Investigations of nuclear decay half-lives relevant to nuclear astrophysics

129 Pages·2011·1.41 MB·English

DE TTK
1949
Investigations of nuclear decay half-lives
relevant to nuclear astrophysics
PhD Thesis
Egyetemi doktori (PhD) ´ertekez´es
J´anos Farkas
Supervisor / T´emavezeto˝
Dr. Zsolt Fu¨l¨op
University of Debrecen
PhD School in Physics
Debreceni Egyetem
Term´eszettudom´anyi Doktori Tana´cs
Fizikai Tudom´anyok Doktori Iskola´ja
Debrecen
2011
Prepared at
the University of Debrecen
PhD School in Physics
and
the Institute of Nuclear Research
of the Hungarian Academy of Sciences
(ATOMKI)
K´eszu¨lt
a Debreceni Egyetem Fizikai Tudom´anyok Doktori Iskola´ja´nak magfizikai
programja keret´eben a Magyar Tudom´anyos Akad´emia Atommagkutato´
Int´ezet´eben (ATOMKI)
Ezen ´ertekez´est a Debreceni Egyetem Term´eszettudom´anyi Doktori
Tana´cs Fizikai Tudom´anyok Doktori Iskola´ja magfizika programja
keret´eben k´esz´ıtettem a Debreceni Egyetem term´eszettudom´anyi
doktori (PhD) fokozata´nak elnyer´ese c´elj´abo´l.
Debrecen, 2011. Farkas Ja´nos
Tanu´s´ıtom, hogy Farkas Ja´nos doktorjel¨olt a 2010/11-es tan´evben a
fent megnevezett doktori iskola magfizika programja´nak keret´eben
ira´ny´ıta´sommal v´egezte munka´j´at. Az ´ertekez´esben foglalt eredm´e-
nyekhez a jel¨olt o¨na´llo´ alkoto´ tev´ekenys´eg´evel meghata´rozo´an hozza´-
j´arult. Az ´ertekez´es elfogad´asa´t javaslom.
Debrecen, 2011. Dr. Fu¨lo¨p Zsolt
t´emavezeto˝
Investigations of nuclear decay half-lives relevant to
nuclear astrophysics
E´rtekez´es a doktori (PhD) fokozat megszerz´ese ´erdek´eben a fizika
tudom´any´agban
´Irta: Farkas J´anos, okleveles fizikus ´es programtervezo˝ matematikus
K´eszu¨lt a Debreceni Egyetem Fizikai Tudom´anyok Doktori Iskola´ja
magfizika programja keret´eben
T´emavezeto˝: Dr. Fu¨lo¨p Zsolt
A doktori szigorlati bizottsa´g:
elno¨k: Dr. ........................................ ........................................
tagok: Dr. ........................................ ........................................
Dr. ........................................ ........................................
A doktori szigorlat ido˝pontja: 2011. ..............................
Az ´ertekez´es b´ıra´lo´i:
Dr. ........................................ ........................................
Dr. ........................................ ........................................
Dr. ........................................ ........................................
A b´ıra´lo´bizottsa´g:
elno¨k: Dr. ........................................ ........................................
tagok: Dr. ........................................ ........................................
Dr. ........................................ ........................................
Dr. ........................................ ........................................
Dr. ........................................ ........................................
Az ´ertekez´es v´ed´es´enek ido˝pontja: 2011. ..............................
