Carbon cluster mass calibration at SHIPTRAP [Elektronische Ressource] / vorgelegt von Ankur Chaudhuri

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• Physik

Informations

• Publié par: ernst-moritz-arndt-universitat_greifswald
• Publié le: 01 janvier 2007
• Nombre de lectures: 14
• Langue: English
• Poids de l'ouvrage: 7 Mo

Extrait

Carbon-cluster mass calibration at SHIPTRAP
Inaguraldissertation
zur
Erlangung des akademischen Grades
doktor rerum naturalium (Dr. rer. nat)
an der Mathematisch-Naturwissenschaftlichen Fakult˜at
der
Ernst-Moritz-Arndt-Universit˜at Greifswald
vorgelegt von
Ankur Chaudhuri, M.Sc.
geboren am 1. Juli, 1975
in Raigunj, Indien
Greifswald, 2007Dekan: Prof. Dr. Klaus Fesser
1. Gutachter: Prof. Dr. Lutz Schweikhard
Ernst-Moritz-Arndt-Universit˜at Greifswald
2. Gutachter: Prof. Dr. Kumar Satish Sharma
University of Manitoba, Canada
Tag der Promotion: 10.12.2007Abstract
A carbon-cluster ion source has been installed and tested at SHIPTRAP, the
Penning-trap mass spectrometer for mass measurements of heavy elements
at GSI/Darmstadt, Germany. A precision mass determination is carried out
by measuring the ion cyclotron frequency ! = qB=m, where q=m is thec
charge-to-mass ratio of the ion and B is the magnetic fleld. The mass of the ion
of interest is obtained from the comparison of its cyclotron frequency ! withc
that of a well-known reference ion. Carbon clusters are the mass reference of
choice since the unifled atomic mass unit is deflned as 1/12 of the mass of the
12 12C atom. Thus the masses of carbon clusters C , n=1,2,3,... are multiples ofn
the unifled atomic mass unit.
12 +Carbon-cluster ions C , 5 • n • 23, were produced by laser-inducedn
desorption and ionization from a carbon sample. Carbon clusters of various
+ + + + + + + + [phone] 12 12sizes ( C , C , C , C , C , C , C , C , C ) were used for7 9 10 11 12 15 18 19 20
an investigation of the accuracy of SHIPTRAP covering a mass range from
84 u to 240 u. To this end the clusters were used both as ions of interest
and reference ions. Hence the true values of the frequency ratios are exactly
known. Themass-dependentuncertaintywasfoundtobenegligibleforthecaseof
¡8(m¡m )<100u. However,asystematicuncertaintyof4:5£10 wasrevealed.ref
In addition, carbon clusters were employed for the flrst time as reference
ions in an on-line studies of short-lived nuclei. Absolute mass measurements
[phone]of the radionuclides Dy, Dy and Ho were performed using C as11
reference ion. The results agree with measurements during the same run using
85 +Rb as reference ion. The investigated radionuclides were produced in the
92 58fusion-evaporation reaction Mo ( Ni,xpyn) at SHIP (Separator for Heavy Ion
147reaction Products) at GSI. Among the measured nuclei Ho has the lowest half
¡8life (5.8 s). A relative mass uncertainty of 5£10 was obtained from the mass
measurements using carbon clusters as calibrants.Contents
1 Introduction 1
2 Radioactive ion beam facilities and mass measurements 3
2.1 Production and separation of radioactive nuclei . . . . . . . . . . 3
2.1.1 The in- ight method . . . . . . . . . . . . . . . 3
2.1.2 The ISOL method . . . . . . . . . . . . . . . . . . . . . . 4
2.2 Mass-measurement techniques . . . . . . . . . . . . . . . . . . . . 5
2.2.1 Penning trap mass spectrometry : Time-of- ight Ion Cy-
clotron Resonance (TOF-ICR) . . . . . . . . . . . . . . . . 5
2.2.2 Revolution-frequency measurement at ESR . . . . . . . . . 5
3 Penning trap Theory 9
3.1 The ideal Penning trap . . . . . . . . . . . . . . . . . . . . . . . . 9
3.2 The real P trap . . . . . . . . . . . . . . . . . . . . . . . . 13
3.2.1 Imperfection of the electric-quadrupole fleld . . . . . . . . 13
3.2.2 Misalignment . . . . . . . . . . . . . . . . . . . . . . . . . 15
3.2.3 Magnetic fleld inhomogeneity and temporal stability. . . . 15
3.2.4 Storage of more than one ion . . . . . . . . . . . . . . . . 17
3.3 Excitation of the ion motion . . . . . . . . . . . . . . . . . . . . . 18
3.3.1 Dipolar excitation . . . . . . . . . . . . . . . . . . . . . . . 18
3.3.2 Quadrupolar excitation . . . . . . . . . . . . . . . . . . . . 19
4 The SHIPTRAP experiment 21
4.1 Experimental set-up . . . . . . . . . . . . . . . . . . . . . . . . . 21
4.1.1 Stopping cell . . . . . . . . . . . . . . . . . . . . . . . . . 23
4.1.2 RFQ buncher . . . . . . . . . . . . . . . . . . . . . . . . . 26
4.1.3 Reference ion source . . . . . . . . . . . . . . . . . . . . . 27
4.1.4 Quadrupole de ector . . . . . . . . . . . . . . . . . . . . . 28
4.1.5 Penning traps . . . . . . . . . . . . . . . . . . . . . . . . . 29
4.2 Bufier-gas cooling and mass selection . . . . . . . . . . . . . . . . 32
4.3 Time-of- ight Ion Cyclotron Resonance (TOF-ICR) detection
