Experimental Fission Studies at Intermediate Energies

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(12) To my family, Anna, Elsa, Tova, and.... “Don’t Panic” written in large friendly letters on the cover to the (fictitious) ‘Hitchhiker’s Guide to the Galaxy’ in the book with the same name by Douglas Adams..

(13) The thesis is based on the following papers, which will be referred to in the text by the Roman numerals I-V. I. O. Batenkov, K. Elmgren, M. Majorov, J. Blomgren, H. Condé, S. Hultqvist, N. Olsson, J. Rahm, E. Ramström, S. Smirnov, and A. Veshikov. High-precision spectrometer for studies of ion-induced and spontaneous fission dynamics. c Nucl. Instr. Meth. Phys. Res. A394 (1997) 235. 1997 by Elsevier Science. II. K. Elmgren, O. Batenkov, H. Condé, M. Majorov, J. Blomgren, N. Olsson, J. Rahm, E. Ramström, S. Smirnov, and A. Veshikov. Experimental study of the fission time scale for the p+209 Bi, α+206 Pb, and 12 C+197 Au, 232 Th, and 248 Cm reactions at 100 MeV Internal report INF 02#03 (2002). III. V.P. Eismont, A.V. Prokofyev, A.N. Smirnov, K. Elmgren, J. Blomgren, H. Condé, J. Nilsson, N. Olsson, T. Rönnqvist, and E. Tranéus. Relative and absolute neutron-induced fission cross sections of 208 Pb, 209 Bi and 238 U in the intermediate energy region. c Phys. Rev. C 53 (1996) 2911. 1996 by the American Physical Society. IV. J. Rahm, J. Blomgren, H. Condé, S. Dangtip, K. Elmgren, N. Olsson, T. Rönnquist, R. Zorro, A. Ringbom, G. Tibell, O. Jonsson, L. Nilsson, P.-U. Renberg, T.E.O. Ericson, and B. Loiseau. np-scattering measurements at 162 MeV and the πNN coupling constant. c Phys. Rev. C 57 (1998) 1077. 1998 by the American Physical Society. V. J. Rahm, J. Blomgren, H. Condé, S. Dangtip, K. Elmgren, N. Olsson, T. Rönnquist, R. Zorro, O. Jonsson, L. Nilsson, P.-U. Renberg, A. Ringbom, G. Tibell, S.Y. van der Werf, T.E.O. Ericson, and B. Loiseau. np-scattering measurements at 96 MeV. c Phys. Rev. C 63 (2001) 044001. 2001 by the American Physical Society. Reprints were made with the permission from the journals.. iv.

(14) Other related publications not included in this thesis: 1. T.E.O. Ericson, B. Loiseau, J. Nilsson, N. Olsson, J. Blomgren, H. Condé, K. Elmgren, O. Jonsson, L. Nilsson, P.-U. Renberg, A. Ringbom, T. Rönnqvist, G. Tibell, and R. Zorro. πNN coupling from high precision np charge exchange at 162 MeV. Phys. Rev. Lett. 75 (1995) 1046. 2. N. Olsson, J. Blomgren, H. Condé, S. Dangtip, K. Elmgren, J. Rahm, T. Rönnqvist, R. Zorro, O. Jonsson, L. Nilsson, P.-U. Renberg, A. Ringbom, G. Tibell, T.E.O. Ericson, and B. Loiseau. Uppsala neutron-proton scattering measurements and the πNN coupling constant. Phys. Scripta T87 (2000) 7. 3. S. Dangtip, A. Ata¸c, B. Bergenwall, J. Blomgren, K. Elmgren, C. Johansson, J. Klug, N. Olsson, G. Alm Carlsson, J. Söderberg, O. Jonsson, L. Nilsson, P.-U. Renberg, P. Nadel-Turonski, C. Le Brun, F.-R. Lecolley, J. -F. Lecolley, C. Varignon, Ph. Eudes, F. Haddad, M. Kerveno, T. Kirchner, and C. Lebrun. A facility for measurements of nuclear cross sections for fast neutron cancer therapy. Nucl. Instrum. & Meth. A452 (2000) 484. 4. G.A. Tutin, I.V. Ryzhov, V.P. Eismont, A.V. Kireev, H. Condé, K. Elmgren, N. Olsson, and P.-U. Renberg. An ionization chamber with Frisch grids for studies of high-energy neutroninduced fission. Nucl. Instrum. & Meth. A457 (2001) 646. 5. J. Klug, J. Blomgren, A. Ata¸c, B. Bergenwall, S. Dangtip, K. Elmgren, C. Johansson, N. Olsson, S. Pomp, A. Prokofiev, J. Rahm, U. Tippawan, O. Jonsson, L. Nilsson, P.-U. Renberg, P. Nadel-Turonski, A. Ringbom, A. Oberstedt, F. Tovesson, V. Blideanu, C. Le Brun, J.F. Lecolley, F.R. Lecolley, M. Louvel, N. Marie, C. Schweitzer, C. Varignon, Ph. Eudes, F. Haddad, M. Kerveno, T. Kirchner, C. Lebrun, L. Stuttgé, I. Slypen, A.N. Smirnov, R. Michel, S. Neumann, U. Herpers. SCANDAL - A facility for elastic neutron scattering studies in the 50–130 MeV range. Accepted for publication in Nucl. Instrum. & Meth. A (2002). In addition, there are more than 25 contributions to international conferences published in conference proceedings.. v.

(15) Contents Preface. 1. 1 Introduction. 4. 2 Theory and phenomenology 2.1 Fission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 np-scattering . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 11 11 15. 3 Beams at the The Svedberg Laboratory. 19. 4 Fission dynamics experiment 4.1 The scattering chamber . . . . . . 4.2 The neutron detector array . . . . 4.3 Triggering and data acquisition . 4.4 Data reduction . . . . . . . . . . 4.5 Data analysis . . . . . . . . . . . 4.6 Estimate of the fission time scale 4.7 Results and discussion . . . . . .. . . . . . . .. . . . . . . .. 22 22 26 31 31 32 36 39. 5 Measurements of neutron-induced 5.1 Thin-film breakdown detectors . . 5.2 Data analysis . . . . . . . . . . . 5.3 Results and discussion . . . . . .. fission cross . . . . . . . . . . . . . . . . . . . . . . . .. sections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 44 45 45 47. 6 np-scattering studies 6.1 The Lisa magnetic spectrometer . . . . . . . . . . . . . . . . . . 6.2 Data reduction and analysis . . . . . . . . . . . . . . . . . . . . . 6.3 Results and discussion . . . . . . . . . . . . . . . . . . . . . . . .. 51 52 53 55. 7 Outline of the papers. 60. 8 Conclusions and outlook 8.1 Fission dynamics . . . . . . . . . . 8.2 Fission cross sections . . . . . . . . 8.3 np-scattering . . . . . . . . . . . . 8.4 Other neutron-induced experiments. . . . . . . .. . . . .. . . . . . . .. . . . .. . . . . . . .. . . . .. . . . . . . .. . . . .. . . . . . . .. . . . .. . . . . . . .. . . . .. . . . . . . .. . . . .. . . . . . . .. . . . .. . . . . . . .. . . . .. . . . . . . .. . . . .. . . . . . . .. . . . .. . . . . . . .. . . . .. . . . . . . .. . . . .. . . . . . . .. . . . .. . . . . . . .. . . . .. . . . . . . .. . . . .. . . . .. 62 62 63 64 65. Acknowledgments. 67. References. 70. vi.

