Radiation and Smoke Detectors

In the late 1930s, a smoker inadvertently made a discovery for detecting smoke. The Swiss physicist Walter Jaeger tried to invent a sensor for poison gas. His device failed: small concentrations of gas had no effect on the sensor’s conductivity. Frustrated, Jaeger lit a cigarette—and noticed that a meter on the instrument registered a drop in the current. Smoke particles had apparently done what poison gas could not. Jaeger’s experiment was one of the advances that paved the way for the modern smoke detector.

Here’s something else surprising: Smoke detectors work because of radiation. They are an example of the beneficial uses of radiation and radioactive materials.

The first significant installations of commercial smoke detectors started in the US around 1969. Since then, the installation of smoke detectors has saved thousands of lives, numerous injuries, and millions of dollars. It has been reported that smoke detectors are installed in 93 percent of US residences. However, it is estimated that more than 30 percent of these alarms don’t work, as users remove the batteries or forget to replace them in a timely manner.

In the US, while smoke detector manufacturers and distributors are subject to NRC regulation, end users of smoke detectors (consumers) are typically not because of the small amount of radioactive material used in each detector.

The most common type of smoke detector consists of an ionization chamber, electronic circuitry, a power source that is usually a battery, an alarm mechanism, and an outer case. The ionization chamber is the main component. It consists of a source of ionizing radiation, usually Americium (Am-241) positioned between two oppositely-charged electrodes. The radiation source is a very small metallic foil disc about 3 to 5 millimeters in diameter.

To give you an idea of the small amount of radiation that is emitted by this disc, a person flying coast-to-coast gets more radiation from cosmic sources in one trip than a person sitting in the close proximity of an ionization smoke detector gets in a whole year.

Here is how the device works: Particles emitted during radioactive decay of the Am-241 interact with neutral air molecules flowing through the chamber and convert them to positive ions by removal of electrons. The removed electrons then form negative ions by attachment to other neutral molecules. The resulting positive and negative ions are attracted toward the electrodes, causing a small, reasonably steady current between the electrodes. The electronic circuitry monitors this current and, if the current drops below a preset level, which it will if the air entering the chamber contains enough smoke, it triggers an audible alarm.

If you are interested in the technical evaluations the NRC has done on smoke detectors and other consumer products containing radioactive material please see NUREG-1717 “Systematic Radiological Assessment of Exemptions for Source and Byproduct Materials” .

Ujagar Bhachu
Mechanical Engineer

Medical Use Harnesses Radioactive Material for Good

The magic pill that cures cancer has not been invented yet. However, a radioactive “pill” is already in use by physicians to destroy certain cancers from the inside out.

Many people fear radiation because it can cause damage to living cells, but modern medicine has learned to harness that characteristic for good use. If the radiation can be focused on the cancer cells, then the healthy cells can be spared.

There’s a device used by oncology departments across the country called a high dose-rate remote (HDR) afterloader. If the cancer meets specific criteria, like certain breast tumors, it may be a candidate to receive the HDR treatment. The device looks like a slimmer, sleeker version of R2D2 of Star Wars fame. Except instead of holding holograms from Princess Leia inside, it safely stores a small radioactive source with shielding around it.

On the day of the procedure, multiple tiny tubes connected to the HDR device, are inserted into the breast tissue where the tumor is located. The physicist programs the HDR device to automatically crank its source from the shielded position inside the HDR device, through the connecting tubes, and finally settling at a precise position inside the tumor. For the few seconds or minutes that it is there, the source irradiates the tumor tissue directly surrounding it, sparing most of the healthy breast tissue that is further away. When its job is complete, the source retracts into the HDR device, where its shield keeps radiation inside, and it is safe to re-enter the treatment room.

The NRC inspector discusses the procedure with the oncology staff. They talk about interlocks and emergency procedures. While malfunctions are rare, the staff is always ready to respond if the source were to be stuck outside the HDR device or fails to retract when required. The inspector verifies that the physicist uses a radiation meter to check that the source returned to its proper place inside the HDR device. Even though the goal of the treatment is to irradiate the tumor with a large amount of radiation, this must be balanced with the need to reduce the radiation that all the other tissues in the body receive. After treatment, the physicist demonstrates to the NRC inspector how the amount of radiation prescribed by the physician matches the amount of radiation actually deposited in the tumor during treatment.

This is only one example of how radioactive materials are used in medicine. There are many others uses in industrial, commercial and academic applications that the NRC inspects to ensure the safety and security of workers and the public.

Jason Razo
Region IV