Preparing an Isotopically Pure 229Th Ion Beam for Studies of 229mTh

We present a protocol for the generation of an isotopically purified low-energy ²²⁹Th ion beam from a²³³U source. This ion beam is used for the direct detection of the ^229mTh ground-state decay via the internal-conversion decay channel. We also measure the internal conversion lifetime of ^229mTh as well.

Transcript

This method describes the production of an isotopically pure Thorium-229 ion beam following the alpha decay of Uranium-233. The main advantage of this technique is that it allows us to study Thorium-229 ions in a low-energy nuclear-excited state that is relevant to nuclear club development. Demonstrating the procedure will be Ines Amersdorffer, a student from our laboratory.

A Uranium-233 source is mounted in this set up for creating an isotopically pure Thorium-229 ion beam. The vacuum chamber's have been evacuated and baked out to prepare for the experiment. The set up is behind the electronics used to monitor and control the system.

Researchers interact with the electronics primarily through computers which also collect and display the data. This cutaway schematic depicts elements of the apparatus. Consider the steps from generating the Thorium-229 isomers and the buffer gas stopping cell to detecting them with a CCD camera.

A 290 kilo Becquerel, large area Uranium-233 source produces the alpha decay Thorium-229 nuclei including 2%of first-excited state isomers. Fast thorium nuclei escape the source and are thermalized in an ultra-pure helium atmosphere buffered-gas stopping cell. Next they encounter an electric funnel system.

Its radio frequency and direct current electric fields guide them towards an extraction nozzle. The supersonic gas jet from the nozzle takes the nuclei into a vacuum chamber with a radio frequency quadrupole structure. The structure acts as an ion guide, face space cooler and potentially a pole trap.

The next vacuum chamber has a quadrupole mass separator to isolate isotopically pure Thorium-229 in selectable charge states. A triodic-electrode system with three-ring electrodes focuses the ions on the detector. Interaction with a micro channel plate detector, causes meta stable ions to decay and release electrons that are multiplied and detected on a phosphor screen with the CCD camera.

This is a cross sectional schematic of the vacuum chamber and associated equipment. Begin the experiment by starting the catalytic gas purifier and waiting 20 minutes for it to reach its operating temperature. Next, ensure the bypass valve is closed before opening the helium gas cylinder.

Open the pressure-reducer valve until a pressure of about 0.5 bar is measured. Then, open the valve from the pressure reducer to the gas tubing. Open the gas flow control by setting a cell pressure of 32 millibar.

Flush the gas tubing for about 10 minutes. Then close the valve connecting the pressure reducer to the gas tubing and wait a few minutes as the helium is removed. For higher purity-buffer gas fill the cryotrap with liquid nitrogen.

Set the gate valve between the buffer gas cell and its turbo molecular pump to remote operation, then close the gate valve remotely. Open the valve connecting the pressure reducer to the gas tubing. At this point the buffer-gas stopping cell is filled with about 30 millibar of helium gas.

The radio frequency quadrupole chamber pressure is approximately 10 to the minus four millibar. The quadrupole mass-separator chamber pressure is about 10 to the minus five millibar. Adjust the rotary speed of the turbo molecular pump attached to the RFQ vacuum chamber to set an ambient pressure of 10 to the minus two millibar.

This updated schematic includes representations of the equipment required for applying the guiding electric fields. Use a DC voltage supply to apply a DC potential to the uranium source. Next, prepare the segmented funnel electrode system.

With the DC power supply and a 24-channel DC offset supply, apply a DC potential gradient of four volts per centimeter and a three volt offset. Apply a DC potential of about two volts to the extraction nozzle. Follow this by applying the DC potentials to the 12 volt segmented extraction radio frequency quadrupole.

Create the gradient with the 24 channel DC offset supply. Voltages for each of the 12 segments of the quadrupole can be applied individually. Apply 1.8 volts to the segment closest to the extraction nozzle.