(16) Preface The Comprehensive Nuclear-Test-Ban Treaty (CTBT), which prohibits all nuclear test explosions in all environments, was opened for signature in New York on September 24, 1996, when it was signed by 71 countries, including the five confirmed nuclear-weapon states at that time. At present (April 17, 2002), 165 out of 193 states have signed the treaty. Of these 165 states, 90 have also ratified it. If the CTBT will be ratified by all the 44 states which are listed in annex II of the treaty, which is mandatory for the treaty to enter into force, there must be methods of verifying that no clandestine tests are performed. For the purpose of verification the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) is building a network with stations all over the world, which uses different techniques to detect tests, irrespectively of where on earth or in the atmosphere the tests might be performed. The International Monitoring System (IMS) comprises a network of 321 monitoring stations and 16 radionuclide laboratories that screen the earth for evidence of nuclear explosions in all environments. The methods planned to be used to trace nuclear tests are: - Noble gas acquisition systems, which measure possible traces of radioactive xenon, which could be an indication of nuclear bomb tests. - Radionuclide particle detection, which is performed by filtering large quantities of air, and then measure radioactivity which has got caught on the filter. This is a well known method which has been operational at FOI in Sweden for many years. When the network is complete there will be 80 such stations around the world. At the same locations xenon detection systems are planned to be installed. - Seismic methods, which also have been used for this purpose during many years at FOI. About 50 primary and 120 secondary stations will be incorporated in the network. The principal use of the seismic data in the verification system is to locate seismic events and to distinguish between an underground nuclear explosion and the numerous earthquakes that occur around the globe. - Hydroacoustic monitoring detects acoustic waves produced by different phenomena in the oceans. Due to the very efficient propagation of acoustic energy in the oceans, only relatively few such stations are needed to cover the 70% of the surface of the earth which is covered by oceans. Eleven such stations are planned. - The infrasound network of 60 stations uses microbarographs (acoustic pressure sensors) to detect very low-frequency sound waves in the atmosphere. Each station has 4–8 sensors located 1–3 km apart. 1.

(17) At present 80 radionuclide detection stations are planned. The locations of all the stations are shown in Fig. 1. The outcome of the plans is of course dependent. Figure 1: The location of all CTBT stations in the IMS network. on whether the treaty will be ratified or not. The preparations are continuing, and much of the IMS network is already operational. At present, after the tragic events at September 11th, 2001, the situation in the world changes quickly, and with formerly unexpected alliances arising, the politics is somewhat unpredictable. It seems as if old verities are taken under new considerations. Sooner or later, however, there will be an end on nuclear bomb tests. There will certainly be a need for tracing clandestine activities irrespectively of the ratification. At FOI (Swedish Defence Research Agency, formerly known as FOA) I have been involved in the commissioning of the Swedish AUtomatic Noble gas Acquisition system, SAUNA. The purpose of SAUNA is to disclose violations of the CTBT by detecting radioactive xenon resulting from fission due to nuclear explosions. It was completed in October 2001, and then transported to Freiburg in Germany for comparative tests together with similar systems built at PNNL, Wa., USA, CEA, Bruyères-le-Châtel, France, and KRI, S:t Petersburg, Russia. After several months of tests SAUNA was approved. It was then dismantled and transported to Svalbard, where some minor upgrade was done, before it was put to work again in May 2001. 2.

(18) Nuclear weapons are based on nuclear fission, and that is what the techniques, motivated by the CTBT, I have been involved in at FOI are designed to detect traces of. For a project of this kind to reach its full potential, broad and deep knowledge of the respective aspects and components are required. In particular, experience in various fission detection techniques are very valuable when striving to accomplish our mission. This has motivated the involvement in a range of fission experiments. The neutron-induced fission experiments all heavily depend on neutron-proton (np) scattering experiments for normalization of the data. This made it logical to participate in studies of np scattering, and some of the results from these experiments are also part of this thesis. Let me finish by a few personal reflections. I am fortunate to be a part of these disarmament efforts, which is a global concern. It has been a challenging and interesting task, which has been completed with great determination. Moreover, I have got many interesting and exciting experiences, and I have had a lot of fun upon completing this part of the work. I think that it is of great importance for an organization such as FOI to have a close link into the more fundamental research at the university, which in turn have very much to benefit from cooperation with other partners. For FOI, to get insight into new experimental methods and scientific ideas, and to acquire contacts with scientists and potential employees are essential. In addition, comparisons of different aspects of management and leadership, besides mutual benefit via joint financing of projects and PhD-students, should indeed be attractive.. Uppsala, May 31, 2002. Klas Elmgren. 3.

(19) 1. Introduction “Oh what idiots we all have been! Oh but this is wonderful! This is just as it must be! Have you and Lise Meitner written a paper about it?” Niels Bohr to Otto Frisch upon being told about the discovery of nuclear fission.. This thesis comprises five papers which span the field of nuclear physics from one extreme to the other in terms of complexity, i.e., from nucleon-nucleon scattering to heavy ion-induced fission. The papers are, however, related to each other, which will be shown later in this summary. In order to get the subject into perspective, a short historical background is given. A hundred years ago, radioactivity had just been discovered by Bequerel [1]. In 1897, Sir J.J. Thomson detected the electron [2] which was the first sub-atomic constituent to be discovered.1 At this time, there were some phenomena that contemporary theories could not explain: Black-body radiation, photo-electric effect, and masses of atomic elements. The first of these, the black-body radiation, got its explanation by Planck, who, although in great hesitation, proposed the idea of light quanta [3]. This was the beginning of an entirely new field of physics, quantum mechanics, which provides necessary tools for the description of many atomic and sub-atomic phenomena. Einstein suggested that light has particle properties and could thereby explain the photo-electric effect, a work he published in 1905 [4], the same year as he published his work on Brownian motion [5] and the theory of special relativity [6], in which, among other things, the equivalence of energy and mass was first discussed. In 1911, Rutherford analyzed the scattering of α-particles by a thin foil of 2 gold [7–10]. The scattered particles had an angular distribution which could not be explained by the predominant model at that time in which the atomic mass was uniformly distributed throughout the whole atomic volume. Rutherford could show that almost the whole atomic mass is concentrated in the center of the atom, forming an atomic nucleus [11]. This lead Bohr to the first quantum mechanical description of an atomic spectrum and to a new3 atomic model in which he could give a theoretical derivation of the Rydberg constant. Two 1 Sir J.J. Thomson got the Nobel prize in 1906 for his discovery of the electron as a particle. In 1937 his son, Sir G.P. Thomson got the same distinction for showing that the electron acts like a wave. 2 The experiment was done by Rutherford’s assistants, Geiger and Marshden. 3 Bohr’s atomic model superseded the ‘Plum pudding’ model which was suggested by Lord Kelvin and further elaborated by J.J. Thomson.. 4.

(20) years later, J.J. Thomson could demonstrate the existence of atoms which was chemically identical, but with different masses, i.e., he had discovered that there are isotopes. During this time, and during the following decades, a lot of important work was done on quantum mechanics (by Bohr, Born, Dirac, Heisenberg, Pauli, Schrödinger, Sommerfeld, and others) and, as will be mentioned later, radioactivity. In 1919, Rutherford managed to knock out protons from nitrogen nuclei by α-particle bombardment, and they were correctly interpreted as hydrogen nuclei [12]. A few years later, in 1924, de Broglie defended his thesis in which he presented the idea of wave properties of matter. In 1930, Pauli proposed the existence of a neutral particle which he called “neutron” [13]. It was, however, not the neutron we know as of today, but rather a particle needed to explain the continuous-energy β-radiation, i.e., the neutrino. The theory of β-decay was later elaborated by Fermi and published in 1934 [14]. In this theory, Fermi introduced the weak interaction and explicitly used neutrinos. In 1932 the positron, the first anti-particle, was discovered by Andersson [15]4 . But, far more important for this thesis, was another discovery in the same year. There was evidence for a highly penetrating kind of neutral radiation in the beginning of the thirties. In Berlin, Bothe and Becker had found what they interpreted as high-energy electromagnetic radiation when they irradiated light elements with alpha-particles [18, 19]. Their observation was confirmed by I. Curie in Paris the following year [20]. Curie and Joliot made transmission experiments on a wide range of materials [21, 22]. They found that the radiation could cause ejection of protons from paraffin, which they misinterpreted as a type of Compton effect. At Cambridge, Webster observed that the energy of the “photons” emitted by the irradiated beryllium varied according to the direction of emission [23]. When Chadwick read about the findings of Joliot and Curie, he realized that this must be the particle he had been looking for during several years, inspired by a lecture held by Rutherford already in 1920 [24]. Chadwick made an extensive series of similar experiments, but managed also to show that the mysterious penetrating radiation was not very high energy γ-rays (∼ 50 MeV), but a massive particle — the neutron [25]. The discovery of the neutron made it possible to explain why atomic masses were not directly proportional to the atomic charge, and the existence of isotopes, none of which were previously quite understood. Models of nuclei with a mass excess from additional protons neutralized by electrons had been suggested, but failed to correctly account for parity and spin. Now an intense period of using the new particle to irradiate virtually all known elements started. Several groups around the world, amongst which the groups led by Fermi in Rome, by Curie and Joliot in Paris, and by Meitner and Hahn and later also Strassmann in Berlin, were particularly important for what 4. The existence of the positron had been predicted by Dirac already in 1931 [16] on the basis of relativistic quantum mechanics [17].. 5.

