Fluorescent Crystal Display Stand

Back when I was graduating from the university, I was working on an experiment for dark matter direct detection called CRESST. This experiment uses particle detectors based on scintillating calcium tungstate (CaWO4) crystals. I stilI have a broken crystal as souvenir and always wanted to build a display stand that excites the crystal's fluorescence.

I realize that people probably will not copy this exact build since calcium tungstate crystals are not commercially available and also the UVC LEDs I used are quite expensive. However, it might help you if you are planning to build a display stand for other fluorescent minerals like amber or fluorite.

Step 1: Gather Materials

• fluorescent CaWO4 crystal
• small project box (e.g. conrad.de)
• 278 nm UVC LED (e.g. Crystal IS)
• LED starboard (metal core PCB) (e.g. Lumitronix)
• thermal pad (e.g. Lumitronix)
• heatsink (e.g. Lumitronix)
• step up module (e.g. ebay.de)
• LED boost driver (e.g. ebay.de)
• LiPo battery (e.g. ebay.de)
• slide switch
• 0.82 Ohm 1206 SMD resistor

Fluorescence in calcium tungstate can be excited at wavelengths < 280 nm. This is quite far in the UV and LEDs at this wavelength are usually quite expensive (~150 $/pc). Luckily, I got some 278 nm SMD LEDs for free as they were left over engineering samples from the company I work at. These type of LEDs are usually used for disinfection.

WARNING: UV light can cause cause harm to the eyes and skin. Make sure to have proper protection, e.g. UV goggles.

According to the spec sheet the LEDs have an optical output power of ~25 mW, an operating current of 300 mA and a high forward voltage of ~12 V. Since this means the LEDs dissipate about 3 W of heat they need to be mounted to a proper heat sink. Therefore, I purchased a metal core PCB (starboard) with the right footprint, a thermal pad and a small heat sink. As LEDs can easily be damaged by too high currents they should be operated with a constant current driver. I got a very cheap constant current boost driver board based on the XL6003 IC which also steps up the output voltage. According to the datasheet the output voltage should not be higher than 2x the input voltage. However, since I wanted to power everything from a 3.7 V LiPo battery, I added another step up converter that increases the battery voltage to ~6 V before the LED driver. The output current of the LED driver is set by two SMD resistors connected in parallel on the board. According to the XL6003 datasheet the current is given by I = 0.22 V/Rs. By default there are two 0.68 Ohm resistors connected in parallel which amounts to ~650 mA. In order to lower the current, I had to replace these resistors with an 0.82 Ohm resistor which will give ~270 mA.

Step 2: Mounting the LED

In the next step I soldered the LED onto the starboard. As already metioned it is important to get a PCB with the matching footprint of your LED. Soldering on a metal core PCB can be difficult as the board dissipates the heat rather well. To make soldering easier it is recommended to put the PCB on a hot plate but I also managed to do without. The LED should be coupled to the board with thermal paste. After soldering I attached the starboard to the heatsink using the thermal pad.

Step 3: Connect Electronics

I glued all electronic components to the bottom plate of my enclosure. Note that the heatsink gets quite hot so it is useful to use a glue which can withstand high temperatures. The battery gets connect to the step up module which increases the voltage to about 6 V. The output is then wired to the LED boost driver which is connected to the LED. A slide switch was added after the battery but you might want to do the soldering only after you have mounted the slide switch in the next step.

Step 4: Modify Enclosure

I made some modifications to the encluse using my dremel tool. A slit-shaped hole was put into the top for the LED light to escape. In addition, I put some openings in the side for ventilation. Another hole was made for the slide switch which was fixed with hot glue. I am not very happy with the look of the enclosure as the holes look pretty rough. Luckily most of them are not visible. Next time I will probably make a custom box using a laser cutter.

Step 5: Finished!

After closing up the enclosure the project was finished. The crystal can be placed on the slit at the top and is excited by the LED from below. The fluorescence emission is quite bright. Note that all light is really coming from the crystal as the UVC light is invisible.

The build can certainly be improved in a few ways. First of all the thermal managment of the LED is not great and the heat sink gets quite hot. This is because there is very little ventilation since the heat sink was mounted inside the enclosure. So far I did not dare to run the LED longer than a few minutes. Secondly, I would like to make a nicer enclosure next time using a custom laser cut box made from black acrylic. In addition, a LiPo charger module with microUSB plug can be added so that you will not need to open the box for recharging.

Comments

[Fusepopper]

Awesome. Thanks for posting this. I'm all about lighting rocks and minerals. I'd like to build something like this with multiple UV LEDs or lightsources within the same platform, and have some type of turntable as a top surface so that you could rotate the specimens over the different light sources and see how each is affected by different frequencies of light. I like the idea of using a slit or hole in an opaque material - it's so simple, I hadn't thought of it. I've been lighting thru glass, plexi, or directly on the LEDs. Maybe the turntable idea could have different or interchangeable slit or hole shapes to put the specimens on, as different pieces would look different with light coming differently shaped/sized openings. Maybe an adjustable aperture... Hmmm...

