Measurements of Isomeric Yield Ratios of Proton-Induced Fission of nat U and nat Th

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1 Vasileios Rakopoulos Measurements of Isomeric Yield Ratios of Proton-Induced Fission of nat U and nat Th at the IGISOL-JYFLTRAP facility

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3 Abstract This thesis presents the measurements of isomeric yield ratios of fission products in 25 MeV proton-induced fission of nat U and nat Th, performed at the Ion Guide Isotope Separator On-Line (IGISOL) facility at the University of Jyväskylä. Knowledge of the relative intensities of metastable states produced in fission is of importance for various fields of nuclear physics, both fundamental and applied. The angular momentum of fission fragments is regarded as an important quantity in order to understand the fission mechanism because it can provide information on the scission configuration. One of the means to deduce the angular momentum of highly excited nuclei is by determining the yield ratio of low lying isomeric states. Isomeric yield ratios are also important themselves for simulations of processes such as the r-process, which is believed to be terminated by the fission of very neutron-rich heavy nuclei, and the neutronics and decay heat of nuclear reactors. In addition, proper simulation of the effect of delayed neutrons in a reactor requires accurate knowledge of the population of isomeric states, since the β-delayed neutron emission probability from the isomeric state can be an order of magnitude different from that of the ground state. The measurements were performed from 2010 to 2014, both at IGISOL-3 and at the recently upgraded and relocated IGISOL-4 facility. With the IGISOL method short-lived fission product yields can be measured and, by employing the high resolving power of the Penning trap JYFLTRAP, isomeric states separated by a few hundred kev from the ground state can be observed. Thus, a direct determination of the isomeric yield ratios by means of ion counting, registering the products in less than a second after their production has been accomplished for the first time. In addition, γ-spectroscopy was employed in order to verify the consistency of the experimental method. Isomeric yield ratios of fission products were measured in a wide mass range (A = 81 to 130) for 25 MeV protons on nat U and nat Th. Specifically, six isomeric pairs ( 81 Ge, 96 Y, 97 Y, 97 Nb, 128 Sn and 130 Sn) with suitable half-lives were measured and indications of a dependence of the production rate on the fissioning system were observed. A 25 MeV proton beam was selected as there are experimental data available in the literature, determined by means of γ-ray spectroscopy, so that a comparison of the results could be performed.

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5 "survive another winter" to Katerina and Elias-Sebastian

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7 List of papers List of papers is not included in this thesis.

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9 Contents Preface Introduction A (brief) introduction to fission Isomers The importance of isomeric yields Fission yields measurements techniques Experimental Facility The IGISOL technique combined with JYFLTRAP Description of experimental elements The fission ion guide Mass separator Radio-Frequency cooler and buncher Isobaric purification with JYFLTRAP Timing Structure of the measurement Chemical effects of IGISOL and JYFLTRAP Data Analysis Penning Trap Data Time of flight selection Peak intensity determination Corrections due to radioactivity γ-spectroscopy Data Efficiency calibration Decay corrections Transport efficiency Uncertainties Results and Discussion Presentation of the results Discussion and comparison Mass Mass Mass Mass Mass General remarks

10 5 Summary and Conclusions Summary Conclusions References

11 List of Figures Fig. 1.1: Nucleus deformation in terms of a liquid drop model Fig. 1.2: Potential energy surface of deforming nucleus Fig. 1.3: Double-humped fission barrier Fig. 1.4: Time scale of fission fragments de-excitation Fig. 1.5: Decay paths of nuclides Fig. 1.6: De-excitation of the fission fragments Fig. 1.7: Decay scheme of mass chain A= Fig. 2.1: IGISOL and JYFLTRAP facility Fig. 2.2: The fission ion guide at IGISOL Fig. 2.3: Ion s trajectory in the Penning Trap Fig. 2.4: Conversion of the ion s motion in the trap Fig. 2.5: Timing structure of the measurement Fig. 3.1: Time of flight distribution of mass A= Fig. 3.2: Mass spectrum without and with TOF gating Fig. 3.3: Frequency distribution of mass A= Fig. 3.4: γ-ray spectrum of mass A= Fig. 3.5: HPGe intrinsic efficiency curve Fig. 4.1: Isomeric yield ratios Fig. 4.2: Frequency spectrum for mass A= Fig. 4.3: γ-ray spectrum for mass A= Fig. 4.4: The case of multiple results Fig. 4.5: Investigation of the IYR dependence on the fissioning system Fig. 4.6: IYR as a function of the spin difference of the states

