Talk:Linear energy transfer

Definition

The definition of linear energy transfer should be changed to correspond to that defined by the International Commission on Radiation Units and Measurements: the difference between LET and stopping power is not due to emission of secondary radiation, but rather to the emission of high energy secondary electrons. —Preceding unsigned comment added by HPaul (talk • contribs) 17:40, 23 March 2009 (UTC)

Something doesn't work in the current definition. I don't have ICRU reports 37 and 60 on hand, so I can't verify those at the moment, but I am dubious that they define LET significantly differently from modern ICRP or ICRU reports. In ICRP report 92, almost all references to LET are for unrestricted LET, either explicitly or implied. The "L" quantity used in pre-1991 dosimetry is definitely unrestricted, with delta = infinity. ICRU report 51 defines linear energy transfer as synonymous to linear collision stopping power, both of them unrestricted. It notes that "a more general concept of linear energy transfer involves an energy cutoff. For the present report, however, the simpler definition of the 'unrestricted' energy transfer, as given above, is adequate." I could not figure out in what context the restricted LET is used, or why LET is considered separate from stopping power.--Yannick (talk) 00:00, 25 November 2012 (UTC)

Electronic effects

I wonder if LET is really used to describe the effects of space radiation on electronics, since on earth, it is neutrons/cm2/hour.HPaul (talk) 11:22, 2 April 2009 (UTC)

Yes it is. The LET of incident cosmic rays in space varies widely (from low LET particles such as electrons, mid LET particles such as protons, and very high LET heavy ions), and the LET an effect on the probability of a bit flip, and (very strongly) on that of a destructive Single event Latch up.AE1978 (talk) 22:16, 5 June 2011 (UTC)

Merger proposal

I propose that stopping power (particle radiation) (SP) be merged into linear energy transfer. (LET) The present articles claim a distinction in that SP is unrestricted, while LET is restricted. (I.e., LET excludes delta radiation and bremsstrahlung) However, my research has been unable to verify that claim. Here's what I found:

• ICRU 16 (1970) says unrestricted LET is the same thing as SP, except mass SP is divided by density and some definitions may exclude secondary gamma.
• ICRP 26 (1977) and 60 (1991) defined quality factors in terms of unrestricted LET
• ICRU 51 (1993) claims LET is synonymous to linear collision SP, and is "unrestricted" by default, except they both exclude bremsstrahlung
• ICRU 60 (1998) claims LET is restricted by default, and is synonymous to restricted linear SP. Linear SP appears to be unrestricted by default. ICRU 60 also gives more emphasis to definitions of mass SP and LET than for linear SP, which suggests to me that the term "LET" is preferred over "linear SP", while "SP" means "mass SP" by default.
• ICRP 92 (2003) claims LET is unrestricted by default, and is just the radiobiology term for SP.
• IAEA's "Radiation Oncology Physics" textbook (2005) describes the difference between SP and LET as subjective: SP is the loss of energy of the radiation, LET is the absorption of energy by the medium. It's left to the reader to reason that LET must equal SP. The textbook appears to refer to unrestricted mass SP by default, though it also defines collision SP (no bremsstrahlung) and restricted SP. (no bremsstrahlung or delta)
• I do not have copies of ICRU 37 and 73, which are currently listed as references for definitions

In summary, it looks like both LET and SP are both normally unrestricted, or at least both normally "collision," meaning they both exclude bremsstrahlung. If there is a functional difference between LET and SP, it would appear to be that SP equals LET divided by density. I think that both concepts can easily be explained in the same article without adding much article length. Merging the two will help remove redundancy and make it easier to maintain the articles. As to merge direction, I recommend going with LET to avoid the naming conflict with bullet stopping power.--Yannick (talk) 03:25, 26 November 2012 (UTC)

Looking over a greater variety of sources, beyond the ICRP/ICRU, it seems to me that stopping power is usually discussed as a property of the material for a fixed radiation, while LET is is usually discussed as a property of the radiation for a fixed material. But they're the same thing. There are some asymmetries when you look at mass stopping power, but there is an analogous of mass energy transfer coefficient defined for gamma.--Yannick (talk) 12:21, 30 November 2012 (UTC)

I think your proposal is a fine idea, but PLEASE merge LET into SP, not vice versa, since the LET article is not scientifically sound (e.g.: LET for gammas: that's nonsense!) The problem is simplified by the fortunate fact that the recent ICRU Report 85a is available in the Internet, and this is clearly the law to which we must abide.HPaul (talk) 03:58, 12 December 2012 (UTC)

Thanks for replying! I would really appreciate your oversight to make sure that our final product is scientifically accurate. With that aim, there are couple of points that I would like to discuss with you. First, the merge direction should be determined by which title we prefer to end up with, not the current content of the articles. The merge direction does not indicate which material will be given preference.

