Health Risks of Radon and Other Internally Deposited Alpha-Emitters BEIR IV (1988) / Chapter Skim
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I Dosimetry of Alpha Particles
I Dosimetry of Alpha Particles
Pages 397-414
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From page 397... ...
He found that very small deflections, a degree or so, are frequent and that larger deflections do occur but are quite rare. Consequently, along a typical track, each time an alpha particle scatters it changes direction only slightly and in a random direction, with the result that the track winds back and forth a small amount around a straight line.
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From page 398... ...
is the quotient of the stopping power by the density of the material. The experunental and theoretical determinations of the alphapa~ticle stopping powers are in good agreement.~4 Figure I-1 shows the stopping power or alpha particles In soft tissues of unit density.
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From page 399... ...
In either case it is important to specify the limit; this is usually done by writing it as a subscript to the symbol L The energy limit is the current preference, because the LET can be calculated from theoretical equations for the stopping power by simply excluding energy losses to secondary electrons above the selected limit.
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From page 400... ...
STRAGGLING Random variations in the energy lost and in the change in direction in individual interactions with the molecules of the material produce distributions in the actual distances traveled by different alpha particles; this is known as range straggling. Similarly, there is a distribution in the energy remaining after traveling a given distance; this is known as energy straggling.
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From page 401... ...
This relation shows that, in air, most of the actual distances traveled are within a few percent of the mean distance (the range) ; theoretical analyses suggest sunilar narrow distributions for the distances in tissue.
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From page 402... ...
In these circumstances the average absorbed dose, (D>, in the particular tissue equals the product of the number of alpha particles emitted within it and their energy, E, divided by the mass of the tissue.
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From page 403... ...
First, S gives the energy lost by the alpha particle, not the energy imparted to the medium in the target. Secondary radiations, electrons (called delta rays)
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From page 404... ...
Use of the stopping power or an average stopping power does not allow for particles emitted in dV and headed for the target element that do not- get there because they scatter away from it; it also does not allow for those not headed for it but scattered so that they hit it. The data in a Bragg curve are converted to e(T)
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From page 405... ...
In general, for uniform distributions in C, the absorbed dose does not exceed CE anywhere. If the sphere (or any other small volume)
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From page 406... ...
. Absorbed dose is a quotient of a quantity with dimensions of energy by one with dimensions of mass; therefore, its unit in the SI is joules per kilogram (J kg-~.
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From page 407... ...
Making the absorbed dose distributions the same during the irradiations may be difficult. There is seldom difficulty in in vitro cell experiments where the absorbed dose can ordinarily be made uniform throughout the exposed population.
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From page 408... ...
SPECIFTC ENERGY Dosimetry deals with the absorbed dose, the mean energy per unit mass imparted to matter by radiation; microdosimetry deals with the actual energy per unit mass. The latter is given another name (specific energy)
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From page 409... ...
According to Equation I-15, the variance is the product of the absorbed dose and a factor that is independent of dose; the factor depends only on the characteristics of single events. Table I-1 lists values of this factor and of the mean specific energy for single events for a number of radiations.
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From page 410... ...
To cover a wide range of the abscissa, the probability density is multiplied by z and then plotted on a logarithmic scale, because equal areas anywhere under such a curve represent equal probabilities of occurrence. On this logarithmic plot the two distributions differ only slightly in shape, but there is a large distance along the abscissa between them due to the difference in the stopping powers of the particles (discussed earlier in this appendix)
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From page 411... ...
The following expression relates, approximately, the mean specific energy in single events, (z: 1) , the mean stopping power of the particles, (S)
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From page 412... ...
for different average numbers of alpha particles per particulate. To get the same absorbed dose, the number of particulates per unit volume is changed in inverse proportion to the number of alphas
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From page 413... ...
A site close to a particulate that emits so many alpha particles stands a good chance of receiving energy from more than one alpha particle, with the result that its fizz is pushed to higher specific energies. But, when so many alpha particles are emitted from each particulate, there are fewer particulates for a given dose, with the result that there is an increased chance that some sites will not be close enough to any particulate to be hit by any alpha particles.
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From page 414... ...
1978. Similarity treatment of phase effects in stopping power for low energy heavy charged particles.
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