The ion-optical design of the MARA recoil separator and absolute transmission measurements of ...

190 Pages·2011·3.26 MB·English

DEPARTMENT OF PHYSICS
UNIVERSITY OF JYVA(cid:127)SKYLA(cid:127)
RESEARCH REPORT No. 7/2011
The ion-optical design of the MARA recoil
separator and absolute transmission
measurements of the RITU gas-(cid:12)lled recoil
separator
by
Jan Sar(cid:19)en
Academic Dissertation
for the Degree of
Doctor of Philosophy
To be presented, by permission of the
Faculty of Mathematics and Science
of the University of Jyv(cid:127)askyl(cid:127)a,
for public examination in Auditorium FYS-1 of the
University of Jyv(cid:127)askyl(cid:127)a on December 9, 2011
at 12 o’clock noon
Jyv(cid:127)askyl(cid:127)a, Finland
December 2011

Preface
The work reported in this thesis has been carried out during the years 2004{
2011. While the work concerns modelling and design of recoil separators the
author had the priviledge of participating in the research of the nuclear struc-
ture during the years.
I wish to express my most sincere thanks to my supervisors, prof. Matti Leino
andDr. JuhaUusitalo,fortheirguidanceduringtheseyearsandforgivingme
theopportunitytodomyPhDthesisworkintheRITUgroup. Theyhaveread
the manuscript of this thesis very carefully through and have given valuable
commentsbutstillleavingmethefreedomtoful(cid:12)lmyself. Iwouldliketothank
Prof. Rauno Julin who has valued my work sometimes more than I do myself
andProf. JuhaA(cid:127)yst(cid:127)oforsupportingmyworkrelatedtotheFAIRprojectand
this thesis study.
I thank Ulrika Jakobsson and Dr. Cath Scholey for many common years we
have spent around the RITU focal plane. I would like to thank Ulrika also for
numerous fascinating conversations about work and free time. Taneli Kalvas I
would like to thank for our long common journey in the physics wonderland. I
thank Taneli and Dr. Olli Tarvainen for great lunch times and especially the
highly analytic chat we have had.
Physicsisusuallymucheasierthanthebureaucracyortheadministrativetasks.
I would like to express my gratitude to Soili Leskinen, Marjut Hilska, Anna-
Liisa Bl(cid:23)a and Ritva V(cid:127)ayrynen for making my life easier.
Outside the topic of the thesis work and the research group, I would like to
thank my long-time friends for whole-hearted relationships. With you I can
simply be myself. I want to express my gratitude to my parents for all the
support and encouragement. I thank my wonderful children, my daughter Iida
and my son Altti. You have given me a great insight into humanity. Finally, I
thank my loving wife Raisa for both understanding me during the years of the
PhD studies and pushing me to write this book.

