Why is the heavy ion test fluence 10,000,000 ions per square centimetre?

It is where the smallest cross section you want to be able to exclude meets the largest total dose you can afford to put into the part. The fluence is set from below by statistics. If a part is irradiated and nothing happens, the result is not "the cross section is zero", it is "the cross section is below some bound". For a Poisson process with no observed events, that bound is a constant divided by the fluence. JESD57A writes it as 3.7 divided by the fluence of no errors times cos(theta). At 1e7 ions/cm2 that gives 3.7e-7 cm2, which is roughly a third of a micron squared per device. For a memory that is well below one bit, and for a discrete it is a small fraction of the die. Run only 1e5 ions/cm2 and the bound is 3.7e-5 cm2, which excludes almost nothing worth excluding.

Why 3.7 for a null result, when other people use 3.0 or 2.3?

All three are the same Poisson calculation. They differ only in the confidence level and in whether the interval is one sided or two sided. For a Poisson process with zero observed events, the upper limit on the mean at a one sided confidence level CL is minus the natural log of one minus CL. That gives 2.303 at 90 percent, 2.996 at 95 percent and 4.605 at 99 percent. This is where the familiar 2.3 and 3.0 come from.

Why is the beam flux held between roughly 100 and 100,000 ions per square centimetre per second?

Too high and the ions stop being independent single events; too low and the run does not finish. The whole cross section formalism assumes each ion acts alone on an undisturbed device. Push the flux up and that assumption fails in several separate ways: two ions land in the same cell between reads and are counted as one multiple-bit upset; charge from one strike has not been swept out before the next arrives; the die heats; the dosimetry system, which is usually counting individual ions, saturates. ASTM F1192 calls these collective effects and allows higher flux only if each of them is shown not to matter.

Why do the standards ask for three parts, and what does three parts actually buy?

Three is the smallest number that lets you see part to part disagreement at all. It does not characterise a distribution, and the documents do not claim it does. The counts differ by document because they are answering different questions. ESCC 25100 asks for three samples and permits two for non-destructive effects, and requires three as a minimum for gate rupture and burnout so that compliance can be checked at each test condition. JESD234 asks for five for proton work on a homogeneous lot, and more when the lot is not homogeneous. JESD57A declines to give a number at all and says to use the largest sample practical, on the reasoning that confidence comes from events and devices together.

Why does the lid have to come off?

Because the horizontal axis of a cross section plot is the LET at the sensitive volume, and package material changes it by an unknown amount. An ion loses energy continuously as it travels. Every millimetre of lid, every micron of coating, every layer of overglass shifts the ion down its stopping power curve, so the LET that reaches the die is not the LET the facility quoted, and the shift depends on material and thickness that are usually not documented. The result is not a small correction, it is an unknown correction, and the cross section curve it produces cannot be integrated against a mission spectrum.

Why does ion range matter, and where do 40 and 60 microns come from?

LET is only constant over part of an ion track. The range requirement is there to keep the sensitive volume inside the flat part. Quoting a single LET for an ion is a convenience that holds only while the stopping power is roughly constant. Near the end of the track it is not: the ion slows, the stopping power rises to the Bragg peak and then collapses. If the sensitive volume sits in that region, the effective LET across it is neither the surface value nor any single number, and the horizontal axis of the plot is fiction again.

Why is effective LET equal to LET divided by cosine of the angle, and when does that stop being true?

It is a path length argument that holds for a thin, wide sensitive volume. It fails as soon as depth is comparable to width, which is most modern parts and all vertical power devices. The derivation is one line. If the sensitive volume is a thin sheet, an ion arriving at angle theta crosses a path longer by one over cosine theta, so it deposits more charge in the same proportion, so it behaves like a normally incident ion of higher LET. That is the whole content of the effective LET concept.

Why is gate rupture worst at normal incidence, the opposite of upset?

Upset is about charge collected along a track, so a longer track is worse. Rupture is about field across a thin dielectric at one point, so a track aimed straight down the axis is worse. The two effects are driven by different integrals. Upset depends on the charge deposited within a collection volume, which grows with path length, so tilting the part helps the ion and hurts the device. Gate rupture depends on the transient field established across the gate dielectric by carriers moving in the underlying structure, and that is maximised when the ion track lies along the vertical axis of the cell, straight through the neck of the device and into the epitaxial layer beneath.

