Answer to Question #7153 Submitted to "Ask the Experts"

Category: Instrumentation and Measurements

The following question was answered by an expert in the appropriate field:

Q
I am using dosimeter badges (from JP Laboratories) to quickly measure the dose experienced by a silicon memory chip during x-ray inspections. The tube operates at 90 kV.

I need to convert a dose in units of Gray (tissue) to a dose in units of Gray (silicon).

Do you know of a general equation, or a specific conversion factor, that I can use?

A

When you know the photon-induced dose in one material you can calculate the dose from the same photon field in a second material, under conditions of secondary charged particle equilibrium, by multiplying the dose in the first material by the ratio of mass energy absorption coefficients for the second material relative to the first for the photons of interest—i.e.,

D2 = D1(µen/?)2/(µen/?)1.

The condition of secondary charged particle equilibrium is necessary in order to perform this calculation; in general, it is met if the material to be irradiated is covered by a thickness of similar material at least as great as the range of the most energetic electrons produced by the photon interactions. For the maximum energy electron (90 keV) from 90 kVp x rays, the range in silicon is about 65 micrometers.

The effective energy of the x-ray beam from the 90 kVp machine depends on the degree of filtration in the x-ray machine; for a typical filtration of about 2.5 mm aluminum equivalent, the effective energy will probably be between 30 and 40 keV. If we assume 35 keV as the effective energy and that the dosimeter you are using is providing an acceptable measure of the soft-tissue dose (depending on what the badge characteristics are, the dose you are interpreting may be at some fixed depth—e.g., the deep-dose element in a personnel badge usually is positioned to measure dose at an effective depth of 1 cm in soft tissue), then we can estimate the dose to silicon at the same depth:

DSi = Dtissue(0.723/0.105) = 6.9 Dtissue.

Thus, the expected dose to silicon is about seven times that to soft tissue. The values of 0.723 and 0.105 for the respective values of the mass energy absorption coefficients for silicon and soft tissue were obtained from interpolation in the tables of values available from NIST. Table 3 contains silicon and Table 4 contains soft tissue (ICRU 44 equivalent). As noted above, this dose is based on the tissue dose at the depth determined by the badge properties. The calculated dose also takes no account of self attenuation in either the tissue dosimeter or in the silicon. The calculated dose is assumed to be that at the incident surface of the silicon. Depending on your requirements, more sophisticated calculations may be necessary to account for nonuniform dose distribution through the silicon thickness.

Hope this helps.

George Chabot, PhD, CHP



 

Answer posted on 21 January 2008. The information posted on this web page is intended as general reference information only. Specific facts and circumstances may affect the applicability of concepts, materials, and information described herein. The information provided is not a substitute for professional advice and should not be relied upon in the absence of such professional advice. To the best of our knowledge, answers are correct at the time they are posted. Be advised that over time, requirements could change, new data could be made available, and Internet links could change, affecting the correctness of the answers. Answers are the professional opinions of the expert responding to each question; they do not necessarily represent the position of the Health Physics Society.