I have been working on improving the LET article and trying to do a slow merge, and I'm disappointed that my work hasn't met your standards. Could you provide more detailed comments? I have been trying to follow reliable sources, and I'd like to understand where I've gone astray. For example, ICRP 92 has a graph of LET versus photon energy in figure 2.3, page 20. I too was surprised by that and had to do considerable reading to figure out what that meant. ICRU 16 speaks of the LET "of the charged particles originally set in motion by" photons or neutrons, and table 1 page 14 gives single average LET values for various x-rays and gamma rays. It's fairly easy to find RBE-LET curves on the internet for Co-60 and Cs-137 gamma emitters. Many radiation protection documents refer to x-rays and gamma as low-LET radiation, and to neutrons as high LET radiation.

I recognize that speaking of gamma LET is a misleading shorthand, but it seems to be a widely used one that deserves explanation. I thought I had managed to present this complexity in the article without contradicting ICRU 85. I wrote, for example, that "stopping power has little meaning when applied to a photon or neutron." Could you suggest how we could make this clearer without contradicting the numerous articles out there that refer to gamma LET or neutron LET?--Yannick (talk) 15:21, 12 December 2012 (UTC)

I am sorry I hurt your feelings concerning gamma LET, and I am shocked that some official documents speak of LET for gammas. Actually, charged particles are mentioned right at the beginning of the LET article. I might ask Steve Seltzer, the main author of ICRU85a, what he thinks about that since I know him personally. HPaul (talk) 11:33, 13 December 2012 (UTC)

That could be interesting. But unless he's written his opinion down somewhere, it would be original research, which is not allowed on Wikipedia.--Yannick (talk) 14:38, 13 December 2012 (UTC)

Misunderstanding: if you tell me a definite reference where experts speak of LET for photons, I'll ask Steve Seltzer's opinion to help my understanding, not to quote him.

But let's leave this point for the moment in thinking of the direction for merging. Medical physicists speak of LET, and most non-medical physicists speak of SP, so both terms are equally important. Is it legal to have a title with both terms?? HPaul (talk) 11:33, 13 December 2012 (UTC)

Wikipedia's policy on this says we pick one name, redirect the other one, and mention both in the lede.--Yannick (talk) 14:38, 13 December 2012 (UTC)

My idea would be to start from the SP article (which is quite a bit larger than the LET article) and include some of the information about alphas and betas in it. The part about the slowing down of ions in solids (probably mostly due to Prof. Nordlund) certainly belongs there, not into LET. Personally, I think I should make sure that the SP article corresponds to ICRU85a. Best regards,HPaul (talk) 11:33, 13 December 2012 (UTC)

Could you be more specific as to which part you mean, and why it is not relevant to LET? If you check the article history, you'll find that I've rewritten most of the LET article myself, while User:Knordlun has made no edits. He has only worked on the stopping power article.--Yannick (talk) 14:38, 13 December 2012 (UTC)

The section "slowing down of ions in solids" by Nordlund in SP is not relevant to LET, nor is nuclear stopping relevant to LET.

I just checked one of your suggestions: Fig. 2.3 in ICRP 92. But that clearly speaks of the LET of electrons liberated by photons, not of the LET of photons!!!HPaul (talk) 11:42, 13 December 2012 (UTC)

I thought I tried to convey that in the article: "However, many authors speak of gamma LET anyway, where they are actually referring to the LET spectrum from secondary beta radiation." Could you suggest a better way to get this point across to readers?--Yannick (talk) 14:38, 13 December 2012 (UTC)

Misunderstanding: I was not talking about your LET article. You claim that there are expert statements about LET for photons, and I just showed that in this one case, this is not so.HPaul (talk) 22:47, 13 December 2012 (UTC)

I've separated out the gamma LET issue in its own section below.--Yannick (talk) 06:10, 14 December 2012 (UTC)

• I wouldn't support this, although the concepts are fundamentally the same, they are used in quite different ways and in different subfields of physics. But more crossreferencing would certainly be in order 128.214.7.97 (talk) 08:31, 13 December 2012 (UTC)

I agree.HPaul (talk) 22:47, 13 December 2012 (UTC)

• Sorry, I forgot to sign the preceding comment in my own name. Anyways, to elaborate a bit on Helmut Pauls comments on nuclear stopping power above: indeed nuclear stopping power for ions (or quasi-neutral recoils in solid) is not in any way LET, at least not in the way LET is defined as a "charged ionizing" particle. Nuclear stopping power does not involve any ionization, and it is also present for neutral recoils. E.g. a recoil inside a metal when moving at velocities below the Fermi velocity is (due to the Born-Oppenheimer approximation) essentially neutral, yet it experiences nuclear stopping power. Hence merging the stopping powerarticle into the LET one would be scientifically incorrect. Knordlun (talk) 18:40, 26 December 2012 (UTC)