(21) was about to follow. Curie–Joliot found that they could produce radioactive phosphorous by irradiating aluminium with α-particles. This was the first example of artificially produced radioactivity. Fermi and co-workers irradiated many elements with neutrons, starting from the very light, and working their way through the periodic table. They observed many new activities, but more important: they found to their surprise that the probability for a neutron to react with the irradiated material increased if the neutrons were slowed down (i.e., moderated ) by putting a slab of material with a high content of hydrogen between the neutron source and the sample [26–30]. In collaboration with Rasetti and D’Agostini, Fermi made a study of (n, γ) reactions in thorium and uranium [27]. They found that the thorium reaction indicated a mixture of at least two different half-lives, while uranium gave at least five different half-lives. By chemical precipitation, they could rule out that the origin of an activity with a 13 minutes half-life came from any of the elements with atomic number 82–83, or 86–92. They found it reasonable to assume that a transuranic element, 93, had been formed. A German chemist, Ida Noddack, pointed out that the chemical procedures used by Fermi did not rule out that the uranium nucleus had split up into smaller rare-earth nuclei [31]. This remark apparently attracted little attention at that time, but it was the first suggestion of nuclear fission. At this time, as a result of neutron bombardment of different nuclei, the existence of extremely narrow and closely spaced nuclear resonances was established by several groups [32–38]. This was not in accordance with the quantum mechanical models of that time, which were based on the idea that the neutron was trapped in a deep nuclear potential well. This led Bohr to propose the concept of the compound nucleus, in which the energy of the incoming particle immediately was shared among all the nucleons [39]. It was an entirely new idea, which became an immediate success.5 Until 1938, when Meitner had to flee due to the hardening situation in Germany – she was Jewish – Meitner, Hahn, and Strassmann concentrated their work on irradiating uranium. Hahn and Strassman continued their work, and at Christmas time 1938 they identified radioactive barium from an irradiated uranium sample [40, 41]. The result was sent to Meitner, now a refugee in Sweden, to get a physicist’s point of view – both Hahn and Strassman were chemists. Meitner and her nephew Frisch managed to give a physical explanation by applying the Liquid Drop model, and realizing that it would be energetically favourable to split the heavy nucleus into two halves. They had discovered nuclear fission 6 . 5. The importance of the idea of a compound nucleus can be illustrated in V.F. Weisskopf’s words: “Rarely has a single paper dominated a field of physics as has Bohr’s address to the Copenhagen Academy in 1936, in which he proposed the idea of a compound nucleus.” 6 It was Frisch, in Copenhagen at that time, who coined the new process “fission” – a denotation which got an immediate acceptance – after having consulted a biologist.. 6.

(22) a. b. Figure 2: An example of the fission process. a) A neutron impinges on a heavy nucleus. b) The nucleus has split into two fission fragments and three free neutrons, i.e., two more neutrons are available to, e.g., sustain a chain reaction.. A year later, spontaneous fission was discovered by two Russian physicists, Petrzhak and Flerov [42, 43]. Spontaneous fission is what limits the size of nuclei to 300 nucleons (with the exception of neutron stars, which are heavy enough for gravitation to become important, and which have around 1057 nucleons). Spontaneous fission occurs with larger probability for heavier nuclei, where the strong interaction, which tends to hold the nucleus together, gets saturated, and the electrostatic repulsive Coulomb interaction gets stronger. This can be visualized as a fission barrier whose height decreases with increasing size and charge of the nucleus. Quantum mechanically, it is possible for a system to tunnel through a barrier, but the probability decreases exponentially with the square root of the barrier height. For elements lighter than thorium, spontaneous fission has not been observed. Soon after the discovery of fission, it was realized that the emitted neutrons could be used to maintain a chain reaction (see Fig. 2). Since this discovery coincided with the outbreak of World War II, extraordinary efforts were put on utilizing the fission reaction for weaponry, resulting in the nuclear weapons used during the last weeks of the war. After the war, an unprecedented armaments race took place until the demise and fall of Eastern European communism around 1990. During this entire period, there have been unsuccessful attempts to ban nuclear weapons altogether. What has been achieved are agreements on limitations, and test ban treaties. The present work is motivated by the need to understand fission in detail, as a knowledge basis for work in monitoring of such a test ban treaty. Fission chain reactions can also be of peaceful use. Although the first reactors were intended for plutonium production for weapon use, it was conceived already at that time that reactors based on self-sustaining fission reactions should be 7.

(23) possible to use for production of useful energy in general, and for electricity in particular. The first reactors for electricity production came into operation around 1950, and the first electricity to public power grids was delivered around 1955. This was the starting point for an extraordinary rapid development of this new energy technology. It should be remembered that fission to this very day remains the only useful energy source discovered in historic time. All other sources have been known since pre-historic time, whilst technologies for utilizing them obviously have been developed more recently. At the end of 2000, there were 438 nuclear power reactors in operation, providing about 16% of the global electricity generation. The total installed capacity was equal to 351 GW(e) [44] of greenhouse-free power. The installed power is still increasing, and nuclear power is the fastest growing electricity supplier worldwide, when counting delivered energy in absolute numbers. Reactors loaded with enriched 235 U and moderated with light water dominate the world market for nuclear power, and it seems unlikely that another concept will outperform it for base load electricity production in a near future. One drawback of light-water reactors, however, is the production of long-lived waste. Spent fuel contains radioactive elements with halflives spanning from very short times up to millions of years. For a typical spent fuel, time spans of the order of 100 000 years are needed for the radiotoxicity to come down to match natural uranium ore (which has a very low radiotoxicity though). Although direct geological storage of spent fuel seems to fulfill the requirements imposed by the political community, there is an interest in finding other strategies for waste incineration. New prospects opens with accelerator-driven systems, which in the future might transmute some of the long-lived nuclear waste into more short-lived, which might facilitate geological storage and at the same time contribute to additional electric power generation. Intensive international research is devoted to this field, and the present work should partly be seen in this context. The underlying idea is to use a subcritical core, loaded with the material to be transmuted, and to create neutrons with spallation reactions induced by an external accelerator. The reason to use an accelerator-driven system is that some of the elements to be transmuted have very low fission cross sections for low energy neutrons, and hence, they only consume neutrons while building even heavier actinides. Furthermore, those heavy actinides are very difficult to run in reactors based on self-sustained fission chain reactions since the number of delayed neutrons is much smaller than for, e.g., uranium, and in this respect an externally controlled system provides a more robust solution. Like for light-water reactors of today, accelerator-driven systems will be based on fission, but with an important difference. While critical reactors have neutron energies up to about 10 MeV, very high energy neutrons will be present in accelerator-driven systems. The maximum energy will be the incident beam energy, which in most design concepts lie in the 1000–2000 MeV range. Very little information exist about neutron-induced fission above 20 MeV; in fact, be8.

