Table of Contents ECSS Model Page
Background Information Radiation sources and effects
Short-term SEU rates

Overview

Single event upset rates are derived from the ion energy spectra produced by trapped particle and/or solar particle and/or galactic cosmic ray environment which have to be generated before running the upset rate model. Before calculating the upset rates the ion energy spectra are corrected for spacecraft shielding and the corresponding LET spectra are computed.

The input parameters and options for calculating single event upset rates are described below. When the input form has been completed, pressing the button will start the calculation and bring up the "Results" page.

The button calls up the model selection page for consecutive runs of multiple models. This feature is available for advanced users only.

Warning: using these buttons deletes all existing output from the upset rate model and from any model that uses this output, in order to ensure consistency in the outputs.

Input parameters

Before setting the characteristics of the sensitive volume and the cross section computation method, the user first has to define the particle environment and the shielding thickness. The models for the particle environment must be run first before they are selectable. For the trapped particles, only the generated orbit-averaged fluxes are used and not the peak fluxes. Since SEU's are short-term effects, only the solar particle flux models are considered.

To propagate the fluxes through the spacecraft wall, the CREME software is used. The thickness of the wall is set by the shielding thickness which must to be defined in units of mils, cm or g/cm2.

In a single run, the user can do calculations for as many as 15 devices with different parameters. After defining the number of devices, the user can start to set-up device by device. First the material of the sensitive volume has to be defined: silicon or gallium-arsenide. For both materials the stopping powers are derived from different sources. For silicon: the creme86 data or SRIM(2008) data, for gallium-arsenide: SRIM(2008) data or GEANT4.9 data. Next the user has the option to define a new device (user defined) or to select a device from a library. In the latter case, the device data (retrieved from the literature) are implemented and cannot be changed. They are given in the report file. For the 'user defined case' the user has to provide:

In case of a rectangular parallellopiped, the dimensions (X, Y, Z) have to be entered in µm or the cross section XY with the depth Z in µm. For all other shapes, the user has to provide the top area (=cross section area), the total area and volume of the cell and the differential path length distribution. We remark that a path length distribution can be generated with GEMAT.

Two mechanisms can create SEUs: direct ionisation by ions transported through the spacecraft skin, or ionisation by secondary particles produced by proton nuclear interactions. The parameters for each mechanism have to be supplied if the the particle spectrum contain protons. For both mechanisms either model parameters are required or experimental cross section data.

Direct ionisation effects

The cross sections for direct ionisation can be specified by a table of cross sections (cm2/bit) vs. LET values (MeV⋅cm2/mg) or by a four-parameter Weibull function (S, W, L0, σlim). If no cross sections are available, the critical charge of the device has to be specified in pC.

Proton nuclear interaction induced effects

The upset rates resulting from nuclear reactions caused by protons use an operational SEU cross section function. If cross section (cm2/bit) vs. energy (MeV) values are available, the data are fitted by a two-parameter Bendel function, a four-parameter Weibull function or linearly interpolated. Alternatively, the parameters for the Bendel or the Weibull function can be entered directly by the user. If neither of them are available, heavy ion cross section data can be used through the PROFIT method.

By default the direct ionisation upset rate is calculated using the LET spectra whereby the maximum LET of the stopping ion applies over its entire path length in the sensitive volume (= CREME method). This method may result in calculated energy depositions that exceed the residual energy of the ion and thus overestimates the upset rate. In SPENVIS two alternative algorithms have been implemented: the variable LET method and the slowing and stopping method. The first method accounts for the variation of the LET during passage and the second makes use of the ion energy spectrum instead of the LET spectrum.

The SEU rates (direct ionisation, proton induced and total) are computed either ″per mission segment″ or ″along the orbit″ in units of bit-1, bit-1day-1 or bit-1s-1. The choice is set by the output resolution which is also an input parameter.

Results

The upset rate model produces the files listed in the table below. A description of the file formats can be brought up by clicking on their description in the table.

The report file spenvis_nuop.html contains the input parameters and summary tables. The upset rate file spenvis_nuoo.txt contains, for each orbital point, the direct ionisation and proton induced nuclear reactions upset rates, as well as the combined upset rates. The corresponding LET and proton fluxes per orbital point are given in the file spenvis_nloo.txt. These files is only produced when the output resolution was set to full spectra for each orbital point. The experimental cross section data supplied by the user are stored in the spenvis_nuof.txt file. Also a file spenvis_nlol.txt with the spacecraft shielded ion energy and LET spectra is generated. In case the user imported a path length distribution, a file spenvis_nuol.txt with the imported values is created for plotting. The following files come along with the variable LET method for direct ionisation: spenvis_nlof.txt (particle averaged LET vs. energy), spenvis_nloq.txt (particle averaged LET vs. energy values along the orbit).

Output files generated by the upset rate model
File name Description
spenvis_nuop.html Report file
spenvis_nuoo.txt Upset rates for each orbital point
spenvis_nloo.txt LET and proton fluxes for each orbital point
spenvis_nloq.txt Particle averaged LET values for each orbital point
spenvis_nlol.txt Spacecraft shielded ion energy and LET spectra
spenvis_nlof.txt Particle averaged LET values
spenvis_nuof.txt Experimental cross section data and fits
spenvis_nuol.txt Imported path lentgh probability distribution

Plots are available only when the output resolution was set to full spectra for each orbital point, or when experimental cross section data or path length distribtions are supplied by the user.

To generate plots, select the plot type(s), options and graphics format, and click the or button. The current page will be updated with the newly generated plot files.

The button calls up the output page for consecutive runs of multiple models. This button only appears when the upset rate model has been included in the combined model run selection. This feature is available for advanced users only.

Last update: Mon, 12 Mar 2018