Why are 15, 37, 60 and 75 MeV cm2/mg all quoted as immunity thresholds, and where does each one actually come from?

Several independent arguments land near the same numbers. Recoil physics fixes 15 and contributes to 37. Mission risk tolerance and, for vertical power devices, what a beam line can actually deliver also produce 37. Only 60 and 75 are about the cosmic ray environment itself. Start with the one that is genuinely physics. A high energy proton does not usually upset a part directly; it undergoes a nuclear reaction and the recoiling fragment does the ionising, so the recoil spectrum sets a ceiling on the effective LET a proton can deliver. In silicon, recoils from 200 MeV protons reach roughly 11.5 to 15 MeV cm2/mg, which is the basis for the ESA rule that proton testing is required below a heavy ion threshold of 15. Modern parts are not only silicon, and tungsten, copper and tantalum in the interconnect and the package produce heavier recoils, which is why the NASA proton guidance uses about 37 as approximately the highest LET secondary obtainable from a 500 MeV proton and waives proton single event testing above it.

Why take cross section data out to LET 80 when the curve looks saturated at 40?

Because saturation has to be demonstrated, not assumed, and deep submicron parts stopped saturating where they used to. A rate prediction integrates the cross section curve against the environment spectrum. If the curve is still rising where the data stop, the integral is truncated and the rate is understated. JESD57A therefore asks for data out to twice the saturation LET, and where that is not achievable, out to 80 MeV cm2/mg, which is close to the practical limit of most heavy ion facilities.

Why does the part sit inside a lead and aluminium box during a cobalt 60 total dose test?

To restore charged particle equilibrium. Without it, low energy scattered photons deposit a locally enhanced dose that the dosimeter does not see and the part does. A cobalt 60 cell is not a clean 1.25 MeV field. Photons scatter off the source housing, the walls, the fixture and the part itself, producing a low energy tail. Low energy photons interact strongly through the photoelectric effect, whose cross section rises steeply with atomic number, so at an interface between a high atomic number material and a low one the secondary electron population is out of balance. Near the gold or tungsten of a package, or the metallisation of the die, the silicon and the gate oxide can receive substantially more dose than the free field reading implies. This is dose enhancement, and it means the number written in the test report is wrong in an unknown direction.

Why is the standard total dose rate 50 to 300 rad(Si) per second?

The lower bound is where enhanced low dose rate sensitivity starts. The upper bound is a practical limit that the documents state without justifying. The 50 rad(Si)/s floor is not arbitrary and it is not about beam economics. MIL-STD-883 Method 1019 defines enhanced low dose rate sensitivity as enhanced damage below 50 rad(Si)/s, and requires bipolar and BiCMOS linear or mixed signal parts destined for applications below that rate to be handled under the low dose rate provisions. Condition A therefore sits exactly at the boundary of the regime where the standard test is known to be non-conservative for bipolar technology.

Why is the low dose rate condition 10 millirad(Si) per second, and why will an accelerated test plus an anneal not substitute?

Because in bipolar oxides the damage is not simply a function of accumulated dose. Method 1019 says outright that the fast test plus anneal cannot simulate it. In a thick, poor quality isolation oxide of the kind found in bipolar linear parts, the holes generated by irradiation move slowly and the space charge they build up affects how many of them are trapped and how many interface traps form. Deliver the same total dose quickly and the space charge shields part of the oxide; deliver it slowly and it does not. The result is that the slow exposure does more damage, which is the opposite of the intuition that a fast test is the conservative one.

Why 100 degrees C for 168 hours after the irradiation?

Because the two kinds of trapped charge in an irradiated oxide anneal on different timescales, and the worst moment for the part is after the fast one has gone and the slow one has not. Irradiation of a MOS oxide leaves two things behind: positive charge trapped in the oxide, and negative charge in interface traps at the silicon boundary. In an n-channel transistor they shift the threshold in opposite directions, so immediately after a fast irradiation they partly cancel. The trapped positive charge anneals relatively quickly. The interface traps do not, and can continue building. So a part measured an hour after the irradiation can look acceptable, and the same part weeks later can be out of specification. That is rebound, and ESCC 22900 defines it in those terms, as net degradation from changes in trapped oxide charge and interface state density over periods of the order of several weeks.

