3.2              Terms specific to the present standard

3.2.1                      absorbed dose

energy absorbed locally per unit mass as a result of radiation exposure which is transferred through ionisation, displacement damage and excitation and is the sum of the ionising dose and non-ionising dose

NOTE 1      It is normally represented by D, and in accordance with the definition, it can be calculated as the quotient of the energy imparted due to radiation in the matter in a volume element and the mass of the matter in that volume element. It is measured in units of gray, Gy (1 Gy = 1 J kg-1 (= 100 rad)).

NOTE 2      The absorbed dose is the basic physical quantity that measures radiation exposure.

3.2.2                      air kerma

energy of charged particles released by photons per unit mass of dry air

NOTE              It is normally represented by K.

3.2.3                      ambient dose equivalent, H*(d)

dose at a point equivalent to the one produced by the corresponding expanded and aligned radiation field in the ICRU sphere at a specific depth on the radius opposing the direction of the aligned field

NOTE 1      It is normally represented by H*(d), where d is the specific depth used in its definition, in mm.

NOTE 2      H*(d) is relevant to strongly penetrating radiation. The value normally used is 10 mm, but dose equivalent at other depths can be used when the dose equivalent at 10 mm provides an unacceptable underestimate of the effective dose.

3.2.4                      bremsstrahlung

high energy electromagnetic radiation in the X-ray energy range emitted by charged particles slowing down by scattering off atomic nuclei

NOTE              The primary particle is ultimately absorbed while the bremsstrahlung can be highly penetrating. In space the most common source of bremsstrahlung is electron scattering.

3.2.5                      component

device that performs a function and consists of one or more elements joined together and which cannot be disassembled without destruction

3.2.6                      continuous slowing down approximation range (CSDA)

integral pathlength travelled by charged particles in a material assuming no stochastic variations between different particles of the same energy, and no angular deflections of the particles

3.2.7                      COTS

commercial electronic component readily available off-the-shelf, and not manufactured, inspected or tested in accordance with military or space standards

3.2.8                      critical charge

minimum amount of charge collected at a sensitive node due to a charged particle strike that results in a SEE

3.2.9                      cross-section

<single event phenomena> probability of a single event effect occurring per unit incident particle fluence

NOTE              This is experimentally measured as the number of events recorded per unit fluence.

3.2.10                  cross-section

<nuclear or electromagnetic physics> probability of a particle interaction per unit incident particle fluence

NOTE              It is sometimes referred to as the microscopic cross-section. Other related definition is the macroscopic cross section, defines as the probability of an interaction per unit path-length of the particle in a material.

3.2.11                  directional dose equivalent

dose at a point equivalent to the one produced by the corresponding expanded radiation field in the ICRU sphere at a specific depth d on a radius on a specified direction

NOTE 1      It is normally expressed as H¢(d, Ω), where d is the specific depth used in its definition, in mm, and Ω is the direction.

NOTE 2      H¢(d,Ω), is relevant to weakly-penetrating radiation  where a reference depth of 0,07 mm is usually used and the quantity denoted H¢(0,07, Ω).

3.2.12                  displacement damage

crystal structure damage caused when particles lose energy by elastic or inelastic collisions in a material

3.2.13                  dose

quantity of radiation delivered at a position

NOTE 1      In its broadest sense this can include the flux of particles, but in the context of space energetic particle radiation effects, it usually refers to the energy absorbed locally per unit mass as a result of radiation exposure.

NOTE 2      If “dose” is used unqualified, it refers to both ionising and non-ionising dose. Non-ionising dose can be quantified either through energy deposition via displacement damage or damage-equivalent fluence (see Clause 8).

3.2.14                  dose equivalent

absorbed dose at a point in tissue which is weighted by quality factors which are related to the LET distribution of the radiation at that point

3.2.15                  dose rate

rate at which radiation is delivered per unit time

3.2.16                  effective dose

sum of the equivalent doses for all irradiated tissues or organs, each weighted by its own value of tissue weighting factor

NOTE 1      It is normally represented by E, and in accordance with the definition it is calculated with the equation below, and the wT is specified in the ICRP-92 standard [RDH.22]:

               (1)

For further discussion on E, see ECSS-E-HB-10-12 Section 10.2.2.

NOTE 2      Effective dose, like organ equivalent dose, is measured in units of sievert, Sv. Occasionally this use of the same unit for different quantities can give rise to confusion.

3.2.17                  energetic particle

particle which, in the context of space systems radiation effects, can penetrate outer surfaces of spacecraft

3.2.18                  equivalent dose

See 3.2.41 (organ equivalent dose)

3.2.19                  equivalent fluence

quantity which represents the damage at different energies and from different species by a fluence of monoenergetic particles of a single species

NOTE 1      These are usually derived through testing.

