Ultra fast timing study of exotic nuclei around ⁷⁸Ni: the β decay chain of ⁸¹Zn

207 Pages·2016·37.55 MB·English

by  Vadym Paziy

UNIVERSIDAD COMPLUTENSE DE MADRID
FACULTAD DE CIENCIAS FÍSICAS
DEPARTAMENTO DE FÍSICA ATÓMICA, MOLECULAR
Y NUCLEAR

TESIS DOCTORAL

Ultra fast timing study of exotic nuclei around ⁷⁸Ni:
the β decay chain of ⁸¹Zn

Estudio de coincidencias ultrarrápidas en núcleos
exóticos alrededor del ⁷⁸Ni: la cadena de desintegración
β del ⁸¹Zn

MEMORIA PARA OPTAR AL GRADO DE DOCTOR
PRESENTADA POR
Vadym Paziy

DIRECTOR

Luis Mario Fraile Prieto

Madrid, 2017
© Vadym Paziy, 2016
Facultad de Ciencias F´ısicas
Departamento de F´ısica Ato´mica, Molecular y Nuclear
Dissertation for the Degree of Doctor of Philosophy in Physics
ULTRA FAST TIMING STUDY OF

78
EXOTIC NUCLEI AROUND Ni:

81
THE β DECAY CHAIN OF Zn

´
ESTUDIO DE COINCIDENCIAS ULTRARRA PIDAS
EN NU ´CLEOS EXO ´ TICOS ALREDEDOR DEL 78Ni:
LA CADENA DE DESINTEGRACI O´ N β DEL 81Zn
Vadym Paziy

Supervisor

Dr. Luis Mario Fraile Prieto

April 2016

Посвящаю моим родителям
Людмиле и Александру, и моим научным руководителям
Луису и Генриху

Contents
Table of contents iii

Summary v

Resumen en castellano vii

Introduction 1

1 Theoretical Fundamentals 5

1.1 Beta decay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

1.1.1 β-delayed neutron emission . . . . . . . . . . . . . . . . . . . . . . 9

1.2 Gamma Decay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

1.2.1 Lifetimes of excited states . . . . . . . . . . . . . . . . . . . . . . . 12

1.2.2 Transition probabilities and Weisskopf estimates . . . . . . . . . . . 12

1.3 The Shell Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

1.4 The r-process path . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

2 Half-life measurements of excited states around 78Ni 23

2.1 Exotic nuclei in the vicinity of 78Ni . . . . . . . . . . . . . . . . . . . . . . 23

2.2 Motivation of the experiment . . . . . . . . . . . . . . . . . . . . . . . . . 28

2.3 Gamma-ray spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

2.4 Advanced Time Delayed βγγ(t) method . . . . . . . . . . . . . . . . . . . 33

2.4.1 The Convolution Technique . . . . . . . . . . . . . . . . . . . . . . 34

2.4.2 The Centroid Shift technique . . . . . . . . . . . . . . . . . . . . . 37

2.5 Summary of the chapter . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

3 Experiment and data analysis 41

3.1 The ISOL method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

3.2 ISOLDE facility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

3.2.1 The CERN accelerator complex . . . . . . . . . . . . . . . . . . . . 44

3.2.2 Target ion-source system and mass separators . . . . . . . . . . . . 47

3.3 Experimental setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

3.3.1 Detectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

3.3.2 Electronics and Data Acquisition System . . . . . . . . . . . . . . . 54

3.4 Data Analysis Technique . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

3.5 Calibrations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64

3.5.1 Energy Calibrations and stability . . . . . . . . . . . . . . . . . . . 65

3.5.2 Efficiency Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . 69
iv CONTENTS
3.5.3 Time calibrations . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72

3.6 Summary of the chapter . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78

4 Nuclear structure of 81Ga and 80Ga 81

4.1 Production of 81Zn . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83

4.2 Previous studies on the decay of 81Ga . . . . . . . . . . . . . . . . . . . . . 83

4.3 β decay of 81Zn to 81Ga . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84