Why must the electrical measurement start within one hour of the irradiation?

Because annealing starts the moment the beam stops, so the delay is part of the test condition whether it is controlled or not. The quantity being measured is changing while it is being measured. Trapped charge begins to anneal at room temperature immediately, so a part measured at 20 minutes and the same part measured at 6 hours give different numbers, and neither is more correct than the other. Fixing the interval is what makes results comparable between laboratories and between lots.

Why 1.5 times the specification dose?

Overtesting is a way of trading dose margin against sample size. The statistics behind that trade are worked out; the specific value of 1.5 is a convention. If a lot is irradiated to a level above the requirement and passes, that is stronger evidence than a pass at the requirement, and it is stronger in a quantifiable way. MIL-HDBK-814 works the relation through for a lognormal failure distribution and shows that overtesting reduces the sample size needed to reach a given acceptance confidence. That is the actual justification for the practice, and it is a statistical one.

Why is 100 mils of aluminium the default shielding assumption?

Because it is roughly what a real spacecraft wall provides, and it can be assumed before the structure exists. It is a convention of convenience, and the documents say so. Typical spacecraft shielding falls in the range of about 100 to 250 mils of aluminium equivalent, so 100 mils is a defensible lower end of the real range. The NASA avionics RHA guidelines describe the practice plainly: the environment is transported through an assumed nominal level of shielding, often 100 mils of aluminium equivalent, using an idealised slab or solid sphere geometry, and the analysis is refined toward real geometry once there is a model to refine it with.

Why not just shield with lead, and what is graded Z actually for?

Because stopping an electron in a high atomic number material converts it into penetrating x-rays. Lead is a good photon absorber and a bad electron absorber, and the order of the layers matters. When an energetic electron decelerates in matter it emits bremsstrahlung. The TREE handbook gives the scaling directly: the intensity depends linearly on the atomic number of the material and on the square of the initial electron energy, and the mean photon energy is about a third of the electron energy. So a lead first surface facing a trapped electron belt turns a stoppable particle flux into a photon flux that is much harder to stop. Aluminium is a poorer absorber per unit thickness but it produces far less bremsstrahlung, which is why spacecraft are built out of it.

Why does more shielding barely help against cosmic rays when it works so well against solar protons?

Because solar particles are soft enough to range out in practical thicknesses and galactic cosmic rays are not. The two populations sit at different places on the energy axis. A large solar particle event is dominated by particles of modest energy, so bulk material genuinely removes them: Schwank and co-authors report that going from 25 mils to about 1570 mils of aluminium cuts the flare flux above LET 30 by five orders of magnitude. Over the same span, the galactic cosmic ray LET spectrum is barely touched, because those particles carry hundreds of MeV to many GeV per nucleon.

Why are solar particle environments quoted as a worst week, a worst day and a peak 5 minutes?

Because they are three different engineering questions, and CREME96 built all three from the same event, October 1989, at about the 99 percent confidence level. The three models in CREME96 are constructed from measured data during the October 1989 event: about 180 hours of it for the worst week, an 18 hour interval for the worst day, and the highest 5 minute average proton flux for the peak, with heavy ion fluxes scaled from the proton data. All three are described as worst case at roughly the 99 percent confidence level.

Why is the sensitive volume modelled as a rectangular box, and why a Weibull?

Because a box is the shape whose chord length distribution has a closed form, and the Weibull is a curve that fits. Neither is a physical claim, and the literature is explicit about that. The rate integral needs the distribution of path lengths through the sensitive volume, because deposited charge is LET times chord length. A rectangular parallelepiped is used because Bradford derived a closed form chord length distribution for it, which makes the integral tractable. That is the entire reason for the shape.

How accurate is a single event rate prediction, and why is it not better?

