the charged particle beam, the energy loss of the charged particle beam in the deuterium or tritium targets, the angular distributions of the neutron beam, the angular distribution of the (n,p) elastic scattering cross sections, the proton energy loss in the PP foil, and the energy resolution of the Si telescope. When the simulated proton spectrum is in agreement with the experimental result it means that the simulation includes all the effects that influence the neutron path up to the PP foil as well as the tracks of recoil protons and their detection in the telescope. We can then deduce the neutron spectrum, the mean value of the (n,p) elastic scattering cross section, and the proton detector efficiency as a function of neutron energy. The (n,p) elastic scattering anisotropy has been taken into account in the calculations of the proton detection efficiencies.

 
The fission detectors were composed of two sets of photovoltaic cells. The photovoltaic cells allowed a complete separation between α particles and fission fragments. The double-humped structure was not observed in the fission fragment spectrum because the energy resolution was spoiled by the target thickness. The photovoltaic cells have no sensitivity to the neutron beam and an intrinsic efficiency of 95(±1) %. Fission detectors were placed in front of each Am target in a very compact geometry to obtain a geometrical efficiency of about 70 %.

 

Cross-section measurements of 243Am(n, f) relative to 238U(n, f) were performed at the new 3.54 MV facility (AIFIRA) at the CENBG (Figure). The fast neutron flux with energy over the range of 4 to 6 MeV was produced by the D(d,n)3He reaction using a deuterium gas target. Back-to-back targets, consisting of 243Am [546 μg/cm2 thick] and 238U [462 μg/cm2 thick], were placed at a distance of 40 mm from the neutron source and perpendicularly to the incident-neutron beam. The fission detectors consisted of two sets of photovoltaic cells in a very compact geometry. The ensemble “Am target + fission fragment detector” was the same as the one used for the cross-section measurements relative to the (n,p) elastic scattering. The ensemble “ 238U target + fission fragment detector” formed our second neutron flux detector. Consequently, the determination of the neutron flux was completely independent of the previous method.

The cross-section measurements of 243Am(n, f) relative to 235U(n, f) were performed at the 4MV Van de Graaff facility of the CENBG using the same method above . For these measurements, two targets of 243Am [106 μg/cm2] and 235U [409 μg/cm2] were used. Thus, these data are independent of all other measurements.

 

Our results are represented in the last Figure in comparison with earlier measurements and the existing evaluations.

Results of 243Am neutron-induced fission cross sections in comparison with the evaluated data files and the experimental data from Knittera and Laptevf.

In this figure, the error bars of our data correspond to the maximum standard deviations, which are obtained when no solid-angle correlations are considered. As can be seen, our measurements contradict those of Laptevf, which are about 15 % higher than the existing evaluations. Our additional measurements (relative to 235U and 238U) are fully compatible with these findings. Our results are in close agreement with the data of Knitter [a] as well as the evaluated data files.

A complete description of our results involves a calculation of the variance-covariance matrix which are detailed in G. Kessedjian et al..

 

[a] H.H. Knitter et al., Nucl. Sci. Eng. 99, 1 (1988)
[b] B.I. Fursov et al., Atomnaya Energiya. 59, 339 (1985)
[c] P. A. Seeger et al., Fission Cross Sections from Pommard (LANL), p. 138 (1970)
[d] J. W. Behrens and J. C. Browne, Nucl. Sci. Eng. 77, 444 (1981)
[e] A. A. Goverdovsky et al., Proc. Int. Conf. Nuclear Data for Basic and Applied Science (Santa Fe, New Mexico, 1985)
[f] A. B. Laptev et al., Nucl. Phys. A 734, E45 (2004)
[g] P. Talou et al., Nucl. Sci. Eng. 155, 84 (2007)

 

Related publications:

Mesures de sections efficaces d’actinides mineurs d’intérêt pour la transmutation
G. Kessedjian, PhD, Université Bordeaux 1, Nov. 2008
 
243Am neutron-induced fission cross section in the fast neutron energy range
G. Kessedjian et al., Physical Review C 85, 044613, 2012
 

 

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