229Th isomer detection via radiative decay of 233U in the CaF2 environment
Citation
Morawetz, I. (2024). 229Th isomer detection via radiative decay of 233U in the CaF2 environment (Master's thesis, TU Wien).
Keywords
- Thorium Nuclear Clock
- 229Th Isomer (229mTh)
- CaF2 Crystal Environment
- Radiative Decay of 233U
- VUV Spectrometer
- Wavelength Measurement
- Cherenkov Radiation
- Micro Channel Plate (MCP) Detector
- Calibration
Brief
This article investigates a new method for detecting the 229Th isomer in a CaF2 crystal environment and measuring its nuclear transition wavelength for the development of a thorium nuclear clock, finding the wavelength to be 150.37± 1.31 nm.
Summary
This master's thesis (dated February 10, 2024) by Ira Morawetz focuses on the detection and wavelength measurement of the nuclear transition in Thorium-229 within a Calcium Fluoride (CaF2) crystal environment. This research is a crucial step towards developing a highly precise nuclear clock using Thorium-229.
Here's a summarized breakdown of the thesis:
- Objective: The primary goal is to precisely measure the wavelength of the gamma photon emitted during the decay of the Thorium-229 isomer (229mTh). This is essential for building a nuclear clock, which relies on the extremely precise frequency of this nuclear transition.
- Methodology:
- Crystal Growth and Doping: Morawetz used the vertical gradient freeze method to grow a CaF2 crystal doped with Uranium-233 (233U). This isotope decays into 229Th, with a small percentage of decays populating the desired isomeric state (229mTh).
- VUV Spectrometer: A highly sensitive Vacuum Ultraviolet (VUV) spectrometer was built to detect the faint signal of the decaying 229mTh. This involved:
- Focusing Mirrors: Both a CaF2 mirror with a dielectric coating and an aluminum mirror were used. The CaF2 mirror filters specific wavelengths, while the aluminum mirror offers broader reflectance.
- Diffraction Grating: A blazed grating, designed for maximum efficiency at the expected signal wavelength, separates the light by wavelength.
- MCP Detector: A Micro Channel Plate (MCP) detector coupled with a phosphor screen and CMOS camera enables single-photon detection with spatial resolution.
- Calibration and Data Analysis:
- Calibration: The spectrometer's wavelength scale was calibrated using known emission lines of Nitrogen and Xenon. However, the thesis notes that there might be systematic errors in the calibration procedure due to differences in the optical paths during calibration and actual measurement.
- Data Processing: The captured images were processed to:
- Remove noise from hot pixels and high-energy particles.
- Correct for the "banana-shaped" distortion in the spectrum, an artifact of the spherical diffraction grating.
- Key Findings:
- Successful Detection: The thesis reports successful detection of a signal attributed to the 229mTh decay. Measurements with the CaF2 mirror suggest a wavelength of 150.37 ± 1.31 nm. Initial data from the aluminum mirror setup indicates a signal at 149.07 ± 0.18 nm.
- Calibration Challenges: The author acknowledges that refining the calibration process is crucial for improving the accuracy of the wavelength measurement.
- Future Work:
- Improved Calibration: A new vacuum chamber design is proposed to allow for in-situ calibration, eliminating the need to disturb the crystal and mirror alignment. This should mitigate systematic errors in the calibration.
- Enhanced Detector: Replacing the partially damaged MCP detector is expected to improve data quality.
In essence, Ira Morawetz's master's thesis describes a significant step towards developing a solid-state nuclear clock based on Thorium-229. The successful detection of the isomeric transition is a promising result. However, the thesis highlights the importance of refining the calibration process to obtain a more precise and reliable measurement of the crucial transition wavelength.