AW: [ RadSafe ] Mistake on the UMRC web site

[Rainer.Facius at dlr.de]

Roger:

The problem which you/we/they appear to be struggling with is the fact that absorbed dose is the expectation value of specific energy, i.e., the ratio of energy absorbed divided by the mass which is absorbing it. 

A whole sub-discipline, microdosimetry, has attempted for many years to spell out what this implies for the variance of "specific energy" in small, microscopic volumes for different "radiation qualities", i.e., LET values at low doses. For common radiation protection practices ICRP has decreed that the mass of the tissue in which energy is deposited should be the denominator for the tissue (equivalent) dose which enters into the calculation of the effective dose. Since decades the usability of this has been questioned by people assigned to the task of assessing space radiation hazards (see quotations below). 

If - as you reasonably might argue - the energy deposited by the uranium alpha particles has to be assigned to those cells, which actually did absorb (at least some fraction of) it, then the table below would result for the alpha decays of the U isotopes (here I declared a sphere with the diameter of the alpha particle range as the absorbing volume):

A    |    B      |      J       |   C    |       D       |   E
238  |   6.47915 |  1.03807E-12 |  46.25 |   5.83791E-11 | 0.01778
235  |   4.679   |  7.49658E-13 |  27.97 |   1.29122E-11 | 0.05806
234  |   4.859   |  7.78498E-13 |  29.61 |   1.53193E-11 | 0.05082
with the column headers:
A: U-isotope; B: alpha energy/ MeV; J: alpha energy / J; 
C: range/micro-m tissue equivalent plastic A-150
D: irradiated spherical mass / kg; E: dose in Gy per decay

With the claimed numbers of decays per year (which are in the same ball park where my numbers for a plain uranium pellet of the specified size lie) substantial doses will be absorbed by those cells which actually were affected by the radiation. 

My personal solution to this problem as I encounter it in space radiation protection against heavy ions (HZE particles) is to get rid of (the nuisance parameter) absorbed dose (with the implied LET dependent fudge factors RBE or quality factor) altogether and to use instead the combination of flux and reaction cross section as a measure of exposure and risk (see e.g. last quotation below).

Best regards, Rainer


<quote>Established approaches to radiation dosimetry are inadequate to assess or to predict the possible hazard from exposure to HZE particles. At the microscopic level, the deposition of energy in tissue along the path of an HZE particle is very different from that encountered with conventional radiations available in the laboratory.<end quote, [1] p. 4>

<quote>The ICRP [2] expressly states that for radiations with very high LET, the concept of dose as energy absorbed per unit mass of tissue breaks down, and the rad unit cannot be applied. <end quote, [1] p.26>

<quote>When, as is the case with heavy nuclei, nonuniformity of energy distribution becomes so great that a group of cells in the critical volume is heavily irradiated while the rest are untouched, the concept of mean dose to a critical tissue or organ breaks down and has no meaning. If dose has no meaning, then obviously QF and biological dose equivalent have no meaning either. It is not possible, therefore, to assess the potential significance of heavy galactic nuclei to manned-space-flight operations by the usual approach. <end quote, [3] p. 38>.

<quote>Consideration of the dosimetric problems of exposure to HZE particles leads to the conclusion that some other way than conventional specification of dose must be found to quantify the exposure. <end quote, [1] p. 12>

[1] National Research Council, Space Science Board, Committee on Space Biology and Medicine, Radiobiological Advisory Panel (Grahn D, ed.), HZE-Particle Effects in Manned Spaceflight. National Academy of Sciences, Washington D.C. 1973

[2] ICRP, Recommendations of the International Commission on Radiological Protection. Publication 9.Pergamon Press, Oxford, UK, 1966

[3] Langham, W H (ed.), Radiobiological Factors in Manned Spaceflight, Publication 1487, National Academy of Sciences/National Research Council, Washington, D.C., 1967

Dr. Rainer Facius
German Aerospace Center
Institute of Aerospace Medicine
Linder Hoehe
51147 Koeln
GERMANY
Voice: +49 2203 601 3147 or 3150
FAX:   +49 2203 61970

-----Ursprüngliche Nachricht-----
Von: radsafe-bounces at radlab.nl [mailto:radsafe-bounces at radlab.nl] Im Auftrag von Roger Helbig
Gesendet: Sonntag, 12. März 2006 14:15
An: radsafelist
Cc: vet2
Betreff: [ RadSafe ] Mistake on the UMRC web site

 To: du-list at yahoogroups.com
> From: "marktwain403" <marktwain403 at yahoo.com>
> Date: Sat, 11 Mar 2006 06:05:55 -0000
> Subject: [du-list] A mistake on the UMRC web site
> 
> Just for the fun of it, I was looking at the UMRC 
> http://www.umrc.net/default.aspx web site tonight and  found a very 
> signicant mistake. They have a page > titled "Radiation and  the human 
> body" http://www.umrc.net/radiation_and_the_human_body.aspx which 
> contains the following text quoted below. But  before I present the quoted text, I will comment on  the mistake. They  say that a tiny pellet of uranium emits approximately 37 alpha  particles per year and from that they calculate a dose of 17 rads per year, far above federal guidelines. I checked their calculations and  did not get exactly the same number of alphas but the number was close to their calculation at about 15 alphas per year.