A good prediction agrees with the observed on-orbit rate within a factor of two to five. The residual comes from four sources and only one of them is fixable by more testing. The NASA avionics RHA guidelines state the expectation directly: a good rate estimate should agree with the observed on-orbit rate within a factor of two to five, citing Petersen's short course and the data workshop literature. They also list where the error comes from: the environment model, part to part variation, departures from the assumed sensitive volume geometry, and Poisson counting error on the cross section measurements. Only the last of those gets better by running more beam time.

Why is the derating 75 percent in one document and 25 percent in another?

Because they are derating from different things. 75 percent is of a measured failure voltage. 25 percent is of a manufacturer's rating on a part nobody has tested. The JPL gate rupture guideline derates from the test result. Its worked example takes a device rated 100 V that ruptures at 80 V under beam, and arrives at 60 V for use, which is 75 percent of the measured threshold and 60 percent of the nameplate. The NASA avionics RHA guidelines do the same thing, deriving an operating limit of 58.5 V from a 78 V measured survival voltage. In this construction the 75 percent covers part to part variation and test uncertainty around a number that was actually measured on that part type.

Why are design margins of 2, 10 and 100 the breakpoints for part categorisation?

They are historical engineering conventions that set how much ongoing lot control a part needs, not statistical thresholds. The design margin breakpoint method sorts parts by the ratio of the level at which they fail to the level the mission requires. Below about 2 the part is unacceptable. Between 2 and 10 it is hardness critical in the strict sense and every purchased lot has to be tested. Between 10 and 100 it is hardness critical in a weaker sense and occasional confirmatory testing suffices. Above 100 it is hardness non-critical and can be bought like any other part.

Why are protons tested at about 200 MeV, and why is 63 MeV the other common point?

200 MeV is where the indirect ionisation cross section has saturated and the dose per proton is lowest. 63 MeV is roughly the median of the trapped spectrum and is what many cyclotrons deliver. Above roughly 100 MeV the proton upset cross section of most parts has stopped changing, because the nuclear reaction cross sections that produce the ionising recoils have levelled off. A single point above that captures the saturated value, and JESD234 states that testing above 200 MeV is not generally necessary for non-destructive upset because higher energy protons are rare enough in the environment that the extra contribution to the rate is small.

Why is the proton fluence 1e10 or 1e11 per square centimetre, and why does the part sometimes die first?

Because that is the fluence needed to bound a small cross section, and at low proton energy it is also enough dose to destroy the part. The statistics are the same as for heavy ions: with no events, the bound on the cross section is a constant over the fluence. JESD234 gives a nominal 1e10 protons/cm2 for soft devices and 1e12 for hard ones, with a rule of thumb of a hundred events where events are available. The 2002 lessons learned guidance is more specific about the null case: stop either at 1e11 protons/cm2 or when the part is destroyed, whichever comes first.

Why did low energy protons suddenly start mattering?

Because critical charge fell far enough that a proton can upset a cell by direct ionisation, without any nuclear reaction at all. That threshold was crossed at around the 65 nm node. For most of the history of the field, protons upset parts indirectly: a nuclear reaction produced a recoil, and the recoil did the ionising. A proton's own stopping power was simply too low to deposit the critical charge. As feature sizes shrank the critical charge fell, and at 65 nm and below the direct ionisation from a slow proton near its Bragg peak became sufficient on its own.

Why is latchup tested hot and burnout tested cold?

Because the two mechanisms depend on temperature in opposite directions, so the worst case for one is the benign case for the other. Latchup is a parasitic thyristor turning on and holding itself on. Higher temperature raises the well and substrate resistances and raises the current gains of the parasitic bipolar transistors, both of which make the structure easier to trigger and easier to sustain. ASTM F1192 states the effect directly, that latchup susceptibility and soft error cross sections increase with temperature, and JESD57A and ESCC 25100 both require latchup testing at maximum rated operating voltage and maximum operating temperature.

Why does it matter whether a destructive event was latchup or burnout?