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13 Preface This thesis reports on the experimentally deduced isomeric yield ratios from proton-induced fission on nat U and nat Th. All the measurements were performed at the IGISOL-JYFLTRAP facility at the University of Jyväskylä over a span of four years, from April 2010 to May This work was accomplished as part of the collaboration between Uppsala University and the University of Jyväskylä that aims at high precision measurements of fission yields. Since the fission yields are an important characteristic of the fission process, a brief introduction of the latter is attempted in Chapter 1, where a description of the time evolution of the fission and a definition of the fission yields are given. In addition, the importance of the knowledge of the population of the isomeric states for both fundamental and applied physics is emphasised, as motivation for the present work. At the end of the chapter different techniques of measuring fission yields are described. During this period of four years, a lot have changed at the IGISOL facility since a major upgrade was realised, both in the IGISOL and JYFLTRAP facilities. Chapter 2 gives an overview of the renewed facility, highlighting the most important elements along the beam line. In Chapter 3, the analysis procedure which was developed and followed in order to deduce the yield ratios of the acquired data is presented. The results of this analysis are presented in Chapter 4, together with some remarks that could be drawn as an outcome from the comparison of the results with each other and with experimental data available in the literature. Last, in Chapter 4 the conclusions of this study are summarised, and some plans for the future are mentioned as well. I was involved in one of the performed experiment, in April My contribution to this sequence of measurements was to develop the analysis routine, deduce the isomeric yield ratios and compare the obtained results. 1

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15 1. Introduction 1.1 A (brief) introduction to fission Fission was first observed experimentally in January 1939 by Otto Hahn and Fritz Strassmann [1]. The reaction in which fission was studied was neutroninduced fission of Uranium. In the process a heavy nucleus decays into two fragments of comparable mass. It was first discussed quantitatively by Lise Meitner and Otto Frisch [2], who discussed the process in terms of a liquid drop, which becomes deformed and which beyond a critical deformation is breaking apart into two pieces, the fission fragments. The same year, N. Bohr and J. A. Wheeler in their prominent work gave an extended theoretical description of fission [3]. The mechanism of fission is a very complicated process and we are still far from a detailed understanding. That s why although fission induced by neutrons in the actinides is perhaps the most studied reaction of nuclear physics, a complete picture of the process is still lacking. From the perspective of the liquid drop model (LDM) description, a neutron that enters into the actinide target adds excitation energy to the compound nucleus, resulting in shape deformations and change in the potential energy, which increases above the level of the ground state. In the landscape of deformation, there is a critical deformation of no return, which is called the saddle point, as can be seen in Fig Afterwards, with further deformation, a situation is reached where the neck joining the two nascent fragments is no longer stable but is breaking apart. The snapping of the neck is called scission. Thereafter, the fragments 1 are created in general with unequal masses. The available energy in fission is approximately 200 MeV and is shared between the total kinetic energy (TKE) and total excitation energy (TXE) of the fragments. In order to de-excite, the fragments will emit neutrons (secondary prompt neutrons) and the process is completed with γ-rays emission. The binding energy (B) of a nucleus can be described by the semi-empirical formula of Bethe-Weizsäcker, based on the liquid drop model (LDM) [4]: B = α ν A α s A 2/3 α c Z(Z 1)A 1/3 α sym (A 2Z) 2 A 1 + δ (1.1) the first term stands for the volume term, which increases with increasing number of nucleons. It is referred to as the volume energy (E ν ) and it is the most important factor of the formula, especially for the lighter nuclei. 1 The formation of two fragments (binary fission) is much more likely than the formation of three (ternary fission), which occurs with a low probability of only a few events per 1000 fissions. 3

16 n Saddle Scission Fragment I secondary neutrons Fragment II Fig Visualisation of the nucleus deformation in terms of a liquid drop. 4 the second term gives the surface energy (E s ), which describes the fact that nucleons on the surface have less neighbours to interact with, resulting in a decrease in the binding energy. the third term, the Coulomb term (E c ), expresses the repulsive interactions between the protons in the nucleus due to the Coulomb force, and it contributes to the split of the nucleus. In cases, like fission, where the nucleus deforms considerably, this might become an important term because the Coulomb repulsion might overcome the short range of the strong nuclear force. the fourth term, which is called the symmetry energy (E sym ), describes the tendency of the nucleus to become symmetric in protons and neutrons in order to be stable. This term is more important for light nuclei, since for the heavier ones the increase in Coulomb repulsion requires additional neutrons for nuclear stability. the last term, which is called the pairing term (E p ), expresses the tendency of nucleons to couple pairwise in order to form stable configurations. Specifically this term does not contribute when A = odd (odd Z and even N and vice versa), δ = 0. However the relatively high stability of even-even nuclei is taken into account by a positive contribution (+δ) to the total binding energy E B of the nucleus, and the relatively low stability of odd-odd nuclei by a negative contribution ( δ).

