FAQ: How hot is it? What do we tell them?

[Richard Hull]

I have had any number of neophytes ask me, " how hot is hot"? Is it dangerous? Why does my friend get totally different readings off the same item than I get?

These questions will never be answerable to the satisfaction of the uniformed stuff on this planet. Why? You can't answer them or have them understand your answer without a small course in radiation measurement and health physics. Finally, there is so much subjective information, imaginitive hype and prejudice brought to the radiation table that you might never, even after a long, well delivered diatribe, give an cognized answer.

However, we will speak for the moment about scientific answers to some of the above questions.

The obvious reason that two persons measure the same item and obtain vastly different results are they have used different instruments and often even different detection methodologies within the different instruments.

One must consider two things when measuring something that is radioactive.

1. What is the source. What are the expected radiation types
2. What is the detection method. What radiations are the method applied sensitive and insensitve to.

The source could be uranium or thorium ore or refined uranium and thorium in the form of a chemical or elemental metal. (Two equal masses of each form will have vastly different radiation readings!) The source could be a refined daughter product like radium with its host of confusing and confounding daughters along for the ride. The source could be a long-lived, man made radionuclide like Co60, Cs137, etc.

Many and complex are the issues here and only a bit of study will help you figure out what you are looking at in a radiation measurement based on the instrumentation at hand. Knowing your source and the specific radiation you wish to measure will certainly help you select the best instrument for the job or, at least, choose an instrument that will not ignore the specific radiation reading you seek.

Instruments are equally far ranging in type and even using a single specific detection method will not warrant identical readings between different instruments using identical methods. Sound complicated? You bet....It is.

An example is given:

Many rad mineral collectors insist, perhaps wisely, that they exchange readings using a stock CDV700 series GM counter. The wise point is that there are millions of this common instrument out there and using so common a device should keep some modicum of understanding about relative values of how "hot" one mineral is versus another. This is nice for collectors in trading and sales deals, but is scientifically rather silly from a radiological standpoint. Why?

The CDV 700 can really count only Beta radiation and even then only a significant fraction of it, maybe 80-90%. It can also detect maybe only 5% of the Gamma radiation and 0.0% of Alpha radiation. This is because the GM tube inside the probe is made out of metal for ruggedness sake.

Let us say a piece of U ore measures 35,000 cpm on a CDV700 with the tube's grill opening in contact with the piece of ore being measured.

U and Th ores are seething with many forms of radiation and a large number of daughter products in them.

Using a 2" pancake GM detector with a thin mica window, we could easily count 270,000 cpm on this same sample!! Why?!!

You are counting a huge fraction of the alpha radiation, (over 95%) now and virtually 100% of the beta radiation and you are counting over a much larger surface area of the sample. You are also counting just a fraction more of the gamma rays, as well. If you now grabbed a scintillator which counts only gamma radiation, for the most part, you might find it is counting at the rate of 150,000 cpm! If you add the two pancake and scintillator readings, you get a much better idea of the amount of disentegrations that would actually be striking or entering the palm of your hand if holding this item. (About 420,000 particles per minute.) OUCH!

Did the CDV700 lie to you? NO! It did a fine job of giving a reading of most of the beta radiation from the ore impinging on the tube's smallish surface area. Did the use of both the pancake and GM counter give a highly accurate indication of radiation from the piece? NO! They gave yet another under estimate of the radiation from the piece, though far closer to some sort of realistic idea than the CDV.

One can never really get a firm fix on the precise number of particles being emitted without a great deal of effort and the use of several instruments and any number of techniques in using them and applying good mathematical data intergation techniques to arrive at an answer.

Thus, in effect, all instruments will not tell a true tale of particle emission.

If we change over to scientific radiation health physics we have a better base for determining radiation damage in the form of "absorbed dose" and biologically adjusted damage factors for various radiations from instrumentation. Now, we can determine how dangerous a source is with some precision based on dose factors using a single instrument and intelligent interepretation of readings.

The CDV instruments often contain a matching mr/hr scale superimposed on the CPM scale of their meter. This scale has been calibrated using a known source of gamma radiation so that area surveys of large amounts of localized penetrating radiation can be detected even at 5% efficiency, as noted above. So, for general sweeps in a post holocaust scenario, this meter can serve as a sort of instant indicator of relative dose rates of penetrating gamma radiation. It can also be used in close contact to determine direct fallout radio nuclide beta emitters as contaminants on food or other items that might be handled or consumed.

