High Accuracy Measurement of the Nuclear Decay of U-235m and Search for the Nuclear Decay of ...

108 Pages·2017·24.09 MB·English

LLNL-TH-734604
High Accuracy Measurement of the
Nuclear Decay of U-235m and Search for
the Nuclear Decay of Th-229m
F. Ponce
July 12, 2017
Disclaimer

This document was prepared as an account of work sponsored by an agency of the United States
government. Neither the United States government nor Lawrence Livermore National Security, LLC,
nor any of their employees makes any warranty, expressed or implied, or assumes any legal liability or
responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or
process disclosed, or represents that its use would not infringe privately owned rights. Reference herein
to any specific commercial product, process, or service by trade name, trademark, manufacturer, or
otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the
United States government or Lawrence Livermore National Security, LLC. The views and opinions of
authors expressed herein do not necessarily state or reflect those of the United States government or
Lawrence Livermore National Security, LLC, and shall not be used for advertising or product
endorsement purposes.

This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore
National Laboratory under Contract DE-AC52-07NA27344.
A High Accuracy Measurement of the Nuclear Decay of 235mU and Search for the Nuclear
Decay of 229mTh
By
Francisco Ponce
B.A. (University of California, Berkeley) 2007
M.S. (San Francisco State University) 2012
DISSERTATION
Submitted in partial satisfaction of the requirements for the degree of
DOCTOR OF PHILOSOPHY
in
Physics
in the
OFFICE OF GRADUATE STUDIES
of the
UNIVERSITY OF CALIFORNIA
DAVIS
Approved:
Rena Zieve, Chair
Stephan Friedrich
Daniel Cebra
Committee in Charge
2017
i
A High Accuracy Measurement of the Nuclear Decay of 235mU and Search for the Nuclear
Decay of 229mTh
Francisco Ponce
Davis, California
2017
Among all nuclear decays, there exist two isomeric states with very low energy that belong
to 229Th (7.8 ± 0.5 eV) and 235U (76.8 ± 0.5 eV). Of particular interest is 229Th, because
the decay energy is in the ultraviolet and therefore in the range of modern tunable lasers.
The isomer can potentially be used as the basis for a nuclear clock that is expected to be
two orders of magnitude more precise than atomic clocks. However, the 229mTh nuclear
decay energy is not sufficiently well known to design the necessary laser system for a nuclear
clock. This work describes the development of a new technique using superconducting tunnel
junction (STJ) detectors to directly measure the nuclear decay of low energy isomers with
a high level of accuracy. The strength of the technique is demonstrated by measuring the
decay energy of the 235U isomer at 76.737 ± 0.018 eV, over an order of magnitude more
accurately than the current literature value. The technique is then applied to search for the
transition in 229mTh and measure its energy with comparable accuracy. These experiments
are unsuccessful and are discussed in light of the recent measurement of the 229mTh half-life
of 7 ± 1(cid:181)s. Modifications to our experimental technique to enable an accurate measurement
of229mTharesuggestedasthenextsteptowardstheultimategoalofbuildinganuclearclock.
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ACKNOWLEDGMENTS
This work was in large part possible through the collective efforts of many individuals
that guided me along throughout the years. This will be an attempt to thank everyone but
I am likely to miss someone.
On the LLNL side I would like to thank my research advisor Stephan Friedrich, he taught
me everything he knew about cryogenic detectors. His positive attitude always helped when
morale was low and nothing was working, particularly at the beginning of this project.
On the UC Davis side I would like to thank Rena Zieve my faculty advisor, for being
willing to work with me. She always provided me with timely feedback to my questions.
I would like to thank all of the supporting researchers, and staff at LLNL. Particularly,
Jason Burke and Erik Swanberg for their advice in everything nuclear related, help getting
me started working with ROOT, setting up the pulsed laser, and wonderful conversations
throughout the years. To Roger Henderson and Sherry Faye for making our sources without
which this project would have been impossible. Marianne Ammendolia for helping to wire
bond our detectors on short notice. To Jonathan Dreyer, Owen Drury, and Camaron Bates
for teaching me the inner working of the ADR and laboratory. To Matthew Carpenter and
Simon George for their help and training at the ALS. To Jan Batteux, for all the help in
machining parts that we needed for the experiment and early morning conversations.
Finally, I would like to thank my family and friends for getting me out of the lab and
have a social life. Finally last but never least, my partner in crime Nicole, who puts up with
me on a daily basis and loves me with or without data.