17 The surface and the Coulomb energy terms in Eq. 1.1 are the only ones which are affected when a liquid drop becomes deformed. Specifically, the surface energy term is smaller for a sphere, but any deformation away from this shape is associated with larger potential energy from this effect. The Coulomb energy term is decreased with a deformed nucleus, because of the larger average separation between the charge elements. A radius vector R(θ) of the nucleus can describe the small axially symmetric deformation: R(θ) = R 0 [1 + α 2 P 2 cos(θ)] (1.2) where θ is the angle of the radius vector, α 2 is a coefficient that describes the amount of the deformation, R 0 is the radius of the non deformed nucleus and P 2 is the second order Legendre polynomial [5]. The surface (E s ) and the Coulomb (E c ) energies for small deformations, were calculated by Bohr and Wheeler: E s = E 0 s ( a2 2) and E c = E 0 c (1 1 5 a2 2) (1.3) where E 0 s and E 0 c are the surface and Coulomb energies respectively without any deformation. In order for a spherical nucleus to be stable against deformation the decrease in Coulomb energy E c =- 1 5 α2 2 E0 c must be smaller than the increase in the surface energy E s = 2 5 α2 2 E0 s. The drop will become unstable when the absolute values of the aforementioned terms equal each other, when E 0 c=2e 0 s. Thus, as introduced by Bohr and Wheeler, the notion of the fissility x can be defined: x = E0 c 2E 0 s (1.4) For x less than unity the nuclei are stable, while for x bigger than unity there will be no potential energy barrier to inhibit spontaneous fission of the drop. In order to describe the large deformations that are encountered on the top of the fission barrier, higher order polynomials must be included in Eq. 1.2: R(θ) = (R 0 /λ)[1 + α n P n cos(θ)] (1.5) n=1 where the parameter λ ensures that the volume remains constant. The potential energy of the deformation is increasing as expected for small deformation, as can be seen in Fig There is though, a crucial point which is reached for specific values of the deformation parameters. This point is called saddle, and a fissioning nucleus moving along the path of deformation has to overcome the potential barrier at this point, the fission barrier B f. The height of this fission barrier for the actinides is about 6 MeV above the ground state. Beyond this, a path of minimum energy slopes downwards until the nucleus is breaking apart at scission [6]. 5

18 Energy E * E n saddle point S n Bf scission point Deformation Fig The potential energy as a function of the deformation of a nucleus. The fission barrier (B f ), as classically approached by the LDM model with a single humped barrier, is also noted. On the left side the neutron separation energy (S n ) and the additional excitation energy, added to the compound nucleus from the extra neutron, appear as well. On the top of the figure, the deformation of the nucleus from spherical shape to scission point is illustrated. The addition of a neutron to the target nucleus contributes to the binding energy of the formed compound nucleus, so that the latter might excite above the fission barrier, as illustrated in Fig The amount of excitation energy which needs to be added to the system in order to overcome the fission barrier is strongly case dependent. Nuclei with even mass number A, such as 238 U, exhibit a higher fission barrier than the energy required to separate a neutron (neutron separation energy), S n, so more energy from the incident particle is required in order to fission. This happens because the extra neutron will not be paired to the nucleus, as all nucleons are already in pairs, so extra energy is needed for the compound nucleus 239 U to undergo fission. For 238 U, the neutron separation energy and the fission barrier of the formed compound nucleus 239 U are S n = 4.8 MeV and B f = 6.3 MeV respectively. On the other hand, nuclei with odd mass, such as 235 U which is very well studied for its use in nuclear reactors, exhibit a neutron separation energy higher than their fission barrier. That means that the extra neutron will readily pair with another nucleon, so that the excitation energy will increase above the fission barrier. 6

19 For 235 U, the neutron separation energy and the fission barrier of the formed compound nucleus 236 U are S n = 6.5 MeV and B f = 5.6 MeV respectively. The simple liquid drop model, although it describes the general properties of the fission process relatively well, has certain limitations when it is called to explain experimental results. For example, it fails to explain the two distinct modes of symmetric and asymmetric mass division in fission, which was suggested from experiments. Maria G. Mayer, in her work in 1948, traced the asymmetric fission back to nuclear shell effects, in particular to the stabilising influence of the 50 proton shell and the 82 neutron shell that coincide in the spherical doubly magic 132 Sn [7]. Moreover, it s not possible to predict the isomeric fission with the single fission barrier of the LDM description. Shell effects "corrections" were introduced to the liquid drop model by Strutisky [8], thus all the processes that could not be explained by the single-humped model could be attributed to shell effects. Energy Mass Distribution Normal Fission EA EB II Isomeric Fission I Spontaneous Fission εi εii Deformation Fig Illustration of the double-humped fission barrier as introduced by shell corrections. Humps at A and B result in minima in potential energy at deformation of ε I and ε II. States in these wells are designated class I and class II, respectively. In Fig. 1.3, the "shell-corrected" description of the fission process is visualised, where it can be noticed that the fission barrier becomes double-humped, instead of single. On the top of the same figure the different stages of the shape deformation of the nucleus along the fission path are visualised. As can be seen, the shape of the compound nucleus is already deformed in the ground 7

