rzh@lll-lcc.UUCP (Roger Hanscom) (04/27/89)
I'm not a radiation physicist, but I have had some experience with some
of the issues discussed in "Radiation Detectors/Counters" by neal@lynx.UUCP
(Neal Woodall)
The radiation sciences have an interesting problem. While particles
and radioactive decay are relatively easy to measure and the units used
in these measurements can be readily defined, the effects of these
phenomena on human tissue are much more complex and poorly known. REMs
vary with the tissue that is exposed to radiation. Hands, limbs, etc
can take higher levels of REMs than gonads, for instance. It is my
understanding that there are government defined threshold levels of
radiation dosage, but there is often no real knowledge, proven over
extended periods of time, of whether these levels are appropriate --
or even safe. There is probably some variability, between organisms,
as to what "safe" levels are. So, when you are in the realm of particles
and radiation, it is a hard science. When considering the effects
on living organisms, things are less concrete. However, one can say
(bogus numbers here, just an example) that 100 gigachurps will kill
a human, but 10 won't. At least you won't die on the spot, but will
exposure to 10 kill you in 30 years? Does anybody *really* know?
And, even worse, where does the government draw the line between 10
and 100 as the "safe" threshold?
On radioactive mineral specimens: The radiation from them is one
thing to be careful of, but I'd be more concerned with the by-products
of their radioactive decay .... radon and other nasties, for example.
When I was a graduate student, Cliff Frondel (one of *THE* experts
on rare radioactive minerals) had a row of cabinets in his office
(in the Museum there at Harvard University) that housed one of the
world's most complete collections of naturally radioactive materials.
He was in his office a lot. The Health Services people looked it over,
and only forced him to install an exhaust fan nearby! Are there any
mineralogists out there?? Is Cliff still alive (he was approaching
retirement age at the time -- 20 years ago)?
roger rzh%freedom.llnl.gov@lll-lcc.llnl.gov
ucbvax!lll-lcc!freedom!rzh
Upstairs, Over a Vacant Lot, Inc.john@stiatl.UUCP (John DeArmond) (04/27/89)
In article <2449@lll-lcc.UUCP> rzh@lll-lcc.UUCP (Roger Hanscom) writes: > > I'm not a radiation physicist, but I have had some experience with some >of the issues discussed in "Radiation Detectors/Counters" by neal@lynx.UUCP >(Neal Woodall) > Well I am a health-physicist (or was in my previous profession), so let's bring some science to this discussion. First some definitions. Roentgen - (pronounced ren-ken) The unit of measure of radiation exposure. It is defined as that quantity of X-Ray or Gamma radiation necessary to deposit 1 statcoulomb of charge per cubic centimeter of air at STP. In terms of energy, this corresponds to an absorption of 87.7 ergs per gram of air at STP. Note that this unit is a measurement of energy deposition and is in no way related to biological effects. Radiation exposure meters and dosimeters are normally roughly calibrated in units of R/hour or mR/hour. Also note that this unit is defined only for Gamma or X-Ray; it has no meaning for particle radiation (alpha, beta, neutron). RAD - Radiation Absorbed Dose - The unit of measure of absorbed dose. This unit takes into account the absorption coefficient of the target material. For water and tissue, the conversion factor of Roentgens to RADS is roughly 1. The RAD is applicable to all radiation including particle fluence such as alpha or beta. This unit measures deposited energy in human tissue but does not indicate effect. REM - Radiation Equivilent Man - This is the unit of measurement of the effect of radiation on living tissue. It takes into account the relative biological effects of the different types of radiations. The REM is simply defined as follows: REM = RAD * QF * DF Where QF is a dimentionless quality factor of the radiation that specifies the radiation's relative effect on living tissue. Some QFs are: Gamma from Radium 1 X-Ray 1 Bata or electron > 0.03 Mev 1 Beta or electrons < 0.03 MeV 1.7 Thermal neutrons 2 Fast neutrons 10 Protons 10 Alpha (internal exposure 20 Heavy ions (Accelerat- ors) 20 DF is the distribution factor which deviates from 1 only for specific bone or organ seeker isotopes which can cause locally heavy exposure to critical organs such as bone marrow. REMs are NOT directly measurable and must be calculated or estimated from available information on the radiation composition and energy. For the purposes of radiation protection, external exposure is evaluated with a QF of 1 unless neutrons are present, which are evaluated at 10. (fast neutrons are very rare outside a fission reaction environment). You should also know that there has been a move to adopt SI units of measure for these quantities. These new units such as the Seivert and Gray have not to date been accepted in the community and at this point serve only to cause confusion. Therefore I will make no more mention of these other than to make you aware of them in case you see them on some instrument or paper. > The radiation sciences have an interesting problem. While particles >and radioactive decay are relatively easy to measure and the units used >in these measurements can be readily defined, the effects of these >phenomena on human tissue are much more complex and poorly known. This is false. The effects of radiation on humans are more precisely known than almost any other toxin. The effects have been characterized over a range of perhaps 12 orders