[sci.electronics] Radiation Detectors/Counters

neal@lynx.uucp (Neal Woodall) (04/26/89)

I have recently decided to purchase a geiger counter, for general use and for
possible survival applications. I have seen several different types of devices
offered for sale, including some that are old US Civil Defense surplus, some
that are based on solid-state sensors that are obviously commercial-grade
units, and some that are advertised as being European/NATO surplus (brand new,
never opened).

Now for questions/confusion:

I believe that the geiger counters that I have seen can be broadly classed
"radiological health" units (measure in the .5 to 500 milli-roentegen/hour
range and are not "hardened" or weather-proof) and those that are intended
for military/war conditions (measure in the 1-500 roentgen/hour range and are
"hardened" and weather/water proof).

How exactly is a "roentgen" defined? Is it in terms of energy dose, or just
in terms of "counts" (ie, gamma striks regardless of energy, also do
beta/alpha strikes count, even though they are obviously not as bad as
high-energy gammas)? What is considered normal "background radiation" in
roentgens?

Also, do any of these detectors count neutrons (which are very bad)? Is a
geiger tube detector better than solid-state (or vise versa) and under which
conditions/uses is either "better" than the other?

How is a roentgen (one measure of radiation) related to a REM (which I
believe stands for Radiation Effective to Man)? What dose of roentgens/REMs
in bad/worse/fatal? (I have been told that more than 350 roentgens is
bad/fatal)

As you can tell, I have lost of questions that perhaps one of you can answer
for me. I have used a solid-state commercial-grade unit (belongs to a friend)
and it was quite interesting to use. One of the "funniest" things occured
when we tested another friend's rock/mineral collection....we found that two
of his rocks (uranannite and cuprosklodouskite (both probably spelled wrong))
gave off MUCH more radiation than the normal background. This device measured
the radiation in "counts" or "events" (which I understand to be the detection
of an alpha/beta/gamma of greater than some threashold energy). Normal
background was about 12 counts/minute....these rocks were on the order of 7000
counts/minute! The rocks now reside in a metal box in the garage. My friend's
wife went through a nite of severe depression when she was just sure that they
would never have kids! She has completely forgotten the whole episode now.

Anyway, all practical answers/info is appreciated. I undertand the difference
between alphas (ionized helium nuclei), betas (high energy electrons) and
gammas (very high energy photons) and why fast neutrons are so VERY bad (affect
the nucleus of an atom, rather than just the electron shell). Any info
concerning the "practical" aspects of radioactivity, radiologcal health, geiger
counters, etc. will be appreciated.

All follow-ups have been directed to sci.electronics.


Thanx in advance!




Neal

bourland@godot.radonc.unc.edu (J. Daniel Bourland) (04/26/89)

A geiger-muller counter (GM) will count any ionizing radiation that is
able to penetrate the wall of the detector.  This includes gamma and x
rays down to 10's of keV, higher energy beta (not from tritium) and
few alphas.  Some GMs have a thin window in the end of the tube (a thin-
end window GM) which allow much more beta and more alphas to enter
the tube and be counted.

In terms of hazard, gamma, x, beta, alpha, and neutrons can all be
of sufficient danger if the exposure route is the "right" one as well
as the dose rate.  For instance, with gammas the risk is for external
exposure although internal exposures can occur.  With alphas the risk
is for internal exposure of the lung through inhalation.  So it can
vary - you should be careful about saying this one is safe and this one
is bad.

The roentgen is a measurement of ionization per mass of air, specifically
2.58 x 10^-4 coul/kg of air (a coulomb is a lot of charge) and is valid
only for photons.  Your GM counter's output is one click per ionizing
event, regardless of the amount of energy deposited, thus the GM
really just gives you counts per minute or hour or something like that.
The GM does not discriminate between photon, beta or alpha radiation
in terms of output - if the particle makes and ionizing event in the
GM, it is counted.  For this reason, the R/hr scale is true only for
the isotope for which the GM is calibrated (usually Cs-137, Co-60 or
Ra-226), but it is still accurate for isotopes having energies near
to that of the calibration isotope.