20 state, which lies in the first minimum and it is designated as ε I in Fig The second minimum, designated as ε II, can explain the isomeric fission. The shape of the nucleus in this second barrier well is elongated due to repulsion of the number of protons. It has a higher energy than the ground state, so it is metastable. In this case, the isomerism is observed due to the difference in the shape of the nucleus, and not to be confused with the spin isomers, produced due to the differences in the spins of the states, and measured in the present work. The nucleus is trapped into retaining its elongated shape since its energy is not sufficient to surmount either barrier. Although the decay of the nucleus is not possible with the classical approach, since its energy is below the fission barrier, it still may occur by tunnelling through the barrier according to quantum mechanics. Actually, this can happen both ways. It can tunnel back to its more spherical ground state by emitting a γ-ray, as an ordinary nuclear isomer, leaving its identity isotopically unchanged. It can also tunnel to the other direction, by producing two separate fragments by fission. Spontaneous fission may occur when the nucleus is on the ground state and tunnel through the whole fission barrier, without needing any additional excitation energy from a particle. Between the two barrier wells, discrete excited states exist (referred to as class-i and class-ii), which become less resolved the higher excitation reached, until as always they enter the continuum level densities region, where they are unable to be resolved. The excited states on the saddle points are referred to as "transition states" and they have their characteristic spin and parity. Time scale of the de-excitation of the fission process A mention explicitly must be done here on the distinction between fission fragments and products, since this is closely related to the time evolution of the fission process itself. The fission fragments are highly excited so, in order to cool down they emit neutrons and γ-rays. The time scale involved in the fission process is shown in Fig The evaporation times for neutrons are much shorter than the emission time of γ-rays, so they first de-excite by neutron emission. After prompt neutron emission the "primary" fragments are called "secondary" or just "products" and their remaining excitation energy is below the neutron separation energy. The only way for the nucleus to de-excite more thus, is by emitting γ-rays. The transition from neutron to gamma emission happens on a time scale of s, while the emission of prompt γ-rays may last for several ms. After this de-excitation process, fission products reach their ground states, but they are still too neutron-rich and hence unstable and liable to β -decay. This decay may last from several ms up to years. The radioactivity of fission products is part of the activity of fuel remnants from nuclear power stations. It is worth mentioning that once the saddle has been passed the fission process is very fast, while it takes comparatively long time to evaporate a neutron. 8

21 Fission Fragments Fission Products Primary Fragments Secondary Fragments Time scale (s): t saddle to scission prompt neutrons prompt γ-rays β-decay: β-particles, delayed neutrons, γ-rays and fission Light charge particles Scission neutrons Fig Time scale in the de-excitation of fission fragment Definition of fission yields Measurements of the fission observables such as the mass yield distributions of the fission products can provide important information about the fission process itself, either for fundamental or applied physics. In Fig. 1.5 part of the chart of nuclides is illustrated, where the possible decay paths of each nuclide can be seen. The β -decay is denoted with the black arrows and the neutron emission with the red arrows. Based on this figure and the time scale presented in the previous paragraph the fission products are categorised as follows [9]: 1. Independent fission yields: a measure of the number of atoms of a specific nuclide produced directly in the fission process before any radioactive decay. 2. Cumulative fission yields: describes the total number of atoms of a specific nuclide produced directly in fission and after the decay of all of its precursors. 3. Total chain yields: expresses the cumulative yield of the last, either stable or long-lived member of an isobaric chain. 4. Mass number yields: is the sum of all independent yields of a particular isobaric chain. The difference between the total chain yield and the mass number yield emerges from the contribution of β-delayed neutrons. The former contains the produced nuclides after the β-delayed neutrons, while the latter does not. If there is no emission of delayed neutrons, the two yields concur. 1.2 Isomers In Fig. 1.6 the de-excitation process of the fission fragments is illustrated. The fragments produced in fission can be characterised by their distribution of excitation energy and initial angular momentum. As explained earlier, the highly excited fragments first de-excite by emitting prompt neutrons and then prompt γ-rays. As long as the excitation of the fragments is high the neutron emission prevails, until the excitation energy is reduced below the neutron emission threshold. Afterwards, the emission of γ-rays prevails, at the beginning with statistical E1 emission. The change in the angular momentum due to the emis- 9

