In my never-ending endeavor to bring light to jewelry through combining science with craftsmanship, I have resorted to the nuclear option. No, really.

If you don't know me by now, I'll have you know I love glowy, sparkly things, especially rings. As far as jewelry goes, rings are a favorite, and I have done ones that glow in the past; my last blog post about a ring was glow-in-the-dark, as well. It is actually rare that a piece I make does not feature a form of luminescence, fluorescence, or some other unusual optical property. I've obtained an obsession with optical oddities over the years. As far as compact lighting solutions that can be incorporated into jewelry go, there are only so many options available. It was only a matter of time before I turned to tritium lighting for a project.

Tritium [³H]

Tritium (³H) is the atomic isotope of Hydrogen containing 2 neutrons and 1 proton. This means it is approximately 3× as massive as the most common isotope of Hydrogen with only a single proton at its core (aptly named Protium). It also means that tritium is unstable, with a half-life of about 12.3 years. Tritium is a low-beta emitter, decaying into ³He (Helium with 2 protons and 1 neutron) and releasing a single free electron (a β-particle) with relatively low penetrating power.

The beta radiation from tritium decay is not particularly dangerous as it can travel only about 6mm in open air, and is blocked completely by the outermost layer of dead skin cells in humans. The only true hazards tritium poses are when ingested as a tritiated compound or when inhaled directly. When stored in a glass capsule, the beta radiation released is contained by the glass, making it safe to handle. Thus, a neat source of safe radioluminescence can be obtained by lining a glass vial with a cathodoluminescent phosphor and sealing tritium gas inside. As the tritium decays and releases beta particles, they interact with the phosphor coating and the absorbed energy is re-emitted as visible light. The result is a light-source that will glow for many years, continuously, unlike photoluminescent 'glow-powders' that must be 'charged' in sunlight or under UV. Tritium vials also do not contain enough of the gas to be hazardous, even when broken, as the quantity is so small that it quickly becomes diluted in a normal room.

TL;DR: Even though tritium is radioactive, handling or even wearing tritium vials for extended periods of time poses essentially no threat of radiation exposure. Just don't go ingesting them.

Lutetium Aluminum Garnet [LuAG]

One of the hardest parts about dealing with invisible radiation in the scientific and engineering fields is the very invisible nature of the stuff. An X-ray emitter is just a dangerous source of radiation without any way to 'see' the X-rays. One of the methods devised for detecting radiation like that is by absorbing the radiation and re-emitting it in the visible spectrum, then using well-understood optical sensors to collect the light emitted. And so, scintillator crystals were eventually devised; materials that are highly efficient at converting high-energy radiation into lower-energy visible light.

One such scintillator is Lutetium Aluminum Garnet (LuAG), formulated for PET/CT scanners. It is one of my favorite industrial lab-grown gemstones as it is not only good at scintillating but also just fluorescing! LuAG is one of the most intensely green materials you will ever see, due to its strong ability to absorb and re-emit even just higher-energy photons like those in blue and UV light. Simply looking at the stuff conjures the sense that LuAG is radioactive all on its own, despite being completely benign. It's my very own Kryptonite. LuAG's high hardness of 8.5 on the Mohs scale also makes it perfect for durable jewelry. It can generally be treated as any other garnet would in stone setting.

Glorious Glowing Synergy

As if scintillators and radioluminescent light sources weren't each already fantastic enough on their own, I wanted to combine the two into a piece of ultimate scientific jewelry. While the beta radiation from tritium decay is all contained within a tritium vial, the light emitted by the blue variants has enough energy to excite a LuAG crystal, and so placing the two side-by-side results in a truly tantalizing combination. I was excited by the possibility of fabricating a ring with a perpetually-glowing gemstone set within.

Design Considerations

Before embarking on my design journey, I devised a set of objectives the finished piece should accomplish.

Aesthetics

I knew from the start that with the high-tech concept of a nuclear-powered ring, I would want to aim for a retro-futuristic aesthetic. If my final piece could look at home on the finger of a Senator of the Galactic Republic on Coruscant, I would be satisfied. I also wanted to maintain a sense of symmetry. Given the unique shape of the fancy cut LuAG I chose for this project, it was not too much of a challenge, though I had never done anything like it, before.

Maintainability

Of course, with tritium's half-life of a mere 12.3 years, I could only reasonably expect a considerable glow out of any given set of tritium vials to last for about a decade. Assuming a ring would last that long (and I hope it would live long past the 10-year mark) the owner would want to be able to replace the tritium tubes when the original ones 'die out' so-to-speak. The finished design should allow the wearer to service their own ring by replacing the tritium vials (assuming vials of near-identical dimensions can be obtained) without the need for a jewelry professional to perform operations such as unsetting and resetting the stone.

Durability

The design should be robust enough to sufficiently protect both the stone and the tritium vials from reasonable harm despite residing on the hand, a high-wear location on the human body. Reinforcement should be added to wherever the tritium vials will be housed so as to ensure they are not crushed or fractured under everyday use. They should preferably be tucked behind the stone for this reason, as well.

Light-Transport

The design should take the transportation of light into account to maximize the glow effect while minimizing the visibility of the tritium vials themselves. The vials should be featured prominently in the design ethos, but should not detract from the attractiveness of the stone or the setting when viewed under natural lighting (sunlight/daylight). As much of the light from the tritium vials as possible should be directed into the stone by nature of the geometry of the setting; additionally, no stray light from the vials should be visible without first passing through the stone itself. My intention was to accentuate the fluorescence of the stone using the light from the tritium vials, not to feature the vials, themselves.

