Re: Rad Silicon

["Gary T. Pepper" <gpepper@capitalnet.com>]

At 12:10 PM 2/17/1998 -0600, you wrote:
>Good morning from soggy California.
>
>Some time ago there was some discussion about "rads silicon" vs. "rads
>air". I did a search in the Radsafe archives, but didn't get good
>results. Do any of you know what "rads silicon" is and the conversion
>factor from silicon to air? Is it the ratio of the mu/rhos?
>
>Thanks for your help, and send us some sunshine, please! I was going to
>buy a house on a hillside, but I just let it slide.
>
>
>Donald P. Mercado
>Radiation Safety Officer
>Lockheed Martin Missiles & Space
>O/47-20, B/101
>1111 Lockheed Martin Way
>Sunnyvale, CA 94089
>Ph. (408) 742-0759
>Fx. (408) 742-0611
>Email: Don.Mercado@lmco.com
>
>

Hi Everyone,

The term "rad silicon" has found wide-spread usage in the radiation effects
in electronics research community, but IMHO, is actually incorrect
terminology.  A more correct terminology would be something like "the dose
in silicon is 'x' rad".

A rad is "defined" as 0.01 J/kg, i.e. the absorbed dose is the energy
absorbed per unit mass of material.  The unit of rad is thus independent of
both the type of radiation and the material.  Like they say, a rad is a rad
is a rad!  ;-)

For gamma-rays in a material other than air, the absorbed dose in the
material (Dm) is related to the absorbed dose in air (Da) by

Dm(rad) = Da(rad) * [ Am / Aa ]

where Am = mass energy absorption coefficient for the material 
      Aa = mass energy absorption coefficient for air

The mass energy absorption coefficients are energy-dependent, so you must
know the energy spectrum of gamma-rays in order to be able to convert the
absorbed dose in air to the absorbed dose in silicon (or vice versa).

[Strictly speaking, in order to apply the above dose conversion formula to
a physical measurement, one must be sure that conditions of charged
particle equilibrium exist in the material, but this is beyond the scope of
this quickie explanation.]

Check out a dosimetry text like "Concepts of Radiation Dosimetry" (Kenneth
R. Kase and Walter R. Nelson, Pergamon Press, ISBN 0-08-023161-6) for a
more complete discussion on this topic.  Hope this helps!

Regards,
Gary T. Pepper, Ph.D.
Radiation Physics Consultant
Ottawa, Ontario, Canada

e-mail: gpepper@capitalnet.com