Abstract
Sar(cid:19)en, Jan
The ion-optical design of the MARA recoil separator and absolute
transmission measurements of the RITU gas-(cid:12)lled recoil separator
Jyv(cid:127)askyl(cid:127)a: University of Jyv(cid:127)askyl(cid:127)a, 2011, 190 p.
Department of Physics Research Report No. 7/2011
ISSN: 0075-465X; 7/2011
ISBN: 978-951-39-4511-4 (paper version)
ISBN: 978-951-39-4512-1 (electronic version)
Diss.
In this thesis work, the use of two complementary recoil separators for studies
of nuclear structure via fusion-evaporation reactions are discussed. The design
and the main ion-optical properties of the vacuum-mode recoil-mass separator
MARA,intendedforstudiesofnucleiwithN (cid:25)Z closetotheprotondrip-line,
are presented. MARA (Mass Analysing Recoil Apparatus) consists of a mag-
netic quadrupole triplet followed by an electrostatic de(cid:13)ector and a magnetic
dipole. The working principle of MARA is discussed and the reasons for the
choiceof the optical parameters of the elementsare given. The performance of
MARAfordi(cid:11)erentkindoffusionreactionshasbeenstudiedwithMonteCarlo
simulations. The results of a sensitivity analysis especially of the electrostatic
de(cid:13)ectorarereported. Therequirementsfortheinstrumentationarediscussed.
The absolute transmission values of the RITU gas-(cid:12)lled recoil separator have
been measured for several fusion evaporation products in the mass region
A = 170{190 using di(cid:11)erent reaction asymmetries. It was observed that the
angular distribution of products after the target and the angular acceptance
of RITU determine the transmission value accurately. The e(cid:11)ects of (cid:12)lling-gas
pressure on the average charge state and focal plane image properties as well
as on transmission values have been studied experimentally. The transmission
probability for 254No synthesized in the 48Ca+208Pb reaction has been calcu-
lated and is in agreement with the experiment. Simulations of transportation
of 186Hg fusion products through RITU have been carried out. The simulated
behaviouroftransmissionandimagepropertiesagreeswellwithanexperiment.
6
Keywords: ion optics, recoil separator, gas-(cid:12)lled recoil separator, recoil-
separator modelling
Author’s address Jan Sar(cid:19)en
Department of Physics
University of Jyv(cid:127)askyl(cid:127)a
Finland
Supervisors Prof. Matti Leino
Department of Physics
University of Jyv(cid:127)askyl(cid:127)a
Finland
Dr. Juha Uusitalo
Department of Physics
University of Jyv(cid:127)askyl(cid:127)a
Finland
Reviewers Dr. Teng Lek Khoo
Argonne National Laboratory
9700 S. Cass Avenue
Argonne, IL 60439
USA
Dr. Maurycy Rejmund
GANIL
Bd Henri Becquerel
BP 55027 - 14076 CAEN Cedex 05
France
Opponents Prof. Gottfried Mu(cid:127)nzenberg
GSI
Planckstra(cid:25)e 1
64291 Darmstadt
Germany
8
Contents
1 Modelling of an in-(cid:13)ight recoil separator 13
1.1 Basic concepts of the optics of charged particles . . . . . . . . . 14
1.1.1 About ion optical simulation methods . . . . . . . . . . 14
1.1.2 Coordinate system . . . . . . . . . . . . . . . . . . . . . 15
1.1.3 Interplay between (cid:14) , (cid:14) and (cid:14) . . . . . . . . . . . . . 17
m K t
1.1.4 Lorentz force and the rigidity of a charged particle . . . 18
1.1.5 Transfer matrices . . . . . . . . . . . . . . . . . . . . . . 19
1.1.6 Properties of an ion-otical system and aberrations . . . 22
1.2 Ion optical elements used in a recoil separator . . . . . . . . . . 25
1.2.1 Magnetic dipole . . . . . . . . . . . . . . . . . . . . . . 26
1.2.2 Electric dipole (electrostatic de(cid:13)ector) . . . . . . . . . . 28
1.2.3 Magnetic quadrupole . . . . . . . . . . . . . . . . . . . . 30
1.3 Charge state distributions . . . . . . . . . . . . . . . . . . . . . 31
1.3.1 Solid targets . . . . . . . . . . . . . . . . . . . . . . . . 33
1.3.2 Gaseous targets . . . . . . . . . . . . . . . . . . . . . . . 34
1.4 Fusion evaporation reactions. . . . . . . . . . . . . . . . . . . . 38
1.4.1 Fusion barrier . . . . . . . . . . . . . . . . . . . . . . . . 38
9
10 CONTENTS
1.4.2 Kinematics . . . . . . . . . . . . . . . . . . . . . . . . . 39
1.4.3 Evaporation of light particles and cooling down . . . . . 40
1.4.4 Elastically scattered target atoms. . . . . . . . . . . . . 43
1.5 JIonO|code for in-(cid:13)ight separator modelling . . . . . . . . . . 45
1.5.1 Simulation of fusion-evaporation reactions and ion inter-
actions in a target . . . . . . . . . . . . . . . . . . . . . 46
1.5.2 Transfer matrices . . . . . . . . . . . . . . . . . . . . . . 48
1.5.3 Transporting particles through a gas-(cid:12)lled separator . . 51
1.5.4 Other computer programmes for separator modelling . . 54
1.6 Angular acceptance and transmission . . . . . . . . . . . . . . . 54
2 Vacuum-mode mass separator MARA 57
2.1 Motivation to build MARA . . . . . . . . . . . . . . . . . . . . 58
2.2 Existing and planned recoil separators employing electric and
magnetic (cid:12)elds . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
2.3 Ion optics and the working principle . . . . . . . . . . . . . . . 62
2.3.1 Fixed energy focus with electrostatic de(cid:13)ector and mag-
netic dipole . . . . . . . . . . . . . . . . . . . . . . . . . 67
2.3.2 Quadrupole triplet . . . . . . . . . . . . . . . . . . . . . 72
2.3.3 Transmission . . . . . . . . . . . . . . . . . . . . . . . . 76
2.3.4 Image aberrations . . . . . . . . . . . . . . . . . . . . . 79
2.4 MARA performance . . . . . . . . . . . . . . . . . . . . . . . . 86
2.4.1 Observed quantities of an ion . . . . . . . . . . . . . . . 87
2.4.2 Time of (cid:13)ight from the target to the focal plane, (cid:1)t . . 88
1
2.4.3 The focal plane position and angles. . . . . . . . . . . . 89
2.4.4 Estimating (cid:1)t from focal plane information . . . . . . 91
1
2.4.5 Estimating the initial horizontal angle (a ) . . . . . . . 93
0