ISO 23038:2006
(Main)Space systems — Space solar cells — Electron and proton irradiation test methods
Space systems — Space solar cells — Electron and proton irradiation test methods
ISO 23038:2006 specifies the requirements for electron and proton irradiation test methods of space solar cells. It addresses only test methods for performing electron and proton irradiation of space solar cells and not the method for data analysis.
Systèmes spatiaux — Cellules solaires spatiales — Méthodes d'essai d'irradiation d'électrons et de protons
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Standards Content (Sample)
INTERNATIONAL ISO
STANDARD 23038
First edition
2006-10-01
Space systems — Space solar cells —
Electron and proton irradiation test
methods
Systèmes spatiaux — Cellules solaires spatiales — Méthodes d'essai
d'irradiation d'électrons et de protons
Reference number
ISO 23038:2006(E)
©
ISO 2006
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ISO 23038:2006(E)
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ii © ISO 2006 – All rights reserved
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ISO 23038:2006(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies
(ISO member bodies). The work of preparing International Standards is normally carried out through ISO
technical committees. Each member body interested in a subject for which a technical committee has been
established has the right to be represented on that committee. International organizations, governmental and
non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the
International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. Draft International Standards
adopted by the technical committees are circulated to the member bodies for voting. Publication as an
International Standard requires approval by at least 75 % of the member bodies casting a vote.
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. ISO shall not be held responsible for identifying any or all such patent rights.
ISO 23038 was prepared by Technical Committee ISO/TC 20, Aircraft and space vehicles, Subcommittee
SC 14, Space systems and operations.
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INTERNATIONAL STANDARD ISO 23038:2006(E)
Space systems — Space solar cells — Electron and proton
irradiation test methods
1 Scope
This International Standard specifies the requirements for electron and proton irradiation test methods of
space solar cells. It addresses only test methods for performing electron and proton irradiation of space solar
cells and not the method for data analysis.
2 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
NOTE Physical constants are given to four significant figures only and reflect current knowledge.
2.1
differential energy spectrum
spread of energies of some specific group
NOTE In this document, this refers to the number of particles possessing an energy value that lies in the infinitesimal
range E, E+dE divided by the size of the range (dE). Integration of the differential particle spectrum over all particle
energies yields the total number of particles. This quantity is given in units of particles per unit area per unit energy.
2.2
electron
−
e
−31 −19
elementary particle of rest mass m = 9,109 × 10 kg, having a negative charge of 1,602 × 10 C
2.3
flux
number of particles passing through a given area in a specified time
NOTE Flux may also be specified in terms of the number of particles per unit time passing through a unit area from
2
source directions occupying a unit solid angle. Typical units are particles per cm per second per steradian (sr) (1 sr is the
solid angle subtended at the centre of a unit sphere by a unit area of the surface of the sphere).
2.4
fluence
total number of particles per unit area in any given time period
NOTE Fluence is also known as time-integrated flux.
2.5
integral energy spectrum
total number of particles per unit area in a specified group that possess energies greater than, or equal to, a
specified value
2.6
irradiation
exposure of a substance to energetic particles that penetrate the material and have the potential to transfer
energy to the material
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ISO 23038:2006(E)
2.7
non-ionizing energy loss
NIEL
rate at which the incident particle transfers energy to the crystal lattice through non-ionizing events
−2 −1
NOTE Typical unit is MeV ⋅ cm ⋅ g .
2.8
omnidirectional flux
number of particles of a particular type which have an isotropic distribution over 4π steradians and that would
2
traverse a test sphere of 1 cm cross-sectional area in 1 s
2
NOTE Expressed in units of particles per cm per second.
2.9
proton
+
p
−27
positively charged particle of mass number one, having a mass of 1,672 × 10 kg and a charge equal in
magnitude but of opposite sign to that of the electron
NOTE A proton is the nucleus of a hydrogen atom.
3 Symbols and abbreviated terms
eV electronvolt
19
−
NOTE A unit of energy commonly used for ions, electrons, elementary particles, etc. (1 eV ≈ 1,602 × 10 J.)
4 Space radiation environments
4.1 Space radiation
Primarily electrons and protons with a wide range of energies characterize the space radiation environment.