With regard to the rem, the rem is a unit for radiation protection purposes,
and is the dose in rads times the quality factor (QF) for the radiation
in question.  QF is an indicator of biological effect, relative to the
effect for photons or electrons (which have a QF of 1).  For neutrons
QF = 10, for alhpas QF = 10 to 20.  As it turns out, 1 rad is about
1 roetgen.  So a rem about equals a roetgen if the QF is 1, which works
for photons and electrons.  For alphas and neutrons, it will depend on
the QF, which does vary with energy.

Your GM will detect neutrons if the interact in the wall of the detector.
Neutron detectors are usually surrounded by a sphere of plastic to
stop the neutrons for counting - it is very tricky dosimetry.

All for now.  For those participating in this mini-series on radiation
instrumentation and dosimetry, please send $10.00 to the address below.
The honor code is in effect.  Thank you.

Dan Bourland, Physicist		bourland@godot.radonc.unc.edu
Dept of Radiation Oncology
CB 7512
Chapel Hill, NC 27599

martens@wasatch.utah.edu (William G Martens) (04/26/89)

> I have recently decided to purchase a geiger counter, for general use and for
> possible survival applications. I have seen several different types of devices

Heath kit sells a general purpose radiation monitor that features a small
geiger-mueller tube with a mica window. It is suppose to detect soft x-rays,
alpha, beta and gamma rays, (will not detect neutrons). It comes as a built
it yourself kit, (piece of cake to put together), and resales for around
$130.00, (the exact same model is featured, pre-built, from Edmunds Sci. for
another $100.00 if money is no object).

William Martens
UofUtah Comp Sci

henry@utzoo.uucp (Henry Spencer) (04/27/89)

In article <5499@lynx.UUCP> neal@lynx.UUCP (Neal Woodall) writes:
>How exactly is a "roentgen" defined? Is it in terms of energy dose, or just
>in terms of "counts"...
>How is a roentgen (one measure of radiation) related to a REM (which I
>believe stands for Radiation Effective to Man)?

There are actually three units involved.  The roentgen is simply a unit
of radiation energy, as I recall.  The rad is, I think, "roentgen absorbed
dose", which is more related to how much of that energy gets absorbed.
And the REM is "roentgen equivalent [for] man", which corrects absorbed
energy for the biological effectiveness of different types of radiation.
REMs are what you really want to know about for health purposes, but the
instruments usually measure roentgens or rads and you have to know
the type of radiation to get REMs.

> What dose of roentgens/REMs
>in bad/worse/fatal? (I have been told that more than 350 roentgens is
>bad/fatal)

Background levels are very small, normally measured in milliREMs.  Hundreds
of REMs are very bad news.  Humans vary, so the most you can get is a
statistical measure, the LD50 -- how big a dose will be lethal to 50% of
a population.  That's something like 250-300 REMs, as I recall.  Some
sources give a tentative rule of thumb that it is reasonable to accept
a once-in-a-lifetime dose of 100 REMs in an emergency with human life
at stake.  That is unlikely to kill you but likely to make you sick (later).

All this is based on rather fuzzy memory; corrections welcome.
-- 
Mars in 1980s:  USSR, 2 tries, |     Henry Spencer at U of Toronto Zoology
2 failures; USA, 0 tries.      | uunet!attcan!utzoo!henry henry@zoo.toronto.edu

mandel@uxe.cso.uiuc.edu (04/27/89)

> /* Written  5:49 pm  Apr 25, 1989 by neal@lynx.uucp in uxe.cso.uiuc.edu:sci.physics */
> /* ---------- "Radiation Detectors/Counters" ---------- */
> 
> 
> I have recently decided to purchase a geiger counter, for general use and for
> possible survival applications. I have seen several different types of devices
> offered for sale, including some that are old US Civil Defense surplus, some
> that are based on solid-state sensors that are obviously commercial-grade
> units, and some that are advertised as being European/NATO surplus (brand new,
> never opened).
> 
> Now for questions/confusion:
> 
> I believe that the geiger counters that I have seen can be broadly classed
> "radiological health" units (measure in the .5 to 500 milli-roentegen/hour
> range and are not "hardened" or weather-proof) and those that are intended
> for military/war conditions (measure in the 1-500 roentgen/hour range and are
> "hardened" and weather/water proof).

