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4hv.org :: Forums :: General Science and Electronics
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Americium 241 fluorescence

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Bjørn
Fri Mar 07 2008, 07:59AM Print View

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Location: Hyperborea
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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?


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Proud Mary
Fri Mar 07 2008, 05:06PM
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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

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Bjørn
Fri Mar 07 2008, 10:40PM

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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.
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Proud Mary
Sat Mar 08 2008, 12:05AM
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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!
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Tesladownunder
Sun Mar 09 2008, 04:55PM
Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
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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.

TDU





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Proud Mary
Sun Mar 09 2008, 05:46PM
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TDU, you don't say what the dopant in the ZnS is.

ZnS:Cu, Cl phosphors will emit blue-green at low copper values, and shift to yellow green as the Cu value is increased.
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Tesladownunder
Mon Mar 10 2008, 12:12AM
Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
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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.

TDU




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Bjørn
Mon Mar 10 2008, 02:47AM

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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.


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Proud Mary
Mon Mar 10 2008, 08:58AM
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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.
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Bjørn
Mon Mar 10 2008, 11:04AM

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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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Tesladownunder
Mon Mar 10 2008, 11:38AM
Joined: Thu Feb 02 2006, 09:45AM
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A quote from my website on Alpha radiation from Am 241.

"The left photo above shows an Americium sample (1 mCurie) showing a count of 80,180 CPM (counts per minute). The centre photo shows the majority of the radiation being blocked by a sheet of paper and the count dropped to 1250 CPM. The right photo shows thin Aluminium sheet dropping the count to 750 CPM. There is some gamma radiation from the Americium which may be accounting for some of this."

TDU
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Proud Mary
Mon Mar 10 2008, 12:54PM
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Some neutrons and positrons will be emitted when aluminium is bombarded with alpha particles. Aluminum-27 (13 protons plus 14 neutrons) will end up as phosphorus-30 (15 protons plus 15 neutrons), which has a half-life of 2.498 mins. The P-30 gives off a positron as it decays.

So it might just be possible to detect this transmutation at home, if you have the means to detect positrons such as a beta-sensitive GM tube. I guess experiments could start by wrapping the Am-241 source tightly in aluminium foil for a goodly period (what would be the optimum time?) and then immediately bringing the foil to the GM tube at the end of the irradiation period. A count graph plotted against time would then show if the decay fell off in the known half-life period of P-30 or not.
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Bjørn
Mon Mar 10 2008, 01:02PM

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I found this:
Thickness of aluminium to absorb half of radiation
Alpha 0.005 mm
Gamma 80 mm

So my foil should halve the radiation 0.015 / 0.005 = 3 times. That means 1 / (2^3) = 1/8 of the radiation should get through. Which is close enough to my 1/10 reduction in brightness to be well within the accuracy of my measurements.

The density of glass and aluminium is quite similar, meaning that 2 mm would not pass a detectable amount of alpha radiation. My calculator just displays -E- when I try to calculate it.
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Proud Mary
Mon Mar 10 2008, 02:29PM
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Perhaps for the sake of certainty, the photographic experiment should be repeated exactly as it is, but without the phosphor material on the plastic film.
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Tesladownunder
Mon Mar 10 2008, 09:56PM
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Location: Bunbury, Australia
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I've tried the aluminum foil experiment to see if P30 decay occurs.
I left Al foil on the front of the detector and put the Am on top for 5 mins. There are 25cps through the Al. Removing the Am should allow any residual alpha, beta or gamma to be observed.
However, the count drops within 1 second to the usual 0-4 cps background. So I don't appear to be seeing this effect.
TDU
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Proud Mary
Tue Mar 11 2008, 01:13AM
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When the Joliot-Curies discovered induced artificial radioactivity in aluminium bombarded wth alpha particles in 1934, they first used a cloud chamber to witness the effect, and confirmed this finding with a Geiger counter.

I don't know what sort of tube they used exactly (which they perhaps made especially for the experiment) but I do know that tubes used in that period were generally quite large - 30 cm or so long and 2 cm in diameter, making them much more sensitive than the much smaller types which are easy to get hold of today. I have a fairly sensitive mica end-window type, and will set it up over the next weekend to see if I can detect any residual activity from the aluminium myself. I'll see what effect I can get with a few different scintillators and my D50 whilst I'm at it.

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Bjørn
Tue Mar 11 2008, 06:54AM

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Perhaps for the sake of certainty, the photographic experiment should be repeated exactly as it is, but without the phosphor material on the plastic film.
I am all for increasing the certainty. Can you describe your idea in more detail?

I have the same effect as TDU when it comes to detecting radiation through a shield but I used a CCD chip instead of a geiger counter.
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Proud Mary
Tue Mar 11 2008, 10:00AM
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Bjørn Bæverfjord wrote ...

Perhaps for the sake of certainty, the photographic experiment should be repeated exactly as it is, but without the phosphor material on the plastic film.
I am all for increasing the certainty. Can you describe your idea in more detail?


I mean a control experiment that would use all the same procedures and methods, including the presence of the Am-241 source, but without the phosphor. This would help rule out any effect due to the direct irradiation of the CCD device in the camera, for example. You might also substitute the phosphor with known non-phosphors to be even more accurate.
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Bjørn
Tue Mar 11 2008, 10:23AM

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Location: Hyperborea
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A thick IR/UV blocking filter had no significant effect on the light. A thin visible blocking filter blocked all the light. Finally the radiation source alone showed no effect with the same exposure time.