22 Proton Number Z 130 Te 131 Te 132 Te β Sb 130 Sb 131 Sb 132 Sb n 128 Sn 129 Sn 130 Sn 131 Sn 132 Sn 128 In 129 In 130 In 131 In 132 In 128 Cd 129 Cd 130 Cd 131 Cd 132 Cd Neutron Number N Fig In figure the decay paths of part of the chart of nuclides is illustrated. The black arrows represent β-decay and the red arrows represent the delayed neutron emission. sion of the prompt neutrons and the statistical γ-rays is small. In the region close to the yrast line, non-statistical (mostly E2) photons carry away the remaining angular momentum. Most of the isomeric states are formed in this region. Isomeric states are metastable states that occur when the angular momentum difference between two states is large. Under these circumstances, the electromagnetic transition probabilities from these states are reduced, because of their high multipole order, resulting in an unusually long lifetime, compared to other excited states. In the shell model picture, these states are formed because major shells are occupied by particles of high angular momentum, while their close in energy sub-shells are occupied by particles of low-angular momentum. Specifically the major shell closures occur at Z, N = 50, 82 and 126 particles on the levels 1g 9/2, 1h 11/2, and 1i 13/2 respectively, while the adjacent lower sub-shells are occupied by 2p 1/2, 2d 3/2 and 3p 1/2 [10]. 1.3 The importance of isomeric yields Isomeric states encompass a wide range of lifetimes due to several reasons: their state transitions occur in various multipole type and order (E3, M3, E4, M4,...), while their decay modes compete between internal conversion and 10

23 E * Primary Fission Fragments E * n n Secondary Fission Fragments E (Yrast) Sn γ γ γ γ γ } Discrete levels Statistical γ J Fig Illustration of the de-excitation of the fission fragments first by prompt neutrons emission and then by statistical (E1) γ-rays. Afterwards, the emission of non-statistical photons takes place to the region close to the yrast line. β-decay. Similarly, the ground states to which the metastable states will decay, unless a β-decay occur either from these states or from an intermediate lower lying level along their decay path, exhibit a similar span of lifetimes, from milliseconds to stability. Thereafter, it becomes evident that a description of the time evolution of the excited states must take into account this variation in half-lives where the population of the isomeric states cannot be ignored. In Fig. 1.7 an example of the decay path of mass A=115 is given in order to visualise how the presence of the metastable states can complicate the decay path and branching ratios of an isobaric chain. As can be noticed, 115 Rh decays by β-particle emission to 115 Pd. In 115 Pd, the isomeric state at 89.2 kev can decay either by the dominant in this case β-particle emission (probability of 92%) to 115 Ag or to the ground state by internal conversion with a smallest probability of 8%. In addition, the isomeric state in 115 Ag complicates further the situation since its de-excitation competes between β-decay (79%) and internal conversion to the ground state (21%). As mentioned in the previous section, the fission products are either stable or unstable to β-decay and/or to delayed neutron emission. Unstable products can decay to either nuclides where no isomeric states exist, or to species occupying their (stable or not) ground state or their metastable isomeric state. The time development of the energy release in the latter case depends crucially on the initial relative populations (branching fractions) between isomeric and ground states. For example, in the case of thermal-neutron fission of actinide nuclei, such as 233,235 U or 239,241 Pu, roughly 800 primary products are formed. Approximately 700 of these products are unstable and about 150 have known 11

24 7/ Rh 0.99 s 0 kev β - : 100% 11/2-50 s 89.2 kev IT: 8% 5/ s 0 kev β - : 92% 115 Pd β - : 100% 7/ s 41.2 kev IT: 21% 1/ m 115 Ag 0 kev β - : 79% β - : 100% Fig Decay scheme of part of the mass chain A=115. The complications that arise in the decay path due to the different decay modes of the metastable states can be seen. The ground states are illustrated with the thick lines, while the isomeric states with the thinner ones. isomeric states with half-lives τ 0.1 s [11]. The importance of isomeric states in calculations of fission products decay energy release, such as the decay heat calculations, is thus clear. In addition, the β-delayed neutron emission probability from the isomeric state can be up to an order of magnitude different from that of the ground state (e.g. 0.33% for 98 Y, 3.5% 98m Y according to NuDat2 [12]). Therefore, a proper simulation of the effect of delayed neutrons in the nuclear reactors requires accurate knowledge of the population of isomeric states in fission. Moreover, the knowledge of the population of the isomeric states is important in yield measurements of fission products. In such studies, close-lying isomeric states to the ground state of a nuclide might create peak multiplets that are difficult to resolve. Thus, the yields of isomeric and ground states are often summed together. In order to apply corrections for the population of the metastables states knowledge of their intensity relative to the ground state intensity is needed. It is worth mentioning that the shorter the isomer s half-life is compared to the ground state s lifetime, the more significant the correction is. So far the importance of isomeric states to applications has been described. Nevertheless the isomeric yield ratios are important for simulation of processes such as the astrophysical r-process. The r-process is believed to be 12

