Particle Detectors: (4.26) Smoke Detector: (2) Design: (2. 2) Ionization: An ionization type smoke detector is generally cheaper to manufacture than an optical smoke detector; however, it is sometimes rejected because it is more prone to false (nuisance) alarms than photoelectric smoke detectors. It can detect particles of smoke that are too small to be visible. Americium-241, an alpha emitter, has a half-life of 432 years. Alpha radiation, as opposed to beta and gamma, is used for two additional reasons: Alpha particles have high ionization, so sufficient air particles will be ionized for the current to exist, and they have low penetrative power, meaning they will be stopped by the plastic of the smoke detector or the air. About one percent of the emitted radioactive energy of 241Am is gamma radiation. The amount of elemental americium-241 is small enough to be exempt from the regulations applied to larger sources. It includes about 37 kBq or 1 µCi of radioactive elementamericium-241 (241Am), corresponding to about 0. 3 µg of the isotope. This provides sufficient ion current to detect smoke, while producing a very low level of radiation outside the device. The americium-241 in ionizing smoke detectors poses a potential environmental hazard. Disposal regulations and recommendations for smoke detectors vary from region to region. Some European countries[which?] have banned the use of domestic ionic smoke alarms