[hackerspaces] Geiger counters

[john arclight]

I have actually had some training on  this topic, and have a pretty
good knowledge of radiation instruments.

What your friend needs is a low-range instrument, i.e. something
capable of detecting at least beta/gamma radiation from background up
to 100mRem/hr or so. The big, yellow civil defense units like the
CDV-715 are high-range detectors that will only register on very high
levels of radiation, such as the fallout from an actual nuclear
detonation.

Unless they are working around that nuclear plant, the worry is not
direct exposure to radiation, but contamination from particles of
radioactive material.  Short-term exposure to high levels of radiation
can cause burns and sickness, and long-term exposure (such as from
eating contaminated food or breathing in radioactive vapors from a
steam explosion) can increase cancer risks or damage certain organs,
such as the thyroid.

Ionizing radiation comes in three major forms: gamma rays (high-energy
photons), beta (high energy electrons), and alpha (heavy particles
with two protons and two neutrons).   Microwaves, radio-frequency
energy and EMI from power lines are not considered ionizing radiation
and not hazardous in the same way.

Radiation exposure can be limited in 3 ways: time (minimize the time
you are outside, for instance), distance (evacuate the immediate area)
and shielding (stay inside, put stuff between you and the source of
radiation).  Gamma rays require mass to stop. Beta rays can be stopped
by thin metal, and alpha particles are actually stopped by the first
few layers of your skin. None of these types of radiation will make
you or anything else radioactive, but you can be harmed if you absorb
them into your body and they continue to decay and release radiation
at zero distance and with nothing shielding you from them,

The type of radiation that turns normal matter into radioactive
isotopes is only found inside the reactor while it's running (neutron
radiation) or in a particle accelerator.

There are several ways to detect low levels of radiation as you might
find in downwind contamination from the plant. One would be the
already-mentioned Geiger-Mueller  counter.  It needs a factory-made
tube with an electrode and a special quench gas sealed inside,  along
with a 500-750VDC power supply. When a ray or particle of ionizing
radiation passes through the tube and intercepts a molecule of the
gas, it will ionize it and cause a spark to jump from the electrode
inside to the grounded outside, registering a "click" or count.

All of these tubes will detect Beta and Gamma rays, and some with a
thin mica window (called a "pancake" or "end window" tube) will also
detect alpha particles . A pancake G-M counter would be the gold
standard to acquire.

A serviceable alternative to the G-M tube is the ion chamber. This
uses a sealed container full of gas, but it operates at a much lower
voltage and the gas can be air. It operates on the principle that the
air inside will become more conductive and pass more current if there
is ionizing radiation passing through it.  You can in fact make one
out of a soda can and some amplifier circuitry.

Unfortunately, the amount of current that flows from small amounts of
radiation is measured in units like femtoamps-nanoamps, i.e. not very
much.

Another detector that can be made at home is the electroscope that was
also mentioned above. This is the old high-school physics experiment
with the two gold leaves. You charge them up, and the rate at which
the charge leaks off (and they come back together) is proportional to
the amount of radiation present.


Here are some links that may help:

Ludlum Measurements - They sell 100% excellent, new detectors. I'd
recommend a pocket-sized detector with a panckae or end-window tube:

http://www.ludlums.com/

Homemade Ion chamber:
http://www.edcheung.com/automa/radon.htm

Homemade dosimeter:
http://en.wikipedia.org/wiki/Kearny_Fallout_Meter

The best place to find discussions on radiation instruments - homemade
and otherwise:
http://tech.groups.yahoo.com/group/CDV700CLUB/

FEMA course on how to monitor radiation (free self-study online)
http://training.fema.gov/EMIWeb/IS/is3.asp


Arclight


On Tue, Mar 15, 2011 at 9:09 PM, Sean Bonner <sean at seanbonner.com> wrote:
> Anyone know anything abut geiger counters? Specifically building them?
> Have some folks in Japan who are trying to get their hands on them but
> failing and considering DIY options...
>
> --
> Sean Bonner
> http://www.seanbonner.com - homebase
> http://www.metblogs.com - get local
>
> *** Please check your address books, the best e-mail for me is
> sean at seanbonner.com
> _______________________________________________
> Discuss mailing list
> Discuss at lists.hackerspaces.org
> http://lists.hackerspaces.org/mailman/listinfo/discuss
>