At 02:03 PM 8/14/97 -0500, you wrote:

>There is allways a probability of interaction depending on the matter that
>the gamma passes through (i.e. lead has a higher probability than
>aluminum).

The higher the frequency the lower the level of interaction, the memory
cell in EPROMS is *designed* to be erased by a particular band of UV,
lower frequencies have less effect and so do higher frequencies - the
trick with X-rays is a frequency that is higher than UV such that it
wll penetrate the packaging but low enough to STILL have some value
in erasing the cell, this will taper off radically as frequency goes up.

Lead (like most things) are less effective the higher frequency
you go. For very high frequency gamma rays (ie high energy) you need
progressively thicker lead (or other dense material) for shielding,
like spent U238 (just make sure there are no neutron sources ;-)

>Gammas are not electromagnetic in the normal fasion.

Rubbish - from low frequency RF, through visible light, UV, Xray, Gamma
its ALL electromagnetic radiation - there is no distinction.

  There is a wide
>energy band of gammas that differ in energy (and frequency) depending on
>the isotope that generated them.

They differ in frequency hence they differ in energy, if you are talking
about beta decay then thats different - thats an electron which can
be expelled with differing energies - but its not EM.

>>- be careful - there is no safe dose
>>of Gamma OR UV for that matter
>
>Sure there is according to the US 10CFR20 regulations from DOE.

I reject the regulation - it is only a reference for convenience, there
is NO safe DOSE of radiation.

>  Dose is
>something like 5 REM (radiation equivalent to man) per year for adults over
>18 years of age (the formula is a little more complex).  BTW that is above
>background, you allready get some exposure just from naturally occuring
>radioactive decay, and from outerspace.

The rule is DON'T add to background radiation, any ionising radiation
will be harmful given enough time. You never know when that one particular
gene gets hit to force a cell to go cancerous - its a statistical phenomena,
therefore there is NO SAFE dose - don't be misled by an out of date
idea about what radiation is. Heck they still transport strong magnets
in lead boxes for crying out loud !

>Well, everything does a little.  Polyethelye does offer some sheilding.

In practical terms the package of a PIC will provide NONE AT GAMMA
frequencies - it provides near 100% shielding against UV since its
opaque and UV is just above visible. Gamma is way above visible, it
so happens the attenuation is probably less that 0.0001% comments ????

>  I
>belive ceramic IC packages offer more than plastic packages (that's why we
>uses them in radiation hardened products).

Hang on - there are other issues here about radiation hardening, ceramic
packages provide negligible shielding - that is NOT radiation hardening.
Ceramic packages are generally more useful for better sealing and better
heat transfer - shielding is definitely negligible for GAMMA rays, though
it 'might' have some benefit against low energy beta particles.

>Enough nuclear physics, lets get back to PIC's.

We've only had enough when its cleared up Norm, otherwise we will
all end up glowing in the dark and coughing up blood due to ignorance ;-)

Rgds

Mike
Perth, Western Australia

There is no a'priori reason that the ultimate truth will be interesting
or even useful, those moments of frustration during philosophical debate
would be replaced by the sheer terror which accompanies true knowledge.

Unsubscribe message should go here - please :)