Current studies of neutron induced reactions regard essentially two mass regions, identified in the chart of nuclides: isotopes in the region from Fe

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1 The talk gives an overview of the current reseach activity with neutron beams for fundamental and applied Nuclear Physics. In particular, it presents the status and perspectives of neutron studies in the field of Nuclear Astrophysics and for advanced nuclear technologies. Nicola Colonna 1

2 Current studies of neutron induced reactions regard essentially two mass regions, identified in the chart of nuclides: isotopes in the region from Fe up to Pb are of interest both for Nuclear Astrophysics (stellar nucleosynthesis) and for nuclear energy (fission fragments). The region of the actinides, above U, is of interest for advanced nuclear systems. 2

3 The vast majority of nuclei above Fe are produced in the Universe by neutron capture. There are two types of neutron capture process: the s process occurs mostly in Heburning stars, in an environment characterized by relatively low neutron densities and temperatures. Due to the long capture times (of the order of years), mostly stable isotopes are involved, and the process proceeds along the beta stability valley. The s process is a subject of investigation with neutron beams. The second type of capture is the r process, (from rapid), which occurs in explosive scenarios like Supernovae explosions, characterized by very high neutron densities. Being the capture time much shorter (as short as ms), unstable isotopes far into the neutron rich region are produced. This process is not directly accessible to studies with neutron beam, but it is the subject of intense investigation with radioactive beam facilities. 3

4 The s process nucleosynthesis proceeds through a succession of neutron captures and beta decays. The key quantity in determining the reaction flow, together with the thermodinamic conditions, is the neutron capture cross section. This quantity is therefore fundamental to model the stellar nucleosynthesis in the Universe, and to obtain information on the stellar evolution. Nicola Colonna 4

5 Several decades of intense studies of neutron capture cross sections, at various neutron facilities around the world, have led to a satisfactory understanding of the main features of the s process. However, for some isotopes, neutron cross sections are still characterized by an uncertainty too high. This is the case, in particular, for neutron magic nuclei, which have a low cross sections, for rare isotopes or for radioactive isotopes. In all those cases, current neutron facilities are inadequate, due to the low neutron flux. Nicola Colonna 5

6 The main quantity of Astrophysical interest is the so called Maxwellian averaged crosssection, i.e. the average neutron capture cross section weighted over the Maxwell Boltzman energy distribution of neutrons in the stars. This quantity can be determined directly, by using a neutron beam with a Maxwellian shape, or can be calculated starting from the energy dependent neutron capture cross section. In both cases, improvements over current data require high flux facilities and optimized detection techniques. 6

7 Maxwellian averaged cross sections (MACS) have been extensively measured in the past, in particular at FZKarlshrue, Germany. A maxwellian like neutron beam at a given temperature is obtained from the near threshold 7Li(p.n) reaction. Typically, activation measurements are performed. The main problems is that the method has large uncertainties when the cross section deverges from the 1/v behavior (due to the presence of resonances). Furthremore, the activation technique cannot be applied if a stable or a short lived isotope is produced in the neutron capture reaction. 7

8 A second, more powerful method for determining the MACS is to measure the energydifferential cross section, in time of flight facilities (the neutron energy is determined from their time of flight). The problem in this case is the lower flux available, especially if a long flight base is used for high resolution, the need of a pulsed neutron beam and of pure samples. Difficult measurements in this case do require high flux facilities. 8

9 An example of the power of high flux facility is the 151Sm(n,gamma) reaction measured at n_tof. 151Sm is an unstable isotope, whichactsasa branching point in the s process in stars: it can either undergo a neutron capture or it decays. The branching ratio depends on the stellar neutron density and temperature, as well as on the neutron capture cross section. Up to a few years ago, no reliable data were available on this isotope, due to the large background associated with the natural radioactivity of the sample. At n_tof it was possible to obtain high accuracy capture cross sections, which led to the determination of the MACS and to a refinement of the stellar models for this region. Nicola Colonna 9

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11 Another field in which neutron studies are extremely important is nuclear energy. This has recently become a priority due to the increasing energy demand and the need of minimizing the production of greenhouse gases. Current nuclear reactors, however, have two limitations, i.e. the inefficient use of the U resources and the production of longlived nuclear waste. To overcome these problems, studies are now being conducted towards the design of new revolutionary systems that would implement a closed fuel cycle. 11

12 In a nuclear reactor, fission and capture reactions induced by neutrons lead to the production of long lived fission fragments and to a series of actinides, in particular to Pu, as well as Np, Am and Cm, also referred to as Minor actinides. The slides indicates the amount of varius elements produced yearly in a 1 GWe light water reactor. Nicola Colonna 12

