Elements Included in LET Spectra Calculations

You must specify the range of elements to be included in your LET spectral calculations. It is not necessary to include in your LET spectrum all of the elements which are contained in your input particle flux file (ie., the "something.tfx" file from the TRANS program or the "something.flx" file from the FLUX program).

For example, your input particle flux file will probably include protons (Z=1).

However, it is generally recommended that protons should not be included in the LET spectra which are used to calculate direct-ionization induced SEE rates in the HUP program.

• Proton-induced SEEs in most devices are due to nuclear recoils from the target material. Because the underlying physical mechanism is different, proton-induced SEE rates are calculated with a different program, PUP. In fact, including protons in the LET spectrum can - especially in some large, low-threshold devices - lead to gross overestimates of the SEE rates, particularly in solar particle ("flare" ) environments. This overestimate occurs because the standard LET/RPP SEE-rate formalism does not properly handle very low-energy protons which range-out in the bit's sensitive volume.

One important exception to this general guidance is optocouplers, which appear to be sensitive to direct ionization by protons. For these devices, protons should be included in the LET spectrum .

• See K.A. LaBel et al., "Proton-Induced Transients in Optocouplers: In-Flight Anomalies, Ground Irradiation Test, Mitigation, and Implications", IEEE Transactions on Nuclear Science 44, 1885-1892 (1997).

On the LETSPEC input page:

• By default, the minimum atomic number in LETSPEC is set to 0 (zero), which instructs the program to begin with the lowest atomic number found in the input particle flux file.
• If you wish to specify some other minimum atomic number, you must overwrite this default value.
• If the minimum atomic number you specify is less than the minimum value found in your input .tfx/.flx file, the calculation will start with the lowest value in the file.
• If you enter 0 for the maximum atomic number, the program will include elements up to and including the highest atomic number found in the input particle flux file. This is the default, which you must overwrite to obtain some other maximum.
• If the maximum atomic number you specify is greater than the maximum value found in your input .tfx/.flx file, the calculation will include only up to the highest value in the file.