5.1 The neutron test spectrum must be known in order to use a measured device response to predict the device performance in an operational environment (Practice E1854). Typically, neutron spectra are determined using a set of sensors with response functions sensitive over the neutron energy region to which the device under test (DUT) responds (Guide E721). 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 making its determination of critical importance.

5.2 When dosimeters with a significant response in the 0.01 to 2 MeV energy region, such as fission foils, are unavailable, silicon transistors can provide a dosimeter with the needed response to define the spectrum in this critical energy range. When fission foils are part of the sensor set, the silicon sensor provides confirmation of the spectral shape in this energy region.

5.3 Silicon bipolar transistors, such as type 2N2222A, are inexpensive, smaller than fission foils contained in a boron ball, and 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 spatially map 1-MeV(Si) equivalent displacement damage fluence. The proper set of steps to take in reading the transistor-gain degradation is described in this test method.

5.4 The energy-dependence of the displacement damage function for silicon is found in Practice E722. The major portion of the response for the silicon transistors will generally be above 100 keV.