NOTE 2      Damage coefficients are used to scale the effect caused by particles to the damage caused by a standard particle and energy.

3.2.20                  extrapolated range

range determined by extrapolating the line of maximum gradient in the intensity curve until it reaches zero intensity

3.2.21                  Firsov scattering

the reflection of fast ions from a dense medium at glancing angles

NOTE              See references [2].

3.2.22                  fluence

time-integration of flux

NOTE              It is normally represented by Φ.

3.2.23                  flux

<unidirectional incident particles> number of particles crossing a surface at right angles to the particle direction, per unit area per unit time

3.2.24                  flux

<arbitrary angular distributions> number of particles crossing a sphere of unit cross-sectional area (i.e. of radius 1/) per unit time

NOTE 1      For arbitrary angular distributions, it is normally known as omnidirectional flux.

NOTE 2      Flux is often expressed in “integral form” as particles per unit time (e.g. electrons cm-2 s-1) above a certain energy threshold.

NOTE 3      The directional flux is the differential with respect to solid angle (e.g. particles-cm-2steradian-1s-1) while the “differential” flux is differential with respect to energy (e.g. particles-cm-2MeV-1s-1). In some cases fluxes are treated as a differential with respect to linear energy transfer rather than energy.

3.2.25                  ICRU sphere

sphere of 30 cm diameter made of ICRU soft tissue

NOTE              This definition is provided by the International Commission of Radiation Units and Measurements Report 33 [12].

3.2.26                  ICRU Soft Tissue

tissue equivalent material with a density of 1 g/cm3 and a mass composition of 76,2 % oxygen, 11,1 % carbon, 10,1 % hydrogen and 2,6 % nitrogen.

NOTE              This definition is provided in the ICRU Report 33 [12].

3.2.27                  ionising dose

amount of energy per unit mass transferred by particles to a target material in the form of ionisation and excitation

3.2.28                  ionising radiation

transfer of energy by means of particles where the particle has sufficient energy to remove electrons, or undergo elastic or inelastic interactions with nuclei (including displacement of atoms), and in the context of this standard includes photons in the X-ray energy band and above

3.2.29                  isotropic

property of a distribution of particles where the flux is constant over all directions

3.2.30                  L or L-shell

parameter of the geomagnetic field often used to describe positions in near-Earth space

NOTE              L or L-shell has a complicated derivation based on an invariant of the motion of charged particles in the terrestrial magnetic field. However it is useful in defining plasma regimes within the magnetosphere because, for a dipole magnetic field, it is equal to the geocentric altitude in Earth-radii of the local magnetic field line where it crosses the equator.

3.2.31                  linear energy transfer (LET)

rate of energy deposited through ionisation from a slowing energetic particle with distance travelled in matter, the energy being imparted to the material

NOTE 1      LET is normally used to describe the ionisation track caused due to the passage of an ion. LET is material dependent and is also a function of particle energy and charge. For ions involved in space radiation effects, it increases with decreasing energy (it also increases at high energies, beyond the minimum ionising energy). LET allows different ions to be considered together by simply representing the ion environment as the summation of the fluxes of all ions as functions of their LETs. This simplifies single-event upset calculation. The rate of energy loss of a particle, which also includes emitted secondary radiations, is the stopping power.

NOTE 2      LET is not equal to (but is often approximated to) particle electronic stopping power, which is the energy loss due to ionisation and excitation per unit pathlength.

3.2.32                  LET Threshold

minimum LET that a particle should have to cause a SEE in a circuit when going through a device sensitive volume

3.2.33                  margin

factor or difference between the design environment specification for a device or product and the environment at which unacceptable behaviour occurs

3.2.34                  mean organ absorbed dose

energy absorbed by an organ due to ionising radiation divided by its mass

NOTE              It is normally represented by DT, and in accordance with the definition, it is calculated with the equation (35) in ECSS-E-HB-10-12 Section 10.2.2. The unit is the gray (Gy), being 1 Gy = 1 joule / kg.

3.2.35                  mean range

integral pathlength travelled by particles in a material after which the intensity is reduced by a factor of e ≈ 2,7183

NOTE              In accordance with the above definition, it is not the range at which all particles are stopped.

3.2.36                  multiple bit upset (MBU)

set of bits corrupted in a digital element that have been caused by direct ionisation from a single traversing particle or by recoiling nuclei and/or secondary products from a nuclear interaction

NOTE              MCU and SMU are special cases of MBU.

