Smoke alarms with ionization technology utilize a small Americium-241 dioxide button as the ionization source for the smoke chamber detector that is more sensitive to small particles than the optical LED or laser units. I use this button as a test source for my electronic Geiger counter!
Americium emits 5x more alpha particles than Radium 226, & relatively little game radiation. With a half life of more than 432 years, it decays into 3% neptunium after 9 years & 5% neptunium after 32 years. A typical ionization type smoke detector contains only 0.29 micrograms (1/3 mass of a grain of sand) with 1 micro curie of 37 kBq. Americium 241 slowly decays to form neptunium 237, an element with a 2.14 mill year half-life. Inside the smoke detector, the radiated alpha particles emitted by the Americium 240 created a constant current of DC electricity between two metal plates, such that they form a capacitor where any smoke entering between these plates causes a drop in current flow of electricity, which via a transistor activates the smoke alarm speaker output to alert people nearby with a very loud audible tone that smoke is detected by the smoke alarm. The core ionization chamber so sensitive to particle interference that this type of smoke alarm sometimes creates a false alarm.
Am-241 dioxide button Manufacturing Process
Americium dioxide mix with gold & shaped into a briquette, pressure heated to 800 deg C. A backing of silver & additional covering of gold or gold platinum alloy applied to the briquette & sealed in a hot forging setup. Cold rolling of this briquette done with several passed to achieve a final thickness of 0.2 mm. This foil strip of about 20mm wide, is cute into sections that are 1 meter long. 5.1 mm disc's are punched from this strip of foil. Then the disc mounted inside an aluminum holder, which makes up the majority of what you see if you take apart a smoke alarm & its ionization chamber & look at the button containing the Americium ionization source. A press then rolls the edge of aluminum around the disc to tightly contain the disk inside the aluminum, by sealing the edge around the disc.
Space Probe RTG source
As a long life core of future probe RTG operating in 5-50 w power consumption range, the radioisotope thermoelectric generator in spacecraft can use Am-241 as the heat source, even though it produce less heat & electricity than plutonium 114.6 mW/g vs 570mW/g respective, it has better potential for long duration issues with a significantly longer half life 432.2 years vs 87 for plutonium 238. Important because of a global shortage of Pu 238 & abundance of Americium 241 in existing nuclear waste reprocessing output. Americium 241 almost isotonically pure & can give 2-2.2 Watts electrical per kg for 5-50 watts electrical RTG designs, especially given the mass of an RTG dominated by the thermoelectric converters, radiations & isotope containment enclosure.
Testing your Geiger-Mueller Counter
I purchased an ionization type smoke alarm, took it apart carefully & harvested the Am-241 source to put next to the detector tube of my geiger-mueller electronic counter unit. It shows 2 micro-sieverts per hour or 300 CPM counter per minute when parked right next to the detector tube!
Am 241 from Nuclear Waste
Americium 241 the most prevalent isotope in nuclear waste from older 2nd Gen. fission reactor fuel bundles, that are processed (recycled) to recover energetic isotopes & valuable metal elements used in medical imagine & other high value added applications, some of which require further neutron bombardment inside experimental reactors where neutron flux can activate fertile elements to fissile elements. Am 241 fissile itself with a critical mass sphere of 58-56 kg with a diameter of 19-20cm. With a specific activity of 3.43 curies per gram (3.43 Ci/g) or 126.8 gigabecquerels (GBq) per gram. Older plutonium 241 sources contain a buildup of Am 241 removed with reworking of plutonium pits with an expensive chemical process that results in Am 241 prices of around $1500 USD per gram. This makes the ionization chamber of the typical smoke alarm the most expensive single component other than the printed injection molded packaging materials.
No comments:
Post a Comment