BS-ISO-23038-2006.pdf

BRITISH STANDARD BS ISO 23038:2006 Space systems Space solar cells Electron and proton irradiation test methods ICS 49.140 ? Licensed Copy: sheffieldun sheffieldun, na, Sun Nov 26 14:49:07 GMT+00:00 2006, Uncontrolled Copy, (c) BSI BS ISO 23038:2006 This British Standard was published under the authority of the Standards Policy and Strategy Committee on 31 October 2006 © BSI 2006 ISBN 0 580 48703 2 National foreword This British Standard was published by BSI. It is the UK implementation of ISO 23038:2006. The UK participation in its preparation was entrusted by Technical Committee ACE/68, Space systems and operations, to Subcommittee ACE/68/-/1, Space systems and operations Engineering Design production. A list of organizations represented on ACE/68/-/1 can be obtained on request to its secretary. This publication does not purport to include all the necessary provisions of a contract. Users are responsible for its correct application. Compliance with a British Standard cannot confer immunity from legal obligations. Amendments issued since publication Amd. No. DateComments Licensed Copy: sheffieldun sheffieldun, na, Sun Nov 26 14:49:07 GMT+00:00 2006, Uncontrolled Copy, (c) BSI Reference number ISO 23038:2006(E) INTERNATIONAL STANDARD ISO 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 BS ISO 23038:2006 Licensed Copy: sheffieldun sheffieldun, na, Sun Nov 26 14:49:07 GMT+00:00 2006, Uncontrolled Copy, (c) BSI ii Licensed Copy: sheffieldun sheffieldun, na, Sun Nov 26 14:49:07 GMT+00:00 2006, Uncontrolled Copy, (c) BSI iii 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. BS ISO 23038:2006 Licensed Copy: sheffieldun sheffieldun, na, Sun Nov 26 14:49:07 GMT+00:00 2006, Uncontrolled Copy, (c) BSI blank Licensed Copy: sheffieldun sheffieldun, na, Sun Nov 26 14:49:07 GMT+00:00 2006, Uncontrolled Copy, (c) BSI 1 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 elementary particle of rest mass m = 9,109 × 1031 kg, having a negative charge of 1,602 × 1019 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 source directions occupying a unit solid angle. Typical units are particles per cm2 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 BS ISO 23038:2006 Licensed Copy: sheffieldun sheffieldun, na, Sun Nov 26 14:49:07 GMT+00:00 2006, Uncontrolled Copy, (c) BSI 2 2.7 non-ionizing energy loss NIEL rate at which the incident particle transfers energy to the crystal lattice through non-ionizing events NOTE Typical unit is MeV cm2 g1. 2.8 omnidirectional flux number of particles of a particular type which have an isotropic distribution over 4 steradians and that would traverse a test sphere of 1 cm2 cross-sectional area in 1 s NOTE Expressed in units of particles per cm2 per second. 2.9 proton p+ positively charged particle of mass number one, having a mass of 1,672 × 1027 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 NOTE A unit of energy commonly used for ions, electrons, elementary particles, etc. (1 eV 1,602 × 1019 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 1 MeV electrons and 10 MeV protons, these typical but not inclusive fluence limits are 1015 and 1013 particles per cm2, 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 Earths 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. BS ISO 23038:2006 Licensed Copy: sheffieldun sheffieldun, na, Sun Nov 26 14:49:07 GMT+00:00 2006, Uncontrolled Copy, (c) BSI 3 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 BS ISO 23038:2006 Licensed Copy: sheffieldun sheffieldun, na, Sun Nov 26 14:49:07 GMT+00:00 2006, Uncontrolled Copy, (c) BSI 4 performing any radiation testing. Similarly, it should be noted that this International Standard gives guidelines on how to perform radiation testing on a space solar cell independent of the device technology. Differing cell technologies might exhibit differing radiation response characteristics that need to be understood in order to perform a meaningful test. On the basis of practical limitations, the recommended energy range for proton irradiations is 30 keV to 30 MeV. The recommended energy range for electron irradiations is 200 keV to 3 MeV. In special cases, lower energies might be achievable. Damage comparisons are usually performed with 10 MeV protons and 1 MeV electrons. It can be convenient in some cases to perform particle transport calculations before performing the irradiations. These calculations can tell one how far the incident irradiation particle will travel within the solar cell before it stops. For example, if one wanted to irradiate a silicon solar cell, the irradiating particle would need to travel some distance on the order of 100 µm to reach the active region of the solar cell and therefore cause significant damage. Therefore, it is necessary to perform the transport calculation to determine the particle energy required to cover this distance. For proton transport calculations, the Monte Carlo code SRIM 12 should be used; for electron irradiation, ITS TIGER 13 should be used. Post-irradiation annealing is one specific example. Silicon (Si) solar cells have been observed to anneal over time at room temperature after irradiation. It was found that the cell electrical output stabilized after a 24 h, 60 °C anneal, so such a post-irradiation anneali