I have a reprint of Army Technical Manual TM9-1105-249-10 for the M16A1 Rifle. Any army manuals that end with -10 are crew/operator manuals. Manuals for weapons ending in -20 are for unit armorers. And -30 or -40 are for direct support and organizational support. I've never seen a -50 manual but that would be depot level where they do complete rebuilds, like re-riveting machine gun back together.
This manual says, "Some rifles are equipped with the Low Level Sight to increase effectiveness during periods of limited visibility." Instead of regular and Long range apertures, the rear sight has 7mm and 2mm apertures. The front sight post contains a vial of luminous material, and the element is seen from 2 sides only on the 4 notch sight. No surprises so far, but it's not a Tritium night sight. The front sight post contains a small glass vial of radioactive Promethium 147, and the manual warns about bumping, abusing, tampering with or altering it in any manner.
Promethium 147 is so rare, there's
total of 1.1 pounds of it in the Earth's crust, compared to
2,535,290,000,000 pounds of iron
mined per year. So they make it in nuclear reactors.
Promethium-147 (Pm-147) is a radioactive isotope of promethium with a half-life of approximately 2.62 years. It undergoes beta decay, emitting low-energy beta particles without significant gamma radiation, which makes it suitable for certain industrial and scientific applications. Pm-147 does not occur naturally in significant amounts; it is typically produced artificially in nuclear reactors as a fission product of uranium or through neutron activation of neodymium-146.
Pm-147 is widely used in industry and technology. Its low-energy beta emissions make it an excellent source for thin-film thickness measurement and quality control in manufacturing processes. It is also used in the production of phosphorescent materials for luminescent dials, signs, and gauges. Some signal lights use a luminous paint containing a phosphor that absorbs the beta radiation emitted by promethium-147 and emits light.
This isotope does not cause aging of the phosphor, as alpha emitters do, and therefore the light emission is stable for a few years. Originally, radium-226 was used for the purpose, but it was later replaced by promethium-147 and tritium (hydrogen-3).
Additionally, Pm-147 serves as a power source in radioisotope thermoelectric generators (RTGs) for space applications and other remote power needs, as its beta radiation can be efficiently converted to electricity.
Pm-147 is not naturally occurring in measurable quantities due to its relatively short half-life. It is produced as a byproduct in nuclear reactors, particularly during the fission of uranium-235 or plutonium-239. Pm-147 is found in controlled environments, such as research laboratories, industrial settings, and facilities involved in the production of radiopharmaceuticals or specialized instruments. Its production and use are carefully regulated due to its radioactive nature.. Promethium-147 is also used, albeit in very small quantities (less than 330nCi), in some Philips CFL (Compact Fluorescent Lamp) glow switches in the PLC 22W/28W 15mm CFL range.
Promethium-147 is also used, albeit in very small quantities (less than 330nCi), in some Philips CFL (Compact Fluorescent Lamp) glow switches in the PLC 22W/28W 15mm CFL range.
Promethium-147 has a half-life of 2.62 years, and is a fission product produced in nuclear reactors via beta decay from neodymium-147. The isotopes 142Nd, 143Nd, 144Nd, 145Nd, 146Nd, 148Nd, and 150Nd are all stable with respect to beta decay, so the isotopes of promethium with those masses cannot be produced by beta decay and therefore are not fission products in significant quantities (they could only be produced directly, rather than along a beta-decay chain). 149Pm and 151Pm have half-lives of only 53.08 and 28.40 hours, so are not found in spent nuclear fuel that has been cooled for months or years. It is found naturally mostly from the spontaneous fission of uranium-238 and less often from the alpha decay of europium-151.
Promethium-147 is used as a beta particle source and a radioisotope thermoelectric generator (RTG) fuel; its power density is about 2 watts per gram. Mixed with a phosphor, it was used to illuminate Apollo Lunar Module electrical switch tips and painted on control panels of the Lunar Roving Vehicle.
Out of several dozen isotopes, only promethium-147 has uses outside laboratories. It is obtained as the oxide or chloride, in milligram quantities. This isotope has a relatively long half-life, does not emit gamma rays, and its radiation has a relatively small penetration depth in matter.
Some signal lights use a luminous paint containing a phosphor that absorbs the beta radiation emitted by promethium-147 and emits light. This isotope does not cause aging of the phosphor, as alpha emitters do, and therefore the light emission is stable for a few years. Originally, radium-226 was used for the purpose, but it was later replaced by promethium-147 and tritium (hydrogen-3). Promethium may be favored over tritium for nuclear safety.
In atomic batteries, the beta particles emitted by promethium-147 are converted into electric current by sandwiching a small promethium source between two semiconductor plates. These batteries have a useful lifetime of about five years. The first promethium-based battery was assembled in 1964 and generated "a few milliwatts of power from a volume of about 2 cubic inches, including shielding".
Promethium is also used to measure the thickness of materials by measuring the amount of radiation from a promethium source that passes through the sample. It has possible future uses in portable X-ray sources, and as auxiliary heat or power sources for space probes and satellites (although the alpha emitter plutonium-238 has become standard for most space-exploration-related uses).
Promethium-147 is also used, albeit in very small quantities (less than 330nCi), in some Philips CFL (Compact Fluorescent Lamp) glow switches in the PLC 22W/28W 15mm CFL range.
The half-life of Tritium is 12.33 years, 4.7 times longer than Promethium. Each one is more dangerous than the other in different ways, but tritium is more useful for self-illuminated sights, switches and buttons, etc. because it lasts longer.
https://www.youtube.com/watch?v=764_8VxNrgg 
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