Strålningsskador vt 2014 Säteilyvauriot kl 2014 Radiation Damage Spring term Macroscopic/application consequences of irradiation

Transcription

1 Strålningsskador vt 2014 Säteilyvauriot kl 2014 Radiation Damage Spring term Macroscopic/application consequences of irradiation

2 10.1 Consequences of irradiation at what scale? Most of the consequences of irradiation mentioned on this course were microscopic in nature Defects nm-sized, dislocations nm or μm-size scale From a scientific point of view, these are very interesting But from the how does this affect the price of milk in the store? point of view, irrelevant by themselves. The key question from this practical-application point of view is how the radiation effects affect practical applications which ultimately work on the macroscale and on human timescales (seconds to years)

3 10.1 Even nanoscale effects affect modern technology Things are, however, not so easy as that one could say that there is a huge gap between the nm scale of defect generation and the macroscopic scale [ Cons[ider for instance a modern Old.summaries/01abstracts/ykchoi.3.html] high-end computer chip. The transistors that make it work are already now on the ~ 10 nm size scale (so called 20 nm generation of chips actually contains feature sizes even less than 20 nm in size A single heat spike can have dimensions ~ 10 nm A single primary damage effect can affect destroy a transistor This is a macroscopically meaningful outcome: one bit in your memory may swap state or become unusable! Fortunately probability is small but error correction needed

4 Aim of this section This section of the course gives some brief examples of (at least) fairly well understood macroscopic outcomes of irradiation and how they come about from the nanoscale original damage production dealt with in the earlier sections This is not a comprehensive list of known outcomes attempting to make one is virtually impossible as there are at least hundreds of known distinct effects Rather, the aim is to present a selection of some commonly dealt with and important effects

5 10.2 Transistor single event upset effects Radiation can (as noted before) cause errors in electronic circuits This can be caused by cosmic rays and natural radioactive decay in any circuit anywhere, and is impossible to avoid completely anywhere on earth! The situation is more severe in high-radiation environments like space, nuclear reactors and particle accelerators And as an intermediate case, flights: cosmic radiation is much higher 10 km up then down at earth due to less protection from the atmosphere Known for a good while [e.g. J. F. Ziegler and W. A. Lanford, "Effect of Cosmic Rays on Computer Memories", Science, 206, 776 (1979)] but the probability is increasing with reduced circuit dimensions

6 Soft and hard errors A single particle-induced disturbance is called a single event upset (enskild händelse-förändring/yksittäistapahtumahäiriö??) or single event effect (SEE) These can be further subdivided into: Soft errors = state is changed once, but transistor continues to work Hard errors = transistor is permanently broken This possibility is well known, and for space applications there is even a standard for testing against them MIL-STD-883 specification available for download from the Defense Logistics Agency (730 pages ) Also particle accelerators often used for testing against them [Partial source: ]

7 Mitigating the effects: ECC s As transistor sizes have become smaller, also other factors reduce the reliability of single transistors E.g. atom-level fluctuation in number of dopants Processor design can be tailored to have lower error rates A (non-objective commercial) example: But there are limits to what one can achieve with reducing dimensions Due to all Error correcting codes (ECC) (felrättande kod / virhekorjaava koodi? ) are routinely used in memories Standard computer science: to store e.g. 8 bits, use a few extra redundant bits in some smart algorithm such that an error can be detected if one or a couple of bits flip state Common variety: Hamming code, allows for 1-2 bit error detection

8 Mitigating the effects: active means Modern flash memories have evolved to have very advanced error correction algorithms, running within the memory chip itself the memory is no longer a passive sets of 0 s and 1 s but have computing power by themselves! [Kim and Sung EURASIP Journal on Advances in Signal Processing 2012, 2012:195] Example: a report already from 2009 discusses 8 kilobyte ECC data blocks Note the Target Bit Error Rate region: Flash mem s are not even expected to work perfectly nowadays [From: Jim Cooke (August 2007) The Inconvenient Truths about NAND Flash Memory Micron MEMCON '07 presentation.] [

9 10.3 Mechanical properties As discussed in the previous section, dislocation activity tends to lead to hardening and embrittlement of metals High dislocation concentration => lots of dislocations act as obstacles to each other => material harder and more brittle Example data for Cu: shows a very strong effect [B.N. Singh, A.J.E. Foreman, H. Trinkaus, Radiation hardening revisited: role of intracascade clustering, Journal of Nuclear Materials 249, (1997) ]

10 Mechanical properties example: Fe Example quantitative data for Fe:

11 Plastic properties of irradiated silica Silica when irradiated with swift heavy ions shows a peculiar flow behaviour known as ion hammering : the material flows perpendicular to the beam [Snoeks et al, Appl. Phys. Lett. 65 (19) (1994)]

12 Plastic properties of irradiated silica Basic idea: swift heavy ion spike causes a horizontally expanding shock wave - Analytical model by Trinkaus [J. Nucl. Mater. 223 (1994) 196] 1. Heating in ion track 2. Onset of pressure wave 3. Pressure wave expansion a-sio 2 ~ 100 fs a-sio 2 ~ 3 ps a-sio2 ~ 10 ps Swift heavy ion path

13 Plastic response of Si A plain wafer of Si when irradiated for a long time undergoes a complex stress state evolution First the stress increases with interstitial formation and amorphous pocket formation. When the sample surface region is finally fully amorphous, the stress can start to relax due to radiation-enhanced flow of the amorphous material The measurement was done by measuring macroscopically the curvature of a piece of Si with a laser reflection technique [Cynthia Volkert, J. Appl. Phys. 70, 3521 (1991)]

14 Organic materials When organic materials are irradiated, the basic effect is usually that hydrogen tends to leave the sample, reacting with the oxygen and/or nitrogen in it if present to form gases H 2, H 2 O, etc. Since the carbon is bound in a network, it is less likely that it leaves Ergo: C content in sample increases => material carbonizes (förkolnas / hiiltyy) End result can be expressed simply: the paper turns brown And becomes mechanically brittle (because amorphous carbon is brittle)

15 10.4 Macroscopic surface effects Many materials, when irradiated to really high fluences by gas atoms, show a blistering (blåsning / rakkoutuminen) effect: micronsized bubbles form which make the surface swell locally Looks like a blister on the skin, hence the name Blister is filled with gas, which eventually ruptures G.-N. Luo et al. / Fusion Engineering and Design 81 (2006)

16 Macroscopic surface effects If the irradiation is done somewhat deeper in and to a high fluence, one may get a smooth layer of gas which can eventually pop up a big segment of the surface This is called exfoliation (exfoliation / exfoliaatio)

17 Smart-Cut In 1994 Michel Bruel realized that if a big Si wafer is irradiated homogeneously, this can be used to extract an entire macroscopic film of Si [US Patent , Bruel, Michel, "Process for the production of thin semiconductor material films",] and e.g. bonded on another piece of oxidized Si to make a buried silica layer, Silicon-on-insulator ) This is sold as Smart-Cut and in wide industrial use!