Decay spectroscopy of the very neutron-deficient lead and thallium isotopes 178,179Pb and ¹⁷⁹Tl

This thesis reports on the production of the very neutron-deficient isotopes 178,179Pb and 179Tl, using heavy-ion fusion-evaporation reactions. The gas-filled separator RITU and the GREAT spectrometer at its focal plane position were used for the decay spectroscopy study. The α decay of 178Pb and its α-decay chain were studied through α-α correlations. The α-particle energy and half-life were measured for 178Pb as, Eα= 7610(30) keV and t1/2 = 0.21−+0.21 0.08 ms, respectively. The half-life is consistent with recent theoretical calculations using the Coulomb and Proximity Potential Model (CPPM). The hindrance factor for 178Pb was deduced and corresponds to an unhindered Δl = 0 transition. The α-decay reduced width was deduced as well and put into a systematic context advancing the systematics of the even-A Pb isotopes to further neutron deficiency. The decay properties of 179Pb were studied through α-α and α-γ correlations, which has allowed the ground-state of 179Pb to be assigned as Iπ = 9/2−. In comparison with the literature, a more precise α-particle energy and half-life were measured for the ground state of 179Pb to be, Eα = 7348(5) keV and t1/2 = 2.7(2) ms, respectively. A search for a νi13/2 state in 179Pb was performed, but only a limit of excitation energy and half-life was obtained. In addition, improved α-decay data were also measured for 179Tl. Evidence for an isomeric state at an excitation energy of 904.5(9) keV was identified for the first time in 179Tl, with a half-life of t1/2 = 114−+18 10 ns and is tentatively assigned to be a proton (9/2−) intruder state

A position sensitive β - γ coincidence technique for sample analysis with the upgraded PANDA device.

PANDA-laitteiston laajentaminen beeta-aktiivisten aineiden analyysiin Radioaktiivisten näytteiden analyysissa käytettävän laitteiston päivitys Radioaktiivisten näytteiden analyysiketjua voidaan parantaa ottamalla käyttöön uudenlaisia ilmaisimia ja tiedonkeruujärjestelmiä. Mittalaite, joka tunnetaan nimellä PANDA (Particles And Non-Destructive Analysis), kehitettiin aiemmin säteily- ja ydinturvallisuuteen ja ydinmateriaalivalvontaan (3S) liittyvien radioaktiivisten näytteiden analysointiin. Päätavoitteena oli parantaa alfahajoavien nuklidien näyteanalyysiketjua käyttämällä ainetta rikkomattomia tekniikoita. PANDA-laitteiston ominaisuuksia on nyt laajennettu mm. lisäämällä siihen uusi säteilyilmaisin, jonka myötä laitteistolla voidaan havaita myös beetahiukkasia. Laitteistossa on jo aiemmin ollut käytössä ilmaisimet alfa- sekä gammasäteilyn havaitsemiseen. Uusi ilmaisin mittaa näytteistä beetahiukkasia Tehtyjen parannusten myötä PANDA-laitteella on mahdollista mitata yhdestä näytteestä alfa- ja beetahiukkasia sekä gammasäteilyä samanaikaisesti. Mitattaessa useampaa eri säteilylajia yhtäaikaisesti, eli tehtäessä koinsidenssimittauksia, näytteestä saadaan enemmän tietoa verrattuna yksittäisiin eri menetelmillä tehtyihin mittauksiin. Tutkimuksessa tarkasteltiin myös PANDA-laitteen kykyä määrittää radioaktiivisen aineen jakautuminen näytteessä. Laitteella voidaan määrittää mm. onko kyseessä yksittäinen säteilevä hiukkanen vai ovatko radioaktiiviset aineet levinneet laajemmalle alueelle näytteeseen. Tämä määritys voidaan nyt tehdä sekä alfa- että beeta-aktiivisille aineille. Laitteiston uusia ominaisuuksia ja suorituskykyä testattiin mm. cesium- ja radiumlähteiden avulla. Uusia työkaluja radioaktiivisten näytteiden analyysiin STUKin laboratorioissa analysoidaan vuosittain tuhansia radioaktiivisia näytteitä käyttäen erilaisia spektrometrisia menetelmiä. Tarkoitus on mm. seurata säteilyn tasoa ympäristössä ja havaita kaikki merkittävät muutokset ympäristön säteily- ja radioaktiivisuustasoissa, joille ihmiset altistuvat. PANDA-laitteisto suunniteltiin, rakennettiin ja testattiin STUKissa yhteistyössä Jyväskylän yliopiston kiihdytinlaboratorion kanssa. Osana yhteistyötä PANDA-laitteisto on sijoitettu uudelleen STUKista Jyväskylän yliopistoon, jossa laitteiston kehitetystä on jatkettu.PANDA (Particles And Non-Destructive Analysis) is measuring system developed for non-destructive analysis of samples for safety, security and safeguards. The capabilities of the PANDA device were expanded by the addition of a thick silicon detector for particle detection. The upgraded device can now be used for position-sensitive and coincidence measurements of various kinds of radioactive samples. The capability of the PANDA device in using the coincidence technique was tested using a mixed source of 134Cs and 226Ra. In addition, the ability of PANDA to locate nuclides emitting particles from samples was tested using a combined sample containing a mixed 134Cs and 226Ra source and a pure 226Ra source placed a few centimeters apart from each other. The construction, commissioning and testing of the upgraded PANDA device is discussed