Step wise, decrease the voltages in subsequent segments by 0.2 volts to achieve a DC gradient of 0.1 volts per centimeter. Now, employ a function generator and linear RF amplifier to apply a frequency of about 850 kilohertz, 220 volts peak-to-peak amplitude to the funnel ring electrode system. With another frequency generator and two RF amplifiers, apply an 880 kilohertz, 120 to 250 volts peak-to-peak amplitude to the extraction radio frequency quadrupole and the individual bunching electrode.

When applying the RF voltage to the funnel ring electrodes, if the helium buffer gas is not sufficiently pure, sparks will occur in the buffer-gas stopping cell. In this case, interrupts the procedure and perform bake-out for a day to re-obtain full extraction efficiency. Use a DC voltage supply to apply a potential of minus one volt to the extraction electrode of the extraction radio frequency quadrupole.

Set the offset voltage of the quadrupole mass separator to minus two volts with DC offset modules. Turn to the function generator and RF amplifier associated with the quadrupole mass separator to start it. After selecting the mass to charge ratio and quadrupole mass separator acceptance use a four-channel power supply to apply potentials to the focusing triodic electrode structure.

After setting up the guiding fields work with the equipment required to tune the quadrupole mass separator. Begin by applying voltages to the double-plate micro channel plate detector, which has a front plate, a back plate and a phosphor screen. Use a high-voltage module to apply an attractive potential of negative 1, 000 volts to the front plate of the double-plate micro channel plate detector.

With a second high-voltage module, apply positive 900 volts to the backside of the second plate of the detector. Use a third high-voltage module to apply positive 3, 000 volts to the phosphor screen behind the micro channel plate detector. Switch on the CCD camera and the light-tight housing behind the phosphor screen and configure its exposure parameters.

Observe the camera output and skin the quadrupole mass separator mass overcharge ratio from below the expected value for Thorium-229 two plus until there is a signal. About 10, 000 Thorium two plus ions are extracted per second corresponding to about 3.5%total efficiency. After finding the Thorium signal scan for the Uranium-233 tube plus signal by again increasing the mass over charge ratio.

Once the Thorium signal disappears, the uranium signal should become evident. Set the quadrupole mass separator to extract only the Thorium-229 two plus ion species. With the mass separator tuned, continue to detect the isomeric decay.

Switch off the quadrupole mass separator pressure sensor to reduce background from ionized helium and light. Adjust the separator parameters to extract the chosen Thorium ion. Then, reduce the surface potential of the front plate of the micro channel plate detector to minus 30 volts.

Apply an accelerating potential to the second plate of the micro channel plate detector, typically 1, 900 volts. Apply an accelerating potential to the phosphor screen behind the detector, typically 4, 000 volts. Start the acquisition sequence of CCD images.

The count rate amounts to about three counts per second. Store the data for image evaluation and post-processing. This mass scan is in units of atomic mass over electric charge and represents counts measured over five seconds.

There are three groups of extracted ion species in singly, doubly and triply charged states. Note the relative number of triply ionized Thorium compared to triply ionized Uranium. These micro channel plate detector signals for triply charged states of Thorium and Uranium reflect experiments with three separate Uranium sources.

Two Uranium-233 sources of different strengths produced clear signals for Thorium and not for Uranium. Tests using a Uranium-234 source produced no signals providing evidence the signals generated with the Uranium-233 source are from nuclear de-excitation not atomic shell processes. The micro channel plate detector images for doubly charged Thorium and Uranium are consistent with this interpretation.

For this data, the attractive plate of the micro channel detector varies from a voltage-favoring electrons from ionic impact, down to zero volts. There is a considerable count rate for doubly-charged Thorium until the threshold of zero volts, unlike the count rate for doubly-charged Uranium. This provides further evidence the signal is from decay of the nuclear isomer.

This technique paved the way for the measurement of the lifetime and excitation energy of the thory-myz-imer as well as for a measurement of its hyperfine structure. Ultimately it may lead to the development of an ultra-precise optical nuclear-clock.

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Preparing an Isotopically Pure 229Th Ion Beam for Studies of 229mTh

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Preparing an Isotopically Pure ²²⁹Th Ion Beam for Studies of ^229mTh