of magnitude. This compares very favorably to the more common 3 to 4 orders of magnitude for chemical toxins. At one end are the effects of massive acute overexposure which leads to delayed or prompt death. At the other end are environmental (natural) radiation levels which produce no known effects as documented by statistical studies of the population unrivaled by any other effect. When Health-Physicists say that the effects of low level radiation are unknown, they are refering to those levels approximately equal to the naturally occuring radiation. A proof of the negative is difficult if not impossible, so HPs hedge toward the unknown in this area. These studies are the equivilent of throwing one grain of cyanide into a river and then trying to evaluate its effect on the population. REMs >vary with the tissue that is exposed to radiation. Hands, limbs, etc >can take higher levels of REMs than gonads, for instance. Extremities are permitted to receive higher dose because there is no significant blood producing tissue in the hands or feet. The concern is absorbed dose to the bone marrow. One one form of cancer that has been linked to radiation at high exposure levels is leukemia. Male gonads are not of particular concern for radiation protection (tough little suckers :-). Female gonad exposure is of concern because of the possibility of unknown pregnancy. First trimester fetuses (feti?) are known to be suceptable to radiation-induced mutations. It is my >understanding that there are government defined threshold levels of >radiation dosage, but there is often no real knowledge, proven over >extended periods of time, of whether these levels are appropriate -- >or even safe. There is probably some variability, between organisms, >as to what "safe" levels are. So, when you are in the realm of particles >and radiation, it is a hard science. When considering the effects >on living organisms, things are less concrete. Again, false. Risks are extremely well characterized. I refer interested parties to the National Academy of Sciences Committee on Biological Effects of Ionizing Radiation known as the BEIR III report. This report deals with both the somatic effects (cancer) and genetic effects of radiation. The committee agreed unanimously that any somatic effects for radiation approximating background levels (100 mrads/year) are masked by other factors such as the normal cancer rate of the population. The committee also agreed that no radiation-induced genetic effects have been observed in man and none are expected. Some of the Neo-Luddite Safty Nazis have tried to discredit this report because it destroys their cause against low level radiation but the report is considered by most health-physicists to be the authorative work in this area. Other agencies that have evaluated the risks are the International Council on Radiation Protection (ICRP) and a United Nations agency UNSCEAR. The BEIR report contains extrapolated risk coefficients for cancer and genetic effects of radiation. The coefficients were extrapolated based on statistics on a population of high exposure people. This extrapolation is based on an assumption that the effects of radiation scale linearly with rate down to 0. We know that this assumption is false and excessivly conservative but the concensus is to err in the direction of safety. It is important to realize when evaluating the effect of radiation that there IS a threshold effect. This means that there is a rate of exposure below which there is no effect. For example, the amount of radiation exposure necessary to kill about half the exposed population is about 600 RADs if delivered in a short time. If, on the other hand, 600 RADs are delivered to a population over 60 years (10 RADS/yr), no one will die from acute radiation poisioning. Or to put this in common terms, consider the common bullet. The unit of measure of the energy delivered by a bullet to a target is the foot-pound. A typical rifle bullet can deliver perhaps a thousand foot-pounds of energy. If you are shot and this energy is deposited in your chest, you will probably die immediately. If on the other hand, you drop the bullet from 1 foot onto your chest and repeat it several thousand times, you will not be killed or hurt (you might be bored to death :-) even though you will still have received 1000 ft-lbs of energy. This is because the RATE of delivery is much lower. Radiation is the same. For the purposes of protection, however, we assume that a bullet dropped a thousand times from a foot will kill you just as if you had been shot. We call this "conservatism". However, one can say >(bogus numbers here, just an example) that 100 gigachurps will kill >a human, but 10 won't. At least you won't die on the spot, but will >exposure to 10 kill you in 30 years? Does anybody *really* know? >And, even worse, where does the government draw the line between 10 >and 100 as the "safe" threshold? > Well, if you want an excrutiatingly (sp) detailed answer to this question, order a copy of the BEIR III report or a copy of 10 CFR 20 from the US GPO. You should also obtain a copy of one of the reference texts listed at the end of this article which will teach you how to interpret this data. > On radioactive mineral specimens: The radiation from them is one >thing to be careful of, but I'd be more concerned with the by-products >of their radioactive decay .... radon and other nasties, for example. These specimens are completely safe. The uranium or thorium in the ore does emit some atoms of radon or thoron but the quantity is so tiny as to be insignificant. This is equivilent to worrying about