Tissue Equivalent Proportional Counter (TEPC) - 05.13.15

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Science Objectives for Everyone
The Tissue Equivalent Proportional Counter (TEPC) is a microdosimetric instrument that measures radiation dose and dose equivalent in complex radiation fields (fields containing a mixture of particle types).
Science Results for Everyone Radiation, NASA has your number. Since 2001, the Tissue Equivalent Proportional Counter (TEPC) has been used on the ISS to measure exposure of crew members to real-time, low-Earth orbit radiation. A new generation TEPC currently in development features improved materials, resolution, and sensitivity and a lighter weight. These dosimeters must be highly reliable, compact, and take into account different radiation types and changes in the radiation environment within the ISS. The next-generation TEPC uses a multi-detector arrangement and a laminated spherical design that detects gamma rays, high-energy protons, as well as iron ions from cosmic radiation which can damage artery walls.

The following content was provided by Fadi M. Riman, and is maintained in a database by the ISS Program Science Office.
Facility Details


Facility Manager(s)
Fadi M. Riman, Engineering and Science Contract Group, Houston, TX, United States

Facility Representative(s)
Information Pending

Battelle Pacific Northwest National Laboratory, Richland, WA, United States

Sponsoring Space Agency
National Aeronautics and Space Administration (NASA)

Sponsoring Organization
Human Exploration and Operations Mission Directorate (HEOMD)

ISS Expedition Duration
March 2001 - March 2010

Expeditions Assigned

Previous ISS Missions
The ISS TEPC is a fifth generation detector system which has been used on Mir and several Space Shuttle Missions.


  • Onboard
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    Facility Description

    Facility Overview

    • The Tissue Equivalent Proportional Counter (TEPC) consists of a spectrometer and cylindrical detector with which to measure external radiation doses.
    The Tissue Equivalent Proportional Counter (TEPC) is a gas proportional counter used to characterize the radiation environment. 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. 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 microdosimetry 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. When TEPC is powered on, it is either in standby or acquire mode. Standby can only be achieved by 1553b command. It is use to put the unit in a Mode that will support the data download protocol of the command and data handling (C and DH) system and support unit commanding. It also will store the current file to memory. In Acquire the unit is collecting data.

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    Facility Operations

    • The TEPC is to be in the acquire Mode continuously. It is taken to standby weekly to transfer detailed records to the ground or as needed to send other commands (infrequent).

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    Results/More Information

    Tissue-equivalent proportional counters (TEPCs) are radiation detectors being continuously used on the International Space Station (ISS) to measure real-time low-Earth orbit radiation doses that space crews are exposed to while living and working on the space station. The first TEPC was launched to ISS during Expedition 2 in 2001; it operated successfully for 5 years before failing on orbit; it was powered off on 10/12/2006. The TEPC was returned on STS-117/13A, a new TEPC was delivered to the ISS on STS-118/13A.1. The new TEPC had several improvements; the toxicity level 4 battery was removed, and a new software version was implemented. The characteristics of the improved detector exceed those of any other TEPC detector that has flown, and this version has been used on ISS through 2011. However, advancements in technology now allow for further improvements in materials, weight reduction, resolution and sensitivity by developing the next generation of TEPCs which will replace existing ones currently operating on the space station. ISS onboard radiation dosimeters must be highly reliable, compact, and take into account different radiation types as well as changes in the radiation environment due to different levels of shielding at different locations inside the ISS. The next generation TEPC uses a multi-detector arrangement and a laminated spherical detector design to increase the range of radiation being detected to include gamma rays, high-energy protons and iron ions, and to provide better tolerance for detector’s placement and orientation. The new design has been ground tested in simulated radiation conditions and results indicate that, with new technologies and materials, desired performance objectives are achievable for the next generation of TEPCs (Perez-Nunez and Braby 2011).

    Results Publications

      Perez-Nunez D, Braby LA.  Replacement Tissue-Equivalent Proportional Counter for the International Space Station. Radiation Protection Dosimetry. 2011; 143(2-4): 394-397. DOI: 10.1093/rpd/ncq409.

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    Ground Based Results Publications

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    ISS Patents

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    Related Publications

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    Related Websites

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    image NASA Image: ISS015E12110 - View of the Tissue Equivalent Proportional Counter (TEPC) Radiation Detector (gold cylinder) and the TEPC Spectrometer (gold box) in the U.S. Laboratory, Destiny during Expedition 15. The TEPC monitors radiation doses at the cellular level.
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