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DISCLAIMER
This document was prepared as an account of work sponsored by an agency of the United
States government. Neither the United States government nor Lawrence Livermore National
Security, LLC, nor any of their employees makes any warranty, expressed or implied, or as-
sumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any
information, apparatus, product, or process disclosed, or represents that its use would not
infringe privately owned rights. Reference herein to any specific commercial product, pro-
cess, or service by trade name, trademark, manufacturer, or otherwise does not necessarily
constitute or imply its endorsement, recommendation, or favoring by the United States gov-
ernment or Lawrence Livermore National Security, LLC. The views and opinions of authors
expressed herein do not necessarily state or reflect those of the United States government or
Lawrence Livermore National Security, LLC, and shall not be used for advertising or product
endorsement purposes.
This work was performed under the auspices of the U. S. Department of Energy by
Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. It was
funded by the LLNL LDRD Grant 14-LW-073 and by U.S. DOE Grants DE-SC0004359 and
DE-SC0006214.
LLNL-TH-734604
iv
TABLE OF CONTENTS
1 Introduction 1
1.1 Nuclear Clocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.2 The Low-Energy Nuclear State in 229Th . . . . . . . . . . . . . . . . . . . . 3
1.3 Direct Measurements of the Decay of 229mTh . . . . . . . . . . . . . . . . . . 6
2 Superconducting Tunnel Junction Single-Photon and Particle Detectors 9
2.1 Superconducting Tunnel Junctions . . . . . . . . . . . . . . . . . . . . . . . 9
2.2 High Resolution Photon Detectors . . . . . . . . . . . . . . . . . . . . . . . . 11
2.3 STJ Fabrication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
2.4 STJ Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
2.5 Characterization with Soft X-Ray . . . . . . . . . . . . . . . . . . . . . . . . 20
2.6 Characterization with UV Light . . . . . . . . . . . . . . . . . . . . . . . . . 22
3 Experiment 30
3.1 Experimental Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
3.2 Radioactive Sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
3.3 Data Acquisition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
4 235mU Measurement 43
4.1 Drift Corrections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
4.2 Offset Corrections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
4.3 Measurements of 235mU . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
4.4 Laser Calibration Uncertainty . . . . . . . . . . . . . . . . . . . . . . . . . . 56
4.5 Detector Calibration Uncertainty . . . . . . . . . . . . . . . . . . . . . . . . 56
4.6 Partition Width Uncertainty . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
4.7 Bin Width Uncertainty . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
4.8 Fit Range and Background Uncertainty . . . . . . . . . . . . . . . . . . . . . 60
4.9 Uncertainty Budget . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
5 229mTh Measurements 66
5.1 Alpha-Recoil Identification . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
5.2 Reduction of 233U Background . . . . . . . . . . . . . . . . . . . . . . . . . . 73
5.3 Search for 229mTh . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
5.4 Future Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
6 Summary 91
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LIST OF FIGURES
1.1 (From [1]) Energy levels and gamma transitions for 229Th . . . . . . . . . . . 3
1.2 (From [2]) Energy levels and gamma transitions for 229Th . . . . . . . . . . . 3
1.3 229Th gamma spectrum from Beck et al. . . . . . . . . . . . . . . . . . . . . 5
1.4 (From [3]) Half-life of neutral 229mTh . . . . . . . . . . . . . . . . . . . . . . 7
2.1 Excitation energy of STJ vs wave number . . . . . . . . . . . . . . . . . . . 10
2.2 DOS of superconductor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
2.3 Outline of a basic STJ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.4 I(V) for an ideal STJ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.5 DOS of basic STJ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.6 Outline of an STJ w traps . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
2.7 DOS of STJ w traps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
2.8 STJ Amplifer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
2.9 SEM image of STJ detector chip. . . . . . . . . . . . . . . . . . . . . . . . . 15
2.10 Elemental composition of newly fabricated STJ surface . . . . . . . . . . . . 17
2.11 Elemental composition of old STJ surface . . . . . . . . . . . . . . . . . . . . 17
2.12 ADR schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
2.13 STJ I(V) curve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
2.14 STJ responsivity vs voltage bias . . . . . . . . . . . . . . . . . . . . . . . . . 21
2.15 STJ operation at the ALS . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
2.16 STJ ALS characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
2.17 STJ ALS FWHM vs energy . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
2.18 STJ resolution vs count rate . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
2.19 Characterization of STJ with pulsed laser . . . . . . . . . . . . . . . . . . . . 27