(24) fore the work presented in this thesis, essentially no information at all had been published. The present work should therefore be regarded as a starting point for thorough investigations of the fission process induced by neutrons at considerably higher energies than for the established applications. As can be found in the author lists of Papers I–III of this thesis, there is considerable competence in Russia on fission. This is in fact also a motivation for the present work. This competence was previously motivated by the nuclear weapons programme of the Soviet Union. When the Soviet Union was dissolved, there was a dramatically reduced need for nuclear weapons expertise, and fear was spread in the Western world that experts on nuclear weaponry might put their competence at the disposal of countries imposing threats on the Western world. Thus, it made sense to financially support such scientific key personnel to allow re-direction of their work into civil arenas. An international organization, the International Science and Technology Centre (ISTC) was established for this purpose, and a large fraction of the present work has been financed via ISTC. The fall of communism in Russia happened to coincide in time with a rapid growth in transmutation technology research. Accordingly, transmutation research is a strong discipline within ISTC. For me, working at FOI, it was natural to be involved in ISTC projects, because of their importance for international security. As mentioned above, it was realized at an early stage that fission is the result of competition between repulsion of the protons in nuclei due to the already wellknown electromagnetic force, and an attractive nuclear force, unknown before the discovery of nuclei. Understanding the properties of the latter therefore became a pre-requisite for understanding the fission process in detail. A model to explain the nature of the strong, but short-range, interaction, which is responsible for keeping nuclei together, and also is the most important force in nuclear scattering and reaction processes, was proposed by Yukawa in 1935 [45]. He postulated that the interaction was mediated by the exchange of a heavy boson, whose mass is given by the range of the nuclear force, the Heisenberg uncertainty principle [46], and the speed of light (m /Rc). When the muon, µ, which has a rest mass of 105.7 MeV, was discovered in cosmic radiation 1937 [47], it was first thought to be the Yukawa particle, but it was later realized that this must be wrong [48, 49]. The muon had too long range in solid matter, which was contradictory to a strongly interacting particle [50]. The µ is a member of the lepton family of particles, to which the electron also belongs. They also have an even heavier cousin, the τ . To each of these, a massless (or nearly massless7 [51]), neutrino is associated. In 1947, the proper Yukawa particle, the π-meson, or the pion (π), was found using new photo-emulsion techniques [52]. The neutral pion was, however, not found until it was produced in an accelerator in 1950. There are three kinds of pions; the positive π + , its anti-particle, the negative π − , and the neutral π 0 , 7. Recent experimental data indicates that the neutrino has a small, but finite mass.. 9.

(25) which is its own anti-particle. Mesons are built up by forming quark–anti-quark pair combinations. Pions are a combination of two of the lightest quarks, up and down. These are also the quarks that, together with gluons build up protons and neutrons. In addition to the up and down quarks, there are four more, strange, charm, beauty, and top. Later, other types of mesons were found (there are about 140), but they are not of primary interest for this thesis, so therefore the subject is considered exhausted for this purpose. To conclude this introduction, the discoveries of the neutron, the fission phenomenon, and the pion have been described, as well as the scientific routes that led there. These three findings can be considered as cornerstones of this thesis. All the papers included in the thesis deal with neutrons, either as beam particles as in Papers III, IV and V, or as a probe for the dynamics of the fission process, as in Papers I and II. Fission is treated in Papers I–III, while the scattering experiment of Papers IV and V has been used to monitor the neutron flux in the fission cross section measurement of Paper III. Finally, pions play a key role in the nucleon-nucleon interaction, as described in Papers IV and V. Papers I and II are part of my main work. As a natural consequence, I have been involved in all aspects of the project; preparations for the experiment, data analysis and interpretation, as well as the preparation of the manuscripts. They are a result of the Fidyc (FIssion DYnamics Collaboration) project. The fission cross section measurements is an extension of Fidyc, and I was deeply involved in the experimental preparations, and I have also been involved in the preparation of a number of manuscripts, of which Paper III is one. The last experiment is the one on neutron-proton scattering (Papers IV and V), where my contribution has been more limited, and mostly focused on practical experimental issues.. 10.

(26) 2. Theory and phenomenology “Progress does not consist of replacing a theory that is wrong with one that is right. It consists of replacing a theory that is wrong with one that is more subtly wrong.” Unknown thinker.. Some of the theoretical and phenomenological concepts concerning fission and np-scattering will be described in this section. The intention is to give a brief background in the scientific areas covered by this thesis.. 2.1. Fission. Energy. Nuclear fission is an extremely complex reaction. Fission can, in some heavy nuclei, occur spontaneously, or, as the fission reactions treated in this thesis, be induced by an incoming particle of a certain energy so as to overcome the fission barrier, see Fig. 3. The energy release in a single fission reaction of a heavy nucleus amounts to around 200 MeV, which can be compared to the energy release in a chemical reaction, typically a few eV, i.e., seven or eight orders of magnitude lower.. E*c. Saddle. Scission. Bf Deformation. Figure 3: Potential energy during the fission process vs. the deformation.. 11.

(27) The binding of nuclei is governed by the competition of two forces. The electromagnetic interaction acts repulsively between the protons, while it leaves the neutrons unaffected. The strong interaction acts primarily attractively, and it makes little difference between protons and neutrons, i.e., attraction can be present in all pairwise combinations of protons and neutrons. The strong interaction outperforms the electromagnetic interaction at typical distances between two nucleons in a nucleus (about 1 fm), but it has a short range, and vanishes already at a few fm. The electromagnetic interaction, on the other hand, is intrinsically weaker in magnitude, but has an infinite range (it varies as 1/r). As a consequence, nuclei in the mid-mass range (50-60 nucleons) are the most bound (about 8.5 MeV per nucleon) while in heavier nuclei, the large repulsion between the protons, which increases as Z 2 , results in less bound nuclei (about 7.5 MeV/A in the uranium region). In addition, for large nuclei, the distance between the nucleons at opposite ends of the nucleus is so large that the strong interaction barely acts between them, while the electromagnetic interaction can act at any distance. This gives profound consequences for heavy nuclei. Since the strong interaction works between any nucleon combination, but electromagnetism works repulsively between protons, the most bound nuclei in this mass range have a sizeable neutron excess, which “dilutes” the proton repulsion. Moreover, since the nuclei can gain binding energy by splitting into two halves, where each can increase its binding by almost 1 MeV/A, fission opens up as a possibility, and this ultimately limits the size of nuclei to about 300 nucleons. Fission can be simplistically understood if considering the textbook semiempirical mass formula, where two terms make major changes in fission; the surface tension (17.8 A2/3 ) and the Coulomb repulsion (0.71 Z 2 /A1/3 ). Very simple computations result in that energy can be gained by splitting a nucleus with Z 2 /A = 17.6, which corresponds to that nuclei from about mass 70 should fission. Obviously, they do not, so something must be missing in the description. The first missing aspect is that stretching a nucleus in shape in general costs energy, because the surface tension increases without energy gain somewhere else. For heavy nuclei, however, there can be a gain in a lower Coulomb repulsion when stretching a nucleus. If including a simple relation between deformation and surface tension energy into the description, it turns out that a spherical shape is stable against fission up to Z 2 /A = 50. No nucleus reaches this limit. As an example, 235 U has Z 2 /A = 36. We know uranium nuclei fission, so still something must be overlooked. The final piece of information required is nuclear structure. Binding energy differences due to nuclear structure effects can be large enough to allow fission although the bulk properties, described by the mass formula, do not reach fission conditions. From the considerations above, it is evident that Z 2 /A is a useful first-order figure of merit for fission probability. For a given A, the first-order binding energy changes due to shape distortion and Coulomb repulsion goes as A2/3 and 12.

