Final answer:
Fixed-temperature heat detectors are triggered by the melting of heated material, unlike smoke detectors which rely on ionization. Heat detectors use a thermal fusible link or materials that change in resistance or expand when subjected to high temperatures to detect fires.
Step-by-step explanation:
Fixed-temperature heat detectors are activated by expansion of heated material, changes in resistance of heated material, and the melting of heated material. Unlike smoke detectors that use ionization for the detection of smoke particles interfering with a current created by radioactive materials such as americium, heat detectors rely on physical changes induced by high temperatures. For instance, certain detectors use a thermal fusible link that melts at a specific temperature, causing a mechanical action to trigger the alarm. Other detectors might employ materials that undergo a drastic change in electrical resistance or physical expansion to sound an alarm.
In terms of radiation detectors mentioned in the provided information, they operate on a different principle, largely involving the ionization of materials and conversion of this ionization or other radiation interaction into electrical signals. This can be seen in smoke detectors and solid-state radiation detectors used for various applications.