NASA ISS IV-TEPC Instrument: Tissue-equivalent proportional counter

The Tissue Equivalent Proportional Counter (TEPC), in situ on the ISS, consists of a spectrometer and cylindrical detector with which to measure external radiation doses.

The purpose of the TEPC is to collect a record of the International Space Station (ISS) environment to construct exposure history records for the crew.

Credit: NASA, JPL

Description
The Tissue Equivalent Proportional Counter (TEPC) is a gas proportional counter used to characterize the radiation environment.

TEPC will also provide near real-time measurements to ground personnel during radiation events and make survey measurements in different parts of the ISS for shield verifications.

TEPC collects data as a function of time to measure the dose and estimate the dose equivalent by making spectral measurements of the lineal energy loss of the radiation as it passes through the detector volume.

The omni-directional detector is surrounded by a tissue equivalent plastic and the internal gas (propane) provides an energy deposition response similar to human tissue. The detector gas is at a very low pressure such that the mass of the gas is approximately that of a cell.

The 512 channel spectrometer stores the lineal energy data in energy bins ranging from approximately 25 keV/micron through channels exceeding 1000 keV/micron. The crew is able to read the current level through an electronic display and has the capability to telemeter data to the ground every 10 seconds.

TEPC is a portable piece of equipment, integrated with numerous ports in various modules to support the survey function of the equipment.

TEPC is an automatic micro-dosimetry system. Each TEPC consists of two main components, the spectrometer unit and the detector unit.

The spectrometer unit contains a powerful computer that allows real-time analysis of the data and provides calculations of total dose, total dose equivalent and incremental dose, as a function of linear energy transfer (LET) and time, for penetrating radiation in space.

The detector unit is attached directly to the multi-channel analyzer (MCA) card in the spectrometer.

Different size detectors can be attached to the TEPC depending on the desired task.

The radiation data that is measured can be stored inside the spectrometer unit for later analysis or communicated via RS-232 to a host computer.

The TEPC is calibrated in terms of lineal energy, by exposing it to fission neutrons and 137Cesium sources.