(28) Z 2 /A1/3 , respectively (see above). If the surface energy increase is larger than the Coulomb energy decrease, the nucleus is stable against deformations. Thus, if the two terms balance each other, the nucleus can be elongated at no cost in energy, and fission becomes viable. It is commonplace to define the fissility of a nucleus, which is defined as the ratio of these two terms, i.e., the fissility x is proportional to (Z 2 /A1/3 )/(A2/3 ) = Z 2 /A. If x = 1, there is no hindrance for the nucleus to fission. Another way of envisioning the fissility parameter x is to note that it corresponds to the ratio of Z 2 /A for a given nucleus to Z 2 /A = 50 for the fission limit of a spherical nucleus. Returning to the example above, 235 U, which has Z 2 /A = 36, thus has a fissility of 36/50 = 0.72. Obviously, no nuclei in nature have x = 1, because then there is nothing preventing them from splitting immediately. The fissility is a simple estimate of how close the nucleus is to fission. In the present work, nuclei in the fissility range 0.7–0.9 are investigated. Without nuclear structure effects, all nuclei would be spherical in shape. The presence of structure effects, however, makes a deformed shape energetically more favourable for some nuclei, in particular far from closed shells. This is true for most actinide nuclei, i.e., nuclei in the uranium region. Thus, these nuclei are already deformed in their ground states, and thereby already part of the way to rupture, and this is part of the reason for their relatively large probability to fission. In Fig. 3, the potential energy during the fission process is shown. The minimum in energy, where the nuclear ground state is located, appears at a moderate deformation of the nucleus. A further increased deformation, however, will not be fully compensated by Coulomb or structure effects, which results in an energy increase, and this is called the fission barrier. In order to be able to fission, the nucleus has to overcome the fission barrier, denoted by Bf in the figure. When a certain degree of deformation, known as the saddle point, has been reached, the Coulomb forces, striving to separate the protons and the attractive nuclear forces are equally strong. Passing this point, the Coulomb forces get the advantage, and the nucleus goes to fission. Na¨ıvely, one could imagine that if a fissile nucleus was excited to an energy far above the fission barrier, it would fission immediately, but this is not the case. Fission is - with nuclear conditions - a rather slow process. A highly excited nucleus can emit neutrons very rapidly, which results in a quickly reduced excitation energy. Thus, for two nuclear reactions leading to the same compound nucleus at very different excitation energy, the starting point for the actual rupture might not be very different. It is also possible to envisage this from the fact that the number of neutrons emitted prior to fission increases more or less linearly with increasing initial excitation energy. When designing a technical system based on fission, many different aspects have to be investigated. Obviously, the probability that fission will occur, i.e., 13.

(29) the fission cross section has to be known, but this is far from sufficient. Fission is a statistical process, which leads to many different combinations of final nuclei. Literally, hundreds of new isotopes can be created in the fission of a certain isotope. These residual nuclei are almost all of them radioactive, and their yields have to be known for assessment of the final radiotoxicity as well as the residual heat generated by the radioactivity. Some of the products have properties influencing the technical system, and therefore their production has to be known. An example is 135 Xe, which is produced by fission8 . It has an extremely large neutron capture cross section (2.65 × 106 b), so once being created, it removes neutrons from a reactor very efficiently, thereby lowering the criticality. A well-known feature of neutron-induced fission at low energies is that it typically does not split the nucleus into two equal parts, but instead the two fragments differ significantly in mass, so that one fragment constitutes about 40 % and the other about 60 % of the mass of the parent nucleus. This has long been recognized as a nuclear structure effect, although a full quantitative explanation of the effect did not appear until about ten years ago. In fission induced by highenergy nuclear beams, however, symmetric fission is common, because the large amounts of energy introduced into the system before fissioning tends to wash out the relatively subtle structure effects. One route to understanding of the product yield in fission is to study the evolution of the fission process, i.e., the fission dynamics. In the present work, one such aspect of ion-beam induced fission has been investigated; the time evolution. As was discussed above, the fission process takes some finite amount of time. As a consequence, before rupture, light particles in general, and neutrons in particular, are evaporated from the excited nucleus. After scission (scission = the moment when the elongated fissioning nucleus ruptures into two fragments), two new highly excited nuclei have been born, and they can also emit neutrons. The ratio of neutrons emitted prior to versus after scission gives a measure on how long time the process takes. This is the principle of the ‘Neutron clock’, which is used in Paper II. The excitation energy is distributed over different energy degrees of freedom. After scission, all available energy does not necessarily appear as kinetic energy of two nuclei in their ground states. Instead, the two fragments typically are excited, and they lose their excitation energy via many different processes. In addition, the huge energy release in fission makes it possible that the product nuclei can rotate with significant angular momentum. The distribution of excitation energy is not static, but changes due to energy dissipation, i.e., one form of excitation energy can be transformed into another type. One example is that excitation energy in the form of a single particle-hole pair can dissipate its energy into two particle-hole pairs, each with lower internal excitation energy than the original pair. One of the two pairs can correspond 8. In fact,. 135. Xe is also one of the Xe isotopes that the SAUNA system is capable to detect.. 14.

(30) to a low-lying vibration mode, i.e., single-nucleon degrees of freedom can be transformed into collective excitations. It has been known since about ten years that fission seems to be a much slower process than statistical model considerations alone would suggest. This has led to speculations that friction due to the viscosity of nuclear matter influences the dissipation of excitation energy, resulting in a slower transition from the excited compound nucleus to the elongation followed by rupture. Different models have been used to describe the nuclear dissipation, of which most are based either on one-body or two-body dissipation. The former models the interaction of individual nucleons with a mean field of the nucleus, i.e., the model assumes a single nucleon moving in a nuclear potential, whereas the latter uses nucleon-nucleon interactions to describe the process of dissipation. Thus, a thorough understanding of the fission process requires all the tools available in nuclear physics. Both bulk effects and subtle details of nuclear structure are needed to describe whether a nucleus can fission at all, and the evolution of the fission process requires both macroscopic parameters, like viscosity and friction, as well as microscopic aspects, like the nucleon-nucleon interaction, to obtain a quantitative description.. 2.2. np-scattering. The np-scattering process is widely used as a reference to monitor the neutron flux in other experiments and applications. It is the most important standard cross section for neutron flux determination. For instance, it is utilized in connection with the fission cross section measurements which is a part of this thesis. When performing high-precision (n,p)-reaction experiments some years ago [53–56] with the supposedly well-known np-scattering cross section as reference, the Uppsala neutron group revealed discrepancies in both shape and absolute normalization of the cross section. This called for a dedicated experiment, and the need for such an experiment was further supported by the confusion on the pion-nucleon coupling constant that begun to propagate as described below. The interaction between nucleons is fundamental, yet some aspects are not well known. Even such a basic property as the strength of the interaction has been under vivacious debate [57–60]. A reason for uncertainty in the determination of the interaction strength is two-fold: Firstly, it is not possible to measure it directly, but it has to be extracted out of, e.g., nucleon-nucleon scattering data in a rather complicated manner. Secondly, NN -scattering is, although a seemingly simple process, not trivial to measure. Many groups have had problems, especially with the absolute normalization, as have been shown by Blomgren, Olsson, and Rahm [61]. The strong interaction is complex, since it is built up as a superposition of strengths coming from many different mediating particles with different weight in the superposition, see Fig. 4. Conservation and invariance laws restrict the 15.

(31) interaction, i.e., the energy, momentum and parity are conserved and the potential stays invariant under reflection, rotation, time reversal and translation of momentum or space coordinates. All terms in the interaction must consequently be scalars. In practice, the long-range part of the strong interaction between nucleons is mediated by pions, and this aspect dominates the np scattering data presented in Papers IV and V.. Potential V(r). Two nucleons overlap. Hard core. Pion exchange. Scalar meson exchange. Nucleon-nucleon distance r Figure 4: An outline of the NN -potential as a function of the distance between the nucleons. In nucleon-nucleon scattering at large distances, i.e., small momentum transfers, the interaction potential is well described by a one-pion exchange potential (OPEP): . . 4π 1 f 2 e−mπ± r − 2 δ 3 (r) σ 1 · σ 2 τ 1 · τ 2 Vπ± (r) = 3 4π r mπ ± 2 e−mπ± r 3 1f 3 + 2 2 S12 (ˆ + 1+ r )τ 1 · τ 2 3 4π mπ ± r mπ ± r r. (1). with rˆ ≡ r/|r|, and σ i and τ i are the Pauli spin and isospin matrices, respectively. The absolute strength of the interaction, i.e., the coupling constant, is. 16.