   This is basically correct.  The high range instruments will not be useful
to you unless you happen to be near a nuclear explosion :-)

> 
> How exactly is a "roentgen" defined? Is it in terms of energy dose, or just
> in terms of "counts" (ie, gamma striks regardless of energy, also do
> beta/alpha strikes count, even though they are obviously not as bad as
> high-energy gammas)? What is considered normal "background radiation" in
> roentgens?

  The Roentgen is defined as the amount of x or gamma radiation which will
produce 1 E.S.U. of ionization in a cc of air at STP.  The Roentgen is NOT
DEFINED FOR beta, neutron or alpha radiation.  Normal background radiation
depends on where you live.  An average for the U.S. is about 50 milliroentgens
per year, but that's only from external gamma and x-rays.  
> 
> Also, do any of these detectors count neutrons (which are very bad)? Is a
> geiger tube detector better than solid-state (or vise versa) and under which
> conditions/uses is either "better" than the other?
> 
   G-M detectors are insensitive to neutrons.  G-M are best for detecting 
> beta radiation.  Solid detectors such as NaI are best for x and gamma rays.
> How is a roentgen (one measure of radiation) related to a REM (which I
> believe stands for Radiation Effective to Man)? What dose of roentgens/REMs
> in bad/worse/fatal? (I have been told that more than 350 roentgens is
> bad/fatal)
> 
  One roentgen (1 R) of x or gamma radiation deposits about 87 ergs per gram
of air.  The same exposure will deposit about 93 ergs per gram of soft tissue.

The roentgen is a unit of EXPOSURE.  In order to relate x or gamma exposure 
to ABSORBED DOSE, we use the rad unit (Radiation Absorbed Dose).  ANY type
of ionizing radiation dose can be characterized by the amount of energy 
deposited per unit mass of material (ANY material).  One RAD is equal to 
an absorbed dose of 100 ergs per gram.

Since some type of radiation are more effective than gamma rays in producing
biological effects, we need a way to "normalize" the absorbed dose units.
We do this by converting the absorbed dose in RADS into a "dose equivalent"
in REMS.  In general, it is assumed that 1 RAD of x or gamma ray dose yields
1 REM of dose equivalent.  On the other hand 1 RAD of thermal neutron dose
yields 3 REM of dose ezuivalent.  1 RAD of fast neutron dose yields 10 REM
dose equivalent.  One RAD of alpha dose yields 20 REM of dose equivalent.
One RAD of beta dose yields 1 REM of dose equivalent.

The conversion factor used for converting from RADS to REMS is called the
Quality Factor (QF).  The QF is defined in terms of the linear energy transfer
of the type of radiation in question (amount of energy transfered to the 
medium per unit path length).

Regardless of the type of radiation in question or the means of exposure, a 
REM produces the same amount of risk.  (Actually there may be other 
"modifying factors" which might be applied in the conversion of absorbed 
dose to dose equivalent in certain situations, but it would be beyond the scope
of this note to discuss them here.)

Great care should be excercised when using geiger counters for determination
of dose or dose equivalent.  G-M's are, by nature, only flux-measuring devices-
they do not measure energy deposition.  G-M's should only be used for 
qualitative assessments such as contamination surveys unless specifically
calibrated (or "energy-compensated") for specific situations.