4.3.1 Half-life of 81Zn and 81Ga . . . . . . . . . . . . . . . . . . . . . . . 86

4.3.2 Level scheme of 81Ga . . . . . . . . . . . . . . . . . . . . . . . . . . 87

4.3.3 Absolute β feeding . . . . . . . . . . . . . . . . . . . . . . . . . . . 92

4.4 Level scheme of 80Ga populated in the βn decay of 81Zn . . . . . . . . . . 97

4.5 Lifetime measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101

4.5.1 Half-lives of the excited states of 81Ga . . . . . . . . . . . . . . . . 101

4.6 Shell-model calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105

4.7 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107

4.7.1 Ground state feeding and ground-state spin-parity of 81Zn . . . . . 107

4.7.2 Low-lying structure of 81Ga . . . . . . . . . . . . . . . . . . . . . . 108

4.7.3 Positive parity states . . . . . . . . . . . . . . . . . . . . . . . . . . 110

4.8 Summary and conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . 112

5 Nuclear structure of 81Ge 113

5.1 Previous information about 81Ge and considerations on the systematics . . 113

5.2 Level scheme of 81Ge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115

5.2.1 Half-life of 81Ga . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125

5.3 Fast Timing measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . 126

5.4 Discussion of the results . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130

5.5 Summary of the chapter . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132

6 Nuclear structure of 81As 133

6.1 Previous studies of 81As . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133

6.2 Level scheme of 81As . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135

6.2.1 Decay scheme of the 9/2+ ground state of 81Ge . . . . . . . . . . . 140

6.2.2 Decay scheme of the 1/2+ isomeric state of 81Ge . . . . . . . . . . 146

6.2.3 Absolute and relative intensities . . . . . . . . . . . . . . . . . . . . 148

6.3 Lifetime measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155

6.4 Discussion of the results . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161

6.5 Conclusions of the chapter . . . . . . . . . . . . . . . . . . . . . . . . . . . 163

Conclusions and Outlook 165

Publications and conference presentations 171

List of figures 175

List of tables 181

Bibliography 183
Summary
The evolution of single-particle states in neutron-rich nuclei provides a key information
on their nuclear structure and is an important ingredient for the development of nuclear
models that can be applied to predict the structure at the borderline of nuclear map. The
role of neutron excitations across shell gaps and the evolution proton-neutron interaction
can be studied in these exotic nuclei. In particular, magic nuclei are key players for the
mapping of the single-particle degrees of freedom around closed cores. A special region
of interest is found around the doubly-magic 78Ni . In addition, gross properties of
28 50
these nuclei play a role in the astrophysical rapid neutron capture process. Nuclei in the
vicinity of 78Ni have motivated recent experimental and theoretical studies, aimed at the
understanding of the nuclear structure in this region with a large neutron excess.
In this thesis we investigate the nuclear structure of 81,80Ga, 81Ge and 81As, populated
in the β decay chain of 81Zn, which was produced at ISOLDE, CERN in the framework
of a systematic fast-timing investigation of neutron-rich nuclei populated in the decay
of Zn. The selectivity and efficiency of the production of Zn ion beams had been
previously optimized in order to guarantee the most pure beam of 77−82Zn nuclei. The
estimated yield of 81Zn was 600 ions/µC giving an average activity of β particles during
the experiment of about 10000 counts per second. The experimental setup included two
HPGe detectors, two LaBr (Ce) detectors and a NE111A plastic scintillator for β particle
3
detection. Coincidences with the β detector were used for γ-ray background suppression,
and γ-γ coincidences between the HPGe detectors to determine the level schemes. For
half-life measurements the combination of LaBr (Ce) scintillator crystals and Time-to-
3
Amplitude Converters was employed. The signals from the detectors were processed by a
digital data acquisition (DAQ) system composed by four Pixie-4 Digital Gamma Finder
cards, specially designed for γ-ray spectroscopy which was used for decay level schemes
and the Advanced Time Delayed (“fast-timing“) βγγ(t) method employed to measure the