3.2.37                  multiple cell upset (MCU)

set of physically adjacent bits corrupted in a digital element that have been caused by direct ionisation from a single traversing particle or by recoiling nuclei from a nuclear interaction

3.2.38                  (total) non-ionising dose, (T)NID, or non-ionising energy loss (NIEL) dose

energy absorption per unit mass of material which results in damage to the lattice structure of solids through displacement of atoms

NOTE              Although the SI unit of TNID or NIEL dose is the gray (see definition 3.2.34), for spacecraft radiation effects, MeV/g(material) is more commonly used in order to avoid confusion with ionising energy deposition, e.g. MeV/g(Si) for TNID in silicon.

3.2.39                  NIEL or NIEL rate or NIEL coefficient

rate of energy loss in a material by a particle due to displacement damage per unit pathlength

3.2.40                  omnidirectional flux

scalar integral of the flux over all directions

NOTE              This implies that no consideration is taken of the directional distribution of the particles which can be non-isotropic. The flux at a point is the number of particles crossing a sphere of unit cross-sectional surface area (i.e. of radius 1/) per unit time. An omnidirectional flux is not to be confused with an isotropic flux.

3.2.41                  organ equivalent dose

sum of each contribution of the absorbed dose by a tissue or an organ exposed to several radiation types, weighted by the each radiation weighting factor for the radiations impinging on the body

NOTE 1      The organ equivalent dose, an ICRP-60 [11] defined quantity, is normally represented by HT, and usually shortened to equivalent dose. In accordance with the definition, it is calculated with the equation below (for further discussion, see ECSS-E-HB-10-12 Section 10.2.2):

            (2)

NOTE 2      The organ equivalent dose is measured in units of sievert, Sv, where 1 Sv = 1 J/kg. The unit rem (roentgen equivalent man)  is still used, where 1 Sv = 100 rem.

3.2.42                  personal dose equivalent (individual dose equivalent)

dose equivalent in ICRU soft tissue at a depth in the body

NOTE 1      The personal dose equivalent, and ICRU quantity, is normally represented by HP(d) for strongly penetrating radiation at a depth d in millimetres that is appropriate for strongly penetrating radiation. A reference depth of 10 mm is usually used. It varies both as a function of individuals and location and is appropriate for organs and tissues deeply situated in the body.

NOTE 2      It is normally represented by Hs(d) for weakly penetrating radiation (superficial) at a depth d in millimetres that is appropriate for weakly penetrating radiation. A reference depth of 0,07 mm is usually used. It varies both as a function of individuals and location and is appropriate for superficial organs and tissues which are going to be irradiated by both weakly and strongly penetrating radiation.

3.2.43                  plasma

partly or wholly ionised gas whose particles exhibit collective response to magnetic or electric fields

NOTE              The collective motion is brought about by the electrostatic Coulomb force between charged particles. This causes the particles to rearrange themselves to counteract electric fields within a distance of the order of the Debye length. On spatial scales larger than the Debye length plasmas are electrically neutral.

3.2.44                  projected range

average depth of penetration of a particle measured along the initial direction of the particle

3.2.45                  quality factor

factor accounting for the different biological efficiencies of ionising radiation with different LET, and used to convert the absorbed dose to operational parameters (ambient dose equivalent, directional dose equivalent and personal dose equivalent)

NOTE 1      Quality factor, normally represented by Q, are used (rather than radiation or tissue weighting factors) to convert the absorbed dose to dose equivalent quantities described above (ambient dose equivalent, directional dose equivalent and personal dose equivalent). Its actual values are given by ICRP-60 [11] (see 11.2.3.2).

NOTE 2      Prior to ICRP-60 [11], quality factors were synonymous to radiation weighting factors.

3.2.46                  radiation

transfer of energy by means of a particle (including photons)

NOTE              In the context of this Standard, electromagnetic radiation below the X-ray band is excluded. This therefore excludes UV, visible, thermal, microwave and radiowave radiation.

3.2.47                  radiation design margin (RDM)

<cumulative process> ratio of the radiation tolerance or capability of the component, system or protection limit for astronaut, to the predicted radiation environment for the mission or phase of the mission

NOTE              The component tolerance or capability, above which its performance becomes non-compliant, is project-defined.

3.2.48                  radiation design margin (RDM)

<non-destructive single event> ratio of the design SEE tolerance to the predicted SEE rate for the environment

NOTE              The design SSE tolerance is the acceptable SEE rate which the equipment or mission can experience while still meeting the equipment reliability and availability requirements.

3.2.49                  radiation design margin (RDM)

<destructive single event> ratio of the acceptable probability of component failure by the SEE mechanism to the calculated probability of failure

NOTE              the acceptable probability of component failure is based on the equipment reliability and availability specifications.

