Ionization smoke detector

Smoke detector with americium ²⁴¹ Am from the inside

Ionisation , ionization , briefly I-detectors consist of a chamber the air with a radioactive substance, usually Americium is ionized -241. Smoke particles slow down the ions, which is noticeable as a voltage change. They have long been smoke alarms of choice. Due to the radioactivity, they are gradually being replaced by optical smoke alarms .

Working principle

Usually two identical chambers are used, both of which are provided with electrodes to which a high voltage is applied. The chambers are electrically connected in series and form a voltage divider .

The reference chamber is almost completely tight. The measuring chamber is open so that smoke can penetrate (one electrode is designed as a grid).

The air in the chambers is ionized by an alpha or beta emitter, causing a current in the nanoampere range to flow.

If smoke particles penetrate the open measuring chamber, they attach to the ions and slow them down. The current decreases, which corresponds to an increase in resistance. This increases the voltage at the measuring chamber, and an alarm is triggered from a certain voltage level .

Various disturbance variables such as B. Air pressure and humidity change only slowly, have the same effect on both chambers and are therefore not measured.

amplifier

The ionization currents are very small. In the beginning, electron tubes were used. They could measure currents in the range of a few nanoamps . These were later replaced by field effect transistors ( MOSFET ). With them it is possible to measure currents of less than 10 pA. This enabled the necessary radioactivity to be reduced accordingly.

isolation

When measuring small currents, isolation is of great importance. Water on the surface , dust and moisture lead to leakage currents. The isolation gaps therefore had to be large and made of suitable materials.

history

As early as the 1920s, there were experiments on smoke gas detection using an ionization chamber.

At the end of the 1930s, the physicist Walter Jäger wanted to use an ionization chamber to detect poison gas, and found that cigarette smoke was much better recognized. Then in 1941 in Bad Ragaz , the Cerberus GmbH founded, there seemed Walter Hunter (founder), Ernst Meili (technical director) and Hans Lutz von Gugelberg (developers of cold cathode tubes) along a high-voltage measurement with sensitive cold cathode tubes to explore and build. Finally, in 1946, a glow relay with a starter anode was developed, which can measure the voltage and amplify the current millions of times. This made it possible to build ionization smoke detectors.

A total of 5 million smoke alarms were sold under the name F2 (from 1946) and improved again as F3 (from 1951). From 1967 onwards they made some with transistors. This made small, inexpensive, battery-operated detectors possible for the household.

Cerberus was later taken over by Siemens Building Technologies. In 1954, Gugelberg founded the Elesta company, which has continued to this day.

Advantages and disadvantages

Ionization detectors are particularly sensitive to small smoke particles , such as those that occur mainly in flaming fires , but also in diesel soot . In contrast to this, optical fire alarms are better suited for the early detection of smoldering fires with relatively large and light smoke particles. The detection behavior of both detector types should therefore rather be viewed as complementary to each other. A clear advantage in terms of security against false alarms (from water vapor , kitchen fumes, cigarette smoke, etc.) cannot be identified for any of these detector types. For use as life-saving smoke alarm devices, as they are already required in some German federal states, only optical smoke alarms (standard equipment) or heat alarms (additional equipment, e.g. in kitchen areas) are used.

Alpha emitters used

Radium -226

The first ionization smoke detectors used radium as an alpha emitter. Radium also emits a large amount of gamma radiation . Only relatively large currents could be measured. The spotlight had to be correspondingly strong. In the past, the radium was applied by hand with brushes to a small area in the smoke detector; with corresponding radiation exposure of the workers. Later the radioactive material was rolled on as an oxide matrix under a very thin but hermetically sealed precious metal cover. The panes or foils were built into the detector as so-called "enclosed radioactive substances". A type approval, and thus general usability, if the installer had a handling permit , was therefore possible.

Xenon , krypton ( ⁸⁵ Kr) and tritium were later used in glass ampoules. However, the gases also harbored risks, particularly during production.

Americium-241

The substances were later replaced by the americium isotope 241 ( ²⁴¹ Am). It has a much lower gamma component. To do this, however, the activity against radium must be about five times as much.

Problem

Due to strict requirements, ionization detectors are only used in special cases. However, the hazard potential of an individual detector is low when used and disposed of as intended. Normally, the ionization detectors are completely harmless due to their low level of activity .

After a fire, however, the debris must be searched for missing fire alarms. If not all detectors are found, the entire fire debris must be disposed of as hazardous waste in accordance with the radiation protection regulations (at least in the EU area ) , which can cause considerable additional costs after the fire brigade has been deployed . Finding the detectors is not always easy. With Geiger counters , you hardly have a chance of finding them under a layer a few centimeters thick. A visual search is also possible. The approximate location of the fire alarm can be estimated using the fire alarm installation plan.

The regulations regarding radioactive emitters have been tightened over time. Manufacturers offer to properly dispose of the radioactive emitters. If a house with a fire alarm system is demolished, probably nobody thinks anymore that there is a radioactive emitter in every room, which has to be properly disposed of. The owner must ensure that the ionization detectors are properly disposed of. Even the installer of the fire alarm system must document every replacement of an ionization detector during maintenance and provide evidence of disposal. For this reason, the use of ionization smoke detectors is prohibited in the technical connection requirements for fire alarm systems in more and more rural districts.

Another sensitive point is the corrosion of the sources. The radioactive materials cannot be encapsulated well because otherwise the alpha radiation would be absorbed by the envelope. In practice, the radiator was often only covered with a very thin layer of gold or not at all. If the source corroded over time, radioactive material could escape. The radiation exposure of an intact detector mounted at some distance is low. Only a small proportion of gamma radiation reaches the resident. However, if radioactive material escapes and is z. B. inhaled, this can have serious consequences because alpha emitters have a highly damaging effect on the body.

Ionization detectors are most widespread in Anglo-America and Australia , where they can usually be disposed of with household waste.