So it is 100% sure it is red visible light coming from the phosphor.
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Tesladownunder
Tue Mar 11 2008, 12:01PM
Joined: Thu Feb 02 2006, 09:45AM
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Posts: 1424
I have set up 3 Americium disks. The larger upper one is uncovered. The left lower has Strontium aluminate:Europium 2% and the right lower has ZnS:Ag? Each disc is 0.9 - 1.0 microCurie.

Second pic shows the bright smears of color. ie the whole of the fluorescent material is being activated. This has to be due to phosphorescence not irradiation. Exposure was 466 seconds ie nearly 8 mins. F 3.5 plus a magnifying lens to make the 3 disks fit on a full screen.
However the ZnS disk does have a clear ring where the Americium is.

I will repeat this after leaving it in the dark for a few hours.

I like pretty colors....

TDU



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Bjørn
Tue Mar 11 2008, 12:50PM

Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
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I repeated mine with 100 times more material on the side of the sample to double check that something similar was not happening.

Even if the powder had been in bright light all the time up to the exposure it does not emit any light except where it is excited by the radiation.

The only other light was a reflection on the foil showing light that got around the door. I made an overlay that shows where the light was.



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Proud Mary
Tue Mar 11 2008, 02:09PM
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Very interesting results, both of you. I will set up an experiment at the weekend so I can contribute my own results.
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Tesladownunder
Tue Mar 11 2008, 05:51PM
Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
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Here is a 416 second shot with the phosphors having been covered for some hours. Now more clear red and green discs with only a little phosphorescence. These are 2mm across.

I have a ZnS:Ag sheet that picks up alpha well but will not fluoresce/phosphoresce with my violet laser (yet UV does pass through). The green ZnS I used below above behaves quite differently so may have a different activation. It has bright prolonged phosphorescence under my violet laser.

TDU


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Proud Mary
Tue Mar 11 2008, 08:54PM
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Here is an interesting paper about strontium aluminate if you blokes haven't already seen it:

The Luminescence from a Long Lasting Phosphor Exposed to
Alpha, Beta, and Gamma Rays
Munehiko KOWATARI1, Daisuke KOYAMA2, Yoshiyuki SATOH3, Kouichi IINUMA3 and Shunsuke UCHIDA3. Journal of NUCLEAR SCIENCE and TECHNOLOGY, Vol. 39, No. 12, p. 1251–1259 (December 2002)

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Bjørn
Thu Mar 13 2008, 08:20AM

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I set up my CCD chip and there are two distinct types of detection. One that is weak but very common and a different one that happens a few times a second that often saturates a pixel.

Whatever causes the bright flashes can some time work its way through 6 mm of shield (glass and ferrite). It seems to take a mm or so of shield to halve the intensity. So that makes me suspect it is beta radiation.

The weak detections are quite deep in the noise so it seems like I have to build a PC with a capture card to caputure uncompressed video to study it.
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Proud Mary
Sat Mar 15 2008, 05:57PM
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I used reagent-grade ZnS (>99.8%) and got a null result after an exposure of 300 secs, at f32 on my Nikon D50.

I had hoped that there would be sufficient Ag/Cu impurities in the ZnS to act as dopants, but it seems not.
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Bjørn
Sat Mar 15 2008, 06:40PM

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f/32?
If so that would be (32/1.4)^2 = 522 times less light gathering power than what I used. Mine would have looked black at f/32.
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Proud Mary
Sun Mar 16 2008, 12:37AM
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Bjørn Bæverfjord wrote ...

f/32?
If so that would be (32/1.4)^2 = 522 times less light gathering power than what I used. Mine would have looked black at f/32.


Dear me! I must have been half asleep when I wrote that! I meant 3200 ISO speed, [thinking of the fastest speed that I am used to] but now I see I meant 1600 on the D50 in any case! The aperture was f3.5, not nearly so bright as your lens, so perhaps I should give it another go with an even longer exposure.
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Bjørn
Fri Jun 06 2008, 09:49PM

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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.
I finally got around to test my 150 mm macro lens and it goes from f/2.8 at infinite to about f/5 at closest focus distance. So the difference is dramatic.

I think this is because of the internal focusing design and it would not happen if the lens change size during focusing.
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Hon1nbo
Sat Jun 14 2008, 12:41AM
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I just have to say something, even though this thread is old (and b/c I would like some feed back on the matter)... it does not seem that a mirror or leaded glass was used for the initial shot, therefore the ionizing radiation could have directly struck the image sensor... main reason I am posting this is because I did a project a while back in which the radioactive source (U-238, as Am-241 is illegal to remove from smoke detectors now), without any chemicals, produces an image due to the charges generated... I planned to make a geiger counter, but it would be ineffective due to the fact that some of the "counts" lasted for too long: I suspected it was due to the very high charge the ions can create... BUT, after experimenting with Betavoltaics, I wondered if I could use the image sensor off the camera to make a usable voltage... anyone familiar with these sensors?

since the betavoltaics hasn't worked out, I successfully made a psuedo-ionization/electrolysis chanmber that powered the clock off of a thorium lantern mantle... never got enough for the iPod -_0
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