Design Execution

I feel as though I was able to properly realize all of the criteria I set for myself, given the finished design:

As far as aesthetics are concerned, I feel as though I was able to achieve that retro-sci-fi look by combining lots of circular arcs and features with a prominent use of Boolean geometry. I decided to do a double-band ring for added retro-futurism and to make the whole piece feel more secure on the finger. The stone setting completely encloses the rear of the stone, leaving only the table of the stone visible; I like doing bezel settings like this for garnets since while 8.5 Mohs is hard, it isn't as hard as a sapphire or a diamond. For durability's sake, garnets can benefit from the extra protection a bezel setting provides over other setting types like prong or pinch.

I added two little capsule-like features protruding to either side of the stone setting, preserving the radial symmetry of the piece. I think of these as tritium vial nacelles, as they will house the 'fuel' for the ring (the tritium vial light sources). I added ribs to each nacelle to increase thickness for strength while also contributing to the retro-futuristic aesthetic; I drew inspiration from the cooling fins on heatsinks. I also modeled in set screws to be used for retaining the tritium vials in each nacelle, also allowing for easy user servicing at home with the right hex-key.

To facilitate light transport from the tritium vials into the stone, I cut windows between the nacelles and the back of the LuAG stone's setting. These windows allow for the light from the tritium vials to be delivered directly into the LuAG through either side of the pavilion, preventing any stray light from escaping without first going through the stone. I hoped this configuration would hide the tubes by the means of total internal reflection while still allowing the light to be absorbed and re-emitted by the LuAG scintillating medium. The hope was that the stone itself would appear to glow steadily from within.

Fabrication

I had my model sent out to a fabrication house to have them 3D-print it in resin, then cast it in sterling silver via the lost-resin technique.

Since I had to model the LuAG's cut myself, I was quite happy with how close the fit was on the casting. It was slightly under-sized in the bezel but plenty deep enough to house the entire back of the stone. It is always better to have a slightly under-sized bezel, so long as you leave yourself enough material to shave away and still have enough left for a secure setting. Luckily, this was just the case, and I only needed to shave off a miniscule amount of material from either end of the setting to get the stone to seat properly. I achieved this using a flex-shaft rotary tool and a fine carbide cross-cut cone square burr.

Post Processing

Once the stone would seat, I drilled out the nacelles by hand to allow the tritium vials to slide in and out smoothly. Once the stone is set, the only way to remove the tritium vials will be by dumping them out, so they must be allowed to slide freely and may not be allowed to get wedged in place. The tolerances for this feature of the piece were the most critical.

Thread Tapping with a Jig

I also needed to cut the threads for the retaining set-screws before going any further. I had decided not to attempt to model the threads directly into the part, fearing the printing and casting would not be capable of rendering that level of detail/precision. Instead, there's a tool for that and I used it:

I held a properly sized thread tap in a pin vise and tapped each hole by hand, carefully chasing with the same drill bit as before to remove any burrs pushed up by the thread-cutting process. This was my first time cutting threads in sterling silver, the piece was delicate, and I only had one shot at the operation. I was not confident I could get everything lined up and held in place correctly, unaided, so I decided to design and 3D-print a dedicated work-holding fixture to align the tap with the hole and hold the ring in place while thread cutting was performed. My solution was essentially a special collet with a built-in guide-hole.

This strategy worked extremely well, and I was able to tap both threads with ease. The fit of the set screws was quite satisfying, and getting everything to come out right on the first try proved all of the planning worthwhile.

Next, I used the same pin vise to hold a slightly undersized drill bit and clean out the treads and the tritium vial channels by hand. This needed to be done with care, as I mentioned before that the fit of the tubes in their nacelles is critical. Once complete, I had a perfect fit for both sides.

Stone Setting

With the first hurdle past me, it was time to take on setting the stone. Since I had already chipped this stone once before (and had it recut) I was extra cautious when handling it. Setting has always been a struggle for me but since getting a flex shaft with reciprocating handpiece, I have much improved. This tool converts rotary motion delivered by the flex shaft into a tapping motion, and different 'anvils' may be attached to allow for things like engraving, texturing, and setting.

Results

After the stone was set, the ring just needed a good polish and then it was complete! Miraculously, the final piece turned out just as I had wanted.

The ring looks great under a wide variety of lighting conditions, and shines especially bright under UV, but it should be noted that it does not get 'charged up' by exposure to light like other glow-in-the-dark objects do. It is just always on, all the time.

The Tritium Difference

While phosphorescent items can be excited by exposure to bright light, especially higher-energy wavelengths like blue, violet, and ultraviolet, this ring glows differently. The core concept uses the tritium vials as the light source, so while the glow may not be as bright as other things that use lume at first, the LuAG Lantern Ring's glow outlasts all other glowing objects I own, and will continue to do so for nearly a decade. The LuAG looks great under a UV lamp, but it is only fluorescent and thus does not hold on to the light after the lamp is removed, returning to the baseline level of glow induced by the tritium vials.

The comparison images demonstrate that while the lume ring glows more brightly immediately after the light source is removed, the residual glow fades to a level dimmer than that of the tritium ring within a matter of minutes. Meanwhile, the tritium-powered glow does not change appreciably.

I am so happy to be able to say I have made a nuclear-powered ring, in all seriousness. I have some more stones and tritium vials I would like to experiment with pairing together in settings exploiting the same mechanism. Now, I'm interested in doing similar things with Gadolinium Aluminum Gallium Garnet (GAGG) and Cerium-doped Yttrium Aluminum Garnet (Ce:YAG).

Gallery

Thanks for reading,
~Joseph

Links

• https://www.ehs.harvard.edu/sites/default/files/radioactive_materials_reference_sheet_h3.pdf

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