technique . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35ii CONTENTS
5 Carbon-cluster ion source 39
5.1 Motivation for a carbon-cluster ion source . . . . . . . . . . . . . 40
5.2 Experimental set-up . . . . . . . . . . . . . . . . . . . . . . . . . 41
5.3 Carbon-cluster ion source characterization . . . . . . . . . . . . . 43
5.3.1 Optimization of electrode voltages . . . . . . . . . . . . . . 43
5.3.2 Energy spread of the ions . . . . . . . . . . . . . . . . . . 47
5.4 Experimental procedure . . . . . . . . . . . . . . . . . . . . . . . 48
5.4.1 Timing sequence for measurement cycle . . . . . . . . . . . 48
5.4.2 Time-of-Flight mass spectrum . . . . . . . . . . . . . . . . 49
5.4.3 Cooling resonance . . . . . . . . . . . . . . . . . . . . . . . 51
5.4.4 Cyclotron frequency determination of cluster ion . . . . . . 51
6 Study of the accuracy of SHIPTRAP 55
6.1 Statistical uncertainty . . . . . . . . . . . . . . . . . . . . . . . . 56
6.2 Count-rate-class analysis . . . . . . . . . . . . . . . . . . . . . . . 57
6.3 Time dependence of resonance frequencies . . . . . . . . . . . . . 59
6.4 Investigation of the systematic uncertainty of SHIPTRAP . . . . 61
6.4.1 Cross-reference measurements . . . . . . . . . . . . . . . . 61
6.4.2 Mass-dependent systematic efiect . . . . . . . . . . . . . . 63
6.4.3 Systematic uncertainty . . . . . . . . . . . . . . . . . . . . 64
6.4.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
7 On-line mass calibration by carbon-cluster ions 67
7.1 On-line mass measurements around A=147 . . . . . . . . . . . . 67
7.2 Carbon-cluster ions for ofi-line mass comparisons . . . . . . . . . 69
7.3 Carb ions for on-line mass . . . . . . . . . 71
7.4 Discussions of mass measurements around A=147 . . . . . . . . 75
7.4.1 Two-neutron separation energies. . . . . . . . . . . . . . . 77
7.4.2 Proton separation energies . . . . . . . . . . . . . . . . . . 78
8 Summary and Outlook 81
A Principles of Time-of- ight mass spectrometry 83
B Values for used auxiliary data 85List of Figures
2.1 Overview of the Separator for Heavy Ion reaction Product (SHIP)
facility at GSI, Darmstadt. . . . . . . . . . . . . . . . . . . . . . . 4
2.2 Overview of Experimental Storage Ring at GSI, Darmstadt . . . . 7
3.1 Schematic drawing of a hyperbolic Penning trap.. . . . . . . . . . 10
3.2 Eigenmotions of ion inside a Penning trap. . . . . . . . . . . . . . 11
3.3 Eigenfrequenciesofionmotionandparametricfrequency(normal-
ized to cyclotron frequency) as a function of trapping parameter. . 12
3.4 Relative magnetic fleld deviation as a function of time measured
at SHIPTRAP. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
3.5 Dipolar and Quadrupolar excitation scheme . . . . . . . . . . . . 19
3.6 Conversion of the radial motions of ion in a Penning trap due to
the external radiofrequency quadrupole excitation. . . . . . . . . . 20
4.1 Photograph of SHIPTRAP facility at GSI. . . . . . . . . . . . . . 22
4.2 Schematic layout of SHIPTRAP. . . . . . . . . . . . . . . . . . . 23
4.3 Technical drawing of the set-up. . . . . . . . . . . . . 24
4.4 Stopping cell and extraction RFQ of SHIPTRAP. . . . . . . . . . 25
4.5 Segmented RFQ cooler and buncher. . . . . . . . . . . . . . . . . 26
4.6 Schematic diagram of the quadrupole de ector. . . . . . . . . . . 27
4.7 Photograph of the quadrupole de ector. . . . . . . . . . . . . . . 28
4.8 Schematic diagram of the SHIPTRAP Penning trap electrode sys-
tem. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
4.9 Photograph of the SHIPTRAP Penning trap system. . . . . . . . 32
4.10 Ion motion in a bufier gas fllled P trap. . . . . . . . . . . . 33
4.11 Schematic of the TOF-ICR detection technique. . . . . . . . . . . 35
4.12 Theoretical line shape of time-of- ight ion cyclotron resonance. . . 36
4.13 Resonant and non-resonant ions in a time-of- ight ion cyclotron
12 +resonance of C . . . . . . . . . . . . . . . . . . . . . . . . . . . 3811
5.1 Nuclear chart indicating reference ions . . . . . . . . . . . . . . . 40
5.2 Schematic diagram of carbon-cluster ion source. . . . . . . . . . . 41
5.3 Photograph of carbon-cluster ion source at SHIPTRAP. . . . . . . 42iv LIST OF FIGURES
5.4 Photograph of rotatable sample-holder of carbon-cluster ion source. 42
?R5.5 of Sigradur target after and before laser ablation. . 43
5.6 Ion-optical simulation of cluster-source using Simion . . . . . . . . 44
5.7 Ion counts from the cluster source as a function of extraction elec-
trode voltage. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
5.8 Ioncountsfromtheclustersourceasafunctionofcentralelectrode
voltage. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
5.9 Ioncountsfromtheclustersourceasafunctionofouterelectrodes
voltage. . . . . . . . . . . . . . . . . . . . . .