Confirmation of the new isotope 178Pb

The extremely neutron-deficient isotope 178Pb has been produced. The GREAT spectrometer at the focal plane position of the gas-filled separator RITU was used to study the α decay of 178Pb and its α-decay chain through α-α correlations. The α decay was measured to have an energy and half-life of Eα= 7610(30) keV and t1/2 = 0.21+0.21 −0.08 ms, respectively. The half-life is consistent with recent theoretical calculations using the Coulomb and proximity potential model. The α-decay reduced width and hindrance factor for 178Pb were deduced and correspond to an unhindered l = 0 transition. In addition, the mass excess of 178Pb and the α-decay Q value were calculated from the experimental results and compared to theoretical values.peerReviewe

Decay spectroscopy of 179 82 Pb 97 and evidence for a 9/2− intruder state in 179 81 Tl 98

The very neutron-deficient isobars 179Pb and 179Tl have been produced using the fusion-evaporation reactions 104Pd(78Kr,xpyn), where x≤1 and y≥2. The gas-filled separator RITU was employed to transport and separate the recoiling nuclei of interest from the scattered beam and unwanted products. The GREAT spectrometer was used to study the decay properties through α−α and α−γ correlations, which has allowed the ground state of 179Pb to be assigned as Iπ=9/2−. The decay of 179Pb was measured to have an α-particle energy and half-life of Eα=7348(5)keV and t1/2=2.7(2) ms, respectively. A search for a νi13/2 state in 179Pb was performed, but only a limit of excitation energy and half-life was obtained. In 179Tl a t1/2=114+18−10 ns isomeric state, likely at an excitation energy of 904.5(9) keV, was identified and is tentatively assigned to be a 9/2− proton intruder state.peerReviewe

Experimental study of isomeric intruder ½+ states 197,203At

A newly observed isomeric intruder ½ + state [ T ½ = 3.5 ( 6 ) ms ] is identified in 203 At using a gas-filled recoil separator and fusion-evaporation reactions. The isomer is depopulated through a cascade of E 3 and mixed M 1 / E 2 transitions to the 9 / 2 − ground state, and it is suggested to originate from the π ( s ½ ) − 1 configuration. In addition, the structures above the ½ + state in 203 At and 197 At are studied using in-beam γ -ray spectroscopy, recoil-decay tagging, and recoil-isomer decay tagging methods. The ½ + state is fed from 3 / 2 + and 5 / 2 + states, and the origin of these states are discussed.peerReviewe

