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Registered Member #27
Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058
I sprinkled some flurecent material from the inside of a energy saving bulb onto an americium sample from a smoke detector. I have done this before and it looked greenish in colour but that is quite comon when the light level is too low for the eye to register the colour.
So I decided to use a camera to record the colour. The light output is very weak so I used a thirty second exposure at f/1.4, ISO 1600. The true colour turned out to be red.
So what material is responsible for the red light in a fluorecent tube and why is it the only material that is excited by the radiation?
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Bjørn, the phosphors inside a low energy light bulb are a composition designed to give varying degrees of colour correction and 'warmth' to the light emitted. Such compositions often include mixtures containing some or all of the following: calcium silicate, calcium halophosphate, strontium magnesium phosphate, calcium strontium phosphate, and magnesium fluorogermanate.
In many lamps, to generate white light, use is made of special phosphors or phosphor mixtures whose radiation is particularly intense in the red, green and blue spectral ranges, resulting in the light being perceived as "white" light. For example, conventional tri-phosphorus fluorescent lamps contain the triphosphors BaMgAl10 O17 :EU(BAM) with an emission band at 450 nm, CeMgAl11 O19 :Tb(CAT) with an emission band at 545 nm and Y2 O3 :Eu ('YOX') with an emission band at 612 nm.
Your light bulb might have contained the Europium-doped Yttrium trioxide - 'YOX' - which fluoresces red under UV, and might well do the same with the low energy gamma (or alpha) of americium.
A second question relates to the exposure of colour film for 30 secs. With an exposure of this length, there will certainly be some colour shift due to what is called 'reciprocity law failure', about which you will find much written with a Google query. So you must not assume that the light that exposed your film was the same colour as that which it appears to be after development
Registered Member #27
Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058
I am fairly confident that the colour is correct. I used a good digital camera that is capable of very long exposures without colour shift or other problems.
I will try to add something between the americium and the fluorecent material to see what is needed to stop the radiation.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Ah Ha! You didn't say it was a digital camera, Bjørn, so I assumed your reference to film speed meant it was silver halide chemistry. So the colour shift caused by reciprocity law failure I mentioned will not apply!
Registered Member #10
Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424
Bjorn had the same camera as me, a Nikon D70s so I got inspired. I used Activated Zinc sulpide on sticky tape over the aperture of the Americium holder disk. I used my 180mm lens that gives 1:1 macros. The 1cm disk takes up 30% of the screen width.
I put some wire cross hairs to give the positioning. After 468 seconds (almost 8 mins) this is what I got. There was a little light leakage from the corridor (warm fluorescent) and a few green LED's that I didn't cover or turn off. This probably accounts for the red tinge and green reflection on the tape. The activated ZnS however is clearly a blue in this exposure (I have not even changed the brightness or contrast). It only lights up over the centre of the disc but it actually spreads a lot further on the tape. ie it is not stray UV. Normally ZnS is a fluorescent/phosphorescent green.
Registered Member #10
Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424
The activator for the ZnS was likely to have been silver. It was just a little bag of powder bought off eBay.... The fluorescent output can diminish with ion bombardment. Although the flux is low from the 1 mCurie Americium, it has been there for about a year and the 5MeV alpha radiation is certainly powerful. The ZnS still fluoresces and is briefly phosphorescent under violet laser though. Maybe I should replenish it or try some Europium doped Strontium Aluminate that I have which has a red glow.
Here are a couple more shots at higher magnification but focus is more difficult. Focal distance is less than the wire diameter and I tried to focus where I thought the ZnS was.
Bjorn's camera wasn't a Nikon D70s as he used f 1.4.
Registered Member #27
Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058
I used a Sigma 30mm f/1.4 lens That is 6.25 times faster than a f/3.5 lens. So a lot less waiting which is good news.
At the moment I have both a D200 and a D70. In this shot I used the D200 because it is easier to operate in the dark.
My 30 second exposure would equal a bit over 3 minutes with a f/3.5 lens. So it looks like mine glows brighter. The complication is that macro lenses can have significantly reduced brightness when focusing close so without knowing if that happened here we don't know for sure if there is a difference.
Edit: This time with 0.015 mm kitchen aluminium foil covering the radiation source. 300s, f/1.4, ISO 1600
There seems to be slightly less fluorecent material this time so it appear dimmer than it probably would have been. So very close to 90% drop in brightness after passing through 0.015 mm aluminium and a tiny bit of air since it impossible to get the foil all the way into the little aperture without tearing it.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Bjørn Bæverfjord wrote ... So very close to 90% drop in brightness after passing through 0.015 mm aluminium and a tiny bit of air since it impossible to get the foil all the way into the little aperture without tearing it.
Well, that suggests that most, if not all, the effect is due to alphas.
Registered Member #27
Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058
Because the gamma radiation will pass through such a thin foil quite easily, right? So if I place a couple of millimetre thick plate of glass over the source then only the gamma radiation would pass through.
Average energy of most abundant emission of Am-241: Gamma & X-rays - 0.05954 MeV Alpha - 5.49 MeV
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