[hackerspaces] Geiger counters

john arclight arclight at gmail.com
Wed Mar 16 07:59:10 CET 2011

This is sorta true. The assumption behind the CDV-715/717/720 is that
when fallout from an A-bomb or H-bomb comes, radiation levels outside
may be thousands of REM/hr.

A percentage of people will die from a 500RAD exposure (that's REM* a
factor for the type of radiation exposed to).  And people can get sick
at levels of 50-100R.

So if you were exposed to 1,000R/hr for only 10 seconds while you
checked it, that would only be 1000/3600 or 270mR.  The goal of these
instruments was to help people make choices that kept their exposure
under 500R for the 2-3 weeks required for much of it to dissipate or
decay away.  The tasks they expected to be performed include:

1. Measuring the levels inside your shelter to determine the safest
place for people, and the safest place for individuals most at risk,
such as pregnant women.

2. Using it to take brief measurements outside, so that you will know
when it is safe to go outside and for how long.

A separate instrument, called the CDV-700 was also made that could
detect much lower levels, down to background. This was to be used for
checking food, clothes, etc for contamination.  You can sometimes find
these on eBay. I would skip the CDV-715 series unless you're worried
about bad, nuclear-capable neighbors attacking you.

Just for perspective, the levels that are permissible in a work
environment are hundreds of times less than the levels that will make
you sick, and much less than the levels documented to  increase cancer
risk.  And all of us are exposed to a few R every year from cosmic
rays and naturally radioactive materials.

So the risk from weapons fallout is immediate and life-threatening,
and those instruments were built to work with that risk. The risks
from the nuclear power plant accident are more long-term and require
the low-range measuring instruments.


On Tue, Mar 15, 2011 at 11:42 PM, Matt Joyce <matt at nycresistor.com> wrote:
> From my reading on the CDV-715...
> It seems as if... if you register anything with that... you have already
> taken a lethal dose of radiation.
> On Tue, Mar 15, 2011 at 11:31 PM, john arclight <arclight at gmail.com> wrote:
>> 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
>> > _______________________________________________
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