Because they call for different mitigations, and because a dataset that lumps them together will give the wrong derating curve. Both present the same way on the bench: current goes up sharply. What distinguishes them is what happens next. Latchup is a self-sustaining state that clears when power is removed, and it becomes destructive only if the current is allowed to run. Burnout in a power device is thermal destruction and does not clear. The JPL gate rupture guideline addresses this directly and says that a high current condition that recovers on power cycling should be noted but not recorded as burnout or gate rupture.

Why can a pulsed laser not replace heavy ion testing?

Because it deposits charge by a different process with no intrinsic LET equivalence, and because light does not go through metal. A focused laser pulse creates carriers by photon absorption, not by the coulomb interaction of a charged particle track. The spatial profile of the deposited charge is different, so there is no first principles conversion between pulse energy and LET. The desk reference is explicit that either an empirical correlation factor, which itself requires heavy ion data, or a tuned laser equivalent LET must be used, and that in most cases heavy ion and proton data are still required for a rate prediction.

Why does a radiation result on one lot not transfer to another lot of the same part number?

Because radiation response depends on process details that are not part of the electrical specification, and manufacturers change them without telling anyone. Total dose response in particular depends on oxide growth conditions, hydrogen content and thermal history, none of which appear on a datasheet and none of which the manufacturer is obliged to hold constant. A part can meet every electrical parameter across a process change and behave completely differently under irradiation. The NASA avionics RHA guidelines state that design mask or fabrication process changes can invalidate archival data while form, fit and function are maintained, and that manufacturers are not necessarily required to give notice.

Standards, Guidelines and Radiation Data Sources

What the documents require, and why they require it. A requirement without its reasoning gets followed wrongly: people skip the anneal step, qualify at one gate bias, or read a wear-out derating table as though it covered single events. Every designation, revision and access term below was checked against the issuing body or the document itself in September 2026, and where that check failed the entry says so rather than guessing. Below the standards is a catalogue of radiation test data sources, including the ones behind paywalls and logins, because knowing what exists matters even when you cannot reach it.

Why the numbers are the numbers Standards and test methods Test facilities Radiation data sources

Why the numbers are the numbers

Every convention in this field started as somebody's reasoning, and most of that reasoning is still written down in the primary documents. It just does not travel with the number. These entries go back to the source and reconstruct it: what the value is, which clause states it, what physical or statistical argument produced it, and which paper the standard or its authors cite as the origin. Where a document states a rule and gives no reason, the entry says so instead of supplying one. Searching filters these along with everything else, so a query for anneal or bremsstrahlung or 1e7 lands here. Expand all

Standards, test methods and guidelines

Single event and derating documents first, then total dose, then the process-level and terrestrial ones. Access is marked honestly: two of the most cited documents here are behind an ASTM paywall, and the two ECSS standards need a free registration. Several widely quoted revisions are out of date, and those are flagged.

Test facilities

Where the beam actually comes from, with the parameter that decides whether a facility can test your part at all: ion range in silicon. A high LET beam that stops before it reaches a buried well or the drift region of a power device gives a falsely reassuring answer, which is why energy per nucleon appears here next to LET. Operational status was checked against each facility's own pages in September 2026 and is flagged honestly, including where the check failed. This matters more than usual right now: one major facility is shut for the whole of 2026, and several widely cited sites have closed or changed identity in the last few years.

Still listed in places, no longer available

Radiation test data sources

Where part-level radiation data actually lives. This includes sources you cannot use: a database that needs a CERN account, a government library open only to vetted stakeholders, proceedings behind an IEEE paywall. They are listed because a search that comes up empty should tell you whether the data does not exist or merely is not reachable. Beam facilities are listed too, marked as such, because they are frequently mistaken for data repositories: the results of a campaign belong to whoever paid for it. Several specialist commercial databases now exist alongside the agency ones, which is a real change in the last few years: a decade ago the only route to radiation data on a commercial part was to test it yourself or find a paper. Their coverage of parts nobody else tests is genuinely useful, and the usual caution applies, which is to check the failure definition and the test protocol behind a number before adopting it.

Corrections are welcome through the community forum or by getting in touch. If a revision has moved on or an access term has changed, say so and it will be fixed. Nothing on this page reproduces the content of a paywalled document; the requirement and reasoning summaries are written from the publicly stated scope and from standard practice.