(32) denoted by f in Eq. 1. There are two conventions for how to express the coupling, which differ just by a multiplicative factor, and by the notation (f or g, see below).9 The differential cross section, dσ(θ, φ)/dΩ, is strictly equal to the square of the scattering amplitude, which in turn is related to the potential in Eq. 1. In a nucleon-nucleon collision, the coupling term comes in twice, in both vertices of the Feynman diagram in Fig. 5. It is obvious that in a one-pion exchange process, only the π 0 is possible as mediating particle if the nucleons involved have the same charge. With different charges, e.g., neutron-proton, exchange of π + and π − is also possible. . . . . . . . . . . . . . Figure 5: Feynman diagrams over the two possible one pion exchange np scattering processes.. There are theoretically a number of reactions that can be utilized to explore ¯ ,N ¯N ¯ , πN ¯ . Due to practical constraints, the πN coupling strength; πN , NN , NN however, the latter two reactions are not possible to study experimentally today. Here π denotes either a π + or a π − , and N could be either a proton or a neutron. The neutral π 0 plays also an important role in some cases, it even dominates the nucleon-nucleon interaction when either both nucleons have the same charge or the incoming nucleon is scattered in the forward direction. It is, however, too short-lived (τ = 8.4±0.6×10−17 s, corresponding to a flight path of 25.1 nm at the velocity of light [62]) to be used experimentally otherwise than indirectly. The least complicated (i.e., experimentally) of these reactions is the nucleon-nucleon and, in particular, the neutron-proton scattering process if the interest is focused on gπ2 ± . As mentioned above, it is not possible to measure the coupling strength directly. By means of dispersion relation methods, which were originally developed 9. The pseudovector, denoted with f 2 , and the pseudoscalar, g 2 , are two different conventions on how to define the coupling strength. They are related by a multiplicative factor; g 2 = f 2 (2MN /mπ± )2 , where MN and mπ± are the nucleon and charged pion masses, respectively.. 17.

(33) in electromagnetic theory [63, 64], and later generalized to be valid also for massive particles [65–69], Chew [70] suggested a method to extrapolate to the pion pole using backward np scattering data to determine the coupling constant. The method was applied to np data for the first time by Cziffra and Moravcsik [71]10 . A more recent determination of the charged coupling constant was made in 1980 by Koch and Pietarinen [73], who obtained gπ2 ± = 14.28 ± 0.18 using pion-proton scattering data. In 1981, Kroll [74] obtained a value for the neutral coupling constant of gπ2 0 = 14.52 ± 0.40 using forward dispersion relations and pp-scattering data. About ten years ago, however, the theoretical group from Nijmegen presented substantially lower values of the coupling constants, which had been calculated by means of Partial Wave Analysis (PWA) and a large fraction (≈ 50 %) of the global NN -scattering database [75–77]. The values they presented were gπ2 0 = 13.47 ± 0.11 and gπ2 ± = 13.54 ± 0.05, several standard deviations lower than the previous results. A similar result was obtained by the Virginia Tech group which made a similar analysis, but also included π ± N -scattering data in their determination [78–80]. These discrepancies led Torleif Ericson to draw the attention of the Uppsala neutron group to the fact that a dedicated high-precision np-scattering experiment would be suitable to perform at the unique neutron beam facility at TSL. The measurements started as a separate experiment in 1993. As new data have been produced, Ericson and Benoit Loiseau have used the data employing their difference method, which they have developed from the Chew method [70, 71], in order to make a better determination of the πNN coupling constant. A short description of how to extract information on the coupling strength from scattering data is found in Section 6.3 of this thesis. The results of this cooperation have been presented at conferences, as well as in publications [57, 81–84], two of which constitutes Papers IV and V of this thesis. 10. For an orientation in the theories on dispersion relations and their utilization in nucleon scattering, see Appendix 12 in Ref. [72], and references therein.. 18.

(34) 3. Beams at the The Svedberg Laboratory “Lugn grabbar, l¨ onen kommer radioaktivt.” Rolf Zetterlund, fotbollstränare.. The various experimental setups for studies of fission dynamics, and measurements of fission, as well as np-scattering cross sections are described in the forthcoming sections. But first, a brief account of the The Svedberg Laboratory, where all the experiments of the present thesis were performed, will be given. nt. e rim xpe e s tic ic am nos dyn d diag wn) n o an t sho ssi o o fi ical m t m opt s are n a e b bea ent n ( lem Io e. The The Svedberg Laboratory Experimental facility overview Bending magnets. II. III. Scattering chamber surrounded by neutron detectors. Collimators Proton deflecting Lithium magnets target. I. (n,p) target. r ete. Neutron beam. m. ctro. Spe Clearing magnet. b on ot Pr. Thin-film breakdown counter. m ea am be to p m du. 5m. Figure 6: The TSL neutron beam facility and the fission dynamics experimental area. The neutron production target, beam dump, and collimators are located in an area shielded from the experimental hall with 2 m of concrete. Both the magnetic spectrometer and the TFBC’s are shown along the neutron beam line. The location of the scattering chamber and neutron detector array at the ion beam line is also indicated. The Gustaf Werner Cyclotron at the The Svedberg Laboratory is a multipurpose accelerator with variable energy (∼14–180 MeV protons) and with three 19.

(35) different ion-sources: an internal source for protons, H+ 2 -ions, deuterons and αparticles, an external ECR ion-source for heavy ions and another external source for polarized protons and deuterons [85]. For 100 MeV protons from the cyclotron, each beam burst is typically 2–3 ns long. The interval between the pulses is ∼60 ns, thus allowing neutron time-of-flight (TOF) measurements. Three different beam particles were used in the fission dynamics experiment, namely protons, α-particles, and 12 C-ions. The first two were produced in the internal ion-source, while the latter was produced in the external ECR-source. The beam currents at the target position were between a few hundred nA for the protons down to a few tens of nA for the carbon beam.. Figure 7: The neutron spectrum as reconstructed from the np-experiment at 162 MeV, and as measured by Byrd et al. [86] for an incident proton energy of 160 MeV. The latter spectrum has been shifted by 2 MeV. The spectra have different energy resolution, and therefore they are normalized to the area under the high-energy peak.. A necessary prerequisite for both the neutron-induced fission cross section measurements and the np-scattering experiments which will be treated in the following sections is the unique neutron beam. An overview of the TSL neutron beam facility is shown in Fig. 6. The neutron beam is produced using protons from the Gustaf Werner Cyclotron which impinge on a 99.98% enriched 7 Li target, and by the 7 Li(p,n)7 Be charge exchange reaction a quasi-monoenergetic beam of 20.

(36) neutrons at 0◦ is produced. The ground state and first exited state at Ex = 0.43 MeV in 7 Be are not resolved and they both contribute to a high energy peak at an energy a few MeV lower than the proton beam. The difference is due to the negative Q-value of −1.64 MeV, and to the energy loss in the Li-target. The latter also contributes to the width of the peak. A 100 MeV proton loses ∼2.5 MeV in 8 mm (400 mg/cm2 ) of Li, which is a common target thickness. After the Lithium target, the proton beam is bent away, into a well-shielded beam dump. A system of three collimators defines a narrow neutron beam at 0◦ which enters the experimental hall. The neutron beam is approximately 7 cm in diameter at the np target position, which is located 8 m from the neutron production target. The neutron yield in the experimental hall is about 5 × 104 s−1 (µA)−1 protons per mm Li-target (1 mm corresponds to 50 mg/cm2 of Li). The maximum intensity that is available is partly limited by radiation protection considerations. In practise 5 − 8 µA of protons out from the cyclotron is the upper limit when operating the cyclotron in isochronous mode, i.e., up to 100 MeV. In frequency modulated mode (100–180 MeV), the intensity is an order of magnitude lower. In addition to the high energy peak, there is a tail of lower energy neutrons, extending all the way down to zero energy. The integrated number of neutrons in the peak and in the tail, respectively, are about the same. The neutron spectrum shape is shown in Fig. 7.. 21.

