Max R. P. Grossmann: Ionization Smoke Detectors and Nuclear Reactors.

Ionization Smoke Detectors and Nuclear Reactors.

The U.S. are known for investing heavily in personal safety. For example, about 93 % of all households are equipped with 120M smoke detectors¹. In the United States, most of them are so-called ionization smoke detectors². Those contain little amounts of radioactive substances, nowadays mostly Americium-241³, that has a half life of 432½ years. It emits very weak gamma radiation, different from Radium-226, which has been used in the past and emits gamma radiation of about four percent. All percentage figures that are mentioned here in connection with various types of radiation refer to the proportion of a specific isotope's total radiation.

Gamma radiation (γ) is a very strong penetrating electromagnetic radiation, which occurs at every decay of a radioactive nuclide as mostly unwanted by-product due to physical laws. It can – mathematically – not be completely shielded and will always be absorbed in parts⁴. The so-called half-value layer specifies the needed thickness of a specified material to reduce the gamma radiation by half. Different materials have different efficiencies.

Nearly 100 percent of Americium-241's total radiation is alpha radiation⁵. The rest consists of spontaneous fission or cluster decay by emission of a silicon-34 atom⁶ (rarely).

This alpha radiation (α) is used in ionization smoke detectors. Alpha particles are Helium nuclei. They each contain two protons and two neutrons, but no electrons. However, electrons are not important in nuclear chemistry. Thus, this emitted atom is a doubly ionized cation. A small amount of air passes through milled slots in the body of the smoke detector, becomes radiated and thus ionized. After a few millimeters, two electrodes measure the electrical conductivity of the ionized air. The electrical conductivity of smoke differs significantly from the normal value (air). Once such a change is detected, an electric current flows in a noise-generating device emitting a tone.

In the U.S., ionization smoke detectors are very common since they may be disposed of as household waste. However, only one device is allowed per bin to keep the level of radioactive waste low. In addition, those smoke detectors are very cheap. All these reasons contribute to the decision to use radioactive smoke detectors.

Americium is the only nuclide from the actinide group that has found its way into households. But not only in the U.S. smoke detectors containing radioactive substances are widely used. My long experience shows that hotel companies in France and partly in Germany⁸ still rely on corresponding devices. This is supported by the wide availability and low cost: the largest online sales portal eBay offers ionization smoke detectors for a song. However, it is possible or even likely, that corresponding objects will be confiscated at customs, depending on the decision maker's expertise or knowledge.

Ionization smoke detectors can become dangerous if the radioactive content is removed by circumventing the security precautions. This will become a very serious risk factor, if the Americium is lost, incorporated or incorporated. Also there is a risk if the smoke detector is disposed of as normal waste, even the Anglo-American space allows this practice.

The real problem is that radioactive sources are available at all. There are people who have extracted the Americium and used it for own experiments - not only from one smoke detector, but from hundreds. One of these people is David Hahn, also known as the "Nuclear Boy Scout"^{9, 10}. In 1994, Hahn created a nuclear reactor in his garden shed. First, he contacted several manufacturers of ionization smoke detectors and asked them to donate a few devices for a school project¹⁰. One of those companies sold him a hundred of non-salable items for a few dollars. After he removed the americium under the help of an electronics company, he built a neutron gun - that is not very difficult - and extracted Thorium from lantern mantles and Lithium from batteries¹¹, he had a working reactor. David Hahn is sure that he has produced a few Plutonium atoms during the lifetime of the reactor¹⁰.

Hahn's reactor was discovered as FBI agents found a tool box sealed with duct tape in his car. Hahn told them that it was radioactive. Finally, the reactor was dismantled, the garden shed demolished and disposed of as radioactive waste in Utah. David Hahn's face shows severe damage from the months of radiation exposure¹⁶.

In theory, everyone can buy ionization smoke detectors. They are not particularly suitable for nuclear bombs, since the critical mass of Americium-241 is 55-77 kg¹². It should be impossible to purchase such an amount of smoke detectors. Since a gram of americium-241 costs about $ 1500, such an amount cannot be procured without further notice ($ 115,500 for 77kg). But there are still so-called dirty bombs, whose job is to use a conventional explosive device to spread the radioactive nuclides in the environment and thus make it virtually impossible for decontamination. There would be neither nuclear fission nor nuclear fusion.

Americium-241 can be produced in nuclear reactors. The mother nuclide, Plutonium-241, decays by emitting an electron and an electron antineutrino¹³ to become the long-living americium-241. Thus, Beta rays (β−) are released. Beta radiation cannot penetrate a thin aluminum plate.

Thus, several kilograms of Americium-241 are produced annually, belonging either to nuclear waste or used more extensively. Besides, Americium-241 can also be used for physics education in schools¹⁴, as neutron source or radionuclide¹⁵.

However, there are already alternatives to ionization smoke detectors. Optical smoke detectors can replace the radioactive devices almost seamless as they are at eye level. In addition, they are not as prone to false positive alarms.

Ionization smoke detectors are only justified if a strong distribution of smoke particles has to be expected. For example, this could be the case in warehouses. But even here are alternatives: CO₂ detectors, flame detectors, or so-called multi-criteria detectors, which combine several techniques in themselves and thus provide a highly secure application. Thus, there are hardly any areas in which the use of ionization smoke detecors is justified and acceptable in accordance with the risks.

Bibliography

(partly German)