Observation of the proton emitter 116 57 La59

The quantum tunneling and emission of a single constituent nucleon provide a beautifully simple and unique window into the complex properties of atomic nuclei at the extreme edge of nuclear existence. In particular, for odd-odd proton emitting nuclides, the associated decay energy and partial half-life can be used to probe the correlations between the valence neutrons and protons which have been theoretically predicted to favour a new type of nuclear superfluidity, isoscalar neutron-proton pairing, for which the experimental “smoking gun" remains elusive. In the present work, proton emission from the lanthanum isotope 11657La59, 23 neutrons away from the only stable isotope 13957La82, is reported. 116La nuclei were synthesised in the fusion-evaporation reaction 58Ni(64Zn, p5n)116La and identified via their proton radioactivity using the mass spectrometer MARA (Mass Analysing Recoil Apparatus) and the silicon detectors placed at its focal plane. Comparisons of the measured proton energy (Ep = 718 ± 9 keV) and half-life (T1/2 = 50 ± 22 ms) with values calculated using the Universal Decay Law approach indicate that the proton is emitted with an orbital angular momentum l = 2 and that its emission probability is enhanced relative to its closest, less exotic, odd-even lanthanum isotope (11757La60) while the proton-emission Q-value is lower. We propose this to be a possible signature for the presence of strong neutron-proton pair correlations in this exotic, neutron deficient system. The observations of γ decays from isomeric states in 116La and 117La are also reported.peerReviewe

Evolution from γ-soft to stable triaxiality in 136Nd as a prerequisite of chirality

The level structure of 136Nd has been investigated using the 100Mo(40Ar, 4n) reaction and the JUROGAM II+RITU+GREAT setup. The level scheme has been extended significantly. Many new bands have been identified both at low and high spin, among which are five nearly degenerate bands interpreted as chiral partners. Excitation energies, spins, and parities of the previously known bands are revised and firmly established, and some previously known bands have been revised. Configurations are assigned to the observed bands based on cranked Nilsson-Strutinsky calculations. The band structure of 136Nd is now clarified and the various types of single-particle and collective excitations are well understood.peerReviewe

Spectroscopy of 161Hf from low to high spin

Excited states in the neutron-deficient nucleus 161 72Hf89 have been populated using the 118Sn(48Ti,5n) 161Hf and 110Pd(56Fe,5n) 161Hf fusion-evaporation reactions at 240 and 270 MeV, respectively. The level scheme for 161Hf has been extended with the observation of new band structures and an I π = (13/2+) isomeric state with a half-life of 4.8(2) μs has been identified. The decay path from this isomer to the (7/2−) ground state is established. The yrast band, based on the (13/2+) isomeric state, is extended up to (73/2+) and side band structures are identified up to (69/2−) and (55/2−). Quasiparticle assignments for these rotational structures are made on the basis of their alignment properties and Woods-Saxon cranked shell-model calculations.peerReviewe

Reassigning the shapes of the 0(+) states in the Pb-186 nucleus

Across the physics disciplines, the 186Pb nucleus is the only known system, where the two first excited states, together with the ground state, form a triplet of zero-spin states assigned with prolate, oblate and spherical shapes. Here we report on a precision measurement where the properties of collective transitions in 186Pb were determined in a simultaneous in-beam γ-ray and electron spectroscopy experiment employing the recoil-decay tagging technique. The feeding of the 02+ state and the interband 22+→21+ transition have been observed. We also present direct measurement of the energies of the electric monopole transitions from the excited 0+ states to the 0+ ground state. In contrast to the earlier understanding, the obtained reduced transition probability B(E2;21+→02+) value of 190(80) W.u., the transitional quadrupole moment ∣Qt(21+→02+)∣=7.7(33) eb and intensity balance arguments provide evidence to reassign the 02+ and 03+ states with predominantly prolate and oblate shape, respectively. Our work demonstrates a step-up in experimental sensitivity and paves the way for systematic studies of electric monopole transitions in this region. These electric monopole transitions probe the nuclear volume in a unique manner and provide unexploited input for development of the next-generation energy density functional models

Evidence of chiral bands in even-even nuclei

Evidence for chiral doublet bands has been observed for the first time in the even-even nucleus 136 Nd . One chiral band was firmly established. Four other candidates for chiral bands were also identified, which can contribute to the realization of the multiple pairs of chiral doublet bands ( M χ D ) phenomenon. The observed bands are investigated by the constrained and tilted axis cranking covariant density functional theory (TAC-CDFT). Possible configurations have been explored. The experimental energy spectra, angular momenta, and B ( M 1 ) / B ( E 2 ) values for the assigned configurations are globally reproduced by TAC-CDFT. Calculated results support the chiral interpretation of the observed bands, which correspond to shapes with maximum triaxiality induced by different multiquasiparticle configurations in 136 Nd .peerReviewe