Contents
1 Introduction 1
2 Radioactivity and lifetimes 5
2.1 The discovery of radioactivity . . . . . . . . . . . . . . . . . . 5
2.2 General decay characteristics . . . . . . . . . . . . . . . . . . 7
2.3 Common decay modes . . . . . . . . . . . . . . . . . . . . . . 10
2.3.1 Nucleon emission decay . . . . . . . . . . . . . . . . . 10
2.3.2 β decay and electron capture . . . . . . . . . . . . . . 13
2.3.3 Nuclear de-excitation . . . . . . . . . . . . . . . . . . 15
2.4 Measuring lifetimes . . . . . . . . . . . . . . . . . . . . . . . . 18
2.4.1 Detection of the decay products . . . . . . . . . . . . . 18
2.4.2 Other technique . . . . . . . . . . . . . . . . . . . . . 20
2.5 Radioactive decay and astrophysics . . . . . . . . . . . . . . . 20
2.6 Recent questions . . . . . . . . . . . . . . . . . . . . . . . . . 22
3 Experimental investigations of the lifetime of embedded 74As 25
3.1 Nuclear reaction measurements for astrophysics . . . . . . . . 26
3.2 Electron screening of nuclear reactions and decay . . . . . . . 28
3.3 The Debye – Hu¨ckel screening model . . . . . . . . . . . . . . 30
3.3.1 The foundations of the Debye – Hu¨ckel model . . . . . 30
3.3.2 Lifetime predictions based on the Debye – Hu¨ckel model 33
3.4 Experimental results in the literature . . . . . . . . . . . . . . 33
3.5 The outlines of our experiment . . . . . . . . . . . . . . . . . 35
3.5.1 The decay of 74As . . . . . . . . . . . . . . . . . . . . 36
3.5.2 Host materials . . . . . . . . . . . . . . . . . . . . . . 37
3.5.3 Model predictions . . . . . . . . . . . . . . . . . . . . 37
3.6 Experimental instrumentation . . . . . . . . . . . . . . . . . . 39
i
ii CONTENTS
3.6.1 Target evaporation . . . . . . . . . . . . . . . . . . . . 39
3.6.2 Irradiation at the cyclotron of Atomki . . . . . . . . . 40
3.6.3 γ detection and electronics . . . . . . . . . . . . . . . 41
3.7 The course of the measurement and its results . . . . . . . . . 44
3.7.1 Measurement runs . . . . . . . . . . . . . . . . . . . . 44
3.7.2 Data analysis . . . . . . . . . . . . . . . . . . . . . . . 45
3.7.3 Results . . . . . . . . . . . . . . . . . . . . . . . . . . 45
3.8 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
74
4 The lifetime of embedded As at low temperatures 51
4.1 The predicted temperature dependence . . . . . . . . . . . . . 51
4.2 Experimental results in the literature . . . . . . . . . . . . . . 52
4.3 The outlines and instrumentation of the experiment . . . . . 53
4.3.1 Cooling system . . . . . . . . . . . . . . . . . . . . . . 54
4.3.2 Target preparation and irradiation . . . . . . . . . . . 54
4.3.3 γ detection . . . . . . . . . . . . . . . . . . . . . . . . 55
4.4 The course of the measurement and its results . . . . . . . . . 58
4.4.1 Measurement runs . . . . . . . . . . . . . . . . . . . . 58
4.4.2 Data analysis . . . . . . . . . . . . . . . . . . . . . . . 58
4.4.3 Results . . . . . . . . . . . . . . . . . . . . . . . . . . 59
4.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
133m 154m
5 Lifetime determination of Ce and Tb with γ spec-
trometry 65
5.1 The astrophysical γ-process . . . . . . . . . . . . . . . . . . . 65
5.2 Cross section measurements for the γ process . . . . . . . . . 67
5.2.1 The activation method . . . . . . . . . . . . . . . . . . 68
5.2.2 When the half-life is incorrect . . . . . . . . . . . . . . 69
133m
5.3 The determination of the half-life of Ce . . . . . . . . . . 70
5.3.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . 70
5.3.2 Target preparation . . . . . . . . . . . . . . . . . . . . 70
5.3.3 The activation chamber . . . . . . . . . . . . . . . . . 71
5.3.4 Irradiation . . . . . . . . . . . . . . . . . . . . . . . . 73
5.3.5 γ detection . . . . . . . . . . . . . . . . . . . . . . . . 73
5.3.6 Data analysis . . . . . . . . . . . . . . . . . . . . . . . 75
5.3.7 Results . . . . . . . . . . . . . . . . . . . . . . . . . . 78
CONTENTS iii
154m
5.4 The determination of the half-life of Tb . . . . . . . . . . 81
5.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85
6 The Newcomb –Benford law as a test of decay models 89
6.1 A historical review of the Newcomb – Benford law . . . . . . . 89
6.2 The Newcomb – Benford law and nuclear half-lives . . . . . . 90
6.2.1 The distribution of the first digits . . . . . . . . . . . 90
6.2.2 Ones scaling test . . . . . . . . . . . . . . . . . . . . . 92
6.3 Mathematical explanation . . . . . . . . . . . . . . . . . . . . 92
6.4 Is the law applicable to test nuclear decay models? . . . . . . 95
7 Summary 99
7.1 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
¨
7.2 Osszefoglala´s . . . . . . . . . . . . . . . . . . . . . . . . . . . 103
Publications 108
Acknowledgements 110
Bibliography 111