• I now added a short paragraph on the nuclear stopping issue in the main article, with references. Knordlun (talk) 20:08, 26 December 2012 (UTC)

Gamma LET

Since the gamma LET discussion is getting messy, let me start this new section by clarifying my position. I agree that LET is not defined for uncharged indirectly ionizing radiation such as gamma. However, it is common for experts to talk about the LET of electrons set in motion by gamma or even neutron radiation, and it is common for experts to shorten their phrases or graph labels by simply talking about the LET of gamma, x-ray or even neutron radiation. Here are some examples of what I'm talking about:

• "Typical LET values for commonly used radiations are: 250 kVp X rays: 2 keV/µm; Cobalt-60 γ rays: 0.3 keV/µm; 3 MeV X rays: 0.3 keV/µm; ... X rays and γ rays are considered low LET (sparsely ionizing) radiations, ..."Podgorsak, E. B., ed. (2005). Radiation Oncology Physics: A Handbook for Teachers and Students (PDF). Vienna: International Atomic Energy Agency. pp. 486–487. ISBN 92–0–107304–6. Retrieved 25 November 2012. {{cite book}}: Check |isbn= value: invalid character (help)
• "Les photons, les électrons et les muons sont des rayonnements à faible TEL avec des valeurs de TEL inférieures à 10 keV/µm." (Photons, electrons and muons are low-LET radiations with LET values below 10 keV/µm.)"The 2007 Recommendations of the International Commission on Radiological Protection". Annals of the ICRP. ICRP publication 103. 37 (2–4). 2007. ISBN 978-0-7020-3048-2. Retrieved 17 May 2012.
• "For alpha particles, the LET is much greater than for gamma radiation ..."Allisy-Roberts, P J (1 March 2005). "Radiation quantities and units—understanding the sievert". Journal of Radiological Protection. 25 (1): 97–100. doi:10.1088/0952-4746/25/1/008.
• I can list dozens more if you need...

This was baffling to me when I was first learning about LET, until I found a reasonably direct explanation in an FAQ from the Health Physics Society: "When the term 'stopping power' is used in reference to photons, as seems to be the case for the example you give, it is not really being used for the photons themselves, but for the electrons set free by the photon interactions." That FAQ indicates that this usage is common, and I am not alone in getting confused by it. Wikipedia aims to write technical articles for the widest possible audience including those who are unfamiliar with the topic before reading. I think it would be helpful to those readers if we gave an explanation of gamma LET similar to the one the Health Physics Society has given. That is what I tried to do in the Application to Radiation Types section. I welcome help in making this explanation clearer or more accurate.--Yannick (talk) 06:10, 14 December 2012 (UTC)

Thank you, Yannick, for this clear exposition. I am shocked, and I have to think. I'll probably ask Steve Seltzer what he thinks about this apparent contradiction between ICRP and ICRU concerning LET for photons.HPaul (talk) 09:28, 16 December 2012 (UTC)

Yay, mutual understanding! I would be interesting in hearing Seltzer's view when you find out. Are there other parts of the current LET article that might need corrections?--Yannick (talk) 22:38, 17 December 2012 (UTC)

Hi Yannick! I have just sent an email to Steve Seltzer about the disturbing difference between ICRU and ICRP. We'll see what he says.HPaul (talk) 11:04, 18 December 2012 (UTC)

Hi Yannick! Please give me your email address so that I can forward you Seltzer's short answer, or send it to me from my German Benutzer page.HPaul (talk) 16:29, 18 December 2012 (UTC). In the meantime, I have made many changes to the LET article. I hope you are not shocked.HPaul (talk) 18:37, 18 December 2012 (UTC)

Hi Yannick! Could you please add the ICRP reference that you quote above, also to the section on Gamma in the present LET article?HPaul (talk) 11:53, 19 December 2012 (UTC)

Article on NIEL?

• The above discussion on nuclear stopping made me realize there is no wikipedia article on "Non-ionizing energy loss" NIEL, which is similar to nuclear stopping roughly as LET is similar to electronic stopping. Although again they are used confusingly in different communities and are not exactly the same. Should we make a wikipedia article on NIEL? I added a mention here and will at least add a short mention to the stopping power page. Knordlun (talk) 20:18, 26 December 2012 (UTC)

No advantage in merger

I think it has become clear in the meantime that the two articles should not be merged but should be connected by appropriate cross referencesHPaul (talk) 10:30, 16 January 2013 (UTC)