excited level lifetimes. From the structural point of view, the isotopes under study are
relatively simple systems with a few particles and/or holes outside the doubly-magic core,
and thus can be treated rather successfully within the nuclear shell model.
The semi-magic 81 Ga is the most interesting case. It has only 3 valence protons
50 31
outside of 78Ni core with the lowest proton orbits being p and f . The M1 transition
3/2 5/2
between these states should be l-forbidden and therefore significantly slow (tens of
picoseconds). After data analysis we have significantly expanded the level scheme with 47
new levels and 70 transitions in the energy range up to 6.5 MeV. The 290(4)-ms half-life of
81Zn measured in this work is in good agreement with the previous studies [Pad10, Xu14].
The direct β feeding to the 81Ga ground state measured in our experiment is compatible
with zero, and much lower than proposed previously by Padgett et al. [Pad10], thus
compatible with both 5/2+ and 1/2+ assignments for the 81Zn ground state. We do not
vi Summary
observe β nor γ population to the 9/2+ state seen in other N = 50 isotones. The level
scheme of 80Ga populated in the β-delayed neutron emission from 81Zn was built for the
first time, containing 11 γ transitions that de-excite 9 energy levels. The P branching
n
was measured to be 23(4)%. For the first time we have measured the half-life of the first
excited state in 81Ga to be T = 60(10) ps, which indicates a l-forbidden M1 transition
1/2
of 351 keV between πp to the πf configuration. For the second excited state a half­
3/2 5/2
life of 23(16) ps was found. This allows to propose a π(f )3 cluster configuration built
5/2
on the 78Ni core, and a 3/2− spin-parity assignment for this state.
The level scheme of 81Ge populated from the β decay of 81Ga includes 15 new γ
transitions and 11 new energy levels, giving in total 111 transitions depopulating 47
energy levels. Fast-timing measurements have allowed to measure three precise half-lives
of the 711-, 896- and 1723-keV states in 80Ge, yielding 3.48(8) ns, 257(13) and 31(7)
ps respectively. Systematic considerations in the A = 81 region combined with the E1
multipolarity of 216-keV transition confirms the assignment of the pair of intruders of
1/2+ and 5/2+ at 679 and 711 keV respectively. Additionally, the low B(E2) value of
711-keV γ-ray determined with its half-life of 711-keV state indicates weak collectivity,
and thus confirms the single-particle configuration of 5/2+ intruder.
The energy levels of 81As were simultaneously populated in the β decay of both long­
lived 1/2+ isomer and the 9/2+ ground state of 81Ge. In total, 5 energy levels 12 not
previously observed γ transitions were placed in the decay scheme 1/2+ and 7 energy
levels with and 12 new γ transitions were assigned tot he decay scheme of 9/2+ ground
state of 81Ge. We do observe the 9/2+ state at 2625 keV, but no de-exciting transition to
5/2− levelwasdetectedandnoconclusioncanbemadeon πg → πf protonexcitation
9/2 5/2
energy investigation. For the first time, five half-lives of the low-lying 93-, 290-, 336-, 738­
and 1129-keV states in 81As were measured, yielding 113(9)-, 53(13)-, 170(5)-, ≤30- and
33(10)-ps values. Our results are consistent with the 9/2− spin of 1129-keV level, the
7/2− spin of 738-keV level and the 5/2− spin of 336-keV level. The new half-lives of the
93- and 290-keV states provide strong arguments for spin-parity assignments of 1/2− and
3/2−. We confirm the E2 behaviour for 792- and 738-keV line, and the mixed M1+ E2
nature for 336- and 197-keV transitions. From our measurements a pure or almost pure
M1 character of the 93-keV transition connecting the first-excited state to the ground
state is deduced. This points towards a dominating single-proton configuration for the
93-keV state.
In conclusion, the results obtained in this thesis provide new information about
structure of three nuclei around the doubly-magic 78Ni: 81Ga, 81Ge and 81As. Our half-life
measurements of the excited states give rise to clarify the nuclearstructure of exotic nuclei
far away from the stability line. The new properties of 81Ga (and 80Ga) were summarized
in an article submitted to Physical Review C journal, while the results on 81Ge and 81As
are under preparation to be submitted. Our comprehensive measurement of the 81Zn
β-decay to 81Ga is the closest approach to date to the spectroscopy of the β-decay of 79Ni
to 79Cu, which has only one proton above the 78Ni core. It can be a future challenge for
fast timing experiments.
Resumen en castellano