So, back to some of the original questions.

How hot is hot? It depends on what you are looking at and what you want to call hot.
Anything above background (30-100cpm) is radioactive, but we are back to, what is hot? Is it 5 times background, 20 times, 100 times? There is no non-subjective answer here. We assume that you are not an ALARAist, but a reasonable being who is determined to work with radiation and seeks reasonable working level scenarios. Plus, we assume you are not a nuclear worker and will only rarely encounter radiation sources. We move on........

Is it dangerous? To what? Will it kill you? If you are talking unwarranted exposure then all radiation by the ALARA doctrine is dangerous. If you are talking about probabilities of long term side effects from very limited exposures this is another area of contention. However, if you are talking about rather noticable exposure effects in the short term, then we can help out. Beta burns for moderate exposure periods might be seen on samples measuring greater than 500mr/hr if in contact with the skin. (You should never encounter such a source!) If serious burns are the criteria then beta samples of 10R/hr or more are really nasty within 1-3 feet of you.

Whole body effects like blood changes occur with whole body exposures to over 10R but under 50R. So, is it dangerous?

Certainly a piece of U ore reading 30mr/hr is not dangerous at all even if handled for a minute or two. Putting the item in your pocket for 12 hours might see a small redness patch develop on your skin due to a very mild beta burn the next day. Who among us is so stupid as to do this, though?

In the end, know your source and respect its particular characteristics that might prove harmful. Always store any small calibration or test sources in a safe place and in a tightly sealed, shielded container, especially if they are what you might consider heavy emitters. Otherwise, just store smaller sources in a manner where the worst of their radiation will not penetrate to where you or other people spend a large fraction of daily time.

Learn not only about your sources, but about all the radiation instruments that are used to quantify and measure radiation. Tell others who ask questions like the above, that you just can't answer them well based on insufficient understanding on their part and your inability to know what they call hot or dangerous. Most of my questions along this line come from folks shown fiestaware of U ore for the first time with a GM ocunter. To many of them, clicking is bad; for they have no reference point and no knowldge to fall back on, but instead, lots of hyped fear and misconceptions.

None of us is ever likely to have a really dangerous or harmful source, thank goodness. However, knowledge is a powerful tool in making radiation measurements and appreciating our responsibility in working with it.

Richard Hull

[John Futter]

Richard
I like to think about disintegrations per sec ie nucleus decay to another lighter nucleus.
ie becquerels. This defines how hot or not something is.
Measurement of of the disintegrations is where it gets hard with no detector showing 100% quantum efficiency or anywhere near it for that matter.

Nice FAQ

to really scare the the uninformed a good detector will find the natural decay of potassium and carbon that we all ingest so yes we humanoids are also hot --- well --maybe luke warm.

[Richard Hull]

Four pi alpha and beta detection can approach near 100% detection efficiency inside special proportional chambers. The thinest mica windowed GM detectors can far exceed 96% efficiency for all alphas impinging on it. (note source needs to be in virtual contact.)

There are lots of special provisos and conditions for the above statements. For alpha and beta counting over 90% efficiency is very easy in many common counter situations. Such efficiencies are based on the number of particles impacting the window and entering the sensitive volume versus the counts made, as long as the number of particles do not exceed an arrival rate anywhere even close to the detector's dead time. For most mica windowed GM counters this would be on the order of 100 to 500Bq.

Gamma counting is far more difficult if efficiency is what you are after. Neutron counting is far worse, still.

Richard Hull

[AFW]

A few years ago, I went on a trip round our local nuclear power station ("Sizewell B"). At the end of it, we were shown a tub of potash fertiliser, bought that morning from a garden centre. A probe, placed against it, showed that it was detectably radioactive (I'm not sure what kind of probe it was, probably one with a mica window). We were told that it would have to be disposed of as low- level nuclear waste, now that its activity had been demonstrated!

Tony Webb

[Richard Hull]

This is all too typical. I was told by the folks at the local nuclear power plant that anything over a certain count found on site is disposed of as waste. They could not bring in fiestaware for training class demo's or it would be taken and disposed of. Rules are rules, I guess.

They had to purchase sources and have them logged as teaching aids with full accountablitiy in order to have class room demos.

Richard Hull