(37) 4. Fission dynamics experiment “Watch this lamp!” The only instruction on how to run the experiment the rest of the night a new Ph.D.-student got from O. Batenkov.. In the fission dynamics experiment, neutron versus fission fragment angular correlations have been measured in the 209 Bi(p, f ), 206 Pb(α, f ), 197 Au(12 C, f ), 232 Th(12 C, f ), and 248 Cm(12 C, f ) reactions, all with an incident beam energy of 100 MeV. The intention with the experiment was to make a new, high quality determination of the fission time-scale over a wider angular momentum span than previously had been explored, since heavy-ion experiments have dominated this field during a long time. The principle of the fission dynamics experiment is rather simple. Neutrons can be emitted both before and after scission. The idea is to use the ratio of neutrons emitted before and after scission to estimate how long time it takes from the impact of the incident particle to the scission to occur. The prefission neutrons are experimentally distinguished from the postfission neutrons by measuring the angular correlations between neutrons and fission fragments. The separation method is based on the assumption that prefission neutrons are emitted isotropically from the compound nucleus, while the postfission neutrons are emitted isotropically in the centre-of-mass systems of the fully accelerated fission fragments. This gives a kinematical focusing in the direction of motion of the fragments, and thus the number of neutrons as well as the neutron energy spectra are strongly correlated to the angle relative to the fragment direction. This is illustrated in Fig. 8. In order to be able to measure this correlation, both fragments and neutrons have to be detected. To that end, a scattering chamber with target and fission fragment detectors, and surrounded with neutron detectors was designed. The design emphasized light weight so as to reduce neutron scattering. The following sections are devoted to a description of the experimental setup, the analysis of the data, and to a presentation of the results.. 4.1. The scattering chamber. The cylindrical scattering chamber is made of 1 mm thick stainless steel, and has the dimensions Ø27 × 20 cm3 . Inside the chamber, there are two telescope arms to detect fission fragments emitted from the target. Each arm consists of two assemblies of aluminium oxide foils (60–80 µg), each viewed by a position sensitive 22.

(38) Figure 8: The effect of kinematical focusing of neutrons as measured in the detectors. At 90◦ , almost all neutrons emanate from the compound nucleus, which emits neutrons isotropically (shaded area). At 0◦ , however, the compound nucleus still contributes with a significant part, but the main contribution comes from the fragments.. microchannel plate (MCP) and one silicon surface barrier detector (SSB), see Fig. 9. The MCP s have excellent timing properties, and are used to obtain the fragment flight time over a certain distance, variable between 60 and 130 mm, while the SSB s give the energy of the fission fragments. The target frames are mounted on a wheel, which can be manœuvered remotely, allowing to change targets (up to 5) without breaking the vacuum. The angle between the TOF arms can, also remotely, be varied in the range 140◦ –180◦ with a smallest step of 0.1◦ . The targets used in this experiment are thin layers (∼130–300 µg/cm2 ) of fissile material evaporated onto ultra-thin (∼20 µg/cm2 ) Al2 O3 backings. The targets have a diameter of 10 mm, and are mounted in a frame which consists of a thin aluminum ring supporting the backing. The inner diameter of the frame is 15 mm, and since the impact angle of the beam is 45◦ , it exposes an elliptical surface to the beam with minor axis ∼10 mm and major axis ∼15 mm. The beam has to be smaller than this surface at the target position, and by minimizing the beam halo, the background is reduced. The microchannel plate (MCP) detectors, registering δ-electrons from the aluminium oxide foils, were used for the fragment velocity measurements. When. 23.

(39) SSB MCP. Al2O3-foils MCP. Al2O3-foil. Al2O3-foil. MCP. SSB. MCP. Target. Figure 9: The scattering chamber lay-out showing the two fission fragment telescopes, each consisting of two aluminium oxide foils with adjacent microchannel plate detector, and a silicon surface barrier detector at the end. The target wheel is in the middle. passing through a thin Al2 O3 foil, a fission fragment produces a shower of ∼ 100 δ-electrons, whereas an α-particle produces only about 20. The rejection of αparticles by applying a threshold in the pulse-height spectra, is therefore reliable without compromising with the fragment detection efficiency. Less than 2% of the fragments are lost due to the α-rejection threshold. Thus, the fragment detection efficiency, calculated as the ratio between the number of detected fragments and the estimated number of fragments, were 98%. The pulses from the two MCP s of each arm are used as start and stop signals, respectively, for the fragment TOF. To improve the time resolution and the collection efficiency, the electrons are accelerated over a short distance in an electrostatic field of 2–5 kV/cm, before hitting the MCP (See Fig. 9). The time resolution was tested using the coincidence peak of two MCP s mounted on opposite sides of a foil which was bombarded with fission fragments from a 252 Cf source. A typical time resolution was 80 ps for each TOF-channel, which made it possible to increase the counting rate by shortening the flight path, and thus increase the solid angle. A typical velocity distribution of 252 Cf SF fragments is shown in Fig. 10.. 24.

(40) Figure 10: Measured velocity distribution of fragments from spontaneous fission of 252 Cf. The velocities have been corrected for losses in the Al2 O3 foils. The peak positions were used to calibrate the fission fragment flight times [87].. Heavy ion-induced fission experiments involve problems of rejecting processes with incomplete momentum transfer to the compound system. By measuring the outgoing kinematic observables of the fission fragments, the missing momentum in the reaction can be estimated. The momentum carried away by the neutrons is relatively small and can be neglected. Tests show that it is possible to distinguish out-of-plane reactions from those in-plane by simultaneous observation of the fragment velocities and emission directions, making use of the intrinsic position sensitivity of the MCP s. Previously, position sensitive MCP s have used a resistive anode read-out, whereas in this experiment the resistive dynode (the back side of the MCP) has been used. Tests have shown that a uniform resistive dynode read-out is more attractive for two main reasons: (i) All MCP s have a metallic coating acting as a uniform resistive sheet (∼ 150 Ω/cm2 ), which can be used as a position sensitive detector without using the anode; (ii) The spatial resolution of the dynode is, in principle, only limited by the dimensions and spacing of the individual channels, whereas there is an additional position spread due to the spray of output electrons between the MCP and the anode. The total acceptance angle, when triggering an event with the fragment detectors close to the target only, is 40◦ , corresponding to a solid angle of 0.76 sr. When using the harder triggering condition with both MCP s on each arm, the 25.

