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Author: Subject: Radiation induced Thermo-Phosphorescence in inorganic Crystals

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[*] posted on 29-3-2024 at 08:06
Radiation induced Thermo-Phosphorescence in inorganic Crystals

Hi everyone, today I'm reporting about recent experiments I made regarding the interaction of radiation and crystalline substances, in particular, I used high doses of X-Rays to induce luminescence in different types of minerals.
Maybe you are familiar with F-centers (, they are created when radiation strikes a crystal and makes it change color, the radiation used can be deep UV, x/y rays or beta, the high energy radiation strikes the crystal structure and the result is an electron trapped inside the vacancy left by the anion ejection or by other mechanisms that you can find in the wiki page.
Sometimes this effect can cause the crystal to gain colors in the visible region that before it didn't have, that is due to the electron being inside the energy gap of the material, which now has energy level low enough to interact with visible light that has energy<3eV.
In the experiments I'm going to show you I recreated this phenomenon, I also observed a loss of coloration instead of a gain and other cool effects, for my experiments I used a high dosage of low energy xrays, in the range of 10-50KeV, which were generated from the machine I described in this earlier post:
And these are my results:
The first candidate was NaCl, it is easily available so I gave it a try without expecting anything, but the result was very clear as you can see:

From left to right: after 40min exposure (tube parameter 30kV 3mA), the salt on the side exposed, after 2h of resting, after several more hours of resting, not easily seen by photo, but as time progressed a clear change in color of the salt was taking place, it was going from the deep brow, towards green-brown, and at the end stopped at gray.
After seeing this work I tested other crystals, which you can see in this photo before and after 1h exposure:

They are, Fluorite CaF2 (the bigger one), NaCl (the 3 that changed color), and calcite CaCO3 at the center. As you can see the biggest color change happened in NaCl crystal, calcite didn't seem to change that much, and fluorite will be seen later.
I then proceeded to the most interesting part of the experiment, which I had never seen online, and that was to heat them and see what would happen, so the minerals were placed on top of a metal plate, that was heated from the underside by a bunsen burner, these are the image I was able to record:

After some time they started glowing a distinct color on the left we have Calcite with a blue glow, and on the right, NaCl started to glow white-yellow and very soon changed to orange, I will leave fluorite for last because of its very peculiar behavior.
I then thought that having this very peculiar phenomenon, maybe the crystal would also interact with UV light, so I tried to expose a NaCl crystal under such light, I used a crystal that was half irradiated, to see if there was any difference between the part that changed color and the one that didn't and this is the outcome:

As you can see there was a difference between the part exposed to x-rays and the one that didn't, the normal part didn't do much under UV, but the one exposed to radiation had a slight purple/dark emission, I didn't understand if that was the UV light fluorescing, reflecting or just being absorbed, this interaction would need further investigation.
But returning to the main point of this post, I then repeated the experiment of irradiating and then heating, with other types of crystal, pyrite (FeS2), and rose desert with unknown composition.
They both didn't undergo any apparent change when irradiated and also when heated over the metal plate.
Lastly, I'll show the main reason for this post, and that is the very interesting behavior of fluorite.

These photos represent the changes that the fluorite crystal got through: the first is before being irradiated, and the second one is after 2h of irradiation, so after taking a really high dose of x-rays, it can be seen a little bit discolored at the center, and at the right is a photo of the crystal after the heating process which releases the light, it was really discolored and almost white/transparent.

This photo is of the same crystal while being heated on the metal plate over the bunsen, it was releasing a really strong (compared to the other one) green light that was emanating from all the parts of the crystal, but more strongly from the center.
This continued for several (2-3) minutes, after this time the crystal emission started shifting color, and now I was really amazed to see the crystal all of a sudden changing color from green to this:

The photo is not the best because my phone ability to see in the dark is really bad, but the strong light emission of the crystal could be seen even from my crappy camera, what you see on the lower side is the reflection of the light emitted by the crystal itself being reflected by the metal plate.
This continued for another 2 minutes and then started to get dimmer.
Until now the bunsen was heating the plate from far away under it, with the flame not touching the plate, so the temperature of the metal must have been between 100°C-300°C, after the light output started to dim, I tried lowering the plate closer to the flame to see if this would increase the light output, and that was able to maintain emission going for some more second :

On the top right corner of the crystal, the red glow is the metal turning red hot because of the vicinity of the flame to it, but I saw that even heating the crystal strongly like that, the color would increase a little bit but then steadily die out, so I raised the metal plate and let the light die by herself:

And after that, no other particular thing happened, besides the crystal becoming not colorated anymore.
These were the minerals that I had ready on hand, but I think that it would be very interesting to see the behaviour of others, that maybe ill buy in the future to try out.
I think we can gather some conclusions from these experiments: mainly that Ca++ ion is really important for this to happen because pyrite and quartz (I'm not really sure about quartz because some footage was lost but I remember it not working) at low x-ray energy didn't emit any light when heated and also didn't change color. I said low energy X-rays because it is widely known that glass (amorphous quartz with impurities) can be colored by y radiation. Also rose desert (usually CaSO4) didn't undergo any transformation, so maybe a halogen ion needs to be present.
Also I think there is almost certainly a correlation between the exposure dosage, and phosphorescence time and intensity, in fact, the smaller crystal light emission, lasted for no more than 30s, but the larger one lasted several minutes after being exposed to a much higher dose (higher exposure time) compared to the smaller one.
After seeing this I started researching literature more deeply about the phenomena and found some papers from 1928 that did some similar experiments, mainly on the coloration of crystal by radiation, more information can be found here, the same author then also briefly discussed phenomena similar to the one I experimented, but they studied kunzite, besides this source I didn't find any more modern one.
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[*] posted on 29-3-2024 at 08:13

Wow! I had never heard of this phenomenon before. This is really neat, thanks for sharing!

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[*] posted on 29-3-2024 at 13:28

Years ago, heliodors, yellow and honey-colored beryls from the Zolotaya Voda site in Tajikistan, were sold in our country, then it was discovered that they were aquamarines from Pakistan that had somehow been radioactively irradiated, and the Zolotaya Voda site was an invented fake site. I was fooled too and bought one beautiful specimen for a lot of money, and I have been interested in the mineral beryl and its gem varieties all my life, and even prominent mineralogists in academia were fooled...I think these aquamarines turned into heliodors in the way you describe here!
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