2.20 Long range calibration of STJ . . . . . . . . . . . . . . . . . . . . . . . . . . 28
2.21 STJ response to laser over hours . . . . . . . . . . . . . . . . . . . . . . . . . 29
3.1 Outline of isomer implantation . . . . . . . . . . . . . . . . . . . . . . . . . . 31
3.2 Experimental setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
3.3 Micro-machined Si collimator . . . . . . . . . . . . . . . . . . . . . . . . . . 32
3.4 Setup for depositing microdroplets . . . . . . . . . . . . . . . . . . . . . . . 34
3.5 Induced flux trapping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
3.6 Large source holder . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
3.7 Alpha spectrum of Pu source . . . . . . . . . . . . . . . . . . . . . . . . . . 37
3.8 Alpha spectrum of U source . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
3.9 233U decay branch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
3.10 232U decay branch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
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3.11 XIA electronics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
3.12 Coincidence laser separation . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
4.1 STJ response to laser with 239Pu source (Raw) . . . . . . . . . . . . . . . . . 45
4.2 Fit to 5 minute partition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
4.3 Drift corrected and calibrated STJ response to laser . . . . . . . . . . . . . . 46
4.4 Centroid uncertainties to fully calibrated laser . . . . . . . . . . . . . . . . . 46
4.5 Variable laser illumination of (208(cid:181)m)2 STJ detector . . . . . . . . . . . . . 47
4.6 Centroid shift of three photon peak with laser intensity . . . . . . . . . . . . 47
4.7 Laser calibration versus average number of photons . . . . . . . . . . . . . . 48
4.8 Responsivity for (208(cid:181)m)2 STJ vs energy for variable laser intensities . . . 48
4.9 Responsivity for (138(cid:181)m)2 STJ vs energy for variable laser intensities . . . . 49
4.10 Centroid shift for (138(cid:181)m)2 STJ vs laser intensity (Right) . . . . . . . . . . 49
4.11 Laser response of (138(cid:181)m)2 STJ detector to indirect illumination . . . . . . 50
4.12 Area of indirect illumination . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
4.13 Daily 235mU decay measurements . . . . . . . . . . . . . . . . . . . . . . . . 52
4.14 235mU isomer signal and fitted spectra . . . . . . . . . . . . . . . . . . . . . . 54
4.15 Residuals for 235mU fitted spectra . . . . . . . . . . . . . . . . . . . . . . . . 55
4.16 Calibration of pulsed 355nm laser . . . . . . . . . . . . . . . . . . . . . . . . 57
4.17 Laser dependence on temperature . . . . . . . . . . . . . . . . . . . . . . . . 57
4.18 Detector Calibration Uncertainty . . . . . . . . . . . . . . . . . . . . . . . . 58
4.19 O and C fluorescence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
4.20 235mU partition width uncertainty . . . . . . . . . . . . . . . . . . . . . . . . 59
4.21 235mU energy bin width uncertainty . . . . . . . . . . . . . . . . . . . . . . . 59
4.22 Background fits to 235mU . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
4.23 235mU uncertainty due to background and fit range . . . . . . . . . . . . . . 63
4.24 Cumlative fits for 235mU decay measurements . . . . . . . . . . . . . . . . . . 65
5.1 233U Low energy background . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
5.2 Alpha-Substrate interaction . . . . . . . . . . . . . . . . . . . . . . . . . . . 68
5.3 ADC trace of alpha event on multiple detectors . . . . . . . . . . . . . . . . 69
5.4 ADC trace of recoil event on multiple detectors . . . . . . . . . . . . . . . . 69
5.5 Filtered trace of recoil event with crosstalk . . . . . . . . . . . . . . . . . . . 70
5.6 TOA of high energy events (with recoils) . . . . . . . . . . . . . . . . . . . . 72
5.7 Alpha particle only setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
5.8 TOA of high energy events (without recoils) . . . . . . . . . . . . . . . . . . 74
5.9 High energy spectra of time cut showing recoils . . . . . . . . . . . . . . . . 75
5.10 STJ detector laser calibration at low energies . . . . . . . . . . . . . . . . . . 76
5.11 233U signal with different data cuts . . . . . . . . . . . . . . . . . . . . . . . 77
5.12 Detector response after high energy alpha impact, long time scale . . . . . . 78
5.13 Detector response after recoil impact . . . . . . . . . . . . . . . . . . . . . . 78
5.14 Detector response after recoil impact in another detector . . . . . . . . . . . 79
5.15 Detector response after high energy impact, short time scale . . . . . . . . . 79
5.16 233U signal at 20 to 50 ms before and after recoil impact . . . . . . . . . . . 82
5.17 Reduced isomer signal on identical background . . . . . . . . . . . . . . . . . 83
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5.18 Projected isomer signal for collection time . . . . . . . . . . . . . . . . . . . 84
5.19 STJ spectra before and after recoil impact . . . . . . . . . . . . . . . . . . . 85
5.20 Difference Method relative to recoil impact . . . . . . . . . . . . . . . . . . . 86
5.21 Projected isomer response . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
5.22 Projected acquisition time for a 10 ms half-life . . . . . . . . . . . . . . . . . 88
5.23 36-Pixel array of (208(cid:181)m)2 STJ detector . . . . . . . . . . . . . . . . . . . . 88
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