[109450] in Cypherpunks
About Alpha radiation...
daemon@ATHENA.MIT.EDU (Jean-Francois Avon)
Wed Mar 24 17:53:35 1999
From: "Jean-Francois Avon" <jf_avon@citenet.net>
To: "Cypherpunks" <cypherpunks@toad.com>
Date: Wed, 24 Mar 1999 16:38:03 -0500
Reply-To: "Jean-Francois Avon" <jf_avon@citenet.net>
On Wed, 24 Mar 1999 09:13:34 PST, Nobuki Nakatuji wrote:
>He said that an alpha particle from space
> can knock out a transistor in the chip and modify the cipher. Then
> again it is a trivial matter to have not one but many CPUs
Alpha ?!? Duhhhhhh!
Alphas are helium nucleus. Two protons and two neutrons.
They have a very shallow penetration and are usually stopped by a sheet of
paper.
Here, quoted from: _Element_of_Nuclear_Physics_, Rasetti, Prentice-Hall, 1936.
---------- quoted figures (not the text) -----------------
[from table on p.116-117]
Alphas from various sources of radioactively decaying atoms have various
energy. One of such measure is the Mean Air Range. It goes from, typically,
2.63 cm for Alphas issued from Uranium I, to 11.5 cm from alphas issued from
Thorium C'. The majority of values are into the 4 to 6 cm range*.
[from p. 48]
Each substances have their own stopping power.
Here is a table of the stopping power of selected substances
Substance Relative Stopping Power
Air 1
O2 1.07
H2 0.21
He 0.17
Ne 0.62
A? 0.98
Kr 1.52
Xe 1.98
Al 1700
----------- end of quoted figures -----------------
----------- quoted text p.48----------------
Bragg has given he approximate enpirical rule: the stopping power is
proportional to the square root of the atomic weight.
------------end quoted text ------------
You have to figure out how much mass a given particle has to traverse. It is
thus function of Bragg's rule, density and tickness of the material.
Crossing one square inch of air that weight a given number of grams is not
that far from crossing one sq.in. of lead that weight the same. It follows
that for air, it has to be much thicker than lead... This is why lead is
surrently used as a shield, for it pack a lot of mass in a thin sheet at a
relatively low price.
Back to silicon chips
Thus, alphas from space having to go through any computer casing, chip ceramic
package, etc, roof and ceiling of the builging, etc would never hit the
silicon chip.
Alpha from other sources might be another story, but still, the slightest
material is enough to shield from alphas. Alphas from Radon, a commonly found
radio-isotope seeping out of concrete in our basements have an Air Mean Range
of 4.014 cm, (approx 1.58 inches) (figures from Rasetti, P116)
Betas are high speed electrons (I'm not talking about NSA employees here...),
with slightly more penetration.
Gammas are high energy photons, just like incredibly short wavelenght radio
waves or light rays. They are ionizing, which mean that they could change the
conductive state of semiconductors.
My father who was a radiologist made experiments with digital watches put
under X-ray beams: he found that they were usually not affected by the
ionizing energy beam.
Cosmic rays are gamma of even higher energy.
Generally speaking, from the medical X-ray technician standpoint, the higher
energy X-ray are less dangerous than the lower energy ones because they keep
going their way through instead of being stopped within and leaving energy in
to wreak havoc.
When you shoot a patien with X-ray, the dangerous one are not as much the
direct ones as the low-energy ones that gets re-radiated after the body
mattere has been excited with higher energy one. The re-radiated (like
fluorescence, but in the X-ray spectrum)ones are of much lower energy, thus
more likely to get stopped and absorbed within body tissue. This is what
creates dammage (usually by releasing a shower of free electrons that roams
around and disrupt cellular and DNA processes).
This is the reason why low energy (low kilovolts) X-rays (like the one of
mammographs) are more dangerous than the high KV ones.
Back to silicon chips, unless I misunderstood my physics classes, Alphas never
get there...
Bêtas are unlikely to get there...
X are Gammas that smoked pot and drank too much, and Cosmics are Gammas on
steroids...
As far as flipping bits are concerned, X could possibly, Gammas do and Cosmics
too. But don't quote me on that last paragraph... :-)
[Rasetti was Enrico Fermi's student and was teacher to Jean Lefaivre, one of
my physics teachers at U.of Sherbrooke)
Ciao
jfa