El estudio de la evolucio´n de los estados de particula independiente en nu´cleos ricos en
neutrones, adem´as de proporcionar valiosa informaci´on sobre la estructura nuclear, es
de gran importancia para el desarrollo de modelos nucleares te´oricos que predicen con
exactitud la estructura nuclear en nu´cleos elejados del valle de estabilidad. El estudio
de estos nu´cleos permite tambi´en determinar el papel que desempen˜an las excitaciones
de neutrones a lo largo de las diferentes capas nucleares junto con la evoluci´on de las
interacciones que se producen entre los protones. Son de especial inter´es las especies
situadas en regiones cercanas a los nucleos doblemente ma´gicos, como el 78Ni , ya que
28 50
presentan el escenario perfecto para determinar los grados de libertad de las part´ıculas
independientes en las capas cerradas. Asimismo, ciertas propiedades de estos nu´cleos
juegan un papel importante en las procesos de nucleos´ıntesis como el proceso r (captura
r´apida de neutrones). Debido a estos hechos, estos nu´cleos han sido durante las u´ltimas
d´ecadas la razo´n de amplios estudios, tanto experimentales como te´oricos, cuyo objetivo
era entender las modificaciones en estrucutra nuclear en la regi´on con gran exceso
neutr´onico.
Esta tesis describe el estudio de los nu´cleos de 81,80Ga, 81Ge y 81As, poblados en la
cadena de desintegraci´on β del 81Zn. El nu´cleo padre, 81Zn, fue producido en ISOLDE,
CERN aprovechando la gran pureza de los haces radiactivos de Zn, cuya selectividad y
eficiencia habian sido optimizadas con anterioridad. El montaje experimental consta de
dos detectores de HPGe, dos detectores de centelleo ultrarra´pidos de LaBr (Ce) y un
3
centelleador pl´astico (NE111A) que actu´a como el detector β. Los esquemas de niveles
se construy´ıan basa´ndose en las coincidencias γ-γ entre los HPGe aprovechando su buena
resoluci´on energ´etica, mientras que las medidas de vidas medias se hicieron con el uso
del centelleador pl´astico y los detectores de LaBr (Ce), junto con los TACs (Time-to-
3
Amplitude converters). El m´etodo de coincidencias ultrarra´pidas o "fast-timing" βγγ(t)’
fue utilizado para determinar las vidas medias de los estados excitados. Las sen˜ales de
cada detector fueron procesadas por un sistema de acquisici´on digital formado por cuatro
tarjetas Pixie-4 Digital Gamma Finder, disen˜adas especialmente para la espectroscop´ıa
γ.
Desdeelpuntodevistaestructural, losisotoposestudiadosenestetrabajosonsistemas
relativamentesencillos,contans´olounaspocaspart´ıculas/huecosfueradelcierredecapas,
siendo por tanto sistemas que pueden ser descritos por el modelo de capas nuclear con
bastantee ´exito. El caso ma´s interesante es el 81Ga , con solo 3 protones de valencia fuera
50 31
del 78Ni y ´orbitas ma´s bajas en p y f . La transicio´n de cara´cter M1 que conecta
3/2 5/2
estos estados tiene que ser l prohibida y, por lo tanto, considerablemnte lenta (decenas de
picosegundos). Tras realizar el an´alisis de datos el esquema de niveles del 81Ga se ampli´o
con 47 niveles y 70 nuevas transiciones en el rango de energ´ıa de hasta 6.5 MeV. La vida