ASTM E1855-10

The neutron spectrum in a test (simulation) environment must be known in order to use a measured device response in the test environment to predict the device performance in an operational environment (see Practice E1854). Typically, neutron spectra are determined by use of a set of sensors that have response functions that are sensitive over the neutron energy region to which the device under test (DUT) responds (see Guide E721). In particular, for silicon bipolar devices exposed in reactor neutron spectra, this effective energy range is between 0.01 and 10 MeV. A typical set of activation reactions that lack fission reactions from nuclides such as ²³⁵U, ²³⁷Np, or ²³⁹Pu, will have very poor sensitivity to the spectrum between 0.01 and 2 MeV. For a pool-type reactor spectrum, 70 % of the DUT electronic damage response may lie in this range. Often, fission foils are not included in the sensor set for spectrum determinations because their use must be licensed, and they require special handling for health physics considerations. The silicon transistors provide the needed response to define the spectrum in this critical range.

If fission foils are a part of the sensor set, the silicon sensor provides confirmation of the spectrum shape.

Bipolar transistors, such as type 2N2222A, are inexpensive, are smaller than fission foils contained in a boron ball, and are sensitive to a part of the neutron spectrum important to the damage of modern silicon electronics. They also can be used directly in arrays to map 1-MeV(Si) equivalent fluence. The proper set of steps to take in reading the transistor-gain degradation is the primary subject of this test method.

1.1 This test method covers the use of 2N2222A silicon bipolar transistors as dosimetry sensors in the determination of neutron energy spectra, and as silicon 1-MeV(Si) equivalent displacement damage fluence monitors.

1.2 The neutron displacement damage is especially valuable as a neutron spectrum sensor in the range 0.1 to 2.0 MeV when fission foils are not available. It has been applied in the fluence range between 2 × 10 ¹² n/cm² and 1 × 10¹⁴ n/cm² and should be useful up to 10¹⁵ n/cm². This test method details the steps for the acquisition and use of silicon 1-MeV equivalent fluence information (in a manner similar to the use of activation foil data) for the determination of neutron spectra.

1.3 In addition, this sensor can provide important confirmation of neutron spectra determined with other sensors, and yields a direct measurement of the silicon 1-MeV fluence by the transfer technique.

1.4 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.

1.5 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory requirements prior to use.