25 terminated by the fission of very neutron rich heavy nuclei, while the fission fragments return to the r-process path. Furthermore, the neutron capture of the high spin isomeric state can be very different compared to the one of the low spin ground state. Hence, these simulations need as accurate knowledge as possible of the population of isomeric states. The fissioning systems that terminate the r-process are more neutron rich than any of those that can currently be reached in the experimental frame, so their yields are estimated based on theoretical calculations. In order to test the ability of the theories to reproduce the isomeric yield ratios of the fissioning system, they have to be determined experimentally. Besides all the aforementioned reasons for which isomeric yields are important themselves, they can also be used in fundamental physics, in the effort for a better understanding of the fission process. The angular momentum of the fission fragments can provide better information on the scission configuration. One of the possible means to deduce the angular momentum of the fragments is the independent isomeric yield ratio of fission products, which can be used to study the collective rotational degrees of the fissioning system at the scission configuration. In [13], and the references therein, more information can be found on the deduction of the root mean square angular momentum of the primary fragment (J rms ), while in [14] information on the deduction of the properties of scission configuration can be found. In the first efforts to deduce the J rms, time consuming radiochemical separation techniques were used, resulting in limitation on the isomeric pairs, because these had to be located close to the valley of stability and shielded by stable or long-lived isotopes from production via the beta decay of more neutron-rich isotopes. In more recent works, [15] [16], direct γ-ray counting was employed and the deduced production via β-decay is simply subtracted from the total yield. 1.4 Fission yields measurements techniques The measurement of fission yields is a complicated process because the fission fragments are not formed in a single way. Therefore several different techniques have been developed over the years aiming at measuring the cumulative or the independent yield, each one with its own advantages and drawbacks. They can mainly be distinguished in two main categories: measurements of stopped fragments. measurements of unstopped fragments. The oldest technique in the first category is radiochemical separation of the longest lived isotopes of fission products. Then, the activity could be determined via β- or γ-spectroscopy. At the beginning, the method was time consuming, so it was able to measure only long-lived isotopes. After some development, short-lived isotopes with a life-time of the order of seconds or less could be identified as well, and even isomers in some cases. 13

26 Fission products can also be measured by means of mass spectroscopy, making use of radioactive beams. Specifically with the Isotope Separator On-Line (ISOL) technique, a thick target is irradiated, and the created products are introduced to an ion source, where they are ionised. Afterwards the ions are mass separated by means of magnetic separation, resulting in pure ions beams, which can be detected by ion counters. The fission products can also be detected by means of γ-spectroscopy, since the β-decay of the products is followed by γ-ray emission. If products are adequately long-lived so that their decay occurs after the mass separation, the unique γ-ray spectrum of each isobaric chain can be employed. The drawback of this method is that it is slow since a thick target is used in order to create a sufficient yield of a product. Moreover the universal use of this method is hindered by the limitations that arise from the use of the ion source. Although in fission a large variety of nuclides is produced, there is not an ion source that can be used for all of them since the chemical selectivity of the ion source is governed from the different ionisation properties of the elements. In order to measure independent fission yields a variant of this technique has been developed. Specifically with the Ion Guide Isotope Separation On- Line (IGISOL) technique, a thin target is connected directly in the ion source so that the method is fast and ions of all chemical elements can be produced. However, one of the biggest issues of this method is its limited mass resolving power, resulting in longer irradiation time in order to overcome this problem. In addition the decay scheme of the most exotic nuclei is not well known in some cases. Direct measurement of γ-rays can be performed as well in case of very exotic targets, since with this method a very small amount of the sample is required. On the other hand, the data analysis is complicated and accurate knowledge of the decay scheme is necessary. Moreover in the case of independent yield measurement, the decrease in the irradiation time, results in reduced statistics and consequently in larger uncertainties. The methods described above regard measurements of stopped fission products. Measurements of fission products without stopping them can be achieved as well. In these techniques, by measuring the kinetic energy and velocity of one fission fragment, the (E,υ) method, or of both fission fragments, the (2E, 2υ) method, and based on the conservation of momentum in fission, the masses of the fission fragments are calculated. A low-energy and light particleinduced fission is one of the requirements of the method, so that the momentum of the inducing particle can be ignored. Another requirement is a thin target, in order to minimise as much as possible the energy loss of the fission fragments in the target. For the velocity measurement the time of flight technique is applied, while for the energy measurement surface barrier detectors may be used. Recoil spectrometers, such as Lohengrin at the Institut Laue-Langevin (ILL) in Grenoble [17], use the (E,υ) technique for studying unstopped fission frag- 14