The LD-50 dose (lethal within 30 days in 50% of those exposed) is about
400 rads (can vary greatly depending on the individual and medical attention).
The average annual NATURAL background dose equivalent is on the order of 
300 millirem per year (this includes RADON).

ron@hpfcmgw.HP.COM (Ron Miller) (04/27/89)

> 
> How exactly is a "roentgen" defined? Is it in terms of energy dose, or just
> in terms of "counts" (ie, gamma striks regardless of energy, also do
> beta/alpha strikes count, even though they are obviously not as bad as
> high-energy gammas)? What is considered normal "background radiation" in
> roentgens?

REM is Roentgen Equivalent Man which is a correction factor for health
affecting dosage. For example, if you were a worker in a plutonium handling
facility, your exposure to gammas would be predictable via a Roentgen
measuring device and the exposure calculated by a correction factor for
the known energy of the gammas from Pu. (if any, I don't have references
handy) Nuclear powerplants utilize enough dosimeters that the correction
factor is already scaled in and many of the personal dosimeters read in
REM.

BTW, the Federal limit for exposure for Radiation Workers is 5 REM per 
year accumulation. The maximum allowable for lifetime dosage is
5 REM per year for every year of age over 18. Extremely localized body
parts have their own limits but 5 REM is the whole-body allowed dosage.
(Hands and fingers have higher limits because the risk to bone marrow
and blood producing organs is reduced.)

Alpha strikes in a G-M detection are non-events. The particle can't 
get into the tube!

Beta can be screened from alpha if you  insert a piece of paper between
the radiation source and the counter. (paper stops beta, counter is 
a scintillation detector with a mylar window)


> 
> Also, do any of these detectors count neutrons (which are very bad)? Is a
> geiger tube detector better than solid-state (or vise versa) and under which
> conditions/uses is either "better" than the other?

Neutrons are very hard to count. Typically they are "fast" and can't be
counted until they are slowed. Polyethylene is used to slow (thermalize)
fast neutrons and then typically a boron-triflouride detector is used.

My training was with G-M detectors so I don't know about "solid state" types.

> 
> How is a roentgen (one measure of radiation) related to a REM (which I
> believe stands for Radiation Effective to Man)? What dose of roentgens/REMs
> in bad/worse/fatal? (I have been told that more than 350 roentgens is
> bad/fatal)

350 REM in a single exposure ought to be fatal. 100 induces radiation sickness
for most people. Of course these are statistical conclusions. Your 
exposure may vary! (Makes you wonder what happened to the statistical people)

> 
> 
> All follow-ups have been directed to sci.electronics.
> 
> 
> Neal

I used to know a lot more when I was a practicing Nuke. Of course my
knowlege was simply about the conditions and equipment in the powerplant.
Given a nuclear attack, I'd be only slightly more knowlegeable than 
anyone else about what radiological problems exist.

Ron Miller 

strong@tc.fluke.COM (Norm Strong) (04/29/89)

In article <1667@wasatch.utah.edu> martens@wasatch.utah.edu (William G Martens) writes:
}> I have recently decided to purchase a geiger counter, for general use and for
}> possible survival applications. I have seen several different types of devices
}
}Heath kit sells a general purpose radiation monitor that features a small
}geiger-mueller tube with a mica window. It is suppose to detect soft x-rays,
}alpha, beta and gamma rays, (will not detect neutrons). It comes as a built
}it yourself kit, (piece of cake to put together), and resales for around
}$130.00, (the exact same model is featured, pre-built, from Edmunds Sci. for
}another $100.00 if money is no object).

I was unaware that Beta particles would penetrate mica.  Incidentally, how
does one measure neutron radiation?
-- 

Norm   (strong@tc.fluke.com)

neal@lynx.uucp (Neal Woodall) (04/29/89)

In article <1240005@hpfcmgw.HP.COM> ron@hpfcmgw.HP.COM (Ron Miller) writes:

>Alpha strikes in a G-M detection are non-events. The particle can't 
>get into the tube!

I have seen Geiger tubes with mica end windows for detecting alphas...the
alphas will indeed pass through the mica.



Neal