3.2.50                  radiation design margin (RDM)

<biological effect> ratio of the protection limits defined by the project for the mission to the predicted exposure for the crew

3.2.51                  radiation weighting factor

factor accounting for the different levels of radiation effects in biological material for different radiations at the same absorbed dose

NOTE              It is normally represented by wR. Its value is defined by ICRP (see clause 11.2.2.2).

3.2.52                  relative biological effectiveness (RBE)

inverse ratio of the absorbed dose from one radiation type to that of a reference radiation that produces the same radiation effect

NOTE 1      The radiation type is usually 60Co or 200-250 keV X-rays.

NOTE 2      In contrast to the weighting or quality factors, RBE is an empirically founded measurable quantity. For additional information on RBE, see ECSS-E-HB-10-12 Section 10.2.2.

3.2.53                  sensitive volume (SV)

charge collection region of a device

3.2.54                  single event burnout (SEB)

destructive triggering of a vertical n-channel transistor or power NPN transistor accompanied by regenerative feedback

3.2.55                  single event dielectric rupture (SEDR)

formation of a conducting path triggered by a single ionising particle in a high-field region of a dielectric

NOTE              For example, in linear devices, or in FPGAs.

3.2.56                  single event disturb (SED)

momentary voltage excursion (voltage spike) at a node in an integrated circuit, originally formed by the electric field separation of the charge generated by an ion passing through or near a junction

NOTE              SED is similar to SET, but used to refer to such events in digital microelectronics.

3.2.57                  single event effect (SEE)

effect caused either by direct ionisation from a single traversing particle or by recoiling nuclei emitted from a nuclear interaction

3.2.58                  single event functional interrupt (SEFI)

interrupt caused by a single particle strike which leads to a temporary non-functionality (or interruption of normal operation) of the affected device

3.2.59                  single event gate rupture (SEGR)

formation of a conducting path triggered by a single ionising particle in a high-field region of a gate oxide

3.2.60                  single event hard error (SEHE)

unalterable change of state associated with semi-permanent damage to a memory cell from a single ion track

3.2.61                  single event latch-up (SEL)

potentially destructive triggering of a parasitic PNPN thyristor structure in a device

3.2.62                  single event snapback (SESB)

event that occurs when the parasitic bipolar transistor that exists between the drain and source of a MOS transistor amplifies the avalanche current that results from a heavy ion

3.2.63                  single event transient (SET)

momentary voltage excursion (voltage spike) at a node in an integrated circuit, originally formed by the electric field separation of the charge generated by an ion passing through or near a junction

3.2.64                  single event upset (SEU)

single bit flip in a digital element that has been caused either by direct ionisation from a traversing particle or by recoiling nuclei emitted from a nuclear interaction

3.2.65                  single word multiple bit upset (SMU)

set of logically adjacent bits corrupted in a digital element caused by direct ionisation from a single traversing particle or by recoiling nuclei from a nuclear interaction

NOTE              SMU are multiple bit upsets within a single data word.

3.2.66                  solar energetic particle event (SEPE)

emission of energetic protons or heavier nuclei from the Sun within a short space of time (hours to days) leading to particle flux enhancement

NOTE              SEPE are usually associated with solar flares (with accompanying photon emission in optical, UV and X-Ray) or coronal mass ejections.

3.2.67                  stopping power

average rate of energy-loss by a given particle per unit pathlength traversed through a given material

NOTE              The following are consequence of the above definition:

•       collision stopping power: (electrons and positrons) average energy loss per unit pathlength due to inelastic Coulomb collisions with bound atomic electrons resulting in ionisation and excitation.

•       radiative stopping power: (electrons and positrons) average energy loss power unit pathlength due to emission of bremsstrahlung in the electric field of the atomic nucleus and of the atomic electrons.

•       electronic stopping power: (particles heavier than electrons) average energy loss per unit pathlength due to inelastic Coulomb collisions with atomic electrons resulting in ionisation and excitation.

•       nuclear stopping power: (particles heavier than electrons) average energy loss per unit pathlength due to inelastic and elastic Coulomb collisions with atomic nuclei in the material.

3.2.68                  tissue weighting factor

factor that accounts for the different sensitivity of organs or tissue in expressing radiation effects to the same equivalent dose

NOTE              It is normally represented by wT, and its actual values are defined by ICRP (see clause 11.2.2.3).

3.2.69                  total ionising dose

energy deposited per unit mass of material as a result of ionisation

NOTE              The SI unit is the gray (see definition 3.2.34). However, the deprecated unit rad (radiation absorbed dose) is still used frequently (1 rad = 1 cGy).