27 ments. These spectrometers are coupled to the intense neutron flux of a reactor, and they achieve a mass separation by electromagnetic separation based on the energy-charge state ratio (E/q). Specifically the spectrometer at ILL, performs an additional separation based on the mass-charge state ratio (A/q). Therefore, one of the disadvantages of this method is that in order for an independent yield distribution to be observed, the measurements have to be repeated for several kinetic energies and charge states. Spectrometers, such as the Cosi-fan-tutte spectrometer at ILL, that make use on the (2E,2υ) technique, by measuring the time of flight and energy of each fission product, also exist, in this way avoiding the need of electromagnets. More spectrometers have been constructed recently, like VERDI (VElocity for Direct particle Identification) [18] constructed for the Joint Research Centre IRMM, Geel, Belgium, or are planned to be installed in the near future, like the FALSTAFF (Four Arm clover for the STudy of Actinide Fission Fragments) spectrometer [19], to be installed in the Neutrons for Science (NFS) facility in SPIRAL2 [20], or the SPIDER spectrometer (SPectrometer for Ion DEtermination in fission Research), located at the Los Alamos National Laboratory [21]. For identifying the nuclear charge the ( E, E) technique is employed. The energy loss (de/dx) of an ion passing through a material of known thickness, due to interactions of the ions with the shell electrons of the matter, depends on the kinetic energy and charge Z of the ion. By measuring the energy loss E and the kinetic energy E, the nuclear charge Z of the nuclides can be deduced. Another method for measuring fission yields, relatively recent, is based on the inverse kinematics. The nuclide of interest is used as a projectile, accelerated to relativistic energies, and impinging on a stationary target. Because of its interaction with the target, the projectile excites, resulting in fission. The fission fragments are then separated in fragment separation, as in the case of the FRS (FRagment Separator) at GSI [22]. Studies of fission of almost any nuclide in the region above lead is possible with this method, which is one of its strongest advantages. The list of the different experimental techniques described in this section is far from complete. New techniques with improved instruments are added to the list all the time, aiming to a better understanding of the fission proocess. The IGISOL technique A new method in order to improve the fission yield measurements in terms of speed and simplicity, has been developed at the University of Jyväskylä. This method couples the chemical non-selectivity of the ion guide, in the sense that ions of all chemical elements can be produced, with the superior mass resolving power of the Penning Trap located at the University of Jyväskylä (JYFLTRAP), which allows identification of ions based only on their mass. Thus it is possible to measure isotopic yield distributions for a wide range of fission products, based simply on their masses and by means of ion counting, 15

28 a clear distinction from the aforementioned techniques. It is worth mentioning that a high precision in the nuclide mass determination is not considered necessary, since these are used only for identification in the mass spectra. The method cannot be used merely for deducing independent fission yields distributions because of the chemical reactions introduced by the ion guide and the JYFLTRAP facility. However, if one independent yield of the isotopic yield distribution is known, the isotopic distribution can be directly converted to the fission cross-sections for the other members of the isotopic chain. This is happening because the rate of production of a particular ion in the secondary beam is directly proportional to its production cross section, since there is no significant accumulation and re-ionisation of the decay products, unlikely to the classical ion sources [23], [24]. One of the advantages of this technique is that even nuclides with very low yields can be detected thanks to the very low background. Hence, measurements of fission products can be performed, at the regions of the higher uncertainties, such as the tails or the valleys in the fission yield distribution of n-induced fission on 235 U [25]. On the other hand determination of more than one fission observables is not possible with this technique, since the production of the fission fragments is integrated over several milliseconds before the yield is registered, and not on an event-by-event basis [26]. The experimental setup and procedure is described in detail in chapter 2. 16

29 2. Experimental Facility 2.1 The IGISOL technique combined with JYFLTRAP In June 2010, the Ion Guide Isotope Separator On-Line (IGISOL) facility at the Accelerator Laboratory of the University of Jyväskylä closed down for a major upgrade, in order to be re-commissioned as IGISOL-4 in a new experimental hall. A new 30 MeV cyclotron (MCC30/15) was housed, which has the possibility to accelerate protons (18-30 MeV) and deuterons (9-15 MeV), and is equipped with two beam lines thus offering access to a possible extraction of two beams simultaneously. The maximum intensities that have been measured inside the cyclotron are for protons 200 µa and 140 µa for 18 and 30 MeV respectively, and for deuterons 62 µa for 15 MeV, exceeding design specifications. However, it still has to be demonstrated what can be delivered and indeed handled on target [27]. IGISOL has a long tradition of experiments on both neutron-rich fission products and neutron-deficient nuclei, produced in light and heavy ion fusion reactions. Measurements of ground state properties, such as charge radii and masses, and decay spectroscopy were covered in these experiments [28]. In Fig. 2.1, a schematic overview of the facility is presented. The elements of the facility that are described in the text are denoted with numbers. In the case of fission related experiments, the charged particle accelerated beam (protons or deuterons), denoted with the red arrow in Fig. 2.1, impinges on a fissile target which is placed inside the fission chamber in order for the fission reaction to occur. Neutron-induced fission is planned to be realised as well, where the accelerated beam will bombard a Be target, placed in the reaction chamber, in order to produce the neutron flux which will induce fission [29],[26]. The thin target (14 mg/cm 2 and 15 mg/cm 2 for nat Th and nat U respectively) is one of the key features of the IGISOL technique, as a significant fraction of the products have enough recoil energy to pass the target and not stop within it. Helium gas is flowing into the ion guide in order to slow down the fission products. In addition due to the high ionisation potential of the buffer gas the charge of the highly charged ions is reduced to the most probable +1 state. The He gas flow and the ion guide are denoted with the green arrow and 1 in Fig. 2.1 respectively. Afterwards, the ions are transported out of the ion guide with the help of the gas flow, and then accelerated with a voltage of 30 kv and guided to the mass separator through a radio-frequency SextuPole Ion Guide (SPIG), indicated with 2 in Fig. 2.1, and electrostatic elements. The motivation for using 17