13 The main contribution to the radiotoxicity soon after disposal of spent fuel comes from fission fragments. However, their radiotoxicity decays in a few hundreds of years to an acceptable level. Therefore, man made barriers are enough to avoid dispersion in the environment. On the contrary, the radiotoxicity of the actinide content in the nuclear waste remains high for hundreds of thousands of years, so that they have to be disposed in geological repositories. Nicola Colonna 13

14 The Th/U fuel cycle offers an attractive solution to the problem of nuclear waste disposal. In this cycle, in fact, a very limited amount of Pu and minor actinides are produced, with the exception of 237Np. This is one of the reason for considering the use of Th/U fuel cycle in future generation reactors. Nicola Colonna 14

15 The main principle at the basis of the Generation IV nuclear reactors is the recycling of the actinides from the spent fuel, so that they can be burned and at the same time produce energy. The problem is that most of the minor actinides have a fission threshold, i.e. their neutron induced fission cross section becomes important only above a given energy, typically around 1 MeV. Therefore, their burn up can only occur in fast reactors. Clearly, the design of new generation reactors require accurate data on minor actinides, in particular in the fast neutron energy range. Nicola Colonna 15

16 There is a urgent need of new and accurate data on serveral isotopes that constitute the fuel of next generation reactors, as well as on long lived fission products that could be incinerated in transmutation devices, and on structural material. The list of requests, made by NEA and other international organization, is rather long, and include capture and fission cross sections on several U and Pu isotopes, as well as on the most important Minor Actinides. The interest of the international community on new data is also testified by the recent call of the EC FP VII (Euratom), in which nuclear data for actinides present in advanced reactor fuels are asked for. 16

17 As shown in the table, the current uncertainty on some reactions are too high relative to the requests. In some cases, uncertainties as high as 50 % should be reduced to a few percent, cleary a challenging task. Nicola Colonna 17

18 For neutron induced reactions of interest for advanced reactor systems, new and accurate time of flight measurements are necessary, in a wide energy range and with high resolution (since the accurate determination of resonances in the cross sections are fundamental for calculations of self absorption effects in fuel elements). At present, the major neutron time of flight facilities operating in Europe in the field are n_tof at CERN and GELINA, at JRC IRMM, Geel, Belgium. Nicola Colonna 18

19 n_tof is an innovative facility that was recently built at CERN to fulfil at least some of the needs of new and accurate neutron data. The facility is based on the spallation of 20 GeV/c protons from the PS accelerator onto a large Pb block. The most important feature of n_tof is the very high instantaneous neutron flux, up to 3 orders of magnitude higher than other facilities, the wide energy range, the high resolution and the low background. In particular, the very high instantaneous neutron flux make n_tof unique in the world, and extremely useful for the measurement of radioactive isotopes, such as the actinides involved in advanced reactor fuels. Nicola Colonna 19

20 Some example of the improvements over previous data, that were recently obtained at n_tof, are the capture cross section of 232Th and the fission cross section of 233U, the two major isotopes involved in the Th/U fuel cycle. The high instantaneous neutron flux allowed to collected data on the 232Th(n,gamma) reaction of clearly superior quality relative to GELINA, while the high resolution of the n_tof neutron beam allows to measure resolved resonances at a much higher limit that previous evaluations. Nicola Colonna 20

21 Other accurate data recently collected at CERN regard the fission cross section of 243Am and 245Cm, two of the most important minor actinides that could be used in the fuel of Gen IV reactors. In both cases, thanks to the very high instantaneous neutron flux, the n_tof data allow to reduce the uncertainty on the cross sections, being a step forward towards the requested improvement in the nuclear data for advanced reactor systems. Nicola Colonna 21

22 The renewed interest in neutron studies for Nuclear Astrophysics and advanced nuclear technologies is at the basis of several projects and ideas for new neutron facilities, mostly characterized by higher flux over existing facilities. The second experimental area proposed at n_tof would have a factor of 100 times more flux, relative to the current n_tof neutron beam. A high flux neutron facility, but only for fast neutrons, is being constructed at Ganil, within the SPIRAL2 projects, and will be used mostly for studies of inelastic reactions for nuclear technology. Two high flux facilities have been proposed for Astrophysics related measurements: Franz in Germany (under construction), and LENOS at LNL, which relies on the 5 MeV RFQ built for the BNCT project. Another possibility, although with lower flux relative to GANIL, would be to use the SPES Cyclotron for mostly nuclear energy applications. Finally, a neutron test facility is proposed at LNF. Nicola Colonna 22

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