(41) solid angle reduces to 0.2–0.3 sr depending on flight distance. Some results from the calibration of the position sensitive MCP s are presented in Fig. 11. The position was calculated with the method presented in Ref. [88]. The experimental distribution along the X-axis was fitted with a step function modulated with a Gaussian (the smooth line in Fig. 11.), with the standard deviation as a variable parameter. The best fit was obtained with σ = 0.9 ± 0.2 mm, which corresponds to a resolution of 2.1 mm (FWHM). The angular resolution was found to be 0.6 ± 0.1◦ over a flight path of 10 cm. The equipment was developed during the course of those experiments. Except for the last 12 C-induced experiment, the fission fragment energy was determined indirectly using the TOF information from the MCP’s. However, in the last experiment, Silicon Surface Barrier detectors were used to measure the energy of the fragments. The detectors were made of n-doped Si with a resistivity of 1 kΩ/cm2 , having a diameter of 30 mm and a thickness of 0.5 mm. The peak-tovalley pulse height relation, as described in Ref. [89], is 2.8 for 252 Cf spontaneous fission (SF). It corresponds to an accuracy of better than three mass units in the fragment mass determination. Pulse-height defects, which deteriorate the performance, were monitored regularly.. 4.2. The neutron detector array. The neutron detector array is used to determine the absolute multiplicity, the energy and angular distributions of the neutrons emitted in spontaneous or ioninduced fission. It consists of 8 stilbene-crystal scintillators of different dimensions: 50×30, 50×20, 70×30 and 70×20 mm (diameter × thickness). The detectors are mounted at different angles, both with respect to the beam and to the fission fragment detector telescopes, i.e., the fission fragment direction, as can be seen in Fig. 12 and in Table 1. The detector time resolution, determined as the half-width of the γ-peak, varies between 0.5 and 1.0 ns, depending on the size, threshold setting and inherent time resolution of the photo-multiplier tubes. The flight paths were between 35 and 50 cm, and the relative energy resolution, ∆E/E, were 4.5–10% and 7.4–20% for neutrons with energies of 0.5 and 5.0 MeV, respectively. The energy of the neutrons is determined by TOF techniques. A coincidence signal from the two MCP s closest to the target is used as start signal for the TOF, and a pulse from a neutron detector as stop. The fact that a coincidence signal is used diminishes the number of events from other types of neutron-producing reactions, such as direct, deep inelastic, and compound reactions, and thus establishes a fission event. The separation of neutrons and γ-rays is based on pulse-shape discrimination techniques. The fast and slow scintillation components are measured event-byevent, by a peak-sensitive ADC and a pulse-integrating QDC, respectively. The n − γ-discrimination is made on-line. 26.

(42) Figure 11: Results from the calibration procedure of the fragment detector position sensitivity with a circular copper shield with 5 holes, 4 mm in diameter each, and separated by 10 mm, mounted close to the fragment detector assembly. The upper panel shows a two-dimensional hit pattern. The lower panel solid histogram displays the experimental counting rate projected onto the X axis. The smooth line is the calculated response function.. The experimental set-up has been optimized to reduce the influence of background effects on the neutron spectra. Special effort has been spent on reducing scattering effects by minimizing the amount of material in the vicinity of the detectors. As mentioned above, an extended beam spot at the target position generates background. Furthermore, the beam dump must be well shielded to reduce the neutron background. To fulfill these requirements, special arrangements. 27.

(43) o. o. 90 -90. o. 30. o. o. 120 -90. o. 60. 60 -90. o. 0 o. o. o. o. 30 -90. n. tio. am. ec dir. 0. o. Be. Figure 12: Schematic drawing of the scattering chamber connected to the beam pipe and surrounded with the neutron detectors. The angles with respect to the fragments are indicated. In the cases when two angles are shown, the first refers to the angle versus the beam-line, and the second to the angle versus the fragments.. Table 1: Neutron detector positions Neutron detector# 1 2 3 4 5 6 7 8. Angle respective to: fragment direction the beam 0 90 0 90 30 90 60 90 90 30 90 60 90 90 90 120. had to be made. In particular, the beam optics and the shielding of the beamdump were carefully elaborated in order to reduce the background produced by the ion beam. A problem in this context is that the emittance of the beam out of the cyclotron is not well defined. There are two adjustable collimators in the cyclotron hall which are used to reduce the spread in transverse momentum and position. This was, however, not enough to limit the size of the beam at the target po28.

(44) sition. A beam optics calculation showed that it would be possible to make a 1:1 projection of a collimator at the target position if the collimator was located at position I in Fig. 6, and consequently, a collimator was installed. The beam optical elements, which were situated between the collimator and the target, were adjusted accordingly so as to make a 1:1 image of the collimator at the target position (II). The collimator was made of graphite, thick enough to stop 100 MeV protons, although tungsten was used around the circular opening (5 mm in diameter) to reduce edge scattering. Directly after the collimator a 10◦ bending magnet and a 2 m thick concrete wall prevented background produced in the collimator from reaching the detectors. The beam stop arrangements had also to be adjusted for different types of accelerated particles in the beam. Protons produce much more background than alpha-particles and carbon ions of the same energy. In all cases the beam dump was well shielded, but the beams were dumped at different positions. The 12 Cdump was located approximately at position III in Fig. 6, the α-dump 5 m further downstream with additional shielding, and the protons were dumped another 7 m downstream, and with even more shielding. The beam current was monitored continuously. In the case of protons, the beam pipe could not be extended all the way to the dump without introducing additional beam optical elements. Instead, the vacuum pipe was terminated with a 0.1 mm thick kapton foil at position III and two joined plastic tubes, 8 m and 5 m long with diameters of 20 and 30 cm, respectively, were used as an extension. The plastic tubes were filled with helium gas in order to reduce scattering of and activation by the beam. The beam was dumped in a graphite block with a diameter of 50 cm. The graphite block was covered with fluorescent ZnS, thereby allowing continuous monitoring of the position of the beam in the dump using a TV-camera. The influence of scattering effects on the neutron spectra has been studied in detail, both experimentally and by Monte Carlo simulations. To this end, the 252 Cf fission neutron spectrum has been measured with conical shadow bars inserted between the target and the neutron detectors for three different flightpaths, viz. 50, 75 and 100 cm. The bars were made of 10 cm lead and 25 or 45 cm polyethylene. Monte Carlo simulations, made with the same geometry, agreed approximately with the experimental data. Thus, it is possible to conclude that the background from scattered neutrons is understood, and that scattering from walls and floor is not a problem in this case. Scattered γ-rays are predominantly of low energies. Hence, they could be suppressed using a thin (1 mm) screen of lead, which was mounted on each neutron detector. The neutron TOF spectrum for flight times longer than 110 ns is dominated by scattered neutrons. To avoid interference of high-energy γ-rays in the high-energy neutron part of the spectra, a low-amplitude software threshold was set in the two-dimensional pulse-height vs. time spectrum. The threshold was set at a pulse-height cor29.

(45) responding to about one tenth of the maximum pulse-height a neutron with a certain flight time can attain at full neutron energy deposition. Similarly, a threshold was set to avoid pulse-heights corresponding to higher energies than given by the TOF, thus rejecting scattered events with high energies (see Fig. 13a). The resulting TOF-spectrum is shown in Fig. 13b.. Figure 13: a) The variable threshold in a pulse-height vs. TOF plot. Data are from measurements of the 209 Bi(p,f)-reaction. The background in the two following beam bunches is also measured. The beam pulses are repeated once every 400 channels in the figure. b) The resulting TOF spectrum with (crosshatched histogram) and without cuts (empty histogram) applied.. 30.

(46) During the ion induced fission measurements, the TOF time window was extended so as to cover two additional cyclotron beam-bunches, either the two following the one that triggered the fission event, or one before and one after. In Fig. 13, one additional beam bunch is included on the left side. Using this information, a prompt-to-random subtraction in the off-line analysis could be performed. The background was very dependent on the type of projectile particle. For protons the background was rather high, whereas for the other types of projectiles it was negligible. The neutron detector efficiencies were measured using a small ionization chamber with a 252 Cf neutron source which was placed in the target position. The detector efficiencies were then determined as the ratio between the measured spectrum and that of a standard reference [90] for this reaction. The “effective” efficiency, including neutron absorption, scattering and reactions, was measured before and after the actual data taking.. 4.3. Triggering and data acquisition. During the experiment, a number of different event types were accumulated on an event-by-event basis. The various coincidence combinations used as triggers in the experiment are shown in Table 2. The fragment detectors were used pairwise. The two MCP-detectors close to the target is present in all trigger conditions, and they give the start signal for both the neutron and the fragment TOF. The MCP s far from the target gives the TOF and the SSB s the energy of the fragments. Table 2: Triggers used in the experiment. Trigger # 1 2 3 4 5. MCP pair close to target X X X X X. MCP pair far from target X X X X. SSB pair (energy). Neutron TOF. X X X. X X. Standard NIM and CAMAC electronics were used to process the detector signals. The data were recorded to disc on an event-by-event basis using a PCbased data acquisition system running under Windows 95.. 4.4. Data reduction. The data were analyzed off line on an event-by-event basis. The events had to fulfill certain conditions before they were accepted. Depending on which triggering 31.

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