30 a multipole ion guide was to reduce the energy spread, and a higher order multipole than the common quadrupole is preferred as it can deliver higher current beam before becoming unstable (of the order of ions s 1 ). More information about the SPIG can be found in [30] and the references therein. The differential pumping system is another key feature of the technique, as it allows efficient removal of the high gas load from the target chamber, while at the same time keeping a sufficiently high vacuum along the beam line. In the dipole magnet, denoted with 3 in Fig. 2.1, the first mass selection of the produced fragments takes place based on their charge to mass ratio (q/m). The mass resolving power of the magnet is m/ m 500. The desired beam is selected by slits located at the focal plane of the dipole and through the electrostatic elements in the beam switchyard it is transferred either to the β- γ spectroscopy station or to the RadioFreQuency (RFQ) cooler and buncher, denoted with 7 and 4 in Fig. 2.1 respectively. From the RFQ cooler and buncher, the isobarically purified beam can be distributed either to the Penning Trap or the laser spectroscopy set up. In the RFQ, the preparation of the beam which will eventually enter into the Penning Trap starts. The continuous ion beam is accumulated over a period of time of several ms, and cooled with the help of helium buffer gas. The cooled ions will enter the Penning Trap in bunches with an energy spread reduced to a few ev so that a better precision can be realised in the measurement. In Fig. 2.1, the Penning Trap is denoted with 5, while the laser spectroscopy beam line is not shown since it was not used in the present work. Inside the traps, a sequence of dipole and quadrupole excitations, as will be explained in the next section, achieve a selection of the nuclides based on their charge over mass ratio (q/m) with a resolving power up to , which is enough to resolve the elements of an isobaric chain, and sometimes even isomeric states. After the extraction from the Penning Trap, the ions are counted by a MultiChannel Plate (MCP) detector which is located at the end of the beam line, denoted with 6 in Fig Description of experimental elements In this section, the most important components along the beam line are presented. More information can be found in a series of studies related to the IGISOL and JYFLTRAP facility ( [31], [32], [33], [34]) The fission ion guide One of the most important elements of the IGISOL method is the fission ion guide and the thin target which is placed within, as has been mentioned earlier. In Fig. 2.2 a schematic view of the fission ion guide, as seen from the top, is presented. The cyclotron beam, olive green arrow in the figure, irradiates the 18

31 Fig Schematic overview of the IGISOL and JYFLTRAP facility, adapted from [26]. tilted fissile target, depicted in red colour. The thickness of the target is a crucial parameter for fission yield measurements, since a too thick target hinders the products of escaping the target, while a too thin one allows the products to leave the target with too high energy, thus decreasing the probability of the fragments to be stopped by the buffer gas. By placing the target in a tilted position, it is possible to use a thin target, while its effective thickness is increased by a factor of almost ten. One of the key features of the method is the He buffer gas, which flows into the fission guide, aiming to slow down the products and cooling the target at the same time. The energy of the highly charged fission products is decreased due to a sequence of collisions with the buffer gas atoms, while their charge states are reduced via charge exchange reactions. Because of the high ionisation potential of the buffer gas a considerable fraction of the ions end up at a +1 charge state. The fission products enter the stopping chamber, which is separated from the small target volume by a thin Ni foil (0.9 mg/cm 2 ) in order to prevent plasma effects caused by the primary beam. It is possible to use such a window as the angular distribution of the fission fragments is almost isotropic and the stopping effect of the foil on the fission fragments is negligible. The He gas flow, typically at a pressure of 200 torr ( 267 mbar), guides the fission products to the mass separator, through a 1.2 mm aperture in the exit nozzle, resulting in an evacuation time of a few tens of ms. The design of the stopping chamber is an important parameter of the measurements since it can affect both the extraction time and the stopping efficiencies. Since the time needed to evacuate the gas chamber is typically of the order of a few tens milliseconds, it is evident that the design of the fission guide generates a constrain in the ability of the method to measure properties 19