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27 November 2013

Americium-241 as Gamma Ray Source

The ionization type smoke detector has an interesting nuclear radiation source which I have removed to make a simple spintariscope earlier. Because I have mentioned how I got my Am-241 radioisotope, here I will just describe what I was doing with it using my newly bought Geiger-Muller detector

The GM assembly was a low-cost unit bought from GQ electronics, USA. The detector was 99% completed (even included a rechargeable battery) but without the tube itself, so I bought a US Navy surplus CBON 6107/BS-212 Geiger Muller Tube manufactured by Anton Electronic Laboratories during the 1950's as the radiation detecting element. Naturally, I suspect there might be some degradation of gaseous mix inside the tube over the inactive years especially when the tube features an alpha radiation permeable mica window but I currently have no resources to test this hypothesis.

Well, the availability of the window was actually the reason why I bought the tube in the first place since Am-241 sources emits mostly alpha radiation. My previous attempts on taking long-exposure photographs on a phosphor screen scintillated by alpha particles at different distance from the radioactive source have failed to give any substantial data (or any photographs actually showing variation of scintillation intensity for that matter). What I really want to do was simple - to observe the inverse square law in ionizing radiation. 

In fact, the set up was so simple, there is really nothing more simple it can go. Here's a photograph of my actual set up:

As you can see, two wooden blocks of same dimension was placed linearly away from each other with the block on the left sticky-tacked to the table, securing its position. The block on the right was free-moving and it is where the radioactive source was tacked onto. The block on the left is where the detector GM-tube was placed, held firm by masking tape and connected to the Geiger Counter circuit with crocodile clips. The circuit features an audio data cable which transfers the audible "clicks" to my laptop's sound card which a software was provided by the vendor was installed to count the "clicks" at a given time. 

I understand this kind of set-up was crude but I have not a linear translational stage for precise distance measurement. So this is really the most I can do now. The measurement was to record the number of "clicks" the counter register over a predetermined exposure time, then average the results for a quick radiation measurement unit called [CPM] or counts per minute, which describes the average number of nuclear events registered by the GM tube. 

First up, I measured the background radiation (i.e. without the radioactive source nearby the detector) which over 15 minutes gives me 11 CPM. Next, I measured the CPM when the source was directly 1 mm away from the GM tube, then at 2 mm, 3 mm, so on until 30 mm. The results were then subtracted 11 CPM to compensate background radiation, here was what I got:

Radioactivity at increasing source distance using Am-241 unattenuated 0.8 microcurie source. 

Notice instead of an exponential decay, the rate of decrease seems to fit Boltzmann sigmoidal function. There are apparently two "interesting" regions observed from the plot. For one, at source distance closer than 5 mm, the counts seems to "flat out" slightly less than 1200 CPM. This seems to suggest the existence of a saturation limit for count discrimination in the software provided by GQ electronics. Since the software interpret the threshold of audio signal "clicks" coming from the GM circuitry, if two successive clicks was loud and close enough, the software might not be able to resolve the coincidence thus limits the CPM resolution at higher activity levels. This I have yet again to verify, perhaps in the future with x-ray photons

The next anomaly is when the source distance range between 10 to 14 mm. The rate of decrease of activity seems to "slow down" which suggest a drastic drop in alpha radiation being absorbed (attenuated) by air but retains a fraction of gamma radiation which is still relatively "bright" at that point. But then again, it could just be a statistical anomaly because of the short exposure time (3 minute average).

I guess more importantly, the data possibly showed the process of nuclear decay is complicated and "dirty". A pure sample of Am-241 was known to be both alpha and gamma ray emitter, although with the latter far less significant than former. Nevertheless, I believe it could be separately measured and detected. Before I went on discussing how I do so, let us study the decay scheme of Am-241:

The figure above graphically illustrate the first decay product of Americium-241 to Neptunium-237. It shows the probability of decay channels which for alpha particles we can see it is with kinetic energy about 5.5 million electron volts. On gamma rays, Am-241 nucleus relaxes mostly on 13.9, 17.6 and 59.5 thousand electron volts, these photon energies are comparable to those of medical x-ray machines which can be shielded with appropriate measure. 

As previously mentioned, the GM tube has a fragile mica window (not seen in photos), which was protected with a black metallic cap (see photo below) having a small hole about 1.2 mm diameter measured with vernier caliper to prevent accidental puncture, thus destroying the tube. The tube itself has an outer and inner diameter of 8.8 mm and 6.4 mm respectively. To register signal for alpha particles, it will have to enter the tube through the small hole on the metal cap. Gamma rays however, depending on its energy, have the capability to penetrate the metal cap, thus the detection area will be a wider, 6.4 mm.

The GM tube was protected by a black "top-hat" metal cap, with a small hole letting alpha particles through.

With the previous result, I decided to check if I could measure significant counts after "filtering" out the alpha particles emitted from Am-241 by placing plastic sheets (as attenuator) in front of the tube. The sheet was a simple plastic projector slide cut out into 25 square centimetre pieces, I weighed them with a sensitive scale to obtain its density. Then I measured the background-compensated counts by successively adding attenuators at a fixed source-to-detector distance (10 mm), this was what I obtained:

When a "filter" was introduced, the counts drastically reduced but stay relatively the same for increasing thickness.

Without any attenuation, the GM counter reads slightly less than 800 CPM. When a piece of plastic was introduced between the source and detector, the count reduced dramatically to about 120 CPM, which is still relatively (about 10 times) higher than background radiation. I followed up by stacking up to 4 sheets of plastic, which persisted in similar counts. This suggest the radiation has relatively high penetration power, possibly gamma ray photons. 

With this in mind, I proposed finding the inverse square relation using attenuated Am-241 source simply by placing a plastic "filter" in front of the radioactive source. By measuring the counts starting 7 mm, in 3mm increment steps, it took a total of 2.5 hours to get a full set of data and here was the plot:

Inverse square law for gamma radiation emitted from Am-241, 0.8 uCi source. 

It fits nicely to an exponential decay profile. According to theory, for every linear multiple of distance away from the source, the intensity of radiation, in our case, CPM, should drop by the factor of the squared distance. Which is quite a dramatic drop (a very steep decay curve) and my data is far from adhering this idealized case. 

In addition to the different penetrating strength of low-energy gamma rays emitted from Am-241 sources, an intelligent guess would be the intensity of gamma radiation was tempered by the absorption and scattering mechanism on the plastic sheet. Besides, as mentioned, the energy range of gamma rays emitted by Am-241 falls within 100 kiloelectron volt, not to mention some are in the soft x-ray range which readily interacts with low density materials such as polymers. 

For the purpose of this study, every gamma ray photon matters (pun unintended), therefore choosing the right attenuator I think, plays a critical role in demonstrating the inverse square law relationship. I reckon fully shielding the GM tube with a 3 mm thick aluminium plate to block away other low-energy gamma rays allowing the majority 59.5 keV photons to pass through, probably will get a more theory-consistent curve with that configuration. 


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