Gamma rays can be used as a substitute for electron irradiation with the proper transformation. Some
reasonable electron and proton fluence limits usually attained in typical space conditions are given below. For
15 13
1 MeV electrons and 10 MeV protons, these typical but not inclusive fluence limits are 10 and 10 particles
2
per cm , respectively. Alpha particles and other charged particles are usually of negligible quantity as far as
solar-cell damage is concerned. The particles come from the solar wind and are trapped by the Earth’s
magnetic field to form radiation belts with widely varying intensities [1]. Solar wind is usually associated with
particles of low energy (typically below 100 keV), whereas the particles of concern for solar cells are generally
of higher energies. The inner portion of the belts consists mainly of protons and of an inner electron belt,
whereas the outer portion consists primarily of electrons. Outside of these radiation belts, there is a likelihood
of sudden bursts of protons and electrons originating from coronal mass ejections from the Sun, referred to
generally as solar flares. Thus, the differential spectrum of electrons and protons for any given mission is
dependent on the specific mission orbit. Owing to the large variability of the involved phenomena, the
prediction of the particle spectrum for a given mission is affected by a significant uncertainty. The most widely
accepted tools for its calculation are the AP8 (protons) and AE8 (electrons) codes developed by NASA for the
trapped particles, whereas the solar flares are modelled with other tools such as the JPL 91 code.
4.2 Shielding effects
Space solar cells are typically flown with some material covering the cell surface, most typically a piece of
glass (coverglass), and are mounted on some support structure. These front and rear covering materials act
to shield the solar cell from some of the incident irradiation. Because of this, the solar cell in space is actually
irradiated by a modified particle spectrum, usually referred to as a slowed down spectrum. An example
showing such a slowed down spectrum calculation can be found in item [2] of the bibliography.
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ISO 23038:2006(E)
5 General radiation effects in solar cells
5.1 Solar-cell radiation damage
Solar cells, like all semiconductor devices, are subject to electrical degradation when exposed to particle
irradiation. In terms of radiation damage to solar cells used in space, the primary particles of interest are
electrons and protons. When these energetic particles are incident upon the solar-cell material, they collide
with the atoms of the crystal lattice of the solar cell. In these atomic collisions, energy is transferred from the
incident particle to the target atom. This energy can be transferred in several ways. The majority of the energy
is transferred through ionization of the target atom, where electrons of the target atom absorb the transferred
energy and are promoted to higher energy levels. Another energy transfer mechanism is through non-ionizing
events, which result in displacement of the target atom. If enough energy is transferred in a non-ionizing event,
then the displaced target atom may, in turn, displace other atoms, creating a cascade of displaced atoms. The
displacement damage induced by the non-ionizing interactions is the primary cause of most solar-cell
degradation.
When an atom is displaced in a lattice, the electron energy band structure of the material is disturbed, and
localized energy levels can be created near the site of the defect. These defect energy levels can act to trap
electrical charge carriers, thus restricting their ability to move through the material, which is characterized by a
reduction in the minority carrier diffusion length. Since solar-cell operation depends on the motion of
photogenerated charge carriers through the material, these defect sites tend to degrade the solar-cell
performance.
The amount of displacement damage caused by an incident particle is a function of the type of incident
particle (i.e. electron or proton), the particle energy, and the composition of the crystal lattice. The rate at
which the incident particle transfers energy to the crystal lattice through non-ionizing events is referred to as
the non-ionizing energy loss (NIEL). Electrons become more damaging as the incident electron energy
increases. The opposite is true for protons, where the lower energy protons are the most damaging. Also,
protons are significantly more damaging in comparison to electrons, primarily due to the increased proton
differential scattering cross-section for atomic displacements. There is a lower limit to displacement damage
corresponding to the threshold energy for atomic displacements.
5.2 Radiation effects on solar-cell cover
Although not specifically a solar-cell radiation effect, it is appropriate in this International Standard to note the
effects of irradiation on solar-cell coverglass material. Certain solar-cell coverglass material has been shown
to darken under irradiation, thereby absorbing some of the incident light [4]. This increased light absorption
can reduce the solar-cell output in one of two ways: reduction of the amount of light that reaches the cell, or
increase in operating temperature of array that reduces the cell electrical conversion efficiency.
NOTE Testing cells with attached coverglass or different geometries require special care (see items [7] and [8] in the
bibliography).
6 Radiation test methods
6.1 General
As described in Clause 5, the space radiation environment consists of a spectrum of particle energies, and as
described in this clause, solar-cell radiation damage is energy dependent. Irradiation by a spectrum of
particles in a laboratory is not typical, so most ground radiation testing is done using a monoenergetic beam of
particles. Therefore, any space solar-cell radiation testing shall be done in such a way as to enable
extrapolation from monoenergetic radiation damage to damage produced by irradiation by a particle spectrum.
This is typically done by using the ground test data to reduce the particle spectrum to a fluence of
monoenergetic particles that produce an equivalent amount of damage. The determination of the equivalent
fluence can be achieved in different ways, the primary ones being the JPL and NRL methodologies [2, 3, 5].
Although it is beyond the scope of this International Standard to discuss these data analysis methods, it is
important that the method to be used for a specific experiment be chosen and well understood prior to
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ISO 23038:2006(E)
performing any radiation testing. Similarly, it should be n
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