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Americium 241 fluorescence

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LAN_403

Tesladownunder

Mon Mar 10 2008, 11:38AM

Registered Member #10 Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424

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

Proud Mary

Mon Mar 10 2008, 12:54PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

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.

Bjørn

Mon Mar 10 2008, 01:02PM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

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.

Proud Mary

Mon Mar 10 2008, 02:29PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

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.

Tesladownunder

Mon Mar 10 2008, 09:56PM

Registered Member #10 Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424

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

Proud Mary

Tue Mar 11 2008, 01:13AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

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.

Bjørn

Tue Mar 11 2008, 06:54AM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

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.

Proud Mary

Tue Mar 11 2008, 10:00AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

BjÃ¸rn BÃ¦verfjord 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.

Bjørn

Tue Mar 11 2008, 10:23AM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

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.

Tesladownunder

Tue Mar 11 2008, 12:01PM

Registered Member #10 Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
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

1205236599 10 FT40676 Spinthariscope3redgreenlightson

1205236599 10 FT40676 Spinthariscope3redgreen

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