the tiny amount of cyanide in almonds. >When I was a graduate student, Cliff Frondel (one of *THE* experts >on rare radioactive minerals) had a row of cabinets in his office >(in the Museum there at Harvard University) that housed one of the >world's most complete collections of naturally radioactive materials. >He was in his office a lot. The Health Services people looked it over, >and only forced him to install an exhaust fan nearby! I suspect this is a typical example of the hysterical reactions from those not trained in the profession to perceived but nonexistent radiation hazards. Instead of consulting a professional to evaluate the risk, they knee-jerk. (if this guy had a couple of tonns of yellowcake in the room, I'll retract the above statement but I don't think I'm at much risk here). Now to answer the orginal question of the poster. As I recall, you had access to a Civil Defense survey meter and some other more sensitive meter. Both of these meters use Geiger-Mueller tubes as the sensitive element. These tubes are sensitive to beta and gamma radiations. Because these tubes produce the same signal regardless of the energy or type radiation, they cannot be directly calibrated in units of dose. They can be calibrated in arbitrary counts per minutes or if the effeciency and geometry of the tube is known, in radioactive disintegrations per second. The display is also likely graduated in units of mR/hr. Since the tube produces the same signal in response to all types of radiation, this cannot be a direct calibration. What is commonly done is relate the mR/hr scale to the gamma radiation produced by Co-60 or Cs-137. Other types or energies of radiation will produce erroneous reading. The reading will generally be accurate within a factor of 2 which is enough for the intended purpose. Note that this calibration is only valid if the window on the detector is closed. This window screens beta particles which have no meaning in terms of mRs. If the window is open, then the display can only display counts per minute. You have discovered the natural radioactivity in pitchblende. There are many other things around the house that are radioactive. A Coleman Lantern Mantle is coated with Thorium Oxide to enhance its light output and is radioactive. They make good checksources for such instruments. Many orange ceramic glazes contain uranium oxide to produce the rich orange color. These are fairly hot. The Fiestaware china that used to come in Fab detergent is very hot. You will find this stuff at antique flea markets and is considered collectible. If you want to be nasty, take your geiger counter to a flea market and demonstrate to the seller how radioactive it is. He'll probably give it to you! I have a couple of orange flower vases that are hot. Red brick usually contains uranium. I can place the probe of a geiger counter in the firebox of my fireplace and about triple the reading over background. Your smoke detector is, of course, radioactive, though you would have to get the geiger tube inside the housing to detect it. Many old clocks and compasses still have radium dials. I have a couple of old Big Ben clocks that are fairly hot. Neon lightbulbs contain thorium to aid in firing in the dark. The electrodes in these are mildly radioactive. Any WWII-era military relic that contains luminous paint contains radium. Compasses and aircraft instrumentation are prime candidates. This stuff is fun to play with and harmless. You can get all kinds of reactions from your friends when you go snooping around with your geiger counter. You should not, simply as a matter of practice, eat this stuff :-) but as long as you don't, there is absolutely no hazard. In order to scale the readings consider these numbers. Your geiger counter is probably calibrated up to perhaps 500 mR/hr. You take the indicated rate and multiply it by the hours of exposure to determine accumulated exposure. Some interesting statistics relating to accumulated dose: Maximum annual exposure to the civilian population 0.005 REM (5 mREM) Average annual background radiation exposure 0.5 REM Maximum annual occupational exposure to radiation workers 5.0 REM (The rest of the numbers assume acute exposure, typically less than a day) Minimum detectable radiation effect using blood tests: 10 REM Onset of macro changes in blood composition 25 REM Mild lethargy and ill-feelings 100 REM Mild nausea 200 REM Onset of classic radiation sickness (acute nausea, diarrhea, hemmorage) 300-400 REM LD-25 (lethal dose to about 25% of the exposed population) 500 REM LD-50 600-700 REM LD-95 1000 REM immediate death ~3000-5000 REM (est) (scrambled nervous system) If you would like to know more about radiation protection and instrumentation than you ever though possible, obtain a copy of the book "Introduction to Health Physics" by Herman Cember (Pergamon Press, ISBN 0-08-030936-4). This is the seminal work on health-physics and is used in most classroom situations. It should be available from your nearest university bookstore. Other texts you might want to look at are: Radiological Health Handbook Public Health Service Publication 2016 From the US Government Printing Office. (full of all kinds of rules of thumb) Radiation Detection and Measurement Glenn F. Knoll John Wiley & Sons publisher ISBN 0-471-49545-X (seminal work on the subject of radiation detection and equipment) Several publications from the NCRP relating to radiation protection. John -- John De Armond, WD4OQC | Manual? ... What manual ?!? Sales Technologies, Inc. Atlanta, GA | This is Unix, My son, You ...!gatech!stiatl!john **I am the NRA** | just GOTTA Know!!!