[nqtronix]

That project would make a good instructable :)

[mosivers]

That sounds like a great project. If you have multiple minerals it makes sense to put them on a rotating platform. Let me know if you make this I would really like to see it in action.

[Indytim22]

First of all, thanks for sharing this display. I'm extremely envious of both your CaWO4 crystal sample and 278nm UVC LEDs. Those are unobtainable for just about anybody outside of exotic physics research. You're lucky to walk away with those samples.

I'm curious about a few aspects of this display piece. What other kinds of radiation sources will cause CaWO4 to scintillate? The piece you have is probably worth a fortune if it can detect and resolve gamma or beta spectra when coupled to an appropriately designed photo-multiplier tube or a sensitive photo-diode based photodetector. Since Tungsten is so dense, you probably wouldn't need much of it to make a nice portable scintillation detector or gamma spectrometer.

With a secondary scintillation screen and a fiber-optic micro-channel plate, you could close couple this crystal to a CCD camera for radiation imaging. I have a quantity of fiber-optic micro-channel plates about 6-7mm thick and 40mm square if you want to make a trade for a slice of your crystal. My plates would need to be polished, but so would a slice of the CaWO4 crystal, so some finishing work would be needed. I also have plates that are pre-polished into a Plano-convex configuration.

The other thing that stands out to me is the UVC light source. Does the CaWO4 absorb all of it? If not, may I suggest a glass shield around the whole display to at least block the stray UVC and UVB rays from a casual observer. I'm also wondering if these UVC LEDs create any ozone in open air?

[mosivers]

Micro-channel plates are also not that easy to obtain ;-) I thought about making a scintillation detector. I did some test where I coupled the crystal to an APD but did not see any signals, probably due to the small active area of the APD. Another problem is that my crystal has some cracks and, as you pointed out, it would need to be polished but I like to keep the crystal they way it is. I found this website which sells old NaI(Tl) crystals and photomultipliers relatively cheap but you would still need a HV power supply.
Regarding your questions, the CaWO4 crystal will completely absorb the UVC light but adding a shield made from transparent acrylic around the box sounds like a good idea. I did not measure the spectrum of the UVC LED but ozone is only produced for wavelengths below 240 nm so it is probably not an issue.

[PeterNLewis]

Be careful with UV LEDs less than 315nm. From a recent instructable comment (UV-Fluorescence Steampunk Lamp - https://www.instructables.com/id/Steampunked-Fluorescence-Reactor-by-Junophor/) "Just a safety tip for anyone who wants to build this project:UV wavelengths below 315 nm are harmful to the skin (not just the eyes) ! And the shorter they get the more damaging they are.If at all possible try using an LED for this build without the small peak at 245 nm, this is UV-C radiation and can be very harmful. Curing LED's (like the one used in this project seems to be) often have several peaks because its helpful in speeding up the curing process, it is however not good for your skin. (That's why they are only allowed to be used in closed environments where the harmful light can't escape.) Even at the low energy levels of one or two LED's long term exposure will still do damage. A black light LED (400 nm) or an LED that produces only one wavelength between 385-415 nm would be fine for this project and way safer. Anything below 380 nm isn't visible anyway, so unless your trying to get a tan, try to stay away from the shorter wavelengths. I hope, even though we all like shiny things with a nice patina, we also like to keep our DNA in one piece ;)"

[mosivers]

I agree that UVC light is dangerous and you should take proper safety precautions. To excite the fluorescence in CaWO4 wavelenghts below 280 nm are needed so there was no way to avoid it. As I see it during operation no LED light can get out since it is either blocked by the housing or absorbed by the crystal.
Regarding the health risks of UVC please see the exempt below from the EU health agency’s safety guidelines.
“UVC is strongly attenuated by chromophores in the upper epidermis (Young, 1997) and UVC-induced DNA damage in the dividing basal layer of human epidermis is not readily detected (Campbell et al, 1993; Chadwick et al, 1995) which may explain why the dose response curve for UVC erythema in human skin is very much less steep than for UVB (Diffey and Farr, 1991). It is unlikely that UVC from artificial sources presents an acute or long-term hazard to human skin. However, UVC is likely to cause acute photokeratitis… UVC exposure is unlikely to cause acute or long-term damage to the skin but can cause severe acute damage to the eye“

[Shinymetalass]

CaWO4 is the mineral scheelite and it and many other fluorescent minerals are available from local or on-line mineral shops.

[audreyobscura]

This a cool little project, and I like that I even learned some materials science tips along the way! Thanks for sharing this tutorial and teaching us something new!